Effect Of Pre-oxidation Of C/C Composites And SiC Nanowires On The Properties Of Silica-based Ceramic Coatings

Coating technology has been used as an effective way to protect Carbon/Carbon（C/C）composites against oxidation.Silicon-based ceramics（such as SiC）has been proved to be an promising coating material for C/C due to the formation of self-sealed SiO₂ glassy protective layer in static air environment at high temperature（below1600°C）.However,due to the CTE mismatch,the thermal cycling would induce the thermal stress,resulting in the cracking or peeling of the coating.To solve the above-mentioned problem,a three-step method was used to construct a SiC nanowire-toughened inlaid transition layer between C/C substrate and coating in the present work.Firstly,C/C composites were pre-oxidized to obtain a porous surface.Then,SiC nanowires were in-situ generated on the pre-oxidized C/C composites by CVD.After that,the porous structure with SiC nanowires was filled with the coating material during coating process.The influence of the nanowire-toughened inlaid transition layer on the properties of the coating for C/C composites were investigated.The main contents are listed as follows:The pre-oxidation temperature is 850°C,the pre-oxidation time are 3 min,6 min and 9 min.The pore size,pore depth and surface roughness of C/C composites formed by pre-oxidation,which showed a clear increasing trend.The pore size and the depth of the pore structure which are constructed by the pre-oxidation treatment time of 6min are relatively moderate,and the nanowires can be evenly distributed in the pore structure.Preparation of CVD-SiC nanowires on the surface of C/C composites after6 min of pre-oxidation treatment at 850°C.Controlling the distance between the reaction powder and the underside of the sample were 4 cm,3 cm,2 cm and 1 cm.With the decrease of the distance between the reaction powder and the underside of the sample,the thickness and density of the nanowires increases gradually.When the distance between the reaction powder and the underside of the sample is 2 cm,the thickness of the prepared nanowires is moderate,and the nanowires are uniform.SiC nanowires were grown on the pre-oxidized C/C composites via CVD.After that,the SiC ceramic was deposited into the porous structure with SiC nanowires by CVD.A CVD-SiC coating with SiC nanowire-toughened transition layer,was fabricated on the surface of C/C composites.After the introduction of SiC nanowires in CVD-SiC coating,the hardness,elastic modulus and fracture toughness of the SiC coating increased by 50.5%,70.8%and 76.6%,respectively.The nanowires mainly through bridging,pull-out and crack initiation mechanism to realize the coatings toughening and reinforcing effect.The SiC nanowire-toughened transition layer enhanced the interface bonding between the coating and C/C substrate,reduced the size and number of cracks in the coating,and improved the thermal shock resistance of the coated samples.The bonding strength between C/C substrate and SiC coating was increased from 6 N to 19 N.The weight loss of the sample after thermal cycles between room temperature and 1500°C for 15 times decreased by 67%.SiC nanowires were grown on the pre-oxidized C/C composites via CVD.After that,the porous structure with SiC nanowires was filled with the coating material during pack cementation.A SiC-MoSi₂-ZrB₂ coating with SiC nanowire-toughened inlaid transition layer,was fabricated on the surface of C/C composites.The synergistic effect of the inlaid structure of the transition layer and SiC nanowires plays positive role on alleviating the mismatch of thermal expansion coefficient between C/C and SiC-MoSi₂-ZrB₂ coating.The oxidation resistance of the coatings is obviously improved,after oxidation of 124 h at 1500°C in static air,the weight loss is only 0.5%.After thermal cycles between room temperature and 1500°C for 15 times,the weight loss of the coated sample with pre-oxidation and SiC nanowires compared to the coated sample with SiC nanowires decreased by 45%,compared to the coated sample without pre-treatment decreased by 76%.The inlaid transition layer at the interface can significantly reduce the numbers and sizes of cracks.The flexural properties of the coated samples were tested before and after the thermal cycling test,and the bending strength of the coated samples with nanowire-toughened inlaid transition layer was only decreased by 6%after thermal cycling test.After thermal cycling test,the bending strength of the coated samples without pre-treatment and the coated samples with SiC nanowires decreased by 41%and 12%,respectively.