Research On The Preparation Of Ultrafine Grained Cemented Carbide And Wear And Oxidation Resistance

In this thesis,the ultrafine-grained WC-Co cemented carbide with excellent mechanical properties was prepared by optimizing the sintering process and using discharge plasma sintering for the purpose of improving the performance of cemented carbide.The oxidation resistance and wide temperature range friction wear properties were also investigated to explore the damage mechanism,which provides technical support for the further expansion of the application field of the ultrafine-grained cemented carbide.The degree of influence of the sintering process（sintering temperature,holding time,and sintering pressure）on the sintered samples was investigated using an orthogonal test.The results showed that the sintering process influenced the sintered samples in the order of sintering temperature,holding time and sintering pressure in the discharge plasma sintering process.When the sintering temperature was 1350℃,the holding time was 4mins,and the sintering pressure was 30 MPa,the best mechanical properties were obtained for the ultrafine-grained WC-6wt.%Co carbide with the relative density,average grain size,hardness,and fracture toughness of 98.84%,320.9 nm,220.31±19.67 HV₃₀,and 11.43±0.35 MPa·m^1/2.The effect of the agglomeration behavior of ultrafine powders on cemented carbide was investigated using ultrafine powders stored for different times as raw materials.It was found that the ultrafine powders formed agglomerates under the action of foreign substances（e.g.air,water,etc.）during long-term storage and led to a decrease in their crystallinity and sintering activity,which in turn led to a wider grain size distribution in the sintered samples.Meanwhile,the agglomeration behavior of ultrafine powders reduces the densification rate of the samples during sintering and adversely affects their mechanical properties,the extent of which depends on the relative relationship between sample density and grain size.The effect of grain size on the antioxidant properties of WC-6Co carbide was investigated.It was found that the effect of grain size on the antioxidant properties of cemented carbide acted differently in different temperature intervals.When the oxidation temperature was relatively low（600℃,700℃）,the grain refinement led to an increase in the number of grain boundaries in the samples,which reduced the antioxidant properties of the samples.In contrast,during oxidation at high temperatures（800℃）,the antioxidant properties of WC-6Co carbide increased with the decrease of WC grain size,and the dense oxide layer microstructure was the main reason for the enhanced antioxidant properties.The increase in the average grain size of WC in the sintered samples led to a decrease in its antioxidant properties,which was mainly related to the greater intergranular fracture between the oxide grains and the"disintegration"of the grains themselves.It was also found that with the increase of WC grain size,the carbide oxide layer color changed from dark green to light blue,which was caused by the combination of the formation of oxygen-defective tungsten oxide and the increase of oxide grain size.The effects of grain size and different pairs of grinding ball materials（SiC,TC4,GCr15）on the frictional wear properties of WC-6Co cemented carbide under wide temperature ranges（25℃,300℃,500℃,700℃）were investigated.It was shown that the wear rate of the samples all increased with the increase of the ambient temperature,with the difference that the wear mechanism of the samples changed with the change of the para-grinding ball material.When the material of the grinding ball is SiC,the wear mechanisms are grain cracking,abrasive wear,plastic deformation,grain pullout,and oxidation wear in order with the increase of ambient temperature.When the grinding ball material is TC4,the wear mechanism is adhesion wear and oxidation wear in the order of increasing ambient temperature,and when the grinding ball material is GCr15,the wear mechanism is grain wear and oxidation wear in the order of increasing ambient temperature.It was also found that grain refinement could significantly improve the wide temperature domain frictional wear performance of the sintered samples.