Q. We manufacture aluminum saw guides out of 6061-T6 aluminum.These are sent out for Type III (hardcoat) anodizing to a local shop to get 2 mils of coating thickness. The problem is that we are finding a variation of 0.0004–0.0008 inches in coating thickness. Before sending the parts out for anodizing, we precision grind them to a dimensional tolerance of 0.0002 inches. I have heard of the Martin hardcoating process and was told that when using this process one could get up to 8 mils of coating thickness, thus allowing plenty for honing to our final tolerance of 0.0002 inches. My question is, is it possible for the parts to be anodized to more precision than 0.0004–0.0008 inches? What is the Martin process, and who does it? P.K.
A.The Martin process was developed in the late ’40s and early ’50s, and was bought by Alcoa soon after. It is a pretty versatile process improved upon by Alcoa a bit. The “nominal” hardcoat process my company generally uses when providing a hardcoat system for our customers is a slight variation of the Alcoa process. I don’t think it is accurate to say that the Martin process itself is capable of an 8-mil coating. It was developed to put 1–3 mils on most alloys. High-copper 2000-series alloys were more of a problem, but that problem has been solved with the process variation. For the most part, at least 2 mils can easily be produced on most high-copper alloys, easily 3 mils on 7075. Processing 6061 falls somewhere in between 2024 and 7075 in degree of difficulty.
The MIL-A-8625 specification allows +20% variation in coating thickness, but the coating thickness and variation that is allowed on a particular job depends on what the customer and the anodizer agree upon. Under carefully controlled conditions, a variation of 0.0002 inches should be attainable. That can also depend on the size and shape of the part, but it sounds like your part is pretty easy to anodize. If the prospective new vendor you mention can put on 3 mils uniformly, as you say, you should be able to hone off what you need to get your required tolerance. Perhaps no honing will be necessary.