Matjaž Gams Bo štjan Kaluža, Erik Dovgan.. +10 Jožef Stefan institute, Slovenia
1 Jožef Stefan Institute, Slovenia 2 Center for tribology and technical diagnostics, University of...
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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- Properties and wear behavior of step- graded alumina graded alumina - - ZTA composites ZTA composites S. Novak 1 , M. Kalin 2 , S. Beranič 1 , P. Lukas 3 , G. Anne 4 , O. Van Der Biest 4 INTRODUCTION 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 AIM Effect of the stresses on the wear behavior of step-graded alumina-ZTA composites in comparison to alumina EXPERIMENTAL 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 PROFILE STRESS PROFILE SURFACE SURFACE Neutron diffraction STRESSES STRESSES: X-ray diffraction WEAR ANALYSIS Reciprocating sliding - Stroke 7 mm - frequency 1 Hz - water bath (pH 6.8) - 100 m (2h) RESULTS RESULTS Acknowledgement: Acknowledgement: The work was performed within the framework of the project BIOGRAD, supported by the European Commission (contract No. G5RD-CT2000-00354) 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. and the Ministry of High Education, Science and Technology of the Republic of Slovenia. Mrs. N. Petkovič, dipl. i i ng. is acknowledged for her valuable contribution in the work. ng. is acknowledged for her valuable contribution in the work. Step-graded disc (3 layer) Step-graded disc (3 layer) Step-graded disc (7 Step-graded disc (7 layers) layers) CONCLUSIONS CONCLUSIONS 50 N WEAR WOLUME WEAR WOLUME COEFFICIENT OF FRICTION COEFFICIENT OF FRICTION Stress profile across ball- head sample after machining and polishing -300 -200 -100 0 100 200 300 0 1 2 3 4 5 6 7 8 9 distance (m m) S tress (M P a) Polished cross-section (SEM) Step-graded ball-head (7 Step-graded ball-head (7 layers) 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 0 0.1 0.2 0.3 0.4 0.5 0.6 0 175 241 260 R esidual C om pressive Stress (M Pa) C oefficient of friction 0 0.001 0.002 0.003 0.004 0.005 0.006 0 175 241 260 Residual Com pressive Stress (M Pa) W earvolum e (m m3 Alumina (A) Alumina (A) : ~0 MPa A-ZTA-A: A-ZTA-A: Sample 1-3: - 175 175 MPa MPa Sample 2-3: - 260 260 MPa MPa Sample 1-7: - 241 241 MPa MPa Al 2 O 3 Step-graded Step-graded Al 2 O 3 Compressive Compressive stresses stresses at the surface at the surface Hidden cracks in ZTA core Polishe Polishe d d surface surface Worn Worn surface surface 10 m Worn Worn surface surface Polishe Polishe d d surface surface 10 m 2 m 2 m
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Transcript of 1 Jožef Stefan Institute, Slovenia 2 Center for tribology and technical diagnostics, University of...
1 Jožef Stefan Institute, Slovenia2 Center for tribology and technical diagnostics, University of Ljubljana, Slovenia3 Nuclear Physics Institute, Czech Republic4 Katholieke Universiteit Leuven, Belgium
Properties and wear behavior of step-graded Properties and wear behavior of step-graded aluminaalumina -- ZTA compositesZTA composites
S. Novak1, M. Kalin2, S. Beranič1, P. Lukas3, G. Anne4, O. Van Der Biest4
INTRODUCTIONINTRODUCTION 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.
AIMAIM Effect of the stresses on the wear behavior of step-graded alumina-ZTA composites in comparison to alumina
EXPERIMENTALEXPERIMENTALSAMPLES PREPARATION
-well dispersed suspensions (attritor)-sequential slip-casting (plaster)-Controlled drying in wet atmosphere-Sintering at 1550°C, 2h-Polishing
STRESS ANALYSIS
STRESS PROFILESTRESS PROFILE SURFACESURFACE Neutron diffraction STRESSESSTRESSES: X-ray diffraction
WEAR ANALYSIS
Reciprocating sliding - Stroke 7 mm- frequency 1 Hz- water bath (pH 6.8)- 100 m (2h)
RESULTSRESULTS
Acknowledgement:Acknowledgement: The work was performed within the framework of the project BIOGRAD, supported by the European Commission (contract No. G5RD-CT2000-00354) 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. and the Ministry of High Education, Science and Technology of the Republic of Slovenia. Mrs. N. Petkovič, dipl. iing. is acknowledged for her valuable contribution in the work.ng. is acknowledged for her valuable contribution in the work.
Step-graded disc (3 layer) Step-graded disc (3 layer) Step-graded disc (7 layers)Step-graded disc (7 layers)
CONCLUSIONSCONCLUSIONS
50 N
WEAR WOLUMEWEAR WOLUME COEFFICIENT OF FRICTIONCOEFFICIENT OF FRICTION
Stress profile across ball-head sample after machining and polishing
-300
-200
-100
0
100
200
300
0 1 2 3 4 5 6 7 8 9
distance (mm)
Str
ess
(MP
a)
Polished cross-section (SEM)
Step-graded ball-head (7 layers)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
0
0.1
0.2
0.3
0.4
0.5
0.6
0 175 241 260
Residual Compressive Stress (MPa)
Coeffi
cient o
f frict
ion
0
0.001
0.002
0.003
0.004
0.005
0.006
0 175 241 260
Residual Compressive Stress (MPa)
Wea
r vo
lum
e (m
m3)
Alumina (A)Alumina (A): ~0 MPa
A-ZTA-A:A-ZTA-A:
Sample 1-3: -175 MPa175 MPa
Sample 2-3: -260 MPa260 MPa
Sample 1-7: -241 MPa241 MPa
Al2O3 Step-graded
Step-graded
Al2O3
Compressive Compressive stresses stresses
at the surfaceat the surface
Hidden cracksin ZTA core
Polished Polished surfacesurface
Worn surfaceWorn surface
10 m
Worn surfaceWorn surfacePolished Polished surfacesurface
10 m
2 m 2 m
