Abstract
Aiming at shortage of metal materials, ceramic is increasingly applied in biomedicine due to its high strength, pleasing esthetics and good biocompatibility, especially for dental restorations and implants, artificial joints, as well as synthetic bone substitutes. However, the inherent brittleness of ceramic could lead to serious complications, such as fracture and disfunction of biomedical devices, which impede their clinical applications. Herein, several toughening strategies have been summarized in this review, including reinforcing phase addition, surface modification, and manufacturing processes improvement. Doping metal and/or non-metal reinforcing fillers modifies toughness of bulk ceramic, while surface modifications, mainly coating, chemical and thermal methods, regulate toughness on the surface layer. During fabrication, optimization should be practiced in powder preparation, green forming and densification processes. Various toughening strategies utilize mechanisms involving fine-grained, stress-induced phase transformation, and microcrack toughening, as well as crack deflection, bifurcation, bridging and pull-out. This review hopes to shed light on systematic combination of different toughening strategies and mechanisms to drive progress in biomedical devices.
Highlights
With the aging of the population, the demand for maintaining the quality of life is in needed worldwide
The results indicated that the density increased as the sintering temperature was higher, which lead to improved mechanical properties, reaching a maximum fracture toughness (5.7 ± 0.3 MPa m1/2) and hardness (18.4 ± 0.4 GPa)
The phase transformation, which is accompanied by a volume expansion of 4% and shear strain of 6%, creates a compressive stress that slows and eventually stops the crack propagation, while the strain energy associated with any net shear component of the transformation strain in the transformation zone contributes an effective increase in the energy of fracture (Wang and Stevens, 1989)
Summary
With the aging of the population, the demand for maintaining the quality of life is in needed worldwide. Toughness can be enhanced by increasing the microstructural resistance, such as by changing the nature and distribution to suppress damage in the form of microcracking or microvoid formation ahead of the crack tip, which is termed intrinsic toughening This approach is largely ineffective with brittle materials such as ceramic (Evans, 1990), which invariably must rely on extrinsic toughening. Extrinsic toughening involves microstructural mechanisms that act primarily behind the crack tip to effectively reduce the crack-driving force experienced at the crack tip; this is termed crack-tip shielding and can occur by such mechanisms as in situ phase transformations and crack bridging (Launey and Ritchie, 2009) Brittle materials, such as ceramics, are invariably toughened with extrinsic mechanisms (Evans, 1990; Becher, 1991), which depend on crack size and to some degree specimen geometry. Toughening mechanisms are introduced briefly to deepen understanding of these strategies
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