Anodizing creates a dense oxide film on the metal surface through an electrochemical process, significantly altering the metal's hardness properties. The oxide film isn't simply a surface coating; rather, it's a chemical reaction between the metal substrate and oxygen ions in the electrolyte, producing a highly hard metal oxide. For example, aluminum alloys typically have a relatively low substrate hardness. However, after anodizing, the resulting aluminum oxide film can significantly increase its hardness, creating a hardness gradient: a high-hardness oxide film on the surface and a gradually transitioning substrate material within. This structure enhances the surface's scratch and wear resistance while maintaining overall toughness.
The increased hardness of the oxide film stems from its unique crystal structure. The aluminum oxide film is composed of countless tiny unit cells, each densely packed in a hexagonal prism shape, forming a dense honeycomb-like structure. This structure not only imparts high hardness but also excellent wear resistance. When external forces act on the surface, the stress is distributed across multiple unit cells, preventing localized stress concentrations that could lead to cracking. Furthermore, the oxide film and the metal substrate are tightly bonded through intermolecular forces, forming an atomic-level transition layer, further enhancing the film's adhesion and ensuring the durability of the increased hardness.
Compared to untreated metal, the increased hardness of anodized plate significantly alters its application scenarios. In applications requiring high surface hardness, such as aerospace components and automotive engine cylinders, anodized metals offer improved resistance to fretting wear and particle erosion. For example, anodized aluminum alloy components for aircraft landing gear can withstand impacts from sand and gravel without scratching, extending the maintenance cycle of critical components. Furthermore, the high-hardness oxide film enhances the metal's fatigue resistance and reduces the risk of crack propagation caused by surface damage.
The oxide film's hardness is also reflected in its environmental adaptability. In high-temperature environments, the aluminum oxide film's melting point reaches 2320K, far higher than that of the aluminum alloy substrate, maintaining structural stability and preventing softening. This property makes anodized plate suitable for applications such as engine compartments and high-temperature industrial equipment. In corrosive media, the dense structure of the oxide film effectively blocks the penetration of aggressive ions such as chloride and sulfate, resulting in salt spray corrosion resistance lasting thousands of hours without failure, far exceeding that of untreated metal.
This increased hardness does not sacrifice other key metal properties. Through process optimization, anodizing can achieve a balance between high hardness and excellent electrical conductivity and insulation. For example, in the electronic component industry, controlling the oxide film thickness to 5-20 microns ensures gem-grade surface hardness while maintaining the inherent electrical conductivity of aluminum alloys, meeting the comprehensive material performance requirements of 5G communication equipment. Furthermore, the porous structure of the oxide film can be converted into a sealing layer through pore sealing, further enhancing corrosion resistance without compromising hardness.
The hardness of anodized plate also offers significant advantages during processing. The high surface hardness reduces burr formation during processing and improves the precision of forming processes such as stamping and bending. Furthermore, the oxide film acts as a natural lubricant, absorbing trace amounts of lubricant, reducing the friction coefficient between the mold and the metal and extending the mold's service life. These properties make anodized plate an ideal material for precision parts manufacturing, significantly improving production efficiency for high-precision products such as mobile phone midframes and laptop cases.
Over long-term use, anodized plate's hardness stability surpasses that of traditional coating technologies. Surface treatments like spraying and electroplating are prone to loss of hardness due to film detachment, but the anodized film is chemically bonded to the substrate, eliminating the risk of flaking. Even under prolonged friction or impact, the film gradually wears away rather than flaking, maintaining its effective hardness. This characteristic gives anodized plate an irreplaceable advantage in applications requiring long-term durability, such as rail transportation and marine engineering.
