The universal response of metal surfaces exposed to oxygen-bearing atmo-spheres is to oxidize. The oxidation product may be a thin adherent film that protects the underlying metal from further attack, or a thicker porous layer that may flake off and offer no protection. In this section, discussion is limited to oxidation via high-temperature exposure; aqueous corrosión oxidation phenom-ena are already the subject of a broad and accessible literature. From the standpoint of thermodynamics all of the structural metáis exhibit a tendency to oxidize. As noted in Chapter 1, the driving forcé for oxidation of a given metal depends on the free-energy change for oxide formation. What thickness of oxide will form and at what rate are questions dependent on complex Icinetics and microstructural considerations, and not on thermodynamics. two simultaneous processes occur during oxidation.
At the metal-oxide interface neutral metal atoms lose electrons and become ions that migrate through the oxide to the oxide-ambient interface. The released elec-trons also travel through the oxide and serve to reduce oxygen molecules to oxygen ions at the surface. If metal cations migrate more rapidly than oxygen anions (e.g., Fe, Cu, Cr, Co), oxide grows at the oxide-ambient interface. On the other hand, oxide forms at the metal-oxide interface when metal ions diffuse more slowly than oxygen ions (e.g., Ti, Zr, Si). An important implication is that highly insulating oxides, such as A1203, Si02, do not grow readily because electrón mobility, so central to the process, is low. This is what limits their growth and results in ultrathin protective native oxide films.
The model of growth Icinetics developed for oxidation of Si, is applicable to other systems. Both parabolic oxide growth under diffusion-controlled conditions, as well as linear oxide growth when interfaciai reactions limit oxidation, are frequently observed. However, not all oxidation processes fit the aforementioned categories, and other growth rate laws have been experimentally observed in various temperature and oxygen pressure regimes.
Thermal Coatings
Ever-increasing demands for improved fuel effíciency in both civilian and military jet aircraft has continually raised operating temperatures of turbine engine components. Among those requiring protection are turbine blades, stators, and gas seáis. The metáis employed for these critical applications are Co-, Ni-, and Fe-base superalloys, which possess excellent bulk strength and ductility properties at elevated temperatures. A widely used cost-effective way to achieve yet higher temperature resistance to degradation in the hot gas environment is to employ an additional thermal barrier coating (TBC) system. This consists of a metaUic bond coat and a top layer composed primarily of Zr02. The bond coating, as the ñame implies, is the glue layer between the base metal and the outer protective oxide. Its function is not unlike that of a bond or primer coating used to prepare surfaces for paintíng. Typical bond coatings consist of MCrAlY or MCrAlYb, where M = Ni, Co, Fe.
Original bond coating compositions such as Ni-26Cr-6Al-0.15Y (in wt%) have been continually modifíed in an effort to squeeze more performance from them. The role of Y or other rare earth substitutes is critical. These elements apparently protect the bond coat from oxidation and shift the site of failure from the base metal and coat interface to within the outer thermal barrier oxide. Just why is not known with certainty; it appears that these reactive metáis easily diffuse along the boundaries of the plasma-sprayed particles of the bond coating, oxidize there, and limit further oxygen penetration.
The use of Zr02 is based on a desirable combination of properties: melting point = 2710 °C, thermal conductivity = 1.7 W/m-K, and thermal expansión coefficient = 9 X 10~6 K~'. However, the crystal structure under-goes transformation—from monoclinic to tetragonal to cubic—as the tempera-ture increases, and vice versa, as the temperature decreases. A rapid, diffu-sionless martensitic transformation of the structure occurs in the temperature range of 950-1400 °C accompanied by a volume contraction of 3-12%. The thermal stresses so generated lead to fatigue cracking, which signifies that Zr02 alone is unsuitable as a TBC.
The ZrOz overlayers are generally stabilized with 2-15 wt% CaO, MgO, and Y203. Through alloying with these oxides, a partially stable cubic structure is maintained from 25 °C to 2000 °C. Actually the tetragonal and monoclinic phases coexist together with the cubic phase, whose stabilization depends on the amount of added oxide. Cubic phase stabilization results in stress-induced transformation toughening, which can be understood as follows. If a crack front meets a tetragonal particle, the latter will transform to the monoclinic phase a process that results in a volume increase. The resultant compressive stresses blunt the advance of cracks, toughening the matrix.
Ronellys Flores---CRF---libro the materials science of thin films
No hay comentarios:
Publicar un comentario