High-Temperature Oxidation

Abstract

The oxidation of zone melted specimens of ZrB2 and HfB2 was studied at temperatures of 1200°–2000°K and at oxygen partial pressures of 1–700 Torr in helium. Parabolic rate behavior was observed. For ZrB2 above 1400°K an activation energy of 77±5 normalkcal/normalmole was determined. Below 1400°K the activation energy is much lower, and the mechanism of oxidation is different, probably because of the presence of boron oxide in the condensed phase. Below 1300°K, the parabolic rate constant appears to be directly proportional to oxygen partial pressure; at 1830°K, the rate constant is independent of oxygen pressure. For HfB2 in the range 1480°–1870°K an activation energy of 47±4 normalkcal/normalmole was observed. Around 1970°K, the transition temperature for the monoclinic to tetragonal phase change in HfO2 , there is a sharp increase in the rate of oxidation. The parabolic rate constant for total oxygen consumption increases as the 0.4±0.1 power of the oxygen partial pressure at 1630° and 1760°K.

In situ studies of oxidation of ZrB2 and ZrB2–SiC composites at high temperatures

“…13 At temperatures less than 1100 • C, the B 2 O 3 forms a continuous liquid layer which limits the transport of oxygen to the ZrB 2 surface, and as a consequence diffusion controlled, parabolic, oxidation kinetics are observed. [14][15][16][17][18][19] The ZrO 2 formed on oxidation of ZrB 2 is porous and therefore non-protective towards further oxidation:…”

Oxidation of ZrB2 powder in the temperature range of 650–800°C

“…Thus, a large number of studies on oxidation of ZrB 2 and its composites in air have been carried out in order to better understand the oxidation mechanisms and enhance oxidation resistance [3][4][5][6][7][8][9][10][11][12]. The reaction of ZrB 2 with O 2 is as follows: …”

Corrosion of Ceramic Materials

“…The monoclinic to tetragonal phase transition for zirconia and hafnia is accompanied by a volume changes which results in less protective scales (Berkowitz-Mattuck, 1966;Kaufmann et a1.,1967). In addition, the titania grown on TiB2 is usually highly textured and cracked resulting in linear kinetics at longer times .…”