Effect of heat treatment on microcrack healing behavior of a machinable dental ceramic

The purpose of this study was to evaluate the effect of annealing in air on the crack healing behavior of a machinable dental ceramic (Vita Mark II). The glass transition temperature and the softening point were determined by dilatometry. Polished Vita Mark II blocks (n = 12) were indented with a Vickers indenter under a 9.8 N load. Optical micrographs were taken immediately after indentation, and the crack lengths were measured. The specimens were annealed at 900 degrees C for either 30 min, 1 h, or 2 h. One group was indented and heat treated at 400 degrees C for 2 h as control. The ratio of crack length after annealing to crack length before annealing treatment was calculated for each indentation. Scanning electron microscopy was performed before and after annealing at 900 degrees C for 1 h. Scanning electron microscopy revealed shortening and blunting of the cracks after annealing. Annealing in air at 900 degrees C for either 30 min, 1 or 2 h significantly reduced the mean crack length of an indented machinable dental ceramic compared to the control group. However, the mean biaxial flexural strength was not significantly affected by an annealing treatment.