Understanding contact-induced damage is of paramount importance in the analysis of the lifespan and performance of surface coatings. In this work, we investigate the effects of dopants and interlayers on the structural durability of diamond-like carbon coatings (DLCs) and molybdenum disulfide (MoS₂) coatings on stainless steel via microscratch tests. The analysis of X-ray photoelectron spectroscopy (XPS) survey spectra and Raman spectra of the DLCs shows that the ratio of sp²/sp³ (i.e., the intensity ratio of sp² to sp³ obtained via XPS) is proportional to I_D/I_G, where I_D and I_G are the intensities of the D and G bands of the Raman spectrum, respectively. The analysis of the scratch tests reveals that there are three critical loads for the scratch-induced damage of the DLCs and MoS₂ coatings, corresponding, respectively, to the initiation of periodic V-cracking, the minimum load for periodic semicircle cracking or peel-off, and the minimum load for partial and periodic delamination. Dopants can reduce the friction coefficient of DLCs and have a negligible effect on Ti/MoS₂ coatings. The Cr interlayer can better enhance the bonding strength between the DLCs and the steel substrate than the Si interlayer. Doping Cr and H can reduce the hardness of DLCs; doping Si can increase the hardness of DLCs; doping Ti, Pb, and PbTi can reduce the hardness of MoS₂ coatings. The deep symbolic optimization (DSO) algorithm is used to establish nominal-mathematical formulations between the critical variables for the scratch test and the material parameters of the surface coating. The DSO analysis demonstrates the feasibility of using “deep learning” to establish “quantitative” relationships between the critical variables for mechanical deformation and material parameters.

In modern machinery, electrified contacts present novel lubrication challenges for sliding components. Understanding the electrified tribological characteristics of tribomaterials is vital. This work studied the electrified tribological changes at a diamond-like carbon (DLC)/steel sliding interface when lubricated with base oils. The results showed that electric current induced sticking friction, resulting in a friction reduction of approximately 5%–20% when mineral, PAO6, and castor oils were used in short-duration tests; conversely, there was a slight increase in friction with rapeseed oil. The electric current triggered the growth of a graphite-like tribo-layer on the DLC surface, particularly at the ester-lubricated interfaces, which mitigated the wear of the DLC. As sliding progressed, the DLC film experienced peeling wear under electrified conditions, especially at high currents and loads. The tribo-layer, formed from tribo-oxidation of the steel pair and lubricant degradation, was correlated with electrified tribological behavior. The enhanced adhesive and molecular interactions caused by the electric field across the contact were deemed to contribute to friction under electrified conditions. These findings validate the electrically caused tribological changes in lubricated DLC/steel contacts and indicate the necessity of a novel DLC film design to counteract electrified-induced damage.

Diamond-like carbon (DLC) and graphite-like carbon (GLC) coatings have good prospects for improving the surface properties of engine parts. However, further understanding is needed on the effect of working conditions on tribological behaviors. In this study, GLC and two types of DLC coatings were deposited on GCr15 substrate for investigation. The friction and wear properties of self-mated and steel-mated pairs were evaluated. Two temperatures (25 and 90 ℃), three lubrication conditions (base oil, molybdenum dithiocarbamate (MoDTC)-containing oil, MoDTC+zinc dialkyldithiophosphate (ZDDP)-containing oil), and high Hertz contact stress (2.41 GPa) were applied in the experiments. The results showed that high temperature promoted the effect of ZDDP on steel-mated pairs, but increased wear under base oil lubrication. The increased wear for steel-mated pairs lubricated by MoDTC-containing oil was due to abrasive wear probably caused by MoO₃ and β-FeMoO₄. It was also found that in most cases, the tribological properties of self-mated pairs were better than those of steel-mated pairs.