Multilayer Diamond-like Carbon (DLC) Coatings Deposited by Magnetron Sputtering to Enhance Wear Resistance at High Contact Stresses
Student thesis: Doctoral Thesis
Related Research Unit(s)
The tribological coatings can generally be divided in two broad categories: the soft coatings for good solid lubrication (including polymers, soft metals, or lamellar solids) exhibit low friction coefficients but high wear rate; the hard coatings for protection against wear (including nitrides, carbides and oxides) exhibit low wear rates but high friction coefficients. It seems difficult to combine low friction with high wear resistance in one coating. Nevertheless, a diverse family of carbon-based materials known as diamond-like carbon (DLC) coatings naturally combines low friction and high wear resistance. These exceptional tribological abilities explain the increasing production of DLC coatings and the global share of DLC coatings keeps increasing from $0.8 billion in 2005 to $1.7 billion in 2015. DLC has found its way into a wide variety of applications such as, automobile parts, magnetic hard disks, medical implants, razor blades, and microelectromechanical systems (MEMS). The harsh service condition in heavily loaded automobile parts, such as gudgeon pins, cam followers, gears and bearings, demands high load capacity from the DLC coatings. However, the DLC often fails where high contact stresses are required due to poor adhesion, high residual stresses and poor toughness of the DLC coating. Typically, under lower contact stresses (< 2 GPa) the specific wear rate of hydrogen-free carbon coatings (a-C) is in the range of 10-9-10-7mm3/Nm, but most coatings fail rapidly at higher contact stresses. From existing literature, the studies on DLC films working under high stresses (> 2 GPa) are rare and a few groups can only push the wear rate into the range of ~10-7mm3/Nm which is at least one order of magnitude higher than that of coatings under low stresses. Thus, it is an ongoing challenge for the coating community to design DLC films working under high contact stresses with low wear rate.
To tackle the issue, the multilayer coatings with alternate hard phase and soft phase inspired from natural materials have been developed in this study. Biologists found that most hard biological materials can achieve both high stiffness and high strength, which are currently unachievable by man-made ceramic composites. For example, mollusks are composed up to 95% of minerals such as calcium carbonate, yet by comparison with these brittle materials, mollusk shells are about 1,000-times tougher, at the expense of a small reduction in stiffness. One of the key reason relating to such exceptional mechanical properties is the hierarchical structure which incorporates nanometre thick ductile phase into the hard mineral matrix to form alternate multilayer structure. Drawing from this natural architecture, the bio-inspired DLC multilayer structure was made by incorporating the designed ductile material into the hard DLC matrix. The ductile phase material can either be of the same carbon-based material to form the structural multilayer, or it is made from the dissimilar material to form the compositional multilayer. This study aims to give new insight into what the key factors are related to design coatings working under high stressed tribological contacts, and how these factors play a role in the wear behavior, and what mechanisms are responsible for the high-contact-stress tribological behavior. The research outcomes are briefly outlined as follows.
First, the coatings were deposited by closed field unbalanced magnetron sputtering ion plating (CFUMSIP) technique. The deposition system is equipped with six targets that consist of one silver target, two chromium and three graphite targets. The arrangement has allowed the deposition of gradient adhesion layer and actual DLC in a single process without breaking the vacuum to guarantee good adhesion. By using the CFUMSIP technique, the deposition occurs under high density bombardment with low energy ions, which is the ideal condition to grow dense film without introducing high residual stresses to coatings. Then, the tribological behavior of single layer DLC coatings was investigated. At the low stress (2.0 GPa), the hard-DLC (with hardness of 36 GPa) showed lower wear rate than that of the soft-DLC (with hardness of 10 GPa). When the stress was increased to 2.5 GPa, the hard-DLC showed brittle cracks and the wear rate was almost one order of magnitude higher than that of the soft-DLC. At a higher stress than 2.5 GPa, the hard-DLC coating peeled off from the substrate in the early run-in stage. On the other hand, the soft-DLC can survive without delamination but the cost is high wear rate. It was found that at a low stress, the wear rate is dominated by hardness. The harder coating shows the better wear resistance. However, when the stress is increased, besides the hardness, the toughness is also an important factor which is relevant to wear resistance.
Second, being inspired by the natural structure of alternate hard phase and soft phase found in mollusk shells and abalone shells, the structural multilayer DLC coating with alternate soft-DLC layer and hard-DLC layer was deposited. The effect of two structural factors, the thickness ratio of hard-DLC layer to soft-DLC layer (2:1, 1:1, 1:2) and the different bilayer thickness (132, 61, 15 nm), on the tribological behaviour of multilayer coatings at different applied stresses was investigated. It was found that the wear volume of coatings with the soft-DLC as the top layer is decreased by an average of ~70% during the run-in stage compared with that of coatings without this soft-DLC. Thus, all multilayer coatings were capped with the soft-DLC to enable the successful run-in stage. As expected, by incorporating the soft phase, the multilayer coating has reduced the overall residual stress compared with the single layer hard-DLC and the amount of reduction in residual stress depended on the ratio of hard-DLC to soft-DLC. For coatings with different thickness ratios, the hardness was increased with the thickness ratio of hard-DLC to soft-DLC. The coating with the thickness ratio of 1:1 showed the lowest wear rate, which implied that the combination of the hardness and the toughness for the coating with thickness ratio of 1:1 was in favour of good wear resistance. For coatings with decreasing bilayer thickness while the fixed thickness ratio was fixed at 1:1, it was found that the hardness was slightly increased with the decreased bilayer thickness. The wear rate of the coatings showed the decreasing trend with the reduced bilayer thickness, which may be associated with the increased hardness. Under the extreme contact stress of 4 GPa, the wear rate of the coating with 61 nm bilayer thickness was marginally lower than that of the coating with 15 nm bilayer thickness but both wear rate fell within the same range. The coating with the thickness ratio of 1:1 and the bilayer thickness of 61 nm showed the lowest wear rate of ~ 3.7 × 10-8 mm3/Nm under 4.0 GPa. The improvement of tribological performance of the multilayer coating is mainly attributed to the combination of reduced residual stress, high toughness and the lubrication effect of the transfer layer in the run-in stage.
Third, the physical structure of the soft phase itself also affects the multilayer design. It is known that in a tribological system many internal properties and external parameters (environment-test conditions) interact with each other in a complicated way. In other words, a coating which is optimized for a certain tribological application could perform below expectation under other tribological conditions. To extend the applicability over a broader range of test conditions, doping other elements into the DLC coating was explored. Silver (Ag) was chosen as the doped element because it has the unique properties such as a low shear stress, high electrical conductivity, and antibacterial effects, and therefore it could probably satisfy the multifunctional needs of future applications. The single layer Ag doped DLC (Ag-DLC) coatings were deposited at different substrate bias voltages (-40 V, -60 V, -80 V, -100 V, -120 V, -140 V) and different the Ag contents (0, 3.1 at.%, 4.0 at.%, 19.1 at.%, 30.0 at.%) respectively. It was found that with the bias increased, the wear rate was decreased to the minimum value at bias of -120 V and then was increased. When the bias was fixed at -120 V, the Ag content should be from 3.1 at.% to 4.0 at.% to enable the coating working under high contact stress. The higher Ag content softened the Ag-DLC coating and resulted in the high wear rate. The Ag-DLC at bias of -120 V with 4.0 at.% Ag content showed the lowest wear rate and thus was used as the ductile phase in the following multilayer coating design.
Finally, the Ag-DLC/DLC multilayer consisted of alternate Ag-DLC layer and hard-DLC layer was prepared and the coating had the wear rate of ~ 2.2 × 10-8 mm3/Nm at 4.0 GPa, which was 40% lower than that of the best multilayer DLC coating in our previous research. It was observed that the Ag nanocrystals with the size of 2-4 nm were embedded into the DLC matrix to form the amorphous/nanocrystalline dual-phase structure. The Ag nanocrystals may block the propagation of brittle deformation in the amorphous carbon and release stress so it enables the coating to survive under high stresses. To satisfy the typical industrial request for thicker coatings, the multilayer Ag-DLC/DLC with the thickness of 1.5 µm was prepared to study the applicability of the thick coating. The thick multilayer Ag-DLC/DLC did not delaminate after wear and provided the wear rate as low as ~2.0 × 10-8 mm3/Nm. It shows potential industrial applications for such multilayer DLC coatings.
In summary, we have successfully pushed the boundary of the wear performance of DLC coatings at high contact stresses from ~10-7 mm3/Nm to ~10-8 mm3/Nm. The newly designed DLC coatings show the wear rates in the range of 2.0 × 10-8 mm3/Nm to 5.0 × 10-8 mm3/Nm at the stress from 2.0 GPa to 4.0 GPa. From this study, the basic rules, for designing coatings for different-contact-stress regimes, including: good adhesion strength, successful built-in lubrication during the run-in stage, alternate the soft-layer and the hard-layer structure, the proper thickness ratio of the hard-layer to the soft-layer and the proper bilayer thickness to achieve both high toughness and high hardness, have been formulated.
- diamond-like carbon, adhesion, residual stress, toughness, multilayer, magnetron sputtering, silver doping, tribological behaviour, high contact stress