Oxidation performance of spark plasma sintered Inconel 625-NbC metal matrix composites

The study aimed to answer the question of previously obtained Inconel 625-NbC metal matrix composites (MMC) applicability in high-temperature and/or oxidative environments, that were proven to possess improved mechanical properties. To achieve that goal, a similar manufacturing process was performed, utilizing spark plasma sintering (SPS) at 1100 °C under 45 MPa, and in a medium vacuum (under 10-3 MPa). The total time for each sample equaled 60 min. Four sets of samples were obtained with ceramic reinforcement additions equaling 0, 5, 10, and 20 wt%. Hot rolling at 1100 °C with a 10% reduction step was applied to sintered materials in order to fully replicate previous technology. Oxidation tests were performed in furnaces with free flow of air for three total time periods: 24, 49, and 100 h at 800 °C. It was revealed that all samples kept their ability to form passive layers by matrix, but those layers were locally breached due to grain growth and thermal stresses of the oxidized reinforcement phase. Two different passive layer formations were observed and discussed: (Cr/Ni)-rich covering composite surface and (Cr/Nb)-rich forming around ceramic reinforcement. Total mass gain during oxidation was kept under 1 mg/cm2 for each sample series, with samples containing 5 wt% of NbC performing similarly to reference material. Hardness loss (up to 15%) was observed in near-surface zones of obtained MMC due to composite oxidation.