Study program to develop and evaluate die and container materials for the growth of silicon ribbons

The development and evaluation of proprietary coatings of pure silicon carbide, silicon nitride, and aluminum nitride on less pure hot pressed substrates of the respective ceramic materials, is described. Silicon sessile drop experiments were performed on coated test specimens under controlled oxygen partial pressure. Prior to testing, X-ray diffraction and SEM characterization was performed. The reaction interfaces were characterized after testing with optical and scanning electron microscopy and Auger electron spectroscopy. Increasing the oxygen partial pressure was found to increase the molten silicon contact angle, apparently because adsorbed oxygen lowers the solid-vapor interfacial free energy. It was also found that adsorbed oxygen increased the degree of attack of molten silicon upon the chemical vapor deposited coatings. Cost projections show that reasonably priced, coated, molten silicon resistant refractory material shapes are obtainable

Тверді плазмохімічні a-SiCN-покриття

Аморфні SiCN-покриття було осаджено на кремнієві підкладки методом плазмохімічного осадження (PECVD) з використанням гексаметилдісилазану в якості основного прекурсора. Досліджено вплив температури осадження на структуру, хімічний склад і механічні властивості покриттів. Встановлено, що при температурах до 400 °С мало місце осадження гідрогенізованих аморфних SiCN (a-SiCN:H)-покриттів, твердість яких не перевищує 23 ГПа. З подальшим підвищенням температури розподіл міцних зв’язків Si–C, Si–N і C–N в покриттях практично не змінюється, а кількість слабких водневих зв’язків С–Н, Si–H і N–H зменшується. Як наслідок такого перерозподілу хімічних зв’язків при температурах 650–700 °C осаджуються a-SiCN-покриття з твердістю понад 32 ГПа. Відпал у вакуумі при 1200 °С не впливає помітно на структуру, твердість і модуль пружності a-SiCN-покриттів.Аморфные SiCN-покрытия были осаждены на подложки из кремния методом плазмохимического осаждения (PECVD) с применением гексаметилдисилазана как основного прекурсора. Исследовано влияние температуры осаждения на структуру, химический состав и механические свойства покрытий. Установлено, что при температурах до 400 °С происходит осаждение гидрогенизированных аморфных SiCN (a-SiCN:H)-покрытий, твердость которых не превышает 23 ГПа. При дальнейшем повышении температуры распределение сильных связей Si–C, Si–N і C–N в покрытиях практически не изменяется, а количество слабых водородных связей С–Н, Si–H и N–H существенно уменьшается. В результате такого перераспределения химических связей при температурах 650–700 °С осаждаются a-SiCN-покрытия с твердостью до 32 ГПа. Отжиг в вакууме при 1200 °С не оказывает заметного влияния на структуру, твердость и модуль упругости a-SiCN-покрытий.Amorphous SiCN coatings have been deposited on silicon substrates by plasmochemical technique (PECVD) using hexamethyldisilazane as basic precursor. The effect of deposition temperature on structure, chemical composition and mechanical properties of coatings has been studied. It was found that deposition of hydrogenated amorphous SiCN (a-SiCN:H) coatings take place at temperatures lower 400 °С with hardness < 23 GPa. Distribution of strong Si–C, Si–N і C–N bonds in coatings at further increase of temperature is not changed practically and number of weak hydrogen bonds С–Н, Si–H and N–H decreased considerably. The hardness of a-SiCN coatings increase up to 32 GPa at deposition temperatures from 650 to 700 °С in results of such rearrangement of chemical bonds. Annealing in vacuum at 1200 °C doesn’t effect appreciably the structure, hardness and elastic modulus of a-SiCN coatings

Вплив потоку азоту на властивості тонких аморфних Si–C–N-плівок, отриманих магнетронним розпиленням

Тонкі плівки Si–C–N осаджено на кремнієві підкладки реактивним магнетронним розпиленням кремній-вуглецевої мішені на постійному струмі та при різних співвідношеннях потоків азоту FN2 і аргону FAr. Для дослідження структури, хімічних зв’язків, морфології поверхні та механічних властивостей отриманих Si–C–N-плівок використано рентгенівську дифракцію, інфрачервону спектроскопію, рентгенівську фотоелектронну спектроскопію, атомно-силову мікроскопію та наноіндентування. Встановлено, що всі отримані тонкі плівки Si–C–N є рентгеноаморфними. Шорсткість поверхні плівок слабо залежить від FN2 і складає 0,23–0,28 нм. Збільшення FN2 призводить до появи і посилення Si–N-і C–N-зв’язків та послаблення Si–C-зв’язків. Тонкі плівки містять невелику кількість кисню, який утворює зв’язки Si–O і C–O, причому останні послаблюються з ростом FN2. Нанотвердість аморфних тонких плівок SiC дорівнює 23 ГПа, пружний модуль – 207 ГПа. Нанотвердість та модуль пружності тонких плівок Si–C–N зменшуються зі збільшенням потоку азоту, що пов’язано з ослабленням Si–C-зв’язків.Тонкие пленки Si–C–N были осаждены на кремниевые подложки реактивным магнетронным распылением кремний-углеродной мишени при постоянном токе и при различных соотношениях потоков азота FN2 и аргона FAr. Для исследования структуры, картины химических связей, морфологии поверхности и механических свойств полученных Si–C–N-пленок применяли рентгеновскую дифракцию, инфракрасную спектроскопию, рентгеновскую фотоэлектронную спектроскопию, атомно-силовую микроскопию и наноиндентирование. Установлено, что все пленки являются рентгеноаморфными. Шероховатость пленок слабо зависит от FN2 и составляет 0,23–0,28 нм. Увеличение FN2 приводит к появлению и усилению Si–N- и C–N-связей и ослаблению Si–C-связей. Пленки содержат небольшое количество кислорода, который образует связи Si–O и C–O, причем последние ослабляются с ростом FN2. Нанотвердость тонких аморфных SiC-пленок равна 23 ГПа, упругий модуль – 207 ГПа. Нанотвердость и модуль упругости тонких аморфных Si–C–N-пленок уменьшаются с увеличением потока азота, что связано с ослаблением Si–C-связей.Si–C–N thin films were deposited on silicon substrate by reactive DC magnetron sputtering of silicon-carbon target at different ratios of nitrogen FN2 and argon FAr. Xray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and nanoindentation were used to study the structure, chemical bonds picture, surface morphology and mechanical properties of obtained Si–C–N thin films. It was found that all films are X-rayamorphous. The roughness of the films is 0,23–0,28 nm and depends weakly on the FN2. Increase FN2 leads to formation and strengthening of Si–N and C–N bonds and to weakening Si–C bonds. Films containing a small amount of oxygen that forms Si–O and C–O bonds, moreover the last ones weakened with increasing FN2. Nanohardness of amorphous SiC thin films is 23 GPa, elastic modulus – 207 GPa. Nanohardness and elastic modulus of amorphous Si–C–N thin films decreases with increasing nitrogen flow, due to weakening of Si–C bonds

Erosion Resistance of Tungsten-Carbide Coatings for Steel Pipes in Fluid Catalytic Cracking Units

In petroleum processing, the flow of catalyst readily leads to erosion of piping in a fluid catalytic cracking unit. Advances in coating materials and processes necessitate a re-evaluation of current protection methods. Commercially available tungsten-carbide (WC) claddings and nanostructured WC-W CVD coatings were investigated as potential alternative erosion-resistant coatings. Erosion tests by solid particle impingement were conducted on 2 variations of claddings and 1 variation of WC-W coatings following ASTM standard G76. A36 steel coupons were used as reference samples. For statistical validation, 2-3 replications of the tests were performed for the claddings and WC-W coating. Testing was conducted using a sandblasting apparatus with 70 μm alumina powder at an impingement angle of 90° and 25°. Data obtained for each cladding, as well as A36 steel, was used to plot mass loss versus time, resulting in erosion rates. Average erosion values were reported as volume loss per gram of abrasive with units of mm3/g. The claddings displayed similar erosion values at 90°, but both were less resistant than bare A36 steel. Optical and electron microscopy of the cladding interface revealed coherent bonding across the interface and no obvious voids in the coating or at the interface. The microstructure of the steel substrate consisted of large regions of pearlite embedded in a ferrite matrix; therefore, improper heat treatment cannot be used to explain the superior erosion resistance. At 25°, the WC-W coating and claddings performed similarly and both resisted erosion better than A36 steel

НОВЫЕ МЕТАЛЛОРГАНИЧЕСКИЕ ПРЕКУРСОРЫ И ПРОЦЕССЫ ХИМИЧЕСКОГО ОСАЖДЕНИЯ ИЗ ГАЗОВОЙ ФАЗЫ В ТЕХНОЛОГИЯХ НАНОМАТЕРИАЛОВ

Разработаны процессы химического осаждения из газовой фазы (CVD) металлических и диэлектрических (high−k и low−k) пленок с применением нетрадиционных исходных веществ (летучих комплексных и элементоорганических соединений). Проведено комплексное исследование химического и фазового состава, структуры пленок двойных оксидов (HfO2)1−x(Ме2O3)x(где Ме = Al, Sc), а также пленок карбонитридов и оксикарбони-тридов кремния. Показано, что полученные материалы обладают комплексом уникальных функциональных свойств, что делает их перспективными для применения в микро−, нано− и оптоэлектронных устройствах.Разработаны процессы химического осаждения из газовой фазы (CVD) металлических и диэлектрических (high−k и low−k) пленок с применением нетрадиционных исходных веществ (летучих комплексных и элементоорганических соединений). Проведено комплексное исследование химического и фазового состава, структуры пленок двойных оксидов (HfO2)1−x(Ме2O3)x(где Ме = Al, Sc), а также пленок карбонитридов и оксикарбони-тридов кремния. Показано, что полученные материалы обладают комплексом уникальных функциональных свойств, что делает их перспективными для применения в микро−, нано− и оптоэлектронных устройствах

Synthesis of high performance ceramic fibers by chemical vapor deposition for advanced metallics reinforcing

The chemical vapor deposition (CVD) synthesis of fibers capable of effectively reinforcing intermetallic matrices at elevated temperatures which can be used for potential applications in high temperature composite materials is described. This process was used due to its advantage over other fiber synthesis processes. It is extremely important to produce these fibers with good reproducible and controlled growth rates. However, the complex interplay of mass and energy transfer, blended with the fluid dynamics makes this a formidable task. The design and development of CVD reactor assembly and system to synthesize TiB2, CrB, B4C, and TiC fibers was performed. Residual thermal analysis for estimating stresses arising form thermal expansion mismatch were determined. Various techniques to improve the mechanical properties were also performed. Various techniques for improving the fiber properties were elaborated. The crystal structure and its orientation for TiB2 fiber is discussed. An overall view of the CVD process to develop CrB2, TiB2, and other high performance ceramic fibers is presented

Workshop on an Assessment of Gas-Side Fouling in Fossil Fuel Exhaust Environments

The state of the art of gas side fouling in fossil fuel exhaust environments was assessed. Heat recovery applications were emphasized. The deleterious effects of gas side fouling including increased energy consumption, increased material losses, and loss of production were identified

Stability of high temperature ceramics under corrosive environments

Thesis (Ph.D.)--Boston UniversityCurrently, ceramics are being used under increasingly demanding environments. This research involves the study of high-temperature stability of ceramic materials in two diverse applications. The first application involves the use of ceramic materials in gas turbines. SiC/SiC ceramic matrix composites (CMCs) are increasingly being used in the hot-sections of gas turbines; and they are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al2O3•2SiO2) deposited by chemical vapor deposition (CVD), the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The hybrid coating system demonstrated excellent physical and chemical stability under severe thermal shock and exposure to an aggressive hot-corrosion environment. Finite element modeling showed that through-thickness cracks reduce the tensile stresses in the TBC, but also reduce the beneficial compressive stresses in the EBC, and may actually lead to the propagation of the vertical cracks into the EBC. The second application involves the formation of solar-grade silicon by an inexpensive and environmentally friendly electrochemical process using an YSZ solid oxide membrane (SOM) at elevated temperature (~1100°C). The SOM membrane is exposed to a complex fluoride flux with dissolved silica, which is then electrochemically separated into silicon and oxygen. Membrane stability is crucial to ensure high efficiency and long-term performance of the SOM process. A failure model of the SOM membrane by the formation of "inner cracks" was studied, and attributed to yttrium depletion in the YSZ, which leads to phase transformation from the cubic to tetragonal phase. A series of systematic experiments were designed and performed to understand the synergistic roles of silica and YF3 in the flux in membrane degradation. It was shown that silica attacks the SOM membrane, while YF3 in the flux slows down the attack. The mechanism of the yttria depleted layer (YDL) formation was attributed to grain boundary attack by the silica in the flux, which was the rate-controlling step. This led to rapid ingress of the flux into this attacked grain boundaries, and the out diffusion of Y from the cubic YSZ grains to the grain boundary. This depletion of the Y from the cubic grains transformed them into tetragonal. Once all of the cubic grains in the YDL region converted to tetragonal YSZ grains, no further diffusion occurred. Based on the stability test results, a new flux design was proposed and tested. The flux composition did not attack the SOM membrane, and successful separation of silica in the flux to phase pure Si crystals was demonstrated without apparent damage to the SOM membrane, thereby demonstrating the viability of the Si-SOM process

Materials review for improved automotive gas turbine engine

The potential role of superalloys, refractory alloys, and ceramics in the hottest sections of engines operating with turbine inlet temperatures as high as 1370 C is examined. The convential superalloys, directionally solidified eutectics, oxide dispersion strenghened alloys, and tungsten fiber reinforced superalloys are reviewed and compared on the basis of maximum turbine blade temperature capability. Improved high temperature protective coatings and special fabrication techniques for these advanced alloys are discussed. Chromium, columbium, molybdenum, tantalum, and tungsten alloys are also reviewed. Molbdenum alloys are found to be the most suitable for mass produced turbine wheels. Various forms and fabrication processes for silicon nitride, silicon carbide, and SIALON's are investigated for use in highstress and medium stress high temperature environments

Micro and nanomecanical behavior of mullite-based environmental barrier coatings

Mullite coatings deposited by chemical vapor deposition (CVD) have been introduced and proven as excellent candidates to protect silicon carbide (SiC) from severe pitting corrosion and recession, becoming part of the new generation of environmental barrier coatings (EBCs). In these coatings mullite columns nucleate from a thin vitreous silicon layer in contact with the substrate and can grow over a wide range of increasing Al/Si ratios in constant or graded compositions. This feature allows for obtaining Al-rich coatings at the outer surface (conferring superior corrosion protection to the substrate) while keeping the stoichiometric mullite composition at the interface (representing great adhesion because of the good match with SiC). Although the excellent performance of these coatings in corrosive environments has been proved, information about the mechanical behavior of these systems is quite limited. The mechanical properties and structural integrity of CVD mullite coatings on SiC substrates are key issues facing the implementation of these systems in real applications. The study of such aspects constitutes the basis of this investigation. Considering the compositional variations, microstructure, and thickness of CVD mullite coatings, it is essential to evaluate their mechanical properties, and the structural integrity of the coated system, from a local perspective. In this regard, nanoindentation and nanoscratch appear as the most suitable techniques for this purpose. This investigation is based on implementing nanoindentation and nanoscratch tests, together with advanced characterization techniques, to evaluate the main local mechanical properties of mullite-based EBCs as well as to investigate the structural integrity of the coated systems. In doing so, columnar mullite coatings composed of stoichiometric (¿ 3) and increasing (¿ 5, 6, 7, 8, 11) Al/Si ratios, as well as compositionally graded coatings, were accounted for. Regarding stoichiometric coatings, main mechanical properties; i.e. hardness (Hf), elastic modulus (Ef), yield strength (sy) and fracture toughness (Kf), are asessed by means of nanoindentation. As a consequence of the columnar nature of coatings as well as the vitreous silicon layer from which they grow, properties were found to be slightly lower than the ones reported for bulk stoichiometric mullite. Also, properties ascribed to the coated system such as energy of adhesion (Gint) and interface fracture toughness (Kint), are assessed. Nanoscratch tests demonstrate great resistance of coatings to the sliding contact, as considerable plastic deformation ocurrs without significant damage. The effect of coating composition on its mechanical behavior is studied through evaluation of specimens with increasing Al/Si ratios. Enrichment in Al produce hardening, stiffening and enbrittlement of mullite coatings. Nevertheless, nanoscratch tests show that structural integrity of the systems is satisfactory since no complete loss of the coating material is registered. An enhancement in Hf and Ef is also evidenced through the thickness of coatings with graded compositions. Nanoscratch tests performed in the cross section of compositionally graded mullite coatings show an optimum combination of stiffness/hardness and cohesive/adhesive scratch strength, as compared to coatings with stoichiometric or extreme Al-enriched compositions. Finally, temperature and corrosion effects on the mechanical behavior are investigated. In stoichiometric mullite, the effect of high temperature is an increase in Hf and Ef, accompanied by a decrease in Kc, whereas none of these properties are altered for Al-rich and the compositionally graded coatings. In addition, nanoscratch tests show that the effect of temperature and hot-corrosion on the structural integrity of tested coatings may be considered as negligible. This is an interesting finding as CVD mullite coatings are expected to be used in gas turbines, under samLos recubrimientos de mullita depositados mediante deposición química de vapor (CVD) son excelentes candidatos para proteger el carburo de silicio (SiC) de la corrosión en ambientes de combustión, configurando la nueva generación de recubrimientos catalogados como barreras ambientales (EBCs). Estos recubrimientos de carácter columnar nuclean a partir de una fina capa vítrea rica en silicio y pueden crecer en un amplio rango de composiciones, siendo posible incrementar la proporción Al/Si de manera constante y gradiente. De este modo se obtienen recubrimientos ricos en aluminio en la superficie exterior (confiriendo al sustrato una alta protección frente a la corrosión) y a su vez se mantiene la composición estequiométrica en la intercara (lo que supone gran adhesion dado el buen ajuste mullita-SiC). A pesar de que se ha demostrado el excelente desempeño frente a la corrosion de estos recubrimientos, la información existente en la literatura acerca de su comportamiento mecánico es limitada. Las propiedades mecánicas e integridad structural de los recubrimientos de mullita depositados sobre SiC son factores cruciales a la hora de implementar dichos sistemas en aplicaciones reales. El estudio de estos aspectos constituye la base de esta investigación. Teniendo en cuenta las características de los recubrimientos, resulta esencial evaluar sus propiedades desde una perspectiva local. En este sentido, la nanoindentación y el nanorayado son las técnicas más apropiadas para tal propósito. Esta investigación se basa en la implementación de ensayos de nanoindentación y nanorayado, en complemento con técnicas avanzadas de caracterización, para evaluar a escala local las propiedades mecánicas de recubrimientos de barrera ambiental basados en mullita, así como para evaluar la integridad structural de los sistemas recubiertos. Para ello, se dispuso de recubrimientos con composiciones de Al/Si estequiométricas (≈ 3) e incrementales (≈ 5, 6, 7, 8, 11), además de recubrimientos de composición gradiente. En cuanto a los recubrimientos de composición estequiométrica, su dureza (Hf), modulo elastico (Ef), límite elastico (σy) y tenacidad de fractura (Kf), se estiman mediante nanoindentación. Como consecuencia del carácter columnar de los recubrimientos, así como de la capa vítrea de silicio a partir de la cual crecen, las propiedades mecánicas encontradas son ligeramente menores que las reportadas para mullita estequiométrica policristalina. La energía de adhesión (Gint) y la tenacidad de fractura de la intercara (Kint) son también estimadas. Estos recubrimientos presentan gran resistencia al contacto deslizante, dado que soportan deformaciones plásticas considerables sin sufrir un daño significativo. El efecto del enriquecimiento en aluminio es el endurecimiento, el aumento en la rigidez y la disminución de la tenacidad en los recubrimientos de mullita. No obstante, los ensayos de nanorayado demuestran una integridad estructural satisfactoria de los sistemas recubiertos estudiados, ya que no se observan pérdidas significativas de material del recubrimiento. Los nanorayados sobre secciones transversales indican que las capas de composición gradiente presentan una combinación óptima de rigidez/dureza y resistencia adhesiva/cohesiva al nanorayado. Finalmente, se investigan los efectos de la temperatura y corrosión sobre el comportamiento mecánico de los recubrimientos. Las altas temperaturas ocasionan un incremento de Hf y Ef, acompañado por una caida en Kc, en recubrimientos estequiométricos, mientras que dichas propiedades permanecen practicamente constantes en recubrimientos ricos en aluminio y gradientes. Adicionalmente, el efecto combinado de temperatura y corrosión tiene un efecto menor sobre la integridad estructural de los recubrimientos ensayados tras los ensayos de nanorayado. Éste es un resultado interesante dado que estos recubrimientos se espera que sean utilizadas en aplicaciones bajo condiciones similares a las aqu