The Effect of Ultrasonic Surface Modification of Nanocrystals on the Surface, Microstructure, and Mechanical Properties of a WC-12Co Coating Deposited by Detonation Spraying

This study investigates the effect of a static contact load applied during ultrasonic nanocrystal surface modification (UNSM) on WC-12Co coatings deposited by detonation spraying. Surface roughness, surface and cross-sectional microstructure, mechanical properties, and adhesion–cohesion strength were evaluated using contact profilometry, scanning electron microscopy (SEM), instrumental indentation (ISO 14577-1), and progressive-load scratch testing (ASTM C1624) under static contact loads of P = 10, 20, 30, and 40 N at fixed amplitude, frequency, and travel speed. Among the treated samples, the arithmetic mean roughness decreased monotonically with increasing load—from Ra = 1.468 ± 0.098 μm at 10 N to 1.158 ± 0.244 μm at 40 N (−21%); pairwise differences fell within the margin of error (p ≥ 0.26), but the linear trend with load was significant (p = 0.039). Maximum profile height (Rz) did not depend on load. The asymmetry Rsk remained negative at all loads, reaching its most negative value at 30 N. Quantitative analysis of the cross-sections yielded coating thicknesses of 230 ± 12, 246 ± 9, 40 ± 6, and 82 ± 7 μm for samples processed at 10, 20, 30, and 40 N, respectively; this variation in thickness is taken into account when interpreting the scratch tests. The average Vickers hardness and indentation modulus exceeded the values for the sprayed material at any load; the EIT peaked at 20 N; and the coefficient of friction in the range Fn = 0–30 N decreased by 17% between 10 and 40 N. We determined P = 30 N as the optimal static contact load: it provides the smallest residual scratch depth (5.5 ± 1.8 μm) and the highest cohesive critical load (Lc1 = 50.2 N), whereas 40 N further reduces roughness but decreases Lc1 to 38.3 N. This conclusion is preliminary because coating thickness varies across the series, and at the final scratching load the indenter extends beyond the coating boundaries on the thinnest samples. The results identify a provisional useful load range for this specimen series. Establishing whether the deposition route changes the UNSM processing window requires a controlled comparison with coatings deposited by other methods, together with measurements of residual stress and near-surface microstructure.