3.2 - Hardcoat Anodizing - General Properties

Hardcoat anodizing is an electro-chemical process of converting aluminum to aluminum oxide. This process allows the production of a thicker wear and corrosion resistant coating on a variety of aluminum alloys. These hard coatings allow the use of aluminum in many design applications formerly restricted to steel.

Thickness
Hardcoat anodizing requires higher electrical power and produces thicker coatings than regular anodizing. Regular anodizing ranges in thickness from .0002 to .0003”, Type II Hardcoat is .0007 to .001”, and Type III Hardcoat is .0005 to .0045”. Food Grade Hardcoat ranges from .001 to .015”.

Tolerances
A hardcoat application will always penetrate below as well as above the surface of a part, no matter which alloy is used. This uniform growth requires that tolerances be correctly identified and considered, leaving no room for error. For example, A .001 Hardcoat application will increase the size of a part by .0005, since the other .0005 grows into the surface. Thus hardcoat anodizing requires 100% quality control. In addition, parts stripped of hardcoat are most likely ruined, due to the fact that the thickness of the coating below the surface will result in altered dimensions.

On threaded surfaces, a hardcoat application will cause the pitch diameter to increase by approximately 4x the thickness of the above-surface coating. In deep recessed holes, the coating results may be less than expected and may diminish towards the bottom. Also, the Aluminum Oxide film produced by hardcoat anodizing actually grows perpendicular to the surface of the metal, suggesting that sharp corners be rounded so that voids do not appear in the thicker coating.

Finally, surface roughness will generally become increased as a result of hardcoat anodizing. Typically, a 10 micro-inch surface on wrought aluminum alloy would increase to 16 micro-inch.

Colors
The color of the hardcoat application depends on both the alloy and the coating thickness. It will normally range from dark brown to grey to dark black. The color can also be affected by the mill production process or heat treating, so that any welds usually cause problems and should be considered in the design of a part requiring hardcoat anodizing.

Teflon
Hardcoat anodizing has progressively become the alternative in manufacturing to hard chromium plating, due mainly to the scientific data proving the superiority of hardcoat’s wear resistance over hardened steel. Hardcoat is filehard, a good insulator, and exhibits a dielectric strength of 500 volts. Also, it is able to withstand higher temperatures than the base metal, making the part resistant to direct flame impingement.

Hardcoat anodizing will not peel, however, the surface may crack under heat-stress conditions such as bending or sliding. This is easily resolved through the application of Teflon, which microscopically forms a honeycomb pattern that seals the part, making it exceptionally corrosion and wear resistant.

Applications
Hardcoat anodizing has proven its resilience on gears, computer parts, hydraulic gears, valves, pump housings, molds, cams, rollers, impellers, sprockets, heat sinks, as well as food industry and OEM parts.

For Hardcoat anodizing as well as any other aluminum anodizing requirements, call us and we’ll be happy to run samples or schedule a visit at your convenience.

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