Oxidation behaviors of the TiNi/Ti2Ni matrix composite coatings with different contents of TaC addition fabricated on Ti6Al4V by laser cladding

• The composite coatings with TaC addition were fabricated on Ti6Al4V by laser cladding.
• Effect of TaC addition on microstructural evolution of the coatings was investigated.
• Oxidation resistance of the coatings were investigated by isothermal oxidation test.
• Oxidation behaviors and mechanisms were investigated by XPS and thermodynamic calculation.

The TiNi/Ti2Ni matrix composite coatings were fabricated on Ti6Al4V by laser cladding the mixtures of NiCrBSi and different contents of TaC (0 wt%, 5 wt%, 15 wt%, 30 wt% and 40 wt%). Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffractometry (XRD) were used to examine the microstructures of the coatings. Oxidation behaviors of these coatings were also investigated at 800 °C for 50 h in air. The results showed that the coating without TaC addition was mainly composed of TiNi/Ti2Ni as the matrix and TiC/TiB2/TiB as the reinforcement. TaC was dissolved completely and precipitated again during laser cladding. Ta and C from the added TaC mainly existed as the solute atoms in the solid solutions of TiC, TiB2 and TiB in the coatings with TaC addition. The addition of TaC refined the microstructures of the coatings. In the oxidation test, the oxidation process was divided into the violent oxidation stage and the slow oxidation stage. The oxidation rates of the substrate and the coatings with different contents of TaC (0, 5, 15, 30, 40 wt%) were 0.644, 0.287, 0.173, 0.161, 0.223 and 0.072 mg cm−2 h−1 in the first stage, 0.884, 0.215, 0.136, 0.126, 0.108 and 0.040 mg2 cm−4 h−1 in the second stage, respectively. The weight gain of these samples were 6.70, 3.30, 2.86, 2.64, 2.41 and 1.69 mg cm−2, respectively after the whole oxidation test. The oxidation film formed on the surface of the coating without TaC addition mainly consisted of TiO2, Al2O3, and a small amount of NiO, Cr2O3 and SiO2. Moreover, Ta2O5 was also formed on the surfaces of these coatings with different contents of TaC. The oxides formed during the oxidation test were supposed to be responsible for the improvement in oxidation resistance of these coatings.