Plasma-sprayed glass-ceramic coatings on ceramic tiles: microstructure, chemical resistance and mechanical properties

This article reports the characterisation and optimisation of glass-ceramic coatings plasma-sprayed on traditional ceramic substrates, dealing with microstructures, chemical resistance, and superficial mechanical properties. A CaO-ZrO2-SiO2 (CZS) frit, capable of complete crystallization after proper thermal treatment, has been employed: due to its refractory nature, its firing temperature in a traditional process would be unbearable for common substrates. The frit was plasma-sprayed onto ceramic tiles and a post-process thermal treatment has been developed in order to sinter and crystallize the coating, employing much lower temperatures than a traditional ceramic firing cycle. The microstructure of both as-sprayed and treated coatings has been evaluated with SEM and EDS, acid resistance tests have been performed. Vickers microhardness, superficial fracture toughness, deep abrasion resistance, elastic modulus and coating adhesion have been evaluated. Unglazed stoneware and an industrial glaze were also tested for comparison and an experiment was made to produce a CZS-based glaze to ascertain the inadequacy of traditional firing. As-sprayed coatings have a typical plasma-spraying defective microstructure, but a thermal treatment with 30′ isotherm at 850 °C plus 15′ isotherm at 1050 °C allows good densification, excellent adhesion and complete crystallization, with formation of wollastonite-2M and many small Ca2ZrSi4O12 grains. Thus, the coating outperforms stoneware by 50% in abrasion resistance, possessing higher fracture toughness, thanks to the continuous crack deflections due to the numerous crystals. Fracture toughness appears to be the property most correlated to abrasion resistance, because brittle fracture is the dominant abrasion mechanism. Hardness and elastic modulus reflect quite well the coating inner cohesion. Treated coatings porosity is lower than industrial glazes and stoneware.