Engineering the friction-and-wear behavior of polyelectrolyte multilayer nanoassemblies through block copolymer surface capping, metallic nanoparticles, and multiwall carbon nanotubes

In previous work, it was demonstrated that coating a surface with polyelectrolyte multilayers (PEMs) composed of polyallylamine hydrochloride (PAH) and poly(acrylic acid) (PAA) resulted in increased friction forces at low normal loads. At stress levels sufficient to cause system wear, however, PAH/PAA PEMs provided significant wear protection for the underlying substrate. This report evaluates three strategies for reducing the coefficients of friction associated with PEMs at low normal stress, while retaining their wear-retarding properties at high normal loads. Anchoring polystyrene-block-poly(acrylic acid) to the PAH surface of a multilayer film, less than 10 nm thick, enhanced the hardness, and hence the load-bearing capacity of these structures. The effect of surface capping was most pronounced at high normal stresses, where substantial wear prevention was observed, accompanied by low friction forces. These films are well suited for systems where wear particle generation hinders smooth operation. The second strategy used the in situ synthesis of silver nanoparticles in the PEM matrix for friction reduction. Optimum levels of silver clusters are required at the surface to reduce the friction forces. Finally, multilayer composites were constructed using PAH and multiwall carbon nanotubes. These assemblies exhibited the lowest values of friction among the strategies studied, at all levels of stress; in addition, substrate wear prevention was also achieved. The tribological behavior of the PEM-modification strategies was correlated with the mechanical properties of the films, elicited by nanoindentation. (C) 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 439-448, 2004.