High-Temperature Stability of Non-Oxide Structural Ceramics
- PDF / 1,059,774 Bytes
- 6 Pages / 576 x 777.6 pts Page_size
- 7 Downloads / 200 Views
High-Temperature Stability of Non-Oxide Structural Ceramics Richard E. Tressler Introduction Oxide-based ceramics have long been used as linings in containment vessels for hot materials (metals, glasses, cement, etc.) and hot gases, at temperatures often in excess of 1500°C, because of their chemical compatibility with these hot materials and with the process ambient —conditions where metals and polymers simply can't perform. However, their low thermal conductivities and generally high thermal expansivities cause poor thermal shock resistance. In addition, their creep resistance (resistance to permanent deformation under load) is generally poorer than the more covalently bonded ceramic materials such as nitrides and carbides which also have excellent thermal shock resistance. Two classes of non-oxide structural ceramics — silicon carbide and silicon nitride—are emerging as acceptable engineering materials from which to fabricate components for use in heat-recovery equipment, heat engines, and high-temperature industrial equipment. Composites containing a silicon phase or molybdenum silicides are also available or are being developed for high-temperature components. These materials are all thermochemically unstable in air or oxidizing environments and owe their long life to the formation of a passivating silicon dioxide layer which has among the lowest permeabilities for oxygen of any oxide. Because their suitability for application in corrosive environments depends so critically on the formation of a passivating scale, there have been hundreds of studies focused on the general goal of defining the envelope of conditions under which these ceramics are "kinetically stable." Recent conferences have dealt with summaries of the results,12 and Jacobson3 has recently published a
58
comprehensive summary of the findings and critical issues remaining for the corrosion of these materials in combustion environments. Along with the purely thermodynamic and kinetic aspects of the corrosion reactions, a recognition is emerging that the selective attack of certain microstructural features can lead to nucleation of new, strength-limiting flaws, and contamination of secondary phases and grain boundaries can lead to modified time dependence of the mechanical properties. The degradation processes of crack growth and creep damage accumulation are controlled by grain boundaries and, thus, the long-term mechanical reliability of these ceramics can be altered by exposure to corrosive environments. The deleterious effects on the mechanical properties can occur within this region of apparent "kinetic stability."4 The long-term reliability issues for these silicon-compound-based ceramics are now being addressed on a broad front, and improved materials are emerging as we understand how to tailor these materials to resist chemical and mechanical degradation. My intent here is to focus on the thermochemical aspects of degradation, with a few comments at the end on the effects of corrosion on the mechanical properties. Thermochemical Degradat
Data Loading...
