Nanoindentation testing of time-dependent materials is becoming increasingly relevant with advances in areas such as polymer science and the study of materials in biological systems. However, mechanical characterization at micrometer or nanometer length scales is not trivial in such materials that may be highly compliant, heterogeneous, or possess unique morphological characteristics. Although many researchers have made progress in overcoming key challenges that are involved in testing non-traditional materials, significant advancements must still be made to optimize testing methodologies and analytical techniques.

There is no one “right” way to perform mechanical testing of viscoelastic materials. The information that is desired drives selection of the testing methodology. The varied testing methodologies and analytical approaches are nicely summarized by Oyen (2006) and in a related discussion on IMechanica in a forum on viscoelastic contact. The exact testing and analytical methodologies are not the intended focus of this Journal Club discussion, rather the intent is to highlight specific challenges that face our community in furthering our ability to study increasingly complex materials using nanoindentation. Plus an additional topic for discussion is proposed with the goal of improving the ability of the larger scientific community to obtain high quality indentation data on time-dependent materials.

Polymers and hydrogels can be engineered in layers or with a gradient in composition, porosity, and/or modulus. The inherent hierarchical nature of biological materials enables substantial variations in composition and structural organization at length scales ranging from nanometers to millimeters. The interface of engineered and biological materials, such as a metallic or ceramic implant that contacts both hard and soft tissues, demonstrates a common and interesting problem where modulus varies significantly at small length scales, say between a titanium implant material and the adjacent bone or soft tissue. Functionally graded materials can be constructed of conventional hard materials, polymers, or hydrogels and possess graded porosity, composition, or modulus permit matching or ingrowth of native tissue in applications such as bone replacement (Kromova et al., 2001). A key issue arises in that a functionally graded material, a sample containing an interface of bone to metal to soft tissue, or even diseased hypermineralized tissue within a more compliant and less viscoelastic region of bone may be examined in a single indent array (shown here in figure 4) without any alteration in the testing parameters between compliant (and time-dependent) versus stiff (and less time-dependent) regions.

Heterogeneous and composite materials. The use of conventional analytical techniques in nanoindentation requires assumption of material homogeneity. In truth, most materials are heterogeneous and can be considered as composites. Nanoindentation measurements in bone, for example, are commonly analyzed using the Oliver-Pharr method. However bone is made up of many materials: collagen, mineral, and water. It contains pores that range in size from nanometers to millimeters. Bone mineral is most often assumed to consist of hydroxylapatite when it is instead a conglomeration of many forms of tricalcium phosphate, octacalcium phosphate, and other minerals that may exist at length scales that are large enough to influence nanoindentation measurements. The complications posed by indentation of nanocomposites are examined by Constantinides et al. (2006) in titanium-titanium boride (Ti-TiB). Arrays of uniformly spaced indents that randomly ‘selected’ Ti, TiB, and TiB2 in four different samples provided a reasonable indication of material composition and an approach for testing and analyzing indentation data from composite materials is presented.  This paper opens the door for discussion of nanoindentation of composites containing a single or multiple time-dependent phases such as presented by Ko et al. (2006) for a biomimetic bone replacement material of hydroxylapatite in a gelatin matrix.

Is a ‘primer’ on indenting soft materials needed? Papers published as recently as this year continue to present indentation data on time-dependent materials that demonstrate creep during the unloading period and analyzed using the Oliver-Pharr method. While disappointing, this common scenario presents an opportunity to discuss how to effectively educate non-experts to perform high quality indentation testing despite a lack of detailed contact mechanics knowledge. It appears that many ‘casual’ nanoindentation users exist in the general scientific community who are want to quickly obtain modulus or hardness data on their specific materials. This sounds trivial until one considers the wide range of both candidate materials for nanoindentation testing as well as the range of potential users’ areas of expertise. Nanoindentation systems are marketed to and used by research physicians, biologists, and others who may not have a strong grasp on mechanics as well as physicists, materials scientists, and engineers!

A key issue is thus raised for discussion in that the mechanics community serves as the educators for the general scientific community. Do we have a responsibility to establish standard approaches for indentation of soft materials to enable general educated user to readily select an appropriate testing approach and method of analysis? Should we establish standards designed to improve data quality and critical evaluation of another’s work? We stand to gain tremendously from the potential knowledge that can be gained by indenting unprecedented and unique materials that may come from scientists in varied disciplines. However our gain is limited by the knowledge that we disseminate, how effectively it is conveyed to the general scientist, and their ability to readily produce high quality data.

While it may be argued as to how well such an effort might catch on, there are many long-standing and effective cases that set precedence for key publications aimed to guide a field of non-experts. One highly successful example lies in the bone research community where histomorphometry, a technique to study bone tissue growth in 2-D, was relatively inaccessible to non-experts prior to the publication of a standard for nomenclature, symbols, and units (Parfitt et al., Journal of Bone and Mineral Research, 1987). While this paper is not suggested for specific review in this Journal Club, it serves as an example where a group of experts enabled the growth of a highly valuable technique by making it more accessible through the sharing of information and standardization of key factors. To date, this article has been cited 2069 times. A similar approach may positively benefit those interested in indentation of time-dependent and complicated materials by creating a set of standards that, say, call for inclusion of a representative force-contact depth plot. Such simple additions would readily reveal critical characteristics, such as the presence of creeping on unloading, and thus aid in the interpretation of the quality of the work presented.

The papers included here for discussion are linked to below: 

(1)  Microhardness studies on functionally graded polymer composites. M Krumova, C Klingshirn, F Haupert and K Friedrich. Composites Science and Technology: 61 (4): 557-563, 2001. 

(2) Grid indentation analysis of composite microstructure and mechanics: Principles and validation. G Constantinides, KS Ravi Chandran, FJ Ulm and KJ Van Vliet. Materials Science and Engineering: A 430 (1-2): 189-202, 2006. 

(3) Mechanical properties and cytocompatibility of biomimetic hydroxyapatite-gelatin nanocomposites. CC Ko, ML Oyen, AM Fallgatter, JH Kim, J Fricton, WS Hu Journal of Materials Research: 21 (12): 3090-3098, 2006. 

Initial points to consider include:

-        Current gaps in our knowledge and ability to adequately test materials with complicated functional properties or structures that also possess time-dependence

-        How are we currently limited by experimental capabilities (i.e., instrumentation and methods) and analysis techniques?

-        Is it reasonable or even feasible to consider the development of standard approaches? Or is this a gross oversimplification that would not be applicable to the broad range of materials that are candidates for analysis using nanoindentation?

Links to additional references are included above. Please feel free to request pdf versions of files from me via email.