Arthur J. Moorhead

Oxidation behaviour and effects of oxidation on the strength of SiC-whisker reinforced alumina

The oxidation behaviour and effect of oxidation on the strength of a SiC-whisker-reinforced-alumina composite material (Al2O3-SiCw) were investigated. The oxidation mechanism of the composite material was determined by thermogravimetric analysis (TGA) and compositional analysis. Changes in the fracture strength and surface morphology were also determined and related to the oxidation mechanism. Weight changes of samples exposed to flowing Ar with various levels of oxygen partial pressure, PO2 at 1400 °C were monitored continuously with a microbalance. Changes in strength were measured after exposure to flowing Ar with different PO2 at 1400 °C for various periods of time. The PO2-range employed in this experiment was from 5×10−7 to 1×10−3 MPa. In contrast to the oxidation behaviour of monolithic SiC materials, weight gains were detected for the whole PO2-range investigated. However, despite the weight gains in the low PO2-region (PO2 < 1 × 10-5 MPa), significant reductions in strength were observed which were apparently due to the loss of SiC whiskers from the sample surface via the formation of volatile SiO. This SiO gas reacted with the Al2O3 matrix to form a non-protective aluminosilicate glass on the surface, resulting in a linear weight gain with time. In the high PO2-region, typical parabolic weight gains were observed as a result of the formation of an aluminosilicate glass on the surface by a reaction between SiO2, formed by the oxidation of SiC whiskers, and the matrix alumina. The observed increases in strength of the specimens with exposure are believed to be due to blunting of existing surface flaws with a product oxide.

Oxidation behaviour of titanium-containing brazing filler metals

Brazements on alumina or partially stabilized zirconia (PSZ) of four silver- or copper-based brazing filler metals that contain titanium to promote wetting of and adherence to structural ceramics, were exposed in a thermogravimetric analyser at temperatures up to 700°C to atmospheres of 100% O2, Ar-20% O2 and Ar-3 p.p.m. O2. The alloys included Cu-41.1Ag-3.6Sn-7.2Ti, Ag-44.4Cu-8.4Sn-0.9Ti, Ag-41.6Cu-9.7Sn-5.0Ti and Ag-37.4Cu-10.8In-1,4Ti, at%. All formed external oxides that were more or less protective under all of the test conditions studied. The growth of the oxides followed a parabolic time law. The gains in weight due to oxidation observed were small, ranging (for 45 h exposure at 400 °C to Ar-20% O2) from 0.20 mgcm−2 for the Ag-37.4Cu-10.8In-1.4Ti alloy to 0.46 mgcm−2 for Cu-41.1Ag-3.6Sn-7.2Ti. As expected, weight gain increased with increasing temperature or {ie4067-01}. Unexpectedly, the titanium played a minor role in the scale formed on any of the filler metals with a titanium oxide, TiO2, being found on only one alloy — Ag-41.6Cu-9.7Sn-5.0Ti. The brazements on PSZ gained weight at a higher rate than comparative brazements on alumina. We attribute this behaviour to oxygen transport through the zirconia resulting in the growth of an interfacial layer of titanium oxide.

Fabrication Of XLWIR Window Materials

A technique was developed for fabrication of strong, fully dense, polycrystalline windows of CsI by hot-pressing in a steel die. We began with ultra-pure, ultra-dry CsI powder and conducted all processing operations within a glove box purged with dry N2. We were able to hot-press 5.08 cm CsI discs having transmittance of about 80% in the extra long wavelength (≥ 10 μm) infrared range--equivalent to that of single crystal CsI. The flexural strength of CsI bars fabricated by hot-pressing small-particle-size powder was 18 MPa, about 5 times higher than that of single crystal CsI.