KIM et al. CYCLIC FATIGUE OF BRITTLE CERAMICS 4715 late any mechanical contribution to the damage ad n). The indentation damage evolution was ulation. In most cases the tests were conducted nitored on a video recorder in distilled water to enhance the fatigue effect; other tests were conducted in laboratory air (relative humidity≈50%) 4. RESULTS In some cases the WC spheres themselves showed signs of deformation, especially in extended tests on 9.1. Strength des data and silicon nitride [35]. and so were rotated periodically es of the strength degradation tests are plotted for glass/water, porcelain/water and silicon 3.3. Strength tests nitride air in Figs 2-4. Data points with error bars Strength tests were conducted on the contact- are experimental means and standard deviations of damaged bar specimens in four-point flexure, with a minimum five specimens at each specified number the contact surface on the tensile side. The outer of cycles n or maximum contact load P. and inner span dimensions used in the flexure tests symbols represent failures from natural flaws, filled e 61 and 27 mm for the glass laths, 20 and symbols failures from indentation sites; in the latter 10 mm for the other ceramics to obtain "inert strengths" the contact sites were first dried in war air and covered with a drop of dry silicone oil, and he specimens then broken in fast fracture(<20 ms) (a)Glass/water 3,18mm [33]. All specimens were examined in a low-power ermine the source of failure. i.e damage site or"natural"flaw. Any edge failures 200N (more common in the glass specimens) were dis- carded from the data poo P=500N Selected surface contact damage sites in all ma terials were subjected to close examination by high- power optical microcopy, both before and after I (b) Porcelain/water flexure testing. Most observations were made in gold coating the indented surfaces. In the transpar- 6 Iso damage;and similarly in the translucent porcelain, a o reflection mode. in Nomarski illumination. after ent glass the damage sites were also examined irectly in transmission mode to observe subsurface after grinding and polishing the specimens from the P=500N back surface to a final thickness <500 um Subsurface side views were also obtained by grinding and polishing cross sections down to the 1500 indentation mid-planes [51] and viewing in optical (c)SiN//air icroscopy. Again, in the glass the damage was amenable to viewing in both reflection and trans 1000 mission microscopy, in the porcelain, by grinding 000N and polishing the specimens from both sides down to the mid-plane, to net thickness <500 um. 500 (Bonded-interface specimens [52, 53], in which spe- P=2200N prior to indentation, were not employed here because of concern that environmental specie might penetrate the interface and produce artifacts) Number of contact cycles, n the cross sections were etched of 12% HF acid to highlight damage features In situ observations of the damage zone crack cycles n, indentation with WC spheres of radius r, at maxi uring the indentation-strength tests These obser. porcelain in water;(e) silicon nitride ng ass id awater: (b) growth were made in selected glass specimens re means and standard deviations, minimum five speci ations were made using a stereo-zoom microscope mens per point. Filled symbols indicate failures fro with back lighting, either directly through the side indentation sites-grey symbols fro surface or from below using a mirror to redirect the mbols from radial cracks. Unfilled symbols indicate fail- res from natural flaws-box at left axis and horizonta light source (in the latter case with the con- dashed line are"laboratory"strengths (unindented speci tact surface precoated with gold to enhance reflec- mens). Solid curves are theoretical fits to datalate any mechanical contribution to the damage ac￾cumulation. In most cases the tests were conducted in distilled water to enhance the fatigue e€ect; other tests were conducted in laboratory air (relative humidity 150%). In some cases the WC spheres themselves showed signs of deformation, especially in extended tests on silicon nitride [35], and so were rotated periodically. 3.3. Strength tests Strength tests were conducted on the contact￾damaged bar specimens in four-point ¯exure, with the contact surface on the tensile side. The outer and inner span dimensions used in the ¯exure tests were 61 and 27 mm for the glass laths, 20 and 10 mm for the other ceramics. To obtain ``inert strengths'' the contact sites were ®rst dried in warm air and covered with a drop of dry silicone oil, and the specimens then broken in fast fracture (<20 ms) [33]. All specimens were examined in a low-power microscope to determine the source of failure, i.e. damage site or ``natural'' ¯aw. Any edge failures (more common in the glass specimens) were dis￾carded from the data pool. 3.4. Damage morphology Selected surface contact damage sites in all ma￾terials were subjected to close examination by high￾power optical microcopy, both before and after ¯exure testing. Most observations were made in re¯ection mode, in Nomarski illumination, after gold coating the indented surfaces. In the transpar￾ent glass the damage sites were also examined directly in transmission mode to observe subsurface damage; and similarly in the translucent porcelain, after grinding and polishing the specimens from the back surface to a ®nal thickness <500 mm. Subsurface side views were also obtained by grinding and polishing cross sections down to the indentation mid-planes [51] and viewing in optical microscopy. Again, in the glass the damage was amenable to viewing in both re¯ection and trans￾mission microscopy; in the porcelain, by grinding and polishing the specimens from both sides down to the mid-plane, to net thickness <500 mm. (Bonded-interface specimens [52, 53], in which spe￾cimens are pre-sectioned and restored into contact prior to indentation, were not employed here because of concern that environmental species might penetrate the interface and produce artifacts). Some of the cross sections were etched in a solution of 12% HF acid to highlight damage features. In situ observations of the damage zone crack growth were made in selected glass specimens during the indentation±strength tests. These obser￾vations were made using a stereo-zoom microscope with back lighting, either directly through the side surface or from below using a mirror to redirect the light source (in the latter case with the upper con￾tact surface precoated with gold to enhance re¯ec￾tion). The indentation damage evolution was monitored on a video recorder. 4. RESULTS 4.1. Strength degradation data and analysis Results of the strength degradation tests are plotted for glass/water, porcelain/water and silicon nitride/air in Figs 2±4. Data points with error bars are experimental means and standard deviations of a minimum ®ve specimens at each speci®ed number of cycles n or maximum contact load P. Un®lled symbols represent failures from natural ¯aws, ®lled symbols failures from indentation sites; in the latter Fig. 2. Inert strength s as a function of number of contact cycles n, indentation with WC spheres of radius r, at maxi￾mum loads P indicated: (a) soda-lime glass in water; (b) porcelain in water; (c) silicon nitride in air. Data points are means and standard deviations, minimum ®ve speci￾mens per point. Filled symbols indicate failures from indentation sitesÐgrey symbols from cone cracks, black symbols from radial cracks. Un®lled symbols indicate fail￾ures from natural ¯awsÐbox at left axis and horizontal dashed line are ``laboratory'' strengths (unindented speci￾mens). Solid curves are theoretical ®ts to data. KIM et al.: CYCLIC FATIGUE OF BRITTLE CERAMICS 4715