Formation and oxidation characteristics of the protective coatings of the CaO-MgO-SiO2 system on siliconized graphite

Abstract

The important problems encountered when producing structural materials from siliconized graphites include their protection against high-temperature oxidation. A possible method of improving the oxidation resistance of the surface of the siliconized material is to apply gas-tight (impermeable) refractory coatings possessing a fairly high degree of adhesion with the substrate.Such coatings include the silicate compounds forming a glassy phase [I]. This paper examines the possibility of forming such glassy coatings using the oxides CaO, MgO and SiO=.Complex analysis of the ternary and binary phase diagrams of this system makes it possible to evaluate the concentration ranges (regions) in which refractory glassy phases exist [2, 3].In the regions adjacent to CaO in the ternary phase diagram (Fig. I), the CaO + MgO eutectic, two congruently melting chemical compounds, viz., 2CaO'SiO 2 (melting point 2150~ and CaO'SiO 2 and the relatively low-melting eutectics consisting of SiO 2 + CaO'Si02 and CaO" SiO 2 + 2CaO'SiO 2 (melting points 1420 and 1400~ respectively) can form. In the Si02-CaO binary system, the incongruently melting compounds 3Ca0~ and 3CaO'Si02 are also present besides the aforementioned constituents.One of the major shortcomings of the compounds belonging to the CaO-SiO 2 binary system is the presence of polymorphic transformations in the solidified compound (for example, the transformation of larnite 8-2CaO'SiO 2 into bredilite ~-2CaO'SiO= with a change in the lattice parameters [2]).In the region adjoining SiO= in the ternary diagram, the compounds characterized by complex polymorphism are present. When viewed from the CaO-SiO 2 side of the diagram, the eutectic of tridymite + pseudowollastonite crystallizes (transforms) initially into tridymite + wollastonite and, subsequently, into quartz + wollastonite as the mixture of cristobalite + liquid is cooled down to form a solidified system. From the SiO=-MgO side of the diagram also, the cooling process is accompanied by multistage crystallization of MgOoSiO2 having three modifications, viz., enstatite, protoenstatite, and clinoenstatite. The values of the thermal expansion of the modifications of MgO'SiO 2 significantly differ from those of fused quartz. The difference in the parameters of thermal expansion increases with increasing temperature (Fig. 2). This situation does not permit one to use the compositions belonging to the region being considered in the ternary diagram for developing protective coatings on the carbonaceous materials whose specific service conditions most frequently involve cyclic heating and cooling. The presence of fused quartz and polymorphic MgO.SiO 2 that exhibit strikingly different indices of thermal expansion can lead to crack formation in the coating and, consequently, to a loss of their protective properties.The compositions located adjacent to MgO in the ternary diagram (see Fig. i) are of particular interest.In this case, shifting the concentration zone from MgO to CaO can be hardly treated as a positive direction of sear...