Surface Mucus Regulates Friction and Wear of Crucian Carp Scales under GCr15 Sliding

Fish-scale surface mucus exhibits a lubricating effect; however, its specific role in regulating friction and wear during direct solid–solid sliding remains unclear. In this study, crucian carp scales were selected as model biological surfaces. Dried mucus-retained and mucus-removed samples were prepared. Reciprocating sliding tests against a GCr15 steel ball were then conducted in three representative regions. Scanning electron microscopy, X-ray photoelectron spectroscopy, and water contact angle measurements were used to characterize the surface properties of the two sample groups. Their frictional behavior and wear damage were also compared. The steady-state coefficients of friction in the three regions were reduced from 0.18, 0.59, and 0.62 to 0.16, 0.45, and 0.49, respectively, after mucus retention. These values corresponded to reductions of 11.1%, 23.7%, and 21.0%, respectively. Wear damage was also markedly alleviated. Taken together, the XPS results showed that polar oxygen- and nitrogen-containing organic functional groups were present on the mucus-retained surface. The wettability results further showed that mucus retention increased the hydrophilicity of the fish-scale surface. After sliding, mucus-related residues were retained in the worn regions and were involved in the sliding contact. Direct solid–solid contact between the GCr15 steel ball and the fish-scale substrate was reduced by the residue-mediated interface. The effective interfacial shear resistance was also lowered, resulting in reduced friction and wear. The friction-reducing and wear-protective roles of mucus-derived material retained on the fish-scale surface after drying were elucidated under solid-sliding conditions. These findings provide a basis for further research on bio-derived materials for interfacial lubrication.