Diffusion barriers for silicon carbide particle reinforcements by ion-beam assisted deposition: Effects on interphase stability in silicon carbide(p/beta)-nickel aluminide and silicon carbide(p/gamma)-nickel aluminide composites

In this study, aluminum nitride and aluminum oxide films were used as diffusion barriers for SiC particles that were consolidated with β-NiAl and γ-Ni₃Al matrices at temperatures of 1673 K and 1373 K, respectively. The focus of this study was to understand factors influencing the effectiveness of the diffusion barriers during the consolidation processes of the two composite systems.; The barrier films were deposited on SiC particles by ion-beam assisted vacuum evaporation during which the SiC particles were radiantly heated and acoustically levitated. The nitride film formed reactively on SiC particles, and consisted of 95% aluminum nitride (balanced with aluminum nitrate and oxide). The oxygen content in the nitride film was a result of the impingement of residual oxygen and water molecules in the deposition environment.; A voided globular structure of fine-grained clusters was found in a nitride film deposited on SiC particles at 593 K, which was attributed to the levitation of the particles and the deposition temperature. Nitride films deposited at a higher temperature of 793 K consisted of a fine-grained dense structure with few voids. The oxide film deposited at room temperature had a fine-grained dense structure with some globular features.; This study found that film material affected film's ability of retaining integrity during compositing process, which was important for the success of the barrier films. Annealed at 1673 K, grains in a nitride film (deposited at 793 K) coalesced to an average size of 0.5 μm that was comparable to the film thickness. Grain boundaries in the film were widened by the pore agglomeration, resulting in micro-cracks. The oxide film exhibited a similar phenomenon of uninhibited grain growth and micro-crack formation at a lower temperature of 1273 K. Both films failed to be an effective barrier in SiC/β-NiAl composite during the compositing process at 1673 K.; This study showed the influence of film structure on grain growth in films at high temperatures. With a voided globular structure, the nitride film deposited at 593 K experienced a grain growth that was primarily restricted within each individual grain clusters at elevated temperatures. Solid-state sintering at 1673 K densified the nitride film because of the film's originally voided structure. After annealing at 1673 K for 4 hours, an average 0.5 μm thick nitride film deposited at 593 K retained film integrity on SiC particles, and succeeded as a diffusion barrier in the SiC/β-NiAl composite.; This study indicated that the chemical reaction potential in a composite system was a critical factor in determining the effectiveness of diffusion barriers. SiC/γ-Ni₃Al composite had a higher chemical reaction potential at 1373 K, comparing to SiC/β-NiAl system at 1673 K. Although being effective as a diffusion barrier in the SiC/β-NiAl composite at 1673 K, the same 0.5 μm thick nitride film (deposited at 593 K) did not protect the SiC particles in the SiC/γ-Ni₃Al composite at 1373 K.; A cursory study performed in this research indicated that Ni has low diffusivities in aluminum nitride lattice. From a γ-Ni₃Al source, the upper-bound lattice diffusivity was estimated to be 2.2 × 10−14cm2/sec at 1473 K. From a β-NiAl source, the upper-bound lattice diffusivity at 1673 K was estimated to be 5.8 × 10−14cm2/sec.