A this paper the influence of surface roughness on contact angles in the system of liquid Al wetting solid surfaces of Al2O3 has been studied. It was observed that contact angles of liquid Al vary significantly on different rough surfaces of Al2O3. A model is proposed to correlate contact angles with conventional roughness measurements and wavelengths by assuming a cosine profile of rough grooves with a Gaussian distribution of amplitudes. In comparison with the experimental results, the model provides a good estimate for describing the influence of surface roughness on contact angles of liquid Al on Al2O3.

Abstract-This paper deals with a well known puzzling observation that sometimes the wetting is improved by a chemical reaction between a liquid and a solid substrate and sometimes just the opposite effect takes place. Here, contact angles of liquid Al on SiO, and liquid Ti on A&O, have been measured. The surface and interface structures have been explored by scanning electron microscopy and energy dispersive X-ray spectrometry. According to the experimental observations, it turns out that the volume change of ceramic substrates during reaction plays a key role in the effect of chemical reaction on wetting. If the volume of ceramic substrate decreases after reaction, the wettability is not improved by the chemical reaction. This is the case of liquid Al wetting on SiO,. If the volume of ceramic substrate increases after reaction, the wettability is improved by the chemical reaction. This is the case of liquid Ti wetting on Al,O, . Besides our experimental observations, results from literature have been reviewed as well, which are in good agreement with the predictions based on the volume change criterion of ceramic substrate proposed in this paper.

Abstraet-A novel process was developed to firmly coat an aluminium alloy, A16061, with ~t-Al203 by means of laser processing. In this approach a mixture of SiO2 and A1 powder was used to inject in the laser melted surface of aluminium. A reaction product ~t-Al203 layer of a thickness of 100/~ m was created which was well bonded to the aluminium surface. Various interfaces, AI#t-A1203, Al/mullite and ~-A1203/mullite, were studied by conventional transmission electron microscopy (CTEM) and high resolution electron microscope (HREM). It turns out that the presence of the A1/mullite interface may be essential to form a well bonded oxide layer and the high Si-content ct-A1203 intermediate layer may be wetted better by liquid A1. Investigations of the interface structures and wetting phenomena during laser processing are presented and a simple correlation between wetting phenomena and interface strength is derived.

-This paper reports an approach to coat a ceramic layer on aluminium alloys by means of chemical reaction. The reaction product of AlP3 layer of 100 µm in thickness has been formed using a powder mixture of silicon oxide and aluminium by laser processing. It turns out that the large amount of heat from the exothermic reaction has a predominate effects on the formation of the oxide layer. Further, the negative free energy of the reaction may promote the metal/oxide wetting. The micro-hardness and wear tests proved that the oxide layer is hard and strong. A mullite intermediate layer between aluminium and oxide have been identified by TEM. The atomic structure and defects of the reaction layer have been imaged at high resolution electron microscope.

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