Preparation,Microstructure And Properties Of Cemented Carbides And Tungsten-based Materials With Special Composition And Structure

WC-Co based cemented carbides are composed of WC hard phase and Co bonding phase.They are traditionaly produced by powder metallurgy process.WC-Co cemented carbides have been widely used in many industrial fields,such as cutting,turning,rock drilling,and milling areas due to their unique combination of high hardness,strenthgh and wear resistance,good electrical and thermal conductivity,resistance to acid corrosion,alkali corrosion and oxidation,and many other excellent properties.For traditional WC-Co cemented carbides with uniform composition and microstructure,there exists a contradiction between the hardness and the toughness.When the hardness of alloys increases,the toughness decreases,and vice versa.With the development of modern industries,the WC-Co based cemented carbides sustain more rigorous service conditions and higher comprehensive properties have been required for WC-Co cemented carbides such as a combination of high hardness and good toughness.In order to realize this,many attempts have been made,include improving the performances by preparing ultrafine-or nano-crystalline cemented carbides,improving the binder phase,doping cemented carbides with rare earth elements and preparing graded cemented carbides,etc.This dissertation mainly uses WC-Co based cemented carbide as the research object.and tries to prepare high-performance cemented carbides by using novel sintering method,partial substituting of Co in the binder phase,design of alloy structure and doping rare earth element in the alloys.Besides,based on the research results of rare earth element doping in WC-Co based cemented carbides,W-based materials doped with rare earth oxides were also prepared to investigate the effects of rare earth elements doping on the microstrue and properties of the W based materials.The main results obtained as follows:?1?Preparation of cemented carbide by sintering through microwave rapid heating.The WC-10Co powder mixtures were prepared by ball milling.After compaction,debonding,and microwave sintering,sintered WC-10Co cemented carbide samples were obtained.The results show that by ball milling for 24 h,WC-10Co powder mixtures with uniform composition and fine particle size were obtained.And sintered WC-10Co samples with relative density of 99.5%and mean WC grain size of 291 nm were also obtained by microwave sintering at 1350�C for 20 minutes.Hardness,transverse rupture strength and fracture toughness of the sintered samples are 90.7HRA,2044 MPa and 11.3 MPa?m^1/2,respectively.?2?Co-Cu composite powders were prepared by the solution combustion synthesis method,and then mixed with WC powders by ball milling.After pressing and microwave sintering at 1350�C,WC-10?Co/Cu?cemented carbides were obtained.The microstructure and performance test results show that addition of a small amount of Cu in the WC-Co cemented carbides can facilitate sintering densification,restrain WC grain growth and improve the mechanical properties of the alloys.Cu exists in the form of elemental or solid solution in the Co-rich binder phase in the sintered.The Rockwell hardness and fracture toughness of the WC-10?Co/Cu?alloys with a 0.5 wt%Cu addition reach 90.7 HRA and 14.3 MPa?m^1/2,respectively.And the transverse rupture strength reaches 2544.7 MPa with 1 wt%Cu content.?3?By controlling the ball milling time,WC-Co powders mixtures with different particle sizes and Co contents were prepared.WC-Co graded cemented carbides with layered structure were fabricated by lamination pressing and microwave sintering.The experimental results show that increasing the ball milling time from 6 to 24 h,the particle size of WC-Co powder mixtures decreases from 0.31 to 0.11?m.After sintering,the relative density of the sintered samples is higher than 99.7%and no?-phase was detected.In addition,the Co in the sintered samples still as a layered distribution,and the interface between the layers is well bonded.In the inner layer of sintered sampls,the mean WC grain size is 529 nm,while in the outer layer,it is only274 nm.Because of the difference in Co content and WC grain size,the graded WC-Co sampls show a Rockwell hardness of 90.75 HRA at the surface and a fracture toughness of 18.12 MPa?m^-1/2 in the core.The alloys have gradient properties of high hardness in the surface and high toughness in the core.?4?La₂O₃/WC-Co powder mixtures were prepared by ball milling.After pressing and microwave sintering at 1350�C,the sintered samples were obtained.The microstructure and performance test results show that addition of an appropriate amount of La₂O₃ can improve WC-Co alloys structure uniformity and decrease WC grain size.La₂O₃ exists in the sintered samples with the nano-size single particles or agglomerated particles of about 200 nm.Two types of interface relationships were observed between the La₂O₃ and WC phases in the samples.One is that of the semi-coherent interface,and the other is that of the poorly coherent interface where structural disorder is found in the 1-3 nm wide regions between the La₂O₃ and WC phases.However,this structural disorder does not occur in the interfaces between Co and La₂O₃ phases.Addition of a minor amount of La₂O₃ has less effect on the relative density of sintered samples.When the La₂O₃ content is 0.8 wt.%?the mass ratio of La₂O₃ in Co?,the Rockwell hardness and fracture toughness reach a maximum value of 91.8 HRA and 13.44 MPa?m^1/2,respectively.The transverse rupture strength reaches a maximum value of 2359 MPa when the La₂O₃ content is 1.2 wt.%.?5?W coated La₂O₃ composite powders with core-shell structure were prepared by a wet chemical method.The composite powders have polyhedral shape with particle size about 100-200 nm.After pressing and sintered at 2200�C for 120 min in Ar atmosphere,La₂O₃/W composites were obtainded.The measurement results show that with the increase of La₂O₃ contents,mean size of W grain in the sintered La₂O₃/W samples decreases and some core-shell structures still remain in the sintered samples.Maximum relative density,transverse rupture strength and Rockwell hardness of96.93%,434 MPa and 70.3 HRA,respectively,were achieved for sintered La₂O₃/W composites containing 0.6 wt.%of La₂O₃.