Study On Gradient Substrate With Ultra-fine Grain For Coated Cemented

China is one of the largest cutting tools producer and consumer in the world, but most high performance cutting tools are imported from other countries at present. Ultra-fine cemented carbide and coated cemented carbide are the development of cemented carbide in the future. Ultra-fine cemented carbide has higher strength and hardness; coated cemented carbide has higher strength and wear resistance because of depositing hard material on the cemented carbide substrate. Due to the difference in the thermal expansion between coating and substrate, cracks are formed when tools are produced and used which cause the failure of tools, it’s of interest to create a tough surface zone on the surface of substrate using gradient sintering technology to prevent the emergence and growth of cracks. Base on the above reason, gradient substrate with ultra-fine grain for coated cemented carbide has been intensively investigated.In the work, two sintering methods, which include vacuum pre-sintering+gradient sintering and low pressure pre-sintering+gradient sintering, were used to produce gradient cemented carbide with enriched binder phase on the surface. The influences of alloy composition and sintering process on the thickness of gradient layer microstru-cture and mechanical properties are studied. The results are as follows:(1) The surface zone enriched binder phase is gotten by both vacuum pre-sintering+ gradient sintering and low pressure pre-sintering+gradient sintering. There are larger gradient layer thickness of cemented carbide with average grain sizes of WC being smaller than 0.5μm and remarkable improvement in mechanical properties by means of low pressure pre-sintering+gradient sintering.(2) The gradient layer thickness of alloys with (Ta,Nb)C or (Ti,W)C are a little thinner than that of alloy with Ti(C,N). Little Ti(C,N) phase is left and the distribution of carbide is uniform in the alloy with (Ta,Nb)C.(3) With increase of gradient sintering temperature and holding time, both the thickness of gradient layer and WC grain size become larger. When holding 2h at 1460℃, the average thickness is 79μm in the alloy with Ti(C,N).