Boron carbide is an advanced structural ceramic and ranked the third hardest material after cubic boron nitride and diamond. Boron carbide is attractive for a number of industrial applications owing to its attractive features such as high chemical stability, low density, high melting point, and high hardness. It has been applied in lightweight armors, blasting nozzles, grinding wheels, and abrasive powders. Unfortunately, consolidating boron carbide to attain its full theoretical density is challenging owing to its covalently bonded structure.
A number of researchers have made several attempts to improve the relative density of sintered boron carbide. They have found sintering parameters such as heating rate, holding time, and sintering temperature had significant impact on the ultimate density and mechanical features of the sintered samples. In addition, it has also been found that the number of graphite foil layers to pack the powder effectively changes the microstructure of the sintered boron carbide sample. However, the mechanism by which these sintering parameters and number of graphite foil layers affect the sintering of boron carbide are not yet well understood.
Typical commercial boron carbide has impurities such as free carbon and oxygen. Oxygen impurity exists as boron oxide on the surface of boron carbide. Boron oxide reacts with the free carbon during the sintering process. Owing to this reaction, several gases are emitted during the process. These gases then act as sources of decreasing the density of the sintered boron carbide samples.
Florida International University researchers investigated the effects of sintering parameters on the properties of sintered boron carbide samples. They adopted a rapid sintering method where the boron carbide powder could be sintered by passing a pulsed direct current through the sample. Their research work is published in peer-reviewed journal, Ceramics International.
In their studies, the authors looked at the effect of the number of graphite foil layers on the features of the sintered boron carbide samples. They explained the experimental results as well as the sintering behaviors dictated by the predictions of computational thermodynamics. They also studied the effect of total pressure of the system on the carbon-boron oxide reaction with the help of CALPHAD (CALculation of PHAse Diagrams) approach. The ratio of free carbon to boron oxide, which was introduced by the authors as a novel and critical criterion for boron carbide sintering, played a pivotal role in determining the mechanism of sintering parameters’ effect on the properties of final samples.
The authors observed that a 50 °C increase in the sintering temperature from 1750 °C increased the relative density of the sintered samples. They also found that denser samples could be realized by lowering the heating rate from 100 °C/min to 75 °C/min. This provided more time for the carbon-boron oxide reaction. Lowering the heating rate was also important to give more time for the emitted gases to exit the system. Increasing the holding time from 13 to 20 minutes led to higher sintering density by providing more time for the carbon-boron oxide reaction.
Analyzing the effect of the number of graphite foil layers suggested that increasing the number of graphite foil layers could increase the driving force for the carbon-boron oxide reaction, which was necessary for oxide removal process and enhance the sintering density.
The results of Mohammad Asadikiya and colleagues study provided deeper understanding of the boron carbide sintering mechanism. Their results will give way for the understanding sintering mechanism of complex boron carbide systems particularly ones with multi-component dopants.
Mohammad Asadikiya, Cheng Zhang, Christopher Rudolf, Benjamin Boesl, Arvind Agarwal, Yu Zhong. The effect of sintering parameters on spark plasma sintering of B4C. Ceramics International, volume 43 (2017), pages 11182–11188.
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