Coating the truth

A new addition to our ever-growing list of treatments brings to mind another adage: what was old is new again. In this case, I’m referring to the application of a foreign substance to a gemstone or diamond.

Diamond and gemstone coatings have been around for more than a century and were initially devised to impart or enhance a unique colour onto a stone. If one wanted to make a yellow sapphire or diamond look more intense, that could be accomplished by adding colour to the stone along the culet or girdle in the same hue.

Today, this coating treatment is experiencing a comeback in the form of diamond-like carbon (DLC) applied to gemstones and cubic zirconia. The treatment involves applying a thin coat of DLC ranging from 5 to 50 nm (1 nm is 0.000000001 metre) through one of several methods.

The DLC coating imparts some important physical properties, such as increased hardness, improved wear resistance, and a reduction in surface friction. In the case of stainless steel, for instance, an application of DLC roughly 1 μm thick (i.e. 0.000001 mm) can increase the lifetime of a low-impact piston from one week to one year. If the application’s thickness were doubled, the lifetime would increase from weeks to well over 50 years.

The key factor determining the effectiveness of a DLC application is the quality of the coating. Although there are several coatings that can be classified as DLC, there is a wide range of ‘recipes’ at equally varying costs.

All DLC coatings comprise a few principal ingredients: carbon (in both diamond crystal and graphite form known as sp3 and sp2, respectively), hydrogen, and a wide range of metals. Therefore, the amount of sp3 present determines the purity of DLC and how it is graded.

The second part of the recipe is the method of application. These can range from chemical vapour disposition (CVD), where a carbon atom is precipitated out of plasma onto a surface, to physical vapour disposition (PVD), where charged sp3 and sp2 particles adhere to a surface through an electrical field. (There are several other processes that we can leave out of the discussion for now.) The common factors to all the processes is that the application of the DLC coating is even and, if possible, converts sp2 carbon into sp3 diamond crystal.

Overall cost of application is determined by these two factors, since it is a diamond crystal’s physical properties that are desired, as opposed to that of the graphite. To be effective, therefore, the DLC material must have a high concentration of sp3-formed carbon. It is this form of carbon that costs more.

In order to yield a high concentration of diamond crystal, the method used may be a combination of many different technologies and processes. In addition, they would eventually be determined by the industry and its needs.

Prices can vary greatly, given the range of possibilities where the resulting appearance is concerned and what the industry dictates. More importantly, the DLC coating’s durability is also a consideration. The main problem is determining what kind of DLC coating has been used, since all treatments carry the same diamond-like carbon identification. This creates a significant amount of confusion among consumers and retailers who are unfamiliar with the material. In addition, it is likely manufacturers will come to market with newer economical methods of applying a DLC coating, such as a diluted treatment that uses a lower concentration of diamond crystal in the recipe or a less energy-intensive method of application.

In recognizing this fact, engineers in Germany were able to mandate that all DLC-coated products related to their industries follow distinct classification standards based on the concentration of sp3 carbons and the application process used to adhere the coating to a product.