Assessment of carbon contamination in MgAl₂O₄ spinel during spark-plasma-sintering (SPS) processing

Carbon contamination caused during spark-plasma-sintering (SPS) processing was investigated in the MgAl2O4 spinel by Raman spectroscopy. Although the carbon contamination became remarkable around the sample surfaces directly contacting the carbon paper, it sensitively changes with the SPS conditions, particularly for the heating rate. For the slow heating rate of 10°C/min, the carbon contamination can be detected around the surface regions rather than inside. For the high heating rate, however, a large amount of the carbon contamination was detected even inside in addition to the significant contamination around the surfaces even though the sintering temperature is the same and the processing time is shorter as compared to those of the slow heating rate. The present results suggest that the carbon contamination is not caused by diffusion processes, but caused by evaporation of the carbon phase from the carbon paper/dies, which were used in the SPS process. For the high heating rates, the carbon evaporation is enhanced due to the rapid heating and goes into the samples through open pore channels. The evaporated carbon is encapsulated into the closed pores during the heating process and remains along the grain junctions as glassy carbon.