Understanding The Nanomechanical Properties And Deformation Mechanism Of Single Crystal Iron Carbide

Thanks to its superior properties including ultra-high hardness,high thermal conductivity and large band gap,diamond plays an irreplaceable role in many fields such as cutting tools and optoelectronics.Specifically,smooth surface and damage-free sub-surface is required when applied in the field of optoelectronics.It is difficult to control the surface roughness for natural and synthetic diamonds during their growth process.Therefore,a series of machining processes are needed to prepare the high-quality diamond substrates that meet the requirements.The existing diamond processing technology has the disadvantages of poor precision and low efficiency that would limit the further applications of diamond.Recently,reactional friction between pure metal and diamond was developed to achieve efficient removal of diamonds.In this process,carbides are generated during friction and the carbides are subsequently removed by abrasives.However,removal mechanism of diamond using this technology has not been clearly understood.In particular,the mechanical properties and removal mechanism of iron carbide（Fe₃C）,which is formed by the frictional reaction between pure iron and diamond,need to be studied urgently.In addition,steel has been the dominant structural material for long time.In recent years,ultra-high-strength pearlite steel has attracted intensive attention.Iron carbide,as the main hard phase of pearlite steel,is essential for maintaining high strength and toughness.Therefore,the study of the mechanical properties and deformation mechanism of single crystal iron carbide is beneficial to understanding and optimization of the processing technology for removal of diamond in frictional reaction,and it is also of guiding significance for understanding and manipulating the mechanical properties of pearlite steel.In this thesis,single crystalline iron carbide grains were obtained by polishing hypereutectic white cast iron,nanoindentation and nanoscratch experiments were performed on different crystal planes of single crystalline iron carbide.The mechanical properties were obtained by nanoindentation,and the nanoscratch-introduced surface and subsurface damage were observed by scanning electron microscopy and transmission electron microscopy,the deformation and removal mechanism and anisotropic characteristics were analyzed.Molecular dynamics simulation was adopted to study the mechanical properties of single-crystal iron carbide and further revealed its deformation mechanism.The main research contents and conclusions in this thesis are as follows:（1）The hardness was measured on the crystalline planes of single crystal iron carbide with low indexes of（100）and（110）,and the values were much lower than that of diamond.From the perspective of mechanical properties,iron carbide as the frictional reaction product is easy to remove by abrasive particles,therefore pure iron might be suitable as a reaction abrasive in the composite grinding wheels for machining diamond.It was also found that iron carbide had plastic deformation characteristics and material accumulation under the nanoindentation at room temperature.（2）Nanoscratch experiments were carried out on different crystal planes of iron carbide.It was found that（100）crystal plane exhibited extremely strong deformation anisotropy:when scratching along the[010]direction,there were clear slip lines on the surface,and slips on the{103}planes were observed in the subsurface;while not significant slippage appears when scratching along the[001]direction.When scratching along the[11 0]direction on（110）plane,slip lines which are different from that on（100）crystal planes were found,and slips of the{113}crystal planes were observed in the subsurface.Obvious anisotropy in deformation was exist between the（100）and（110）crystal planes of iron carbide.Through the analysis of high resolution transmission electron microscopy images,polycrystalline was found in the severely deformed area of iron carbide.Variable load scratches on（100）and（110）planes were also carried out,and the critical loading conditions for each deformation were clarified.The results of nanoscratch experiments show that the deformation of single crystalline iron carbide has strong anisotropic characteristics along different crystal orientations,as well as between different crystal planes.（3）Molecular dynamics simulations were conducted for both nanoindentation and nanoscratch.The orientations of single crystal iron carbide in simulation were in line with that in experiments.Slips of{103}planes were found in both indentation and scratch simulation of（100）-oriented iron carbide,while there is no slip on（100）planes,indicating that the slips of iron carbide on the（103）planes were more easily to activate.