Ultra-high temperature ceramics are ceramics that can undergo temperature, chemical, and erosive extremes and still reliably perform its purpose. Extreme temperatures, in the case of ceramics, are typically considered at around 3000C.
This table provides an example of ceramics with a high temperature resistance.
These materials form two primary groups: borides and carbides. Borides typically have a flex strength of 300-500MPa at room temperature. When mixed with other materials, they can double that. However, as temperature is increased, a lower strength will be observed. One manufacturing process to date has shown strength retention up to 1500C. That process is called “spark plasma sintering.”
Sintering is the process by which a ceramic is held at just below melting point to allow the ceramic powder to adhere to each other. Small pores will be formed initially but as the process continues, the pores will close up or disappear entirely. This will lead to a much higher mechanical strength.
Spark plasma sintering is similar except instead of a furnace, an electrical current is instead applied. Plasma arcs will jump from each ceramic particle to another which will heat them up to several thousand degrees Celsius. As each particle adheres to each other, a DC current will flow. This generates more heat through current losses which will finish the heating process. Finally, the material will undergo plastic deformation as it is under a uniaxial stress in the die. This is a difficult process to implement as the direct current generator is extremely expensive. Also, only simple shapes that are symmetrical can be created.
The other type of UHTC is carbides. Carbides typically have a slightly higher melting temperature but are less electrically conductive than borides. Carbides are also slightly more susceptible to oxidation. However, they are much more resistant to erosion which has made them the material of choice for rocket nozzles. Another common problem with carbides is a breakdown temperature in the manufacturing process where density will actually lower due to grain coarsening leading to pore formation.