【作者】 周洪；

【导师】 孙宝德； 王俊；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2008， 博士

【摘要】 钛合金是重要的结构材料和功能材料,具有低密度、高比强度、较宽的工作温度范围和优异的耐腐蚀性能等特性。受高温氧化的影响,钛合金最高使用温度范围只有600℃左右。热障涂层是先进的高温防护涂层,在航空航天、轮船、能源等领域有着广泛的研究与应用背景。在钛合金表面制备热障涂层,不仅可以使零件轻型化,而且可以通过热障涂层的有效隔热和抗氧化保护,在高温下阻止钛合金表面的氧化和氧、氮元素向钛合金内部的扩散,从而提高钛合金抗氧化能力,阻挡高温侵袭,显著提高钛合金短期的使用温度和组织稳定性。本论文从这一角度出发,针对钛合金表面纳米热障涂层的制备和组织性能进行了系统的实验研究与分析,并以常规热障涂层为参照进行了对比。对等离子喷涂后纳米热障涂层系统的微观组织与性能、钛合金表面纳米热障涂层的隔热效果,热震失效形式与机制,残余应力分布以及激光重熔技术对纳米陶瓷层的影响等方面进行了深入的研究与分析。采用大气等离子喷涂技术在TC11钛合金表面制备了纳米热障涂层,并对其微观组织和性能进行了表征。研究表明:等离子喷涂后,TC11钛合金内部组织没有产生明显变化。基体/粘结层/陶瓷层之间主要以机械物理形式结合。与常规陶瓷层相比,纳米陶瓷层的层片较薄,层片间结合紧密。纳米陶瓷层中的纳米结构来自未熔的纳米喷涂粉末以及熔融颗粒快速冷却重结晶析出的纳米柱状晶。纳米和常规陶瓷层均表现出双态孔隙大小分布,纳米陶瓷层的孔隙率较低,并且孔隙更加细小,分布均匀。力学性能上,等离子喷涂陶瓷层表现出明显的各向异性,截面显微硬度平均值高于表面;纳米陶瓷层的显微硬度高于常规陶瓷层,两者截面的Weibull分布均呈现双态分布,纳米陶瓷层中显微硬度的分散性低于常规陶瓷层。钛合金表面纳米热障涂层结合强度高于常规涂层。粘结层与钛合金基体的结合强度好于陶瓷层与粘结层之间的结合。对等离子喷涂纳米陶瓷层的热物理性能和隔热效果进行了测试与分析。常规陶瓷层的热膨胀系数约为10.4×10-6 K-1;纳米陶瓷层的热膨胀系数约为11.3×10-6 K-1,提高了约9%。纳米陶瓷层的导热系数低于常规陶瓷层。两者导热系数均随着温度的上升,略有升高。隔热实验表明:各试样均在约180 s达到了稳定热流状态;纳米涂层试样表现出良好的隔热性能,其最大的隔热效果达到了约130℃;而常规涂层试样的最大隔热效果不到80℃。隔热效果与陶瓷层厚度基本呈线性比例增长关系。基于力平衡、弯矩平衡和梁的弯曲理论,采用逐层模式对钛合金表面等离子喷涂后的热障涂层系统残余应力进行了分析,并与实际测试结果相比较。综合考虑淬火效应与热物理性能失配导致残余应力的计算结果更接近测试值。热失配产生的压应力在陶瓷层残余应力中占优势地位;而淬火拉应力则由于陶瓷层中微裂纹的产生得到了有效的释放。相对于常规陶瓷层,纳米陶瓷层具有更低的残余压应力;陶瓷涂层表面的残余压应力绝对值随着厚度的减薄而增加。分析了钛合金表面纳米热障涂层的热震失效形式与机制。钛合金表面纳米热障涂层的失效以陶瓷层表面出现剥落或大块凸起为特征,薄弱环节存在于粘结层与陶瓷层界面。随着热震循环的进行,在粘结层界面的钛合金基体也会因热膨胀系数失配而形成孔洞等缺陷,降低了界面强度。纳米涂层试样的抗热震性优于常规涂层试样。热震循环过程中,因热膨胀系数失配首先在陶瓷层/粘结层界面形成垂直界面的内部微裂纹。随着热震的进行,垂直裂纹不断扩展,粘结层的波动振幅逐渐加剧,在陶瓷层/粘结层界面附近形成水平微裂纹。陶瓷层内靠近粘结层界面的水平微裂纹扩展与合并,并沿着平行于陶瓷层/粘结层界面在陶瓷层中传播,最终导致了陶瓷层的剥落失效。随着热震温度的升高,常规和纳米涂层试样的热震循环次数显著减少;不同热震温度下,纳米涂层试样的热震循环次数始终高于常规涂层试样;钛合金表面热障涂层的热震寿命与陶瓷层厚度呈负向变化。热震后,钛合金基体与粘结层界面形成扩散反应层。扩散层主要由Ti和Ni元素组成。由于Ni、Ti元素的扩散与反应,从与钛合金基体的接触界面开始,扩散层中依次生成了Ti2Ni,TiNi,AlNi2Ti和Ni3(Al,Ti)等新相。对等离子喷涂纳米陶瓷层表面进行了激光重熔以及激光重熔试样的抗热震性能研究。研究表明:激光重熔后,陶瓷层表面形成网状微裂纹与凹坑,表面粗糙度明显下降。随着激光扫描速度的减小,陶瓷层的网状微裂纹分布逐渐稀疏,龟裂程度逐渐变大,表面凹坑呈增加和增大的趋势,表面粗糙度上升。激光重熔区域中心处熔深随着激光平均能量的增加而近似线性比例增加,熔宽也随着激光平均能量的增加而增加,在相同工艺条件下,纳米陶瓷层的熔深和熔宽始终大于常规陶瓷层。陶瓷层重熔区形成致密的柱状晶组织,消除了等离子喷涂层中常见的孔隙、未熔颗粒等缺陷。与常规陶瓷层比较,纳米陶瓷层中柱状晶粒直径稍大。激光工艺参数对重熔层组织形貌影响较为显著。扫描速度高时,陶瓷层重熔组织为单一的柱状晶结构;扫描速度降低后,则得到表面等轴晶+柱状晶双层结构的重熔层。激光重熔导致纳米和常规陶瓷层内出现分割裂纹。分割裂纹始于重熔层,向涂层内部延伸,但并没有全部贯穿整个陶瓷涂层。随着激光扫描速度的增加,陶瓷涂层内分割裂纹密度近似线性比例提高。激光重熔后的纳米陶瓷层更易获得垂直表面的分割裂纹,其密度高于常规陶瓷层。激光重熔对纳米试样热震性能的影响是双重的。在较高扫描速度下,激光重熔试样的热震寿命高于等离子喷涂态涂层试样;但在较低扫描速度下,激光重熔试样的热震寿命反而低于等离子喷涂态试样。激光重熔纳米试样的热震次数随着扫描速度的降低而降低。更多还原

【Abstract】 Titanium and its alloys play important roles in new structural materials. The applications of titanium are mainly focused on its high specific strength at room and elevated temperatures, unique corrosion resistance and nonmagnetic properties. This fact has predetermined the wide use of its alloys in aircraft and space industries. However, poor oxidation resistance and oxygen induced embrittlement deteriorate the application of titanium alloy at high temperatures. This poor oxidation resistance results from the formation of a non-protective oxide scale consisting of a heterogeneous mixture of alumina and titania on high temperature exposure. Deposition of protective and thermally insulating coatings is considered as an effective means to reduce the substrate temperature and suppress both oxidation and oxygen induced embrittlement.Thermal barrier coatings （TBCs） have been widely used to provide thermal protection to metallic components from the hot gas stream in gas-turbine engines due to their low thermal conductivity and thermal diffusivity combined with proper chemical stability at high temperatures. The TBC system allows conventional metals to be reliably used at high temperatures because the ceramic layer provides thermal stability to the base metal due to insulation from the heat, while the metallic bond coat provides oxidation resistance, and sufficient toughness. Consequently, plasma spraying thermal barrier coatings on titanium alloy substrates is a shortcut to improve the short-term properties of the titanium alloy at high temperatures.Based on this, a systematic study of nanostructured TBCs on titanium alloy has been carried out. The characterization of nanostructured TBC, the mechanical and thermophysical properties of nanostructured TBCs, heat insulating effect, residual stress in the coating system after plasma spraying, thermal shock behavior and failure mechanism, and surface lazer-glazing have been investigated.Nanostructured TBCs were applied on the titanium alloy by air plasma spraying. Conventional counterparts which were used for comparison were fabricated as well. The microstructure of the titanium alloy inside the substrate keeps unchanged after plasma spraying. Neither interaction nor atomic diffusion evidently takes place at the bond coat/substrate interface. However, there exists a thin layer of plastic deformation zone in the substrate beneath the bond coat/substrate interface and its thickness is non-uniform. The as-sprayed ceramic coatings show a typical lamellar structure in an overlapping and interlocking fashion. Compared to the conventional ceramic coatings, the nanostructured ones have thinner laminas, and which are banded together more tightly. The nanostructure in the ceramic coating is originated from the unmelted powders and the nanostructured columnar grains derived from the melted powders after cooling rapidly. The surface connected porosities in both conventional and nanostructured ceramic coatings present a typical bimodal pore size distribution. The porosity in the nanostructured coating has a lower value than the conventional one. The micropores in the nanostructured coating are smaller and distributed evenly as well. Micro-mechanical property results show that anisotropy in microhardness between the cross section and the top surface of the ceramic coating is examined because of the lamellar structure. The nanostructured ceramic coatings have higher microhardness and adhesive strength compared to the conventional one. Both coatings demonstrate a bimodal distribution of microhardness values, as evidenced by their weibull plots. The nanostructured coating presents a better adhesive strength than the conventional one.Thermophysical properties and heat insulating test are measured. Coefficient of thermal expansion （CTE） for the conventional ceramic coating is 10.4×10^-6 K^-1, while it is 11.3^-6 K^-1 for the nanostructured one. The conventional ceramic coating has the higher thermal conductivity. In general, the thermal conductivities of all the coatings increase slightly with increasing temperatures. The heat insulation test results reveal that all samples reach a state of thermal stability within 180 s, and the nanostructured TBC with smaller micropores presents a better heat insulation effect with a temperature drop of about 130℃. The heat insulation is proportional to the TBC’s thickness.An analytical model of distribution of thermal stress in TBCs on the substrate of titanium alloys has been derived based on force, moment balances and classical beam bending theory. The quenching stresses set up due to the progressive deposition process are considered, followed by those due to CTE mismatch during final cooling to room temperature. The calculated values are compared with the measured residual stresses at the ceramic surface, which shows a reasonable agreement. The stresses due to the mismatch of CTE are dominant since the quenching stresses are released largely by generating microcracks. The nanostructured ceramic coatings present lower residual stresses than the conventional counterparts. The absolute values of residual stress at the ceramic surface are increased with the decrease of ceramic coating thickness.The behaviors of the resistance of TBCs on titanium alloy to thermal shock have been studied, and the corresponding failure modes have been put forward. The failure of TBCs on titanium alloy is characterized by spallation and delamination at the ceramic coatings. The weak link exist at the ceramic coating/bond coat interface. During thermal shock cycles, some pores also appear at the substrate/bond coat interface due to the mismatch of CTE. The vertical microcracks are formed firstly near the bond coat, and then propagating in the ceramic coatings. During thermal shock cycling, the undulation of bond coat get amplified, and the horizontal microcracks are generated at the ceramic coating/bond coat interface as well. The horizontal microcracks propagate parallel to the interface in the ceramic coating, and coalesce, which result in the failure of TBCs on titanium alloy. The nanostructured samples have exhibited promising lifetime than the conventional ones. With the increase of temperature, the numbers of thermal shock cycling for both kinds of sample decrease obviously. The lifetime of samples is proportional adversely to the ceramic coating thickness during thermal shock test. After thermal shock test, a diffusion area is developed near the bond coat/substrate interface in the substrate. The diffusion area is concentrated of Ni and Ti elements. Because of the reaction of Ni and Ti, new compounds such as Ti2Ni,TiNi,AlNi2Ti, and Ni3（Al,Ti） have been come into being along the thickness of the diffusion area.Plasma-sprayed conventional and nanostructured ceramic coatings have been subjected to laser-glazing processes which provide a remelting and subsequent solidification of the surface. The results revealed that laser-glazing carried out on plasma-sprayed zirconia coatings has brought in a smooth and dense glazed surface with craters and a network of microcracks. The microcrack network in the conventional coating is sparser than that in the nanostructured coating. For both kinds of ceramic coatings, with the decrease of laser scanning speeds, the microcrack networks turn sparser, and the craters at the surfaces increase. A reduction on surface roughness is achieved after laser processing. The surface roughness increases with the decrease of laser scanning speed for both kinds of coatings. The nanostructured coatings have a deeper and wider molten region than the conventional coatings at the same laser processing conditions. For both kinds of ceramic coatings, the track depths increase with increasing the mean laser energy density in an approximately linearly proportional way; the track widths increase with the increase of mean laser energy density as well. The laser-glazed regions consist of a columnar microstructure. The columnar grain sizes in the nanostructured coating are bigger slightly than that in the conventional coating. With the decrease of scanning speed, for both kinds of coatings, the columnar grain sizes increase, and pores within the column grains is diminished. There are segmentation microcracks in the laser-glazed ceramic coatings. Most of segmentation microcracks don’t run through the coatings along thickness. Some of the segmentation microcracks across the densified layers are perpendicular to the surface and start to deviate from the vertical direction within the porous plasma-sprayed coating. The segmentation microcrack density is more dependent on the laser scanning speed, resulting in higher densities of segmentation microcrack with the increase of the scanning speed. The laser-glazed conventional ceramic coatings have lower Ds values than the nanostructured one. The thermal shock resistance of laser-glazed nanostructured ceramic coating under high laser scanning is improved, which has more thermal shock cycling times than that of the plasma–sprayed TBCs on titanium alloy. However, for the samples which are laser-treated at low scanning speed, their thermal shock resistances are deteriorated.更多还原