Nanoindentation

The nanoindentation (also instrumented indentation testing ) is a method of material testing to determine the hardness of materials at small length scales (nanometers, nm). Main area of ​​application is the determination of hardness of thin coatings.

Historical Summary

The investigation of the material properties of metals has been since the Middle Ages of interest. At least in the course of industrialization, it has become essential to be able to produce high-quality metallic materials for a variety of tasks. In order to satisfy the ever growing demands on the materials, is a good understanding of the material parameters, such as hardness, brittleness and roughness, imperative. All of these material properties were early by using so-called Indentationstests, where initially only the plastic impression after Opening a hammer was measured, examined. Conceptually, the present method for the measurement of hardness correspond still the procedure of Friedrich Mohs: Using standard materials of different hardness is a material to be tested is plastically deformed. The hardness is then determined in relation to the standard materials Based on the resulting plastic deformation. On the Mohs scale of 1 (talc ) to 10 ( diamond ) the materials used in the present work, copper and aluminum have a hardness of 3 or 2.3 ... 2.9. It turns then as now the question of how certain macroscopic material properties are influenced. Often desirable properties are mutually exclusive: It is, for example, a particular challenge is to find a simultaneously tough and ductile material. Today we can look at the atomistic level for the causes of the macroscopic material properties and there are increasingly more options are available to edit on the smallest length scales materials. At least since the seminal speech of Feynman miniaturization of consumer goods (especially electrical ) has been used. This is of course entirely new demands on the mechanical properties of the materials. At the same time the resolution of the measuring equipment has increased significantly. Was it 1968 yet groundbreaking, perform stress tests on the micron scale, so today are measurements on the Å - scale feasible.

Method

The nanoindentation hardness is derived from the classical test, but takes place in a much smaller scale. It is pressed with known geometry in the surface to be tested a diamond tip. Due to the miniaturization of the structure it is impossible to measure the area of remaining in the specimen hardness impression, as is done in the conventional methods for measuring hardness. Therefore, the penetration force applied and the penetration distance of the tip can be measured simultaneously in the nanoindentation during the experiment. Due to the known geometry of the probe and the measured data for penetration and penetration distance, the hardness, the contact area, and subsequently be calculated.

The measuring head for nanoindentation ( Hysitron Triboscope ) consists of a three-plate capacitor and is mounted on an atomic force microscope. An electrical voltage applied to the capacitor, produces a force on the central capacitor plate that presses a pin with a diamond tip into the surface to be tested. The displacement of the central capacitor plate causes a change in capacitance of the capacitor to give the required strength and penetration data for determining the hardness. As a boundary between micro -and nano- range 14577 0.2 microns are determined according to DIN EN ISO. Most devices for instrumented indentation measure but also in the micro range up to the border to Makroprüfbereich of 2 N.

Why nanoindentation?

Why nanoindentation? First, it is of course in general a target material properties measured with ever higher resolution - and thus to minimize the relative error of the measurements. The smaller are also the groups of samples, the more accurate they can prepare and therefore the smaller the fluctuations due to impurities. The influence of each parameter of a material can therefore be better separated. During the indentation is on the micron scale now quite well understood, many questions are still open in the nanoindentation (on the nm scale ). This means in particular that the atomistic processes and their effects on the macroscopic material properties are not yet understood in detail. Nanoindentation in metals is a complex process that can be studied with the both elastic and plastic properties. When the indenter is pressed into the material, this first deformed elastically. With further indentation up to the critical penetration depth dyield the resulting plasticity, leading to a drop in the load force. Once the first dislocations were generated spread them out, react with each other and there is material transport instead. Most of these atomistic processes have direct consequences for the force Find, acting on the indenter; So the indenter penetrates no longer an ideal, but a hardened through the Indentationsprozess a material ( Workhardening ). While an ideal crystal is characterized by a pure, perfect crystal lattice, a real crystal is interspersed with various types of lattice defects. Induces the crystal lattice have a length scale of the substrate, then a further length scale determined by the size and the interaction range of the lattice defects. This length scale is inherently greater than that of the lattice constant. However, because further vary the macroscopic properties significantly with the microscopic lattice structures, crystals are real so dominated by a coupling of the different length scales. It is therefore an understanding of the macroscopic material properties and an understanding of the atomistic processes necessary. The method of nanoindentation is particularly well suited to investigate the relationship between microscopic material parameters and macroscopic material properties. While it is closed in a classical hardness test of macroscopic observations to microscopic properties, we follow here a reciprocal approach, are examined in the direct microscopic properties. An investigation of the atomic structure of materials makes it possible to develop new theoretical continuum models. For example, the geometric model of the required displacements due nanoindentation was developed. The reaction of a material to the load by the indenter can be very different. The easiest to understand is the linear elastic region - even if it is to describe complicated by the multi-axial stress in detail. For he is not only reversible, but it is also retained the crystal structure. With respect to its crystal lattice structure in the ideal elastic range remains homogeneous. It becomes more complicated when the crystal structure - ie the bonds between the atoms under the load changes, the substrate is thus inhomogeneous. Thus phenomena were discovered by the nanoindentation, which have not been described by the continuum theory: It may occur, for example, phase transformations of the lattice structure, which lead to ( reversible ) load interruptions in the power of Find. This load dips then stir ago but not from the onset of defect formation and are therefore not included in conventional continuum theory models. Next may occur for other classes of materials very different phenomena. Thus, shear bands can be observed for example in the amorphous silicon. General binding changes can lead to dramatic effects: So recently, a crystal structure was found, which is harder than diamond with lower density. Although the method of indentation actually acts directly on the free surface, as yet affected the activity within the substrate, the reaction of the material on the burden of the indenter considerably. The obvious conclusion that the measured properties are dominated by the free surface is deceptive, because the indenter acts like a magnifying glass and focus the maximum shear stress in the material. The long reach in the interactions of the tension in the substrate including conclusions about the bulk properties can be drawn by means of indentation. This makes the method of indentation for the investigation of single crystals of interest. While a real monodisperse crystal in its properties is largely determined by dislocations and grain boundaries, we only consider ideal single crystal materials here.

Indenterform

The shape of the indenter tip Indenterform is usually either pyramidal, rectangular or spherical; it strongly influences the results obtained. For it is obvious that a sharp indenter will leave an impression other plastic as a spherical indenter. Moreover, since the tension under the indenter are very different, other slip systems are activated each. When the indentation method so there is a strong influence on the observables by the measurement. It is therefore necessary to a measured value always include the method used.

Indentation hardness test as

Indentation in general is the classic endurance test of forging, nanoindentation is the acid test of materials scientists. Because the method of hardness measurement by indentation is basically quite simple. Thus, the indenter is pressed with a certain force in a material and the acting force Find measured. Then the plastic impression can be measured and from this the contact surface to be approximated. The hardness is then calculated on the correlation.

Material properties at the nanoscale

Through atomistic movements, material properties can change dramatically. This motivates the production and analysis of so-called nano- materials. Such materials may be harder than diamond or plastic deformation can heal. On the other hand, are not yet fully understood for classic materials, the causes of their material properties. These include in particular the mechanisms of plastic deformation of metals that are the subject of current scientific debates.

Application

Besides the ability to determine the hardness equipment for instrumented indentation are usually also able to identify a the modulus size related. This is usually determined from the first third of the unloading curve. In this range, purely elastic deformation will be accepted. Through modifications to the meter hardness and elastic modulus can be determined also deeply resolved by the monotonic increasing force superimposed on a high frequency signal and the response of the penetration depth is evaluated for each interval. This method is particular to the graded layers are used. Can also be done through the integration of heating and cooling systems, a temperature- dependent determination of the material parameters. Due to the high sensitivity of the measurement on thermal drift, however, the temperature field must be stationary during the measurement to a great extent. There are already methods to gain by coupling of nanoindentation and FEM simulations information about the anisotropy of the layer properties as well as for the plastic flow behavior.

The practical applications of the indentation are very diverse. Of very practical applications towards, such as that a substrate directly in the system - ie in situ - can be measured without it expand on the investigation of thin layers to the investigation of the surface of hard drives are the obvious areas of application already covers a very wide. Since defects occur in a variety of crystalline structures - for example, in ice - Indentationsversuche can also be performed on these systems. For the semiconductor industry, the mechanical properties of silicon ( mixtures ) are interesting. Thus, for example, indene benefits under voltage standing silicon wafer and then the load-dependent resistance to be measured. In addition to the change of resistance by a macroscopic deformation and defects affect the electrical properties. Similarly, the indentation is the method of nano- scratching, which again similar to the investigation with an Atomic Force Microscope ( AFM) is even. A completely different field of application is biomedicine. It may be for example indene advantage in polymers. Renewed investigations have shown that diseases also be harmful to an organism by altering the mechanical properties of blood cells so that they are not stretchy enough to slip through the thin vessels. Therefore, the study of the elastic properties of cells or viruses is interesting.

Swell

• Nanoindentation, TU -Graz
• Nanoindentation, Fraunhofer Institute for Surface and Surface Technology IST
• Nanoindentation, University of Erlangen -Nuremberg (PDF, 6.3 MB)
• Nanoindentation, Max Planck Institute for Iron Research