In situ synthesis of cobalt silicide particle reinforcing and coloring Si3N4 ceramics

In this paper, silicon nitride (Si3N4) ceramics with black color and high toughness were fabricated by gas pressure sintering and characterized by X-ray diffraction, Raman, scanning electron microscopy, EDS, and transmission electron microscopy. The in situ formed cobalt silicide was confirmed to contribute to the black color through the introduction of CoO. Due to the addition of CoO, the growth of beta-Si3N4 grains is promoted, forming elongated grains, and eventually forms the self-reinforcing microstructure. However, with adding excessive CoO, interfacial debonding is found between cobalt silicide and Si3N4 matrix and a decrease in strength was resulted. The optimum composition is 1 mol% CoO in Si3N4, with the fracture toughness of 9.9 +/- 0.3 MPa m(1/2), flexural strength of 826.1 +/- 46.0 MPa, and a much darker black color. The mechanism of color formation is discussed where the black color derives mainly from the metallic silicon and additionally the porosity.