Role of Indentation Load on ISB Fracture Toughness Measurement

Objective: To investigate the influence of indentation load and type of bending test applied on the determination of the indentation-strength-in-bending (ISB) fracture toughness of dental porcelains.

Methods: Two dental porcelains, Duceram Plus and Sintagon Zx, were used to make bars (2 mmx3 mmx25 mm) for the 3-point and 4-point bending tests, and discs (Ø:21mm, thickness: 1.9mm) for the biaxial flexural test. The tensile surfaces and edges were wet-polished with sandpaper (up to grit 1200). An indentation load of 19.6N or 9.8N was exerted on the specimens. Fracture strength was determined with one of the three bending strength tests (cross-head speed 0.05 mm/min). There were at least 10 specimens per group which fracture passed the indentation. The ISB fracture toughness values were calculated and analyzed with ANOVA.

Results: Duceram Plus showed to be tougher than Sintagon Zx for all bending techniques and indentation loads. Independent of the bending technique applied all values for Duceram Plus indented at a load of 19.6N were higher than those at 9.8N, while for Sintagon Zx this was only for the biaxial bending group the case. For Sintagon Zx no effect for bending technique was found, while for Duceram Plus the 4-point bending test resulted always in a lower toughness value than with the other two bending tests. The indentation load of 9.8N produced more non-acceptable specimens whose fracture surfaces hadn't passed artificially introduced indentations, than the load of 19.6N, especially when 4-point bending was employed for Duceram Plus.

Conclusion: The fracture toughness values of dental porcelains measured by the ISB test can be influenced by the magnitude of indentation load exerted and by the bending test technique used.