Ying Liu

The lubrication condition of friction pairs in water or aqueous solutions is crucial to the performance and lifespan of key mechanical components in engineering applications. In our previous research, a promising in-situ self-sensing method was proposed based on liquid-solid triboelectric signals generated from the rotation-induced streaming current. However, the comprehension of the interfacial charging mechanism and the enhancement of the signal outputs from the viewpoint of zeta potential are deficient. Besides, the influence of a convergent channel on zeta potential needs further investigation, which fits most engineering conditions for seals or bearings due to the hydrostatic or hydrodynamic effect. Herein, electrode coated silicon nitride (Si₃N₄) composites are prepared by laser machining and magnetron sputtering. The interfacial charging mechanism is proposed based on the comprehensive investigation of electrode area ratio, surface roughness, and liquid ion concentration on zeta potential. The zeta potential originates from the combined efforts of the dissolution of metallic electrodes and the hydration effect of Si₃N₄. The interfacial charging mechanism can be interpreted by the ion absorption of ceramic substrate and the conduction behaviors of electrode layers. Moreover, samples with convergent profiles along the fluid flow exhibit a higher absolute value of zeta potential than divergent ones, and a negative relationship between zeta potential and convergent slope is observed. Finally, design criteria for self-sensing friction pairs are proposed for enhancement of interfacial charging performances: optimal electrode area ratio of 40%, surface roughness of 0.1–0.2 μm, aqueous solution with a moderate number of hydrated ions. Working conditions with a large variation of film gap are more advisable in terms of discernibility and sensitivity. This work builds a bridge between the interface charging mechanism and the self-sensing application of liquid-solid triboelectric behaviors based on streaming current input.

Impregnated graphite is a common material for friction pairs in aeroengine seals, especially at high temperatures. For the convenience of the application of graphite materials in aeroengines, an SRV-4 tribometer and a synchronous thermal analyzer are employed to study the tribological properties and thermal stability of pure, resin-impregnated, metal-impregnated, and phosphate-impregnated graphite against stainless steel from room temperature to 500 °C. The results indicate that impregnations can improve the wear resistance and thermal stability of graphite at high temperatures. Compared with other impregnated graphite materials, the resin-impregnated graphite shows a good friction coefficient and poor wear rate and thermal stability over 300 °C, due to the degradation and oxidation of the resin-and-graphite matrix. The metal- and phosphate-impregnated graphite materials exhibit excellent wear resistance and thermal stability under 500 °C as a result of the protection of the impregnations, while the average friction coefficient of the metal-impregnated graphite is much greater than the phosphate-impregnated graphite, and even reaches 2.14-fold at 300 °C. The wear rates for the graphite impregnated with resin, metal, and phosphate are 235 × 10⁻⁷, 7 × 10⁻⁷, and 16 × 10⁻⁷ mm³N⁻¹m⁻¹ at 500 °C, respectively. Considering all aspects, the phosphate-impregnated graphite exhibits excellent tribological properties and thermal stability.

This paper is the second part of a two-part report studying the responses of a typical point-contact elastohydrodynamic lubrication (EHL) system to multiscale roughness mimicked by wavy surfaces. The wavy surfaces are defined by three key parameters: amplitudes, frequencies, and directions. The previous Part I paper focuses on the full film lubrication condition, while the current paper focuses on the partial film regime where asperity contacts occur. A transient thermal EHL model simulates lubrication problems with different waviness parameters, loads, and speeds. The total number of simulations is 1600. Performance parameters, including the asperity contact ratio, minimum film thickness, maximum pressure, central point film thickness, central point pressure, mean film thickness, coefficient of friction (COF), and the maximum temperature rise, are obtained for each simulation. These performance parameters are post-processed in the same manner as those in the previous Part I paper. The influences of the waviness parameters, load, and speed values on the eight performance parameters are discussed.

This paper is the second part of a two-part report studying the responses of a typical point-contact elastohydrodynamic lubrication (EHL) problem to multiscale roughness that is mimicked by artificially generated waviness with different amplitudes, frequencies, and directions. The previous Part I paper (doi:10.3390/lubricants10120368) focuses on the full film regime, while the current paper focuses on the partial film regime. The amplitudes and frequencies are related to the feature geometry of smooth EHL problems. Generated waviness is input to a transient thermal EHL model. The simulation is conducted 1,600 times for different waviness parameters, loads, and speeds. Eight performance parameters are extracted: the asperity contact ratio, minimum film thickness, maximum pressure, central point film thickness, central point pressure, mean film thickness, coefficient of friction (COF), and the maximum temperature rise. The ratios of the first seven parameters with and without waviness and the asperity contact ratio values are plotted on the frequency–amplitude coordinate plane as contour maps. The influences of the amplitude, frequency, wave direction, load, and speed on the eight performance parameters are analyzed and summarized. The simulated data and plotted contour maps are provided to the readers in the Supplementary Material.