T. Chudoba

Investigation of creep behaviour under load during indentation experiments and its influence on hardness and modulus results

To improve the accuracy and comparability of hardness and modulus results from nanoindentation experiments an evaluation of the creep behaviour is required. Creep depends on the material and normally diminishes to very low values within some seconds. Nevertheless, it influences the maximum depth and the upper part of the unloading curve in a way that measurement errors of more than 20% may occur. In this work, a detailed analysis of the creep behaviour for different film and substrate materials is done. In addition, the influence of loading time and hold period at maximum load on the hardness and modulus results is investigated. The results show that especially for materials with low hardness-to-modulus ratio (mostly metals), the modulus results are not reliable if the hold period is chosen too low. Hold periods are proposed in dependence on the material type that should be kept for high accuracy measurements.

Determination of elastic properties of thin films by indentation measurements with a spherical indenter

Abstract Indentation is an important method for the determination of mechanical properties of surfaces and thin films. It is well known that the measurement results from thin layers are strongly influenced by the substrate properties. For hardness measurements it is frequently quoted that the indentation depth should be less than one-tenth of the film thickness (1/10th rule). This rule is often not practicable for thickness values below 1 μm. Therefore a correction method is required that allows the separation of substrate and film properties from the load-depth data. Moreover, the calculation is complicated if plastic deformation occurs. The use of a spherical indenter allows one to remain completely within the elastic range if the indenter radius is large enough and the load is low enough. In this case a novel analytical solution for the elastic deformation of a film on a flat substrate can be used to simulate the load-depth data. With this solution the determination of Young’s modulus of thin layers is possible independent of indentation depth and film thickness. Measurement data from a UMIS-2000 indentation system for different film substrate combinations are compared with theoretical results. It is shown, that a separation of the elastic film properties is possible. For metal films on Si the load-depth data did not differ from that of uncoated substrates. This can be explained mainly by delamination.

Steps towards a mechanical modeling of layered systems

Up to now the search for suitable film substrate combinations with respect to mechanical applications is mainly based on empirical approaches using results of hardness, scratch, wear or other tests. It is desirable to cut down this time consuming process by a higher degree of modeling. Steps towards this aim are presented using a novel methodology for the evaluation of the response of coated substrates to mechanical contact. This approach is based on the combination of a recently developed theoretical method (the method of image loads) and high-accuracy indentation experiments using spherical indenters. For different film substrate combinations it is shown how accurate and reliable values of mechanical film parameters like Youngs modulus and yield strength can be obtained, even for films below 100 nm thickness. Then the theoretical method is used for the calculation of critical loads to failure of film-substrate combinations in dependence on the radius of the counterpart. It is shown how the addition of an intermediate layer can improve the whole compound and which properties it should have. Finally the influence of an additional tangential load due to friction in a sliding motion on the stress fields is investigated. The conventional scratch test is discussed in the light of the latter results.

New possibilities of mechanical surface characterization with spherical indenters by comparison of experimental and theoretical results

Abstract The topic of this work is a novel approach for the determination of mechanical properties of thin films on a substrate based on the theoretical modeling of spherical indentation into a film substrate system together with its adequate experimental realization. First, some results of a novel analytical solution were compared to results of finite element (FE) calculations and to flat punch models. Then, SiO2 layers on silicon were investigated as an example. The measured load-displacement curves for spherical indentations were compared to curves simulated by means of the theoretical model. When using appropriate elastic parameters of the film and substrate a complete agreement can be achieved. Finally, the onset of plastic deformation within SiO2 was determined by a multiple partial unloading procedure with a 4 μm sphere. It was found that the load necessary to start plastic deformation in a 538 nm SiO2/Si system is only 60% of the value that was obtained for a 2007 nm SiO2 layer, although the plastic hardness is the same. Using the theoretical model it can be shown that the plastic deformation starts within the film and that - despite the different critical loads measured - the critical von Mises stress is the same.

Determination of mechanical film properties of a bilayer system due to elastic indentation measurements with a spherical indenter

Abstract A recently developed theoretical model represents the generalization of the indentation of a sphere into an infinite homogeneous halfspace to the problem of a Hertzian load acting on a halfspace covered with one or more films having different elastic properties. The model allows the analytical calculation of the complete elastic stress field and the deformations within the films and the substrate. Some results of the model shall be confirmed by nanoindentation experiments using an UMIS-2000 nanoindenter into Si3N4/SiO2 and SiO2/ Si3N4 double layers on BK7 glass and Si(100) single crystal. The materials used allow accurate measurements due to their homogeneous, amorphous structure as well as low surface and interface roughness. After the determination of the instrument compliance and the real, depth dependent indenter radius the measured load–depth data are compared with calculated results. It is shown that measurement results can be correctly interpreted by the model. The onset of plastic deformation is investigated for the same samples by multiple partial unloading experiments with a 4-μm radius diamond sphere. The critical load at which a first deviation from a wholly elastic response occurs is used for a stress calculation with the model. The mechanical behavior of the different film combinations is interpreted by means of the von Mises comparison stress. The measured results, together with the analytical modeling, allow an optimization of the thickness and modulus of the individual layers to get a maximum mechanical stability.

Investigation of coating substrate compounds using inclined spherical indentation

The complete analytical approach for both the elastic normal and tangential Hertzian loading of a coating substrate compound is presented in this paper. This theory is intended to act as a tool for experimental applications. Here, the analytical model will be applied to a hard coating substrate compound. Its validity will be discussed with the help of finite element calculations of a spherical/plane stress model. In addition, experimental data obtained for an aluminium coating on glass and measured with an UMIS 2000 indentation system will be discussed.

Contact modelling in the vicinity of an edge

In this paper the problem of a spherical contact in the vicinity of an edge will be considered by analytical modelling and nanoindentation experiments. This problem attracts interest because of the increasing use of nanoindentation as a tool for the investigation of mechanical property profiles of cross-sectioned samples. The paper is separated into three parts: first, a short introduction will be given into the problem of the theoretical modelling and the mathematical procedures necessary to solve load problems for so called quarter spaces, which deal as a model for rectangular bound bodies. In a second part the effect of the edge and its distance from the contact centre on the measurable parameters indentation depth and force are discussed with experimental results obtained using a UMIS-2000 nanoindenter system. Finally an illustration of the elastic stress and displacement field resulting from a Hertzian load in the vicinity of an edge is given.

Neue Möglichkeiten zur Charakterisierung dünner Schichten mit Indentermethoden

A novel methodology for the evaluation of the response of coated substrates to mechanical contact is presented which is based on the combination of a new theoretical method and high-accuracy indentation experiments using a spherical diamond indenter, The concept may be extended in many respects and will be illustrated here only with few special examples: Using low loads, i.e. staying completely in the elastic region, the elastic parameters of film and substrate can be measured with high accuracy. When the indentation experiments is extended until failure of the coating substrate compound, the full stress and strain fields in three dimensions in the very moment of failure can be calculated. The knowledge of those fields enables one to draw conclusions on the relevant mechanisms. Once the failure mode has been identified, layer systems with optimum resistance to that failure mechanism can be found by theoretical simulations.