The driving comfort and safety of the automotive powertrain are significantly related to the performance, lifetime, and functionality of the lubricant. The presented study focuses on investigating the performance loss of the lubricant due to water contamination resulting from environmental influences and iron particles originating from the wear of different machine elements. The main purpose is to determine critical factors that contribute to the degradation of the lubricant, and increase the tendency to NVH behavior, leading to adverse comfort losses to the respective user. Therefore, this performance loss is evaluated by test rig-based analysis of the friction behavior of wet clutches. Due to physical adsorption, a significant impact of water and iron contamination on the degradation of the lubricant is found, while the influence of the energy input is secondary.

Tribolayer build-up was investigated on wet clutch steel disks with experiments in three different test levels of the tribological verification chain via X-ray photoelectron spectroscopy. A new characteristic value of the tribolayers was developed, i.e., the lateral elemental distribution uniformity. This value together with the parameters of additive elemental concentration as well as the thickness and the vertical composition of the layers were the basis for the subsequent correlation of the tribolayer properties with the tribological performance of clutch systems from different tests. One important finding was a non-zero threshold concentration of the additive elements calcium and phosphorous below which the clutch performance was unsatisfactory, even with a uniformly covered disk, concluding that the presence of some slightly higher additive element concentration seemed to stabilize the clutch performance. The importance of representative investigation area selection was demonstrated.