Wear Resistant Carbide Surfacing
Tungsten Carbide Applied by the High Velocity Thermal Spray Processes
In the continuous-combustion high-velocity oxyfuel (HVOF) thermal spray process, a fuel gas and oxygen are combusted in a high-pressure chamber, reaching temperatures of 4500 to 5600°F. The hot, high-pressure gas is ejected through a small-diameter nozzle and accelerated down a long barrel at supersonic speeds. Powder is also injected into the nozzle, where the particles mix such as Tungsten Carbide with the speeding gases and reach velocities of 2500 ft/s to more than 3000 ft/s.
The high speeds impart tremendous amounts of kinetic energy to the particles. Remember that kinetic energy equals half the mass of the particle times the square of its velocity (KE= ½ mv2). When these highly energetic particles strike the substrate, the velocity goes to zero and the kinetic energy is absorbed by the particle, causing it to melt in a millisecond. Subsequent particles arrive almost simultaneously, rapidly building up the coating. The result is a very dense with high adhesive/cohesive strength . It has comparatively very high bond strength, low residual stress, low porosity (typically less than 0.5%), and high wear resistance.
Here at ASB Industries, the most frequently applied powder is tungsten carbide with a cobalt binder. Tungsten carbide melts at 5200 F, and is extremely hard, with Vickers hardness number 2242. On the other hand, cobalt has a melting point of 2460°F, and Vickers hardness number is 1043. Coating properties such as hardness, wear resistance, and strength depend primarily on the grain size and volume fraction of the tungsten carbide particles. Therefore, WC-Co coatings can be designed to provide the specific properties needed for an application by controlling the grain size of the tungsten carbide particles and their volume relative to cobalt. In general, cobalt content ranges from 5 to 30 wt%, and WC grain sizes range from one to eight microns, although for some applications grain size may reach 30 microns.