New strategy to grow TiC coatings on titanium alloy: Contact solid carburization by cast iron

Abstract

A contact solid carburization method to fabricate TiC coatings on titanium alloy by a “carbon sponge” – cast iron is proposed. When titanium alloy (Ti6Al4V) and cast iron contact in atomic scales at high temperatures below the melting point of the cast iron (e.g. 1100 °C), the interstitial carbon atoms in the cast iron diffuse into titanium alloy forming a TiC layer. Meanwhile, due to the ignorable interdiffusion of metallic atoms, the iron can be easily taken off, leaving a TiC coated titanium alloy. The coating is composed of equiaxed TiC grains and it is completely dense (no porosity). The microstructure is gradient in TiC grain size which increases exponentially from about 100 nm to 1 μm with depth. The inward growth of the coating is diffusion-controlled, and the coating thickness reaches 23 μm when annealed for 10 h. The coatings exhibit high hardness (2400 HV0.05) and excellent coating-substrate adhesion strength. This strategy could have a wide application: to grow ceramic coatings on carbide-forming metals by solids having good carbon solubility.