Preparation Of Siliconized Graphite By Liquid Silicon Infiltration And In-situ Reaction Method

As a kind of silicon carbide reinforced graphite composite material,graphite silicide not only has high hardness,high wear resistance,high temperature oxidation resistance of silicon carbide materials,but also has good self-lubricating performance,heat conduction ability and low thermal expansion coefficient of graphite materials,which can be applied to high-precision parts such as mechanical seals.The performance of silicified graphite sealing material largely depends on the physical and chemical characteristics of graphite matrix and siliconizing process.Therefore,it is necessary to study the effects of graphite matrix density and porosity,high temperature treatment process and siliconizing method on the mechanical properties,thermophysical properties and friction properties of silicified graphite,which can provide a theoretical basis for improving silicified graphite performance.Taking commercial graphite with different density as matrix,the structure and properties of commercial graphite were improved by heat treatment and graphitization.With silicon carbide and silicon powder as silicon source,carbon powder as carbon source and alumina as sintering additive,the silicified graphite was obtained by liquid silicon infiltration and in-situ reaction.The effects of density and porosity of graphite matrix,high temperature treatment and siliconizing method on mechanical properties,thermophysical properties and friction properties of silicified graphite were studied.The results show that:1.The silicon carbide in the silicified graphite obtained by liquid silicon infiltration mainly exists in the surface and internal pores of the graphite,and there is a thick silicon carbide layer on the surface of the silicified graphite obtained by in-situ reaction.The depth of siliconized graphite or the thickness of Si C layer is affected by the density and open porosity of graphite matrix.The graphite matrix with low density and high open porosity is easier to obtain deeper depth of siliconized graphite or thicker Si C layer.2.The results show that the porosity size and distribution characteristics of No.4 graphite are more suitable for siliconizing.The silicified graphite silicon carbide prepared with No.4graphite as matrix has small grains,uniform distribution and high mechanical strength.The maximum compressive strength is 106.7 MPa,which is 70.1% higher than the original graphite,and the maximum bending strength is 109.9 MPa,which is 142.6% higher than the original graphite.3.Compared with the heat treatment at 1200 ℃,the graphitization degree of graphitized graphite at 2400 ℃ is significantly improved,which can maintain its volume stability in the process of siliconizing and make the mechanical properties of silicified graphite more stable.4.After graphitization and liquid silicon infiltration,the coefficient of thermal expansion of graphite decreases to a certain extent,and the thermal diffusivity of graphite increases obviously.5.The results show that the liquid silicon infiltration method can retain the good selflubricating property of graphite,while the introduction of silicon carbide network can enhance the wear resistance of the material,and its wear mechanism is mainly fatigue wear;the in-situ reaction method has no obvious improvement on the wear resistance of graphite,and its wear form is mainly abrasive wear.