Processing development of 4TaC-HfC and related carbides and borides for extreme environments
Carbides, nitrides, and borides ceramics are of interest for many applications because of their high melting temperatures and good mechanical properties. Wear-resistant coatings are among the most important applications for these materials. Materials with high wear resistance and high melting temperatures have the potential to produce coatings that resist degradation when subjected to high temperatures and high contact stresses.
Among the carbides, Al4SiC4 is a low density (3.03 g/cm3), high melting temperature (>2000˚C) compound, characterized by superior oxidation resistance, and high compressive strength. These desirable properties motivated this investigation to (1) obtain high-density Al4SiC4 at lower sintering temperatures by hot pressing, and (2) to enhance its mechanical properties by adding WC and TiC to the Al4SiC4.
Also among the carbides, tantalum carbide and hafnium carbide have outstanding hardness; high melting points (3880˚C and 3890˚C respectively); good resistance to chemical attack, thermal shock, and oxidation; and excellent electronic conductivity. Tantalum hafnium carbide (Ta4HfC5) is a 4-to-1 ratio of TaC to HfC with an extremely high melting point of 4215 K (3942˚C), which is the highest melting point of all currently known compounds. Due to the properties of these carbides, they are considered candidates for extremely high-temperature applications such as rocket nozzles and scramjet components, where the operating temperatures can exceed 3000˚C.
Sintering bulk components comprised of these carbides is difficult, since sintering typically occurs above 50% of the melting point. Thus, Ta4HfC5 is difficult to sinter in conventional furnaces or hot presses; furnaces designed for very high temperatures are expensive to purchase and operate.
Our research attempted to sinter Ta4HfC5 in a hot press at relatively low temperature by reducing powder particle size and optimizing the powder-handling atmosphere, milling conditions, sintering temperature, and hot-pressing pressure. Also, WC additions to Ta4HfC5 were found to improve densification and increase microhardness. The ability to process these materials at relatively low temperature would save energy and reduce cost.
Boron-based hard materials are used in numerous applications such as industrial machining, armor plating, and wear-resistant coatings. It was often thought that in addition to strong bonding, super-hard materials must also possess simple crystallographic unit cells with high symmetry and a minimum number of crystal defects (e.g., diamond and cubic boron nitride (cBN)). However, one ternary boride, AlMgB14, deviates from this paradigm; AlMgB14 has a large, orthorhombic unit cell (oI64) with multiple icosahedral boron units. TiB2 has been shown to be an effective reinforcing phase in AlMgB14, raising hardness, wear resistance, and corrosion resistance. Thus, it was thought that adding other, similar phases (i.e., ZrB2 and HfB2) to AlMgB14 could lead to useful improvements in properties vis-à-vis pure AlMgB14. Group IV metal diborides (XB2, where X = Ti, Zr, or Hf) are hard, ultra-high temperature ceramics. These compounds have a primitive hexagonal crystal structure (hP3) with planes of graphite-like boride rings above and below planes of metal atoms. Unlike graphite, there is strong bonding between the planes, resulting in high hardness. For this study two-phase composites of 60 vol. % metal diborides with 40 vol. % AlMgB14 were produced and characterized.