When Mother Nature wants to protect a delicate seed, she coats it with a tough, hard shell. When engineers want to protect lightweight aluminium components, they coat them with Hard Anodising – a hard wearing finish, with high electrical insulation properties.
What is Hard Anodising?
Hard Anodising (Hardcoat) is an electrochemically produced layer of aluminium oxide on the surface of aluminium. A controlled thickness can be obtained from just a few microns up to 120 microns depending upon the alloy used.
The process is used to produce much thicker, harder and corrosion resistant coatings than the Sulphuric Acid process. The thickness can be very closely controlled, and the cool operating temperature of the process prevents thermal or physical distortion of precision engineered parts or equipment.
Secondary treatments such as impregnation with solid film lubricants and coloured dyes are available.
We are founder members of the International Hard Anodizing Association.
ASE was the first company in England to introduce the Hard Anodising process.
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.
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).
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.
Depending on alloy chosen and film thickness the colour varies from light grey to black.
The aluminium oxide coating produced by anodising is a good insulator 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.
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.
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 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.
Hard Anodising is a cost-effective choice for wear and corrosion resistant surfaces on aluminium alloys. Abrasion resistance can be equal to that of tool steel or hard chrome around 1,000 VPN, with the actual hardness of the Hard Anodised film up to 350-500 micro Vickers Hardness. The process is environmentally beneficial due to the chemicals used and its low cost for recycling.
The number of applications for Hard Anodised aluminium continues to grow, and any manufacturer or user of engineering equipment should contact us to discuss whether hard anodic coatings could profitably and advantageously replace existing parts.
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