Preparation And Mechanical Properties Of High Emissivity MoSi₂-borosilicate Glass Gradient Coatings On Mullite Fibrous Insulation Tiles

The high emissivity coating applied on the surface of rigid thermal insulation tile is an important part of spacecraft thermal protection system.Studies showed that MoSi₂-borosilicate glass coating with interface gradient structure had better mechanical properties and greater application potential than mono-layer coating.However,at present,there were few studies on gradient interface and mechanical properties of high emissivity coatings with excellent temperature resistance prepared by in-situ reaction.Therefore,this study is devoted to the preparation and mechanical properties of in-situ reaction MoSi₂-borosilicate glass gradient coating.Gradient coating is also prepared by the combination of one-step rapid sintering method and in-situ reaction method.In this study,a double-layer gradient MoSi₂-borosilicate glass coating with a porous interface layer and a dense surface layer was prepared on mullite fibrous insulation tiles by in-situ reaction method.Compared with mono-layer dense coating and high-density gradient coating,double-layer gradient coating showed higher bearing capacity and avoided radial crack initiation and propagation in Hertzian indentation test.Besides,the double-layer gradient coating endured the most thermal shock cycles between 1500°C and 20°C and presented substrate fracture instead of interface fracture.In addition,double-layer gradient coating and high-density gradient coating possessed much higher interface bonding strengths than mono-layer dense coating.In order to inlarge the range of raw materials ratio and sintering temperature,and to reduce the difficulty of coating preparation,MoSi₂-borosilicate glass gradient coating,using fused quartz and B₂O₃ as raw materials of borosilicate glass of surface layer,was prepared through rapid sintering.Nano silicon and boron oxide were used as raw materials of borosilicate glass of interface layer.The coating infiltrated into the substrate and fill part of the pores to form porous interface layer.In this study,the reaction process of raw materials was analyzed.It was proved that one-step sintering enabled the coating pass through the middle and low temperature stage rapidly,during which MoSi₂ oxidized drastically.The effects of Si/B on the properties of MoSi₂-borosilicate glass gradient coating were studied.With the increase of Si/B,the temperature resistance of the coating was improved by the protection of Si O₂,but the contact damage resistance and thermal shock resistance were decreased because of residual stress and interface stress.