Importance of surface preparation on the nano-indentation stress-strain curves measured in metals

Abstract

In this work, we investigated experimentally the various factors influencing the extraction of indentation stress-strain curves from spherical nanoindentation on metal samples using two different tip radii. In particular, we focused on the effects of (i) the surface preparation techniques used, (ii) the presence of a surface oxide layer, and (iii) the occurrence of pop-ins at the elastic-plastic transition on our newly developed data analysis methods for extracting reliable indentation stress-strain curves. Rough mechanical polishing was shown to introduce a large scatter in the measured indentation yield strengths, whereas electropolishing or vibropolishing produced consistent results reflective of the pristine sample. The data analysis techniques used were able to discard the portions of the raw data affected by a thin oxide layer, present on most metal surfaces, and yield reasonable indentation stress-strain curves. Experiments with different indenter tip radii on annealed and cold-worked samples indicated that pop-ins are caused by delayed nucleation of dislocations in the sample under the indenter.

High Throughput Assays for Additively Manufactured Ti-Ni Alloys Based on Compositional Gradients and Spherical Indentation

“…The protocols used in this study to produce the indentation stress-strain curves [6,8,9] are briefly summarized next (see Fig. 2).…”

“…The sample preparation involved a specific sequence of steps that proceed through grinding (P240, P1200 SiC paper) and then finished with polishing steps with decreasing abrasive particle size (9, 3, 1 μm diamond suspension), with each step ensuring that the surface deformation introduced is removed in the next step. The importance, particularly in indentation, of removing the damaged layer from mechanical polishing without introducing further mechanical deformation has already been expounded [9]. This is mainly because of the very small indentation volumes involved in the indentation protocols employed in this work.…”

“…In this regard, recent advances in spherical indentation stress-strain protocols [6][7][8][9][10][11][12][13] offer many advantages. The conventional practices in indentation testing rely on sharp indenters and analyze the unloading segments after imposing substantial amounts of indentation loads (or depths) to arrive at estimates of modulus and hardness [14][15][16].…”

Microstructure and local mechanical property evolution during high strain-rate deformation of tantalum

“…Typically, the default protocols in the vendor-provided analyses software do not perform adequately in this respect, mostly resulting in the overestimation of the sample surface [4]. The problem is often made worse by surface roughness issues, non-ideal geometry of the indenter and the effects of the surface preparation techniques [17]. The load and displacement corrections (P * and h * respectively) are calculated by performing a linear regression analysis of the expected relationship between the measured load signal,P, the measured displacement signal,h, and the continuous stiffness measurement, S. For the initial elastic loading segment in the spherical indentation of a flat isotropic sample surface this relationship is given by Hertz theory as:…”

Mechanical Characterization of Mesoscale Interfaces Using Indentation Techniques

“…40,41 Modern indenters with their impressive load and displacement resolutions and continuous stiffness measurement (CSM) capabilities have now made it possible to study systematically the mechanical responses of regions near mesoscale interfaces as a function of the distance from the interface. While the experiment itself is relatively simple (and capable of high throughput), the central challenges lie in the sample preparation 42 and in the post-test data analysis protocols employed to recover meaningful properties intrinsic to the material volume probed in the experiment. This review will summarize the main advances already made in studies of mesoscale interfaces in both metals and polymer composites using indentation methods.…”