Densification and Properties of Hot Pressing Sintered Ultrafine WC-Co Cemented Carbide
Ultrafine WC-Co cemented carbide exhibited increased hardness,toughness,and wear resistance,and is an essential avenue for future cemented carbide research.However,the WC grains were exceedingly easily grown during the sintering process,which was not favorable to the fabrication of ultrafine WC-Co cemented carbide.Hot-press sintering(HP)allows to produce highly density materials at lower sintering temperatures,effectively inhibiting grain growth.The densification process of the alloys at various sintering temperatures was investigated using the traditional pressure-assisted sintering model in this experiment to discuss the impacts of sintering temperature on microstructure and mechanical characteristics.The nano WC-10Co composite powders were chosen as the sintering material,and WC-Co cemented carbide was prepared in a hot press furnace with different sintering temperatures(1100,1150,1200 and 1250 ℃as variables and held for 1 h.The morphology of WC-Co composite powder was observed by transmission electron microscopy(TEM).Scanning electron microscopy(SEM)was used to observe the microstructural features of the samples,and optical microscopy(OM)was used to count the crack length of the indentation.The density of the alloy was determined using the Archimedes principle,its hardness was determined using Vickers hardness tester(HYHV-30Z)measurements,the fracture toughness was calculated using the statistical indentation length,and its average grain size of WC was measured in SEM images using the line intercept method.The results showed that as the sintering temperature was raised,the density of WC-Co cemented carbide increased progressively,with the density exceeding 99％at 1200 and 1250 ℃,indicating that a specific sintering temperature was required to produce high-density samples.In addition,the stress index nvaried between 1 and 3 at sintering temperatures ranging from 1100 to 1200 ℃.In the pre-sintering stage n≈1,corresponding to the low-stress state,diffusion dominated the densification process.As the temperature rose and the holding time extended,n value changed,corresponding to different densification mechanisms,with n corresponding to 1.98 and 2.95 at the late stages of holding at 1150 and 1200 ℃,respectively,where the densification mechanism changed from grain boundary sliding to plastic deformation.It was also discovered that at 1250 ℃,which corresponded to liquid-phase sintering,the pressure promoted the flow of the liquid phase and rapidly filled the pores,leading to rapid densification of the samples.SEM images showed that the number of pores in WC-Co cemented carbide gradually decreased as the sintering temperature increased,and no pores were observed when the sintering temperature was raised to 1250 ℃.The distribution of the binder phase also became more uniform as the temperature increased.In addition,when the temperature increased from 1100 to 1250 ℃,the average WC grain size increased from 138 to 390 nm.The results showed that the sintering temperature was too low in the solid phase sintering stage at a lower temperature,the binder phase diffusion distance was short,many Co pools existed,and WC grains grew in aggregate.When the temperature was increased to 1250 ℃ for liquid phase sintering,the organization was homogeneous,but WC grains produced abnormal growth phenomena.When the temperature was increased from 1100 to 1250 ℃,the hardness of WC-Co cemented carbide increased from 1100 to 2080 MPa and the fracture toughness increased from 9.5 to 13.4 MPa·m1/2.At the solidphase sintering temperature,the number of pores decreased significantly with increasing temperature and the hardness of the samples increased significantly.And,at the liquid phase sintering temperature of 1250 ℃,the flow of liquid phase Co was effectively accelerated by pressure,causing the liquid phase Co to fully diffuse and fill the pores,therefore boosting the interfacial bonding capacity and strengthening the interfacial bonding.Furthermore,WC grains had improved the skeletal strength of the alloy through a particle rearrangement and solution-precipitation process,which suggested that liquid-phase sintering could further enhance hardness.The increased fracture toughness was primarily the result of an increase in sintering temperature,which caused an increase in WC grain size and a gradual reduction in pores at grain boundaries.The temperature of hot-pressing sintering had a significant impact on the densification behavior,microstructure,and mechanical properties of WC-Co cemented carbide.At 1250 ℃,the best overall performance of ultrafine WC-Co cemented carbide was successfully manufactured,with a grain size of 390 nm,hardness of 2080 MPa,and fracture toughness of 13.4 MPa·m1/2.The hot-pressing holding time also had a great influence on the properties of cemented carbide,and the different holding times for ultrafine grain WC-Co cemented carbide was subject to further study at a later date.
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