C / C Composites Of Mosi <sub> 2 </ Sub> -cr-si/sic Department Of High Temperature Anti-oxidation Of The Composite Coating,

Carbon/Carbon composites(C/C) are mainly used as high temperature structural materials in aerospace field.The range of application is limited because it is very easily oxidized at high temperature in the presence of oxygen.In order to solve this problem,we are going to prepare coatings which can protect C/C composites from oxidation for a long time at high temperature.A SiC coating and MoSi2-Cr-Si coating for C/C composites were produced by pack cementation,chemical vapor reaction and slurry paint method,respectively. The phase composition and microstructure were studied by XRD and SEM.The oxidation and mechnical properties are also investigated.The main research contents and conclusions are as follows.(1)The parameters of pack cemention method were investigated,such as the sintering atmosphere,Si content of pack cementation and the kind of C/C matrix.The results show that vacumm and high Si content is conductive to infiltrating silicon.The residual Si filled pores and cracks within the coating.The free Si of the coating is higher,the more compack.But high Si content of coating was not beneficial antioxidant properties.Further,The amount of Si infiltrated in the low density C/C composites is more than high density C/C composites.We got the best SiC coating,the weight loss of which is 1.86% after oxidation at 1500℃for 30h.(2) The article studies preliminarily the microstructure and property of the SiC coating by chemical vapor reaction method.The results show that the distribution of SiC coating is graded.The coating integrates well with the matrix,and the surface is smooth.Another.However,the defects of the matrix will influence the quanity of the coating obtained by this method.(3) The MoSi2-Cr-Si coating was prepared by slurry method for the C/C composites with SiC coating. Effect of the content of MoSi2,sintering atermosphere,temperature and the kind of the binder on the coating were studied.The results show that when the conten of MoSi2 is 45%, the coating has the best anti-oxidation performance.We obtained the MoSi2-Si2N2O-CrSi2 by the addition of N2 and SiO during sintering. The main phase of the coating by slurry sintering method were Si2N2O and a few CrSi2,SiC,MoSi2.Compared with the SiC/MoSi2-SiC-CrSi2 coating with no Si2N2O, the SiC/MoSi2-Si2N2O-CrSi2 coating showed improved oxidation resistance.In addition, compared with the Ludox using PVA as binder, the performance of the coating was better.In the end,we obtained double-layer coating using the best processing parameter,and the weight loss of the sample is 1.37% after oxidation at 1500℃for 108h.(4) The effect of the coating processing,thermal shock and oxidation on the mechanical properties of carbon/carbon composites were investigated by three point bending tests.The results show that after coating processing,the fracture behabior of C/C composite was changed.The sample with double-layer coating's flexure strength increases.The fracture behavior changes from the pseudo-plastic fracture to brittle fracture. After enduring thermal cycling between 1500℃and room temperature for 30 cycles, The weight loss of the composites coated double-layer coating is 1.1% and the flexural strength reduced to 82.2%. The weigh change of the sample with composite coating is increasing 0.32% after oxidation for 9 hour at 1500℃,but the flexural strength is decreased to 98.5%,the fracture pattern is brittle fracture mode.