1. 1 Jožef Stefan Institute, Slovenia 2 Center for tribology and technical diagnostics, University of Ljubljana, Slovenia 3 Nuclear Physics Institute, Czech Republic 4 Katholieke Universiteit Leuven, Belgium Properties and wear behavior of step-graded alumina - ZTA composites S. Novak 1, M. Kalin 2, S. Beranič 1, P. Lukas 3, G. Anne 4, O. Van Der Biest 4 INTRODUCTION Functionally graded materials (FGM) provide a reasonable compromise in terms of the properties of materials that would not be possible to achieve otherwise. The variation of the composition from a tough zirconia-rich core to a hard, chemically and wear-resistant surface layer of alumina, opens a wide range of applications where ceramics retain all the advantages of the particular materials thereby providing very valuable synergy. In ceramic hip-joints with graded composition, increased burst strength and reliability may be expected. Further, from differential shrinkage due to sintering and thermal contraction of the ZTA core and alumina surface layer, residual compressive stresses result at the surface, that may affect the wear behavior of the composite. Functionally graded composites with ZTA core and alumina surface layer with three to seven layers were prepared by sequential slip-casting of aqueous suspensions with compositions enabling minimal mismatch in the shrinkage between the layers. The resulting residual compressive stresses beneficially effect wear resistance of the FGM, while tensile stresses in the zirconia-containing core may cause formation of hidden cracks in large components. Both, the wear and the friction coefficient decrease with an increase in the compressive stress level. The wear mechanism in FGM was found to be the same as for the homogeneous alumina. In water (pH 6.8) a thin tribolayer is formed that protects the alumina from severe wear. The observed beneficial effect can be ascribed to reduction of crack nucleation and propagation as a result of compressive stresses at the outermost alumina layers, which are generated in combination with a tensile-stressed ZTA core. AIM Effect of the stresses on the wear behavior of step-graded alumina-ZTA composites in comparison to alumina EXPERIMENTAL SAMPLES PREPARATION - well dispersed suspensions (attritor) -sequential slip-casting (plaster) -Controlled drying in wet atmosphere -Sintering at 1550°C, 2h -Polishing STRESS ANALYSIS STRESS PROFILESURFACE STRESSES STRESS PROFILE SURFACE Neutron diffraction STRESSES: X-ray diffraction WEAR ANALYSIS Reciprocating sliding - Stroke 7 mm - frequency 1 Hz - water bath (pH 6.8) - 100 m (2h) RESULTS Acknowledgement: The work was performed within the framework of the project BIOGRAD, supported by the European Commission (contract No. G5RD-CT2000-00354) and the Ministry of High Education, Science and Technology of the Republic of Slovenia. Mrs. N. Petkovič, dipl. ing. is acknowledged for her valuable contribution in the work. Step-graded disc (3 layer) Step-graded disc (7 layers) CONCLUSIONS 50 N WEAR WOLUME COEFFICIENT OF FRICTION Stress profile across ball-head sample after machining and polishing Polished cross-section (SEM) Step-graded ball-head (7 layers) SEM micrographs of the worn surface of homogeneous alumina sample Worn alumina surface with compressive stress 260 MPa at the surface of A-ZTA-A sample Alumina (A) Alumina (A): ~0 MPaA-ZTA-A: 175 MPa Sample 1-3: -175 MPa 260 MPa Sample 2-3: -260 MPa 241 MPa Sample 1-7: -241 MPa Al 2 O 3 Step-graded Al 2 O 3Compressivestresses at the surface Hidden cracks in ZTA core Polished surface Worn surface 10  m Worn surface Polished surface 10  m 2  m