A study of Biolox® delta subject to water lubricated reciprocating wear

Biolox® delta is the latest commercial ceramic material for artificial hip-joint replacements manufactured by CeramTec AG, Germany. This zirconia toughened alumina composite employs several toughening mechanisms and has been successfully implanted over the past 8 years. Although extensive in vitro testing has been carried out for ceramic-on-ceramic artificial joints, the wear mechanisms that occur in Biolox® delta are not fully understood, in particular, the role of the zirconia. Reciprocating sliding wear tests were undertaken in ultra pure water according to ISO 6474 using a ball-on-flat configuration. Loads were varied in the range of 0.5–4 N and the reciprocating frequency was varied through 8.3–11.7 Hz over a stroke length of 10 mm in order to construct the Stribeck curve. The Stribeck curve indicated that there were two lubrication regimes for these test conditions, namely full fluid-film lubrication and mixed lubrication. The worn surfaces were characterised in detail using scanning electron microscopy (SEM), atomic force microscopy (AFM) and focused ion beam (FIB) cross-section milling. Damage, which comprised dislocation flow, inter- and transgranular fracture, was limited to one grain depth below the worn surface. The zirconia grains distributed in the alumina matrix near the worn surface had almost completely transformed to monoclinic symmetry. The observations of surface damage accumulation are discussed in relation to the current understanding of the wear mechanisms of alumina based ceramics.