Nano-indentation and nano-scratch of flexible intraocular lens material at the molecular scale

In this paper, the deformation and material removal mechanism of acrylate intraocular lenses (IOLs) are studied, which lays the foundation for ultra-precision polishing of this flexible material. The indentation experiment based on an atomic force microscope (AFM) shows that the material has a pronounced surface effect under the action of nano-scale indentation. With the change of loading rate, the loading curve deviates from P-h(2) relationship to some extent. The material has a pronounced size effect, and the nano-hardness of the material decreases from 7.7 MPa to 6.4 MPa within 300-800 nm indentation depth. The nano-scratch test shows that the friction coefficient decreases drastically and then remains stable as the normal load reduces due to the transition of friction within the system from plastic removal of inclusion elastic deformation to interfacial friction. The mechanism of loading rate dependence and inhomogeneous phenomena of surface adhesion are verified by molecular dynamics simulation. In addition, the fluctuations in load-displacement curves and friction coefficients are due to a combination of different movement patterns of material molecular chains and uneven cutting depths caused by material accumulation.