The effect of surface treatments on dental zirconia: An analysis of biaxial flexural strength, surface roughness and phase transformation

Abstract

Objectives: This study investigates the effect of selected surface finishing techniques on the biaxial flexural strength, surface roughness and phase transformation of a zirconia dental restorative material.

Materials and methods: Fully-sintered zirconia discs (⌀19.5 mm × 0.85 mm) were treated on one side with a single or a combination of the following treatments: diamond and/or tungsten-carbide burs without water coolant in an air-turbine handpiece, air-particle abrasion, rubber-point polishing in a contra-angle handpiece, or no treatment (control). Biaxial flexural strength (BFS) (eleven groups, n = 10) was determined using a universal testing machine and surface roughness (thirteen groups, n = 6) was assessed using a profilometer. Results were analysed using one-way ANOVA and Student-Newman-Keuls Post-hoc test (α = 0.05) with Bonferroni correction. Specimens were observed under scanning electron microscopy (SEM) and x-ray diffraction (XRD) for their microstructure and crystalline phases respectively.

Results: Grinding with diamond burs did not weaken zirconia (p > 0.0045) but produced rougher surfaces than the control group (p < 0.0038). Tungsten-carbide burs smoothened diamond ground specimens (p < 0.0038) for both grits of diamond. Specimens ground by tungsten-carbide burs have significantly reduced mean BFS (p < 0.0045) by up to two-thirds and SEM revealed fine surface cracks. Air-particle abrasion restored the mean BFS of tungsten-carbide ground specimens to control levels (p > 0.0045) and surface cracks were not observed. Phase transformation was not detected by XRD.

Conclusions: Dental zirconia ground dry with tungsten-carbide burs has a significantly reduced BFS and a smooth but defective surface. These defects may be removed and BFS restored by air-particle abrasion.

Clinical significance: The use of tungsten-carbide burs for grinding dental zirconia should be cautioned. Diamond grinding does not weaken zirconia but requires further polishing.

Keywords: Abrasion; Air particle abrasion; Bi-axial flexural strength; Grinding; Sandblasting; Surface roughness; Zirconia.