The Vickers hardness of AlCoCrFeNi coating was 4.13 ± 0.43 Gpa (HV300gf, n=30), which was higher than that for supersonic APS AlCoCrFeNi (1.7 GPa) and cold sprayed AlCoCrFeNi coatings (3.8 GPa). This can be attributed to the additional strengthening of the alloy phases with relatively harder oxide phases generated by IFO. The elastic modulus of coating was measured as 106 ± 7 GPa and used in calculating the coating thermal stresses.
Nanoindentation is the appropriate technique for analysis of nanomechanical properties of multi-phase APS AlCoCrFeNi HEA coating. Each of the indentation matrices of 6×6 indents was observed under SEM to locate each indent on individual phases, followed by an evaluation of their nanomechanical properties.
Table 1 lists the results of contact depth (in nanometers), reduced elastic modulus, Er (in GPa) and hardness, H (in GPa) for the three phases (white (W), grey (G) and black (B)). The average hardness and average reduced elastic modulus for all 324 indents was 9 ± 6 GPa and 153 ± 33 GPa, representing a coefficient of variation, CV, of 63% and 21%, respectively.
Table 1
Average contact depth (in nanometers), reduced elastic modulus, Er (in GPa), hardness, H (in GPa) values with their standard deviation and coefficient of variation (CV), H/Er
and H3/Er2 ratios measured during nanoindentation of individual phases observed in AlCoCrFeNi coating.
Fig. 1(a) and Fig. 1(b) show the SEM images of nanoindent impressions on the individual G, Wand B phases. Fig. 1(c) represents the load-depth curves corresponding to phase-wise indents as indicated in Fig. 1(a) and (b). Under identical loading conditions (5000 mN), for an average of 25 indents per phase, the indentation depth for B was found to be lowest, followed by G, and W. This implies that phase B (alumina) is harder and more resilient to plastic deformation, than G (mixed oxides); while W (alloy) is the softest phase. The higher indentation depth and wider indent impression for the softer phase W (Fig. 1(a) and (b)) substantiate these results. Thus, there is a significant difference in hardness distribution for different phases. This variation in nanohardness with the indentation depth is associated with the indentation size effect (ISE), which implies that a decrease in indent size corresponds to both an increase in hardness and decrease in plasticity. The variation in reduced elastic modulus is not as radical as hardness, as evident from the lower CV values shown in Table 1.
Fig. 1. (a) Precise nanoindentation impressions on G and W phases and pile-up around the indent of W phase imaged by SEM, (b) indent impression on B phase, (c) load-depth curve of the corresponding indent impressions of G, B andWphase mentioned in (a) and (b) and, (d) Vickers indentation impression covering a large volume of the microstructure, including multiple phases, porosity and microstructural defects imaged via optical microscopy.
The wider impression of the white phase (W) also exhibited material pile-up around the indents, see Fig. 1(a). The appearance of material pile-up during nanoindenation has been attributed to highly localized plastic deformation of the phase. Further, the ratio H3/Er2 signifying the material’s resistance to plastic deformation, was found to be lowest in the case of the white phase (Table 1). A simulation study conducted by Muthupandi et al. predicted that nanoindentation of material with low H/Er ratio results in pile-up along the indent due to progression in plasticity and proliferation of the plastic strain across the indenter tip. Theobserved low H3/Er2 ratio and material pile up around indents in the W phase conformed to this behaviour.
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