Ceramics at Elevated Temperatures

The most outstanding feature of ceramics in this category is the various degrees of plasticity that occur following the transition from brittle to ductile condition. This is the first transition to be discussed here, since aU the other features are consequences of the brittle-to-ductile transition (henceforth BDT). 

The classic method for evaluating the transition temperature from a ductile to a brittle state is by impact testing. The basic reasons for using such a test are the high strain rate that can be achieved by impact and its simplicity. Though there are currently many other ways to vary strain rate, those who choose to perform impact tests can enjoy the use of modem, instmmented impact machines. For most ceramics which are brittle at room temperature (henceforth RT), ductility is a high-temperature feature thus, it is more meaningfiil to discuss BDT, rather than ductile-to-brittle transition (DBT), the more common nomenclature. 

Relatively few impact strength data are available in the literature on ceramics and there are even fewer recorded experimental reports. The major limitations of performing such impact tests are the brittleness and low impact strength of ceramics at low and ambient temperatures, especially when the focus is on their applications at elevated temperatures, in light of their high strength properties. 

For an early work on the determination of the transition temperature by impact, one may consult the paper by Kingery and Pappis [29]. Above a critical transition temperature, ductility increases markedly and ductile fractures are observed in ceramics. An illustration of the transition temperatures for a few ceramics may be seen in Fig. 2.1. 

The experimental set-up for impact loading is shown in Fig. 2.2. The samples are cylindrical, 6 in. long and V2 in. in diameter, supported on dense, sintered alumina knife edges across a 4V2 in. span. 

---

Sintering process by which it is possible to densify gels to glasses and ceramics at elevated temperatures. 

C1211-02 Test Method for Flexural Strength of Advanced Ceramics at Elevated Temperatures... 

The mechanisms responsible for fracture in structural ceramics at elevated temperatures have been reviewed [154]. Sensitivity to flaws or microstructural inhomogeneities which nucleate microcracks are among the failure mechanisms. The flaws which control failure under creep conditions are different from those responsible for fast fracture at room temperature. A common feature is the development of cracks through gradual damage accumulation, depend on the microstructure. The role of cracks in the deformation and fracture behavior of polycrystalline structural ceramics have been reviewed [155]. 

In many polycrystalline ceramics at elevated temperatures, GBS contributes significantly to the total strain. GBS can be markedly reduced by introducing additional phases, which form precipitates (such as nitrides, carbides, borides, etc.)... 

The Creep modeling procedure uses finite element methods to predict the time dependent deformation of structural ceramics at elevated temperatures. User-made creep subroutines using the theta projection method are under development. 

ASTM D1829 is a test for the electrical conductivity of ceramics at elevated temperatures. 

D. Chen, K. Shirato, M.W. Barsoum, T. EL-Raghy, R.O. Ritchie, Cychc fatigue-crack growth and fracture properties in TisSiC2 ceramics at elevated temperatures, J. Am. Ceram. Soc. 84 (2001) 2914-2920. 

Conductors TiO, ReOj, and Cr02 possess conductivity comparable to metals. VO2, VO, and Fe304 have good conductivity at elevated temperatures. NiO, CoO, and Fe203 have less conductivity than the previous group of ceramics at elevated temperatures. All these ceramics possess conductivity due to the movement of electrons. 

C1366-97 Test method for tensile strength of monolithic advanced ceramics at elevated temperature... 

Molybdenum hexafluoride is used in the manufacture of thin films (qv) for large-scale integrated circuits (qv) commonly known as LSIC systems (3,4), in the manufacture of metallised ceramics (see MetaL-MATRIX COMPOSITES) (5), and chemical vapor deposition of molybdenum and molybdenum—tungsten alloys (see Molybdenumand molybdenum alloys) (6,7). The latter process involves the reduction of gaseous metal fluorides by hydrogen at elevated temperatures to produce metals or their alloys such as molybdenum—tungsten, molybdenum—tungsten—rhenium, or molybdenum—rhenium alloys. 

Ceramic—metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. Knowledge of the chemical reaction products and, if possible, their properties are needed. It is therefore imperative to understand the thermodynamics and kinetics of reactions such that processing can be controlled and optimum properties obtained. 

Beryllium Nitride. BeryUium nitride [1304-54-7], Be N2, is prepared by the reaction of metaUic beryUium and ammonia gas at 1100°C. It is a white crystalline material melting at 2200°C with decomposition. The sublimation rate becomes appreciable in a vacuum at 2000°C. Be2N2 is rapidly oxidized by air at 600°C and like the carbide is hydrolyzed by moisture. The oxide forms on beryllium metal in air at elevated temperatures, but in the absence of oxygen, beryllium reacts with nitrogen to form the nitride. When hot pressing mixtures of beryUium nitride and sUicon nitride, Si N, at 1700°C, beryllium sUicon nitride [12265-44-0], BeSiN2, is obtained. BeSiN2 may have appHcation as a ceramic material. 

Carbon disulfide is normally stored and handled in mild steel equipment. Tanks and pipes are usually made from steel. Valves are typically cast-steel bodies with chrome steel trim. Lead is sometimes used, particularly for pressure reUef disks. Copper and copper alloys are attacked by carbon disulfide and must be avoided. Carbon disulfide Hquid and vapor become very corrosive to iron and steel at temperatures above about 250°C. High chromium stainless steels, glass, and ceramics maybe suitable at elevated temperatures. 

Ceramic-matrix composites are a class of materials designed for stmctural applications at elevated temperature. The response of the composites to the environment is an extremely important issue. The desired temperature range of use for many of these composites is 0.6 to 0.8 of their processing temperature. Exposure at these temperatures will be for many thousands of hours. Therefore, the composite microstmcture must be stable to both temperature and environment. Relatively few studies have been conducted on the high temperature mechanical properties and thermal and chemical stability of ceramic composite materials. 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

---