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Properties and wear behavior of step-graded
alumina - ZTA composites
S. Novak1, M. Kalin2, S. Beranic1, P. Lukas3, G.
Anne4, O. Van Der Biest4
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
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.
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 1550C, 2h
• Polishing

STRESS ANALYSIS 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
COEFFICIENT OF FRICTION
WEAR WOLUME
Step-graded disc (7 layers)
Step-graded disc (3 layer)
Step-graded ball-head (7 layers)
Worn alumina surface with compressive stress 260
MPa at the surface of A-ZTA-A sample
SEM micrographs of the worn surface of
homogeneous alumina sample
Polished cross-section (SEM)
Stress profile across ball-head sample after
machining and polishing
Hidden cracksin ZTA core
CONCLUSIONS
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.
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. Petkovic, dipl. ing. is
acknowledged for her valuable contribution in the
work.