【作者】 王璟；

【导师】 张长瑞； 白书欣；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2009， 博士

【摘要】 热障涂层具有良好的隔热和抗氧化效果,是目前最为先进的高温防护涂层之一,广泛应用在航空、航天、汽车和大型火力发电等行业。目前最常用的热障涂层材料是8mol.%Y2O3-ZrO2（8YSZ）,但这种材料的长期使用温度低于1200℃,已经不能满足未来技术发展的需要。研究能替代YSZ用在更高温度下的热障涂层材料是今后工作的重点。锆酸镧（La2Zr2O7,LZ）由于具有熔点高、相结构稳定、导热系数低等特点而被认为是一种非常有潜力的新型高温热障涂层材料。本文从LZ粉末制备入手,通过离子掺杂改性、造粒和大气等离子喷涂（APS）技术在Ni基合金表面制备出新型锆酸镧基热障涂层,在此基础上,首次深入系统地研究了涂层在1250℃下的抗烧结、抗氧化和抗热震性能。文章最后对锆酸镧基/8YSZ双陶瓷层热障涂层及Mo基体上锆酸镧基热障涂层的性能和失效机制进行了探索性研究。主要内容和结论如下:采用化学共沉淀法,分别利用氨水和草酸铵作为沉淀剂制备了LZ粉末,研究了沉淀剂对反应条件、过程、产物成分及形貌的影响。结果表明,与草酸铵相比,利用氨水作沉淀剂,虽然抽滤和洗涤效率较低,但可以在更低温度（1200℃）下合成出成分控制更精确、均匀度更高的单相烧绿石结构LZ。利用稀土离子掺杂对LZ粉末进行改性。首次考察了Nd³⁺、Ce⁴⁺复合掺杂对LZ热膨胀系数和烧结行为的影响。发现适量稀土元素共掺杂可以显著提高LZ粉末的热膨胀系数,并改善其抗烧结性能。其中La1.6Nd0.4Ce1.0Zr1.0O7（LNCZ）的热膨胀系数在25¹200℃可达到10.4×10^-6/K,高于目前热障涂层中应用最广泛的8YSZ陶瓷的热膨胀系数,与Ni基合金的热膨胀行为更匹配。通过喷雾造粒和1200℃热处理得到了粒径和流动性能均满足等离子喷涂要求的LNCZ造粒粉体。利用该粉体采用APS法在Ni基合金表面制备了LNCZ热障涂层,通过考察喷涂工艺与涂层结构及性能的关系,得到了优化的工艺参数:功率40kW,喷涂距离9cm,送粉速率12g/min。高温热处理能显著改善涂层的结合强度和抗热震性能,1200℃氩气气氛下保温2小时后,涂层的结合强度从1.326MPa提高到7.048MPa,1250℃下热震寿命从15次提高到65次（涂层剥落50%）。研究了LNCZ陶瓷涂层的高温烧结行为及其对涂层显微结构和热、力学性能的影响。发现LNCZ涂层在1250℃会发生烧结,但孔隙率反而从11.35%增至15%,片层层间裂缝、垂直方向微裂纹的粗大化以及三维大空洞的不断增多是孔隙率增大的主要原因。受孔隙率的影响,保温5h后,涂层的导热系数从0.88W/（m·K）减小至0.75W/（m·K）,硬度从1.97GPa增至2.73GPa。重点考察了LNCZ热障涂层在1250℃下氧化和热震过程中的失效行为,通过裂纹分析手段和对粘结层氧化产物（TGO）内部元素扩散行为的研究,结合文献报道的涂层热疲劳失效模型理论,得出了涂层在1250℃下的氧化和失效机制。结果表明两种考核方式下涂层的失效部位一样,都发生在LNCZ层、LNCZ/TGO界面反应层以及TGO层的内部,但是失效形式和失效机制有所不同。LNCZ热障涂层的氧化失效形式是边缘分层,涂层的失效属于单源失效,裂纹源主要是由烧结应力和热失配应力诱发的位于陶瓷层内部靠近粘结层凸起的平行Ⅰ型裂纹。该裂纹在随后的氧化过程中,受TGO生长所引入的热生长应力和烧结应力共同作用,不断粗化、扩展直至涂层失稳破坏;促使裂纹失稳扩展的原因是TGO生成以及TGO与LNCZ反应在涂层内部引入的大量空洞、应力和疏松氧化产物的出现。LNCZ热障涂层的热震失效形式是开裂和皱曲分层,涂层的失效属于多源失效,裂纹源包括:热喷涂残余应力诱发的涂层内部垂直开口Ⅰ型裂纹,热失配应力诱发的片层间平行Ⅰ型裂纹、位于TGO余弦型界面中部偏上位置处的Ⅱ型裂纹和LNCZ/TGO界面波峰位置处的平行Ⅰ型裂纹,以及随TGO厚度的增加,由热失配应力分布发生变化而诱发的位于陶瓷层内部波谷位置的Ⅰ型裂纹。上述裂纹在热失配应力和热生长应力的作用下,沿缺陷较多的LNCZ/TGO界面或者穿过疏松的TGO内部发生扩展、相连,多处微裂纹的贯通最终导致陶瓷层的剥落失效。陶瓷层材料的低断裂韧性、TGO外侧NiO和（Cr,Al）2NiO4等脆性且不致密氧化物的过早生成以及TGO与LNCZ之间的化学不稳定性是导致涂层快速失效的主要原因。为缓解LNCZ涂层内部的应力积累并阻止LNCZ与TGO的化学反应,设计并制备了LNCZ/8YSZ双陶瓷层热障涂层（DCL-TBCs）。该涂层与相同厚度LNCZ热障涂层相比,隔热性能略有降低,但抗热震性能明显提高,1250℃下热震45次后涂层表面仅剥落5%。涂层皱曲剥落扩展和剥落加深是LNCZ/8YSZ TBCs失效的主要形式,导致LNCZ层和8YSZ涂层先后剥落的原因分别是热失配应力和热生长应力。首次在Mo基体上制备了以Mo与La_1.4Nd_0.6Zr₂O₇（LNZ）的混合物（ML）为粘结层、以LNZ为陶瓷层的热障涂层。该涂层具有良好的结合强度,但是1200℃下的热震寿命非常短。Mo在高温下的氧化以及氧化产物与涂层极差的化学相容性是导致LNZ热障涂层快速失效的主要原因。避免Mo基体氧化或者氧化产物与锆酸镧接触是保证锆酸镧热障涂层在高温环境下安全使用的必要条件。更多还原

【Abstract】 Thermal barrier coatings （TBCs） are one of the most advanced high temperature protective coatings and being wildly used in aeronautic, astronautics, motor industry and heat power station, for their good performance at thermal barrier and oxidation resistance. 8 mol. % yttria partially stabilized zirconia （8YSZ） is currently used as the commercial materials of TBCs. However, the major disadvantage of YSZ is the limited operation temperature of 1200oC for the long-term application. And the search for new candidate materials that can work at even higher temperature has been intensified in future. Among the interesting candidates for TBCs, lanthanum zirconate （La2Zr2O7, LZ） has been proposed as a promising TBCs material for its high melting point, more stable structure and lower thermal conductivity than YSZ. In this work, the preparetion of LZ powder was studied. Different rare earth elements doped LZ powders were researched and LZ-based powders with higher thermal expansion coefficient and better sintering resistance were obtained. Then the powders with good spraying performance were prepared by spray pelletization. And the LZ-based TBCs on Ni super alloy with high bonding strength and thermal shock resistance were fabricated by Air Plasma Spraying （APS）. On this basis, the sintering behavior, oxidation resistance and thermal shock resistance of TBCs at 1250 oC were studied in detail for the first time. Besides, the LZ-based/YSZ double ceramic layer （DCL） TBCs deposited on Ni super alloy substrate and LZ-based TBCs deposited on Mo substrate were prepared and their failure mechanism were researched primarily. Details are as follow:The single-phase La2Zr2O7 powders were synthesized by co-precipitation method using ammonia and oxalate ammonium as precipitators, respectively. And the synthesis procedure, calcination temperature, compositon and morphology of products were studied. The results show that, compared with oxalate ammonium, co-precipitation using ammonia is prone to obtain better composition controllable and more homogenous La2Zr2O7 powders with pyrochlore structure at lower temperature （1200oC）, despite with lower filtering efficiency.The properties of LZ were modified by rare earth elements adoption. The thermal expansion coefficient （TEC） and sintering behavior of Nd³⁺, Ce⁴⁺ doped LZ powder were investigated in this work. It is observed that the proper addition of Nd and Ce into LZ can largely increase its TEC and improve the sintering-resistance. The TEC of La1.6Nd0.4Ce1.0Zr1.0O7 （LNCZ） is 10.4×10^–6/K at 25¹200oC, which is higher than that of commercial 8YSZ and approaches that of Ni super alloy.The LNCZ powder with good spraying performance was obtained by spray pelletization and being heat treatment at 1200oC. Using this kind of powder, APS LNCZ TBCs were fabricated on the surface of Ni super alloy. The effects of spraying parameters on the microstructure and properties of APS LNCZ TBCs were examined and the optimized technological conditions are given out: power is 40kW, stand off distance is 9cm and powder feeding rate is 12g/min. APS LNCZ TBCs with post argon shield heat treatment were employed to improve their properties. According to the experimental results, post argon shield heat treatment could improve the average bonding strength and thermal shock resistance of coatings. The bonding strength value of LNCZ TBCs increases from 1.326MPa to 7.048MPa, and the thermal shock life increases from 15 to 65 cycles （while 50% of the ceramic coat was delaminated） at the same time, after being heated at 1200℃for 2h.The sintering behavior of LNCZ coat and its effects on the microstructure, thermophysical and mechanical properties of coat were investigated. The porosity of coat increases from 11.35% to 15% after being calcined at 1250℃for 5h. The increasing of intersplat gaps, intrasplat cracks and three-dimensional coarse pore may be the reasons. Thermal conductivity and hardness are more sensitive to the changes in microstructure. After an exposure to a temperature of 1250℃for 5h, the coat’s thermal conductive decreases from 0.88W/（m·K） to 0.75W/（m·K）, while the hardness increases from 1.97GPa to 2.73GPa.The oxidation and thermal shock behavior of LNCZ TBCs at 1250oC were studied in detail. The failrue mechanisms in LNCZ TBCs were discussed by cracks analysis and the investigation of element’s diffusion, combined with the theory of failure on thermal fatigue of PS TBCs as reported. The analysis indicates, during isothermal oxidation and thermal cycling, the breakage all occurs at the LNCZ coat interior primarily with a part of the interface between LNCZ and the thermally grown oxidation （TGO） and the TGO interior. But the failure mode and mechanism during these two processes are different.The failure mode of LNCZ during isothermal oxidation is edge-delamination. Its failure is dominated by one type of crack——parallel mode I crack which is initiated in LNCZ coat near the crests of bond coat undulations by tensile sintering stress and residual stress. The main cause of crack propagation and thickening is the sintering stress and the thermal growth stress caused by TGO product between bond coat and LNCZ coat. The reaction between TGO and LNCZ results in the loose microstructure near the interface of TGO/LNCZ and compressed stress in LNCZ coat. The parallel crack fast propagates along the weak area and induces the ultimate failure of the TBCs.The failure mode of LNCZ TBCs during thermal cycling is cracking and bucking- delamination. Its failure is dominated by two types of cracks——mode I crack and modeⅡcrack, including the vertical mode I crack initiated in the LNCZ coat by residual stress, the parallel mode I crack between the ceramic splat, the mode I crack along the LNCZ/TGO interface at undulation crests, modeⅡcrack within the LNCZ coat in the troughs adjacent to TGO and mode I crack within the LNCZ coat in the troughs of undulations which are induced by the thermal mismatch stress dues to the differences in thermal expansion between LNCZ coat and substrate. As the TGO being thicken, the thermal mismatch stress increases. The increased thermal mismatch stress and thermal mismatch stress can accelerate the propagation of all cracks along the weak area, such as the interface of LNCZ/TGO or TGO interior. The coalescence of transverse cracks induces the ultimate failure of the TBCs.The reasons for fast failure of LNCZ TBCs include the lower fracture toughness of LNCZ, rapid formation of brickle and porous oxides, such as NiO and （Cr,Al）2NiO4, and worse thermochemical compatibility between LNCZ and TGO. In order to release the thermal stress between bond coat and LNCZ coat and prevent the reaction between LNCZ and TGO, LNCZ/8YSZ DCL-TBCs were designed and fabricated. After being combined with 8YSZ, the thermal barrier ability of TBCs degrades slightly but its thermal shock resistance has been improved significantly. Just only 5% of the ceramic coat was delaminated after thermal shock for 45 cycles under 1250oC in an air furnace. The failure mode of DCL-TBCs during thermal cycling is bucking-delamination near the interface of LNCZ/8YSZ and 8YSZ/TGO by sequence. The failure mechanisms of LNCZ coat and 8YSZ coat are in the light of thermal mismatch strain and thermal growth stress, respectively.La_1.4Nd_0.6Zr₂O₇ （LNZ） TBCs on Mo substrate with high bonding strength was prepared for the first time. The mixture of Mo and LNZ powder was used as bond coat materials. The behaviors of LNZ TBCs during thermal cycling under 1200oC were examined. The result shows that the thermal shock life is very short. The reaction between LNZ and MoO3 from the oxidation of Mo results in the failure of LNZ TBCs on Mo substrate. Avoiding the oxidation of Mo or the osculation between molybdenum oxide and ceramic coat is favorable for the integrality of LZ-based TBCs on Mo substrate.更多还原