Tribology, DLC, Dry lubrication, Wear
Multifunctional DLC coatings have been extensively studied and recognized as a promising solution to avoid wear and friction problems. However, the effect of the environment on the tribological behaviour, in particular in the refrigeration industry, is little studied yet. The present work aims to study the influence of refrigerant gases (HFC134a and R600a) on the tribological behaviour of multifunctional CrN-Si rich DLC coatings, in particular on the tribo-chemical reactions. Tribological behaviour was evaluated using constant and incremental load reciprocating sliding tests. The constant load tests were conducted to access the friction coefficient and wear rates of specimen and counter-bodies whereas the incremental load tests (increments of 2 N at 15 min. intervals) allowed the assessment of the scuffing resistance of the coatings. The scuffing resistance was defined as the work (N.m) at which the value of the friction coefficient first rose above 0.20 (lubricity effect). The characterization of wear scars (tribolayers) was performed by optical interferometry, energy dispersive X-ray spectrometry associated with scanning electron microscopy, and micro Raman spectroscopy. It was verified a strong influence of the atmosphere via the formation of a tribolayer at the interface between body and counter body. This influence resulted in some disturbance of friction coefficient and contact resistance values. Tests performed in R600a atmosphere presented lower (36%) friction coefficient and lower (40%) scuffing resistance (surface durability) than those performed in HFC134a refrigerant gas. The observed differences were justified in terms of the tribo-chemical reaction between the multifunctional coating, the counter body and the refrigerant gases. Both results presented traces of Oxygen, but in the tests with HFC134a gas it was observed the presence of Fluorinated compounds in the tribo-layer. In the tests performed with R600a gas atmosphere it was found only Carbon and Silicon, indicating different interactions of the environment. It is worth noting that these results can serve as a guide to the application: the low friction induced by the presence of R600a recommend it for applications where sustainability and energy saving are required while the use of HFC134a is recommended for applications requiring increased reliability (longer life).