Surface Hardening of Hard Tungsten-Carbide Alloys: A Review

Russian and non-Russian research on the surface hardening of hard tungsten-carbide alloys to improve the wear resistance is reviewed. There is great scope for improving the wear resistance and durability of hard-alloy components by surface strengthening on the basis of various coatings, including coatings with 100-nm structural components. On hard tungsten-carbide alloys, the most common coatings consist of titanium carbide TiC and nitride TiN, characterized by high lattice binding energy and high melting point. If such coatings are applied to hard-alloy tools, the frictional coefficient is reduced by a factor of 1.5–2.0 when cutting steel. The use of a TiN + ZrN ion-plasma coating reduces the frictional coefficient by a factor of 5.9. At present, multilayer coatings are widely employed. The most widespread are TiN + TiC and Al₂O₃ + TiC coatings. Their wear is proportional to the coating thickness. These multilayer coatings still leave room for improvement in the wear resistance of hard alloys. In Russia, the potential of hard alloys with a strength gradient from a ductile and high-strength core to a wear-resistant surface is being explored. At the Research Institute of Refractory Metals and Hard Alloys, a method has been developed for producing alloys with variable cobalt content over the thickness of the cutting insert. That permits change in alloy composition from VK20 to VK2 over the sample thickness. Correspondingly, the wear resistance of the insert’s working section is equivalent to that of VK2 alloy, while the base is able to withstand considerable flexural stress. Recently, cutting tools with a diamond coating on hard alloys have been adopted in practice. To increase the durability of hard-alloy VK inserts, strengthening based on concentrated energy fluxes may be employed. Examples include treatment of hard-alloy surfaces by γ quanta, ion beams, and laser beams, electroexplosive alloying, and electrospark strengthening.