Investigation To Mircro-Structure And Interfacial Mechanical Properties Of Metallic Nitride Superhard Nano-Multi-Layer Coatings

With the development of modern manufacturing and metal-cutting technology, themore and more rigorous working-environment bring forward higher and higherrequirements for the performance of cutting-tools, such as, super-hard hardness, wearresistance, thermal resistance, toughness, strength and life, etc. Surface coating is one ofthe most effective and economic methods which could meet above requirements,especially, the super-hard metallic nitride nano-multilayered coatings, which couldincrease not only the life of cutting-tools but also its applicability in practicalengineering, due to its tailorable capability. With the improvement of technology for thefabrication of multilayered coatings, the varieties of multilayered coatings increaserapidly, from a single mono-metallic nitride coating to binary-or multi-componentmultilayered coatings. Meanwhile, the thickness of each layer becomes thinner andthinner, and approaches to nano-scale, which brings about much more interfaces. Sincethe mechanical properties of the interfaces play the key role in achieving super-hardnessand excellent mechanical properties of nano-multilayered coatings, in this dissertation,the mechanical properties of metallic nitride nano-multilayered coatings areinvestigated at atomic scale. The main contribution is listed as follows:(1) For TiN/VN multilayered coatings, a total of36candidate interfaces are firstlybuilt with the consideration of various terminals of TiN and VN at atomic scale. Themost preferred stacking sequences of interfacial atoms are determined with interfacialadhesion energy. Making use of somr analytical approaches, such as planar-averagecharge density difference, partial density of states (PDOS), charge density anddifference, etc, it is found that the interfacial bonds are mainly ionic, incorporatingsmall amount of covalent bonds. Smooth transitions of both atomic structure andelectronic property are found when cross the interface between TiN and VN,demonstrating that the interfaces in TiN/VN nano-multilayered coatings are wellbonded.(2) During the fabrication of the multilayered coatings, the inevitably existing orpurposively introduced alloy element often introduces additional changes in theproperties of the materials. It is expected that the interfaces may play key roles in themechanical properties of multilayered coatings, especially the super-hardness atnano-multilayered coatings, and some doping elements may strongly affect the adhesion property of the interfaces. The effects of doping atoms of element Cr, Mo, Ta,Y, Al, Nb, Zr, or Sc on the structure and the adhesion energy of TiN/VN interfaces areanalyzed with the first-principles calculation. It is found that the effects of Al, Nb andZr on the adhesion energy are insignificant, Sc can slightly increase the adhesionenergy, while Cr, Mo, Ta, Y can substantially reduce the interfacial adhesion. Theeffects of the doping element on the properties of the interfacial bonds are alsoanalyzed with several different methods, and it is shown that the above mentioneddoping elements do not change distinctly the stacking pattern and bond properties ofthe interfacial atoms, the transition across the interfaces keeps smooth, and the bondsare still mainly ionic incorporating a certain covalent，indicating that the interfaces arewell bonded after introducing the doping elements.(3) With the conclusions from the above analysis for the interfacial properties ofthe TiN/VN multilayered coatings with or without introducing doping elements, anapproach is developed for the evaluation of the local residual stress with the concept oflattice mismatch between neighboring layers and the assumption that each layer is linearelastic. It is shown that the calculated result is in good agreement with experimentalresult, demonstrating the validity of the proposed approach. The effects of ratio of thethicknesses and the ratio of Young’s modulus of neighboring layers are analyzed. It isfound that the lattice mismatch can account reasonably for the extremely large residualstress in nano-multilayered coatings.(4) The adhesion energies of TiN coatings and TiN/CrN multilayered coatings atatomistic scale are calculated, taking into account the effect of residual stress. Theobtained adhesion energy can be applied in the cohesive zone model implemented in theFEM to reproduce the initiation and the growth of cracks. It may not only validate themethodology for calculating the adhesion energy at atomic scale but also shows that theresidual stress may affect strongly the adhesion energy. The tensile deformation ofTiN/CrN multilayered coatings is simulated at an atomic scale. The abrupt drop of stressduring the axial tension is found, which could be attributed to the slide of the layer of Natoms in the CrN coatings.(5) Based on the alternating adsorption of Al and N atoms on the TiN surface, thegrowth of AlN on TiN surface is simulated. The variation of AlN structure on the TiNsurface is studied in the view of adsorption energy and the absorbed AlN atomicstructure. It is shown that when the thickness of the absorbed AlN layer approachesabout0.6nm, the stacking pattern of AlN may change from A-B-C-A-B-C---, as that of TiN, to A-B-A-B---, indicating that the templating effect may disappeared, whichagrees well with that observed in experiments.