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Chapter 3

EXPERIMENTAL PROCEDURES This chapter describes the experimental procedures followed and processing

parameters selected in the present study. The procedures and parameters are selected on the

basis of literature review presented in Chapter 2.

On the basis of literature consulted [Refer section 2.2], the rotating beam bending

fatigue test was selected owing to its simplicity and versatility, as the loading in this test

closely resembles to that encountered by real life machine elements, such as axles and

shafts etc. Further, since the present work involves comparative study of the uncoated and

coated materials, it was decided to make use of unnotched specimens, as they provide better

insight into the fatigue properties of the material [96].

A detailed description of the experimental procedures followed is presented in the

following sub-sections.

3.1 Material selection

In view of the extensive applications of low-alloy steels for making automotive

components, such as shafts, gears and cams etc., four different types of commonly used

low-alloy steels had been selected for the present study [Refer Table 3.1]. The raw material

was procured in the form of φ14mm (for fatigue test specimens) and φ20mm (for tensile

test specimens) round bars of 6 meter length each. The use of long bars for preparing the

specimens ensured minimal compositional variations among various specimens obtained

from a single bar. Subsequent to procurement, the chemical composition of each grade of

steel was evaluated through spectroscopic analysis, the results of which are given in Table

3.1.

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Table 3.1: Composition of various grades of steel (wt. %).

Wt. % of elements Steel Grade

C Ni Si Mn Cr Mo

SAE 8620 0.18 0.60 0.16 0.72 0.59 0.24

20MnCr5 0.18 - 0.32 1.02 1.28 -

EN353 0.16 1.38 0.23 0.68 1.14 0.15

SCM420 0.19 0.18 0.29 0.74 0.93 0.21

3.2 Preparation of fatigue and tensile test specimens

Based on the recommendations provided in ISO1143 standard [Refer section 2.2.2],

dumbbell shaped round test specimens having nominal neck diameter of 6±0.05mm and

18mm transition fillets were prepared for carrying out fatigue tests. The drawing of the

specimen is given in Figure 3.1. A short overall length of specimen is helpful in minimizing

bending load during the machining process, besides facilitating fabrication of large number

of test specimens from a single lot of raw material.

The fatigue test specimens were machined on a CNC lathe with the help of a coated

tungsten carbide tool. The machining involved gradual reduction in diameter of raw

material through successive cuts of 0.3mm depth for roughing and 0.1mm for semi-

finishing. The finish cut removed 0.05mm layer in a single go. In order to minimize

stressing of the material due to cutting forces, the cutting tool was traversed towards the

chuck during the machining operation. This helped to transmit the cutting forces through

the uncut portion having larger diameter section, instead of transmitting the forces through

machined portion with smaller diameter.

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Figure 3.1: Drawing of fatigue specimen

The tensile test specimens [Refer Figure 3.2] were prepared according to the

dimensions prescribed in ASTM E 8M [258]. Owing to their large length-to-diameter ratio,

these specimens were carefully machined on a manual lathe machine.

Figure 3.2: Tensile test specimen as per ASTM E 8M

Subsequent to machining, the tooling marks on fatigue as well as tensile test

specimens were removed by polishing with SiC paper of 120 grit.

The specimens were case carburized at 920°C for three hours, followed by soaking at

850°C for 30 minutes and quenching in oil at 120°C. Finally, the quenched specimens were

tempered at 180°C for 2 hours, followed by cooling in still air, so as to achieve a final

hardness in the range of 59-61 HRc.

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After heat treatment, the surface finish of the fatigue specimens in the neck and

shoulder regions was further improved by polishing with successive grades of SiC papers of

grit sizes from 320 to 1200, followed by final polishing with alumina paste. A low surface

speed of 0.6 to 1.2 m/s was used during these polishing operations to avoid introduction of

residual stresses, viz. those originating due to abusive grinding. The initial finishing was

performed by holding the specimens in a rotating chuck, while the subsequent finishing

operations involved rubbing the specimens in a direction parallel to their axis, in

accordance with the procedures outlined in ISO1143 standard [112]. While the surface

finish in the neck region was improved owing to its importance during subsequent fatigue

tests, the finish was also improved on the flat end of the specimens, which was later used

for measurement of residual stress through X-ray diffraction. The finished fatigue

specimens, as shown in Figure 3.3, exhibited mirror-like finish with surface roughness

better than Ra = 0.2 µm.

Figure 3.3: Polished fatigue test specimens

The ends of the case carburized tensile test specimens were ground on a cylindrical

grinding machine to remove their entire case, up to a radial depth of 0.8mm, so as to expose

64

the soft core. This ensured good gripping and prevented slippage of the case-carburized

specimens in the jaws of tensile testing machine.

Subsequent to finishing operations, all tensile and fatigue specimens were cleaned and

immersed in SAE20W40 oil to prevent rusting.

Half of the case carburized specimens were sent to Oerlikon Balzers Coating India

Limited, Bangalore (India) for coating them with a 2µm thick layer of WC/C (Balnit®C)

coating. The coating process involved de-greasing, Argon-ion etching and sputter-coating at

220°C. Details of the coating process have already been explained in section 1.4.4.4.

The coating unit comprised of a number of modular, tube-shaped component holders

for accommodating the components to be coated [Refer Figure 3.4]. In order to achieve a

uniform deposit over the entire surface of the components during the coating process, the

component holders are given complex spinning motion by means of gear-arrangements.

Figure 3.4: Coating unit at Oerlikon Balzers

A characteristic of the process is that the deposit does not reach into deep crevices on

the substrate. Thus, the coating does not reach the portion of the specimens that is held

inside the tubular holder [Figure 3.5]. Accordingly, the specimens were held in the holders

65

from the end with centre-mark, while the flat, polished side was kept facing upwards, so as

to obtain coating on the flat portion of the specimens. These flat coated ends were

subsequently employed for estimation of residual stresses.

Figure 3.5: Fatigue test specimen after WC/C PVD coating

3.3 Mechanical testing

In order to assess the effect of coating on mechanical properties of the steels used,

tensile and hardness tests were conducted on uncoated as well as PVD coated specimens.

The details of procedure adopted for these tests are described in the following sub-sections.

3.3.1 Tensile testing

As mentioned earlier, the tensile tests were carried out on standard round specimens

of 6mm and 9mm diameter, in accordance with the practices prescribed in ASTM E 8M

standard [258].

For determining the Young’s modulus, tensile tests were conducted within the elastic

limit of the steel. For this purpose, an extensometer (of Mitutoyo make) having a least

count of 0.01mm was attached to the specimen for recording elongation, as shown in Figure

3.6, while the specimen was held in the grips of a tensile testing machine. Load on the

specimen was applied gradually by manually turning the load-wheel at a very slow rate.

Extreme care was taken during the test to ensure proper alignment of the load, as prescribed

in ASTM E 1012 [259].

66

The load and extension were recorded during loading as well as unloading of the

specimen. The mean of these readings was used for calculation of stresses and strains

induced in the specimen. The Young’s modulus was estimated from the slope of best-fit

line, fitted over elastic portion of the stress-strain graph.

Figure 3.6: Extensometer fitted over tensile test specimen.

In order to determine the yield and ultimate strength of the material, the specimen was

loaded on a computerized servo controlled universal testing machine, [Refer Figure 3.7] till

it fractured [Figure 3.8].

Figure 3.7: Specimen mounted on a universal testing machine

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Figure 3.8: Specimens failed under tensile tests.

3.3.2 Hardness measurement

The hardness of the specimens was measured on bulk as well as micro-scale. The

bulk hardness of the specimens was measured on a Rockwell hardness tester on C-scale,

while the micro-hardness was measured on a Vickers microhardness tester. Wherever

required, hardness conversion from one scale to other was carried according to ASTM E

140 standard [260].

The specimens for measuring microhardness (Vickers) profile across depth were

prepared by mounting the fatigue test specimens on magnetic bed and grinding them flat on

both the sides. The ground specimens, shown in Figure 3.9, were further polished manually

to improve their surface finish in the region where micro-indentations were to be made.

68

Figure 3.9: Specimens for determination of microhardness profile across depth.

A Metatech make digital microhardness tester, shown in Figure 3.10 was employed

for measuring Vickers microhardness across depth of the specimens. The digital

microscope provided with the hardness tester was first calibrated according to ASTM E

1951 standard [261]. For this purpose, movement of the reference points, as seen on the

monitor screen, was recorded against the displacement shown by micrometers controlling

the movement of mechanical stage along X and Y directions.

Figure 3.10: Digital microhardness tester

69

Subsequent to calibration, the microhardness measurements were made according to

the procedures prescribed in ASTM E 92 [262] and ASTM E 384 [263] standards by using

a diamond pyramid indenter. In order to determine Vickers hardness across depth of the

specimens, the indentations were made at a spacing of 200µm, while applying a load of

200g for a dwell period of 15 seconds. The diagonals of the impressions, created at various

locations on the specimen cross-section [Figure 3.11], were measured to estimate the

microhardness across depth of the specimens.

Figure 3.11: Indentation mark created by Vickers microhardness tester.

The specimen for measurement of coating hardness was prepared by cutting a button-

shaped cylindrical section from the end of fatigue specimen. The indentations were made at

different radial locations on the outer (coated) surface. Owing to very small thickness of the

coating (~2µm), the indentations were made at relatively lower load of 10g, so as to avoid

influence of substrate properties on the hardness measurement.

3.3.3 Surface roughness measurement

The average value of surface roughness (Ra) of the fatigue specimens was measured

using a surface roughness tester (Mitutoyo SJ201) as shown in Figure 3.12.

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Figure 3.12: Surface roughness tester

Owing to restrictions imposed by the geometry of fatigue test specimens [Refer

Figure 3.1], it was difficult to employ long sampling lengths in the neck portion of the

specimens. Therefore, the roughness measurements were made either over short sampling

lengths in the neck region or along the longitudinal direction in the holding region.

3.4 Fatigue testing

The design of fatigue test rig and procedure adopted for conducting fatigue tests is

described in the following sub-sections.

3.4.1 Fatigue test rig

The fatigue test rig was designed to subject the specimens to a constant amplitude

load under four point rotating bending, as described in section 2.2.2. The schematic for

holding and loading the cylindrical fatigue specimens in the experimental rig and the

resulting stress induced in the specimen are shown in Figure 3.13. The load is applied over

the outer (stationary) casing of ball-bearings. The loading points are separated by a distance

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of 250mm, while the fulcrums F1 and F2, which hold the support bearings, are spaced at

410mm. The design provides a longer beam span for inducing large bending moment (BM)

in the specimen. Besides, the design employs specimens of much smaller overall length,

which facilitates their easy machining owing to lower length to diameter ratio.

The fatigue test rig is shown in Figure 3.14, in which the specimen is held between a

pair of 3-jaw chucks C1 and C2. The chucks, in addition to rotation about their axes (X-

axis), are also free to rotate about fulcrums F1 and F2 in XZ plane. The fulcrum-holding

brackets K1 and K2 were machined simultaneously in the same setup, while their mounting

pads M were machined in a single setup before their assembly to the base plate, B. The

mounting bolts of the brackets holding the fulcrums F1 and F2, were initially kept slightly

loose to facilitate adjustment of brackets during mounting of the specimen. A cylindrically

ground specimen was then mounted in the chucks C1 and C2, so as to align the rotation

axes of both the chucks and thereafter, the mounting bolts were tightened. In order to

maintain alignment of the brackets during the course of fatigue tests, dowels were engaged

between the brackets and the base plate, B.

Figure 3.13: Loading of cylindrical specimen in the test rig

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The chucks C1 and C2 are assembled respectively with bearings B1 and B2, to which

a load pan is attached symmetrically with the help of flexible steel chains to provide the

required flexibility in the event of specimen failure. The load is applied mechanically by

putting dead weights in the pan, which induces a uniform bending moment in the span B1-

B2, as is evident from Figure 3.13.

Figure 3.14: (a): Front and (b): Top views of the test rig with casing removed

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In order to subject the specimen to rotating bending fatigue, the chuck C1 holding one

end of the specimen is driven with the help of a flexible torsion wire W. The other end of

wire W is connected to the shaft carrying a V-pulley (P). The shaft is mounted on two ball-

bearings, so as to support the belt tension. The pulley is coupled to a 0.5 HP AC motor with

the help of a V-belt. The mounting for motor is provided with oblong holes to adjust

tension in the belt. The torsion wire (W) provides the required flexibility to the chuck in the

event of specimen failure. The entire arrangement of belt, pulley and flexible coupling is

housed inside a metallic casing to ensure safety of operator from fast moving parts.

A small metallic screw S is attached to the pulley, P [Refer Figure 3.14 (b)], so as to

provide capacitive pick-up to the proximity sensor X, which provides input to a 10 digit

electronic revolution counter [Refer Figure 3.15] mounted on the casing. The counter is

capable of storing the number in memory, in the event of power failure.

Figure 3.15: Revolution counter

A limit switch, L is fixed adjacent to the chuck C1 and is actuated by an adjustable

stopper S1. In the event of specimen fracture, both the chucks (C1 and C2) fall apart and

rest on their respective adjustable stoppers S2 and S3, while the limit switch disconnects the

power supply to the electric motor.

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3.4.2 Fatigue tests

With reference to Figure 3.13, the stress induced in the fatigue test specimen can be

calculated from the following relationship:

+

+== Cb

gammdZ

BMS p

2)(32

3π

Equation 3.1

where d is the neck diameter of the fatigue test specimen, m is the mass in the load pan, mp

is mass of the pan, a is the distance between loading point and support on either side of the

simply supported beam and C is the weight of each chuck at distance b from the fulcrum

[Refer Figure 3.13]. Substituting respective values of these parameters for the fatigue test

rig, i.e. mp = 1.872kg, a = 0.08m, b = 0.185m and C = 1.58kgf and re-arranging the terms,

we get:

872.1867.232

55.232

2 33

−

−=−

−=

dSmCbdSga

m pππ

Equation 3.2

If stress is expressed in N/m2 and specimen neck diameter in meters, the value of

mass will be calculated in kilograms. Before carrying out the fatigue test, the diameter of

fatigue test specimen was measured to an accuracy of 1µm with the help of a digital

micrometer. The value of measured diameter was substituted in Equation 3.2 to calculate

the dead weight to be put in the pan for inducing the desired magnitude of maximum

alternating stress in the specimen.

The specimen was then mounted in the machine and the calculated load was applied

by putting dead weights in the load pan. Before the start of actual test, the chucks were

rotated by hand to check their proper movement. The revolution counter was then set to

zero by pressing re-set button on its console, following which, power supply to the electric

motor was switched on.

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In case of infinite life fatigue tests, the machine was stopped after completion of

2×106 cycles, while for finite life tests, the machine was stopped automatically by limit

switch in the event of specimen failure. In line with the recommendations made in ASTM E

739 standard [121] for conducting research experiments, a replication ratio of 50% was

maintained for the fatigue tests [Refer section 2.3.1.2]. For this purpose, two identical

fatigue tests were conducted at each level of maximum alternating stress.

3.5 Metallographic examination

The specimens for metallographic examination were prepared according to the

guidelines prescribed in ASTM E 3 [264]. Owing to high hardness of case carburized

specimens, a 2mm thick abrasive slitting wheel was used for sectioning of specimens for

metallographic investigation. The sectioning was done intermittently in the presence of

cutting fluid to prevent microstructural changes due to heating of the specimen being cut.

Wherever possible, the section of interest was cut first, so that the remaining bulk material

could act as an efficient heat-sink. In order to prevent over-heating just ahead of final

sectioning of the specimen, the time interval between intermittent cutting was increased

further. At times, the metallographic specimens were prepared simply by polishing the

specimens fractured under fatigue, so as to completely avoid abrasive cutting. These

specimens were employed for examining the microstructural features right in the vicinity of

fatigue fracture.

The sectioned specimens were first ground flat with a grinding wheel and then

polished with successive grades of SiC paper, followed by polishing with lavigated alumina

paste. In order to keep plastic deformation within the surface layer to minimal, the final

finishing of the metallographic specimens was performed through skid-polishing, wherein a

thick paste of polishing abrasive was placed on the polishing cloth and the specimen was

held lightly against surface of the paste without touching the fibers of the polishing cloth.

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In case when the region of interest was too close to the outermost periphery, it

became imperative to avoid edge-rounding during the grinding and polishing processes. For

this, the specimens were first cleaned with organic solvents to remove grease/oil, and

subsequently mounted in epoxy resin. The epoxy was allowed to cure for a few hours and

therafter, the section of interest was ground and polished.

Depending on the analysis of interest, the polished specimens were etched with

various types of etchants in accordance with the procedures outlined in ASTM E 407 [265].

Etching involves a controlled corrosion on the surface, whose rate depends on the

electrolytic action between surface areas having different potential. For example, a potential

difference may exist between grains having different orientations; between grain boundaries

and interiors; between impurity phases and the matrix; or as a result of concentration

gradients in single phase alloys. For multiphase alloys, a potential difference also exists

between the different phases present. The differential potential alters the rate of attack,

thereby revealing the microstructure.

For general purpose analysis, the polished specimens were etched with 2% Nital (2%

Nitric acid in ethanol) for a few seconds, followed by rinsing with ethanol. Besides Nital, a

number of chemical tint-etchants, viz. Klemm’s – I and Beraha’s Reagent were also

employed for revealing certain microstructural features. Contrary to ordinary chemical

etchants such as Nital, where the corrosion products from etching get dissolved into the

etching medium, these standard tint etchants are chemically balanced to produce a stable

film on the surface of specimen.

The tint etchants produce colours by means of interference of light in a thin film that

is deposited on the surface. As the thickness of film is different for different constituents,

the resulting interference leads to enhancement of contrast. As the colour produced through

this method depends on the thickness of film, proper timing has to be established in order to

get similar colours every time. Figure 3.16 demonstrates the formation of colours with

changing thickness of the deposited film. The patterns were obtained by applying the tint

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etchant and then allowing the surface dry out its own, without rinsing it with distilled water

or alcohol. In this way, the residual etchant continued to adhere on the surface for some

time in the form of microscopic droplets, which evaporated in due course of time. As a

result, only a thin film was deposited near the outermost edges of the droplets, while a

thicker deposition took place in the middle portion, which remained in contact with the

etchant for longer duration. Therefore, proper care needs to be taken while tint-etching, as

similar microstructures may yield different colours. The white spots appearing in Figure

3.16 represent carbide particles, which are not covered by the anodic film.

Figure 3.16: Interference fringes in thin sulfide film.

The general mechanism of these chemical tint etchants involves formation of a sulfide

film through decomposition of the metabisulfite ion present in the aqueous solution when it

comes in contact with the metallic surface, thereby yielding sulfur dioxide (SO2), hydrogen

sulfide (H2S) and hydrogen (H2) gases. The SO2 depassivates the surface and promotes

formation of sulfide film, while H2S provides the required S2- ions to form the sulfide film

in the presence of iron, nickel, or cobalt ions. Beraha's reagent works much like

Klemm's – I, but with slightly less aggressive colouring of ferrite.

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A summary of etchants employed in the present study, along with etching procedure

and application, is given in Table 3.2 [265].

Table 3.2: Summary of etchants used in the present study.

ASTM Ref. No.

Etchant Etching procedure Application

74a Nital: 2-3% Nitric Acid in Ethyl Alcohol

Immerse for a few seconds to a minute

General structure; Ferrite grain boundaries

78 10g potassium metabisulfite in 100mL water

Immerse for 1-15 seconds. Better results may be obtained by first etching with Nital.

Carbides and phosphides (Matrix darkened, carbides and phosphides remain bright)

210 Klemm’s Reagent (I): Stock Solution: 50 mL cold water saturated with sodium thiosulfate. (Maximum solubility of Na2S2O3.5H2O per 100mL of water is 79.4g at 0°C and 291.1g at 45°C) Add 1 g potassium metabisulfite

Immerse face up, gently agitate until coloration begins, allow to settle. Stop etch when surface is red-violet. Etch time ~3 minutes; varies with material.

Colors ferrite and martensite in cast iron, carbon and low alloy steels; reveals segregation.

211 Beraha’s Etchant: 10 g anhydrous sodium thiosulfate in 100 mL water. Add 3 g potassium metabisulfite

Immerse specimen face up, gently agitate solution until coloration begins, allow to settle for 1 to 15 min. Stop etch when surface is red-violet. Etch time varies with material.

Colours matrix phases – ferrite, martensite, pearlite and bainite. Sulfides are brightened.

216 8-15g sodium metabisulfite in 100mL water

Immerse face-up. Agitate solution gently until colouration begins; allow to settle for ~20s. Stop when surface is dark. Use crossed polarized light and sensitive tint to improve colouration.

Darkens lath martensite in low carbon high alloy steels.

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The post-etching procedure for all the tint-etchants includes washing with warm

water, followed by spraying with ethanol, and drying. The Klemm’s – I reagent was often

stored for several days by covering the beaker with an aluminium foil to prevent vapour-

loss and associated crystal formation. The reagents and apparatus used for etching process

are shown in Figure 3.17.

Figure 3.17: Reagents and apparatus used for etching

The finish-polished and etched metallographic specimens were mounted in precisely

machined aluminium cup-holders with the help of clay-dough so that the specimen’s top

face became co-planar with the top face of the specimen holder, as shown in Figure 3.18.

This helped in obtaining good quality metallographs by placing the entire field of view in

focal plane of the microscope’s objective.

Figure 3.18: Aluminium cups for mounting of metallographic specimens

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The specimen holder containing the specimen was then placed on the stage of

Almicro trinocular metallographic microscope shown in Figure 3.19. Technical

specifications of the microscope are given in Appendix D. The microscope was calibrated

at various magnifications according to the procedures described in ASTM E 1951 [261].

Micrographs were recorded at various magnifications, depending upon the features of

interest. Wherever required, a sensitive tint was employed for contrast enhancement in

colour-etched specimens. In situations where colour information was not important, the

images were often white-balanced to improve their appearance.

Figure 3.19: Almicro trinocular metallographic microscope with digital camera.

3.6 Fractographic examination

The aim of fractography is to analyze the fracture surface features and to relate the

observed topography of the fracture surface with the associated causes and/or basic

mechanisms behind fracture. The fractured surfaces of failed specimens were viewed at

various magnifications under digital camera, digital optical microscopes and scanning

electron microscope. Light microscopy offers magnifications of upto 1000×, while

scanning electron microscopy provides more than hundred times the magnification possible

with an optical microscope. Typical resolution achievable in light microscopy is around

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0.2µm, while an average scanning electron microscope can generally achieve a resolution

of 3nm. Another limitation of light microscopy is the restriction on depth of field, which is

inversely proportional to the square of total magnification. Scanning electron microscopes

on the other hand, offer depths of field that are two orders of magnitude higher than those

achievable with light microscopes. However, despite all these limitations, light microscopes

are extensively used as they are easily available, easy to operate, do not require vacuuming

and can provide information regarding colour, which adds an extra dimension to the

observation and understanding.

The fracture surface contains a wealth of information concerning the causes and

modes of failure. Interpretation of the fracture features depends on the condition of fracture

surface. However, this interpretation can get obstructed as the fracture surface is quite

fragile and vulnerable to get damaged by chemical or mechanical action of the

environment, thereby obscuring or obliterating the fracture features. Both these damages

can occur during the fracture event as well as during subsequent storage and handling.

Though, not much can be done to prevent damage during the course of fracture, care must

be exercised during subsequent storage and handling of the fractured specimens. As the

fracture surface is usually not examined immediately after it is produced, it becomes

imperative to adopt some suitable means for its preservation.

In the present work, the short-term storage of fractured specimens was done by

storing them in air-tight plastic containers in the presence of a desiccant (silica gel). Care

was taken to avoid contact of the fracture surfaces of the specimens with any other metallic

object during handling as well as storage. For long-term storage, the specimens were

immersed in SAE 20W40 oil, which, if required at a later stage, was cleaned with the help

of organic solvents, viz. acetone and ethanol.

Fractographic observations of relatively large features, viz. ratchet marks, topography

of regions failed under different modes of crack propagation etc., were made with the help

of Celestron Digital Microscope, shown in Figure 3.20. Technical specifications of the

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microscope are given in Appendix D. The microscope was calibrated at various

magnifications according to the procedures described in ASTM E 1951 [261].

Figure 3.20: Celestron digital microscope employed for fractographic studies

The fracture surface itself does not require any preparatory steps for making

fractographic observations. However, it is often required to size the specimen appropriately

for facilitating its placement under the microscope. In instances when the mean fracture

surface was not normal to the specimen axis, the specimen was often supported on clay-

dough or epoxy, so as to make the fracture surface roughly parallel to focal plane of the

microscope.

As the fracture surface is characterized by jagged features, it often becomes

impossible to bring the whole area within the field of view into sharp focus owing to the

limited depth of field achievable in the light microscope, especially at higher

magnifications. In order to overcome this problem, multiple fractographs were recorded by

shifting the focal plane of microscope, which were digitally stitched to make final

fractographs.

Another difficulty encountered during fractography arises from the presence of

numerous planes, inclined at various angles on the fracture surface, which act as catchlights

83

when illuminated with a point light sources. The catchlights that do not fall within the focal

plane of the frame appear as blurred circles and serve to degrade the quality of image. In

order to offset this problem, the fractographic images were recorded by either keeping the

microscope in open daylight, or by placing a cylinder of tracing paper around the specimen

to diffuse the light coming from the point source(s). At times, a second light or a reflecting

white sheet of paper was also used to act as a secondary source of light for illuminating the

regions under shadow. In addition to these arrangements, the specimens were rotated to

obtain the best contrast for highlighting the features of interest before recording the

micrographs. Exposure adjustments available in the digital microscope offered additional

control during fractography.

Scanning electron microscope (SEM) can easily resolve the fine fractographic

features, such as fatigue striations and tire tracks, which are often too small to be resolved

with an optical microscope. Besides, the higher depth of field offered by an SEM is an

additional advantage while viewing the rough fracture surface.

The only pre-requisite for observing a specimen under SEM is that it should be

electrically conductive. Metallic specimens, cut to suitable size, can be placed directly into

the SEM. In instances when the specimens were secured to the aluminium stubs with the

help of glue [Figure 3.21], electrical continuity was checked with a multimeter and if

required, silver paint was applied to ensure electrical continuity.

The specimens thus mounted were placed in Joel, JSM 6100 scanning electron

microscope [Refer Figure 3.22] available at Regional Scientific Instrumentation Centre,

Panjab University, Chandigarh. The electron microscope was capable of viewing the

specimens at accelerating voltages in the range of 15 kV to 30 kV. The images were

recorded in slow scanning mode on to an SLR camera attached to the microscope or

captured directly from the display screen with a digital camera.

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Figure 3.21: Mounting of specimens onto aluminium stubs and application of alumina paste for electrical continuity.

Figure 3.22: Scanning electron microscope employed for fractographic studies

85

3.7 X-ray diffraction analysis

Crystallographic studies on the specimens were made with a PANalytical X-ray

diffraction machine, available at National Facility of OIM and Texture, IIT Bombay.

Technical specifications of the machine are summarized in Appendix D.

3.7.1 Specimen preparation

The specimens for X-ray diffraction (XRD) analysis were prepared by cutting 4.5mm

thick cylindrical portions from the flat, polished ends of the case carburized (uncoated) and

case carburized – WC/C coated fatigue test specimens. While preparing the specimens for

XRD analysis, the cutting operation was performed by holding the fatigue test specimens in

vertical position [Refer Figure 3.23] so that the force exerted by the holding vise did not

affect the face on which residual stress measurement was to be carried out. The cutting was

done slowly and intermittently with the help of a 2mm thick slitting wheel, while the

specimen was cooled with water, so as to avoid the chances of rise in temperature, which

could affect the distribution of residual stresses.

Subsequent to cutting, the uncoated specimens were first chemically etched using 2%

Nital, followed by washing with ethanol. The objective of etching was to remove the thin

plastically deformed layer from the top surface of specimens, which could have formed

during the abrasive polishing, carried out during the specimen preparation stage [Refer

section 3.2]. On the other hand, the coated specimens were simply washed with ethanol

without etching, as their ends had already been polished and etched prior to the coating

process. The amount of material removed through chemical etching was determined by

measuring the thickness of specimens before and after etching. For most of the specimens, a

surface layer of around 10 µm was dissolved by chemical etching.

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3.7.2 Characterization

For the purpose of carrying out XRD analysis, the specimens were mounted in an

aluminium holder with the help of clay-dough, so that their top planar face became co-

planar with the top face of the specimen holder, as shown in Figure 3.24.

The specimen holder was then mounted into the goniometer head of the diffraction

machine [Refer Figure 3.25] and a 2θ scan was performed. In case of coated specimens, the

X-rays were able to penetrate through the 2 µm thick layer of amorphous carbon matrix to

produce the diffraction pattern of the substrate underneath. The 2θ scans for steel specimens

made of different grades employed in this study were found to be similar to the one

presented in Figure 3.26 for SAE8620 steel.

Figure 3.23: Specimen preparation for X-ray diffraction analysis.

Figure 3.24: Mounting of specimens for XRD

87

Figure 3.25: Mounting of specimens in PANalytical X-ray diffraction machine

88

Figure 3.26: 2θ Scan of SAE 8620 specimen.

The lattice planes and the corresponding BCC structure were identified by applying

extinction rules to the first three peaks, as illustrated in Table 3.3. The average value of the

lattice parameter (a), as estimated from the expression given below, was found to be 2.87Å,

which corresponds to the lattice constant of iron reported in the literature [266]:

( )2222

2

sin4lkha ++=

θλ

Equation 3.3

Table 3.3: Determination of lattice structure

i

2θi

sin θi

sin2 θi

Ratios

12

2

sinsin

θθi

Normalized

Ratios

Lattice

Planes

1 44.55 0.3791 0.1437 1 2 (110)

2 64.65 0.5347 0.2859 1.99 3.98 ~ 4 (200)

3 82.35 0.6584 0.4334 3.02 6.03 ~ 6 (211)

89

On the basis of location and intensity, the peak corresponding to (211) reflection was

found to be suitable for the purpose of estimating lattice strain. For most of the specimens, a

total of eleven X-ray diffraction measurements were made in the tilt range of

°+≤≤°− 4040 ψ for estimation of residual stress. The peaks were generally recorded at a

step size of 0.0098° (255 steps in the 2θ range from 80.99° to 83.50°) and count time of 4s

per step or 75s per frame in case of area detector, while the divergent slit was kept at 2mm.

Parabolic regression fit was employed for determining the peak positions. From the

positions of peak, the residual stresses were computed by applying sin2ψ method by using

the following relationship:

( )

∂∂

+

=ψν

σ φψ

φφ 2

0)( sin1

1d

dE

hkl

Equation 3.4

This relationship is similar to Equation 2.20, except that d0 has been replaced by dΦ0.

The value of dΦ0 is determined by recording peak corresponding to ψ = 0, while the value

of term ( )

∂∂

ψφψ

2sind

in the expression is computed from the slope of d vs sin2ψ plot.

3.8 Coating characterization through Raman spectroscopy

In the present study, Raman spectroscopy was employed for characterizing the

bonding nature in WC/C PVD coating. In order to characterize the nature of bonding in

WC/C PVD coating, the Raman spectrum was recorded by irradiating flat end of the coated

specimen with laser light of wavelength 5145 Å. The measurements were performed in the

Department of Physics at Indian Institute of Science, Bangalore on Dilor XY Laser Raman

spectrometer [Refer Figure 3.27] equipped with triple monochromator and liquid nitrogen

cooled charge coupled device (CCD) detector in near backscattering geometry.

Raman spectroscopy is based on inelastic Raman scattering of radiation. The Raman

effect is the result of interaction between incident light and electron cloud of a molecule. A

molecule, which is in a low energy state, goes to a higher energy state by absorbing a

90

photon. It will relax from this excited state back to the low energy state by emitting a

photon. However, as the ground state consists of a number of closely spaced vibrational

levels, the molecule may return back to an energy level that is slightly different from the

one from which it was excited. This difference in the energy level between the original state

and the new state after relaxation will lead to a shift in the emitted photon's wavelength, as

compared to that of the exciting photon. In case where the energy of emitted photon is less

than that of the exciting photon, the shift is designated as Stokes shift, while if the emitted

photon possesses more energy than the exciting one, the shift is called, Anti-Stokes shift

[267].

The intensity of Raman scattering is a function of the degree of deformation of the

electron cloud with respect to the vibrational coordinate. The Raman shift spectrum is thus

determined by the rotational and vibrational states of the sample and provides information

pertaining to various low frequency modes in the material.

Figure 3.27: Dilor-XY Laser Raman Spectrometer

91

The technique involves irradiating the target with high intensity, highly

monochromatic laser radiation of a given wavelength. The photons from laser light interact

with the phonons or other excitations within the target, which results in an upward or

downward shift in the energy of the laser photons. Scattered light from the specimen is

collected with an optical system and the wavelengths close to the laser line (arising from

elastic Rayleigh scattering) are filtered out, while rest of the light is dispersed onto a

detector. The shift in energy provides information regarding phonon modes in the system.