Surface characterization and mechanical properties of eggshell-derived calcium-deficient hydroxyapatite and graphene-nanoplatelet coatings on titanium alloys

Titanium alloys are widely employed in orthopaedic and dental implant applications because of their favourable corrosion resistance, strength-to-weight ratio, and biocompatibility. However, inadequate surface hardness and wear resistance may limit long-term performance under repeated mechanical loading. Hydroxyapatite-based coatings have received considerable research attention for improving the biological response of titanium surfaces, although brittleness and limited mechanical stability remain important concerns. In the present study, sustainable eggshell-derived calcium-deficient hydroxyapatite (CDHA) and CDHA reinforced with graphene nanoplatelets (GNPs) were applied to Ti6Al4V and Ti6Al7Nb substrates using a low-temperature pack-based thermal treatment at 800 °C for 2 h under argon. Surface morphology, coating thickness, elemental composition, surface roughness, Raman characteristics, and microhardness were evaluated. The coatings exhibited heterogeneous particle-based morphologies with thicknesses of approximately 25–50 µm. Surface roughness increased after coating, whereas the incorporation of 0.5 wt.% GNPs produced a modest reduction in the mean Ra values. The highest measured microhardness values were 362 ± 11 HV for Ti6Al4V-CDHA-GNP and 354 ± 8 HV for Ti6Al7Nb-CDHA-GNP. Raman spectroscopy confirmed the retention of characteristic graphitic features in the GNP-containing systems. The results demonstrate that eggshell-derived CDHA-GNP coatings can modify the surface and mechanical response of titanium alloys; however, further optimization of coating uniformity, topography, and long-term biological and mechanical performance is required.