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.

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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.

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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.

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