Gangqin Shao

PREPARATION OF WC-Co POWDER BY DIRECT REDUCTION AND CARBONIZATION

A new approach to produce superfine WC-Co powder by direct reduction and carbonization is proposed. Water-soluble salts containing W and Co were used as raw materials. Tungsten and cobalt oxide powder (CoWO4/WO3) was first formed by a spray-pyrolysis technique, which was then mixed with carbon black and converted to WC-Co composite powder at 950°C for 4 h in N2 atmosphere. The resulting powder has a particle size of 100–300 nm.

Study on the diamond/ultrafine WC-Co cermets interface formed in a SPS consolidated composite

Nanocrystalline WC-Co composite powder and coated tungsten diamond by using vacuum vapor deposition were consolidated by the spark plasma sintering (SPS) process to prepare diamond-enhanced WC-Co cemented carbide composite materials. The interface microstructures between coated tungsten diamond and WC-Co cemented carbide matrix were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The results showed that there is a transitional layer between the diamond and the matrix, in which the carbon content is 62.97wt.%, and the content of cobalt in the transitional zone is 6.19wt.%; the content of cobalt in the WC-Co cemented carbide matrix is 6.07wt.%, in which the carbon content is 15.95wt.%, and the content of cobalt on the surface of diamond is 7.30wt.%, in which the carbon content is 80.38wt.%. The transitional zone prevents the carbon atom of the diamond from spreading to the matrix, in which the carbon content does coincide with the theoretical value of the raw nanocomposite powders, and the carbon content forms a graded distribution among the matrix, transitional zone, and the surface of diamond; after the 1280°C SPS consolidated process the diamond still maintains a very good crystal shape, the coated tungsten on the surface of the diamond improves thermal stability of the diamond and increases the bonding strength of the interface between the diamond and the matrix.

RESEARCH ON SHAPING TECHNOLOGY OF NANOCOMPOSITE WC-6Co POWDER AND PROPERTIES OF SINTERED COMPACTS

The influences of powder extrusion molding (PEM), die pressing and cold isostatic pressing (CIP) on the green compacts and the sintered compacts of nanocrystalline WC-6Co composite powder produced by spray pyrogenation-continuous reduction and carburization technology were researched. The results showed that the pore volume distribution, density and scanning electron microscopy (SEM) morphologies of fractured surface of powder extrusion molding or die pressing followed by the cold isostatic pressing consolidation green compacts were better than that of powder extrusion molding or die pressing. The green compacts were sintered by using vacuum sintering plus hot isostatic pressing (HIP), the sintered specimens were characterized by testing density, Rockwell A hardness, saturated magnetization, coercivity force, transverse rupture strength (TRS) and atomic force microscope (AFM) images, the results showed that sintered specimen of the green body that prepared by powder extrusion molding or die pressing followed by cold isostatic pressing had excellent properties of high strength and high hardness, transverse rupture strength of sintered specimen was more than 3100 MPa, Rockwell A hardness of sintered body was more than 93. Ultrafine WC-6Co cemented carbide rods with excellent mechanical properties and fine microstructure were obtained.

FABRICATION AND PROPERTIES OF Ni-YSZ AND Ni-Co-YSZ CERMETS

Ni-YSZ and Ni-Co-YSZ cermets were fabricated by hot-press-sintering from nanocomposite powders. The structure, morphology, relative density, hardness, specific heat capacity and thermal conductivity were measured. Results show that the crystalline grains are much homogeneous. Their size distribution is quite narrow and between 0.2–0.4 μm (200–400 nm). The Ni-Co-YSZ cermet reaches a maximum relative density (96.2%) at 1350°C and a maximum Rockwell hardness (70.5) at 1400°C. The specific heat capacity of Ni-YSZ cermet is larger than that of Ni-Co-YSZ cermet. The thermal conductivity of Ni-YSZ cermet is smaller than that of Ni-Co-YSZ cermet.