LIST OF FIGURES Figure 1.1: Types of surface coatings....4 Figure 1.2: PVD processing techniques...8 Figure 1.3: Schematic of PVD process... 11 Figure 2.1: Formation of intrusion and extrusion marks on the material surface.... 19 Figure 2.2: Phases of fatigue life... 20 Figure 2.3: Mechanism of fatigue crack propagation... 21 Figure 2.4: Various types of fatigue test specimens... 24 Figure 2.5: (a), (b) and (c): Cylindrical fatigue test specimens under single, 2-point and 4-point bending respectively; (d), (e) and (f): Toroidal fatigue test specimens in single, 2-point and 4-point bending respectively.... 25 Figure 2.6: S-N testing according to JSME standard... 27 Figure 2.7: S-N data along with ASTM regression fit and confidence bands.... 30 Figure 2.8: Step test for approximating the fatigue limit using a single specimen... 32 Figure 2.9: Probit method for determining the endurance limit... 33 Figure 2.10: Staircase method for determining endurance limit.... 34 Figure 2.11: Oxidation potentials of alloying elements and iron in steel, heated in endothermic gaseous environment... 40 Figure 2.12: Diffraction from a set of hkl planes for residual stress measurement... 49 Figure 2.13: Relationship between equipment coordinate system, principal stresses and stress to be measured, σ Φ... 52 Figure 2.14: Various forms of d vs sin 2 ψ plots; (a): Under uniaxial or biaxial stresses in isotropic material, (b):under triaxial stresses and (c): For textured material... 54 Figure 3.1: Drawing of fatigue specimen... 62 Figure 3.2: Tensile test specimen as per ASTM E 8M... 62 Figure 3.3: Polished fatigue test specimens... 63 Figure 3.4: Coating unit at Oerlikon Balzers... 64 Figure 3.5: Fatigue test specimen after WC/C PVD coating... 65 Figure 3.6: Extensometer fitted over tensile test specimen... 66 Figure 3.7: Specimen mounted on a universal testing machine... 66 Figure 3.8: Specimens failed under tensile tests... 67 Figure 3.9: Specimens for determination of microhardness profile across depth... 68 Figure 3.10: Digital microhardness tester... 68 Figure 3.11: Indentation mark created by Vickers microhardness tester... 69 Figure 3.12: Surface roughness tester... 70 xii
Figure 3.13: Loading of cylindrical specimen in the test rig... 71 Figure 3.14: (a): Front and (b): Top views of the test rig with casing removed... 72 Figure 3.15: Revolution counter... 73 Figure 3.16: Interference fringes in thin sulfide film... 77 Figure 3.17: Reagents and apparatus used for etching... 79 Figure 3.18: Aluminium cups for mounting of metallographic specimens... 79 Figure 3.19: Almicro trinocular metallographic microscope with digital camera.... 80 Figure 3.20: Celestron digital microscope employed for fractographic studies... 82 Figure 3.21: Mounting of specimens onto aluminium stubs and application of alumina paste for electrical continuity.... 84 Figure 3.22: Scanning electron microscope employed for fractographic studies... 84 Figure 3.23: Specimen preparation for X-ray diffraction analysis.... 86 Figure 3.24: Mounting of specimens for XRD... 86 Figure 3.25: Mounting of specimens in PANalytical X-ray diffraction machine... 87 Figure 3.26: 2θ Scan of SAE 8620 specimen... 88 Figure 3.27: Dilor-XY Laser Raman Spectrometer... 90 Figure 4.1: Stress-Strain graph for SAE8620 steel in green state... 93 Figure 4.2: Effect of coating on microhardness profiles of various steels... 94 Figure 4.3: Laser Raman spectra of WC/C coating at two different locations... 96 Figure 4.4: Metallographs of case carburized SAE8620 specimen s cross-section: (a): Martensitic structures in the case, revealed by etching in 3% Nital for 10s; (b): Chunks of carbides at a depth of 300µm, (c) and (d): Pictures reproduced in true colour to reveal carbide segregates along prior austenitic grain-boundaries. Specimens in (b), (c) and (d) etched face-up for 3 min in Klemm s - I reagent;... 98 Figure 4.5: Metallographs showing the presence of lath martensite in the core of SAE8620 specimen etched with 3% nital for 5 seconds, viewed using: (a): 10X objective and (b): 40X objective.... 98 Figure 4.6: Mosaic of metallographs showing variation in microstructure with depth in case carburized and tempered specimens made of SAE8620 specimen, etched in 3% Nital for 10 seconds... 100 Figure 4.7: (a): Cross-section of case carburized, tempered and coated specimen etched in 3% nital for 2 h, followed by light polishing; (b): Magnified view of region A in figure (a) showing the presence of lower bainite in martensitic matrix; (c): Magnified view at location B in figure (a); and (d): Result of EDAX elemental analysis performed at location marked with cross-hair in (c).... 101 Figure 4.8: d vs sin 2 ψ plot for uncoated and coated specimens made of SAE8620 steel... 103 Figure 4.9: S-N graphs for SAE8620 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states.... 107 xiii
Figure 4.10: Percentage change in fatigue strength of uncoated and coated SAE8620 steel specimens with respect to specimens in green state... 110 Figure 4.11: Fatigue fracture surfaces of SAE8620 steel specimens in green state, tested at (a): 262 MPa (b): 279 MPa (c): 300 MPa (d): 327 MPa (e): 365 MPa (f): 396 MPa (g) and (h) 425 MPa.... 112 Figure 4.12: Optical micrograph of specimen tested at 279MPa, showing ratchet marks and transgranular crack propagation.... 113 Figure 4.13: SEM image of fatigue specimen tested at 279 MPa, showing transgranular crack propagation, along with ratchet mark and extrusion sliver on the outer surface (identified with arrow-mark)... 113 Figure 4.14: SEM image of fractured specimen tested at 279 MPa, showing fatigue striations on multiple plateaus... 114 Figure 4.15: SEM image of crack geometry under mode-ii in fatigue specimen tested at 396 MPa.... 117 Figure 4.16: SEM image of fractured specimen tested at 396 MPa, showing presence of tire tracks... 117 Figure 4.17: Fractographs of specimens made of SAE8620 steel: (a): Optical fractograph of uncoated fatigue specimen tested at 810 MPa, showing crack initiation site; (b): Scanning electron micrograph of the location marked by arrow in (a); (c): Optical fractograph of uncoated specimen tested at 1000 MPa, showing crack initiation site; (d): Scanning electron micrograph showing striations and tire tracks at a location within the regions marked by rectangles in (c); (e): Top and side views of coated fatigue specimen tested at 860 MPa; (f): Scanning electron micrograph showing adherence of coating at the failed section marked with rectangles in (e); (g): Optical fractrograph of coated fatigue specimen tested at 910 MPa, showing ratchet marks and crack initiation site; (h): Scanning electron micrograph at a location within the region marked with rectangle in (g), showing tire tracks... 120 Figure 4.18: Magnified view of crack initiation region of specimen shown in Figure 4.17 (e), depicting intergranular initiation and transgranular propagation.... 122 Figure 4.19: Optical fractograph taken on side-wall of coated specimen shown in Figure 4.17 (g), revealing the formation of multiple cracks under low-cycle fatigue.... 123 Figure 4.20: Close-up view of specimen shown in Figure 4.17 (g), revealing crack formation on multiple planes... 123 Figure 4.21: Metallograph of 20MnCr5 steel (etched in 3% Nital for 4s) in green state revealing the presence of pearlitic microstructure.... 125 Figure 4.22: Mosaic of metallographs, showing variation of microstructure with depth in case carburized and tempered specimens made of 20MnCr5 steel, etched in 3% Nital for 8 seconds.... 126 Figure 4.23: Metallograph of case carburized and tempered specimen made of 20MnCr5 steel, etched in Klemm s - I reagent for 3 minutes, revealing the presence of carbide particles in the carbon-rich case.... 127 Figure 4.24: d vs sin 2 ψ plot for uncoated and coated specimens made of 20MnCr5 steel... 129 xiv
Figure 4.25: S-N graphs for 20MnCr5 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states.... 134 Figure 4.26: Optical fractographs showing macroscopic features on uncoated / coated specimens made of 20MnCr5 steel, fatigued at various stress levels: (a): Uncoated specimen tested at 1000 MPa, (b): Uncoated specimen tested at 935 MPa, (c): Uncoated specimen tested at 920 MPa (d): Uncoated specimen tested at 900 MPa (e): WC/C coated specimen tested at 941 MPa (f): WC/C coated specimen tested at 888 MPa (g): WC/C coated specimen tested at 842 MPa and (h): WC/C coated specimen tested at 765 MPa.... 136 Figure 4.27: Magnified views of regions marked with rectangles in the corresponding fractographs given in Figure 4.26... 138 Figure 4.28: Micrograph indicating intergranular crack initiation and small region of stable transgranular growth (marked by dashed-line), followed by dominantly intergranular propagation in the specimen shown in Figure 4.26 (d)... 139 Figure 4.29: Scanning electron micrograph showing intergranular cracking in the specimen shown in Figure 4.26 (b)... 139 Figure 4.30: Magnified view of specimen shown in Figure 4.26 (a), depicting three different regions of crack propagation: (A): Region dominated by intergranular fracture, (B): Region of cleavage-like transgranular fracture, characterized by river pattern and (C): Region of ductile fracture... 140 Figure 4.31: Magnified views showing the presence of curved-cracks in various specimens whose fractographs are shown in Figure 4.26: (a): Non-radial cracks in uncoated specimen shown in Figure 4.26 (d); (b): Looping cracks in coated specimen shown in Figure 4.26 (e); (c): Completely looped cracks in coated specimen shown in Figure 4.26 (f); (d): Composite micrograph showing chipped-off material in specimen shown in Figure 4.26 (c), and (e): Curved-cracks in coated specimen shown in Figure 4.26 (h)...141 Figure 4.32: Micrographs indicating the presence of internal oxidation in 20MnCr5 specimens: (a) and (b): Specimens etched in 3% Nital for 5 seconds; (c): Specimen polished after etching to reveal the depth of penetration of oxides; (d): Micrograph taken in the vicinity of chipped-off portion appearing towards left end in Figure 4.27 (b); the image is digitally processed for enhancing depth of field by stitching together portions of various photographs taken by shifting the focal plane of the microscope... 143 Figure 4.33: (a): Scanning electron micrograph showing the presence of oxide precipitates within a grain near the surface of a case-carburized 20MnCr5 steel specimen; (b): EDAX spectrum, confirming the presence of oxygen in the region identified with a rectangle in (a)... 143 Figure 4.34: d vs sin 2 ψ plots for case-carburized, uncoated specimens made of 20MnCr5 steel at various depths below surface... 145 Figure 4.35: Residual stress profile in case carburized 20MnCr5 steel showing variation of residual stress with depth below surface... 146 xv
Figure 4.36: Optical fractographs of uncoated specimen, polished to remove surface layers affected by internal oxidation, fatigue tested at 1014 MPa. (a): Fracture macrograph showing the formation of fish-eye; (b): Magnified view of the fish-eye appearing in (a).... 147 Figure 4.37: Optical fractograph of ODA at the crack initiation site within the fish-eye... 148 Figure 4.38: Mosaic of metallographs, showing variation in microstructure with depth in case carburized and tempered specimens made of EN353 steel, etched in 3% Nital for 5 seconds, followed by immersion in potassium metabisulfite solution for 20 seconds.... 151 Figure 4.39: (a): Metallograph indicating the presence of internal oxidation in case carburized and tempered specimens made of EN353. The specimen was lightly polished after etching with nital; and (b): Magnified view of region marked with rectangle in (a).... 153 Figure 4.40: Metallographs of the cross section of case carburized and tempered specimen made of EN353 steel. (a) and (b): Lath martensite in the core; and (c): Composite metallograph showing martensite, retained austenite and chunks of carbide precipitates (marked with arrows) within the carburized and tempered case. Specimens in (a) and (b) etched with 3% nital for 5s, specimen in (c) were etched with 3% nital for 4s, followed by immersion in potassium metabisulfite for 12s... 154 Figure 4.41: d vs sin 2 ψ plot for uncoated and coated specimens made of EN353 steel... 155 Figure 4.42: S-N graphs for EN353 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states.... 160 Figure 4.43: Percentage change in fatigue strength of uncoated and coated EN353 steel specimens with reference to specimens in green state... 161 Figure 4.44: Optical fractographs showing macroscopic features in uncoated and WC/C coated specimens made of EN353 steel, fatigued at various loads: (a): Coated specimen tested at 843 MPa, (b): Uncoated specimen tested at 925 MPa, (c): Coated specimen tested at 726 MPa (d): Uncoated specimen tested at 860 MPa (e): Coated specimen tested at 655 MPa, and (f): Uncoated specimen tested at 765 MPa.... 163 Figure 4.45: Micrograph of specimen shown in Figure 4.44 (d), indicating crack initiation by intergranular cracking (marked with arrow)... 164 Figure 4.46: Scanning electron micrograph of specimen shown in Figure 4.44 (a), indicating a mix of transgranular and intergranular cracking... 165 Figure 4.47: Optical fractograph of specimen shown in Figure 4.44 (c), indicating various regions of crack propagation. The fractograph is constructed as a mosaic by stitching together four individual fractographs.... 165 Figure 4.48: Mosaic of metallographs, showing variation of microstructure with depth in case carburized and tempered specimens made of SCM420 steel, etched in 3% Nital for 4 seconds, followed by immersion in potassium metabisulfite solution for 9 seconds... 168 Figure 4.49: Metallograph of SCM420 specimen, colour etched with Klemm s I reagent for 2 minutes to reveal the presence of carbides (marked with arrows). Green tint employed for contrast enhancement... 169 xvi
Figure 4.50: d vs sin 2 ψ plot for uncoated and coated specimens made of SCM420 steel... 171 Figure 4.51: S-N graphs for SCM420 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states.... 175 Figure 4.52: Percentage change in the fatigue strength of uncoated and coated SCM420 steel specimens with respect to specimens in green state... 176 Figure 4.53: Optical fractographs of SCM420 steel specimens. (a): Uncoated specimen tested at 980 MPa, (b): Coated specimen tested at 940 MPa, (c): Uncoated specimen tested at 780 MPa, and (d): Coated specimen tested at 820 MPa. Fractomicrographs given in the right column provide magnified views of crack initiation sites, captured by holding the specimen in same orientation as in the left column... 179 Figure 4.54: Magnified optical fractograph of specimen shown in Figure 4.53 (d), depicting crack initiation site... 180 Figure 4.55: Optical fractograph covering entire cross-section of fractured specimen shown in Figure 4.53 (d). The fractograph is constructed as mosaic by joining together six individual fractographs.... 180 Figure 4.56: Optical fractograph of a stage-i crack propagation site in the specimen shown in Figure 4.53 (d).... 181 Figure 4.57: Optical fractograph showing cleavage-like crystallographically oriented stage-i fatigue fracture exhibiting factory-roof morphology, recorded at the region marked with rectangle in Figure 4.56... 181 Figure 4.58: Composite optical fractograph showing formation of multiple cracks in specimen shown in Figure 4.53 (b), tested under low-cycle fatigue.... 182 Figure 4.59: Optical fractograph showing multiple-plane cracking for specimens tested under low-cycle fatigue... 182 xvii