Abstract
Mo-W-C coatings with three different C/(Mo+W) ratios (5:1, 2.8:1 and 2.2:1) were deposited by using combined unbalanced magnetron sputtering (UBMS) and high-power impulse magnetron sputtering (HIPIMS) technology. The influence of the C/(Mo+W) ratio on coating microstructure and related tribological properties at ambient temperature and at 200 °C were studied in lubricated condition (up to 7500 m and 1800 m of sliding distances, respectively). Results showed that a decrease in the C/(Mo+W) ratio could be correlated with an increase in coating thickness, adhesion strength, hardness and elastic modulus values, and a decrease in the degree of graphitization. At ambient temperature, outstanding tribological properties (very low friction and negligible wear) were observed irrespective of the C/(Mo+W) ratio. At 200 °C, low C/(Mo+W) ratios (2.8:1 and 2.2:1) were found particularly beneficial to achieve excellent tribological properties. The keys to significant friction reduction at 200 °C were (i) in situ formation of MoS2 and WS2 due to tribo-chemical reactions and (ii) presence of amorphous carbon debris particles in the protective tribolayer. With an increase in sliding distance, the tribolayer gradually lowered the friction coefficient by protecting both the coating and counterpart from severe wear. On the other hand, a high C/(Mo+W) ratio (5:1) led to low friction but noticeable abrasive wear at 200 °C.
Highlights
IntroductionDiamond-like carbon (DLC) coatings are extensively used in automobile industries for wear-resistant applications in a number of engine components, such as ball and journal bearings, piston and piston rings, pins, cylinders and brakes, face seals, and gearbox and valve-train components [1]
An opposite trend in coating behavior was noticed with a further increase in W content up to 17.2% [20]. These findings indicate that the amount of metal content in a carbon-based coating significantly controls the coating composition and microstructure; it influences the tribological properties
Mo and W doped graphitic carbon-based coating (Mo-W-C) coatings were deposited in a combined unbalanced magnetron sputtering (UBMS) + high-power impulse magnetron sputtering (HIPIMS) mode, using a 4target
Summary
Diamond-like carbon (DLC) coatings are extensively used in automobile industries for wear-resistant applications in a number of engine components, such as ball and journal bearings, piston and piston rings, pins, cylinders and brakes, face seals, and gearbox and valve-train components [1]. Extensive research studies were conducted in order to achieve better tribological properties of DLC coatings, using a number of dopants, such as B [2], Cr [3], W [4], Si [5,6], Ti [7,8,9], Al [10], Ti-Al [11] and Mo [12,13]. A very low range of mean friction coefficients (μ~0.03–0.055) was observed for magnetron sputtered Ti-DLC, Mo-DLC and W-DLC coatings during lubricated sliding in engine oil containing a friction modifier
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