The Growth Behavior And Surface Performance Enhancement Of Diamond Film Deposited On Polycrystalline Diamond Compact

As a mature superhard material tool,polycrystalline diamond compact(PDC)has been widely used in precision machining tool manufacturing,geological drilling,petroleum and natural gas exploration,and other industries.PDC is prepared by sintering diamond micro powder with cemented carbide WC-Co under high temperature and high pressure(HTHP).It has the ultra-high hardness and abrasive resistance of diamond and the impact fracture toughness and weldability of cemented carbide.Under the condition of HTHP,the fluid cobalt diffuses continuously to the diamond micro-powder layer and form a polycrystalline diamond layer with the diamond skeleton-cobalt microstructure.The strong graphite catalysis of cobalt on the diamond and their thermal mismatch will make the PDC suffer from thermal abrasive wear and impact damage when working in harsh downhole environments.The decobaltization pretreatment is used to reduce the damage of diamond at high temperature.However,as a binder of PDC,the removal of cobalt will expose the voids of diamond skeleton and deteriorate the mechanical behavior of PDC.As a kind of surface protection material with excellent properties of superhard and wear resistance,diamond film can be deposited on the PDC surface to reduce its surface porosity and prevent the further diffusion of cobalt,so as to improve wear resistance and prolong PDCs' service life.What's more,few people have systematically studied the growth behavior of diamond film on this substrate and its effect on surface performance enhancement.Here,epitaxial growth conditions were adopted to deposit the polycrystalline diamond film on PDC by the hot filament chemical vapor deposition technique(HFCVD),which can firmly adhere to the PDC under severe working conditions and enable to enhance its wear resistance.A systematic study is presented into the effects of deposition parameters like substrate temperature,methane concentration,ethanol/methane flow ratio,deposition pressure,and hydrogen flow rate on film growth behavior,which involved the evolution of crystal morphology,deposition rate,crystallization quality,and the most important,growth mode of the film on the surface of the PDC.A unified theoretical explanation is given for the corresponding results.Beside the SEM and Raman spectra,fluorescent penetrant inspection and vertical turret lathe(VTL)was used to characterize the feasibility of film deposited on PDC with different decobaltization pretreatment and deposition time for PDC surface performance enhancement.As a result,under deposition conditions of low temperature,high methane concentration,high pressure,and small hydrogen flow,the crystal grain surfaces have a large number of secondary nucleation and multiple-twins with a relatively high growth parameter ?.In this case,the film and the PDC substrate are loosely bonded with relatively poor adhesion,which is easy to cause edge chipping and delamination when grinding and not proper to surface performance enhancement.While under the deposition conditions of high temperature,low methane concentration,low pressure,and large hydrogen flow,the crystal grain surfaces change to flat and smooth with a relatively low growth parameter ?,and the film is dominated by epitaxial growth,which is closely bonded with PDC without cracks and delamination.It is found that the more cobalt removed from PDC,the fewer cracks will the film have under the same deposition time.After synthesizing the adhesion of the film,the integrity of PDC,and the quality of crystallization,the optimized epitaxial conditions for the CVD of PDC were concluded and tested.The test suggested that a longer deposition time will aggravate the formation of cracks in films or between the polycrystalline diamond layer and the WC-Co cemented carbide layer.It is found that after 21 days of decobaltization pretreatment,the diamond films deposited on PDC by the optimized epitaxial growth conditions for 16 hours can firmly adhere to the substrate under the severe VTL testing conditions and provide the best surface performance enhancement.