Papers by Author: Hau Chung Man

Abstract: Titanium alloys have successfully been used as surgical implants. Laser gas nitriding (LGN) is an effective method to improve the wear and corrosion resistance of titanium alloys. A nitride strengthened uniform layer, free of cracks, was obtained on the Ti6Al4V alloy by irradiating with a continuous wave Nd-YAG laser in a N₂ environment. The microstructure, electrochemical corrosion and tribological behavior of the alloy before and after LGN were studied by SEM, XRD, potentiostat, microhardness tester and tribometer. The results show that the performance and composition of the surface of the Ti6Al4V alloy were significantly improved by LGN, with better corrosion resistance in simulated body fluid and better wear resistance.

Abstract: A nitrided layer was formed on the NiTi shape memory alloy (SMA) after being irradiated by a continuous wave Nd-YAG laser in a N₂ environment. With optimum process parameters, a compact laser modified gradient layer reinforced with fine TiN particles was achieved. Electrochemical measurements of the laser gas nitrided layer on the NiTi SMA showed that the corrosion potential and the breakdown potential were increased while the corrosion current was decreased as compared with the untreated the NiTi SMA. The polarization resistance of the laser gas nitrided layer on the NiTi SMA was increased significantly while the capacitance was decreased. Based on the EIS spectra, a simple model and an equivalent circuit were proposed to describe the electrode-electrolyte interfaces.

Abstract: Laser gas nitriding of Ti-6Al-4V alloy was carried out using a Nd:YAG pulsed laser under pure nitrogen environment at a flow rate of 30 l/min. The microstructure and corrosion behavior of the nitrided samples were examined using scanning electron microscopy, XRD, XPS, and anodic polarization tests in 2M HCl solution. For comparison, untreated samples were tested under the same conditions. After laser treatment, samples showed a relative flat surface with no problems of cracks or delamination of the alloyed tracks. Laser nitriding produced dendritic structures. The microstructure of the laser melted zone consisted of a thin continuous layer followed by a nearly perpendicular growth of dendrites. Below this a mixture of small dendrites and large needles with random orientation was produced. X-ray spectrum and XPS analyses from the surface of the laser nitrided specimen at different depth, confirmed that the thin top layer and large dendrites close to the surface corresponded to TiN. It can be also seen that the strong TiN peaks on the top surface gradually decrease with depth which suggests that the structure beneath the top surface is likely TiN0.3 and ά-Ti mixtures. In general, the corrosion potential of laser gas nitrided specimens was relatively nobler than the untreated sample. Furthermore, the proper laser nitrided specimen exhibited less corrosion current density, passivated more readily and also maintained a lower current density over the duration of the experiment. This was correlated with the formation of very thin, continuous TiNxOy film in an oxidation state that was confirmed by XPS analyses of the passive layers.

Abstract: Laser surface modification of nine tool steels, namely, plastics mold steels (PMSs), high-speed steels (HSSs) and cold/hot-work steels (CHWSs), was achieved by means of a CW Nd:YAG laser. The microstructure and the phases present in the surface of the specimens were analyzed by optical microscopy, scanning-electron microscopy and X-ray diffractometry. The surface hardness of the specimens was measured using a Vickers microhardness tester. The corrosion characteristics of the laser surface-melted steels in 3.5 wt% NaCl solution at 25 oC were studied by potentiodynamic polarization technique. The microstructures of the surface of the steels were changed completely after laser surface melting. Some steels showed improved corrosion resistance compared with the conventionally hardened specimens due to dissolution of the alloying elements in solid solution. The hardness and corrosion characteristics of all the laser surface-melted specimens are strongly dependent on the amount of passivating elements in solid solution and also on the morphology of the re-precipitated carbides. Both these factors depend on the laser processing parameters and the substrate compositions.

Abstract: A layer of bioceramic HA was coated on laser gas nitrided and grit-blasted pure Ti substrates using plasma-spraying technique, respectively. X-ray diffraction analysis showed that the microstructures of the coating were mainly composed of HA, amorphous calcium phosphate and some minute phases of tricalcium phosphate, tetracalcium phosphate and calcium oxide. The experimental results showed that the 3-D TiN dendritic scaffold structure produced on the surface of pure Ti using laser gas nitriding technique in advance could anchor the HA coating and improved the interfacial adherence significantly as compared with those on the grit blasted surfaces.

Abstract: To accelerate the bone growth around a metallic implant and to achieve the mechanical characteristics needed for biomedical applications, a HA/Ti composite coating was produced on NiTi alloy substrate by laser cladding. The chemical compositions, microstructures and surface morphology of the cladded layer were analyzed using energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). The experimental results showed that an excellent metallurgical bonding between the coating and the substrate was obtained. Owing to decomposition of HA under irradiation of high power laser, the microstructures in bioceramics coating were mainly composed of CaO, CaTiO3, Ti4P3, and HA phases. In vitro experimental result showed that HA/Ti composite coating made the bioactivity of NiTi alloy improve remarkably, which would promote the bone growth and could restrain Ni ion releasing from NiTi alloy.