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Ryan’s MFP-3D Procedural Operation ‘Manualette’ Version 10 (v080501; Igor 6.04A); 11.27 NovaScan SiNx cantilever (k= 0.32 N/m). Sample? Fitting compliance curves with basic Herztian mechanics found in the MFP-3D software is described in Section 11.5.XX– the author is aware that analysis can get pretty involved using more complex mathematical models, but this is a good place to start for those in the crowd that want to dabble with this to make a relative measurement. 11.4.5: Cantilever based indentation: Cantilever based indentation is a nice (relative) way to look at compliance of materials, but it is subject to great error due to the concomitant longitudinal and torsional cantilever movements as the piezo pushes the cantilever towards the surface- this additional movement cannot be easily characterized, ultimately compromising the quantitative modulus property value in the analysis. Never the less, is very commonly used and can yield a lot of information about samples. Depending on cantilever spring constant and the compliance (modulus) of the sample, it can be difficult to determine which dimension the ‘tip’ experiences the most movement in. Another significant issue is deconvolving the contact area of the tip (one of the larger sources of error in nanoindentation experiments). Other considerations are the possibility of the tip piercing the sample, and determining the torsional cantilever constant. All that being said, it can be rather useful for some relative / approximations for sample compliance. In addition to using the force panel for acquisition, the vertical nanoindentation panel can also be used- here the surface is ‘found’ with a low trigger force, then the indenter panel kicks in to apply a load under constant load, or force (system works under closed loop operation). More on the indentation panel is seen in Section XX. An example of the tip moving in the X (or Y) dimension as force is applied to the surface can be seen in Figure 11.4.5.1- a ~80nN load was applied with a 1.45 N/m Si cantilever into some PDMS. Notice the slipping of the cantilever in the extension curve (red)- this is the tip slipping on the surface as the load is applied- hardly a uniform load. De-convolving this would be very challenging and laden with error. nN 80 60 40 20 0 pdms0002Force_Ext pdms0002Force_Ret 500 400 300 200 100 0 nm Figure 11.4.5.1: example of cantilever based indent on PDMS where tip stick-slips in the approach (red), while retract looks like a smooth unload (blue). Force (Y) vs. distance (X).
Ryan’s MFP-3D Procedural Operation ‘Manualette’ Version 10 (v080501; Igor 6.04A); 11.28 Instances where the cantilever gets pinned (arrows) as the Z piezo continues to push against the material often results in negative deflection (cantilever becomes bowed under tension), which give negative slope in the Force vs. Indentation plots (Figure 11.4.5.2). This will confuse the fitting algorithm because negative slope tricks the software into thinking the cantilever is moving away from the surface (remember extending piezo). In this example, some cantilever based indentation was occurring on an active ingredient of a common pharmaceutical (data courtesy of Jonas, <strong>University</strong> of Iowa). 50nN Force (nN) 40 30 20 10 0 -60nm -50 -40 -30 -20 -10 0 Indentation (nm) Figure 11.4.5.2: Effects of negative deflection that can occur when cantilever gets ‘pinned’ as piezo continues to push towards surface: Indenation vs. Force gives negative deflection- AFM thinks cantilever is movi n g away as piezo Figure 11.4.5.3 shows and where the cantilever spring constant was too floppy to show a difference between the infinitely hard calibration surface, and the material. This is apparent due to the relative slopes of the two force curves- they are very similar. TAKE HOME MESSAGE: cantilever selection is very important. 60nm 40 Deflection (nm) 20 0 soft material Hard Surface -20 -40 0 -5 -10 Figure 11.4.5.3: Example of too soft a lever being used- hard surface contact regime very similar to force on material- means cantilever cannot apply enough force to push into it. Empirical Considerations for acquisition- Aside from the empirical considerations mentioned in Section 11.1. X, here are some additional considerations- • Tip material vs. sample material hardness- if the tip material is similar in hardness/ modulus to the sample, a two spring system is in play, further adding to the error- especially is soft. In indentation, the indenting tip should be much harder than the sample (Si has a Moh’s hardness scale of ~2- 3). • sample mounting is important- use a good glue to eliminate this intended adhesion layer as a possible spring under the substrate. This can manifest itself as a hysteresis between the approach and retract curvesmost noticeable in the contact regimes. • Elastic vs. plastic deformation: this will affect the load and unload a great deal. The shape of curve and give qualitative sense to what the material is doing during a loading cycle.
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