Fretting wear behavior of calcium phosphate-mullite composites in dry and albumin-containing simulated body fluid conditions

In a recent work, it has been shown that it is possible to achieve a better combination of compressive strength, flexural strength and toughness properties in calcium phosphate (CaP) composites containing 20 and 30 wt% mullite (3Al₂O₃·2SiO₂). In view of their potential application as load bearing implants, the present work reports the friction and wear properties of the newly developed composites against zirconia under dry ambient as well as in simulated body fluid (SBF) containing bovine serum albumin (BSA) protein. For comparison, experiments were also conducted on monolithic hydroxyapatite (HAp, Ca₁₀(PO₄)₆(OH)₂) and mullite under identical conditions. Under the investigated fretting conditions, the mullite-containing composites exhibited higher coefficient of friction (COF) of 0.4–0.6, compared to pure HAp (COF ~ 0.25–0.3). Although the wear resistance of the composites containing 20 or 30 wt% mullite was better in dry conditions, higher wear rate was measured in SBF conditions. The difference in tribological properties has been analyzed in reference to the difference in phase assemblage and mechanical properties. A comparison with some competing biomaterials reveals good potential of the investigated CaP-mullite composites for application as wear resistant implants.