【作者】 李冠群；

【导师】 李戈扬；

【作者基本信息】 上海交通大学 ， 材料学， 2011， 博士

【摘要】 硬质薄膜是一类在现代工业中具有重要应用的表面涂层材料。纳米多层膜的超硬效应以及材料组合的多样性和性能可裁剪性,使其具有广阔的应用前景。而这类材料通过人工设计微结构而获得高硬度的强化机制则具有更为重要的理论研究价值。经过二十多年的研究,在高硬度纳米多层膜的微结构特征和材料体系拓展等方面都已取得了明显的进展。然而强化机制和设计准则等基础性研究的滞后成为了这类材料进一步发展的障碍。本论文研究了VC、HfC单层膜以及VC/TiC、VC/TiN、VC/TiB₂、VC/SiC、VC/AlN、VC/Si₃N₄和HfC/Si₃N₄等多种碳化物为基的纳米多层膜。主要的内容包括:碳化物为基的纳米多层膜中的生长结构和超硬效应;立方结构碳化物对非晶材料晶体化的模板效应;采用反应溅射方法获得高硬度碳化物基纳米多层膜的制备技术;讨论了纳米多层膜中的强化机制和调制结构参数对多层膜性能的影响。论文在纳米多层膜材料体系的新拓展、设计准则的新补充和制备技术的新方法三个方面取得了一些创新性的成果,得出的主要结论如下:1.单相VC薄膜可以在Ar气体中磁控溅射VC陶瓷靶制备获得。溅射气压对薄膜的化学成分、相组成、微结构、沉积速率以及相应的力学性能有较大的影响,通过调整VC靶材的化学计量比也可以显著改变所得薄膜的生长结构和力学性能。等化学计量比VC靶所得薄膜的结晶程度较差,最高硬度为28.0GPa;而富金属元素的VC靶所得薄膜在低的溅射气压下即可获得良好的结晶和较高的力学性能,最高硬度可达31.5GPa。2. VC/TiC纳米多层膜在小调制周期时可获得硬度的显著升高。其中由富金属元素VC和TiC组成的纳米多层膜形成了良好的共格外延生长结构,并获得了硬度增量为40%的超硬效应,其最高硬度达到41.9GPa;而由等化学计量比VC和TiC组成的纳米多层膜形成的共格界面结构则相对较差,多层膜的硬度增量仅约为19%,其最高硬度为31.0GPa。3.不同VC、TiN层厚的VC/TiN纳米多层膜均可以在小调制周期时获得共格外延的生长结构和硬度显著升高的超硬效应。其中,固定调制比的VC/TiN纳米多层膜获得了硬度增量高达62%的超硬效应,其最高硬度达到45.8GPa;而固定VC层厚和固定TiN层厚的VC/TiN纳米多层膜也分别取得了41%和42%的硬度增量。调制周期或可变调制层厚度增大后,多层膜的共格界面结构遭到破坏,其硬度也逐步降低。4. VC/TiB₂纳米多层膜也可以在合适的调制结构下获得硬度明显升高的超硬效应。立方结构VC和六方结构TiB₂在小调制周期或可变调制层厚度很小时形成了共格界面结构,其共格关系为:{111}VC// {0001 }TiB< 110 >VC //<1120>TiB。在不同调制结构参数的VC/TiB₂纳米多层膜中,固定调制比的多层膜获得了硬度增量为26%的超硬效应,其最高硬度为38.3GPa;而固定VC层厚和固定TiB₂层厚的多层膜则分别取得了39%和41%的硬度增量。调制周期或可变调制层厚度增大后,多层膜的共格生长逐渐遭到破坏,硬度也逐步降低。5.由立方晶体的VC和非晶态的SiC或AlN组成的纳米多层膜也能获得超硬效应。在模板层VC的模板作用下,沉积态为非晶的SiC和AlN都在厚度小于约0.7nm时被强制晶化成NaCl结构的亚稳晶体,多层膜呈现共格外延的生长结构并获得硬度的明显升高。其中VC/SiC纳米多层膜获得的最高硬度为36.0GPa;VC/AlN纳米多层膜获得的最高硬度可达40.1GPa。SiC或AlN层随厚度增大后又重新转变为以非晶态生长,多层膜的共格界面迅速遭到破坏,其硬度随即降低。6.在Ar-C₂H₂混合气体中,利用溅射金属V和Hf靶材的反应溅射方法可以高效率地合成碳化钒和碳化铪薄膜。薄膜的化学成分、相组成、微结构以及力学性能对C₂H₂分压非常敏感。在C₂H₂分压为混合气体总压的约3%时可以获得力学性能较高的立方结构VC薄膜,其硬度可达35.5GPa;在C₂H₂分压为混合气体总压约2.5³.0%的范围内才可获得力学性能较高的单相HfC薄膜,其最高硬度为27.9GPa。7.由于在Ar-C₂H₂的混合气氛中,溅射的Si₃N₄粒子不与C₂H₂气体发生反应,因而采用反应磁控溅射技术可以高效地制备VC/Si₃N₄和HfC/Si₃N₄纳米多层膜。在VC或HfC的模板作用下,原为非晶态的Si₃N₄在层厚小于1nm时被强制晶化,并与模板层形成共格外延生长结构,相应多层膜获得了硬度显著升高的超硬效应。其中VC/Si₃N₄获得的最高硬度达45.8GPa,而HfC/Si₃N₄获得的最高硬度为38.2GPa。Si₃N₄层随厚度增加超过1nm后又转为以非晶态生长,多层膜的共格生长结构随即遭到破坏,其硬度亦相应降低。由于反应溅射具有很高的沉积速率,这种采用反应溅射技术制备高硬度纳米多层膜的方法在工业上具有很好的应用前景。8.根据VC/TiN、VC/TiB₂和VC/SiC纳米多层膜的实验结果总结出了立方晶体/立方晶体、立方晶体/六方晶体、立方晶体/非晶体三种典型结构组合纳米多层膜的调制结构参数与薄膜硬度的具体关系图。由于这三种结构类型组合涵盖了大多数陶瓷纳米多层膜的结构类型,该参数图在纳米多层膜的调制结构设计中具有普遍的指导价值。更多还原

【Abstract】 Hard ceramic films, which play an important role in the development of modern manufacturing industry, have been widely used in the field of surface modification and protection. Nanomultilayer shows promising potential practical application in these fields, for it can achieve high hardness due to the superhardness effect and obtain special physical or mechanical properties through tailorability of component properties. Furthermore, the strengthening mechanism of obtaining high hardness through proper microstructure design shows more significant theoretical research value.Over the past two decades, numerous experimental studies have been conducted leading to significant progress in the extending of high hardness nanomultilayer systems and understanding of their microstructure characterization. However, in contrast to experimental research, relatively slow progress has been achieved in theoretical studies to explain the strengthening mechanism of these nanomultilayers, which impedes the further development of this kind of material.In this thesis, VC, HfC films and VC/TiC, VC/TiN, VC/TiB₂, VC/SiC, VC/AlN, VC/Si₃N₄, HfC/Si₃N₄ nanomultilayers were prepared by magnetron sputtering. The microstructure and superhardness effect in carbide-based nanomultilayers have been investigated, and the template-induced crystallization phenomenon of naturally amorphous materials on different template materials has also been studied. Besides, this thesis also developed a new method of reactive sputtering to synthesize high hardness carbide-based multilayers. Finally the strengthening mechanism and the effect of modulation structure parameters in nanomultilayer have also been researched. Some innovative results of this thesis can be summarized as below: first, extending high hardness nanomultilayer systems from nitride-based to carbide-based. Second, elaborating the modulation structure parameters to supplement the design criterion. The third, coming up with a new method to synthesize high hardness carbide-based nanomultilayers. The main conclusions drawn from these studies are listed as follows:1. Vanadium carbide films can be easily synthesized by direct VC compound target sputtering in Ar atmosphere. The Ar pressure significantly affected the composition, phase, microstructure and mechanical properties of the films. The growing structure and mechanical properties hold close connection with the chemical composition of sputtering targets either. The films prepared by equal stoichiometry VC target showed poor crystallization and gained the highest hardness of 28.0GPa, while the films synthesized by metal enriched VC target could obtain good crystallization and mechanical properties under low sputtering pressure, whose highest hardness was 31.5GPa.2. Superhardness effect has been found in VC/TiC nanomultilayers. The multilayers combining with metal enriched VC and TiC exhibited excellent epitaxial growing structure and obtained highest hardness of 41.9GPa which was 40% higher than the rule-of-mixture hardness of components. While in the multilayer with equal stoichiometry VC and TiC, the highest hardness of 31.0GPa with only 19% enhancement had been obtained as a result of relatively poor coherent interface.3. VC/TiN nanomultilayers with small modulation periods exhibit epitaxial growth and superhardness effect, too. The multilayers with fixed modulation ratio obtained the highest hardness of 45.8GPa, which was 62% higher than rule-of-mixture hardness of components. In the multilayers with fixed VC layer thickness or TiN layer thickness, 41% and 42% hardness enhancement had also been achieved respectively. With the increase of modulation period or variable layer thickness, the coherent interface of multilayer was disrupted, resulting in a quick decline of hardness.4. Superhardness effect has also been found in VC/TiB₂ nanomultilayers. When the modulation or the variable layer thickness in multilayers were very small, cubic VC and hexagonal TiB₂ formed coherent interface with orientation relationship of {111}VC// {0001 }TiB< 110 >VC //<1120>TiB₂< 110 >VC //<1120>TiB. The VC/TiB₂ nanomultilayers with fixed modulation ratio achieved the highest hardness of 38.3GPa, which was 26% higher than the rule-of-mixture hardness. While in the multilayer with fixed VC layer thickness or TiB₂ layer thickness, 39% and 41% hardness enhancement had been achieved respectively. When further slightly increasing the modulation or variable layer thickness, film’s hardness decreased gradually because of the disruption of coherent interface.5. Superhardness effect also appears in the multilayers synthesized by VC and naturally amorphous SiC or AlN. Under the template effect of cubic VC, as-deposited amorphous SiC and AlN were forced to crystallize when their layer thicknesses were less than about 0.7nm. The crystallized SiC or AlN formed a metastable structure identical to that of VC and grew epitaxially with VC layer. Consequently, film’s hardness was enhanced significantly to a maximum of 36.0GPa in VC/SiC multilayers and 40.1GPa in VC/AlN multilayers. A further increase in SiC or AlN layer thickness resulted in the amorphization of SiC or AlN, which caused the destruction of coherent interface. The film’s hardness dropped quickly accordingly.6. Vanadium carbide and hafnium carbide films can be synthesized conveniently with reactive magnetron sputtering method in Ar and C₂H₂ mixture. The composition, phase, microstructure and mechanical properties of the films showed the sensitivity to the partial pressure of C₂H₂. The single phase VC films with optimistic hardness could be obtained when the proportion of C₂H₂ partial pressure was only about 3.0% of the mixture, the highest hardness was 35.5GPa. When C₂H₂ partial pressure was only about 2.5³.0% of the mixture, single phase HfC films with high mechanical properties could be synthesized with the highest hardness of 27.9GPa.7. VC/Si₃N₄ and HfC/Si₃N₄ nanomultilayers can be prepared efficiently by multi-target reactive magnetron sputtering as Si₃N₄ does not react with C₂H₂ in the sputtering condition. Under the template effect of VC or HfC layers in the nanomultilayers, as-deposited amorphous Si₃N₄ was crystallized at the layer thickness of less than about 1nm and grew coherently with VC or HfC. Correspondingly the multilayer achieved significantly enhanced hardness with maximum values of 45.8GPa in VC/Si₃N₄ multilayers and 38.2 GPa in HfC/Si₃N₄ multilayers. Further increasing the thickness, Si₃N₄ layers transformed into amorphous and then blocked the coherent growth of multilayers, resulting in a rapid decrease in hardness. The expected high deposition rate resulting from the reactive sputtering technology provided this kind of high hardness nanomultilayers promising application in the industrial mass productions.8. Based on the experiment results of VC/TiN, VC/TiB₂ and VC/SiC multilayers, the hardness distribution maps in three typical structure combination multilayers had been concluded. As including the main structure types in ceramic multilayers （cubic/cubic, cubic/hexagonal, cubic/amorphous）, these maps could provide a practical reference for the actual modulation structure parameters designing.更多还原