Tianchang Hu

An explanation for laser-induced spallation effect in a-C:H films: Altered phase evolution route caused by hydrogen doping

The laser-induced spalling effect has been recognized as a unique phenomenon for amorphous carbon (a-C) films during laser processing. In this work, the origin of spalling effect was investigated by ablating two different types of a-C film: hydrogenated a-C (a-C:H) and nonhydrogenated a-C with an Nd-yttrium aluminum garnet laser system. Comparisons of ablating results demonstrated that the spalling effect only occurred in a-C:H rather than nonhydrogenated a-C. Laser heating simulation indicated that the temperature distributions in both films after laser pulse are similar with a high temperature gradient in depth direction. Annealing test results, Raman spectra and nanoindentation show that with the increase in annealing temperature, a-C film transforms into grassy carbon directly, while a-C:H experiences two subprocess under heating: the hydrogen mobilization and rearrangement of CC network at a relatively low temperature range resulting in a denser CC network and raised film density; the graphitizatio...

The effect of laser surface texturing on the tribological behavior of Ti-6Al-4V:

This article reports the surface modification of Ti-6Al-4V, which is widely used for mechanical engineering applications by laser texturing in order to enhance its tribological properties. Surface patterns with three different diametered dimples were prepared for Ti-6Al-4V. Two types of oil with different viscosities were evaluated as lubricants using a pin-disc tribometer. The results show that the textured surfaces exhibit lower friction coefficients and wear compared with the untextured surface. However, as the load increases, the effect of friction reduction for all the textured surfaces decreases when lubricated with lower viscosity oil. The beneficial effects of the dimples are more pronounced at higher speeds and loads when lubricated with higher viscosity oil. The influence of dimple sizes on lubricating-regime transitions is investigated in this study, and the mechanism for friction reduction is discussed.

Mechanical and wear characteristic of Y-TZP/Al2O3 nanocomposites

Purpose – The purpose of this paper is to investigate the mechanical and wear properties of Y-TZP/Al2O3 nanocomposites. Design/methodology/approach – In this paper, the Y-TZP/Al2O3 nanocomposites with different crystal-sizes are designed and fabricated by hot pressing. Their mechanical and anti-wear properties are investigated, and the wear mechanism is studied by scanning electron microscopy, X-ray diffraction, and transmission electron microscope and so on. Findings – The experimental results indicate that the wear of Y-TZP/Al2O3 nanocomposites can be divided into two regimes: mild wear regime and normal wear regime. In mild wear regimes, the relationship between wear resistance (ɛ) and hardness (H) of the material can be described as: ɛ−1∝H−1. The corresponding relationship among wear resistance, hardness and toughness (KIC) of the material in normal wear regime can be described as: ɛ−1∝H−1K IC −4. Originality/value – In this paper, the mechanical and anti-wear properties of Y-TZP/Al2O3 nanocomposites ...

Surface Engineering Design of Alumina/Molybdenum Fibrous Monolithic Ceramic to Achieve Excellent Lubrication in a High Vacuum Environment

Al2O3/Mo fibrous monolithic ceramics are potential candidates for space applications because of their excellent mechanical properties and low density. This study aims at achieving low friction and long life of this material in a high vacuum environment. Three-dimensional composite-lubricating layers were fabricated by considering texture pattern as storage dimples and MoS2 synthesized via hydrothermal method as lubricant. The tribological properties were studied sliding against Si3N4 ceramic and GCr15 bearing steel balls under high vacuum condition. Results showed that the lubricating properties of the Al2O3/Mo fibrous monolithic ceramics were improved greatly by the micro-texture and MoS2 solid lubricant; the friction coefficients were as low as approximately 0.08 and 0.04, respectively, when Si3N4 ceramic and GCr15 bearing steel balls acted as the pairing materials. It was also demonstrated that the low friction coefficient can be realized with various normal loads and sliding speeds, indicating the composite-lubricating layers have good adaptation of working conditions. This excellent performance of the material is mainly because of MoS2 stored in dimples can be easily dragged onto the friction surface to form lubricating and transferring films during the friction process. This work is an extension of studies that were previously published in Tribology Letters journal.