Comparative investigation on the adhesion of ... · Comparative investigation on the adhesion of...

Comparative investigation on the adhesion of hydroxyapatitecoating on Ti–6Al–4V implant: A review paper

E. Mohseni, E. Zalnezhad, A.R. Bushroa n

Center of Advanced Manufacturing and Material Processing, Department of Engineering Design and Manufacture, Faculty of Engineering,University of Malaya, Kuala Lumpur 50603, Malaysia

a r t i c l e i n f o

Article history:Accepted 17 July 2013Available online 7 October 2013

Keywords:AdhesionHydroxyapatiteCoatingTi–6Al–4V implant

a b s t r a c t

Hydroxyapatite (HA) has been used in clinical bone graft procedures for the past 25 years. Although abiocompatible material, its poor adhesion strength to substrate makes it unsuitable for major load-bearing devices. Investigations on various deposition techniques of HA coating on Ti–6Al–4V implantshave been made over the years, in particular to improve its adhesion strength to the metal alloy and itslong-term reliability. This review comprehensively analyzes nine techniques mostly used for depositionof HA onto Ti–6Al–4V alloys. The techniques reviewed are Plasma sprayed deposition, Hot IsostaticPressing, Thermal Spray, Dip coating, Pulsed Laser deposition (PLD), Electrophoretic deposition (EPD),Sol–Gel, Ion Beam Assisted deposition (IBAD), and Sputtering. The advantages and disadvantages of eachmethod over other techniques are discussed. The adhesion strength and the factors affecting theadhesion of HA coating on Ti–6Al–4V implants are also compared.

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1. Introduction

Biological fixation is defined as the process where prostheticcomponents become firmly bonded to the host bone by ongrowthor ingrowth without the use of bone cements [1–3]. In the late1960s, the concept of biological fixation of load-bearing implantsusing bioactive hydroxyapatite (HA) coatings was proposed as analternative to cemented fixation. Hydroxyapatite (HA: Ca10(PO4)6(OH)2), a pure calcium phosphate phase, is a preferred biomaterialfor both dental and orthopedics use due to its favorable osteo-conductive and bioactive properties [4,5]. HA has a similarchemical composition and crystal structure as the apatite in thehuman skeletal system, and is therefore suitable for bone sub-stitution and reconstruction [6]. Furthermore, HA has shownsignificant success in implants due to its favorable in vivo behavior[7,8] and the presence of HA films prolongs the lifetime ofprostheses [9]. However, HA coatings are susceptible to fatiguefailure, making it unsuitable for load bearing implants [10,11].

Nevertheless, there is a large demand for implants withexcellent mechanical properties. These implants should possesssimilar properties to the human bones, such as in the value of itsYoung's modulus, which result in less stress shielding effect [12]and extends its service life. The implants can be made into

different shapes such as plates, rods, screws and pins [13].Historically, titanium-based alloys are the most common materialfor this purpose since it is known to be a tolerable metal in thehuman body [14].

Titanium (Ti) and its alloys are the most commonly usedmetallic materials for medical implants in orthopedic and dentalapplications, due to their low density, high strength, non-toxicityand excellent corrosion resistance [15]. However, there have beenreports on inflammatory reaction around these implants as aresult from the creation of an avascular fibrous tissue thatencapsulated the implants [16,17]. A coating of hydroxyapatitelayer can be deposited on the metal alloy to assist the osseointe-gration of these implants with surrounding tissues [16].

The bond strength between the coating layer and the metalsubstrate is a very critical factor. Separation of the coating layerfrom the implant during service in the human body results inadverse effects on the implants and the surrounding tissue causedby detached particles [18]. The main reason of using HA coating onmetallic substrates is to keep the mechanical properties of themetal such as load-bearing ability and, at the same time, to takeadvantage of the coating's chemical similarity and biocompatibil-ity with the bone [19].

According to Blind et al. the HA coating allows rapid osteointe-gration as a result of bone tissue bonding properties [20]. The firstclinical results from HA coatings on titanium dental implants werepromising, showing excellent results, even with poor bone quality.However, after a long period, mechanical failure would occur atthe interface of HA and metallic substrate [21]. The HA coating

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International Journal of Adhesion & Adhesives

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n Corresponding author. Tel.: þ+603 7967 5239; fax: þ60379675330.E-mail addresses: [email protected] (E. Mohseni),

[email protected] (E. Zalnezhad), [email protected] (A.R. Bushroa).

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dissolves as a result of poor crystallized structure [22,23], decreaseof adherence with the titanium surface and dramatic late implantfailure [23,24]. Moreover, HA itself has poor mechanical proper-ties, with a bending strength of less than 100 MPa [25]. Thus, it canbe concluded that the stability of the HA coating is the mostcritical factor to ensure the success of this type of implant.Furthermore, the method used to deposit HA powder onto thesubstrate could influence the coating characteristics such as itsadhesion strength and reliability.

Several techniques have been used to create the HA coating onmetallic implants, such as plasma spraying process [26], thermalspraying [27], sputter coating [28], pulsed laser ablation [29],dynamic mixing [30], dip coating [31], sol–gel [32], electrophoreticdeposition [33], biomimetic coating [34], ion-beam-assisted-deposition [35], and hot isostatic pressing [36]. Amongst thetechniques listed, plasma spraying is the only process which isapproved by the Food and Drug Administration (FDA), USA forbiomedical coatings due to its excellent coating properties ascompared to other coating processes [37]. However, plasmasprayed hydroxyapatite coatings suffer from poor mechanicalproperties on tensile strength, wear resistance, hardness, tough-ness and fatigue. Improvements in plasma spraying techniquesover the years have addressed many of these limitations. However,other coating methods are available which can be used as analternative to conventional techniques.

Limitations such as high porosity, poor uniformity in thickness,phase impurity, limited crystallinity, and poor adhesion are com-mon in HA coating. However, low coating adhesion seems to bethe major issue, limiting its extensive use for implants at acommercial scale [38–40]. Hence, improvement of bondingstrength between the metallic substrate and ceramic coating is ageneral requirement regardless of the techniques used.

This review focuses on adhesion strengths between HA coatingand Ti–6Al–4V substrate, fabricated using various techniques suchas plasma sprayed deposition, hot isostatic pressing, thermalspray, dip coating, pulsed laser deposition (PLD), electrophoreticdeposition (EPD), sol–gel, and ion beam assisted deposition (IBAD).Parameters affecting the adhesion of coating and other factorsinfluencing the enhancement of bonding strength of coating sur-face and the substrate are also discussed in detail.

2. Coating techniques

2.1. Plasma sprayed coating technique

Plasma spraying process involves melting of ceramics or metalpowders using the heat of ionized inert gas (plasma). The moltenpowders are then sprayed onto the surface to be coated, formingthe protective layer which provides a barrier against corrosion,wear or high temperatures. The technique offers advantages suchas low cost and rapid deposition rate [41,42]. In addition, the riskof thermal degradation of the coating and substrate is much lessthan other high-temperature processes since the gas in the plasmaflame is chemically inert and the target can be kept relatively cool[43]. However, plasma sprayed coatings suffers from poor adhe-sion between the coatings and substrates [44], and the processmay induce structural changes in the microstructure of the coatingmaterial [45,46].

2.1.1. Plasma sprayed hydroxyapatite (HA) coatingsPlasma spray was the first method used for the production of

calcium phosphate coatings, such as HA coating, due to its ease ofapplication [26]. Plasma sprayed hydroxyapatite (HA) coatings arebiocompatible and able to bond directly to the bone [38], thus makingplasma spraying a favorable choice amongst the many techniques

available for coating HA layers onto metallic substrates [47]. Recentstudies on plasma sprayed HA coatings (HACs) on titanium haveshown encouraging results in orthopedic implant applications. Thesestudies reported that the new bone could appose directly onto the HAcoatings and very good adhesion between the HACs and the new bonecan be obtained [48–51]. The plasma sprayed HA coatings have alsoassisted in overall quick bone recovery [52].

Nevertheless, the brittle nature of the HA coating makes itprone to crack and fracture, non-uniformity in density of coating[53], wear of the coated layer, weak mechanical adhesion to thesubstrate [44,54], and alteration of structure [55].

Overall, plasma sprayed coating did not show significantimproved long-life performance, better mechanical integrity andreliability over uncoated implants [56,57]. An alternative to plasmaspraying is the pulsed laser deposition (PLD) which enables thestoichiometric transfer of sintered HA yields to form a thin andadherent bioactive coating on titanium substrate surface [58].

2.1.2. Adhesion of plasma-sprayed hydroxyapatite (HA) coatings onTi–6Al–4V

It is well understood that, the determination of the adhesionbetween the substrate and coating is one of the main concernswhen using plasma spraying techniques [59]. It is quite compli-cated that how coating adheres to a substrate and by today it is notcompletely understood. Many theories describe the mechanism ofadhesion, although, there is no single clear interpretation for alladhesion behaviors [60]. Many factors seem to affect the adhesion:(1) Van der Waals physical interaction forces mechanical ancho-rage; (2) mechanical anchorage; (3) metallurgical processes and(4) chemical interaction [59].

Recent reports on alternative orthopedics implant fixationutilizing plasma sprayed HA coatings (HACs) on Ti–6Al–4V haveshown that the new bone was able to appose directly onto the HAcoatings, which resulted in a very good adhesion between theHACs and the new bone [48–51]. From the viewpoint of materialsscience, characteristics of HACs are varied with the sprayingparameters such as phase composition, the microstructure, OH-ion content, crystallinity, and the ration of calcium to phosphorusfor the HACs. Among these parameters, high bonding strength ofHACs can be achieved by high spraying power due to a densermicrostructure caused by the greatest extent of coating melting.

Yang et al. experimented on six plasma sprayed HA on Ti–6Al–4V substrates by varying the cooling conditions and thesubstrate temperatures [61]. The residual stresses and bondingstrengths were measured by XRD “sin2 φ” technique and astandard adhesion test (ASTM C-633). Results of the bondingstrength evaluation shows that the HA coating with the lowestresidual stress exhibited a higher bonding strength (9.1870.72 MPa).

The deposition stress and thermal stress are the two majorsources of residual stresses in plasma sprayed coating. Depositionstresses are produced during the cooling of sprayed particles aftersolidification. Thermal stresses are generated from differentialthermal contraction during the post-fabrication cooling phaseafter coating [62,63]. The residual stresses are present near theinterface of metal substrate and coating [64–66], due to thedifference of thermal expansion coefficients between both materi-als [62,63]. These stresses may vary with substrate cooling effects,parameters of spraying [62,63], and coating thickness [67,68].Generally, it is believed that the increased thickness of coatingand the temperature of the specimen during plasma spraying arethe main reasons for the rise in the residual stress.

In addition, high-powered, dense plasma sprayed HA coatingswould have stronger bonding strength than those sprayed usinglow power. The result is not solely due to the difference in

E. Mohseni et al. / International Journal of Adhesion & Adhesives 48 (2014) 238–257 239

adhesive strength of HA coating. The value for bonding strengthreflects the combination of both cohesive (within the coatinglayers) and adhesive (coating to substrate) strengths of a coating[61]. In a similar study, Tsui et al. claimed that the cohesive andadhesive integrity of the coatings influence the long term perfor-mance of HA coated implants considerably [69]. The adhesivestrength is usually evaluated based on surface roughness, coatingproperties, residual stress, and the mechanical interlockingbetween the coating and the substrates, whereas the cohesivestrength is determined by coating properties, such as microstruc-ture and crystallinity [61].

The bonding strength of HA coatings on metallic substrates canbe evaluated using several techniques such as the standard tensileadhesion test [69], interfacial indentation test [58], tensile adhe-sion strength (TAS) [61], and indentation method [63]. However,there are limitations on these techniques to accurately measurethe adhesion strength, such as a probability of penetration of glueinto the coating layer, and a dependence of coating failure to theflaw distribution at the edge of specimen [69]. However, Moham-madi et al. have demonstrated that the tensile adhesion strengthtest measured by the standard adhesion test ISO 13779-4, can beused in conjunction with the interface indentation test to predictthe effects of different parameters on the adhesion properties ofthe HA coating by plasma spraying [70]. In general, the HAcoatings with the densest structure (i.e. lowest porosity, andpredominantly amorphous phase) have a higher tensile adhesionstrength than those of lower density [61,71]. The report byMohammadi et al. [70] also showed that the tensile adhesionstrength was in the range of �25 MPa for HA coated on theTi–6Al–4V.

2.2. Hot isostatic pressing technique

Hot isostatic pressing (HIP) is an enabling technology providingan efficient method for the densification of ceramic powderswhich allows production of net-shape ceramics with superiorand consistent properties [72]. HIP is an alternative method ofproducing an HA coating on a Ti substrate in which pressurizedgas is used to exert the required load at the desired temperature.This requires a gas-tight metal or glass encapsulation around theporous HA coated implant [73]. In the HIP process, pressure andtemperature are applied to the workpiece simultaneously [74–77].

In hot isostatic pressing, high-pressure levels can be obtainedsince there is no dependency on rigid tools with limited strength(such as graphite tools in uniaxial hot pressing) to transmit thepressure to the body. Typical operating pressure ranges are100–320 MPa (15–50 ksi), with temperatures exceeding above2000 1C conducted in large industrial equipment [72]. The advan-tages of HIP are better temperature control as compared touniaxial hot pressing, and a resultant homogeneous materialstructure and properties. The reduced sintering temperatureenables control or even avoidance of grain growth and undesirablereactions. A very high uniformity of properties as well as freedomfrom directionality can also, if desired, be obtained [72]. Someresearchers have used HIP treatments to densify plasma sprayedcoatings, and results have shown that HIP is useful in reducing theporosity and improving the physical and mechanical properties ofceramic coatings [78].

Thus, the most important advantage of the hot isostatic press-ing is the ability to control the size and shape of the product to avery high precision without costly diamond machining operations.Under ideal conditions no change of shape (just a change of scale)of the body occurs. It has an inherent ability to produce parts withexceptionally accurate shape, virtually with no dimensional orshape limitation [72].

2.2.1. Hot isostatic pressing of hydroxyapatite (HA) coatingsReports shows that, sort of problems such as porosity and crack

appearance are conducted with existing dc plasma sprayed Hacoating on Ti–6Al–4V [79]. In medical applications some amountof porosity is needed for bony tissue to grow into the coating forefficient fixation. In addition, the crack propagation needs to behealed for the composite coating to have reasonable mechanicalstrength during usage. In this sense, HIP introduces its profoundadvantages by improving the adhesion and physical properties ofthe plasma sprayed HA coatings as a post- treatment [79].

2.2.2. Adhesion of hot isostatic pressing of hydroxyapatite (HA)coatings on Ti–6Al–4V

Khor et al. [79] investigated the effect of post-sprayed HIP onplasma sprayed HA on Ti–6Al–4V. Fig. 1 shows the bond strengthsof HA coated Ti–6Al–4V for the plasma sprayed samples, and afterHIP treatment at different temperatures with respect to the coat-ing thickness. In general, it was shown that the bonding strengthgenerally improves after HIP. It is also shown that the adhesionstrength decreases with increasing coating thickness. Theenhancement of the adhesion strength in the 20 wt% HA coatingafter HIP is apparent for coating below 160 μm. However, theresult of adhesion strengths for coatings thicker than 160 μmshow that HIP may have adverse effects on the coating strengths.

2.3. Thermal spray coating technique

Thermal spray technology is a group of coating processes thatprovide functional surfaces to protect or improve the performanceof a substrate or component. Many types and forms of materialscan be thermal sprayed to provide protection from corrosion,wear, and heat; to restore and repair components; and for a varietyof other applications [80]. Thermal spraying of biomedical coatingis a relatively new class of applications for thermal spray coating ascompared with other industrial applications, [81]. Thermal sprayprocesses are grouped into three major categories: flame spray,electrical arc spray, and plasma arc spray. These energy sources areused to heat the coating material (in powder, wire, or rod form) toa molten and semi-molten state. The resultant heated particles areaccelerated and propelled towards a prepared surface by eitherprocess gases or atomization jets. A schematic diagram of thermalspray coating is illustrated in Fig. 2.

Fig. 1. Tensile bond strength result of plasma sprayed Ti–6Al–4V/20 wt% hydro-xyapatite coating (as sprayed and HIPed) [79].

E. Mohseni et al. / International Journal of Adhesion & Adhesives 48 (2014) 238–257240

2.3.1. Thermal spray deposition of hydroxyapatite (HA) coatingsThermal spraying of HAP on implant devices can be compared

with plasma spray coating technique, having the advantage of highdeposition rate and low cost [82,83]. Thermal spray technique hasthe ability to produce HA layer with thickness from 30 to 200 mmdepending on the coating condition However films deposited bythermal spraying suffers from poor coating–substrate adherenceand non-uniform crystallinity which reduces the lifetime ofimplants [84,85]. In addition, thermal spray requires high sinteringtemperature which may result in crack propagation on the surfaceof the coating [86–90].

2.3.2. Adhesion of thermal spray deposition of hydroxyapatite (HA)coatings on Ti–6Al–4V

Hsiung et al. [91] have evaluated the applications and char-acterizations of biological coating such as hydroxyapatite ontitanium alloy, particularly Ti–6Al–4V, in artificial knee joint bythermal spray coating technology. The process involves melting ofHA powder and guiding the molten mass via a jet stream of air toform a coating on the substrate, as shown in Fig. 3. The thermalspray process conditions of the three coating materials are shownin Table 1, highlighting the important parameters affecting thequality of the coating, such as inert gas compositions, currents,voltage levels, powder feeding rates, and spraying distances.

The tensile test is commonly used to evaluate the bondstrength in accordance to ASTM C-633 standard method [92].A bonding strength of 33.2 MPa was obtained by Hsiung et al. [91]for the HA coating on Ti–6Al–4V by thermal spraying technique. Incomparison, this result is not satisfactory when compared to othercoatings for the same application such as Al2O3, ZrO2. In addition,results of microstructure analysis shows that the HA coatingssuffers from spalling, interface separation and high levels ofporosity.

Several pre and post-treatments of HA coating were alsoinvestigated by Hsiung et al. [91]. Treatment conditions includehigh pressure cleaning, ultrasonic cleaning and cryogenic treat-ments [92]. Table 2 shows the result of the bond strength testusing ASTM C-633 [93], indicating the bond strengths of samplescleaned with high pressure air are lower as compared with thoseultrasonically cleaned, and the bond strengths with cryogenictreatments are better than those without cryogenic treatments.The result shows that the inclusion of ultrasonic cleaningand cryogenic treatments can effectively improve the coatingproperties.

2.4. Dip coating technique

Dip coating involves the deposition of a wet liquid film bywithdrawal of a substrate from a liquid coating medium. Thecomplete process of film formation involves several stages, asshown in Fig. 3. The process starts by immersion of the substratein the solution of the coating material. When the substrate iswithdrawn from the coating fluid, a coherent liquid film isentrained on the surface of the substrate. A thin layer of coatingis formed upon evaporations of solvents and any accompanyingchemical reactions in the liquid film. Normally an additional post-treatment such as curing or sintering is required to obtain the finalcoating. Dip coating technique is similar to sol–gel coating tech-nique, although the process is significantly faster in which acomplete transition can be achieved within a few seconds ifvolatile solvents are used [94]. Dip coating is fairly popular inthe industry and in laboratory applications due to its low cost,simple processing steps and high coating quality.

2.4.1. Dip coating of hydroxyapatite (HA) coatingsHA can be homogenously coated onto metal substrates to

obtain coating thickness in the range of 0.05–0.5 mm. The surfaceuniformity of HA can be controlled well using this technique, ascan be seen in Fig. 4. In addition, the processing time for dipcoating can be very short, even for substrate with complex shapes.The coating layer is deposited on the surface of the substratewithout decomposition or reaction with the metal substrate.

Fig. 3. Fundamental stages of dip coating (the finer arrows indicate the flow of air) [94].

Table 1Thermal spray condition of HA powders [92].

Parameters Argon (l/min) Helium (l/min) Current Voltage Powder rate (g/min) Spray distance (mm) Surface speed (m/min) Travers speed (mm) Cooling

Setting 41 60 700 52 30 115 75 8 yes

Table 2Bond strength test results with different pretreatment and cryogenic treatment[92].

Coating Bonding strength ( MPa)

Without cryogenic treatment With cryogenic treatment

Ultrasonic High pressure air Ultrasonic High pressure air

HA 26.56 18.91 36.65 29.30

Fig. 2. A schematic diagram of thermal spray coating [82].


However, this technique requires high sintering post-treatmentswhich may induce crack formations on the surface of the substrate[95].

2.4.2. Adhesion of dip coating of hydroxyapatite (HA) coatings onTi–6Al–4V

Mavis et al. [96] had developed several compositions of theliquid coating medium for the dip coating of HA on Ti–6Al–4Vsubstrates, using chemically precipitated hydroxyapatite precursorpowders. To evaluate the adhesion strength, two steel cylinders5 mm in diameter were attached to both sides (coated anduncoated after the coating layer was ground off) of the dippedstrips by a thin layer of glue. The adhesive strengths weredetermined by measuring the tensile stress needed to separatethe cylinders from the strips [97]. It is reported that, the HAcoatings obtained were highly porous, with bonding strengths ofmore than 30 MPa.

2.5. Pulsed laser deposited coating technique

Laser processing is a rapid and clean process which can be usedfor surface modification and controlled micro-structuring ofmaterials. In biomedical applications, laser has been used tomodify the surface texture of materials to improve its bio-functionality [98–102]. Pulsed laser deposition (PLD) techniquecan be used to grow ceramic thin films. By using appropriate laser,thin films such as semiconductor films [103], cuprate supercon-ductor films [103,104], and ferroelectric films [105] can be depos-ited onto substrates. PLD process involves using high power laserenergy to vaporize the bulk coating material from a target. Thevaporized material is ejected from the target and condenses on thesubstrate. Repeated laser pulses will result in the deposition of thethin film as a coating on the substrate [106].

The formation of thin film by PLS can be separated into thefollowing three stages [103,107,108]:

1. Laser radiation interaction with the target.2. Dynamic ablation of the materials.3. Deposition of the ablation materials with the substrate, nuclea-

tion and growth of a thin film on the substrate surface.

One of the main advantages of PLD technique is the ability toretain the stoichiometry of the target in the deposition films [107].This is due to the high ablation rate which causes all compounds

or elements to evaporate at the same time [106]. Conversely,limitations of PLD include the splashing of the particulatesdeposition on the film. Some methods have been developed todecrease splashing problem since it is a major issues of the PLD[109]. One method is to apply a mechanical particle filter thatincludes a velocity selector acting as a high-velocity pass filter toeliminate slow-moving particulate. The second method is using asmooth, high-density target which can be obtained by polishingthe target surface before each coating run. The third method is byapplying a lower deposition rate or low energy density. Further-more, the deposited films have only a small area of structural andthickness uniformity, due to the angular distribution of theablation plume. Several methods have been proposed to scale upthe PLD process for large area thin films, such as laser beamrasterizing across a rotating target [106].

High quality hydroxyapatite thin films deposited by the PLDwas first reported in 1992 [105,110] and since then the processhave been improved significantly to obtained well adhered andhighly crystalline HA thin films under certain conditions [29,111–113].

2.5.1. Pulsed laser deposited hydroxyapatite (HA) coatingsPreparing hydroxyapatite thin films by pulsed laser deposition

allows accurate control of hydroxyapatite growth parameters atlow deposition temperatures and the ability to produce highlycrystalline HA coatings [16,112]. In-vitro evaluations shows thatthese HA coatings are stable and osteoinductive [114,115]. Nanos-tructured hydroxyapatite layer having unique biological propertiescan be obtained by selection of suitable parameters for thedeposition process [16].

2.5.2. Adhesion of pulsed laser deposited hydroxyapatite (HA)coatings on Ti–6Al–4V

Adhesion strength of HA coating on metals depends on themicrostructure of the substrate, the surface chemistry and the PLDprocess parameters such as laser power density and substratetemperature. [58,116–118]. Various surface modification techni-ques have been used to improve the metal–ceramic interface suchas nitridation, surface oxidation and ion implantation [119–123].

Blind et al. reported that adhesion of pulsed laser deposited HAfilms on titanium alloy is due to the existence of an oxide,specifically titanium dioxide, at the interface between the sub-strate and the coating layer [20]. Another report suggests thatthere may be some effects of epitaxy between the oxide andcoating [124]. Fernández-Pradas et al. [54] commented whetherthe presence of a titanium oxide interface would favor adhesion ofthe HA coating to the Ti–6Al–4V substrate is still a cause fordebate. Some authors consider that such a layer favours adhesion[125,126]. Other studies have attributed the weak adhesion in thefirst calcium phosphate coatings deposited by PLD at high tem-peratures to the formation of a titanium oxide layer during theprocess of pressure stabilization [105]. A study of the adhesionstrength in coatings deposited by ion bombardment on passivatedand non-passivated substrates, suggest that this oxide layer shouldbe as thin as possible [127].

Koch et al. [16] investigated pulsed laser deposition of hydro-xyapatite on Ti–6Al–4V for medical and dental applications.A pull-off testing method was used to determine the coating-to-substrate adhesion strength. Garcia-Sanz et al. had also examinedhydroxyapatite films prepared using pulsed laser deposition usinga pull-off test based upon a modified ASTM C-633 procedure [128].The measured tensile strength of hydroxyapatite grown at 480 1Cwas 58 MPa and failure was observed at the coating–substrateinterface. Wang et al. obtained tensile bonding strength valueswithin the range of 30 MPa and 40 MPa for hydroxyapatite

Fig. 4. SEM micrographs from cross-sectional view of HA coatings (via SOL 2) onTi–6Al–4Vsubstrates after heating at 840 1C [96].


coatings grown on Ti–6Al–4V in an argon–water atmosphere at500–600 1C [129]. Zeng et al. determined the bond strength valuesfor hydroxyapatite films grown using 3rd harmonic YAG:Nd lasers(λ¼355 nm), and 4th harmonic YAG:Nd lasers (λ¼266 nm) onTi–6Al–4V substrates in an argon–water atmosphere at 500–520 1C [116]. Films grown on unpolished titanium substrates hadtensile strength values of �30 MPa while films grown on polishedtitanium possess lower tensile strength values of �20 MPa.

In a study to enhance the bonding strength of HA, Nelea et al.[110] utilized a TiN interfacial layer between the Ti–6Al–4Vsubstrate and HA coating. The study reported that the adhesionwas improved due to better bonding of HA to TiN, which is aceramic, and then to the surface of metallic substrate. Man et al.[40] and Chen et al. [130] described the utilization of a pre-treatment process which included etching and laser surfacenitriding on titanium to produce a TiN dendritic scaffold networkstructure. This coralline-like structure provides additional surfacearea for interlocking of the coating material.

Man et al. [119] reported the influence of pre-treatments on theadhesion of the HA coating to the substrate. Five types of pre-treatments, shown in Fig. 5 were: (i) mirror finished specimen, (ii)60 grit grinded SiC paper (specimen 2), (iii) 320 grit grinded SiCpaper (specimen 3), (iv) mirror finish with 1-μm diamond paste(specimen 1), and (v) 10 s etching with Knoll solution afterpolishing (specimen 4). The surface roughness of the specimenswere determined using a profilometer (Taylor Hobson Surtronic25) and the adhesion strengths between HA coatings and thesubstrates were evaluated in accordance to ASTM C-633 [131]. Themaximum adhesion strength obtained was �16 MPa for specimen5 (nitridedþetching).

Fig. 5 shows the adhesion strength of deposited HA on differentpre-treated specimens and surface roughness. Generally, anincrease in surface roughness increases the adhesion strength.Based on these results, it can be concluded that significantenhancement in the adhesion strength of pulsed laser depositedHA on Ti–6Al–4V can be obtained by laser surface nitriding andsubsequent etching [119].

A related study has concluded that a controlled surface micro-structure can be obtained by using few laser pulses withoutaffecting the bulk mechanical property of titanium substrate[132]. Fig. 6 plots the average surface roughness values, measuredafter laser treatment and after HA coating versus their initialroughness.

Fig. 7 compares the adhesion strengths of HA coating onsubstrates treated with 500–18,000 laser pulses with those ofuntreated, polished titanium. The adhesion of HA to the substrateis examined in accordance to ISO 20502:2005(E) [133] using a

micro-scratch tester (micro-combi tester; CSM Instrument Swit-zerland) equipped with a diamond Rockwell tip of 100 μm [132].It was found that in all cases, the laser treated substrates wouldhave higher bonding strengths, which imply that the surfaceroughness directly influences the adhesion strength. Varying thelaser pulses would affect the surface morphology. Fig. 6 shows thatthe roughness increases with the increase in the number of laserpulses, which starts from �0.4 μm at 500 laser pulses/min upto�1 μm at 12,000 pulses/min. However, there is a significantdecrease in the roughness value for laser pulses in the range of12,000–18,000 laser pulses/min. Low rate of laser pulses (500,1000, and 3000 pulse/min) would only etch the surface and maynot be able to control the surface roughness. The surface rough-ness is under control only after �3000 pulses/min. A surface withcontrolled structure/pattern is obtained using 18,000 pulses [132].

The polished surface of specimen does not have much adhesionstrength to the coating. However, once the surface is treated withlaser, the surface roughness increases which results in increasedadhesion (from 0 to 1000 pulses/min) due to the initial materialremoval from the surface. However, at this stage, certain regionsare unaffected by the laser and a control over the adhesion at thisstage is not predictable. Once the laser pulses reaches �3000pulses/min, the surface attains a certain level of smoothness sincethe large, number of pulses would completely remove the original

Fig. 5. Comparison of adhesion strength for HA on substrates with different pre-treatments [119].

Fig. 6. Average surface roughness of titanium substrates treated with differentlaser pulses and HA coating compared with control sample [132].

Fig. 7. Failure values obtained by scratch test (Lc1, Lc2 and Lc3) for the HA coatingson different irradiated and non-irradiated titanium substrate [132].


top surface to uncover a fresh coating surface. Therefore, themorphology and adhesion can be controlled by the number oflaser pulses (3000–18,000) [132]. Fig. 7 shows the trend ofadhesion strength versus number of laser pulse, showing thatthe adhesion strength would gradually increase until 1000 pulse/min, then decreases in between 1000 and 3000 pulse/min, andincreases again past 3000 pulse/min. The highest adhesionstrength obtained was 10.87 N and 11.21 N at 2000 and 18,000laser pulses respectively, while untreated substrate showed alower adhesion strength value of 4.57 N [132].

HA coatings by PLD exhibit good biocompatible and mechanicalproperties making it suitable for medical implants. PLD HA coat-ings, on titanium alloy such as Ti–6Al–4V, resulted in higheradhesion between the coating and substrate and have only minorundesirable phase under optimal conditions [54,106].

2.6. Electrophoretic deposition coating technique

Electrophoretic deposition (EPD) is a process in which particlesin a suspension is coated onto an electrode under the effect of anelectric field [134]. The colloidal particles suspended in a liquidmedium migrate under the influence of an electric field (electro-phoresis) and are then deposited onto an electrode. Electrophore-tic deposition (EPD) is particularly advantageous for ceramic filmand coatings as well as laminar ceramic composites applications[134–137]. Furthermore, the method used low-cost equipment,easy to set-up, and is able to coat complex shapes and patterns.A high degree of control on the coating results can be achieved byregulating the deposition conditions and the ceramic powder sizeand shape [138]. EPD is a cheaper method than chemical vapordeposition, sol–gel deposition, and sputtering for producing filmsof a wide range of thickness, from less than 1 mm to more than100 mm thick [139]. However, limitations of the technique includeslow adhesion strength, and cracking on the coated surface due topost-process shrinkage.

EPD has shown its potential use in biomedical applications inrecent years [140–142]. The interest in electrophoresis for biome-dical applications [143–147] stems from a variety of reasons suchas the possibility of stoichiometric deposition, high purity materialto a degree not easily achievable by other processing techniquesand the possibility of forming coatings and bodies of complexshape [140]. Considering all advantages and disadvantages of thistechnique, electrophoretic deposition is one of the favorable coat-ing techniques which can be utilized for hydroxyapatite coating.

2.6.1. Electrophoretic deposition hydroxyapatite (HA) coatingsThere is a growing interest in processing of HA powders using

EPD technique, owing to its uniformity and good sinterability ofthe deposits, possibility of impregnation of porous substrates, andcomposite consolidation [142,148]. However, reports on the use ofEPD for depositing HA on titanium substrate are thus far, relativelylimited. Nie et al. [149] and Soares et al. [150] have used EPD todeposit HA on Ti–6Al–4V substrates and have obtained uniformthin coating with good mechanical strength. Stoch et al. [146] havealso coated HA on titanium implants with intermediate layer ofsilica. EPD process of HA is a colloidal process where HA powdersare deposited directly from a stable colloid suspension by using aDC electric field [25].

Electrophoretic deposition of HA can be processed at roomtemperature or lower, which avoids problems related to formationof amorphous phases. The nature of the bond is more metallurgi-cal rather than mechanical, thus HA coatings using EPD areexpected to have improved adhesion strength as compared tothermal sprayed techniques. However, a major drawback is thepresence of porosities which may later on leads to corrosion and

delamination of the titanium caused by penetration of body fluidsinto the substrate. Post-treatment high temperature sintering canbe utilized to minimize the porosity by increasing the coatingdensity. Unfortunately, cracks in the coating can form during hightemperature sintering due to the difference in the thermal expan-sion coefficients and large reduction of the pore volume betweenthe titanium and HA [151].

For nanostructured materials, the mismatch in thermal expan-sion coefficient is not a significant problem [152]. In nano-ceramics, the thermal expansion coefficient is fairly matched withthe metal alloy because the large quantity of atoms located at thegrain boundary improves mobility [152–154]. However, the suc-cess of electrophoretic deposited HA has been limited to conven-tional materials in the range of micron-sized grains [134,140,154].Limitations on the mechanical properties of the micron size HA arepoor fracture toughness, adhesion, and compressive strengths.There is a need for the HA coating and the substrate to havesufficient interfacial bond strength since the coating would endurehigh interfacial stresses during in vivo service.

2.6.2. Adhesion of electrophoretic deposited hydroxyapatite (HA)coatings on Ti–6Al–4V

Zhang et al. [151] have developed a unique room temperatureEPD process to deposit nanostructured HA coating having adhe-sion strength of 50–60 MPa, which is 2–3 times better thanthermal-sprayed HA coating. The interfacial bond strength wasmeasured in accordance to ASTM Standard F 1501-95 using atensile tester [151]. The corrosion resistance of this nanostructuredHA is 50–100 times higher than conventional HA coating. Fig. 8shows the corrosion resistance results for both EPD coatings andthermal sprayed coatings, where the corrosion current of n-HAcoating is 50–100 times smaller than the thermal sprayed coatingin simulated human body fluid at room temperature.

High quality HA nano-coating can be produced using EPDtechnique. The adhesion stress obtained was 60 MPa, measuredusing a direct-pull-tests, which exceeds the 50 MPa requirementsof the food and drug administration (FDA) [155]. A 2 monthsin vitro testing also showed that the bonding strength of the EPDn-HA coating on the titanium alloy was able to be maintained inthe range of 50–60 MPa, which is significantly better than plasmasprayed HA coatings [151].

Ma et al. [139] reported that HA particles were successfullydeposited onto a titanium substrate via a single electrophoreticdeposition. Good adhesion between the coating and substrate wasverified by scanning electron miscopy examination and shear

Fig. 8. Electro-polarization corrosion curves for both EPD n-HA coating and HAthermal sprayed coating [151].


strength tests, following methods outlined by Wei et al. [148] andASTM standard F1044-87. The shear stress of the HA coating aftersintering at 1000 1C was 3.34 MPa, indicating a good adhesion ofthe coating has been obtained. Figs. 9 and 10 show SEM micro-graphs of the cross-section and the surface deposit of the 1000 1Csintered HA coating, respectively. It can be seen that a layer of HAcoating as thick as 400 mm has adhered well into titaniumsubstrate and no delamination or crack was observed at both theinterface and the surface. The deposition was found to be uniformwith the coating thickness maintained consistently along thesurface of the sample. No observable crack, which is one of thecommon problems of EPD, was detected. It is believed that thegood deposition result is due to the stable and dispersed HAsuspension used for the deposition [148].

Studies on EPD coating of HA on titanium alloys show thatparticle size is an important factor for the process as the mobilityof the charged particles is proportional to the size of the particles[156]. Ferrari et al. [157] have also reported that the charges, hencethe conductivity of the suspension, play an essential role and has anoptimum value for the process. Nevertheless, the colloidal stability ofthe suspension could also be a main factor to obtain good coatinguniformity and bonding strength in the EPD process [142].

Like many similar techniques for coatings involving ceramics,EPD coating of HA requires a densification stage involving thesintering of the coated implants. This requirement poses a

dilemma, especially since high sintering temperature is sometimenecessary. Low sintering temperatures results in weak bond withlow-density coatings whereas high sintering temperatures canlead to the degradation of the HA and the metal substrate(oxidation and impaired mechanical properties) as a result of themetal substrate catalyzing decomposition of the HA to anhydrouscalcium phosphates [158,159].

A high sintering temperature may also lead to phase transfor-mation and grain growth of the metal substrate, causing signifi-cant decrease in mechanical properties. It has been demonstratedthat the mechanical properties of these titanium alloys degradesignificantly when heated above 1050 1C [138]. Therefore, it isrecommended to keep the densification temperatures below1000 1C to minimize degradation of the HA and the metalsubstrate.

The sintering phase for EPD implants improves densificationand the bonding of the coating. However, HA may decompose inthe process [160]. An interlayer can be used in between the HAand the metal substrate to moderate the problem of HA decom-position. Nie et al. deposited a dense layer of titanium dioxide(TiO2) as the inner layer between HA top layer and titanium alloysubstrate to achieve a very good combination of mechanicalintegrity, chemical stability and bioactivity [149].

Kumar and Wang [161] investigated the coating of TiO2 powderson Ti–6Al–4V substrates as the first layer, followed by the HA–TiO2

composite layers of different weight ratios, coated onto the TiO2 layer.Wei et al. [138] studied on the adhesion strength of HA coating inwhich HA powders are used as both inner and outer layer. Hence, nochange occurred in the structure of coating layers. Sintering was alsoapplied after the deposition of every single layer. In the HA coating onTiO2 deposited substrate, the decomposition of HA is decreased; andgenerally adhesion of coating, which is tested according to ASTMF1044-99, was enhanced with the reduction of voltage value used forTiO2 coating [160]. Table 3 shows the result of adhesion strengths ofHA coated samples with and without TiO2 inner layer deposited usingdifferent voltages.

2.7. Sol–gel derived coating technique

The sol–gel method is one of the simplest technique tomanufacture thin films which can produce almost any single ormulticomponent oxide coating on glass or metals [162,163].Sol–gel derived coating can be used for optical, electronic, mag-netic or coating with chemical functions [164]. Sol–gel derivedceramic films are widely used as a protective layer againstcorrosion and oxidation of stainless steel [165], Ag [166], and Al[167] substrates. The sol–gel process involves the formation ofsolid materials, mainly inorganic non-metallic materials fromsolution. This can be a solution of monomeric, oligomeric, poly-meric or colloidal precursors [168].

The sol–gel process, shown in Fig. 11 [169], consist of:(i) producing a homogeneous solution of purified precursors inan organic solvent which can be mixed with the reagent used inthe next step or water; (ii) shaping the solution to the ‘sol’ form byusing treatment with a suitable reagent, e.g. water for oxide

Fig. 9. Cross section SEM micrograph of the EPD deposited under the identifiedoptimum suspension condition [140].

Fig. 10. SEM micrograph of the uncrack deposit surface [140].

Table 3Adhesion strengths of HA coated samples with and without TiO2 inner layerdeposited using different voltages [160].

Samples (substrateþ inner layerþHa) Shear strength (MPa)

Ti–6Al–4Vþ⋯þHA 13.8 (s¼1.8)Ti–6Al–4VþTiO2 (50 V)þHA 11.9 (s¼1.8)Ti–6Al–4VþTiO2 (20 V)þHA 13.1 (s¼1.8)Ti–6Al–4VþTiO2 (10 V)þHA 21.0 (s¼1.8)

s: standard deviation.


ceramics; (iii) changing the sol to a ‘gel’ by polycondensation; (iv)converting the gel to the finally preferred shape like thin film,fiber, and (v) finally converting( sintering) the shaped gel to thedesired ceramic material at temperatures (�500 1C) much lowerthan those required in the conventional procedure of the meltingthe oxides together [168,170–172].

Olding et al. [172], reports that sol–gel techniques has con-siderable advantages such as:

1. Ability to produce thin bond-coating to provide excellentadhesion between the metallic substrate and the top coat.

2. Corrosion resistance performance due to ability to form thickcoating.

3. Ability to shape materials even complex geometries in thegel state.

4. Production of high purity samples.5. Low temperature sintering, usually in the range of 200–600 1C

[173].6. A simple, economic and effective method to produce high

quality coatings.

However, the sol–gel technique has disadvantages such as highpermeability, low wear-resistance, and difficult porosity control,which has limited its utilization in the industry. For crack-freecoating, the maximum thickness of the coating is only 0.5 μm[172]. Furthermore, trapped organics during the thermal processwould result in coating failure. Recent advancement in highsubstrate sensitive sol–gel also suffers from thermal expansionmismatch. Nevertheless, there is a wide room for improvement inthe technique and further investigation should be done to improvethis highly potential method for biomaterial coating.

2.7.1. Sol–gel derived hydroxyapatite (HA) coatingsThe sol–gel is a low temperature process, thus does not suffer

from the implications of structural instability of hydroxyapatite atelevated temperatures [174–177]. A major processing stageinvolves solution chemistry, whereby a sol is produced fromsuitable alkoxides or salts to yield a hydroxyapatite compositionupon heating [178].

Gross et al. [178] described that the production of sol–gelhydroxyapatite coatings on titanium substrates using alkoxideprecursors requires more control on firing temperature and theaging time. X-ray diffraction of the coatings heated to varioustemperatures, as illustrated in Fig. 12, indicated that the titaniumsubstrate would start to oxidize at temperatures starting at 800 1C.Thus for sol–gel hydroxyapatite coating, it is suggested that theprocessing temperature should be around 800 1C to reduce possi-ble phase transformation in the metallic substrate as well asthe occurrence of oxidation. [178]. Nanograined hydroxyapatite

coating with an average grain size of 50 nm was achieved usingthis technique. Fig. 13 shows a scanning electron micrograph of acoating fired to 800 1C for 10 min. Densification of the coating canthen be obtained with a longer duration of firing at 800 1C.

Fig. 11. Steps in the sol–gel process for ceramic materials [169].

Fig. 12. X-ray diffraction of sol–gel coatings preferred to 500 1C on titaniumsubstrates and then fired at various temperatures [179].

Fig. 13. A scanning electron micrograph of a coating fired to 800 1C for 10 min, thefield of view is 250 nm�250 nm [179].


Fabrication of sol–gel deposited HA on implants HA [173,179,180] requires extremely stringent processing parameters,particularly for the thermal processing phase such as the durationand calcining temperature, chemical compositions of the precur-sor, types of substrate, and number of HA-coated layers. Majorissues include the crystalline phases, adhesion strength andbiocompatibility of the resulted coatings.

2.7.2. Adhesion of sol–gel deposited hydroxyapatite (HA) coatingson Ti–6Al–4V

Tests have shown that pure HA suffers relatively high dissolu-tion rate in simulated body fluid that would affects its long-termstability. High dissolution may lead to disintegration of the coat-ings and hinder the fixation of implant to the host tissue [181,182].To address this issue, Zhang et al. [183] incorporated fluorine ion,which exists in human bone and enamel, into HA crystal struc-tures. Mixing of fluorine into HA, or fluoridation, decreases thesolubility of HA while still maintaining its biocompatibility [184].

Zhang et al. [183] have successfully deposited dense, crack-freefluoridated hydroxyapatite (FHA, Ca10(PO4)6(OH)2�xFx) coatings(�1.5 μm) through sol–gel dip coating on Ti–6Al–4V substrates.Scratch testing has shown an increase of over 35% in the adhesionstrengths of the coating to Ti-alloy. The increase in adhesion ismore prominent for high annealing temperatures. This increase ismost likely due to the formation of chemical bonding at theinterface and the incorporation of fluorine in HA which providedrelief of thermal mismatch.

Fig. 14 illustrates the coefficient of friction in terms of relativevoltage as a function of normal load while scratching (a) pure HAcoating; (b) fluoridate HA (FHA6) coating on Ti–6Al–4V. At thebeginning of the scratch and because of the “soft” nature of thecoating, coefficient of friction increases as load increases. Thefluctuation in the diagram, before point 1, is caused by the surfaceroughness. After point 1, the indenter would start to advance intothe coating, resulting in a sharp increase in friction coefficient. Theindenter would completely peel off the coating and scratches thesubstrate as the load increases to point 2, or 370 mN for pure HA(shown in curve a), which results in a sudden increase in friction atabout 470 mN for FHA6.

Comparison of curves (a) and (b) in Fig. 14 shows that curve “b”appears to have less fluctuation before the indenter completelydigs in and the adhesion of coating and substrate is better sincethere is a slower gradient rise after the indenter digs in. A sharpincrease of friction would indicate a brittle peeling-off of thecoating from the substrate surface. Since curve “b” lacks the sharp

change in friction, it is thus a more ductile interface and subse-quently have better coating–substrate bonding than those ofcurve “a” (pure HA) [185].

Fig. 15 shows the “upper critical load”, Lc, of all FHA coatings asa function of firing temperature and fluorine. Both firing tempera-tures and fluorine content seems to have a significant effect on theadhesion strength of the coating. Increasing firing temperature orfluorine concentration results in a dramatic raise of the criticalload. For coatings with the same amount of fluorine content,higher adhesion is due to higher annealing temperatures. Simi-larly, at the same firing temperature, adhesion strength increaseswith fluorine content.

Zhang et al. [186], in similar studies [183], reported that FHA isa potential replacement for pure HA coating on metallic implantsdue to FHA's significant biocompatibility and resistance to biode-gradation [184,187]. Ding et al. [188] identified two critical aspectsas the main contributors for long-term stability of the ceramic-coated implants: high adhesion strength of substrate to coatingand low solubility of the coating. Incorporation of fluoride ionsinto HA lattice structure results in reduction of HA solubility.However, reports on adhesion improvements, especially on adhe-sion studies after in vitro dissolution test have yet to be studiedextensively. In vitro dissolution tests can be used to investigate theinfluence of dissolution behavior on the adhesion. Zhang et al.[186], evaluated the adhesion of FHA coated on Ti–6Al–4V usingsol–gel technique before and after dissolution tests. The dissolu-tion tests were conducted by soaking FHA coatings in a Tris-buffered physiological saline solution (TPS) (0.9%NaCl, pH7.4) at afixed temperature of 37 1C for a duration of 3 weeks (Fig. 16). Itworth to mention that the “P” value in Fig. 16 is one-way ANOVAtest was conducted to assess the statistical significance of theadhesion and toughness results.

Fig. 16 shows the nominal adhesion strength between thecoating and the Ti–6Al–4V substrate. “Adhesion failure” and“cohesion failure” cannot be recognized by “nominal”. Withoutfluoridation (sample F0), the adhesion strength is about 19 MPa.Fluoridated samples (F1 and F2) show significant increase inadhesion strength to about 26–27 MPa. Zhang et al. [186] con-cluded that, the strength range starts from about 19 MPa for purehydroxyapatite (x¼0) up to about 26 MPa for x¼1. However, after21 days of soaking the coating in Tris-buffered physiological salinesolution, the adhesion strength increases to about 30 MPa for pureHA and to over 40 MPa for FHA.

Fig. 14. Coefficient of friction in terms of relative voltage as a function of normalload while scratching: (a) pure HA coating and (b) fluoridate HA (FHA6) coating onTi–6Al–4V [174].

Fig. 15. Adhesion strength of pure HA and fluoridated HA coatings on Ti-6Al-4Vsubstrates as indicated by upper critical load in scratch test. Firing temperatures areindicated [174].


Comparing the sol–gel and thermal spraying methods for thesame FHA coatings on Ti–6Al–4V, Gu et al. [189] described thatafter soaking, the adhesion strengths of thermal sprayed speci-mens tends to decline, with reductions up to 75%. For example theadhesion strength had decreased from 27 MPa before soakingdown to 19 MPa after soaking in synthetic body fluid (SBF) for2 weeks. The reduction in adhesion strength of thermal spraydeposited HA coatings is probably due to the presence of cracks inthe coating [83].

Cheng et al. [190] used a pull-out method and scanning scratchtechnique to evaluate the bonding strength of FHA coatings onTi–6Al–4V. Fig. 17 shows the result of measurements by pull-outstrength, showing the strength is about 11 MPa for pure HAcoating (FHA0), with considering of F content, the strengthintensifies up to about 22 MPa, and then decreases to around17–18 MPa. Coating peeling-off value is about 390 mN for pure HA.In contrast, the coating peeling- off increases with increasing Fcontent, 447 mN for FHA1, 450 mN for FHA2, 449 mN forFHA3 and 478 mN for FHA4. The result of the study confirms thatthe presence of F in FHA coatings has improved the adhesionstrength [190].

2.8. Ion beam assisted deposition technique

Surface modification techniques based on the bombardmentmethod have been used since the mid-1970s, and many have beendeveloped and are now widely used for surface engineering ofmaterials such as ceramics, bioceramics, and metals. Examples ofsuch methods are ion beam deposition, ion beam mixing and ionbeam assisted deposition (IBAD) [191–195].

IBAD is a vacuum deposition process based on the combinationof ion beam bombardment and physical vapor deposition. Themajor characteristic of IBAD is the bombardment with a specificenergy ion beam during coating deposition. Many parameters canaffect the composition, mechanical properties, chemical proper-ties, and structural properties of the deposited coating in the IBADprocess. The most important processing parameters in IBAD areevaporation rate or sputtering rate, coating materials, ion species,ion beam current density and ion energy [196].

IBAD has the ability to prepare bio-coatings with considerablyhigher adhesive strength as compared to traditional coatingmethods. The high adhesive strength is the result of interactionbetween the substrate and coating atoms, assisted by ion bom-bardment. This results in an atomic intermixed zone in thesubstrate–coating interface [196]. IBAD process is highly reliable,reproducible and is conducted at low substrate temperature,without unfavorably affecting the bulk substrate characteristics.Furthermore, the process has superior control over coating micro-structure and chemical composition [197].

2.8.1. Ion beam assisted deposition of hydroxyapatite (HA) coatingsAs it mentioned earlier, there are several methods to make HA

coating on Ti–6Al–4V, among which plasma spraying is the mostfrequently used [198,199]. However, long-term clinical follow-uphas demonstrated that there are significant deficiencies in theplasma-sprayed HA coatings. The limited cohesive strength of thecoatings and the limited strength of the coating–metal substrateinterface are the main problem with plasma-sprayed cotingtechnique. Moreover, heat treatments in plasma-sprayed HA coat-ings results in cracks in the coating layer because of thermalexpansion mismatch between the metal substrate andcoated layer. This leads to a severe decreasing in bond strength[200–203]. In order to produce more permanent bone-bondingcalcium phosphate coatings, ion beam assisted deposition (IBAD)is introduced as an alternative technique for plasma sprayingtechnique. Previous studies shows that implants coated with HAby the IBAD method demonstrate a very good adhesion to thesubstrate [204].

2.8.2. Adhesion of ion beam assisted deposition of hydroxyapatite(HA) coatings on Ti–6Al–4V

In the IBAD process, a wide atomic intermixed zone betweenthe coatinsg material and the substrate can be created, assisted bythe bombardment with energetic ions during deposition. Thiscreates a strong adhesion of the coating to the substrate[205,206]. Ohtsuka et al. first used 50 keV Caþ implantation intoTi, followed by Caþ IBAD to deposit HA coating on Ti substrate andhas obtained higher adhesive strength than conventional methods[204]. It has been demonstrated that Caþ implantation alone intoTi was unable to provide the bioactive surface.

Cui et al. [207] proposed using Arþ IBAD to form highlyadhesive hydroxyapatite coatings on titanium alloy. The coatingsprepare by IBAD was compared to those formed by ion beamsputtering deposition (IBSD) of calcium phosphate coatings.Scratch test is used to investigate the adhesive strength of theIBSD and IBAD coatings on the substrates. Fig. 18 shows the typicalFz–Fy curves of scratch test results for the specimens prepared by

Fig. 16. Pull-out adhesion strength of FHA coating before and after soaking in TPSsolutions. *Indicates a significant increase of adhesion strength with respect to F0(as prepared coatings); **Indicate a significant increase of adhesion strength withrespect to F0 (after soaking in TPS for 21 days) [186].

Fig. 17. Pull-out strength of coatings with different F content [190].


IBSD and IBAD. Markers “A” and “B” indicate the points of the firstoccurrence of coating detachment from the substrate. Fz and Fy, asthe normal and tangential forces respectively, are affecting thediamond indenter during the test. A load speed of 2000 gf/minwas chosen for the tests. The results have shown that the criticalloads were 660 gf for IBSD and 1050 gf for IBAD samples. Gen-erally, it was seen that the adhesive strength of the coatingsprepared by IBAD technique is almost twice that of the IBSDcoatings.

It has been shown that the adhesion strengths of coatingsprepared by IBSD and plasma sprayed technique are generallysimilar [127]. Thus, it can be deduced from the comparative resultsbetween IBSD and IBAD that the adhesive strength of IBADcoatings would be reasonably higher than that of plasma sprayeddepositions. The main benefit of IBAD is the improved adhesionstrength due to the wide atomic intermixed zone at the interfaceof the coating and substrate [204,206]. Thus, the issue of lowadhesion strength, which exists in plasma sprayed coatings can besignificantly eliminated by using the IBAD technique [207].

Choi et al. [35] have used an Ar ion beam in the coating of HAon Ti–6Al–4V deposited by IBAD technique. Fig. 19 illustrates thebonding strength as a function of the ion beam current, before andafter the heat treatment. Increasing the current would increase theion bombardment and broadens the atomic intermixed zoneduring the deposition. This results in the increase of adhesionstrength between the substrate and coating layer [207].

Several studies have shown that heat treatments would decreasethe bond strength [200–203]. Fig. 20 shows the SEM micrographs ofthe coating layer before and after heat treatments. The morphologieswere found to be relatively similar regardless of the current level.Before heat treatment, the layer was rather featureless, as shown inFig. 20(A). The lines at the interface are Wallner lines frequentlyobserved when hard coating layers are detached from a metalsubstrate [208]. However, after heat treatment, the layer becameseverely cracked, as shown in Fig. 20(B). This is probably due to thethermal expansion mismatch between the coating and the substrate[209]. These cracks are the main reason for the reduction in bondstrengths. The micrograph also reveals that the metal surface wasslightly oxidized, presumably by OH in the coating layer [209]. Overallsignificant improvement in the bond strength is resulted by Choi et al.[35] using an Ar ion beam while deposition.

Hamdi and Ide-Ektessabi [197] have proposed the deposition ofhydroxyapatite layer using a combination of technique of IBAD andsimultaneous vapor deposition (SVD), namely ion-beam-assistedsimultaneous vapor deposition (IBASVD). Fig. 21 illustrates the

Fig. 18. Fz–Fy curve of scratch test from specimen prepared by (a) IBSD and(b) IBAD [207].

Fig. 19. Layer–metal substrate bond strengths, before and after heat treatment, as afunction of ion beam current [207].

Fig. 20. SEM micrographs of the coating layer (A) before and (B) after the heattreatment [208].


result of coating detachments for two sets of IBASVD samples asfunction of different annealing temperatures. Both types of sam-ples resulted in similar curve patterns with the minimum detach-ment forces recorded at 700 1C annealing temperature and themaximum adhesion strength at 1200 1C. In all cases the adhesionstrength for the 260 mA/cm2 sample was higher than the 180 mA/cm2 sample. In general, the recorded data for both samples areextremely higher than the maximum adhesion strength obtainableby the SVD samples, which was less than 100 mN [210]. It issuggested that the increase in adhesion strength was the result ofthe formation of a mixed layer between the substrate and the HAfilm, consisting of a gradient fill of Ca, P and the element of thesubstrate [207,211]. Hamdi and Ide-Ektessabi [197] described thatthe energetic ions assisted the reactions between the migratedatoms and the substrate atoms to generate an intermixed layer,which have specific properties different from the deposited filmsand the substrate. It was also understood that high current densityof ion beam resulted in a wider atomic intermixed zone, whichconsequently improved the overall adhesion strength.

2.9. Sputter coating technique

Sputter deposition is a physical vapor deposition (PVD) methodof depositing thin films by sputtering. This involves ejectingmaterial from a source, known as a “target”, onto a “substrate”such as a silicon wafer. It was reported that initial sputtering usingmulti-component ceramic targets such as superconducting oxides,HA and other CaP materials would produce coatings whosechemistries were different upon deposition than the bulk target[212,213]. Sputtering utilizes a gas plasma (argon, neon, krypton orxenon) to remove material from a negatively charged target whichis then deposited as a thin film coating onto the substrate. Studieshave shown successful deposition of thin HA layers on titaniumsubstrates using RF magnetron sputtering [214].

2.9.1. Sputter coating of hydroxyapatite (HA) coatingsSputtering techniques have been used to deposit homogeneous

thin films coatings of high adhesion strength with thicknessesranging from 0.5 to 3 μm. However, sputter coated HA films onmetals were found to be of low crystallinity [214–216]. The lowcrystallinity increases the rate of dissolution of the coating in theliving body. Post-treatment thermal process can be used tocrystallize the film, hence reducing the possibility of dissolution.However, conventional thermal treatment in the electric furnaceincreases the likely formation of cracks and may degrade theHA films.

2.9.2. Adhesion of sputter coating of hydroxyapatite (HA) coatingson Ti–6Al–4V

Ozeki et al. [217] compared the thermal treatments of the HAcoated on titanium alloy substrate prepared by sputter coatingwith those prepared by plasma spraying technique. The substrateswere sandblasted using Al2O3 (125–180 mm) abrasive before coat-ing. The specimens were post-treated with a hydrothermal processfor 24 h. The film thickness obtained for sputter coating was1.2 mm while the thickness for plasma spraying was 60–100 mm.

Fig. 22 shows the shear strength results of the sputter coating,the plasma sprayed coatings and the non-coated columns over aperiod of time. The sputter coating showed the highest bondingstrength overall with recorded strengths of 3.370.2, 5.770.5, and8.671.6 MPa after two, four, and 12 weeks, respectively. Theplasma sprayed coatings resulted in strength values of 1.970.25,4.070.3, and 6.670.7 MPa, respectively, for the same period oftime. The strength values of the non-coated columns were0.470.3 and 1.170.3 MPa after four and 12 weeks, respectively.The strength of the sputter coating exceeded that of the plasmasprayed coating by more than 70%, 40%, and 30% after a period oftwo, four and 12 weeks, respectively. De Groot et al. reported thatcoating thicknesses above 100 μm were associated with fatiguefailure under tensile loading [218]. According to Hasegawa et al.thin plasma sprayed coatings are bound more strongly than thickcoatings [219].

Ding et al. [220] investigated on a series of thin (o10 μm),single layered HA/Ti coatings deposited on Ti–6Al–4V substrateusing an RF magnetron-assisted sputtering system. For the experi-ments, six HA/Ti targets with different compositions (95HA/5Ti,90HA/10Ti, 85HA/15Ti, 75HA/25Ti, 50HA/50Ti, and 25HA/75Ti)were prepared. Generally it was found that the coating withhigher Ti contents resulted higher adhesion strengths. The highestadhesion strength (of the 25HA/75Ti coating), evaluated using aSebastian adhesion test system (Sebastian Five, Quad Group,Spokane, WA) [127] was even higher than 80 MPa, whichexceeded the maximum value achievable using the bonding resinin the pull-out test. Table 4 reports the adhesion strength and their

Fig. 21. Adhesion strength of the IBASVD samples at different elevated tempera-tures [197].

Fig. 22. Bone bonding strengths of sputtered films [217].


corresponding failure point for different compositions and Fig. 23shows the adhesion strength for each composition.

The high adhesion strength of sputtered monolithic HA coatingis higher than most plasma sprayed HA coatings [221,222], and isbelieved to be attributed to the sputter cleaning and ion bombard-ing processes. The sputter cleaning process would remove con-taminants and adsorbed gas molecules from the surface of thesubstrate to produce a clean, highly active surface [223]. The ionbombarding process during sputtering would enhance atomicdiffusion and mixing near the interface region [207,224]. Mechan-ical interlocking effect may have contributed to the higher averageadhesion strength of coating sputtered on the rougher surface(Ra¼0.7 mm) as compared to the lower value obtained for thesmoother surface (Ra¼0.06 mm). However this effect was not assignificant for sputtering with Ti-containing targets.

Results from Ding et al. [220] have shown that all coatings hadadhesion strengths between 60 and 80 MPa. Furthermore if thesputtering uses a target comprising of more than 15 vol% Ti, theresulting coating adhesion strength and hardness were signifi-cantly higher than those of monolithic HA coating.

3. Discussion

There have been numerous studies on coatings of hydroxyapa-tite (HA) onto Ti–6Al–4V because of its significant utilization inorthopedic prostheses and implants. Table 5 summarizes theprevious discussion on the various techniques for coating of HAon Ti–6Al–4V, with comparison on their advantages anddisadvantages.

Plasma spraying is the most frequently investigated method tocoat HA onto Ti–6Al–4V specimen [198,199]. Plasma spray is thefirst method used for HA coating, owing to its ease of application

[26]. Moreover, the determination of the adhesion between thecoating and the substrate has been always a main concern whenusing plasma spraying technique [59]. High spraying power resultsin high adhesion strength of HACs due to significant melting of thecoating material which forms dense microstructure. However, thehigh-temperature process can lead to phase transformation andgrain growth of the metal substrate which may cause significantdecrease in the mechanical properties of the metal.

Results of the study [61] has established the relationshipbetween residual stress and bonding strength especially forplasma sprayed hydroxyapatite coatings. This stress in the coatingis influenced by the spraying parameter, coating thickness [67,68],and substrate cooling effect (i.e. temperature of substrate) [62,63].Generally, the residual stresses increase with the increase in thethickness of coating and the temperature of the specimen duringplasma spraying. Moreover, high-power sprayed HA coatingsgenerally possess higher adhesion strength than those sprayedwith lower power. In some cases, the adhesion of the plasmasprayed HA can be significantly improved by a subsequent hotisostatic pressing operation.

The adhesion strength is a reflection of the combination ofcohesive (within the coating layers themselves) and adhesive(coating to substrate) strengths of a coating [61]. The cohesivestrength is obtained by coating properties, such as the micro-structure and crystallinity, but the adhesive strength is mostlyinfluenced by coating properties, such as surface roughness,residual stress, and the mechanical interlocking between substrateand HACs [61].

Overall, it was found that plasma sprayed coating has notimproved the service-life performance of uncoated implants. Inaddition, there are issues with poor reliability and mechanicalintegrity [56,57]. The pulsed laser deposition (PLD) is a betteralternative than the plasma spray technique because the PLDtransfers sintered HA stoichiometrically to deposit a thin adherentcoating onto titanium substrate surface [58]. The substrate tem-perature is lower in PLD as compared to plasma spray anddifferent calcium phosphate compositions can be deposited bychanging the parameters of deposition [112,114,225]. In addition,undesirable phases of HA coatings by PLD are reduced underoptimal conditions and generally have better coating to substrateadhesion [54,106].

TiO2 and TiN layers can be used as an interfacial layer betweencoating and the metal substrate as reported in studies related tothe adhesion of crystalline PLD HA thin films on Ti–6Al–4Vsubstrates [20]. Some authors consider that this interfacial layerfavours adhesion due to better bonding of HA to TiN which is then,directly bonded to the substrate [125,126]. These layers can be

Table 4Adhesion strength and failure mode of coatings [220].

CoatingCode

Adhesion strength (MPa) Failure mode(Ra¼0.06 lm)

(Ra¼0.06 mm) (Ra¼0.7 mm)

HA 59.9712.4 (41) 71.8714.7 (25) R/C, C/S95HA/5Ti 59.576.5 (20) 60.775.8 (23) R/C, C/S90HA/10Ti 58.476.2 (18) 54.576.1 (12) R/C, C/S85HA/15Ti 64.876.2 (17) 69.5710.3 (19) R/C, C/S75HA/25Ti 64.076.9 (32) 65.376.5 (50) R/C, C/S50HA/50Ti 75.175.5 (22) 72.975.4 (28) R/C25HA/75Ti 81.975.2 (19) 79.876.3 (17) R/CTi 79.579.1 (15) 85.175.1 (34) R/C

Fig. 23. Adhesion strength of coatings [220].


created using pre-treatment processes, such as laser surfacenitriding and etching on titanium, which have been reported toimprove the bonding strength of the coating. Thus, laser surfacenitriding and subsequent etching of the substrate is an effectivepre-treatment method for improving the adhesion strength of HAcoated onto Ti–6Al–4V by PLD [119].

EPD is a technique which is gaining attention due to its abilityto economically produce films of a wide range of thicknesses ascompared to conventional methods such as thermal spraying, sol–gel deposition, and sputtering [139]. Moreover, EPD of HA hasability to be processed at room temperature, reducing the possi-bility of formation of the amorphous phase in HA. The gooduniformity and bonding strength results is mostly due to thecolloidal stability of the suspension [142]. The EPD technique canalso produce nanostructured HA coating having bond strength 2–3times better than thermal sprayed HA coating.

Similar to PLD, studies have shown that an intermediate layer,such as silica or TiO2, improves the adhesion strength of coatingfabricated using EPD [146]. Dense titanium dioxide (TiO2) filmspossess a very good combination of bioactivity, chemical stabilityand mechanical integrity [149]. A TiO2 inner layer would alsoreduce the decomposition of HA and increases the and overalladhesion strength of coating [160].

The sol–gel technique is a simple technique which can createsingle or multicomponent oxide coating on glass or metals[162,163]. However, there is a coating thickness limit of 0.5 μm[172]. Fluoridation of HA can enhance the coating's resistance to

biodegradation while still maintaining good biocompatibility[184,187]. An increase in fluoridation ratio would increase theadhesion strength by about 40%. The strength range for FHA isabout 26 MPa which is higher than the value of the bondingstrength of 19 MPa for pure hydroxyapatite. The fracture tough-ness increases about 200–300% and the scratch test results inadhesion improvement of 35% for fluoridated HA coatings ascompared to pure hydroxyapatite coating [183,186,190]. Theenhancement in adhesion strength is believed to be caused bythe formation chemical bonding at the interface and the relief ofthermal mismatch resulting from the incorporation of fluorine(F) into the HA structure.

Dip coating can be generally compared with sol–gel coatingtechnique. The technique is simple, economical and is able togenerate high coating quality. Dip coating process is rapid, wherethe complete transition can be completed within a few seconds orless if volatile solvents are used.

IBAD technique can deposit highly adhesive HA coating on Ti–6Al–4V due to atomic interactions between the substrate andcoating materials, assisted by ion bombardment [196]. The mainadvantage of IBAD compare to other methods, such as IBSD orplasma spraying, is that there is a wide atomic intermixed zone atthe coating–substrate interface which significantly improves theadhesive strength of the coating. Heat treatment of IBAD coatedsamples reduces the adhesion strength, due formation of cracks inthe layer and the thermal expansion mismatch between thecoated layer and the metal substrate [200–203].

Table 5Different techniques to deposit HA coating.

Technique Thickness Advantages Disadvantages

Plasma spraying o20 mm Rapid deposition; sufficiently low cost; fast bonehealing, less risk for coating degradation

Poor adhesion, alternation of HA structure due tocoating process; non-uniformity in coating density;extreme high temperature up to 1200 1C, phasetransformation and grain grow of substance due tohigh temperature procedure; increase in residualstress; unable to produce complete crystalline HAcoating

Thermal spraying 30–200 mm High deposition rates; low cost; Line of sight technique; high temperatures inducedecomposition; rapid cooling produces amorphouscoatings; lack of uniformity; crack appearance; lowporosity; coating spalling and interface separationbetween the coating and the substrate

Sputter soating 0.5–3 mm Uniform coating thickness on flat substrates;dense coating; homogenous coating; high adhesion

Line of sight technique; expensive time consuming;produces amorphous coatings; low crystallite whichaccelerates the dissolution of the film in the body

Pulsed laserdeposition

0.05–5 mm Coating with crystalline and amorphous; coating withdense and porous; ability to produce wide range ofmultilayer coating from different materials; ability toproduce high crystalline HA coating; ability to restorecomplex stoichiometry; high degree of control ondeposition parameters

Line of sight technique; splashing or particledeposition; need surface pretreatment; lack ofuniformity

Dip coating o1 mm Inexpensive; coatings applied quickly; can coatcomplex substrates; high surface uniformity; goodspeed of coating;

Requires high sintering temperatures; thermalexpansion mismatch; crack appearance

Sol–gel 0.1–2.0 mm Can coat complex shapes; Low processingtemperatures; relatively cheap as coatings are verythin; simple deposition method; high purity; highcorrosion resistant; fairly good adhesion

Some processes require controlled atmosphereprocessing; expensive raw materials; not suitable forindustrial scale; high permeability; low wearresistance; hard to control the porosity;

Electrophoreticdeposition

0.1–2.0 mm Uniform coating thickness; rapid deposition rates; cancoat complex substrates; simple setup, low cost, highdegree of control on coating morphology and thickness,good mechanical strength; high adhesion for n-HA

Difficult to produce crack-free coatings; requires highsintering temperatures; HA decomposition duringsintering stage

Hot isostaticpressing

0.2–2.0 mm Produces dense coatings; produce net-shape ceramics;good temperature control; homogeneous structure;high uniformity; high precision; no dimensional orshape limitation

Cannot coat complex substrates; high temperaturerequired; thermal expansion mismatch; elasticproperty differences; expensive; removal/interactionof encapsulation material

Ion beam assisteddeposition

o0.03 mm Low temperature process; high reproducibility andreliability; high adhesion; wide atomic intermix zoneare coating-to-substrate interface

Crack appearance on the coated surface


Fig. 24 shows adhesion strength values of HA coatings on Ti–6Al–4V coated using various techniques. The sputtering techniquehas the highest adhesion of coating to the substrate compares toother methods which can be attributed to the sputter cleaning andion bombardment processes.

4. Conclusion

Adhesion strength of HA on Ti–6Al–4V substrate has beenreviewed in detail. Nine common techniques of deposition such asplasma sprayed deposition, hot isostatic pressing, thermal spray, dipcoating, pulsed laser deposition (PLD), electrophoretic deposition(EPD), sol–gel, ion beam assisted deposition (IBAD), and sputteringwere evaluated and discussion were made on the coating parametersaffecting the adhesion strength of the coating. Advantages anddisadvantages of each method were discussed and a quantitativecomparison was made on the different techniques of HA coating onTi–6Al–4V substrate. Based on this review, the best adhesion of HAcoating to substrate is obtained by sputtering deposition techniquewhile the worse bonding strength was obtained by PLD at 1000 laserpulses. Using an interfacial layer (such as TiO2 or TiN) as the initialcoating layer on the substrate followed by HA coating layer canenhance the bonding strength. Pretreatments such as nitriding,followed by etching, can enhance the adhesion strength in PLD.Moreover, post-treatments also have similar effects on other techni-ques such as IBAD and thermal spray.

Acknowledgment

The authors would like to acknowledge the University ofMalaya for providing the necessary facilities and resources for thisresearch. This research was fully founded by the Ministry of HigherEducation, Malaysia with the high impact research Grant no. ofum.c/625/1/HIR/MOHE/ENG/27.

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