Structure Characteristics and Failure Behavior of Ceramic Coatings Fabricated by Plasma Electrolytic Oxidation on Aluminized Steel
|School||Institute of Mechanics|
|Keywords||hot-dipped aluminum plasma electrolytic oxidation micro-mechanical properties thickness ratio in-situ tension test|
Plasma electrolytic Oxidation （PEO） is a new method of forming ceramic coatings on light metals such as Al, Mg, Ti, etc. and their alloys. But, it is very difficult in fabricating ceramic coatings on steel using a single PEO technique. To overcome this problem, hot-dipped aluminum （HDA） was treated by PEO technique in this paper, and multilayer composite coatings which consisting of Fe-Al layer, Al layer, and Al2O3 layer were formed on steel substrate. Structures, micro-mechanical properties, and failure behavior of in-situ tension of composite coatings were investigated. Moreover, the structure system of composite coating was also analyzed by finite element method （FEM）. These studies are of great significance for exploring the evolution of Al layer transformed into ceramic coating, optimizing coating properties and revealing failure mechanism of these coatings under in-situ tension test.Characteristics of the average anodic voltage, growth regularity of ceramic thickness, surface and cross-sectional morphologies and element distribution were studied by using many test techniques. Results show that the anodic voltages of aluminized steel and pure Al vary similarly at the early PEO stage, but the voltage of aluminized steel decreases at later PEO stage. The thickness of ceramic coating increases approximately linearly with the Al consumption. Once hot-dipped Al layer is completely transformed, the aluminized steel voltage descends. Ceramic coatings are mainly composed ofγ-Al2O3, mullite, and a-Al2O3 phases, but a-Al2O3 appears only at the end PEO stage.There are many discharging pores with micron scale or sub-micron scale within ceramic coatings. When Fe-Al layer participates in PEO reaction, PEO process becomes more complicated than before. Some big discharging holes, which get to Al2O3/Fe-Al interface, are within ceramic coatings and many micro-cracks are also observed at the interface. EDS results reveal that the atomic percent of Fe element of these big discharging holes near interface is about 7.6 at.%. It is much larger than that the regions far from discharging holes at interface. Element distribution at the surface of ceramic coating shows that when Fe-Al layer partic ipates in PEO process, and the percent of Al element of the oxides near the big discharging hole decreases. At the same time, the percent of Si element increases and the percent of Fe increases from 0.5at.% to 1.52.6at.% because of the shortage of Al atoms.Mechanical parameters of coatings such as hardness, elastic modulus, fracture toughness, etc. were measured by using a microhardness tester and a nanohardness instrument. The effects of discharging pores on mechanical properties were discussed. Results show that nanohardness of ceramic coating is about 19.6GPa which is 15 times that of the substrate. Due to the effect of discharging pores, some abnormal curves of elastic modulus vs. depth and hardness vs. depth appear and radial and lateral cracks also occur at nanoindentation. It can be found that the crack path of ceramic layer is deflected and the crack tip becomes dull. The stress intensity at crack tip decreases. Many multiple mirco-cracks are also generated by these discharging pores. Therefore, indentation cracks of ceramic layer can be restrained and the fracture toughness of them is improved to a certain extent. Fracture toughness of ceramic coating can be calculated on Ansits formula by measuring the length of radial cracks, which is about 1.75MPa·m1/2 .FEM models of composite coating on aluminized steel were established by using FEM software. And stress fields of these coating/substrate system subjected to uniform normal contact load were computed and investigated. A triangle of normalized layer thickness was created for describing thickness ratios of composite coatings. Then, the effect of thickness ratio on stress fields at the surface or the interfaces was analyzed. It is seen that every layer of composite coating has a special function. For example, the thickness of Al2O3 layer determines the location of maximum shear stress within coating. The advantage of Al layer is that interfacial stresses can be reduced greatly. As Al2O3 layer and Al layer have the same layer thickness, shear stress at the Al2O3/Al interface is minimal. As the thickness Al2O3 layer increases, the support performance of coating is improved and tensile stresses at the surface are relieved. When thick ceramic layer and thin Al, Fe-Al layers are chosen, there are fine stress distributions at surface and interface of coating and the supporting performance of composite coatings under contact load can be improved. It is found that the data of supporting test are in accord well with FEM results.As Fe-Al layer thickness was invariable and the thickness ratio of ceramic coating to aluminium layer （tAl2O3/tAl） was changed, tension failure and propagating behavior of tensile cracks of composite coating on aluminized steel were studied. The results show that the transverse cracks firstly appear in Fe-Al layer. The thickness ratio tAl2O3/tAl can affect the propagating direction of these transverse cracks. When thickness ratio tAl2O3/tAl is small, these transverse cracks favor to grows along the Fe-Al/substrate interface. However, when thickness ratio tAl2O3/tAl is large, Al layer is very thin and these transverse cracks are easy to penetrate Al layer and arrive at the surface of coating. In this thesis, the critical crack opening displacementδc was introduced to express quantitatively the crack resistance of Al layer, which increases as the Al thickness increases. Through observing the distribution of tensile cracks of samples, we also found that the space between tensile cracks decreases as the thickness ratio tAl2O3/tAl increases. It can be concluded that thickness ratio of coating has an important influence on its tension failure and the propagation of tensile cracks is resisted or delayed by Al layer.