Research On Preparation And Characteristics Of NiCoCrAlY Laser Cladded Coatings Reinforced By Nano-particles

Based on the analysis of the oxidation behavior of NiCoCrAlY coatings and the overview of the related researches from home and abroad, a novel idea to improve its high temperature protect properties was proposed in this work. That is to obtain good high temperature protect properties by introducing nano-particles into the coating, which gives full play to the synergism of the"high-power & fast-cooling"of laser cladding and the"nanometer effect"of nano-particles. A novel laser cladding coating technology namely the squash presetting laser cladding was presented in the work, which could well meet the requirement in the preparation of high quality NiCoCrAlY cladded coating reinforced by nano-particles. A preparation technology of highly dispersed metal matrix micro-nanometer composite powders and a presetting technique of cladded powders that is different with the chemical bonding method were as key techniques in the novel technology. NiCoCrAlY cladded coatings reinforced different nano-particles were successfully prepared based on the theoretical analysis and the experimental study of the technical process. The microstructure and the high temperature protect properties including oxidation resistance, hot corrosion resistance and thermal shock resistance of the NiCoCrAlY coatings reinforced by three kinds of nano-particles with different contents were investigated, and the influencing mechanism of nano-particles was discussed.The major work done and the results obtained are as follows:Firstly, a novel preparation technology of highly dispersed metal matrix micro-nanometer composite powders was proposed based on the discussions on the additive technique of nanometer powders, the additive content of nanometer powders, and the ball-milling parameters. The technology possess excellent powder quality, simple process, short processing time, controlled reaction, continuous production and other good characteristics.Secondly, a novel powders presetting method namely squash presetting method was presented to overcome the shortcomings of existing methods. The method could meet the needs in the preparation of high quality NiCoCrAlY cladded coating after introducing nano-particles. Compared with the thermal spray technology and the chemical bonding method, it has many good characteristics, such as high powder utilization ratio, good adaptability to powders size, avoiding importing impurity into the coating, etc.Thirdly, the theoretical analysis and the experimental study of the technical process of the squash presetting laser cladding, considering it is a novel laser cladding coating technology. The temperature distribution and stress distribution of NiCoCrA1Y cladded coatings on Ni-based superalloys in the laser cladding process were simulated based on the finite element analysis model. The optimized process parameters such as laser power of 1000 W, spot diameter of 2 mm and scanning speed of 5 mm/s were obtained based on the simulation results and the orthogonal test. The adhension performance of the NiCoCrA1Y cladded coatings on Ni-based superalloys adopted the optimized process parameters was investigated.Fourthly, the microstructure of the NiCoCrAlY coatings reinforced by three kinds of nano-particles with different contents was investigated by using SEM, EDS, XRD and other testing methods, and the influencing mechanism of nano-particles on microstructure was discussed. With the addition of nano-particles, the growth pattern of interface grain was changed; the microstructure of cladding layer was refined; and cracks in the substrate close to the interface were inhibited. Among the three kinds of nano-particles, the SiC nano-particles showed the most improvement on microstructure, while the CeO₂ nano-particles were insufficient. It is suggested that the improvement of nano-particles on the microstructure mainly depends on their dispersiveness among micro-nanometer composite powders and their stability in the melt-solidification process.Finally, the isothermal oxidation behavior, the hot corrosion behavior and the accelerated thermal shock behavior of the NiCoCrAlY coatings reinforced by three kinds of nano-particles with different contents were investigated, and the influencing mechanism of nano-particles on these high temperature protect properties was discussed. The high temperature protect properties of the NiCoCrAlY coatings after adding nano-particles were obviously improved, among which the one with 1 wt.% nano-SiC presented the best oxidation resistance and thermal shock resistance and the one with 2 wt.% nano-CeO₂ presented the best hot corrosion resistance. It is suggested that the improvement of nano-particles on the high temperature protect properties mainly be attributed to the refinement of microstructure.To sum up, the preparation technology presented in this work has a good engineering adaptability and a widely applied prospect. Moreover, the NiCoCrAlY cladded coatings reinforced by nano-particles adopting the technology has good high temperature protect properties, which could meet better the service demand of advanced gas turbines.