Preparation And Failure Mechanism Analysis Of Non-ablative Thermal Protective Coatings On Aluminum Alloys

Thermal barrier coatings(TBCs)could be used to increase the service temperature of high-strength aluminum alloy usually applied as the main material of lightweight weapons and equipments.However,the aluminum alloy substrate is easily oxidized,and its melting point and thermal expansion coefficient(CTE)is very different from those of TBCs,leading to the difficulty in preparing,poor uniformity and low bonding strength of the coatings.This paper has explored suitable spraying materials and effective cooling methods during atmospheric plasma spray(APS)as well as designed functional gradient(FG)structured coatings to reduce the thermal mismatch of materials and improve the thermal shock resistance and bonding strength of TBCs on the surface of aluminum alloy substrates.In addition,the failure mechanisms of the coatings were analyzed.New TBCs'ceramic materials were further explored to replace traditional 8YSZ(8wt.%Yttria-stabilized zirconia)material which was difficult to be used at higher temperatures due to its phase transition and sintering.By means of studying the effects of APS cooling process(compressed air and dry ice blasting),aluminum-cladded nickel-chromium alloy(Al@Ni Cr)bond coat material,and FG structure design on the microstructure,elastic modulus,bonding strength and thermal cycling performance of TBCs on the surface of the aluminum alloy substrate,the results show that dry ice blasting process can significantly reduce the substrate temperature,the oxides and pores inside the bond coat,which significantly improves the coating density and increases the bonding strength of the coatings.Similar to traditional Ni Cr Al Y bond coat,Al@Ni Cr bond coat undergoes selective oxidations of Al and Cr during spraying.But the aluminum oxide always wraps around Ni Cr in Al@Ni Cr bond coat and the bonding strength of Al@Ni Cr/8YSZ TBCs is higher than that of Ni Cr Al Y/8YSZ TBCs,implying that Al@Ni Cr is more suitable as a bond coat material for aluminum alloy substrates.In particular,the improvement of bonding strength of the Al@Ni Cr/8YSZ TBCs prepared by APS with dry ice cooling was more obvious,and the porosity and oxide content of the bond coat show a greater reduction.The bonding strength of Al@Ni Cr/8YSZ FG coating system(up to 38.2±2.78 MPa)is almost twice that of the double-layer coating system,showing superior thermal protection performance.The feasibility of La₂Hf₂O₇(LH)as a new type TBCs'materials was explored by studying the thermophysical properties,infrared radiation,and thermal cycling properties of their bulks and coatings.The results show that the LH ceramic coating has better phase stability than traditional 8YSZ coating and maintains the pyrochlore structure for a long time at 1600°C.In particular,LH coatings have lower thermal conductivity(0.5W m^-1K^-1?0.6W m^-1K^-1),comparable mechanical properties to those of 8YSZ coatings,but their CTEs are lower.LH coatings have an infrared emissivity of 0.886 in the 3-20?m band at 600°C.Single ceramic layered(SCL)LH TBCs have low thermal cycling lifetime under the conditions of 1100°C and 1000°C,and the failure occurs at the interface of LH ceramic layer and bond coat or the interior of LH ceramic layer near the bond coat.Double ceramic layered(DCL)LH/8YSZ TBCs can significantly prolong the lifetime.The failure of DCL LH/8YSZ TBCs is mainly caused by the mismatch of thermal expansion of LH and YSZ coatings.LH may be more suitable as a TBCs'material on the surface of ceramic matrix composites with a low CTE.