Preparation,Oxidation Resistance And Thermal Shock Resistance Proporties Of SiOC Based Composite Coatings

C/C composites material has the advantages of low density,high specific strength,high thermal stability,high specific modulus,high specific heat capacity,good thermal shock resistance,good friction and wear resistance,etc.,and it is one of the high temperature structural materials which are mainly research and develop emphatically in the field of new materials.However,C/C composites are easily oxidized in high temperature oxygen-containing environment.Therefore,the problem of high temperature oxidation protection of C/C composites is a major obstacle to restrict its development and application.High temperature anti-oxidation coating is the most direct and effective means to solve the problem of anti-oxidation.In this thesis,SiOC multiphase composite coatings and SiO₂/SiOC composite coatings were prepared by chemical vapor deposition（CVD）using hexamethyldisilazane（HMDSN）and tetraethyl orthosilicate（TEOS）as precursors.The effects of different deposition temperatures on the microstructure,composition and nano-hardness and elastic modulus of SiOC multiphase composite coatings were studied.At the same time,the effects of different deposition temperatures on the oxidation resistance and thermal shock resistance of SiOC multiphase composite coatings were also studied.To further improve the oxidation resistance and thermal shock resistance of C/C composites,the oxidation resistance and thermal shock resistance of SiO₂/SiOC composite coatings were studied.The main conclusions were listed as follows:（1）SiOC multiphase composite coatings mainly contained Si C,SiO_xC_4-x（x=1,2 or 3）,SiO₂ and free carbon,and also contained a small amount of Si₃N₄ and Si CN.At 1300 K and 1373 K,the SiOC multiphase composite coatings were mainly amorphous.At 1438 K and 1513 K,both2H-Si C and 3C-Si C existed in the coatings,while at 1576 K,only 3C-Si C existed in the coating.The coating morphology was smooth and compact at low temperature,but with the increase of deposition temperature,the coating gradually become rough and loose.Meanwhile,with the increase of deposition temperature,the deposition rate of the coating increased first and then decreased,reaching the maximum of 42.2μm/h at 1513 K.Nano-hardness and elastic modulus of the coatings reached the maximum of 28.8GPa and 234.2 GPa at 1373 K,which were attributed to the high content of amorphous Si C as well as Si₃N_4.（2）With increase of deposition temperature,the oxidation resistance and thermal shock resistance of SiOC multiphase composite coatings increased first and then decreased.Among them,SiOC multiphase composite coating prepared at 1438 K had a dense structure and good bonding with C/C matrix,so it had the best oxidation resistance and thermal shock resistance.After 30 cycles of thermal shock at room temperature to 1773 K,the mass loss rate was only 2.64%.At low deposition temperature（1300 K and 1373 K）,the prepared SiOC multiphase composite coatings were prone to thermal cracking during cooling,and its oxidation resistance was poor.（3）SiO₂ coatings were deposited with tetraethyl orthosilicate as precursor on the surface of SiOC multiphase composite coatings prepared at different deposition temperatures,to obtain SiO₂/SiOC composite coatings.The oxidation resistance and thermal shock resistance of SiO₂/SiOC composite coatings were improved largely.Among them,the SiO₂/SiOC composite coating with inner deposition temperature of 1438 K had the best oxidation resistance and thermal shock resistance,and the oxidative mass loss ratio was only 1.96%after 36 h isothermal oxidation at 1773 K,and only 0.78%after 30 thermal shocks from room temperature to 1773 K.