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Niobium carbide composite coatings with Nb 2 C, NbC, Nb 3 Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb 3 Si region, nano-Nb/Nb 3 Si cellular eutectic region, and equiaxed Nb 2 C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb 3 Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb 3 Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb 3 Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb 2 C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system. [ABSTRACT FROM AUTHOR]

Niobium carbide composite coatings were prepared on titanium alloy surface by plasma spraying NbC–Al 2 O 3 , Nb–SiC and Nb–SiC–Al composite powders, respectively. The phase composition, microstructure and formation mechanism of the three composite coatings were analyzed and their microhardness, toughness and scratch resistance were compared. The phases of the NbC–Al 2 O 3 system did not change during the plasma spraying process, and new phases (Nb 2 C, NbC and Nb 3 Si) were formed in the Nb–SiC and Nb–SiC–Al systems. TEM results of the Nb–SiC composite coating indicate that the new phases nanocrystalline Nb 2 C, submicron NbC and nanocrystalline Nb 3 Si were formed during the plasma spraying process. Compared with the NbC–Al 2 O 3 composite coating, the microstructure of the Nb–SiC and the Nb–SiC–Al composite coatings were uniform, and the porosity were relatively low, and the hardness was higher. The Nb–SiC–Al composite coating was denser than the Nb–SiC composite coating, the lamellar structure was obvious and the number of pores in the coating was the least, which is attributed to the better molten state of the composite powder by the addition of the Al to the Nb–SiC system. The Nb–SiC–Al composite coating had better toughness and scratch resistance. [ABSTRACT FROM AUTHOR]

Yttria-stabilized zirconia coatings were prepared by atmospheric plasma spraying, and the bonding strength of the coatings was determined by tensile testing. The microstructure of the coating, including grains and pores, was characterized using scanning electron microscopy and electron backscatter diffraction. The results show that the higher the proportion of columnar grains, the higher the bonding strength. The large misorientation at the small grains leads to large stress, which is easy to fracture in the tensile process, so the existence of small grains leads to poor bonding strength. By using image processing, pores were classified as holes, cracks, and microcracks, and their proportions were calculated. It was found that both holes and transverse and vertical cracks in pores linearly affected the bonding strength; the influence of holes was the greatest, and the influence of microcracks was the smallest. In conclusion, the smaller the porosity, the greater the bonding strength of the coating. [ABSTRACT FROM AUTHOR]

To improve the sliding wear resistance of plasma sprayed Mo coating, Mo–SiC and MoO 3 –Al–SiC composite powders were used to prepare Mo composite coatings by plasma spraying. The results show that the introduction of SiC improved the mechanical properties and sliding wear resistance of plasma sprayed Mo coating. The reactivity of Mo and SiC was low during plasma spraying Mo–SiC composite powder. The thermite reaction for MoO 3 and Al was not conducive to the densification of the composite coating prepared by MoO 3 –Al–SiC composite powder, but improved the reactivity of in-situ generated Mo and SiC, and the Nowotny phase Mo 4.8 Si 3 C 0.6 was observed in the coating. The coating prepared by Mo–SiC composite powder displayed better mechanical properties and sliding wear resistance than that of the coating prepared by plasma spraying MoO 3 –Al–SiC composite powder. The coating obtained by Mo–SiC composite powder revealed a combination of adhesive wear, abrasive wear and oxidation wear. [ABSTRACT FROM AUTHOR]