A new strategy to efficiently fabricate tungsten carbide coating on tungsten: Two-step interstitial carburization

Abstract

A tungsten carbide coating on tungsten was fabricated by hot-pressing tungsten with high‑carbon steel. The interstitial carbon atoms in the high‑carbon steel diffused into the tungsten, forming a WC coating. The coating is completely nonporous, and the volume fraction of the carbide phases is close to 100%. By comparing the microstructures of the coatings fabricated by one-step hot-pressing at 1100 °C and 1150 °C, we found that increasing the temperature by 50 °C can markedly increase the growth rate of the coating as desired, but this increase of temperature generates undesired η carbide (Fe6W6C) at the coating/substrate interface. Based on the inference that Fe atoms are diffusible in tungsten but indiffusible in WC at 1150 °C, we proposed a two-step hot-pressing method to fabricate coatings at 1150 °C without introducing η carbide. The sample was first hot-pressed at 1100 °C for 4 h to prepare a pure WC layer and then hot-pressed at 1150 °C for further carburization. The WC layer prepared at 1100 °C was utilized as a semipermeable film to filter out Fe atoms and allow the diffusion of carbon atoms at 1150 °C. The two-step hot-pressing method worked as desired, and the obtained coating exhibits a distinct gradient microstructure with columnar WC grains. Compared to that of the one-step hot-pressing method at 1100 °C, the total carburization time for obtaining a coating with a saturated hardness of 2400 HV is shortened from 8 h to 6 h. Therefore, the two-step interstitial carburization can be used to efficiently fabricate high-quality and high-performance tungsten carbide coatings.