MaterialPedia

This process produces a coating that is normally 50 m (2 mils) thick, although other
thicknesses can be specified. The coating is extremely hard. It is described as file hard (equal to about 60 to 70 HRC). The
color of the class I coating ranges from gray to bronze to almost black, depending on the alloy coated, the coating
thickness, and the electrolyte temperature. The coating can be dyed to produce a class 2 coating. Because thick coatings
are naturally very dark, only colors darker than natural are possible. Generally, this limits the dying of hardcoat to black
in common processes. If a more extensive color choice is required, there are several proprietary hardcoat processes
available to accomplish this.
Hardcoat penetrates the base metal for one-half of its thickness and builds above the original base metal dimension for
one-half of its thickness. Thus, for a thickness of 50 μm (2 mils) per side, the dimensional change of the workpiece would
be 25 μm (1 mil) per side. Commercially available coating thickness tolerances are the greater of ±5 μm or ±10% of the
total targeted thickness. The corrosion resistance of the unsealed class 1 coating is very good and comparable to the other
types of anodize. When the hardcoat anodize is sealed, as in a class 2 coating, it becomes the most corrosion-resistant type
of anodize.
Advantages. Hardcoat anodize, because of its variety of desirable properties, has found widespread use in
manufactured products. First, because of its extreme hardness, it is used in situations in which wear resistance is required.
Applications include valve/piston assemblies, drive belt pulleys, tool holders and fixtures, and many other items requiring
wear resistance.

Second, because of its excellent resistance to corrosion, hardcoat is used on aluminum components in harsh environments.
These include outside exposure in salt air, marine components, automobile wash equipment, components for the aircraft
and aerospace industries, and food preparation machines.
Third, because hardcoat is an excellent electrical resistor, it can be used to insulate heat sinks for direct mounting of
electrical or electronic equipment. Also, it is used in welding fixtures where some areas may need to be insulated from
work.

Fourth, because hardcoat is a naturally porous substance, it is used in many areas in which the bonding or impregnation of
other materials to aluminum is needed. This coating bonds very well with paints and adhesives. Also, it can be
impregnated with teflon (polytetrafluoroethylene, or PTFE) and many dry film lubricants to impart lubricating properties
to the coating.

Lastly, because of its desirable properties and also because it produces a buildup of coating, it is widely accepted as a
salvage coating to restore worn or improperly machined parts to usable dimensions. Coating thicknesses in excess of 250
μm (10 mils) per side are possible on some alloys with certain proprietary hardcoat processes.
Suitable Alloys. Although almost all alloys can be coated, the 6000-series aluminum alloys produce the best hardcoat
properties. As with the other anodize types, high-silicon die castings produce the lowest-quality coatings. Also, because
the hardcoat process is sensitive to copper, alloys in the 2000 series should be avoided if possible. Alloys containing
copper can be hardcoated, but only a relatively few commercial sources have the ability to coat these alloys with
reliability.

Relative Costs. Hardcoat anodize is the most expensive type of anodize. It is generally twice the cost of H2SO4 anodize
and 50% more than CrO3 anodize.