Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic.

Dental ceramics generally fail because of the growth of microscopic surface flaws that form during processing or finishing or that result from surface impact during service. The ion-exchange process has been shown to be effective in improving the flexural strength of most dental porcelains through the development of a compressive surface layer. The Vickers hardness and crack resistance of several commercial dental ceramics were determined by indentation techniques. The results of this investigation indicate that ion-exchange reinforcement can significantly improve the resistance of the ceramic surface to crack propagation with little effect on apparent surface hardness.

Effects of ion exchange on hardness and fracture toughness of dental ceramics.

This study measured apparent fracture toughness (K c ) and elastic modulusto-hardness ratio (E/H) of dental restorations made with five all-ceramic crown systems. An indentation technique was used whereby K c was calculated from the length of radial cracks formed on the release of a Vickers Diamond. Twenty-four crowns were made by using five different crown systems: Cerestore, DICOR, Hi-Ceram, Renaissance, and Vitadur-N. After the crowns were fired and glazed, they were embedded, polished, gold-coated, and indented under oil. In addition Knoop indentations were made for E/H calculation. Cerestore had a mean K c of 1.79MPa · m 1/2 and E/H of 13.9, DICOR 2.10 and 26.8, Hi-Ceram 1.96 and 17.8, Renaissance 2.02 and 24.2, and Vitadur-N 1.96 and 22.3. Statistically significant differences ( p 0.05) were found between DICOR and Cerestore materials for K c and between DICOR, Renaissance, and Cerestore materials for E/H. For comparison, previous work found Vita VMK metal ceramic crowns to have a mean K c of 1.91MPa · m 1/2 and E/H of 20.2

Apparent fracture toughness of all-ceramic crown systems

Although ceramics present high compressive strength, they are brittle materials due to their low tensile strength so they have lower capacity to absorb shocks. This study evaluated the fracture toughness of different ceramic systems, which refers to the ability of a friable material to absorb defformation energy. Three ceramic systems were investigated. Ten cylindrical samples (5,0mm x 3,0mm), were obtained from each ceramic material as follows: G1- 10 samples of Vitadur Alpha (Vita-Zahnfabrik); G2- 10 samples of IPS Empress2 (Ivoclar-Vivadent); G3- 10 samples of In-Ceram Alumina (Vita-Zahnfabrik). Fracture toughness values were collected upon indentation tests that were performed under a heavy load. A microhardness tester (Digital Microhardness Tester FM) utilized a 500gf load cell during 10seconds to perform four impressions on each sample. Statistically significant results were observed (ANOVA and Kruskal-Wallis tests). In-Ceram Alumina presented the highest median toughness values (2,96N/m3/2), followed by Vitadur Alpha (2,08N/m3/2) and IPS Empress2 (1,05N/m3/2). It may be concluded that different ceramic systems present distinct fracture toughness values, thus In-Ceram is capable of absorbing superior stress when compared to Vitadur Alpha and IPS Empress2.

/pdf/relative-fracture-toughness-of-differents-dental-ceramics-1zay6hh37l.pdf

Relative fracture toughness of differents dental ceramics

Dental ceramics, like all brittle materials, suffer from an inability to absorb appreciable quantities of plastic strain energy prior to fracture. This liability is manifested in such behavior as flaw sensitivity, low tensile strength, and catastrophic failure. One measure of the strain energy absorbing ability of a brittle material is the critical stress intensity factor “fracture toughness” or K . The fracture toughness of a material is related to the level of tensile stress which must be attained in the vicinity of a crack tip before a catastrophic fracture process is initiated. Mechanical properties as diverse as strength, thermal shock resistance, and susceptibility to erosive wear are all controlled by this parameter. Unlike fracture strength, K is intrinsic to the material and is less sensitive to such variables of specimen preparation as the size and density of surface flaws. Because of the importance of fracture toughness in determining all aspects of brittle material mechanical behavior, knowledge of K for dental ceramics is an essential starting point if the resistance to fracture of ceramic base dental prostheses is to be improved. K for dental materials is usually determined through the use of 1

Relative fracture toughness and hardness of dental ceramics

Apparent fracture toughness of dental porcelain with a metal substructure.

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Apparent fracture toughness of dental porcelain with a metal substructure.