Deproteinization-Induced Deterioration of the Mechanical and Tribological Behaviors of Mature Giant Panda Enamel

The giant panda (Ailuropoda melanoleuca) relies on bamboo as its principal food source, exposing its enamel to sustained indentation, shearing, and abrasive loading. Mature enamel is highly mineralized but retains a small residual organic phase, whose contribution to wear resistance remains insufficiently defined. A gradient deproteinization model of mature giant panda enamel was established using KOH treatment. Thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were used to verify organic-phase removal and mineral-framework preservation. Nanoindentation, indentation creep, nanoscratch testing, atomic force microscopy (AFM), and SEM were then used to evaluate mechanical response, scratch resistance, and surface morphological evolution. The mass loss within 200–450 °C decreased with increasing KOH exposure and reached a plateau after 9 days. FTIR showed reduced organic-related or surface-sensitive bands, whereas phosphate bands were retained. XPS revealed decreased C and N contents; increased O, Ca, and P contents; and a higher Ca/P ratio. Deproteinization decreased the elastic modulus, shifted load–displacement curves toward greater indentation depth, and reduced the relative creep index. The critical load in nanoscratch testing decreased from 12.53 ± 0.35 mN to 6.07 ± 0.77 mN, while scratch depth, width, and residual depth increased. AFM showed a rightward shift in aggregate-size distribution and increased roughness. SEM revealed a transition from shallow plowing grooves to complex damage involving cracks, debris accumulation, and local spallation. Therefore, KOH treatment for 9 days provided an effective deproteinization endpoint for mature giant panda enamel. These findings suggest that the residual organic phase may contribute to surface interfacial continuity, local deformation accommodation, scratch-damage resistance, and reduced permanent damage accumulation. This material role may contribute to the adaptation of giant panda enamel to the mechanical demands of a bamboo-based, high-wear diet.