Wettability Of Silicon Carbide By Molten Aluminum Or Aluminum Alloys And Their Interfacial Microstructures

In recent years, great interests have been focus on the SiC–reinforced aluminum matrixcomposites due to due to their low density, high thermal conductivity, good mechanicalproperties, and low fabrication cost. In the fabrication of the SiCp/Al composites, thewettability of SiC by liquid Al or Al–base alloys is of vital importance. However, it isusually reported to be poor at relatively low temperatures (T<1173K). Increasingtemperature would improve it but give rise to the formation of a brittle Al4C3phase at theinterface, which greatly deteriorates the property of the composites. In order to produce theSiCp/Al composites with excellent performance, it is necessary to improve the wettabilityand simultaneously control formation of Al4C3.In order to improve the wettability and inhibit the formation of Al4C3, alloying Al havebeen widely investigated. However, bescuse of ready oxidation of the Al drop and SiCsubstrte and trantional sessile drop method they used, the intrinsic contact angle for moltenAl on the polycrystalline SiC surface was usually inaccurate. In this dissertation, weinvestigated the wetting behavior in an Al/a-SiC system and the role of Si addition using adispensed sessile drop method in a high vacuum. Moreover, other chemical elements such asCu, Cr, Ti, La, Ce on the wettability and the interfacial microstructrues of the Al/a-SiCsystem were also researched. The major results obtained in the present study are as follows:(1) The effect of the sessile drop method was investigated on the wettability of Al/a-SiCsystem. Using the conventional sessile drop method, the SiC and Al were heated togetherin a contact mode from room temperature to the desired testing temperature and it isimpossble to accurately measure the initial wetting stage. The most significant feature ofthe modified and dispensed sessile drop methods lie in the separate locating of thewetting couple before the testing, thus enabling the isothermal wetting kinetics to befully monitored and avoiding the preinteraction of the wetting couple during heatingprocess. Moreover, using the dispensed sessile drop method, the oxide film on the moltenAl surface can be disrupted by mechanical extrusion of the liquid, making the contactangle measurement more accurate and reliable. In this experiment, the initial contactangle is much smaller obtained using the dispensed sessile drop method than the oneobtained using the modified sessile drop method and the difference of the the final contact angles obtained from the three sessile drop methods is negligible.(2) The time dependence of contact angle was monitored and representative wetting stageswere identified. The liquid spreading was initially controlled by the deoxidation of theSiC surface and then by the formation of Al4C3at the interface. The intrinsic contactangle for molten Al on the polycrystalline SiC surface was suggested to be lower than90oprovided that the oxide films covering the Al and SiC surfaces were removed, i.e.,the system is wetting in nature.(3) An increase in the Si concentration in liquid Al weakened the interfacial reaction butimproved the final wettability. The role of the Si addition on the wetting was presumablyattributed to its segregation at the interface and the formation of strong chemical bondswith the SiC surface.(4) The effects of alloying elements such as La, Ce, Cu, Ti, Cr on the wettability ofpolycrystalline SiC substrates by Al were investigated. Addition of La, Ce, Cu, Cr hadlimited effect on the improvement of the wettability. Addition of Ti greatly promoted thewettability and the spreading can be driven by both the reaction and adsorption, and thelatter plays an important role.