Microstructure And Superhardness Effect Of The Hard Nano-multilayer Films

The ceramic hrad films, such as TiN, have been widely applied in some fields of surface modification and protection inclunding coatings of cutting tools. The development of dry and high-speed cutting demands protective cutting-tool coatings with not only high hardness but also good thermal stability at high temperatures. Nanomultilayer films show much potential in the coatings fields of cutting tools, for they can obtain high hardness due to the superhadrness effect and especially can present flexiable performace modification by changing their constituents. Additionally, the nanomultilayer films can obtain high hardness by designing their structure, which shows more research value. However, it's impossible to design the superhard nanomultilayer only according to the theory now, which leads that the experiments are still the main intestigation method to obtain the nano-multilayer films with high hardness.In this work, VN/SiO₂, VN/AlON, ZrO₂/TiN and TiAlN/Si₃N₄ nanomultilayer films were synthesized by magnetron sputtering method, and their microstructure and mechanical properties were characterized by XRD, TEM, EDX, SEM and nanoindentation. The main studies in this work are described as follows: It was investigated that the reactive sputtering method to synthesize the oxide-composed nanomultilayer films, the template effect in nanomultilayer films with different structural template layer that the crystal template can affect the growth of the other layer, the high-temperature thermal stability of nanomultilayer films, and the strengthening models of nanomultilayer films. Finally, the present designing rule for nanomultilayer films with high hardness were modified and supplemented based on the research results.The main conclusions in this work are summarized as follows:1. The studies on the VN/SiO₂ and VN/AlON multilayers indicate that the reactive sputtering method with higher deposition rate can be successfully used to synthesize the oxide-composed multilayers with high hardness. The results show that, under the template effect of Nacl-type VN, normally amphorous SiO₂ and AlON with very low thickness (<1nm) can be crystallized and further grow epitaxially with VN layer, accompanied by a remarkable increase in hardness of multilayers. On further increase of SiO₂ and AlON layer thickness, the SiO₂ and AlON gradually transform into amorphous structure, resulting in a rapid decline in hardness. On the other hand, the change of VN layer thickness shows a relatively small effect on the growth structure and mechanical properties of the nanomultilayers.2. The studies on ZrO₂/TiN multilayers show that the tetragonal structural ZrO₂ also presents the template effect like the NaCl-type structural TiN. Under this template effect, TiN with layer thickness lower than 1.8nm can grow into metastable tetragonal structure and grow epitaxially with ZrO₂. With further increase of thickness, TiN gradually transforms into NaCl-type structure and blocks the epitaxial gowth of multilayers, resulting in a rapid decline in crystal integrity. In addition, ZrO₂/TiN multilayers don't present the superhardness effect, for the coherent strain in multilayers can change the modulus of the modulation material.3. The studies on TiAlN/Si₃N₄ multilayers show that under the t1emplate effect of TiAlN, normally amorphous Si₃N₄ with low layer thickness (<⁰.6nm) can be crystallized and grow epitaxially with TiAlN layer, accompanied by a remarkable increase in hardness. On further increase of layer thickness, Si₃N₄ gradually changes into amorphous structure, resulting in a quick decline in hardness. The research about the thermal stabilities of nanomultilayer films indicates, TiAlN/Si₃N₄ nanomultilayer films present good high-temperature structural stability, and their crystal structure and modulation structure can still be stable even at 900℃, which results in their higher hardness than TiAlN single film. However, TiAlN/Si₃N₄ nanomultilayer films don't show apparently higher oxidation resistance than TiAlN single film.4. The studies indicate that it's necessary to form a coherent structure for nanomultilayers in order to obtain high hardness, The modulus-difference strengthening dominates the superhardness effect compared with the alternating stress field strengthening, but it should be taken into account that the effect of alternating stress field on the modulus of modulation layers in nanomulatilayer films. Based on these studies, the present designing rule for nanomultilayer films with high hardness were modified and supplemented in this work, which were mainly described as follows:1) The two modulation layers should form the coherent structure;It's necessary to form a coherent structure between the two modulation layers for nanomultilayer films to obtain high hardness, but it doesn't mean that the lattice parameters of two modulation layers have to be nearly equal. Under the template effect, the nanomultilayer films can also grow into coherent structure even if their lattice parameters have a large difference.2) The modulus difference of two modulation layers under the coherent strain should be as large as possible;A large difference between the modulus of two modulation layer with coherent structure is the main reason for nano-multilayer films to obtain high hardness. However, herein the modulus isn't the modulus of the bulk constituents of multialyers, but the modulus of two modulation layer under the coherent strain in nanomultilayers.The above modification and supplement to the present designing rule can expand the scope of material combination for nanomultilayer films with high hardness.