Abstract
A coating that was made of peppermint essential oil was obtained on different metal substrates: Ti6Al4V alloy (mechanically polished and chemically etched) and 316L stainless steel (mechanically polished and mechanically ground). The final aim is to get a multifunctional (chemical and mechanical) protection of metal surfaces in contact with water media. The coatings were characterized by means of fluorescence microscopy, contact angle measurements, and Fourier Transformed Infrared spectroscopy (FTIR) spectroscopy. The chemical stability of the coatings was tested by means of soaking in water for different times (up to seven days) and washing with different alkaline or acidic solutions. The mechanical adhesion of the coating was tested by tape adhesion test (before and after soaking) and scratch tests to verify whether it has protection ability with respect to the metal substrate. All of the performed characterizations show that the coatings are chemically stable on all of the substrates and are nor dissolved or removed by water during soaking or by alkaline solutions during washing. The adhesion is high and classified as 4B or 5B (on the chemically etched or mechanically ground substrates) according to ASTM D3359-97, depending on the substrate roughness, both before and after soaking. In the case of scratch test (up to 10 N), the coating is not removed and it has a protection action that is able to avoid the surface damage, even if the substrate has a plastic deformation.
Highlights
Lubrication and protection of metals is devoted to synthetic fluids, coatings, and paints
The synthesis of a coating of peppermint essential oil previously described by the authors [13]
The Ti6Al4V alloy chemically treated (Ti64-CT) substrate is featured by a multiscale roughness, both at the micro and nano scale and by a surface chemistry characterized by the presence of a surface oxide layer with a high density of OH groups, as already described by the authors [15]; the macroscopical appearance of the Ti64-CT surface is of a green-violet color tone, instead of metal grey as the other substrates, because the surface titanium oxide layer is partially transparent in the visible range
Summary
Lubrication and protection of metals is devoted to synthetic fluids, coatings, and paints. Natural (fatty and essential) oils can be considered as an environmentally friendly alternative, because they are biodegradable, not explosive, non-toxic (they do not irritate the skin), renewable, low cost, and available from organic waste [1] The essential oils, such as the peppermint oil used in this research, contain terpenes and terpenoids, and, even if they were less explored than the vegetable ones because of the higher costs, are gaining increasing interest for the realization of corrosion inhibitors and green polymers [2], even if their application in this field is still poorly explored. Vegetable fatty oils, such as monounsaturated oils (mainly ester-based), are in use as biodegradable lubricants in applications that include engine oils, hydraulic fluids, and transmission oils [3]. Better lubricant ability was obtained in the case of the oils rich in polar compounds (kernel peach oil, grape seed oil, camomile oil, laurel oil, and carrot seed oil), with higher density and viscosity
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