This study investigates the effect of changes in substrate morphology induced by coating manganese phosphate (MnPh) on surface fatigue crack formation, which can help clarify mechanisms for improving rolling contact fatigue (RCF) life under MnPh coating and establish design guidelines for friction surface textures that can suppress RCF damage. Surface fatigue crack formation trends of ground steel (Grinding) and steel from which only the MnPh coating was removed (MnPh-removed) were compared. After sufficient friction progression, the MnPh-removed sample, characterized by isolated asperities with small curvature radii induced by the MnPh coating process, exhibited fewer surface fatigue cracks than Grinding, even with increasing surface roughness. To better understand the delayed formation of surface fatigue cracks observed in the MnPh-removed sample, contact behavior under dry conditions and surface morphological changes during rolling-sliding conditions were analyzed. The MnPh-removed sample revealed an early reduction in the mean surface contact pressure, which was attributed to asperity deformation. Therefore, this proposed mechanism is unique to its surface morphology, which is specific to the MnPh treatment. In addition, it is expected to contribute to the suppression of macroscopic RCF damage such as pitting.

View full abstract

This study investigated the water-lubricated tribological properties of coatings made of a novel polysiloxane-containing polyimide (si-PI) material that was recently developed for the aerospace industry and can be diluted with the harmless and environmentally friendly ethanol or water. The si-PI coatings were deposited on stainless steel (JIS SUS304) substrates at curing temperatures ranging from 160°C to 275°C. Their water lubrication properties were measured by rubbing the coatings against each other in water at room temperature. The coatings exhibited lower friction than conventional polyimide materials, with a minimum friction coefficient of 0.04, which was lower than that of polytetrafluoroethylene (PTFE) measured under the same sliding conditions. Unlike the conventional polyimide, the coatings did not exhibit any obvious wear or damage. The results demonstrate that the si-PI coating is a promising low-friction and highly durable coating for water lubrication.

View full abstract

Friction-induced stick–slip motion is a complex phenomenon in tribological systems. Accurate prediction and analysis of this behavior are crucial for understanding and controlling frictional dynamics. In this study, we integrated long short-term memory (LSTM) networks with rate- and state-dependent friction models to predict stick–slip behavior. A simulated dataset from a one-degree-of-freedom (1-DOF) friction system was used to train the proposed model. LSTM outperformed recurrent neural networks (RNNs) and gated recurrent units (GRUs) in univariate predictions, accurately capturing nonlinear dynamics. To address the unobservable variables in the experiments, we employed multivariate prediction using the spring force to estimate the velocity, acceleration, friction coefficient, and other state variables. Gaussian noise was added to the training data to enhance the robustness of the model, enabling accurate predictions even with noisy experimental data. Additionally, the inverse analysis capability of LSTM was used to estimate past stationary contact times based on the maximum friction coefficient observed before sliding. Validation against experimental data showed a strong consistency between the predicted and actual trends, demonstrating the potential of the model for analyzing stick–slip motion.

View full abstract

To overcome the high friction and wear limitations of pure polyether ether ketone (PEEK) under elevated-temperature sliding, this study develops novel PEEK composites by incorporating ultra high molecular weight polyethylene (UHMWPE). Composites with UHMWPE contents ranging from 1.0% to 10.0% were fabricated via hot pressing and tested at an elevated temperature of 100°C using a ball-on-disk tribometer. Results show that a low-content addition of 2.0% UHMWPE to PEEK results in optimal tribological performance, achieving reductions of 87%, 98%, and 71% in friction coefficient, wear rate, and surface roughness, respectively, compared to pure PEEK. This study presents an effective approach to enhancing the tribological performance of PEEK composites, with significant implications for elevated-temperature engineering applications, particularly in PEEK-based sliding bearings.

View full abstract

Gas foil bearings (GFBs), which employ gas, typically air, as the lubricating medium, have garnered significant attention in high-speed and high-temperature applications. GFBs are renowned for their superior speed, damping, and environmentally friendly attributes compared to traditional bearings. Despite numerous advantages, GFBs have insufficient load-carrying capacity, which restricts their usage in turbomachinery. Despite multiple research papers exploring GFBs, only a few review papers have addressed the structural advancements in GFBs over the years. These GFBs may be of journal or thrust bearings depending upon the load application. The research carried out on gas foil journal bearings (GFJBs) are more abundant than the gas foil thrust bearings (GFTBs). This review aims to bridge this gap by providing an in-depth analysis of the GFTBs. This analysis is based upon the modification of structural design and implementation of advanced materials to enhance the static characteristics. It also elucidates the effect of alterations in foil design, manufacturing methodologies, and coating materials on the behavior of GFTBs. These aspects have consistently captivated the attention of researchers and academicians in their quest to enhance the performance of GFTBs. Furthermore, the review encapsulates insights gained from diverse fabrication methodologies and experiments from the work of different researchers and scientists worldwide.

View full abstract