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Elevated Crosslinking Alone Does Not Explain Polyethylene Wear Resistance
Furman, B. D., Maher, S., Morgan, T., Wright, T.
Abstract
New polyethylene materials have been introduced that utilize elevated radiation doses combined with post-irradiation heat treatments. The elevated radiation dose creates higher levels of crosslinking and is reported to cause improved abrasive wear resistance. The heat treatment quenches free radicals that result from irradiation, thus preventing oxidative degradation. Although abrasive wear occurs between conforming bearing surfaces, macroscopic wear damage is more commonly cited than abrasive wear as a limiting factor with non-conforming surfaces. These new materials demonstrate reduced mechanical properties, yet recent studies revealed that some of these materials performed well under non-conforming (total knee-type) in vitro wear conditions. Our aim was to understand why materials with reduced fracture toughness and yield strength would be more resistant to damage.
Polyethylene materials with varying radiation doses and heat treatments were examined. We measured modulus, yield strength, ultimate stress, and ductility (ASTM-D638), fracture toughness (J-integral), and wear. Crystalline morphology was evaluated using transmission electron microscopy. A relation was found between modulus, morphology and wear behavior. Materials with modulus less than 800 MPa and lamellar structure smaller than 4000 angstroms demonstrated no pitting, delamination or cracking in wear tests. Even though these materials showed reduced toughness, they demonstrated good damage resistance, suggesting that elevated crosslinking alone may not explain wear and damage resistance.
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