Direct microscopic evidence of shear induced graphitization of ultrananocrystalline diamond films

The origin of ultralow friction and high wear resistance in ultrananocrystalline diamond (UNCD) films is still under active debate because of the perplexed tribochemistry at the sliding interface. Herein, we report a comparative study on surface topography and nanoscale friction of tribofilms, in wear tracks of two sets of UNCD films having different structural characteristics. Despite both the films display ultralow coefficient of friction, the UNCD films grown under Ar atmosphere (UNCDAr) exhibit a high wear resistance while the wear rate is higher for the films grown in N2 (UNCDN). Frictional force microscopic (FFM) investigations clearly reveal the manifestation of shear induced graphitization on both the films. However, the wear track of UNCDAr films have a large network of a few layer graphene (FLG) structures over the amorphous carbon tribofilms while only isolated clusters of FLG structures are present in the wear track of UNCDN films. Here, we demonstrate the direct micro-/nanoscopic evidence for the formation of large network of ~ 0.8 - 6 nm thick FLG structures, as a consequence of shear induced graphitization and discuss their decisive role in ultralow friction and wear.Comment: 33 pages, 8 figures, 1 Tabl

Tribochemistry of TaN, TiAlN and TaAlN coatings under ambient atmosphere and high-vacuum sliding conditions

© 2019 Elsevier B.V.Tribochemical analysis of monolithic TaN, TiAlN, and TaAlN coatings deposited by reactive magnetron sputtering onto 316LN stainless steel (SS) substrates are described. Tribology experiments were carried out in ambient atmospheric and high-vacuum sliding conditions to investigate the tribo-atmospheric dependent friction and wear characteristics of these coatings. The lower friction coefficient and improved wear-resistant properties were observed for TaN and TiAlN coatings in the humid atmosphere than in high-vacuum testing condition. Interestingly, lower friction and wear resistance properties of TaAlN coated SS are significantly enhanced in atmospheric as well as high-vacuum sliding conditions because of their highly dense and fine-grained microstructure with stable cubic B1 TaAlN phase. Energy dispersive X-ray spectroscopy elemental mapping and micro-focused X-ray photoelectron spectroscopy were carried out on the wear tracks to explore the comprehensive tribo-environment dependent tribochemistry. Formations of alumina (Al2O3) rich tribolayer reduced the friction and enhanced the wear resistance of TaAlN/SS sample tested in atmospheric condition; whereas this coating is highly stable in the high-vacuum condition with higher wear resistance11sciescopu

Tribological Properties of Ultrananocrystalline Diamond Films: Mechanochemical Transformation of Sliding Interfaces

Improving the tribological properties of materials in ambient and high vacuum tribo-conditions is useful for inter-atmospheric applications. Highly-hydrogenated and less-hydrogenated ultrananocrystalline diamond (UNCD) films with distinct microstructural characteristics were deposited on Ti-6Al-4 V alloy, by optimizing the plasma conditions in the chemical vapor deposition. Both the UNCD films showed less friction coefficient in ambient atmospheric tribo-contact conditions due to the passivation. This provides chemical stability to UNCD films under the tribo-mechanical stressed conditions which limits the transferlayer formation and conversion of UNCD phase into graphitization/amorphization. However, in the high vacuum tribo-conditions, highly-hydrogenated UNCD films showed low friction value which gradually increased to the higher magnitude at longer sliding cycles. The low friction coefficient was indicative of passivation provided by the hydrogen network intrinsically present in the UNCD films. It gradually desorbs and the dangling bonds are progressively activated in the contact regime, leading to a gradual increase in the friction value. In contrast, less-hydrogenated UNCD films do not exhibit low friction regime in high vacuum conditions due to the lack of internal passivation. In this case, the conversion of UNCD to amorphized carbon structure in the wear tracks and amorphous carbon (a-C) tribofilm formation on ball scars were observed. © 2017 The Author(s)1

Tribological Properties of Ultrananocrystalline Diamond Films in Inert and Reactive Tribo-Atmospheres: XPS Depth-Resolved Chemical Analysis

Tribological properties of diamond films are sensitive to the chemically reactive and inert tribo-atmospheric media, and therefore, it is difficult to understand the underlying tribological mechanisms. In the present work, tribological properties of surface modified ultrananocrystalline diamond (UNCD) thin films were investigated in four distinct tribo-environmental conditions of ambient humid-atmosphere, nitrogen (N-2), argon (Ar), and methane (CH4) gases. The in situ depth-resolved X-ray photoelectron spectroscopy (XPS) showed the desorption of oxygen and oxy-functional additives and sputtering of weakly bonded amorphous carbon species from the UNCD film surface after the Art-ion sputtering process. After desorption of these chemical entities, friction and wear were decreased and run-in regime cycles became shorter in UNCD films. Friction in the ambient humid-atmosphere was higher compared to other tribo-environmental conditions, and it was explained by the oxidation mechanism of the sliding interfaces and the formation of the oxidized carbon transferfilm. However, low friction and wear in the N-2 atmosphere was associated with the adsorption of N-2 species, forming nitrogen-terminated carbon bonds at the sliding interfaces. This was directly investigated by XPS and energy dispersive X-ray spectroscopy techniques. Furthermore, low friction in the Ar atmosphere was explained by the physical adsorption of Ar gaseous species, which tend to avoid the covalent carbon bond formation across the sliding interfaces. Moreover, ultralow friction in the CH4 atmosphere was governed by the passivation of dangling carbon bonds by dissociative CH4 complexes, which creates hydrogen-terminated repulsive sliding interfaces. More importantly, a shorter run-in regime with low friction and wear in Art-ion sputtered UNCD films were explained by desorption of the oxygen and oxy-functional groups, which are inherently present in the UNCD films © 2018 American Chemical Society

Tribofilm formation in ultrananocrystalline diamond film

Friction and wear properties of ultrananocrystalline diamond (UNCD) films are found to be superior, and therefore it could be useful for various applications. However, understanding of the tribological properties with respect to boundary phase composition in this material is not yet well understood. Here, the grain boundary phases such as graphite and amorphous carbon (a-C) of UNCD films were tailored during the chemical vapor deposition process by altering the Argon and Nitrogen gases in CH4 plasma medium. The significance of these grain boundary phases in UNCD film was discussed to explain the tribological properties. In run-in, friction coefficient was high in UNCDAr film deposited in CH4 (1%)/Ar plasma condition and it was decreased to lower value after longer sliding distance. However, ultrahigh wear resistance of this film was observed. Here, graphite and a-C phases were insignificant in the grain boundary region as evident from high resolution transmission electron microscope (HRTEM). Further, chemical bonding of these phases was quantitatively described by electron energy loss spectroscopy (EELS). In contrast, ultralow value of friction coefficient with significantly shorter run-in high friction regime was observed in UNCDN film deposited in CH4 (6%)/N2 plasma. Such a unique characteristic was described by the nanographite phase encasing the needle-like diamond grain of UNCDN film, forming core-shell granular structure. Atomic force microscopy (AFM) showed nucleation of two dimensional (2D) nanographite particles in the deformed wear track after run-in. This was possible due to the presence of core-shell granular structure in UNCDN film. Graphitic nature of the shell for needle-like diamond grains in the wear track was investigated by micro-Raman spectroscopy. Moreover, graphite and a-C tribofilm phase in the wear track was investigated by X-ray photoelectron spectroscopy (XPS) having spatial resolution micrometer scale. Needle-like diamond grains and graphite phase of tribofilm could be one of the primary reasons for the marked reduction in the friction coefficient. © 2017 Elsevier B.V3

Dynamic friction behavior of ultrananocrystalline diamond films: A depth-resolved chemical phase analysis

© 2019 Elsevier Ltd and Techna Group S.r.l.Investigation of dynamic changes in friction behavior of ultrananocrystalline diamond (UNCD) films is a complex mainly because of the rapid change in chemical composition at the sliding interfaces. To address this issue, for the first time, we report chemical phase analysis of transferfilm using the depth-resolved X-ray photoelectron spectroscopy (XPS) technique. The friction coefficient of the UNCD films was high during the initial run-in regime, but it gradually decreased to an ultralow value after longer sliding cycles at the ambient atmospheric tribo-condition. Depth-resolved XPS analysis showed a higher sp3/sp2 carbon ratio during the initial run-in regime. This ratio decreased with increasing sliding cycles and consequently the friction coefficient decreased. However, a higher value of the friction coefficient throughout the run-in regime persisted at the high-vacuum tribo-condition. In this case, the sp3/sp2 carbon ratio inside the transferfilm was quite high and no considerable changes were observed in the depth-resolved XPS analysis. This investigation confirmed that the dynamic friction behavior in UNCD films was manipulated by the sp3/sp2 carbon ratio inside the transferfilm which showed tribo-atmospheric dependence11sciescopu

Triboenvironment Dependent Chemical Modification of Sliding Interfaces in Ultrananocrystalline Diamond Nanowall Film: Correlation with Friction and Wear

Tribological properties of ultrananocrystalline diamond nanowall (UNCD NW) films were investigated quantitatively in three different and controlled triboenvironmental conditions, proposing the passivation and graphitization mechanisms. However, these mechanisms are rather complicated and possibly can be understood in well-controlled tribological conditions. It was shown that the friction and wear of these films were high in high-vacuum and room temperature (HV-RT) tribo conditions where the passivation of carbon dangling bonds were restricted and frictional shear-induced transformation of sp3 carbon into amorphous carbon (a-C) and tetrahedral amorphous carbon (t-aC) were noticed. However, the friction coefficients were reduced to the ultralow value in ambient atmospheric and room temperature (AA-RT) tribo conditions. Here, both passivation of dangling bonds through atmospheric water vapor and graphitization of the contact interfaces were energetically favorable mechanisms. Furthermore, the conversion of diamond sp3 into hydrogenated-graphitized phase was the dominating mechanism for the observed superlow friction coefficient and ultrahigh wear resistance of films in high-vacuum and high temperature (HV-HT) tribo conditions. These mechanisms were comprehensively investigated by micro-Raman and X-ray photoelectron spectroscopy analyses of the sliding interfaces. © 2017 American Chemical Society11sciescopu

Tribological Properties of Ultrananocrystalline Diamond Films in Inert and Reactive Tribo-Atmospheres: XPS Depth-Resolved Chemical Analysis

Tribological properties of diamond films are sensitive to the chemically reactive and inert tribo-atmospheric media, and therefore, it is difficult to understand the underlying tribological mechanisms. In the present work, tribological properties of surface-modified ultrananocrystalline diamond (UNCD) thin films were investigated in four distinct tribo-environmental conditions of ambient humid-atmosphere, nitrogen (N₂), argon (Ar), and methane (CH₄) gases. The in situ depth-resolved X-ray photoelectron spectroscopy (XPS) showed the desorption of oxygen and oxy-functional additives and sputtering of weakly bonded amorphous carbon species from the UNCD film surface after the Ar⁺-ion sputtering process. After desorption of these chemical entities, friction and wear were decreased and run-in regime cycles became shorter in UNCD films. Friction in the ambient humid-atmosphere was higher compared to other tribo-environmental conditions, and it was explained by the oxidation mechanism of the sliding interfaces and the formation of the oxidized carbon transferfilm. However, low friction and wear in the N₂ atmosphere was associated with the adsorption of N₂ species, forming nitrogen-terminated carbon bonds at the sliding interfaces. This was directly investigated by XPS and energy dispersive X-ray spectroscopy techniques. Furthermore, low friction in the Ar atmosphere was explained by the physical adsorption of Ar gaseous species, which tend to avoid the covalent carbon bond formation across the sliding interfaces. Moreover, ultralow friction in the CH₄ atmosphere was governed by the passivation of dangling carbon bonds by dissociative CH₄ complexes, which creates hydrogen-terminated repulsive sliding interfaces. More importantly, a shorter run-in regime with low friction and wear in Ar⁺-ion-sputtered UNCD films were explained by desorption of the oxygen and oxy-functional groups, which are inherently present in the UNCD films