Understanding Type 3 Hard Coat Anodize
Type 3 (III) Anodize aka “Hardcoat” Anodize, is an electrochemical process used to improve the characteristics of the aluminum surface, usually for a functional reason such as corrosion resistance or electric insulation. This differs from type 2 anodize, also known as clear coat in that it is usually just used for cosmetic applications such as the Macbook.
Hardness: The hardness of a Type III anodized aluminum surface is typically around 60-70 on the Rockwell C scale (350-450 vickers), which is comparable to case-hardened steel. For comparison, untreated aluminum is generally around 15 on the Rockwell C scale.
Coating Thickness: The coating generated by hard anodizing typically measures between 25 and 75 micrometers (0.001 to 0.003 inches). It can be tailored to application requirements and is substantially thicker than the 0.8 to 25 micrometers (0.00003 to 0.001 inches) achieved with Type II anodizing.
Wear Resistance: Type III anodize demonstrates a wear resistance up to 100 times better than untreated aluminum. The Taber Abrasion Test, for example, measures weight loss in milligrams, and hard anodized surfaces often perform at less than 5 milligrams lost, compared to untreated aluminum, which can see up to 500 milligrams lost under similar conditions.
Thermal Properties: Hard anodized surfaces have an excellent thermal conductivity of around 6 W/m·K (watts per meter-kelvin) vs aluminum that is around 200. This conductivity improves heat dissipation in high-performance engineering applications such as in heatsinks and electrical battery packs. Thermal emissivity is around .8.
Corrosion Resistance: Hard anodized coatings exceed the minimum standard of 336 hours in a 5% salt spray (ASTM B117) without showing signs of corrosion. This makes them extremely valuable for applications in harsh or marine environments. In the semiconductor industry, the HCL bubble test is used to measure corrosion resistance. Good quality anodize can last 2-4 hours. Semano’s AX200 and AX300 coatings can last 12-36 hours.
Dielectric Strength: Hard anodize coating acts as an excellent electrical insulator with a breakdown voltage. Good anodizing can achieve around 600 volts per mil of thickness. Some proprietary coatings such as Semano’s AX200 coating can achieve around 1200 volts per mil with the total insulation over 4000 volts.
Compensating for Anodize Dimensional Change: You can typically expect 50% buildup and 50% penetration when the anodize is built up. As a result, if you need a 2 mil coating, then plan for 1 mil of dimensional change.
Anodize is often chosen as a metal finish based solely on the rich appearance alone. While typically type 2 anodize is used for purely cosmetic purposes, type 3 anodize also has a very distinctive and visually appealing look.
Color: The standard color of a hardcoat anodized surface is a deep, matte gray to charcoal black. However, the exact color can depend on the specific aluminum alloy being anodized and the thickness of the hardcoat (thicker anodize typically results in darker color). With regard to alloys, 5052 and 6061 tend to look matte black. 7075, 2k series, and 3k series will usually look more gray.
Texture: Hardcoat anodizing creates a harder surface that feels substantially smoother than raw or Type II anodized aluminum. This finish also imparts a slight matte effect due to the thicker oxide layer, which helps reduce glare and enhance its aesthetic appeal. It lacks the glossy finish seen on many Type II anodized parts.
Uniformity: The anodizing process results in a uniform and consistent surface finish, even on complex geometries. The coating penetrates into the material as well as builds up on the surface, ensuring a consistent appearance across the part. The only exception to this, is typically when surface finish is different from one area to the next.
It should be noted that the appearance can vary based on the specific hardcoat anodizing process used and the conditions under which it is done. For instance, while dying is common with Type II anodizing, it is less frequently used with hardcoat anodizing due to its inherent dark color. However, if dye is used with hardcoat anodizing, it can create different shades and appearances.
How is Anodizing Done:
Anodizing is a process that starts by taking a part and placing it on a rack. The rack, with the part now on it, is placed in a series of chemical preparation tanks. These preparation tanks usually include soap, an etch tank to remove surface contamination, and an acid tank to remove other contaminants. After these prep tanks, the surface of the part should be aluminum rich and ready to anodize.
At this point the part is placed in an acid bath (usually sulfuric acid) and an electric current is passed through the part and the tank resulting in the creation of the anodize. The part will remain in the tank until the desired thickness is reached.
The end result is anodize on all the parts that made contact with the acid. In some applications, particularly semiconductor, it may be desirable for conductivity or some other purpose to leave a portion of the part bare aluminum. As such, the part can be masked off to prevent contact of these areas with the acid leaving the aluminum bare.
Once the anodizing is complete, the anodize can be dyed or sealed. Typically if type 3 is a hard coat, dye is not used as the pores are not large enough to accept the dye well. The only exception to this is typically black dye which will look just like a darker version of the hard coat.
The seal process is almost always used as it greatly increases the dielectric and seal properties of the anodize by literally sealing off the pore preventing attack on the bare aluminum.
Semiconductor Anodize: Anodize in the semiconductor equipment industry has existed since the birth of the modern industry in the late 70s/early 80s with the primary use being in the etching process. The principles of Semano, Frank Largusa, Hans Sellge, and Jose Dacorro were early pioneers creating many of the processes still used today. Semano further advanced the industry with its AX200 and AX300 coatings, specifically designed for the tough electrochemical process environments. As a result, the semiconductor process equipment market remains the largest for Semano to this day.
Anodizing for Military Applications: Perhaps the oldest of all applications, come from the military. The anodize Milspec 8625 is well known by almost all anodizers and is used in many commercial applications as well. Almost all small arms are hardcoat anodized as well as weapon systems used in corrosive environments such as the ocean.
Anodizing for the Automotive Industry: Some of the most interesting new applications of hard coat anodize are seen in the automotive industry. Evolving from what was usually purely cosmetic, car manufacturers and battery pack makers need more capable anodizing to support their projects that often require both corrosive strength as well as highly electrically insulative anodize.
Anodizing for the Medical Devices: Anodizing has become popular recently in medical devices for a couple of reasons. Firstly, anodized surfaces are inherently non-porous and smoother, making them resistant to microbial growth and easier to sterilize — essential attributes for medical environments where hygiene is paramount. Furthermore, the anodized layer enhances wear resistance, protecting instruments from the rigors of daily use and the corrosive nature of repeated sterilization cycles. The process can also improve the aesthetic appeal of devices, with the ability to incorporate colors for branding or functional differentiation. Given that medical devices often have tight dimensional and performance specifications, the anodizing process chosen must ensure minimal dimensional changes while providing the necessary corrosion resistance and durability. All these factors combined make anodizing an invaluable tool in the realm of medical device manufacturing.