The effect of surface roughness on the flexure strength of an alumina reinforced all-ceramic crown material

Abstract 

Objectives. The aim of the current study was to assess the contribution of surface defects to early failures. Specific surface flaw distributions were introduced to a series of nominally identical standard disc shaped specimens to assess the contribution of surface flaws to early failures rather than depending on average fracture strengths that cannot be used reliably as a design parameter for brittle all-ceramic crown materials.

Methods. Sets of 30 Vitadur-Alpha core porcelain discs (15mm diameter, 1.7mm thickness) were ground on silicon carbide abrasive paper to induce specific surface flaw distributions to a series of nominally identical standard disc shaped specimens. Mean flexure strengths, standard deviations and associated Weibull moduli (m) were determined using bi-axial flexure (ball-on-ring).

Results. A significant increase in mean bi-axial flexure strength and associated Weibull modulus was identified for the ‘fine’ compared with the ‘coarser’ ground nominally identical disc shaped specimens. A reduction in the degree of coarseness of the fit surface of specimens led to an increase in the mean bi-axial flexure strength compared with a control group of samples and therefore an associated increase in the survival probability at low strength levels.

Conclusions. The use of a relevant testing method (bi-axial flexure) combined with the Weibull analysis as a method of predicting survival rates from the strength data could aid in identifying the failure mechanisms involved in crown fracture. This would result in an improved validation of the strength data rather than depending on the average fracture strength that cannot be used reliably as a design parameter for brittle all-ceramic crown materials.

Keywords:  Alumina reinforced porcelain, Weibull statistics, Bi-axial flexure strength, Flaw distribution