The wear mechanisms of a 3% mol yttria stabilized tetragonal zirconia ceramics (TZP)/52100 steel reciprocating sliding couple under water lubrication were investigated in this paper. The wide ranges of normal loads and sliding speeds were chosen in order to study the relationship between the tribological performance of this sliding couple and the testing conditions. Different wear regions were identified as a function of normal load and speed. The wear surfaces were carefully examined by scanning electron microscopy (SEM). The mild wear and severe wear of TZP ceramics are respectively related to plastic deformation and surface fracture. The mild and severe wear of a 52100 steel ball are respectively related to slight polishing and abrasive wear. At very high normal load and low sliding speed, TZP ceramics experience negative wear due to steel transfer caused by the strong adherence between TZP ceramics and the 52100 steel ball. The wear mechanisms of TZP ceramics and 52100 steel under different sliding conditions are illustrated by their respective wear transition diagrams.Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996