The relationships between fabrication variables, microstructure and selected properties of carbon bonded magnesia-graphite refractory composite materials have been investigated. A novel optical microscope method of characterizing the morphology of flake graphites was developed and used to determine distributions of length and thickness and average aspect ratios for the four graphite samples used in the study. The compaction behaviour of magnesia alone and in combination with the flake graphites has been studied in some detail and the microstructures of the products elucidated. It is shown that the amount of magnesia of small particle size plays a significant role in determining the graphite-graphite contact area in the structure. An irreversible volume expansion is observed on firing composites, the magnitude of which can be related to the microstructure and the graphite content. A phenolic resin binder restricts this expansion. It is shown that the carbon binder does not bond to the graphite phase and only weakly, if at all, to the magnesia. Consequently the strengths and moduli are low and show only a small variation with graphite type. The effect of adding graphite to carbon-bonded magnesia is to lower the strength slightly, but increasing the graphite content from 20-30% causes a small increase in strength. Increasing the amount of carbon bond from pitch has little effect on strength at levels of 5-15% whereas over the range 5-13% the resin binder has a more pronounced effect. The most significant factor affecting the strength and modulus of fired composites is the amount of silicon or aluminium, added as oxidation inhibitors, which react to form carbide and nitride phases. Finally, a brief study of slag penetration shows that this can be reduced by decreasing the amount of oxide fines in the composite because of the changes in microstructure that, result.

published_or_final_version / Physics / Master / Master of Philosophy

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High performance machines such as gas turbine engines demand efficient solid lubricants at high temperature and in vacuum. The current conventional solid lubricants need to be further improved. This research evaluates carbon based solid lubricants using a high vacuum, high temperature pin-on-disc tribometer. The objectives of this research were to develop an understanding of the tribological properties of solid lubricant coatings under extreme operating conditions, and to determine whether using a carbon based solid lubricant would be acceptable for use in those conditions. Experimentally, two solid lubricant coatings on tungsten carbide substrate were tested against two different materials. The coatings were carbon based and molybdenum disulfide based. The other materials were 440C stainless steel and tungsten carbide. The temperature, pressure, and relative humidity are independent variables. The results showed that the carbon based coating increases friction and wears out quickly due to high temperature, high vacuum, and low humidity. Abrasive wear is the dominating mechanism. At elevated temperatures and in dry environment, the carbon based coating underwent significant oxidation and phase transformation. This research is beneficial for future design and development of solid lubricants for aerospace applications, as well as other industries requiring lubricants that must operate in extreme conditions. This thesis includes five chapters. Chapter I is an introduction to tribology and to the materials being used in this research. Chapter II describes the motivation and objectives behind this research. Chapter III discusses the experimental procedure and further explains the materials used. Chapter IV presents and discusses the results obtained. Chapter V discusses the major conclusions obtained from the results and offers some future work that may be conducted concerning this research.

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Carbons and graphites have many industrial applications e.g. synthetic graphite (as moderators in the nuclear industry), natural flake graphites (for application in the manufacture of anti-piping agents) and metallurgical coke (for use in the blast furnace). The overall objective of this Thesis is to study effects of changes in properties of graphites and cokes by (i) radiolytic gasification of graphite, (ii) intercalation of natural flake graphites by sulphuric acid and (iii) intercalation of metallurgical cokes by potassium. (i) Radiolytic gasification Methods of image analysis have been developed to study the pore structure of graphite. These methods have been used to investigate the change in pore structure of a series of radiolytically gasified graphites. To examine the pore structure of the graphites by optical and scanning electron microscopy, each sample was vacuum-impregnated with a slow-setting resin containing a yellow dye. A semi-automatic image analysis system (Micromeasurements VrDS) linked to the optical microscope, enabled data on porosity to be obtained. The pore outline data, so obtained, were used by programmes on the controlling microcomputer to provide pore parameters such as cross-sectional areas, perimeters, Feret's diameters and shape factors. The results show that pores less than 100 ~m2 cross-sectional area are gasified because of the inability of the inhibitors (carbon monoxide and methane) to deactivate activated CO2* species before reaching the pore wall. Pores >1000 ~m2 cross-sectional area show only small changes in size and shape because of the deposition of carbon from methane inhibitor in these pores and are only developed at weight losses >17.0 wt.% by coalescence of open porosity <100 ~m2 cross-sectional area. (ii) Intercalation of natural flake graphite's Techniques have been developed to distinguish between natural flake graphite's and establish those suitable for use as anti-piping agents. Techniques used to examine the structure of natural flake graphite's include EDAX analysis to monitor amounts and distributions of elements, bromine intercalation to assess crystallographic ordering and image analysis to examine size and shape of the natural flake graphite's before and after intercalation. Results indicated that performances of the natural flake graphite's for use in intercalation studies can be predicted by assessing morphology and extents of fissures, bromine uptake, and mineral distribution of the flakes. Flakes suitable for intercalation studies have a mean flake thickness of ~25 ~m. Bromine uptake can be used to give an indication of the perfection of stacking. A high bromine uptake is desirable indicating a high stacking order i.e. good crystal perfection. Fissures in the natural flake graphite's are advantageous particularly in flakes of 40-70 ~m thick, by facilitating, a mean flake thickness of ~25 ~m. Fissures in the intercalated flake are detrimental as they may allow an 'escape route' to desorbing intercalate. Mineral impurities in the graphite flakes are of importance as they influence the flake thickness and cleavage properties. (iii) Intercalation of metallurgical cokes by potassium It is considered that the alkali metals, particularly potassium, have a crucial role in the breakdown of coke material during blast furnace operation. Extents of degradation, related to coke structure (optical texture) are examined to identify those structural aspects of cokes which are susceptible to alkali attack. The mechanism of potassium entering into metallurgical coke is investigated, ~. solid state diffusion, intercalation, absorption and adsorption. Metallurgical cokes, with a range of heat-treatment temperatures, graphitic carbon, and a shot-coke of small sized optical texture were heated with potassium vapour, either from direct addition of metal, or formed by heating a mixture of potassium carbonate with carbon black.Results of the study indicate that the rank of coking coal, and hence the optical texture of the derived coke, influenced the extents of degradation of the metallurgical cokes. Cokes from high rank coals (204 and 30lb) were consistently less degraded than those from lower rank coals (401 and 502 rank). Optical texture studies indicated that those optical textures most resistant to degradation by potassium vapour were of single component textures (flow anisotropy and isotropic). Multi-component textures as found in metallurgical cokes were less resistant to alkali attack. Heat-treatment of metallurgical cokes increased their resistance to degradation (2800 > 2400 > 2000 > 1500 > 11000C HTT). Degradation of metallurgical coke is thought to be due to mixed staging (yellow/blue/black colouration) of intercalates in graphitizable carbons because of non-uniform concentration of potassium causing high stresses and leading to break-up by macro-crack formation.

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