Research On Synthesis And Properties Of AlN/Si₃N₄ Nanostructured Multilayers And Zr-Si-N Composite Films

AlN/Si3N4 multilayers with different modulation periods and a series of Zr-Si-N composite films with different Si contents were prepared by reactively magnetic sputtering. The crystallization behavior of amorphous Si3N4 layers under the effects of w-AlN is studied. The superhardness effects as well as the surface topography are also investigated. The critical thickness of Si3N4 layers, which changes from crystalline to amorphous, has been calculated through thermodynamic and elastic considerations. The hardening mechanisms of AlN/Si3N4 multilayers have also been discussed. The effect of Si content on the microstructure and oxidation resistance and mechanical properties of Zr-Si-N films was studied. The hardening mechanisms of Zr-Si-N films have also been investigated.The results are showed as follows: The crystal structure of Si3N4 layers in the multilayers and the hardness of AlN/Si3N4 multilayers depend on the thickness of Si3N4 layers. When the thickness of AlN is 4.0 nm and Si3N4 layer is 0.4 nm, Si3N4 layers grew epitaxially with AlN and formed a strong columnar crystals and extended over several modulation periods. A large portion of the hardness enhancement of the multilayer was happened, the hardness and elastic modulus of the multilayers reach the maximum value of 37 GPa when Si3N4 are totally crystallized, which are 1.4 times the values as calculated by the rule of mixture for AlN and Si3N4 films showing a superhardness effect. The microstructure of Si3N4 was changed from crystalline to amorphous as Si3N4 thickness increases, leading to blocking the epitaxial growth. The superhardness effect disappeared. However, the surface topography of AlN/Si3N4 multilayers rapidly decreased and was gradually close to the Si3N4 films'. It is suggested that the coherent stresses and modulus difference between crystallized w-Si3N4 layer and w-AlN layer is the main reason for hardness enhancement when Si3N4 layer thickness is less than 0.7 nm.The maximum hardness values and elastic module of Zr-Si-N composite films is 37.8 GPa and 363 GPa respectively when Si/Zr atomic rate is 0.03. The oxidation resistance obviously improves with Si content increases. With a further increase of silicon content in the film, crystalline Zr-Si-N composite films turn into amorphous films, its mechanical properties decrease gradually. Zr–Si–N composite films with Si/Zr atomic rate of 0.198 cannot be used to protect efficiently substrates against air oxidation at temperature higher or equal to 850℃. The solid solution hardening mechanism and modulus difference between crystallized c-Si3N4 layer and ZrN layer is the main reason for hardness enhancement in composite films.