Effect of Heat Pressing on Low Fusing Willemite-based Glass-ceramic

Objectives: To evaluate the effect of heat pressing on the texture development and bending strength of willemite-based glass-ceramics using commercially available dental equipment. Methods: The glass investigated in the study was based on the formula: SiO2-B2O3-R2O-Al2O3-ZnO-MgO-F. Glass rods and plates were prepared by melting the glass composition at 1400 °C for 2 h. In heat pressing group, ingots were cut from the rods. After pre-heated at 500°C for 1 h, the glass-ceramic ingots were heat-treated from 600°C to 860°C by 5°C/min. Before heat-pressed the ingots were hold for 20 min at 860°C and heat-pressed for 10 min under 0.4 Mpa to form bar-shaped specimens using commercially available dental equipment. In heat treatment group, bars were cut from plates and underwent identical heat treatment procedures without heat pressing. SEM was used to investigate the microstructure of the glass-ceramics. X-ray diffraction was used to identify the crystal phases in the glass-ceramics. Flexural strength was measured using a three-point bending test. The Vickers hardness was measured using indentation strength technique. Results: XRD results showed the main crystalline phases were willemite and quartz. SEM examination indicated crystal was fine grain size (1-2 micrometer) and randomly interlocked in heat treatment group while crystal alignment along the pressing direction was found in heat pressing specimens. There was a significant increase in mean bending strength (179.35±18.91 MPa ) (p<0.01 ) in the heat pressing group compared with the heat treatment group(127.36±11.41 MPa), while no significant difference was found in Vickers hardness between the two groups. Conclusion: Heat pressing leads to a significant increase in bending strength compared with heat treatment in glass-ceramics because of crystal alignment along the direction of pressing.