| CVD thick diamond films are widely used in the fields of cutting tools and abrasive tools for their extremely high hardness,good wear resistance and high thermal conductivity etc.However,due to its high surface energy and chemical inertness,diamond is difficult to be wetted by ordinary metals during brazing or packaging,resulting in low adhensive strength for brazing and packaging.In order to improve the wettability of CVD thick diamond films as well as bonding strength of brazing and packaging,Ta coatings with good carbon affinity and low thermal expansion coefficient are prepared on unpolished and polished CVD thick diamond films by double glow plasma surface metallurgy technology.The composition,structures,morphologies and the bonding strength between Ta coatings and CVD thick diamond films,which were effected by metallization temperature and time,were studied by means of XRD,SEM,EDS,and scratch test.Then the Ta metallized CVD diamond films were brazed to the cemented carbides.Shear strength of these samples detached from cemented carbides was measured.Finally,the behavior of Ta atoms adsorbated on the diamond surfaces as well as the thermal stress between Ta coatings and diamond substretes were analysed by first principles calcution.The results show that:?1?After metallization,dense and uniform Ta coatings formed on the surfaces of the unpolished and polished CVD thick diamond films,respectively.The grain size of these Ta coatings increased with the metallization temperature.The Ta coatings were composed of fine nanocrystals at low temperature.But columnar crystals of Ta coating microstructures formed during the temperature increased.As the temperature rising,the Ta coatings formed as the coarse equiaxed crystals with defacts,looseness,voids,and cracks appeared.The thickness of Ta coatings increased exponentially with the substrate temperature below 950°C,but the thickness decreased when the temperature reached 950°C.?2?Interdiffusion zone formed between Ta coatings and CVD thick diamond film substrates.Compounds TaC and Ta2C were formed in the interdiffusion zone.The content of compounds increased with the metallization temperature.When holding time was 30 min,preparing Ta coatings on the surface of polished CVD thick diamond films at 850°C was the best solution.The maximum shear strength for samples peeled off from cemented carbides after brazing was 209 MPa.?3?The thickness of Ta coatings prepared on the surfaces of the polished CVD thick diamond film increased linearly with the metallization holding time.The average deposition rate was 0.05?m/min.In addition,as the holding time increased,the grain size of the Ta coatings increased either,but the content of the compounds TaC and Ta2C was substantially unchanged.When the temperature was at 850°C,the optimal time for preparing Ta coatings on the surfaces of CVD thick diamond films was 20 min.The maximum shear strength for the sample shedding from cemented carbides after brazing was 218 MPa.?4?The first-principles calculation results showed that the adsorption energy of Ta atoms on surfaces of diamond?110?,?111?and?311?:E?311?<E?111?<E?110?<0.Ta-C chemical bonds were formed after Ta atoms adsorbed on the three diamond surfaces,which lowered the energy of the system.Since the largest binding energy was attained when Ta atoms was adsorb on the?311?surface of diamond,the more?311?oriented grains on the CVD thick diamond film were,the more advantageous it was to enhance the bonding strength of Ta coatings.It can be seen from thermodynamic analysis that Ta2C had better high temperature stability than TaC,but TaC had lower formation temperature,and both TaC and Ta2C were brittle compounds.The maximum theoretical thermal stress between Ta coatings and diamond substrates was 20 MPa at the experiment conditions. |
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