Microstructure And Superhardness Effect In VC-based Nanomultilayers

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.