Fatigue Life Evaluation of Heat Treated and PVD Coated Low...
Transcript of Fatigue Life Evaluation of Heat Treated and PVD Coated Low...
60
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.
61
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.
62
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.
63
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
67
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.
70
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
71
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
72
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
73
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.
74
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.
75
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.
76
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
77
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.
78
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.
79
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
80
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
81
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
82
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.
84
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.
86
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.