Microstructures And Properties Of Gradient Cemented Carbide With Nano-sized Grain Prepared By Two-step Method

With the increasing application of difficult-to-cut material in the industry,performance requirements of the cutting tools are higher and higher.A coating with high hardness and wear resistance is deposited on the cemented carbide substrate by PVD or CVD to achieve higher wear resistance and longer service life,as the wear resistance and strength of common cemented carbide tool material can’t meet the requirements of difficult-to-cut material.Due to the difference in the thermal expansion between coating and substrate,microcracks are formed in the coating when tools are produced and used.Microcracks will expand to the substrate in the process of using,then make failure of the cutting tools and reduce the service life of cutting tools.The gradient cemented carbide with the surface zone enriched in binder phase can effectively prevent the growth of cracks because of the high toughness of the surface zone enriched in binder phase,so it is widely used as the substrate of the coated cutting tools.Refining grain is an effective way to improve the strength of cemented carbide.Nowadays gradient cemented carbide with ultra-fine grain was prepared by refining WC grain.In this paper,gradient cemented carbide with nano-sized grain is prepared to futher improve the properties of the substrate.In this paper,nano-sized mixed powder was prepared with high-energy ball milling in the first place,then gradient cemented carbide with nano-sized grain was prepared with two-step sintering.To research the influences of temperature,Co contents and Ti(C,N)contents on the mircostructures and properties after pre-sintering,1300℃,1350℃ hot pressing pre-sintering and 1100℃,1150℃,1200℃ SPS pre-sintering was carried out respectively.Then,gradient cemented carbide with nano-sized grain was prepared by gradient sintering.And the influences of pre-sintering process and Ti(C,N)contents on the thickness of gradient layer,grain sizes and properties were studied.The results are as follows:(1)The average particle size of the mixed powder is about 120 nm through high-energy ball milling.The nano-sized cemented carbide whose average grain size is from 200 to 220 nm is prepared by hot pressing pre-sintering.And the average grain size of cemented carbide prepared by SPS pre-sintering is from 150 to 180 nm.(2)Through 1300℃ hot pressing pre-sintering + gradient sintering,10Co2Ti,10Co3Ti and 10Co4Ti alloys can obtain a gradient layer whose thickness is from 8 to 10 microns.The average grain size is from 280 to 300 nm after gradient sintering.(3)10Co4Ti alloys can obtain gradient structure after 1100℃ SPS pre-sintering + gradient sintering and 1150℃ SPS pre-sintering + gradient sintering.After 1100℃ SPS pre-sintering + gradient sintering,the thickness of gradient layer is 36 μm,and the average grain size is 255 nm.