复合溶胶凝胶法制备TiAl表面防护涂层及其抗氧化性能研究

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	复合溶胶凝胶法制备TiAl表面防护涂层及其抗氧化性能研究
	田晓
学位类型	硕士
导师	杨锐 ; 崔玉友
	2010
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	复合溶胶-凝胶法 Tial合金 Y溶胶高温氧化
摘要	"TiAl合金由于其具有低密度、高的比强度和比模量以及优异的抗高温蠕变性能，使其成为新一代航空航天高温轻质结构材料。但在800oC以上长期使用时TiAl合金出现抗氧化性能严重不足，成为制约其实际应用的瓶颈。如何提高TiAl合金的高温抗氧化性能成为目前研究是热点问题之一。本文采用复合溶胶-凝胶法在TiAl合金表面制备一种复合陶瓷涂层，研究了粘结剂Y溶胶的胶凝特性和热性能、溶胶成分配比、涂层热处理工艺等因素对涂层的影响；通过等温氧化和循环氧化实验来评价涂层对TiAl合金高温氧化性能的影响，并对涂层的高温抗氧化机理进行了初步讨。粘结剂Y溶胶的胶凝特性和热分解机理对确定涂层的最佳涂覆时机、制定合理的涂层热处理制度至关重要。实验表明，Y溶胶的胶凝时间随加水量的增加呈指数递减的规律迅速缩短，对于复合溶胶-凝胶法制备涂层，应选择稳定性在2-5天的溶胶比较合适。通过TG-DSC-MS、XRD、FTIR分析了Y溶胶转变为凝胶后在加热过程中发生的物理化学变化。结果表明：115oC强烈吸热峰对应于凝胶中大量的有机溶剂和自由水逸出；180-280oC为凝胶分子间进行缓慢的缩合反应并脱去结合水；280-600oC之间有两个放热峰，其中380oC放热峰是由羧酸盐分解为碱式碳酸盐引起的，440oC放热峰是由碳酸盐分解并开始析出Y2O3微晶引起的；600oC时Y凝胶已分解完全并转化为Y2O3。根据Y溶胶的热分解过程即可制定合理的涂层热处理制度，并为涂层热处理工艺的优化提供理论指导。制备高质量的溶胶是复合溶胶-凝胶法制备理想涂层的重要前提。本文通过设计正交实验考察了Y3+ 浓度、加水量、二乙烯三胺含量和甲酰胺含量对Y溶胶性能的影响，通过最终涂层的表面形貌评价溶胶性能的优劣。结果表明，正交5#溶胶的成分配比最为合适，能制备出无裂纹的涂层。此外，溶胶的陈化时间对涂层形貌有一定影响，实验发现正交5#溶胶陈化22 h后涂覆能够制备出均匀致密的涂层，且多次涂覆得到的涂层更加致密。合理的热处理制度是复合溶胶-凝胶法制备理想涂层的关键技术，尤其是升温速率直接关系着涂层的质量。实验发现，在室温至280oC之间采用0.1oC/min升温速率能有效地避免涂层在此阶段的开裂，280oC以上即使以0.3oC/min升温速率涂层也不发生开裂。涂层经高温Ar气热处理后表面致密均匀，单层厚度大约10μm，且与基体之间结合良好；涂层主要由Y3Al5O12、TiN、少量α-Al2O3和Ti2AlN四相组成。对无涂层TiAl样品和涂层样品进行1000oC等温氧化和循环氧化实验，来考察涂层对TiAl合金高温氧化性能的影响。结果表明，涂层对TiAl基体具有良好的高温防护作用，能显著降低TiAl合金的高温氧化速率。涂层氧化产物Y3Al5O12和Y2(Ti2O7)的热膨胀系数与TiAl基体非常接近，大大提高了涂层和基体之间的相容性，从而避免了涂层在氧化过程中的剥落。通过BSE/EDS分析，发现在涂层与基体界面处形成一层致密的Al2O3保护膜，有效地阻挡了氧扩散到合金基体引起的高温氧化。"
其他摘要	"TiAl-based alloys become new type high-temperature structural materials in aerospace and aircraft industry because of their excellent mechanical properties such as low density, high specific modulus and strength and good creep resistance. With the solution of their poor ductility at room temperature by alloying and microstructure controlling, it is the main obstacle to the practical application of TiAl alloys due to their insufficient oxidation resistance above 800oC. Thus how to improve their high-temperature oxidation resistance is one of the hot issues in the current study. In this paper, the protection coating on TiAl alloys was prepared by composite sol-gel method composted of Y sol and TiAl3/TiN nano-powders. The gelling characteristics and thermal decomposition properties of Y sol were investigated in order to provide scientific guidance for dipping time and heat-treatment. Also, the different ingredient proportion of Y sol and heating rate were studied how to influence the surface morphology of coatings. The effect of coatings on the high-temperature oxidation behavior and the oxidation mechanism of TiAl alloys were discussed on the base of high temperature oxidation kinetics, oxides morphology and microstructure, etc. It was found that the gelation time of Y sol decreased exponentially with the increasing of water content from the gelation experiment. Thus, the stability of Y sol between 2 to 5 days should be chosen for the preparation of coatings by composite sol-gel. The physical and chemical changes of Y sol were investigated during heating process by TG-DSC-MS, XRD and FTIR. The results show that Y gel loses lots of organic solvents and free water at 115 oC. The hydroxyl condensation between the molecules takes place slowly from 180 to 280 °C. DSC curve of Y gel has two exothermic peaks in the range of 280-600 oC, which are contributed to the transformation from carboxylate to subcarbonate at 380 oC and the formation of Y2O3 crystallite from the decomposition products of subcarbonate at 440 oC. Y gel completely transforms into α-Y2O3 at 600 oC. The reasonable heat-treatment can be drawn based on the thermal decomposition process of Y gel. The influence of Y3+ concentration, the content of water, DETA and formamide on the properties of Y sol was investigated by orthogonal experiments. It was shown that orthogonal 5# ingredient proportion was most suitable which could prepare compact coatings. Besides, the aging time of Y sol had an effect of the surface morphology of coatings to some extent. The coatings prepared after aging 22 h were uniform and compact, especially for multilayer. The reasonable heat-treatment is the key to the preparation of coatings by composite sol-gel method, above all, the heating rate. It was found that 0.1oC/min heating rate can effectively avoid the cracking of coatings below 280 oC, and the coating was non-crack even if the heating rate was 0.3 oC/min. The single coating of 10 μm thickness was composed of Y3Al5O12, TiN, α-Al2O3 and Ti2AlN and formed a good combination with the substrate. The oxidation kinetic curves of uncoated and coated samples at 1000 oC showed that the coating can obviously decrease the oxidation rate of TiAl alloys. The coating did not crack and spall during oxidation due to the thermal expansive coefficient of Y3Al5O12 and Y2 (Ti2O7) similar to TiAl substrate. It was found by SEM/EDS analysis that a compact Al2O3 film was formed at substrate/coating interface during oxidation and effectively prevented oxygen diffusion to TiAl substrate."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64222
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	田晓. 复合溶胶凝胶法制备TiAl表面防护涂层及其抗氧化性能研究[D]. 北京. 中国科学院金属研究所,2010.