In the universe of wear resistant thin films, diamond-like carbon (DLC) coatings have emerged as the ideal solution for demanding physical applications, such as medical devices and instruments, where components are under high loads or subject to extreme friction, wear, and contact with other parts.
In these types of environments, only the high hardness of a DLC coating — along with a corresponding low coefficient of friction — can prevent parts from pitting, galling, seizing, and ultimately failing in the field. To examine the benefits of DLC coatings for medical applications, this article discusses the advantages of this approach.
Because of the broad range of customizable attributes possible within the category, DLC coatings can play an important role in medical component engineering from the earliest steps of the design process.
A coating is very much a sophisticated design element with highly engineered properties. The properties can be tailored specifically to meet the medical performance requirements of different operating conditions. As such, coatings should not be a design afterthought but rather a critical element of how a component is initially engineered to fully utilize possibilities for maximum system performance.
Hydrogenated Amorphous Carbon Coatings
DLC Coatings are hydrogenated amorphous carbon (a-C:H) coatings, but this categorization can be a misconception. Coatings within the DLC family can be highly engineered based on hydrogen content (hydrogenated or hydrogen-free), the selection of additional metallic and non-metallic doping elements, the presence of sublayers, and choice of deposition and bonding methods.
Together, these factors can be precisely controlled to create a broad range of thinly applied (typically 1–5 m) DLC coatings with a hardness of 8–80 GPa or higher (diamond is the hardest known material at 70–150 GPa). In addition, the desired coefficient of friction, surface finish, and even application temperature can also be manipulated.
The most widely known DLC coating type, hydrogenated amorphous carbon, is most often applied through a process called plasma-assisted chemical vapor deposition (PACVD). This deposition method causes a chemical reaction through plasma excitation and ionization, creating a coating hardness of approximately 15–30 GPa, which is on the lower end of the DLC family. A hydrogenated amorphous carbon coating can be manipulated further through doping, which is a process of adding chemical elements to alter the performance properties. Silicon, oxygen, or metals can all be used as doping elements to achieve different results.
When a lower coefficient of friction is required for mated or sliding parts, or to assist in releasing items from cavities or molds, silicon doping can be a suitable approach. This silicon doping approach creates an a-C:H:Si coating with a coating hardness of 15–20 GPa. With silicon and oxygen-doping, high electrical resistivity and chemical inertness can also be achieved. All of Oerlikon Balzers’ DLC coatings, for example, are biocompatible, which makes them an ideal solution for medical instruments.
An alternative to hydrogenated DLC coatings is a family of hydrogen-free based coatings that provide even higher hardness along with a very low coefficient of friction. Most hydrogen-free coatings are applied using a method of physical vapor deposition (PVD) by arc evaporation, which produces tetrahedral amorphous carbon, or ta-C. With a high level of tetrahedral bonds (mostly 50–60 percent), substantially higher abrasive wear resistance is achieved compared to a-C:H alternatives.
With a typical hardness up to 60 GPa, ta-C coatings are an excellent choice for components that are exposed to extreme operating forces over the long term, including shafts and seals that must work in tribological environments where friction can cause them to overheat and fail.
The challenge historically with hydrogen-free coatings, and with ta-C deposition in particular, is that the application process produces small droplets that contribute to a rougher surface finish. As a result, coating manufacturers must complete secondary polishing processes to smoothen the surface. Because of its hardness, it is a time-consuming and expensive process the requires specialized equipment.
To address this concern, some hydrogen-free DLCs are produced using a filtered cathodic arc deposition method in which an electromagnetic filter removes most droplets. Although this creates a smoother surface, a secondary polishing step is still often required, and process times are longer for the same coating thickness.
Coatings as a Design Element
When an even smoother surface is needed, hydrogen-free DLC coatings can be applied utilizing a proprietary Scalable Pulsed Power Plasma (S3p) technology developed by Oerlikon Balzers. S3p is a unique type of high-power impulse magnetron sputtering (HiPIMS) technology, which can be seen as combining the advantages of the arc evaporation and sputtering methods. The very dense plasma yields hard coatings with high adhesion (at a level comparable to arc evaporation). At the same time, it results in smooth coatings due to the nature of the sputtering process, in which atoms are ejected from a target or source material.
The coating is applied at relatively low temperature, well below 200° C as compared with up to 350° C for other DLC coatings, which enables its application to a much wider panel of materials, effectively bonding to aluminum and steel substrates. Hydrogen-free DLC coatings applied using the S3p process BALIQ CARBOS include medical instruments and other small, precision tools.
At the top end of the scale are diamond coatings applied by a chemical vapor deposition (CVD) process in both micro and nanocrystalline options that are rated at 80–100 GPa. Such coatings are mainly used for highly specialized tools for cutting demanding materials such as carbon-reinforced fiber materials and do not offer the benefit of low friction anymore.
Given the number of variables involved with DLC coatings, it is important that medical OEMs better understand the range of options so they can select the ideal solution for the application while also taking into consideration the economics.
Coatings are effectively an architecture of layers engineered to achieve specific properties. A coating is built layer by layer focused on bonding, hardness, and the surface. Modifying the properties of each creates an extremely wide range of medical surface solutions within the DLC coating family.
By applying DLC coatings to medical components, not only is the surface hardness and durability increased, but essential parts are far less likely to fail, if at all. As a result, maintenance and unexpected downtime is drastically reduced, even in the most demanding medical environments with high friction, wear, and contact pressure.
This article was written by Dr. Florian Rovere, a coatings expert for Oerlikon Balzers, Scchaumburg, IL. For more information, visit here .