Depth sensing nanoindentation

In this chapter, we overview basic techniques for making nanoscale adhesion and mechanical property measurements. Both quasi-static and dynamic measurements are addressed. In Section 2 of this chapter, we overview basic AFM instrumentation and techniques, while depth-sensing nanoindentation is overviewed in Section 3. Section 4 addresses recent advances in instrumentation and techniques... 

Indentation has been used for over 100 years to determine hardness of materials [8J. For a given indenter geometry (e.g. spherical or pyramidal), hardness is determined by the ratio of the applied load to the projected area of contact, which was determined optically after indentation. For low loads and contacts with small dimensionality (e.g. when indenting thin films or composites), a new way to determine the contact size was needed. Depth-sensing nanoindentation [2] was developed to eliminate the need to visualize the indents, and resulted in the added capability of measuring properties like elastic modulus and creep. 

Fig. 9. (a) Depth-sensing nanoindenter model and (b) simple mechanical model for force controlled indentation assuming purely elastic contact mechanics. 

Depth-sensing nanoindentation is one of the primary tools for nanomechanical mechanical properties measurements. Major advantages to this technique over AFM include (1) simultaneous measurement of force and displacement (2) perpendicular tip-sample approach and (3) well-modeled mechanics for dynamic measurements. Also, the ability to quantitatively infer contact area during force-displacement measurements provides a very useful approach to explore adhesion mechanics and models. Disadvantages relative to AFM include lower force resolution, as well as far lower spatial resolution, both from the larger tip radii employed and a lack of sample positioning and imaging capabilities provided by piezoelectric scanners. 

Nanoscratch tests have been used to simulate the effect of third-body particulate wear debris on component surface scratching during use. The load at which the co-efficient of friction or friction force suddenly increases is identified as the critical load, and is used to evaluate scratch resistance and adhesion strength. The depth-sensing nanoindenter, usually equipped with a conical indenter, can elucidate the mode of failure, whether elastic/plastic deformation, cracking, or delamination. 

Fang Z et al (2009) Mechanical properties of porous silicon by depth-sensing nanoindentation techniques. Thin Solid Films 517(9) 2930-2935... 

For depth-sensing nanoindentation, a controlled, variable force is applied to a sample by the indenter and the resulting displacement of the indenter is measured. The resulting load vs. displacement data, together with the indenter geometry, can be analyzed to obtain hardness and elastic modulus using well established mechanical models (14). The simultaneous measurement of load and displacement also allows study of creep (time dependent strain response due to a step change in stress) (15,16). 

Experimental techniques most commonly used to probe the plastic properties of thin film materials involve direct tensile loading of either a freestanding film or a film deposited onto a deformable substrate material, microbeam bending of films on substrates, substrate curvature measurement or instrumented depth-sensing nanoindentation. Sahent features of these methods, as well as specific examples of the adaptation of these methods for the study of mechanical properties in thin films, are briefly addressed in the following subsections. 

Mechanical Properties Using Depth Sensing Nanoindentation... 

Depth sensing nanoindentation is a viable alternative to AFM beam bending for mechanical property measurements on polymer surfaces. The technique provides quantitative results owing to careful calibration of the system compliance and tip area/shape. It suffers from the high minimum load (about 1 iN) necessary for imaging prior to indenting. A new approach, which combines the best elements of both approaches, is still needed. 

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