Template And Superhardness Effects In Two-phase Nanostructured Films

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.