Study of wear mechanisms at high temperature scratch testing
Introduction
Wear at high temperature (HT) is of major concern in industry, as many applications operate at elevated temperatures. Hence the knowledge of wear resistance of applied alloys is of great interest. The simulation of abrasive wear phenomena by scratch testing is an adequate solution for addressing single interactions with abrasive particles. Thereby wear mechanisms such as material deformation, work hardening, micro-cutting, etc. can be studied at a fundamental level [1].
Scratching is done by a hard indenter, like Rockwell or Vickers diamond tips. Nevertheless, scratch tests for elevated temperature investigation are scarce [2], [3]. A novel scratch test, also utilised for this work, was introduced by Varga et. al in [4] allowing for testing up to 1000 °C. Post-test analysis of the scratch can give useful information on the present wear mechanisms. Topography measurements depict the plastic deformation behaviour, cross- and length-sections can help to identify material changes [5], [6].
Correlation of hardness with scratch and abrasion resistance is often done in literature, nevertheless one must keep in mind, that abrasion is the cumulative action of many single scratching events. Especially wear resistant metal matrix composites (MMC) generally feature two wear levels: very low wear rates at conditions, when the abrasive is softer than both, the matrix and the hard phases. This changes to severe wear when the abrasive becomes even harder than the hard phases [6]. Scratch testing with a diamond indenter is always done in this very high wear regime, as the “abrasive” simulated by the diamond indenter is the hardest material known. Temperature influence on different phases in MMCs and abrasives was studied by Berns [7]: hard abrasives like Al2O3 or SiC keep a higher hardness, higher than most hard precipitations in steels, over a wide temperature range, while quartz features a significant hardness drop even in the low temperature range <500 °C. Hence, scratch testing can simulate attack by hard abrasive particles.
Simulations provide a suitable tool in order to gain a higher insight into the mechanisms of scratch testing. In conventional computer simulations scratch tests can be modelled using either mesh-based continuum mechanical methods [8] or classical molecular dynamics [9]. The meshless continuum methods, like the smooth particle hydrodynamics (SPH) [10] applied here, have the main advantage that arbitrarily large deformations and instability phenomena like fracture can be handled. Since indentation and scratching is accompanied by the removal of material, SPH is a well suited method for this purpose. Recently, SPH was used to numerically study the deformation of solid bodies. Good correlations were found e.g. for cutting [11] or impact [12] applications. The present work relies on the numerically stable and efficient Total Lagrangian SPH (TLSPH) implementation of Ganzenmüller [13] into the open-source-code LAMMPS. For the case of copper the simulated scratch test topographies and forces have been in excellent agreement with experiments [14].
The aim of this work was to study the influence of microstructure and matrix lattice on the scratch behaviour at different temperature levels. Thereto an austenitic material without hardphases was compared to a ferrite and an austenite with carbides. A SPH simulation was undertaken for the pure austenite to get insight in the deformation mechanisms occurring during scratching.
Section snippets
Materials investigated
Three materials were selected for investigation with the aim to cover a broad spectrum of technological alloys used at HT. The selected materials comprise a Fe- and Ni-based alloy with different phases in order to evaluate the role of microstructure on scratch resistance. The chemical compositions of the materials and the hardness values at room temperature (RT) are given in Table 1. Hardness was evaluated at a high load of 98.1 N (HV10) to average over the phases within the various
Hot hardness of the materials investigated
Fig. 3 gives the hardness-temperature curves of the three materials investigated. The austenite shows the lowest hardness at low temperatures. It starts at ~160 HV10 at RT and decreases linearly to ~80 HV10 at 800 °C. The Ni-based MMC has a hardness with similar temperature dependence, but at higher hardness values due to the presence of hardphases within the material. It starts at 250 HV10 and decreases down to ~145 HV10 at 800 °C. Both alloys have a fcc matrix, which do not undergo phase
Conclusions
High temperature (HT) scratch experiments were carried out on materials for HT usage, namely an austenite, a ferritic MMC and a Ni-based MMC. The influence of load and temperature on the wear behaviour was studied. Numerical simulations with smooth particle hydrodynamics (SPH) were conducted to get insight in the wear process.
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Ploughing is the dominant wear mechanism for the materials investigated at almost all load and temperature conditions. This changes to cutting at the Ni-based MMC at 800 °C
Acknowledgements
This work was funded by the Austrian COMET Program (Project K2 XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology”. SJ. Eder is acknowledged for depicting numerical results.
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