Materials and Design 23 Ž2002. 291᎐296
Influence of consolidation parameters on the microstructure and hardness of bulk copper samples made from nanopowders T.S. Srivatsan a,U , B.G. Ravi a,1, A.S. Narukaa , M. Petraroli b, R. Kalyanaraman c , T.S. Sudarshan c a
b
Department of Mechanical Engineering, The Uni¨ ersity of Akron, Akron, OH 44325-3903, USA Timken Research, The Timken Company, 1835, Dueber A¨ enue, S.W., P.O. Box 6930, Canton, OH 44706-0930, USA c Materials Modification Inc., 2721-D, Merrilee Dr., Fairfax, VA 22031, USA Received 23 January 2001; accepted 13 September 2001
Abstract This paper describes the consolidation of copper powders by the technique of plasma pressure compaction. Microstructural observations and density measurements provide evidence of the presence of residual porosity even after consolidation at 820 ⬚C. Vicker’s microhardness measurements revealed the sample consolidated at temperatures as high as 820 ⬚C to have the maximum hardness of 0.79 GPa. The influence of processing variables on microstructure, including the presence and distribution of processing-related artifacts, density and microhardness are presented and discussed. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Copper; Nanopowders; Consolidation; Microstructure; Hardness
1. Introduction Achieving refinement in microstructure to the nanometer scale has been shown to result in commendable improvements in the mechanical properties of materials spanning ceramics, metals, and including intermetallics w1᎐7x. A material is considered to be nanocrystalline when its average grain or crystallite size is less than 100 nm w8x. The property improvements of nanocrystalline metals have been attributed to the novel characteristics of grain boundaries coupled with dramatic refinements in grain size. For example, the hardness and strength of nanostructured metals was believed to be exceptionally high owing to strengthening by the U
Corresponding author. E-mail address:
[email protected] ŽT.S. Srivatsan.. 1 Presently with Materials Modification Inc., Fairfax, VA, USA.
grain size, as exemplified by the Hall᎐Petch relation w9,10x. The unique feature of these materials contributing to improved mechanical properties is that a large fraction of the atoms Ž20᎐50%. are located at the grain or intercrystalline boundaries w11x. Another important microstructural feature contributing to improved properties of nanocrystals is the triple junction, i.e. the intersection line of three adjoining crystals. The triple lines have been described as line defects, i.e. disinclination, which completes the representation of a polycrystalline material as a balanced network of dislocations w12,13x. The overall structure of the triple junction is dependent on the specific crystallographic arrangement of the adjoining crystals, with increased randomness in the orientation of adjoining crystals enhancing the defect character w12,14x. As a result of the attractive properties observed in fine grained materials, processing techniques such as gas phase condensation w15᎐17x,
0261-3069r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 0 1 . 0 0 0 7 8 - 4
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severe plastic deformation w18x, solution phase synthesis w19x, and sonochemical synthesis w20,21x have been successfully used to prepare nanocrystalline copper. These methods have provided viable processing routes to produce conventional materials of refined grain sizes. However, the primary limitation of all of these processing methods is that the output per day is limited to a maximum of few grams, thus preventing the production of large quantities. Scaling up the production rate of powders is of acute importance for the purposes of consolidation, mechanical property evaluation, and potential end applications. However, some of the problems associated with the large-scale production of powders are: Ža. controlling the size of the powder particles; Žb. particle distribution; and Žc. occurrence of contamination. The consolidation of fine nanoscale powder particles into bulk samples, which is important for structural applications, continues to be a challenge and is not as yet a welldeveloped technology. Conventional methods of consolidation often result in significant growth of the starting powder particle. This results in the loss of the submicron to nanoscale structure and the associated benefits of enhanced material properties. It is essential to minimize grain growth through careful control of consolidation temperature and time. To this end, the technique of plasma pressure compaction Žreferred to henceforth in this manuscript as P 2 C. has shown the capability of being a cost-effective and efficient fieldactivated sintering technique that can successfully densify metallic, intermetallic and ceramic powders to near-theoretical density w22x. The purpose of this paper was to examine the influence of consolidation parameters on microstructure, density and hardness of bulk samples of copper made by consolidating nanocrystalline copper powders. The results are interpreted in light of intrinsic microstructural features, including processing related artifacts such as porosity.
2. Experimental procedures 2.1. Sample preparation by consolidation High purity copper nanopowders, with an average powder particle size of 100 nm, were procured from ULTRAM ŽDenver, CO, USA. and consolidated by the P 2 C technique. Approximately 30 g of nanocrystalline copper powder was poured into a graphite die. The fine powder particles were initially compacted, without any additive or binder, to approximately 45% density using a uniaxial hand press. The graphite die and the powder compact were contained within the P 2 C apparatus between metallic electrodes. The compaction procedure was carried out in two steps: Ža. the application of
a pulse current; and Žb. the application of constant current. 2.1.1. Application of pulse current A pulse current Žf 900 A. was applied between the metal electrodes once the vacuum level in the chamber reached a value of 10y3 torr. During this time, the voltage was kept constant. The powder compact was heated to a temperature of approximately 650 ⬚C so as to eliminate adsorbed gases and moisture. A marginal drop in the vacuum level revealed this. Concurrently, a pressure of 20 MPa was applied so as to provide an ample current path through the powder compact. The pulsed current was applied until the vacuum level reached its initial value of 10y3 torr. 2.1.2. Application of constant current In this stage, the specimen was heated to the desired temperature by the application of a constant current Žf 1400 A.. Densification occurred at the desired temperature upon the application of pressure for a time period of 5 min. The densification of copper powders was carried out at four different temperatures Ž650, 720, 820 and 900 ⬚C.. Throughout densification of the copper powders, the compaction pressure was held constant. The compaction pressures examined were 25, 40 and 48 MPa. The consolidated samples measured 25 mm in diameter and 12 mm in thickness. 2.2. Microstructural characterization The as-consolidated samples were prepared for examination, both by light optical and scanning electron microscopes, to identify the presence and distribution of microscopic defects that exert an influence on mechanical behavior. Typically, these defect features include: Ža. irregular surface cracks; Žb. string-like and angular features; and Žc. small cavities such as microscopic pores and voids. Sample preparation involved an initial wet grind and coarse polish on progressively finer grades of silicon carbide impregnated emery paper using copious amounts of water both as a coolant and lubricant. The samples were then fine polished using 5 m and 1 m alumina suspended in distilled water. Finish polishing was achieved using a 1- m diamond paste with water as the lubricant. Polishing aids in reducing the size and abundance of surface flaws, but does not remove all of the flaws that are larger than nano-grain size. The as-polished samples were chemically etched using a solution mixture of potassium dichromate Ž1 g., sulfuric acid Ž4 ml. and distilled water Ž5 ml.. The etched samples were observed in an optical microscope and photographed using standard bright-field illumination. The polished and etched samples were also examined in a scanning electron microscope ŽSEM. to determine the presence, distribu-
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tion and morphology of surface cracks, the size and morphology of grains and the distribution of pores larger than grain size. Precision density measurements, based on Archimedes’ principle, were made according to ASTM Standard B3800-90 w23x, on the consolidated bulk copper sample. The measured density for a given sample is reproducible within 5%. 2.3. Indentation hardness Using a Buehler Micromet II microhardness tester Vickers microhardness Ž H¨ . measurements, on polished surfaces of the as-compacted sampleŽs., were made using a load of 50 g and a dwell time of 15 s at room temperature. Each hardness value represents the average of five such measurements and is reported as pressure, both in units of kgrmm2 and GPa. 3. Results and discussion 3.1. Initial microstructure Fig. 1 is a scanning electron micrograph of Sample 噛2 consolidated at 650 ⬚C for 5 min and reveals the presence of porosity and oxide phases. Sample 噛4 consolidated at 820 ⬚C revealed colonies of agglomerates of varying size ŽFig. 2.. Each agglomerate consisted of smaller grains, less than 500 nm, intermingled with the larger grains. Two competing factors which exert a profound influence on densification of the nanocrystalline copper powder particles are: Ža. degree of agglomeration; and Žb. green density of the compacts. The effect of agglomeration has been the subject of several recent studies focussed on understanding the densification of nanocrystalline materials w12᎐14x. In a recent study, copper powders having particles of size: Ža. 100 nm; Žb. 2᎐3 m; and Žc. 13 m, were consolidated at a temperature of 900 ⬚C to near full
Fig. 2. Scanning electron micrograph of Sample 噛4 consolidated at 820 ⬚C for 5 min.
density using the P 2 C technique w24x. In this present study, the nanocrystalline powders were not consolidated to full density. This is attributed to the high degree of agglomeration of the powders. Both microstructural observations and density measurements provide evidence for the presence of residual porosity even after consolidation at 900 ⬚C ŽFig. 3.. It has been observed that in powder systems having a relatively large agglomeration of fine pores, the end result is a porous microstructures even after consolidation at high temperatures w12,13x. 3.2. Density and microhardness
Fig. 1. Scanning electron micrograph of Sample 噛2 consolidated at 650 ⬚C for 15 min.
The process conditions of consolidation, density and microhardness are summarized in Table 1. To study the influence of grain growth on densification, four different temperatures were chosen for the purposes of consolidation. Because graphite dies were used for consolidation, the pressure was limited to a maximum of 48 MPa. At a constant pressure of 48 MPa, the density of the as-consolidated samples was found to increase with an increase in consolidation temperature.
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Fig. 3. Scanning electron micrograph of Sample 噛6 consolidated at 900 ⬚C for 5 min and pressure of 40 MPa showing the presence and location of microscopic pores.
Consolidation of the copper nanopowders at 650 ⬚C for 5 min resulted in a sample having 82% of its theoretical density. However, increasing the isothermal holding time from 5 to 15 min at 650 ⬚C resulted in increasing the density to 86%. At a temperature of 820⬚ and 48 MPa pressure, the bulk sample had a density of 94%. The maximum temperature used for consolidating the copper powders was 900 ⬚C for 5 min. However, an increase in temperature and a concurrent decrease in compaction pressure were found to have a detrimental influence on density. The sample consolidated at 900 ⬚C at a pressure of 25 MPa had the lowest density of 70%. Increasing the compaction pressure to 40 MPa at a temperature of 900 ⬚C was found to increase density of the consolidated sample to 84%. The experimental measurements clearly indicate that the magnitude of applied pressure during consolidation has a profound influence on density of the compacted bulk sample.
Vicker’s microhardness measurements were made from edge-to-edge across the center of each specimen. The microhardness values and the averages are summarized in Table 1. At a maximum pressure of 48 MPa and consolidation at 650 ⬚C: Sample 噛1 had a microhardness of 0.62 GPa; while Sample 噛2 revealed a 10% increase in microhardness for an increase in holding time at the consolidation temperature from 5 to 15 min. At the highest temperature of consolidation Ž900 ⬚C., a decrease in pressure from 40 to 25 MPa resulted in a 160% decrease in microhardness, i.e. from 0.70 Ž40 MPa. to 0.27 GPa Ž25 MPa.. Microhardness values measured at different locations on the surface of the specimen consolidated at 820 ⬚C ŽSample 噛4. are shown in Fig. 4. A decrease in temperature of consolidation at constant pressure Ž48 MPa. resulted in an increase in both density and microhardness. However, an increase in consolidation temperature with a concurrent decrease in pressure was found to have a detrimental influence on both density and hardness. The conjoint influence of temperature and pressure on density and microhardness of the bulk samples is well depicted in the bar graph shown in Fig. 5. It is fairly well established that a finer grain size results in a higher hardness for the as-consolidated bulk samples of materials w4᎐6x. Suryanarayanan and co-workers w19x have reported microhardness values of 0.8 and 0.98 GPa for nanocrystalline copper samples prepared by solution-phase synthesis and having a density of 95.1% and 97.8%, respectively. Alexandrov and co-workers w18x have reported a microhardness of 1.8 GPa for a severely plastically deformed nanocrystalline copper sample having a density of 96 " 2%. Identical andror higher values of microhardness were reported for samples of nanocrystalline copper produced by in-
Table 1 Summary of consolidation conditions, density and microhardness results Sample No.
Consolidationa conditions
Density Ž%.
Microhardness indentation results Ž H¨ .b Trial 1
650 ⬚C; 5 min 48 MPa 650 ⬚C; 15 min; 48 MPa 720 ⬚C; 5 min 48 MPa 820 ⬚C; 5 min 48 MPa 900 ⬚C; 5 min 25 MPa 900 ⬚C; 5 min 40 MPa
1 2 3 4 5 6 a
Trial 2
Trial 4
Trial 5
Average H¨
GPa
82
73.6
71
55
57.7
57.7
63.00
0.62
86
66.6
71.9
76.1
70.4
76.1
72.22
0.71
90
71
73.8
67.2
83.4
81.4
75.36
0.74
94
76.1
82.9
79.3
81.4
82.9
80.52
0.79
70
25
26.2
25.9
35.9
23.7
27.34
0.27
84
74
70.4
72.3
73
69.4
71.82
0.70
Prior to consolidation, all samples were pulsed at 800 ⬚C for 5 min. All measurements at 50 g load.
b
Trial 3
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Fig. 4. Schematic diagram of Sample 噛4 showing the location and value of microhardness measurements. Typical dimensions of a consolidated sample are also shown.
ert gas condensation ŽIGC. w2x. The microhardness of Sample 噛4, consolidated at 820 ⬚C, compares well with the value reported by Suryanarayanan and co-workers w19x. However, the overall hardness value is still low when compared with typical hardness values recorded for nanocrystalline copper w19x. In an earlier study w24x, an almost four-fold increase in the value of microhardness Ž1.88 GPa. was obtained for nanocrystalline copper powders Žinitial powder particle size s 100 nm. consolidated by P 2 C at 900 ⬚C, in comparison with samples made by consolidating powders having an average starting size of 13 m. He and Ma w25x examined the effect of porosity on microhardness of Fe᎐29Al᎐2Cr. They observed that the hardness decreased with an increase in residual porosity. A similar observation can be made in this study. Although the samples were consolidated at different temperatures with the objective of improving the
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density, the presence of internal porosity is responsible for the observed low values of microhardness. Model calculations for hot isostatic pressing ŽHIP. w26x predict that conventional copper powder can be densified to near full density at a temperature of 500 ⬚C and at a pressure of 120 MPa. Since P 2 C is based on fieldassisted rapid consolidation and the isothermal holding time is only a few minutes, the energy concentrated andror available at the interparticle regions of an agglomerated powder compact is not fully sufficient to facilitate complete particle contact. The highly agglomerated nature of the starting copper powders and the resultant density attained during consolidation is a plausible reason for the observed microhardness values.
4. Conclusions Based on the results obtained on a study on the influence of consolidation parameters on microstructure and hardness of bulk samples made by consolidating nano-sized copper powders, our findings are as follows: 1. The nanocrystalline copper powders were consolidated using a novel field assisted technique known as Plasma Pressure Compaction to density of 84% at temperature of 900 ⬚C and a pressure of 40 MPa. 2. The microstructure of the consolidated samples revealed agglomerates consisting of smaller grains Žless than 500 nm. intermingled with larger grains. Residual porosity was evident even after consolidation at the highest temperature Ž900 ⬚C..
Fig. 5. Bar chart showing the conjoint influence of temperature and pressure at consolidation on microhardness of bulk samples.
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3. Vicker’s microhardness measurements revealed that for a consolidation pressure of 48 MPa, the sample consolidated at 900 ⬚C had a maximum hardness of 0.79 GPa. A decrease in pressure at constant consolidation temperature Ž900 ⬚C. resulted in a decrease in both density and hardness. At constant temperature and pressure, microhardness increased with an increase in holding time at temperature. At constant pressure of consolidation Ž48 MPa., the density increased with an increase in consolidation temperature.
Acknowledgements This research was jointly supported by the National Science Foundation ŽGrant Number: NSF-CMS: 9802185., the State of Ohio: Board of Regents ŽColumbus, Ohio., and The University of Akron ŽAkron, Ohio.. Sincere thanks to the ‘unknown reviewers’ for their comments, corrections and suggestions to help strengthen the manuscript. References w1x Cottrell AH. Trans Minerals Metals Mater Soc 1958;212:192. w2x Armstrong RW. In: Baker TN, editor. Yield, flow and fracture of polycrystals. London: Applied Science Publishers, 1983. w3x Choksi AH, Rosen A, Karcdh J, Gleiter H. Scripta Metallurg Materialia 1989;23:1679. w4x Fougere GE, Weertman JR, Siegel RW, Kim S. Scripta Metallurgica Materialia 1992;26:1879.
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