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Nanocrystalline iron layers produced by the pulse plasma method
NanoStruchuedMatuials. Vol. 8. No. 6, pi. 725730.1997 Elsevia Science.Ltd @ 1997 Acta MetallurgicaInc. Printedin the USA. All rights reserved 0965-9773197 $17.00 + .OO
Pergamon
PII SO9659773(97)00223-7
NANOCRYSTALLINE IRON LAYERS PRODUCED PULSE PLASMA METHOD
BY THE
A. Michalski Warsaw 1Jniversityof Technology, Department of Material Science and Engineering, 02-524 Warsaw, Poland {AcceptedJuly 25,1997) Abstract - Using crystallization from the gaseous phase, layers with a nanocrystalline structure and stronglydistortedcrystallinelatticewere obtained. The layers were deposited at temperatureswithinthe rangefrom 373 to523 K. The layersproduced at 373 K were composed of the two phases: Fea and Fey. When deposited at higher temperatures, the layers were monophase and containedthe Feaphase alone. Depending on the temperatureof the substrate, the hardness of the layers was greater by a factor of 4.8 to 2.5 than that of iron with a coarse-grained structure. 0 1997 Acta MetallurgicaInc. INTRODUCTION Many investigators (l-3) have reported that, compared to conventional materials, the materials of ultrafine gmined structure, in which the grain size is below 100 nm, show a greater mechanical strength, a higher hardness and an increased resistance to brittle fracture. Fougere et al. (4) examined sinters of Fe grains 4- 15 nm in size. The hardness of these sinters was 3 to 7 times greater than that of iron with the 110 pm grain size. The increase in hardness by a factor of 2 to 10 was also observed by Jang and Koch (5) in iron flakes with a nanocrystalline structure produced by the ball milling technique. The present paper discusses the results of examinations of nanocrystalline iron layers produced by pulse plasma deposition (PPD). The PPD methoddiffers from other plasma assisted processes in that the crystallization from the gaseous phaseand the growth of the layer proceeds under specific conditions. The metal vapors from which the layer is synthesized are obtained by evaporating the material of an electrode placed in a high-current pulse discharge (the pulse duration t c 100 ps, the current magnitude - 100 kA). The vapors thus produced, in the form of a plasma pack, are accelerated towards the substrate by an electrodynamic force at a velocity of about 10km/s. Under these conditions, the layer grows from portions: of ionized metal vapor (plasma packs) (6-8). The plasma pack incident upon the substrate during the growth of the layer plays a double role. It is a pulsed high energy source of heat that heats up the substrate and a source of the metal vapors from which the layer nucleates and grows on the isubstrate. Because of the pulse character of the plasma-substrate interaction (t c 100 ps) and the incomparably long pauses between the consecutive plasma pulses (from 1 to several 725
726
A MICHALSK~
II
*
I
/ j j / -___-_.j . . . . . . .._.__._..... i.‘._ . . . p. . ..I.... _ .._.._. j. i .... .._... ------..---; -i --~----.i---.-.---.~-.-.---..t i
--cITem~erst&eof~~ ,,
0 Ii
;
,,
; lOO@,
7‘
t=1+1oo
>
Tim8 Figure 1.Diagram showing the variation of the temperature of the substrate surface during the PPD deposition process. seconds), the heat state on the substrate varies considerably (Figure 1). During the plasma action, the substrate surface and the near-surfaceregion to a depth of the order of several micrometers are heated to a temperature of about 2000 K (9), whereas the temperature of the substrate, settled as a result of the action of the successive pulses of the plasma packs, is constant and substantially lower. This substrate temperature depends on the energy of the pulse discharge, the repetition frequency of the pulse discharges and the intensity of heat removal from the substrate.
EXPERIMENTAL DETAILS The iron layers were deposited in a hydrogen atmosphere at a substrate temperature within the range from 373 to 873 K using the PPD apparatusdescribed in (7). Table1gives the deposition process parameters. The layers were deposited at a constant energy of the pulse discharge using an iron electrode (of chemical composition given in Table2) as the source of the vapors of which the layer grows. 20 pm thick layers were deposited on iron samples 40 mm in diameter. The of the substratewas obtained deposition rate was about 0.01pm per pulse. The desired temperature by adjusting appropriately the frequency of the pulse discharges. ln the layers thus obtained we examined the phase composition, the chemical composition, the hardness and the structure (microscopically). The phase composition was determined using aPhilips PW-170diffractometer,and the chemical composition by the atomic emission spectrometry using a PGS-2 spectrograph. The contents of carbon and sulfur were determined with LECO analyzers. The hardness was measured using a Vicker ‘s penetrator under a load of 50 G. The size
NANOCZRYSTAUINE IRON LAYERSPRODUCEDBY THE PULSE PLASMAMETHOD
727
TABLE 1 Parameters of the PPD Process Energy Released in a Pulse kJ1
Pulse.Repetition Frequency [see’]
Substrate Temperature
2.5~ 2.5 2.5
0.1 0.2 0.5
313 423 523
Pressure
[PaI
El 100 100 100
of the crystal&s in tbe iron layers and the average percent of the lattice distortions in them were determined by the Williamson and Hall method (10). The calculations were based on the diffraction refllexes for the (110) and (220) planes. The Kal radiation component was found analytically by approximating the Ka diffraction line profile using the Gauss function.
RESULTS AND DISCUSSION Figure 2 shows a microphotograph of the iron layer deposited on an iron substrate preanneakd at a temperature of 873K. Table 2 compares the chemical composition of the iron layers produced by the pulse plasma technique with the chemical composition of the material of the electrode used for producing these layers. We can see that the chemical composition of the layers does not differ significantly from that of the electrode material: the only differences are the slightly increased (by about 0.01%) carbon content and the reduced contents of Si, Mn and S. Figure 3 shows the KRD patterns obtained for the iron layers deposited at temperatures of 373 - 573 K. The KRD examinations have shown that the phase composition of the layers depends on the temperature of the substrate. The layers produced at 3’73K contained Fea iron and Fey iron. The Fey content decreases with increasing substrate temperature so that the layers deposited at a temperatureabove423 K have a single-phase composition tlhat corresponds to the Fea phase. In the layers deposited at 373 K, the amount of Fey estimated from the proportion of the intensities ofFeyandFea diffraction lines was 23%. The TABLE 2 Chemical Composition of Electrode and Layer Materials Chemical composition [wt%] Fe
C
Mn
Si
P
S
CU
Electrode Material
99.74
0.03
0.15
0.02
0.01
0.02
0.03
Layer Material
99.77
0.04
0.13
0.01
0.01
0.01
0.03
A MICHALSKI
728
Figure 2. Microphotograph of the cross-section of the iron layer deposited on an iron, after etching by 2% HN03.
?=373K
60.00
60.00
70.00
80.00
Ztheta
Figure 3. XFUI patterns for the layers deposited at various temperatures.
NANOCRYSTALLINE IGON IAYEFS PRODUCEDBY THE PUS
PLASMAMETHOD
373
423
523
373
423
523
729
Tempemt~ of the substrateW]
Figure 4. Average crystallite sizes and percent of the lattice distortions in layers deposited at various temperatures. 800
800
700
B 6
600
is
500 4#0 300 200
100 0 373 K
423 K
Teqemtu~ofthesu~
523 K
Iron
at 873 K
Figure 5. Hardness of the layers deposited at various substrate temperatures. presence of the metastable phase Fey in the layers deposited at 373 K indicates that the growing layers are cooled down fkoma temperature within the range from 1184to 1666 IS,where the Fey phase is stable, at a very high rate. At higher substrate~rn~~~es, F~underg~ ~sfo~~on
730
A
MICC~ALSKI
into the stableFea phase. These observationsare consistentwith the results reported by Hayakawa and Iwama (1l), who examined nanocrystallinepowders that crystallized from the gaseous phase and were cooled down from a temperature within the range of stability of the Fey phase at a very high rate. The powders obtained under these conditions contained about 40 % of the Fey phase. Figure 4 shows the calculated values of the average size of the crystallites and the percent of the average lattice distortions in the iron layers deposited at tern~~t~~ between 373 and 523 K. We can see that these values depend on the temperature of the substram. As the substrate temperature increases, the average size of the crystal&s increases from 1.4 nm in the layers deposited at 373K to 12.3 nm in the layers deposited at 523 K. The average lattice distortions decrease with increasing temperature. They are 6.3%in the layersdeposited at 373 K and decrease to 0.7% in those deposited at 523 K. Figure 5 shows the hardnesses of the layers deposited at various subs~te tern~~t~. A maximum hanhtess of 800 HV is exhibited by the layers deposited at a temperature of 373 K, in which the average size of the crystallites is 1.4 nm. The hardness of the layers deposited at 523 K decreases to 390 HV,and this is accompanied by an increase of the average crystallites size to 12.3nm. The observed changes in the hardnessof the layers am in agreement with the Hall-Petch theory, which describes the variation of the hardness of metals depending on their grain size. CONCLUSIONS The PPD layers have a nanocrystallinestructure and a strongly distorted crystalline lattice. The average size of their crystallites and the percent of the lattice distortions depend on the temperature of the substrate. The layers deposited at a ~rn~~~e of 373 K are composed of the two phases Fea and Fey. At higher temperatures,the layersare single-phaseand only contain Fea. An increase in the substrate temperature decreases the percent of the lattice distortions and increases the size of the crystallites, but decreases the hardness. Depending on the substrate temperature, the hardness of the PPD layers is greaterby a factor of 4.8 to 2.5 than that of iron with a coarse-grained structure. ACKNOWLEffiMENTS This work was supportedby the CommitteeforScientificResearchby GrantNo. 3P40704007. ‘REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Yamagchi, M., Mater~~s Science and ~chnofo~, 1989,24,1599. Siegel, R.W., Annual Review in Materials Science, 1991, 21,559. Gell, M., Materials Science and Engineering, 1995, A204,246. Fougere, G.E., Weertman, J.R. and Siegel, R.W.,Nanostructured Materials, 1995,5,127. Jang, J&C. and Koch, CC., Scripta Metallurgica and Materialia, 1990,24,1599. Mieh~~, A., Sokolowska, A., Jour~l of~ateri~s Science, 1985,20,1842. Miehalski, A., Olszyna, A., Sz@ace Coating Technology, 1993,59,287. Michalski, A., Journal of Materials Science Letters, 1991,10,503. Romanowski, Z., Wronikowski, M., Journal of Materials Science, 1992,27,2619. Williammson, G.K., Hall, W.H., Acta Metallurgica, 1953, 1,22. Hayakawa, K. and Iwama, S., Journal of Crystal Growth, 1990,99,188.
Pergamon
PII SO9659773(97)00223-7
NANOCRYSTALLINE IRON LAYERS PRODUCED PULSE PLASMA METHOD
BY THE
A. Michalski Warsaw 1Jniversityof Technology, Department of Material Science and Engineering, 02-524 Warsaw, Poland {AcceptedJuly 25,1997) Abstract - Using crystallization from the gaseous phase, layers with a nanocrystalline structure and stronglydistortedcrystallinelatticewere obtained. The layers were deposited at temperatureswithinthe rangefrom 373 to523 K. The layersproduced at 373 K were composed of the two phases: Fea and Fey. When deposited at higher temperatures, the layers were monophase and containedthe Feaphase alone. Depending on the temperatureof the substrate, the hardness of the layers was greater by a factor of 4.8 to 2.5 than that of iron with a coarse-grained structure. 0 1997 Acta MetallurgicaInc. INTRODUCTION Many investigators (l-3) have reported that, compared to conventional materials, the materials of ultrafine gmined structure, in which the grain size is below 100 nm, show a greater mechanical strength, a higher hardness and an increased resistance to brittle fracture. Fougere et al. (4) examined sinters of Fe grains 4- 15 nm in size. The hardness of these sinters was 3 to 7 times greater than that of iron with the 110 pm grain size. The increase in hardness by a factor of 2 to 10 was also observed by Jang and Koch (5) in iron flakes with a nanocrystalline structure produced by the ball milling technique. The present paper discusses the results of examinations of nanocrystalline iron layers produced by pulse plasma deposition (PPD). The PPD methoddiffers from other plasma assisted processes in that the crystallization from the gaseous phaseand the growth of the layer proceeds under specific conditions. The metal vapors from which the layer is synthesized are obtained by evaporating the material of an electrode placed in a high-current pulse discharge (the pulse duration t c 100 ps, the current magnitude - 100 kA). The vapors thus produced, in the form of a plasma pack, are accelerated towards the substrate by an electrodynamic force at a velocity of about 10km/s. Under these conditions, the layer grows from portions: of ionized metal vapor (plasma packs) (6-8). The plasma pack incident upon the substrate during the growth of the layer plays a double role. It is a pulsed high energy source of heat that heats up the substrate and a source of the metal vapors from which the layer nucleates and grows on the isubstrate. Because of the pulse character of the plasma-substrate interaction (t c 100 ps) and the incomparably long pauses between the consecutive plasma pulses (from 1 to several 725
726
A MICHALSK~
II
*
I
/ j j / -___-_.j . . . . . . .._.__._..... i.‘._ . . . p. . ..I.... _ .._.._. j. i .... .._... ------..---; -i --~----.i---.-.---.~-.-.---..t i
--cITem~erst&eof~~ ,,
0 Ii
;
,,
; lOO@,
7‘
t=1+1oo
>
Tim8 Figure 1.Diagram showing the variation of the temperature of the substrate surface during the PPD deposition process. seconds), the heat state on the substrate varies considerably (Figure 1). During the plasma action, the substrate surface and the near-surfaceregion to a depth of the order of several micrometers are heated to a temperature of about 2000 K (9), whereas the temperature of the substrate, settled as a result of the action of the successive pulses of the plasma packs, is constant and substantially lower. This substrate temperature depends on the energy of the pulse discharge, the repetition frequency of the pulse discharges and the intensity of heat removal from the substrate.
EXPERIMENTAL DETAILS The iron layers were deposited in a hydrogen atmosphere at a substrate temperature within the range from 373 to 873 K using the PPD apparatusdescribed in (7). Table1gives the deposition process parameters. The layers were deposited at a constant energy of the pulse discharge using an iron electrode (of chemical composition given in Table2) as the source of the vapors of which the layer grows. 20 pm thick layers were deposited on iron samples 40 mm in diameter. The of the substratewas obtained deposition rate was about 0.01pm per pulse. The desired temperature by adjusting appropriately the frequency of the pulse discharges. ln the layers thus obtained we examined the phase composition, the chemical composition, the hardness and the structure (microscopically). The phase composition was determined using aPhilips PW-170diffractometer,and the chemical composition by the atomic emission spectrometry using a PGS-2 spectrograph. The contents of carbon and sulfur were determined with LECO analyzers. The hardness was measured using a Vicker ‘s penetrator under a load of 50 G. The size
NANOCZRYSTAUINE IRON LAYERSPRODUCEDBY THE PULSE PLASMAMETHOD
727
TABLE 1 Parameters of the PPD Process Energy Released in a Pulse kJ1
Pulse.Repetition Frequency [see’]
Substrate Temperature
2.5~ 2.5 2.5
0.1 0.2 0.5
313 423 523
Pressure
[PaI
El 100 100 100
of the crystal&s in tbe iron layers and the average percent of the lattice distortions in them were determined by the Williamson and Hall method (10). The calculations were based on the diffraction refllexes for the (110) and (220) planes. The Kal radiation component was found analytically by approximating the Ka diffraction line profile using the Gauss function.
RESULTS AND DISCUSSION Figure 2 shows a microphotograph of the iron layer deposited on an iron substrate preanneakd at a temperature of 873K. Table 2 compares the chemical composition of the iron layers produced by the pulse plasma technique with the chemical composition of the material of the electrode used for producing these layers. We can see that the chemical composition of the layers does not differ significantly from that of the electrode material: the only differences are the slightly increased (by about 0.01%) carbon content and the reduced contents of Si, Mn and S. Figure 3 shows the KRD patterns obtained for the iron layers deposited at temperatures of 373 - 573 K. The KRD examinations have shown that the phase composition of the layers depends on the temperature of the substrate. The layers produced at 3’73K contained Fea iron and Fey iron. The Fey content decreases with increasing substrate temperature so that the layers deposited at a temperatureabove423 K have a single-phase composition tlhat corresponds to the Fea phase. In the layers deposited at 373 K, the amount of Fey estimated from the proportion of the intensities ofFeyandFea diffraction lines was 23%. The TABLE 2 Chemical Composition of Electrode and Layer Materials Chemical composition [wt%] Fe
C
Mn
Si
P
S
CU
Electrode Material
99.74
0.03
0.15
0.02
0.01
0.02
0.03
Layer Material
99.77
0.04
0.13
0.01
0.01
0.01
0.03
A MICHALSKI
728
Figure 2. Microphotograph of the cross-section of the iron layer deposited on an iron, after etching by 2% HN03.
?=373K
60.00
60.00
70.00
80.00
Ztheta
Figure 3. XFUI patterns for the layers deposited at various temperatures.
NANOCRYSTALLINE IGON IAYEFS PRODUCEDBY THE PUS
PLASMAMETHOD
373
423
523
373
423
523
729
Tempemt~ of the substrateW]
Figure 4. Average crystallite sizes and percent of the lattice distortions in layers deposited at various temperatures. 800
800
700
B 6
600
is
500 4#0 300 200
100 0 373 K
423 K
Teqemtu~ofthesu~
523 K
Iron
at 873 K
Figure 5. Hardness of the layers deposited at various substrate temperatures. presence of the metastable phase Fey in the layers deposited at 373 K indicates that the growing layers are cooled down fkoma temperature within the range from 1184to 1666 IS,where the Fey phase is stable, at a very high rate. At higher substrate~rn~~~es, F~underg~ ~sfo~~on
730
A
MICC~ALSKI
into the stableFea phase. These observationsare consistentwith the results reported by Hayakawa and Iwama (1l), who examined nanocrystallinepowders that crystallized from the gaseous phase and were cooled down from a temperature within the range of stability of the Fey phase at a very high rate. The powders obtained under these conditions contained about 40 % of the Fey phase. Figure 4 shows the calculated values of the average size of the crystallites and the percent of the average lattice distortions in the iron layers deposited at tern~~t~~ between 373 and 523 K. We can see that these values depend on the temperature of the substram. As the substrate temperature increases, the average size of the crystal&s increases from 1.4 nm in the layers deposited at 373K to 12.3 nm in the layers deposited at 523 K. The average lattice distortions decrease with increasing temperature. They are 6.3%in the layersdeposited at 373 K and decrease to 0.7% in those deposited at 523 K. Figure 5 shows the hardnesses of the layers deposited at various subs~te tern~~t~. A maximum hanhtess of 800 HV is exhibited by the layers deposited at a temperature of 373 K, in which the average size of the crystallites is 1.4 nm. The hardness of the layers deposited at 523 K decreases to 390 HV,and this is accompanied by an increase of the average crystallites size to 12.3nm. The observed changes in the hardnessof the layers am in agreement with the Hall-Petch theory, which describes the variation of the hardness of metals depending on their grain size. CONCLUSIONS The PPD layers have a nanocrystallinestructure and a strongly distorted crystalline lattice. The average size of their crystallites and the percent of the lattice distortions depend on the temperature of the substrate. The layers deposited at a ~rn~~~e of 373 K are composed of the two phases Fea and Fey. At higher temperatures,the layersare single-phaseand only contain Fea. An increase in the substrate temperature decreases the percent of the lattice distortions and increases the size of the crystallites, but decreases the hardness. Depending on the substrate temperature, the hardness of the PPD layers is greaterby a factor of 4.8 to 2.5 than that of iron with a coarse-grained structure. ACKNOWLEffiMENTS This work was supportedby the CommitteeforScientificResearchby GrantNo. 3P40704007. ‘REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Yamagchi, M., Mater~~s Science and ~chnofo~, 1989,24,1599. Siegel, R.W., Annual Review in Materials Science, 1991, 21,559. Gell, M., Materials Science and Engineering, 1995, A204,246. Fougere, G.E., Weertman, J.R. and Siegel, R.W.,Nanostructured Materials, 1995,5,127. Jang, J&C. and Koch, CC., Scripta Metallurgica and Materialia, 1990,24,1599. Mieh~~, A., Sokolowska, A., Jour~l of~ateri~s Science, 1985,20,1842. Miehalski, A., Olszyna, A., Sz@ace Coating Technology, 1993,59,287. Michalski, A., Journal of Materials Science Letters, 1991,10,503. Romanowski, Z., Wronikowski, M., Journal of Materials Science, 1992,27,2619. Williammson, G.K., Hall, W.H., Acta Metallurgica, 1953, 1,22. Hayakawa, K. and Iwama, S., Journal of Crystal Growth, 1990,99,188.