HIGH TEMPERATURE TESTING

It is well known that borides of the fourth group possess better oxidation resistance than carbides, thanks to the formation of a protective layer based on B2O3. On the contrary, when carbides come in contact with oxygen at high temperature, the formation of CO and porous MO2 (where M= Hf or Zr) provokes the loss of strength of the materials.

Furnace oxidation studies showed HfB2 and ZrB2 to be more oxidation resistant than other diboride materials.¹ Based on these results, ManLabs chose HfB2 and ZrB2 from the list of borides, as the best candidates for continued research into high temperature applications. A comparison of oxidation rates of HfB2 and ZrB2 with other high temperature ceramics is shown in Fig. 8.12.^6,27-31 At lower temperatures it is apparent that materials such as SiC and Si3N4 have lower oxidation rates than either HfB2, ZrB2

or their mixture with SiC. But these Si based materials are only applicable in temperature ranges less than ~1700°C. At higher temperatures the materials are either

Property Unit ZrC HfC ZrB2 HfB2

Structure Cubic (NaCl) Cubic (NaCl) Hex. (AlB2) Hex. (AlB2)

Lattice parameter Nm 0.4698 0.4636 3.17, 3.53 3.139, 3.473

Composition ZrC0.55 to ZrC0.99 ZrC0.60 to ZrC0.99 - -

Molecular weight g/mol 104.9 190.50 112.82 200.11

Color Silver gray Silver gray Silver gray Silver gray

X-ray density g/cm³ 6.59 12.67 6.119 11.212

unstable and dissociate, and/or reach a temperature regime where active oxidation predominates. During active oxidation the protective layer is disrupted by evolution of SiO, exposing the surface below to continued oxidation. Thus, at these ultra high temperatures, materials that are primarily Si-based are currently no longer suitable.

Based on their favourable oxidation resistance, the diborides are one family of materials that make well thinking of some promise for use in ultra high temperature applications.

As shown in Fig. 8.12 HfB2 has a lower oxidation rate than ZrB2 and that the HfB2/SiC mixture has improved oxidation resistance compared to pure HfB2.

Fig. 8.12: Comparison of oxidation rates of several engineering ceramics.

Fig. 8.12 also shows that HfB2 and HfB2/SiC have lower oxidation rates than pure HfC.

Oxidation studies also showed that metal rich compositions of HfB2 or ZrB2 (i.e. HfB1.9) were more oxidation resistant and possessed higher thermal stability than boron rich compositions (HfB2.1) of the same diboride.¹ To determine the effect of SiC content on diborides oxidation, ManLabs conducted a series of furnace oxidation experiments at 1800°, 1950° and 2100°C on HfB2 specimens with 10 vol% SiC and ZrB2 specimens with 50 vol% SiC.^5,10,27,32 Samples were heated in argon to the respective test temperature at which time the dry air was introduced for periods of 30–60 minutes, then argon was reintroduced for cool down. They found that 5 vol% SiC additions provided little improvement in oxidation resistance while samples containing 50 vol% SiC

showed improvements after one hour at 1800°C and 1950°C but were completely oxidized after one hour at 2100°C. They concluded that additions of 35 vol% SiC to both ZrB2 and HfB2 provided the best protection for temperatures up to 2100°C during a furnace test, but that reasonable oxidation protection can be achieved with SiC contents as low as 15 vol% for ZrB2 and 10 vol% for HfB2 .^2,27 A comparison of the oxidation scale thickness as a function of temperature for pure HfB2, SiC and HfB2-SiC is shown in Figure 8.13. This figure clearly shows that at temperatures >2100 K the oxide scale thickness on pure HfB2 is thinner than that on pure SiC, whose scale thickness is rapidly increasing with temperature above 2050 K. But at all temperatures >2050 K, the HfB2 -SiC materials have the thinnest oxide layers.

Fig. 8.13: Summary comparison of a 1-hour oxidation study of SiC and SiC.

The response of the refractory UHTC materials to high temperature oxidizing conditions imposed by furnace heating has been observed to differ markedly from the behaviour observed in arc plasma facilities that provide a simulated re-entry environment.²⁷ Furnace evaluations are normally performed for long times at fixed temperature and slow gas flow with well defined solid/gas-reactant/product chemistry.

Arc jet tests, on the other hand, are carried out under high velocity gas flow conditions in which energy flux, rather than temperature, is defined. Furthermore, furnace studies employ air at 1 atmosphere and O2 as diatomic species. But during a typical re-entry profile for a manned space vehicle, the pressures will generally be much less than 1 atmosphere and a significant portion of the gas molecules will be dissociated into highly reactive monatomic species as they cross the bow shock formed during re-entry. The

resulting monatomic species may recombine at the surface giving up some of their energy to the material and, depending on the catalycity of the substrate, this recombination can add a significant fraction to the overall heating of the articles’

surface. Arc jet testing provides the best ground based simulation of the re-entry environment, although there are a number of differences between the arc jet environment and the actual re-entry environment that must be accounted for when designing an arc jet experiment and when interpreting the data. For example, catalycity can play a more significant role during arc jet testing than in flight because a higher proportion of the air molecules are dissociated in the arc jet than in flight. Because of the differences between static or flowing air oxidation experiments and experiments in the arc jet, correlation of material responses from the two test situations is difficult -if not impossible- in many cases. For example, if material A performs better in the furnace test than material B it does not necessarily hold that the same trend will occur in the arc jet.³³

8.6 APPLICATIONS

The need for high temperature materials that can operate with no or limited oxidation or ablation at temperatures greater than 3000K has driven the development of UHTC materials. The potential applications for UHTCs span a wide number of needs arising from future military, industrial and space based projects. Potential industrial applications for UHTCs include use in foundry or refractory processing of materials.

Their chemical inertness makes them ideal for molten metal crucibles, thermowell tubes for steel refining and as parts for electrical devices such as heaters and igniters.³⁴

The military and aerospace applications for UHTCs range from rocket nozzle inserts and air augmented propulsion system components to leading edges and nose caps for future hypersonic re-entry vehicles.^34,35 Early space vehicle designs, such as the space shuttle, were designed with a large radius, blunt body design to reduce aerothermodynamic heating to maintain moderate temperature limits on all parts of the vehicle. However, the larger the leading edge radii, the higher the vehicles drag, which reduces manoeuvrability and cross range during re-entry. Therefore, to improve performance, hypervelocity vehicle concepts have been proposed using slender aerodynamic shapes with sharp leading edges.^36,37 Development of sharp body vehicles increases the lift-to-drag ratio thereby improving the vehicles’ re-entry cross range. A

higher lift to drag ratio also has the potential to improve the overall vehicle system safety in a number of ways. Design of a high L/D vehicle would increase the window during ascent in which a launch could be aborted and the vehicle safely recovered on land, reducing the need for crew to bail out and reducing the possibility of having to ditch the vehicle in the ocean. Secondly, as mentioned previously, the high L/D increases the vehicles cross range during descent from orbit. This provides the vehicle more opportunities to initiate descent while on orbit, and safely land the vehicle at a desirable location.

However, the temperature of the leading edge is inversely proportional to the square root of the leading edge radius, i.e. as the leading edge radius decreases the temperature increases.³⁷ Therefore the successful design of a sharp hypersonic vehicle requires the development of new materials with higher temperature capabilities than the current state-of-the-art materials can provide. Ultra High Temperature Ceramics are a family of materials that are promising candidates for meeting such requirements.^36,37

Concerning carbides, hafnium carbide is, with tantalum carbide, the most refractory compound available. In spite of its excellent properties, it has only limited industrial importance, possibly because of its high cost (see Ch. 16). Some experimental applications are as follows:

• Oxidation resistant coatings for C-C composites (co-deposited with SiC)

• Production of whiskers (with nickel catalyst)

• Coating for superalloys

• Coating on cemented carbide

Then, zirconium carbide is a highly refractory compound with excellent properties but, unlike titanium carbide, it has found only limited industrial importance except as coating for nuclear-fission power plants.⁵ ZrC coatings replaced SiC coatings, as the least loose their mechanical integrity at temperatures >1700°C and are chemically attacked by the fission product palladium. In contrast, ZrC coating layers have much higher temperature stability and are more resistant to Pd chemical attack.

Besides, the high hardness of the carbides, opens the possibility of production of cutting tools. Kyocera Inc. currently manifactures hot-pressed ZrC knives with sharp edges that are extremely hard, wear resistant and chemically inert.³⁸

This relatively few applications for borides and carbides of Zr and Hf may be due to the high price and difficulty in obtaining them dense and free of impurities.