Recent Advancements in Nanomechanical Characterization Techniques and Associated Technologies

Abstract

Nanoindentation [1] is a widely used technique for quantifying mechanical properties of microscale to nanoscale volumes of materials of relevance to shrinking technologies such as microelectronics, hard disk drives, and microelectromechanical systems. A particularly successful class of nanoindentation instrumentation is the scanning nanoindenter, which is capable of conducting scanning probe microscopy using the indentation probe as the imaging stylus. Although extracting elastic modulus and hardness from nanoindentation tests does not require direct observation of the residual indentation impression for the purpose of measuring its size, the ability to visualize surfaces in the sub-optical regime has proven to be empowering nonetheless. Intimate knowledge of the position of the indentation probe in relationship to specific surface features enables site-specific mechanical characterization of engineered microstructures, individual grains and phases of materials, and nanoparticles, as examples. Moreover, re-examination of the surface after completing the nanoindentation test can provide valuable insight into the deformation mechanism at hand.In recent years, nanoindentation technology has evolved from relatively simple instruments dedicated solely to quasistatic operation (slow loading and unloading) to nanomechanical / nanotribological test platforms supporting nanoscratch and nanowear (or nanomachining) experimentation, plus a host of novel mixed-mode techniques. Techniques involving dynamic forces superimposed onto quasistatic forces include the nanoscale analogue of dynamic mechanical (thermal) analysis [2] for determining complex modulus (storage and loss components) as a function of depth, frequency, or temperature, and a method of complex modulus imaging [3] for differentiating material phases on the basis of their spatial distribution of complex modulus. Figure 1 shows loss tangent (ratio of loss modulus to storage modulus) in an image format for a phasesegregated polymer blend. For certain classes of materials (e.g., ceramics and metals), a more appropriate mixed-mode technique is in-situ acoustic emission monitoring of nanoindentation [4] for detecting high-speed deformation transients associated with the onset of plasticity, phase transformation, fracture, or delamination. The manner of controlling the indentation process also has evolved. Recent work in augmenting the familiar PID control algorithm with a predictive feedforward loop [5] has considerably improved the performance of both load-controlled and depthcontrolled operation.For any important technique, the process of growing from infancy to maturity eventually leads to a major paradigm shift in usage. In the early years, nanomechanical testing typically involved a single sample mounted on the instrument and perhaps ten nanoindentation tests per sample. Today, nanoindenters are being exploited as screening tools in a number of combinatorial materials science workflows. Such high-throughput research environments demand reconsideration of the defi...