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Thèse doctorat : Génie des Matériaux : Villeurbanne, INSA : 1997. / Titre provenant de l'écran-titre. Bibliogr. p. 144-151.

The strive to increase the engine efficiency in terms of fuel consumption and lower emissions have lead to higher demands on materials. In this thesis five different thermal barrier coatings applied using air plasma spraying to three materials commonly used for exhaust application are evaluated. This thesis work was done at Scania CV in Södertälje with main focus on evaluation during thermal cycling. The goal of this thesis is to evaluate the coatings and correlate their behaviour to their characteristic microstructure. The coatings were evaluated through their stability in thermal conductivity, fracture toughness, hardness, porosity and failure modes. The parameters where obtained using laser flash, Vickers indentation, Vickers indentation fracture toughness and microscopic evaluation methods. The evaluation shows that conventionally used zirconia based materials exhibits low thermal conductivity, high hardness, and stable fracture toughness compared to other evaluated materials. One material that can be applicable in diesel exhaust application is mullite, which showed similar performance to zirconia based materials. For the use of TBC together with SiMo51 a different bondcoat than conventional NiCrAlY needs to be evaluated. / Strävan efter att konstruera effektivare motorer för att generera minskade utsläpp och bättre bränsleekonomi har genererat högre krav på konstruktionsmaterialen som används idag. I detta examensarbete som utförts på Scania CV i Södertälje utvärderas fem olika termiska barriärsskikt som belagts med plasma sprayning på tre vanligt förekommande konstruktionsmaterial för grenrör. Målet med detta examensarbete är att utvärdera beläggningarnas beteende under termisk cykling och koppla deras beteende till karakteristiska mikrostrukturer. Beläggningarna utvärderades genom att jämföra deras värmeledningsförmåga, hårdhet, brottseghet och porositet. Materialparametrarna utvärderades genom laser flash, Vickers hårdhetsmätning, Vickers brottseghet samt mikroskopiska bildanalyser. Resultaten visar att kommersiellt använda zirkonium baserade material uppvisar låg värmeledningsförmåga, hög hårdhet och hög brottseghet i förhållande till övriga material. Ett annat material som analyserats, mullit, visar på liknande beteenden som zirkonium baserade material men behöver utvärderas ytterligare. Om SiMo51 används som substratmaterial finns behovet att utvärdera användningen av andra typer av bindskikt än det austenitiska NiCrAlY nu använt.

In the iron ore pelletizing, the sintering of the pellets takes place in a rotary kiln which is lined with refractory bricks. During the process, deposited materials such as iron ore slag, additives, binders and fly ash stick on the surface of the lining, and with time, by infiltration and diffusion, a corroded layer is formed. Some of the reactions occurred in that layer are promoted by the presence of alkalis. Refractory/deposited materials reactions and infiltration of deposited materials were studied at laboratory scale. In this study, techniques such as differential scanning calorimetry (DSC), x-ray diffractometry (XRD) and scanning electron microscopy (SEM) were used. Reaction temperatures, phase transformations and infiltration depth of deposited materials were determinated. Additions of alkalis into the materials involved were used to enhance the reactions between them. Using powder mixtures the results of DSC show that in addition of K2CO3 the reaction temperature range is 850-950°C showing formation of new phases around 850°C with kalsilite and around 950°C with leucite; in addition of the mixture Na2CO3 and K2CO3, there is a broader temperature interval of 600-925°C with formation of kalsilite at lower temperatures and nepheline at higher temperatures. In the characterization of the infiltration of deposited materials into the brick it was observed that nepheline was formed mainly in the corroded brick layer but K feldespathoids (leucite, kalsilite, kaliophilite) were formed beneath the corroded brick layer. / <p>Validerat; 20110705 (anonymous)</p>

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Results are presented of an investigation into the effect of the anion and cation of various catalytic additives on the mullite development in a Georgia Kaolin. The sodium and potassium compounds investigated were found to be poor catalysts. The calcium compounds were very good catalysts while the lithium compounds ranged from poor to very good for lithium chloride. It was found that the catalysis of mullite is affected by the anion although its effect is not as pronounced as that due to the cation. / Master of Science

Solid-state reaction and sol-gel, processing techniques were used extensively to form chemically-modified mullite solid solutions in an effort to lower their thermal expansion coefficients. TiO₂, B₂O₃, CrPO₄, P2O₅, Ga₂O₃, Cr2O₃, and WO₃ and the half-breed SiO₂ compounds AlPas, BPO₄, GaPO₄, BAsO₄, AIAsO₄, GaAsO₄, and GeO₂ were chosen as the modifiers. The results indicate that, apart from TiO₂, none of the substitutions made in mullite significantly change the thermal expansion properties. The solubility of 3 wt% TiO₂ in mullite reduces the coefficient of thermal expansion by about 10%; That corresponds to a reduction in AAl₂O₃/SiO₂ molar ratio ( < 1.5) compared to stoichiometric mullite (3Al₂O₃·2SiO₂). The formation of TiO₂-modified mullite depends on processing condition and heat treatment. The possible mechanism of lowering the CTE of mullite by the addition of TiO₂ is discussed in terms of the bond strength. The axial expansion of a Ga2O₃-modified mullite was measured' up to 1200°C to show that the expansion is increased along the c-axis compared with that of the stoichiometric mullite. / Master of Science

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The purposes of this study were to determine as a function of temperature the thermal diffusivity and/or thermal conductivity of SiC-Mullite and SiC-SiC, and to explain the observed behavior in terms of changes in temperature, microstructure, composition, and/or orientation. Materials used in the SiC-Mullite study consisted of single crystal SiC whiskers (prepared from rice hulls or by the vapor-liquid-solid process) dispersed within a polycrystalline mullite matrix. During measurement of thermal diffusivity, the samples were heated to l500°C and cooled back to room temperature. No hysteresis occurred. However, both thermal diffusivity and conductivity exhibited maximum values at room temperatures, perpendicular to the hot pressing direction, at high volume percentages of SiC whiskers, and when VLS whiskers were employed. The SiC-SiC samples consisted of a crossweave of polycrystalline SiC fibers that were coated with phenolic resin and surrounded by a chemically-vapor-deposited matrix of SiC. The two types of samples examined were prepared with different amounts of resin. The matrices of the high resin samples were found to be dominated by the presence of char. Samples were cycled to 1000, 1400, and l800°C; hysteresis occurred on some of the cycles. Thermal diffusivity was highest parallel to one set of fibers. These results allow the qualitative tailoring of the heat flow properties of these ceramic composites, for particular applications, and set forth limitations on the use of the SiC-SiC composites at high temperatures. / Master of Science

A near-net-shape process for the production of mullite matrix ceramic composites below 1300 °C has been achieved by reaction bonding AI2O3, silicon, mullite seeds and a eutectic of A12O3-SiO2-mixed rare earth oxide. The roles of the transient liquid phase from the eutectic and the mullite seeds are examined. The approximate eutectic composition was estimated from the A12O3-SiO2-Y2O3 phase diagram as 22 wt % AI2O3/46 wt% S102/3 2 wt% Y2O3. The fusion temperature of the mixed-rare-earth-based eutectic composition was lower (1175 °C vs. those of pure, rare earth oxides based eutectic composition; Pr6On/A12O3/SiO2: 1224 °C, EU2O3/AI2O3/S1O2: 1259 °C and Y2O3/Al2O3/SiO2: 1345 °C). The densification characteristics of the reaction-bonded mullite (RBM) mixture were investigated. Density increased with eutectic, and decreased with mullite seed contents. Oxidation and volume expansions due to Si and mullite formation are examined by thermogravimetric analysis (TGA) and dilatometric measurement. TThe measured weight gain and maximum volume expansion were lower than theoretical values due to preoxidation of the Si powder. Dilatometric curves indicated sintering shrinkage is compensated by the oxidation-induced volume-expansion. AI2O3 + SiO2 mixtures of the mullite composition exhibited shrinkage exclusively. X-ray diffraction of the RBM sinters display major mullite peaks and minor residual a-A12O3. Mullite develops with low residual AI2O3 when 7.5 wt% mixed-rare-earth-oxide eutectic and 5 wt% mullite seeds are incorporated into the mix. The final sinter is > 90 % theoretical density, > 90 % mullite, and suffers 2.2 % sintering shrinkage. Transmission-electron-microscopy (TEM) and Energy dispersive X-ray spectra (EDX) were employed to follow mullite evolution. Model samples were utilised to study diffusion-, and reaction-, rates. The highest reaction rates at the lowest temperature occur when the eutectic penetrates an AI2O3/S1 layer. Bimodal pellets with and without eutectic (or with and without mullite seeds) directly illustrate their roles. Mullite seeds promote mullite formation, but the transient liquid accelerates Si oxidation, mullite formation and densification. / Thesis / Master of Engineering (ME)