13

CHAPTER 2

LITERATURE SURVEY

2.1 INTRODUCTION

The objectives of the present study calls for a closer review of the

following topics:

Biomaterials

Surface engineering

AISI 316L Stainless Steel

Ion implantation

Quench Polish Quench process

Corrosion studies

Hardness studies

Wear studies

Design of Experiments

Genetic Algorithms

A literature review was carried out to understand and assess the

current status of the above areas. This chapter is organized in the order

mentioned above.

14

2.2 BIOMATERIALS

William (1964) defines biomaterials as “nonviable materials used

in medical devices, intended to interact with the biological systems”. Sujata

V. Bhat (2002) stated that the Romans, Chinese, and Aztecs used gold in

dentistry over 2000 years ago.

Hench and Ethridge (1982) discussed the usefulness of biomaterials

in replacing a part or a function of a body in a safe, reliable, economic and

physiologically acceptable manner. There are several types of biomaterials

used in orthopedics, density, neurological and etc. Jagielski et al (2006)

discussed the uses of biomaterials in detail. Biomaterials are used for making

devices that can interact with biological systems to coexist for longer service

with minimum failure. Materials used for transplantation have to fulfill very

strict requirements, the most important being those imposed by biological

reasons like blood and tissue compatibility and mainly, the mechanical

properties.

Artificial materials used for implants in medicine and dentistry,

traditionally employ titanium and its alloys, as well as stainless steel because

of its high fracture and corrosion resistance, easy handling and cost

effectiveness. Surface modification of biomaterials is being widely practised

to improve their performance since biomaterials are intended to expose to a

variety of aggressive body liquids. Pramatarova et al (2007) have dealt with

this problem in detail.

Chu et al (2002) discussed the importance of designing biomaterials

with the best surface properties to be used for implants. These properties must

possess along with the desired bulk properties that meet other requirements,

especially mechanical properties in order to function properly in a bio-

15

environment. To enhance the surface properties it would be desirable to arrive

at adequate bulk properties followed by a special surface treatment. In this

way, it allows one to make ideal biomaterials with surface attributes that are

decoupled from the bulk properties. Further, the surface properties can be

selectively modified to enhance the performance of the biomaterials. Jagielski

et al (2006) dealt with the surface treatments of biomaterials. Advanced

biological materials require an appropriate surface treatment ensuring the best

possible interface between implant and human body and optimum functional

properties. Pramatarova et al (2007) indicated that the material surfaces play a

critical role in biology and medicine since most biological reactions occur on

surfaces and interfaces. Xuanyong Liua et al (2004) have dealt with the

surface characterization. The surface characterization of biomaterials is

particularly important when biomaterials are going to be designed. This is not

an easy task considering different surface properties that are most likely to

play an important role in the reaction of the host’s body to the artificial

material.

2.3 SURFACE ENGINEERING

Since most of the reactions occur on surfaces and interfaces,

Pramatarova et al (2007) studied the role of material surfaces in biology and

medicine. There are many examples to demonstrate the fact that the surface

properties of the materials control and are directly involved in biological

reactions and processes. Cui and Luo (1999) insisted on the importance of

surface-engineered biomaterials. The importance of surface-engineered

biomaterials to the longevity of medical implants has been recognized by both

major medical device companies and by more and more patients. In bone

replacement, especially long bone and joint replacements, metal implants are

widely used to take the unsubstituted position. Although the metallic

orthopedic implants might have excellent bulk properties such as ideal

16

strength and elasticity, it has relatively poor surface properties, e.g. poor wear

resistance and limited biocompatibility. It is therefore necessary to make a

compromise between bulk and surface properties. Bharat Bhushan (1999)

showed the commonly used coating deposition techniques and surface

treatments.

2.4 AISI 316L STAINLESS STEEL

Zbeka et al (2002) discussed the selection of the material type for

biomedical applications. Metals are the most favored materials in orthopedic

surgery and particularly in total hip replacement, because of their good

mechanical stability. On the other hand, metals corrode in contact with

aggressive body fluids or tissue. Therefore, the designer must be careful while

selecting materials of this type.

Tomonori Nakanishi et al (2007) have dealt with the metallic

materials for biomedical application. Metal materials are chosen as

osteosynthesis implants for internal fixation in fractures nowadays: in

particular, austenitic stainless steel (e.g. AISI 304, AISI 316L) has been

widely used for many cases because of the following qualities: Excellent

mechanical properties, corrosion resistance, sufficient formability, and cost

effectiveness. However there are many problems related to plate breakage and

irritation with respect to the fixation devices for osteosynthesis treatment.

Gurappa (2002) dealt with the problems in metallic materials. Although the

metallic orthopedic implants might have excellent bulk properties such as

ideal strength and elasticity, it has relatively poor surface properties, e.g. poor

wear resistance. It has been reported that stainless steel corrodes in vivo. Cui

and Luo (1999) explored the problems related to wear. In case of hip

replacement, the wear debris from the implant is one of the important factor

for the aseptic loosening, which is a frequent cause of failure of the prosthetic

17

implants. It is generally accepted that improving the wear resistance and

biocompatibility by surface engineering is an optimal option.

Liu Chenglong et al (2005) found some of the applications of AISI

316L stainless steel to be useful in biomedical implants, such as orthopedic,

cardiovascular and dental devices. Metin Usta et al (2004) did considerable

research on the use of 316L austenitic stainless steel for implant fabrication

because of its greater resistance to corrosion than carbon and low-alloy steel,

primarily due to the presence of chromium. Surface modification methods are

widely used in order to improve corrosion resistance, wear resistance, and

fatigue strength of 316L stainless steel.

The properties of AISI 316L have been dealt with in detail by

Zbeka et al (2002). Austenitic stainless steel, as their name implies, has an

austenitic microstructure at room temperature and cannot be hardened to any

great extent by heat treatment, although it can be appreciably strengthened by

cold work. Gurappa (2002) studied the applications of AISI 316L SS in

reconstructive surgery, heart valve parts, wire leads, aneurysm clips and

dental uses. He further stated that AIS1 316L stainless steel is the most widely

used stainless steel for medical and dental applications. Metin Usta et al

(2004) studied the applicability of 316L SS in situations where corrosion

resistance is required and found out that the inclusion of molybdenum

enhances resistance to pitting corrosion in salt water.

Liu Chenglong et al (2005) have dealt with the corrosion of

implants inside the body and listed the applications of AISI 316L SS. In

recent years, the application of protective coatings for implants has been

attracting considerable attention. TiN film is an interesting choice for

implants due to some of its useful properties, such as chemical stability, high

hardness, excellent wear properties, electrical properties and intrinsic

18

biocompatibility. Zhu and Lei (2005) concluded that the austenitic stainless

steel and its alloys are successfully used in surgical implants, because of their

ease of fabrication and reasonable resistance to corrosion. Pramatarova et al

(2007) studied the use of titanium and stainless steel in medicine and dentistry

because of its high fracture and corrosion resistance, easy handling and

comparatively low cost.

2.5 SURFACE MODIFICATION TECHNIQUES

Various surface modification techniques have been applied to

improve the properties of AISI 316L SS. Some of these studies are

summarized below: De Oliveira et al (2003) studied the effect of the

temperature of plasma nitriding on AISI 316L austenitic stainless steel.

Double layers of nickel and aluminum were electroplated on 316L stainless

steel by Kannan et al (2005). Fossati et al (2006) studied the properties of

glow discharge nitrided AISI 316L austenitic stainless steel in NaCl solutions.

Kemin Zhang et al (2006) investigated in detail the surface modification by

HCPEB and the pitting corrosion behaviour of AISI 316L stainless steel in

saline environment. Tomonori Nakanishi et al (2007) described the

mechanical properties and corrosion resistance of fully and partially solution-

nitrided 316L stainless steel plates.

2.6 ION IMPLANTATION PROCESS

Among numerous methods of surface treatment, Jagielski et al

(2006) found that certain unique advantages of ion implantation enable it

specifically well-suited for medical applications. Halit Dogan et al (2002)

have dealt with this technique in detail. Ion implantation is a surface

modification technique that modifies the materials surface characteristics.

Micro structural effect due to the interaction between the ions of the ion beam

19

and the atoms of the target are produced in the surface. These effects may be

responsible for modifications in the wear and corrosion resistance.

Cui and Luo (1999) investigated the biomaterials modification by

ion beam processing. His study deals with the techniques and applications of

surface modifications of ion beam process, in particular, the technologies of

ion implantation and ion-beam assisted deposition. In order to improve the

corrosion and wear resistance for the austenitic stainless steel and its alloys,

different surface modification techniques such as ion implantation was used

by Zhu and Lei (2005). Newly designed polymers using ion beam

modification methods to improve blood and tissue compatibility have been

studied by Yoshiaki Suzuki and Hiroshi Ujiie (2004).

A review of literature reveals studies related to the implantation of

various ions in biomaterials. The extensive study on nitrogen implantation has

been done. Nitrogen was implanted in polymers and various biomaterials for

different applications. Xu et al (1999) performed the ion irradiation of helium,

nitrogen and oxygen on different materials. A study by Halit Dogan et al

(2002) focused on the friction and wear behaviour of N2 and Zr implanted and

TiN coated 316L stainless steel and compared it with a substrate. A study by

Rodriguez et al (2007) aims at evaluating the effect of light ions (nitrogen,

helium) implanted in く-irradiated UHMWPE samples. In their study, Micro

hardness changes are correlated with the implantation dose in order to identify

the optimum treatment conditions.

In his research of surface characterization and wear behaviour of

ion implanted NiTi shape memory alloy, Neonila Levintant-Zayonts and

Stanislaw Kucharski (2009) studied the effects of nitrogen implantation on

NiTi shape memory alloy. Ion implantation of oxygen and nitrogen in CpTi

was researched upon by Munoz-Castro et al (2009) and nitrogen implantation

20

in 316L stainless steel was also studied by Ozturk (2009). Implantation of

helium also seems to have drawn a considerable degree of attention from the

researchers. Helium was implanted in polymers for biomedical applications.

Jagielski et al (2006) conducted an experiment on the ion implantation

process for surface modification of biomaterials. Furthermore, Rodriguez et al

(2007) found that helium implantation led to a considerable increase in the

surface mechanical properties of polymers.

Xu et al (1999) worked on the surface modification of polymers by

ion implantation for biocompatible materials. The ion beam mixing technique

was discussed by Nastasi and Mayer (1994). Johnson et al (2001) described

the oxygen and hydrogen profiles in metal surfaces following plasma

immersion ion implantation of helium. Valenza et al (2004) studied the

characterization of ultra high molecular weight polyethylene (UHMWPE)

modified by ion implantation. Argon was implanted in ceramics by Jagielski

et al (2006) and polymers by Valenza et al (2004). Nitrogen and oxygen

implantations in stainless steel were studied by Anandan et al (2007). Kripton

was implanted in Ti-6A1-4V alloy by Budzunski et al (2009). Singh et al

(2002) conducted studies using a mixture of ions for implantation with

nitrogen and argon, while Chih-Neng Chang and Fan-shiong Chen (2003)

used nitrogen and carbon, Pramatarova et al (2007) used calcium and

phosphorus for their study.

2.7 SALT BATH NITROCARBURIZING AND POST

OXIDATION-QPQ PROCESS

The QPQ salt bath treatment is a type of advanced technology used

for surface strengthening. Nitrocarburizing has got its wide applications, since

improved surface hardness, fatigue strength and corrosion resistance at

elevated temperatures are achieved at minimal distortion. However since

21

additional processes of oxidizing of nitrocarburized components are also

applied, it significantly improves the corrosion resistance, while the wear

resistance and dimension tolerance are maintained. Franjo Cajer et al (2003)

have dealt with this in detail. Yeung et al (1997) stated that QPQ process is

used widely to increase surface wear resistance, to enhance fatigue strength

and to improve corrosion resistance of the treated components. Li et al (1997)

applied the QPQ surface-treatment process to AISI 1045 steel and studied the

influence of the nitriding temperature and nitriding time on the depth of the

nitriding layer. Hong et al (2000) performed the plasma post-oxidation

process after a plasma nitrocarburizing step in a comparison with a

conventional post-oxidation process and found plasma oxide film showing

improved anti-corrosion properties. Gui-jiang Li et al (2008) studied the

mircrostructure of AISI 316L stainless steel under QPQ process.

Though the QPQ is a combination of nitrocarburizing and oxidizing

process, the process was tried separately as well. Czarnowska et al (2000)

studied the surface layers produced by nitriding and nitrocarburizing to

improve the wear resistance and hardness of titanium alloy for biomedical

applications. The effects of carbon ion implantation into Ti-6Al-4V as a pre-

treatment process prior to deposition of Ta-C coatings were studied by Tong

et al (2001). Chih-Neng Chang and Fan-shiong Chen (2003) conducted the

wear resistance evaluation of plasma nitocarburized AISI 316L stainless steel

and Chun-che Shin et al (2004) studied the effect of surface oxide properties

on corrosion resistance of 316L stainless steel for bio-medical applications

and both of them found an improvement in the properties. Metin Usta et al

(2004) studied the effect of nitriding on surface properties of surgical AISI

316L stainless steel and found that the hardness of the treated material

improved significantly. Ani Zhecheva et al (2005) analyzed the effects of

nitriding in the mircrohardness and corrosion resistance of titanium and

titanium alloys.

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2.8 CORROSION STUDIES

The human body is a harsh environment for metals and alloys to be

in an oxygenated saline solution. When an orthopedic implant is surgically

installed into the human body, it is constantly bathed in extracellular tissue

fluid. All the surgically implanted metallic materials, including the most

corrosion-resistant materials, undergo chemical or electrochemical dissolution

at some finite rate due to the complex and corrosive environment of the

human body. An orthopedic implant is considered to be a failure if it is

prematurely removed from the body due to severe pain and due to the

occurrences of other reactions with the body like corrosion and wear. Thus

the corrosion study of biomaterial becomes very important. Kamachi Mudali

et al (2003) have dealt with this in detail.

In the recent years, several attempts have been made to improve the

corrosion properties of metals alloys used in orthopedic surgery. Youngjoon

Moon and Dokyol Lee (2003) formed a corrosion-protective surface layer of

Ni2Al3 compound on a 316L SS plate and studied the corrosion behaviour of

those plates. They observed that the passive currents are lower than observed

for a base SS plate.

Franjo Cajer et al (2003) observed that the corrosion as well the

hardness was improved by applying the salt bath nitrocarbunizing with post-

oxidation. The corrosion and wear-corrosion behaviour of NiTi modified by

plasma source ion implantation was studied by Tan et al (2003) and he

concluded that better wear-corrosion resistance was observed in the case of

oxygen implanted samples. Chun-che Shin et al (2004) studied the effect of

different passivative process on the in vitro corrosion resistance of 316L

stainless steel wire. To substantiate the further results, the SEM and XPS

results were used. The nitrogen plasma immersion ion implantation (PIII)

23

treatment of austenitic steels 1.4301 and 1.4571 was performed and the

influence of process conditions on the corrosion properties was studied by

Mandl et al (2005). Kannan et al (2005) stated that in case of coatings induced

by surface treatments, a passive layer is present in the ceramic metal interface

contributing to the reduction in the corrosion of coating. Since the bio

materials are used inside the body, Liu Chenglong et al (2005) prepared

Ti/TiN- multilayered film on surgical AISI 316L stainless steel and did the

corrosion study in simulated body fluid. Kemin Zhang et al (2006) found the

pitting corrosion resistance of AISI 316L SS treated by high current pulsed

electron beam was improved, from this study he observed the Ecorr, icorr and

Epit values for analisis. Murugan and Kannan (2007) conducted an

experimental study to analyze the effects of various flux cored arc welding

process parameters on pitting potential in dublex stainless steel. Mathematical

model was developed using the observed Epit values for analysis. To perform

the corrosion tests of biomaterials in simulated nature tissues environment,

Linda Gil et al (2006) suggested electrochemical tests be conducted in an

electrolyte solution of NaCl (9 g/l of H2O) at pH=6.3 and a temperature of

37 °C.

Fossati et al (2006) studied the effect of the nitriding temperature

on the pitting and cervic corrosion resistance of glow-discharged nitrided

AISI 316L SS. Tomonori Nakanishi et al (2007) made an attempt to improve

the mechanical properties and the corrosion resistance of commercial 316L

SS plates by means of solution nitriding. Saravanan et al (2007) examined

how low energy of high current density PIII affects the corrosion, wear and

hardness behaviour of 316L SS. Anandan et al (2007) observed that nitrogen

implementation affects corrosion resistance.

Ion implementation of oxygen and nitrogen in CpTi was done by

Munoz-castro et al (2009) and corrosion studies were conducted. The

24

Ecorr, icorr and Epit observations from the electrochemical study were used to

evaluate the corrosion behaviour of the treated samples. The electrochemical

behaviour of an AISI 304C SS implanted with nitrogen was studied by Abreu

et al (2008). The corrosion resistance of pulsed laser-treated Ti-6A1-4V

implant in simulated body fluids was found to be increased by Nikita Zaveri

et al (2008).

2.9 HARDNESS STUDIES

Guemmaz et al (1998) discussed that AISI 316L is widely used as a

biomaterial, due to its high corrosion resistance in biological environments.

However, in the case of applications in the domain of orthopedic prosthesis, a

major limitation is its weak wear resistance. As a matter of fact, an increase in

hardness is usually correlated with a corresponding improvement in wear

resistance. Schmidt et al (1997) measured the micro hardness of the Ti64l4V.

The hardness values were determined by measuring the depth of the

indentation while increasing loads from 0.4 to 104 N. In their discussion of

QPQ treatment, Yeung et al (1997) stated that when the temperature of the

process exceeds 560 °C, the peak hardness shifts from the outer most layers to

the inner nitride layer. Tong et al (2001) measured the Vickers microhardness

with a range of varying loads from 5 g to 20 g for carbon pre-implanted, ta-C

coated Ti-6A1-4V substrate material. He further observed that the micro

hardness increased at 5 g load and decreased to microhardness equal to that of

the substrate material at 20 g load. The surface hardness of the substrate and

nitride samples were measured using knops indenter and load of 1 N, and it

was found by Singh et al (2002) that the hardness of nitride samples were

increased. De Oliveira et al (2003) did plasma nitriding on AISI 316L SS and

observed that the treated layers presented an accentuated increase in their

hardness in relation to the hardness of the substrate. The microhardness

distribution from the surface of the plasma nitride 316L SS was measured by

25

Chih-Neng Chang and Fan-shiong Chen (2003). Metin usta et al (2004)

observations on hardness measurements indicated that the hardness of the

nitrides is much higher when compared to the base metal. The load dependent

micro hardness of nitrogen ion implanted Ti-6A1-7Nb was displayed by

Gokul Lakshmi et al (2004).

2.10 WEAR STUDIES

Li et al (2004) stated that wear characteristics constitute an

important aspect of the performance of biomaterial alloys. Failure generally

occurs due to excessive wear of the components. Serra et al (2002) stated that

total joint replacements are highly reliable medical devices, but their

long-term performance is limited by several factors which includes wear.

Bills et al (2005) found that the main cause of failure is aseptic loosening due

to the cascade of events started by the tissue reaction to wear debris. The joint

with higher wear rate typically leads to reduced function and premature

failure. Many studies have been done on the wear studies to estimate the wear

and hardness of the materials. De Oliveira et al (2003) observed a major

limitation of applying AISI 316L SS in those cases where high hardness and

wear resistance were required.

In the field of wear study, the pin-on-disc plays an important role,

due to it’s simplicity and ease in simulating the natural wear environment.

Teoh et al (1998) reported the functional properties of a triplastic titanium

graphite composite which is used in biomedical applications. Daisaku Ikeda et

al (2002) performed the friction test using pin-on-disc test machine on

nitrogen-based ion implanted joint prosthetic material and the coefficient of

friction was measured. Md Ohidul Alam and Haseeb (2002) subjected pin-on-

disc type arrangement to dry sliding wear and studied the wear performance.

Aluko et al (2002) described the design and development of an inexpensive

26

rig for carrying out rapid comparative evaluation of the wear performance of

electrode fill materials. The wear tests were carried out on carburized,

carbonitrided and borided AISI 1020 and 5115 steels with pin-on-disc

configuration and weight losses were determined as a function of sliding

distance and applied load by Selcuk et al (2003). Met et al (2003) carried out

rotating pin-on-disc friction tests at room temperature in ambient air using

ringers solution and synthetic serum to analyse the in vivo wear conditions.

The friction coefficients and wear rates of different materials were studied.

Paulo Davim and Nuno Marques (2004) used a pin-on-disc tribometer to

study the effect of sliding distance, sliding velocity and contact stress on the

friction coefficient and wear of cancellous bone.

The wear studies were conducted on various kinds of biomaterials

with various types of test rigs. Chih-Neng Chang and Fan-Shiong Chen

(2003) employed a ring-on-disc wear resistance test to evaluate the

nitrocarburized layers, derived from various treatment conditions and

observed the Vickers microhardness of the treated materials. Poliakov et al

(2004) reported on the experimental data of wear and hardness of hard DLC

thin film on UHMWPE and compared it with other biomaterials such as

Co-Cr-Mo, Ti and stainless steels. Amit Roychowdhury et al (2004)

conducted wear studies of UHMWPE, ceramics, Co-Cr-Mo alloy and

titanium which are frequently used as implant materials. The effect of

nitriding on wear and hardness properties of surgical AISI 316L stainless steel

was investigated by Metin Usta et al (2004). Saduman Sen et al (2006)

investigated the wear behaviour of borided and borided with oxidized for a

short duration AISI 4140 steel. It was observed that the friction coefficient

dropped after short-duration oxidizing.

27

Hardness is quite often used in the field of wear resistance as the

criteria for judging metals. The general conception is that the harder the

material, the greater it’s wear resistance.

2.11 DESIGN OF EXPERIMENTS

It is essential to design the experiments on a scientific background

basis rather than on the commonly employed trial and error method. Apart

from the trial and error methods of investigations, to analyse the effect of

process parameters on responses, the other methods adopted by researchers

include the theoretical approach, qualitative approach, qualitative cum

dimensional analysis approach, specific qualitative approach and general

quantitative approach. Box and Hunter (1978) have dealt with in length about

this.

DOE is a statistical method which is used to perform the

experiment work in a planned manner and to investigate the interaction

effects between the various process parameters considered. Hamad et al

(2010) stated more specifically that the Response Surface Method (RSM) has

been utilized to perform the parametric studies and to develop a statistical

method.

Kwak (2005) observed that the RSM is a collection of statistical

and mathematical methods which are useful for modeling and analyzing

engineering problems. RSM also quantifies the relationship between the

controllable input parameters and the obtained response surface. The design

procedure for RSM was discussed by Gunaraj and Murugan (1999) in detail.

The experimental design techniques commonly used for process

analysis and modeling are the full factorial, partial factorial and central

composite rotatable designs. Aslam (2008) have discussed this in length. An

28

effective alternative to factorial design is central composite rotatable design

(CCRD), originally developed by Box and Wilson (1951), and improved upon

by Box and Hunter (1961). CCRD gives almost as much information as a

three-level factorial, requires fewer tests when compared to the full factorial

design and has been proved to be efficient in describing the majority of

steady-state process responses. Obeng et al (2005) have discussed this in

detail.

A substantial research work has been done in the field of process

modeling and optimization with the application of DOE and RSM. Ghezal

and Kamal Khayat (2002) optimized the self-consolidating concrete with

limestone filler using the statistical factorial design method. Byungwhan Kim

et al (2006) used a statistical experimental design which includes the six

process parameter to analyze the temperature effect on deposition rate of

silicon nitride films.

Gaitonde et al (2008) developed second order mathematical models

for assessing burr height and burr thickness using RSM. Aslam (2008)

established the fact that the RSM and central composite rotatable design could

efficiently be applied for the modeling of multi-gravity separator for

chromites concentration. Darwin et al (2007) used Taguchi method to

optimize cryogenic treatment to maximize the wear resistance of 18% Cr

martensitic stainless steel. The RSM was employed to optimize the

microencapsulation condition of sun flower oil as typical seed oil by Jang-

Hyuk Ahn et al (2008). Guaracho et al (2009) established the fact that the

central composite design could be applied to the removal of lead and nickel

ions from sand and it was also found be an economical way of obtaining the

maximum amount of information with the fewest experiments. Aliofkhazraei

et al (2009) applied RSM to optimize the operating conditions for small

nanocrystallite sizes of coatings. He used a three factor three level matrix.

29

El-Tayeb et al (2009) reported the usage of DOE and utilization of RSM in

the study of the effect of main and mixed independent variables such as speed

and sliding distance in wear study, on the friction coefficient. The bake

hardening behaviour of AI7075 was investigated using the RSM by Kamran

Dehghani et al (2010). Hamad et al (2010) performed the laser nitriding of

commercial pure titanium using RSM in the DOE statistical approach to

determine the optimum processing parameters.

Gunaraj and Murugan (2002) developed regression equations using

the method of least squares. F–ratio and R–ratio were used to test the

adequacy of these equations. Using student t–test, the significance of each

coefficient of different parameters were tested and final models were

developed using only those significant coefficients without sacrificing

accuracy of the models. In general, this approach helps in minimizing the cost

and time of testing while at the same time increasing the probability of

success. The use of commercially available software for carrying out all the

above analysis has also been reported. Gunaraj and Murugan (1999) found the

central composite rotatable type is favored for the exploration of quadratic

response surfaces. Response surface methodology is a technique which is

used to determine and represent the cause and effect relationship between true

mean responses and input control variables as a two or three-dimensional

hyper surface.

2.12 GENETIC ALGORITHM

Most of the traditional optimization methods such as sequential

quadratic programming are not applicable for solving a large scale, highly non

linear, non convex, discrete optimization problems. This is because they are

typically highly dependent on the initial design point and tend to be tightly

coupled to the solution domain. They are essentially local search techniques,

30

as opposed to the global search methods. Fu et al (2004) dealt with this in

detail. Probabilistic algorithms, such as genetic algorithms (GAs) are suitable

for such problems. This algorithm offers significant savings, when compared

to exhaustive searches in computational costs, by selectively searching a

much smaller fraction of the solutions space for problem involving a large

number of discrete variables.

Genetic Algorithms form a class of adaptive heuristics based on

principles derived from the dynamics of natural population genetics was

discussed by Goldberg (1989). The GA combines the Darwinian principle of

natural selection “survival of the fittest” strategy to eliminate unfit solutions,

resulting in a search mechanism with surprising power and speed. Man et al

(1999) has dealt with this in detail. GA is found it’s application in many

fields. Hajela (1989) applied GA in truss design. While Dunn (1997) applied

GA in structure design, Price (1997) applied it in market simulation.

To optimize the combustion process, the genetic algorithm was

used by Ryoji Homma and Chen (2000). Suresh Kumar Reddy and

Venkateswara Rao (2006) used Genetic Algorithm to select the optimal

parametric combination for achieving a better surface finish in dry milling.

The combination of DOE and GA was used in many applications. The DOE

was used to develop the mathematical model and GA was used to optimize

the model. Ozcelik and Erurumlu (2005) attempted to determine the effecting

dimensional parameters on warpage of the thin shell plastic parts using

integrated response surface methodology and GA. To predict the temperature

effect on deposition rate of silicon nitride films, Byungwhan Kim et al (2006)

used the statistical experimental design and Genetic Algorithm.

Gaitonde et al (2008) developed the second order mathematical

model for burr height and burr thickness using RSM. The developed models

were employed with GA, to determine the optimal process parameters.

31

Combined optimization algorithm was also used by Shen Changyu et al

(2007). The combination of artificial neural network and genetic algorithm

method was used to optimize injection molding process parameters. GA is

known to provide an optimization platform method capable of treating highly

non-linear and complex behaviour problems, thereby making it an appealing

option. Jing Ying Zhang (2007) developed a new constraint handling strategy

combined with population initialization. Xiu-Juan Zhang et al (2008)

developed a detailed approach using an improved Genetic algorithm for

selecting optimal material constituent compositions according to the

optimized material properties. A C-code was developed and used to simulate

the GA by Kalyanmoy Deb et al (2009) for optimization process.

2.13 CONCLUDING REMARKS

Although research has recorded several studies conducted ion

implantation experiments on AISI 316L stainless steel, it does seem to

indicate a gap-the combination of implantation parameters and specific test

parameters like microhardness, electrochemical corrosion tests, SEM and

XRD analysis. The present study attempts to fill this gap. The following ions

were selected for implantation based on the literature survey.

Helium

Nitrogen

Oxygen

Argon

For Implantation the following parameters were considered:

Ion beam energy: 100 keV

The dose: 11017

ions/cm2

Beam current density: 1たA/cm2

32

From the survey, it can be observed further that not much of

investigations have been covered out in the following areas also:

The application of DOE in the QPQ process and development of

mathematical models involving QPQ process variables has not been much

researched. While there is considerable amount of literature available on the

application of GA in optimization of engineering problem, the investigation

on the application of GA in QPQ process parameters with the aim of required

responses is still found to be inadequate. The summary of the literature review

about the experimental designs is given below:

Use of the statistical experimental techniques based on central

composite rotatable design is well suited for QPQ process and

investigations.

The second order quadratic mathematical models can be used

in predicting the mechanical behaviour of QPQ processed

AISI 316L SS.

Response surface methodology can be employed in empirical

study of relationships between one or more measured

responses and a number of independently controllable process

variables.

It was decided to conduct experiments to predict the effects of

process parameters on responses.

Five factors, five levels central composite rotatable

experimental design was used for the present study.

33

The five factors selected for investigation were:

Bath Chemistry (C)

Process Temperature (T)

Process Time (t1)

Oxidizing Time (t2)

Post oxidizing Time (t3)

The responses were:

Pitting corrosion resistance-Epit (mV)

Hardness (HV0.1)

Wear volume loss (cm3)

Coefficient of friction-µ