Influence of operating conditions on the degradation of co-annular Si₃N₄-MoSi₂ glow plugs

Silicon-nitride-based composites are widely used in ceramic heaters for combustion engines due to their high strength and wear resistance. Recent developments in engine designs, focusing on using alternative jet fuels with lower auto-ignition tendencies, have increased the demand for ignition-assistance devices. However, extended operation of glow plugs at high temperatures, beyond their intended use, accelerates degradation processes, thereby reducing durability. To design improved materials for this application, investigating the degradation modes of state-of-the-art glow plugs is crucial. This paper presents experimental results on co-annular glow plugs tested in three operating environments: ambient air, constant airflow chambers, and combustion engines. XRD, SEM microscopy, and Micro-CT were used to characterize microstructural changes alongside resistance behavior. Observed degradation modes in co-annular designs included surface oxidation, volatilization, sintering aid migration, and internal cracks caused by thermal stress from CTE mismatch. The primary degradation mode of this glow plug design was surface oxidation, as the resistive heating occurred at the outermost surface, making it particularly susceptible to surface degradation.