【作者】 李国栋；

【导师】 黄伯云； 熊翔；

【作者基本信息】 中南大学 ， 材料学， 2006， 博士

【摘要】 为提高C／C复合材料的抗烧蚀性能，以适应新一代高性能固体火箭发动机（SRM）对喷管材料更高的要求，本文首次用化学气相沉积的方法在C／C复合材料上成功地制备了TaC涂层、TaC／SiC多层复合涂层、TaC-SiC共沉积涂层，并对其工艺、结构特点及结构形成动力学进行了研究，采用了激光烧蚀和氧炔焰烧蚀等试验对涂层的抗热震、抗烧蚀性能和烧蚀机理进行了研究，建立了TaC、TaC-SiC共沉积涂层烧蚀模型。较全面地研究了TaC涂层化学气相沉积（CVD）工艺。研究包括沉积温度、沉积压力、沉积位置、稀释气体Ar与反应气体H₂用量等方面对涂层的成分、结构的影响。探明了CVD TaC涂层的结构形态及其形成基本规律，沉积温度与沉积压力对涂层结构形态、致密度有着决定性的影响。低于1250℃快速沉积为形态1型结构——针状晶体拱形排列；高于1250℃慢速沉积为形态2型结构——柱状晶纵向排列。2500℃热处理时CVD TaC涂层的原子体扩散能力大大提高，涂层出现了明显的烧结收缩与再结晶现象，涂层结构与性能全面退化。探索了CVD-TaC涂层的结构形态的形成动力学机制。由于沉积相对温度低，TaC涂层在950～1400℃之间只出现针状和柱状两种典型结构。ES势直接控制着层间质量传输，也就控制了涂层的生长模式。二维岛的形成与形貌演变对涂层初期形貌起着决定性的作用，从而影响涂层的最终结构和涂层的性能，岛的紧致程度主要是由原子沿岛边扩散和跨越角的快慢程度所决定。制备了TaC／SiC多层复合涂层及TaC—SiC共沉积涂层，研究了其结构特点，发现并初步研究了TaC—SiC共沉积规律。无梯度TaC／SiC多层复合涂层内有明显的层间界面，热失配使得界面易出现应力集中和层间裂纹。C／C基体表面的驰豫结构对沉积气体TaCln（n=1～5）具有选择性吸附特性，在无H₂或H₂很少的情况下TaC-SiC共沉积涂层主要是TaC。只有在H₂适量并提高CH₃SiCl₃的浓度的情况下才能得到TaC-SiC共沉积涂层。在稳定的TaC-SiC共沉积涂层工艺中，发现了结构与成分周期性波动的自组织结构花样，这种结构的抗热震性能和抗氧化性能优于单一TaC涂层。发现了低应力、无裂纹、抗热冲击性良好的TaC涂层结构——纳米介孔针状拱形结构；发明了具有低应力、无裂纹特征的双重梯度共存的TaC／SiC多层复合涂层技术，突破了陶瓷涂层抗热冲击性能差的世界性难题。CVD涂层中均匀分布的纳米介孔结构具有低弹性模量、低膨胀特点和吸收、缓解内部应力应变的功能，具有抵抗2300℃／s氧炔焰热冲击能力。从基体到涂层表面形成由致密到疏松的TaC涂层结构，能够获得无裂纹、抗热冲击性能良好的单一成分TaC涂层。成分波动式C-TaC梯度复合涂层工艺是一种有效的低应力、无裂纹涂层的制备方法。TaC／SiC双梯度涂层不仅减小了涂层间和表面涂层的热应力，而且通过第二重梯度将界面应力分散到整个涂层，显著地降低了应力方向的单一性所导致的应力应变的递增传递集中。提高基体的均匀性是高质量涂层的保障；C-TaC或C-SiC的梯度过渡层能够有效提高涂层与基体的连接强度，降低界面应力，消除裂纹。激光烧蚀和氧炔焰烧蚀等试验探索了TaC、TaC／SiC涂层的烧蚀过程、烧蚀机理。致密TaC涂层与基体间的热失配大，涂层的烧蚀为热冲击脱落，瞬间失效，烧蚀机制为热冲击剥蚀控制机制；结构为针状的多孔TaC、C-TaC涂层无裂纹，抗热冲击性能好，但涂层自身的强度低，易被高速气流冲蚀，其烧蚀机制为机械冲蚀控制机制。较理想的抗烧蚀的涂层应在满足抗热冲击性能的前提下，尽量提高涂层的致密度、厚度，提高涂层的抗冲蚀性能。探明了TaC涂层的氧化界面存在由Ta、O、C组成的多孔氧化过渡层，TaC涂层的超高温抗氧化性能是由氧化性气体在表面的Ta₂O₅的液膜的溶解扩散机制控制，当TaC涂层全部被氧化后，在表面的Ta₂O₅的液膜易与C／C基体发生活化氧化，管蚀效应使活化氧化沿基体内的缝隙向内部快速渗入腐蚀，大大加快了对C／C基体的整体腐蚀和烧蚀。具有双梯度的TaC／SiC多层复合涂层抗热冲击性能和抗氧化性能优异。从热力学和试验两方面证明了TaC、TaC-SiC涂层在烧蚀过程中无活化氧化反应。建立了TaC涂层、TaC—SiC共沉积涂层涂抗氧化、抗烧蚀模型。在小型C／C、石墨喷管基体上成功制备出厚度大、低应力、无裂纹、均匀的TaC、C／TaC、TaC／SiC涂层，为下一步制备高性能喷管的研究与应用打下了坚实的基础。更多还原

【Abstract】 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 patternⅡwas 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.更多还原