【作者】 孔明；

【导师】 李戈扬；

【作者基本信息】 上海交通大学 ， 材料学， 2009， 博士

【摘要】 硬质薄膜是一类具有广泛用途的表面涂层材料。近年来的研究发现,陶瓷纳米多层膜和纳米复合膜均具有硬度异常升高的超硬效应。这些两相纳米结构薄膜因材料组合的多样性而带来的性能的可裁剪性,展示出广阔的应用前景,而它们通过纳米尺度微结构,而不是通过传统的原子间强键能获得高硬度的强化机制,更具理论研究价值。纳米多层膜和纳米复合膜已成为近年来超硬材料和薄膜材料的研究热点。本论文从二维结构纳米多层膜和三维结构纳米复合膜两方面研究了两相纳米结构薄膜产生超硬效应的微结构本质和强化原因。在纳米多层膜方面,论文设计、制备了TiN与非晶SiC、AlON以及AlN与非晶Si₃N₄组成的纳米多层膜,研究了立方结构TiN和六方结构AlN晶体对各非晶层晶体化的模板效应,以及非晶层生长模式的转变对纳米多层膜微结构与力学性能的影响。在纳米复合膜方面,论文揭示了TiN/Si₃N₄纳米复合膜的微结构特征,并进一步采用二维结构TiN/Si₃N₄纳米多层膜实验模拟的方法研究了非晶Si₃N₄在TiN晶体模板层上的晶化现象及其结构变化对薄膜微结构和力学性能的影响。论文得出的主要结论如下:1.立方结构的纳米晶TiN和非晶SiC组成的多层膜显示了一种晶体生长的互促效应:由于B1-NaCl结构的TiN晶体层的模板作用,SiC层在厚度小于0.6 nm时晶化为同样结构的晶体并与TiN共格外延生长;SiC晶化后对TiN层的晶体生长亦有促进作用,使TiN层晶体生长的完整性显著提高,TiN/SiC多层膜因而形成强烈（111）择优取向的柱状晶,并产生硬度异常升高的超硬效应,最高硬度值可达60.6 GPa。SiC层厚增大到0.8 nm后将逐步由晶体结构变为非晶结构,阻碍了纳米多层膜的共格外延生长,多层膜的硬度随之降低。在SiC晶化并与TiN形成共格外延生长结构的纳米多层膜中,TiN厚度的改变对多层膜的力学性能影响不甚明显。2.采用在Ar, N2混合气氛中溅射金属Ti靶和化合物Al2O3靶的反应溅射方法可以制备TiN/AlON纳米多层膜。在TiN/AlON纳米多层膜中,由于TiN晶体层的模板作用,原为非晶态的AlON层在厚度小于约0.6 nm时被强制晶化,并与TiN层形成共格外延生长结构,多层膜获得硬度显著升高的超硬效应,最高硬度达到40.8 GPa。AlON层随厚度的增加又转变为以非晶态生长,多层膜的共格外延生长结构受到破坏,其硬度也相应降低。由于反应溅射TiN有很高的沉积速率,这种采用反应溅射制备高硬度纳米多层膜的方法为超硬多层膜的工业化生产提供了新的思路。3.在AlN/Si₃N₄纳米多层膜中,非晶态的Si₃N₄当其层厚小于0.8 nm时在六方结构AlN晶体层的模板作用下晶化为六方结构的赝晶体,并与AlN形成以（0001）为择优取向的共格外延生长结构,多层膜产生硬度异常升高的超硬效应,最高硬度达32.8 GPa。Si₃N₄随层厚的增加又转变为以非晶态生长,多层膜的共格外延生长结构遭到破坏,其硬度也随之降低。这一六方结构的AlN也具有使非晶材料晶化的现象为纳米多层膜晶体生长模板效应的普遍性提供了新的例证。4.对高硬度TiN/Si₃N₄纳米复合膜的微结构分析发现,复合膜中的TiN呈直径约10 nm,高度约数百纳米的柱状晶,Si₃N₄界面相的厚度约为0.5-0.7 nm,呈现晶体态,并与相邻的TiN晶粒形成共格外延生长结构,复合膜具有若干TiN纳米柱状晶通过Si₃N₄界面相共格相连而形成集束柱状晶的微结构特征。5.二维结构TiN/Si₃N₄纳米多层膜的实验模拟表明,由于TiN晶体层的模板作用,非晶的Si₃N₄在厚度小于0.7 nm时被晶化,并与相邻的TiN晶体形成共格外延生长结构,多层膜获得38.5 GPa的高硬度,Si₃N₄随厚度的进一步增加又转变为非晶态,多层膜的共格生长结构遭到破坏,其硬度也随之迅速降低。6.通过对TiN/Si₃N₄纳米复合膜和多层膜微结构特征和力学性能的对比可以发现,两相纳米结构超硬薄膜中高硬度的获得都与非晶相晶体化并与模板相形成共格界面结构有关,而它们硬度的迅速降低也都与非晶界面相随厚度的增加由晶体态转变为非晶态而导致共格结构遭到破坏有关。论文基于以上研究,揭示了模板效应在两相纳米结构薄膜特殊微结构形成和产生超硬效应中的关键作用和普遍性;提出了TiN/Si₃N₄纳米复合膜微结构和强化机制的新观点;以及利用纳米多层膜中晶体生长的模板效应通过非晶体晶化的方法拓展高硬度两相纳米结构陶瓷薄膜材料组合范围的设计路线。更多还原

【Abstract】 Hard ceramic films have been widely used as surface protection materials. It was found recently that ceramic nanomultilayers and nanocomposites exhibit superhardness effect, i.e. significant enhancement in hardness in contrast to their bulk counterpart. The virtue of investigating these two-phase nanostructured materials lies not only in the aspect of application in that their physical or mechanical properties can be specially tailored, but also in the aspect of theoretical as their high hardness has been achieved via proper design of microstructure in nanometer scale, rather than intrinsically exhibiting strong atomic bondings. Therefore, nanomultilayers and nanocomposites are being more and more popular nowadays.In this thesis, both 2-D laminar structured nanomultilayers and 3-D network structured nanocomposites were synthesized and studied to understand their unique microstructure and hardening mechanisms related to their superhardness effect. In the aspect of nanomultilayers, TiN/SiC, TiN/AlON as well as AlN/Si₃N₄ systems were designed and synthesized, with an aim to investigate the template-induced crystallization phenomenon of naturally amorphous materials （SiC, AlON, and Si₃N₄） on various template materials different in crystal structure （cubic crystalline TiN and hexagonal crystalline AlN）, as well as how the change of microstructure in amorphous layer affect corresponding mechanical properties of nanomultilayers. In the aspect of nanocomposites, the microstructure and mechanical properties of superhard TiN/Si₃N₄ nanocomposites as well as its parallel 2-D laminar TiN/Si₃N₄ counterpart were subjected to a thorough scrutinize, with an aim to understand the existing state of Si₃N₄ interfacial phase in nanocomposites and its influence on the resultant film’s mechanical properties. The conclusions drawn from these studies are as follows:1. A mutual promotion effect that affects crystallinity of both TiN and SiC was found in TiN/SiC nanomultilayers: Due to the template effect of B1-NaCl structured TiN nanocrystals, SiC crystallized into the same structure and grew epitaxially with TiN when its thickness was less than 0.6 nm. Meanwhile, the formation of crystalline SiC reversely promoted the crystal integrity of TiN and nanomultilayers formed large columnar crystals with intense （111） preferred orientation. Correspondingly, the nanomultilayers showed a superhardness effect with highest hardness reaching 60.6 GPa. When SiC thickness was further increased to 0.8 nm, it turned into amorphous growth mode, resulting in the destruction of the coherent structure and a significant decrease in film’s hardness. However, the change of TiN thickness does not have a significant influence in mechanical properties of nanomultilayers.2. It is viable to synthesize TiN/AlON nanomultilayers through reactively sputtering the metallic Ti and ceramic Al2O3 targets in a mixture atmosphere of Ar and N2. Due to template effect of TiN layer, AlON layer was forced to crystallize when its thickness was less than 0.6 nm. The crystallized AlON formed a pseudomorphic structure identical to that of TiN and grew epitaxially with TiN layer. Consequently, film’s hardness was enhanced significantly to a maximum value of 40.8 GPa. A further increase in AlON thickness resulted in the amorphization of AlON and dramatic decline in multilayer’s hardness. The expected high deposition rate resulting from the adoption of reactive sputtering technology together with its superior mechanical properties provides this kind of nanomultilayers high promising in the industrial mass productions.3. For AlN/Si₃N₄ nanomultilayers synthesized by reactive magnetron sputtering, wurtzite-typed hexagonal AlN also showed‘template effect’. Under this effect, Si₃N₄ assumed hexagonal pseudo-crystal structure and grew epitaxially with AlN at an intense（0001）preferred orientation when its thickness was less than 0.8 nm. Superhardness effect with highest hardness reaching as high as 32.8 GPa appeared as a result. When further increasing its thickness, Si₃N₄ transformed into amorphous and destructed the coherent interfaces, resulting in the degradation of film’s mechanical properties. This study provides an example that hexagonal crystallization of amorphous structure is also possible when the template material is hexagonal structured, implying that template-induced crystallization in nanomultilayers is a universal phenomenon no matter what kind of template material is employed.4. Researches on superhard TiN/Si₃N₄ nanocomposites reveals that: TiN are columnar-like nanocrystals separated by Si₃N₄ interfacial phases, with dimensions of less than 10 nm in width and several hundred nanometers in height. The thin Si₃N₄ tissue with thickness of 0.5-0.7 nm exists in crystalline state and forms coherent interfaces with the adjacent TiN nanocrystals. Several TiN nanocrystalline columnar grains are glued by coherent Si₃N₄ interfaces and form a bundle-like columnar grain, making them primary structural units of TiN/Si₃N₄ nanocomposites with superhardness effect.5. Experimental simulation from parallel 2-D laminar TiN/Si₃N₄ nanomultilayers reveals that when Si₃N₄ thickness is less than 0.7 nm, affected by the template effect of TiN crystal layers, Si₃N₄ is forced to crystallize and grows epitaxially with TiN. Correspondingly, the hardness of nanomultilayers is significantly enhanced to a maximum value of 38.5 GPa. A further increase in Si₃N₄ thickness will, however, cause Si₃N₄ to return back into amorphous growth mode, as a result, epitaxial structure is destructed and film’s hardness is declined.6. The comparison between TiN/Si₃N₄ nanocomposites and nanomultilayers reveals that the achievement of superhardness effect in these two-phase nanostructured films is closely related to the crystallization of amorphous layer and the formation of epitaxial coherent interfaces. And the hardness degradation is caused primarily by the amorphization of interfacial phase and the destruction of coherent interface.Based on these results, the thesis revealed the key role that template effect plays and its universality feature in generating unique microstructure and superhardness in two-pahse nanostructured films; suggested a new opinion different from the well known nc-TiN/a-Si₃N₄ model on hardening mechanism of TiN/Si₃N₄ nanocomposites; and proposed a new design principle to strengthen two-phase nanostructured films: i.e. employing template effect to induce crystallization of naturally amorphous interfacial phase.更多还原