Micromechanical characterization of small volumes by means of nanoindentation

Abstract

Mechanical characterization of micro-volume systems, as thin films or micro-sized phases embedded in multiphase materials, has attracted special interest in the last decades since different micromechanical techniques have been developed to characterize microdevices and materials at the micro and nano scale and it has become apparent that mechanical properties may depend on the analysis scale. An example is the way a crack grows in a bulk material that is likely to be different from crack propagation in a micro-volume where crack and microstructural dimensions are comparable. Consequently, there is a need of a detailed knowledge of material properties at micro and nano scale to design materials with advanced mechanical properties. In this way, micro and nanoscale science and technology enables to improve new materials and applications at macroscopic scale through a sound micromechanical design. The accuracy of test methodologies will depend on the size scale in which specific mechanical properties are studied. Micro scale is usually defined as the length scale in the range of 1-1000 microns, whereas nanoscale is usually defined as smaller than a one tenth of a micrometer in at least one dimension, although this term is sometimes also used for materials of larger dimension but smaller than one micrometer. Efforts to characterize the mechanical response of small volumes have led to the development of a variety of test methodologies, as uniaxial micro testing machines, micro beam cantilever deflection or nanoindentation devices. Challenges of testing at the micro scale include micro specimen preparation and handling, the application of small forces, and stress and strain measurement. Nanoindentation appears as the easiest way to study local behaviour on thin films or micro-sized phases, since no special sample preparation is required and tests can be performed quickly and inexpensively. Nanoindentation tests consist in the application of a controlled load on the specimen surface through the direct contact with a sharp diamond indenter and recording the evolution of the load versus the penetration depth of the indenter. The use in engineering of thin films, advanced coatings and materials with small tailored microstructures has led to the analysis of mechanical properties of very small volumes in which size effects might be important. Efforts to design and model the reliability of small-scale devices are directly dependent on the availability of accurate and reliable measurements of relevant mechanical properties at small scales. In designing structural or machine components an important step is the identification of the main micromechanical damage mechanisms. It is particularly interesting to determine the first fracture step, i.e., the crack nucleation in order to optimize the material resistance to crack nucleation. Stable brittle fracture takes place easily by the contact of a hard indenter on a brittle surface; this methodology is known as indentation fracture. Indentation fracture yields valuable information on the fundamental processes of brittle fracture in covalent-ionic solids, and detail on subsidiary deformation processes in the contact region; it provides ‘controlled flaws’ for systematically evaluating fracture properties, and it serves as a simple microprobe for determining material fracture parameters, toughness, crack-growth exponent, etc. For materials that exhibit R-curves behaviour, it affords a much needed bridge between the short-crack domain of microstructural flaws and the long-crack domain of traditional toughness testing; mainly in the study of the first regimes of crack propagation. The great appeal of the indentation methodology is its versatility, control and simplicity, requiring only access to routine hardness testing apparatus. In order to study the mechanical behaviour of small-volumes and micro-sized phases, nanoindentation has become a suitable technique for the mechanical characterization of small-volumes and micrometer – sized phases, in terms of hardness (H), elastic modulus (E) and fracture toughness (Kc). While H and E can be routinely measured by nanoindentation from the load – displacement curves, the evaluation of Kc of hard micro-sized phases can in principle be measured from the length of the cracks at the corners of the indentation. This method of evaluation of Kc is known as Indentation Microfracture (IM) and it was proposed in the 1970s for Vickers indentation cracks in bulk materials. However, the design of new materials leads to ever smaller microstructures, hence lower loads and sharper indenters has to be used in order to concentrate the deformation and fracture only in the very small volume of phases of interest. Mechanical characterization of small volumes, has recently received much attention, and many works have focused on the determination of Kc by nanoindentation following the IM method. Nanoindentation allows using low loads needed for accurate micromechanical characterization with high spatial resolution. However, the use of a different kind of tip geometry and load range in nanoindentation technique raises some questions about the applicability of the existent fracture toughness equations which were developed in the past mainly for Vickers tips and for loads typically more than two orders of magnitude higher. Therefore, for a better knowledge of the micromechanical behaviour of brittle materials, this work is directed to the study of indentation microfracture applied to small volumes, focussing on the understanding of the fracture behaviour of brittle materials in terms of indenter tip geometry, applied load and crack morphology generated. On the other hand, since it is of a scientific and technological interest to understand the mechanical response of micro-volume systems, the feasibility of extending the IM developed for brittle bulk materials to engineering systems formed by micro-sized hard phases in multiphase materials or thin films will be also studied.