KIM et al. CYCLIC FATIGUE OF BRITTLE CERAMICS 5. DISCUSSION a 5./ General We have shown that intrinsically brittle materials, specifically soda-lime glass, porcelain and silicon nitride, are susceptible to fatigue in contacts with spheres. The strength degradation plots in Section 4. I usefully quantify this fatigue, thereby providing a basis for materials evaluation in applications involving concentrated oscillating loads. For this purpose, it is convenient to condense data from such plots onto a "master diagram-accordingly in Fig. 8 we replot some of the curves from Fig. 2, along with companion curves for a glass-infiltrated 250m alumina and yttria-tetragonal-polycrystal zirconia from a study on dental ceramics [26]. It is clear that materials like silicon nitride and zirconia, by virtue of their intrinsic toughness, occupy a high position (b) in such a diagram, and present themselves as perior candidates for bearing materials Nevertheless, even the strongest and toughest ma- terials are susceptible, to a greater or lesser extent to some degree of degradation moreover there are other considerations such as cost and aesthetics (e. g. dental restorations), that may in some cases constrain selection to the lower portions of the plot. We shall return to the issue of materials design We have also demonstrated the existence of at least two modes of damage in nominally brittle cer- amics. The first mode is classical cone fracture. dri- ven in moist environments by chemically-enhanced slow crack growth. Cone fracture accounts for the first stage of degradation in the strength plots-the 500um Fig. 7. Half-surface(refiected light) and side-section(trans 2000 of hertzian indentati soda-lime glass/water (P=140 N, r=3.18 mm. n=I Y-TZP(P= 1000N n=5x 10). Deep penetrating cracks form beneath the near-surface damage zone. Bars denote outer and inner Si3N4(P=1000N ontact diameters secondary cracks appear to correlate with the inner radius of the Hertzian contact, associated with the 3 400 minimum load used in the cyclic tests--inserting Alumina(P= 1000N Hertzian relation between contact radius a and load P, aaPl/s [58] yields ainner/ Porcelain(P= 500 with the relative scale of the annular zone for the 0 Glass(P= 500N indent on glass in Fig. 7(a). Interestingly, these deep cracks do not appear to participate in the sub- sequent flexural failure, possibly because they extend down into the compressive zone of the flex- Number of cycles, ure field in the strength testing: however, they may Fig. 8. Master diagram comparing contact fatigue re- be the cause of failure during extended contact fati- sponses a(n) for selected materials, at specified contact gue testing at high P and n(arrows, Fig. 2) load P: data for soda-lime glass, porcelain and silicon nitride from Fig. I: data for a glass-infiltrated alumina such analogous subsidiary cone cracks and zirconia(Y-TZP)from a companion study [26]. Solid observed in comparative static loading lines denote brittle region. (Note break in strength axis.secondary cracks appear to correlate with the inner radius of the Hertzian contact, associated with the minimum load used in the cyclic testsÐinserting Pmin=Pmax10:14 (20 N/140 N) into the classical Hertzian relation between contact radius a and load P, aAP1=3 [58] yields ainner=aouter10:5, consistent with the relative scale of the annular zone for the indent on glass in Fig. 7(a). Interestingly, these deep cracks do not appear to participate in the sub￾sequent ¯exural failure, possibly because they extend down into the compressive zone of the ¯ex￾ure ®eld in the strength testing; however, they may be the cause of failure during extended contact fati￾gue testing at high P and n (arrows, Fig. 2). No such analogous subsidiary cone cracks were observed in comparative static loading. 5. DISCUSSION 5.1. General We have shown that intrinsically brittle materials, speci®cally soda-lime glass, porcelain and silicon nitride, are susceptible to fatigue in contacts with spheres. The strength degradation plots in Section 4.1 usefully quantify this fatigue, thereby providing a basis for materials evaluation in applications involving concentrated oscillating loads. For this purpose, it is convenient to condense data from such plots onto a ``master diagram''Ðaccordingly, in Fig. 8 we replot some of the curves from Fig. 2, along with companion curves for a glass-in®ltrated alumina and yttria-tetragonal-polycrystal zirconia from a study on dental ceramics [26]. It is clear that materials like silicon nitride and zirconia, by virtue of their intrinsic toughness, occupy a high position in such a diagram, and present themselves as su￾perior candidates for bearing materials. Nevertheless, even the strongest and toughest ma￾terials are susceptible, to a greater or lesser extent, to some degree of degradation. Moreover, there are other considerations, such as cost and aesthetics (e.g. dental restorations), that may in some cases constrain selection to the lower portions of the plot. We shall return to the issue of materials design later. We have also demonstrated the existence of at least two modes of damage in nominally brittle cer￾amics. The ®rst mode is classical cone fracture, dri￾ven in moist environments by chemically-enhanced slow crack growth. Cone fracture accounts for the ®rst stage of degradation in the strength plotsÐthe Fig. 8. Master diagram comparing contact fatigue re￾sponses s(n) for selected materials, at speci®ed contact load P: data for soda-lime glass, porcelain and silicon nitride from Fig. 1; data for a glass-in®ltrated alumina and zirconia (Y-TZP) from a companion study [26]. Solid lines denote brittle region. (Note break in strength axis.) Fig. 7. Half-surface (re¯ected light) and side-section (trans￾mitted light) views of Hertzian indentation sites in: (a) soda-lime glass/water (P = 140 N, r=3.18 mm, n=104 ); (b) porcelain/water (P = 500 N, r=3.18 mm, n ˆ 5 104). Deep penetrating cracks form beneath the near-surface damage zone. Bars denote outer and inner contact diameters. 4720 KIM et al.: CYCLIC FATIGUE OF BRITTLE CERAMICS