Shape Recovery after Nanoindentation of NiTi Thin Films

Transcription

1 Shape Recovery after Nanoindentation of NiTi Thin Films W. C. Crone *, G.A. Shaw œ, D. S. Stone, A. D. Johnson φ, A.B. Ellis œ Department of Engineering Physics, œ Department of Chemistry, Department of Materials Science and Engineering University of Wisconsin-Madison, Madison, Wisconsin φ TiNi Alloy Company, San Leandro, California ABSTRACT The shape memory effect is a visually striking phenomenon, whereby a material is able to recover its initial shape after significant deformation by heating past the material s transformation temperature. In the case of shape memory alloys, the effect arises from a thermally-induced atomic-level structural change. This solid-state phase change is known as a martensitic transformation and occurs in a wide variety of metal alloys, including NiTi. In our research, a series of nanoindentations were made on NiTi shape memory alloy thin films at mn load levels with a Berkovich indenter. Mapping of the indentation topography using Atomic Force Microscopy (AFM) revealed direct evidence that the thermally-induced martensitic transformation of these films allows for partial indent recovery on the nanoscale. Indeed, recovery is nearly complete at indentation depths of less than 100 nm. INTRODUCTION Shape memory alloys (SMAs) are a class of materials that can recover their shape after relatively large deformations. The mechanism of this behavior involves a solid-state phase change known as a martensitic transformation. In the case of NiTi (nitinol), this phase change is a transition between a cubic CsCl structure (austenite) and a monoclinic B19 structure (martensite). Pseudoelasticity occurs when the application of stress causes the austenite to transform to martensite; in NiTi fully reversible strain levels of 8-10% can be achieved. This phase transformation can also be induced thermally. If the temperature of a martensite sample is increased, it will begin to transform at what is called the austenite start temperature, A s, finishing the transformation at the austenite finish temperature, A f. As it is cooled, it will begin the reverse transformation at the martensite start temperature, M s, finishing the transformation at the martensite finish temperature, M f. If the martensite is prestrained to induce martensitic twin rearrangement, the temperature-induced transformation to austenite will allow it to regain its original shape; this is commonly called the shape memory effect (SME). A host of applications have been developed for NiTi, including, for example, the use of the shape memory effect as a microvalve actuator [1] and the use of pseudoelasticity for improving arterial stents [2]. Improvement of existing technology and development of new applications requires continuing research into these materials properties. In the research described below, the shape memory effect is examined on a very localized scale. Surface deformation of NiTi Plastic Deformation Shape Memory Deformation Elastic Deformation Figure 1: Idealized elastic half-space representation of indentation of an SMA. 1

2 SMA thin films is examined on the scale of 3 µm to 30nm. Through the use of nanoindentation and atomic force microscopy (AFM) methods, the shape memory effect is observed in the nanoscale regime. Our nanoscale work shows good agreement with the microscale data on bulk NiTi and extends the measurements to thin film materials as well as to different kinds of deformation, including nanoscratch and nanowear. We also find that much of the surface deformation induced can be recovered at indentation depths less than 100nm. BACKGROUND INFORMATION Nanoindentation is a testing method in which a pyramidal diamond indenter tip is pressed into a material s surface. Process starts at zero load and displacement, just touching the material s surface. An increasing load is then applied to the indenter tip causing it to penetrate the sample surface until a maximum load or depth is reached, after which the load is decreased back to zero. Load and displacement of the indenter tip are recorded throughout. Recently, several studies have used indentation to characterize the properties of NiTi SMAs, including Young s modulus and hardness [3-10]. Shape recovery has also been demonstrated on the microscale using Vickers and spherical indentation [11]. In order to understand the twin rearrangement and pseudoelastic transforamtion during indentation, it is necessary to develop a conceptual picture of the processes occurring under the indenter tip as it applies a load to the surface being studied. Figure 1 illustrates the processes occurring during indentation for a martensite sample. Deformation of the solid occurs by elastic deformation, followed by martensite twin rearrangement, and finally plastic deformation. In the region nearest the indenter tip, the stresses will be greatest and will decrease further away from the tip. Thus, the indentation process for SMAs can be conceptually described by a series of concentric shells. The innermost shell is in the highest stress state, and will plastically deform. Further from the indenter tip, as the stress level decays, deformation will occur by martensite twin rearrangement. Still further away from the tip, the stress will be even lower and can be accommodated by elastic deformation. A hemispherical cavity model can be used to locate the plastic/shape memory and shape memory/elastic boundaries within the solid [12,13]. Similarly, when an austenite film is indented, the material nearest to the tip is still considered to be plastically deformed, but as the stress level decays, the deformation can be accommodated by the stress-induced austenite-to-martensite transformation (pseudoelasticity). Finally, the outmost region contains only elastic deformation. 1 µm 1 µm Figure 2: AFM topographical maps of a nanoindent made at 0.5 mn before (left) and after (right) heating past the A f temperature. Insets below show profiles taken along a line from the indent s deepest point to its vertex, as shown by the arrows. 2

3 EXPERIMENTAL NiTi thin films were deposited at TiNi Alloy Co., San Leandro, CA. DC magnetron sputtering was used to coat substrates of oxidized Si to depths of approximately 1.7 µm or 10 µm with Nitinol. Both austenite and martensite films were created, and their crystal structures verified by x-ray powder diffraction. The average grain size in the 10 µm austenite film was estimated to be 2-5 µm using electron backscatter diffraction (EBSD). The experiments in this study were performed using a Digital Instruments Nanoscope AFM in conjunction with a Hysitron Triboscope nanoindenter. The apparatus was set up such that the Berkovich diamond tip used for indentation could be used to image the sample by using the AFM scanner tube to raster it across the surface. This configuration also allowed precise positioning of the indenter tip to within ~10 nm of the desired indentation area, as well as the ability to conduct nanoscratch and nanowear experiments. Indentations were made at 8, 3, 1, and 0.5 mn loads on both the austenite and martensite film samples. The nanoindenter head was then exchanged for a standard AFM head, and the impressions left by indentation were imaged in contact mode with a cantilever tip. Next, the martensite sample was removed and heated to approximately 200 C, well past A f, with a heat gun for 30 seconds. The film transformed to the austenite phase and then was allowed to cool past M f, returning to the martensite phase. This transformation could be viewed macroscopically as a change in the film s reflectivity (the martensite is cloudy, and the austenite more shiny). The film was then returned to the AFM, and the indents imaged again. The change in the depth of the remnant indentations due to the SME was quantified using a profiling feature of the Digital Instruments software. Nanoscratch and nanowear experiments were conducted on the 10 µm martensite film in a similar manner. To make nanoscratches, a 20 µn load was applied to the Berkovich tip while the AFM scanned the sample in single lines. The nanowear was produced by doing a normal AFM scan using the Berkovich tip at a load of 10 µn. This created an array of 256 parallel nanoscale scraches on the material surface. The samples were then imaged by conventional AFM before and after heating past the material s transformation temperature. Figure 3: AFM topographical maps of a UW -shaped nanoscratch before (left) and after (right) heating past the A f temperature. A marker is placed at the bottom left corner of the W in reach image. Although recovery of the nanoscale scaratch is nearly complete, some faint remnant scar does remain after transforamtion. RESULTS AFM profiles of a nanoindent in the martensite thin film is shown in Figure 2 before and after heating past the A f temperature. The depth profiles of the indentations were used to determine the indent depth before heating, d br, and after heating, d ar. 3

4 Profiles were taken along a line from the indent s deepest point through the vertex of the pyramidal impression to ensure the same section was being examined. These data clearly shows that the indent recovers substantially after heating. Nanoscale scratches were applied in the shape of UW to the surface of a martensitic film with a 20 µn load. Figure 3 shows an AFM image from the nanoscratch experiments before and after heating past the A f temperature. The recovery of the scratches is nearly complete after transformation. The nanowear experiments were conducted by creating an array of 256 parallel horizontal nanoscratches. Figure 4 shows a 7 µm by 7 µm area that has been interrogated by the Berkovich tip under load. The surface roughness induced by the nanoscale scratches repairs itself when the film is transformed. It should be noted that the particles to the right and left of the wear area are surface contaminants that have been moved to the edge of the scan by the indenter. Figure 4: AFM topographical maps of a nanowear test on a 50 µm 2 area of NiTi thin film before (left) and after (right) heating past the A f temperature. Top left and bottom right corners of the wear region are identified with markers. DISCUSSION In figure 1 the region that deforms through martensite twin rearrangement is labeled the shape memory region. If this region is heated past the A f temperature, the transformation to austenite will allow the material to revert back to its original shape which is retained by self-accommodating twins upon cooling. In our case, this means the impressions left from nanoindentation will recover the non-plastic deformation, thus becoming shallower. The region in which plastic deformation occurs, however, remains deformed. We have used an expression called the recovery ratio, R, to quantify the indent recovery, where R = (d br - d ar ) / d br. (1) In the case of the martensite films, (d br - d ar ) can be taken as a measure of the amount of shape memory recovery, and d ar as the amount of plastic deformation. R is therefore the fraction of total deformation due to the shape memory effect. According to our model [13], R should remain constant at 0.38 with indentation depth for martensite. In the austenite samples, the martensitic transformation is caused by the stress from indentation, so the recovery ratio has a slightly different meaning. In this case, d br is the maximum indentation depth and d ar is the depth of the residual indent. The value (d br d ar ) quantifies the combined effects of elastic and pseudoelastic recovery, and d ar is still a measure of plastic deformation. As figure 5 shows, good agreement is seen between the data in the current study and that of Ni et al. for the bulk martensite NiTi samples [11]. The recovery ratio is linear with depth until d br is less than about 100nm, where it steadily increases to over 0.8 for the shallowest indents. If d br is plotted vs. d ar, the martensite data measured here by nanoindentation agree well with the previous study. The austenite data show trends similar to those of the martensite, with recovery ratio increasing markedly at depths less than 100 nm. We attribute the increase in recovery ratio for indentation depths less than 100 nm to be a result of the shape of the Berkovich indenter. The tip is not perfectly sharp, but rounded somewhat at the apex. Using a blind reconstruction technique [14], it was determined that the tip had a spherical character at the apex, with a radius of curvature of roughly 100nm. In previous work [11], it has been shown that the remnant impressions from micro-scale spherical indenters recover more completely than do those from sharp indenters due to the lower stress levels inherent in spherical contact. 4

5 1000 log (d ar / nm) µm mart. 1.7 µm mart. 10 µm aust. 1.7 µm aust. Ni et al. Vickers Ni et al. Spherical log (d br / nm) Figure 5: Comparison of current results for nanoscale indents on thin film NiTi to those of Ni et al. [11] for microscale indents on bulk NiTi. CONCLUSIONS Nanoindentation, nanoscratch, and nanowear tests conducted on NiTi thin films have shown evidence of both pseudoelastic and shape memory recovery on the nanoscale. In the case of indents deeper than 100 nm, the amount of recovery was constant and agreed well with a hemispherical cavity model. As indent depths decreased from 100 nm, the recovery ratio increased, presumably reflecting decreased stresses caused by the spherical apex of the indenter. The results observed in nanoscratch and nanowear experiments demonstrate that the ability of shape memory alloys to recover from large deformations extends to the nanoscale. ACKNOWLEDGEMENTS The authors wish to thank Professors John Perepezko and Robert Carpick at UW-Madison for valuable discussion and AFM expertise. This research was funded by the Energy Efficiency Science Initiative of the Department of Energy (DE-FC36-01GO11055). REFERENCES 1. Johnson, A. D.; Busch, J. D.; Ray, C. A.; Sloan, C., Fabrication of Silicon-Based Shape Memory Alloy Micro- Actuators. Mater. Res. Soc. Symp. Proc., , Duerig, T.; Pelton, A.; Stöckel, D., An Overview of Nitinol Medical Applications. Mat. Sci. Eng., A, , Fu, Y.; Huang, W.; Du, H.; Huang, X.; Tan, J.; Gao, X., Characterization of TiNi SMA Thin Films for MEMS Applications. Surf. Coat. Technol., , Gall, K.; Dunn, M. L.; Liu, Y.; Labossiere, P.; Sehitoglu, H.; Chumlyakov, Y. I., Micro and Macro Deformation of Single Crystal NiTi. J. Eng. Mat. Tech, , Cheng, F. T.; Shi, P.; Man, H. C., Correlation of Cavitation Erosion Resistance with Indentation-Derived Properties for a NiTi Alloy. Scr. Mater., ,

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