Ceramic films on cast iron for high temperature treatments.

The research aims the deposition of low thickness ceramic films on the surface of cast iron forms that are used for recycled Pb moulding. The films will establish a thermal insulating coating to delate the metal solidification for a short time in order to complete an unforced mechanical scorification. Moreover the bars will assume a more uniform and market acceptable surface. The thermal conductivities of the cast iron (Meehanite HE type) and the ceramic material are 55 W/m°C and 2.2 W/m°C respectively. High temperature ceramic coating not suited to thin film production so the sol gel process seems appropriate to homogeneously apply a mixture of metal oxides at low temperature. To avoid the film shrinkage due to the phase evolution of the inorganic components, at first, we prepared the inorganic oxide mixture by the conventional frit manufacturing process; the mixture was homogenized in a water suspension, dried and melted at temperature between 1000 and 1200°C. Then the glass was quenched in air or poured in water; after it was milled and sieved at 325 mesh (< 40 µm). The powder was added to the sol-gel acid mixture (hydrolysis step) prepared from tetralkylalkoxysilanes, and applied on the cast iron surface by dipping, spraying or spreading with a blade. The cast iron surface was prepared by alternate washings with acids and bases followed by drying. To improve the adhesion largely influenced by the differences between the thermal expansion properties of the substrate and the film (the CTE, linear is 13.0 µm/m-°C for the cast iron and 2-5 µm/m-°C for the ceramic film) and corrosion resistance, the surface was treated with an activating agent like fluoro-metal acid (metal = Zr, Si) before the sol-gel colloid application. Thermal treatment at temperature between 750 and 930°C were adopted and the film evolution was investigated by SEM. The frits were characterized by XRD analysis checking the influence of annealing temperature on the film structure and the phase evolution was investigated, so as to obtain films with superior performance. GDOES analysis was used to characterized the film composition. The thickness was 100-150 µm with an adhesion value measured with the ASTM 3359-02 method of about 2B-3B probably due to an high hardness characteristic. More investigation on the metal oxide mixture and film properties are in course.