Microelectrode Technology for Spatial Resolution of Tribocorrosion Process
Hydrogen is involved in some main issues of tribological components such as bearings and gears, especially the ones which are employed in harsh environments like wind turbines in offshore applications. Hydrogen promotes surface rolling contact fatigue, micropitting, and hydrogen embrittlement in steel which is decreasing steel ductility and consequently leads to premature failure at lower stress concentrations. Damage of metals due to hydrogen is prevalent and cannot be easily detected which makes it even more catastrophic. Despite all investigations into the role of hydrogen, the question “what is the main contributing factor to hydrogen generation from the lubricated contact?” has not yet been answered.
In this project, a new rig was designed and built at University of Leeds which could monitor hydrogen evolution from a tribological contact online and continuously. This is the first facility of it is kind that could do in-situ measurement of hydrogen permeation from a lubricated tribological contact. In this method, hydrogen entry and permeation from a lubricated metal-metal tribological contact into steel were monitored in-situ using a modified Devanathan-Stachurski technique in which a tribological charging cell is incorporated. The electrochemical charging cell was removed from the conventional DS setup and replaced by a metal rubbing counterpart sliding on the membrane surface in an oil bath. the below schematic figure shows the rig. The load and friction torque was constantly measured during the test using a load cell implemented into the shaft. This method is expected to provide new insight regarding both the lubricant’s chemical and tribological parameters influences on the hydrogen uptake into steel.
The results so far indicate that detectable hydrogen was successfully generated from a lubricated metal-metal rubbing contact and its permeation into the steel was monitored online. Reproducibility of the results was good when precautions were taken to avoid contamination of surfaces and keep it free from mechanical damage. The results from tribocontact may emphasize that the hydrogen permeation rate is strongly dependent on the lubrication condition. These results indicate that a small amount of hydrogen is generated from PAO base oil, while hydrogen uptake promotes in presence of ZDDP additive and water contamination in the oil. According to the results, it can be concluded that hydrogen uptake from the tribological contact into the steel increased up to four times higher in presence of 5% water injection into the PAO base oil. The effect of additives and contaminants in accelerating hydrogen uptake into the steel could lead to higher crack formation on steel surface. These cracks would be expected in a fatigue mechanism and are suggested to constitute the initiation phase of micropitting.
Furthermore, results showed that sliding contact is neccassary for hydrogen permeation into the steel; no permeation was detected in absence of sliding contact. Comparison of variation of maximum friction torques as a function of time for different lubricants showed that the friction torqueses are same and did not change during the sliding process. Therefore, the generated friction heat is almost constant. Moreover, maximum friction torques for all the lubricants are almost similar. In short, the friction could not be the reason of higher hydrogen permeation in presence of water contamination in the oil.
Project Aims and Objectives
Journal of Thermal Spray Technology, December 2012, Volume 21, Issue 6, pp 1195-1202
Institute of Functional Surfaces
Mechanical Engineering Department
University of Leeds
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