Tribological characteristics of polyelectrolyte multilayers

Abstract

Polyelectrolyte multilayers (PEMs), formed by the sequential adsorption of poly(acrylic acid) and poly (allylamine hydrochloride) possess architectures that can be tuned as a function of polyelectrolyte solution pH. PEMs are easy to fabricate, requiring little or no substrate pretreatment, and provide conformal nanocoatings that are resistant to scotch-tape peel tests on metallic, plastic, and glass substrates. Their tribological behavior has received little or no attention. We have studied the friction and wear behavior of PEM-coated materials primarily using a meso/micro-scale flexure-based apparatus; a 1 or 2 mm diameter flat pin was made to articulate against a larger sliding surface. The friction coefficient of film-coated steel substrates against glass was higher than that exhibited by the bare substrates at all stresses; the average value decreased with an increase in the applied normal load, a trend predicted for polymeric materials. PEM constructs prevented wear of steel substrates in the dry state, and also in the presence of water and bovine calf serum, used to simulate synovial fluid in human joints. At these higher stresses, the delaminated film fragments prevent contact between the mating surfaces, thus eliminating substrate wear. The adhesion, deformation, and dragging of the fragments causes the friction force to be higher than the bare substrates (prior to the onset of their wear). Wear can be prevented without a substantial increase in friction force by using thinner films on the larger slider surface or coating the smaller pin counterface. The capacity for PEM-induced wear reduction was confirmed at larger scales of testing in the dry state using a pin-on-disk tester.