Hard anodising (Hard coat) is a term used to describe the production of anodic coatings with film thickness, corrosion and abrasion resistance as their primary characteristic. The nominal thickness of coatings applied is 25 - 50 microns but a controlled thickness can be obtained from just a few microns up to 120 microns depending upon the alloy chosen.
Depending on alloy chosen and film thickness the colour varies from light grey to black.
Hardness & Wear Resistance
The wear characteristics of hard anodised aluminium are most favourable at low loads and compare most favourably with hard chromium and high speed steel, in most cases better. The Microhardness of the coating varies significantly depending on the alloy chosen. (Also castings of the same chemical makeup of wrought alloy's and different casting methods).
Generally the high strength alloys i.e. 2000 and 7000 alloys produce a Microhardness figure ranging from 250 to 350 micro Vickers where as the 6000 alloy range in between 400-500 micro Vickers. A note of caution should be made when comparing micro hardness to wear resistance, the hardness measured by micron-indentation method does not represent the performance of the material. It has been found that a hard anodised coating measuring 450 mv will have a wear resistance of tool steel of 950 mv and even an anodic coating of high copper alloy such as 2014 measuring 250-280 mv will often offer as much wear resistant as tool steels.
The wear resistance of the coating is closely related to the surface finish. The rougher the surface finish the more likely under high local pressure, for the coating to break away in minute particles and get trapped or impinged into the sliding surface, hence causing an abrasive medium. The surface finish of the component should be better than the final finish required as the hard anodising process increases the surface roughness. Where a fine finish is required, honing or grinding is recommended.
The properties of the hard anodised aluminium as a bearing material are good and are improved by impregnating the coating with lubricants such as "NITUFF".
The thermal conductivity of the anodic coating is between one tenth and one thirtieth of that of aluminium. The emissivity of aluminium increases rapidly as the thickness of the anodic layer is built up increasing to 80% for a 10 micron coating. A thick hard anodic coating is therefore well on the way to being a "black body" for heat dissipation, and there is very little advantage in dying it black as is sometimes done.
The aluminium oxide coating produced by anodising is a good insulator and this may be a disadvantage where good heat dissipation is required. Meanwhile hard anodised aluminium shows a good heat resistance and can stand short exposure to temperatures in the order of 2000oC. Thus it is suitable for example. For rocket venturis.
The breakdown voltages of Hard anodic coatings are highly insulating. The best results are obtained from a homogenous alloy composition , impurities in the film will lower the insulating properties. The advantage which anodised coating possess over most other media for electrical insulation is that they can operate up to temperatures of 500oC. The combination of high dielectric strength and good thermal conductivity makes hard anodised aluminium superior to most other materials as an insulating medium.
Due to the fact the hard anodic film is an integral part of the aluminium you will find that adhesion properties are superior to that of any other coating applied to aluminium.
Hard anodic coatings have better corrosion resistance than conventional coatings of the same thickness. Corrosion resistance is further improved by sealing in potassium dichromate, but with some loss in the abrasion resistance (about 20%). For excellent corrosion resistance without loss of abrasion our NITUFF process has been tested in excess of 2000 salt spray resistance.
Hard anodising will reduce fatigue strength considerably, more than the Equivalent thickness from a conventional sulphuric acid process, and can be as much as 50%. But this can be largely restored by potassium dichromate sealing but this will also reduce abrasion resistance or by using the Nituff process without loss of abrasion.
Applications Of Hard Anodising
With the many new aluminium alloys now being produced and especially using the hard anodising process, the replacement of traditional metals are substituted for the lightness of aluminium and the superior properties of the hard anodic process, also the process is environmentally friendly both by the chemicals used and it's low cost for recycling. The number of applications for hard anodised aluminium continues to grow rapidly and any manufacturer or user of engineering equipment would be well advised to examine whether hard anodic coatings could not profitably and advantageously replace existing parts because of it's unique properties.