Wear Resistant Coatings with a High Friction Coefficient Produced by Plasma Electrolytic Oxidation of Al Alloys in Electrolytes with Basalt Mineral Powder Additions

Abstract

To achieve a better performance of engineering components, modern design approaches consider the replacement of steel with lightweight metals, such as aluminum alloys. However, bare aluminum cannot satisfy requirements for surface properties in situations where continuous friction is needed. Among the various surface modification techniques, plasma electrolytic oxidation (PEO) is considered as promising for structural applications, owing to its hard and well-adhered ceramic coatings. In this work, the surfaces of two Al alloys (2024 and 6061) have been modified by PEO coating (~180 µm) reinforced with basalt minerals, in order to increase the coefficient of friction and wear resistance. A slurry electrolyte, including a silicate-alkaline solution with addition of basalt mineral powder (<5 µm) has been used. The coating composition, surface morphology, and microstructure were studied using X-ray diffraction, scanning electron, and optical microscopy. Linear reciprocating wear tests were employed for the evaluation of the friction and wear behavior. It was found that the coatings reinforced with basalt mineral showed that the wear and friction coefficients reached values 10−6–10−7 (mm3 N−1 m−1) and 0.7–0.85, correspondingly (sliding distance of 100 m). In comparison with the characteristics of resin-based materials (10−5–10−4 (mm3 N−1 m−1) and 0.3–0.5, respectively), the employment of thin inorganic frictional composites may bring considerable improvement in the thermal stability, durability, and compactness, as well as a reduction in the weight of the final product. These coatings are considered an alternative to the reinforced resin composite materials on steel used in frictional components, for example, clutch disks and braking pads. It is expected that the smaller thickness of the active frictional material (180 μm) reduces the volume of the wear products, extending the service intervals associated with filter and lubricant maintenance.

Fabrication and Characterization of Ceramic Coating on Al7075 Alloy by Plasma Electrolytic Oxidation in Molten Salt

“…In PEO, a target metal is subjected to a high voltage which leads to the discharge appearance on the surface with the extremely high temperature and pressure that both provide oxidation of the surface. Usually, PEO treatment is conducted in an aqueous electrolyte made of silicates, phosphates, aluminates, fluorides, and other [7][8][9][10][11][12]. Recently, we have showed a possible implementation of the electrolyte of the molten salt in the PEO process [13][14][15].…”

Wear of AlCrN and CrAlSiN Coatings Applied to Nonstandard Involute Gears

“…One criterion of the Niemann test evaluation corresponds to the weight loss of 10 mg in two successive load stages [48]. This criterion is suitable mainly for gears without applied coatings [53,54]. The maximal roughness of worn gear tooth surface Rz exceeding a value of 7 µm is the other criterion for the evaluation of the Niemann test.…”

Tailored surface textures to increase friction—A review

“…In a world without friction, humans would not be able to walk or, once in motion, they would not be able to slow down and stop their movement. A closer look into brakes [75,76] and clutches [77,78] reveals that they rely on friction; a certain value of the acting coefficient of friction (COF) must be achieved to ensure their proper function. When aiming at improving their performance, the COF rather needs to be increased, which demonstrates that it can be partially beneficial to intentionally increase friction.…”