Study On The Growth Structure, Superhardness Mechanism Of AlN-included Nano-multilayer Films

The application of hard ceramic thin films, such as TiN, as coatings of cutting tool have gained huge success and significantly promoted the development of manufacturing industry. The advancement of machining techniques, such as high speed, dry cutting, demands better properties for the coatings of cutting tools. The nano-multilayer makes itself a promising candidate coating for its high hardness and the diversity, from which various superior properties could be obtained. Meanwhile, of more theoretic value is its hardening mechanism, where high hardness is obtained by designing its microstructure.It is already known that in some multilayers, one modulation layer could be phase transformed and grow epitaxially with the other layer. In TiN/SiO2 mulitlayers, for example, when the thickness is less than 1nm, the amorphous SiO2 layer is transformed into the pseudo-crystal under the"template effects"of TiN modulation layer, which shares the structure with cubic TiN layer. However, current researches on crystallization of amorphous are mostly focused on the cubic nitride based nano-mulitlayers. Few, if any, investigations have been done on the multilayer series, whose template layers have different structures.A series of TiN/AlON nano-multilayers are synthesized with chemical sputtering method; a series of AlN/TiB2 nano-multilayers are synthesized with physical sputtering method. The crystallizing conditions are investigated, under which amorphous AlON and TiB2 modulation layers are crystallized for"the template effects"of cubic TiN and hexagonal AlN. The effects of the crystallization of amorphous layers on multilayers'microstructure and mechanical properties are also studied. A method of measuring internal stresses in the metal based hard thin films is proposed. Results are as follows:1. When using Al₂O₃ target to synthesize TiN/AlON multilayers in the gas mixture of Ar and N₂, the O atom in the Al₂O₃ will be partially replaced by N atom, forming amorphous AlON.2. When the thickness of AlON layer is less than 0.6nm, AlON is forced to crystallize and grow epitaxially with TiN and the hardness of multilayers are enhanced with the highest value reaching 40.6GPa. Further increasing the thickness, the AlON layer transformed into amorphous, destroying the crystal structure of multilayers, and the hardness also decreases. Due to the high deposition rate, this technique, which uses chemical sputtering method to synthesize TiN/AlON multilayer, is of potential industrial application.3. In AlN/TiB₂ nano-multilayers, due to the template effects of AlN, the amorphous TiB2 layers are forced to crystallized and grow epitaxially with h-AlN at small thickness. At the same time, the hardness of multilayers increases up to 30.5GPa. Further increasing the thickness of TiB2 layers, the TiB2 layers turn into amorphous, and destroying the epitaxial structure of multilayers, accordingly the hardness decreases.4. Based on Stoney equation, a method is proposed to measure the internal stresses in the metal based thin films, which releases the stresses by cutting metal substrate with eletro discharging machining (EDM). With this method, the internal stresses can be conveniently and rapidly measured.