Oxide structural ceramics are a type of ceramic material that was developed relatively early and is widely used. They generally refer to various simple oxide ceramics with a melting point higher than that of SiO2 crystal (1730℃), such as Al2O3, MgO, ZrO2, BeO, ThO2, TiO2, or composite oxide ceramics, such as mullite(Al6Si2O13), spinel(MgAl2O4), cordierite (2MgO·2Al2O3·5SiO2), etc.
Oxide ceramics are typical ionic crystals, whose cations and anions are bound by strong ionic bonds, so they have excellent properties such as high strength, high temperature resistance, oxidation resistance, good chemical stability, and electrical insulation. Among them, Al2O3 ceramics are the most widely used oxide ceramics due to their excellent comprehensive properties and relatively low manufacturing costs; while phase-change toughened ZrO2 ceramics (such as Y-TZP) and ZrO2 toughened Al2O3 ceramics (ZTA) have the best mechanical properties among existing ceramic materials, with a bending strength of up to 2.0 GPa and a fracture toughness of more than 15 MPa·m1/2 (Masaki, 1986), thus being widely used in modern science and technology and industrial fields.
The thermal expansion coefficients of oxide ceramics vary greatly, such as the expansion coefficients of MgO and ZrO2 are close to or greater than 10×10-6/℃; while the expansion coefficients of ceramics such as cordierite, lithium aluminum silicate (Li2O-Al2O3-SiO2), and fused quartz are very low, usually less than 2×10-6/℃, and some are even zero expansion; while the expansion coefficients of mullite and zirconium silicate are in the middle, about (4~5)×10-5/℃. Therefore, according to the size of the thermal expansion coefficient, oxide ceramics can be divided into three categories: low thermal expansion coefficient (<2.0×10-6/℃), medium thermal expansion coefficient (2.0~8.0)×10-6/C], and high thermal expansion coefficient (>8.0×10-5/℃).
Thermal conductivity is an important property of oxide ceramics because it directly involves the diffusion and transfer speed of heat, thus affecting the thermal stability of the product. BeO ceramics are currently the ceramic material with the highest thermal conductivity, and Al2O3 ceramics also have good thermal conductivity; while ZrO2 has a lower thermal conductivity and has better thermal insulation properties, and can be used as a thermal barrier material or thermal barrier coating. Usually, the thermal conductivity of oxide ceramics decreases with increasing temperature. In addition, the complexity of the ionic crystal lattice will cause the thermal conductivity to decrease, so the thermal conductivity of mullite and spinel is relatively small.
As structural materials, oxide ceramics are widely used not only in the fields of machinery, chemical industry, electronics, energy, environmental protection, aerospace, etc. as heat-resistant, wear-resistant, corrosion-resistant, insulating and anti-oxidation structural materials; but also some oxide ceramics, such as Al2O3, ZrO2, mica microcrystalline glass ceramics, have been widely used as bioceramics since the 1970s due to their good biocompatibility, chemical stability, wear resistance and strength matching. For example, they are used as artificial joints, artificial bone screws, artificial middle ear bones, dental implants, etc.