LIST OF FIGURES Figure 1.1: Figure 1.2: Figure 1.3: Figure 2.1: Figure 2.2: Figure 2.3: Figure 2.4: Figure 2.5: (a) (b) (c): (d) (e) (f):

Transcription

1 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 Figure 2.1: Formation of intrusion and extrusion marks on the material surface Figure 2.2: Phases of fatigue life Figure 2.3: Mechanism of fatigue crack propagation Figure 2.4: Various types of fatigue test specimens 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 Figure 2.6: S-N testing according to JSME standard Figure 2.7: S-N data along with ASTM regression fit and confidence bands Figure 2.8: Step test for approximating the fatigue limit using a single specimen Figure 2.9: Probit method for determining the endurance limit Figure 2.10: Staircase method for determining endurance limit Figure 2.11: Oxidation potentials of alloying elements and iron in steel, heated in endothermic gaseous environment Figure 2.12: Diffraction from a set of hkl planes for residual stress measurement Figure 2.13: Relationship between equipment coordinate system, principal stresses and stress to be measured, σ Φ 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 Figure 3.1: Drawing of fatigue specimen Figure 3.2: Tensile test specimen as per ASTM E 8M Figure 3.3: Polished fatigue test specimens Figure 3.4: Coating unit at Oerlikon Balzers Figure 3.5: Fatigue test specimen after WC/C PVD coating Figure 3.6: Extensometer fitted over tensile test specimen Figure 3.7: Specimen mounted on a universal testing machine Figure 3.8: Specimens failed under tensile tests Figure 3.9: Specimens for determination of microhardness profile across depth Figure 3.10: Digital microhardness tester Figure 3.11: Indentation mark created by Vickers microhardness tester Figure 3.12: Surface roughness tester xii

2 Figure 3.13: Loading of cylindrical specimen in the test rig Figure 3.14: (a): Front and (b): Top views of the test rig with casing removed Figure 3.15: Revolution counter Figure 3.16: Interference fringes in thin sulfide film Figure 3.17: Reagents and apparatus used for etching Figure 3.18: Aluminium cups for mounting of metallographic specimens Figure 3.19: Almicro trinocular metallographic microscope with digital camera Figure 3.20: Celestron digital microscope employed for fractographic studies 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 Figure 3.23: Specimen preparation for X-ray diffraction analysis Figure 3.24: Mounting of specimens for XRD Figure 3.25: Mounting of specimens in PANalytical X-ray diffraction machine Figure 3.26: 2θ Scan of SAE 8620 specimen Figure 3.27: Dilor-XY Laser Raman Spectrometer Figure 4.1: Stress-Strain graph for SAE8620 steel in green state Figure 4.2: Effect of coating on microhardness profiles of various steels Figure 4.3: Laser Raman spectra of WC/C coating at two different locations 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; 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 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 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) Figure 4.8: d vs sin 2 ψ plot for uncoated and coated specimens made of SAE8620 steel Figure 4.9: S-N graphs for SAE8620 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states xiii

3 Figure 4.10: Percentage change in fatigue strength of uncoated and coated SAE8620 steel specimens with respect to specimens in green state 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 Figure 4.12: Optical micrograph of specimen tested at 279MPa, showing ratchet marks and transgranular crack propagation 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) Figure 4.14: SEM image of fractured specimen tested at 279 MPa, showing fatigue striations on multiple plateaus Figure 4.15: SEM image of crack geometry under mode-ii in fatigue specimen tested at 396 MPa Figure 4.16: SEM image of fractured specimen tested at 396 MPa, showing presence of tire tracks 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 Figure 4.18: Magnified view of crack initiation region of specimen shown in Figure 4.17 (e), depicting intergranular initiation and transgranular propagation 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 Figure 4.20: Close-up view of specimen shown in Figure 4.17 (g), revealing crack formation on multiple planes Figure 4.21: Metallograph of 20MnCr5 steel (etched in 3% Nital for 4s) in green state revealing the presence of pearlitic microstructure 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 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 Figure 4.24: d vs sin 2 ψ plot for uncoated and coated specimens made of 20MnCr5 steel xiv

4 Figure 4.25: S-N graphs for 20MnCr5 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states 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 Figure 4.27: Magnified views of regions marked with rectangles in the corresponding fractographs given in Figure 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) Figure 4.29: Scanning electron micrograph showing intergranular cracking in the specimen shown in Figure 4.26 (b) 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 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) 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 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) Figure 4.34: d vs sin 2 ψ plots for case-carburized, uncoated specimens made of 20MnCr5 steel at various depths below surface Figure 4.35: Residual stress profile in case carburized 20MnCr5 steel showing variation of residual stress with depth below surface xv

5 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) Figure 4.37: Optical fractograph of ODA at the crack initiation site within the fish-eye 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 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) 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 Figure 4.41: d vs sin 2 ψ plot for uncoated and coated specimens made of EN353 steel Figure 4.42: S-N graphs for EN353 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states Figure 4.43: Percentage change in fatigue strength of uncoated and coated EN353 steel specimens with reference to specimens in green state 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 Figure 4.45: Micrograph of specimen shown in Figure 4.44 (d), indicating crack initiation by intergranular cracking (marked with arrow) Figure 4.46: Scanning electron micrograph of specimen shown in Figure 4.44 (a), indicating a mix of transgranular and intergranular cracking 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 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 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 xvi

6 Figure 4.50: d vs sin 2 ψ plot for uncoated and coated specimens made of SCM420 steel Figure 4.51: S-N graphs for SCM420 steel specimens in green, case carburized (uncoated) and case carburized - WC/C coated states Figure 4.52: Percentage change in the fatigue strength of uncoated and coated SCM420 steel specimens with respect to specimens in green state 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 Figure 4.54: Magnified optical fractograph of specimen shown in Figure 4.53 (d), depicting crack initiation site 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 Figure 4.56: Optical fractograph of a stage-i crack propagation site in the specimen shown in Figure 4.53 (d) 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 Figure 4.58: Composite optical fractograph showing formation of multiple cracks in specimen shown in Figure 4.53 (b), tested under low-cycle fatigue Figure 4.59: Optical fractograph showing multiple-plane cracking for specimens tested under low-cycle fatigue xvii