The purpose of this study was to investigate the friction behaviour of human tooth enamel irradiated by Nd:YAG laser. Forty specimens were randomly divided into four groups, and received the Nd:YAG laser irradiation with different energy densities (without irradiation; 20, 40, and 80 J/cm2). Subsequently, nano-scratch tests were conducted on the treated enamel surfaces, with progressive loads from 0.1 to 80 mN. The morphologies of the treated enamel surfaces and the scratch scars were observed via a field emission scanning electron microscope and an AMBIOS XP-2 stylus profilometer. The results indicated that the Nd:YAG laser promoted slight morphological alterations under a lower energy density (20 J/cm2), but different morphologies occurred under a higher energy density (40 and 80 J/cm2). The irradiated enamel surfaces had inferior nano-scratch resistance, and the nano-scratch resistance decreased as the energy density increased. Nd:YAG laser irradiation decreased the nano-scratch resistance of the enamel. The main damage mechanisms of the irradiated enamel under nano-scratch consisted of brittle delaminations and formation of cracks.
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