Oxidation of Hafnium Carbide and Titanium Carbide Single Crystals with the Formation of Carbon at High Temperatures and Low Oxygen Pressures

Abstract

The isothermal oxidation of the 200 face of HfC and TiC single crystals was performed at temperatures of 700°-1500°C and at oxygen pressures of 0.08 -80 kPa for 4 h. The weight gain by oxidation of the two crystals was followed using an electromicrobalance. A polished cross section of the oxidized crystals was observed using backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Hf, Ti, O, and C was performed by wavelength-dispersive X-ray microanalysis. The early-stage oxidation kinetics of HfC crystals were described by the contracting volume equation, followed by slowed reaction in the latter stage, whereas the same equation was applied to the oxidation of TiC over the entire oxidation time. The preferred {200} orientation of monoclinic HfO 2 occurred on the oxidized surface of the HfC crystal. The oxide scale on the HfC crystal consisted of a compact and pore-free black inner scale (zone 1) and a white/gray outer scale that contained many pores (zone 2). Zone 1 contained ϳ25 at.% unoxidized carbon, and zone 2 contained 6 -11 at.% carbon. The oxide scale of TiC was composed of an inner dense lamella subscale (zone 1) with a carbon content of 7-23 at.% and an outer region with laminations that was separated by pores and cracks (zone 2). The Ti 3 O 5 phase, which exhibits a strong 020 line, was formed at depths of >40 m in the scale obtained at 1500°C. Treatment with a concentrated HF solution allowed zone 1 to be separated from the HfC crystal in the form of carbon-containing films, which were characterized using Raman spectroscopy and transmission electron microscopy.

Laminated HfC–SiC ceramics produced by aqueous tape casting and hot pressing

“…It has attracted considerable attention as potential candidate material for a variety of ultra-high temperature applications, including atmospheric re-entry, hypersonic flight, and hightemperature electrodes, nozzles and armor, because of its high melting point, high thermal and electrical conductivity and excellent chemical and physical stability at high temperatures [4][5][6][7][8][9]. However, the low fracture toughness and poor thermal shock resistance limit its use in structural applications to date [10,11].…”

Thermal activation of fatigue crack growth: Analysing the mechanisms of fatigue crack propagation in superalloys

“…In Nb containing superalloys, the oxidation of NbC particles near grain boundaries has been cited as the cause for environmental sensitivity of FCG [19]. As other TiC and HfC carbides can also oxidise readily [47,48,49], the mechanism related to oxidation of carbides of various kinds can potentially cause environmental sensitivity of FCG in most superalloys.…”

Effect of interlayer on the ablation properties of laminated HfC–SiC ceramics under oxyacetylene torch

“…Ultra-high temperature ceramics (UHTCs) have recently attracted significant research as potential candidates in these extreme environments because of the high melting points [2,4,5]. Hafnium carbide (HfC) is considered as one of the most attractive candidates for these thermal protection systems due to the highest melting point ($3900°C) among UHTCs, high hardness, high thermal conductivity, and excellent chemical stability [6][7][8][9][10][11]. However, its wide scale use has been limited due to the low fracture toughness, poor sinterability and thermal shock resistance [12,13].…”