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SiC structures tested by microtensile testing
NanoStluctured Materials, Vol. 6, pp. 897-g@& 1995 Copy@ Q 1995 Elsevia Science.Ltd Printed in the USA. All rights resened 096S-9773/95 $9.50 + .OO
09659773(95)002049
NANOLAYERED A1203/SiC STRUCTURES TESTED BY M ICROTENSILE TESTING R. Lappalainen, P. Toni and J.-P. Hirvonen* Dept. of Physics, P.O. Box 9, FIN-00014 University of Helsinki, Finland * VTT, Manufacturing Technology, FIN-02044 VTT, Finland Abstract Elastic properties of nanolayered A1203ISiC structures were examined using microtensile testingfor self supportedfiber specimensmade of thin films. Multilayered thin film structures (0.86 pm) of A1203 and Sic were prepared with physical vapor &position using e-gunfor evaporation and ion gunfor sputtering. The compositionand the purity of the sampleswere studied with RBS and nuclear reactions. Photolithography was used to prepare fher specimensfrom the thin films deposited.The stability of the multilayered structure and crystallization in heat treatment was studied by bachscattering,X-ray difiaction and SIMS. Mechanical properties such as hardness,elastic moaVulus and ultimate tensile strength values were compared with the sameparametersfor similar single componentfilms of A1203. The preliminary results show that the A1203ISiC multilayer structure is maintained at high temperatures with a nanocrystalline microstructure which results in the significant enhancementof mechanicalcharacteristicsas comparedto Al203 .
INTRODUCTION Alumina coatings have been increasingly used for tribological applications such as cutting tools, seal rings, bearings etc. due to their chemical inertness and high mechanical strength and good wear resistance at high temperatures.Alumina has several stable and metastablephasesincluding alpha (hexagonalcorundum), gamma (cubic spinel) and kappa (orthorhombic). When alumina films are depositedusing physical vapor depositon(PVD), the structure of as depositedfilms is typically amorphous.If theseamorphousfilms are annealed at temperatures around 1000 OC, the films crystallize and transform typically by route amorphous + gamma + theta+sigma+ alpha phase.During the early stagesof nucleation and grain growth the films are nanocrystallinewith a uniform grain size. However, the phase transformation to the alpha alumina leads to anomalous grain growth and the grain size distribution becomes bimodal consisting of fine gamma-aluminagrains and large (several micrometers) alpha-alumina grains. Due to this abnormal grain growth alumina has poor superplasticpropertiesand low fracture toughness.The mechanicalpropertiesof alumina can be significantly enhancedby the addition of submicron Sic particles as pointed out by Niiham [l]. By using a 5 8 Sic addition, fracture toughness was increased about 40 % and the strength from 380 to 1000 MPa. This improvement was explained by measurementsand a theoretical model for the microstressesin both of the componentsof the composite 123. 897
898
R LAPPALAINEN, P Tow ANDJ-P HIRVONEN
In this work, alumina/silicon carbide composite is studied in the form of multilayers deposited using physical vapor deposition (PVD). This layered structure is assumed to be stable at high temperaturesand should lead to enhancedmechanicalproperties as comparedto alumina films due to modified m icrostructureand internal stress.
EXPERIMENTAL
METHODS
AND
RESULTS
Multilayered thin film structures of alumina/silicon carbide were deposited on various substrates using electron beam evaporation for alumina and sputtering for silicon carbide, Substratesincluded silicon wafers, unalloyedcarbon steel, photoresistpatternedSi wafers and graphite. Prior to deposition the steel and graphite samples were mechanically polished and washedwith chemicals using standardpractice. Depositionson silicon and graphite were used to determine the composition and m icrostructure of the coatings using backscattering, XRD and SIMS. The patterned silicon wafers (31 were used to test thin film specimensin the form of free standing fibers. Coated steel specimenswere prepared for wear tests and oxidation StUdieS.
Deposition was carried out using the system illustrated in fig. 1. Prior to deposition a chamber was pumped down to a pressure of 1O-5Pa. Alumina was evaporatedfrom a high purity alumina source using e-gun with a stable deposition rate of 0.5 rim/s. Silicon carbide layers were deposited by sputtering with ion gun (750 eV Ar+ ions) from a high purity Sic target. The layered AlzO$SiC structure schematically illustrated in fig. 2 was prepared by cyclically passing the samples beneath the two deposition sources. The thicknesses were measuredusing calibrated thickness monitors and a nominal layer thickness of alumina and silicon carbide was 40 and 3 nm, respectively. The total thickness of the samples was 860 nm. Single layer alumina sampleswith the samethickness were depositedas well. AI,% 4Onm SIC 3nm
AI -ion gun
~ e-beam evaporator to vacuum pumps
LI
40 layers, total 860 nm
I
Figure 1. A schematicof a dual-sourcedeposition Figure 2. A schematicof a multilayer A1203/SiC thin film specimens. system for multilayers.
899
NANOLAYERED Ai20K3iC STRUCTURES TESTEDBYMICROTENSILE TESTING
TABLE 1 TensileTest ResultsFor Fiber Specimens E (( 660 A1201 93 ! I 7tx.P Al+#iC 88 I I I*) dtimw. tensilestrength.#I au = 1450Mpa for 10 pm wide specimens TABLE2 Vickers HardnessFor Thin Film SamplesOn Silicon 1OOOOCforlh 6OOOCforlh asdeposited 1030 1470 Al’LOIJ 1820 I 1980 , AlzOq/SiC 1290 1206 hardnm are mensuredwith a 10 g load and given in units of @ f/mm2
. I I
I
Both types of the sampleswere fit analysedwith 2 MeV He+ and 2.4 MeV H+ backscattering.Alumina and silicon carbidewere found to be stoichiometricand high purity containing’only minor amounts of impurities such as W , Cu and Fe. Fig. 3 shows a backscatteringspectrum for a A120$SiC structure deposited on a high purity graphite substrate.The simulationcalculatedusinga programGISA 141is includedand that agreeswell with the thicknessesmeasuredduring depositionandcompositionof the sourcematerials. Free standingfilm specimens(fibers) were prepared from the films on photoresist patternedsilicon wafersusing the lift-off processdevelopedin our earlier studies131.Tensile propertiesof the fibers were testedin a custombuilt microtensiletester.The specimenwas 8 m m long with a cross sectionof 10 or 50 pm x 0.86 pm in the 4 m m long gagesection.The shoulder region of a 10 pm wide A1203/SiC fiber is shown in fig. 4. The specimenis smooth and the lift-off process has worked well. Fig. 5 shows a stress-straincurve for a Al203/SiC specimenmeasuredat room temperaturewith a strain rate 10-4. The average elasticmodulusand ultimatetensilestrengthvaluesof severalspecimensare given in table 1.
Channel
number
Fig. 3. Backscatteringspectrum for a Al$$SiC sampleon graphite.
Fig. 4. S E M photographof a 10 pm wide Al@$SiC fiber specimen.
900
R ~PPAUINEN,
P Tom
AND
J-P HIRVONEN
One set of Al203 and A1203/SiCsamples 8M) on siliconand steelwere annealedfor one hour at temperaturesof 600, 800 and 1000 OC. These sampleswere used for structuralchacterization,o hardnessmeasurementsand wear tests.Vickers nma I hardnessvalues for the sampleson silicon are 6 given in table 2. After annealing, the - 440 composition of the sampleswas studied with E - 88.6 GPa backscatteringand SIMS measurements and the 6t microstructureof the sampleswas characterizedQ)2oo using XRD. It turned out that the composition 2 of both the Al203 and A1203/SiC samplesw remainedunchanged and the layeredstructurewas cI-I I I maintained.In both samplesAl& crystallizedat omo 0.005 0.010 0.0 =800 Oc and the y-phasebecameclearly visible True Stmin, t at 1000OC.It is assumedthat Sic remained amorphousalthough some peaks related to a Figure 5. A stress-straincurve for a certain nanophase were visible in a XRD Al203/SiC fiber specimenat RT. spectrumfor a 1000OCA1203/SiCsample. DISCUSSION
Multilayered A1203/SiCthin film sampleswith a good stoichiometryand purity were depositedusinga dual-sourcesystem.No significantdifferencein elasticmodulusand tensile strength values at RT for Al203 and Al203/SiC samplesof the samesize were observed (table 1). However,after annealingat high temperaturesthin film sampleson silicon showed a different behavior: Al203 films becamesofter vs. temperaturewhereasthe hardnessof A1203/SiCsamplesincreased50 % (table 2). Resultsfor aluminaagreewell with a general behavior of the ceramicsas they crystallize and grains grow. Al203/SiC results can be explained by the diffusion at the interfaces, internal stress change and the very fine microstructureevenat 1000OC. Basedon the preliminaryresults,especiallyat high temperaturesnanolayeredAl203/SiC structureshouldgive a significantimprovementas comparedto aluminain wearand oxidation resistanceand in superplasticpropertiesdue to a stablenanosixemicrostructute.Furthermore, the fibers with this structurehad a Weibull modulusof about 15 indicatinga good reliability. REFERENCES
1. 2. 3. 4.
K. Niihara and A. Nakahira,Ann. Chim. Fr. 16,479 (1991) I. L.evin,W .D. Kaplan,D.G. Brandonand T. W ieder,Acta metall.mater.a,1147 (1994). R. Lappalainenand R. Raj, Acta metallmater.3,3125(1991). J. Saarilahtiand E. Rauhala,Nucl. Instr. Meth. E&, 734(1992).
09659773(95)002049
NANOLAYERED A1203/SiC STRUCTURES TESTED BY M ICROTENSILE TESTING R. Lappalainen, P. Toni and J.-P. Hirvonen* Dept. of Physics, P.O. Box 9, FIN-00014 University of Helsinki, Finland * VTT, Manufacturing Technology, FIN-02044 VTT, Finland Abstract Elastic properties of nanolayered A1203ISiC structures were examined using microtensile testingfor self supportedfiber specimensmade of thin films. Multilayered thin film structures (0.86 pm) of A1203 and Sic were prepared with physical vapor &position using e-gunfor evaporation and ion gunfor sputtering. The compositionand the purity of the sampleswere studied with RBS and nuclear reactions. Photolithography was used to prepare fher specimensfrom the thin films deposited.The stability of the multilayered structure and crystallization in heat treatment was studied by bachscattering,X-ray difiaction and SIMS. Mechanical properties such as hardness,elastic moaVulus and ultimate tensile strength values were compared with the sameparametersfor similar single componentfilms of A1203. The preliminary results show that the A1203ISiC multilayer structure is maintained at high temperatures with a nanocrystalline microstructure which results in the significant enhancementof mechanicalcharacteristicsas comparedto Al203 .
INTRODUCTION Alumina coatings have been increasingly used for tribological applications such as cutting tools, seal rings, bearings etc. due to their chemical inertness and high mechanical strength and good wear resistance at high temperatures.Alumina has several stable and metastablephasesincluding alpha (hexagonalcorundum), gamma (cubic spinel) and kappa (orthorhombic). When alumina films are depositedusing physical vapor depositon(PVD), the structure of as depositedfilms is typically amorphous.If theseamorphousfilms are annealed at temperatures around 1000 OC, the films crystallize and transform typically by route amorphous + gamma + theta+sigma+ alpha phase.During the early stagesof nucleation and grain growth the films are nanocrystallinewith a uniform grain size. However, the phase transformation to the alpha alumina leads to anomalous grain growth and the grain size distribution becomes bimodal consisting of fine gamma-aluminagrains and large (several micrometers) alpha-alumina grains. Due to this abnormal grain growth alumina has poor superplasticpropertiesand low fracture toughness.The mechanicalpropertiesof alumina can be significantly enhancedby the addition of submicron Sic particles as pointed out by Niiham [l]. By using a 5 8 Sic addition, fracture toughness was increased about 40 % and the strength from 380 to 1000 MPa. This improvement was explained by measurementsand a theoretical model for the microstressesin both of the componentsof the composite 123. 897
898
R LAPPALAINEN, P Tow ANDJ-P HIRVONEN
In this work, alumina/silicon carbide composite is studied in the form of multilayers deposited using physical vapor deposition (PVD). This layered structure is assumed to be stable at high temperaturesand should lead to enhancedmechanicalproperties as comparedto alumina films due to modified m icrostructureand internal stress.
EXPERIMENTAL
METHODS
AND
RESULTS
Multilayered thin film structures of alumina/silicon carbide were deposited on various substrates using electron beam evaporation for alumina and sputtering for silicon carbide, Substratesincluded silicon wafers, unalloyedcarbon steel, photoresistpatternedSi wafers and graphite. Prior to deposition the steel and graphite samples were mechanically polished and washedwith chemicals using standardpractice. Depositionson silicon and graphite were used to determine the composition and m icrostructure of the coatings using backscattering, XRD and SIMS. The patterned silicon wafers (31 were used to test thin film specimensin the form of free standing fibers. Coated steel specimenswere prepared for wear tests and oxidation StUdieS.
Deposition was carried out using the system illustrated in fig. 1. Prior to deposition a chamber was pumped down to a pressure of 1O-5Pa. Alumina was evaporatedfrom a high purity alumina source using e-gun with a stable deposition rate of 0.5 rim/s. Silicon carbide layers were deposited by sputtering with ion gun (750 eV Ar+ ions) from a high purity Sic target. The layered AlzO$SiC structure schematically illustrated in fig. 2 was prepared by cyclically passing the samples beneath the two deposition sources. The thicknesses were measuredusing calibrated thickness monitors and a nominal layer thickness of alumina and silicon carbide was 40 and 3 nm, respectively. The total thickness of the samples was 860 nm. Single layer alumina sampleswith the samethickness were depositedas well. AI,% 4Onm SIC 3nm
AI -ion gun
~ e-beam evaporator to vacuum pumps
LI
40 layers, total 860 nm
I
Figure 1. A schematicof a dual-sourcedeposition Figure 2. A schematicof a multilayer A1203/SiC thin film specimens. system for multilayers.
899
NANOLAYERED Ai20K3iC STRUCTURES TESTEDBYMICROTENSILE TESTING
TABLE 1 TensileTest ResultsFor Fiber Specimens E (( 660 A1201 93 ! I 7tx.P Al+#iC 88 I I I*) dtimw. tensilestrength.#I au = 1450Mpa for 10 pm wide specimens TABLE2 Vickers HardnessFor Thin Film SamplesOn Silicon 1OOOOCforlh 6OOOCforlh asdeposited 1030 1470 Al’LOIJ 1820 I 1980 , AlzOq/SiC 1290 1206 hardnm are mensuredwith a 10 g load and given in units of @ f/mm2
. I I
I
Both types of the sampleswere fit analysedwith 2 MeV He+ and 2.4 MeV H+ backscattering.Alumina and silicon carbidewere found to be stoichiometricand high purity containing’only minor amounts of impurities such as W , Cu and Fe. Fig. 3 shows a backscatteringspectrum for a A120$SiC structure deposited on a high purity graphite substrate.The simulationcalculatedusinga programGISA 141is includedand that agreeswell with the thicknessesmeasuredduring depositionandcompositionof the sourcematerials. Free standingfilm specimens(fibers) were prepared from the films on photoresist patternedsilicon wafersusing the lift-off processdevelopedin our earlier studies131.Tensile propertiesof the fibers were testedin a custombuilt microtensiletester.The specimenwas 8 m m long with a cross sectionof 10 or 50 pm x 0.86 pm in the 4 m m long gagesection.The shoulder region of a 10 pm wide A1203/SiC fiber is shown in fig. 4. The specimenis smooth and the lift-off process has worked well. Fig. 5 shows a stress-straincurve for a Al203/SiC specimenmeasuredat room temperaturewith a strain rate 10-4. The average elasticmodulusand ultimatetensilestrengthvaluesof severalspecimensare given in table 1.
Channel
number
Fig. 3. Backscatteringspectrum for a Al$$SiC sampleon graphite.
Fig. 4. S E M photographof a 10 pm wide Al@$SiC fiber specimen.
900
R ~PPAUINEN,
P Tom
AND
J-P HIRVONEN
One set of Al203 and A1203/SiCsamples 8M) on siliconand steelwere annealedfor one hour at temperaturesof 600, 800 and 1000 OC. These sampleswere used for structuralchacterization,o hardnessmeasurementsand wear tests.Vickers nma I hardnessvalues for the sampleson silicon are 6 given in table 2. After annealing, the - 440 composition of the sampleswas studied with E - 88.6 GPa backscatteringand SIMS measurements and the 6t microstructureof the sampleswas characterizedQ)2oo using XRD. It turned out that the composition 2 of both the Al203 and A1203/SiC samplesw remainedunchanged and the layeredstructurewas cI-I I I maintained.In both samplesAl& crystallizedat omo 0.005 0.010 0.0 =800 Oc and the y-phasebecameclearly visible True Stmin, t at 1000OC.It is assumedthat Sic remained amorphousalthough some peaks related to a Figure 5. A stress-straincurve for a certain nanophase were visible in a XRD Al203/SiC fiber specimenat RT. spectrumfor a 1000OCA1203/SiCsample. DISCUSSION
Multilayered A1203/SiCthin film sampleswith a good stoichiometryand purity were depositedusinga dual-sourcesystem.No significantdifferencein elasticmodulusand tensile strength values at RT for Al203 and Al203/SiC samplesof the samesize were observed (table 1). However,after annealingat high temperaturesthin film sampleson silicon showed a different behavior: Al203 films becamesofter vs. temperaturewhereasthe hardnessof A1203/SiCsamplesincreased50 % (table 2). Resultsfor aluminaagreewell with a general behavior of the ceramicsas they crystallize and grains grow. Al203/SiC results can be explained by the diffusion at the interfaces, internal stress change and the very fine microstructureevenat 1000OC. Basedon the preliminaryresults,especiallyat high temperaturesnanolayeredAl203/SiC structureshouldgive a significantimprovementas comparedto aluminain wearand oxidation resistanceand in superplasticpropertiesdue to a stablenanosixemicrostructute.Furthermore, the fibers with this structurehad a Weibull modulusof about 15 indicatinga good reliability. REFERENCES
1. 2. 3. 4.
K. Niihara and A. Nakahira,Ann. Chim. Fr. 16,479 (1991) I. L.evin,W .D. Kaplan,D.G. Brandonand T. W ieder,Acta metall.mater.a,1147 (1994). R. Lappalainenand R. Raj, Acta metallmater.3,3125(1991). J. Saarilahtiand E. Rauhala,Nucl. Instr. Meth. E&, 734(1992).