Hardness versus Wear resistance

The first post about the hardness of the aluminum oxide formed by the hard anodizing process described the various types of hardness test, Vickers, Knoops, Brinell and Rockwell.

This second one will give an idea of the differences between the hardness of the hard anodized coating (hard anodizing) and the wear resistance of the hard anodize anodic process. This text will also have referrals to presentations at the International Hardanodizing Association´s symposium in September 2010 in Las Vegas.

Aluminum oxide is almost as hard as diamond (1200 HV or more) but in general too thin to increase the hardness of the aluminum metal itself. It will not protect against strong pressure but it will resist surface scratches and therefore protect the overall appearance of the surface.

The hardness of the aluminum oxide layer formed by anodizing, increases by decrease in temperature of the electrolyte and in the acid concentration. The hardness is also increased by an increase in the homogeneity of the microstructure and by an increase in the current density used to form the aluminum oxide film.

Prof. Allan Matthews of the University of Sheffield, England pointed out in a presentation that “Wear = constant load/hardness is a commonly accepted relationship. However this equation is nonsense, because it ignores the many different types of wear, such as impact, fretting, abrasion, friction sliding and others.”

In fact, the Elastic modulus is also very influential, and the ratio of hardness (H) to modulus (E) gives a better indication of wear resistance than either alone.

If the hardness is too high, the coating is susceptible to cracking. However, ductility allows a coating to accommodate deformation. When the ration H/E is high, then wear resistance is good.

Mr. Leonid Lerner from Sanford Process Corp., US showed in his presentation at the IHAA symposium a great slide of the two different directions which we expose the oxide layer for external stresses depending on if we test or use it in normal applications.

When testing the aluminum oxide film formed by the hard anodizing process it is normally done on a cross, shown in Image A and is explained more in the first post about how to define the hardness of aluminum oxide formed by hard anodizing.

This leads to a stress horizontal and perpendicular to the hexagonal oxide cell structure (Cross-sectional View, see right bottom of the slide).

Whereas the mechanical stresses in normal applications will be vertical and perpendicular to the hexagonal oxide cell structure (Top View, see left bottom of the slide).

So even though the hardness of the aluminum oxide film itself is very hard, it is way to thin to increase the hardness of the aluminum metal itself.

The hardness of the aluminum material is most often proportional to the abrasive wear resistanc but as explained above,the hardness of the aluminum oxide film formed by hard anodizing will not always be proportional to abrasive wear resistance.

Maximum abrasion resistance of the aluminum oxide is found on pure aluminum and aluminum-magnesium alloys for the same hard anodizing process parameters.

Sealing decreases the wear resistance of oxide film formed by hard anodizing up to 50 – 70 % of the unsealed value.

If you find this article useful and you would like to know please contact me at [email protected]__________________________________________________

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Summery of two important anodizing conferences

This year two anodizing conferences took place in a row.

The International Hard Anodizing Association, IHAA, 13th Technical Symposium in Las Vegas,NV September 29 – October 1, 2010 and the Aluminum Anodizers Councils, AAC, 19th AnnualInternational Anodizing Conference & Exposition, October 5-7, 2010 in Montréal, Québec, CA.

The International Hard Anodizing Association symposium
72 hard coat people from all over the world listened to the most experienced and knowledgeable industry and university experts, talking about all different topics regarding hard anodizing. Topics as plasma electrolytic oxidation, advanced anodizing using process control technology, Interfacial Phenomena, Hard Anodizing 7000 alloys, blistering, flaking and pitting, hardanodizing – what is hard?, Hardness vs. Wear resistance, Oxalic Acid Anodizing in Japan, FDAapproval of Hard Anodizing, Dyeing Anodic Coatings and much more – plus the very importantpart of this symposium – the networking between the sessions and during the lunches and dinners.
The Aluminum Anodizers Councils Conference

Some of the people who attended the IHAA symposium chose to fly up to Canada to meet with140 people from the anodizing industry. General sessions and three different focus sessions gave a lot of opportunities to hear what ever you thought interesting, and meet a lot of different people.

If you find this article useful and you would like to know more please contact me [email protected] __________________________________________________

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Aluminium 2010

8th World Trade Fair & conference in Essen has started. I am a part of the Danish Pavillion in Hall 5. This is three days with a lot of great aluminium talk and very interesting companies from all over the world.If you find this article useful and … Continua a leggere

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Just Grand!

Just Grand! is a very informative article from this months Product Finishing newsletter which gives a great view of the finishing market right now.Read the article hereIf you find this article useful and you would like to know more please contact me bl… Continua a leggere

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Less than two months to the next Anodizing Workshop in San Diego

Click here for registration

Register by July 14th and save $100

The anodizing workshop will be held at Holiday Inn Express San Diego Old Town

Make sure to let them know you will be attending the Surface Finishing Academy’s Introduction to Anodizing Course to get the best possible rate.

Holiday Inn Express Old Town
3900 Old Town Avenue, San Diego – Old Town, CA 92110
Tel: 619-299-7400

If you want to know more about the anodizing workshop, please send me an email at [email protected] __________________________________________________

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How to define the hardness of the aluminum oxide film formed by hard anodizing

The term “Hard Coat” or Hard Anodizing gives the impression of an anodizing process which gives a very hard anodic layer. The values below show that the anodic layer formed by this process really is harder but it is still important to remember that saying “Hard Coat” to an anodizer doesn’t give him enough information to process the metal.

HARDNESS COMPARISON between different materials
  • Untreated Aluminum Alloy 6082 - HV 100 – 120
  • Hard Anodized Alloy 6082 - HV 400 – 460
  • Stainless Steel - HV 300 – 350
  • Mild Steel - HV 200 – 220

The values are measured in (VPN) = Vickers pyramid number, also referred to as the Vickers hardness number (HV or VHN).

The Vickers hardness is the amount of force applied to the diamond divided by the area of the indentation the diamond makes in the material; in practice the diagonal of the pyramidal indentation is measured and the result is read from a table and is stated as an empirical measurement, without units.
Other hardness measurement numbers are found, such as, Brinell, Rockwell and Knoops. The Vickers hardness is up to about HV 500 about 1.04 times the Brinell hardness but most of the time hardness of the anodic oxide layer is measure by Vickers.
Knoops hardness is almost identical with Vickers hardness except for the form of the diamond. In this testing method the diamond has a rhombic-based pyramidal shape. The form of this
indentation makes it possible to measure the hardness of aluminum oxide more accurate but it is still not widely used.

The anodic oxide layer is very brittle and to obtain the best reproducibility of the measurements the Knoops diamond should be used. Using Vickers hardness measurements causes cracks in the oxide layer, so only measurements in the middle of the oxide layer are possible.

HARDENSS TESTING on anodic coatings should be carried out on the edge of the film so that the effect of the underlying, soft, aluminum is eliminated but not to close to the edge so the

softness of the resin influence the results.

The image to the right shows a cross section of a hard anodic coating with Vickers indentations and thickness measurements.
The pyramid has to be square formed to be sure of hardness value measured.
The light blue to the left is the aluminum alloy and the dark to the right in the picture is the resin.

THE MILITARY SPECIFICATION, MIL-A-8625F, for “Anodic Coatings for Aluminum and Aluminum Alloys” says nothing about any requirements of the hardness of Type III – Hard Anodic Coatings, see earlier post about this subject here.
The European Standard EN 12373 ”Aluminium and aluminium alloys – Anodizing” has none either. Both of them have on the other hand requirements of a wear resistance of the coating.
The maximum wear index for coatings on aluminum alloys having a copper content of 2 % or higher is of 3.5 mg/1000 cycles and 1.5 mg/1000 cycles for all other alloys.
Next post will discuss the wear resistance versus the hardness of hard anodic oxide coatings.

For more information on how to define your hard coat for your product please contact me [email protected]


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Why is Hexavalent Chromium so hazardous

The reason for the toxicity of hexavalent chromium is explained great on Wikipidia

Hexavalent chromium is transported into cells via the sulfate transport mechanisms, taking advantage of the similarity of sulfate and chromate with respect to their structure and charge.

Trivalent chromium, which is the more common variety of chromium compounds, is not transported into cells.

Inside the cell, Cr(VI) is reduced first to metastable pentavalent chromium (Cr(V)), then to trivalent chromium (Cr(III)). Trivalent chromium binds to proteins and creates haptens that trigger immune response. Once developed, chrome sensitivity can be persistent. In such cases, contact with chromate-dyed textiles or wearing of chromate-tanned leather shoes can cause or exacerbate contact dermatitis. Vitamin C and other reducing agents combine with chromate to give Cr(III) products inside the cell.

Hexavalent chromium compounds are genotoxic carcinogens. Chronic inhalation of hexavalent chromium compounds increases risk of lung cancer (lungs are especially vulnerable, followed by fine capillaries in kidneys and intestine). It appears that the mechanism of genotoxicity relies on pentavalent or trivalent chromium. According to some researchers, the damage is caused by hydroxyl radicals, produced during reoxidation of pentavalent chromium by hydrogen peroxide molecules present in the cell. Strontium chromate is the strongest carcinogen of the chromates used in industry. Soluble compounds, like chromic acid, are much weaker carcinogens.

Chromic acid is used in various surface treatments on aluminum, such as chrome plating, chromating and Chromic Acid Anodizing.

In the U.S., the OSHA PEL for airborne exposures to hexavalent chromium is 5 µg/m3 (0.005 mg/m3).


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Next Anodizing Workshop in Chicago

Join us for the next Anodizing Workshop in Chicago from May 11 – 12, 2010.

If you simply just want to know more about anodizing, or why some parts are rejected and others not, or enjoy a great network opportunity with other people who love to talk about anodizing, then join us by clicking here.

Hear what one of the former attendees said about the Anodizing Workshop:

“I have attended Anne Deacon Juhl’s Anodize Workshop and would recommend it without any reservations. Her workshop is well structured and informative along with her excellent communication skills. Anne’s knowledge made the workshop interesting and enabled her to answer all of my questions to my satisfaction.”

Rick Webster, Nelson Nameplate

Perhaps you want to build a new anodizing line, or just want to improve the old, see how a fully automatic line can look after working with me by taking a look to the right column where a short video shows the automatic anodizing line in Denmark.

If you are interested in knowing more about my products, please feel free to contact me or push the consulting button on top of the site.
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