| 1. |
- Barrirero, Jenifer, 1981-
(författare)
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Eutectic Modification of Al-Si casting alloys
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aluminum alloys with silicon as the major alloying element are the most widely used aluminum casting alloys. The eutectic phase in these alloys is formed by hard and brittle silicon plates in an aluminum matrix. Such silicon plates can act as crack propagation paths deteriorating the toughness of the material. To enhance ductility, silicon can be modified to a coral-like microstructure by addition of a modifying agent. Amongst the elements proposed as modifiers, only strontium, sodium and europium induce a plate-tocoral transition, while others such as ytterbium, only refine the silicon plates. The exact mechanism for the remarkable plate-to-coral change, and the reason why certain elements only refine the structure, is still not completely understood.In this investigation, atom probe tomography and transmission electron microscopy were used to analyze and compare the crystal structure and the distribution of solute atoms in silicon at the atomic level. An unmodified alloy and alloys modified by strontium, sodium, europium and ytterbium were studied. Elements inducing silicon plate-to-coral transition were found to contain nanometer sized clusters at the defects in silicon with stoichiometries corresponding to compounds formed at the ternary eutectic reaction of each system. In contrast, the addition of ytterbium, that only refines the silicon plates, is unable to form clusters in silicon. We propose that the formation of ternary compound clusters AlSiNa, Al2Si2Sr and Al2Si2Eu at the silicon / liquid interface during solidification restrict silicon growth. The formation of clusters on silicon facets create growth steps and increase growth direction diversity. The incorporation of clusters in silicon explains the high density of crystallographic defects and the structural modification from plates to corals.The parallel lattice plane-normals 011Si // 0001Al2Si2Eu, 011Si // 610Al2Si2Eu and 111Si // 610Al2Si2Eu were found between Al2Si2Eu and silicon, and absent between Al2Si2Yb and silicon. We propose a favorable heterogeneous formation of Al2Si2Eu on silicon. The misfit between 011Si and 0002Al2Si2X interplanar spacings shows a consistent trend with the potency of modification for several elements such as strontium, sodium, europium, calcium, barium, ytterbium and yttrium.
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| 2. |
- Engberg, David
(författare)
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Atom Probe Tomography of TiSiN Thin Films
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the wear resistant coating TiSiN and the development of the analysis technique atom probe tomography (APT) applied to this materials system. The technique delivers compositional information through time-of-flight mass spectrometry, with sub-nanometer precision in 3D for a small volume of the sample. It is thus a powerful technique for imaging the local distribution of elements in micro and nanostructures. To gain the full benefits of the technique for the materials system in question, I have developed a method that combines APT with isotopic substitution, here demonstrated by substitution of natN with 15N. This alters the time-of-flight of ions with of one or more N and will thereby enable the differentiation of the otherwise inseparable isotopes 14N and 28Si. Signs of small-scale fluctuations in the data led the development of an algorithm needed to properly visualize these fluctuations. A method to identify the best sampling parameter for visualization of small-scale compositional fluctuations was added to an algorithm originally designed to find the best sampling parameters for measuring and visualizing strong compositional variations. With the identified sampling parameters, the nano-scale compositional fluctuations of Si in the metal/metalloid sub-lattice could be visualized. The existence and size of these fluctuations were corroborated by radial distribution functions, a technique independent of the previously determined sampling parameter. The radial distribution function algorithm was also developed further to ease in the interpretation. The number of curves could thereby be reduced by showing elements, rather than single and molecular ions (of which there were several different kinds). The improvement of the algorithm also allowed interpretation of signs regarding the stoichiometry of SiNy. With a combination of analytical transmission electron microscopy and APT we show Si segregation on the nanometer scale in arc-deposited Ti0.92Si0.0815N and Ti0.81Si0.1915N thin films. APT composition maps and proximity histograms generated from Ti-rich domains show that the TiN contain at least ~2 at. % Si for Ti0.92Si0.08N and ~5 at. % Si for Ti0.81Si0.19N, thus confirming the formation of solid solutions. The formation of relatively pure SiNy domains in the Ti0.81Si0.19N films is tied to pockets between microstructured, columnar features in the film. Finer SiNy enrichments seen in APT possibly correspond to tissue layers around TiN crystallites, thus effectively hindering growth of TiN crystallites, causing TiN renucleation and thus explaining the featherlike nanostructure within the columns of these films.
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| 3. |
- Shulumba, Nina, 1988-
(författare)
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Vibrations in solids : From first principles lattice dynamics to high temperature phase stability
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis I introduce a new method for calculating the temperature dependent vibrational contribution to the free energy of a substitutionally disordered alloy that accounts for anharmonicity at high temperatures. This method exploits the underlying crystal symmetries in an alloy to make the calculations tractable. The validity of this approach is demonstrated by constructing the phase diagram via direct minimization of the Gibbs free energy of a notoriously awkward and technologically important system, Ti1-xAlxN. The vibrational entropy including anharmonic effects is shown to be large and comparable to the configurational entropy at high temperatures, and with its inclusion, the theoretical miscibility gap of Ti1-xAlxN is reduced from 6560 K to 2860 K, in line with atom probe experiments. A similar treatment of Zr1-xAlxN and Hf1-xAlxN alloys suggests that mass disorder has a minimal effect on phase stability compared with chemical ordering. My method is also capable of demonstrating that Hf1-xAlxN, which is dynamically unstable at room temperature, is stabilised at high temperatures. Moreover I develop a new method of computing temperature dependent elastic constants for alloys from their phonon spectra, and show that for Ti1-xAlxN, the elastic anisotropy is found to increase with temperature, helping to explain the spinodal decomposition.The effects of lattice dynamics on phase stability, mechanical, magnetic and transport properties on other materials are also examined. Four specific systems are discussed in detail. Firstly, in the case of CrN, lattice vibrations are shown to decrease the antiferromagnetic to paramagnetic phase transition temperature from 500 K to 380 K, in line with experimental evidence. Secondly, a temperature/pressure induced phase transition in AlN becomes much more facile than in the quasiharmonic approximation, and the thermal conductivity of the rocksalt phase is shown to be much lower than that of the wurtzite phase, as a result of the increased anharmonicity in the rocksalt structure. Thirdly, the temperature dependence of elastic constants of TiN becomes more isotropic as the temperature increases. Finally, iron carbides are evaluated as potentially important phases at the Earth’s core; specifically, calculating the Gibbs free energy of a recently discovered orthorhombic phase of Fe7C3 demonstrates that it is not stable relative to the known hexagonal phase at extreme pressure and temperatures.
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| 4. |
- Atakan, Aylin, 1984-
(författare)
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Mesoporous material systems for catalysis and drug delivery
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hybrid material systems possess multi-functional properties which make them intriguing for the materials science community since very early dates. However, it is not straightforward to produce such material systems. A smart and efficient approach is necessary to extract the desired properties of each component under the desired conditions. This study evolved to its last form primarily around this notion, where the development of a hybrid material is the core of the work. This hybrid material is then further explored for two different applications in the catalysis and drug delivery fields.A nanoassembly was established around a mesoporous silica support. SBA-15 was picked as this support among the other mesoporous silica due to its well-defined pore structure and accessible pore volume. The silica framework was doped with Zr atoms and the pores were partly infiltrated with Cu nanoparticles resulting in a hybrid material with tunable properties. SBA-15 was synthesized by a sol-gel method where a micellar solution was employed as a template for the silica framework. To achieve the doped version, a Zr precursor was added to the synthesis solution. The effects of different synthesis conditions, such as the synthesis catalyst (F-or a Cl-salt) and the Si source (tetraethyl orthosilicate (TEOS) or sodium metasilicate (SMS)) on the characteristics of the final material were investigated. It was observed that these changes in the synthesis conditions yielded different particle morphology, pore size (11-15 nm), and specific surface area (400-700 m2/g). Cu nanoparticles (NPs) were grown in the (Zr-)SBA-15 support using infiltration (Inf) or evaporation induced wetness impregnation (EIWI) methods. The infiltration method is based on functionalizing the (Zr-)SBA-15 support surfaces before the Cu ion attachment whereas EIWI is based on slow evaporation of the liquid from the (Zr-)SBA-15 - Cu aqueous suspension. Both methods are designed to yield preferential growth of Cu NPs in the pores with a diameter smaller than 10 nm and in oxidized form. However, depending on the loading method used, different chemical states of the final material were achieved, i.e. Zr content and porous network properties are different. Cu-Zr-SBA-15 nanoassemblies produced under various synthesis conditions were used for the catalytic conversion of CO2into valuable fuels such as methanol and dimethyl ether (DME). The effect of different chemical states of the catalyst arising from variations in the synthesis parameters was investigated. It was found that the Si precursor (TEOS or SMS) had a considerable impact on the overall performance of the catalyst whereas the Cu loading method (Inf or EIWI) changed the catalytic selectivity between DME and methanol. The activity of the catalyst was further investigated in a time-evolution study where the accumulation of each product in the gas phase and the molecular groups attached to the catalyst surface were recorded over time. Accordingly, thermodynamic equilibrium was achieved on the 14th day of the reaction under 250°C and 33 bar. The resulting total CO2conversion was 24%, which is the thermodynamically highest possible conversion, according to theoretical calculations. It was also concluded from the experimental results that, DME is formed by a combination of two methoxy surface groups. Additionally, the formation of DME boosts the total CO2conversion to fuels, which otherwise is limited to 9.5%.The design of Cu-Zr-SBA-15 was also investigated for drug delivery applications, due to its potential as a biomaterial, e.g., a filler in dental composites, and the antibacterial properties of Cu. Also, the bioactivity of SiO2and ZrO2was considered to be an advantage. With this aim, Cu infiltrated Zr doped SBA-15 material was prepared by using TEOS as the silica precursor and the Inf-method to grow Cu NPs. The performance of the final material as a drug delivery vehicle was tested by an in-vitro delivery study with chlorhexidine digluconate.The nanoassemblies show a drug loading capacity of 25-40% [mg drug / mg (drug+carrier)]. The drug release was determined to be composed of two steps. First, a burst release of the drug molecules that are loosely held in the voids of the mesoporous carrier followed by the diffusion of the drug molecules that are attached to the carrier surface. The presence of Zr and Cu limits the burst release and beneficially slows down the drug release process. The effect of pore properties of SBA-15 was explored in a study where the antibiotic doxycycline hyclate was loaded in SBA-15 materials with different pore sizes. It was observed that the pore size is directly proportional to the drug loading capacity [mg drug / mg (drug+carrier)] and the released drug percentage (the released drug amount/total amount of loaded drug). The drug release was fast due to its weak interactions with the SBA-15 materials. In summary, this work demonstrates the multifunctional character of a smart-tailored nanoassembly which gives valuable insights for two distinct applications in catalysis and drug delivery.
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| 5. |
- Calamba, Katherine M., 1988-
(författare)
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Phase stability and defect structures in (Ti,Al)N hard coatings
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This study highlights the role of nitrogen vacancies and defect structures in engineering hard coatings with enhanced phase stability and mechanical properties for high temperature applications. Titanium aluminum nitride (Ti,Al)N based materials in the form of thin coatings has remained as an outstanding choice for protection of metal cutting tools due to its superior oxidation resistance and high-temperature wear resistance. High-temperature spinodal decomposition of metastable (Ti,Al)N into coherent c-TiN and c-AlN nm-sized domains results in high hardness at elevated temperatures. Even higher thermal input leads to transformation of c-AlN to w-AlN, which is detrimental to the mechanical properties of the coating. One mean to delay this transformation is to introduce nitrogen vacancies.In this thesis, I show that by combining a reduction of the overall N-content of the c-(Ti,Al)Ny (y < 1) coating with a low substrate bias voltage during cathodic arc deposition an even more pronounced delay of the c-AlN to w-AlN phase transformation is achieved. Under such condition, age hardening is retained until 1100 ˚C, which is the highest temperature reported for (Ti,Al)N films. During cutting operations, the wear mechanism of the cathodicarc-deposited c-(Ti0.52Al0.48)Ny with N-contents of y = 0.92, 0.87, and 0.75 films are influenced by the interplay of nitrogen vacancies, microstructure, and chemical reactions with the workpiece material. The y = 0.75 coating contains the highest number of macroparticles and has an inhomogeneous microstructure after machining, which lower its flank and crater wear resistance. Age hardening of the y = 0.92 sample causes its superior flank wear resistance while the dense structure of the y = 0.87 sample prevents chemical wear that results in excellent crater wear resistance.Heteroepitaxial c-(Ti1-x,Alx)Ny (y = 0.92, 0.79, and0.67) films were grown on MgO(001) and (111) substrates using magnetron putter deposition to examine the details of their defect structures during spinodal decomposition. At 900 ˚C, the films decompose to form coherent c-AlN- and c-TiN- rich domains with elongated shape along the elastically soft <001> direction. Deformation maps show that most strains occur near the interface of the segregated domains and inside the c-TiN domains. Dislocations favorably aggregate in c-TiN rather than c-AlN because the later has stronger directionality of covalent chemical bonds. At elevated temperature, the domain size of (001) and (111)- oriented c-(Ti,Al)Ny films increases with the nitrogen content. This indicates that there is a delay in coarsening due to the presence of more N vacancies in the film.The structural and functional properties (Ti1-x,Alx)Ny are also influenced by its Al content (x). TiN and (Ti1-x,Alx)Ny (y = 1, x = 0.63 and x = 0.77) thin films were grown on MgO(111) substrates using magnetron sputtering technique. Both TiN and Ti0.27Al0.63N films are single crystals with cubic structure. (Ti0.23,Al0.77)N film has epitaxial cubic structure only in the first few atomic layers then it transitions to an epitaxial wurtzite layer, with an orientation relationship of c-(Ti0.23,Al0.77)N(111)[1-10]ǀǀw-(Ti0.23,Al0.77)N(0001)[11-20]. The w-(Ti0.23,Al0.77)N shows phase separation of coherent nm-sized domains with varying chemical composition during growth. After annealing at high temperature, the domains in w-(Ti0.23,Al0.77)N have coarsened. The domains in w-(Ti0.23,Al0.77)N are smaller compared to the domains in c-(Ti0.27,Al0.63)N film that has undergone spinodal decomposition. The results that emerged from this thesis are of great importance in the cutting tool industry and also in the microelectronics industry, because the layers examined have properties that are well suited for diffusion barriers.
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| 6. |
- Engberg, David L. J., 1986-
(författare)
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Atom Probe Tomography of Hard Nitride and Boride Thin Films
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hard ceramic thin films, including TiSiN, ZrAlN, ZrB2, and ZrTaB2, with applications for wear-resistant coatings, have been studied using atom probe tomography and correlated with several other analytical techniques, including X-ray diffraction, electron microscopy, and elastic recoil detection analysis. Outstanding obstacles for quantitative atom probe tomography of ceramic thin films have been surmounted.Mass spectral overlaps in TiSiN, which make 28Si indistinguishable from 14N, was resolved by isotopic substitution with 15N, and the nanostructural distribution of elements was thus revealed in 3-D, which enabled the identification of additional structural elements within the nanostructured Ti0.81Si0.1915N film. Improvements to the growth model of TiSiN by cathodic arc deposition was suggested.A self-organized nanolabyrinthine structure of ZrAlN, consisting of standing lamellae of fcc-ZrN and hexagonal AlN, was investigated with focus on the onset and limits of the self-organization. The local crystallographic orientational relationships were (001)ZrN || (0001)AlN and <110>ZrN || <2-1-10>AlN. Close to the MgO substrates, a smooth transition region was formed, going from segregated and disordered to the self-organized nanolabyrinthine structure. With increased growth temperature, coarse (111)-oriented ZrN grains occasionally precipitated and locally replaced the nanolabyrinthine structure. Significant local magnification effects rendered the Zr and N signals unusable, thereby inhibiting quantitative compositional analysis of the constituent phases, but the nanostructure was resolved using the Al signal.Ceramic materials are often affected by correlated evaporation, which can result in losses due to the detector dead-time/space. A compositional correction procedure was suggested, tested against an established procedure, and applied to ZrB2. The correction was found to be less dependent on the isotope abundances and background correction compared to the established procedure. While losses due to dead-time/space occur in atom probe tomography of all materials, the correlative field evaporation behavior of ceramics significantly increases the compositional error. The evaporation behavior of ZrB2 was therefore thoroughly investigated and evidence of preferential retention, correlated evaporation, and inhomogeneous field distributions at a low-index pole was presented. The high mass resolution, relatively low multiple events percentage, and quality of the co-evaporation correlation data was partly attributed to the crystal structure and film orientation, which promoted a layer-by-layer field evaporation.The evaporation behavior of the related ZrTaB2 films was found to be similar to that of ZrB2. The distribution of Ta in relation to Zr was investigated, showing that the column boundaries were both metal- and Ta-rich, and that there was a significant amount of Ta in solid solution within the columns.In addition, an instrumental artefact previously not described in atom probe tomography was found in several of the materials investigated in this thesis. The artefact consists of high-density lines along the analysis direction, which cannot be related to pole artefacts. The detection system of the atom probe was identified as the cause, because the artefact patterns on detector histograms coincided with the structure of the microchannel plate. Inconsistencies in the internal boundaries of the microchannel plate multifibers from the manufacturing process can influence the signal to the detector and locally increase the detection efficiency in a pattern characteristic to the microchannel plate in question.Altogether, this thesis shows that atom probe tomography of nitride and boride thin films is burdened by several artefacts and distortions, but that relevant material outcomes can nevertheless be achieved by informed choices of film isotopic constituents and analytical parameters, exclusion of heavily distorted regions (such as pole artefacts), and the use of compositional correction procedures when applicable.
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| 7. |
- Kumar Yalamanchili, Phani
(författare)
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Multiscale materials design of hard coatings for improved fracture resistance and thermal stability
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Physical vapor deposited hard coatings comprised of cubic (c) transition metal (TM)-Al-N, and (TM)-Si-N are the current workhorse materials for a large number of metal cutting and wear resistant applications to fight against the extreme conditions of temperature and stress simultaneously. In spite of a high degree of sophistication in terms of material choice and microstructural design, a lower fracture resistance and limited thermal stability of the coatings remains a technological challenge in the field. The lower fracture resistance of the coating is an inherent material property. Limited thermal stability in the TM-Al-N system is associated with the transformation of metastable c -AlN to its stable wurtzite (w)-AlN phase at a temperature above 900 oC resulting an undesirable hardness drop.The current work shows how to overcome these challenges by manipulating the coating material at different length scales, i.e. microstructure, crystal and interface structure, and alloy design. The endeavor of multiscale materials design is achieved by converging a deeper material and process knowledge to result specific structural modification over multiple length scales by alloying transition metal nitrides with AlN and SiNx as following.Microstructure variation is achieved in ZrN coating by alloying it with SiNx, where the surface segregated SiNx breaks down the columnar structure and evolves a selforganized nanocomposite structure with a hardness variation from 37 ±2 GPa to 26 ±1 GPa. The indentation induced fracture studies reveal crack deflection for the columnar coating, likely along the column boundaries. The crack deflection offers additional energy dissipative mechanisms that make the columnar structured coating more fracture resistant, which is not the case for the nanocomposite coating in spite of its lower hardness.Crystal structure of AlN is varied between stable wurtzite structure to metastable cubic structure in the ZrAlN alloy by adapting a multilayer structure and tuning the layer thickness. The multilayer consisting c-AlN layer shows a hardness of 34 ±1 GPa and a twofold enhancement in the critical force to cause an indentation induced surface crack compared to the multilayer containing w-AlN in spite of a lower hardness for the later case. The higher fracture resistance is discovered to be caused by stress- induced transformation of AlN from its metastable cubic structure to its thermodynamically stable wurtzite structure associated with a molar volume expansion of 20% that builds up local compressive stress zones delaying the onset and propagation of the cracks. This is in fact the first experimental data point for the stress-induced transformation toughening in a hard coating.The current work also demonstrates a concept of improving the thermal stability of the TM-Al-N by modifying the interface structure between w-AlN and c-TMN. A popular belief in the field is that AlN in its stable wurtzite structure is detrimental to coating hardness, and hence the current material design strategy is to force AlN in metastable cubic phase that confines the application temperature (~ 900 oC). In contrast, here it is shown that the w-AlN offers a high hardness provided if it is grown (semi-)coherent to c-TMN. This is experimentally shown for the multilayer system of TiN/ZrAlN. The interface structure between the c-TiN, c-ZrN and w-AlN is transformed from incoherent to (semi-)coherent structure by tuning the growth conditions under a favorable crystallographic template. Furthermore, the low energy (semi-) coherent interface structure between w-AlN and c- TiN, c- ZrN display a high thermal stability, causing a high and more stable hardness up to an annealing temperature of 1150 oC with a value of 34± 1.5 GPa. This value is 50 % higher compared to the state-of-the-art monolithic and multilayered Ti-Al-N and Zr-Al-N coating containing incoherent w-AlN.Finally, an entropy based alloy design concept is explored to form a thermodynamically stable solid solution in the TM-Al-N material system that has a positive enthalpy of mixing. Multi-principal element alloys of (AlTiVCrNb)N are formed in a near ideal cubic solid solution. The high configurational entropy in the alloy is predicted to overcome positive enthalpy of mixing, there by an entropy stabilized solid solution formation is expected at a temperature above 1000 K. However, at elevated temperature, optimization between the minimization of interaction energy and maximization of configurational randomness causes precipitation of AlN in its stable wurtzite structure and the cubic solid solution is only confined between TiN, CrN, VN and NbN that have a low enthalpy of mixing.In summary, this work provides technological solutions to the two outstanding issues in the field. A significant enhancement in fracture resistance of the coating is achieved with appropriate material choice and microstructural design by invoking crack deflection and stress induced transformation toughening mechanisms. A remarkable thermal stability enhancement of the TM-Al-N coating is achieved by a new structural archetype consisting c-TMN and thermodynamically stable w-AlN with a low energy (semi-)coherent interface structure.
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| 8. |
- Schramm Benítez, Isabella Citlalli
(författare)
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Defect-engineered (Ti,Al)N thin films
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates the effect of point defects (nitrogen vacancies and interstitials) and multilayering ((Ti,Al)N/TiN) on the phase transformations in cathodic arc-evaporated cubic (Ti,Al)N thin films at elevated temperatures. Special attention is paid to the evolution of the beneficial spinodal decomposition into c-TiN and c-AlN, the detrimental formation of wurtzite AlN and the potential application as hard coating in cutting tools.c-(Ti1-xAlx)Ny thin films with varying Al fractions and N content (y = 0.93 to 0.75) show a delay in the spinodal decomposition when increasing the amount of N vacancies. This results in a 300 °C upshift in the age hardening and a delay in the w-AlN formation, while additions of self-interstitials enhance phase separation. High temperature interaction between hard metal substrates and thin films is more pronounced when increasing N deficiency through diffusion of substrate elements into the film. Low N content films (y = 0.58 to 0.40) showed formation of additional phases such as Ti4AlN3, Ti2AlN, Al5Ti2 and Al3Ti during annealing and a transformation from Ti2AlN to Ti4AlN3 via intercalation. The multilayer structure of TiN/TiAlN results in surfacedirected spinodal decomposition that affects the decomposition behavior. Careful use of these effects appears as a promising method to improve cutting tool performance.
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| 9. |
- Syed, Muhammad Bilal, 1977-
(författare)
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Cathodic arc deposition process
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis aims to expand the knowledge of fundamental mechanisms that govern the cathodic arc process. The first part of this thesis explores and explains the correlations between a rather unexplored process parameter (i.e. cathode microstructure) and the microstructure of the coatings. The second part of the thesis focuses on discovering and explaining the correlations between process parameters (i.e. arc guiding magnetic field and nitrogen pressure), plasma properties (i.e. plasma density, electron temperature, ion saturation density etc.), and the microstructure of the coatings.Two aspects of the cathode microstructure are explored. The first is the cathode grain size and the second is the disparity among parent phases of the cathode in terms of work function (W.F.) and cohesive energy (C.E.).Two material systems are selected to investigate the effects of the cathode grain size on the microstructure of the coatings. In this research evolution of the microstructure of the cathode surface under the influence of arc has also been studied. The results show that for CrN coatings a decrease in average grain size of Cr cathode is beneficial in terms of reduction in macroparticle density of Cr-N coatings. In the case of powder metallurgically prepared Ti-50 at.% Al cathodes, a decrease in grain size from 1800 μm to 100 μm promotes the intermixing of Ti and Al grains at the cathode surface which resulted in lower macroparticle density of TiAlN coatings, a Ti/Al ratio closer to cathode composition, and improved hardness. However, further reduction in grain size from 100 μm to 10 μm, upon arcing favors a self-sustaining reaction between Ti and Al grains whose end product is the γ phase. This self-sustaining reaction and arc-created holelike features on the cathode surface render the coatings rich in Al and high in macroparticle density which results in reduced hardness.The research in the effects of disparity among the parent phases in terms of W.F. and C.E. of the constituents of Ti-50 at.% Al cathodes on the microstructural evolution of the converted layer and the coating's microstructure shows that the phase which has lower W.F. and C.E. suffers higher erosion. It is also shown that irrespective of the cathode type, the arc guiding magnetic field and the surface geometry of the cathode are two significant factors in controlling the microstructure of TiAlN coatings.The research in finding correlations between the arc guiding magnetic field, plasma density and the microstructure of the coatings show that for a particular arc source assembly the plasma density can be altered by just changing the strength of an electromagnet. A weaker electromagnet strength results in higher plasma density of Ti-67 at.% Al cathode which promotes the growth of dual phase TiAlN coatings, while a stronger magnetic field reduces the plasma density and promotes the growth of single phase TiAIN coatings and a reduction in deposition rate.The research in establishing the correlations between N2 pressure, plasma properties and coatings microstructure reveals that for plasma generated from Ti-50 at.% Al cathode the average charge state of Ti shows a stark increase with an increase in N2 pressure from 0 Pa to 0.07 Pa, and upon further increase in N2 pressure the average charge state gradually decreases. Moreover, the ionization of nitrogen takes place at the expense of Al2+. It has also been observed that the electron density increases with increasing the N2 pressure while the effective electron temperature decreases. Furthermore, the energetic ion flux to the coating's growth front decreases as the N2 pressure is increased which leads to the alteration of growth texture from 220 to 111.
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| 10. |
- Yang, Jing
(författare)
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Grinding effects on surface integrity, flexural strength and contact damage resistance of coated hardmetals
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The tribological and mechanical behavior of coated tools depends not only on intrinsic properties of the deposited film but also on substrate surface and subsurface properties – such as topography and residual stress state – as well as on interface adhesion strength. It is particularly true in the case of coated tools based on WC-Co cemented carbides (backbone materials of the tool manufacturing industry, and simply referred to as hardmetals in practice) as substrates. Manufacturing of hardmetals often involves grinding, and in the case of cutting tools also edge preparation, etching and coating. The quality of the shaped components is influenced by how the surface integrity evolves through the different process steps. In this regard, substrate grinding and coating deposition represent key steps, as they are critical for defining the final performance and relative tool manufacturing cost. Within this framework, it is the main objective of this thesis to assess the influence of substrate surface integrity on different mechanical (flexural strength and contact damage resistance under spherical indentation) and tribological (scratch resistance as well as cracking and delamination response under Brale indentation) properties for a TiN-coated fine-grained hardmetal grade (WC-13 wt.%Co). In doing so, three different surface finish conditions are studied: as-sintered (AS), ground (G), and mirror-like polished (P). Moreover, aiming for an in-depth analysis of surface integrity evolution from grinding to coating, a relevant part of the work is devoted to document and understand changes induced by grinding in nude hardmetal substrates. The study is also extended to a fourth surface finish variant (GTT), corresponding to a ground substrate which is thermal annealed before being ion etched and coated. Because residual stress induced by grinding are effectively relieved after this high temperature thermal treatment, GTT condition allows to separate grinding-induced effects associated with surface texture and surface/subsurface damage changes (inherited from the G surface finish) from those related to the referred residual stresses.Surface integrity was characterized in terms of roughness, residual stresses (prior and after coating deposition), and damage at the subsurface level. It was found that grinding induces significant alterations in the surface integrity of cemented carbides. Main changes included relevant roughness variations; emergence of a topographic texture; anisotropic distribution of microcracks within a subsurface layer of about 1 micron in depth; severe deformation, microstructure refinement and phase transformation of binder regions, down to 5 microns in depth; and large compressive residual stresses, gradually decreasing from the surface to baseline values at depths of about 10-12 microns.Additional changes in surface integrity are induced during subsequent ion etching and coating deposition. In general, removal of material from the surface during sputter cleaning and extended low-temperature (during film deposition) treatment resulted in a significant residual stresses decrease (about half its original value). However, damage induced by grinding was not completely removed, and some microcracks were still left on the substrate surface (close to the interface). On the other hand, and as expected, high temperature annealing (GTT condition) resulted in a complete relief of the referred residual stresses, but without inducing any additional change in terms of existing microcracks and depth of damaged layer. This was not the case for the metallic binder phase where thermal treatment induced an unexpected microporosity, development of a recrystallized subgrain structure, and reversion of grinding-induced phase transformation.Flexural strength was measured on both uncoated and coated hardmetals, and complemented with extensive fractographic analysis. It was found that grinding significantly enhances the strength of hardmetals, as compared to AS and P conditions. However, such beneficial effect was partly lost in the corresponding coated specimens. On the other hand, film deposition increases strength measured for GTT surface variant. These findings were analyzed on the basis of the changes on nature and location of critical flaws, induced by the effective residual stress field resulting at the surface and subsurface after each manufacturing stage.The influence of substrate surface finish on scratch resistance of coated hardmetals and associated failure mechanisms was investigated. It was found that coated AS, G and P samples exhibit similar critical load for initial substrate exposure and the same brittle adhesive failure mode. However, damage scenario was discerned to be different. Substrate exposure was discrete and localized to the scratch tracks for G samples, while a more pronounced and continuous decohesion was seen for AS and P ones. Relieving of the substrate compressive residual stresses (GTT condition) yielded lower critical loads and changes in the mechanisms for the scratch-related failure, the latter depending on the relative orientation between scratching and grinding directions.The cracking and delamination of TiN-coated hardmetals when subjected to Brale indentation was studied while varying the microstructure and surface finish of the substrate. In this case, another fine-grained WC-Co cemented carbide with lower binder content (6 wt.%Co) was included in the investigation. It was found that polished and coated hardmetals exhibit more brittleness (radial cracking) and lower adhesion strength (coating delamination) with decreasing binder content. Such a response is postulated on the basis of the influence of intrinsic hardness/brittleness of the hardmetal substrate on both cracking at the subsurface level and effective stress state, particularly regarding changes in shear stress component. On the other hand, grinding was discerned to promote delamination, compared to the polished condition, but strongly inhibits radial cracking. This was the result of the interaction between elastic-plastic deformation imposed during indentation and several grinding-induced effects: remnant compressive stress field, pronounced surface texture, and microcracking within a thin microcracked subsurface layer. It is then concluded that coating spallation prevails over radial cracking as the main mechanism for energy dissipation in ground and coated hardmetals.Contact damage resistance of coated hardmetals with different substrate surface finish conditions was investigated by means of spherical indentation under increasing monotonic loads. It was found that grinding enhanced resistance against both crack nucleation at the coating surface and subsequent propagation into the hardmetal substrate. Hence, crack emergence and damage evolution was effectively delayed for the coated G condition, as compared to the reference P one. The observed system response was discussed on the basis of the beneficial effects associated with compressive residual stresses remnant at the subsurface level after grinding, ion-etching and coating. The influence of the stress state was further corroborated by the lower contact damage resistance exhibited by the coated GTT specimens. Finally, differences observed on the interaction between indentation-induced damage and failure mode under flexural testing pointed in the direction that substrate grinding also enhances damage tolerance of the coated system when exposed to contact loads.
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