The Correlation Between Mechanical Properties And Grain Size Of Ti-Si-N Composite Coatings

Nanoscale polycrystalline materials with unique mechanical properties becomenow a hot point in the field of materials science. One of the research focuses is thegrain size effect on the material mechanical properties. The main purpose of this studyis to obtain the correlation between grain size and strength by strain-stress curve ofdifferent grain size, aimed to find the peak of strength. This paper is divided into twoparts to study.In first part, a series of TiN crystals with different grain size ranging from0.6387nm to2.3419nm were calculated by means of first principles method toinvestigate their mechanical properties. The results show that: as the grain sizeincrease, the yield strength of TiN crystal will decrease. That means the TiN crystaltrend to softening; TiN crystal begins to yield at the tensile strain of5%, the yieldstrength is about21.5GPa. The strain-stress curve indicates that the tensile strengthoccurred in15%of tensile strain, the tensile strength of TiN will decrease followingthe grain size increase. The correlation between the microstructure of TiN and theirhardness and elastic property was discussed, which indicates that besides the grainsize effect, the defects in the TiN film is a crucial factor to have a strong effect ontheir hardness and strength.In second part, firstly, we developed a simulation program of Kinetic MonteCarlo which is used to study mechanical properties of material composites. Then,using this program study a series of Ti-Si-N composite surface with different grainsize ranging from1.272nm to6.36nm (hexagonal grain boundary) and grain sizeranging from0.848nm to4.24nm (diamond grain boundary) were simulated byKinetic Monte Carlo to investigate their mechanical properties. The results show that:the highest intensity of Ti-Si-N nanocomposite surface corresponding to the grain size:Hexagonal grain is3.816nm, and diamond grain of Ti-Si-N materials is in the range of2nm to2.5nm.The tensile strength of hexagonal grain is26.1GPa, and the tensilestrength of diamond grain is higher than26.37GPa.Ti-Si-N composite surfaces of twodifferent shape of grains, the relationship of grain size and tensile strength either haveHall-Petch regime but also have inverse Hall-Petch regime.There are two differentcritical dimensions in this paper; this is not only related with the configuration ofmaterial, but also with the type of atomic bond on the grain boundary. This shows thatnot only grain size affect the strength of material, the shape of grain also affect its strength. The fracture modes of Ti-Si-N composite surface are: tangent fracture,ductilt fracture and transgranular fracture.