ICMCTF2016 Session H2-1: Advanced In-situ Mechanicall Testing of Films and Coatings

Recently, there has been an increased interest in high temperature nanoindentation due to improved instrumentation and better experimental techniques. This has further extended the capability of nanoindentation based techniques to study temperature effects on the mechanical properties of small volumes of materials. Currently, maximum test temperatures are often limited by the indenter tip material and the fastening techniques. Many factors including hardness, elastic modulus, thermal expansion coefficient, fracture toughness, chemical reactivity etc., need to be considered in selecting a material for a high temperature indenters. Recently, several single crystal refractory carbides have been identified for possible use as indenter tip materials such as TiC, NbC, VC, WC, and ZrC. Here, we present mechanical properties measured from nanoindentation on these single crystal materials to assess their suitability for use as an indenter tip. New techniques for fastening the tips are also discussed.

In recent years TiN/CrN multilayered PVD coatings are well known for their unique properties like high hardness and good wear resistance. Large number of studies has been reported, mainly focused on tribology characterization as well as detailed TEM analysis of interfaces. On the other hand, information on the correct determination of the fracture toughness and contact damage is rather scarce. A deeper knowledge is crucial to improve the performance of these materials and to enhance the lifetime of coated systems.

The present work aims to conduct a systematic micro- and nanomechanical study of the mechanical integrity of multilater TiN/CrN systems with different bilayer periods (8, 19 and 25 nm). In doing so, nanoindentation technique is implemented and corresponding deformation/damage mechanisms are also investigated. Work includes the evaluation of the interfacial strength response as well as the determination of the strength and fracture toughness as a function of the bilayer period through ex-situ tests by FIB-milling micropillars and microcantilevers for each multilayer TiN/CrN system.

It is found that the data extracted through ex-situ tests under different stress fields and damage scenarios for the multilayer approach is quite effective on promoting crack growth resistance mechanisms; thus, damage tolerance is clearly enhanced, as compared to the one exhibited by the single layer ones.

Heteroepitaxial microelectronic structures possess usually a complex sandwich morphology which is designed to fulfil not only electronic requirements but must be also mechanically optimized. The heterostructures should exhibit negligible overall residual stresses in order to avoid substrate curvature and guarantee mechanical stability.

In this contribution, we have studied the fracture behaviour of 1.7µm thick Al_xGa_1-xN sandwich heterostructures grown by metalorganic chemical vapour deposition (MOCVD) on a Si (111) substrate. Unnotched and notched microcantilevers with dimensions of 2x2x9µm³ were machined using focused-ion beam technique and controllably loaded using Hysitron PI85 indenter in scanning electron microscope. In displacement-controlled experiments, bending force and the deflection were recorded until fracture. Experimental data were used to evaluate Young’s moduli, fracture stresses and fracture toughness as a function of heterostructure morphology and deposition conditions. The results indicated that the fracture behaviour and especially the morphology of fracture surfaces correlate well with stress concentrations in the individual sublayers, which were characterized using Ion Beam Layer Removal Method (ILR) in as-deposited heterostructures.

The approach opens the way to assess mechanical behaviour of heteroepitaxial structures with sub-micron resolution and optimize residual stress depth-profile as well as stack morphology by tuning deposition process parameters.

Forging and cutting tools for high-temperature applications are often protected using hard nanostructured ceramic coatings. While a moderate amount of knowledge exists for material properties at room temperatures, significantly less is known about the system constituents at the elevated temperatures generated during service. For rational engineering design of such systems, it is therefore important to have methodologies for testing these materials to understand their properties under such conditions. Additionally, small-scale mechanical testing is of inherent importance for thin-films systems.

In this work, we present results on the fracture toughness evaluation of hard ceramic coatings using in situ micro-mechanical measurements at testing temperatures up to 500 °C. The fracture toughness behavior of a model hard CrN coating was first investigated using various micro-geometries and notching parameters. Toughness measurements at high temperatures highlighted the profound effect of the notching ion during small-scale fracture measurements. It was found that gallium ion implantation led to significant toughening of CrN, based on gallium dosage experiments and alternative notching using both xenon and helium sources. The effect of different notching ions was additionally understood through Monte Carlo simulations of energetic ion interactions in a dense ceramic matrix.

Measuring the uniaxial creep response from nanoindentation has been of great interest to the small scale mechanics community. However, several experimental and modeling challenges pose obstacles to direct comparison of indentation and uniaxial results. In this talk, we present new experimental procedures to address some of these issues that improve the precision and accuracy of high temperature nanoindentation tests. Indentation creep results at a number of temperatures up to 550 °C on commercial purity aluminum alloy will be presented. The activation energy for creep was found to be 140 KJ/mol/K, matching the value determined with high temperature tensile creep experiments extremely well.Uniaxial power-law creep behavior (stress exponent and pre-exponential term) is calculated from the indentation data for direct comparison of results to the uniaxial data. The results are in excellent agreement with the uniaxial compression/torsion tests over a wide range of strain rates and temperatures demonstrating the capabilities of the current experimental procedure to study high temperature creep. The relative contributions and interplay of indentation size effect, strain rate and temperature on the creep response will also be discussed. These results indicate that nanoindentation based techniques can be simple, quick and cost effective for high temperature mechanical characterization of small volumes of materials. A similar analysis on grade 91 steel with applications to the nuclear industry will also be presented.

We have integrated a Raman spectrometer with a nanoindenter to produce a valuable tool well suited for the study of carbon-based materials, with many advantages over two separate instruments. This integration allows rapid characterization of the chemical properties of carbon – and other materials - in situ following nanomechanical testing. We demonstrate the utility of combining these techniques for the examination of amorphous Si carbide (SiC), diamond-like carbon (DLC) and highly oriented pyrolytic graphite (HOPG) samples. The analysis of the uniformity, strain, stress and defects of these materials over large areas demonstrates the variations in material’s physical and mechanical properties. These results show the direct correlation between the nanomechanical properties (hardness and modulus, extracted from the nanoindenter) and the chemical information (molecular orientation, stress and conformation, provided by the Raman spectrometer). Researchers are using this important information to better understand growth mechanisms and improve growth techniques for these carbon materials.

The mechanical behavior of three different geometries of Al/a-Si nanostructures is studied using instrumented nanoindentation to investigate the role that geometrical confinement of the Al core plays on the deformation of these structures. Hemispherical Al nanodots of various diameters (100, 200, and 300 nm), horizontally-aligned Al nanorods of various length (200 nm, 600 nm, and 10 μm), and a 100 nm Al thin film were fabricated on Si substrates and then conformally coated with 300 nm of PECVD a-Si to represent structures with 3-, 2-, and 1-dimensional core confinement, respectively. The hemispherical structures display unique deformation-resistant behavior, with a load-displacement response characterized by discrete indentation signatures known as load-drops and load-jumps and no residual deformation after loading. This behavior is hypothesized to be enabled by dislocation activities within the confined Al core. As this confinement is reduced, through either increasing core diameter or decreasing the geometrical dimensionality of the confinement, the indentation signatures and deformation resistance are suppressed. Supporting molecular dynamics simulations show that higher core confinement correlates with improved removal of dislocations during unloading. This study provides a deeper understanding of the mechanisms that contribute to the deformation-resistant properties of Al/a-Si core-shell nanostructures that will enable their use in a variety of applications where the mechanical integrity of nanostructures is important.