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  • Presentation: 2017-02-23 10:15 Jordan-Fermi, Linköping
    Landälv, Ludvig
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Thin Film and Plasma Characterization of PVD Oxides2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The state-of-the-art tools for machining metals are primarily based on a metal-ceramic composite(WC-Co) coated with different combinations of carbide, nitride and oxide coatings. Combinations of these coating materials are optimized to withstand specific wear conditions. Oxide coatings are especially desired because of their possible high hot hardness, chemical inertness with respect to the workpiece, and their low friction.

    This thesis deals with process and coating characterization of new oxide coatings deposited by physical vapor deposition (PVD) techniques, focusing on the Cr-Zr-O and Al-Cr-Si-O systems.

    The thermal stability of α-Cr0.28Zr0.10O0.61 deposited by reactive radio frequency (RF)-magnetron sputtering at 500 °C was investigated after annealing up to 870 °C. The annealed samples showed transformation of α-(Cr,Zr)2O3 and amorphous ZrOx-rich areas into tetragonal ZrO2 and bcc Cr. The instability of the α-(Cr,Zr)2O3 is surprising and possibly related to the annealing being done under vacuum, facilitating the loss of oxygen. The stabilization of the room temperature metastable tetragonal ZrO2 phase, due to surface energy effects, may prove to be useful for metal cutting applications. The observed phase segregation of α-(Cr,Zr)2O3 and formation of tetragonal ZrO2 with corresponding increase in hardness for this pseudo-binary oxide system also opens up design routes for pseudo-binary oxides with tunable microstructural and mechanical properties.

    The inherent difficulties of depositing insulating oxide films with PVD, demanding a closed circuit, makes the investigation of process stability an important part of this research. In this context, we investigated the influence of adding small amount of Si in Al-Cr cathode on plasma characteristics ,process parameters, and coating properties. Si was chosen here due to a previous study showing improved erosion behavior of Al-Cr-Si over pure Al-Cr cathode without Si incorporation in the coating.

    This work shows small improvements in cathode erosion and process stability (lower pressure and cathode voltage) when introducing 5 at % Si in the Al70Cr30-cathode. This also led to fewer droplets at low cathode current and intermediate O2 flow. A larger positive effect on cathode erosion was observed with respect to cleaning the cathode from oxide contamination by increasing cathode current with 50%. However, higher cathode current also resulted in increased amount of droplets in the coating which is undesirable. Through plasma analysis the presence of volatile SiO species could be confirmed but the loss of Si through volatile SiO species was negligible, since the coating composition matched the cathode composition. The positive effect of added Si on the process stability at the cathode surface should be weighed against Si incorporation in the coating. This incorporation may or may not be beneficial for the final application since literature states that Si promotes the metastable γ-phase over the thermodynamically stable α-phase of pure Al2O3, contrary to the effect of Cr, which stabilizes the α-phase.

  • Presentation: 2017-03-03 13:00 ACAS, Linköping
    Kanesund, Jan-erik
    Linköping University, Department of Management and Engineering. Linköping University, Faculty of Science & Engineering.
    Influence of deformation and environmental degradation of Inconel 7922017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Industrial gas turbines are often used as a mechanical drive for pumps and compressors or in power generation as an electric power supply. The gas turbine has for many years been a popular engine due to its flexibility with respect to different types of fuel and due to a design, that enables a high power-to-weight ratio. A simplified description of a gas turbine is that the engine consists of a cold and hot section. The turbo compressor section belongs to cold section and the combustion chamber together with the turbine section belongs to the hot section. In the hot section of a gas turbine, the condition is extremely severe because of an aggressive environment characterized by high temperatures, increased temperature gradients, high pressure and centrifugal forces resulting in large stresses on individual components together with an oxidizing and corroding atmosphere. Materials used in the high temperature section (hot gas path) of a modern gas turbine are different types of superalloys, as single crystal, directionally solidified or polycrystalline alloys, depending on temperature and load conditions. In the first turbine stage, temperature is very high due to exposure to the combustion gas. To handle the problem with creep, single crystal superalloys are often used in this section. In the second row of turbine blades, the temperature of the gas is lower and polycrystalline superalloys are typically used. IN-792 is a cast polycrystalline superalloy with high strength, good resistance to hot corrosion and a cheaper option than single crystals. In the hot section of gas turbine, IN-792 is a suitable material for components such as turbine blades and vans where a complex load condition, high temperature and severe environment prevails. Due to startup and shutdown of the gas turbine engine during service, the components in the hot section are exposed to cyclic load and temperature. This will generate mechanical and thermal fatigue damage in gas turbine components. Steady state temperature gradient arises by the cooling system acting at cold spots during service to introduce tensile stress, which indirectly gives rise to creep damage in the component. This work includes tree studies of deformation and damage mechanisms of superalloy IN-792. The first study is made on test bars exposed to thermomechanical fatigue in laboratory environment, the second and the third study is made on turbine blades used during service. In the second study, the machines are placed off-shore and exposed to marine environment. In the third study the machine is landbased and exposed to an industrial environment. In the second study, the deformation and damage mechanisms are compared between the turbine blades used during service and the test bars exposed to thermomechanical fatigue testing in the first study.

    List of papers
    1. Deformation and damage mechanisms in IN792 during thermomechanical fatigue
    Open this publication in new window or tab >>Deformation and damage mechanisms in IN792 during thermomechanical fatigue
    2011 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, Vol. 528, no 13-14, 4658-4668 p.Article in journal (Refereed) Published
    Abstract [en]

    The deformation and damage mechanisms arising during thermomechanical fatigue (TMF) of the polycrystallinesuperalloy IN792 have been investigated. The TMF cycles used in this study are in-phase(IP) and out-of-phase (OP). The minimum temperature used in all TMF-tests is 100 ◦C while the maximumtemperature is 500 or 750 ◦C in the IP TMF-tests and 750, 850 or 950 ◦C in the OP TMF-tests. Themajority of the cracks are transcrystalline, except for the IP TMF-test at 750 ◦C, where some tendencyto intercrystalline crack growth can be seen. In all tests, the cracks were initiated and propagated inlocations where deformation structures such as deformation bands have formed in the material. In thetemperature interval 750–850 ◦C, twins were formed in both IP and OP TMF-tests and this behaviouris observed to be further enhanced close to a crack. Twins are to a significantly lesser extent observedfor tests with a lower (500 ◦C) and a higher (950 ◦C) maximum temperature. Recrystallization at grainboundaries, around particles and within the deformation structures have occurred in the OP TMF-testswith a maximum temperature of 850 and 950 ◦C and this is more apparent for the higher temperature.Void formation is frequently observed in the recrystallized areas even for the case of compressive stressesat high temperature.

    Place, publisher, year, edition, pages
    Elsevier, 2011
    Keyword
    Nickel based superalloys, Fatigue, Shear bands, Recrystallization, Twinning
    National Category
    Engineering and Technology Other Materials Engineering
    Identifiers
    urn:nbn:se:liu:diva-67259 (URN)10.1016/j.msea.2011.02.063 (DOI)000290004200051 ()
    Note
    Original Publication: Jan Kanesund, Johan Moverare and Sten Johansson, Deformation and damage mechanisms in IN792 during thermomechanical fatigue, 2011, Materials Science & Engineering: A, (528), 13-14, 4658-4668. http://dx.doi.org/10.1016/j.msea.2011.02.063 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/ Available from: 2011-04-06 Created: 2011-04-06 Last updated: 2017-01-11