| With the rapidly developing in nation's aerospace industry and continuously research of various ultra-high-speed vehicles,the demands for reusable,light-weight,high-efficiency thermal insulation is imperative.Therefor,the insulation tile and surface coating structure is playing an important role in current aircraft's large-aera thermal protection for their low density and low thermal conduction.However,for the thermic expansion mismatch between coating and substrate,the interface zone generates thermal stress,which will break the coating/ substract system and threatens the safety of overall thermal protection.In addition,with the shuttle back and forth to the atmosphere the properties and ingredients of coating are generally transforming,which bring instabilities.So,the study of thermal stress in interface and microstructure and phase transition of coating at an elevated temperature is urgent to be put into practice.In this paper,various analytical methods have been employed to characterize the coating's microstructure and phase composition and measure thermodynamic parameters and mechanical parameters,etc.The results from different methods deepen the understanding of borosilicate glass coating/ substrate structure,which will provide a guidance to comprehend the relationship between the coating structure and properties.Adopting the finite element method and taking coating's parameters into the model,a simulation of the effect of substrate thickness,intermediate layer properties and thickness on coating thermal stress was carried out.By fixing the thickness of surface and intermediate layer,the effects of matrix thickness on Von Mises stress and thermal stress of coating were calculated.And the result shows that Von Mises stress decreases with the increase of substrate thickness and then remains unchanged.Changes in matrix thickness have little influence on the thermal stress.Moreover,the effect of intermediate layer thickness in different thermic expansion of coating stress was studied with the ANSYS.It can be concluded when surface coating being 140?m,transition layer with 3.5×10-6?-1 being 260?m,the Von Mises stress and themal stress of coating reach a minimum.Additionally,in order to testing the thermal shock resistance of coating before and after optimization,a thermal shock test from room temperature to 1200? was implemented.One thing to note is that the thermal expansion of intermediate layer was controlled by the filling ratio.The results show that the optimized surface coating remains intact without cracks after undergoing 25 thermal cycles.So,it is obviously that the thermal shock resistance of coating has a significant improvement.In this paper,the cyclic heat treatment at the different temperatures was employed to investigate the influence of heat treatment temperature and times in the microstructure and phase evolution of coating.The cristobalite precipitated in the borosilicate glass coating after heat treatment at 1200 ?,while the interface zone remained favorable combination.When the sample was treated 10 cycles at 1250?,the radiation agent Mo Si2 underwent a complex thermal oxidation reaction progress leaving behind a series of thermally oxidized micro-pores.With the increased number of cycles,crystal precipitation from the surface was intense,and the crystal phase was cristobalite.Part of the intermediate layer zone occurred resintering phenomenon,which generated cristobalite.It is 5 cycles that the surface coating began to be oxidized at 1300?,and the quartz fiber in interface zone appeared an obviously significant resintering,thinner fiber,brittle fracture.In additional,with an increased number of thermal treatment cycles,the amount of cristobalite in both of surface and transition layer was increasing. |
PDF Full Text Request