What are the properties and application characteristics of zirconia ceramics

       Zirconia (ZrO2) is extracted from zirconium ore. The purer zirconia is yellow or gray, and the high-purity zirconia is white. Its melting point is 2715°C, and the maximum operating temperature can reach 1650°C; the thermal conductivity is the lowest among common ceramic materials (RT~1000°C: 1.51W/m.K~2.2W/m.k). The thermal radiation rate is small and the reflectivity is high.

    Zirconia also has good thermal stability and chemical stability, and is the best heat-resistant thermal insulation material. But its hardness is lower than that of alumina. Zirconia is a polycrystalline oxide. It mainly has three stable crystal forms; monoclinic phase (m) in the low temperature section (950℃), tetragonal phase (t) in the middle temperature section (1135~2370℃), and tetragonal phase (t) in the high temperature section (2370~2680℃) is the cubic phase (c). When t-ZrO2 is cooled from high temperature to room temperature, a t-m phase transition will occur. At the same time, with a volume expansion of 3% to 5%, the generated pressure will form cracks. Therefore, zirconia for thermal spraying must be stabilized, usually Y2O37 is used. %~8% of semi-stable zirconia (PSZ) is used as a material for the preparation of thermal barrier coatings. Semi-stabilized zirconia has good high temperature stability and good thermal insulation performance, which is the most important thermal barrier coating material for aerospace applications.

    The zirconia coating reacts chemically with substrate materials such as graphite at temperatures of nearly 2000 degrees Celsius. If tungsten is used as a transition layer, the reaction between solid interfaces at such high temperatures can be suppressed.

    Zirconia is an acidic coating that conducts electricity at high temperatures. Therefore, it is often used poorly as a coating material against alkaline slag corrosion and high temperature insulation. Zirconia has high electrical conductivity and its special crystal structure makes it an important ionic conductive material. If the workpiece base material is not a high temperature anti-oxidation material, the same as the treatment method of the alumina coating, it is necessary to use a nickel-chromium alloy or nickel-clad aluminum as a transition layer to protect the base from oxidation. Pure zirconia cannot be used for spraying, because when pure zirconia is heated to about 1200 degrees Celsius and then cooled to 1000 degrees Celsius, its crystal will change from a monoclinic phase to a square, and the phase transition process of the crystal is accompanied by As the volume changes, and this process is reversible, the coating is easily peeled off from the surface of the substrate when heated and cooled. This phase transition can be suppressed if a small amount of stabilizer (eg, calcium oxide, magnesium oxide, yttrium oxide, etc.) is dissolved in zirconia. For thermal spraying, stable or semi-stable zirconia,

    Zirconia has medium hardness, good resistance to high-speed gas erosion, and its unique phase transformation toughening properties, good mechanical properties and thermophysical properties make it the preferred thermal barrier coating material among many ceramic materials.