*3.10. Nanoindentation Experimentation*

Nanoindentation was performed on oriented single crystals of RIV, MAL and RIV-MAL Co using a Ti-950 TriboIndenter (Hysitron Inc., Minneapolis, MN, USA) using a protocol, which is similar to that of previously reported by our research group [32]. Briefly, the indenter employed was a tripyramidal (Berkovich) tip having an inclined angle of 142.3◦ and a tip radius of ∼150 nm. The fused silica and polycarbonate standards were used to calibrate the tip area function. The tip area function calibration was carried out by performing a series of indents with di fferent contact depths on a standard sample of known elastic modulus (E). A plot of the calculated area against contact depth (h) was created and fitted by the TriboScan software. An optical microscope integrated into the nanoindentation system was used to identify the regions on crystal surface for testing. The "tip to optics calibration" was undertaken by performing 10 indents in an "H-pattern". The testing was carried out at 28 ± 0.5 ◦C temperature and 45 ± 5% relative humidity. For quasi-static analysis of all samples, 10−12 subsequent indents were performed along the length, midline parallel to the longest axis of the crystal on a dominant face with user-specified parameters. The su fficient contact depths, large enough to local surface roughness were estimated to avoid strong e ffect of roughness on the measured mechanical properties. The peak load (P) for these indentations was 1000 μN, and the indent spacing was 55.0 μm. A load function consisting of a 5 s loading to peak force (F) segment, followed by a 2-s hold segmen<sup>t</sup> and a 5-s unloading segmen<sup>t</sup> was used (the loading and unloading rates were 0.2 mN/s). The Oliver and Pharr method [35] was employed to compute the nanomechanical hardness (H) and the elastic modulus (*Er*). The *Er* value is related to the Young modulus of elasticity of the tested sample (*Es*) and the indenter (*Ei*) through the following relationship in Equation (5):

$$\frac{1}{E\_r} = \frac{\left(1 - \upsilon\_i^2\right)}{E\_i} + \frac{\left(1 - \upsilon\_s^2\right)}{E\_s} \tag{5}$$

where υ*i* is Poisson ratio for the indenter material, while υ*s* is Poisson ratio of the substrate material. The values of the elastic modulus and Poisson ratio for the diamond indenter tip are 1140 GPa and 0.07, respectively.
