Ion energy-induced nanoclustering structure in a-C:H film for achieving robust superlubricity in vacuum

Abstract

Hydrogenated amorphous carbon (a-C:H) films are capable of providing excellent superlubricating properties, which have great potential serving as self-lubricating protective layer for mechanical systems in extreme working conditions. However, it is still a huge challenge to develop a-C:H films capable of achieving robust superlubricity state in vacuum. The main obstacle derives from the lack of knowledge on the influencing mechanism of deposition parameters on the films bonding structure and its relation to their self-lubrication performance. Aiming at finding the optimized deposition energy and revealing its influencing mechanism on superlubricity, a series of highly-hydrogenated a-C:H films were synthesized with appropriate ion energy, and systematic tribological experiments and structural characterization were conducted. The results highlight the pivotal role of ion energy on film composition, nanoclustering structure, and bonding state, which determine mechanical properties of highly-hydrogenated a-C:H films and surface passivation ability and hence their superlubricity performance in vacuum. The optimized superlubricity performance with the lowest friction coefficient of 0.006 coupled with the lowest wear rate emerges when the carbon ion energy is just beyond the penetration threshold of subplantation. The combined growth process of surface chemisorption and subsurface implantation is the key for a-C:H films to acquire stiff nanoclustering network and high volume of hydrogen incorporation, which enables a robust near-frictionless sliding surface. These findings can provide a guidance towards a more effective manipulation of self-lubricating a-C:H films for space application.

Genesis of Superlow Friction in Strengthening Si-DLC/PLC Nanostructured Multilayer Films for Robust Superlubricity at Ultrahigh Contact Stress

“…Moreover, the results of the indentation hardness (H), elastic Young's modulus (E), H/E (plastic index [27][28][29]38 ), and recovery rate are summarized in Figure 7b,c. It should be pointed out that the hardness of the monolayer PLC film at a depositing ion energy of 0.3 kV was only 4.64 GPa, 39 which was lower than that of both regions A and B.…”

“…Its originally recorded and fitted Raman spectra with certain characteristic regions are demonstrated in Figure b,c, respectively. From Figure c, each Raman spectrum can be processed by peak deconvolution, namely, a benzene ring peak of 1227 cm –1 , a D-mode of 1360 cm –1 originating from the breathing vibration of ring-sp 2 -based configurations, and a G-mode of 1580 cm –1 resulting from the stretching vibration of aromatic or chain sp 2 -based configurations. ,− As for the peak around 1227 cm –1 , it can be believed that the intrinsic benzene ring sp 2 structure of the outmost PLC layers has the potential for its derivation, which is markedly different from the highly hydrogenated DLC monolayer films using a deposition parameter of low ion energies in previous reports. , The relevant study has noticed that the benzene ring sp 2 structures derive from the retaining of the benzene ring molecular structures of the toluene gases, which can be contained in the films for the deposition voltage at a low level of 0.3 kV. , Moreover, the G -peak location position and band intensity ratios of I D / I G can also be obtained by the Raman fitting results, as shown in Figure c. From the following deduction of the quantitative formula proposed by Casiraghi et al, the hydrogen content of the DLC films can be calculated H [ a t ⁢ % ] = 21.7 + 16.6 0.25em log { m I false( G false) false[ μ normalm false] } where m is regulated as the linear slope of the Raman spectrum in the wavenumber range from 1000 to 1800 cm –1 containing the information of the two bands D-peak and G-peak, and I ( G ) is the intensity of the G-peak, which can be obtained after the peak fitting process by adopting the Gaussian deconvolution method.…”

“…As reported in the previous studies, the highly hydrogenated DLC films have a large amount of hydrogen, which is derived from the incompletely decomposed aromatic sp 2 precursor molecules of the toluene gas. The rational explanation is that the growth of the DLC films could be tailored by the ion impact-induced mechanism as the deposition in a low level of ion energy at 0.3 kV . It has been reported that a monolayered PLC film at the ion energy of 0.3 kV has an extremely high level of up to 50.8 at % hydrogen concentration and possesses a significant fraction of sp 2 -clustering ring-based structures, which are considered as the intrinsic origin for realizing an outstanding superlubricity state in the dry lubrication mode.…”

“…30,36 The relevant study has noticed that the benzene ring sp 2 structures derive from the retaining of the benzene ring molecular structures of the toluene gases, which can be contained in the films for the deposition voltage at a low level of 0.3 kV. 37,39 Moreover, the G-peak location position and band intensity ratios of I D /I G can also be obtained by the Raman fitting results, as shown in Figure 7c. From the following deduction of the quantitative formula 35 proposed by Casiraghi et al, the hydrogen content of the DLC films can be calculated…”

“…The rational explanation is that the growth of the DLC films could be tailored by the ion impact-induced mechanism as the deposition in a low level of ion energy at 0.3 kV. 39 It has been reported that a monolayered PLC film at the ion energy of 0.3 kV has an extremely high level of up to 50.8 at % hydrogen concentration and possesses a significant fraction of sp 2clustering ring-based structures, which are considered as the intrinsic origin for realizing an outstanding superlubricity state in the dry lubrication mode. From the hydrogen concentration calculated by formula (1), it is observed that the hydrogen contents in all regions are larger than 20 at %, which is found in the friction interface of the ball wear scar.…”