Tribology International 93 (2016) 29–35

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Study on erosion–wear behavior and mechanism of plasma-sprayedalumina-based coatings by a novel slurry injection method

Kai Yang a,b,n, Jian Rong a,b, Chenguang Liu a,b, Huayu Zhao a,b, Shunyan Tao a,b,Chuanxian Ding a,b

a Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, PR Chinab The Key Laboratory of Inorganic Coating Materials, Chinese Academy of Sciences, Shanghai 201899, PR China

a r t i c l e i n f o

Article history:Received 27 March 2015Received in revised form4 August 2015Accepted 2 September 2015Available online 12 September 2015

Keywords:Plasma sprayingAlumina-based coatingsErosion wearSlurry jet test

x.doi.org/10.1016/j.triboint.2015.09.0079X/& 2015 Published by Elsevier Ltd.

esponding author at: Shanghai Institute of Cs, Shanghai 201899, PR China.6 21 69906320; fax: þ86 21 69906322.ail address: kaiyang@mail.sic.ac.cn (K. Yang).

a b s t r a c t

In this paper, Al2O3–Cr2O3 composite coating was fabricated by plasma spraying. It has better mechanicalperformances than Al2O3 coating. Erosion–wear resistance of the coatings was evaluated by a new typeof solid particle impact test (slurry jet). Slurry was mixed with compressed air in the nozzle andeventually injected on coating surface at high velocity. Injected slurry on coating surface resulted in awear progression (wear rate) proportionately to the erosion strength of the coating material. Al2O3–

Cr2O3 composite coating possesses better erosion–wear resistance than pure Al2O3 coating.& 2015 Published by Elsevier Ltd.

1. Introduction

Oxide ceramics exhibit high strength, high hardness and anti-wear performance, as well as high temperature and good oxida-tion resistances [1–3]. The corresponding coatings manifestexcellent potential to be employed for special surface protection ofmetal components operating at severe working conditions [4,5].Atmospheric plasma spraying is the most flexible or versatilethermal spray technique, which enables deposition of manyceramic materials such as alumina, chromia, titania, zirconia andrelated mixture [6]. As their quintessential representative, aluminacoatings are good candidates for anti-wear and anti-corrosionapplications, due to their high hardness, chemical inertness andhigh melting point, as well as to their great resistance to abrasionand erosion [7,8]. Al2O3 coatings are able to retain up to 90% oftheir strength at 1100 °C [9]. With respect to oxide ceramics, lowtoughness restricts their practical applications [10]. It is difficult tocombine conventional toughening methods with plasma sprayingtechnology. These traditional toughness improvement meanscontain particle toughening [11], fiber toughening [12], transfor-mation toughening [13] and gradient structure toughening [14].Strengthening and toughening from grain refinement or solidsolution are beneficial to the enhancement of the strength and

eramics, Chinese Academy of

toughness of the ceramic coatings. The addition of TiO2 (3, 13 and40 wt%) allows to increase coating toughness and resistance towear and erosion [15,16]. However, the corresponding coatings areaccompanied by the decrease of hardness and high-temperaturestability, which are essential for tribological and high temperatureerosion applications. Al2O3–ZrO2 composite coatings possess goodfracture toughness and poor thermal conductivity [17]. Cr2O3 andα-Al2O3 enjoy the same crystalline structure. Cr3þ and Al3þ havethe approximate ionic radiuses. Accordingly, Al2O3–Cr2O3 solidsolutions are easily formed. In our previous studies [18–20],Al2O3–Cr2O3 composite coatings were fabricated by plasmaspraying. The phase compositions, microstructures, mechanicaland thermal properties of the coatings were investigated. Thesliding wear performances of the coatings were also evaluatedunder the severe condition. The obtained results suggest thatAl2O3–Cr2O3 composite coatings have better mechanical, thermaland anti-wear properties than Al2O3 coating.

It can be observed that the tribological behavior of the coatingsin sliding wear situation is dominated by the formation of a surfacetribofilm and by its progressive change, which would have sig-nificant effects on the accumulation of wear debris [21]. However,the failure mode of erosive wear is exceedingly different from thatof sliding wear. Erosion wear of materials occurs by the removal oftarget material from the impact zone, owing to repeated impacts ofthe erodent, by a micromechanical deformation/fracture process.Several related wear mechanisms are largely controlled by theparticle material, the particle size, the impact velocity, the impactfrequency per unit area, and the angle of impingement [22]. The

Table 1The plasma spraying parameters for NiCr bond coating and top ceramic coating.

Parameters NiCr bond coating Top ceramic coating

Arc current (A) 590–610 640–650Primary plasma gas (Ar) (slpm) 55–60 40–50Secondary plasma gas (H2) (slpm) 6–8 6–8Carrier gas (Ar) (slpm) 3.0–4.0 3.0–4.0Powder feed rate ( g min�1) 15–20 30–40Spray distance (mm) 110–120 100–110

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properties (material and size) of the eroding particle are regarded asrelevant parameters for this type of wear process. Higher angle ofimpingement causes greater impact. The speed of the particlebrings an extremely strong effect on the erosive wear. Solid particlesare transported by compressed air or high speed liquid. Addition-ally, there are differences between erosive wear behaviors of bulkmaterials and coatings. Hard coatings have been considered to beespecially useful in applications standing up to erosive and abrasivewear. Ceramic or cermet coatings are suitable for the application ofwater turbine blade and sluice gate roller, which requires excellentanti-erosion performance of the workpiece surface. With respect toatmospheric plasma spraying, the coatings are made up of flattenedparticles, known as splats. The splats constitute layers in the coatingand these layers in turn create the lamellar structure of the deposit.Simultaneously, the coating structure always contains unmeltedparticle, porosity, oxidized particle and microcrack. The erosivewear behaviors of coating are not fully understood.

The current study investigated the erosion wear behavior ofplasma sprayed alumina-based coatings by a new type of solidparticle impact test (slurry jet). Combined with the phase com-position, microstructure and mechanical performance of thecoatings, the erosive wear mechanisms were elucidated.

2. Experimental procedure

2.1. Coating preparation

The Multicoat atmospheric plasma spraying system equippedwith a F4-MB plasma gun (Sulzer Metco AG, Switzerland) wasapplied to produce coatings. Commercially fused and crushedAl2O3 and Cr2O3 powders were used as feedstock. The medianparticle sizes are 17.5 μm and 16.7 μm. The corresponding particlesize distributions can be obtained in Ref. [18]. According to a cer-tain mass ratio, the starting Al2O3 and Cr2O3 powders weredirectly mixed in the roller with its rotation speed of 150 rpm.Mechanical mixing time was 120 h. Prior to spraying, the stainlesssteel substrates were degreased ultrasonically in acetone and thengrit blasted with corundum to the roughness (Ra) of 6–8 μm.Moreover, NiCr powder was applied to fabricate bond coating priorto spraying ceramic coatings. According to our previous investi-gations [18], AC70 composite ceramic coating possesses bettercomprehensive mechanical properties. The addition of Cr2O3 isbeneficial to the stabilization of α-Al2O3. With increasing thecontent of Cr2O3 in the original composite powders, the resultingcomposite coatings possess lower porosity, higher hardness, largerbending strength and better thermal conduction performance.Simultaneously, AC70 composite coating possesses the maximumof the bending strength. Consequently, the Cr2O3 weight fractionin mechanically mixed composite powder used in this study was70 wt%. The plasma spraying parameters for NiCr bond coatingand top ceramic coating are displayed in Table 1. In order to obtaingreat coating performance, spray parameters need good matchingrelationships. Namely, these parameters possess mutual restric-tions. Therefore, each parameter has a certain range of variationsin the value. Due to mechanically mixed method and feedstockparticle size, solid solution only exists in the contact surfaces ofmolten Al2O3 and Cr2O3 droplets. This would strengthen phaseinterface and decrease porosity in the coatings. Additionally, Al2O3

and Cr2O3 droplets could not be totally mixed in plasma sprayingprocedure. Mechanical mixing takes 120 h, which ensures dis-tribution homogeneity of the composite powder. Heterogeneousnucleation and partial solid solution are obtained in the compositestructure [19].

2.2. Coating characterization

The phase compositions of as-sprayed coatings were identifiedby X-ray diffraction (XRD) using a Rigaku D/Max2550 Dif-fractometer with nickel-filtered Cu Kα radiation (λ¼0.15406 nm).The XRD measurements were executed in the 2θ range from 20° to80° at a scanning speed of 4°min�1 (the corresponding peakintegral intensity calculation was corrected accounting for struc-ture factors, peak multiplicities, and unit cell volumes). The cross-sectional morphologies of the coatings were observed by a HitachiTM3000 scanning electron microscope. Vickers microhardnessmeasurements were carried out on the polished cross-sections ofthe coatings using an Instron Wilson-Wolpert Tukon 2100BHardness Tester under the load of 200 gf with a dwell time of 10 s.The coating microhardness represented the average of 10 inden-tations. The fracture toughness and bending strength of the coat-ings were measured with a universal testing machine (ModelInstron-5566, Canton, USA) at room temperature and averagedover the values for five specimens. The detailed thick coatingpreparation and mechanical property test methods could beobtained in Ref. [18].

2.3. Erosion–wear testing

The erosion wear performances of the coatings were evaluatedby a new type of slurry jet test. Slurry (water and solid particlemixture) was mixed with compress air in the nozzle and eventuallyinjected on coating surface at high velocity. Injected slurry on thecoating surface resulted in a wear progression (wear rate) pro-portionately to the erosion strength of the coating. It is a new typeof solid particle impact test (slurry jet) to swiftly estimate wearproperties of the hard coatings. The slurry jet tester (MSE TESTERS201, Palmeso Company, Japan) was used to perform the erosionwear tests, which possess four notable features (shown in Fig. 1).These characteristics are: ① about 1 μm in diameter and hold 10–50 nm of wear depth per solid particle; ② accurate control of slurryinjection pressure and flow rate; ③ wear progression by solidparticle collusion with up to 100 m/s in velocity using compress air;④ high velocity wear progression by some hundred million of solidparticle impact per second. Compared to other erosive wear sys-tems, the originalities of the test study are: (a) nanoparticles areused to effectively increase impact frequency and injection energyconcentration degree; (b) the impact velocity is higher; (c) slurryinjection pressure and feed rate could be accurately controlled; (d)the dimension of predefined wear scar is about 1 mm�1 mm,which indicates better erosion area controllability than other ero-sive wear systems. Therefore, the obtained performance evaluationresults of the coatings would be more reliable. The dimension oftest specimen (stainless steel substrate) was Φ28 mm�6 mm. Inthis study, stainless steel substrates were used for better compar-ison test (in our previous investigations, stainless steel substrateswere applied [20]). The test conditions are presented in Table 2. Themeasurement steps were as following aspects: firstly, slurry wasinjected on predefined coating specimen surface; secondly, shapemeasurement was conducted along the center of wear scar of A–A0

Fig. 1. The schematic view of erosion wear by slurry jet test.

Table 2Erosion conditions applied to erosion wear test.

Test conditions Test parameters

Erosive particle type/size (μm) Corundum (Al2O3)/1–2Nozzle air pressure (MPa) 0.36Air flow (L/min) 11.0Nozzle slurry pressure (MPa) 0.30Slurry flow (mL/min) 125Impact angle (deg) 90Test temperature (°C) 25Erosion testing time (min) 60Size of erosion surface (mm) 1�1

Fig. 2. The schematic of wear scar.

Fig. 3. The coating sample installation.

Fig. 4. The observation for wear scar.

Fig. 5. The wear profile measurement.

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line exhibited in Fig. 2; thirdly, wear rate was calculated accordingto projected particle amount and wear depth figure. The coatingsample installation are shown in Fig. 3. The optical microscope(View Solutions, USA) and microprofiler (BMT, Germany) were used

to observe wear scar morphologies and to carry out profile mea-surement of wear scar, respectively (revealed in Fig. 4 and Fig. 5).

3. Results and discussion

3.1. Phase composition

The XRD patterns of as-sprayed alumina-based coatings arepresented in Fig. 6. By calculating the intensities of the greatestdiffraction peaks (namely, γ-Al2O3(440) and α-Al2O3(113)), thecontent of γ-Al2O3 is 90.16 wt% in the Al2O3 coating (shown inFig. 6a). This result suggests that the preferential formation of γ-Al2O3 is attributed to the high quench rate and lower nucleationenergy. With respect to the pure Al2O3 coating, the greatest dif-fraction peak intensity ratio of α-Al2O3 to γ-Al2O3 (I(113)(α-Al2O3)/

Fig. 6. The XRD patterns of as-sprayed alumina-based coatings: (a) Al2O3 coating; (b) Al2O3–Cr2O3 composite coating.

Fig. 7. Cross-sectional morphologies of the coatings: (a) Al2O3 coating; (b) Al2O3–Cr2O3 composite coating.

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I(440)(γ-Al2O3)) is equal to 0.11. In Fig. 6b, Cr2O3, γ-Al2O3 and α-Al2O3 phases appear in the XRD pattern of Al2O3–Cr2O3 compositecoating. The value of I(113)(α-Al2O3)/I(440)(γ-Al2O3) increases up to0.85, which may indicate the positive influence of Cr2O3 on thestability of α-Al2O3 in the composite coating. Cr2O3 and α-Al2O3

possess the same crystalline structure. Cr3þ and Al3þ have theapproximate ionic radiuses. The Al2O3–Cr2O3 solid solutions areeasily formed. Accordingly, the addition of Cr2O3 is conducive tomaintain α-Al2O3 phase in the coating. Due to the heterogeneousnucleation and partial solid solution [18], the stabilizing effect ofCr2O3 on α-Al2O3 could be obtained.

3.2. Microstructure and mechanical performances

The polished cross-sectional morphologies of the coatings aredisplayed in Fig. 7. The thickness of the coatings is about 220–250 μm. Compared to the Al2O3 coating (shown in Fig. 7a), thepores in the structure of Al2O3–Cr2O3 composite coating (shown inFig. 7b) are less and their sizes are also smaller. The cross-sectionof Al2O3 coating is more smoother compared with that of Al2O3–

Cr2O3 based coating. The single phase easily obtains lower surfaceroughness compared with binary phases at the same grinding andpolishing process. This may be attributed to more homogeneousperformance for the single coating. In Fig. 7b, french grey zonesindicate the Cr2O3 phases and dark grey zones denote the Al2O3

phases. A image analysis method was employed to evaluate thecoating porosity. Porosities of Al2O3 and Al2O3–Cr2O3 coatings arerespectively 3.570.46% and 1.8670.28%. Consequently, theaddition of Cr2O3 is conducive to the densification of the coating,

which may be chiefly attributed to phase interface strengtheningby partial solid solution. The similarity of crystalline structure andionic radius would contribute to the heterogeneous nucleation andthe formation of Al2O3–Cr2O3 solid solutions. The heterogeneousnucleation could possess greater nucleation rate and smallernucleation radius, which may be beneficial to decreasing thecoating grain size. In the plasma spray process, Al2O3 and Cr2O3

droplets contacted each other and partially melted. This would beconducive to enforcing phase interface bonding in the compositecoating. The decrease of coating grain size and the strengtheningof phase interface by partial solid solution could contribute toincreasing the coating density (namely, decreasing the porosity).Furthermore, the Al2O3–Cr2O3 coating exhibits finer and moreuniform pore distribution than Al2O3 coating (displayed in Fig. 8aand b). It could be observed that the bonding at NiCr bond coating/steel substrate and top ceramic coating/NiCr bond coating inter-faces is good. No conspicuous coating defects, such as crack, poreand inclusion, are found in the above-mentioned binary interfaces.The main mechanical property comparisons between Al2O3 andAl2O3–Cr2O3 coatings are exhibited in Table 3. Average Vickershardnesses (HV0.2, 200 gf) of the Al2O3 and Al2O3–Cr2O3 coatingsare 9.67 GPa and 12.17 GPa, respectively. Therefore, the coatinghardness increases by 26%, which may be attributed to lowerporosity and tighter lamellar boundaries in the composite coating.The detailed expressions about the mechanical test results can bereferred in our previous investigations [18,19]. Likewise, thesimilar results could be obtained in the bending strength (σ) andfracture toughness (KIC). Especially, the fracture toughness of thecoatings was measured through the single-edge notch bending

Fig. 8. High-resolution SEM images of (a) Al2O3 coating and (b) Al2O3–Cr2O3 composite coating.

Table 3Main mechanical properties of the as-sprayed alumina-based coatings.

Specimens HV0.2 (GPa) σ (MPa) KIC (MPa m1/2)

Al2O3 coating 9.6770.15 169710 3.0870.16Al2O3–Cr2O3 composite coating 12.1770.12 18377 3.2670.11

Fig. 9. The erosion wear scars of Al2O3 coating for different projected particle amount: (a) 0 g; (b) 10 g; (c) 30 g; (d) 50 g; (e) 70 g; (f) 90 g; (g) 110 g; (h) 130 g. (Forinterpretation of the references to color in this figure , the reader is referred to the web version of this article.)

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method. Micro-indentation offers an easy and convenient methodto understand fracture toughness of brittle ceramic materials.However, this estimation method of fracture toughness is unreli-able and inconsistent. The corresponding testing results are onlyabout 60–75% dependable [23]. The loading direction, along theheight direction of the specimen, is parallel to the substrate(namely, vertical to the coating deposition direction). A parametertaking into account the toughness in the direction parallel to thesubstrate and the coating anisotropy must be introduced. In thispaper, the obtained fracture toughness could effectively reflectintersplat adhesion, which may be important for the anti-wearperformance of the coating.

3.3. Erosion–wear testing

Slurry jet tests were carried out to estimate the erosion wearperformance of plasma sprayed alumina-based coatings. Theslurry concentration is 3 wt%. Erosion testing time is 1 h.

The erosion wear scars of Al2O3 and Al2O3–Cr2O3 coatings for dif-ferent projected particle amount are shown in Figs. 9 and 10,respectively. The red squares represent the erosion surfaces and thecorresponding dimensions are about 1 mm�1mm. With respect topure Al2O3 coating, the erosive wear morphologies present inhomo-geneity and obvious erosion damage areas (shown in Fig. 9f, g and h).On the contrary, Al2O3–Cr2O3 composite coating exhibits uniformwearscars and no evident erosion injury spots (shown in Fig. 10f, g and h).As shown in Table 3, the Al2O3–Cr2O3 composite coating has bettermechanical properties than the pure Al2O3 coating, which may indi-cate that the former supplies more defenses against slurry impact. Dueto the strengthened phase interface and grain boundary in the Al2O3–

Cr2O3 composite coating, the formation of intergranular and trans-granular cracks would be effectively restrained. Continuous slurryinjection resulted in the increase of microcracks in the coating. Theaccumulation and propagation of cracks further lead to splat delami-nation. The Al2O3–Cr2O3 composite coating has more advantages in

Fig. 10. The erosion wear scars of Al2O3–Cr2O3 composite coating for different projected particle amount: (a) 0 g; (b) 10 g; (c) 30 g; (d) 50 g; (e) 70 g; (f) 90 g; (g) 110 g;(h) 130 g. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 11. The Longitudinal section profile measurements of erosion wear scars of alumina-based coatings for different projected particle amount: (a) Al2O3 coating; (b) Al2O3–

Cr2O3 composite coating.

Fig. 12. The erosion wear graph of Al2O3 and Al2O3–Cr2O3 coatings.

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the cohesion strength inside the splats and splat-to-splat interfacestrength than the single Al2O3 coating. Accordingly, this may denotethat Al2O3–Cr2O3 composite coating possesses greater anti-erosionperformance. Shape measurement was conducted along the center ofwear scar of A–A0 line shown in Fig. 2. Longitudinal section profilemeasurements of wear scars of plasma sprayed alumina-based coat-ings are exhibited in Fig. 11. With the increase of projected particleamount, coating erosion depth increases. Compared to Al2O3 coating,the Al2O3–Cr2O3 composite coating possesses lower erosion depth atthe same projected particle amount and better erosion resistance.

When the projected particle amount is 0 g, the profile measurement isregarded as datum reference. It can be speculated from Fig. 11a thatthe bulge deformation may appear on the Al2O3 coating surface closeto erosion wear area. However, there is no similar phenomenonobserved in Fig. 11b. This indicates that particle injection has a lessnegative effect on the Al2O3–Cr2O3 coating surface near erosion zone.Fig. 12 shows the relationship between projected particle amount andmaximum wear depth for the as-sprayed alumina-based coatings.Although the wear depth has the same variation tendency withincreasing injected particle amount, the wear rates are conspicuouslydifferent for alumina-based coatings. Wear rate was calculatedaccording to the ratio of wear depth to projected particle amount.Linear fit results are y¼0.67xþ10.67 and y¼0.33xþ3.12. The lineangularity is defined as wear rate (the evaluationmethod is defined byPalmeso Company, Japan). The similar studies for wear rate mea-surement could be obtained in Ref. [24]. Therefore, the wear rates ofAl2O3 and Al2O3–Cr2O3 coatings are 0.67 μm/g and 0.33 μm/g,respectively. This may indicate that the Al2O3–Cr2O3 composite coatingpossesses better anti-erosion wear performance than the pure Al2O3

coating, which may be attributed to greater microhardness, flexuralstrength and fracture toughness of the former.

4. Conclusions

This paper shows the results of an experimental study of theerosion wear behavior of plasma sprayed alumina-based coatings

K. Yang et al. / Tribology International 93 (2016) 29–35 35

by a new type of solid particle impact test (slurry jet). The salientconclusions arising from this work are as follows:

① The addition of Cr2O3 contributes to the densification of thecoating, which may be chiefly attributed to phase interfacestrengthening by partial solid solution. The Al2O3–Cr2O3 compo-site coating possesses better comprehensive mechanical and anti-erosion wear properties than the pure Al2O3 coating.

② Compared to Al2O3 coating, the Al2O3–Cr2O3 compositecoating possesses lower erosion depth and wear rate for differentprojected particle amount.

Acknowledgments

The study is jointly supported by National Natural ScienceFoundation of China (51302299) and Shanghai Nature ScienceFund Project (13ZR1446000). Simultaneously, we are grateful toInorganic Materials Analysis and Testing Center of ShanghaiInstitute of Ceramics for providing the support for this work.

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