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 restricted to steel.
Hardcoat anodizing requires higher
electrical power and produces thicker coatings than regular anodizing. The
coating penetrates into the surface and also builds up on the parts. The
anodized surface is dielectric. All of the alloys show a uniform growth on the
surface equal to that penetrated below the surface of the base metal. This
important factor should be considered in cases of close tolerances. The
aluminum oxide film grows perpendicular to the surface of the metal so that
sharp corners are left void. This is more pronounced in thicker coatings.
Therefore, if possible, sharp edges should be rounded. Other factors in
hardcoat anodizing are that the color of the coating depends on the alloy and
the coating thickness. Colors can range from gray to dark black. Surface
roughness will generally increase by hardcoat anodizing. For example, a 10
micro-inch surface on wrought aluminum alloy would read 16 micro-inch after
hardcoating, and a 60 micro-finish on a sand casting would read a 100
micro-finish. Thread must also be considered when hardcoating, as threaded
pitch diameter will increase approximately four times that of surface coating
thickness (depending on the pitch angle).
Hardcoat anodizing is an
alternative to hard chromium plating. Wear resistance of this aluminum coating
has proven superior to hardened steel. The coating is filehard and is also a
good insulator. The hardcoat film will withstand higher temperatures than the
base metal. It makes the part resistant to direct flame impingement and does
not conduct electricity. Its dielectric strength is 500 volts. Hard anodized
coatings will not peel since the coating is an integral part of the base metal.
It will, however, crack if the parts are bent. In hard anodizing, as the
coating thickness increases, the fine capillary pores seal themselves making
these parts corrosion and erosion resistant.
All alloys of aluminum can be hardcoated. The
2000 and 4000 series contain the most copper and silicon, however, and are more
difficult to hardcoat. There are many applications for this process. Some parts
presently being hardcoated are gears, castings, computer part, hydraulic gear,
valves, pump housings, molds, cams, impellers, sprockets and heat sinks.
The costs are determined by the surface area of
the parts, ease of racking and coating thickness. If masking is necessary in
cases of close tolerances or in cable connectors for electrical conductivity,
it is an added expense to the hardcoat process.
