Friction and Wear of a-C:H and a-C:H:Si Coatings Sliding Against Different Counterpart Materials under Dry and Moist Environments

Abstract Diamond-like carbon (DLC) coatings can significantly reduce friction and wear in machine components across several industries. However, their development is often hindered by a limited understanding of their behavior in real-world scenarios. Factors such as temperature, humidity, and interacting materials deeply affect their tribological performance. Doping of DLCs with elements like silicon can help to tailor their properties for specific applications, enhancing their effectiveness under diverse operational conditions. For this study, soft DLC coatings were deposited on AISI 4140 machine component steel using a semi-industrial PA-CVD system. Acetylene was used as carbon precursor to obtain a-C:H, and HMDSO was added to produce silicon-doped DLC (a-C:H:Si). Tribological behavior was evaluated using pin-on-disk tests applying 12 N normal load on three different 6 mm ball counterparts: bearing steel, alumina, and silicon nitride. The tests were conducted at room temperature under dry and humid environments. The frictional force was recorded, and the wear scars on both sample and counterpart were measured and analyzed using confocal, optical, and scanning electron microscopy with EDS, and Raman spectroscopy. The a-C:H:Si coatings exhibited twice the hardness and elastic modulus compared to a-C:H. When sliding against steel, a-C:H:Si presented a lower friction coefficient but higher wear. However, against ceramic materials, a-C:H showed lower wear and friction coefficients. Under dry conditions, a-C:H:Si demonstrated higher wear and recurrent friction peaks. In contrast, under humid conditions the coatings exhibited significantly reduced wear and stable friction coefficients due to the formation of a lubricative silicon gel compound rather than hard abrasive particles.