Refractory hard metals (RHMs) are a ceramiclike class of materials made from metal-carbide particles bonded together by a metal matrix. Often classified as ceramics and sometimes called cemented or sintered carbides, these metals were developed for extreme hardness and wear resistance.
The RHMs are more ductile and have better thermal shock resistance and impact resistance than ceramics, but they have lower compressive strength at high temperatures and lower operating temperatures than most ceramics. Generally, properties of RHMs are between those of conventional metals and ceramics. Parts are made by conventional powder-metallurgy compacting and sintering methods.
Many metal carbides such as SiC and BC are not RHMs but are true ceramics. The fine distinction is in particle bonding: RHMs are always bonded together by a metal matrix, whereas ceramic particles are self-bonded. Some ceramics have a second metal phase, but the metal is not used primarily for bonding.
Four RHM systems are used for structural applications and, in most cases, several grades are available within each system.
Tungsten carbide with a 3 to 20% matrix of cobalt is the most common structural RHM. The low-cobalt grades are used for applications requiring wear resistance; the high-cobalt grades serve where impact resistance is required.
Tantalum carbide and tungsten carbide combined in a matrix of nickel, cobalt, and/or chromium provide an RHM formulation especially suited for a combination of corrosion and wear resistance. Some grades are almost as corrosion resistant as platinum. Nozzles, orifice plates, and valve components are typical uses.
Titanium carbide in a molybdenum and nickel matrix is formulated for high-temperature service. Tensile and compressive strengths, hardness, and oxidation resistance are high at 2,000°F. Critical parts for welding and thermal metalworking tools, valves, seals, and high-temperature gaging equipment are made from grades of this RHM.
Tungsten-titanium carbide (WTiC≥) in cobalt is used primarily for metal-forming applications such as draw dies, tube-sizing mandrels, burnishing rolls, and flaring tools. The WTiC″ is a gall-resistant phase in the RHM containing WC as well as Co.