Study On Multilayered Diamond/?-SiC Composite Coatings By Hot Filament Chemical Vapor Deposition

Diamond has great application prospect as a super-hard coating material deposited on cemented carbide(WC-Co)because of its excellent physical properties such as high hardness,high modulus,high thermal conductivity and high wear resistance,etc.However,the industrial application of diamond coating has been restricted for long time by the following problems:Firstly,the diffusion of Co bonding phase from WC-Co substrates to diamond coatings to catalyze diamond into graphite,deteriorating the mechanical properties of diamond coatings;Secondly,the exorbitant residual stress occurring between diamond and WC-Co substrates due to the difference of thermal expansion coefficient is destructive the adhesive performances of diamond coatings;Thirdly,the difference of mechanical properties of diamond coating on different WC-Co substrates is aggravated due to the selectivity problem between diamond and WC-Co substrates,restricting the application of diamond coatings.It is more important that the mechanism between structure design of diamond coating and mechanical properties is not clear.Therefore,based on the above problems,combined with the high performance diamond/?-SiC composite coatings having successfully deposited in the early stage of our group,a series of new multilayered diamond/?-SiC composite coatings has been design and synthetized.The structure design and optimization,growth preparation,microstructure analysis,mechanical properties and cutting performance of diamond coatings have been studied in this paper.Firstly,the effects of tetramethylsilane(TMS)flow,deposition power,substrates temperature and pressure on the structure evolution of diamond/?-SiC composite coating have been studied.The results show that the variety of TMS flow(10 sccm?15 sccm)is the most significant for the structure regulation of diamond/?-SiC composite coating at low deposition pressure(?3.6 kW).Furthermore,lower substrates temperature(?850?)and appropriate deposition pressure(5 kPa)is more beneficial for continuous control of high-performance diamond/?-SiC composite coatings.Secondly,the different structural conventional diamond coatings and three new multilayered diamond/?-SiC composited coatings have been deposited on WC-Co substrates by hot filament vapor chemical deposition(HFCVD).The research results indicate that the multilayered diamond/?-SiC composited coating structured with diamond/?-SiC-microcrystalline diamond(MCD)multilayer+nanocrystalline diamond(NCD)top processes an excellent combination of high hardness(83.6 GPa),high modulus(856.1 GPa),high adhesion(52.9 N)and high crack propagation resistance(?164 ?m).Furthermore,the coating performs the longest service life and best machining accuracy in milling tests.Firstly,the improved adhesion and low residual stress are realized by diamond/?-SiC composite as interface and support layers.Secondly,the multilayer structure ensure low surface roughness,high crack propagation resistance,enhanced toughness and plastic deformation resistance.Finally,the high hardness and high modulus of the coating are guaranteed by the continuous deposition of MCD layers with high crystal quality,and NCD top layer ensures coating process low roughness.Finally,a study on the universality of multilayered diamond/?-SiC composite coatings for different WC-Co substrates have been carried out.The study results show that the conventional diamond coatings present selective problem to different WC-Co substrates.The microstructural discrepancy on different substrate surfaces after etching is determined as the main reason leading to the substrate selection problem for the conventional diamond coatings.By contrast,the difference in mechanical performance of diamond/?-SiC composite multilayer coatings on three substrates becomes indistinguishable.The excellent mechanical properties including the lowest residual stress(-0.34 GPa?-0.45 GPa),the smallest delamination(0.13 mm2?0.16 mm2)and the highest critical load(51.8 N?52.3 N)are achieved on all three substrates.The formation of denser interface eliminates the discrepancies in the interface structure on three different substrates during the deposition of diamond/?-SiC composite interlayer due to non-selective growth of ?-SiC phases.The multilayer structure with diamond/?-SiC composite layer and MCD layer ensures that the coating has high adhesive strength and mechanical properties.Diamond/?-SiC composite interface layer acts as a barrier to crack initiation and upward propagation from poor Co layer.The combination of above three aspects work together to eliminate the interfacial microstructure differences of diamond coatings on different WC-Co substrates and improve the universality of diamond coatings.This paper,the diamond/?-SiC composite structure have been introduced into diamond multilayer structure design system.The multilayered diamond/?-SiC composite coating can optimize the structure of diamond coating,strengthens the mechanical properties of diamond coating,and improve the universality of diamond coating,which has great industrial application value.