Processing and properties of metal-ceramic interpenetrating composites

Composite materials consisting of two or more different phases are very extensively used in modern society. If the composite is designed and fabricated correctly, then desirable properties not available in any single conventional material can be achieved. Ceramic reinforced aluminium alloys are desired for high performance applications due to their superior properties compared with the soft, unreinforced metal. However "traditional" particle or fibre reinforced composites suffer from a limited ability to achieve high reinforcement levels. Interpenetrating composites (IPCs) have 3-3 connectivity, with both the matrix and reinforcement phases being fully connected; they are expected to provide truly multifunctional properties. Whilst pressure is normally needed for the processing of IPCs due to the poor wetting between most aluminium alloys and ceramic materials, it raises the risk of ceramic preform damage and limits the component shape. In this research, interpenetrating composites were produced at atmospheric pressure by infiltrating 10 wt% magnesium content Al-Mg alloys into 15-40% dense, gel-cast ceramic foams with average cell sizes from 100 to 500 μm, made from three different ceramics. Previous research at Loughborough University focused on the aluminium / alumina system. In this study, the ceramic foams were made from spinel, mullite and silicon carbide. Effects of processing parameters, including atmosphere, temperature and time, were investigated. The results showed that heating the metal-ceramic couple in Ar and infiltrating in N2 followed by cooling in Ar was a better approach for the infiltration process than heating in N2 during the whole process. The Al(Mg)/spinel system was observed to require the lowest processing temperature and shortest time compared with the Al(Mg)/mullite and Al(Mg)/alumina systems. Microstructures of IPCs were characterised using a series of techniques, including optical microscopy, field emission gun scanning electron microscopy (FEG SEM), X-ray diffraction (XRD), dual beam focused ion beam (DBFIB) and transmission electron microscopy (TEM). A continuous nitride layer consisting of AlN and Mg3N2 was observed at the metal-ceramic interface of the spinel and mullite-based IPCs ABSTRACT ii with MgO and MgAl2O4 observed at localised positions, similar to alumina-based composites. Based on these results, a two-step nitridation infiltration mechanism has been proposed for oxide ceramic foam / aluminium-magnesium IPCs. The infiltration is believed to be dependent on the reaction between Mg and N2 to form Mg3N2, which then deposits onto the oxide ceramic foam surface; once in contact with molten Al, Mg3N2 reacts with the Al to form AlN, which is wetted by the liquid aluminium and induced the infiltration. In the case of mullite-based composites, a small amount of Mg2Si was observed as a result of the reactions between the SiO2 phase in the mullite foam and the liquid metal. The feasibility of fabricating SiC foam / Al-Mg and SiC foam / Al-Si IPCs by pressureless infiltration of molten Al alloys into gel-cast SiC foams has been also evaluated in this research. Serious degradation of the SiC foam was observed in the SiC / Al-Mg IPCs, resulting in the formation of Mg2Si and Al4C3, whilst the SiC foam could not be spontaneously infiltrated by the Al-Si alloy without the presence of Mg. A modified pressureless infiltration technique was developed to allow the manufacture of fully infiltrated SiC foam / Al-Si interpenetrating composites, with little degradation of the SiC foam and very little formation of detrimental phases. Preliminary property characterisation showed that the ceramic-foam based IPCs were up to twice as wear resistant as composites made by infiltrating a bed of ceramic powder. Effects of parameters on wear resistance have been investigated, including the ceramic material, foam density, cell size and degree of sintering. The denser the ceramic foam, the stronger the foam struts, and hence the more effective the composites were in resisting wear. However, a non-linear relationship between the foam cell size and the wear rate was observed; the composites with moderate mean foam cell sizes exhibited better properties than composites with smaller or larger cell sizes. Thermal expansion behaviour of the IPCs has been also studied; a clear hysteresis was observed in the strain curve between heating and cooling. The coefficient of thermal expansion (CTE) was observed to vary as a function of temperature.