Modification of the interface in SiC/Al composites
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I. INTRODUCTION
AN inadequate choice of process parameters and matrix alloys when fabricating SiC/Al alloy composites may cause the formation of Al4C3 at the SiC/Al interface. Formation of Al4C3 has been reported to be undesirable due to its detrimental effect on material properties.[1,2] With this regard, fabrication of SiC/Al alloy composites devoid of Al4C3 has long been one of the major concerns.[3–14] Among various methods that were proven to be effective in suppressing the formation of Al4C3, the addition of Si into Al matrices has been successful to some extent[3–6] and is currently adopted for producing various SiC/Al composites. Examples include compocasting[15] and pressureless metal infiltration processes.[16] The basic principle behind this method is to reduce the activity of Al by dissolving Si within the Al matrix, thereby suppressing a direct reaction between Al and SiC. One drawback of the method, however, is that Al matrices have to contain high Si contents such as 10 to 18 at. pct, depending on the process temperature,[15,16] in order to prohibit the formation of Al4C3 during a prolonged exposure at elevated temperatures. On the other hand, in the case of short-fiber-reinforced Al composites, such as a Saffil/Al composite, thin SiO2 binder layers on the fiber surface react with the matrix to form MgAl2O4 on fiber surfaces, according to the reaction given by Eq. [1].[17,18] MgAl2O4 crystals formed as a result of this interfacial reaction are believed to induce a strong mechanical locking between the matrix and the reinforcement under the applied load.[19,20] 2SiO2 1 2Al 1 Mg 5 MgAl2O4 1 2 Si
[1]
If one could utilize the interfacial reaction mechanism given by Eq. [1] for fabricating SiC/Al composites, the interfacial characteristics of SiC/Al composites, which, in
general, are known to be undesirable, could be improved. Of interest here is how the SiO2 layer, which will later be transformed into MgAl2O4, can be produced on the surface of SiC. One simple method is the use of inorganic binders, such as Na2SiO3 and a silica colloid,[9,13] as a coating material. The main drawback in this method, however, is its difficulty in controlling the uniformity and thickness of the binder layer. Another alternative is the artificial oxidation (or passive oxidation) of SiC at high temperatures to form a SiO2 layer on the surface of SiC.[6–8,21–25] The purpose of the present study is to investigate a methodology to modify the interface in the SiC/2014Al composite in an attempt to improve the interfacial properties of the composite. The interfacial reaction mechanisms and the detailed microstructures of the interfaces were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
II. THEORETICAL BACKGROUND When SiC is oxidized in air at temperatures above 800 8C, SiC reacts with the oxygen to form a thin layer of SiO2 on the surface of SiC, according to the reactions given by Eq. [2].[21–25] This method not only could be effective for prohibiting a direct contact between SiC
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