Based on the concept of multilayer designing of coating systems, the multilayer ceramic metal-DLC coatings were fabricated by Voevodin et al.
[45] using electron-enhanced unbalanced magnetron sputtering for sliding wear applications. Low COF and low wear rates can be achieved by the multilayer coatings with upper Ti20%-DLC and Ti35%-DLC layers. Sui et al.
[50] prepared CrN/DLC/Cr-DLC multilayer coatings with plasma-enhanced chemical vapor deposition, which can significantly improve the lubrication and wear-resistance performance comparing to single component coating. The improved performances of the CrN/DLC/Cr-DLC multilayer coatings can be attributed to the lubrication of DLC layers, the supporting of CrN layers, the enhanced crack propagation inhibition, and the increased elastic recovery governed by the multilayer structure. DLC coatings were also combined with MoS
2 to enhance the lubrication and wear-resistance performances. Pu et al.
[51] prepared a multilayer DLC/MoS
2 coating using medium-frequency magnetron sputtering, which exhibited a low COF of 0.02 and a low wear rate of ~6.5 × 10
−6 mm
3 N
−1 m
−1. The influence of different underlayers on the tribological behaviors of the DLC-based multilayer coatings prepared by magnetron sputtering was investigated by Duminica et al.
[52], where a better adhesion could be achieved with only Cr under layer, exhibiting lower COF compared to other samples.
For single-layer DLC coating, high residual stress would lead to brittle fracture and delamination under high normal load during the friction process. Researchers found that the tribological behaviors of multilayer DLC coatings can be improved through the DLC layers with different properties. Li et al.
[53] fabricated multilayer DLC coatings with alternated soft and hard layers through the alternating of bias during magnetron sputtering. Delamination was observed in monolayer coatings due to high residual stress. The results showed that the bonding structure (sp
3 and sp
2) can be changed by substrate bias. The sp
3 fraction in DLC coating can be increased with increased bias ratio on the two adjacent sublayers from −40 V/−160 V to −80 V/−160 V, leading to increased coating hardness. With the multilayer designing, the hardness of multilayer DLC coating was similar to the coatings deposited at low constant bias, but the adhesion strength and toughness were significantly improved. It can be concluded that alternately biased sputtering deposition is a promising way to fabricate DLC coating with high hardness, toughness, and adhesion strength. With the similar designing concept, Harigai et al.
[54] fabricated multilayer N-DLC coatings with each layer thickness of 10 nm using filtered arc deposition, containing periodic bilayer structures with ta-C:N and soft a-C:N layers. The multilayer coatings showed better wear-resistance performance than monolayer ta-C:N coating and multilayer N-DLC coatings with each layer thickness of 50 nm. Lin et al.
[55] fabricated multilayer DLC coatings with alternated soft and hard layers using unbalanced closed-field magnetron sputtering to enhance wear-resistance performance at high contact stress. It was found that the multilayer coating with a soft top layer had lower wear volume under high contact stress, which can be attributed to the fact that the soft top layer can form a transfer layer to reduce friction and wear.
4. Other Multilayer Coatings for Tribology Applications
MoS
2 coatings exhibit excellent lubrication performance under high dry or vacuum conditions due to the easy shear between lattice layers
[56][57]. However, when rubbed in humid air, the dangling bonds at the edge of MoS
2 react strongly with O, resulting in higher COF and shorter service life
[58][59][60]. Aiming to the shortcomings, MoS
2-based multilayer coatings have been designed to further enhance the performance. The tribological behaviors of multilayer coatings of MoS
2 and metallic including Au, Ni, Pb or PbO were studied in humid air with 50% relative humidity, which exhibited lower and more stable COF compared to pure MoS
2 coating
[61]. The function mechanism of metal for the sputter-deposited metal–MoS
2 multilayer coatings is believed to be the optimization of the MoS
2 structure. Kong et al.
[62] investigated the tribological behaviors of MoS
2/Ti–MoS
2/Si multilayer coatings deposited by magnetron sputtering, indicating that better lubrication performance can be achieved by the multilayer design of coatings. Those results indicated that the multilayer structures have potential to improve the tribological behaviors of conventional MoS
2 coating, but the inherent mechanisms are still worth further investigation to guide the designing of MoS
2-based coatings for future application.
With the development of coating fabrication and characterization techniques, several new findings shed the light on the precision structure design of multilayer coatings from an atomic view
[63][64]. Dwivedi et al.
[65] developed C/SiN
x multilayer coatings with layer thickness of 7–8 nm using an enhanced atomic intermixing (formation of nanocomposite interfaces) approach, leading to 2–10 times better macroscale wear durability compared to conventional coatings with larger thickness of 20–100 nm. The enhanced performance can be attributed to the high sp
3 bonding of the carbon overcoat and increased interfacial strength induced by intermixing, leading to improved adhesion and robustness of the coatings. Khadem et al.
[66] designed discreate periodic nanolayered coatings, which had a different structure compared to conventional multilayer coatings. The discrete periodic nanolayered coatings exhibited better wear-reduction performance compared to conventional multilayer coatings, which can be attributed to the reduced interfacial defects. The tribological performance was further improved by surface-texturing treatment. Advanced research tools make it possible to investigate the fundamental mechanisms of multilayer coatings in tribological application.
Two-dimensional (2D) materials, including graphene-family materials
[67][68][69][70][71][72], MoS
2 [58][59], and black phosphorus
[73][74][75][76] have been used as lubricants because of their low interlayer shear strength. Recently, multilayer coatings with 2D materials have also been designed to promote tribological properties. Most recently, Fan et al.
[77][78][79][80][81][82] fabricated coatings with Ti
3C
2T
x Mxene and achieved excellent self-healing, antiwear, and anticorrosion capacity. Saravanan et al.
[83] fabricated multilayer coatings with graphene oxide and PEI via layer-by-layer assembly technique. Macroscale superlubricity (COF < 0.01) can be achieved with the multilayer coating having a thickness of about 300 nm. The superlubricity mechanism is believed to be the formation of carbon nanoparticles in dry conditions. In the subsequent study, it was found that the formation of transfer layer is also critical for the achieving of ultralow friction
[84]. Achieving macroscale superlubricity is possible with multilayer coatings containing 2D materials, but the environment adaptivity still needs to be improved, and the inherent mechanisms also need to be further investigated.
5. Mechanisms for Controlling Friction and Wear Using Multilayer Coatings
Coatings have been widely used in industrial applications as protection for cutting tools, dies, pistons, etc. However, the performance of monolayer coatings is usually restricted by their poor adhesion with substrate, and the high residual stress induced by the fabrication process. In addition, the mismatch of the mechanical properties between substrates and coatings also suppresses the performance of monolayer coatings. Aiming to solve these problems, the concept of multilayer coating has been proposed; and lots of work has been carried out to enhance the coating performance through the multilayer structure. One of the fundamental concepts of the multilayer design is stress relaxing and crack deflection
[39]. Back in the 1970s–80s, researchers attempted to build multilayer coatings through alternating thin layers with high-shear-modulus and thin layers with low shear modulus based on the models of Hoehler. In 1990, Holleck et al.
[39] found that the multilayer structure of TiC/TiB
2 coatings leads to the deflection of cracks through the interface zones, causing energy dissipation without coating failure. Layer thickness also has influence on the stress distribution of the multilayer coatings. In addition, from an engineering point of view, when a normal force is applied on the coating’s surface, the multilayer coating with thin, soft layers can reduce the maximum bending stress. With the multilayer design with soft and hard layers, the plastic yielding of hard layers can be avoided, especially under the condition with cyclic loading and fatigue. Another fundamental concept for the designing of multilayer coating is the functional design of different layers for purposes such as adhesion, load supporting, lubrication, and wear reduction, etc.
[45]. The wear-resistance and lubrication performances can be enhanced through the multilayer design of the coatings. However, macroscale friction is a complex physical–chemical process. The friction and wear-reduction mechanisms of multilayer coatings with different structures and compositions are different. Hence, various mechanisms for controlling friction and wear using multilayer coatings have been proposed (
Table 1), which can guide the future development of the multilayer coating systems.
Table 1. Friction and wear-reduction mechanisms of multilayer coatings.
* The influence of environment humidity on the lubrication behaviors was investigated.