On The Fabrication And Thermal Damage Mechanisms Of Polycrystalline Diamond Sintered By Coated Diamond Particles

The polycrystalline diamond(PCD)is widely used in machining,geological or petroleum drilling systems due to its excellent properties,such as high hardness,extreme wear performance and great thermal conductivity,etc.However,the application of PCD is severely limited due to its poor thermal stability,including low oxidation and graphitization resistances,as a consequence of catastrophic loss of the mechanical properties.Therefore,this work mainly focuses on the regulation and reinforcement of the thermal stability of PCD.In this work,the titanium(Ti)and boron(B)coating were deposited onto the diamond particles via magnetron sputtering,respectively,and the polycrystalline diamond compacts were synthesized by sintering the coated diamond particles under high temperature and high pressure conditions.The phase compositions,microstructures and thermal stability of the B-PCD and Ti-PCD were studied,while the contributions of the boron and titanium coatings to the oxidation and graphitization resistances enhancement of PCDs were systematically investigated.In addition,the effect of grain size on the thermal stability of PCD was studied.Special efforts had been devoted to fully exploit the role of cobalt content on the thermal damage and tribological behaviors of the annealed PCD,thus achieved an effective regulation on the thermal stability of PCD.The main conclusions of this work are summarized as follows:(1)The B-PCD was successfully synthesized with B-coated diamond particles,which forms a uniform boron carbide barrier.The as-received boron carbide phase enhances the initial graphitization and oxidizing temperatures to 800°C and 780°C,respectively,which are ~100°C and ~30°C higher than those(700°C and 750°C)of the P-PCD sintered with uncoated diamond particles.The Ti-PCD was successfully synthesized with Ti-coated diamond particles,which achieves a uniform titanium carbide barrier.The graphitization and oxidation resistances of Ti-PCD are strengthened due to the existence of titanium carbide phase,which acts as an effective inhibitor.The as-received inhibitor delays the graphitization and oxidation of Ti-PCD,elevating their initial temperature by ~100°C and ~50°C,respectively.The boron carbide and titanium carbide barriers protect the diamond grains from direct contact with the cobalt,prohibiting the cobalt-catalytic graphitization.In addition,the oxidation of the boron carbide and titanium carbide barriers occur prior to that of the diamond grains,which inhibits the PCDs from oxidation.(2)The effect of grain size via cobalt content and distribution form on the thermal stability of the 750℃ annealed PCD was studied.For the PCD with the average diamond grain size of 2 μm and 10 μm,the low amount of cobalt with spherical shape induces no microcrack during the annealing treatment,but the oxygen corrosion.For the specimens with the average diamond grain size of 25 μm and 35 μm,the large amount of cobalt with leaf-like shape leads to the high-level graphitization,microcracks,the ruptured D-D bonds and diamond exfoliation during the annealing treatment,and the PCD was severely damaged.(3)The large amount of cobalt favors the friction reducing and resisting wear behaviors of the 700℃ annealed PCD with big grain size.For the PCD with the average diamond grain size of 25 μm and 35 μm,the large amount of cobalt with leaf-like shape facilitates the diamond graphitization during the annealing treatment,which accelerates the formation of friction reducing carbonaceous transfer film,thus yields to the low friction coefficient.The fully oxidation filling into the spalling inhibits the grain exfoliation,thus prevents the annealed PCD wearing substantially.