Abstract :
[en] Introduction
Sintering of cemented tungsten cobalt carbide by conventional processes (typically vacuum sintering) produces an unwanted grain growth due to the long holding time at high temperature (between 30 min and 1 h at 1400-1500 °C). The use of microwave sintering allows the reduction of the sintering time from 50 to 75 % in comparison to conventional sintering processes [1-2]. With shorter holding time, the diffusion of the species is reduced and the grain growth decreases.
In microwave sintering, the heating of the samples is achieved by the microwaves themselves, which produce oscillations of the free electrons in cobalt, free carbon and ions in WC. The heating occurs instantaneously because of the direct interaction between microwave and matter leading to a volume heating [3].
Tungsten-cobalt carbides were synthetized from WC powder and cobalt powder. The mixing was performed by conventional ball milling, followed by a uniaxial pressing to form a green body.
Results and Discussion
The tungsten-cobalt carbide samples (WC-20Co) were sintered by microwave sintering. The milling parameters were investigated through the grinding balls diameter (10 and 12 mm). The uniaxial pressing was performed with two different compaction pressure (200 and 500 MPa) while the sintering temperatures were set to 1350 and 1400 °C. The green body samples have a circular form with 12 mm diameter. The densification, calculated by the division of the density of the green sample by the theoretical density, is about 50 % for the samples pressed at 500 MPa and 45 % for the samples pressed at 200 MPa.
During the sintering process, the microwave with the frequency 2.45 GHz are used in a single-mode cavity. The use of a susceptor in the cavity leads to a hybrid heating. The samples were put into an alumina support, which ensures the insulation of the system and the atmosphere was a mix of Ar (95 %) and H2 (5 %) to avoid oxidation of the samples. The entire cycle (heating + holding time + cooling down) lasts more or less 1 h. Vacuum sintered samples will be processed to compare the two sintering techniques.
Conclusions
Microwave sintering of cemented tungsten-cobalt carbides reduces the processing time of the sintering of the hardmetals. In comparison to conventional sintering techniques, where the entire cycle lasts between 12 and 24 h, the microwave sintering is only 1 h. There is thus time and energy saving. Moreover, the shorter thermal treatment time should lead to finer microstructure.
References
[1] D. Agrawal, J. Cheng, P. Seegopaul and L. Gao, Powder Metallurgy, 2000, Vol. 43, No 1, 15-16.
[2] B.R. Sunil, D. Sivaprahasam and R. Subasri, International Journal of Refractory Metals and Hard Materials, 2010, 28, 180-186.
[3] E. Breval, J. Cheng, D. Agrawal, P. Gigl, M. Dennis, R. Roy and A. Papworth, Materials Science and Engineering, 2005, A391, 285-295.
