Tribochemistry of Carbon Films in Oxygen and Humid Environments: Oxidative Wear and Galvanic Corrosion

Abstract

The effects of oxidation on wear of carbon/steel tribological interfaces were studied. When mechanical wear was small, the oxidation behavior of hydrogenated diamond-like carbon (H-DLC) and stainless steel (SS) sliding interface varied depending on the nature of the oxidizing environment. In dry air or oxygen, both H-DLC and SS wore readily. The wear debris of SS did not form iron oxide in dry air and oxygen. In humid nitrogen, however, the wear of H-DLC diminished with increasing humidity, and the SS surface showed mild wear and iron oxide debris accumulated around the sliding contact region. These results revealed that different tribochemical reactions occur in dry oxygen and humid environments. In the absence of water, oxygen oxidizes the H-DLC surface, making it susceptible to wear, creating debris, and inducing wear on both H-DLC and SS. In contrast, adsorbed water molecules at less than 40% RH act as a molecular lubricant of the oxidized DLC surface, while multiwater layers adsorbed at near-saturation act as electrolyte inducing electrochemical galvanic corrosion reactions on the SS surface. When hydrogen-free amorphous carbon (a-C) was used in tribo-tests, severe wear of the SS surface occurs, in addition to the tribochemical wear observed for H-DLC, due to the high hardness of the a-C film.

Superlubricity of hydrogenated carbon films in a nitrogen gas environment: adsorption and electronic interactions at the sliding interface

“…During friction processes in a reactive atmosphere, a-C:H lms inevitably experienced varying extents of oxidation, structural changes, and other tribochemical reactions. 27,61 In the absence of oxygen, these processes can also occur through the adsorption of water vapor or organic compounds. Oxygen only acts as an oxidant, oxidizing the surface of carbon-based lm, while water vapor (at low humidity) can be adsorbed at the friction interface and act as a molecular lubricant to reduce the solid-solid contact during friction processes.…”

“…Water vapor at high humidity can lead to oxidation reactions at the interface and even act as an electrolyte to promote electrochemical reactions. 61 Kim et al reported that oxygen atmosphere had a smaller effect on the tribological behavior of a-C:H lm than did an atmosphere containing water vapor. 10 Although an increase in oxygen pressure increased the COF of a-C:H lm, this increase was smaller than that caused by water vapor.…”

Effect of Humidity on Friction and Wear—A Critical Review

“…This transfer layer dominates the friction and wear properties of the a-C:H/steel interface [58,68]. It was further found that adsorbed water molecules at relative low humidity act as a molecular lubricant of the oxidized DLC surface, while multilayers of water adsorbed at near-saturation act as electrolyte inducing electrochemical galvanic corrosion reactions on the steel surface ( Figure 3) [63]. When the counter surface is ceramics, similar to the situation of steel, tribochemical reactions also occur during the rubbing process and the resulting transfer layer controls the friction and wear.…”

“…There are several models to explain this superlubricity; the most highly cited mechanism assumes that the hydrogen-termination of the carbon surfaces leads to little or no chemical or physical interactions during sliding contacts [48][49][50]. However, the superlow COF cannot be maintained in humidity conditions and COF generally increases as the water vapor pressure rises, regardless of the chemistry or type of the counter friction material: a-C:H film [47][48][49]51,52], steel [41,45,[53][54][55][56][57][58][59][60][61][62][63] or ceramics [40,57,61,[64][65][66][67].…”

Galvanically induced potentials to enable minimal tribochemical wear of stainless steel lubricated with sodium chloride and ionic liquid aqueous solution

“…The hypothesis of this work is based on two tribological mechanisms which arise due to the interaction of ionic molecules on a polarized interface. On the one hand the steel surface is electrochemically protected against corrosion reactions [32,33]. On the other hand the polarized surface in combination with ionic molecules can lead to the formation of very stable interfacial layer as it was shown using AFMmeasurements [20].…”