Surface hardening treatment on titanium alloy

      Following the last article, today we continue to learn about the surface hardening treatment on titanium alloy. Carburization on the surface of titanium alloy produces TiC phase with high hardness. However, adhesion between TiC layer and substrate is very poor, which prevents practical usage. Boronisation on the surface of titanium forging pieces produces TiB2 phase with high hardness, too. According to documents, acid-washed titanium components are embedded in mixed powder, consisting of half of unformed boron powder and half of A1203 powder (of which 0.75%-1.0% NH4F*HF is added), and are placed at 1010℃ for one hour; then TiB2 layer is formed. Under above conditions, the coating’s thickness varies from alloy to alloy. The coating’s thickness on industrial pure titanium is 25p, and on TC4 titanium alloy is 20um, with harness between HV2800 and 3450. Boronisation has high requirement on the temperature, so its application is restricted. If iron is electroplated first on titanium plates and then boronisation is carried out, temperature for boronisation could be lowered to 870℃, the coating’s thickness could reach to 40um and hardness could reach to HV2300. Titanium could react with nitrogen, so argon has to be used as the carrier. If gas mixture of oxygen and nitrogen (air) is used as the source of oxygen, then enough nitrides will be formed at temperature (about 850℃) for oxygen diffusion, which will reduce oxygen diffusion. In order to optimize depth and distribution of oxygen diffusion layer, concentration of oxygen has to be high enough to produce maximum diffusion rate. However, if the concentration is too high, it will form a continuous layer of surface oxide film, which can prevent diffusion reportedly.