Several Toughening Methods of Alumina Ceramics

Alumina ceramic is a kind of ceramic Hardness with alumina (Al2O3) as the main body. Its hardness is relatively high and its toughness is poor. In order to meet the application of alumina ceramics in more fields, we will toughen alumina ceramics. The following methods are commonly used in the toughening process of alumina ceramics.

1. Fiber or whisker toughening is achieved by adding fibers (or whiskers) to the ceramic matrix in a certain way. On the one hand, high-strength fibers (whiskers) can share the external load, and on the other hand, the weak interface between fibers (or whiskers) and the ceramic matrix can be used to create an absorption system for external energy, thereby improving the brittleness of ceramic materials. The main mechanisms are crack deflection or bifurcation, pull-out effect, and bridging effect.

2. Self toughening of A12O3 ceramics has been widely studied in the past decade by introducing additives or crystal seeds to induce the anisotropic growth of equiaxed A12O3 grains into grains with plate-like or long columnar morphology. Self toughened alumina ceramic materials are formed by adding raw materials that can generate a second phase to the raw materials, controlling the generation conditions and reaction process, and directly inducing the anisotropic growth of alumina grains through high-temperature chemical reactions or inducing the growth of whiskers uniformly distributed, large grain length to diameter ratios, or wafer reinforcements in the main crystal phase matrix, forming ceramic composite materials. This can avoid the incompatibility and uneven distribution of defects between the two phases, resulting in higher strength and toughness than the same material toughened with an external second phase, thereby further improving the mechanical properties of the material.

3. Phase transformation toughening: When pure ZrO2 (unstable ZrO2) particles are added to Al2O3 to form ZrO2 toughened Al2O3 ceramics, the transition of metastable tetragonal t-ZrO2 to monoclinic m-ZrO2 under stress induction results in a volume change of 3% to 5% and a shear strain effect of about 8%, which can offset external stress and absorb energy, thereby easing the stress concentration at the main crack tip, and significantly improving the toughness of Al2O3 ceramics. Research has shown that the toughening mechanisms of ZrO2 include stress induced phase transformation toughening, microcrack toughening, refinement of matrix grains, crack turning and bifurcation, and surface toughening, among which phase transformation toughening is the main toughening mechanism. There are many factors that affect phase transformation toughening, such as ZrO2 content and particle size, grain size, types and quantities of other additives, and grain orientation. Its disadvantage is that the toughening effect decreases sharply with the increase of temperature, so materials that rely solely on phase transformation toughening to improve toughness are generally only suitable for lower temperature situations.

Usually, the particles of ZrO2 introduced into the Al2O3 matrix are in the micrometer or submicron scale, making it easy for the grain size of ZrO2 in the composite ceramics to exceed the critical size, greatly reducing the effect of reinforcement and toughening. Introducing nanoscale ZrO2 particles as dispersed phases into micrometer scale ceramic matrix to prepare nanocomposite ceramics has achieved good results and attracted people's attention.

4. The main mechanisms of particle dispersion toughening include thermal stress induced microcrack toughening, shear stress hindering microcrack propagation toughening, microcrack deflection and branching, weakening stress concentration toughening, and refining matrix grains. Particle dispersion toughening is temperature independent and can serve as a high-temperature toughening mechanism. In the study of particle toughened alumina ceramics, carbide, nitride, Boride second phase particles with high melting point, high strength and high elastic modulus and metal particles with ductility are the toughening phases.

Affects the second phase particle recombination

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