Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Sol-gel coated enamel for steel: 250 days of continuous high-temperature stability Gundars Mezinskis a , Ilona Pavlovska a, ⁎ , Kaspars Malnieks a , Laimons Bidermanis a , Andris Cimmers a , Pavels Onufrijevs b a Institute of Silicate Materials, Faculty of Materials Sciences and Applied Chemistry, Riga Technical University, Riga, Latvia b Institute of Technical Physics, Faculty of Materials Sciences and Applied Chemistry, Riga Technical University, Latvia ARTICLE INFO Keywords: Enamel for steel A. Sol-gel processes B. Thermal properties C. Chemical properties E. Thermal applications ABSTRACT Two types of thermally durable and heat-resistant enamel coatings were applied to steel alloys. Three-layer TiO 2 -SiO 2 sol-gel films were grown by a dip-coating method on an enamelled metal alloy to maintain and protect their properties from undesired environmental impact. The enamel coatings withstood 6000 h at 600 °C, retaining their colour while staying hard and adhering firmly to the metal surface. A three-layer sol-gel coating improved the chemical resistance of the enamelled metal by 6% (0 h), 29% (3000 h), and 28% (6000 h) at 600 °C. Throughout the whole treatment at 600 °C, the enamel coatings maintained their microhardness and even increased it from 3.6 to 4.4 GPa. 1. Introduction Extensive use of vitreous enamels for domestic uses and in the construction, chemical, and agro-food industries is entailed mainly by such engineering properties as corrosion protection and resistance to heat and abrasion. The hardness and abrasion resistance of enamels permits the use of cleaning methods that are not acceptable for other types of steel coatings. Heat-resistant coatings are designed to increase the life of the underlying alloy during service. The expression “thermal stability” refers to the temperature at which the coating retains its decorative, protective, and physical-mechanical properties within a certain time period. Protective properties in some cases include protection against high-temperature oxidation and chemical corrosion of coated metal. Oxidation mechanisms of enamelled steel were studied and it was concluded that glass coating effectively protects stainless steel against high-temperature oxidation at 600–800 °C [1]. Another way to increase the thermal stability of enamel is to use high-temperature pigments. Although many chemical classes poten- tially fall into this category, an important family of pigments is termed complex inorganic colour pigments, otherwise known as mixed metal oxides or complex inorganic pigments. These inorganic colour pigments are the pigments of choice where coloured objects are subjected to high temperatures, UV light, or harsh chemical environments, either during the manufacturing process or in use. They are called high performance pigments because they are extremely durable in use. Further, these pigments show excellent heat stability and good chemical resistance against attack from solvents, acids, and alkalis; they are also chemically inert and combine well with other inorganic and organic pigment types and so on [2]. Complex inorganic colour pigments can actually enhance the overall lifetime of the coating or coloured object [3]. Durable heat-resistant enamel coatings for protection of parts of corrosion-resistant steels and high-temperature nickel, high-chro- mium, and nickel-iron super alloys regulate the oxidation of metal surfaces and block the route of the components of corrosive gaseous environment and fuel-combustion products. The coatings produced by slurry sintering technology of available primary components possess strong adhesion to the metal sub-layer, gas-tightness, strong chemical bonds, high heat resistance, temperature resistance, low catalytic activity, and thermo-dynamic stability in high-speed corrosive gaseous flows in a wide temperature range (600–1100 °С) and are character- ized by high corrosion resistance, low-temperature stability, and the ability to endure cyclic loads and heat shock non-destructively as well as the immediate high-temperature effect and aggressive fuel combus- tion components (sulphur, chlorine, etc.). The application of coatings makes it possible to reduce the oxidability of steels and alloys by a factor of 6–10, increasing the service life and reliability of working parts by a factor of 1.5–2 [4]. To maintain or enhance the corrosion resistance of enamels, sol-gel derived coatings of SiO 2 , SiO 2 -TiO 2 , and SiO 2 -TiO 2 -ZrO 2 compositions with a thickness between 90 nm and 2 μm were deposited on several technical steel enamels and it was shown that the more stable enamels http://dx.doi.org/10.1016/j.ceramint.2016.11.060 Received 22 June 2016; Received in revised form 9 November 2016; Accepted 9 November 2016 ⁎ Corresponding author. E-mail addresses: gundarsm@ktf.rtu.lv (G. Mezinskis), ilona.pavlovska@gmail.com (I. Pavlovska). Ceramics International xx (xxxx) xxxx–xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Available online xxxx Please cite this article as: Mezinskis, G., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.11.060