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Degradation of silicon nitride glow plugs in electric field-experiments and modeling

University of British Columbia

Silicon nitride (Si₃N₄) based ceramic Glow Plugs (GP) are considered by the automotive industry as a reliable, long-term source of ignition for natural gas and hydrogen internal combustion engines. The commercial GPs investigated in this work comprised of an all-ceramic heater with two U-shaped tungsten carbide heating elements encased in an Yb₂O₃-doped silicon-nitride (Si₃N₄) insulating phase. Upon applying electric potentials of 10-14V, the temperature on the surface of ceramic heater rapidly raises to as high as 1500ºC. This work looks into various modes of deterioration of GPs, particularly resulting from interaction of high operating temperature and the electric field within the GP heaters. An extensive scanning electron microscopy and energy dispersive x-ray spectroscopy investigation was performed to determine the degradation mechanisms of GPs in natural gas-burning rig, electric rig and engine. GP testing has shown that under the influence of constant electric load (DC) the sintering aid (Yb₂O₃) cations continuously migrate away from the high potential side of the heating elements following the electric field pattern. A 2D mathematical model was developed to simulate the redistribution of the sintering additive (Yb₂O₃) cations as a function of time, temperature, and electric field. The damage pattern of the tested GPs suggests synergistic impact of temperature, voltage, and environment on GPs lifetime. For the GPs tested in the burner rig and in engine the internal joule heating, externally applied combustion heat, together with the corrosive nature of the combustion gases, synergistically contribute to the degradation of Si₃N₄-based heaters. The comparison of cross sections for aged GPs revealed an increase in Yb ions migration with increasing temperature, electric field, and test duration. This study confirms that the removal of just one of the failure stimuli may significantly improve the GP performance. For example, applying AC voltage provided a significant improvement of GP durability in electric rig, even without addressing any other damage phenomena.

Text

2015-10-23

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/36256

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/