Abstract
Presented is a study on the original preparation of individual and in situ intimately mixed composite nanocrystalline powders in the titanium nitride-aluminum nitride system, Ti:Al = 1:1 (at.), which were used in high pressure (7.7 GPa) and high temperature (650 and 1200 °C) sintering with no binding additives for diverse individual and composite nanoceramics. First, variations in precursor processing pathways and final nitridation temperatures, 800 and 1100 °C, afforded a pool of mixed in the nanosized regime cubic TiN (c-TiN) and hexagonal AlN (h-AlN) composite nanopowders both with varying average crystallite sizes. Second, the sintering temperatures were selected either to preserve initial powder nanocrystallinity (650 °C was lower than both nitridation temperatures) or promote crystal growth and recrystallization (1200 °C was higher than both nitridation temperatures). Potential equilibration towards bimetallic compounds upon solution mixing of the organometallic precursors to nanopowders, monomeric Ti[N(CH3)2]4 and dimeric {Al[N(CH3)2]3}2, was studied with 1H and 13C NMR in C6D6 solution. The powders and nanoceramics, both of the composites and individual nitrides, were characterized if applicable by powder XRD, FT-IR, SEM/EDX, Vicker’s hardness, and helium density. The Vicker’s hardness tests confirmed many of the composite and individual nanoceramics having high hardnesses comparable with those of the reference h-AlN and c-TiN ceramics. This is despite extended phase segregation and, frequently, closed microsized pore formation linked mainly to the AlN component. No evidence was found for metastable alloying of the two crystallographically different nitrides under the applied synthesis and sintering conditions. The high pressure and high temperature sintering of the individual and in situ synthesis-mixed composite nanopowders of TiN-AlN was demonstrated to yield robust nanoceramics.
Highlights
Artificial main group metal/metalloid and transition metal nitride nanostructured thin films and powders are unparalleled engineering materials with an ever-increasing impact on technology [1,2,3,4]
Commercial hexagonal AlN (h-AlN) (97%) and cubic TiN (c-TiN) (3%) microcrystalline mixtures were subjected to high pressure and high temperature sintering (12 GPa, 1472–1772 K, 3 min) and afforded an unexpected ceramics containing, in addition to h-AlN
We recently reported a related study on the preparation of mixed metal nitride nanopowders via transamination-deamination chemistry applied to selected metal organoamide derivatives in the system AlN-GaN, which was followed by no-additive high pressure and high temperature sintering of the nanopowders [49]
Summary
Artificial main group metal/metalloid and transition metal nitride nanostructured thin films and powders are unparalleled engineering materials with an ever-increasing impact on technology [1,2,3,4]. As can be deduced from this concise overview, both nitrides, individually or as the metastable ternary phase and/or composites, are mostly utilized as thin nanostructured films (most often made via gas phase deposition) or bulk ceramics made, in vast majority of cases, from commercial microcrystalline powders In this regard, the nanopowder materials form appears to offer significant advantages through increased surface-related reactivities and specific dependencies of many properties on crystallite dimensions in this particle size regime. In addition to the binary nitride composites AlN-GaN, the nitrides stable solid solution Alx Ga1-x N, 0 < x < 1, was already partially formed (or, to phrase it alternatively—the nitrides alloyed) in the nanopowder preparation stage and, under suitable conditions, continued to form during the subsequent high pressure and high temperature sintering (7.7 GPa, 1000 ◦ C). Similar syntheses were carried out for the individual organometallic precursors to result in pure nanocrystalline nitrides of TiN and AlN that were sintered towards the pure nitride nanoceramics for reference purposes
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