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The hard anodizing process also uses a sulfuric acid electrolyte, but the bath is maintained at higher sulfuric acid concentration, lower temperature, and higher current density, which allows thicker coatings to be obtained.
Nominal thickness of 1 to 2 mils is common for most applications. The overall thickness of the coating formed is 50 percent penetration in the substrate and 50 percent growth over the original dimension of the part. Parts processed by this method have a hardness of 65 Rockwell “C”. Hard anodic coatings generally have increased surface roughness as well as having the property of being softer on the top surface than down in the core of the coating. Such coatings may be processed oversized and then lapped down to final dimensions. Lapping of a hard anodized surface results in an exceedingly smooth, hard finish resembling polished stone. Hard anodizing is an excellent electrical insulator, and base for paint, dry film lubricants and other coatings.
Sealing of hard coating is not recommended unless corrosion resistance is a factor. Wear resistance is sometimes reduced by sealing. Hard coatings may also reduce fatigue strength. These factors should be considered in proposed use of parts subjected to cyclic loads. Generally, hard coatings should not be used on parts, which during rework would normally require restoring of dimensional tolerances because of wear of hard coated surfaces.
When the hard anodize is sealed with PTFE, the coating not only has great wear and abrasion resistance, but also now has improved lubricating properties. Some applications for the impregnated coating are; valves, sliding parts, hinges, cams, pistons, and gears.
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