Cr2AlC涂层的制备、表征及其性能研究

IMR OpenIR

	Cr2AlC涂层的制备、表征及其性能研究
	李静静
学位类型	博士
导师	李美栓
	2012
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	涂层 Cr2alc 氧化微观结构非晶晶化 Coatings Cr2alc Oxidation Microstrutaure Crystallization
其他摘要	" 施加高温防护涂层是提高合金基体抗高温腐蚀能力的一种有效途径。高温防护涂层良好的高温抗氧化能力主要由其中的Al元素在氧化过程中生成具有保护性的Al2O3氧化膜来实现，而其抗热腐蚀性能则主要依赖于涂层中Cr元素的存在。三元层状陶瓷Cr2AlC，其中既含有Cr元素又含有Al元素。因此，它同时具有良好的抗高温氧化和抗热腐蚀性能。此外，Cr2AlC还具有与高温合金相匹配的热膨胀系数。这意味着当Cr2AlC作为高温合金表面的防护涂层来使用时，匹配的热膨胀系数可以显著降低涂层和基体之间产生的热应力，从而抑制涂层在服役过程中的开裂和剥落。综合以上的特点，Cr2AlC在高温防护涂层方面具有潜在的应用前景。本文采用磁控溅射沉积制备了Cr2AlC涂层；研究了沉积温度对所沉积涂层微观结构的影响；并对不同沉积温度下得到的Cr2AlC涂层的高温抗氧化行为和氧化过程中涂层性能退化进行了研究。最后，为了进一步提高Cr2AlC涂层的沉积速率和致密度，首次尝试了利用多弧离子镀沉积技术和后续热处理来制备Cr2AlC涂层。得出的主要结论如下： 1.) 利用磁控溅射方法以块体Cr2AlC作为靶材分别在370oC和500oC沉积温度下制备出了Cr2AlC涂层。涂层致密、光滑且具有很强的(110)择优取向。实验发现沉积温度对Cr2AlC涂层的微观结构具有显著的影响。370oC下沉积得到的涂层呈现明显的三层结构，分别由α-(Cr,Al)2O3内层，非晶中间层和晶态Cr2AlC层所组成。而500oC下制备的涂层则呈现简单的单层结构，由单相、晶态Cr2AlC组成。对涂层形成机理进行研究发现，370oC下所制备涂层的三层结构的出现主要是由于较低的沉积温度和沉积过程中溅射室内O的污染所导致。 2.）对500oC下制备的单相Cr2AlC涂层在900-1100oC下空气中的抗氧化性能进行了研究。实验发现单相Cr2AlC涂层在900-1100oC空气中氧化时的动力学曲线遵循抛物线规律。在所研究的温度区间内，Cr2AlC涂层的氧化速率小于相应的Ni基高温合金M38G，表明Cr2AlC涂层可以改善M38G高温合金的抗氧化性能。氧化过程中，Cr2AlC涂层表面可形成连续、致密的Al2O3氧化层。Cr2AlC发生Al的选择性氧化，这与其独特的晶格结构和成键特性有关。与此同时，氧化引起的Cr2AlC中Al元素的消耗造成了Cr2AlC的分解和Cr-C的生成。此外，氧化过程中Cr2AlC涂层发生了内氧化和内氮化现象，这主要是由于Cr2AlC涂层中大量存在的柱状晶晶界所造成的。此外，内氧化和内氮化的发生会促进Cr2AlC中Al的消耗，增加涂层的氧化增重，使得实验中所测量到的Cr2AlC涂层的氧化速率高于相应的Cr2AlC块体材料。 3.）对370oC下所制备的三层结构的Cr2AlC涂层在900-1100oC的高温抗氧化性能进行了研究。900-1100oC空气中氧化时三层结构的Cr2AlC涂层的氧化速率要小于相应的M38G高温合金，这意味着在所研究的温度区间三层结构的Cr2AlC涂层可以改善M38G高温合金的高温抗氧化性能。三层结构的Cr2AlC涂层与单层Cr2AlC涂层和块体Cr2AlC的氧化机理相类似。氧化过程中均会发生Al的选择性氧化生成具有保护性的Al2O3氧化膜。而氧化过程中Al元素的消耗则会进一步导致Cr2AlC向Cr-C的相转变。与单相Cr2AlC涂层的氧化结果相类似，三层结构的Cr2AlC涂层的氧化速率要比相应的块体材料高。这主要是由于涂层氧化过程中θ-Al2O3的生成，有限的涂层厚度以及涂层的柱状晶结构所导致的。与单层Cr2AlC涂层不同，三层结构的Cr2AlC涂层中(Cr,Al)2O3内层和中间非晶层的存在、较弱的(110)择优取向和基体的良好的抗氧化性能，使得三层结构的Cr2AlC涂层在氧化过程中Al元素消耗减缓，并抑制了内氮化的发生，进而导致了三层结构的Cr2AlC涂层在1000和1100oC空气中氧化时的氧化速率小于单层结构的Cr2AlC涂层。 4.）370oC下所制备的具有三层结构的Cr2AlC涂层在900-1100oC空气中氧化后，表面发生了非常严重的开裂现象。网状的裂纹穿透整个涂层的厚度形成贯穿性的裂纹。声发射监测发现，涂层的开裂起始于加热阶段，其初始开裂温度约为590oC。拉曼光谱测量的结果表明，Cr2AlC涂层在加热至600oC时会存在一反常的压应力上升。对三层结构的Cr2AlC涂层不同亚层内的热应力状态进行了计算，表明涂层的开裂并非起因于大的热应力。考虑到600oC时非晶层发生晶化会导致该层体积变化，该部分应力远远大于此时存在的热应力。这一结果表明Cr2AlC涂层在氧化过程中开裂主要是由于中间非晶层在氧化过程中晶化产生大的内应力所导致的。而Cr2AlC涂层的初始开裂温度（590oC）也与前人所测量得到的非晶Cr2AlC的晶化温度基本一致，这为非晶层晶化引起的Cr2AlC涂层的开裂提供了另一个佐证。 5.）以反应热压的块体Cr2AlC作为靶材，采用多弧离子镀并结合后续热处理获得了以Cr2AlC为主相的Cr-Al-C涂层。实验发现，采用多弧离子镀在室温下沉积得到的Cr-Al-C涂层为非晶态。与所用Cr2AlC靶材的化学成分相比，Cr-Al-C涂层中存在较多的C缺位，这主要是由于反溅效应所导致的。此外，在沉积态的Cr-Al-C涂层中，还观察到了由于涂层制备过程中样品的旋转所引起的成分调制的纳米片层结构的出现。对沉积得到的Cr-Al-C涂层在620oC退火处理后即可得到晶态的涂层。涂层以Cr2AlC为主相，同时含有AlCr2和少量的Cr7C3。退火处理后的Cr-Al-C涂层中的AlCr2的出现主要是由于Cr-Al-C涂层中的C缺位所导致的。" ; " The application of high temperature protective coatings is an effective way to improve the oxidation and hot corrosion resistance of superalloys. Generally, the oxidation resistance of high temperature protective coatings mainly depends on the presence of Al in the coatings to form an adherent and compact alumina scale, while the incorporation of Cr can promote the formation of Cr2O3 and improve the hot corrosion resistance of the coatings. Ternary layered carbide Cr2AlC contains both of Al and Cr elements. Previous investigations found that this compound has excellent oxidation and hot corrosion resistance. Besides that, Cr2AlC also has matched thermal expansion coefficient to normal alloys, which is beneficial for reducing the thermal stress and then improving cracking and spallation resistance of Cr2AlC as a coating on superalloys. Therefore, Cr2AlC has potential applications as high temperature protective coating. In present work, Cr2AlC coating was preprated using magnetron sputtering method from sintered Cr2AlC target and the effect of the deposition temperature on the microstructure of the as-deposited Cr2AlC coating was investigated. Furthermore, the high temperature oxidation resistance of the Cr2AlC coating deposited at different temperatures was studied. Cracking behavior of the triple-layered Cr2AlC coating, deposited at relatively low temperature, during the oxidation was explained. In order to deposite the dense Cr2AlC coating in the conditions of high deposition rate and room temperature, the composite processes of cathodic arc deposition and subsequent heat treatment was developed. The following conclusions are drawn: 1) Cr2AlC coating was deposited at 370oC and 500oC by D.C. magnetron sputtering method from an as-synthesized bulk Cr2AlC target, and the microstructure of the as-deposited Cr2AlC coating was investigated. The coating deposited at 370 and 500oC have strong (110) preferential orientation. The microstructures of the coatings deposited at different temperatures are distinct. The coating deposited at 370oC had a triple-layered structure, including an α-(Cr,Al)2O3 inner layer, an amorphous intermediate layer and a crystalline Cr2AlC outer layer. Whereas, the coating, deposited at 500oC, revealed a simple single-layered crystalline Cr2AlC structure. It is suggested that low deposition temperature and oxygen contamination contributed to the formation of the triple-layered structure. 2) High temperature oxidation behavior of single-layered Cr2AlC coating was investigated. It was found that the oxidation kinetics of the Cr2AlC coating follows parabolic law at 900-1100oC in air. The oxidation rate of the Cr2AlC coating is lower than that of the Ni-base alloy M38G, meaning that the Cr2AlC coating can improve the oxidation resistance of the M38G alloy. The excellent oxidation resistance of the Cr2AlC coating could be attributed to the formation of the Al2O3 exclusive scale. And the oxidation induced depletion of Al within the Cr2AlC coating results in the transformation of Cr2AlC to Cr-C phases. Furthermore, internal oxidation and internal nitridation happened in the as-deposited Cr2AlC coating during oxidation in air. The main reason may be that a great number of columnar grain boundaries in the Cr2AlC coating provide short paths for inward diffusion of O and N. The internal oxidation and internal nitridation cause the protectiveness degradation of the Cr2AlC coating due to the additional consumption of Al in the coating interior, which also leads to the oxidation rate of the Cr2AlC coating higher than that of the bulk Cr2AlC. 3) The oxidation behavior of the triple-layered Cr2AlC coating was investigated at 900-1100oC. During the oxidation, the oxidation rate of the Cr2AlC coating was smaller than that of the M38G alloy, indicating that the triple-layered Cr2AlC coating can improve the oxidation resistance of the substrate. Similar to the bulk Cr2AlC and single-layered Cr2AlC coating, Al preferntial oxidation happened during the oxidation of the triple-layered Cr2AlC coating, which promoted the formation of Al2O3 oxide scale. However, the oxidation resistance of the triple-layered Cr2AlC coating was worse than that of the bulk Cr2AlC, which could be due to the formation of θ-Al2O3 in the oxide scale, the limted coating thichness and the columnar structure of the triple-layered Cr2AlC coating. Furthermore, different to the single-layered Cr2AlC coating, the absence of internal nitridation was found in the oxidized triple-layered Cr2AlC coating, which could be due to the existence of (Cr,Al)2O3 inner layer and the amorhous intermediate layer, and the relatively weak (110) prefertial orientation 4) Serious through-thickness cracking of the triple-layered Cr2AlC coating was found after oxidation tests. Acoustic emission (AE) tests indicated that the cracking of the coating happened during heating. Raman spectroscopy was used to determine the internal stress evolution of the Cr2AlC outer layer with heating temperature. An abnormal compressive stress increase was detected in the crystalline Cr2AlC outer layer at about 600oC, which corresponds to the crystallization temperature of amorphous Cr2AlC. The calculations of the internal stress in this triple-layered coating during heating showed that a rapid increase of the internal stress in the amorphous intermediate layer happened due to its crystallization at around 590oC, which is the main reason for cracking of the coating. This work was instructive for the application of the triple-layered Cr2AlC coating in high temperature environment. The formation of the amorphous phase should be avoided in the as-deposited Cr2AlC coating. 5) The dense Cr-Al-C coating was prepared through the composite processes of cathodic arc deposition and subsequent heat treatment. It was found that the coating deposited by using cathodic arc deposition at RT from Cr2AlC compound target was amorphous. Compared with the chemical composition of the Cr2AlC target, C-deficiency was detected in the as-deposited coating, which could be due to the re-sputtering effect. Besides, nano-layering structure of the as-deposited coating was introduced by substrate rotation during the coating preparation. After annealing treatment at 620oC in Ar for 20 h, the amorphous Cr-Al-C coating happened to crystallize. Finally, the coating with Cr2AlC as the major phase combining with the co-existence of AlCr2 and Cr7C3 was obtained. Obviously, the homogeneous mixture of the Cr, Al and C at atomic level in the as-deposited Cr-Al-C coating could promote the formation of Cr2AlC. As a result, the synthesis temperature of Cr2AlC was greatly decreased from 1050oC for the sintered bulk to 620oC for the deposited coating. And the C deficiency in the as-deposited coating results in the formation of the AlCr2 in the annealed coating."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64453
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	李静静. Cr2AlC涂层的制备、表征及其性能研究[D]. 北京. 中国科学院金属研究所,2012.