Indentation induced lateral crack in ceramics with surface hardening

Indentation is widely used to probe the fracture toughness of brittle materials such as ceramics or glass. In use, a hardened surface layer is inevitable in ceramics due to grinding and polishing; the surface layer also exists during sample preparation for indentation test. We study the mechanics of indentation induced lateral cracking in TiB2 (titanium di-boride) ceramics, and show that the surface hardened layer has a pronounced effect on the initiation position of lateral crack, thereby affecting the wear resistance rate. The occurrence of lateral crack underneath the surface is detected in situ by an integrated acoustic emission (AE) and corrosion potential fluctuation (CPF) technique. By computing the detailed stress distribution, we discuss the mechanics of lateral crack formation in ceramics under the influence of surface hardening. It is found that when the maximum contact force is low, the lateral crack is confined within the surface layer, whereas when the maximum indentation force is high, the crack prefers to nucleate at the interface. The findings in this paper may be useful to optimize the surface polishing process and improve wear properties of ceramics.