Abstract
The hardness and post-scratch surface morphology of Cr₃C₂–NiCr coatings subjected to ultrasonic nanocrystal surface modification (UNSM) were examined for detonation-sprayed (DS) and high-velocity oxy-fuel (HVOF) deposits. Four treated samples per deposition route were evaluated at static loads of 20 and 30 N and vibration amplitudes of 20 and 30 µm. Instrumental hardness was measured at a load of 300 mN with a dwell time of 10 s. The DS samples exhibited hardness values of 820–970 HV, whereas the HVOF samples covered the wider range of 546–1165 HV. The highest values were recorded for DS-T1 at 20 N/20 µm and HVOF-T4 at 30 N/30 µm. SEM fields acquired after scratch testing showed heterogeneous surface contrast and crack-like discontinuities in both coating series. These observations provide qualitative evidence of the post-test surface condition but do not isolate damage caused by UNSM or establish a ranking of scratch resistance. Available mean friction coefficients are reported as supplementary observations for six treated samples. The results demonstrate a sample-dependent hardness response and emphasize the need to assess local mechanical properties together with surface integrity.