Microstructure and wear properties of niobium carbide particulates gradient-distribution composite layer fabricated in situ

Niobium carbide (NbC) particulates gradient-distribution composite layer was successfully synthesized by in situ techniques. The formation mechanism, microstructure evolution and wear properties of the composite layer were investigated in detail. The results show that the thickness of the composite layer is about 1360 µm after the heat treatment at 1175 °C for 1 h. From the surface to the matrix, the volume fraction of NbC particulates continuously decreases while the average particulate diameter gradually increases from 213 nm to 1.46 µm. Correspondingly, the micro-hardness and relative wear resistance almost gradually decrease from the top surface to the matrix.In order to discuss the wear mechanism clearly, the composite layer was artificially divided into four regions according to the continuously variation of composition and microstructure. The wear mechanism of NbC particulates high-density zone (region A) is mainly characterized by the intergranular micro-cracks and micro-ploughing. The relative wear resistance of this region is 75 times higher than that of HT300. The wear mechanism of NbC particulates partly gathering zone (region B) is micro-ploughing and part of micro-cutting. NbC particulates dispersing zone (region C) is slight micro-cutting and a small amount broken NbC particulates. And near-matrix zone (region D) is severe micro-cutting with broken and re-embeding NbC particulates in the matrix. And the wear resistance of the four zones is higher than that of matrix.