Wei-Yu Ho

Microstructural and tribological characterization of MoS2–Ti composite solid lubricating films

Abstract Sputtered MoS 2 thin films provide lubrication and wear improvements for vacuum and space applications. When exposed to humid environments, however, the MoS 2 films are prone to rapid deterioration by oxidation. In this study, we synthesized a composite coating, which consists of titanium metal and solid lubricating MoS 2 layers by the unbalanced magnetron sputtering process. Experimental results indicate that the Ti interlayers among the MoS 2 –Ti composite films effectively enhance the density and stability of the film structure. Transmission electron microscopy, X-ray diffractometry, and Raman analyses reveal the amorphous nature of the MoS 2 –Ti coating, which contains only a short-range order within the Ti interlayers. Ball-on-disc tribotests of the MoS 2 –Ti composite films show a higher friction coefficient, increased wear life and microhardness, and reduced wear debris as Ti interlayers increased in thickness. In addition to its densification and strengthening effect, the Ti interlayer also reacts preferentially with oxygen to form TiO 2 and thus effectively suppresses adverse oxidation of MoS 2 . Thus, in humid environments, the MoS 2 –Ti composite film is highly promising as a solid lubricant, as evidenced by its prolonged wear life and resistance to oxidation.

Microstructure analysis of MoS2 deposited on diamond-like carbon films for wear improvement

A compound solid lubricating film containing a MoS2 top layer deposited on DLC interlayer by UBM sputtering technique was investigated for its tribological applications in humid environments. TEM, Raman and XRD analysis revealed the amorphous nature of the MoS2 film which contains only short-range order in its lattice structure. The as-deposited MoS2 compound films showed well-bonded interfaces. The MoS2, however, is very susceptible to humidity and oxidation, which resulted in higher friction coefficients and lower wear life. A friction coefficient of 0.05 was measured between steel balls and MoS2 in atmosphere of 90% RH. Excessive abrasive wear was identified, as a result of the wear debris and the oxidized transfer layers between MoS2 and its counterpart. The inclusion of a supportive DLC interlayer has effectively improved the wear behavior of MoS2 films under various loading conditions. The overall wear mechanism of MoS2 was complicated due to its oxidation problem which needs to be resolved for successful usage of MoS2 in humid environments.

Characterization of hydrogen-free diamond-like carbon films deposited by pulsed plasma technology

Abstract Diamond-like carbon (DLC) films demonstrated significant advantages in cutting and forming non-ferrous materials. The ultra-low friction coefficient and high surface hardness make DLC one of the most promising surface modification technologies available for processing advanced structural materials. In this study, Ti-doped and hydrogen-free DLC films (a-C:Ti) were synthesized by unbalanced magnetron sputtering of Ti and graphite targets. The high residual stress of the DLC thin films was dissipated through a compound interface consisting of a series of Ti, TiN, and TiC x N y graded interlayers. The target poisoning problem was resolved with a 2 kHz medium-frequency DC power supply. In addition, a 20–100 kHz variable-frequency DC power supply was used for both arc suppressing and substrate biasing. The effects of deposition parameters on film qualities were investigated by SEM/EDS, XRD, EELS and wear tests. Results demonstrated an improved DLC thin film with superior microhardness and adhesion strength compared with the conventional DLC deposited by PVD or PECVD processes.

Preparation of LaCrO3 Coatings on Stainless Steel by Magnetron Sputtering Process

Chromium-containing stainless steel is a prospective material for use as an interconnect in solid oxide fuel cells (SOFCs). However, during operations at high temperatures, the growth of oxide scales on the stainless steel causes the deteriorated performance of the SOFC. In this study, deposition of La-Cr-O coatings on stainless steel was conducted by using magnetron sputtering process. By using sputtering process, La-Cr-O coating exhibited with amorphous structure in nature, however, after annealing treatment at 800℃ for 1 hour, the deposited coating with LaCrO3 structure was obtained. Dynamic oxidation of La-Cr-O coated stainless steel studied by using thermo-gravimetric analysis (TGA) and differential scanning calorimeter (DSC) confirms that the better oxidation resistance and electrical resistance of the La-Cr-O-coated stainless steel during isothermal heating process. This was attributed to the transformation of coating from amorphous to LaCrO3 perovskite structure, resulting in a better performance after heating up to 800℃.

Structural Characterization of TiAl-doped DLC Coatings by Raman Spectroscopy and X-ray Photoelectron Spectroscopy

In this work, the combinations of TiAl-doped diamond-like carbon (DLC) and TiAlN/TiN double-layered films were designed to deposit on the tool steels using cathodic arc evaporation in a single process. The economic advantage in depositing the combined coating in one production scale PVD coating system is of practical importance. The TiAl-doped DLC as lubricant coatings were synthesized with TiAl-target arc sources to emit ion plasma to activate acetylene (C2H2) reactive gases. Scanning electron microscopy (SEM), micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) techniques were employed to analysis the microstructure properties of TiAl-doped DLC coatings. Vickers hardness tester was used to measure the mechanical properties of TiAl-doped DLC coatings. The results show that the microhardness of TiAl-doped DLC coatings was dependent on the ratio of I(subscript D)/I(subscript G) and sp^3/sp^2, which higher sp^3/sp^2 ratio or lower I(subscript D)/I(subscript G) ratio with higher hardness value.