Microstructures And Properties Of TiC-doped And LaH₂-doped Tungsten-based Coatings Fabricated By Plasma Spraying

In the present work, the pure tungsten coatings, tungsten coatings with 1.5 wt.% TiC（W/TiC） and tungsten coatings with 1.5 wt.% LaH₂（W/LaH₂） were fabricated by Atmospheric Plasma Spraying（APS） and Supersonic Atmospheric Plasma Spraying（SAPS） techniques. Optical Microscope（OM）, Scanning Electron Microscope（SEM）, Energy Dispersive Spectrometer（EDS） and X-ray diffraction（XRD） were used to observe the microstructures of powders and coatings. The properties of the coatings such as the density, oxygen content, porosity and thermal conductivity were tested by the Archimedes drainage method, oxygen and nitrogen analyzer and laser thermal conductivity meter. The mechanical properties of the coatings were tested by nano-indentation tester and the plasticity of the coatings were discussed.The spreading abilities of the W particles in the APS process was poor. Lots of pores and serious oxidation were observed in the APS-W coating, the oxygen content, porosity and thermal conductivity of APS-W coating were 1.44%, 20.88%, 77.62W/m K. In the APS-W/TiC, APS-W/LaH₂ coatings, lamellar structures and columnar tungsten grains were clearly existed. The TiC was distributed in the lamellar gaps or around detrital particles after deposition. The APS-W/TiC coating was denser, the porosity and oxygen content were only 10.39%, 0.65%. The thermal conductivity of the APS-W/TiC coating increased to 100.9 W/mK and the plasticity of APS-W/TiC coating was better than that of the APS-W coating. The La2O3 was observed in the lamellar gaps and around detrital particles of the APS-W/LaH₂ coating. The oxygen content of the APS-W/LaH₂ coating being about 0.38% was lowest in the three APS coatings. The porosity and thermal conductivity of the APS-W/LaH₂ coating were 10.47%, 109.54 W/mK. However, La H2 doping had little effect on the plasticity of the APS-W coating.The lamellar structures and columnar grains could be observed in the three SAPS coatings. The second phases in the composite coatings were also distributed in the lamellar gaps or around the detrital particles, which filled and connected the lamellae and the particles. The porosity, oxygen content and thermal conductivity of the SAPS-W coating were 8.06%, 0.6%, 99.94% W/m K. The pores and gaps of the SAPS-W were decreased after TiC addition. The porosity and oxygen content of the SAPS-W/TiC coating decreased to 5.45%, 0.55%. Meanwhile the thermal conductivity reached to 119.59 W/mK. The thermal conductivity and plasticity of the SAPS-W/TiC coating were the best in the three SAPS coatings. The oxygen content of the SAPS-W/LaH₂ coating was decrease to 0.54%. The thermal conductivity of the SAPS-W/LaH₂ was 103.95 W/mK. However, the lamellae bonding was not dense, there were many detrital particles in the SAPS-W/LaH₂ coating. The porosity of the SAPS-W/LaH₂ coating was increased and the plasticity of the SAPS-W/LaH₂ coating was the worst in the three SAPS coatings.Compared to the APS coatings, the porosities of the SAPS coatings were significantly reduced, and the lamellar was spread smoothly and compactly. The comprehensive performances of the SAPS coatings were better. After laser remelting, the W grains in the SAPS-W/TiC coating were significantly refined compared to the SAPS-W coating, there was no pore in the remelted SAPS-W/TiC coating. Meanwhile, the rearrangement and distribution of the particles were realized by remelting, which made the Ti C precipitate on the tungsten grain boundaries. While a lot of pores were still observed in the remelted SAPS-W/LaH₂ coating and the microstructures of the remelting area were disordered and coarse. After laser irradiation, the microstructures of the SAPS-W/TiC coating was significantly better than that of SAPS-W and SAPS-W/LaH₂ coatings.