Figure 2. Radial crack sequence in glass-infiltrated alumina layer, thickness d = 155 µm, on polycarbonate substrate, from indentation with WC sphere of radius r = 3.96 mm. Sequence during loading cycle at (a) P = 15.1 N (critical), (b) P = 35.1 N (intermediate), (c) P = 56.6 N (peak), and (d) P = 0 N (fully unloaded).