The Microstructure,Superhardness Effect And High Temperature Oxidation Resistance Of VN/SiO₂ And TiAlN/Si₃N₄ Films

Ultra-high hardness and various material combinations ensure nano-scale multilayers extensively applied in surface modification for protective coatings on cutting tools. But compared with the good properties of such films, it is more important and valuable to investigate how superhardness effect is achieved through modifying the microstructure.Recent investigations indicate that amorphous material may be crystallized under the influence of"Template Effects"occurring in the nano-scale multilayers, accompanied with ultra-high hardness achievement. While, examples of specific material combinations for such superhard nano-scale multilayers are still scarce, so it is required to carry out investigations on the preparation approach and the superhardness mechanism. Because the cutting tools need to undertake very high temperature in use, the high temperature stability of nano-scale multilayers is one of the key properties of this kind of materials for production.VN/SiO₂ and TiAlN/Si₃N₄ nano-scale multilayered films are reactively deposited in this paper to investigate the crystallization conditions of amorphous SiO₂ and Si₃N₄ on VN and TiAlN templates under the vapor deposition status and the growth structure, superhardness effects and strengthening mechanism of VN/SiO₂ and TiAlN/Si₃N₄ films. Besides, the high temperature stability of TiAlN/Si₃N₄ multilayered films is also studied in this paper. Investigation results of this paper indicate that:1. VN/SiO₂ nano-scale multilayers can be conveniently prepared by reactively sputtering vanadium and silicon oxide targets in the Ar-N2 atmosphere. In the accomplished VN/SiO₂ multilayers, the SiO₂ layer, used to be amorphous under normal deposition conditions, are crystallized into a NaCl structured pseudocrystal under the template effects of VN crystal layer when the thickness of SiO₂ layer is less than 1nm. And then, it epitaxially grows with VN layer to form column crystals with strong (200) texture. The interface between SiO₂ and VN layers is sharp. Correspondingly, the film hardness is significantly enhanced to the maximum of 34GPa. Crystallized SiO₂ gradually changes back to amorphous with a further increase in SiO₂ layer thickness to more than 1nm. As a result, the epitaxial structure of multilayers is destroyed and the hardness falls.2. The increase in VN layer thickness does not cast a significant influence on the growth structure of such nano-scale multilayers. Epitaxial growth structure maintains during this procedure but the hardness increment gradually decreases. The hardness of VN/SiO₂ film is equivalent to that of VN film when the VN layer thickness is increased to 37nm. As the deposition rate of VN is very high during the reactive sputtering process, the approach to prepare ultrahard nitride/oxide nano-scale multilayers in Ar-N2 atmosphere will be extensively applied in various industrial prospects.3. In the nano-scale multilayers of TiAlN/Si₃N₄ , Si₃N₄ layers with small thickness (<⁰.9nm) are crystallized under the template effect of TiAlN template and then form an epitaxial growth structure TiAlN layers. A tension-compression alternative stress field occurs in the TiAlN/Si₃N₄ multilayers and then the hardness is increased as a result. The maximum value of film hardness will arrive to 39GPa. With the increase in its layer thickness, Si₃N₄ will change into amorphous to block the epitaxial growth structure of multilayers, and then the film hardness decreases correspondingly. 4. The modulation structure and crystal structure can keep stable up to 800℃, but when the annealing temperature is increased above 600℃, the alternative stress field generated by epitaxial growth disappears after the TiAlN/Si₃N₄ multilayers are annealed under high temperature, accompanied with the lack of superhardness effects in films. It is indicated from this investigation that the alternative stress field caused by epitaxial growth in nano-scale multilayers is the reason for film strengthening in the microstructure aspect.