Study On The Fabrication, Structures And Mechanical Properties Of High Performance Co-free Cemented Carbides For Geo-engineering Applications

Because of the high hardness, high flexural strength and excellent fracture toughness, cemented carbides have been widely used in geo-engineering fields as the tool materials of drill buttons and picks. However, the applications of WC-Co in high oxidative/corrosive environments, such as drilling in saline-alkali land or the bottom of sea, were limited due to their relative inferior oxidation/corrosion resistance. What’s more, considering the huge consumption of drill buttons and picks in geo-engineering and the high price of Co, it is significant to reduce the use of Co in drill buttons and picks to lower the drilling cost. Therefore, this dissertation focused on the fabrication of high performance Co-free cemented carbides suitable for geo-engineering applications.This work mainly focused on the improvement of the performance of ultrafine WC-Ni and binderless cemented carbides by compositing them with the super-hard particles, nano-whisker or fibers of high strength, or ceramic nano-powders. By analyzing the sintering behavior, microstructure, mechanical properties and wear behavior of the obtained samples, the following results can be obtained.(1) The graphitization of diamond and the phase transition of cBN during sintering can be reduced by Ti-coating or Cu-coating on the super-hard particles. With 6 wt.% Diamond Ti, 8 wt.% DiamondCu and 15 vol.% cBNTi, the hardness of the obtained samples reached their maximum of about 2000, 2000 and 1820 HV10, respectively. With 2 wt.% Diamond Ti, 2 wt.% DiamondCu and 5 vol.% cBNTi, the flexural strength reached their maximum of about 960, 950 and 1500 MPa, respectively. During the wearing with granite, with appropriate added fraction of super-hard particles, the main wear mechanisms were the extrusion and removal of binder phase, the crush and pull-out of WCgrains, and the worn-out of super-hard particels. With more super-hard particles addition, the fracture and spalling of the cutting edge and super-hard particles became more and more serious.(2) The flexural strength and fracture toughness of the WC-Ni cemented carbides can be improved with 0.53 wt.% SiCw addition. However, with more SiCw addition, the agglomeration of the nano-whisker became more and more serious. The WC-Ni cemented carbides can be toughened by in situ transformed carbon fiber using pre-oxidized polyacrylonitrile fiber(PANf) as precursor. The sample sintered at 1300 ℃ with 10 vol.% PANf had shown the highest hardness and strength and the sample with 20 vol.% PANf had shown the highest fracture toughness. During the wearing with granite, the main wear mechanisms were the extrusion and removal of binder phase, and the crush and pull-out of WC grains.(3) The relative density, Vickers hardness, flexural strength and fracture toughness of the WC-Ni cemented carbides can be improved with 3-5 wt.% ZrC nano-powder addition. In this case, the composites had shown the lowest wear rate during the wearing with granite. However, with more than 7 wt.% ZrC nano-powder addition, the agglomeration of Zr-rich particles became more and more serious, resulting in the reduction of relative density, hardness, strength and toughness of the samples. In this case, the wear rate of composites gradually increased. During the wearing with granite, the main wear mechanisms were the extrusion and removal of binder phase, and the crush and pull-out of WC grains.(4) By adding appropriate content of AlN, La2O3 or ZrC nano-powder(no more than 5 wt.%), the relative density of the binderless cemented carbides can be improved, the WC grain growth can be inhibited, and the Vickers hardness and flexural strength of binderless cemented carbides can be increased. However, with more than 5 wt.% nano-powder addition, the agglomeration of the nano-powder became more and more serious, resulting in the reduction of the relative density, hardness and flexural strength. With the increase in the added fraction of Al N and La2O3 nano-powders, the fracture toughness of the composites gradually decreased. However, with the increase in the added fraction of ZrC nano-powder, the fracture toughness of the composites increased at first and then decreased. The wear rate of the composites decreased at first and then increased with the increase in the addition amount of ceramic nano-powder. When the flexural strength of the composites was higher than about 1000 MPa, the main wear mechanisms were the crush and pull-out of WC grains. But when the strength was lower than about 1000 MPa, the fracture and spalling of the cutting edge became more and more serious.