Fabrication And Properties Of ZrB₂-ZrC-SiC Ultra-high Temperature Coating On C/C Composites

C/C composites have advantages in space flight and aviation field due to its excellent properties.However,it will oxidize at low temperature,which largely limits the application of C/C composites.This subject used reactive melt infiltration?RMI?with ZrSi₂ alloy as row material to form ultra-high temperature coatings on C/C composites.Respectively based on chemical vapor deposition prepared boron carbide pre-coating and slurry dip combined pyrolysis prepared B₄C/C,we adopted RMI to fabricate ZrB₂-ZrC-SiC coating.The purposes are to study the micrograph transformation,phase composition and the formation mechanism during the process of coating fabrication.And then the oxidation resistance,the oxidation resistance mechanism and mechanical property were studied thoroughly.Finally,the coating prepared by slurry dip combined RMI was optimized and its properties of were also studied.The main results were presented by following.?1?The pre-coating prepared by CVD possessed a dense structure.Based on this pre-coating, RMI prepared ZrB₂-ZrC-SiC coating with a three layers structure:outer ZrB₂-ZrC-ZrSi₂ ?about 17?m?,middle SiC layer?about 15?m?,and inner unreacted layer?about 77?m?.?2?CVD combined RMI prepared coating presented a high weigh loss rate at 1500?in the airowning to its superfluous unreacted layer.After 2 h oxidation,its weight loss rate up to 38%and the inner C/C composite substrate was obviously oxidized.?3?B₄C/phenolic resin pre-coating was prepared by slurry dip.This pre-coating was oxidized under the heat treatment at 900?which is unbeneficial to the further process.While it changed to B₄C/C pre-coating with porous structure when the temperature was increased to 1400?.?4?Based on the porous B₄C/C pre-coating,RMI prepared ZrB₂-ZrC-SiC coating also has three layers structures:outer ZrB₂-ZrC layer,middle ZrB₂-ZrC-SiC layer and inner SiC layer. Besides,SiC presented an increasing concentration gradient from outer to inner in the middle ZrB₂-ZrC-SiC layer.The mechanical strength of C/C composites were enhanced by 68.91%and presented extraordinary oxidation resistance at 1500?with this coating.Its oxidation weight loss rate curve was fast increased at the beginning,then gradually slowed down,and finally declined.After 30 h oxidation,the coated sample still had a weight gain of 3.85%,and formed a four layers structure:SiO₂ layer,ZrO₂-SiO₂ layer,ZrB₂-SiO₂ layer and ZrB₂-ZrC-SiC layer from outer to inner.After 20 h oxidation,the bending strength of coated sample was 67.7 MPa with a 62.12%mechanical retention.?5?Slurry dip combined RMI prepared coating with some flaws like cracks on its surface was optimized by CVD to gain ZrB₂-ZrC-SiC/SiC coating.Its surface was deposited by particles in size of 5?m and without any flaws like cracks or holes.This optimized coating had a more integrated appearance than non-optimized one,7%weigh gain rate and increasing weigh gain tendency after 30h oxidation.Besides,its bending strength was 76.5 MPa and the strength retention is up to 70.43%,8.31%increment compared to non-optimized one.