Effect Of Yttrium Barrier on Precursor (Y-Doped Ammonium Paratungstate) of WC-Co Cemented Carbide

WC-Co cemented carbide has widely used in machining, drilling, automotive, military, and aerospace owing to its excellent mechanical performances. Most research on cemented tungsten carbides with a cobalt binder (WC-Co cemented carbides) has focused on improving their strength, fracture toughness, and hardness.

It is well-known that the refinement of WC grain size in WC-Co cemented carbides to the sub-micrometer or nanometer scale can significantly improve their mechanical properties. Doping WC-Co with rare earth elements to refine their grain size and improve the mechanical properties of tungsten and its alloys is quite well studied. There have been several efforts to add small quantities of yttrium (Y) to W, WC, or their cemented carbides to improve their properties. These are considered intermediate or final products during the production of tungsten and its alloys. However, there have been very few studies that have focused on understanding the influence of Y on the raw materials for the preparation of tungsten. These raw materials include tungsten oxide, ammonium paratungstate (APT), and ammonium metatungstate (AMT). These raw materials show microstructure heredity in the intermediate or final tungsten products. Therefore, it is meaningful to discuss the effect of Y on the precursors used in the preparation of WC-Co cemented carbides and the properties of the sintered bulk, which has not been reported earlier.

To study the effect of yttrium (Y) during the hydrogen reduction behavior of tungsten oxide, Y-doped APT was used as the raw material in the liquid-solid doping method. The experimental procedures are as follows:

(a) Materials and precursor powder preparation

APT (particle size: 37.5 μm; purity: 99.97%) was used as the starting material to synthesis tungsten oxide (WO3). Y(NO3)3·6H2O that was easily soluble in water was used as the dopant. Following stoichiometric calculation, 1% yttrium (Y) was dissolved in suitable amounts of deionized water. Before drying at 80 °C in an oven, the solution was mixed with the corresponding amounts of APT. The pure and doped APT powders were then calcined at 600 °C for 2 h in a muffle furnace to remove organic salt components. The obtained W-Y composite oxide powders (3.0–4.0 g), were filled in an alumina ceramic boat of size 60 × 30 × 14 mm and were reduced to tungsten powders under a hydrogen atmosphere at 800 °C for 3 h in a tube reduction furnace. The samples were then cooled under hydrogen gas flow (dew point: −40 °C, flow rate: 0.4 m3/h). The two prepared tungsten powders were designated W and W-1%Y. These W-Y composite powders (12.0 g) were further mixed with 0.76 g carbon black (W and C molar ratio of 1:1) by dry ball milling in a planetary ball mill using agate balls as milling medium. The milling time was 2 h and the rotation speed was 200 r/min. Carbonization was carried out at 1400 °C for 2 h in a hydrogen atmosphere with subsequent cooling (to room temperature) in a H2 atmosphere. All heating rates were controlled at 5 °C/min.

(b) Preparation of sintered bulk

WC-Co composite powders with 6 wt% Co (WC-6wt.%Co) were mixed with paraffin and compacted using a rectangular die with dimensions 5.5 × 6.25 × 20 mm3. The green bodies were subjected to low pressure vacuum sintering at 1410 °C for 1 h under a pressure of 9 MPa, with an average heating rate of 1.45 °C/min.

(c) Characterization

The phase compositions, particle sizes and morphologies of all the components were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy,

Conclusions

The effect of yttrium on every production process of WC-Co cemented carbide was investigated from APT calcinations to alloy sintering. 1%Y-doped tungsten oxide consisted of WO3 phase and Y2WO6 phase. The reduction of Y-doped tungsten oxide was accomplished at 800 °C for 3 h. After reduction with hydrogen, Y-doped tungsten particles were remarkably smaller than undoped W powder particles, and a yttrium barrier was observed in the cross-sectional morphology of 1%Y-doped tungsten powder.

• < Prev
• Next >