After zircon is refined, we can get zirconia (ZRO2), a substance that is rich in color, pure zirconia is yellow or gray, and high purity is white. It has a fairly high melting point of 2715, while in practical applications it can be used up to 1650 °C. It has the lowest thermal conductivity of any ceramic material, at about 100051w m·k to 22w m·k. At the same time, its thermal emissivity is not high, but its reflectivity is quite high.
Zirconia has very good thermal and chemical stability and is one of the best heat-resistant insulation materials on the market. However, it is less hard than alumina. Zirconia is a polycrystalline oxide with three main stable crystal forms. In a low temperature environment, it is mainly monoclinic phase (m); At moderate temperatures, it transforms into a tetragonal phase (t), which ranges from about 1135 to 2370; At high temperatures, it appears as a cubic phase (c). When the zirconia of the tetragonal phase is cooled from high temperature to room temperature, a phase change from the t phase to the m phase will occur, and this phase change process will be accompanied by a volume expansion of 3% to 5%, and the resulting pressure may form cracks. Therefore, zirconia for thermal spraying needs to be stabilized, usually using semi-stable zirconia (PSZ) with 7% to 8% yttrium stability, which is an important material for the preparation of thermal barrier coatings, because of its good high temperature stability and good thermal insulation performance, it has a very important application in the aerospace field.
At high temperatures of nearly 2000 °C, zirconia coatings may chemically react with substrate materials such as graphite. To avoid this reaction, tungsten can be used as a transition layer between the coating and the base material. Since zirconia is an acidic coating that conducts electricity at high temperatures, it is not suitable for use as a coating material for corrosion resistance to alkaline slag and high-temperature insulation. However, due to its high electrical conductivity, the special crystal structure makes it an important ionic conductive material.
If the base material of the workpiece is not a high-temperature oxidation-resistant material, then a nickel-chromium alloy or nickel-clad aluminum can be used as a transition layer in the same way as an alumina coating treatment to protect the substrate from oxidation. Pure zirconia is not suitable for spraying, because when it is heated to about 1200 and then cooled to 1000, its crystal phase will change from monoclinic phase to square, this process is accompanied by a change in volume, and it is reversible, so this coating is easy to fall off from the surface of the substrate during heating and cooling. This phase transition can be inhibited by the addition of a small amount of stabilizer in zirconia, such as calcium oxide, magnesium oxide, yttrium oxide, etc. As a result, zirconia for thermal spraying is either stable or semi-stable.
Zirconia has moderate hardness, good resistance to high-speed gas erosion, its unique phase change toughening characteristics, and good mechanical and thermophysical properties, making it the preferred thermal barrier coating material among many ceramic materials.