Yaran Niu

Microstructure and Tribological Property of TiC-Mo Composite Coating Prepared by Vacuum Plasma Spraying

TiC-based composite coating using Mo as an additive has been fabricated by vacuum plasma-spraying, and then the phase composition and microstructure of TiC-Mo composite coating were investigated. Wear resistance of the TiC-Mo composite coating was comparatively studied with pure TiC coating. The experimental results showed that the microstructure of the TiC-Mo composite coating was relatively homogeneous and compact, exhibiting typical lamellar structure of plasma-sprayed coating. Orientated columnar grains of TiC can be found in the composite coating, and a (Ti, Mo)C transition phase was also observed. Due to the formation of (Ti, Mo)C transition phase, strong interface between TiC and Mo splats was obtained, which positively influenced the wear performance of the composite coating. As compared with pure TiC coating, the TiC-Mo composite coating exhibited improved wear resistance both at low and high loads. Wear mechanisms for the TiC coatings have been changed by adding Mo element.

Influence of Oxidation Behavior of Feedstock on Microstructure and Ablation Resistance of Plasma-Sprayed Zirconium Carbide Coating

Abstract Plasma spray is one of the suitable technologies to deposit carbide coatings with high melting point, such as ZrC. However, in the spray processes performed under atmosphere, oxidation of the carbide powder is inevitable. To investigate the influence of the oxidation behavior of feedstock on microstructure and ablation resistance of the deposited coating, ZrC coatings were prepared by atmospheric and vacuum plasma spray (APS and VPS) technologies, respectively. SiC-coated graphite was applied as the substrate. The obtained results showed that the oxidation of ZrC powder in APS process resulted in the formation of ZrO and Zr2O phases. Pores and cracks were more likely to be formed in the as-sprayed APS-ZrC coating. The VPS-ZrC coating without oxides possessed denser microstructure, higher thermal diffusivity, and lower coefficients of thermal expansion as compared with the APS-ZrC coating. A dense ZrO2 layer would be formed on the surface of the VPS-ZrC-coated sample during the ablation process and the substrate can be protected sufficiently after being ablated in high temperature plasma jet. However, the ZrO2 layer, formed by oxidation of the APS-ZrC coating having loose structure, was easy to be washed away by the shearing action of the plasma jet.

Fabrication and Tribological Evaluation of Vacuum Plasma-Sprayed B4C Coating

B4C coating was fabricated by vacuum plasma spraying and the tribological properties of the coating against WC-Co alloy were evaluated by sliding wear tests. Al2O3 coating, one of the most commonly used wear-resistant coatings, was employed as comparison in the tribological evaluation. The results obtained show that, the B4C coating is composed of a large amount of nanostructured particles along with some amorphous phases. Both of the friction coefficient and wear rate of the B4C coating are much lower than those of the Al2O3 coating, and the tribological evaluation reveals a decreasing trend for the B4C coating in friction coefficient as well as wear rate with increasing normal load, which is explained in terms of the formation of a protective transfer layer on its worn surface. Tribofilm wear is found to be the dominant wear mechanism involved in the B4C/WC-Co alloy friction pair.