Thermal shock has been evaluated qualitatively for ZrC by various means. Susceptibility to failure by thermal shock is lowered in materials with high tensile strength, low elastic modulus, low thermal expansion coefficient, and high thermal conductivity. Gangler’s71 test involved cyclic heating and quenching of hot-pressed ZrC0.83-o.85 between a 1255 K furnace and 300 K air stream. ZrC withstood 22 cycles, though excessive oxidation was noted. Shaffer and Hasselman54 subjected hot-pressed ZrC spheres to thermal shock on heating: room — temperature specimens were drawn rapidly into the hot zone of a tube furnace at a temperature sufficiently high to cause fracture. For ZrC this was determined to be 1725 K, and free carbon was found to improve thermal shock resistance. Lepie96 subjected a pyrolytic ZrC-C alloy to firing in the nozzle-throat section of a solid-fuel rocket; no ill effects from the sudden expo­sure to the 3894 K exhaust flame were reported, and firing for 30 s at 5.5 MPa caused little erosion.

2.13.5 Environmental Resistance

2.13.6.1 Oxidation

Despite excellent refractory properties, ZrC suffers from poor oxidation resistance, with oxidation initi­ating in the range of 500-900 K (Table 5). The kinet­ics and mechanism of ZrC oxidation have been assessed in several studies, between room tempera­ture and 2200 K, at oxygen partial pressures (P02) between 8 x 10~4 and 101 kPa (0.79 x 10~6 and 1 atm), with the oxidation products a function of both parameters.

2.13.6.1.1 Oxidation products

Oxidation resistance is imparted by the formation of a dense, adherent oxide scale which effectively restricts oxygen access to the carbide. Since the oxi­des of carbon are gaseous, protection is only afforded

aBartlett efa/.134

bShimada & Ishii,135 temperature at which sintered ZrC weight gain initiates.

cShimada,136 DTA peaks indicating onset of oxidation of Zr and

C in single crystal ZrC, respectively.

dRama Rao and Venugopal.137

eOpeka et a/.138

Voitovich and Pugach.139

9Shevchenko et a/. ,140 DTA peaks indicating onset of oxidation of Zr and C, respectively. hTamura et a/.141

‘Zhilyaev et a/. ,142 ZrCxOy (x = 0.7-0.85, y = 0.15-0.25). jZainulin et a/. ,143 ZrCxOy (x = 0.43-0.97, y = 0.09-0.36).

by the zirconium oxide. However, low temperatures (<973 K) and P02 insufficient to oxidize C result in preferential oxidation of Zr and precipitation of amorphous carbon at the oxide-carbide interface, as detected by TEM,144 Raman spectroscopy,145,146 and Auger electron spectroscopy.147 Nonisothermal oxi­dation of ZrC by DTA showed two peaks, indicating the onset of appreciable Zr and C oxidation, respec­tively: the peak associated with Zr oxidation appeared between 653 and 763 K, while the C peak appeared at a higher temperature of 773-973 K.136,140

Alternatively, the liberated C may be incorporated into the ZrO2 lattice, stabilizing the cubic fluorite structure of ZrO2, whereas the monoclinic structure is normally stable at room temperature. Cubic ZrO2 nuclei absent in X-ray diffraction (XRD) were iden­tified by electron diffraction and TEM lattice fringes by Shimada and Ishii135 at 653-743 K. Cubic Zr02, with or without trace monoclinic ZrO2, formed in the 723-1013 K range.134,135,143-146,148 Shimada and Ishii135 and Tamura eta/.141 also reported the meta­stable tetragonal ZrO2 phase, based on XRD analysis. While XRD easily distinguishes monoclinic from tetragonal or cubic ZrO2, the latter two are difficult to tell apart, and additional techniques such as Raman spectroscopy or electron diffraction are required for a conclusive identification.

As the oxidation temperature increases, apprecia­ble oxidation of the precipitated or combined carbon occurs in addition to oxidation of Zr. Concurrently, the relative proportions of cubic and monoclinic ZrO2 in the scale shift in favor of monoclinic. As carbon is more readily oxidized at higher tempera­tures, less carbon is available to stabilize cubic ZrO2 and the more stable monoclinic structure forms instead. These transitions from carbon precipitation to oxidation and from cubic to monoclinic ZrO2 are reported at temperatures higher than about 1073 K, with carbon-free, fully monoclinic ZrO2 reported at

1473 1773 K 137,139—141,143,144,146,147

The transformation may be manifested by a carbon concentration gradient with depth, either by a gradual decrease in carbon from the oxide-carbide interface to the free surface, or by the formation of distinct layers in the scale. This is consistent with observations of cubic ZrO2 concentrated at the oxide-carbide interface and monoclinic ZrO2 concentrated at the free surface.139 Wavelength dispersive spectroscopy by Shimada eta/.146 detected 7-10 at.% C combined in an outer ZrO2 scale layer, following oxidation of single-crystal ZrC097 at 773-873 K, with an inner layer characterized by a steep decrease in C content with depth.

Some authors also report the formation of a ZrC^Oj, oxycarbide phase isostructural with ZrC. The ZrC^Oj, phase is reported to exist between ZrC098 on the oxygen-poor side and ZrC0 73O014 on the oxygen-rich side.149 Formation of the oxycar — bide may occur as an intermediate layer between the ZrO2 scale and carbide, by the dissolution of oxygen in ZrC,145,150 or by a reaction between the ZrO2 scale and CO or CO2 gas diffusing outward from the oxide-carbide interface.142 Limited thermodynamic data for the Zr-O-C system (see Quensanga and Dode149) has hindered more thorough assessment of these hypotheses. At high temperatures in air atmosphere, formation of ZrN or ZrCxOj, Nz com­pounds may occur,139 but thermodynamic data for this quaternary system is similarly limited (see Constant et a/.151).