| Carbon/Carbon (C/C) composite possesses advantages as low density, superior high temperature strength, high modulus and heat conductivity, and low thermal expansion coefficient. It is one of the most promising materials successfully used at temperatures over2000℃. However, C/C composite oxidizes easily above380℃, which significantly limits its practical application at high temperatures. Ir/Re coatings are the high-temperature oxidation resistant coating system which fully meets the zero-erosion, long-lifetime requirements for C/C composites at ultra high temperature. At present, researches on Ir/Re coatings are mainly concentrated on coating preparation processes and oxidation resistant properties, while reports on properties of the Ir/Re coatings on C/C composites are rare. In this dissertation, properties of Re transition layer on Ir/Re/C/C composites are the main focus, and characterization of the compatibility between the coating and the substrate, interaction between coating layers, and influencing factors on the coating properties are investigated.A variety of coating performance methods were used in this study, such as microhardness testing, tensile method, shear method, scratch method. Mechanical properties, the bonding strength of the rhenium transition layer to the substrate and the thermal shock resistance of the coating were investigated. The microstructure and chemical composition of the coatings were examined by photography, SEM, and EPMA. The influence of carbon element on mechanical properties of the rhenium coatings and the thermal shock failure mechanism of the coatings were also discussed. Main conclusions are summarized as follows:(1)Carbon element of C/C composites diffused to the CVD Re coating, which led micohardness increasing. The diffusion occurred in the CVD process, high temperature heat treatment (1600℃) showing diffusion speed of carbon in Re is very fast. Carbon diffused uniformly in a short time. Tensile strength and elongation of CVD Re decreased when treatment time prolonged. This mainly attributed to carbon element diffusion along Re grain boundary and Re coarsening columnar structure deformation coordination.(2)Near the substrate of CVD Re coating’microhardness is an average of Hv517.6, far beyond the substrate at an average of Hv406.4. Near and away from the substrate of Re coating’microhardness is basically the same about Hv650after1600℃treatment for1hours,3hours and5hours. Tensile strength of CVD Re was about457.5MPa with carbon diffusion treatment(1600℃3hours), tensile strength of CVD Re was about540.1MPa with no carbon diffusion treatment. (3)The tensile and shear experimental results showed that the fracture of graphite and C/C compositescoatingwith CVD rhenium occurred in the substrate, which dues to the low strength of graphite and the continuity of C/C composites demolished by machining. It demonstrated that CVD Re has good combination with graphite and C/C composites. The results indicated that the highest average value of tensile fracture strength is21.9MPa and shearing fracture strength is24.3MPa on the Z-direction of the fabric-base pierced carbon-carbon composites.(4)Thermal shock failure modes of Ir/Re/C-C composites were crazing, cocking, hunching by cooling rom high temperature (2000℃) to room temperature in vacuum atmosphere. There were no cracking, cocking and hunching in Re/C-C composites by10thermal shock cycles in vacuum atmosphere,and Re coating have good combination with substrate. The results of thermal shock failure of Ir/Re/C-C composites couldattributed to molten salt sandwiched between Ir and Re, Larger grain size of Re columnar crystal, thermal mismatch with Ir, Re and C/C composites. |
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