Control of ZM5 alloy ignition via PEO/aluminum phosphate composite coating

Abstract

A composite coating consisting of the plasma electrolytic oxidation (PEO) and aluminum phosphate layer was fabricated to improve the ignition resistance of ZM5 alloy. The influences of PEO coating and PEO/aluminum phosphate composite coating on the ignition resistance of ZM5 alloy were investigated by torch ignition and non-isothermal oxidation test. The ZM5 alloy with PEO/aluminum phosphate composite coating possesses the best ignition resistance and does not ignite when exposes to flame for 20 min, about 16 min longer than that of ZM5 alloy with PEO coating, and about 18 min longer than that of bare ZM5 alloy. The ignition protection was achieved through synergetic combination increased oxidation resistance, high strengthening and thermal expansion coefficient match of the composite coating. The incombustible ceramic component and metallurgical bond to the substrate endow the PEO coating with a self-extinguishing property. In addition, three characteristic stages (protective oxidation, non-protective oxidation and burning) of ZM5 alloy and its coating were identified based on the non-linear mass gain behaviors at different temperatures. The ZM5 alloy with PEO/aluminum phosphate composite coating had higher non-protective oxidation initial temperature (605 °C) than 437 °C of the bare ZM5 alloy. Meanwhile, the non-protective oxidation energy of the ZM5 alloy coated with PEO/aluminum phosphate composite coating was remarkably improved from 61 kJ/mol to 136 kJ/mol.