Characterization of Invar/Si3N4 Type Composites Obtained by Spark Plasma Sintering

Abstract The use of metal matrix composites (MMCs) in aerospace industry, has shown significant growth recently due to their superior properties compared to conventional alloys. The Invar/Si 3 N 4 MMCs nanocrystalline composites were obtained by spark plasma sintering (SPS) of composite powders prepared by mechanically milling (MM). The Invar alloy nanocrystalline powders were synthesized by mechanically alloying (MA) technique. The influence of α-Si 3 N 4 addition on coefficient of thermal expansion (CTE), microstructure and mechanical properties of Invar matrix composites were investigated. The composite compacts relative density decreases, and hardness increases with increasing amount of α-Si 3 N 4 . The relative density of Invar SPS-ed compacts is 83.7% and decreases to 79.6% for the addition of 5% Si 3 N 4 , respectively to 76.6% if the addition of Si 3 N 4 increases to 10%. Also, the Vickers microhardness values (HV0.5) increases significantly for SPS-ed samples of Invar with the addition of 5% Si 3 N 4 , from 98 to 328 and reaches 364 for samples of Invar with 10% Si 3 N 4 . The CTE value of Invar/Si 3 N 4 is not significantly influenced by the increase in the amount of α-Si 3 N 4 , but the temperature range in which it remains constant decreases. Thus, the CTE value of the SPS-ed Invar + 5 vol.% Si 3 N 4 composite is 0.8 × 10 –6 °C −1 up to 150 °C, while that of Invar + 10 vol.% Si 3 N 4 is 0.69 × 10 –6 °C −1 up to 175 °C. At the interface between α-Si 3 N 4 particle and Invar matrix diffusion occurs, and the Fe 4 N chemical compound is formed.