Preparation And Study Of Thermal Barrier And Ablative Coating Based On Nanozirconia

Along with the increasing need for national defence technology and project on space flight, aerocrafts are being developed to the ones with higher reentry velocity, longer reentry time, smaller weight, miniaturization and mechanization. Consequently, higher demand is put forward to present thermal protective measure. In this paper, thermal barrier and ablative composite coatings used for thermal protection were prepared on the matrix of alloy by atmospheric plasma spraying (APS), which were illuminated by sweat metal ceramic, and a concept of thermal barrier and ablation and a mechanism of thermal protection were both put forward.According to the characterestics that thermal barrier and ablative coating represent, coatings were designed for nano-zirconia doped some ablative materials that could absorb partial caloric at a relatively large range of temperatures when coating were heated, and the porous structures remained after ablation could further diminish conductivity of coatings. In this paper, three kinds of materials were selected, including organic material A, copper and glass whose phase change at different temperature. Composite coatings of thickness of 0.3-0.5 mm were prepared by APS in term of certain process parameters, with agglomerated powder consisting of one or more kinds of ablative material prepared by the method of spray prilling. Performances of thermal protection and thermal insulation were qualitatively evaluated through ablation and heat insulation test.Microstructure and performance of agglomerated powder, coatings before and after ablation were characterized by x-ray diffraction(XRD), scanning electron microscopy(SEM), thermo-gravimetric and differential thermal analysis, infrared analysis(IR), as well as electron probe x-ray microanalyzer(EPMA). In addition, loose density and fluid of agglomerated powder were tested. Conductivity coefficient, porosity and coherent strength were detected and analyzed. It was also discussed about microstructure, composition and thickness influence thermal protective property of coatings. Finally, conclusion was enumerated as follow:(1) Agglomerated powder prepared by spaying prilling tended to form good spherical type, while quite ordinary fluid and loose density were found. Distribution of ablative material was relatively uniform. Composite coatings No.1 and No.2 inclusive of organic substance A and glass occupied excellent features such as wash resistance, thermal shock resistance and thermal protection. During the ablative test that plasma blaze was regarded as thermal resource, thermal protective time of No.2 was accumulated up to 53.6S, which was superior to that of single zirconia coating. This kind of composite coating can satisfy the need of thermal protection for some local position of aerocraft flying in high mach number.(2) After microstructural analysis for composite coatings before and after ablation , it was discovered that ablation of organic and inorganic materials occurred during ablative test. On the contrary, as for glass and copper, ablation was remarkable on the surface of coating of approximately 0.1mm, accompanied with a great deal of pores. Content of ablative material significantly dropped via EPMA by observation, which manifested that ablative materials had been vaporized or volatilized. Nonetheless, vaporization was not evident inside coatings, taken place of by melting. Content of A greatly decreased and was decomposed into carbon after APS, which resulted in definite thermal protection. While it was more important that A as a pore-creating substance could form a great deal of pores and enhance performances of thermal protection. Composite coatings doped with a certain amount of ablative material owned good thermal shock resistance which single zirconia did not possess.(3) Also in this paper, the mechanism of thermal protection for composite coatings was discussed . It was considered that composite coating prevented heat from delivering not only by thermal insulation but also by ablation at different temperatures. Different ablative materials could absorb an enormous collection of caloric, decomposing, melting and vaporing at different temperature. Gas coming from ablative materials not only carried away a part of caloric, but also came into being the effect of convective block. Pores left after material vaporing could further downsize the conductivity of coatings. More than one mechanics of thermal protection exist in composite coatings. Coatings that included low conductivity ablative material were superior to single coating in thermal protection. By contrast, coatings containing high conductivity one was inferior or ordinary, contributed by the effect of ablative thermal protection that was unable to counteract the uprising of heat conductivity. Apart from delaying time to reach melting point of alloy, composite coatings also avoided alloy from burning before melting by the isolation of oxygen from air.