The use of carbon-based coatings has steadily increased over the last decade. It all started with high-tech applications in high-end racing series like Formula 1, but it is gradually finding its way into more and more common and mass-volume applications. As the number of carbon coatings available on the market increases, these coatings not only become more accessible, but also the number of applications is increasing year on year. Whereas the applications for these coatings used to be found mainly in the engine, there are also potential applications in transmission components. In this article we will briefly describe the world of DLC (diamond-like carbon) coatings, their properties, and the benefits and potential applications on gears.
Increasing government regulations, energy prices that are bound to increase, and international competition provide a strong motivation to improve the efficiency and performance of transmission systems. Engineers are always looking to design smaller and lighter systems, using lighter and/or cheaper materials and adding reliability to a current design. Material choice and heat treatment have long been the only instruments that could be used. By adding new surface treatments like carbon coatings, a whole new area of development is accessible.
DLC coatings are a family of coatings made up primarily of carbon chains in an amorphous structure with sp2 (trigonal) and sp3 (tetrahedral) bondings (Figure 1). Sp3 is the cubic form of carbon known as diamond, compared to sp2 that is graphite. There are many different types of DLC coatings: hydrogen-free (a-C), hydrogenated (a-C:H), or (non) metal-doped coatings. These coatings are typically thin-film and are applied during a vacuum deposition process (PVD or CVD). In this article we will concentrate on hydrogen containing coatings (Figure 2), more specifically a-C:H and a-C:H:Me coatings. DLC coatings are characterized by the following properties:
• High hardness, up to 20 GPa (measured by nano-indentation) for a-C:H coatings, 10-15 GPa for a-C:H:Me coating, with the possibility to be tailored in function of the requirements of the application. The high hardness guarantees a high abrasive wear resistance.
• Good adhesion, better than HF2 following the VDI 3198 norm and a critical value (Lc2) exceeding 25 N in the scratch test (coating deposited on M2 steel for the both tests), the critical value (Lc2) can be improved through the use of different interlayers (e.g. TiN).
• The temperature during the deposition process can be kept below 200°C with some DLC deposition technologies. This is particularly important in order to avoid tempering and distortions of parts. Most of the materials that are currently being used for automotive components call for coating temperatures below 200°C in order to avoid distortion.
• The typical coating thickness is between 1 and 5 µm, depending on the type of application, with a spreading of +/-15 percent.
• The impact on the original surface roughness is correlated to the DLC deposition technology used and the nature of the adhesion layer. DLC coatings deposited by a Plasma Assisted Chemical Vapor Deposition (PACVD) process will typically reproduce the original surface roughness. The use of ceramic adhesion layers (e.g. CrN) can lead to an exceedingly large increase in surface roughness, thus requiring a post polishing and increasing the cost.
• DLC coatings show very good coefficient of friction values. In a dry Ball-On-Disc (BOD) test typical friction values are in the range of 0,1 (compared to 0,6-0,8 for an uncoated system). Some DLC coating show even lower values down to 0,05.
DLC Coating Benefits
Because of the unique combination of hardness and friction properties, DLC coatings can be used to solve different wear problems.
Abrasive wear: This wear mechanism leads to the removal of particles from one or both components in the tribological system, typically wearing out the flanks of the gears and ultimately leading to a failure of the system after excessive wear. The hardness of carbon coatings will reinforce the resistance against this type of wear at the surface, increasing the lifetime of the coated component. Depending on the tribological systems and the targeted results, one or both components will be coated. Often the problem can be solved by coating only one component, especially with “softer” (<15 GPa) coatings, and a transfer layer can be formed on the uncoated counterpart. In Figure 3 one can see the important reduction of wear volume (mg). This is especially truth at lower speeds as the lubrification of the system is not working properly. As soon as the system works in hydrodynamic conditions (typically higher speeds), the oil film is evenly spread and the wear—and therefore the impact of the coating—will decrease.
Adhesive wear: This wear mechanism is characterized by a transfer of material from one component to its counterpart, also described as galling or cold welding. Because of the combination of high pressure in the contact zone between the components and friction heat is generated in this contact area, resulting in a “welding” of the surfaces. Through the movement of the parts this weld is broken, and material gets transferred from one component to its counterpart. As soon as this wear mechanism starts the negative impact is amplified as during the subsequent contact the surface geometry has changed, leading to even greater contact pressures and heat. The low friction properties of carbon coatings lower this heat generation and avoid that a sufficient temperature is reached to start this wear mechanism. Also, in this wear mechanism it is often sufficient to coat only one component. As shown in Figure 4, the friction between the components is reduced significantly by coating one of the components in the tribo-system, avoiding any adhesive wear.
Increased load bearing capacity: Because both abrasive wear and adhesive wear can be reduced by applying a carbon coating, one can also choose to translate this benefit in an increased loading of the system. Figure 5 is the result of extensive tests at different loads (Y-axis). For different loads the lifetime of the system was determined (expressed in number of cycles). Designers now have two possible option based on this graph. They can clearly increase the lifetime of the system at a given load. As an example, at a pressure of 2000 N/mm2 the limit is increased from 5*106 to 2*107 cycles, multiplying the lifetime by a factor of four. But they can also allow a higher load to be applied for a given lifetime. The increased load limit at which the system can be considered robust is increased from 1380 N/mm2 to 1780 N/mm2, an impressive 28 percent.
Surface Roughness and Lubricants
Rough surfaces are not suited for thin coatings, as the adhesion of the coating will deteriorate as the roughness increases. On the basis of different tests performed and the long-term experience of coating race engine components, we can state as a general rule that the smoother the surface prior to coating, the better the overall result will be. An example of this correlation is given in Figure 6.
Friction measurements using a ball-on-disc setup were performed on two different samples. The coating applied is exactly the same for both samples, with the only difference being the surface roughness (Ra) before coating. With improved surface roughness, the friction behavior of the coated system is improved. These results have been confirmed at different temperatures and clearly illustrate the importance of a good surface finish on the performance of the total tribological system.
Next to the friction behavior, the wear behavior can also be improved with the use of DLC coatings. As shown in Figure 7, different elements determine the overall impact on the wear improvement through the use of coatings. The results in Figure 7 were obtained trough a “ball-on-disc” test similar to the one previously described. Different oils were used, and surfaces with different roughnesses were tested. The results show a different wear behavior depending on the type of oil used. For some oils the use of DLC coatings has an adverse effect on the wear behavior. But when the surface roughness is improved, suddenly the wear can be reduced for all types of oil. These tests clearly illustrate the need to consider the full tribological system and not only the coated components.
DLC coatings have a thickness of a few microns. This is one of the reasons why the properties of the surface on which the coating is deposited are very important. As illustrated in Figure 8, having a high Rz value will wear out the coating at the highest points, leading to a smoother surface but a surface that contains non-coated areas. These areas are potential spots for occurrence of adhesive wear. The limit for Rz depends also on the application, the type of DLC coating applied, and the Hertzian pressures involved in a particular systems but typical values are between 0,8 and 1,0 µm. Softer DLC coatings which might be applied in a higher thickness can typically accommodate a higher initial surface roughness as they will show more running-in (because of softer coating) and the increased thickness allows more running-in before reaching the substrate material.
In recent years DLC coatings have shown their benefits in various applications. From Formula1 engines where these coatings have increased the performance and reliability of highly loaded and high revving engines to normal diesel engines where carbon coatings have allowed direct injection to be introduced.
These coatings are now increasingly introduced in other applications, as their unique combination of high wear resistance and low coefficient of friction make them an interesting design element. The design engineer can use them for different purposes. An existing system can bear higher loads; a new system can be design with smaller parts, decreasing the overall weight of the system. The coating also adds a guarantee as it will protect the system during abnormal functioning (e.g. bad lubrication), and its lower friction properties will also result in less heat generation, which in turn can have an impact on the oil used and/or its lifetime. As these coatings become more available and affordable we can expect they will become an additional design tool during the development of new systems.