Diamond-Like Carbon (DLC) coatings have been around since the late 1970s. Machining and automotive applications used the coating because of its exceptional durability, high hardness, low coefficient of friction and self-lubricating properties. Not only did the coating reduce wear on frictional components, it also increased the efficiency of motors. The same properties that make DLC an excellent coating for frictional applications also make it a superior coating for optical and electrical applications.
However, most companies using a DLC coating experience four common issues:
- Small defects called pinholes
- Non-uniform coating thickness
- Residual stress between the coating and the base material
- Poor coating adhesion
These defects are often related to process parameters or equipment configurations.
Pinholes are small imperfections in a DLC coating. They appear as circular pockmarks on the surface and can often extend to the substrate surface. These defects are a source of infiltration and point of weakness that can lead to the premature failure of high-end optics. They also create non-uniform optical properties across the coating surface impacting performance.
There are various causes of pinhole defects. If the substrate is not properly cleaned or outgassed, trapped air, liquid or solvents can bubble through the coating. The orientation of the substrate can also increase pinhole creation, where horizontal substrates are more prone. Top-down coating processes are also more susceptible to the occurrence of pinholes.
We address many of the common causes of pinhole defects with proper substrate cleaning and preparation. In addition, the “Coat Up” design, which injects the gas from the bottom of the chamber and evacuates it near the top creates an advantageous gas flow that virtually eliminates pinholes.
Non-Uniform Coating Thickness
In optical applications, single-layer coatings are advantageous for tuning the optical performance. The spectral range is a function of the coating thickness, so a non-uniform coating thickness impacts optical performance. PE-CVD process parameters, part orientation, and even chamber shape and design can affect the uniformity of DLC coating.
Dynasil uses a cylindrical deposition chamber rather than a cubic one. The cylindrical chamber has no corners and helps ensures the gas distribution inside the chamber remains uniform. Furthermore, there are no crevices for contaminates to hide.
While the hardness and amorphous nature of a DLC coating is an attractive property, it also makes it an unforgiving coating when it comes to residual stress within the coating and the substrate. Rather than deform under residual stress, a DLC coating tends to delaminate from the substrate.
The primary cause of residual stress is the thermal expansion mismatch between the substrate and the DLC coating. The higher the process temperature, the more difficult it is to prevent residual stresses. The PE-CVD process requires lower process temperature compared to traditional CVD, which further minimizes residual stresses.
Poor Coating Adhesion
Adhesion problems are somewhat of a catchall for all of the above defects, but poor adhesion is typically a result of improper process parameters or thermal expansion mismatches. The PE-CVD process addresses the latter and carefully set process parameters address the former.
A DLC coating offers excellent mechanical properties in return for minor losses in optical performance. The combination makes it ideal for coating optical substrates like Germanium (Ge), Silicon (Si), Zinc Sulfide (ZnS), Zinc Selenide (ZnSe) and Chalcogenides. Dynasil employs PE-CVD combined with a cylindrical deposition chamber and a unique “Coat Up” technique to accomplish virtually zero pinholes and minimize all processing defects.