Hard Coat Anodizing
Superior Protection for Aluminum Type III
Aluminum anodizing is the electrochemical process by which aluminum is converted into aluminum oxide on the surface of a part. This coating is desirable in specific applications due to the following properties:
- Corrosion Resistance
- Dielectric Properties
- Lubricity
- Bonding
- Wear Resistance
- PTFE Infusion
- Heat Dissipation
- Versatility
Properties
Superior Protection for Aluminum Type III
Aluminum anodizing is the electrochemical process by which aluminum is converted into aluminum oxide on the surface of a part. This coating is desirable in specific applications due to the following properties:
Hardness
The coating is approximately file hard, and cannot be scratched by repeated rubbing with a sharp instrument.
Corrosion Resistant
Hard coated specimens subjected to the standard salt spray test (ASTM B117-49T) have withstood 5000 hours exposure without failure. The coating is resistant to most chemicals, but will not withstand caustic or strong acids.
Electrical Properties
The dielectric strength is proportional to the thickness of coating, and varies with the alloy being coated. A .003" thick Hard Coat on 61S alloy has a breakdown voltage of 2,000.
PTFE Infusion
This process is an impregnation of PTFE into the oxide surface before being sealed for better lubricity and mold release qualities. PTFE can be added to any type of Anodize process.
Coefficient of Friction:
This value depends largely upon the surface finish. Preliminary tests with a three micro-inch finish gave a coefficient of friction of 0.11. Here again a lower coefficient of friction can be obtained through the use of Everlube. Since the film is an insulator, in an assembly in which a Hard Coated part rubs with another, the heat generated should be taken away by a cooling medium or conducted out through the mating part.
Heat Resistance
Under direct flame impingement, the aluminum alloy core will melt before the Hard Coating collapses due to the refractory nature of the coating.
Abrasion Resistance
Wear tests have indicated that the Hard Coating is comparable to case-hardened steel and hard chromium plate. The time to wear through the coating is approximately proportional to the thickness of the coating. An interesting feature of the coating is that in some cases wear is equally slight on the mating metal rubbing against the coating. In all cases, considerable additional wear resistance can be obtained by using a secondary coating of "Everlube" Solid Film Lubricant.
Adherence
The adherence of the Hard Coating to the basis metal is very good. A sheet may be bent 180° on a liberal radius and straightened again without spalling of the coating. Severe bending results in spalling on the compression side.
Cost
The cost for Hard Coating has been estimated to correspond generally with Hard Chrome plating. The Primary factor in "cost to process" is masking, which should be avoided or limited in design consideration, whenever possible. The weight and shape of the part and its adaptability to racking are other considerations that determine cost.
Applications
Many users are replacing heat treated or surface hardened steel, stainless, etc. parts with Hard Coated Aluminum. This not only eliminates the cost of heat treating, cleaning, and finishing, but it also introduces the advantages of machining aluminum instead of a more difficult material. The electrical insulating and corrosion resistant properties of Hard Coatings are introducing many new uses.
Typical Applications:
- Gears, Pinions, Pistons
- Slides, Valves, Impellars
- Textile Machine Parts, Electrical Parts
- Cams, Bearing Races
- Hydraulic Cylinders
- Ratchets Guides and Tracks
Hard Coat Specifications
Hardness: 65 to 70 Rockwell C, 380 to 430 Vickers Hardness, harder than hard chrome plate
Color: Dark Gray to Black
Coating Thickness: 0.002" average, 0.015" for salvage purposes on selected alloys
Dielectric: Hardcoat is non-conductive and will withstand 800 volts per 0.001" thickness.
Machining: Hardcoat can be ground, lapped, honed or polished.
Dyeing: Hardcoat may be dyed most colors but tends to come out dark.
Sealing: Hardcoat may be Dichromate, Ni-Acetate, hot water, or PTFE sealed.
Resistivity: Between 106 to 1012 Ohm-cm
Hard Coat Anodize - Quote Information
Factoring Costs: consider these points when gauging a general price for the part you want Hard Coat Anodized.
Masking (Selective Hard Coating)
Hardcoat may only be required on one surface of the part. All areas not Hard Coated must be masked either by painting, tape, plastic or rubber plugs. This is done by hand and is very costly. If possible, hardcoat the entire part even though you only require one area to be coated. Threads, tapped holes, bores, or other close tolerance areas can be stopped off.
Contact Points
In the process it is necessary to establish positive electrical contact with the part. For this reason, it is necessary to make contact through a tapped hole, bolted connection, or a clamp across opposite edges. These contact points will leave small uncoated spots. It is important to designate the area where the coating is primarily desired and where electrical contact may be made.
Tolerances
It is important to note that "one half of the total thickness is growth". A standard .002" coating on a cylindrical part will increase the over all diameter by .002" (.001" growth on each surface.) Normal blueprint practice is to state the dimensional range before and after coating, allowing maximum tolerance for the Hard Coat dimensions. Where tolerance is not a factor, specify only the minimum coating desired.
Selection of Alloy
Some alloys form better oxide coatings than others because of the influence of the alloying constituents in the structure of the coating.