Traditionally a coating is defined as a material layer applied onto a surface for protection or decoration. Usually it protects the surface from a variety of environmental problems such as corrosion of solder joints, moisture and mildew, fuels and process solvents, service temperatures and dust, dirt and physical damage from handling. Since 2010, the rapid advancement of the micro and nanotechnology has collided with the coatings field resulting in new coatings with novel properties and functions that differ from the traditional ones. Functional coatings consists of organic, inorganic or hybrid materials and can be prepared using a huge amount of techniques depending on the properties of the materials, the substrate and the final application of the coating. Therefore it is possible to find coatings with optical properties, thermal capabilities, structural and mechanical features, physico-chemical properties, magnetic and electric characteristics and biological purposes that cannot be found in the starting materials.
TopIntroduction
Mankind has been using coatings as protective and decorative layers since the beginning of times. Apart from the paints and decorative drawings that Neanderthal and Cro-Magnon men used in the Prehistoric era, coatings have been present in the human society for centuries and even millenniums (Makhlouf & Tiginyanu, 2011). Ancient Egyptian used mixed organic - inorganic coatings based on bees wax, arabic gum and egg whites to develop coatings implemented with asphalt and balsam exudations to waterproof protection of boats, wooden ships and bronze sculptures from year 3000 BC to around 1000 BC (Ghoniem, 2014; Gooch, 2002). Besides this, other Asian cultures utilized varnishes and lacquers to cover artwork, furnishings and buildings during the second century BC in China, Japan and Korea (Stillman, 1924). Since then coatings have been used in different cultures and societies around the world to protect and preserve tools and gadgets with any kind of application.
The coatings technology , as it is known at present, was firstly described in 1773 by Watin and the first patent regarding varnish was released in 1763 (Watin, 1773). However, the first coatings factory (to produce varnish) was established in England in 1790 (Seymour & Mark, 1990). After that pioneer business, other factories started their activity operating in Europe and in the United States. During the twentieth century the coatings industry and research increased continuously, leading thousands of systems that varied from single layers to complex structures applied onto a surface for protection or decoration.
Nowadays, with the development of micro and nanotechnology and their extension both in the academic and in the industrial field of coatings, these coatings have experienced a new revolution, becoming an essential element in the protective layers utilized in commercial devices as well as in the most selected research areas. In this context, the term of functional coatings has appeared. Functional coatings are systems that besides the classical properties of the coatings (protection and decoration) have an additional functionality (Wulf, Wehling, & Reis, 2002). A functional coating can be classified in several categories depending on the nature of the coating, the methodology or techniques used to fabricate it and its functional characteristics. Moreover, whatever application or functionality they have, functional coatings are expected to possess various properties: durability and reproducibility, cost effectiveness and easy application, adequate surface morphology and somehow environmental sustainability (Mathiazhagan & Joseph, 2011). The historical evolution of the coatings is summarized in Figure 1.
Figure 1. Historical evolution of coatings from the beginning of times since nowadays
The increase of interest on functional coatings and their applications can be clearly observed by the number of papers published on this field on the last two decades. The evolution of the amount of papers released on micro- and nanoscale functional coatings is even more impressive. Figure 2 shows the number of papers found on the Web of Science regarding functional coatings (black bars) in general and the ones focused on micro and nanoscale coatings (grey bars). The increase of the articles published on the topic of micro- and nanoscale functional coatings follows almost a linear trend, reaching a maximum of 467 papers on 2013. The number of papers on functional coatings in general is around five times larger; however, this topic seemed to reach the top a few years ago and now the release of research related to this field remains somehow constant. The progression of the number of papers published suggests a large and exciting future of research and development on micro- and nanoscale functional coatings.
Figure 2. Number of articles per year published on micro- and nanoscale functional coatings (grey bars) and functional coatings divided by five (black bars). Source: Web of Science accessed on May 5, 2015