Phase quantification of ß-Si3N4/ß-SiC mixtures by X-ray powder diffraction analysis

X-ray powder diffraction methods of phase quantification were adapted and compared to mixtures of -Si3N4 and -SiC. Multiline mean-normalized-intensity methods and whole pattern analysis (Rietveld) both have advantages and disadvantages over each other. Satisfactory results (less than 3% absolute deviation) can be achieved in minimal time using intensity normalization methods. Phase quantification using the Rietveld method requires significantly longer measuring time, evaluation time and expertise to obtain the same results

In situ generated homogeneous and functionally graded ceramic materials derived from polysilazane

The pyrolysis of cross-linked poly(hydridomethyl)silazane pellets via transient isothermal ammonia gas treatment yields amorphous layered ceramic bulk Si/N-Si/N/C-materials with graded carbon content. By this process the C content in the material can be adjusted with high accuracy in the range between 0 and 14 wt. %. The influence of (1) temperature of reactive ammonia treatment (2) time of reactive isothermal ammonia treatment (3) isothermal holding time under inert atmosphere (Ar) before application of ammonia (4) degree of cross-linking of the polycarbosilazane (5) porosity of green compact (6) volume ratio of NH3 in the reactive atmosphere was examined. The temperature of 525ºC and the reactive atmosphere containing 10 vol. % NH3 were found to be optimum for carbon content adjustment. Higher ammonia contents did not allow suitable control of the process, while higher temperature of transient heat treatment caused crack formation in the specimen due to excessive pressure of gaseous reaction products. High degree of cross-linking as well as the annealing of cross-linked green bodies at transient temperature in inert atmosphere decrease the efficiency of the reactive treatment and increase the ratio of C built in the pre-ceramic continuous random network (CRN). Next to graded materials, samples with bulk homogeneous carbon distribution were generated. However, these require more sophisticated heating schedules and combination of reactive treatment with pre-annealing in inert gas

HYDROTHERMAL CORROSION RESISTANCE OF SILICON NITRIDE WITH O'-Si Al ON GRAIN BOUNDARY PHASE

Dense Si₃N₄-based composites with O'-SiAlON bonding phase were prepared by hot pressing of the mixture of Si₃N₄ matrix and polysiloxane (samples SA) and polysilazane (samples SB) based sintering aids. In both samples α-Si₃N₄ and β-Si₃N₄ were identified as major phases and O'-SiAlON with a chemical formula Si1.84Al0.16O1.16N1.84 as a minor phase. The hydrothermal corrosion tests were performed in subcritical conditions at 250 °C for 100 hours. The weight loss was rather low, 0.96 wt.% and 1.32 wt.% for samples SA and SB, respectively. The corrosion rate of samples SA and SB was 206 mg.m-2.h-1 and 233 mg.m-2.h-1. The higher corrosion rate of sample SB prepared with polysilazane-based sintering aid is most probably due to the hydrothermal dissolution of Si₃N₄ grains producing ammonia, as the pH value of eluate of this sample was higher, 8.5 compared to pH = 8.0 determined for sample SA. In sample SA a continuous SiO₂-based passivation layer was formed, while in sample SB the passivation layer was discontinuous and the corrosion medium could easily attack the Si₃N₄ grains. As the corrosion solution was more basic in sample SB, the SiO₂ passivation layer did not form so effectively, or dissolved faster than in sample SA. The higher corrosion resistance of sample SA was partly improved also by using Y₂O₃ sintering additive, which formed a rather stable Y₂SiO₅ phase

Functional ceramics via precursor chemistry coupled with forming methods

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Corrosion and oxidation behaviour of B-SiAlON via different processing route

The corrosion resistance of B-SiAlON based advanced ceramic materials in molten aluminum, equimolar NaCl-KCl melt and also in the melted mixture of NaF and AlF3 (molar ratio: 1:1.163) were investigated. two kinds of B-SiAlON were tested: B-SiAlON prepared from powder precursor produced by carbothermal reduction and nitridation (CRN) of pyrophyllite and B-SiAlON made from commercila powders (AlN, Al2O3 and Si3N4). Corrosion tests were realized at 760?C for 7, 36 and 72 h under nitrogen atmosphere by dip finger test method. All types of B-SiAlON have great corrosion resistance against molten aluminium and NaCl-KCl melt at these conditions. The results of corrosion test were different for the B-SiAlONs in the in molten fluorides. While the corroded zone was only 100?m deep after 72 h corrosion test in reference B-SiAlON prepared from synthetic powders /SiAlON-R), in sample prepared from CNR pyrophyllite (SiAlON-P) it was almost 230 ?m thick. Additionally, the oxidation resistance of B-SiAlON was tested. The results showed that both SiAlON-P and reference SiAlON-R have comparable good oxidation resistance

In-situ carbon content adjustment in polysilazane derived amorphous SiCN bulk ceramics

The paper is concerned with in-situ carbon content adjustment in amorphous bulk silicon carbonitride ceramic matrices prepared by thermal polymer to ceramic conversion of crosslinked and compacted poly(hydridomethyl)silazane powders (NCP200). Heat treatment under inert (Ar) or reactive (Ar/NH3, NH3) atmosphere was applied for carbon content adjustment. Isothermal annealing steps at various intermediate temperatures were included into the schedule. Optimal annealing temperatures were found to be in the range from 500 - 550°C. Results are presented. Different possible chemical reactions are considered