Synthesis And Adhesion Of HfC-SiC Interlaters For Diamond Coated Cemented Carbides

Diamond coated cemented carbide(WC-Co)cutting tools have vast application in cutting non-ferrous metals and their alloys,particulate or fiber reinforced composite materials and ceramic composites,due to the extented service life by combining the unique properties of diamond and WC-Co substrates.However,the insufficient coating adhesion limits their applications.The main reason of the adhesion problem is that the cobalt(Co)binder phase inside the WC-Co matrix strongly inhibit diamond nucleation and facilitate the formation of graphite,which is detrimental to the coating adhesion.Another reason is that the significant differences in thermal expansion coefficients(CTE)and hardness between diamond and the WC-Co matrix will introduce large residual thermal stress at the coating/substrate interface.In this paper,two new types of interlayers metallurgically bonded to the WC-Co substrate,i.e.SiC/HfC and HfC-SiC/HfC dual-interlayers were prepared by double glow plasma surface alloying(DGPSA)technique to inhibit the detrimental effect of Co and enhance the coating adhesion.The HfC and SiC were selected as the main composition of the interlayers in considering their relatively low CTE values closed to that of WC-Co and high hardness between diamond and WC-Co.The low CTE would help to relax the thermal stress caused by the CTE mismatch,and the high hardness would realize an inherently better matching performance in the coating system.The SiC/HfC dual-interlayer was composed of a HfC dominant inner layer and a SiC dominant outer layer.The HfC-SiC/HfC dual-interlayer had same inner layer as that of the SiC/HfC dual-interlayer,while its outer layer was made up of HfC-SiC mixtures with a gradient structure component.Firstly,the DGPSA device was simulated and modified before the interlayer preparations.Secondly,the effect of the preparation parameters on the microstructure,phase composition,hardness and adhesion of two types of interlayers were investigated.Then,the optimized microstructure of the interlayers was confirmed for adherent diamond deposition.Finally,the effectiveness of the optimized interlayers on the diamond adhesion was discussed.The results of this research would be expected to provide novel approach for enhancing the adhesion of diamond coatings on WC-Co cutting tools.The main contents and results are summarized as follows:(1)The feasibility of SiC synthesis using DGPSA technique was investigated.The gas flow field in the vacuum chamber of the DGPSA device was simulated and optimized by COMSOL software.Thereafter,the DGPSA device was modified according to the simulation results,and then experimental verification was operated.The results showed that SiC coatings could not prepared by the existed DGPSA device even if a high TMS flow rate was adopted.Because the thermal insulation cover in the vacuum chamber prevented the reactive gas to flow into the plasma above the sample.Hence,no gas source was provided for SiC synthesis.To resolve the problems,two types of gas inlet and outlet way were proposed and simulated.After simulation and optimization,it was found that gas flow field with high intensity and good uniformity could be obtained when the gas entered the vacuum chamber from the sidewall of the thermal insulation cover and flew out through the temperature observation hole and a new added outlet hole.It was the most likely gas inlet and outlet way to the synthesis of SiC.To verify the speculation,SiC coatings were prepared by the modified DGPSA device according to the simulations.The results of the verification experiments confirmed that SiC coatings could be successfully produced under different TMS flows and substrate temperatures.However,the density and adhesion of the coatings were very poor due to Co catalysis.It indicated the directly deposited SiC coatings were not suitable to be used as interlayers for diamond deposition.(2)SiC/HfC dual-interlayers were prepared onto the WC-Co substrate by the modified DGPSA device under different substrate temperatures and TMS flow rates.The surface morphologies,phase composition,hardness and adhesion of the interlayers were investigated,and the effect of the optimized interlayers on the diamond adhesion was analyzed.The results demonstrated that the Si C/HfC dual-interlayers prepared under substrate temperature of 800 ? and TMS flow rate of 2.0 sccm had dense structure,relatively highest microhardness and largest adhesion strength among all the interlayers.The outward diffusion of the Co binder was effectively inhibited by the SiC/HfC dual-interlayer,and the diamond coating deposited on this interlayer possessed a excellent adhesion,referred to class HF3.(3)Based on the research results about the SiC/HfC dual-interalyers,gradient-modified HfC-Si C/HfC dual-interlayers were prepared onto WC-Co substrate by increasing TMS flow rate during the outer layer deposition.The impacts of TMS flow rate increments on the composition,microstructure and properties of the interlayers was researched and the optimized interlayer was obtained.Subsequently,diamond coatings were deposited and the effectiveness of the gradient-modified HfC-SiC/HfC dual-interlayers in enhancing diamond adhesion was investigated.The results indicated that the HfC-SiC/HfC dual-interlayers possessed similar microstructures to those of the SiC/HfC dual-interlayers,except that the interfaces between the HfC inner layers and the HfC-SiC outer layers were weakened.Moreover,the HfC and SiC distributed gradually in the out layers.The dual-interlayers synthesized with TMS flow rate increment of 0.20 sccm possessed relatively highest hardness,roughest surface and largest adhesion strength among all the gradient-modified interlayers.The diamond coatings on the gradient HfC-SiC/HfC duo-interlayered substrates had better adhesion strength and lower residual stress than those of the coatings on the SiC/HfC duo-interlayered substrates.Therefore,the gradient-modified HfC-SiC/HfC dual-interlayer is an optimized approach for enhancing the adhesion of diamond coatings.