Residual Stresses evaluation in Functionally Graded Composite Vitreous Enameled Steel Sheets

Functionally graded composite vitreous enameled steel sheets are a special class of metal-ceramic composite materials obtained by a two stages coating process: the deposition of the enamel raw material over the metal surface and the firing of the binary system (metal substrate-enamel material). Enamel raw material is made by a mixture of boron-silicate vitreous scales (frits) and other metal oxides as additives. During the firing process, that is performed at about 870°C in the case of a steel substrate, the enamel raw material melts and interacts with the metal substrate so enabling the formation of a continuous varying structure. As shown in the micrograph of the transversal section reported in figure 1, the interface domain between the substrate and the external layer is made of a complex material system where the ceramic vitreous and the metal constituents are mixed. In particular four main regions can be identified, (figure 1A): (i) the pure metal region, (ii) the region where the metal constituents are dominant compared with the ceramic vitreous components, (iii) the region where the ceramic vitreous constituents are dominant compared with the metal ones, and the fourth region (iv) composed by the pure ceramic vitreous material. It has also to be noticed the presence of metallic dendrites that hinder the substrate and the external layer passing through the interphase region, (figure 1B). Each region of the final composite structure play a specific role: the metal substrate has mainly the structural function, the interphase region and the embedded dendrites guarantee the adhesion of the external vitreous layer to the substrate and the external vitreous layer is characterized by an high tribological, corrosion and thermal shock resistance. Such material, due to its internal composition, functionalization and architecture can be considered as a functionally graded composite material[4]. The knowledge of the mechanical, tribological and chemical behavior of such composites is not well established and the research is still in progress [5]. The object of this paper is the investigation of the residual stress in the vitreous enameled composite. In fact, during the cooling to room temperature thermal mismatch stress develops due to the difference of the coefficients of thermal expansion between the vitreous enamel material ( 10-6 °C-1) and the metal substrate ( 10-5 °C-1). The presence of residual stresses affect cracking, spallation of the coating, shape changes, etc., and in general can change the performance of the entire part. Thus, the knowledge of the stress state is necessary to understand its evolution, assess its impact on the lifetime and function of the coated part, and to enable the control of the stress by modification of the manufacturing process.