Microstructure And Superhardness Effect In TiC-based Nanomultilayers

The superhardness effect and diverse material combinations endowthe nanomultilayers with both high hardness and splendidcomprehensive properties. Widening the system of hard nanomultilayersis an important research aspect, and the multilayer systems have alreadybeen extended based on the technical route of amorphous crystallizationoriginated from template effect, however, the nanomultilayers are mostlynitride based, rarely involving with carbides, bordides or oxides.In order to extend the material systems from nitride to carbide basednanomultilayer, several TiC based nanomultilayers were designed on thebasis of TiC film preparation techniques, and these nanomultilayers areTiC/TiN multilayers composed of TiC and cubic nitride, TiC/WCmultilayers consisted by TiC and hybrid-structure carbide, TiC/Si₃N₄andTiC/SiC multilayers made of TiC and amorphous nitride, carbide,TiC/B₄C multilayers made up with TiC and B₄C which can react witheach other. The microstructure along with mechanical properties has beenstudied for the template effect of TiC with different types of materials andstructures， and the impact of modulation structure on mechanicalproperties of the multilayers has also been further studied. The research results not only reflect the extension of multilayer system, also promotethe understanding of the regulations between multilayer structure andsuperhardness effect.1. For the TiC films prepared by reactive sputtering techniques,modulating the partial pressure of C₂H₂can control the film’scomposition, phase formation, microstructure as well as mechanicalproperties, and single phase, high hardness TiC films with bettercrystalline quality can be obtained in certain partial pressure, the highesthardness is31.6GPa. When TiC films are sputtered by compound target,the sputtering pressure （Ar） and substrate temperature can both influencecontent composition, deposition rate as well as film’s microstructure andmechanical properties, and the sputtering pressure has more obviouseffect on TiC films over substrate temperature.Modulating the composition of TiC targets is an effective measure forobtaining single phase TiC films with better crystalline quality: TiC filmsprepared by stoichiometric TiC target, of which the content is located inTiC+C two phase region in phase diagram, may get poor crystallinity andmechanical properties owing to free carbon; TiC films, which wereprepared by substoichiometric TiC target with higher Ti content, wouldform double phase structure owing to high Ti content in TiC targets; TiCfilms, which were prepared by substoichiometric TiC target with a bit of overdosed Ti content，could obtain single phase TiC with higher hardnessand crystalline quality. By avoiding the reaction with other components inreactive atmosphere, preparing TiC films by substoichiometric TiC targetwith a bit of overdosed Ti is beneficial to match other components toform nanomultilayers.2. The research of TiC_0.8/TiN and TiC_1.0/TiN nanomultilayers preparedby TiC targets using stoichiometric TiC target, substoichiometric TiCtarget with a bit of overdosed Ti content and TiN targets shows, theepitaxial growth structure formed in TiC_0.8/TiN nanomultilayers can beretained to the modulation period of14.4nm, and the multilayer can gainsuperhardness effect with highest hardness of38.6GPa. Athough theepitaxial growth structure formed in TiC_1.0/TiN nanomultilayers can beretained to the modulation period of3nm, the highest hardness inTiC_1.0/TiN nanomultilayers is only30.9GPa.3. The investigation of TiC/WC nanomultilayers consisted of TiC andhybrid-structure WC shows that TiC layer can generate template effectthat can confine WC layer transiting to cubic structure, and undertemplate effect, WC layers can growth coherently with TiC layer in thesame structure when WC layer thickness is less than1.2nm. Meanwhile,crystal perfection is promoted by the mutual effect in the multilayer, andthe multilayer achieved its highest hardness of41.5GPa at0.5nm WC layer thickness. Further increasing the layer thickness, WC layers willturn back to the growth in multi-structure leading to the disruption ofcoherent growth structure and the decrease of hardness.4. The investigation of TiC/Si₃N₄、TiC/SiC nanomultilayers consistedof TiC and amorphous Si₃N₄, SiC shows that, under the template effect ofTiC layers, amorphous Si₃N₄, SiC were forced to crystallized and growcoherently with TiC layers in the same structure when SiC and Si₃N₄layers is quite thin （about0.5nm）, and the multilayers all appear hardnessincreased phenomenon with their highest of36.9GPa and37GPa,separately. Further increasing the layer thickness, the coherent growthstructure will be disrupted, and the hardness will decrease accordingly.5. The research of TiC/B₄C nanomultilayers with interface reactionshows that when B₄C layer thickness is about0.5nm, the layer will allreact into TiB₂modulation layers, and the multilayer obtainsuperhardness effect with the highest hardness of40GPa. Furtherincreasing B₄C layer thickness, the non-reactive amorphous B₄C willdisrupt the coherent growth of the multilayers, the multilyers will formTiCx/TiB₂/B₄C triple layered structure, and the hardness will dropdramatically.Based on the research results, design requirements of thiskind of multilayers for achieving high hardness have been proposed:1.At least one modulation layer should be crystalline layer which is used as template layer for the new building layers;2. The other layer（amorphous layer included） should be as thin as possible （≤1nm） so thatthe interface reaction layers would all turn to the new building layers.3.The new building layers should be immiscible with the template layers;4. The template layers should have quiet large solid solubility to absorbthe excessive atoms generated in the reaction.6. A series of nanomultilayers with different TiC and TiN layerthickness were used to reveal the impact regulation of modulationparameters on multilayers’ microstructure and mechanical properties.The results show that the multilayer can achieve high hardness onlywhen the multilayers can form sharp interface coherent growth structurewith modulation period and modulation layer thickness is quiet small;When modulation structure parameters change, decreasing themodulation layer thickness will lead to the increase of mixing layerthickness, increasing the layer thickness will lose the the amount ofinterface, especially when the coherent growth structure was disrupted,the hardness of the nanomultilayer will drop down quickly. The relationdiagram between modulation structure parameters and hardnessestablished in this paper can provide reference for designingnanomultilayers with superhardness effect.Based on the research results above, new systems with superhardness effect have been obtained in TiC based nanomultilayers; The effect ofcrystallinity state in nanomultilayers has been revealed on the relationshipbetween microstructure and superhardness effect; The requirements fordesigning interface reactive nanomultilayers with superhardness effecthave been proposed; The design map of multilayer modulation structurehas been established.