Elasticity, plasticity and analytical machinability prediction of lithium metasilicate/disilicate glass ceramics.

This paper applied non-linear theory of elasticity (NLTE) to partition indentation-induced deformations into elasticity and plasticity for lithium metasilicate glass ceramic (LMGC), sintered and pressed lithium disilicate glass ceramics (SLDGC and PLDGC). It also used elastic plastic fracture mechanics (EPFM) approach to analytically predict machinability for these materials. Using the Sakai's series elastic and plastic deformation model that applied NLTE, the resistances to plasticity for LMGC, SLDGC and PLDGC were extracted from their respective indentation-extracted plane strain moduli and contact hardness values. Plane strain moduli and resistances to plasticity were used to calculate elasticity and plasticity for these materials. Furthermore, the EPFM approach in the Sakai-Nowak model was applied to deconvolute resistances to machining-induced cracking for these materials. All properties were extracted at 10 mN peak load and 0.1-2 mN/s loading rates to determine the loading-rate influence on these properties. The resistances to plasticity of LMGC and SLDGC were loading rate dependent (ANOVA, p < 0.05) and the resistance to plasticity of PLDGC was loading rate independent (ANOVA, p > 0.05). The strain rate sensitivity model was used to find the intrinsic resistances to plasticity for LMGC and SLDGC. The elastic displacement/deformation components were dominant for LMGC at all loading rates. For SLDGC and PLDGC, the deformation mechanisms were dynamic with the plastic and elastic deformation components dominating at low loading and high loading rates respectively, a phenomenon attributed to indentation energies. The decrease in plastic displacements for all materials with increase in loading rate was due to the strain hardening behaviour. Also, PLDGC revealed the highest absorbed energy followed by SLDGC and LMGC. Finally, PLDGC had the highest resistance to machining-induced cracking followed by SLDGC and LMGC. This study provides a quantitative basis to rank materials in terms of brittleness, ductility and resistance to mechanically-induced cracking.