Simultaneous synthesis and densification of ceramic and intermetallic functionally graded composites

Functionally graded materials (FGMs) vary in composition and properties across one or more dimensions. Ceramic/metal and intermetallic/metal FGMs can withstand large thermal gradients and are desirable for high-temperature structural applications. Synthesis is one large technical challenge preventing widespread utilization of FGMs.; In this work, synthesis of layered FGM composites of molybdenum disilicide (MoSi₂) and silicon carbide (SiC) as well as niobium silicide (Nb5Si3) and niobium (Nb) is demonstrated. The process begins by mixing elemental powders in different stoichiometric proportions, layering and cold compaction of the powder mixtures in a graphite die, and then simultaneous application of electric current (up to 3500A) and uniaxial load (up to 95 MPa). Heat generated in the conductive die and sample initiates a chemical reaction in the sample leading to formation of the desired products and densification of the sample within 20 minutes. Final densities are up to 99% of theoretical. Temperatures reach 1500°C, as measured on the surface of the die. MoSi₂/SiC FGMs with five layers varying in composition from 100 to 60 mol% MoSi ₂ and from 90 to 50 mol% MoSi₂ were synthesized. In the Nb ₅Si₃/Nb system, five-layer FGMs were synthesized with layers varying in composition from 90 to 20 mol% Nb. Compositional profiles reveal no mixing between layers. Microhardness measurements vary with composition and are comparable to literature values. A model of the combustion process in the MoSi₂/SiC system is presented. Simulation results show that in large diameter samples, ignition occurs at the center and a reaction wave propagates toward the outside, while in small diameter samples, ignition occurs simultaneously throughout the sample. It is demonstrated that the relative conductivities of the die and sample affect whether the majority of current is carried in the powder or in the solid graphite die. If enough current is carried in the die, the reaction can ignite at the outside and propagate toward the center. Reaction phenomena are comparable in trend to reactions observed in experiments.