How to toughen zirconia ceramics?

The development of modern new technology is inseparable from materials, and higher and higher requirements are placed on materials. With the development of material science and process technology, modern ceramic materials have evolved from traditional silicate materials to involving force, heat, electricity, sound, light and their combinations. The surface of ceramic materials is metalized to make it A composite material with ceramic properties and metallic properties has attracted more and more attention to its application and research.


Zirconia is a special material

The method of toughening zirconia is mainly achieved by the phase change of zirconia!

Pure zirconia is a white solid, which will appear gray or light yellow when it contains impurities. Various other colors can be displayed with the addition of color developer. The molecular weight of pure zirconia is 123.22, the theoretical density is 5.89g/cm3, and the melting point is 2715°C. It usually contains a small amount of hafnium oxide, which is difficult to separate, but has no obvious effect on the properties of zirconium oxide. Zirconia has three crystal forms: monoclinic, tetragonal, and cubic crystal phases.


At room temperature, zirconia only appears as a monoclinic phase, it transforms into a tetragonal phase when heated to about 1100°C, and it transforms into a cubic phase when heated to a higher temperature. As the monoclinic phase changes to the tetragonal phase, there will be a large volume change, and a large volume change will occur in the opposite direction when cooling, which will easily cause the product to crack and limit the application of pure zirconia in the high temperature field. . However, after adding stabilizers, the tetragonal phase can be stabilized at room temperature, so there will be no sudden changes in volume after heating, which greatly expands the application range of zirconia.


Ceramic materials have the advantages of high melting point, high hardness, high wear resistance, oxidation resistance, etc., and can be used as structural materials, tool materials and functional materials. Among them, common advanced ceramic materials such as alumina, zirconia, silicon oxide, silicon carbide, silicon nitride, etc., are widely used in aerospace, automotive, biomedicine, electronics and mechanical equipment industries. At present, the brittleness of ceramic materials is one of the main factors restricting its development, so toughening has become a core issue in the field of ceramic materials research. 


Why are ceramic materials so brittle?


As we all know, metal materials are prone to plastic deformation because the metal bond has no directionality. In ceramic materials, the bonding bonds between atoms are covalent bonds and ionic bonds. Covalent bonds have obvious directionality and saturation. However, the repulsive force of the ions of the same number of ionic bonds is very large when they are close, so it is mainly composed of ionic crystals and ionic bonds. Ceramics composed of covalent crystals have very few slip systems and generally fracture before slippage occurs. This is the root cause of the brittleness of ceramic materials at room temperature.


According to Griffith theory, the fracture strength of solid materials mainly depends on three basic performance parameters of the material: elastic modulus E, fracture surface energy γ and critical crack size c.


Some main factors affecting the fracture strength of ceramic materials


The toughness of a material can be quantified by the value of fracture toughness. From the point of view of fracture mechanics, the key to enhancing the toughness of ceramic materials lies in: improving the ability of ceramic materials to resist crack propagation; slowing down the stress concentration effect at the crack tip. In addition, the use of advanced preparation and processing technology can also enhance the toughness of ceramic materials. At present, there are roughly six toughening mechanisms in ceramic materials: phase transformation toughening; microcrack toughening; crack deflection and bridging; whisker/fiber toughening; domain switching and twinning toughening; self-toughening. In fact, there is usually more than one toughening mechanism in ceramic materials, but the superposition of the above several mechanisms, that is, cooperative toughening. The following is a detailed introduction to the common toughening mechanism and application of ceramic materials.


The martensitic transformation of zirconia (ZrO2) has greatly improved the toughness of zirconia ceramic materials, and it is one of the most successful toughening methods so far. Pure ZrO2 crystals have three structures: monoclinic phase (m), square phase (t) and cubic phase (c). With temperature changes, the following allotropic transformations occur:


During the cooling process, the t→m phase transition is accompanied by a 4-5% volume expansion, so pure ZrO2 ceramics are prone to breakage during the cooling process. Later, by adding appropriate amount of stabilizers such as CaO, MgO, Y2O3 and CeO to ZrO2, and controlling the heating and cooling conditions, the high temperature phase (t or c or both) partially exists at room temperature, forming a partially stable ZrO2, which is extremely stable. Greatly improve the toughness of zirconia ceramics.


In ZrO2 tetragonal polycrystalline (TZP) or ceramic matrix composites with tetragonal ZrO2 as the second phase particles (such as PSZ, ZTA), the effect of high stress near the crack tip leads to phase transformation of the tetragonal ZrO2 grains (t →m phase transformation). The lattice expansion and shear produced by this martensitic transformation form a shield at the crack tip, releasing the driving force of the crack tip to increase the fracture toughness of the material.


Zirconia ceramics are non-magnetic, non-conductive, non-rusting, and wear-resistant, so they are widely used in the field of biomedical equipment and cutting tools. Partially stabilized zirconia can be used to make artificial bones, artificial joints and artificial teeth. ZrO2 toughened ceramic blades can be used to process alloy steel due to their very high blade strength and wear resistance. In addition, structural ceramic parts made of partially stabilized zirconia, such as optical fiber connectors, sleeves and jumpers, have been widely used in the market.


The following briefly introduces several toughening types of zirconia

Phase change toughening

Toughening by phase change, the second phase consumes a large amount of energy required for crack propagation, which relaxes the stress at the crack tip and hinders further crack propagation. At the same time, the volume expansion caused by the phase change causes the surrounding matrix to be compressed, prompting other cracks to close, thereby improving the fracture toughness and strength. This kind of phase transformation toughening is also called stress induced phase transformation and phase transformation induced toughness.


Microcrack toughening

The fundamental reason for the toughening of micro-cracks is to increase the crack propagation path, that is, to increase the work required to overcome the surface energy to overcome the surface energy during the material fracture process. Micro-crack toughening is a commonly used ceramic toughening mechanism. Between the ceramic matrix phase and the dispersed phase, due to the thermal expansion difference caused by the temperature change or the volume difference caused by the phase change, a dispersed distribution of micro-cracks will occur, which will cause fracture When the main crack grows, these uniformly distributed microcracks will promote the bifurcation of the main crack, make the main crack propagation path tortuous and uneven, increase the surface energy during the expansion process, so that the rapid crack propagation is hindered, and the material toughness is increased.


Crack deflection and bridging

Through the dispersion of the second phase particles with high strength and toughness in the ceramic matrix or the movement of the particles, the crack tip will bend along the particles due to the hindering effect of the dispersed phase particles during the crack propagation. In addition, when residual compressive stress is generated around the boundary between the dispersed phase particles and the matrix phase, and the crack encounters the dispersed particles, the original direction of advancement will be reversed. The thermal expansion coefficient of the particles and the matrix is the main factor that determines the toughening effect.


Crack bridging usually occurs at the crack tip, relying on bridging elements to connect the two surfaces of the crack and generate a closing stress between the two interfaces, which causes the intensity factor to increase as the crack grows. The crack bridging may undergo transgranular failure, or the crack may develop and deflect along the crystal while bypassing the bridging unit. The crack bridging toughness value is proportional to the square root of the size of the bridging unit. The presence of microcracks in the composite material will also cause the main crack to deflect during the propagation process and increase the toughness of the composite material.


Domain switching and twinning toughening

Domain switching and twinning toughening is to add piezoelectric ceramics as the second phase to structural ceramics to achieve the purpose of toughening and strengthening. During the crack propagation process, the piezoelectric second phase in the ceramic matrix not only has a bridging and deflection effect on the crack, but also the piezoelectric effect and electric domain deflection will also consume the crack propagation driving force, thereby playing a toughening effect. Therefore, in the ceramic materials toughened by the piezoelectric phase, after crack bridging and deflection toughening, the energy of crack propagation can also be released through three ways: the piezoelectric effect converts mechanical energy into electrical energy; The electrical phase undergoes a phase change to consume energy; the domain wall motion in the piezoelectric second phase is caused by stress to improve the fracture toughness of the composite material.


Whisker/fiber toughening

Practice has proved that the whisker/fiber reinforced toughening mechanism can greatly improve the strength and toughness of the material, and it is considered to be the most promising toughening mechanism for high-temperature structural ceramics. The characteristics of the whiskers/fibers and the interface bonding characteristics between the fibers and the ceramic matrix are the main factors affecting the toughness of the fibers. Incorporating high-strength and high-toughness whiskers/fibers into the ceramic matrix can block macroscopic cracks when passing through the whiskers/fibers, thereby improving the strength and toughness of the ceramic material. The toughening mechanism is: debonding, pulling out and bridging of the whiskers/fibers in the ceramic matrix.



Self-toughening is also called in-situ toughening, that is, adding raw materials that can form the second phase into the ceramic matrix, controlling the generation conditions and reaction process, and directly growing uniformly distributed crystals in the matrix through high-temperature chemical reactions or phase change processes. The whiskers, the crystal grains with high aspect ratio and the reinforcement of the wafer form form a ceramic composite material. The toughening mechanism of self-toughening is similar to the effect of whisker/fiber toughening, mainly through the pull-out, bridging and crack deflection mechanisms of the self-generating reinforcement. This method can overcome the problems of incompatibility and uneven distribution of the two phases in the toughening of the second phase. Therefore, the strength and toughness of the obtained composite material are higher than those of the same material toughened by the second phase.


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