Surface Densification Modification And Performance Analysis Of Graphite Materials

Graphite materials have been widely used in many fields such as electronics,machinery,and semiconductors due to their excellent properties such as electrical conductivity,lubricity and low density.However,due to graphite’s lubrication characteristics,it is easy to produce powder and debris,which greatly limits its application in machinery.Therefore,the study aims to improve the mechanical properties of graphite without undermining its excellent inherent properties.The thermo-reactive deposition and diffusion（TRD）technology,widely used as a surface treatment process in steel metallurgy,can be applied to the field of graphite surface modification and play a positive role.Our work reports the preparation of Nb-Cr complex coating on the surface of graphite materials by TRD technology.The main research contents are as follows:1.The effects of TRD treatment temperature and time on the morphology and structure of Nb-Cr carbide coatings on graphite were studied at 1073K-1323K for 2-8hours,based on borax（Na₂B₄O₇）and boron trioxide（B₂O₃）as the molten salt system.The results show that the positive correlation of coating thicknesses and the TRD treatment temperature/time shows a diverse pattern.The variation of coating thickness conforms to the classical kinetic theory of film growth.The optimum TRD treatment temperature range was 1273K-1323K and the best treatment time was about 7h.2.The growth mechanism of Nb-Cr composite coatings and the distribution of elements in the coatings were analyzed by thermodynamic calculation.The theoretical feasibility of the TRD treatment method to realize the modification of the metal carbide coating on the surface of the graphite material was confirmed.The results show that the distribution of elements in the coatings is differential at various TRD temperatures.Besides,a rare element distribution pattern of the Nb-Cr multilayer alternating structure coating was observed,and a possible explanation was made based on the zero-solubility theory and amplitude modulation decomposition theory of Nb in chromium carbide（Cr₇C₃）.3.The prepared Nb-Cr coatings showed compactness,uniformity,and good adhesions to the graphite substrates.The microhardness is as high as 3000 HV with gradual cross-sectional microhardness.The overall friction coefficient of the coatings is about 0.25-0.26,and the lowest wear rate is 1.78×10^-4 mm³/（N·m）,and its thermal expansion coefficient（CTE）matches the graphite substrate in the range of 313K-723K.In addition,the resistivity of the coating is about 1.2×10^-7Ω·m,exhibiting excellent electrical conductivity comparable to metal materials.The corrosion current density is as low as about 0.2μA·cm^-2,which is up to 3 orders of magnitude higher than that of pure graphite materials.