Apple granted patent for scratch-proof nitride metal device coating

The U.S. Patent and Trademark Office this week granted ownership of the invention "Nitriding Stainless Steel for Consumer Electronic Products" to Apple. Known as U.S. Patent No. 7896981, it describes a cost-effective system that uses a nitride coating to prevent scratches and blemishes on the metal surface of a mobile device.

Adding a layer of nitride atop a stainless steel exterior would protect a device from damage. But the choice of material would maintain the look and feel of stainless steel, as nitride allows the color of the metal to show through.

Nitride is not to be confused with titanium nitride, a ceramic material that is also used as a coating on metal. Unlike nitride, which is clear, titanium nitride has a metallic gold color that conceals the look of the metal beneath it.

Apple's invention notes that stainless steel, while scratch-prone, is an ideal material to craft electronic devices. Stainless steel is preferable because it is not magnetic, and it is less likely to inhibit wireless technology like a cell phone signal, Wi-Fi or Bluetooth.

The addition of nitride could give Apple the best of both worlds: the advantages of stainless steel would remain, while a nitride coating would address the chief concern of durability.

The invention also includes specifics on how the nitride coating would be applied to a device. It describes using a salt bath nitride process to coat stainless steel with an initial layer at least 15 to 30 microns thick and with a Vickers Hardness value of at least 1,000. Another method would use a nitrogen salt bath with an average temperature of no more than 580 degrees Celsius.

After the stainless steel sits in a superheated salt bath for between 45 and 90 minutes, the material would be removed and the nitride layer would be polished to remove no more than 10 percent, resulting in a smooth exterior.

Apple first filed for the invention in April of 2010. It was discovered last October by AppleInsider, when the filing was made public by the USPTO. The invention is credited to Douglas Weber.