14 research outputs found
Development of DLC coating architectures for demanding functional surface applications through nano- and micro-mechanical testing
No full textDLC coatings can combine high hardness with low friction. However, they are often deposited with high levels of intrinsic stress and display low adhesion strength resulting in poor performance in demanding applications. A highly topical challenge is to develop advanced DLC coatings capable of withstanding more demanding applications in the
automotive, cutting tools, MEMS and oil and gas sectors. The results from several nanomechanical and tribological test techniques - nanoindentation, nano-scratch and nanofretting (nano-wear) - can be used together to aid the design of DLC coating architectures for enhanced durability in specific applications. In this study the behaviour of multilayered DLC coatings (Cr/W-C:H/a-C:H, Cr/W-C:H/Si-a-C:H) was compared to that of CrN/a-C:H:W (WC/C). We have previously reported that in nano-wear tests the coating with the highest hardness and H/E displayed greater wear resistance [T.W. Liskiewicz et al, Surf. Coat.
Technol. 237 (2013) 212]. By employing nano- and micro-scale tribological testing with probes of differing sharpness it is possible to change the sensitivity of the test to probe the response of the coating top layer or the entire multilayer coating-substrate system. In the
nano-scratch tests using a spherical indenter with a 5 m end radius the maximum stresses
are located well within the top layer of the multilayer coatings and consequently the mechanical properties of this top layer dominate the nano-tribological behaviour. In the micro-scratch using a 25 m spherical probe the stress field extends further towards the sublayers
and steel substrate and consequently the behaviour is completely different. Under these conditions the coating with the lowest hardness and H/E showed improved performance with higher critical loads for cracking and total coating failure. High resolution SEM imaging has
been used to investigate this further. A simple contact model strongly suggests that cracking and failure events occur on the harder coatings when the maximum von Mises stress was located close to the interfaces in the multilayer systems
Surface morphology in engineering applications: Influence of roughness on sliding and wear in dry fretting
No full textInfluence of initial surface roughness on friction and wear processes under fretting conditions was investigated experimentally. Rough surfaces (Ra=0.15-2.52 [mu]m) were prepared on two materials: carbon alloy (AISI 1034) and titanium alloy (Ti-6Al-4V). Strong influence of initial surface roughness on friction and wear processes is reported for both tested materials. Lower coefficient of friction and increase in wear rate was observed for rough surfaces. Wear activation energy is increasing for smoother surfaces. Lower initial roughness of surface subjected to gross slip fretting can delay activation of wear process and reduce wear rate; however, it can slightly increase the coefficient of friction
Nano-indentation mapping of fretting-induced surface layers
No full textTribologically modified surface layer results from the energy dissipated in the frictional contact area. This layer usually has a different elastic modulus and hardness from the substrate, and its structure corresponds to the intermediate stage between a material of the first-body and debris of the third-body. Even though, the existence of the tribologically transformed structure in the fretting contact has been well proven, the formation and mechanical transformation mechanisms are still not clear. Hence, in this paper, evolution of mechanical properties of four metallic materials (titanium alloy, stainless steel, carbon steel, copper alloy) induced by fretting was investigated using nano-indentation mapping of the fretting wear scars. It was shown that the tribologically transformed structure formed very quickly within the initial fretting cycles, and its mechanical properties remained almost constant during the entire test duration for tested materials. However, it was observed that all materials responded differently to the frictional energy, exhibiting particular rate of change of the H/E ratio before and after the fretting experiment. Modified XRD technique was used to probe the friction induced changes within the small spots of the fretting scars, and revealed distinctive structural modifications within the transformed layers. The approach proposed in this study can be used to inform the predictive wear models, by providing information about the evolution of the mechanical properties of the tribo-system with time
Deformation of Si(100) in spherical contacts — Comparison of nano-fretting and nano-scratch tests with nano-indentation
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Crack propagation of a thin hard coating under cyclic loading : irreversible cohesive zone model
No full textThe numerical study of the fatigue behaviour of steel and light alloy substrates when coated with thin hard coatings is limited. This paper aims to investigate the fatigue failure mechanism of the coating system by observing the initiation and propagation of cracks within the coating under the cyclic loading. The model coating system is composed of three layers: the TiN coating, a case-hardened diffusion zone and the H11 steel substrate. The cohesive elements were arranged evenly in the horizontal direction and vertically through the thickness of the coating layer in order to observe the crack initiation and propagation. The model coating system was indented by a spherical indenter of 300 μm radius. Both the coating and the substrate were characterised as being homogenous, with elastic properties followed by linearly-hardening plastic behaviour. The irreversible cohesive zone model, allowing for the local degradation of the material properties to be incorporated into the model, was employed to simulate the crack initiation and propagation under cyclic loading. It was observed that the crack was initiated at the edge of the contact area between the indenter and coated surface at early stage of loading cycles, then progressed rapidly through the thickness of the coating layer. The deepest crack was found at 1.4 μm below the top surface. The study has demonstrated that the irreversible cohesive zone model can be used to track the evolution of crack propagation with cyclic loading, therefore, has the capability to predict the loading bearing capacity of the coating system under contact fatigue loading conditions
Influence of roughness on ZDDP tribofilm formation in boundary lubricated fretting
No full textInfluence of initial surface topography on tribofilm formation in ZDDP lubricated contact was analysed. A small displacement fretting tests with sinusoidal motion were carried out in classical sphere/plane configuration. A range of surfaces with different initial roughness were prepared by milling and grinding processes. Tests were carried out using variable displacement method where amplitude of imposed displacement was gradually increased after every 1000 cycles from 2 to 30 µm. The surfaces after tribological tests were measured by interferometric profiler. Main findings confirm that initial roughness has a significant influence on antiwear tribofilm formation in boundary lubricated contact. Tribofilm form faster and require less energy to activate in case of rough surface obtained by milling process than in case of smooth grinded surface. However, in contact lubricated by ZDDP additive a significant transfer of material occurred from plane to sphere specimen
Utilising H/E to predict fretting wear performance of DLC coating systems
No full textDiamond-like carbon coatings have previously been studied as a protective coating for fretting wear protection providing low friction and low wear. H/E ratio has been used as a metric to rank coating performance in sliding wear, but this has not been applied to gross-slip fretting. Three DLC coating systems (a-C:H, Si-a-C:H, a-C:H:W top layers) on hardened M2 tool steel were studied using a bespoke electrodynamic shaker with a 10 mm 52100 steel ball as the counterface. This work has shown that H/E ratio can be used to predict wear performance in gross-slip fretting; the highest H/E ratio a-C:H performed best with low friction and wear
Dynamic evolution of interface roughness during friction and wear processes
No full textDynamic evolution of surface roughness and influence of initial roughness (Sa=0.282 to 6.73 µm) during friction and wear processes has been analyzed experimentally. The mirror polished and rough surfaces (28 samples in total) have been prepared by surface polishing on Ti-6Al-4V and AISI 1045 samples. Friction and wear have been tested in classical sphere/plane configuration using linear reciprocating tribometer with very small displacement from 130 to 200 microns. After an initial period of rapid degradation, dynamic evolution of surface roughness converges to certain level specific to a given tribosystem. However, roughness at such dynamic interface is still increasing and analysis of initial roughness influence revealed that to certain extent, a rheology effect of interface can be observed and dynamic evolution of roughness will depend on initial condition and history of interface roughness evolution. Multiscale analysis shows that morphology created in wear process is composed from nano, micro and macro scale roughness. Therefore, mechanical parts working under very severe contact conditions, like rotor/blade contact, screws, clutch etc. with poor initial surface finishing are susceptible to have much shorter lifetime than a quality finished parts
