| Polycrystalline diamond compact(PDC)is a superhard composite material that utilizes the high temperature and high pressure(HPHT)method to sinter diamond powder onto a cemented carbide substrate.It combines the excellent thermal conductivity,ultrahigh hardness and wear resistance of polycrystalline diamond(PCD)with the high impact toughness and easy weldability of cemented carbide and is therefore widely used as cutting tools and oil and gas drilling bits cutter.In recent years,with the deepening of oil and gas resources exploration and development,drilling work has gradually developed to the deep stratum.Although PDC has good efficiency in drilling soft to medium-hard rock formations,the problem of low rate of penetration and premature failure when drilling hard and strong abrasive and non-homogeneous formations at depth has not been effectively solved.Currently,high-end PDC products used in deep and complex formation drilling still heavily rely on imports,which is mainly due to the imported PDC has higher wear resistance,impact resistance and heat resistance.Additionally,they are differentially customized the PDC composition and interfacial geometry according to the formation conditions.High-end PDC has become a bottleneck restricting the independent development of key materials for extracting deep-seated oil and gas resources in China.In order to solve the above problems and develop polycrystalline diamond compacts with high wear resistance,high heat resistance,as well as high impact toughness and high work stability,a systematic study of the factors affecting the performance of PDCs from the root cause is required to enable the summarization of the methods and mechanisms for performance enhancement.Therefore,this paper mainly focuses on the most basic and important factors affecting the performance of PDC,i.e.raw material diamond powder and cemented carbide substrate.We systematically carried out the research on influence law and mechanism of diamond powder formula and substrate interfacial geometry on PDC performance.The enhancement effect and improvement mechanism of cryogenic treatment and SiC whisker addition on PDC were further investigated.The main research work and related conclusions are as follows:1.Macroscopic properties and microscopic characterization of PDC with different single grain sizesThe microscopic features of eight single-grain-size diamond powders were analyzed,and the results of Raman spectroscopy show that the larger the grain size,the larger the surface compressive residual stress value.The microstructures of the PDC samples prepared from different grain sizes of diamond powders were characterized,and the mechanical properties of coarse-grained PDC including hardness,fracture toughness,abrasion resistance,impact toughness and thermal stability were tested.The results showed that the smaller the initial diamond grain size and the higher the Co content of the polycrystalline layer,the higher the hardness and wear resistance of the PDC,and the lower the fracture toughness and impact toughness.The influence of grain size effect on the performance of PDC is dominant,while the influence of Co content is mainly reflected in the thermal stability.The difference in residual stress between PDCs with different grain sizes and the thickness of the Co transition layer at the interface are the key factors affecting the impact resistance of PDC.2.Cold compaction fragmentation behavior of diamond powders with different grain sizes and size ratiosCold compaction experiments were performed on diamond powders with different single and double grain size formulations,and the results showed that the larger the particle size,the easier it is to be crushed under high pressure,and the added filler particles of fine grain can inhibit the degree of crushing phenomenon.With the increase of the proportion of filler particles,the higher the degree of crushing of coarse particles is inhibited.Ideal formula design requires the particle to be crushed to a certain extent under high pressure but not excessively,in order to consume part of the weak grain boundaries,thus improving the strength of the prepared polycrystalline diamond layer.3.Research on the performance law of multi-grain size PDC and optimization design of diamond particle formulationThrough research on the performance law and microscopic characteristics of singlegrain-size PDC,three systems of double-grain-size formulations with different main particle systems were designed.Three kinds of double-grain-size formulations with were preferred through the analysis of a large number of tests on the vertical turning lathe(VTL)rock-cutting performance of PDC.A preliminary attempt of a double-layer filler scheme was designed,with fine-grained diamond as the upper layer and coarse-grained diamond as the lower layer to achieve PDC material with more balanced wear-resistance and impact resistance.Finally,on the basis of the three double-particle-size formulations,combined with the influence of different filler particle ratios on the performance of PDCs with different main grain size systems,trimodal particle size formulations were further designed.The optimal formulations with three different performance orientations were obtained under the experimental system of the present study: wear-resistant formulation;impact resistant formulation and balanced formulation.The wear resistance and cutting life of the optimized design of the wear-resistant formulation compared to the G12 singleparticle-size PDC increased by 249.2% and 50 P,respectively.The wear resistance and cutting life of the impact resistant formulation compared to the G27 single-particle-size PDC increased by 185.2% and 40 P,respectively.The wear resistance and cutting life of the balanced formulation compared to the G20 single-particle-size PDC increased by206.4% and 50 P,respectively.The optimized particle size ratio reduces the voids between the powders to improve the densities of the PDCs prepared under HPHT conditions and reduces the content of cobalt swept from the substrate,which is the main reason for the overall improvement of the PDCs’ comprehensive performance.The optimized threegranularity PDC was subjected to cobalt removal treatment,and the effects of the formulation optimization design were further verified by performance tests.4.Study on the influence of substrate interfacial geometry on the performance of multigranularity PDC and numerical simulation of residual stressPDC samples were prepared using three particle size formulations and cemented carbide substrates with different interfacial geometries,and the mechanical properties of PDC were tested.The results show that the influence of the substrates’ interfacial geometry on the cobalt sweep path in the HPHT process will affect the composition of the polycrystalline layer to a certain extent,thus affecting the mechanical properties.More importantly,the geometry of the substrate influences the microstructure at the interface,thus affecting the interfacial bonding strength.Moreover,numerical simulation calculations show that the interface geometry has a great influence on the distribution of residual stresses within the PDC.These are the most critical reasons for the difference in the impact toughness of PDCs with different substrate geometries.5.Experimental study on PDC reinforcement by cryogenic treatment and SiC whisker additionPDC was cryogenically treated with liquid nitrogen,and its mechanical properties were tested.The results showed that the cryogenic treatment effectively improved the impact toughness of the PDC.The improvement mechanism can be attributed to the enhancement of the residual stress state of PDC and the change in fracture form of the PCD layer.The improvement in drilling performance of anchor bits equipped with cryogenically treated PDC proves that the performance enhancement of PDC brought by cryogenic treatment is irreversible.Thus it can be considered as a complementary means to improve the impact toughness of PDC after cobalt leaching.The introduction of SiC whiskers into the PDC polycrystalline layer can substantially improve the impact toughness of PDC without reducing its wear resistance.This is mainly attributed to its excellent affinity with WC,which to some extent inhibits the abnormal growth of WC grains at the substrate interface and improves the interfacial bonding strength.At the same time,the addition of SiC whiskers improves the consistency of the elemental composition of the polycrystalline layer,which is beneficial to the working stability of PDC. |
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