Porcelain enamel coatings have their origins in ancient times when they were mainly used for decorative and ornamental purposes. From the industrial revolution onwards, these coatings have started to be used also as functional layers, ranging from home applications up to the use in high-technological fields, such as in chemical reactors.
The excellent properties of porcelain enamel coatings, such as fire resistance, protection of the substrate from corrosion, resistance to atmospheric and chemical degradation, mainly depend and originate from the glassy nature of the porcelain enamel matrix itself. On the other side, the vitreous nature of porcelain enamel coatings limits their application in many fields, where mechanical stress and heavy abrasion phenomena could lead to nucleation and propagation of cracks inside the material, thus negatively affecting the protective properties of this coating. Many efforts have been made to improve the abrasion resistance of enamelled materials.
On this regard, researchers showed encouraging results and proposed many different improvement approaches. Now it is possible to obtain porcelain enamels with enhanced resistance to abrasion. Differently, the investigation of the mechanical properties of porcelain enamel coatings remains a poorly studied topic. In the literature, there are interesting methodological ideas, which could be successfully applied to the mechanical study of enamelled materials and could allow to have further insights on their behaviour. Thus, the path that should be followed in the future includes the mechanical characterization of these coatings and the search for new solutions to address their brittle behaviour.
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With the invention of powder coating about 60 years ago, a lot of work began in the coatings and finishing industries to leverage this new technology. The porcelain enamel industry was an early adopter of electrostatic powder coating, but a lot of development was required from both the material and equipment aspects. By the 1980s, porcelain enamel powder coating was taking place in many factories, primarily for flatware but also in several lines for cavities, like ovens and dishwashers.
The physics of powder coating of porcelain enamel powder for metal is the same as for traditional powder paint, although there are some differences in behavior, etc. One of the most noticeable is the transfer efficiency; porcelain enamel powder for metal is about 40 percent efficient, so much more powder travels through the recovery system in comparison with traditional powder paint. Additionally, porcelain enamel powders for metal are often applied in a thicker film; usually six to eight mils.
Almost all of the electrostatic dry powder porcelain enamel application is for major appliances; for exterior parts it is usually a two coat/one fire process, and for interior (non-appearance) parts it is a one coat/one fire process. In two coat/one fire processing, the first coat is a special ground coat enamel powder for metal(designed to adhere to the steel) which is applied to a thickness of one to two mils, directly followed in a second application booth with a cover coat (for the final color of the part) at a thickness of five to seven mils. Both layers are melted/cured at about 1500 degrees Fahrenheit in a single pass through the furnace. Firing times are typically dependent on the thickness of the metal; for successful enamel coatings the time at peak temperature is two to three minutes.
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