Relationship between fracture toughness and flexural strength in dental porcelains

Abstract

The aim of this study was to investigate the correlation between fracture toughness (K(Ic)) and flexural strength (FS) in dental porcelains. Porcelains with different leucite contents and clinical indications were used (A, B, C, D, and E). K(Ic) was determined by surface crack in flexure method (SCF) and FS was determined by four-point-bending test. Microstructural characterization was also carried out. The leucite contents of porcelains A, B, C, D, and E were, respectively, 22, 22, 6, 15, and 0%. Materials with higher leucite content (A and B) presented significantly higher K(Ic) values compared to materials with lower leucite content (C and E). The Weibull moduli (m) of porcelains A and B were statistically higher than those of the other three materials. Regarding characteristic strength (sigma(0)), porcelains D and E showed similar values and statistically higher than those of the other materials which were statistically different from each other. According to the regression analysis, sigma(0) increased with the increase of K(Ic) until approximately 0.75 MPa m(1/2). After that, the increase in K(Ic) was accompanied by a decrease in sigma(0). However, the Weibull modulus increased with the increase in K(Ic), especially for values greater than 0.80 MPa m(1/2).

Fracture toughness of three heat pressed ceramic systems

“…Then, high fracture toughness of EMP can be attributed to its leucite content (35%), once there is a positive correlation between leucite content and fracture toughness because leucite increases the resistance to crack propagation 19,26 . Additionally, fracture toughness increases with greater particle size 6 , corroborating the results of the present study where greater crystals (5 to 10 micrometer) are present in EMP. A previous study reported that a homogeneous crystal distribution is achieved by heat pressing 5 , possibly increasing fracture toughness because crack propagation is avoided.…”

“…According to some authors, high leucite content, particle size and homogeneous particle distribution can improve ceramic strength 6 . Then, high fracture toughness of EMP can be attributed to its leucite content (35%), once there is a positive correlation between leucite content and fracture toughness because leucite increases the resistance to crack propagation 19,26 .…”

“…In addition, the low fracture toughness of POM is directly affected by its high microhardness while the lowest microhardness of EMP leads to greater fracture toughness than POM. Some authors also argued that some surface flaws (microcracks, scratches and broken edges) of dental ceramics are caused by grinding or usage 6 . All samples were polished for microhardness and fracture toughness evaluation, and polishing may originate surface flaws, especially when materials present greater microhardness (POM) that make difficult polishing.…”

“…Then, several all-ceramic materials and processing techniques have been introduced in the past decade [3][4] . Heat pressing, slip-casting and computer aided design-computer aided machining (CAD-CAM) are the processing techniques developed to offer greater mechanical performance associated with higher crystalline content of new ceramic systems, extending the indications to posterior restorations and fixed partial dentures Common defects, inherent in the processing, such as internal microcracks, pores, inclusions and second-phase clusters, can possibly propagate under tensile stresses until achieving a critical size, fracturing 6 .…”

Effect of temperature and heating rate on the sintering of leucite-based dental porcelains

“…This crystalline phase was initially added only to adjust the thermal expansion coefficient of the porcelain, making it compatible with that of the metal substructure. However, it has been demonstrated that these second-phase particles are also responsible for the improvement of the material's fracture toughness [1][2][3].…”