Terentyev A. Structure formation features of self-fluxing-based alloys with addition of refractory compoundscomposite materials for thermal spray coatings with high wear resistance.

The thesis is devoted to development of new composite materials based on self-fluxing alloy with TiB2 additive for wear resistant thermal spray coatings by liquid-phase sintering with pounding and classification. To choose the reinforcement additive it was investigated the wetting kinetics and contact interaction in systems (Ni-Cr-Si-B) – TiC, TiB2, ZrB2, CrB2. TiB2 was selected as reinforcement additive because of low contact angle (?=34°) and features of interfacial interaction with self-fluxing alloy. It is found that at contact interaction TiB2 is partially dissolved in transition zone, and new high hardness phase CrB(С) formation is performed, that may be a positive in increasing wear resistance of future coatings. Influence features of TiB2 amount on phase formation were investigated using Differential Thermal Analysis of initial alloy Ni-Cr-Si-B and composition of (Ni-Cr-Si-B) + (10, 20, 40) % ТіВ2. It is found that at certain heating modes in Ni-Cr-Si-B alloy formation of CrB and Cr23C6 1 mm grains takes place. Addition of TiB2 particles leads to sharp decreasing of CrB and Cr23C6 grains size (from 1 mm to 20 m) and additional high hardness (up to 25 GPa) reinforcing phases (up to 30 v.%) formation of different composition CrB(С), Cr(Ti)23C(B)6 and TiC. This is explained by TiB2 partial dissolving in self-fluxing alloy and subsequent formation of higher boron concentration local areas. Respectively the number of crystallization centers of reinforcing phased is increased while decreasing their sizes. It was produced the experimental samples of composite material NiCr-TiB2-10, NiCr-TiB2-20, NiCr-TiB2-40. It was found that every particle of powder has heterogeneous structure with metallic matrix with uniform distribution of reinforcement phases grains CrB(С), Cr(Ti)23C(B)6 and TiC with size from 1 mm up to 30 m. Plasma spraying modes were worked out and the coatings were sprayed on obtained composite materials samples. Developed powder plasma-sprayed coatings have heterogeneous structure with metallic matrix with uniform distribution of reinforcement phases grains CrB(С), Cr(Ti)23C(B)6 and TiC. The coatings have high density (94-95%) and good adhesion with substrate (32-35 MPa). Chemical and structural phase composition fully corresponds to the composition of initial sprayed composite materials. It is found that addition of TiB2 into Ni-Cr-Si-B leads to the increasing of coating’s wear resistance under dry sliding conditions in 3-3,2 times compared to initial alloy. Powder composite material NiCr-TiB2-20 has the maximal wear resistance.