The crack propagating behavior of composite coatings prepared by PEO on aluminized steel during in situ tensile processing

Research highlights: → Composite coatings on the aluminized steel were prepared by the plasma electrolytic oxidation (PEO) technique, which comprised of Fe-Al layer, Al layer and Al2O3 layer. → The evaluation method of the crack critical opening displacement δc was introduced to describe quantitatively the resistance of Al layer to the propagation behavior of cracks and evaluate the fracture behavior of composite coatings. → The crack propagating model was established. - Abstract: This paper investigates the in situ tensile cracks propagating behavior of composite coatings on the aluminized steel generated using the plasma electrolytic oxidation (PEO) technique. Cross-sectional micrographs and elemental compositions were investigated by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The composite coatings were shown to consist of Fe-Al, Al and Al2O3 layers. The cracks propagating behavior was observed in real-time in situ SEM tensile test. In tensile process, the cracks were temporarily stopped when cracks propagated from Fe-Al layer to Al layer. The critical crack opening displacement δc was introduced to quantitatively describe the resistance of the Al layer. There was a functional relation among the thickness ratio tAl/tAl2O3, the δc of composite coatings and tensile cracks' spacing. The δc increased with the increasing of the thickness ratio (tAl/tAl2O3). The high δc value means high fracture resistance. Therefore, a control of the thickness ratio tAl/tAl2O3 was concerned as a key to improve the toughness and strength of the aluminized steel.