Study On The Mechanical Properties And Microscopic Mechanisms Of Carbon-based Amorphous Coatings

Diamond-like carbon coatings(DLC)are highly regarded in many fields due to their unique mechanical properties,including high toughness,hardness,and resistance to corrosion and friction.However,the structural instability and mechanical properties shown at high temperatures are easily affected by temperature,density and other factors,which limit the application prospects.At present,most of the research focuses on the DLC characterization after modification of doped elements such as Si and N,and the relevant computational simulation is relatively lacking.In order to optimize DLC for high-temperature use,a more comprehensive understanding of its behavior at a microscopic level is needed.Molecular dynamics simulations are frequently utilized as an effective method to investigate the microstructure of materials,enabling systematic simulation of research objectives and analysis and definition of material properties from the microstructure level that cannot be characterized by macroscopic experiments.Atomic-scale studies are essential in understanding the microscopic mechanisms involved in analyzing mechanical properties from amorphous structures and bonded forms.In recent years,the chemical reaction force field(Reax FF)has garnered widespread attention for its capacity to well describe the bonding and breaking processes of chemical bonds in various systems,basing on the bond-level viewpoint.Additionally,its calculation speed is much faster compared to traditional computational first molecular dynamics methods in simulating chemical reaction processes.The paper establishes a DLC carbon-based amorphous coating using molecular dynamics derived from the Reax FF chemical reaction.Further,it explores the mechanical properties and stability mechanisms from a microscopic perspective.Four sets of amorphous carbon structure bulk models were produced using the liquid phase quenching method,with densities of 2.34 g/cm~3,2.60 g/cm~3,2.87 g/cm~3and 3.01 g/cm~3,respectively.Non-metallic elements such as Si and N in proportions of 5at.%,10 at.%,15 at.%,and 20 at.%were introduced to produce Si-DLC and N-DLC and multi-density pure DLC coatings,and their properties were systematically studied,observing the change law of their mechanical properties in relation to the type and content of doped elements at 300-1500 K.Finally,the paper explores the intrinsic origin of performance stability from the amorphous structure level,revealing its microscopic mechanism.Considering the influence of density and temperature on the mechanical properties of the DLC coating,this study uses the molecular dynamics(MD)method to simulate the uniaxial compression and tensile structures of DLC films,with the purpose of characterizing their deformation mechanisms and mechanical property changes.Surface models were established based on the 2.34g/cm~3,2.60g/cm~3,2.87g/cm~3,and 3.01g/cm~3DLC bulk models to compare deformation mechanisms during compression and tensile processes.(1)Analyzing the change rule of the mechanical properties and microscopic mechanisms of DLC.The mechanical behavior of DLC with different densities at high temperatures was analyzed,and it was found that its mechanical properties were sensitive to high temperatures.The mechanical parameters had a significant dependence on density and temperature,and the higher the density,the more sensitive the performance was.At the same temperature,the Young’s modulus and strength increased with density.When the density was the same,the Young’s modulus and strength decreased as the temperature increased,and the decrease was faster at high density because the proportion of Csp~3decreased more rapidly.This is the reason why the mechanical properties of high-density DLC are more sensitive to temperature.The tensile and compressive deformation mechanisms of DLC are different.During compression at room temperature,the film failed due to the occurrence of relative slip,which accelerated the transformation of hybrid atoms.In contrast,during tensile deformation,there was no slip phenomenon in the same direction,and the reason for the film failure was the rupture of the C-C bond,resulting in the deformation of the film.(2)Analyzing the variations in the thermal stability and microstructural mechanisms of Si-DLC and N-DLC.Different density DLC films were maintained within a temperature range of 300-1500 K,with low-density DLC experiencing slight graphitization at high temperatures,while high-density Csp~2 appeared to increase rapidly at high temperatures,accelerating the conversion of Csp~3-Csp~2 and aggravating graphitization.The Si doping results showed that when the density and temperature were the same,the higher the Si concentration,the better the suppression of Csp~2.Si and C formed Si-C bonds with high thermal stability,and the Csp~3 hybrid atoms that became bonded with Si also may maintain high thermal stability,making C atoms stable in the sp~3 hybrid state,thereby inhibiting their conversion to sp~2 hybrid bonds at high temperatures.Therefore,Si element doping can stabilize atomic structure and reduce the degree of graphitization at high temperatures.The N doping results showed that when the density was the same,as the N concentration increased,Csp~2 continued to decrease.At a doping concentration of 20 at.%,Csp~2 was suppressed more,and the thermal stability was better.After N doping,the strong covalent bond between C and N atoms allowed the C=N bond to suppress Csp~2 growth,leading to improved thermal stability of the N-DLC film.In summary,at the same density and doping element,the higher the doping concentration,the better the suppression of Csp~2 transformation and the better the thermal stability.In addition,compared with N-DLC,Si-DLC exhibited better thermal stability.(3)This study systematically analyzed the effects of temperature,density,and concentration on the mechanical properties of Si-doped DLC and its corresponding microscopic changes.Uniaxial tensile tests were conducted on Si-DLC at operating temperatures ranging from 300 K to 1500 K.The results show that the mechanical properties of Si-DLC doped with Si are sensitive to temperature,and the mechanical properties are similar to pure DLC.With increasing temperature,the tensile strength and Young’s modulus of Si-DLC decrease.At room temperature of 300 K,it was found that Si doping reduces the Young’s modulus compared to pure DLC.The higher the doping concentration,the smaller the Young’s modulus compared to pure DLC.At the same density,the highest doping concentration has the smallest reduction in Young’s modulus when the temperature increases to 1500 K.