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Surface quality and dimensional accuracy of titanium components produced by investment casting are limited by the conventional dip-coating procedure of the lost-wax process. This study presents a novel approach with spraying and centrifugation as alternative coating technologies. Aiming at improved surface quality, calcium zirconate slips and coatings based on alginate gelation were developed. Scanning electron microscopy and computed tomography revealed a homogeneous spray coating. The alginate-based spray coating was used for producing functionally graded calcium zirconate shell molds. These molds exhibited a fine-grained microstructure with pores in the lower μ m range. Applying an intermediate layer between the fine-grained spray coating and the coarse-grained dip-coating layers as well as the alternating application of dip-coating layers prevented cracking. The characteristic microstructure showed a bridging zone and a homogeneous distribution of coarse grain across the diameter. Ti6Al4V cast parts with no presence of an alpha case layer were obtained.

Binders play an important role in the manufacture of carbon-bonded alumina (Al2O3-C) refractory materials. To diminish the currently used coal tar pitch binder Carbores® P, which could release hazardous polycyclic aromatic hydrocarbons during operations, environmental-friendly lactose and tannin are applied as the main binders in this study. Compact cylinders of fine-grained Al2O3-C based on the lactose-tannin (16 wt%) binder system with significantly reduced pitch content (4 wt%) are successfully prepared via slip casting for the first time. The lactose to tannin mass ratio is systematically varied from 5:1 to 1:5 in the raw material formulation. The specimens based on the new binder system are less dense, more porous and contain less residual carbon than the pitch-based reference. Mechanical tests show that the specimens with lactose-tannin ratio of 5:1, 4:1 and 3:1 have considerable cold crushing strengths, although the splitting tensile strengths are low. In contrast to the well-established pressing route, the slip casting route for compact Al2O3-C is more comparable to the replica route for Al2O3-C foam filter due to the pressureless shaping process.

Alkali aluminosilicates are potential materials for high temperature applications under alkali load. This study investigated the corrosion behaviour of potassium aluminosilicates of the nominal compositions of KAlSi2O6 and KAlSiO4 synthesized at different temperatures and dwell times and exposed to alkali load at 1100 °C. For understanding the corrosion behaviour depending on the synthesizing parameters, the produced materials and corroded samples were investigated by XRD and NMR spectroscopy (29Si, 27Al). The combination of both analytical techniques showed that characteristics of the synthesized materials on a structural level played an important role for the corrosion behaviour. It was shown that amorphous and disordered components in the samples occurred which were not registered by XRD. The application of 29Si and 27Al NMR spectroscopy yielded that materials having already after synthesis a structural arrangement similar to the equilibrium target phase of the batch showed a high structural stability under alkali load and gave positive results in the applied corrosion test. Materials with large structural rearrangements during alkali load exhibited volume changes and therefore failed the corrosion test. Based on these observations it should therefore be possible to design materials with high chemical and mechanical stability suitable as furnace linings and to evaluate end-of-life aluminosilicates for an application as alkali corrosion resistant recyclates.

In the present work the solid-state synthesis of phase-pure barium zirconate (BaZrO3) using barium carbonate and monoclinic zirconia as powder raw materials is investigated. Due to the high evaporation tendency of BaO, special attention is paid to the thermogravimetric behaviour of stoichiometric mixtures of BaCO3 and ZrO2, namely without sintering aids. DSC/TG measurements of the powder mixtures were conducted to assess the synthesis reaction using heating rates of 1, 3 and 10 K min−1. In addition to the investigations of the thermogravimetric behaviour in dependence of different heating rates, the influence of synthesis temperature and ceramic carrier grains was examined on pressed specimens of 50 mm in diameter. Moreover, the influence of three different partial pressure relations resulting from the decarbonisation reaction and the assumed BaO evaporation was investigated. It was found that a synthesis temperature of 1200 °C and a heating rate of 1 K min−1 resulted in the least pronounced mass loss and mass loss rate, respectively. XRD analyses revealed phase-purity for specimens of assumed low BaO evaporation. Thus, optimum thermal treatment conditions were identified for the BaZrO3 synthesis that can be applied to the production of phase-pure material for e.g. the refractory industry.

Main objective of the presented research is the life time prediction of self-supporting flame-sprayed Al2O3- and Al2O3 -ZrO2 -TiO2-materials under constant load. The characteristic life time and its scatter were derived from stable crack growth tests and Weibull-statistics including the four-point-bending and ball-on-three-balls method. The potential life time was estimated in order to assess room-temperature handling and long term storage of self-supporting flame-sprayed alumina components. In terms of flexural strength, energy release, and subcritical crack growth parameters, distinct differences between both materials were shown. In turn, the characteristic life times only barely deviated from each other. From that the conclusion was drawn that life time performance under constant load application is governed by the characteristic flame-spray microstructure. However, advantages in the flame-spray processing of Al2O3 - ZrO2 -TiO2 are still given, attributed to its lower melting temperature.

Carbon-bonded alumina is currently applied in metal casting processes due to its excellent thermomechanical performance. Used as ceramic foam filters in steel casting, the surface composition of such material is essential for trapping primary and secondary inclusions in liquid steel, where functionalized coatings are proven to enhance the filtration efficiency considerably. In this paper, compression tests on uncoated and coated Al2O3–C foam filters are performed at high temperatures under argon atmosphere. The macrostructure of the prepared filters before the high temperature compression tests and the microstructure afterwards are examined by computer tomography and scanning electron microscopy, respectively. High temperature compression strength of the studied filters generally exceeds cold crushing strength values thanks to plastification processes. The applied coatings have positive impact on the filter strength. The observed high scatter of the compression strength can be attributed to the inhomogeneity of strut thickness, distribution of the oxide and non-oxide materials within the struts as well as coating thickness.

In this work, cement-free alumina castables with different fiber additives were produced. The effect of commercial polypropylene fibers was compared to that of magnesium borate precursor fibers produced by electrospinning. The drying behavior of different batches was monitored by means of thermogravimetry. Elastic moduli were estimated in dried, sintered and thermally-shocked samples through the impulse excitation technique. The mechanical strength as cold modulus of rupture (CMOR) was measured by three-point bending tests. The open porosity and bulk density after sintering were assessed by water immersion. The thermal shock resistance was evaluated by heating the samples up to 950 ∘C and then quenching with compressed air. After 1 or 5 thermal shock cycles, the residual CMOR was determined. The microstructure of fracture surfaces was analyzed by scanning electron microscopy (SEM). The possible formation of new phases during thermal treatment was also assessed by means of XRD. Finally, the refractoriness under load was measured.

The current paper focuses on the fabrication of yttria and magnesia stabilized zirconia nanofibers via electrospinning from zirconyl chloride octahydrate and polyvinylpyrrolidone precursors with minor additions of yttrium nitrate hexahydrate (3 mol.%) or magnesium nitrate hexahydrate (10 mol.%). The precursor materials were dissolved in an ethanol-water mixture in a ratio of 75:25. After successful fiber preparation, the thermal decomposition behavior of the starting materials and the subsequent phase evolution at elevated temperatures were studied. Pure tetragonal zirconia nanofibers were obtained for the composition stabilized with 3 mol.% yttria when the thermal treatment was conducted with a heating rate of 10 K/min up to 1100 °C. In future research work, these tetragonal zirconia nanofibers will be used as reinforcing material in metal matrix composites based on metastable austenitic steel. The combination of the TRIP/TWIP-effect in the steel matrix with the stress-assisted tetragonal to monoclinic phase transformation in the tetragonal stabilized zirconia will lead to a composite material with outstanding mechanical properties.

Hydroxyapatite (HA) is considered as the most promising biomaterial candidate to replace and regenerate hard tissues. A small amount of β-tricalcium phosphate (β-TCP) phase is advantageous for rapid bonding of the artificial bones to natural ones due to its high solubility compared to hydroxyapatite. Synthesizing HA nanofibers from electrospinning of sol-gel is considered as a widely researched topic. Motivation of the current work was to investigate the influence of polymeric binder in the final phase evolution after heat treatment of electrospun nanofibers. Calcium phosphate nanofibers were fabricated by electrospinning sols using gelatine and polyvinylpyrrolidone as carrier polymers and subjected to heat treatment. It was realized that carrier polymers facilitate preferential calcium phosphate phase formation by forming hydroxyapatite as major phase while PVP was used and β-TCP with HA as secondary phase while gelatine was employed. XRD and thermal analyses were performed to ascertain the reason behind this interesting behaviour.

This study investigates the feasibility of recycling carbon fiber reinforced polymer (CFRP) composites in carbon-bonded alumina refractories. For this purpose, different procedures for pretreatment and separation of recyclable CFRP composites were examined. The influence of pretreatment, dimensions and condition of the CFRPs on physical, mechanical, and thermo-mechanical properties of carbon-bonded alumina refractories were evaluated. The CFRP fractions were classified by sieving and partly treated by means of mechanical and thermal methods. The mechanical approach describes a simple method to obtain short fibers along with a reusable fiber dust. By thermal treatment in an oxidizing atmosphere, the carbon fibers can be easily separated from the polymer matrix. Carbon-bonded alumina bricks of industry standard with CFRPs of different length and treatment and partly silicon as antioxidant were produced and compared to the respective reference without CFRPs. The addition of recycled CFRPs to carbon-bonded alumina refractories resulted in a modified microstructure consistent with open porosity and altered mechanical properties at room temperature. The microstructural change in the refractory material provoked by recycled CFRPs

Open cell ceramic foam filters are used for metal melt filtration to enhance the quality of cast products by reducing non-metallic inclusions and generating calm mold fill with less turbulences. A new generation of filters made of carbon-bonded alumina show an increased thermal shock resistance but a lower strength. The filters have to withstand high thermal and mechanical loads due to the filtration process, whereas a temperature dependent mechanical characterization of the bulk material is required. This contribution concentrates on the influence of carbon content and coking temperature, which are two important manufacturing process parameters. In the ball on three balls test miniaturized disk-shaped specimens are loaded with a spherical tipped punch at temperatures up to 1500 ∘ C until failure occurs. The fracture stress is calculated and used to analyze the material strength by means of Weibull distribution. Two different mechanisms are identified, which change the micro-structure and therewith the strength of the material. Additionally, Raman spectroscopy supports the interpretation of the achieved results of the mechanically tested specimens.

The mixed-mode fracture toughness of a 42CrMo4 high-strength steel under mode I/II loading was investigated. The steel was processed in a steel casting simulator where it was poured through a spaghetti-type filter into a ceramic crucible. The machined compact tension shear specimens were heat treated in order to obtain the quenched and tempered state. The designs of the specimen and the loading device enabled testing at different loading angles leading to different ratios KI/KII. Strain gauges were applied in order to analyze the stress field around the crack tip. It was observed that the strain gauges can be reasonably applied to measure KI. However, large errors occurred during the measurement of KII. The specimens fractured in a brittle manner. However, no valid plane-strain fracture toughness was obtained. With increased loading angles, mode I fracture toughness decreased while mode II fracture toughness increased. Furthermore, it was found that stretch zone was flattened with increased loading angle.

Carbon-bonded magnesia and magnesia-alumina bricks are the state of the art lining materials of the sidewalls and the bottom of steel ladles. Industrial trial tests revealed, that a new generation of MgO–MgAl2O4–C bricks, where pre-reacted spinel is added in form of a new calcium magnesium aluminate (CMA) raw material, exhibit a longer service life caused by an enhanced corrosion resistance due to the formation of a protective slag layer. In terms of this work, laboratory corrosion tests have been performed in order to mimic the protective slag layer formation and to study the kinetics of the formation. It has been found that the formation highly depends on the amount of iron present in the melt, which leads to the precipitation of the brownmillerite-phase. Furthermore, the study was extended to force the formation of protective slag layers of different composition which are reliable at higher temperatures and wider temperature ranges by targeted adaption of the brick composition. Cup tests showed that there is a potential to manipulate the composition of a protective slag layer by targeted adaption of the brick composition. Especially MgO, Al2O3, FexOy, and SiO2 from the brick or slag enrich at the slag/refractory-interface.

The insufficient strength and thermal shock resistance of mullite reticulated porous ceramics (RPC) limited their application in porous media combustion. To strengthen mullite RPC, the struts with three layers were prepared using the combined approaches of polymeric replica and vacuum infiltration. The effects of residual stress within the struts on thermal shock resistance of mullite RPC were investigated by finite element analysis (FEA). The results showed that the strut was composed of the coating of reaction-bonded mullite, middle layer of mullite skeleton and the triangular filling layer. The triangular voids and surface cracks were eliminated in the formed struts, resulting in a higher strength of mullite RPC than that of hollow strut. In addition, FEA results indicated that the residual compressive stress formed in the coating of strut after mullite RPC was sintered at 1450 °C and 1500 °C, while the residual tensile stress formed at 1550 °C. The larger residual compressive stress within the strut coating was beneficial to enhance the thermal shock resistance of mullite RPC.

Calcium zirconate fibers were produced by electrospinning and characterized in this work. The solution was prepared from zirconium and calcium salts, using polyvinyl-pyrrolidone (PVP) as processing aid. The decomposition of the organic fraction and crystallization of calcium zirconate were followed by thermogravimetry and differential scanning calorimetry (TG/DSC). Raman Spectroscopy was used to measure the vibrational modes in the green as well as in the calcined fibers. The final phase composition was studied by means of X-ray diffraction (XRD). The fiber morphology was investigated by confocal laser scanning microscopy (CLS) and scanning electron microscopy (SEM). The formation reaction of calcium zirconate was observed at about 740 °C. Highly crystalline fibers were obtained already at 800 °C, but the crystallinity and calcium zirconate yield improved when the temperature was increased to 1000 °C.

Calcium zirconate refractories exhibit a promisingly high corrosion resistance in contact with titanium alloy melts. In the present article, we improved the thermomechanical properties of calcium zirconate based refractories by altering their microstructure and chemical composition. In a full factorial experimental design we investigated the effects of an insitu phase formation, of the addition of coarse grained MgO aggregates as well as the addition of SrCO3. Based on these factors, the chemical, physical, mechanical and thermomechanical properties as well as the resulting microstructure were thoroughly characterized. Strikingly, the addition of SrCO3 resulted in significantly improved mechanical properties before and after thermal shock. The improved thermal shock resistance can be attributed to a lower determined thermal expansion coefficient, a homogeneous pore size distribution with a reduced pore size and a better bonding between the matrix and the coarse grained aggregates.

The influence of particle size distributions on the density, density gradient and strength of uniaxially two-sided die-pressed cylinders was investigated. To design the particle size distributions with a maximum grain size of 3 mm, a continuous gap-grading model was applied. The bulk densities were measured before and after firing (1600 °C) and the cold crushing strength after firing. Some fired samples were axially and radially cut to analyze the density distribution and gradients. Densities and strength correlated and were highest for increasing amounts of the finest fraction (0–0.02 mm) up to 45 wt%. The density gradients were lowest for high amounts of fines leading to an improved pressure transmission and densest packing. Additionally, the gradients were low for high amounts of coarse grains leading to a pressure transmission through a formed coarse grain skeleton. Summarized, densest compacts with lowest density gradients were achieved for particle packings with a reduced amount of medium sized grains.

Carbon-bonded foam-filters, such as alumina-carbon foams, are widely used for the filtration of metal melts. The aim of the experimental investigations reported in this paper was to examine the possibility of producing alumina coatings with a porous structure on such alumina-carbon foams by an electrophoretic deposition method. This method utilizes the simultaneous gas evolution by electrolysis to generate channel-like pores during the formation of ceramic layers by the electrophoretic deposition from aqueous suspensions. The foam samples to be coated were found to possess a sufficiently high electrical conductivity to function as deposition electrode. Using an aqueous suspension of a submicron alumina powder with an anionic polyelectrolyte as dispersant, coatings containing the desired pore channels were successfully produced on these foam samples. Voltages of 10 or 15 V proved suitable for the pore formation, as illustrated by tomographic 2D slices of the coated foams and by images that were taken by means of a digital microscope. First tests showed that the sintered coatings were able to resist the thermal shock to which they were exposed by immersing them into an aluminium melt.

The three-layered struts within SiC reticulated porous ceramics (RPC) were designed to improve the thermal characteristics and thermal shock resistance of porous burner. The effects of strut structure and its composition on the flame stability limits, flame temperature and thermal damage resistance of porous burner were investigated. The results showed that three-layered struts consisted of continuous mullite coating, middle layer of SiC skeleton and inner layer of reaction-bonded mullite after sintering at 1450 °C. Compared to porous media with single layer, RPC with three-layered struts possessed the combined effects of high thermal conductivity and heat radiation because of its middle layer of SiC skeleton and in-situ formed mullite coating. The three-layered struts in porous media extended the flame stability limits and increased the flame temperature of porous burner. Furthermore, the formation of three-layered struts eliminated strut defects and formed continuous mullite coating, thereby significantly improving the oxidation resistance and thermal shock resistance of RPC, which increased the service life of porous burner.

Due to their unique properties and structure, thermally sprayed ceramics are applied in many fields. Their applications range from engineering ceramics to the use in high temperature units, especially since an improved thermal shock performance was observed. The present study presents an analysis of structural and mechanical properties of thermal sprayed alumina-rich coatings at room temperature. Microstructure and phase composition were analysed by scanning electron microscopy and X-ray diffraction. For pure alumina material, high temperature X-ray diffraction was performed additionally and yielded a phase transition within heating to 1200 °C. Micrographs present typically layered and chaotic microstructure, streaked by microcracks. Cyclic loading test-procedures and the resulting load-displacement-curves revealed a pronounced R-curve-behaviour for coatings of pure Al2O3 and Al2O3 ZrO2 TiO2 compositions.

Within this study the influence of the sample shape, the cell size and the measurement method on the Young's modulus E of open cell foam alumina was investigated. The measurement methods used for the determination of E were the impulse excitation technique, the longitudinal ultrasonic measurement, the quasi-longitudinal ultrasonic measurement and the three point bending technique (3PB). The samples comprising the smallest height, yielded the highest E values for all measurement methods due to its processing, causing higher densities and homogeneity and hence leading to increased E values. Furthermore, its smaller area moment of inertia results in a higher beam deflection for a fixed force applied in 3PB. Thus, in the case of this shape, the forces involved during the 3PB tests were much lower and a smoother test could be performed and resulted in higher E values. Problems were found for the determination of E by the impulse excitation technique at large cell sizes (10 and 20 ppi). This was attributed to a reduction of the vibrating surface beneath the microphone capturing the audio signal. Ultrasonic methods performed robustly, however have to be conducted with care (e.g. Poisson's ratio assumption).

The existence of liquid phases causes difficulties in Ti 4+ -activated sintering of alumina, at which soda is being involved. Knowledge of the system Al 2 O 3 -TiO 2 -Na 2 O is necessary in order to control the manufacturing of alumina-rich and titania-containing ceramics. In order to promote the development of Al 2 O 3 -TiO 2 -Na 2 O ceramic components and high-performance titania-containing alumina refractories, sample bodies containing a different portion of Na 2 Al 2 Ti 6 O 16 phase were prepared by mixing different types of sodium-containing raw materials, alumina, and titania. A double solid state synthesis has been adopted to manufacture sample bodies without deformation. After sintering at 1140 °C, a dense ceramic material with almost single phase of Na 1.97 Al 1.82 Ti 6.15 O 16 was obtained by the usage of Na 2 CO 3 at a molar ratio of 1:6:1 (Al 2 O 3 :TiO 2 :Na 2 O), which has an open porosity of 3.9% and a bending strength of 105 MPa, but a low thermal shock resistance.

Since solid, non-metallic inclusions influence considerably the quality of casted steel products, carbon-bonded alumina foam filters are used in secondary metallurgical treatments to remove these particles from steel melts. In order to attain a significant improvement of the filtration process, five different carbonaceous spinel compounds from the Fe-/Mg-/Mn-Al-O systems are applied on carbon-bonded alumina filters in this study and investigated with regard of their filtration efficiency. However, these spinel compounds decompose partially during sintering at 1400 °C under reducing atmosphere, wherefore the resulting coatings contain not only spinel compounds, but also oxidic and metallic components. The subsequent interaction with molten steel leads to the development of multicrystal structures on the filter surface, which stem from interfacial reactions between coating materials, molten steel, and inclusions. As a result of this procedure, a reduction of almost 60% alumina inclusions is measured with the aid of an automatic SEM, whereby spinel compounds from the Fe-Mn-Al-O system achieve highest filtration efficiencies.

New carbon-bonded alumina filters for steel melt filtration were developed. The carbonaceous matrix was based on a new, environmental friendly binder system based on lactose and tannin. The filter preparation was analogous to the production of conventional foam filters according to the Schwartzwalder process. The processing as well as the rheology of the slurries was investigated. An addition of n-Si increased the carbon yield and the cold crushing strength (CCS) of the samples. Higher values of CCS were obtained after coating of the filters with alumina. The material was characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The applicability of these new filters was assessed in impingement tests with a steel melt, in which three out of four recipes survived the thermal shock.

Various melt refining techniques were developed in the past and led to ongoing improvement of casting quality. One of these processes is the so-called metal melt filtration, which became state of the art especially in aluminum foundry operations by using ceramic foam filters. Attempts to adapt the technique for the continuous casting of steel showed numerous difficulties due to the severe thermomechanical conditions and the high quantity of processed melt. Premature blockage or failure of ceramic foam filters implemented in the tundish promoted the application of flow control devices and multi-hole filters as alternatives. In the present study, a new approach for ceramic foam filters in the continuous casting of steel was tested. Thereby, the main disadvantage of limited filter service lifetime was circumvented by the application of an exchangeable filter system without disturbing the underlying continuous casting process. Cylindrical carbon-bonded alumina foam filters were prepared and mounted on alumina-carbon shafts using different adhesives and slag protection measures. The filters were immersed by a crane into the steel melt in an industrial tundish with 80 t capacity. During each test, one batch with approximately 380 t of steel was cast at over 1550 °C before removing the filter. The application of high-alumina mortar as adhesive and a compact filter geometry without macro channels proved to be essential for the successful operation. Samples of the corresponding filter were analyzed by digital light microscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The complementary investigations revealed the formation of pronounced clogging layers based on aluminum-rich calcium aluminates, which can be clearly distinguished from the synthetic slag material containing silicon and other trace elements. At the bottom of the filter pronounced clusters of coral-like and plate-like inclusions were observed, which approached the filter surface by buoyancy.

In this paper, a unique nano-coating consisting of a mixture of graphene oxide and multi-walled carbon nanotubes was applied on carbon-bonded alumina substrates. Graphene oxide (GO) was prepared according to the modified Tour's method. GO in combination with carbon nanotubes (CNTs) and modified coal tar pitch was spray-coated on conventional carbon-bonded alumina material for investigations in contact with steel. Temperature and time-dependent experiments were performed in a hot stage microscope to analyze the wetting behavior. The microstructures of the substrates before and after heating were compared by scanning electron microscopy. The results showed that the coating containing both GO and CNTs delivered a fast stabilization of the wetting angle with low wettability towards the metal melt. Similar coatings may offer an innovative approach to improve purification of steel melts by filtration in the steel industry.

This study addresses the question of whether mercury intrusion porosimetry is an appropriate measurement devise for the porosity characterization of reticulated ceramic foams which features three kinds of pores: functional pores, material pores and strut cavities remaining after decomposition of the polymeric foam. Reticulated ceramic foam samples made of Al2O3 and Al2O3-C and with functional pore sizes between 10 and 60 ppi (pores per inch) were investigated. The results show that it is feasible to measure the strut cavities and the entryways of the material pores with the help of the mercury intrusion porosimeter. The results for the strut cavity diameter depends strongly on the ppi number of the polyurethane foam used for the preparation of the ceramic foam. For the reticulated Al2O3-C foam samples no extrusion of the mercury is observed. Reasons for the missing extrusion of mercury are discussed. Furthermore, atomic force microscopy (AFM) measurements on Al2O3 and Al2O3-C samples are carried out using a recent AFM method to examine interactions between a mercury droplet and rough surfaces. Factors like approach/retrace speed, reproducibility and applied force are investigated. It is seen that for Al2O3 larger attractive forces are measured than for Al2O3-C.

This paper focused on the nano-functionalization of carbon-bonded alumina using graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs). GO was prepared according to the modified Tour's method. The rheology of suspensions containing GO, CNTs and both substances were analyzed and discussed. Xanthan proved to be a suitable stabilizer for the three systems. The spraying process of the suspensions was investigated with the aid of a high-speed camera. Al2O3-C filters and flat samples were spray coated, for investigations in contact with steel. The ceramic foam filters as well as the starting suspensions were analyzed by several analytic techniques to investigate the microstructure and other properties. Moreover, the hot stage microscope was used to study the steel/coating wetting behavior during operation at high temperatures. The results showed that the formulation containing both GO and CNTs delivered the best performance in contact with the melt. Similar coatings based on these nano-sized materials may offer an innovative route to improve purification of steel melts by filtration.

The investigation of phase transformations in metastable ceramic systems such as zirconia often requires local phase analysis within the areas of interest. Electron backscatter diffraction is a suitable method in combination with focused ion beam sample preparation. The interaction between ion beam and sample has to be carefully considered. In case of metastable Y-PSZ and Mg-PSZ, phase transformations were observed after FIB preparation with 30 kV, 30 nA and 5° incidence angle. Damage was the dominating effect for angles of 72°. The expected local temperature increase due to the ion bombardment with 30 kV and 30 nA is 700 K for ZrO2. Thus, the observed phase transformations can be explained on the basis of the temperature increase in the corresponding Y-PSZ phase diagram. In case of Mg-PSZ, the transition temperature is 1083 °C. The local temperature increase was obviously lower. The excitation energy for the observed phase transformation was smaller than expected from the phase diagrams of the thermodynamic equilibrium. Using 5 kV, 4.8 nA and 5° incidence angle, no phase transformations and no damage were observed. Thus, these conditions are well suited for the FIB preparation of metastable zirconia.

Coarse-grained refractory composites, with grain sizes between 20 and 5000 µm, based on Nb-Al 2 O 3 and Ta-Al 2 O 3 castables were produced for the first time and characterised in terms of shrinkage after sintering, splitting tensile strength, compressive strength, porosity and the measurement of the elastic properties ( E , G and ν ). After sintering at a temperature of 1600 °C, the shrinkage of the composites was 1.5% and 0.3%. Measured values of splitting tensile strength were between 5.5 and 15 MPa and the ones of compressive strength were between 23 and 89 MPa. Values of E and G were between 117 and 45 GPa and 48–17 GPa, respectively, for samples with 11–45 vol% refractory metal. Poisson's ratio was found to be very sensitive to the bonding between the fine matrix and coarse-grained particles of the composites. Its value increased from 0.25 for good bonding to ν = 0.43 in case of poor bonding between the coarse metal particles and the fine ceramic matrix. DTA/TG measurement under air atmosphere showed that the metal-ceramic composites start to oxidise at temperatures above 450 °C.

To investigate possible reactions between differently coated carbon-bonded alumina filters and an AZ91 magnesium alloy melt, immersion tests were carried out. Uncoated as well as MgAl 2 O 4 -, Al 2 O 3 -, nano- (carbon nano tubes/alumina nano sheets) and MgO-C-coated filters were tested. Thermodynamic calculations showed that only magnesia (MgO) and carbon are stable against molten magnesium; alumina (Al 2 O 3 ) and spinel (MgAl 2 O 4 ) will be reduced under the formation of magnesia. Optical and scanning electron microscopy as well as EDX analysis were performed near and at the filter-magnesium alloy-interface of the cooled and sectioned filter samples after their immersion into the AZ91 melt. The results of the thermodynamic calculations were confirmed by the experiments. The MgO-C-coated filter was the only one that did not show an in situ-formed layer on its surface after being in contact with the magnesium alloy melt. The alumina- or spinel-containing filter surfaces displayed platelet-like in situ layers after their contact with the molten AZ91. The results of the EDX analysis of these layers suggest their composition of MgO, since notable respective Mg and O contents were detected, as predicted by the calculations.

Carbon-bonded alumina filters with special design for continuous casting were produced by the replication technique using different coating procedures. The filters were investigated regarding filter strut diameter, porosity and cold crushing strength. The potential filter capacity and the mechanical load expected during the immersion in a continuous casting tundish were calculated. The best compromise between mechanical strength and structural features was achieved by combining the advantages of centrifugation, dip coating and spray coating. This filter type was analyzed by computed tomography and withstood the contact with molten steel in a casting test. After melt contact, the filter material exhibited near-surface decarburization and alumina-based in-situ layer formation. The observed layers were more pronounced in zones with potentially lower melt flow velocity. The results of the study indicated that the produced filters are able to withstand the severe conditions in continuous casting.

The influence of the pH value and of an anionic polyelectrolyte on the zeta potential of different raw materials that are used for producing carbon-bonded alumina refractories was studied by measurements of the electrokinetic sonic amplitude (ESA). The following raw materials were investigated: alumina CT 3000 SG, tabular alumina T60/T64, modified coal tar pitch Carbores® P, graphite AF 96/97, carbon black LUVOMAXX® MT N-991 and RW micro-silica. The isoelectric points of these materials were identified, except for the graphite and the micro-silica, which exhibited a negative surface potential over the entire pH range investigated (pH 3–11). An anionic polyelectrolyte (Dolapix CA) was found to increase not only the value of the zeta potential of the oxides at the inherent suspension pH but also that of the graphite and the carbon black. Its addition to concentrated slurries containing a mixture of the investigated raw materials reduced the viscosity and resulted in a decrease in the apparent porosity of carbon-bonded alumina samples produced by pressure filtration. The suitability of the suspension composition for shaping large-sized samples by pressure slip casting and the high thermal shock resistance of the fabricated alumina-carbon refractory have been demonstrated.

The quality of titanium cast parts depends not only on the corrosion reaction of the mold in contact with the titanium alloy melt, but also on the dimensional accuracy and the surface quality of the investment casting mold. Usually, the ceramic mold is produced by dip-coating. This contribution investigated the coating properties of green calcium zirconate (CaZrO3) coatings produced by the replica technique. Thus, the effect of different coating slurries as well as different coating technologies on important coating properties was analyzed. In all cases, centrifuging and spraying produced thinner coatings compared to dip-coating. Using an appropriate coating slip, particularly centrifuging revealed a cohesive homogeneous coating surface and is therefore a promising technology for the production of functional coatings for titanium casting molds.

MgO is pointed out as an alternative binder for refractory materials, mainly for systems where the presence of CaO might not be desired. Selecting the most suitable magnesia source is an important step as its purity and reactivity should influence the hydration reaction, leading to binding effect or cracks. This work investigated the design of vibratable high-alumina compositions bonded with MgO fumes [which is a very fine powdered oxide (d 2 formation and control the crystal growth of this phase. The green mechanical strength and thermomechanical performance (cold and hot mechanical strength, thermal shock, refractoriness under load, corrosion, etc.) of designed MgO-bonded compositions were analyzed. Improved green mechanical strength and crack-free samples were obtained when adding up to 6 wt% of MgO fumes to the refractories and processing them with aqueous solutions with 3 wt% of formic acid. The compositions with 6 wt% of magnesia fumes resulted in samples with flexural strength in the range of 12.0 MPa after curing at 50 °C/24 h and similar green mechanical strength (12.9 MPa) as the ones bonded with 4.0 wt% of calcium aluminate cement after drying at 110 °C for 24 h, which highlights the great potential of this MgO source. Despite the enhanced green mechanical strength, alumina-based castables containing 6 wt% of MgO (fumes, dead-burnt or their blend) showed low mechanical strength at intermediate temperatures and high linear expansion, as a consequence of the in situ spinel phase formation above 1200 °C. Thus, better densification, improved HMOR, thermal shock resistance and corrosion behavior were obtained for the castables prepared with less MgO fume contents.

Different approaches to develop low-carbon refractories have been recently proposed. Among them, the use of Ti3AlC2 can influence the corroded interface and improve the corrosion resistance of low-carbon containing refractories, but current understanding of the role of Ti3AlC2 is far from complete. In this work, the impact of the single additives Ti3AlC2 and Si as well as their combination on the microstructure and properties of MgO-C refractories were evaluated. The newly formed ceramic bonding phases contribute to superior mechanical properties. However, the high volume expansion of Ti3AlC2 led to adverse effect when using Ti3AlC2 as single antioxidant. The combination of Ti3AlC2 and Si could inhibit the rapid oxidation and excessive expansion of Ti3AlC2.