Apple Invents Coating Method for Scratch Resistant Stainless Steel

A patent application filed by Apple shows the company is looking for ways to manufacture stainless steel with an invisible coating that would make it both scratch and impact resistant. The patent application shows that the company has invented a way of doing so by adding a nitride layer on top of the metal that would protect the metal, even while preserving the “natural surface color and texture of stainless steel.”

In the unusually easy-to-understand and clear abstract for the application, Apple characterized its invention as, “a cost effective system, method and apparatus adapted to provide a nitride layer on stainless steel used for the manufacture of consumer electronic products. In addition to providing a durable, hard surface that is both scratch and impact resistant, the nitride layer allows for the natural surface color and texture of the underlying stainless steel to remain visible to the user. It is this natural surface color and texture of the stainless steel that adds to the aesthetically pleasing appearance of the consumer electronic product thereby enhancing the user’s overall experience.”

In other words, Apple could use this system to make Macs or iOS devices with stainless steel covers without having to put a coating on the metal that changed its appearance.

The patent goes on to describe exactly how Apple would achieve this using a process of nitrogen based salt baths of “no more than 580° C” to give the stainless steel a nitride layer. The resulting layer would be from 15 to 30 .µ (microns) thick, and would give the treated metal a Vickers Hardness Value of 1,000.

And this is where we risk getting in over our heads, because that number needs context to understand its true meaning. Without getting into the boring details of the Vickers Hardness Test, the short version is that it’s a method for testing how hard a metal is by measuring its resistance to being deformed by a standard object.

It is usually express as a hardness value accompanied by the load of the object used to test, as in 140HV30. That’s the Vickers Hardness Value of Stainless Steel 316L, a standard form of stainless steel, and it happens to be the specific kind of stainless steel discussed by Apple in this patent application.

Apple is saying that its invention will take stainless steel 316L from 140HV30 to “at least approximately” 1,000HV30, a 7.14x improvement. And now we can all be suitably impressed.

The following image was provided by Apple in its application and more or less shows how the nitride layer would be applied, one molecule at a time.

Nitride Patent Application Figure

The inventor of the process is Douglas Weber, and the application was filed on April 6th, 2010 and published on October 28th, 2010. The good people at PatentlyApple were the first to spot it. The application is titled, “Nitriding Stainless Steel for Consumer Electronic Products.”