Innovative composite enamel coatings with improved abrasion resistance and mechanical properties

The earliest method employed by ancient civilizations to decorate precious artifacts is vitreous enameling, which has its origins in antiquity. The use of vitreous enamels for technical purposes started in the nineteenth century with the First Industrial Revolution, as these coatings were able to combine aesthetically pleasing colors and surface finishing with superior technical properties, including durability, thermal resistance, and chemical resistance. Nowadays, vitreous enamels, which are silica- based inorganic coatings deposited by high temperature (580-890 °C) vitrification, are usually employed for the protection of metallic components that must show superior durability and resistance in corrosive and harsh environments, such as in chemical reaction vessels, heat exchangers, flue pipes, and gas turbines. Unlike organic coatings, enamel coatings are limitedly affected by UV weathering and corrosion phenomena, therefore their protective properties remain unchanged over time. Moreover, these coatings are resistant to acid and mild alkaline substances as well as organic solvents, and they have strong thermal shock and direct flame resistance. The strong adhesion between the enamel coating and the covered substrate is the reason why enameled materials show optimal resistance to corrosion. All these positive features make enamel coatings first-choice materials in many high-duty technological applications, but some issues still limit their application in some fields. Enamel coatings show non-optimal abrasion resistance and poor mechanical properties due to the brittle-prone behavior of the glassy matrix. Abrasion and tensile stresses are very common mechanical solicitations that these coatings are subjected to in everyday use. Degradation due to abrasive processes negatively affects the corrosion resistance of enameled artifacts, as the removal of material by brittle fracture and the opening of enamel’s intrinsic porosity can put the covered metal in contact with the external aggressive environment. The same problem arises when enamel coatings are subjected to tensional stresses, as cracks nucleate and easily propagate through the whole coating thickness until reaching the metal substrate. Therefore, it is imperative to improve the abrasion resistance and mechanical properties of enamel coatings to extend their durability and application areas. Many recent studies have focused their attention on the development of abrasion resistant enamels by the addition of mill additives, hard ceramic particles, such as WC and SiC, or graphene-based fillers. Nevertheless, there are still several issues to consider, for example, the study of composite Al2O3/enamel coatings, the study of graphene/enamel composite coatings with improved graphene dispersion, and the assessment of the effect of metallic powders on the mechanical properties of these coatings. The study of abrasion resistant enamel coatings is then a hot topic, but the assessment of enamel’s mechanical properties still remains little investigated even though it is a critical aspect. This work represents an attempt in the direction of exploring the effect of different types of fillers, with the aim to develop innovative enamel coatings with improved abrasion and cracking resistance. Moreover, this work also aims at investigating the behavior of composite enamel coatings by standardized testing methods coupled with in-situ techniques, to obtain novel insights into the failure mechanisms of these coatings and the effect of the different fillers. Thus, as this thesis covers a wide range of topics, the results of the research have been divided into independent Chapters, in the interest of a better presentation. Each of them has been provided with an abstract and an Introduction and divided into a Materials and Methods section, a Results and Discussion section, and finally Conclusions. Therefore, the main body of the thesis is organized into five chapters. In the first Chapter, the background to the topics discussed in the subsequent chapters is provided and the relevant literature is reviewed, while in the fifth and last Chapter some conclusions are drawn, and future perspectives are discussed. The core of the work is contained in the three central chapters. Chapter II presents the main aim of the thesis and describes the most important experimental techniques, that will be exploited to charcaterize the different types of composite enamel coatings. In Chapter III, the effect of graphene-based filler on the abrasion resistance of composite enamel coatings is investigated. The main aim of this study is to develop an efficient way to deposit composite graphene/enamel coatings avoiding the agglomeration of the filler. The assessment of the abrasion resistance by Porcelain Enamel Institute (P.E.I.) test coupled with microscopic observations is the core of this chapter. The novelty of this study with respect to the present literature is represented by the deposition of homogenous coatings and the study of the abrasion behavior by means of a surface-limited abrasion method with a particular emphasis on the mechanisms underlying the abrasive damaging processes. Chapter IV presents some investigations related to the development of abrasion resistant Al2O3/enamel composite coatings. This study is inserted in a wider project about the development of chemically resistant enamel coatings with improved abrasion resistance. Here, the effect of the size of the corundum particles on the abrasion resistance and chemical durability is investigated by means of traditional and accelerated tests. The novelty with respect to the present literature is represented by a complete investigation of the functional properties of enamel coatings mixed with corundum particles, which have found limited attention in this field up to now. In Chapter V, the effect of 316L stainless steel flakes (SS-Fs) on the functional properties of enamel coatings is assessed by considering abrasion resistance, chemical resistance, and cracking resistance. The abrasion behavior of the coatings is investigated by the P.E.I. test coupled with microscopical observations to better explain and clarify the damaging mechanisms. The mechanical properties of the composite coatings are assessed by means of different tests, namely scratch test, IF (Indentation Fracture) method, and bending tests. Quantitative data regarding the cracking resistance of the coatings are extrapolated thanks to in-situ Scanning Electron Microscopy (SEM) /Acoustic Emission (AE) measurements, which are helpful in better evaluating the cracking resistance of the coatings and the role of the filler in counteracting the negative effect of the external mechanical stresses. The novelty of this study with respect to the literature is represented by the development of enamel coatings with the addition of 316L SS-Fs, and the study of their properties by means of in-situ techniques.