【作者】 李永华；

【导师】 王煜明；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2004， 博士

【摘要】 NiTi形状记忆合金薄膜是迄今微机械领域中有发展前景的微驱动器材料之一，具有突出的形状记忆性能，是唯一具有单程、双程、全程精确形状记忆效应的薄膜材料，也是一种大应力、应变、高单位功密度、宽功率范围的材料。研究之将有助于最终揭示形状记忆的奥秘；开发之将推动微电子机械系统的发展。形状记忆的关键在于热弹性马氏体相变，以NiTi形状记忆合金薄膜作为微驱动器元件，须首先明确NiTi薄膜在各种因素影响下（如热处理和受力状态）相变行为特点，为提高NiTi形状记忆合金薄膜微驱动器元件的性能稳定性提供可参考的实验依据；NiTi形状记忆合金薄膜用于微驱动器元件时，对相变温度有严格限定，而相变温度又直接受热处理条件，也即晶化行为的影响，故研究在各种热力学条件下的相变行为具有理论和实际方面的意义； NiTi形状记忆合金薄膜微驱动器元件在使用中受力变形，使薄膜产生形变硬化效应；应力还将诱发马氏体相变，进而改变马氏体相变温度，直接影响微驱动器元件的性能稳定性；物理气相沉积方法制备的NiTi薄膜一般为非晶态，而控制晶化温度，以适应多种衬底的特性，也是制备有实际价值的NiTi薄膜的重要方面。本研究以直流磁控溅射方法制备NiTi形状记忆合金薄膜；以广角X射线衍射（XRD）研究了薄膜的晶化行为；以扫描电子显微镜（SEM）、原子力显微镜（AFM）及X射线光电子能谱（XPS）研究了薄膜的表面和界面；以透射电子显微镜（TEM）研究了薄膜显微结构；以差示扫描量热法<WP=151>（DSC）研究了热循环对相变行为的影响。既研究了不同衬底温度和热处理条件对薄膜相变行为的影响，也研究了复合膜的拉伸行为及应变与析出相、位错密度变化的关联。本研究以稳定地制备高质量样品为基础，从NiTi形状记忆合金薄膜的晶化过程切入，探讨了衬底温度对NiTi薄膜的晶化温度、表面和界面的氧化行为的影响；探讨了热处理参数的实用化和规范化问题；探讨了固溶、时效对薄膜中位错的相关性；对热循环影响相变行为及复合膜中应力诱发析出相的现象进行了研究；对薄膜各种相态的显微结构进行透射电子显微镜观察，主要结论如下：随固溶时间增加，NiTi薄膜中的亚晶粒尺寸D逐渐增加，平均位错密度下降，平均位错分布参量基本不变；位错密度的变化与析出相粒度、种类、分布等因素的改变有关；薄膜中的析出相使显微硬度实验值明显高于由母相的平均位错密度和平均位错分布参量得到的显微硬度计算值。室温衬底薄膜的晶化温度约为774K，晶化开始温度约为756K，晶化结束温度约为823K；573K衬底薄膜的晶化温度约为750K，晶化开始温度约为670K，晶化结束温度约为810K；室温衬底薄膜的相变温度区间宽于573K衬底薄膜。室温衬底薄膜进行的是一阶段相变。573K衬底薄膜在升温过程是二阶段相变，即；其降温过程是一阶段相变，即。随热循环次数增加，薄膜基体的平均位错密度不断增加，平均位错分布参量大幅度下降，即位错塞积向林位错分布转化，平均体弹性储能密度<WP=152>也不断下降。不同衬底温度薄膜经X射线光电子能谱检验，表面仅存在钛和氧原子，未发现镍原子，即在常温、常压、长时间时效后，薄膜表面形成了TiO2层，阻止镍原子向表面渗出；室温衬底及573K衬底薄膜表面氧化层的厚度小于6nm，723K衬底薄膜表面氧化层厚度为6~12nm；衬底基片上吸附的氧原子数量随衬底温度升高而降低；衬底温度升高也促进氧原子向薄膜内部的扩散，573K衬底薄膜氧原子百分比含量相对最低；573K及723K衬底薄膜，在表面仅有TiO2层，而在膜与Cu基片的界面上还存在Ti3+（Ti2O3）和Ni2+（NiO）的结合能峰，表明溅射原子与基片上能态较高的氧原子化合。表面氧化层的存在影响薄膜的晶化温度；经763K退火1h，室温和723K衬底薄膜仍是非晶态，而 573K衬底温度的薄膜基本晶化。以扫描电子显微镜能谱分析，室温及573K衬底薄膜成分均为Ti-51.06at.%Ni，而723K衬底温度溅射的薄膜成分则为Ti-52.0at.%Ni。经923K退火30min，在室温下做X射线衍射分析，室温衬底NiTi薄膜基体是马氏体相，573K衬底薄膜基体是奥氏体和马氏体的混合相，723K衬底温度薄膜有析出相Ni3Ti和TiO2。晶化温度与衬底温度相关，经763K退火1h的组样显示，对于3μm厚度的薄膜，衬底温度升高，易于晶化，奥氏体相的转变温度（As）降低。晶化温度与膜厚相关，相同衬底温度673K溅射的薄膜组样，<WP=153>经763K退火1h，厚度为3μm的高于18μm的；经923K退火0.5h后，薄膜奥氏体相的转变温度（As），3μm的高于18μm的。薄膜厚度影响相转变温度，与薄膜表面的氧化层有关，氧化层的生成使薄膜内部Ti原子缺失，也使氧原子更易向薄膜基体内部扩散；薄膜内部Ti和Ni原子百分比含量的变化，和填隙氧原子的存在，都直接影响相转变温度；薄膜越薄，氧化层的影响也越大。用透射电子显微镜观察，673K衬底18μm厚薄膜经743K真空退火1h后，晶化是不均匀的，晶化区周围总伴有Ni4Ti3相的析出，退火温度升高，Ni4Ti3相和奥氏体相都长大；室温衬底18μm厚的薄膜经923K真空退火0.5h，薄膜基体为马氏体相，经热循环后，相态稳定。随拉伸应变量增加，在塑性金属衬底上沉积的NiTi薄膜中，Ni3Ti相不?更多还原

【Abstract】 NiTi shape memory alloy thin film has the characteristics of large stress, strain and high work density. It is a perfect micro-actuator candidate for the micro-electrical mechanical system (MEMS) in recent years. In order to NiTi shape memory alloy thin film in micro-actuator system, the phase transformation behavior of NiTi thin film is important. Since the heat treatment temperature of NiTi thin film has a great influence on the phase transformation behavior. The crystallization of NiTi shape memory alloy thin film is studied in this work. The effects of substrate temperature on crystallization, surface and interface oxidation behavior are discussed. The phase transformation is detected during thermal cycling by DSC. X-ray diffraction (XRD) and SEM which shows the stress induced precipitation. Systematic observations had been made on the microstructure of NiTi thin film after thermal treatment by TEM. The conclusions are summarized as follows:The average dislocation density in NiTi thin films decrease with the increase of solution temperature which the average dislocation distribution parameter <WP=155>unchanged. The precipitation made the microhardness values is greater. The results show that the microhardness values are not in a good agreement between calculated and measured values.The crystalline temperature of the thin film at room temperature substrate is about 774K, the starting temperature is about 756K, the finishing temperature is about 823K. The crystalline temperature of the thin film at 573K substrate is about 750K, the starting temperature is about 670K, the finishing temperature is about 810K. The temperature range of phase transformation is greater. With the increase of thermal cycling times, the average deformation storage energy density dropped.The NiTi thin films deposited on Cu substrate at different substrate temperatures have been studied by X-ray photoelectron spectroscopy (XPS). The effects of substrate temperature on the surface and interface oxidation of NiTi thin films are discussed. After oxidation the samples are covered by an oxide layer with thickness of a few nanometers. This oxide layer is composed of TiO2. The Ni atom has not been detected in surface. In the film /substrate interface there is an oxide layer with thickness of about 12 nm thick with a mixture Ti2O3 and NiO in the films deposited at substrate temperature 573K and 723K, respectively. In the film/substrate interface a layer with thickness of about 12 <WP=156>nm contains Ti suboxides (TiO) and metallic Ni in the films deposited at ambient temperature. The composition of the films deposited at RT and 573K are Ti-51.06 at.% Ni, while that of the film deposited at 723K is Ti-52.0 at.% Ni. In the film deposited at ambient temperature the oxygen atoms concentration is greater than that of the film deposited at 573K. The XRD pattern of the NiTi film at 573K substrate temperature after heat treatment at 923K for 0.5h exhibits the specimen is with B2 austenite and B19’ martensite structure, the thin film at 723K substrate temperature precipitates Ni3Ti and TiO2.After annealed at 673K for 1 h the film deposited on 673K substrate is with B2 and B19’ phase, the As of thickness of 3μm is higher. As the oxidation occurs, titanium atoms diffuse outward while oxygen atoms diffuse inward. The absorption of interstitial oxygen atoms in the lattice during the oxidation and the atoms concentration of TiNi thin film could affect the martensite transformation temperature. The microstructure of TiNi film annealed at 743K for 1 h shows Ni4Ti3 precipitate and B2 phase. The samples grow at room temperature contain fine grain of B2 phase and B19’ martensite phase after annealed at 923K for 0.5 h.Ni3Ti phase precipitate and grow during tensile testing. The average dislocation density and the <WP=157>microhardness values increase with the strain. A series of parallel cracks grow in a concerted fashion across the film. The cracks are equally spaced.更多还原