Study On Preparation Of Ni-coated Mullite Powders And Coatings Performance

With the rapid development of aerospace,aircraft manufacturing,internal combustion engines and other high-temperature fields,higher requirements are put forward for the wear resistance,heat insulation and thermal shock resistance of parts under high temperature conditions.Because of its low thermal conductivity,high temperature resistance,creep resistance and stable chemical properties,mullite is used as a surface protective coating for high-temperature parts and has a good effect,but due to its poor thermal spraying processability,was restricted to a wide range of applications.Therefore,this article first uses the electroless plating method to coat the mullite powders with Ni-P coating,and then uses the atmospheric plasma spraying equipment to prepare four sets of Ni Cr-mullite ceramic composite coatings with different proportions.The microscopic morphology and phase composition of the coating were characterized,and its performance was tested.The influence of different ratios of Ni Cr-mullite powders on the structure and performance of the coating were studied.The main research contents are as follows:(1)The Ni-P coating is coated on the surface of the mullite powders by the electroless plating method.The coating is uniform and dense.The heat treatment improves the adhesion of the coating and the coating with the mullite.At the same time,the formation of the coating Hard phases such as Ni₃P effectively increase the hardness of the powder.During the coating process,Ni²⁺is first adsorbed on the surface of the pretreated mullite,and nucleates and grows near the Pd atom to form an island-like structure.As the reaction proceeds,Ni particles grow and aggregate in a three-dimensional direction,forming on the surface of the mullite the Ni-P coating film,and then the Ni particles generated on the surface of the film are used as the activation center to continue the reaction.The powders coated by electroless plating have the characteristics of good strength and toughness,good fluidity,high density and good wettability,and are suitable for preparation of coatings by atmospheric plasma spraying.(2)Four sets of coatings with different proportions were prepared by atmospheric plasma spraying equipment,and the morphology and microstructure of the four sets of coatings were analyzed.The results showed that the mullite content of the coating was 100%mullite.The powders are stacked on the substrate in a molten or semi-molten state,and the coating is dominated by mullite phase and?-Al₂O₃phase;while most of the mullite powders in the composite coating are embedded in the coating with the original morphology.The layer is mainly composed of mullite phase and Ni Cr alloy phase,and there are also a small amount of hard phases such as Ni₃P and Al Ni₃.The porosity of the four groups of coatings decreases first and then increases with the decrease of the mass fraction of mullite.When the mass fraction of coated mullite is 85%,the porosity of the coating is the smallest,which is 5.67%.In the hardness test,the hardness of the four groups of coatings is higher than that of the substrate,and the hardness value first increases and then decreases with the decrease of the mass fraction of mullite.When the mass fraction of coated mullite is 85%,the hardness of the coating is the largest,which is 542.2 HV.This is because the coating prepared with coated mullite has low porosity and the presence of Ni₃P,Al Ni₃and other hard materials in the coating.Phase to increase the hardness of the coating.(3)The thermal barrier,thermal shock and 800?friction and wear test analysis of the four sets of coatings show that the four sets of coatings have better thermal barrier performance.When the coating is pure mullite,the thermal barrier performance of the coating is the best.Although the thermal barrier of the coating decreases as the quality of the mullite in the coating decreases,the heterogeneous phase formed by the coated mullite and the Ni Cr alloy also blocks the heat transfer and reduces the thermal conductivity.The thermal shock test of the coating shows that the thermal shock resistance of the coating increases as the mass fraction of mullite in the coating decreases.When the mass fraction of mullite in the coating decreases,the alloy content increases and the thermal expansion coefficient of the coating increases.Improve the bonding strength of the coating.Under 800?friction,the coating is mainly abrasive wear.The amount of wear and friction coefficient of the coating both decrease first and then increase with the decrease of the mass fraction of mullite.This is because the coating prepared with coated mullite has low porosity,high degree of metallurgical bonding,and high degree of metallurgical bonding with the substrate.The material has strong bonding force,so it has better wear resistance.