The influence of organic binders on fiber/matrix bonding during the powder metallurgy fabrication of sapphire fiber-reinforced NiAl matrix composites (sapphire/NiAl) was investigated. One composite panel was fabricated using a poly(methyl methacrylate) (PMMA) fiber binder and a teflon matrix powder binder; another panel was fabricated by binderless powder metallurgy consolidation. The effect of the binders on fiber/matrix bonding was evaluated by fiber push-out testing from room temperature to 900 °C. Examination of mating fiber and matrix-trough fracture surfaces by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) revealed differences in interfacial morphology and chemistry, depending on the use of binders in fabrication. The primary difference between the two composites was the much higher concentration of carbon at the fiber/matrix interface in sapphire/NiAl fabricated with binders. This carbon residue from binder burnout prevented clean contact between the sapphire fiber and NiAl matrix surfaces, resulting in a weak, thermomechanically clamped fiber/matrix interface, in contrast to the stronger, less temperature dependent, interfacial bonding observed without binders.

Strength degradation of single crystal Al2O3 fibers due to the effect of fiber/matrix interaction and processing of NiAl and superalloy matrix composites, was investigated. Strength loss was quantified by tensile testing fibers that were exposed to the matrix alloy using two different methods. In one method, the fibers were incorporated into a composite by either the Powder Cloth (P-C) or binderless powder technique. The fibers were then extracted from the composite by chemical dissolution of the matrix and subsequently tensile tested and examined by scanning electron microscopy. In the other method, fibers were sputter-coated with a similar matrix composition and heat-treated to simulate conditions similar to those experienced during composite powder fabrication methods. In the sputter coating method, the contribution of fiber-matrix reaction on fiber strength loss was isolated from the effects of the various mechanical loads which are present during powder fabrication. For all matrices studied, significant strength loss was observed both in fibers extracted from composites and in fibers sputter-coated and annealed. Although surface ridges and pores were observed on the degraded fibers, it remains uncertain whether these features were responsible for the strength loss.

As part of a study to assess NiAl-based composites as potential high-temperature structural materials, the mechanical properties of polycrystalline NiAl reinforced with 30 vol.% continuous single crystal Al2O3 fibers were investigated. Composites were fabricated with either a strong or weak bond between the NiAl matrix and Al2O3 fibers. The effect of interfacial bond strength on bending and tensile properties, thermal cycling response, and cyclic oxidation resistance was examined. Weakly-bonded fibers increased room-temperature toughness of the composite over that of the matrix material but provided no strengthening at high temperatures. With effective load transfer, either by the presence of a strong interfacial bond or by remotely applied clamping loads, Al2O3 fibers increased the high-temperature strength of NiAl but reduced the strain to failure of the composite compared to the monolithic material. Thermal cycling of the weakly-bonded material had no adverse effect on the mechanical properties of the composite. Conversely, because of the thermal expansion mismatch between the matrix and fibers, the presence of a strong interfacial bond generated residual stresses in the composite that lead to matrix cracking. Although undesirable under thermal cycling conditions, a strong interfacial bond was a requirement for achieving good cyclic oxidation resistance in the composite. In addition to the interfacial characterization, compression creep and room temperature fatigue tests were conducted on weakly-bonded NiAl/Al2O3 composites to further evaluate the potential of this system. These results demonstrated that the use of A12O3 fibers was successful in improving both creep and fatigue resistance.