Flexural strength measurement of ceramic dental restorative materials

Flexural strength of a dental material reflects its ability to withstand tensile stresses and as such has an impact of the fracture risk of a filling. There are several methods to measure flexural strength of a material. The flexural strength of a bioceramic calciumaluminate-based dental restorative material (DoxaDent) has been measured in three different methods with a composite (Tetric Ceram), a glass ionomer cement (Fuji 11) and a phosphate cement (Harward) as references. The three test methods were: a) ISO 4049 for dental composites, three-point bend test of 2*2*25 mm rods, non-polished surface, b) EN 843-1 for ceramic materials, three-point bend test of 3*4*40 mm, polished surface, and c) biaxial ball-on-disc for ceramic materials (ASTM F-394), polished surface. The results obtained clearly show the difficulty in performing flexural strength testing of a bioceramic material. By using the ball-on-disc method the defect size was reduced and thus the resulting flexural strength higher. The strength of DoxaDent tested in the ball-on-disc method is close to the theoretical strength based on the microstructure of the material (maximum grain size of 15 mu m). The composite material and the phosphate cement were rather insensitive to the test method whereas the glass ionomer cement also showed sensitivity towards the test method. Based on fracture mechanics the flexural strength of bioceramic materials is discussed. A modified biaxial test method for evaluation of strength of dental materials in a close to real-life component is proposed.