Oxyacetylene torch testing and microstructural characterization of tantalum carbide.

Tantalum carbide samples have been subjected to high-temperature testing at ∼2300°C using an oxyacetylene torch to evaluate their potential for ultra-high temperature applications. While large samples cracked during the rapid heating, indicating their inability to withstand thermal shock, small samples survived the severe test conditions. The oxidation products formed were characterized and found to comprise different phases of Ta2 O5 . The ultra-high temperature experienced by the samples resulted in the formation of many interesting microstructures, including the formation of submicron sized grains, which has not been reported previously in the literature, as well as the expected evidence of melting.

Interface study by dual-beam FIB-TEM in a pressureless infiltrated Al(Mg)-Al2O3 interpenetrating composite.

This paper considers the microstructures of an Al(Mg)-Al(2)O(3) interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al-Al(2)O(3) system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal-ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al-8 wt% Mg alloy and 15% dense alumina foams at 915 degrees C in a flowing N(2) atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin-film samples, specifically produced across the Al(Mg)-Al(2)O(3) interface, were prepared using a dual-beam focussed ion beam and subsequently observed using transmission electron microscopy. XRD scan analysis shows that Mg(3)N(2) formed in the foam at the molten alloy-ceramic infiltration front, whereas transmission electron microscopy analysis revealed that fine AlN grains formed at the metal-ceramic interface and MgAl(2)O(4) and MgSi(2) grains formed at specific points. It is concluded that it is the reactions between Al, Mg and the N(2) atmosphere that improve the wettability between molten Al and Al(2)O(3) and induce spontaneous infiltration.

Reducing chemical vapour infiltration time for ceramic matrix composites.

Conventional routes to producing ceramic matrix composites (CMCs) require the use of high temperatures to sinter the individual ceramic particles of the matrix together. Sintering temperatures are typically much higher than the upper temperature limits of the fibres. This paper details preliminary work carried out on producing a CMC via chemical vapour infiltration (CVI), a process that involves lower processing temperatures, thus avoiding fibre degradation. The CVI process has been modified and supplemented in an attempt to reduce the CVI process time and to lower the cost of this typically expensive process. To this end microwave-enhanced CVI (MECVI) has been chosen, along with two alternative pre-infiltration steps: electrophoretic infiltration and vacuum bagging. The system under investigation is based on silicon carbide fibres within a silicon carbide matrix (SiCf/SiC). The results demonstrate that both approaches result in an enhanced initial density and a consequent significant reduction in the time required for the MECVI processing step. Dual energy X-ray absorptiometry was used as a non-destructive, density evaluation technique. Initial results indicate that the presence of the SiC powder in the pre-form changes the deposition profile during the MECVI process.