The A356 aluminum alloy is a very commonly used alloy in the automotive industry, for parts such as pistons, cylinder heads, and connecting rods, for which the mechanical properties can be effectively increased by anodizing. In this work, oxide layers were formed in oxalic acid solution with defined parameters on A356 aluminum alloy and then studied by using a novel combination of the scanning electron microscope (SEM) and in-situ nanoindentation. The purpose of this research is to understand the relationship between the substrate and the oxide layer by examining its microstructure and nanohardness. Based on the experimental results showing special composite microstructure and corresponding high hardness, this alloy seems to be a good alternative for replacing steel brake disks in an environmentally conscious manner.

The A356 alloy is commonly used in the foundry industry to produce high-stressed automotive components, such as motor frames and cylinder heads. The aim of this work was to investigate how the mechanical and tribological properties of this alloy can be improved by applying an anodizing process. The properties of the oxide layer formed by anodizing using oxalic acid at low temperatures were characterized by different analytical and tribological methods. The combination of tribological methods with standard measurement techniques—such as hardness measurement, layer thickness measurement, as well as the analysis of the morphological characteristics—was used to track the layer evolution during wear developments.

The cross-sectional microstructure, local chemical composition, and mechanical behavior of anodic oxide layers produced by oxalic acid anodizing of cast A356 aluminum alloy were investigated using digital optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and in situ nanoindentation inside the SEM chamber. The anodized specimens contained a compact oxide-side interfacial region adjacent to the substrate–oxide interface and an outer porous oxide layer. The oxide layers formed at the higher nominal charge density were generally thicker than those formed at the lower charge density. Cross-sectional nanoindentation revealed two characteristic interface-related features: localized, unusually high apparent hardness values on the oxide side of the interface and a gradual hardness increase within the adjacent substrate-side region. The highest apparent values exceeded the expected hardness range of anodic oxide layers on A356 alloy and were therefore interpreted as interface-affected responses rather than intrinsic oxide hardness values. Their repeated occurrence at a well-defined position nevertheless supported their association with the oxide-side interfacial region. The substrate-side hardness increase was characterized by the estimated width of an anodizing-affected substrate zone. EDS line scans showed that the oxide-side interfacial region retained a significant oxygen concentration, whereas the substrate-side hardness increase was not accompanied by a systematic elemental concentration gradient. The results demonstrate that the cross-sectional mechanical response of oxalic-acid-anodized A356 alloy is governed not only by the conventional barrier-layer–porous-layer structure, oxide thickness, and porosity, but also by local chemical composition, microstructural heterogeneity, and substrate/interface-related effects.