Nano Tungsten Carbide — SPS Process

Spark Plasma Sintering (SPS) is a kind of new sintering technology based on electrical discharge machining. It belongs to plasma sintering technology, which uses sparks and gap discharge to sinter in pressure. While the other is in a vacuum environment, uses 5000-20000k plasma flame to heat sintering, this called thermal plasma sintering. The basic principle of SPS is that use the instantaneous high temperature produced by pulse energy, pulse discharge pressure and the Joule heat to achieve sintering process and by instantly generate a plasma discharge inside each particle is sintered to produce a homogeneous particle surface activation and self-heating. Furthermore, SPS process can be widely used in ceramics, cermet, Intermetallic compound, FGM (Functional gradient Material), thermoelectric materials, ferroelectric materials, magnetic materials, nano materials, amorphous alloys fabricating.

In brief, SPS process is to inlet pulse current directly into the powder grains for heat sintering, so it always called as plasma activated sintering or Ion-assisted sintering technology. The plasma generated by pulse current and the pressure in sintering process is beneficial for decreasing the sintering temperature of powders. Meanwhile, the low-voltage, high-current characteristics can promote the powder densification. In addition, due to the high rate of heating and cooling and short holding time can effectively avoid surface diffusion stage and hinder the particles growth. It greatly reducing the production cycle, improve the overall efficiency.

Compared with conventional vacuum sintering process, SPS process has many advantages in nano tungsten carbide machining, such as finer granularity, higher densification, more uniform structure and no apparent porosity defects. While the sample by vacuum sintering has loose structure, leaving a large number of inter-particle porosity, grain growth phenomenon is more evident. Viewed from density, Vacuum sintering to full density sintering temperature is higher than the discharge plasma sintering nearly 300 ℃, sintering time is longer than 6h. And in the process of SPS sintering, the densification of the samples will never decrease when the temperature exceeds an optimum value. This is due to the volatile Co does not appear in the SPS phase sintering process, the sample has been densified with sintering temperature increases; from the view of hardness, Rockwell hardness and micro hardness of sample after the sintering SPS are significantly higher than traditional vacuum sintering samples. It is for this reason that the heat source of vacuum sintering is outside of the sample and the thermal conduction process is outside-in. The surface reaches the sintering temperature in advance, but the internal heat can not be effectively conducted, so there is still some porosity and it is difficult to achieve full densification. In the process of SPS sintering, the presence of spark plasma sintered body such that each particle inside their own uniform heating, surface atomic particles are in an activated state. And the whole densification is synchronous, there is no sequence of densification.

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