Silicon Nitride for High‐Temperature Applications

In this paper, a summary of the development of high‐temperature silicon nitride ( T >1200°C) is provided. The high‐temperature capacity of various advanced commercial silicon nitrides and materials under development was analyzed in comparison with a silicon nitride without sintering additives produced by hot isostatic pressing. Based on this model Si 3 N 4 composed of only crystalline Si 3 N 4 grains and amorphous silica in the grain boundaries the influence of various sintering additive systems will be evaluated with focus on the high‐temperature potential of the resulting materials. The specific design of the amorphous grain‐boundary films is the key factor determining the properties at elevated temperatures. Advanced Si 3 N 4 with Lu 2 O 3 or Sc 2 O 3 as sintering additive are characterized by a superior elevated temperature resistance caused by effective crystallization of the grain‐boundary phase. Nearly clean amorphous films between the Si 3 N 4 grains comparable to that of Si 3 N 4 without sintering additives were found to be the reason of this behavior. Benefit in the long‐term stability of Si 3 N 4 at elevated temperatures was observed in composites with SiC and MoSi 2 caused by a modified oxidation mechanism. The insufficient corrosion stability in hot gas environments at elevated temperatures was found to be the main problem of Si 3 N 4 for application in advanced gas turbines. Progress has been achieved in the development of potential material systems for environmental barrier coatings (EBC) for Si 3 N 4 ; however, the long‐term stability of the whole system EBC‐base Si 3 N 4 has to be subject of comprehensive future studies. Besides the superior high‐temperature properties, the whole application process from cost‐effective industrial production, reliability and failure probability, industrial handling up to specific conditions during the application have to be focused in order to bring advanced Si 3 N 4 currently available to industrial application.