Effect of Interlayer Thickness on Fatigue Behavior in A5052 Aluminum Alloy with Diamond-Like Carbon/Anodic-Oxide Hybrid Coating

Abstract

Anodic oxide layer was formed on A5052 aluminum alloy, and subsequently diamond-like carbon (DLC) was deposited to fabricate DLC/ anodic-oxide hybrid coating. Plane bending fatigue testes have been performed using the hybrid-coated specimens with different thicknesses of anodic-oxide interlayer. The interlayer thicknesses were 10 and 50 µm, where the thickness of DLC film was fixed as 3 µm. The specimens without coating, with DLC single layer and with anodic-oxide single layer were also used for comparison. The fatigue strengths of the specimens with anodic-oxide single layer were lower than those of the substrate without coating because the corner edge cracking or pin-hole defects could be fatigue crack initiation sites. However, when DLC was deposited on the anodic-oxide layer, fatigue strengths were improved. The hybridcoated specimens with the interlayer thickness of 10 µm had higher fatigue limit than the substrate and specimens with the interlayer thickness of 50 µm. Thin DLC layer with the thickness of 3 µm could have suppressed the cracking in interlayer, and led to the improvement of fatigue strengths than the substrate. When the interlayer became relatively thicker compared with the DLC film, such as 50 µm, DLC film could not suppress the cracking from the corner edge defects, resulting in the lower fatigue strengths in spite of the presence of hybrid coating.

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

“…The substrate material has the lowest fatigue strengths, in which the fatigue limit is within 145-150 MPa. Compared with the substrate, DLC single layer increased fatigue strengths similar to the previous report by the authors (Uematsu et al, 2015). The fatigue limit is within 160-180 MPa for the DLC single-layered specimens.…”

“…Recently, diamond-like carbon (DLC) attracts attention as hard coating on Al alloys, because defects in DLC coating are much less than those in anodic-oxide layer. The authors have reported that DLC coating on A5053 Al alloy could successfully improve fatigue strengths of substrate (Uematsu et al, 2015). However, it is well known that the elastic modulus of Al alloys is much lower than that of DLC.…”

“…In the previous cases of DLC/WC-Co (Kakiuchi et al, 2013, Okada et al, 2010 or DLC/anodic-oxide (Uematsu et al, 2015) hybrid coatings, the thickness of the interlayers varied in the range of 10 ~ 80 μm. Thus the thickness of the plating in the present study was set to be 20 μm for comparison.…”

“…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. …”

“…However, thermal spraying followed by DLC deposition highly increases the processing cost mainly due the thermal spraying process, because the spraying needs complicated facilities and the material cost of spraying powder is high. In the previous study, therefore, anodization was employed as the interlayer of DLC/anodic-oxide hybrid coating (Uematsu et al, 2015), because anodization process was much cheaper than thermal spraying. However, anodizing layer included porosities in itself, and the cracking of coating occurred at the corner edge of the specimens.…”