The coating produced by the hard anodising process is chemically akin to many gemstones and shares their property of extreme hardness and resistance to abrasion. The hardness of anodised coatings is specified in British Standards using the Vickers Micro Hardness Test in which the hardness is measured on a section through the film giving micro hardness values of 250 to 600; depending on the aluminium alloy, the process and the sealing.
The matrix of aluminium oxide itself has a micro hardness of around 1,100 Vickers. The indentation method of the Vickers Test really indicates the crushability of the brittle and porous anodic film whereas the property often expected by customers is abrasion resistance.
Wear resistance is measured by various methods. ISO 10074 recommends an abrasive wheel test method as defined in ISO 8251. The SUGA Test meets the requirements of this specification and the results are expressed in terms of a wear index.
There is an alternative, the Taber Abraser Test that is quoted in US MIL specifications. This test measures a weight loss under a given loading for a number of cycles; weight loss (mg) for 10,000 cycles at 1 kg loading.
Coatings applied for wear resistance are generally left unsealed or treated with P.T.F.E (Ano-lube)
APPLICATIONS INCLUDE:
Pneumatic Cylinder Tube
Which is anodised internally to resist wear from contact with the piston seal. The coating also provides a useful measure of corrosion resistance.
Pneumatic Valve Spools
In this case, the light loading can permit use without lubricant
Fire Pump Components
Which are hard anodised to resist erosion, particularly since such pumps may be used on water containing sand or other abrasive media.
Automotive Hydraulic Components
Such as brake callipers, pistons and cylinders are hard anodised to give increased service life and again to improve corrosion resistance.
Pulleys
Both 'V' and Timing types are hard anodised to resist wear and to extend working life to approximate to that of cast iron without the weight penalty.
Plasma Etching Equipment
Where the anodising protects the metal surface from erosion by the process.
