The effect of veneering and heat treatment on the flexural strength of Empress® 2 ceramics

Objectives. The aims of the study were to test and compare the biaxial flexural strength and reliability of Empress 2 ceramics after heat treatment and the addition of the veneering material and to characterise their microstructures. Methods. Forty disc specimens (2×14 mm) and forty disc specimens (1×14 mm) were produced by heat pressing in the EP 500 press furnace. Group 1 (2×14 mm Empress 2 core) was as heat pressed and group 2 (2×14 mm Empress 2 core) was subjected to the recommended firing cycles. Groups 3 and 4 (1×14 mm Empress 2 core) were veneered with the dentine material and heat-treated as per group 2. Groups 1, 2 and 3 were lapped to 800 grit silicon carbide paper on the compressive surface only and group 4 on both the compressive and tensile test surfaces. Twenty disc specimens per group were tested using the biaxial flexure test at a crosshead speed of 0.15 mm/min. Specimens were characterised using X-ray diffraction (XRD) and secondary electron imaging (SEM). Results. Mean biaxial flexural strengths (MPa±SD) were group 1: 265.5±25.7; group 2: 251.3±30.2; group 3: 258.6±21.4 and group 4: 308.6±37.7. There was no statistical difference between groups 1, 2 and 3 (p>0.05), but differences for group 4 (p<0.05). XRD and SEM revealed lithium disilicate and lithium orthophosphate in the Empress 2 core material and an amorphous glass and some evidence of a crystalline phase in the dentine material. Conclusions. Veneering or heat treatment of Empress 2 ceramics did not significantly affect the mean biaxial flexural strength (p>0.05) or reliability. Surface modification of the Empress 2 core material increased the mean biaxial flexural strength (p<0.05).