The High‐Temperature Oxidation Behavior of a HfB2 + 20   v / o SiC Composite

Abstract

The high-temperature oxidation behavior of a HfB2 ~-20 v/o SiC composite was investigated in the temperature range 1200~176 and in oxygen partial pressures at 1400~ ranging from 2.6 X 10 -9 to 3.3 X 10 -1 atm. Oxidation kinetics in pure oxygen were parabolic, with the rate constants exhibiting linear Arrhenius behavior. The rate of oxygen consumption is described by kp ----1.62 X 107 exp(--lO6,0OO/RT) g2-cm-4-hr -1 in the range 1350~176The oxide scale formed in this range consisted of an inner HfO2 layer in series with an outer SiO2 layer. The oxidation rate was found to be controlled by initial short-circuit oxygen diffusion through the HfO2, followed by volume diffusion through the SiO2 scaling layer. Below 1350~ rapid oxidation kinetics were observed, HfO~ being the primary scale constituent.Interest in diboride composites stems from their potential use in high-temperature oxidizing environments. These materials hold promise due to their chemical inertness, thermal-shock resistance, retained strength at high temperature, and oxidation resistance.Early studies by Kaufman et al. (1) on the oxidation of undoped diborides showed the oxidation resistance of HfB2 to be superior to that of its Zr, Ti, Ta, and Nb counterparts based on a comparison of the total diboride recession after a 1 hr exposure to oxygen at temperatures ranging from 1300~176Later, Kaufman et al.(2) and Berkowitz-Mattuck (3) studied the oxidation rate of HfB2 and ZrB2 as a function of temperature and oxygen partial pressure in order to determine the mechanism of oxidation. The oxidation kinetics were observed to obey a parabolic rate law, and oxygen diffusion through the scaling layer was found to be rate determining. More recently, Clougherty et at. (4) investigated the oxidation behavior of HfB2 and ZrB2 containing additions of SiC as a function of temperature only, and total diboride recession was measured after 1 hr of oxidation. The SiC addition was found to increase the oxidation resistance; however, kinetic data were not obtained. The oxidation of ZrB2 and ZrB~ -}-20 v/o SiC has been investigated by Tripp et aL (5-7) as a function of temperature and oxygen partial pressure using thermogravimetric techniques. The oxidation reaction was found to be controlled by diffusion through a SiO2 scahng layer. In the present investigation, the oxidation behavior of HfB2 -}-20 v/o SiC was studied as a function of temperature and oxygen partial pressure. The oxidation resistance of the HfB2 -9 SiC composite was compared to that of its constituent components as well as its zirconium counterpart in order to define the rate-determining oxidation mechanism. sure of 3000 psi for 90 rain, and the resulting billet was 88% dense. Coupons (--1 >< 0.5 X 0.1 cm) were 9 Electrochemical Society Active Member. cut from the billet, ground with 600-grit SiC paper, and ultrasonically cleaned in acetone and methanol.Oxidation kinetics were determined using an oxygen-consumption technique (5-7) which is very useful for studying oxidation where one or more of the reaction ...

Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB2+SiC and Oxygen Transport Mechanisms

“…Introduction R EFRACTORY diborides (HfB 2 , ZrB 2 ) with 20-30 vol% SiC additive are prominent ultrahigh-temperature ceramics withstanding temperatures 2000 K and above. 1,2 During operation in air its surface is oxidized, giving rise to a crystalline oxide skeleton (HfO 2 , ZrO 2 ) and a silica-rich borosilicate liquid that wets it, [3][4][5][6][7][8][9][10] produced by the reactions:…”

Toward Oxidation-Resistant ZrB2-SiC Ultra High Temperature Ceramics

“…[21] SiC inclusions remain in the oxide layer. Hinze et al [29] found that the rate-limiting step accounting for the parabolic kinetics observed in these materials was the diffusion of oxygen through the borosilicate glass; this idea has been supported by more recent research. [13] Monteverde and Bellosi [13] studied the oxidation resistance of hot-pressed HfB 2 -SiC composites by isothermal and nonisothermal treatments in air at temperatures £1873 K (1600°C).…”

Thermodynamic Calculation of HfB2 Volatility Diagram

“…The oxidation layer of HfO 2 is a porous scale, and the distribution of HfO 2 and B 2 O 3 in the oxide scale varies with temperatures. [8][9][10][11][12][13] At low temperatures (<$1273 K), a liquid B 2 O 3 film is formed on the top of HfO 2 plus B 2 O 3 scale, and the pores in the HfO 2 are covered or filled with glassy or liquid B 2 O 3 . At intermediate temperatures ($1273 to $2073 K), B 2 O 3 (l) would still partially fill the pores in porous HfO 2 even though the external B 2 O 3 (l) layer evaporates.…”

“…The remaining B 2 O 3 (l) layer acts as a barrier to oxygen diffusion, resulting in passive oxidation with para-linear oxidation kinetics. [10,11] At high temperatures (>$2073 K), B 2 O 3 (l) would be absent owing to its volatilization, yielding a non-protective porous HfO 2 (s) layer.…”

“…The oxidation behaviors of ZrB 2 and HfB 2 , perhaps analogous, will be different to some extent. It was shown that the oxidation resistance of HfB 2 was superior to that of ZrB 2 from 1473 to 2473 K. [2,[8][9][10][11]20,21] Therefore, the volatility diagram may provide insight into the oxidation mechanism of pure HfB 2 and serve as the basis for the investigation of the oxidation behavior of HfB 2 -based ceramics including HfB 2 -SiC. [2,11,20,21] The constructions of volatility diagrams for HfB 2 varying with temperature and equilibrium oxygen pressure are described in this paper.…”