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Contributed Papers Nanoindentation Studies in a Liquid Environment† A. B. Mann* and J. B. Pethica Department of Materials Science, University of Oxford, Parks Road, Oxford OX1 3PH, U.K. Received October 17, 1995. In Final Form: January 30, 1996X Surface and chemomechanical effects are of considerable importance in tribology, wear, and friction. Recently there has been substantial interest in the use of nanoindentation techniques to investigate these phenomena for asperity size indentations. In this paper we report a new type of nanoindentation experiment where tip and sample are immersed. We show that with due care the difficulties due to surface tension when testing in liquid can be overcome, and well-controlled nanoindentation experiments can be conducted for even the shallowest indentation depths. This type of testing, under “model” environmental conditions, has potential utility in the examination of several key mechanisms involved in tribology. This is demonstrated by experimental results for GaAs in distilled water and single-crystal tungsten in aqueous HCl and distilled water. When GaAs is indented in conditions of high atmospheric humidity, the area around the indentation exhibits substantial bulging, reminiscent of lateral cracking. Testing of the same sample under distilled water does not give this result. The implication is that capillary condensation present in atmospheric ambient has a quite different effect to complete immersion in water. This is probably due to the modified forces acting when water has condensed at the tip-sample interface. Nanoindentation curves for electropolished, single-crystal tungsten are almost perfectly elastic for shallow indentations. We have assessed the effects of the passivating, surface oxide film on the elastic behavior by nanoindentation tests in air and under aqueous HCl and distilled water. The results for HCl, which is known to remove the oxide film, indicate that the elastic behavior in BCC metals is modified by the passivating layer, but is not wholly dependent on it.
Introduction The importance of surface effects in the plasticity of materials has been known for sometime,1 but experimental results have generally failed to give a clear insight into the exact processes taking place. This is due in part to the inherent problems of controlling the environment when testing in air and the magnitude of the effects, but of equal importance is the lack of a clear understanding of the generic mechanisms involved. See Hainsworth and Page2,3 for a review of the weaknesses in the current understanding of surface effects. These difficulties have highlighted the need for simple, modelable experiments in controlled environments. The significance of chemomechanical effects should not be underestimated, particularly when the size of the contact between two bodies is reduced to that of a single asperity. In these circumstances the ratio of surface area to volume is increased, and consequently, the importance of the surface is magnified. Many of the mechanisms involved in tribology, wear, and friction are concerned with the deformation of asperities, and hence, the experimental investigation of surface effects is vital to the understanding of these processes. The experiments presented here are aimed at clarifying some of the mechanisms involved and have indicated, as will be seen, that the geometry and presence of a distinct surface film are crucial. * To whom correspondence should be addressed. e-mail:
[email protected]. Fax: +1865 273783. Tel: +1865 273788. † Presented at the Workshop on Physical and Chemical Mechanisms in Tribology, Bar Harbor, ME, August 27 to September 1, 1995. X Abstract published in Advance ACS Abstracts, Sept. 15, 1996. (1) Latanision, R. M., Fourier, J. T., Eds. Surface Effects in Crystal Plasticity; Noordorf: Leyden, 1977; NATO Advanced Study Institutes Series E; Applied Science, No. 17. (2) Hainsworth, S. V.; Page, T. F. J. Mater. Sci. 1994, 29, 5529. (3) Hainsworth, S. V.; Page, T. F. Surf. Coat. Tech. 1994, 68, 571.
S0743-7463(95)00901-2 CCC: $12.00
Advances in instrumentation over the past 10 years now permit indentation techniques to be used in the examination of mechanical properties, such as hardness and elastic modulus, on the same scale as a single asperity.4-7 This has led to considerable interest in the use of these nanoindentation techniques to examine the effects of surface chemistry and the environment on the mechanical properties of ceramics,2,3 semiconductors,8 and metals.9,10 In each case an attempt has been made to control the surface chemistry and environment during the testing. Hainsworth and Page2,3 heated the samples and then quenched them in a liquid, Mann et al.8 placed the indenter and sample in an enclosed space and then modified the relative humidity using silica gel and water, whereas Venkataraman et al.9,10 attempted to control the surface chemistry by placing drops of liquid on the sample in contact with the indenter. Until now no one has fully immersed both the sample and indenter in a liquid and, thus, achieved what are essentially “model” conditions for the testing of chemomechanical effects. Experiments of this type have generally been regarded as problematic (4) Pethica, J. B.; Hutchings, R.; Oliver, W. C. Phil. Mag. A 1983, 48, 593. (5) Oliver, W. C.; Hutchings, R.; Pethica, J. B. In Microindentation Techniques in Materials Science and Engineering; Blau, P. J., Lawn, B. D., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1986; p 90. (6) Oliver, W. C.; Pharr, G. M. J. Mater. Res. 1992, 7, 1564. (7) Page, T. F.; Oliver, W. C.; McHargue, C. J. J. Mater. Res. 1992, 7, 450. (8) Mann, A. B.; Pethica, J. B.; Nix, W. D.; Tomiya, S. In Thin Films: Stresses and Mechanical Properties V; Baker, S. P, Børgesen, P., Townsend, P. H., Ross, C. A., Volkert, C. A., Eds.; Materials Research Society: Pittsburgh, PA, 1995; Vol. 356, p 271. (9) Venkataraman, S. K.; Kohlstedt, D. L.; Gerberich, W. W. J. Mater. Res. 1993, 8, 685. (10) Gerberich, W. W.; Venkataraman, S. K.; Huang, H.; Harvey, S. E.; Kohlstedt, D. L. Acta Metall. Mater. 1995, 43, 1569.
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Figure 1. The Nanoindenter (Nanoinstruments, Inc., Knoxville, TN) applies load to a diamond indenter tip via an electromagnet and measures the tip’s displacement using capacitive plates. The whole indenter assembly was placed inside a semi-air-tight container during the testing to reduce the rate of evaporation. The problems of surface tension were reduced by using a narrow indenter shank and ensuring the contact angle, θ, between the shank and liquid was small. The resolved force due to the surface tension is proportional to cos θ; hence the force is most stable for θ ≈ 0.
because the magnitude of capillary forces on the indenter is comparable to those used during nanoindentation testing (see for instance Hainsworth and Page2,3). In this paper we show that nanoindentation testing under liquids is feasible and that total immersion of the indenter permits capillary forces to be removed from the indentation test zone to a region where they are well defined and controllable. In this way some of the more interesting effects seen during small-scale indentation testing can be examined in an idealized environment. We present two examples of environmental influence: first on metals where the influence of a very thin surface film on the on set of plasticity is addressed, and second on GaAs where the effect of ambient water on crack propagation (an important process in wear) is examined. Experimental Technique A diagram of the Nanoindenter (Nanoinstruments, Inc., Knoxville, TN) used to carryout the testing is given in Figure 1. It is a high-precision instrument capable of applying loads and recording indenter depth during an indentation with accuracies of 0.3 µN and 0.16 nm6, respectively. The Nanoindenter can also apply a small ac load in addition to the normal dc load; this permits the stiffness of the contact between the tip and sample to be recorded continuously.11,12 This has proven to be of great utility when performing very shallow indentations and avoids disturbances due to drift. The accuracy of the data provided by the Nanoindenter is, as pointed out by Hainsworth and Page,2,3 both the strength and, until now, the weakness of the instrument when testing under liquids. That is, the high resolution permits the indentation process to be studied in detail but it also makes the apparatus sensitive to the effects of surface tension when a liquid meniscus is broken or moved while contacting the indenter tip holder. To overcome the problems of surface tension, the sample, the indenter tip, and a purpose built extension to the indenter shank were immersed in liquid to a depth of around 5 mm, as shown by Figure 1. The extension was slightly narrower than the normal indenter shank, 3.2 mm diameter compared with 4.8 mm, to somewhat reduce the effet of surface tension. Everything was enclosed in a semi-air-tight container to help reduce evaporation and hence steady meniscus motion. A protective coating of polyurethane varnish was also applied to the extension and tip mounting when testing was performed in corrosive solutions, such as HCl. However, by far the most important consideration (11) Pethica, J. B.; Oliver, W. C. Phys. Scr. 1987, T19, 61. (12) Pethica, J. B.; Oliver, W. C. In Thin Films: Stresses and Mechanical Properties; Bravman, J. C., Nix, W. D., Barnett, D. M., Smith, D. A., Eds; Materials Research Society: Pittsburgh, PA, 1989; Vol. 130, p 13.
Figure 2. (a) Load/depth curve for a shallow indentation in GaAs(100). There is substantial “drift” probably due to evaporation and the slow capillary rise/fall of the liquid up/ down the indenter shank. (b) Stiffness/load curve for the same indentation as (a). The stiffness data shows that despite the variation in the measured depth the contact between the tip and sample is well controlled. It should be noted that this data has been specifically selected to demonstrate the effects of drift and that for faster indentations the drift is less pronounced (see Figure 4c,d). was found to be the contact angle, θ in Figure 1, between the extension and the liquid surface. If this angle was small so the liquid was “wetting” the extension, then the indenter was found to move in a well-controlled manner, but if the contact angle was greater than 90°, so that there was a net force upward due to the surface tension, the indentation experiments proved all but impossible due to changes in the contact angle with displacement. Hence, it was necessary to “prepare” the extension, usually by careful cleaning, so that the contact angle was small for whatever liquid was to be used. Even with the apparatus set up as outlined above, it was found that surface tension presented a number of problems which meant indentations could not be performed using the normal automated software. Notably it was necessary to manually “find” the approximate position of the specimen’s surface prior to commencing the first indentation. Once this had been done the indenter was able to operate in exactly the same automated way that it would if the testing had taken place in air. Drift. Thermal drift is a well-documented problem during nanoindentation testing,13 the effects of which can be mitigated using the ac modulation technique. When the testing is performed in liquids, there is a similar problem when using long indentation cycles due to evaporation and the slow capillary rise/ fall of the liquid up/down the indenter shank. A particularly acute example (using hold periods of over 100 s) is shown in Figure 2a which is the load/displacement curve for a shallow indentation in GaAs(100) under distilled water. It can be seen (13) Weihs, T. P.; Pethica, J. B. In Thin Films: Stresses and Mechanical Properties III; Nix, W. D., Bravman, J. C., Arzt, E., Freund, L. B., Eds.; Materials Research Society: Pittsburgh, PA, 1992; Vol. 239, p 325.
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from Figure 2b, which is the corresponding stiffness/load curve, that even when the load/displacement curve shows unusual features, the stiffness data is still sensible and the contact is well controlled.
Results GaAs in Distilled Water. In an earlier paper8 we reported the observation of a humidity dependence in the deformation of GaAs and a range of epitaxial II/VI semiconductors when the (100) face was indented. Notably there was considerable bulging of the surface, reminiscent of lateral cracking, around the indentation when the testing had been performed at high humidities (80% Relative Humidity, RH) which was absent at lower humidities (25% RH). Subsequently we have observed the same effect in GaAs(111) and in Si(100), but we have not been able to induce a similar phenomena in sapphire, fused quartz, or metals. It was suggested in a previous paper8 that the bulging may be due to adhesion between the tip and sample “pulling” the surface upward during unloading. The presence of a capillary neck between the tip and sample can increase this adhesion14 and may also aid the propagation of cracks near the surface. Subsequent testing has been undertaken using a range of different indentation conditions (different loading/unloading rates, hold periods, approach rates, humidities, tip geometries, and methods of sample preparation). Some of the effects observed are illustrated by the optical micrographs of Figure 3a,b; more detailed SEM images have been given elsewhere.8 Our conclusions based on this data, which demonstrate a clear time, rate, and humidity dependence, remain that one or more of the following are responsible for the extensive cracking: (i) a capillary neck is formed resulting in tension at the edge of the contact which then acts as the driving force for the cracks or (ii) water actually in the cracks enhances crack growth by lowering the surface energy or alternatively via a chemical interaction occurring at the crack tip.15 Now, by performing indentation tests under distilled water we have been able to determine if the bulging is due to (ii) alone or a combination of (i) and (ii). Tests under distilled water were conducted on n-type, p-type, and intrinsic GaAs(100). All of these showed bulging at high humidities, but there was no sign of bulging when the indentations were performed under distilled water. This at first startling result implies that the presence of a capillary neck and the associated tensile forces are one of the main causes of the bulging. The mere presence of water in the crack is not the primary cause of propagation! It might be thought that this difference in behavior could be due to a protective oxide layer preventing access of liquid to the crack tip during indentation. Such an oxide layer might be formed by full immersion in water. However, we found that performing the experiments under octanol also suppressed the bulging. Thus the formation of a surface film specifically due to immersion in water cannot be the sole cause of the difference in behavior. The conclusion must be that simple exposure to water is not the cause of the bulging, and a capillary neck (i.e. a free water surface) between the tip and sample is necessary for this effect to be observed. This suggests that surface tension forces are critical to the propagation (14) Fogden, A.; White, L. R. J. Coll. Inter. Sci. 1990, 138, 414. (15) Lawn, B. R. Fracture of Brittle Solids, 2nd ed.; Cambridge University Press: Cambridge, 1993; Chapter 5.
Figure 3. (a) Optical micrograph of 4 × 8 array of indentations, spaced 20 µm apart, in GaAs(100). The indentations were performed at high humidities using identical loading cycles; however the unloading rates and hold periods during unloading were varied. The bulging around the indentations is greatest for the slowest unloading rates and longest hold periods. (b) Optical micrograph of 4 × 4 array of indentations, spaced 20 µm apart, in GaAs(100). The environment was the same as for (a), but the loading cycle was varied while the unloading was kept constant. The bulging and the resulting surface damage appear to be due to capillary condensation at the indenter tip. This leads to increased adhesion between the tip and sample. The presence of condensed water may also aid the propagation of cracks.
of the cracks and that water alone inside a crack is not the sole cause of this phenomena. Tungsten in HCl. The almost perfectly elastic behavior of tungsten for very shallow indentations has been widely documented,6,12 and recently evidence has been presented showing a similar phenomena in a range of both FCC and BCC metals.9,10 The latter results have been attributed to the presence of an oxide layer on the metal’s surface which must be ruptured before the sample can deform plastically. Indentations were performed through drops of 5% HCl9 (the Cl ion is known to remove the passivating layer) to show that the elastic response was due to the oxide layer. These results indicate that a number of metals do not show elastic regions when HCl is present. However, the question remains whether the passivating film is the sole cause of the elastic behavior. To address this question an electropolished, single crystal of tungsten was indented in air, under an HCl solution and under distilled water. Figure 4a,b shows the results for indentations in air and Figure 4c,d shows the results for similar indentations under HCl acid and distilled water. Two features stand out from these curves: (1) Tungsten behaves elastically and still shows pronounced discontinuities even when indented under HCl, and (2) The load at which the discontinuity occurs, when compared to testing in air, is lower if the tests are performed in HCl by a factor of 0.6 and higher by a factor of almost 2 in distilled water. These results were found to be highly reproducible with three sets of indentations (six in each of air, HCl, and distilled water) performed on
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Figure 4. Load/depth curves for single-crystal tungsten in a number of different environments. (a) An almost perfectly elastic indentation in air. (b) An elastic/plastic indentation in air. The loading curve exhibits a highly reproducible discontinuity which marks the transition from elastic to elastic/plastic behavior. (c) An indentation performed in HCl solution. The Cl ion removes the passivating, surface oxide layer with the result that the discontinuity occurs at a lower load. (d) An indentation performed in distilled water. The oxide layer is assumed to be thicker in this case, and as a consequence the load at which the discontinuity occurs is increased.
different occasions all showing the same effect. An attempt was made to increase the concentration of the HCl, but this led to the evolution of gas on the metal’s surface which made testing impractical. Since HCl removes the passivating film, the conclusion must be that the film cannot be the sole cause of the elastic behavior and the discontinuity in the loading curve. However, removal of the film reduces the load at which pop-in and plasticity is observed. This might be because the film alters the stress distribution at the indenter/ surface contact and, also, possibly due to changes in the surface roughness.16 Discussion It has been shown that nanoindentation testing under liquids is feasible provided consideration is given to the problems of surface tension. In particular the contact angle between the indenter’s shank and the liquid must be low if a high degree of control over the indenter’s motion is to be achieved. The continuous stiffness (ac loading) technique has proven to be particularly useful for the shallowest indentations when dc loading is susceptible to the effects of evaporation and the liquid moving up or down the indenter shank by capillary action. The results of the tests on GaAs under distilled water have shown that capillary condensation of water vapor causes markedly different effects compared to full immersion in water. The implication is that the modified interaction between the tip and sample when capillary (16) Mann, A. B.; Pethica, J. B. Unpublished results.
condensation has taken place can lead to substantial changes in the near surface stress field and, hence, result in extensive surface damage by lateral cracking. By indenting tungsten in a HCl solution it has been shown that the presence of a passivating layer is not the only cause of the elastic response and subsequent discontinuites in the indentation curves of BCC metals. However, the oxide layer appears to extend the load range giving elastic behavior for tungsten. We conclude that changes in stress field and geometry are important factors in the apparent changes in deformation rather than intrinsic material properties being altered. This has been shown directly in these experiments where we have intentionally controlled these factors. The results for tungsten in HCl and those for GaAs in distilled water have demonstrated that nanoindentation testing under liquids is potentially of considerable utility in the study of chemomechanical and surface effects. In particular the “model” conditions provided by testing of this type offer considerable scope for the investigation of several key mechanisms in tribology, wear, and friction. Acknowledgment. We would like to thank our colleagues at Oxford University and particularly Dr. Paul Warren for useful discussions and the provision of GaAs samples and also S. A. Syed Asif for help in setting up the Nanoindenter. A. B. Mann is supported by the EPSRC through research studentship number 92312138. LA950901Z
