Preparation And Properties Of Diamond/Cemented Carbide Transition Layer By Double Glow Plasma Method

CVD diamond-coated WC-Co cutting tools possess a series of excellent properties,such as high hardness and wear resistance,extremely high thermal conductivity,and low coefficient of friction,which makes them possess a bright future in cutting difficult-to-cut materials（e.g.,carbon plastics,graphite,alumosilicic alloys wood,etc.）.However,graphite can be formed due to the catalysis of Co during the growth of the diamond coatings.Moreover,there are obvious differences in the hardness and thermal expansion coefficient between the diamond and cemented carbide substrates.Therefore,these factors result in the inadequate adhesion property between the diamond coating and cemented carbide materials,which should be solved urgently.Based on the above problems,in this manuscript,the Ti Mo Ta interlayers were first developed on WC-6%Co via the double glow plasma surface alloying（DGPSA）technique.And then the Ta and Ti Mo Ta diffusion layers with gradient composition and structure were prepared as the Co and C barrier layers by the combination with the DGPSA technique and the reverse-sputtering process,which takes advantage of the technical characteristics of plasma reverse sputtering technology and the merits of the application of transition layer method.In this way,the interface stress can be reduced,the nucleation density of the diamond can be increased,and finally,the diamond-coated carbide tool with high bonding strength can be obtained.The main research contents and conclusions of this manuscript are as follows:（1）Through the design of material components and elements,Ti Mo Ta multi-component alloy transition layers were fabricated on the WC-Co substrate,and their effects on diamond nucleation and growth were investigated.The results showed that the Ti Mo Ta interlayer had high compatibility with the WC-Co substrate,and promoted the nucleation and growth of diamond.This was due to the formation of stable compounds at the interface between the Ti Mo Ta interlayer and substrate,which can effectively inhibit the outward diffusion of Co,and the Ti Mo Ta interlayers showed a higher surface hardness(～3633 HV_{0.98 N})and a lower specific wear rate(5.55×10^-6mm³N^-1m^-1),narrowing the gap between the physical properties of the diamond coating and the substrate.Based on this,the effects of deposition time and temperature on the morphology,composition,and mechanical performance of the Ti Mo Ta interlayer were systematically analyzed.The results showed that the Ti Mo Ta interlayer prepared at 830℃for 30 min had a compact structure and good quality diamond coating.Therefore,the Ti Mo Ta interlayer with nanocrystalline structure was an ideal transition layer material to improve the adhesion performance of the diamond coating.（2）The Ta diffusion layer with gradient structure was prepared on the WC-6%Co substrate surface using the plasma reverse-sputtering process for the first time,and the role of the Ta diffusion layer as a cobalt diffusion barrier was investigated.The results indicated that the removal of the Ta deposition layer was beneficial to relieve the difference in physical performance between the substrate and diamond coating.With the increased of reverse-sputtering time,the thickness of the Ta deposition layer and the content of Ta elements decreased.After 30 min reverse-sputtering,the Ta diffusion layer showed a higher surface hardness,～3496.3 HV_1.96N.Meanwhile,the increase in surface defects effectively improved the nucleation rate and quality of a diamond.Diamond coating with compact structure and good quality was deposited on the Ta diffusion layer after 30 min and 60 min reverse-sputtering.This can be attributed to the fact that the Ta diffusion layer effectively inhibited the diffusion of Co,and finally,the diamond coating showed good bonding performance on the substrate with the Ta diffusion layer.（3）Based on conclusions（1）and（2）,the gradient structure formed by combining Ti Mo Ta alloy elements to inhibit Co diffusion and reverse sputtering can promote diamond nucleation.The Ti Mo Ta diffusion layer was prepared on the WC-Co substrate using the process of plasma reverse-sputtering,and the influence of potential difference and time of reverse sputtering on the morphology,microstructure,mechanical performance,and adhesion of the Ti Mo Ta diffusion layer were studied.The results showed that both had significant effects on the structure and properties of the interlayer.The potential difference between the workpiece cathode and source electrode influenced the interlayer properties by adjusting the energy and density of Ar⁺bombardment on the surface of the substrate.When the potential difference was 300 V,the Ti Mo Ta diffusion layer with a gradient structure can be obtained,and its adhesion reached HF-1 grades.Meanwhile,the Ti Mo Ta diffusion layer can effectively inhibit the diffusion of cobalt and obtain diamond coating with good adhesion and high wear resistance.（4）Through the above experiments,it is confirmed that the process of plasma reverse-sputtering is an effective approach to enhance the adhesion between the diamond coating and cemented carbide.As a new process to improve the adhesion of the interface proposed for the first time,the working principle of plasma reverse sputtering needs further discussion and analysis.In this manuscript,the plasma motion,transmission characteristics,and their physical parameters in the double glow discharge region during deposition and reverse sputtering were in situ diagnostic by atomic emission spectroscopy（OES）.The results showed that the plasma during reverse sputtering was composed of a large number of neutral particle clusters（Ar I,Ti I,Ta I,Mo I,W I）and a small number of monovalent ions（Ar⁺and Ti⁺）,and the plasma was stable and uniform,with high repeatability.This is of great significance for exploring the mechanism of ion permeation of metals and controlling the plasma treatment process.In conclusion,Ti Mo Ta interlayer,Ta diffusion layer,and Ti Mo Ta diffusion layer with high interfacial strength were prepared on the surface of WC-Co using DGPSA combined with plasma reverse-sputtering process,which improved the adhesion and tribological properties of CVD diamond coated cemented carbide.In this study,the double glow plasma surface alloying technique combined with the plasma reverse-sputtering process to prepare the gradient diffusion layer provides a new idea for the application of surface modification.