Preparation And Anti-ablation Mechanisms Of TaC Coatings, TaC/SiC Coatings On C/C Composites

In order to improve the resistant-ablation performance of C/C composites and to meet higher requirement for the throat of the latest type of solid rocket motor with high performance, TaC coatings, TaC/SiC composite coatings and TaC-SiC co-deposited coatings were first prepared successfully by means of chemical vapor deposition. The technology, the characteristics in the structure and the kinetics forming coating structure were studied. The thermal shock resistance, ablative resistance and ablative mechanism of the coatings were investigated by ablation tests with low power laser and oxyacetylene flame, and the ablation models of TaC coatings, TaC-SiC co-deposited coatings were built.The CVD TaC coating technology was studied overall. The studies included the influence of deposited temperature, deposited pressure, deposited position and the amount for dilute gas Ar and reaction gas H₂ on the composition and structure of coatings. The basic mechanisms for the structure patterns of CVD TaC coatings and their formation were discovered. The temperature and pressure in CVD have remarkable influence over the structure patterns and density of CVD TaC coatings.The needle-like crystal-----the structure pattern I was formed under1250℃in high speed deposition, or the columnar crystals ----- the structure patternâ…¡was formed over 1250℃in low speed deposition. With thermal treating at 2500℃, the atomic volume diffusion of CVD TaC coatings was noticeably improved and coatings structure and performance degenerated overall while apparent shrinking and re-crystalline in coatings appeared.The kinetic mechanism of formation of CVD TaC structure patterns was explored. Only two structure patterns, needle-like crystal and columnar crystals, occur to TaC coatings because of the lower relative deposited temperature during 950～1400℃. ES energy controls directly the atom transmission between layers, and so does the growth model of TaC coatings. Formation and evolution of two dimension islands play a decisive role in incipience morphology of coatings, and affect ultimate structures and properties of coatings. The density of islands is decided by on diffusion speeds alone island edge and rate crossing angle between islands.TaC/SiC composite coatings and TaC-SiC co-deposited coatings were prepared and their structure characteristics were studied. The deposited mechanisms of TaC-SiC co-deposited coatings have been discovered and studied. TaC/SiC composite coatings with non-gradient have obvious interface between layers. Cracks between layers appear easily due to mismatch of thermal expansion and stress concentration. The relaxation structure of C/C matrix surface has a characteristic of selective adsorption to deposited gas TaCl_n(n=1～5), TaC-SiC co-deposited coating is mostly TaC with no H₂ or scarce H₂. Under the circumstances of suitable rate of H₂ and higher rate of CH₃SiCl₃, the TaC-SiC co-deposited coating with equal ingredient of TaC or SiC may be acquired. Under stable TaC-SiC co-deposited technology, the self-assembly structure patterns with fluctuation of structure and composition were discovered. And its performance of thermal shock resistance and oxidation resistance are superior to TaC coatings.The nanosized pore needle-like structure of TaC coating structure, with crackfree, low stress and good thermal shock resistance was discovered. The technology used to prepare TaC/SiC composite coatings with crackfree, low stress and two gradients was invented. So, the international problem for the low thermal shock of ceramic coatings was solved. Results show that the nanosized pore structure formed well-distributed in the TaC coating interior during CVD process characterizes to reduce the hardness, elastic modulus, linear expansibility and to absorb and relieve the inner thermal stress in coatings with resistance of 2300℃/s thermal shock. The TaC coating structure distributing from the dense matrix towards loose coating surface will result in the thick crackfree coatings with good thermal shock resistance. The technology to prepare C-TaC composite coatings with composition fluctuation is the effective method of preparation of crackfree, low stress coatings. TaC/SiC composite coatings with two gradients not only diminish the thermal stress between coating layers and on coating surface, but also reduce remarkably thermal stress concentration and disperse interface stress to full coating by two-gradient structure. It is necessary to obtain the dense and homogeneous matrix surface for the crackfree and low stress coating. C-TaC or C-SiC gradient transitional coatings can enhance connective strength between coating and matrix, and reduce interface stress, eliminate crack in coatings.Ablation process and mechanism at ultra-high temperature for TaC and TaC/SiC coatings on carbon-carbon composites have been investigated by ablation experiments with low power laser and oxyacetylene flame. Ablation results show that the dense TaC coatings fall from matrix by thermal shock because of mismatch between TaC coating and matrix, losing efficacy instantly. The ablation mechanism is the denuded mechanism by thermal shock. TaC and C-TaC coatings with needle-like pore structure are crackfree and good thermal shock resistance, but low strength and washed out by high speed fluid. The ablation mechanism is the mechanical erosion. The ideal coatings with ablation resistance should improve coating density, thickness, washing-out-resistance under the prerequisite of satisfied thermal resistance. There exists the diffusion transition layer of 1～2μm that consists of pores and fine crystals containing carbon, oxygen and tantalum. The Ta₂O₅ melt occurrence changes the oxidation mechanism for the TaC coating from the interface reaction mechanism to the diffusion mechanism in the way that oxygen is dissolved and diffused in the melt. When TaC coating was oxidized wholly, the Ta₂O₅ melt on the surface of matrix makes a active reaction to C/C composites. The pipeline-like erosion effect speed active oxidization of carbon alone the aperture in C/C composites, so the erosion and ablation of C/C matrix is accelerated greatly. TaC/SiC composites with two-gradient have superior thermal shock resistance and oxidization resistance.Results show from thermodynamics and tests that there are no active oxidization under the ablation of TaC coatings and TaC-SiC co-deposited coatings. The models of oxidization resistance and ablation resistance for TaC coating and TaC-SiC coatings were built. TaC coatings, C/TaC coatings and TaC/SiC coatings with thicker, low stress, crackfree and homogeneous character were prepared successfully on the small throats of C/C matrix and graphite matrix, so the sold foundation on researching and manufacturing high performance throats of SRM was laid.