Fabrication of TaC coating on tantalum by interstitial carburization

Abstract

The interstitial carburization technique is applied to fabricate a TaC coating on tantalum (Ta). High-carbon steel is chosen as the carbon source because of its high content of interstitial carbon atoms. When tantalum is hot-pressed with high-carbon steel, the interstitial carbon atoms diffuse into the tantalum, and then carbides precipitate in the surface, forming a carbide coating. The coating is completely dense, and it grows inward, reaching a thickness of 30 μm after 10 h carburization. The coating is composed of TaC, a small amount of Ta2C and very small remnant Ta. The volume fraction of the carbide phases reaches 99%. The TaC grain size increases from 280 nm to 330 nm with increasing depth, and its grain morphology changes from equiaxed to columnar, displaying a gradient microstructure. These features of the microstructure endow the coating with excellent mechanical properties. The hardness of the coated tantalum is 1900 HV, which is much higher than the hardness of the uncoated tantalum (137 HV). When the coating surface is scratched with a load increasing from 0 N to 100 N, cracks appear at 10 N, and the substrate is locally exposed when the load exceeds 80 N.