DLC film: challenging and elusive, but rich in possibility
Gears, bearings, and other frictional sliding parts are found in numerous machines worldwide, each playing a critical role in their operation. The quality of the parts can make a big difference in the machine’s performance, even if they are not so noticeable. Friction and wear are inherent to these parts, as they come into contact, mesh, drive, and are driven by one another. Controlling these effects to maintain their functionality is a key theme in mechanical engineering.
Dr. Nakamura is pioneering research on innovative surface modifications using Diamond-like Carbon (DLC) film—a material primarily composed of carbon. DLC film offers unique characteristics that can be applied across various fields, paving the way to groundbreaking advancements.
Dr. Nakamura first encountered DLC film in 2005. After working in a company, he returned to Doshisha University, his alma mater, and began his research in the doctoral program. Here, under the guidance of Dr. Takashi Matsuoka, a professor in Doshisha’s Faculty of Science and Engineering, and Dr. Nakamura’s supervisor during his undergraduate and master courses, he pursued research under his chosen topic.
Dr. Nakamura recalls, “In my undergraduate years, I performed surface analysis and then studied carbon-reinforced composite materials in my master’s course. Thus, for many years, I studied design and surface engineering, though my research field was not related with coatings and I did not know DLC films well.At the time, research on DLC was limited, and the material was not widely known. However, even with the limited findings available, I was convinced of its potential for future development.”
As the name suggests, “Diamond-like Carbon” is a material with both a diamond and a graphite structure (Fig. 1). When applied on the frictional sliding parts, it lowers friction and enhances the wear resistance of machine parts. The characteristics of the material can be modified with the addition of elements such as hydrogen, nitrogen, etc., resulting in a wide variety of types, collectively referred to as “DLC film.”
Dr. Nakamura explains, “Applying DLC film to frictional sliding parts helps them to slide more smoothly, reducing wear. These characteristics make it useful for applications such as sliding parts in motor vehicles and cutting tools, etc. The film’s biocompatibility also makes it suitable as a coating material for metallic stents. Additionally, its superior gas barrier functionality means that it can be used in plastic bottles that contain hot drinks. By preventing the penetration of oxygen and other gases, it can ensure the long-term freshness and quality of the liquid (beverage) inside. With so many types of DLC films available, the potential applications are vast. While DLC is a challenging and ‘elusive’ material to work with, its versatility suggests vast potential across multiple industries.”
Experiencing difficulties has given Dr. Nakamura a deep appreciation for the importance of basic research
During his doctoral course at Doshisha University, Dr. Nakamura began his research work as an assistant at a lab of Kyoto Institute of Technology, which studied gears.
Dr. Nakamura reports, “Gears are classical machine elements that everyone knows well, but they are also deeply fascinating. Altering a gear’s tooth shape changes how it interacts with counter gears. Gear tooth flank slide while rotating, generating frictional heat and causing a loss of power. The goal of DLC film application is to reduce this loss of power in gears. However, DLC film has poor adhesive characteristics and easily becomes detached from the gear tooth surface because of slide and contact forces. This led me to focus on developing DLC films that would adhere more effectively to gear flank surfaces.”
The gears Dr. Nakamura initially studied were made of plastic material, which made them largely incompatible with DLC film due to differences in hardness and other mechanical properties. Plastics deform more easily than the hard DLC film, causing the film to crack and separate from the gear flank surface. Dr. Nakamura consulted with his fellow researchers and experts in the coating industry, all of whom acknowledged it was an “extremely difficult challenge,” but could not offer viable solutions. Undeterred, Dr. Nakamura persisted with trial-and-error experiments until one researcher suggested using plating techniques. Acting on this idea, he arranged to have the plastic gears plated with a different material before applying the DLC film. While this led to some positive results, it still fell short of the desired outcome.
Dr. Nakamura continued his research on DLC film even after returning to Doshisha University in 2019. He aimed to find practical applications by studying the coating of plastic gears. “After careful investigations of the materials themselves, I came to realize that the best approach was to apply them on actual machinery, and I continue to actively pursue basic research,” he explains. One such effort involved reducing the residual stress on DLC film through evaluations of “nitrogen-doped DLC film.”
“There is major residual stress occurring in DLC films,” Dr. Nakamura states. “DLC films have large residual stress in themselves. Even when no external force is applied, there is still large stress in the film itself. This might be easier to understand if you imagine the film expanding or shrinking. If we can reduce this residual stress, then the adhesion property can be improved, making detachment less likely to occur.” To achieve this, he conducts experiments by including hydrogen and other different elements into the film. These elements bond with the carbon in the DLC film, loosening the bonds between carbon elements, which causes residual stress. The result is a reduction in residual stress. Dr. Nakamura states that if this basic research proves successful, then he can expand its scope to include applications for actual machines and find uses for further development of plastic gears.
To play a role as a researcher at Doshisha University and to contribute to the industrial development
Dr. Nakamura strives to use his research to address the needs of the broader scientific community and society as a whole. He considers this an important parameter for evaluating DLC film.
“There are many experimental methods to investigate the basic properties of DLC film. However, to study DLC film in actual machine usage, one must account for defaults, breakages, and destruction of experimental prototypes, which can incur large costs! Therefore, I aim to develop evaluation methods for basic testing that can simulate stress conditions on the parts.”
Dr. Nakamura proposed the “cyclic indentation test” to evaluate DLC film on a forming mold. This test method involves repeatedly pressing a sphere onto a DLC-deposited metal plate. This simple experimental approach results in significant contact pressure on the film itself. This can be used to determine how and when DLC fracture and detachment occur. Dr. Nakamura is currently designing and building prototype equipment to evaluate the strength of DLC films (Photo 1).
Prototype equipment for evaluations of
film strength
There is a reason why Dr. Nakamura is so dedicated to research of this nature. “I believe it is the university’s role to delve deeply into basic research, to pursue and discover universal aspects. Yet we must also ask ourselves, ‘Why basic research?’ This is due to our need to return to fundamentals for real-world application and implementation within society. This is the ultimate goal shared by both basic and applied research.”
When it comes to applied research for DLC films, Dr. Nakamura collaborates with various companies. He finds joint research between industry and academia, with clearly defined themes, to be an effective way of fostering social collaboration. Dr. Nakamura sees his mission as “serving industry through research,” which drives him to overcome the challenges he encounters in his research.