Microstructure and mechanical properties of TiC-Fe surface gradient coating on a pure titanium substrate prepared in situ

A TiC-Fe surface gradient coating was prepared on the surface of pure titanium by a simple two-step heat-treatment process (1150 °C × 5 min + 1000 °C × 10 h). The phase composition, microstructure, nanoindentation hardness, elastic modulus, fracture toughness, and adhesion strength of the TiC-Fe surface gradient coating were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), nanoindentation testing, and scratch testing. The results reveal that the TiC-Fe surface gradient coating was composed of dense TiC grains and a small amount of α-Fe. The TiC-Fe surface gradient coating was divided into three zones according to the variation in the volume fractions of TiC particulate and α-Fe phase. These zones were labeled as follows: a columnar TiC zone (I zone), a large particle TiC zone (II zone) and a large bulk TiC zone (III zone). The formation process and mechanism of the TiC-Fe surface gradient coating include the nucleation growth of TiC grains, followed by diffusion and in situ reactions between titanium and carbon. The nanoindentation hardness and elastic modulus for cross section of the TiC-Fe surface gradient coating ranged from 19.1 to 31.7 GPa and from 368.1 to 464.3 GPa, respectively. The fracture toughness values of I zone, II zone and III zone are 3.5, 1.6 and 3.1 MPa m1/2, respectively, and these values largely depend on composition and microstructure. Investigations of crack surface morphologies indicate that radial cracks at the corners of the indentation originate from the crossing of slip bands and that the toughening mechanism was mainly crack deflection and bridging. In addition, the scratch testing indicated that the coating exhibited excellent coating/substrate adhesion strength.