Micromechanical Testing of HM
Session Chairs
Prof Luis Miguel Llanes (Universitat Polytecnica de Catalunya, Spain)
Dr José Manuel Sanchez (CEIT-IK4, Spain)
Introduction
The Special Interest Seminar on MICROMECHANICAL TESTING OF HARD MATERIALS (to be held on Oct 16th as part of the EUROPM 2018 conference) addresses the state-of-the-art on the characterization of mechanical properties of hard materials at microscopic scale. The works to be presented include different ceramic metal composites with special focus in hardmetals and PVD and CVD hard coatings.
The objective of these tests is to obtain quantitative information about the mechanical behaviour of microstructural features with sizes in the order of a few cubic microns including, among others, single Co ligaments in hardmetals, individual WC-WC interfaces, samples extracted from CVD and PVD coatings, etc.
Different methodologies are currently available for obtaining stress-strain curves corresponding to these small volumes of material. In most of the cases, they involve the use of different FIB milling strategies (micropillars, microbeams, bridges, etc. ), instrumented loading systems (“In-situ” and other testing methods), high resolution electron microscopy and finite element modelling including crystal plasticity data. Key issues like metrological methods and the effect of residual stresses will be also discussed.
Presentations
Deformation and Fracture Mechanisms of Nanoscale Metallic and Metal-Ceramic Multilayers
Prof Jon M. Molina-Aldareguia (IMDEA Materials Institute, Spain)
Nanoscale multilayers or nanolaminates are nanostructured materials made up by alternating layers of two or more materials with a layer thickness below ~100 nm. These nanolaminates present very high strengths at ambient temperature. Their unique properties, typically measured by nanoindentation and/or micropillar compression are mainly a result of the high density of interfaces, which change the standard mechanisms of deformation when the layer thicknesses are below ~100 nm. The combination of dissimilar materials together with the small dimensions of the layers are also expected to significantly affect the fracture behavior. However, fracture properties have not been studied in detail so far, mainly due to the lack of appropriate testing techniques to determine fracture toughness at small scales. With the current development of novel nanomechanical testing techniques, to determine the fracture behavior of these heterogenous materials by microcantilever bending and/or micropillar splitting experiments. Examples focusing on a model nanolaminate system, Al/SiC, combining ceramic and metallic layers with thicknesses between 10 and 100 nm, will be shown.
A Metrological Study of Micro-pillar Compression Tests in WC/Co Hardmetals
Dr Helen Jones (National Physical Laboratory, UK)
The work presented assesses the feasibility of the micropillar compression technique for measurement of properties of WC/Co materials. The aspects investigated were the repeatability and consistency of the specimen fabrication and testing methods, dependence of specimen geometry variation on the apparent mechanical properties, and the influence of ion damage. The micro-pillars were fabricated in a fine grained WC, 16vol% Co material with gallium and xenon focused ion beams (FIB). They were compressed with a nanoindenter in situ in a scanning electron microscope (SEM), so that the mechanical deformation could be imaged simultaneously with acquiring load-displacement data. To achieve good statistics, multiple tests were carried out over a large range of micro-pillar sizes between 1 and 25 µm in diameter.
Small-scale Mechanical Response of Cemented Carbides
Prof Luis Miguel Llanes (Universitat Polytecnica de Catalunya, Spain)
The unique combination of hardness, toughness and wear resistance exhibited by heterogeneous hard materials (e.g. cemented carbides, PCD composites, PcBN systems and generic hard coating/ substrate combinations) has made them preeminent material choices for extremely demanding applications, such as metal cutting/forming tools or mining bits, where improved and consistent performance together with high reliability are required. The remarkable mechanical properties of these materials results from a two-fold effectiveness associated with their intrinsic composite character. On the one hand in terms of composite nature: combination of completely different phases (hard, brittle and soft, ductile constituents) with optimal interface properties. On the other hand as related to composite assemblage: two interpenetrating-phase networks where toughnening is optimized through different mechanisms depending on the relatively different chemical nature among them.
Micromechanics of Polycrystalline CVD coatings for Milling Inserts
Dr. Jose García (Sandvik, United Kingdom)
The design of CVD coated milling inserts aims at reducing thermo-mechanical stresses acting at the cutting edge during intermittent machining. In this work we investigate the micro-mechanical and residual stress behavior of Ti(C,N)/α-Al2O3 and Zr(C,N)/α-Al2O3 CVD multilayers deposited on cemented carbides. FIB-Micropillars were compressed using a nanoindenter equipped with a flat punch to analyze deformation mechanisms by SEM, EBSD and TEM. Furthermore, residual stresses were measured in-situ using Synchrotron X-ray diffraction at cycling temperatures ranging from 25-800°C. The connection between micro-deformation mechanisms and residual stresses is discussed based on the experimental work and a phenomenological model for the elasticplastic behavior of Ti(C,N)/α-Al2O3 CVD coatings is presented.
Measuring Mechanical Properties and Residual Stresses at Local Scale in Hardmetals
Dr M. Reyes Elizalde (CEIT-IK4, Spain)
In order to gain understanding on crack propagation through hardmetals it is key to have reliable data on the mechanical properties at the local scale. This work focus on the development of techniques to measure properties at the relevant scale. First, we present the abilityof microbeam testing to assess the mechanical properties of individualmicrostructural features, as interfaces and coatings. Cantilevers withdifferent geometries have been machined by means of a focused ion beam (FIB) and subsequently tested to fracture at a nanoindenter. Repeatableresults have been obtained and modelling has been used to extractquantitative results. Secondly, we present the ability to measure localresidual stresses using micro-hole drilling. This technique is a combination of FIB to machine grooves, digital image correlation (DIC) to measure thedisplacement field produced by the groove and modelling to extract the residual stresses at the microscale.
Time for Questions
Prof Luis Miguel Llanes (Universitat Polytecnica de Catalunya, Spain)
