Microstructure And Long-time Oxidation Properties Of EB-PVD NiAlHf Coatings Treated By Laser Shock Processing

Thermal barrier coating is a key technology for hot end components such as aero-engine turbine blades in aircraft engines,and the metal bonding coating is one of the key components of the thermal barrier coating.The metal bonding coating can rapidly form a dense thermal growth oxide（TGO）mainly composed of Al₂O₃,at high temperatures.During the long oxidation process,TGO protects the alloy substrate and improves the resistance of the entire thermal barrier coating to high temperature oxidation.Therefore,the rate of TGO formation and growth state is critical to the oxidation resistance of the thermal barrier coating.The conventional MCr Al Y metal bonded coating is no longer adequate for the current service environment.In contrast,the new NiAlHf coating offers superior resistance to high temperature oxidation.Laser shock processing（LSP）influences the high temperature oxidation resistance of metal bonded coatings by inducing crystal defects and grain refinement in the metal surface layer and regulating the diffusion of elements during the oxidation process.In this paper,the EB-PVD NiAlHf coating was modified using laser shock processing technology to investigate the microstructure of the NiAlHf bonded coating before and after modification and the mechanism of the effect of LSP technology on the high temperature oxidation resistance of the coating.The following are the main research work and conclusions:（1）Effect of laser shock processing on the microstructure and surface morphology of NiAlHf coatings.The NiAlHf coating was subjected to LSP and its microstructure and surface morphology were characterized.The results show that after LSP the grains are refined and a large number of crystal defects are introduced.At the same time,the proportion of large particles in the initial oxide nucleation on the surface is reduced and the distribution density is increased.（2）Effect of laser shock processing on the resistance of coatings to high temperature oxidation.Characterisation of the surface morphology and composition of transiently oxidised samples at 1200°C and analysis of the effect of LSP on the initial growth of TGO.The results showed that the LSP samples formed a continuous dense oxide film more rapidly than the unmodified samples at the early stage of oxide film formation.No-LSP samples produced a continuous oxide film after 30 min.granular Al₂O₃had grown in sheets after 10 min of oxidation and a continuous dense Al₂O₃film was formed at 30 min.（3）Long-time oxidation properties of laser shock processing NiAlHf coatings.The samples were subjected to a long oxidation study at 1200°C before and after the laser shock processing.The results show that the TGO growth of No-LSP samples is divided into two stages:（I）uniform growth stage（<50 h）with an oxide film growth rate of4.3×10^-5;（II）accelerated growth stage（50 h-150 h）with an oxide film growth rate of1.0×10^-4.During the whole oxidation experiments,the TGO is in the process of growth-exfoliation-re-growth.Therefore the loss of metallic elements in the coating is severe,thus limiting the high temperature oxidation resistance of the NiAlHf coating.The TGO growth of the LSP samples was divided into three stages:（I）short fast growth stage（<10 h）with an oxide film growth rate of 8.51×10^-5,when a continuous dense TGO layer was formed on the surface of the coating;（II）slow growth stage（10h～100 h）with an oxide film growth rate of 2.24×10^-5,which was significantly lower compared to the previous stage;（III）stable growth stage（100 h～150 h）,the oxide film growth rate was 3.88×10^-5,which grew slowly but was lower than that of the No-LSP samples in the same time period.As a result,laser shock processing promoted the rapid generation of continuous dense Al₂O₃,which led to a reduction in the overall growth rate of TGO and improved the resistance of NiAlHf coatings to high-temperature oxidation.