Carbon Contamination Prevention during Spark Plasma Sintering.

Carbon contamination from graphite molds during spark plasma sintering (SPS) considerably affects the properties of the sintered materials, especially transparent ceramics. Herein, transparent YAlO (YAG) ceramics were prepared via SPS using Mo and Ta foils, separately and in tandem, as protective barriers against carbon contamination. The effects of Ta and Mo foils on the transparency and microstructure of the ceramics, and their protection mechanisms were studied. Experimental results show that a reaction layer formed at the Ta-YAG interface with a YTaO-AlO eutectic composition suppresses carbon penetration into the ceramic, increasing its transparency. By contrast, Mo foils, when used as protective barriers, allow carbon diffusion into the ceramic, resulting in the formation of nonuniform microstructural features. However, it does not form a reactive layer and, hence, is removed easily from the YAG surface. Multilayered Ta-Mo barrier exhibits improved outcomes if the Ta thickness is more than ∼100 μm. This behavior is attributed to the interior diffusion-blocking mechanism of Ta. Similar optical performance was demonstrated by both approaches. The results prove that carbon contamination in SPS-derived samples can be effectively prevented. Additionally, this study reports on a novel strategy of bonding oxide ceramics to metals by adding a Ta layer at the joint interface.