Fig. 3.

Post-mortem optical micrographs showing top (occlusal) and side (section) views of (a) glass/zirconia/polycarbonate and (b) porcelain/zirconia/composite trilayers subjected to off-axis loading for single- and multi-cycles. Indentation with tungsten carbide sphere of radius r = 1.5 mm, P = 350 N, in water. Large arrows indicate the sliding direction for the off-axis test. Small black filled arrows highlight the trajectories of partial cone cracks. Small white arrows in (b) indicate the dark permanent markers used to index the contact region. Note in both cases, partial cones form in the first contact–slide cycle and continue to propagate through the veneer layers. However, the partial cone propagates much deeper in porcelain veneer after one contact-slide cycle and only take several cycles to propagate through the porcelain veneer, while several thousands of cycles are required to drive partial cones to the glass/zirconia interface.