Corrosion of Si3N4–MoSi2 ceramic composite in acid- and basic-aqueous environments: surface modification and properties degradation

The corrosion behaviour of an electroconductive Si3N4-35 vol.% MoSi2 composite, hot pressed with the addition of Al2O3 and Y2O3 as sintering aids, was studied in 1.8 M sulphuric acid and 4 M sodium hydroxide aqueous solutions at room temperature, 40 and 70 °C up to 400 h. In acidic environment, the corrosion follows linear kinetics involving the progressive chemical dissolution of glassy grain boundary phases, in the system Al-Y-Si-O-N up to 40 °C. At 70 °C, the dissolution mechanism is hampered by the formation of a barrier that limits the transport of the aggressive cations towards the sites of chemical attack. In basic environment, the rate controlling step for corrosion of this ceramic composite is the chemical dissolution of MoSi2. Si3N4 phase is less affected by the selected corrosive environments. The effect of the corrosion on electrical resistivity and flexural strength were investigated. The electrical resistivity rises after long term corrosion in line with the progressive chemical dissolution of grain boundary phase and of the electroconductive MoSi2 particles. The flexural strength values are about 21 and 13% lower than the as-sintered ones, after a permanence of 400 °C at 70 °C in H2SO4 and in NaOH solutions, respectively.