표면 절삭 및 연마가 지르코니아 상변이와 굴곡강도에 미치는 영향

Abstract

Purpose: The purpose of this in vitro study was to evaluate the effect of surface grinding and polishing procedures using high speed zirconia diamond bur with different grit size on the phase transformation and flexural strength of zirconia.

Methods: Forty five disc shape specimens (15 x 1.25mm) with a cylindrical projection in the center of each disc (1 x 3mm) were fabricated from Yttrium oxide-stabilized zirconium oxide (Prettau, Zirkonzahn, Italy). Each of these specimens was divided into 4 experimental groups (n=10) and control group (n=5) according to the following grinding and polishing procedures : Group 1 – grinding (coarse-grit diamond bur) + polishing, Group 2 – grinding (coarse-grit diamond bur), Group 3 - grinding (fine-grit diamond bur) + polishing, Group 4 – grinding (fine grit diamond bur). The remaining specimens without grinding and polishing were used as a control group. Each specimens analyzed by 3D-OM, XRD analysis, FE-SEM imaging and biaxial flexural strength test.

Results: Based on the surface morphology by 3D-OM and FE-SEM images, polished specimens showed more smooth surface and lower roughness value (Ra). In the result of XRD analysis, control group contained only tetragonal zirconia. However partial phase transformation from tetragonal to monoclinic zirconia occurred in all experimental groups. Group 2, ground with coarse grit diamond bur, showed more t→m phase transformation than Group 1 significantly. In the biaxial flexural strength test, Group 2 showed the lowest mean flexural strength than other groups.

Conclusions: The flexural strength in all specimens after grinding and polishing showed over 500 MPa, and those were clinically acceptable. However, grinding with coarse grit diamond bur without polishing induce the phase transformation and low strength. Therefore, surface polishing is required for the occlusal adjustment using high speed zirconia diamond bur to reduce the amount of monoclinic phase transformation, and increase the flexural strength of zirconia.

Keywords: 3Y-TZP, Phase transformation, XRD, Roughness, Biaxial flexural strength