Depending on their high temperature characteristics, advanced ceramics are widely used in chemical industry, metallurgy, machinery, aerospace and other fields. Its high temperature characteristics include high temperature resistance and heat insulation.

Do you know the differences or connections between the two?
High temperature resistant ceramics are mainly aimed at the "high melting point" of ceramics. In another word, they are not easy to be destroyed at high temperatures, while thermal insulation ceramics are mainly aimed at the "low thermal conductivity" of some special ceramics, that is, they can isolate heat. The discussed "thermal insulation materials"  include "thermal insulation", "cold insulation", "thermal insulation materials", etc. The present research on thermal insulation ceramics is generally focused on thermal insulation at high temperatures. Therefore, within the scope of this application research, high-temperature resistant ceramics may not necessarily be thermal insulation. However, in high-temperature working environments, thermal insulation ceramics must meet the requirements of high temperature resistance and thermal insulation.

High temperature resistant ceramics
Generally speaking, high temperature resistant ceramics refer to the general name of ceramic materials whose melting temperature is above the silicon oxide melting point (1728 ℃). It is an important part of special ceramics, and sometimes also a part of high temperature refractory materials.

According to the main chemical composition of ceramic materials, they can be divided into high-temperature oxide ceramics (such as Al2O3, ZrO2, MgO, CaO, ThO2, Cr2O3, SiO2, BeO, 3Al2O3 · 2SiO2, etc.), carbide ceramics, boride ceramics, nitride ceramics and silicide ceramics. As a high-temperature structural material, it is widely used in aerospace, atomic energy, electronic technology, machinery, chemical industry, metallurgy and many other departments. It is an indispensable high-temperature engineering material for modern science and technology.

Recently, smelting and other thermal equipment have put forward higher and higher requirements for high temperature resistant ceramic materials and products. The rapid development of aerospace industry has also stimulated the development of high temperature resistant ceramics in order to improve its quality and variety. At present, single component high temperature resistant ceramic materials have obvious shortcomings in properties due to their single composition, such as corundum materials, high sintering temperature, large thermal expansion coefficient of sinter, poor thermal shock resistance, and poor oxidation resistance of silicon carbide ceramic materials. In addition, the high temperature resistant ceramic materials are difficult to process. They have poor thermal shock resistance In addition, they are not easy to bond in use, which also promote the development of high temperature resistant ceramic materials composite, such as Sialon materials, Sialon composite materials, high temperature resistant ceramic coating materials, carbide composite ceramic high temperature resistant materials, etc.

Ultra high temperature ceramic materials
Ultra high temperature ceramics (UHTC) refer to ceramic compounds with a melting point of more than 3000 ℃, such as ZrC, HfC, TaC, HfB2, ZrB2, HfN, etc.

They have excellent thermochemical stability and excellent physical properties, including high elastic modulus, high hardness, low saturated vapor pressure, high thermal conductivity and electrical conductivity, moderate thermal expansion rate and good thermal shock resistance. They can maintain high strength at high temperatures, usually including transition metal borides, carbides Nitrides and their composites.

1. Ultra-high temperature boride ceramics
Ultra high temperature boride ceramics mainly include HfB2, ZrB2, TaB2, TiB2 and YB4 ceramics. These ceramic materials have the characteristics of high melting point, high hardness, high strength, low evaporation rate, high thermal conductivity and conductivity, due to their strong covalent bonds. ZrB2 and HfB2 are the most widely studied UHTCs in boride ceramics. However, their poor oxidation resistance limits their wide application.

2. Ultra-high temperature carbide ceramics
Among carbide ceramics, ZrC, HfC, TaC and TiC can be used at ultra-high temperatures. This kind of ceramics has a very high melting point. It does not undergo solid phase transformation during the heating or cooling process, and has good thermal shock resistance and high temperature strength. however, the fracture toughness of carbide UHTCs is low, and the oxidation resistance is poor.

3. Ultra-high temperature nitride ceramics
Ultra high temperature nitride ceramics, such as ZrN, HfN and TaN also have good properties. Transition metal nitrides have high melting points. However, the melting point of such refractory nitrides is also related to the ambient pressure. Not all refractory nitrides are suitable for working in the high-temperature and high-pressure oxidation environment. Transition metal nitrides have important applications in the surface hardened layer of cutting tools.

Thermal insulation ceramics
The present research on thermal insulation ceramics is mostly focused on thermal barrier coating ceramic materials. Thermal barrier coating is mainly used in the aeroengine industry, which has good thermal insulation effect and high temperature oxidation resistance. It is one of the most advanced high temperature protective coatings at present.

Thermal barrier coating has the functions of heat insulation, high temperature oxidation resistance and corrosion resistance. Its typical structure is a double-layer system, consisting of a ceramic thermal barrier layer on the surface and a metal bonding layer in the middle. The ceramic thermal barrier layer actually plays an insulating role in the thermal barrier coating. It can effectively reduce the heat conduction to the metal substrate and protect key components. Suitable ceramic materials for thermal barrier coatings shall meet the requirements of high melting point, low thermal conductivity, better matching of thermal expansion coefficient with metal matrix, good chemical stability at high temperature, high adhesion with metal layer, and no phase change between room temperature and working temperature.

1. Oxide stabilized ZrO2
Oxide stabilized ZrO2 has low thermal conductivity, high thermal expansion coefficient and good high temperature performance. It has been the main ceramic material of thermal barrier coating for a long period of time. There are many kinds of oxides used to stabilize ZrO2, including divalent stabilizers such as CaO and MgO, trivalent stabilizers such as Y2O3, Sm2O3, Nd2O3, Er2O3, and tetravalent stabilizers such as CeO2 and HfO2.

2. ABO3 ceramics with perovskite structure
Among the perovskite structured ABO3 ceramics, SrZrO3, BaZrO3, MgZrO3, etc. were used in thermal barrier coatings in the early stage. The melting point of SrZrO3 is as high as 2690 ℃. However, its phase stability at high temperatures is poor. It is not suitable to be used alone as a thermal barrier coating material at high temperatures. The melting point of BaZrO3 is 2000 ℃. Its expansion coefficient is much lower than that of YSZ. Therefore, its thermal shock resistance is poor.

3. A2B2O7 ceramic materials
A2B2O7 (A is rare earth element, B is Zr, Hf, Ce and other elements) ceramic material has lower thermal conductivity than ZrO2 material. It is equivalent thermal expansion coefficient and good high-temperature phase stability. Meanwhile, it is considered to be the most promising material system to replace ZrO2.

4. Magnetite structure MMeAl11O19 ceramics
The microstructure of hexaaluminate MMeAl11O19 (M is La, Nd, Sr and other elements, Me is alkaline earth metal element, etc.) ceramics with magnetite lead structure is composed of randomly arranged layers. As a thermal barrier coating, it is developed late for maintaining long-term good structure and thermal stability at high temperatures. It has a sintering rate far lower than that of ZrO2 based thermal barrier coating materials. There are many micropores to ensure good thermal insulation effect.

5. Other ceramic materials
In addition to the above ceramic materials of thermal barrier coatings, other ceramic materials with application prospects of thermal barrier coatings have also been developed. Y3Al5O12 (YAG for short) is also a good thermal barrier coating material, belonging to garnet structure. It can not only maintain good thermal stability from room temperature to melting point (1970 ℃), but also have low thermal conductivity. The diffusion rate of oxygen in YAG is 10 orders of magnitude smaller than that in ZrO2. Therefore, YAG can protect the substrate and metal bonding layer well.