Typical crack defects found on ceramic ball surfaces under UV light. (a) Star defect, (b) pressing defect, (c) single ring crack, and (d) concentric ring cracks. 2 Loading configuration of four-ball machine. 3 Loading configuration of five-ball machine. 4 Loading configuration of ball-on-plate machine. 5 Loading configuration of ball-on-rod machine. 6 Japanese type ball-on-rod machine. 7 Loading configuration of disc-on-rod machine. 8 Contacting ring (cylinder on cylinder) machine. 9 Schematic illustration of the process of rolling contact failure in ceramics and metals. 10 Spalling fatigue failure resulting from artificially induced ring /cone cracks (SEM micrographs). (a) Single spall and (b) double spall. 11 Loading configuration and ring crack location on the contact track. (a) Loading system. (b) Geometric location on the contact track. (c) Twelve typical locations within the contact path. 12 Surface fatigue damage resulting from 'natural' ring cracks (Figs. 10(a)-(c)) and line defects (Fig. 10(d)). (a) Ring cracks and wear track after 113 million stress cycles at crack location β = 0 o and δ = 0 (optical micrograph). (b) Incipient failure after 27 million stress cycles at the crack location β = 45 º and δ = 0 (optical micrograph). (c) Spall SEM micrograph after 16 million stress cycles at crack location β = 90 o and δ=0. (d) Spall SEM micrograph after 1.4 million stress cycles at β=90 º. 13 Subsurface observation of spalling fatigue failure (optical micrograph). 14 Surface damage resulting from ceramic/steel contact (SEM micrographs). (a) Lateral crack spall. (b) Radial crack propagation and (c) delamination and (d) Ceramic/ceramic contact at high Hertz contact pressure. 15 Relationship between crushing strength and life ratio. 16 Schematic of the 12.7-mm-diameter C-sphere flexure strength specimen. 17 Diametral compression of the C-sphere flexure specimen causes fracture initiation from a hoop stress at the outer fiber. 18 Nodal first principal stress distribution (left) and element 1 st Principal stress distribution (right) for the 12.7-mm-diameter C-sphere specimen. 19 Maximum 1 st Principal tensile stress (located at outer fiber-see Fig. 17) as a function of diametral compressive load for the 12.7mm-diameter Csphere geometry in Fig. 16 for a Si 3 N 4. v Figure Page 20 Si 3 N 4 12.7mm-diameter C-sphere flexure strength specimens. 31 21 BS-SEM microstructure on finished ball surfaces of NBD200 and SN101C. 22 C-sphere Weibull strength distribution comparison of NBD200 and SN101C. 33 23 95% confidence ratio rings for NBD200 and SN101C C-sphere strengths. 33 24 Example of a surface-located strength-limiting flaw in a SN101C C-sphere flexure strength specimen. This specimen had a strength of 770 MPa. 34 25 Weibull plots of rolling contact fatigue lifetime. 35 26 Elastic properties of NBD200 and SN101C balls measured using Resonance Ultrasound Spectroscopy.