Exploring long-run reciprocating Wear of diamond-like carbon coatings: microstructural, morphological and tribological evolution
Introduction
To realise low friction and long wear life are the chief aims for self-lubricating coatings [1]. Amongst polytetrafluoroethylene (PTFE) [2], MoS2 coatings [3], and graphite [4], diamond-like carbon (DLC) coatings, which consist of metastable amorphous structures containing the hybridised CC sp3 structure of diamond and the CC sp2 structure of graphite, have attracted attention in engineering due to their advantages of high hardness, low friction coefficient and high wear resistance [[5], [6], [7], [8]].
The substrate is known to be a key factor that can influence the adhesive properties, tribological behaviour, and wear life of the DLC coatings due to the indentation response [[9], [10], [11]]. In order to obtain better adhesive properties and tribological behaviour of the DLC coatings, researchers are continuously trying to design a suitable transition layer between the DLC coating and the substrate [[12], [13], [14]]. Sui deposited a CrN/DLC/Cr-DLC multilayer structure coating by a plasma enhanced chemical vapour deposition (PECVD) device with the close field unbalanced magnetron sputtering ion plating (CFUBMSIP) technique, and found that the friction coefficient and adhesive wear of the CrN/DLC/Cr-DLC multilayer coatings are obviously improved compared to those of the single CrN and DLC coatings [12]. Wang deposited DLC coatings on micro arc oxidation (MAO) coated pure Ti substrates, and the results showed that the MAO/DLC coating exhibited an excellent tribological behaviour and corrosion resistance in a simulated body fluid solution [13]. The transition layers minimise the difference between the substrate and the DLC coated layers in terms of elastic modulus and thermal expansion coefficient, which in turn reduces the internal stress so producing better adhesion [14]. Wang designed a more than 50 μm thick DLC coating with the structure of Six-DLC/Siy-(DLC)n/DLC. The results explained that the thickness of the coating can be ascribed to the architecture of the tensile stress/compressive stress structure [15]. Meanwhile, an ultrahigh load-bearing capacity of the super thick DLC coating can be achieved due to its residual stress decreasing to almost zero.
Previous work on the microstructure and mechanical properties of DLC coatings associated with the substrate effects has been well discussed. However, the extreme environments on key components, such as aero-bearing [16], engine sliding unit [17] and machined tool [18] are increased requirement for long-term operation of mechanical assemblies bring out the challenges for mostly-used solid lubrication coatings. The durability and stability of DLC coatings in extreme environments has become a key issue that needs to be solved urgently. Therefore, the long-run test need to be carried out to study the microstructural evolution of the DLC coatings at different stages during the reciprocating wear. Indeed, the synergistic effect between transition layer and substrate influences the various friction coefficients and wear failure forms of the DLC coatings due to the specific contact load for long-run reciprocating wear. Therefore, to determine the synergistic effect between the transition layer and the substrate on the microstructural, morphological and tribological evolution, it is beneficial to better understand the wear mechanism of the DLC multilayer coatings.
In this paper, DLC coatings were prepared by an ionic enhanced physical vapour deposition device with the unbalanced magnetron sputtering. During the deposition, the austenitic stainless steel SUS304 and the cemented carbide YT15 were selected as the substrates, and the AlTiN coated layer was selected as the transition layer. The microstructural, morphological and tribological evolution of the DLC single and multilayer coatings with different combinations of these substrates and transition layer were investigated. In particular, the long-run test was carried out to study the microstructural evolution of the DLC coatings at different stages during the reciprocating wear, and the cross-sectional stress distribution of the DLC coatings under load was simulated by finite element analysis to better understand the wear mechanisms of the single and multilayer DLC coatings.
Section snippets
Coating deposition
The coatings were prepared by a Diamant-VII-340 ionic enhanced physical vapour deposition device with the unbalanced magnetron sputtering system (developed by the Star-Arc Coating New Material Technology (Suzhou) Company, China). The equipment consists of a vacuum chamber, vacuum pumping system, heating system, sputtering system, and air source system. The vacuum pumping system contains a dry compressing vacuum pump and a turbomolecular pump. The ultimate vacuum degree of the vacuum chamber can
Composition and microstructure characterisation
The microstructural characteristics and carbon bonding state of the DLC coatings was analysed by Raman spectroscopy. As shown in Fig. 2, an asymmetrical broad peak with a range from 800 cm−1 to 2000 cm−1 is visible in the Raman spectrum. The Gaussian function was used to fit the wide peak of the test results, and the peak can be decomposed into a D peak and G peak. The G peak shows the stretching vibration mode of the chain or aromatic ring CC sp2 bond, and the D peak shows the breathing
Conclusions
Diamond-like carbon (DLC) coatings were prepared, and the AlTiN transition layer was selected to be deposited for comparation. The microstructure, roughness, hardness, toughness, adhesive properties and tribological behaviour are systematically explored. The results indicate that
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The adhesion of the coatings can be significantly improved from 8.1 N to 12.5 N when the substrate changes from SUS304 to YT15. More importantly, the adhesion of the DLC coating with AlTiN transition layer is better
CRediT authorship contribution statement
Yefei Zhou: Conceptualization, Methodology, Writing - original draft. Weidong Ma: Data curation, Formal analysis, Investigation. Jia Geng: Data curation, Validation. Lixiang Rao: Methodology. Zhengyu Qian: Validation. Xiaolei Xing: Supervision, Project administration, Funding acquisition. Qingxiang Yang: Writing - review & editing.
Declaration of competing interest
The work described has not been submitted elsewhere for publication, in whole or in part, and all the authors listed have approved the manuscript that is enclosed.
Acknowledgement
This work was supported by the National Natural Science Foundations (Grant Nos. 51705447 and 51905466), P. R. China, Youth Top Talent Project of Hebei Province Higher Education (Grant No. BJ2019058) and Natural Science Foundation of Hebei Province China (E2020203184).
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