Adverse Effects Of Tungsten Carbide Grinding On Strength Of Zirconia

Objectives: This study aims to investigate the outcomes of tungsten carbide burs and surface treatments on the mechanical strength and surface characters of dental yttria-stablized zirconia ceramic (Y-TZP) using Weibull distribution.
Methods: Y-TZP blocks (LAVA Plus high translucency zirconia disc, 3M ESPE, Seefeld, Germany) were trephined and cut into discs (N=144) and were subsequently polished and sintered (ø20mm, 0.85mm ± 0.05mm in thickness).Specimens were divided into 8 groups and subjected to different surface treatments: i) Control (C), ii) Round-shaped tungsten carbide burs 20s (TC₁), iii) TC₁ and air-particle abrasion (alumina, 50 μm, 4 bar, 10 seconds, 10mm) (TC₁+ APA), iv) TC₁ and rubber polishing (TC₁+ RP), v) Tungsten carbide bur finishing chamfer 20s (TC₂), vi) TC₂ and air-particle abrasion (TC₂+ APA), vii) TC₂ and rubber polishing (TC₂+ RP), viii) Rubber polishing (RP). Grinding was performed in a dental air turbine handpiece with water cooling. Samples were examined by SEM, surface roughness (Ra), atomic force microscopy (AFM) and X-ray diffraction. A ball-on-three-piston biaxial flexural strength was performed in 15 samples per group and the broken specimen were observed under SEM to investigate the fracture origin pattern. Data were analyzed by one-way ANOVA and Weibull analysis were performed (p<0.05).
Results: A significant difference (p<0.05) in biaxial flexural strength were found among groups RP > C > TC₂+ APA > TC₂+ RP > TC₁+ APA = TC₁ + RP > TC₁ = TC₂. The Weibull plot revealed that RP > C>TC₂+ APA > TC₂+ RP >TC₁+ APA >TC₁+ RP > TC₂> TC₁for the characteristic strength at both 63.2% and 10% (B10). Ra tests showed a rougher surface after APA. Topographic surface analysis by SEM and AFM demonstrates that grinding causes unidirectional scratches parallel to the movement of the tungsten carbide bur with some particulate debris.
Conclusions: These results demonstrate that tungsten carbide burs are not recommended for grinding or finishing dental zirconia alone due to the significant reduction of biaxial flexural strength. While air particle abrasion and rubber polishing were found to increase the mean BFS this appears to be TC bur specific therefore cannot be recommended as a universal polishing treatment for TC burs.