Study On The Pretreatment Of Cemented Carbide To Promote Co Phase Transformation And Its Effect On The Growth Of Diamond Coating

Due to its extremely excellent physical properties,diamond-coated cemented carbide tool has an irreplaceable position in the current machining industry.However,graphite catalytic effect of Co in cemented carbide would weakens the coating adhesion,which is the main reason for the failure of the diamond-coated tool.To overcome the negative influence,the hollow cathode ion nitriding,hot filament ion nitriding,and decarburization heat treatment are adopted to modify the cemented carbide surface in this paper.Then diamond coating is deposited on the as-pretreated cemented carbide surface by the self-design hot filament CVD equipment.Scanning electron microscope,energy disperse spectroscopy,X-ray diffractometer,X-ray photoelectron spectroscopy,Raman spectrometer,Vickers hardness tester and Rockwell hardness tester are used to characterize the phase and organization of the pretreated substrate and the quality of the diamond coating.The main research achievements are as follows:?1?Co on the surface of cemented carbide can be converted to Co₄N by using hollow cathode ion nitriding and hot filament ion nitriding.However,N cannot diffuse to the deep layer of cemented carbide,which is limited by the upper limit of temperature of equipment.The higher the N content in nitriding atmosphere,the more conducive to the formation of Co₄N and the atmosphere of pure N₂ is the best.Hot filament ion nitriding can lead to the etching of Co on the surface of YG6 and YG12 cemented carbide,thus reducing the content of Co on the surface but producing surface pits.?2?The Co on the surface layer of YG6 cemented carbide can be converted into various Co compounds by decarburization heat treatment,almost without the Co of 0 valence state.In the process of decarburization heat treatment of 1000?for 4h at normal pressure,the addition of H₂ will significantly promote the decarburization of cemented carbide.Before and after the addition of H₂,the nearly complete conversion layer of Co of 0 valence state is 2.5?m and 8?m respectively.But the high temperature decarburization heat treatment will cause the enrichment of Co element on the surface.In addition,in the process of decarburization heat treatment of700?for 4h at normal pressure,Co of 0 valence state within 1.5?m of the surface can be converted to Co₃W almost completely by using the optimized working atmosphere:N₂/H₂=50/6.And the Co contents on the surface will not be increased.?3?The etching effect of the hollow cathode ion nitriding at low temperature and low bias voltage on Co was weak,and the Co₄N on surface did not obviously promote the nucleation and growth of diamond coating.Under the pure N₂ atmosphere,in addition to the formation of extremely thin Co₄N layer,hot filament ion nitriding at high temperature and high bias voltage has a strong etching effect on Co.And then diamond coating with better crystal face development can be deposited on the surface of YG6 and YG12 cemented carbide.However,it will produce pits by etching Co,resulting in poor adhesion of coating.?4?As to YG6 cemented carbide,the Co conversion layer formed by decarburization heat treatment of 1000?cannot promote the nucleation and growth of diamond coating.Pretreated by optimized decarburization heat treatment of 700?for 4h at normal pressure under working atmosphere of N₂/H₂=50/6,diamond coating with good adhesion,high crystallinity can be deposited on the as-pretreated cemented carbide surface.The reduction of H₂ content in the atmosphere will enhance the graphitization degree in the subsequent diamond coating process and reduce the quality of diamond coating.