Papers by Author: K.Y. Li

Abstract: Quaternary CrTiAlN hard coatings were deposited by closed field unbalanced magnetron sputtering ion plating technique onto steel substrates, and their structural, mechanical, and tribological properties after heat treatment in air at different temperatures (500-900 oC) were studied and compared by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-indentation, and pin-on-disc (POD) tribometer, etc. The onset temperature of oxidation was determined by thermogravimetric analyser (TGA). The compositional depth profiles before and after the heat treatments were examined by X-ray photoelectron spectroscopy (XPS) in order to study the oxidation mechanism. The experimental results indicate that the CrTiAlN coatings have excellent oxidation resistance and thermal stability, and outperform the traditional hard coatings like TiN and TiAlN in terms of higher oxidation temperature, hardness, adhesion, and wear resistance. It is expected that the CrTiAlN coatings with superior properties should have better performance in dry high speed machining.

Abstract: Carbon ion implantation has often been considered as an additional method to further improve the wear, corrosion and oxidation resistance of hard coatings on tools or machine parts. The present research investigates the effect of carbon implantation on the structural and mechanical properties of the sputter-deposited solid solution Ti-Al-N coatings. The carbon implantation was carried out by using metal vapour vacuum arc ion source (MEVVA) with solid cathode at energies of 5 and 50 keV, and a dose of 6×1017 atoms cm-2. The mechanical and the microstructure properties of the implanted layer were identified by a variety of analytic techniques, such as nano-indentation, x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) etc. Additionally, the wear performance of the samples was evaluated by a typical ball-on-disk tribometer in dry conditions. The results showed that the coatings with high energy carbon implantation exhibited an enhanced hardness. The improved hardness could be attributed to the formation of TiC phase, as indicated in XPS. In the sliding tests, the coatings with the post-treatment of carbon implantation showed an improved tribological property in terms of friction coefficient and wear rate. The friction coefficient could be reduced from 0.6 to 0.1. The coatings had ten-fold better wear resistance than the coating without ion implantation.

Abstract: CrN/CNx nano-scale multilayered films were deposited on Si (100) substrate by closed-field unbalanced magnetron sputtering. Designed experimental parameters enabled an evaluation of the effects of negative substrate bias voltage (Vb), and bi-layer thickness λ (by changing substrate rotation rate) during deposition on the structural and mechanical properties of multilayer films. These multilayers were characterized and analyzed by transmission electron microscope (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and nanoindentation measurements. In all cases, the CNx layers were amorphous and independent of Vb, while the microstructures of the CrN layers were dependent primarily on Vb. The CrN layers showed a mixed structure phase consisting of CrN, Cr2N, and Cr at Vb = -(40-120) V. At higher Vb values (-140 V or above), the Cr2N phase was dominant along with low CrN phase content. AFM measurements revealed that the root-mean-square (rms) surface roughness of the CrN/CNx film was 2 nm at Vb= -200 V whereas the rms values were about 9.5-3.3 nm for lower Vb values of -(40-180 V). By nanoindentation measurements, a maximum hardness of about 36 GPa was observed at Vb= -140 V. The improved mechanical properties of the films are correlated to the phase formation during deposition.