Properties of hard anodized film
The most notable properties of hard anodized films are their exceptional hardness and wear resistance, making them a preferred option for aluminum alloy surface hardening and widely used in components requiring resistance to mechanical wear. The hardness of hard anodized films typically ranges from HV300-500 , and under certain process conditions can reach HV600 or above, significantly exceeding the HV150-200 of conventional anodized films . This high hardness stems from the film’s dense structure and the presence of the α-Al₂O₃ phase, a stable phase of aluminum oxide with exceptionally high hardness (Mohs hardness 9 ). α -Al₂O₃ comprises up to 30%-50% of the film’s surface area . In terms of wear resistance, the wear volume loss of hard anodized films is typically less than 0.1 mm³ ( according to ASTM G99 ), 5-10 times that of conventional aluminum alloys. For example, on the surface of the aluminum alloy piston ring groove in an automobile engine, the hard anodized film can withstand the high-frequency friction of the piston ring, and its service life is more than three times that of the untreated surface; on the surface of the aluminum alloy roller in textile machinery, the wear resistance of the hard anodized film extends the maintenance cycle of the equipment from 3 months to 1 year.
Hard anodized films offer excellent corrosion resistance, effectively protecting aluminum alloy substrates from a variety of corrosive media and maintaining stability in harsh environments such as humidity, acidity, and alkalinity. This performance is attributed to the film’s high density and chemical stability. Hard anodized films typically have a porosity of less than 5% , and the film contains a high concentration of Al₂O₃ , making them chemically inert and resistant to reaction with water, oxygen, and most acids and bases. In a neutral salt spray test, hard anodized films with a thickness of 50μm or more remained rust-free for over 1000 hours, while conventional anodized films exhibited corrosion pitting after only 300 hours under the same conditions. In industrial environments, hard anodized films also exhibit excellent resistance to organic media such as lubricating oils and hydraulic fluids, resisting swelling or dissolution. Aluminum alloy valves from a chemical plant, after hard anodizing treatment, were immersed in a 5% sulfuric acid solution for six months. The surface showed only slight discoloration and no obvious signs of corrosion, while untreated valves developed severe pitting within one month.
Hard anodized films offer excellent heat resistance and insulation properties, making them uniquely valuable for applications in high-temperature and electrical applications. The melting point of Al₂O₃ , the film’s primary component, is as high as 2050 °C. Therefore, hard anodized films can withstand long-term use in environments below 300 °C and withstand temperatures as high as 500 °C for short periods without significant performance degradation. In high-temperature environments, the film also protects the aluminum alloy substrate from rapid oxidation. For example, on aluminum alloy brackets within the engine compartment, hard anodized films can reduce the substrate’s high-temperature oxidation rate by over 50% . In terms of insulation performance, hard anodized films boast a breakdown voltage of 500-1000V and a volume resistivity exceeding 10¹⁴Ω・cm , making them excellent electrical insulating materials. On aluminum alloy heat sinks used in electronic equipment, hard anodized films provide both heat dissipation and insulation, preventing short circuits. Tests by a communications equipment manufacturer have shown that heat sinks coated with hard anodized films meet IP67 protection rating requirements while only reducing heat dissipation efficiency by 5%.
Hard anodized films exhibit extremely high bonding strength with aluminum alloy substrates, a key factor in ensuring long-term reliable operation under complex operating conditions. The bonding strength between hard anodized films and the substrate is typically between 30 and 50 MPa, significantly higher than that of other coatings (such as electroplating and paint). This high bond strength stems from the in-situ growth of the film—the film is formed by direct oxidation of the base metal, forming a gradient transition with the substrate instead of a distinct interface. In a bending test, a 50μm-thick hard anodized film can withstand 180° bending without peeling. In an impact test, it can withstand an impact energy of 5J without flakes. This high bonding strength makes hard anodized films suitable for components subjected to vibration and shock loads, such as aluminum alloy suspension components in automobiles and aircraft engine blades. An airline has demonstrated that hard anodized engine blades experienced no peeling during 1,000 hours of flight testing, achieving a fatigue life exceeding 90% of that of new parts.
The properties of hard anodized films can be flexibly controlled through process parameters to meet the individual needs of different application scenarios, making them widely applicable. Adjusting the electrolyte composition can alter the film’s properties: adding oxalic acid to a sulfuric acid electrolyte improves the film’s toughness and reduces its brittleness; adding an organic acid (such as citric acid) enhances the film’s corrosion resistance. Controlling the oxidation temperature allows for control of film hardness: films prepared at low temperatures (0-5°C) exhibit higher hardness (HV 450-550) but greater brittleness; films prepared at moderate temperatures (10-15°C) achieve a balance between hardness (HV 350-450) and toughness. By varying the oxidation time, films of varying thicknesses can be achieved, ranging from 10μm to 100μm, meeting diverse requirements, from decorative to functional. For example, a thin, dense hard anodized film (10-20μm) is applied to the surface of aluminum alloy parts in precision instruments to improve wear resistance without affecting the dimensional accuracy of the parts. On the guide rails of heavy machinery, a thicker film (50-100μm) is used to provide long-term wear protection. This controllable performance allows hard anodized films to adapt to a wide range of applications, from microscopic electronic components to large-scale industrial equipment, making them an indispensable and important technology in aluminum alloy surface treatment.
