Fabrication,Mechanical And Electrical Properties Of Thick 8YSZ-Al₂O₃ Composite Coatings

With the continuous increase of engine service temperature,higher requirements have been put forward for the thermal resistance of the hot metal components.Meanwhile,the scattering characteristic of the engine is very strong.It is a key problem to improve the thermal resistance and radar absorption synchronously.The deposition of thick coatings on the metal components cannot only improve the thermal insulation performance,but also use as the stratum medium of high-temperature radar absorbing coatings.In this paper,8YSZ-Al2O3 composite coatings were selected as the research objects.The core of this work is the evolution law and influence factors of mechanical and electrical properties,further exploring the mechanism of thermal shock failure,which lays a good foundation for the subsequent preparation of high-temperature radar absorbing coatings.Firstly,the 8YSZ-Al2O3 composite powders with different Al2O3 content(10wt%,20wt%,30wt%and 40wt%)were synthesized by spray-drying process.Experiments show that the composite powders have uniform particle size and good flowability.The thick 8YSZ-Al2O3 composite coatings(870?m)were deposited at different spraying power(34k W,38k W and 42k W)by using the composite powders,and the evolution law and influence factors of thermal shock resistance and bonding strength were studied.Results confirmed that spraying power only has a fine-tuning effect on bonding strength and thermal shock resistance within the selected power range.At 900^oC,with increasing Al2O3 content from 10wt%to 40wt%,thermal shock resistance of the thick composite coatings decreased from 379 cycles to 18 cycles,and increasing test temperature also made the thermal shock resistance decrease obviously.However,the bonding strength increases from 14.4MPa to 17.8MPa by adding more Al2O3.Furthermore,the addition of Al2O3 into 8YSZ results in an obvious decrease of high-temperature conductivity and permittivity.The electrical properties of the composite coatings can be effectively controlled by adjusting Al2O3 content.Then,the thermal shock failure of the thick composite coatings could be determined by many factors,such as phase compositions,microstructure and mechanical properties.By comparison,the addition of Al2O3 resulted in the formation of amorphous phase due to the high-speed annealing of APS.The recrystallization of the amorphous phase occurs between 900^oC to 1000^oC accompanied with a certain volume shrinkage were proved by measuring coefficient of thermal expansion,which adversely affected their thermal shock resistance.In addition,with increasing Al2O3 content,the melting state of the composite coatings increase gradually,and the fully melted splats can fill a part of the pores and defects,thus leading to the decrease of porosity of the thick composite coatings.It is not conductive to the release of the thermal stress and strain energy in the coating with less porosity.Meanwhile,coating surface Young's modulus increased from 99.07GPa to 134.59GPa with increasing Al2O3 content from10wt%to 40wt%,while thermal expansion coefficient(CTE)decreased from9.68×10^-6/^oC to 7.29×10^-6/^oC.As a result,residual stress on the surface of the thick composite coatings changed from tensile stress(+13.3MPa)to compressive stress(-189.0MPa).According to the empirical formula,the change of mechanical parameters may be an important reason for the decrease of thermal shock resistance.Finally,in order to further explore the mechanism of thermal shock failure,the mechanical properties and thermal shock resistance of 8YSZ-Al2O3 composite coatings with different thickness and Al2O3 content were compared.The results showed that increasing Al2O3 content and coating thickness brought increasing residual compressive stress to the composite coatings,which also resulted in the increase of Young's modulus and hardness.Meanwhile,coating surface fracture toughness of 870?m coatings increased from 0.93MPa·m^1/2 to 1.93 MPa·m^1/2,while the fracture toughness of ceramic coatings near the interface increased from 0.51MPa·m^1/2 to 0.83MPa·m^1/2.Therefore,appropriate residual compressive stress is beneficial to the improvement of coating surface fracture toughness.However,increasing coating thickness produces excessive residual compressive stress in the coating thickness direction,which obviously reduces the fracture toughness of ceramic coatings near the interface,further affecting the thermal shock resistance of the composite coatings.