The PhD research presented here is part of the new nuclear manufacturing (NNUMAN) research group, looking into novel materials and manufacturing processes. NNUMAN is an EPSRC funded (EP/J021172/1) program with direct support from Rolls Royce plc. The fabrication of MAX phase coatings by different means and assessing their tribological properties was the original theme of this thesis. However, due to issues with fabrication processes the overriding direction was moved towards assessing the response of bulk MAX phases to irradiating and corrosive environments. It is believed that this research will contribute to the understanding of suitable compositions and applications within nuclear. The MAX phases comprise an early transition metal (M), an A-group element (A) and either carbon or nitrogen (X). They have an inherently nano-layered structure with alternating ceramic (MX) and metallic (A) layers. The unique mix of both ceramic and metallic properties have made MAX phases a proposed material for nuclear applications. Proton irradiation is performed on Ti3SiC2, Ti3AlC2, Ti2AlC and Cr2AlC bulk MAX phases to 0.1 dpa at 350oC. Crystallographic instabilities are observed through x-ray diffraction (XRD) analysis after irradiation. A mechanism for irradiation induced point defect swelling is proposed, by matching XRD and modelling data. Anti-site point defects are proposed to be a major contributor to anisotropic lattice parameter changes in Ti3SiC2. Additional carbon interstitial defects are also proposed in Ti3AlC2, as well as anti-site defects, making them less radiation tolerant. Further proton irradiations on Ti3SiC2 and Ti3AlC2 were performed at elevated temperatures to propose a temperature at which lattice changes are not observed. It is concluded that high proton fluences require temperatures in excess of ~700oC and ~1,000oC for Ti3SiC2 and Ti3AlC2, respectively. Corrosion of bulk MAX phases in simulated primary water also suggests a deleterious response to normal light water reactor (LWR) operations. Advanced scanning transmission electron microscopy (STEM) techniques, such as energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), is employed to understand corrosion mechanisms. Selective oxidation of A-layers is observed, with no evidence of passivation for Ti3SiC2, Ti3AlC2 and Ti2AlC. The response of Cr2AlC was most promising as little oxidation occurred, which is confirmed also with XRD analysis. A comprehensive corrosion mechanism is proposed, whereby compositions of MAX phase in LWR coolants is limited.

This Thesis presents two different deposition techniques for the synthesis of Ti2AlC coatings. First, I have fabricated Ti2AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211TM Ti2AlC powder of size ~38 µm in an H2/O2 gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti3AlC2, TiC, and AlxTiy alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211TM Ti2AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti2AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti2AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti2AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti2AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti2AlC (101̅7) // Al2O3 (0001). / Report code: LIU-TEK-LIC-2008:15.

Ti2AlC coatings have been fabricated by cold-spray deposition. The microstructure evolution as a function of basic spray parameters temperature and pressure onto AA6060 aluminium alloy and 1.0037 steel substrates has been studied. Adherent and dense 50–80 μm thick Ti2AlC coatings were deposited on soft AA6060 substrates under gas temperature and pressure of 600 °C and 3.4 MPa, respectively, whilst comparable results were obtained on harder 1.0037 steel by using higher temperature (800 °C) and pressure (3.9 MPa).

<p>This Thesis presents two different deposition techniques for the synthesis of Ti₂AlC coatings. First, I have fabricated Ti₂AlC coatings by high velocity oxy-fuel (HVOF) spraying. Analysis with scanning electron microscopy (SEM) show dense coatings with thicknesses of ~150 µm when spraying with a MAXTHAL 211^TMTi₂AlC powder of size ~38 µm in an H₂/O₂ gas flow. The films showed good adhesion to stainless steel substrates as determined by bending tests and the hardness was 3-5 GPa. X-ray diffraction (XRD) detected minority phases of Ti₃AlC₂, TiC, and Al_xTi_y alloys. The use of a larger powder size of 56 µm resulted in an increased amount of cracks and delaminations in the coatings. This was explained by less melted material, which is needed as a binding material. Second, magnetron sputtering of thin films was performed with a MAXTHAL 211^TM Ti₂AlC compound target. Depositions were made at substrate temperatures between ambient and 1000 °C. Elastic recoil detection analysis (ERDA) shows that the films exhibit a C composition between 42 and 52 at% which differs from the nominal composition of 25 at% for the Ti₂AlC-target. The Al content, in turn, depends on the substrate temperature as Al is likely to start to evaporate around 700 °C. Co-sputtering with Ti target at a temperature of 700 °C, however, yielded Ti₂AlC films with only minority contents of TiC. Thus, the addition of Ti is suggested to have two beneficial roles of balancing out excess of C and to retain Al by providing for more stoichiometric Ti₂AlC synthesis conditions. Transmission electron microscopy and X-ray pole figures show that the Ti₂AlC grains grow in two preferred orientations; epitaxial Ti2AlC (0001) // Al2O3 (0001) and with 37° tilted basal planes of Ti₂AlC (101̅7) // Al₂O₃ (0001).</p> / Report code: LIU-TEK-LIC-2008:15.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18988号 / 工博第4030号 / 新制||工||1620 / 31939 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 中村 裕之, 教授 安田 秀幸, 教授 吉村 一良 / 学位規則第4条第1項該当

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18988号 / 工博第4030号 / 新制||工||1620(附属図書館) / 31939 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 中村 裕之, 教授 安田 秀幸, 教授 吉村 一良 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

MAX-phase ceramics are a class of ductile ceramic material group with the general molecular formula Mn+1AXn (n = 1, 2, 3….), where M is an early transition element, A is an element from the ‘A’ group of the periodic table and X is either nitride or carbide. One advantage of these materials is that they maintain their strength at high temperatures. In addition these ceramic materials possess the best properties of both ceramics and metals. Some of their important characteristics are low density, high stiffness, machinability, excellent thermal and electrical conductivity and they even exhibit some plasticity at elevated temperature. These amazing combinations of properties have made researchers foresee the technological importance of these materials as a structural ceramic for high temperature application. Since this ceramic is relatively new to the market, only a handful of work has been undertaken on this material and its applications are limited to heating elements. In addition, analysis of the thermodynamic data on this material is incomplete. This PhD work addresses this issue and conducts a complete thermodynamic analysis involved in the formation mechanism of the ternary titanium carbide MAX-phase Ti2AlC ceramic, using Self-propagating High temperature Synthesis (SHS) form of combustion synthesis process, based on the following exothermic reaction: (2+x) Ti + (y) Al + C → Ti2AlC + (x) Ti + (y) Al (i) Where x and y = 0.1. 0.2, 0.3… A thermodynamic model has been formulated to predict the temperature evolution during the reaction (i), for the formation of Ti2AlC using SHS process. In addition the effect of particle size in the elemental reaction has been studied on the formation mechanism of Ti2AlC and methods to control the porosity by fine tuning the particle size has been recognized. Manufacturing processes such as Self-propagating High temperature Synthesis (SHS), foam replication and freeze casting have been developed in this thesis to produce micro and macro porous Ti2AlC ceramic mainly for electrode applications. A systematic material development technique to produce macro porous Ti2AlC ceramic, using a foam replication technique has been established in this research work. The material fabricated by this technique has a uniform pore size (up to 5mm), with open interconnected pores and is ideal for a flow battery application which requires a multifunctional electrode material which is highly porous to allow the flow of electrolyte through it, is corrosion resistant and at the same time being electrically conductive. The mechanical properties of the ceramic produced by this method has been characterised and steps to mitigate the cracks and defects formed during the fabrication process to obtain structurally stable macro porous Ti2AlC ceramic has been reported in this work. This research demonstrates that one of the applications of macro porous Ti2AlC ceramic formed using foam replication technique is as an electrode material in a photo-Microbial Fuel Cell (p-MFC). Graded porosity micro porous Ti2AlC ceramics have also been fabricated using a freeze casting technique, with camphene as the freezing vehicle. A systematic material development process has been tailored for this particular material. A ceramic material with gradient pore size ranging from 27-305µm has been fabricated using this technique. This type of ceramic is a good candidate as an electrode material in micro-redox battery and for sensing applications. A variety of processing parameters such as solid loading (amount of ceramic content in the material), freezing temperature and mould material which affect the pore formation and pore size have been studied in this PhD and the range of porosities achieved by controlling these parameters have been reported.

This thesis reports on synthesis, microstructure and properties of binary and ternary carbide thin films deposited by dc magnetron sputtering. These materials are interesting since they exhibit a wide range of useful properties, such as high hardness, resistance to wear and oxidation, and high electrical conductivity. Here, an early transition metal (M) and carbon (C) have been used as the basis, often with the addition of a second M-element or an A-group element (A). In these systems nanocomposites, metastable solid solutions, multilayers, or Mn+1AXn-phases have been deposited. The Mn+1AXn-phases are a group of nanolaminated compounds with a unique mixture of metallic and ceramic properties. In general X is carbon or nitrogen, although here only carbon has been used. Epitaxial MAX-phase thin films of Ti2AlC, Ti3AlC2 and V2GeC have been deposited for the first time. They have been studied with emphasis on phase stability, phase composition and nucleation characteristics to gain deeper insights into their growth. The microstructure of the films was characterized by electron microscopy and X-ray diffraction. In addition, bond strength characteristics have been studied by soft X-ray spectroscopy and complementary calculations within DFT. Their mechanical and electrical properties have been studied, and the results are discussed on the basis of their electronic structure. Furthermore, by interleaving the Ti3SiC2 MAX-phase with TiC0.67 a multilayer structure has been formed, for which a new intrusion-type deformation behaviour has been described. A new concept in the design of nanocomposite films has been developed, whereby a solid solution of a weak carbide-forming element in the carbide structure creates a driving force for surface segregation of C. This concept has been verified both theoretically and experimentally for the Ti-Al-C and Ti-Fe-C systems. It has been shown by pin-on-disc measurements that this surface segregation leads to graphitization and consequently a very low friction coefficient for these films. Finally, it has been demonstrated that low-friction films with tunable magnetic properties can be achieved in the Ti-Fe-C system.

Les phases MAX forment une famille de carbures et de nitrures nano-lamellaires de formule chimique Mn+1AXn, où M est un métal de transition des premières colonnes, A appartient aux colonnes 13-16 et X est soit C, soit N, ou une combinaison des deux éléments. Ces phases combinent les mérites des céramiques et des métaux, comme une bonne stabilité chimique, l’usinabilité, la résistance aux chocs mécaniques, de bonnes conductivités thermique et électrique, etc. Malgré tout, l’étude de leurs propriétés intrinsèques et de leurs anisotropies a été jusqu’à présent limitée par l’indisponibilité de monocristaux. Cette thèse traite de la réactivité de tels monocristaux de phases MAX. Grâce à la large taille des cristaux produits au LMGP, il a été possible d’évaluer directement l’anisotropie de la réactivité chimique et d’obtenir des données originales. Nous avons montré le rôle prépondérant joué par l’élément A pour initier des transformations chimiques menant parfois à la synthèse de matériaux originaux, et nous nous sommes concentrés sur quatre aspects différents : Tout d’abord, nous avons tenté de synthétiser des MXènes de grande taille, en profitant de la grande taille des cristaux disponibles. Un effort particulier a été porté sur la description de la réactivité chimique de phases MAX plongées dans diverses solutions d’attaque, avec un accent particulier mis sur l’utilisation de HF. En second lieu, nous avons étudié la chloruration de phases MAX : l’objectif initial était de former des MXènes, mais nous avons finalement développé une méthode pour synthétiser des carbures de chrome poreux avec des propriétés intéressantes. Troisièmement, nous avons utilisé des cristaux de grande taille pour évaluer l’anisotropie des propriétés électrochimiques. Une anisotropie significative a été trouvée, soit en mesurant le courant durant la polarisation électrochimique, soit par mesure de spectroscopie d’impédance. Divers mécanismes ont été proposés afin d’expliquer cette anisotropie des propriétés de corrosion. Enfin, nous avons montré que les résultats électrochimiques pouvaient être utilisés pour révéler indirectement la présence de défauts structurels récemment identifiés dans la littérature. De tels défauts, appelés « ripplocations », sont spécifiques aux matériaux nano-lamellaires. / MAX phases are a family of layered ternary carbides and nitrides with chemical formula Mn+1AXn, where M is an early transition element, A is an element of groups 13 to16 and X is either C, N or both. These phases combine the merits of ceramics and metals, such as chemical stability, machinability, shock resistance, good electrical and thermal conductivity, etc. However, the investigation of their intrinsic properties and anisotropies has heretofore been limited by a lack of availability of single crystals. This thesis mainly deals with the chemical reactivity of MAX phase single crystals. Owing to the large size single crystals grown at LMGP, it was possible to directly assess the anisotropy of the chemical reactivity and to obtain original data. We showed that the prominent role played by the A element for initiating chemical transformations could lead to the synthesis of original materials, and we focused on four different aspects. First, we tried to synthesize MXenes from MAX phase single crystals: The purpose was to obtain large-scale MXenes by taking advantage of the large size of the single crystals. Effort was put on describing the chemical reactivity of MAX phases dipped in different etchants, focusing on HF. Secondly, we studied the MAX phase reactivity with chlorination: the initial purpose was to obtain MXenes, but we finally developed a method for synthesizing porous chromium carbides which exhibit several interesting properties. Thirdly, we used large size single crystals in order to assess the anisotropy of the electrochemical properties. A significant anisotropy was found, either by measuring the current during electrochemical polarization or by frequency-dependent impedance measurements. Several mechanisms were proposed in order to explain this anisotropy of the corrosion properties. Eventually, we showed that the electrochemical results could be used to indirectly evidence the presence of structural defects recently identified in the literature. Such defects, called ripplocations, are specific to nano-lamellar materials.

Dans le cadre des recherches menées sur l'amélioration de la résistance à l'oxydation des gaines de combustible en conditions accidentelles, des revêtements Cr-Al-C et Cr2AlC ont été développés dans ce travail. Dans la première partie, nous avons étudié le procédé HiPIMS afin de comprendre l'effet de différents paramètres de dépôt sur le plasma et les propriétés des films obtenus. Il en ressort que malgré un bombardement ionique plus intense, un apport supplémentaire d'énergie est requis pour obtenir un revêtement cristallin. Des recuits à partir de 500°C sous argon de systèmes Cr-Al-C tels que déposés permettent ainsi une cristallisation partielle des revêtements en Cr2AlC à une température suffisamment basse pour être compatible avec la métallurgie des alliages de zirconium. Dans un second temps, l'évaluation du comportement à haute température de ces deux types de revêtements, recuits ou non, a révélé un effet protecteur contre l'oxydation rapide du zirconium jusqu'à 1200°C en atmosphère oxydante grâce à la formation d'une couche d'oxyde continue. Cette couche est constituée d'un mélange d'alumine α et de chromine pour le revêtement de Cr-Al-C tandis que seule l'alumine α est présente pour le revêtement Cr2AlC dans les premiers instants de l'oxydation. Ensuite, en raison de l'appauvrissement en Al, les revêtements se dégradent en formant une couche intermédiaire résiduelle de carbure Cr7C3 servant de réservoir de Cr jusqu'à complète oxydation. Ces résultats ont également montré la perte d'une partie du réservoir d'Al par diffusion dans les alliages Zr. Une architecture multicouche a été développée pour limiter cette diffusion et ainsi prolonger la durée de vie du revêtement. L'ajout d'un intercalaire en Mo pour bloquer la diffusion d'Al dans le substrat s'est avéré peu concluant, le molybdène s'évaporant à haute température. Les systèmes base Cr-Al-C revêtus chrome, présentent quant à eux, un comportement amélioré par rapport aux revêtements monocouches. / This study aims to improve oxidation resistance of nuclear fuel claddings in accident conditions. In this context, Cr-Al-C and Cr2AlC coatings deposition and their behavior were studied. Firstly, we investigated the influence of HiPIMS process parameters on the properties of the plasma and the deposited films. Despite more intense ionic fluxes due to the HiPIMS process, coatings do not crystallize without an additional energy supply. Partially crystallized Cr2AlC thin films were obtained by a 500°C annealing of as-deposited Cr-Al-C coatings. This two-step process is a viable solution to protect nuclear claddings with Cr2AlC coating while maintaining the metallurgical properties of the zirconium-based substrates. Secondly, the assessment of the oxidation resistance of as-deposited and annealed coatings revealed significant protective effect against rapid oxidation under dry and wet air at high temperatures (up to 1200°C) owing to the formation of a continuous oxide layer. During the first stages of oxidation, this layer is made of α Al2O3 and Cr2O3 for as-deposited coating while only α-Al2O3 is present for the annealed one. Because of Al depletion, coatings later deteriorate and form a residual and porous intermediate chromium carbide (Cr7C3) layer which further fully oxidizes. It was shown that the inward diffusion of Al with Zr also accelerates the coating deterioration. To improve the oxidation resistance of these coatings, multilayered architectures were developed. Adding a molybdenum interlayer as diffusion barrier globally decreased the oxidation resistance of the coating. In contrast, topping Cr-Al-C and Cr2AlC with a Cr layer improved oxidation behavior over single-layer coatings.