Effect of thick DLC coating on fatigue behaviour of magnesium alloy in laboratory air and demineralised water

Abstract

A B S T R A C T Rotating bending fatigue tests have been performed using Diamond-like carbon (DLC) coated specimens of a wrought magnesium alloy, AZ80A, in laboratory air and demineralised water and the effect of DLC coating on fatigue and corrosion fatigue behaviour was studied. Three film thicknesses of 3.5 μm, 13 μm, and 25 μm (two-layer film) were evaluated and particular attention was paid to the role of thick DLC coating. In laboratory air, the fatigue strengths of the DLC-coated specimens were higher than that of the substrate specimen and increased with increasing film thickness. This was because hard DLC coating with good adhesion suppressed the crack initiation due to cracking of inclusions or cyclic slip deformation on the substrate surface. In demineralised water, the fatigue strength of the 3.5-μm DLC-coated specimen was the same as that of the substrate specimen due to the penetration of the water through pre-existing film defects, while the 13-μm and 25-μm DLC-coated specimens showed increased corrosion fatigue strength with increasing film thickness and also exhibited nearly the same fatigue strength as in laboratory air except for a few premature failed specimens, indicating a potential of thick DLC coating or two-layer coating for complete improvement of corrosion fatigue strength in aqueous environments.

Fatigue behaviour in AZ80A magnesium alloy with DLC/thermally splayed WC-12Co hybrid coating

“…In this case, however, the tendency of S-N diagram was irrelevant to the stress amplitude calculation methods. In addition, it is known that AZ80 has age hardenability, but it was confirmed that the effect of the thermal history during the DLC deposition on fatigue strength can be neglected [3]. Figures 6 (a) and (b) are SEM micrographs showing fracture surfaces near the crack initiation sites in the substrate and D15, respectively.…”

“…As shown in Fig.6(b), a film defect exists at the crack initiation site of D15. The crack is believed to be generated in the substrate underneath the bottom of the film defects due to their stress concentration [3]. The DLC film around the crack initiation site does not peel off the substrate, indicating very good adhesion of the film.…”

“…Diamond-like carbon (DLC) has recently been considered as a promising candidate of surface coatings for improving both fatigue properties and corrosion resistance because of high hardness, good adhesion, low defect density and chemical inertness [3]. DLC films can be deposited on substrate by various methods such as chemical vapour deposition (CVD) and ion beam assisted deposition (IBAD), and so on.…”

Fatigue behavior of A5052 aluminum alloy with diamond-like carbon/electroless nickel plating hybrid coating

“…3(b). It has been reported that corrosion fatigue strengths largely decreased due to the penetration of corrosive environment through this kind of pin-hole defects of DLC film (Uematsu et al, 2010). Figure 4 shows the surface morphology of Ni-plating single-layered specimen, which macroscopically has smooth surface (Fig.…”

“…However, it should be noted that hybridcoating is not effective if the Ni-plating layer has large cracking in itself. The fractographic analyses of DLC singlelayered sample could be found elsewhere (Uematsu et al, 2010, Uematsu et al, 2015. …”