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CHAPTER 2

LITERATURE SURVEY

2.1 GENERAL

A major challenge in technological development has been the need

to continuously meet the stringent materials requirements for use in

progressively demanding conditions. Exposure to increasingly aggressive

environments has made modern engineering components more susceptible to

rapid degradation due to interactions between the component surface and the

environment. In the material-environment configuration, the surface of a

component serves as the first line defence against the attack by an adverse

environment. Therefore, the component surface is always of vital importance

in determining its performance and durability. This has formed the basis for

improving the material properties by adopting the surface modification

approach. This approach involves the formation of an appropriate protective

coating on the surface to impart the desired property of combating premature

degradation of the component. Generally the surface modification approach,

which is also referred to as “surface engineering”, enables the achievement of

properties that neither the bulk nor the coating is capable of imparting on its

own. Surface engineering is a multi disciplinary subject which deals with the

modification of the surface properties of engineering components in order to

improve their function and service capabilities (Kenneth G. Budinski 2001).

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2.2 SURFACE ENGINEERING

Surfaces are the bounding faces of solids. The interaction of

component surface with the working environment results in wear and

corrosion. Surface engineering is a discipline that develops methods for

combating wear, corrosion and other surface degradation phenomena (Davis

2001; Marchione 2002). Surface engineering is a generic term now applied to

a large field with diverse technologies that can be gainfully harnessed to

achieve increased reliability and enhanced performance of degradation-prone

industrial components. The incessant quest for higher efficiency and

productivity across the entire spectrum of manufacturing and engineering

industries has ensured that most modern-day components are subjected to

increasingly harsh environments during routine operation. Critical industrial

components are, therefore, prone to more rapid degradation as the parts fail to

withstand the rigours of aggressive operating conditions and this has been

taking a heavy toll of the industry’s resources. In an overwhelmingly large

number of cases, the accelerated deterioration of parts and their eventual

failure has been traced to material damage brought about by hostile

environments like high relative motion between mating surfaces, corrosive

media, extreme temperatures and cyclic stresses. Simultaneously, research

efforts focused on the development of new materials for fabrication are

beginning to yield on the development returns and it appears unlikely that any

significant advances in terms of component performance and durability can be

made only through development of new alloys. It is vital in every product or

processes where a component or a system experiences “use” whether an

engine component, a bridge or building, a chemical plant component or power

plant components and it also refers to the control of problems originating

from the surface of engineering components (Bathelor et al 2002).

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As a result of the above, the concept of incorporating engineered

surfaces capable of combating the accompanying degradation phenomena like

wear, corrosion and fatigue to improve component performance, reliability

and life has gained increasing acceptance in recent years. The recognition

that a vast majority of engineering components fail catastrophically in service

through surface related phenomena has further fueled this approach and led to

the development of the broad interdisciplinary subject of Surface Engineering.

Generally, the solution is special treatment of the surface to delay or even

prevent corrosion, erosion, fatigue, abrasion or adhesive wear. This special

treatment may be the application of a coating or a modification of the surface

(Alam and Ion 2002). A protective coating deposited to act as a barrier

between the surface of the component and the aggressive environment that it

is exposed during routine operation is now globally acknowledged to be an

attractive means to significantly reduce and suppress damage to the actual

component by acting as the first line of defence. The increasing utility and

industrial adoption of surface engineering is a consequence of the recent

advances in the field. Surface Engineering today is the best defined as “The

design of surface and substrate together as a system to give a cost effective

performance enhancement of which neither is capable on its own”.

2.2.1 Advantages of Surface Engineering

In brief, surface engineering is relevant to all types of products. It

increases the performance and controls the surface properties independently

of the substrate offering enormous potential for improved functionality. It also

provides solution to previously insurmountable engineering problems, the

possibility to create entirely new parts, conservation of scarce material

resources and reduction in power consumption and waste output. The success

of the surface engineering is demonstrated by the applications of surface

23

technologies like thermo chemical treatments and coatings to a variety of

engineering components.

In the past years the role of surface engineering has become a well

recognized area for ensuring enhanced materials performance particularly in

severe environments (Bhuvaneswaran et al 2003) and also because of the

growing commercial requirements of wide range of cost effective surface

technologies (Bell 1992). Surface engineering of components are subjected to

higher stress and greater fatigue, abrasion and corrosive damages than the

interior. Therefore more than 90% of the service failures of engineering

components initiate at or near the surface (Gopalakrishnan et al 2002)

2.3 SURFACE MODIFICATION / TREATMENT

By virtue of the versatility of this approach, everyone is confronted

daily, either directly or otherwise, with products that have been modified for

functional or decorative reasons. Surface engineering is now applicable to a

wide range of industry sectors and the end purpose of a surface modification

process could be manifold. There is growing realization that the surface

modification approach does not only solve component degradation problems

but can provide added value. This could be in the form of significant

improvement in productivity due to less frequent repairs and consequent

down time. The development of surface engineering has been dynamic largely

on account of the fact that it is a discipline of science and technology that is

being increasingly relied upon to meet all the key modern-day technological

requirements such as materials savings, enhanced efficiencies, environmental

friendliness etc. The overall utility of the surface engineering approach is

further augmented by the fact that modifications to the component surface can

be idealized through metallurgical, mechanical, chemical or physical ways

(Groover 2002). At the same time, the engineered surface can span at least

five orders of magnitude in thickness and three orders of magnitude in

24

hardness. The development of a suitable high-performance coating on a

component fabricated using an appropriate high mechanical strength

metal/alloy offers a promising method of meeting both the bulk and surface

property requirements of virtually all imagined applications. The newer

surfacing techniques, along with the traditional ones, are eminently suited to

modify a wide range of engineering properties. The surface properties of the

metals can be improved by changing the chemical compositions of the metal

matrix at the surface level by thermo chemical processes like carburizing,

nitriding, boriding (Wahl 1996) or by hard coatings on the surface, induction

hardening and ion-impregnation on the surface (Kamat 1995). The properties

that can be modified by adopting the surface engineering approach include

tribological, mechanical, thermo mechanical, electrochemical, optical,

electrical, electronic, magnetic / acoustic and biocompatible properties.

2.3.1 Surface Modification / Treatment Methods

Driven by technological need and fuelled by exciting possibilities,

novel methods for applying coatings, improvements in existing methods, and

new applications have proliferated in recent years. Surface modification

technologies have grown rapidly, both in terms of finding better solutions and

in the number of technology variants available, to offer a wide range of

quality and cost. Typically there are a large number of variants in each of the

coating processes available. The significant increase in the availability of

coating processes of wide ranging complexity that are capable of depositing a

plethora of coatings and handling components of diverse geometry today

ensures that components of all shapes imaginable can be coated economically.

The surface engineering options are ranging from traditional, well established

techniques to the more technologically demanding coating technologies and

surface treatments. Existing surface treatment processes fall under three broad

categories:

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(a) Overlay coatings: This category incorporates a very wide

variety of coating processes wherein a material different

from the bulk is deposited on the substrate. The coating is

distinct from the substrate in the as-coated condition and

there exists a clear boundary at the substrate/coating

interface. The adhesion of the coating to the substrate is a

major issue. (eg. Electroplating)

(b) Diffusion coatings: Chemical interaction of the coating-

forming element(s) with the substrate by diffusion is

involved in this category. New elements are diffused into

the substrate surface. Usually at elevated temperatures, the

composition and properties of the outer layers are changed

as compared to the properties of the bulk. (eg. Nitriding)

(c) Thermal or Mechanical modification of surfaces: In this

case, the existing metallurgy of the component surface is

changed in the near-surface region either by thermal or

mechanical means, usually to increase its hardness.

(eg. Flame hardening)

A variety of surface modification techniques are currently available

and their abilities differ substantially in several aspects like (i) type of

coatings (ii) coating thickness achievable (iii) size of components coated

(iv) component geometries / accessibility of surfaces to be coated and

(v) substrate temperature during coating. Apart from the above processing

issues, the coating characteristics in terms of porosity levels, bond strength,

hardness etc., also vary considerably for each technique. As a consequence, a

single coating technology may not be the most useful, desirable or economical

for all the coating requirements that emerge. This is often dictated as (a)

function for which coating is required (b) location of surface to be coated and

26

its accessibility (c) temperature of the part in use (d) coating thickness

required (e) surface finish desired (f) dimensional tolerances and (g) the cost.

In view of the above, the advantages as well as limitations of all the available

coating options and their utility need to be fully appreciated in order to be

able to make an intelligent choice of the most suitable process to meet any

arising need for a surface coating.

With the present day availability of a wide spectrum of surface

modification technologies, the surface of a component can be virtually

“tailored” by selecting a proper surfacing method in order to meet the

required properties without compromising the mechanical characteristics of

the bulk material. Surface engineering, however, is not confined merely to

improve the component properties against surface related degradation

processes such fatigue, corrosion, wear etc. This approach can also be applied

for cases where specialized surface properties that are not necessarily related

to surface protection and /or performance enhancement of the component, are

desired as exemplified by the category of abradable coatings for engineering

applications which are sacrificial in nature (Das 2003).

Surface modification methods also prove vital in saving /

substituting valuable and scarce bulk materials and for reclaiming damaged

components. Furthermore this approach provides the designers considerable

flexibility in use of materials. All the above mentioned benefits of surface

modification methods translate into significant cost effectiveness. Numerous

surface modification techniques have been developed over the years and the

designer today has the advantage of choosing from a broad array of surfacing

technologies that offer a wide range of quality and cost. Several attempts have

been made in the past decades to modify the surface of the stainless steels to

enhance their surface hardness and tribological properties. Thermo chemical

surface engineering such as nitriding and nitrocarburizing proved to

27

significantly improve the surface hardness and wear resistance of austenitic

stainless steels (Bell 2002).

2.4 NITRIDING

Nitriding is a ferritic thermochemical treatment process which

involves diffusing atomic nitrogen in to surface of metals to produce a hard

case. This diffusion process is based on the solubility of the nitrogen in iron

as shown in the equilibrium diagram (Figure 2.1). The solubility limit of

nitrogen in iron is temperature dependant and at 450˚C the iron base alloy will

absorb upto 5.7 to 6.1% N (Pye, 1939). As the temperature is further

increased beyond the gamma prime (') phase temperature at 490˚C, the

“window” or limit of solubility begins to decrease at a temperature 650˚C.

The equilibrium diagram (Figure 2.1) shows that control of the nitrogen

diffusion is critical to process success. This process holds significant role in

the industrial applications, among the processes available for improving the

surface properties of engineering components (Kurney et al 1983).

Figure 2.1 Iron-Nitrogen equilibrium diagram (Pye 1939)

Tem

pera

ture

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This process of case hardening was first used at the end of 1920s and since then its application has continuously spread among other things

owing to the fact that the process has been further developed and can now be applied to much larger varieties of steels than originally thought possible. The more important effect of nitriding is the ability to form hard compounds with iron. These compounds can give rise to the surface hardness as maximum as possible.

The beneficial properties imported by nitriding to steels can be summarized as follows.

High surface hardness and wear resistance together enhance

anti-galling properties.

Increased high temperature hardness / Enhanced Hot hardness

High fatigue strength and low fatigue sensitivity

Improved corrosion resistance.

High Dimensional Stability

It is well known that nitriding produces a case of moderate hardness due to the diffusion of nascent nitrogen quickly beneath the surface. The

quick diffusion forms hard complex iron-nitrides dispersed to greater depths. Since nitride forming elements have a greater affinity for nitrogen they

prevent the diffusion to a greater depth giving an extremely hard but shallow case (Ashrafizadeh 2003). Nitriding is used to confer both wear resistance and fatigue resistance on engineering components. The advantage of nitriding over the other surface hardening methods is the reduced risk of distortion

through the treatment temperature and the elimination of quenching. The notable limitation is the formation of a brittle white layer which is to be removed for many applications before a nitrided component can be put in service (Clayton and Sachs 1976).

29

Formation of the nitrided layer case begins through a series of

nucleated growth areas on the steel surface. The nucleating growth will

eventually become what is known as the compound layer (Pye 1939). This

layer is usually very hard and brittle and consists of two intermixed phases as

ε (epsilon) phase containing Fe2-3 N and γ' (gamma prime) phases containing

Fe4N (Davis 2002). This layer does not diffuse into the steel, but remains on

the immediate surface and grows thicker with time, temperature and gas

composition. The layer immediately beneath the compound zone is called as

the “diffusion zone” (Figure 2.2). This region is made up of stride nitrides

formed by the reaction of nitrogen with the nitride forming elements such as

Cr, Mo and Va. It is from this layer the fatigue and load bearing strength are

determined and the area below this zone is the core of the metal (Davis 2002).

The area below the diffusion zone is the core of the metal which comprises

formation of nucleation of γ' at the immediate steel surface interface with the

nitriding atmosphere. This nucleation process progresses and continues until

subsequent nucleation of ε at the steel surface interface.

Figure 2.2 Typical nitrided case (Pye, 1939)

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2.5 METHODS OF NITRIDING

Nitriding is effected in a salt bath nature (liquid nitriding), in a

gaseous medium (gas nitriding) or in a plasma state atmosphere (plasma

nitriding).

2.5.1 Liquid Nitriding

Liquid nitriding (or) salt bath nitriding process as an alternative to

the gas nitriding process was sought in the mid 1930s, which would produce a

more uniform and better metallurgically formed case. A liquid would fulfill

the uniformity requirement through surface contact of the liquid to the steel.

The depth and quality of the case would be determined by the chemical

composition of the liquid. A heat source would be necessary to drive the

nitrogen into the steel surface. Salt bath nitriding utilizes the melting of salt

containing rich nitrogen source. When heat is applied from either an internal

or external source the salt melts and liberates nitrogen into the steel for

diffusion. When the steel work piece is introduced into the salt bath and

heated upto a temperature in the molten salt, controlled amounts of nitrogen

are released to diffuse into the surface. The salt bath nitriding technique

gained popularity in the early 1950s, because it required a low capital

investment than gas nitriding.

Liquid nitriding is carried out in a molten salt bath in a temperature

range of 783 – 893K. The case hardening medium in this method is molten

nitrogen bearing fused salt bath containing cyanides and cyanates (Mehrkam

et al 1991). Formation of pores which occurs during salt bath nitriding

increases with the increase in time. This restricts the nitriding time in the

liquid process to maximum of 4 hrs. Salt bath nitriding is a very active

process and more intense than the gas nitriding and plasma nitriding processes

(Funatani 2004).

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The typical commercial bath for liquid nitriding is composed of a

mixture of sodium and potassium salts. The sodium salts which contribute

60-70 wt% of the total mixture consist of NaCN, NaCO3, and NaCNO. The

potassium salts contribute to 30-40% of the total mixture comprising KCN,

K2CO3, KCNO and KCl. The liquid bath initially aged at a selective time-

temperature for combination to increase the cyanates content and then parts

are immersed in the liquid bath for further processing.

Due to the intermetallic composition of the compound layer,

friction and the tendency to weld with the metallic counterpart are reduced.

Salt bath nitrided components exhibit excellent sliding and running in

properties as well as greater wear resistance. Salt bath nitriding improves

wear resistance, lubricity, fatigue strength and corrosion resistance as a result

of the presence of iron nitride compounds formed at the surface in addition to

a zone of diffused nitrogen in solid solution with the base metal adjacent to

the compound layer (Easterday 2001). Both of these zones are metallurgically

discernible, each providing specific engineering properties like antigalling,

antiseizing characteristics and reduced tendency for fretting corrosion. Salt

bath nitriding is an effective and economical means to enhance performance

of engineered components made up of ferrous metals.

2.5.2 Gas Nitriding

Gas nitriding is a casehardening process whereby nitrogen is

introduced at surface of a solid ferrous alloy by holding the metal at a suitable

temperature in contact with a nitrogenous gas, usually ammonia (Knerr et al

1991). The nitriding temperature for all steels is between 763K and 863K.

Because of the absence of a quenching requirement with volume changes, and

the comparatively low temperature employed in this process, nitriding of

steels produces less distortion and deformation than either carburizing or

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conventional hardening. Some growth occurs as a result of nitriding but

volumetric changes are relatively small.

In gas nitriding, anhydrous ammonia is allowed to flow over the

parts to be hardened at 823 K. The ammonia gas dissociates into nitrogen and

hydrogen at the part surface. At the instant of dissociation, nitrogen is

liberated in atomic form and is absorbed by materials as such whereas the

hydrogen becomes the part of atmosphere in the furnace. The depth of

hardening and extent of case hardness developed due to gas nitriding depends

on several factors such as time-temperature schedule of the process, degree of

dissociation of ammonia, concentration and nature of nitride forming

elements etc. The major limitation of gas nitriding is longer process time

which gives rise to the increased cost of the process. Gas nitriding has never

been developed to its fullest potential due to the long process times, increased

material and treatment cost with lack of sufficient control over the process

(Staines 1996).

Gas nitriding becomes extremely handy, when a greater depth of

nitriding is required. For the same depth of hardening, gas nitriding causes

less dissociation of the hardened parts than the liquid nitriding. In terms of

wear resistance and toughness of hardened steels, gas nitriding is believed to

have slight edge over liquid nitriding.

2.5.2.1 Single and Double Stage Nitriding

Either a single or a double stage processes may be employed when

nitriding anhydrous ammonia. In the single stage process, a temperature in

the range of about 768 K to 798 K is used and the dissociation rate ranges

from 15 to 30%. This process produces a nitrogen rich layer known as the

white nitride layer which is extremely hard but very brittle. The double stage

process has the advantage of reducing the thickness of the white nitrided

33

layer. The first stage of the double stage process is a duplication of the single

stage process except for time. The second stage may proceed at the nitriding

temperature employed for the first stage or the temperature may be increased

from 823 K to 838 K. However, at either temperature, the rate of dissociation

in the second stage is increased to 65 to 80% (preferably 75 to 80%).

Generally an external ammonia dissociator is necessary for obtaining the

required high second stage dissociation.

The main purpose of double stage nitriding is to reduce the depth of

the white layer produced on the surface of the case. Except for a reduction in

the amount of ammonia consumed per hour, there is no advantage in using the

double-stage process unless the amount of white layer produced in single

stage nitriding cannot be tolerated on the finished part or unless the amount of

the finishing required after nitriding is substantially reduced.

2.5.3 Plasma Nitriding

The plasma nitriding process patterned by Egan (1931) is a well

demonstrated surface hardening process, which provides wear, fatigue and

corrosion resistant surfaces (Prabhudev 1998 and Baldwin et al 1998). The

plasma nitriding is a method of surface hardening using glow discharge

technology (ionized gas) to introduce nascent nitrogen into the surface of a

metal part for subsequent diffusion into the material (O’Brien and Goodman

1991). Depending on the process parameters and material composition a

diffusion zone is formed with nitrogen penetrating upto 0.7mm into the

surface. Frequently a surface compound zone is also formed on the top of the

diffusion zone with micron range thickness. In the diffusion zone the

microstructure is changed by the introduction of single intensified N atoms in

to solid solution and when solubility limit is reached, very fine coherent

nitride precipitates are formed.

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The hardness is slightly changed by the nitrogen in solid solution,

while the hardness increases substantially when the nitrogen is in the form of

nitride. Precipitates form depending on the nitride forming alloying elements.

In the compound zone, intermetallics and nitrides of the alloying '- Fe4N, -

Fe2-3N, and Cr2N, CrN are formed. The advent of plasma nitriding (Edenhofer

1974) has led to an increased interest in the application of the process to

various alloy steels for obvious advantages offered by these materials. Such

treatment would modify the surface related properties such as resistance to

applied loads, adhesive and abrasive wear, rolling contact fatigue and

corrosion.

2.6 NITRIDNG OF AUSTENITIC STAINLESS STEELS

Nitriding is a proven technology used to improve the surface

properties, providing fatigue life, wear and corrosion resistance enhancement

of components made of austenitic stainless steels. The uptake of nitrogen onto

the surface could produce several modifications. As the nitrogen atomic

concentration grows in the region and solid interstitial solution of N exceeds

the solubility limits, precipitation of iron chromium nitrides would occur. A

more selective formation of some nitrides depends on the process parameters

like gas composition, nitrogen ion density, treatment temperature and time.

Among the most desirable nitrides to be synthesized in the compound layer,

are '-Fe4N, -Fe2-3N, N, CrN, and Cr2N. etc. These nitrides have good

tribological properties like enhanced hardness, reduced wear rate, friction and

corrosion resistance. The nitrogen ions driven by thermally activated diffusion

could penetrate far beyond the compound layer.

2.6.1 Surface Characteristics studies

Zhao et al (2005) have found that nitrogen and carbon atoms can

simultaneously be dissolved into the austenite lattice during the

nitrocarburizing process, forming a nitrogen and carbon supersaturated solid

35

solution, which has a Face Centre Tetragonal (FCT) structure (i.e. the S

phase). Glow Discharge Spectroscopy (GDS) analysis showed that the

maximum nitrogen and carbon concentrations in the nitrocarburised S-phase

layer occur at different depths from the surface. The nitrocarburised layers not

only had high hardness like the nitrided layer, but also high thickness and

gradually reduced hardness profile like the carburized layer.

Picard et al (2001) have reported that the nitrogen implantation

process induces the formation of the metastable γN phase. This γN phase

resulted from the incorporation of nitrogen into the interstitial positions of the

face centered cubic structure of stainless steels. The ion-implanted specimens

showed a significant increase in the hardness value originating from the γN

phase with compressive residual stresses and micro distortions in the high

nitrogen containing layer. From the hardness measurements and Glow

Discharge Optical Spectroscopy (GDOS) profiles, it was evident that nitrogen

diffused into steel to a depth of 3 mm. The presence of nitrogen improves the

corrosion behaviour in acid media and the stainless character was maintained.

In chloride media, the very high nitrogen content (30 at. %) modified the

repassivation process in such a way that the potential range in which

repassivating pitting occurs was broadened.

The effect of plasma nitriding time on the microstructure and phase

composition of nitrided layers on austenitic stainless steel was investigated by

Liang Wang et al (2006). The phase composition and structure of the nitrided

layer have been studied by X-Ray Diffraction (XRD) and Scanning Electron

Microscopy (SEM). The XRD analysis of samples treated at 693 K showed

the presence of γN phase in the nitrided layers for all nitriding times involved

in this study. The lattice parameters calculated based on γN (111) and γN (200)

were different and became larger with time for up to 5hrs of nitriding

treatment. The surface hardness of nitrided layer was also increased with

36

nitriding time. The maximum thickness of the nitrided layer was observed to

be 27 µm.

The microstructure of the layers produced by plasma nitriding

austenitic stainless steel at treatment temperatures 673 and 773 K were

studied by Xu et el (2000) with Scanning Electron Microscopy (SEM)

together with X-Ray Diffraction (XRD) and Transmission Electron

Microscopy (TEM). The results showed that the microstructures were

composed of ‘expanded austenite’ (γn) and α (ferrite) + CrN following plasma

nitriding at lower and higher treatment temperatures, respectively. The former

contained stacking faults and deformed twin substructures, while the latter

was made up of colonies displaying a lamellar structure.

Baranowska and Powel (2007) have investigated the mechanical

properties of thin nitrided layers produced on stainless steels and reported that

a hard nitrided layer of hardness 5 times harder than the matrix was observed

at the surface level. Brief nitriding of low carbon stainless steels makes it

possible to obtain even higher quality case with a higher hardness. The case

hardness varies with dispersity of the nitrides and the maximum hardness is

attained when the nitrides are in the precipitation stage ensuring the largest

distortion of the matrix lattice (Yakhnina and Mescherinova 1974). Plasma

ion nitrides of 316L grade stainless steel for a time of 8 hrs at a process

temperature of 723 K produced nitrogen expanded austenite (N) layer with a

thickness of 6µm and average hardness values of about 1800 Hk0.01 (Linda

et al 2006).

Priest et al (1999) have reported in their study that low pressure RF

plasma nitriding of austenitic stainless steels resulted in a surface layer of

2mm thick with a fairly constant nitrogen concentration and very thin

interface region. The X-ray diffractograms for the samples around (111) and

(200) austenite peaks were barely seen at an incidence angle of 2 theta. The

37

tribological and corrosion resistance properties of austenitic stainless steels

enhanced by the formation of austenite phase was investigated (Gontijo et al

2003). In this work, it was reported that the modified layer consists of an

austenite phase with different nitrogen content and ' - Fe4N and - Fe2-3N

phases were instrumental in enhancing the surface properties of austenitic

stainless steels.

A novel electrochemical surface modification for high nitrogen

stainless steels, which significantly lowered the wear rates under self mating

conditions, was presented (Buscher and Fischer 2003). It was also suggested

that this process might be a suitable treatment for one of the sliding partners

in all metal prostheses.

Nitriding of AISI 304 grade austenitic stainless steels using Plasma

Immersion Ion Implantation resulted in a uniform layer composed of ()

gamma expanded phase and compounds such as Fe3N. The hardness was

observed to be incremented several times compared to the untreated samples

due to the lattice expansion created by the super saturation of nitrogen

(Valencia 2004). The nitriding of austenitic stainless steels at 823K for 3 hrs

resulted in a compound layer of 44 µm thick with a hardness of 1434 Hv0.01,

consisting predominantly of γ'-Fe4N and CrN phase (Baggio-scheid et al

2006).

The effect of plasma nitriding time on the microstructure and phase

composition of nitrided layers of AISI304 stainless steels was investigated by

Liang et al (2006). The XRD analysis of the nitrided layer showed the

presence of γN phase with broad peaks of (111) and (200). An expanded

austenite layer was formed on the surface of substrate with the thickness

ranging from 2µm to 27µm. The hardness was enhanced upto a maximum of

1250Hv by the formation of nitrided layer due to the nitrogen diffusion

into substrate.

38

Plasma nitriding of stainless steels at temperature about 773K

showed nitrogen enriched layers with higher nitrogen content leading to a

significant increase in the hardness (Larisch et al 1999). The XRD patterns of

austenitic steels exhibited that a set of broad peaks associated with a

metastable phase called “expanded austenite” together with the substrate

austenite peaks. The study of sub surface modifications induced by nitrogen

ion implantation revealed that the surface hardness reached upto 600Hv

(Guemmaz et al 1998). From the XRD study it was also observed that

nitriding induced phases like ε – Fe2-3N and α – Fe besides the predominant

γN phase which remains most important.

In the study of pulsed DC glow discharge nitriding with optimal

process conditions, a compound layer of 50 m thickness with a maximum

hardness of 1400Hv0.1 was observed. From the XRD study, it was observed

that in addition to the γ'-Fe4N phases, CrN and Cr2N phases were also seen in

the compound zone of nitrided austenitic stainless steels (Berg et al 2000).

Nitrogen implantation enhanced the mechanical and surface modifications.

The hardness was improved to a maximum value upto 800Hv. The

microstructural analysis through glancing incidence of X-Ray diffraction,

showed iron-nitride (ε – Fe2-3N) formation and high level of residual stresses.

The verification of glancing angle indicated that the residual stress was non

homogeneously distributed through the depth (Pelletier, 2002). The study on

the nitriding of AISI 316 stainless steels by low pressure RF plasma nitriding

(Baldwin et al 1998) exhibited a nitrided layer containing f c c γN (111)

phase. The micro hardness of nitrogen enriched expanded austenite phase was

found to be about 700Hv with a layer thickness of 40µm.

The surface modification of austenitic stainless steel has been paid

close attention by many researchers. Xiaolei et al (2005) reported that the

peaks of phases induced by nitriding were considerably broadened and shifted

39

to lower diffraction angles when compared with the peaks of untreated

samples. This shift and broadening of phases were associated with the

expanded austenite (N) produced by nitrogen super saturation and associated

stress caused by nitrogen present in solid solution in the f c c lattice.

A combined surface modification treatment consisting of ion

nitriding at high pressure and higher temperature followed by a cycle at low

pressure using a gas mixture at H2-N2 was applied to austenitic stainless steel.

The resulting thick modified layer presented mainly the presence of expanded

austenite (N) phase and mixed iron – chromium nitrides, with a surface

hardness of 2.7 times higher of standard untreated surface value (Gomes et

al 2003). Gaseous thermo chemical treatment of austenitic stainless steels at

moderate temperature would give rise to the transformation of the surface

adjacent region into nitrogen expanded austenite (Christiansen and Somers,

2006). The hardness depth profile showed a smooth decrease near the surface

followed by a sharp drop in hardness at a position close to the surface /

substrate interface. A maximum hardness value of about 1585Hv was

obtained directly at the surface. It was evident that the nitrides of nitrided

layer are very hard and the transition from the hardened zone to the soft

substrate occurred within a few microns. The diffractograms for the nitrided

samples exhibited clearly the shift of (peaks) Bragg reflections to lower

angles, mainly due to the dissolution of nitrogen.

The influence of nitriding temperature time and gas mixture ratio of

Nitrogen to Hydrogen on the nitriding behaviour of chromium plated type

316LN stainless steels have been investigated (Kuppusami et al 2002). The

results indicated that the nitriding temperature plays a dominant role in

obtaining hardness and case depth in this material. A reasonable value of

hardness of 550Hv with a case depth of 5µm was obtained when the nitriding

temperature was about 833K. The hardness have reached a maximum value of

40

1800Hv at 1273K with a case depth of 40 m. Nitriding at an intermediate

temperature of 913K produced a peak hardness of about 900Hv with a

uniform decrease in hardness or a function of depth and hard surfaces with

excellent bonding with the matrix.

The microstructure of nitrocarburised and post oxidized 304L

austenitic stainless steels produced by Quench Polish Quench (QPQ) complex

salt bath process was investigated (Gui jiang 2008). The nitrocarburised

coating as a whole composed of 3 sub layers namely the CrN layer, the

intermediate γ'-Fe4N layer and an inner expanded austenite (N') layer.

The XRD analysis of the post oxidized sample revealed the

existence of a small concentration of ε – Fe2N sub layer. A modestly higher

sub surface hardness and an overall superior hardness profile were obtained

by post oxidizing. The measured hardness for post oxidized samples was

fairly uniform and reached a maximum value of 1200 Hv0.1 with a thickness

of 35 m and then decreased sharply with in a further depth of 20m.

The study on the crystal structure and micro hardness of

carbonitrided layers produced by RF plasma processing of austenitic stainless

steels exhibited the predominance of new phases like Fe2-3N and Cr2N (El-

Hossary et al 2001). These phases were responsible for the significant

increase in the hardness which related to the maximum value of about

1715 Hv0.01 with a case depth of 20m. Plasma processing resulted in a

uniform surface layer of hardness about 750 Hv0.05 along with an effective

hardening depth of 40µm (Suh and Lee 1997).

Recco et al (2007) investigated High Temperature Gas Nitriding

(HTGN) of austenitic stainless steel specimen and reported a hardened case of

1.2 mm thick with enhanced hardness. It was also found that the high

temperature gas nitriding reduced the wear rate of austenitic stainless steels

41

by the order of 1.5 magnitudes. The hardness, after Plasma Immersion Ion

Implantation (PIII) treatment on austenitic stainless steels reached the

maximum value of about 1200Hv with better corrosion resistance than the

base material (Mandl et al 2005). The effect of a metastable nitrogen super

saturated fcc phase, when applied as a coating using a reactive magnetron

sputtering on the corrosion wear resistance of austenitic stainless steel was

investigated by Dearnley (2002). The results showed a considerable increase

in surface hardness value to a maximum of 1600Hv. It was also reported that

corrosion wear of metastable phase reduced with the increase in nitrogen

concentration of the coating.

Usta et al (2004) studied the effect of nitriding on the properties of

surgical AISI 316L stainless steel in a fluidized – reactor at a temperature of

833K for 16 hrs. The hardness of the nitrogen rich layer of 40 m thick was

about 1200Hv and the XRD analysis confirmed the presence of nitrides such

as CrN, Fe2-3N and Fe4N on the surface layer. The hardness of nitrided

specimen decreased with distance from surface to the interior of this

specimen. Scanning Electron Microscopic study combined with energy

dispersive X- ray spectroscopy revealed the presence of nitrogen in the outer

layer and in the transition layer but not in the base metal. It was also reported

that surface design with improved hardness and corrosion resistance is

possible by sequential plasma processing process (Tsujikawa et al 2005) with

the hardness of diffused layer increased to a maximum value of about

1050Hk.

Dima and Cazacu (2006) investigated the effect of nitridation on

various grades of austenitic stainless steels. In their experiments they have

carried out the surface hardening in a fluidized bed, where a gaseous

atmosphere of N2 mixed with dissociated 30% NH3 was maintained at a

42

temperature of 823K. They found that the nitridation resulted in the strong

increase in a hardness upto 1314Hv0.05 and case thickness about 40m.

The microstructure of the plasma nitrided austenitic stainless steel

specimen was studied elaborately by Xiaolei et al (1998). The results

indicated that the surface of the modified layer consists of supersaturated

solid solution (N) based on the -Fe4N) phase. Plasma immersion ion

implantation on vacuum melted 316L grade austenitic stainless steels resulted

an increase in surface hardness with better wear resistance and increased

corrosion resistance (Saravanan et al 2007). It was also reported that the

broad XRD peaks were observed in the X-ray diffractograms due to the

presence of the mixed nitrides like FeN, ' – Fe4N.

The incorporation of nitrogen in stainless steel is widely applied to

provide major improvements in material performance with respect to fatigue,

wear, tribology and atmospheric corrosion. These improvements rely on

modification of the surface and the sub surface of the material by the

precipitation of alloying element nitrides or by the development of continuous

layer of iron based nitrides (Somers and Christiansen 2007). Nitriding at

823 K for 48 hrs in ammonia and ammonium chloride environment resulted in

a diffusion layer of 0.25-0.3mm thick with a hardness of about 820 – 850 Hv

and magnetic permeability above 1.05 G/Oe (Nikonerova and Florensova

1965).

2.6.2 Wear Studies

It was reported that ion nitriding improves the wear resistance of

austenitic stainless steel type AISI 316L. The nitrided layer had hardness

around 1200 Hv, appeared to be uniform and formed by two sub layers. These

nitrided layers reduced the subsurface plastic deformation on a rolling-sliding

test at 50 kg normal load. The applied contact stress produced a significant

43

plastic deformation on the surface and subsurface, except on the nitrided

samples tested at 50 kg load. Such deformation, observed through the micro

hardness profiles and micrographs showed that at this load level, the stresses

at the nitrided layer were lower than its yield strength because at the end of

the test, the nitrided layer still remained (Cabo et. al. 2005). The effect of

sliding wear on tribocorrosion behavior of austenitic stainless steels of grade

AISI 304L was studied and reported that the sliding wear affected the

repassivation by increasing the anodic current and greatly influenced by the

increase in normal force (Berradja et al 2006).

Singh et al (2002) processed AISI 316L grade stainless steel using

Intensified Plasma Assisted Processing (IPAP) technique and reported that

IPAP processed samples exhibited a single phase nitrided layer with a four

fold increase in the surface hardness, significantly lowered coefficient of

friction with lesser wear rate. Also it was reported IPAP showed no adverse

effect in the corrosion resistance. The compound layer formed by

corbonitriding with pulsed glow discharge process resulted in increased

hardness upto 1250Hk0.25 and exhibited the presence of -Fe3-2N, ' – Fe4N,

Fe3C and CrN phases (Gontijo et al 2003).

The wear behaviour of gas nitrided austenitic stainless steel in a

corrosive liquid environment showed that wear performance was superior.

The nitrided layers were mainly composed of γN phase. In the samples ,

nitrided at 723K, an increased quantity of nitrides mainly Cr2N and Fe2N were

found in the surface area (Baranowska et al 2007). The ion nitriding of 304L

and 316L grade austenitic stainless steel specimen resulted in a diffused layer

of thickness about 13 µm along with surface hardness of about 1060 Hv0.025

and significant improvement in wear resistance (Poirier et al., 2002). A

substantial reduction in wear by two orders of magnitude and an increase in

hardness of factor of 4 were obtained with 40-kV PIII process. It was also

44

observed from the XRD analysis that the nitrided layer of 10µm thickness

consists of γ'N expanded austenite phases (Mandl et al 1998).

A sizeable reduction of wear by more than two orders of magnitude

and an increase of hardness at the surface upto 1200Hv were obtained as a

result of plasma immersed ion nitriding of austenitic stainless steels (Richter

et al 2000). The corrosion behaviour tested by potentiodynamic measurements

in 0.1N H2SO4 solution with a sweep rate of 10mV/s, demonstrated that the

nitrided layer composed of expanded austenite was the cause for the increase

in the hardness and wear resistance without the loss of corrosion resistance at

moderate process temperature.

Lebrun et al (2002) have reported that many components used in

nuclear power stations have to meet strict requirements in terms of in-service

corrosion resistance. In their work, they plasma processed the components

made of austenitic stainless steels used in nuclear reactor assembly and

reported that the nitriding allowed a considerable extension in the wear life

time and ensured a good corrosion behavior in various media like pressurized

water, aerated water and acid etc.

Increase in the surface hardness showed higher erosion resistance

as a result of compressive stresses which were induced into the surface by a

compound layer produced on the target surface by gas nitriding (Divakar et al

2005). The ion nitriding of AISI 316L grade austenitic stainless steels

exhibited the development of a nitrided layer consisting N phase lattice

(Nosei et al 2004). It was also reported that modified surface layer could

conserve the structure at base material from distortions. The micro hardness

of this nitrided layer was observed to be about 1340Hv0.025.

45

2.6.3 Corrosion Studies

The austenitic stainless steels are widely used in many industrial

fields because of their very high general corrosion resistance; nevertheless

they suffer from pitting in specific environments and their lower hardness

with low wear resistance limit the number of possible industrial applications.

Glow discharge nitriding of AISI 316 austenitic stainless steels with varying

treatment time was investigated. The morphological analysis and surface

characterization of treated samples exhibited that the surface hardness was

much higher than the untreated specimen and varied with the treatment time

up to maximum value of about 1450Hk0.1. The electrochemical corrosion tests

in 5% NaCl aerated solution showed that the pitting corrosion resistance of

nitrided samples was higher than that of untreated samples and increased with

the increase in nitriding time up to an optimal time. Nitride samples showed a

sensibly lower damaging amount in comparison with untreated samples. As

the nitriding time increased, the dimensions and density of pits decreased and

at the end of corrosion studies, samples nitrided for optimum time appeared

almost untouched (Fossati et al 2006).

A series of nitriding experiments were conducted on the AISI 304L

grade austenitic stainless steel samples at temperature ranging from 673 –

873K (Menthe et. al 1995). The maximum Knoop hardness was found to be

1400 Hk0.01 and the maximum thickness obtained was reported as 34µm. The

XRD patterns showed the presence of the S phase at the surface layers. The

corrosion performance was tested in 0.05M H2SO4 solution at pH 3.3 and in

neutral 3.5% NaCl solution at ambient temperature. Potentiostatic and

potentiodynamic experiments yielded slightly higher passive corrosion

currents for nitrided samples. Pitting corrosion in neutral electrolytes

containing chloride was observed only for the untreated specimen. The

46

passive layers formed on the samples were similar in constitution and

thickness as determined from the XPS sputter profiles.

The effect of ion nitriding on the oxidation behaviour of 316LN

grade stainless steels compared with the as received conditions was

investigated (Rajendran Pillai et al 2007). In their study, they have reported

that the oxidation was interrupted at specific time intervals to examine the

mass change of the specimen and the mass of the spalled oxide. The analysis

of surface by GIXRD revealed the nature of different phases formed on the

surface. The corrosion resistance of stainless steels in aqueous environment

was attributed to the formation of passive film, which was extremely thin

oxide / hydroxide layer in which Cr ions are enriched as oxide or hydroxide.

The passive film was possible to be more protective by various additives such

as Mo, N. Attempts to obtain the higher corrosion resistance of metallic

materials have been tried since early 1970s.

Type 304 and 316 grade austenitic stainless steels were ion

implanted with Cr+, N+ and Mo+ in order to obtain improved corrosion

resistance in aqueous environment (Fujimoto et al 1999). The surface alloyed

with Cr+ ions improved general corrosion resistance in acid solution and never

suppressed localized corrosion in chloride environments, whereas the

implantation of N+ and Mo+ ions effectively suppressed (or) inhibited pitting

corrosion in neutral chloride solutions.

Nosei et al (2004) investigated the corrosion susceptibility of ion

nitrided AISI 316L stainless steel and compared with corrosion susceptibility

of the untreated material. Plasma nitriding resulted in the ‘S’ phase with a

thickness of 5 µm and micro hardness of 1300 – 1400Hv0.25, which was 6.5

times higher than the untreated material. Anodic potentiodynamic polarization

curves and immersion tests were performed in 1M NaCl at room temperature

to evaluate the corrosion resistance of both treated and untreated samples.

47

Nitrided samples showed a much better corrosion resistance than untreated

specimen.

In a study on the corrosion - erosion behaviour of austenitic high

nitrogen stainless steels, Lopez et al (2007) reported that, a high temperature

gas nitriding resulted in 1.5mm thick cases with nitrogen content in solid

solution upto 0.5wt%. The results of corrosion – erosion tests performed in

slurry composed of 3.5% NaCl with 10wt% quartz particles, indicated that

nitrogen addition improved the corrosion – erosion resistance owing to the

strengthening effect of nitrogen in solid solution and to the increase of

repassivation ability of the passive layer.

A series of plasma nitriding experiments have been conducted on

austenitic stainless steels at temperatures ranging from 400K to 600K using

pulsed DC plasma with various pulse durations in an N2–H2 gas mixture by

Menthe et al (1995). The maximum hardness after plasma nitriding was

observed about 1400Hk0.01 with a maximum thickness of the compound layer

as 34µm. Potentiostatic and potentiodynamic experiments yielded slightly

higher passive corrosion currents for plasma nitrided specimen.

Glow discharge nitriding treatments performed at temperatures in

the range 673-773K for 5 hrs at 10 kPa on AISI 316L grade austenitic

stainless samples, were able to produce a modified surface layer consisting of

‘S’ phase with small amount of CrN precipitates at the surface (Borgioli et al

2005). In all the nitrided samples, the micro hardness profile showed a higher

hardness values in the modified layer and a steep decrease to matrix values.

The mean hardness values in the layer reached a value of about 1450 Hk001

and a further hardness increase upto 1550Hk001 was also observed when

substantial amounts of nitrides were forming. The modified layer thickness

increased as treatment temperature increased ranging from 4µm to 47µm.

Preliminary corrosion tests performed in 5% NaCl aerated solution with

48

potentiodynamic method showed that glow-discharge nitriding was able to

increase the pitting potential of AISI 316L samples and hence their corrosion

resistance. The microstructure and corrosion properties of austenitic stainless

steels were investigated after plasma nitriding at 773K (Spies et al 2002). The

nitrided layer of austenitic stainless steels consisted of nitrogen expanded

austenitic phase. The enrichment of the surface layer with nitrogen led to a

significant improvement of the resistance against pitting corrosion compared

to the initial state.

The microstructure and phase composition of the layers produced

by the influence of gas nitriding of austenitic stainless steels, were

investigated (Baranowska and Arnold, 2006). The results revealed that during

gas nitriding it was possible to obtain a uniform protective layer composed of

expanded austenite (γN) phase. The potentiodynamic polarization test using

3% NaCl at a temperature of 292K against Ag/AgCl electrode with a scan rate

of 1 mV/s revealed that the nitrided layer exhibited a better corrosion

resistance. The corrosion potential was shifted to higher values and these

values varied with the ammonia content and temperature during the nitriding

process. The corrosion currents were much lower than the current observed in

untreated specimen.

Surface modification of AISI 304 austenitic stainless steels by

plasma nitriding using NH3 gas at substrate temperature up to 793K was

studied (Liang 2003). The modified surface was found to be containing a

compact surface nitride layer composing a γN phase with a thickness around

12µm. The micro hardness measurements showed significant increase in the

hardness from 240Hv for untreated samples up to 1700Hv for nitrided

samples. The potentiodynamic anodic polarization curves for specimen in

3.5% NaCl solution exhibited that pitting (on corrosion potential was higher)

49

and the corrosion current density were lower for treated samples than the

untreated samples.

The microstructures and corrosion properties of austenitic stainless

steels have been investigated after gas nitriding using sensor controlled

furnace at a temperature range of 523 -773K (Spies et al 2002). The corrosion

behaviour was tested with electro chemical methods and the anodic potentio

dynamic polarization curves were recorded using a 0.05M H2SO4 electrolyte

with a scan rate of at 1.8Vh-1 for general corrosion and 0.5M NaCl electrolyte

for pitting corrosion at a scan rate of 0.2 Vh-1. The enrichment of the surface

layer with nitrogen, up to a temperature of 773K resulted in significant

resistance against pitting corrosion compared to initial state. It was also

reported that the resistance against the general corrosion had been improved.

Dearnley and Aldrich-Smith (2004) reported that providing a hard

thin layer of 10 m thick with improved hardness using various surface

hardening techniques could solve the problem of surface material loss caused

by the synergistic combination of mechanical (wear) and chemical (corrosion)

processes. AISI 316 grade austenitic stainless steel as the substrate material of

specimen was plasma processed and the resulting nitrided layer was

investigated with XRD analysis for phase identification, metallography for

layer morphology and microhardness tests for hardness measurements.

Electrochemical corrosion tests have been conducted potentiostatically in a

3wt% NaCl aqueous solution to measure the anodic polarization curves of the

plasma processed surface (Sun 2005). The results exhibited that the nitrogen

enriched surface layer had a hardness of around 1500 Hv0.025 with a thickness

of 30m. The electrochemical corrosion test showed a reduction of current

density in the anodic region for nitrided sample, indicating improved

corrosion resistance. Four zones have been detected (Andreeva and Gurvich,

1959) within the thickness of nitrided layer on stainless steels with different

50

corrosion resistance and electrode potentials. The variation of corrosion

resistance of nitrided layers with the thickness of removed layer corresponded

to the change in electrode potentials.

Baranowska et al (2007) investigated the wear behaviour of gas

nitrided austenitic stainless steels in corrosive liquid conditions appropriate to

the manufacture of soft drinks. The nitrided layers obtained by the gas

nitriding were predominantly composed of n phase nitride and other nitrides

like Cr2N and Fe2N. The wear performance in the aggressive liquid was

superior mainly due to the formation of a lubricating transfer layer on the

contact surface area. Flis et al (2000) investigated the stainless steels which

were plasma nitrided in N2/H2 mixtures with 25 Vol% N2 at 858K. The

corrosion behaviour through depth the nitrided layers was examined in 0.1M

Na2SO4 at pH 3.0 and the corrosion resistance of the nitrided samples was

found to be more than that of untreated samples.

2.6.4 Summary

Several researchers investigated the effect of nitriding on the

mechanical, surface and corrosion behaviour of the austenitic stainless steels.

Some researchers reported that the nitriding of stainless steel improves wear

resistance but impairs corrosion resistance (Staines and Bell 1979; Sedriks,

1979). Others studied the relationship between the structure and the corrosion

properties of nitrided stainless steels (Zhang and Bell 1985). However little

information is available on the relationship between the mechanical and

surface properties of nitrided AISI 316LN grade austenitic stainless steels and

on the comparison of the properties obtained by the different methods of

nitriding. The present study focused in the direction of documenting the

relationship of the mechanical, surface and corrosion properties obtained by

different nitriding methods on the indigenously manufactured AISI 316LN

austenitic stainless steels.