As a tool material, cemented carbide has been widely used in various cutting tools, mining tools and wear-resistant corrosion resistance due to its high hardness, high strength, high modulus of elasticity, wear resistance and corrosion resistance. component. In order to adapt to various service conditions and improve the efficiency of use, for the contradiction of high hardness and low toughness of traditional cemented carbides, such as gradient cemented carbide and ultra-fine cemented carbide materials have been developed. In recent years, with the concept of functionally graded materials, functionally graded cemented carbide is developing into one of the important research contents in the field of cemented carbide. In order to improve the cutting performance and service life of cemented carbide cutting tools, a thin layer of high hardness wear resistant material can be applied to the surface of the alloy. Due to the different coefficients of thermal expansion of different materials, the coating material may be cracked by thermal stress during cooling. Due to the brittleness of the coating material, cracks are more likely to occur on the surface of the coating and expand into the matrix. In order to prevent material failure due to crack propagation as much as possible and to obtain a high-performance cemented carbide cutting tool material, the substrate may be subjected to a gradient treatment to form a cubic phase carbide and carbonitride in the surface region of the substrate. A ductile zone in which the binder content is higher than the nominal binder content of the matrix. When the crack formed in the coating spreads to the region, due to its good toughness, the energy of the crack propagation can be absorbed, thereby effectively preventing the crack from expanding into the interior of the alloy and improving the performance of the cemented carbide cutting tool. Next page
In this paper, the preparation of the gradient cemented carbide coating matrix, the base coating technology, and the control of the surface carbon content of the substrate are reviewed.
Preparation of Gradient Cemented Carbide Substrate
The preparation of a coating matrix is ​​a critical issue in order to obtain a well-performing coating grade cemented carbide product. The coating must be combined with a suitable substrate to achieve the desired properties. The surface cobalt-rich alloy matrix with gradient structure makes the coating cutting edge stronger, improves the crack propagation resistance of the coating, improves the bonding strength between the substrate and the coating, and the bending strength of the tool. The scratch strength test of cemented carbide inserts showed that the matrix-to-coating strength of the gradient-structured coated inserts was stronger than that of the non-gradient-structured coated inserts. The cutting experiments of cemented carbide inserts also show that the cutting performance of cemented carbide inserts with gradient structure coatings is better than that of non-gradient structured coated carbide inserts when the matrix and coating composition are the same.
The gradient cemented carbide substrate can be prepared by a segmented sintering process. In the first stage pre-sintering, the sample is heated under nitrogen protection (heating rate is 5 °C/min), and the temperature is raised to 400 ° C for 1 h dewaxing; when the temperature is 1380 ° C, the alloy is densified for 1 h, then cooled to Room temperature. In the second stage of gradient sintering, the sample after pre-sintering is raised from room temperature to the sintering temperature under vacuum and kept for 2 hours, and then cooled to room temperature with the furnace.
Nitrogen-containing cemented carbide gradient sintering is carried out in a vacuum atmosphere, the nitrogen activity inside the alloy is greater than the surface nitrogen activity, and the internal nitrogen atoms diffuse toward the surface. There is a strong thermodynamic coupling between the N atom and the Ti atom. Therefore, at the liquid phase sintering temperature, the nitrogen atoms in the alloy diffuse to the surface through the liquid phase binder, and the Ti atoms on the surface are also bonded through the liquid phase. The agent diffuses internally, and the diffusion causes decomposition of cubic phase carbides, nitrides, and carbonitrides such as TiC, TiN, (Ti, W) (C, N) on the surface of the alloy. The metal atoms diffused into the interior of the alloy react with internal carbon, nitrogen and other atoms to form some hard phase carbides, nitrides and carbonitrides. Since the metal atoms diffuse into the interior of the alloy to form a volume vacancy in the surface layer of the alloy, the liquid phase binder flows to the surface layer of the alloy, and a surface toughness region having a gradient structure is formed on the surface layer of the alloy, thereby preparing a gradient cemented carbide substrate.