The influence of surface modification techniques on the performance of a Y-TZP dental ceramic

OBJECTIVES: The purpose of the current study was to examine the influence of the pre-cementation surface modification techniques, namely alumina abrasion and surface grinding, routinely employed by dental practitioners prior to cementation and placement of crown and bridge restorations on the performance of a Y-TZP dental ceramic. METHODS: Twelve sets of 30 Lava ceramic discs (13 mm diameter, 1.5mm thickness) supplied by the manufacturer were randomly selected. Six groups were abraded utilising 25, 50 and 110 microm alumina and stored dry or in a water bath at 37+/-1 degrees C for 24 h. Four groups were ground utilising a fine or a coarse grit diamond bur, specimens were ground dry or while using water as a coolant. The mean bi-axial flexure strengths, standard deviations and associated Weibull moduli (m) were determined. The surface roughness, hardness and phase composition were assessed utilising profilometry, the Vicker's hardness indentation and X-ray diffraction, respectively. RESULTS: No significant difference (P > 0.05) was identified in the bi-axial flexure strength of the 25, 50 and 110 microm alumina-abraded and the control specimens stored dry and wet for 24h. However, a significant increase in m was identified for the alumina-abraded specimens stored dry (10.7+/-1.9, 10.6+/-1.9 and 10.6+/-1.9) compared with the control (7.5+/-1.3) and the specimens stored in a water bath. In addition, the alumina abrasion regimes reduced the surface roughness compared with the controls. The coarse grinding regime significantly reduced both the bi-axial flexure strength and the associated m compared with the control whilst no significant difference was identified for the fine grinding regimes. The surface modification techniques initiated a phase transformation mechanism and resulted in the formation of a layer of compressive stresses on the surface of the disc-shaped specimens. CONCLUSIONS: The combination of the reduced surface roughness and the formation of a surface layer of compressive stress as a result of the alumina abrasion regimes investigated increased the reliability of the bi-axial flexure strength. The presence of water in the current study did not detrimentally influence the performance of the Y-TZP ceramic under investigation. Coarse grinding significantly reduced the bi-axial flexure strength and m due to the increased surface roughness. The Y-TZP specimens in the current investigation underwent a toughening mechanism as a result of a phase transformation mechanism which generated a transformation compressive stress that opposes the externally applied, crack-propagating tensile stress.