Brittle or Ductile? Abrasive Wear of Polyacrylamide Hydrogels Reveals Load-Dependent Wear Mechanisms

Abstract: Cartilage and hydrogels are composed of an elastic network that retains a large volume of water allowing them to efficiently maintain smooth sliding interfaces while under high compressive loads. For hydrogels to be viable candidates to replace osteoarthritic cartilage, a study of their robust long-term use and surface failure mechanism is necessary. In this work, a sandpaper covered probe attached to a microtribometer with a reciprocating stage was used to wear 7.5 wt% polyacrylamide hydrogels under a range of speeds (1 mm/s, 2 mm/s, and 3 mm/s) and normal loads (1 mN, 5 mN, 10 mN, and 20 mN). The subsequent wear scars were imaged using a 3D laser scanning confocal microscope. For all sliding speeds of the 1 mN and 5 mN loading conditions, microcutting, which is characteristic of brittle materials, contributed more to the measured wear volume. For the 10 mN and 20 mN loading conditions, microplowing, which is characteristic of ductile materials, contributed more to the measured wear volume. We found that the mechanical wear of hydrogels is a competition between ductile fracture and brittle fracture, and the dominant mechanism is dependent upon load, but not speed for the range of speeds tested. The different wear behavior between the lower loads (1 mN and 5 mN) and higher loads (10 mN and 20 mN) suggests that there is a critical load between 5 and 10 mN that marks the shift from more brittle fracture to more ductile fracture. This work is the beginning of developing more accurate predictions of the wear behavior of hydrogels and cartilage based on the nature of the materials. Graphic Abstract: [Figure not available: see fulltext.].