Effects of colouration mechanism and stability of CoAl2O4 ceramic pigments sintered on substrates

Abstract

CoAl2O4 ceramic pigments produced by common methods have low chroma values, which limit their application in ceramic glazing, enamel glazing, and ceramic product printing. The colour stability of the ceramic pigments and changes upon high-temperature sintering are important factors, especially in 3D printing. In this study, CoAl2O4 ceramic pigments were synthesized using a co-precipitation method and both the colouration mechanism and stability of CoAl2O4 ceramic pigments were investigated. The Co content, Co ion valence, and coordination number were found to be important factors affecting the CoAl2O4 ceramic pigments. When the Co content was excessively high, either Co3+was produced or Co2+ entered the octahedron spaces of the spinel structure. The coordination field of the Co ions was consequently altered and the colour transitioned from blue to green. After sintering on substrates at 1300°C, the brightness and chroma of obtained CoAl2O4 coatings were superior to those of the pigment powders prior to sintering, and the colour of CoAl2O4 with a relatively high Co/Al ratio changed from green to blue. The latter pigment may have formed a partially inverse spinel structure upon sintering at a high temperature. The probability of Co2+ entering the octahedron spaces increased; concomitantly, the number of Co2+ in the tetrahedron spaces decreased, resulting in an increase in the coordination number of Co. In addition, Co3+ in the ceramic pigments was transformed into Co2+ by deoxidation reactions during the coating sintering process, thereby increasing the chroma of blue CoAl2O4 pigments.