Strain glass transition of cobalt phase in a cemented carbide

Cemented carbides are hard materials used for the fabrication of cutting tools. They consist of hard micron or sub-micron carbide grains held together in a matrix of a tough metallic binder such as cobalt. The understanding and control of cobalt characteristics are of crucial importance to improve the mechanical properties of WC-Co cemented carbide materials. Indeed, cobalt controls the toughness of the material and its properties seem to condition the durability of cemented carbides. At low temperature, pure cobalt is normally found in hcp structure, while above 426 degrees C (700 K) it changes to fcc crystalline structure. Remarkably, in cemented carbides, cobalt appears even at low temperature as fcc, otherwise as a mixture of fcc and hcp. It is unsure if these crystal structures are due to the presence of internal stresses or due to W and C soluted atoms. However, this paper demonstrates that cobalt in WC-10wt.%Co cemented carbide may show some characteristics of a glass transition at around 669 degrees C (942 K) in the 1 Hz frequency range, inducing a glassy state to cobalt, which is of great interest from a fundamental point of view, however challenging to control in materials technology.

Optimization of composition and structure of cemented carbide cutting tools

Samy Adjam

WC-Co cemented carbides are composites, which combine a hard phase consisting of WC grains and a metallic ductile phase as a binder. Their excellent mechanical properties, combining high hardness, toughness and refractory properties, make them excellent materials for cutting processes such as turning, drilling and milling. For such applications, cemented carbides are coated with thin ceramic films that increase their resistance to thermal stress as well as to the mechanical and chemical wear inherent in extreme machining conditions. Three main characteristics determine the performance and wear resistance of cutting tools: the cohesion of the hard phase above 1200 K, the ductility of the cobalt binder, and the mechanical and thermal resistance of the coatings. This work focuses on the impact of the microstructural behaviour of the cobalt binder on the tool-life of the cutting tools. The aim is to develop new binders for cemented carbides with improved impact resistance. The techniques of transmission electron microscopy at room temperature and in-situ at high temperature, as well as mechanical spectroscopy in a forced torsion pendulum have proved to be essential for the identification of the microstructure of cobalt and for the study of the relaxation phenomena occurring therein. Mechanical spectroscopy has evidenced the presence of two relaxation peaks, P1 and P2 in the cobalt phase and a peak (P3) related to WC-WC grain boundary sliding. The peak P1 appears to be correlated with the cutting tool life measured in interrupted cutting tests. In this work, the role of the binder is highlighted in particular by the identification of a glass-type transition observed for cobalt. The thermodynamics of the transition is evidenced by the behavior of P1 whose relaxation time diverges according to a Vogel-Fulcher-Tammann model, which is characteristic of glass transitions. The state of the material corresponding to temperatures below this transition shows a strong dependence on its thermomechanical history. This dependence is confirmed by a divergence in plastic deformations over two identical temperature cycles with different thermomechanical histories. Such a divergence on a standard protocol, called (ZFC/FC), seems to indicate a break in the thermodynamic hypothesis of ergodicity, even though it could not be attributed solely to the behaviour of cobalt. Transmission electron microscopy at room temperature has determined that the low temperature phase of the cobalt binder has a face-centred cubic structure and is composed of twinned nanodomains containing nanotwins. This state is therefore characterised by a short-range order without the long-range order, which is characteristic of glassy materials. In-situ observations have highlighted a detwinning process corresponding to the temperatures of the cobalt relaxation peak identified by mechanical spectroscopy. This detwinning process continues until the appearance of a long-range ordered face-centred cubic phase above 1050K, which is favourable to deformation. Mechanically, this transition is similar to a brittle-to-ductile transition, greatly influencing the properties of the material and its toughness. The cutting tool life could benefit from lowering the brittle to ductile transition temperature.

EPFL2021

Highly filled polymer nanocomposite films derived from novel nanostructured latexes

Riccardo Ruggerone

The overall aim of this thesis has been to assess the potential of latex-based technologies for the preparation of polymer/clay nanocomposites. The key feature of latex-based technologies is that they offer the possibility of improved control of the final nanocomposite morphology at significantly higher clay loadings than can be obtained with more conventional processing techniques, such as melt blending or in situ polymerization. The idea is to exploit swelling of the clay in either the aqueous or the monomer phase of a water-based latex, depending on the clay surface functionalization, to produce hybrid polymer/clay latex particles with controlled diameters of the order of 100 nm, which may then be consolidated to produce solid nanocomposite films. The materials considered in this work were based on styrenic matrices, considered to be a model system, and acrylics, which are of more interest for commercial coating applications. Two different polymerization techniques were investigated, namely conventional emulsion polymerization and miniemulsion polymerization. The thermal and mechanical properties of films produced from the resulting latexes were then studied in detail. Conventional emulsion polymerization was found to be particularly suitable for the preparation of particles with a well-defined "armoured" morphology, in which the clay formed a more or less complete shell around a matrix core, providing the focus for the remainder of the project. Clay contents of up to about 50 wt % were obtained for both the styrenic and the acrylic latexes using this approach, with excellent degrees of dispersion, the average clay aggregate thickness not exceeding 10 nm. The armoured morphology of the latex particles resulted in a cellular arrangement of the clay in the consolidated films, which became better defined as the clay content increased. The reinforcing effect of the clay on mechanical properties varied according to the physical state of the matrix. Increases in Young's modulus by a factor of 3 to 4 were observed in styrenic films with the cellular morphology in the glassy state, and the degree of exfoliation of the clay was found to be a critical parameter under these conditions, samples containing 5 to 7 wt % of clay showing increased moduli with respect to those obtained at somewhat higher clay contents, for which aggregation was more apparent. In the rubbery state, on the other hand, the Young's modulus increased by more than 2 orders of magnitude for clay contents above 20 wt % and was strongly correlated with the overall filler content. Thermal analysis showed that a significant proportion of the matrix remained immobilized in the rubbery state, i.e. did not contribute to the glass transition. This was argued to be due to strong physical confinement of regions of the matrix intercalated in the clay aggregates. While the increases in Young's modulus in the glassy state could be accounted for in terms of classical micromechanical models, such as those of Halpin-Tsai and Mori-Tanaka, the same models failed to predict the behaviour in the rubbery state. Models based on foam mechanics were therefore developed incorporating an immobilized matrix fraction in the cell walls, whose elastic properties were treated as fitting parameters. Although somewhat different values of the Young's modulus for this immobilized matrix fraction were required to fit the data, depending on the details of the model, they were consistently found to be two to three orders of magnitude greater than that of the neat matrix in the rubbery state (but not to exceed the Young's modulus of the matrix in the glassy state), providing direct evidence for the importance of this interphase for the overall nanocomposite properties. The importance of the cellular arrangement of the clay was also confirmed through comparison with nanocomposites containing non-cellular morphologies resulting from other preparation techniques or the use of mechanical deformation to break-up the initial cellular structure. Finally, it was demonstrated that the results obtained for the styrenic systems could be extended to acrylic-based nanocomposites with comparable morphologies, underlining their broader significance for formulations of potential commercial interest. A further goal of this thesis was to study the effect of the clay on the microdeformation and fracture mechanisms of the nanocomposites, and it was also of interest to compare these results with those obtained for conventional isotactic polypropylene (PP)/clay nanocomposites prepared by melt blending, whose macroscopic properties have been studied previously in our institute. In situ TEM investigation of deformation in glassy styrenic nanocomposite films of about 200 nm in thickness containing the cellular structure revealed a decrease in local matrix drawability at clay contents above 10 wt %, and extensive coarse cavitation, thought to be associated with the particle cores, which replaced crazing as the dominant deformation mechanism, accounting for the observed decrease in macroscopic tensile strength at intermediate clay contents. Moreover, at the highest clay contents, these mechanisms were replaced by failure of the particle-particle interfaces, leading to an extremely brittle macroscopic response. Above Tg, localized deformation zones were also observed, indicating the network formed by the clay and the immobilized regions of matrix to show yielding behaviour, again consistent with the macroscopic response. The conventional PP/clay nanocomposites also showed a decrease in matrix ductility and an increase in coarse cavitation with increasing clay content. In this latter case, however, the zones of cavitation were clearly identified with breakdown of the clay aggregates or the interfaces between the clay and the matrix, underlining the important role of the clay-matrix interface for the properties of the styrenics, and by inference, those of the acrylics. To provide further insight into the role of the cellular structure of the styrenics, finite element (FE) simulations were used to investigate the distribution in hydrostatic stress in the nanocomposite films under finite deformations. These showed the principal stress concentrations to appear in the clay aggregates aligned in the direction of the applied deformation, while the hydrostatic stress in the matrix remained relatively uniform, suggesting that as long as the interface and clay aggregates remain stable, cavitation may initiate anywhere within the matrix, as in the unmodified polymer. The FE simulations were also used to model the elastic properties in the rubbery state, confirming the need to assume the presence of an immobilized matrix fraction in order to account for the observed Young's moduli. The results of these calculations were found to be consistent with those obtained from the foam-based micromechanical models, confirming the applicability of these latter, and suggesting the local anisotropy of the clay aggregates to play a limited role in the overall low strain elastic properties. The main outcome of this thesis has been the establishment of a general physical basis for predicting structure-mechanical property relationships in a new range of latex-based nanocomposite materials with exceptional properties, thanks to the possibility of incorporating very high clay loadings without compromising processability, and vast potential for fine tuning of stiffness and stiffness related properties. Moreover, the work has highlighted the important role of the "nano" effect, particularly at temperatures above the glass transition of the matrix, where matrix immobilization at the clay-matrix interface is clearly demonstrated to contribute to both thermal and mechanical properties. It is consequently expected to provide solid guidelines for the choice of morphological and materials parameters for the optimization of basic mechanical properties in such materials via controlled synthesis. Moreover, with further advances in synthesis and processing and hence in film quality, it should ultimately be possible to extend the basic physical model developed here to account for the structure dependence of other important applicative properties, such as permeability and fire resistance.

EPFL2010

Electrons leave the flatland: out-of-plane charge dynamics in layered materials

Edoardo Martino

The goal of this thesis is to explore the "neglected" third dimension of layered materials, by studying the interlayer charge dynamics through dc conductivity, optical spectroscopy and ab-initio calculations. This project was designed to respond to the growing interest in layered materials and heterostructures. It is today evident that new properties originate by the interaction between neighbouring atomic planes, but the magnitude of the interlayer coupling has so far been poorly investigated. Reliable and reproducible experiments, in particular for dc conductivity, are today made possible thanks to the recent advancements in the field of materials microfabrication through focused ion beam. In the case of this thesis, it enabled me to review long-standing scientific paradigms and verify that previous experimental results differ by more than 2 orders of magnitude from the intrinsic material property. The subjects of study are metallic layered transition metal chalcogenides, as they offer a very flexible playground of crystalline structure and physical properties. The discussion will start in Chapter 3, where we will discuss the case of simple layered metals (2H-NbSe2, 2H-TaS2) or semimetal (ZrTe5), for which the anisotropic transport properties can be accurately predicted by their simple band structure calculation and Fermi surface topology. The anisotropic response will be treated in a more general term by considering the effective number of carriers which contributes to charge transport along a specific direction. The discussion will continue by looking at the peculiar properties of 1T-TaS2, a prominent layered material for the rich set of charge density waves reconstruction, and the presence of a low temperature metal-insulator transition. Recent results from band structure calculations, photoemission spectroscopy and X-ray diffraction, on the contrary, suggests that strong interlayer interactions need to be accounted to justify the observed properties of this material. The experimental results in this thesis provide a clear evidence from transport properties in support of this new interpretation of a layered material where physical properties are strongly dictated by the way different layers interact with each other. This counterintuitive result is regarded as the consequence of the intertwined relation between the charge density wave and the orbital character of the conduction electrons bands, producing a c-axis oriented orbital texture. The weak bond that holds together the crystalline planes in layered materials also creates the perfect condition for intercalation, or the growth of compounds with different crystalline layers stacked together, creating a natural heterostructure. We will explore the new physical properties that emerged in layered transition metal dichalcogenides when different layers are combined together to form a natural heterostructure, or when magnetic ions are intercalated. Surprising results are reported from the studies of interlayer conduction properties. The interaction between the different types of layers in the natural heterostructure of 4Hb-TaS2, influences the charge density wave and superconducting properties. In this situation, the conduction mechanism is highly anisotropic. In Chapter 6 we will see how conduction anisotropy is affected by the specific long-range magnetic order produced by the intercalated atoms.

EPFL2020