Sandia diamond coatings extending material life

17 Jun 2002

Scientists at Sandia National Laboratories have developed a method to relieve the internal stresses of amorphous, or noncrystalline, diamond films, creating improved protection for metal and plastic materials.

The concept of diamond films as a protective coating is not new, but ordinary amorphous diamond films contain very high levels of compressive stress, placing a pressure of about 70,000 atmospheres within the plane of the film. Thus surfaces had to be coated extremely thinly, no more than a fraction of a micron, or the coating would pull off from the stress.

"It wants to get rid of that type of stress, and to do that it wants to wiggle and it pulls off from the surface," said Neal Signer, a science writer for Sandia National Laboratories. "Plastic can't hold any kind of diamond coating, nor can metal."

The new method is only in the laboratory stage at this time, but success would allow thicker coatings. The process, an outgrowth of a project to provide improved coating for sensors, uses a pulsed laser on a graphite target to deposit an amorphous carbon film at room temperature.

"The purpose of the work on the coating was to improve the life of sensors, to protect damage and lengthen their lives," noted Singer.

The difference between an amorphous diamond and a crystalline diamond, the type found in jewelry stores, is that the amorphous diamond is created from carbon after changing the bonds between atoms. The diamond is not a perfect crystal but sporadic, and the technically correct term is diamond-like film.

Sandia's scientists learned that when the deposited material is heated the films lose the stress but retain the diamond-like properties. It has been measured to 90% of the hardness of that measured for a diamond.

"The big advance is that you have this extremely hard substance that could be used as a coating," said Singer. "It's very unusual to have something that's both harder and you can put more and more layers on it."

Stress-free films of greater than seven microns thick have been made.

Although the amorphous diamond process does not produce a perfect crystal, it provides very little distortion. Since it creates a vacuum while still letting in a beam, it can stand by itself. Free-standing membranes more than an inch thick in diameter and less than 600 Angstroms thick have been produced. The free-standing membranes create the possibility of transparent windows for X-ray or electron optics.

In addition to its stand-alone ability and thickness tolerance, it also allows coating on plastics as well as metals.

Diamond coatings containing hydrogen tend to degrade at temperatures as low as 200 degrees Celsius, but the stress-free coatings have shown negligible degradation up to 800 degrees Celsius.

"Increased thickness means increased protection, but you could never increase the thickness. Now it can be done," stated Singer.