Impact Wear Mechanisms of DLC Coating

Abdollah, Mohd Fadzli bin (2011) Impact Wear Mechanisms of DLC Coating. PhD thesis, Nagoya University.

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Diamond-like carbon (DLC) films have been explored in the past due to their highly attractive properties, such as high hardness; low friction coefficient; chemical inertness and electrical insulation; optical transparency and smoothness; and biological compatibility. In previous studies, most of the impact wear mechanisms of DLC coatings are concerned with the fracture process in the coating due to the crack propagation under severe wear conditions. However, no reports discuss about how the impact wear mechanisms of the DLC coatings act under mild wear conditions. Under impact, it is well known that plastic deformation should occur prior to the impact wear. So, the first objective of this study is to identify the most significant impact parameter that controls the deformation of DLC coating. After cyclic impacts, though under mild wear conditions, it believes that impact wear should be occurring by the phase transformation of DLC coating. Therefore, the second objective is to clarify the impact wear mechanisms of DLC coating based on its phase transformation. Finally, in order to distinguish more clearly between the plastic deformation and impact wear of DLC coating as well as to predict its transition points, a deformation-wear transition map has been proposed. In this present study, DLC films were deposited on the tungsten high speed steel (SKH2) substrate using physical vapor deposition (PVD) method. The impact test was performed by using a horizontal impact tester for more than 102 impact cycles with a frequency of 10 Hz, and a drop-weight impact tester for low impact cycles. The DLC coated SKH2 disc was repeatedly impacted by a chromium molybdenum steel (SCM420) pin. The 90o inclination of impact tests were performed at room temperature under lubricated conditions. Prior to the impact test, both disc and pin were cleaned using acetone in an ultrasonic bath. The maximum normal impact load was obtained from the graph of normal impact load vs. time, generated by a load cell. Besides, the contact impulse was determined from the area below this graph. As for the absorbed energy, this can be determined using a high speed camera. From the experimental analysis, there is no unique relationship between the residual impact crater volume/depth of DLC coating and contact impulse. The highest coefficient of determination R2 for both the Vr and hr (R2 hr = 0.9362 and R2 Vr = 0.9076) obtained from the response of maximum normal impact load. Besides, there is an experimental error in the case of absorbed energy due to the microslip effect. Furthermore, the impact phenomenon in this study can be considered as the quasi-static indentation, where load is a governing parameter, because the impact velocity is very low. By comparing with the analytical solutions, it is easier to predict the residual impact crater volume/depth by static indentation analysis. Therefore, from these reasons, it can be concluded that the residual impact crater volume/depth is more affected by maximum normal impact load than absorbed energy. From Raman spectroscopy analysis, it has been suggested that there is pressure-induced graphitization since the impact testing was performed at the room temperature. The phase transformation of wear debris and transfer layer from sp3 to sp2 induced graphitization process. However, the sp3 fractions of DLC coating on the crater surface are significantly increased with impact cycles as it is evidently shown by decreasing ID/IG ratio approximately from 0.63 (as-deposited) to 0.47 (after 105 impact cycles), accompanied by a widening of full-width at half maximum of G peak FWHMG (approximately from 179 cm-1 to 192 cm-1) with impact cycles. Besides the hardness reaches approximately to 21 GPa that is higher than as-deposited (17.14 GPa) after several impact cycles. This suggests the size of the larger sp2 clusters is reduced due to the mechanical crush of the larger sp2 clusters. Since the impact test was performed under atmospheric conditions, oxidation of iron with the environment also occurs in the wear debris and transfer layer, where two predominant peaks of magnetite (Fe3O4) and hematitie (α-Fe2O3) are observed from its Raman spectrum. For a given material and controlled variables (pin radius, coating thickness, substrate material, environmental conditions and so on), the proposed deformationwear transition map of DLC coating apparently shows that the maximum normal impact load and impact cycles influence this transition. This empirical-based transition map, which presents deformation and wear data in a graphical manner, is able to provide a more global picture of how DLC coating behaves under cyclic impacts systematically. Three main transition zones that graphically distinguish between the plastic deformation and impact wear of DLC coating were identified: (i) Plastic deformation of the substrate, (ii) suppression of plastic deformation of the substrate and (iii) wear of the DLC coating. Beyond the elastic limit, the DLC coating only follows the plastic deformation of the substrate until several impact cycles. Then, a suppression of plastic deformation of the substrate is taking place due to the decreasing contact pressure with impact cycles to the yield point. The hardness of the DLC coating on the crater surface is also increases after numerous impacts and no wear has been observed within these two zones. Wear of the DLC coating becomes dominant when the critical limit of maximum normal impact load and impact cycles is exceeded. Experimental observations show that this wear is associated with some degradation of the DLC coating. This includes the phase transformation of the wear debris/transfer layer and propagation of radial cracks in the DLC film as well as the formation of transfer layer on the counterpart material, which may attribute to the adhesive wear.

Item Type: Thesis (PhD)
Subjects: T Technology > TJ Mechanical engineering and machinery
Divisions: Faculty of Mechanical Engineering > Department of Automotive
Date Deposited: 18 Oct 2013 05:48
Last Modified: 14 Mar 2018 03:52
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