PSI - Issue 47

G. Di Egidio et al. / Procedia Structural Integrity 47 (2023) 337–347 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Therefore, the debonding at the Ni-P/substrate interface results from the compressive stresses developed in the coating due to the difference in the Poisson ratio and young's modulus between coating and substrate [14], which leads to a fast crack initiation and sample failure promoted by T5 microstructure. Even though the Ni-P coating shows good overall adhesion to the L-PBF AlSi10Mg substrate (Figure 3), the higher Si phase density in T5 than in T6R (Fig. 4), as well as more capillary distribution and highly branched morphology of the eutectic-Si network, reduce the Ni-P substrate interface area and hence the adhesion of Ni-P to the substrate (Figure 9(a)). Conversely, the T6 composite-like microstructure provides a larger contact area between the Al matrix and the Ni-P coating, thus increasing both the adhesion and the ability of the coating to follow the plastic deformation of the substrate. This condition is highlighted by the fracture occurring within the substrate, a few microns underneath the Ni-P-substrate interface, with a layer of substrate material remaining well adherent to the Ni-P interlayer (Figure 9(b)).

(a) (b) Fig. 9. High magnification of the fracture zone at the Al substrate/Ni-P coating interface of the T5-C (a) and T6R-C (b). Yellow arrows indicate chevron markings in the Ni-P interlayer typical of a brittle fracture. Red arrows point out submicrometric (a) and micrometric dimples (b). White dashed lines indicate the position of the substrate-Ni-P interface Even though different e f values characterize coated and uncoated samples, they show comparable failure mechanisms in the bulk material, mainly influenced by the heat treatment conditions. In particular, T5-C and T5-U samples (Figure 10(a,b)) show shallow dimples associated with the detachment of Al cells from the edges of the eutectic-Si network. Conversely, the T6-C and T6-U samples show a comparable ductile failure mode, characterized by deep dimples (Figure 10(c,d)) induced by the plastic deformation of the Al matrix around the Si particles.

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