EFFECT OF DIFFERENT SINTERING TECHNIQUES ON THE BIAXIAL FLEXURAL STRENGTH AND CRYSTAL STRUCTURE OF MONOLITHIC ZIRCONIA (IN VITRO STUDY)

INTRODUCTION: For monolithic zirconia restorations to be a viable chair-side treatment option, recent studies are aiming to reduce the sintering time required without altering its properties. OBJECTIVES: This study compared the influence of high-speed sintering, speed sintering and conventional sintering on monolithic zirconia regarding bi-axial flexural strength, phase transformation and dimensional changes after sintering. METHODS: Thirty six monolithic zirconia disc specimens were prepared with the dimensions 12mm x 1.5mm and categorized into 3 groups (n=12) based on the sintering technique: High-speed sintering (HS: 1580 °C, total time is approximately 10 minutes) Speed sintering (SS: 1515 °C, total time is approximately 90 minutes), and Conventional sintering (CS, 1500 °C, total time is approximately 10 h). The ball on ring design was used to determine biaxial flexural strength (BFS). Dimensional changes after sintering was evaluated by digital micrometer (with an accuracy of 0.001 mm). The specimen's crystallography was investigated by x-ray diffraction technique (XRD). RESULTS: All groups exhibited a uniform sintering shrinkage of about 20% in all dimensions. Crystallographic analysis revealed only tetragonal and cubic characteristic peaks. BFS test results showed statistically insignificant difference between conventional (1105.5 ± 55.85MPa), speed (1078.3 ± 56.22 MPa), and high speed (1050.6±53.42 MPa) sintering cycles (p<0.05). CONCLUSION: Speed and high speed cycles can be recommended for sintering of monolithic zirconia in order to reduce fixed prosthetic restoration fabrication times as the changes they induced were within the clinically acceptable ranges .