Load effect on the mechanical behaviour of zirconia-reinforced lithium silicate glass ceramics

Abstract

The mechanical behaviour of pre-crystallized and crystallized zirconia-reinforced lithium silicate glass ceramics (ZLS) using micro-indentation techniques is reported. Vickers micro-indentations were conducted on these materials at the load of 0.245 N–9.8 N and 4.9 N–9.8 N to determine the load effect on the hardness and fracture toughness, respectively. The hardness increased with the load by 78.2% (ANOVA, p < 0.05) and 163% (ANOVA, p < 0.05) on pre-crystallized and crystallized ZLS, respectively, indicating the occurrence of the reverse indentation size effect (RISE) on both materials. The fracture toughness increased with the load by 20% and 12.3% on pre-crystallized and crystallized ZLS, respectively. Hardness data of both materials were analyzed using the Meyer's, indentation-induced cracking (IIC), proportional specimen resistance (PSR), modified PSR, Hays–Kendall and elastic plastic deformation (EPD) models. The Meyer's and IIC models described the RISE behaviour of both materials satisfactorily. The PSR, Hays–Kendall and EPD models provided consistent load-independent hardnesses while the modified PSR model gave erratic load-independent hardnesses for both materials. The continuum model provided intrinsic compressive yield strengths of both materials which were consistent with those obtained from the EPD model and the Tabor relation, implying the EPD model as the most successful. The brittleness index predicted better machinability for crystallized ZLS than pre-crystallized ZLS. Finally, the variations of hardness and fracture toughness with load might account for the different micro-material removal mechanisms in the sharp abrasive machining of these materials.