20.2 Corrosion Protection of Turbine Blades

The development of turbine blade materials to meet the challenge of increasing engine temperatures must satisfy the requirements listed in Table 20.3.

These requirements severely limit our choice of very resistant materials. For example, ceramics, with their high softening temperatures and low densities, are ruled out for aeroengines because they are far too brittle (they are under evaluation for use in land‐based turbines, where the risks and consequences of sudden failure are less severe). Cermets offer no great advantage because their metallic matrices soften at a too much low temperature. The materials that best fill present needs are the nickel‐based superalloys.

The alloy used for turbine blades in the high pressure stage of an aircraft turbofan engine is a classic example of a material designed to be resistant to dislocation (power‐law) creep at high stresses and temperatures. At takeoff, the blade is subjected to stresses approaching 250 MN m⁻², and the design specification requires that this stress be supported for 30 hours at 850 °C without more than 0.1% irreversible creep strain. In order to meet these stringent requirements, an alloy based on nickel has evolved with the rather mind‐boggling specification given in Table 20.4. The point of all these complicated additions of foreign atoms to the nickel is straightforward. It is (i) to have as many atoms in solid solution as possible (the cobalt, the tungsten, and the chromium); (ii) to form stable, hard precipitates of compounds like Ni₃Al, Ni₃Ti, MoC, and TaC to obstruct the dislocations; and (iii) to form a protective surface oxide film of Cr₂O₃ to protect the blade itself from attack by oxygen. These superalloys resist creep so well that they can be used at 850 °C, and they are so hard that cannot be machined easily by normal methods and must be precision‐cast (for example, by investment casting that produces a fine‐grained material that may undergo a fair amount of diffusion creep and that may fail rather soon by cavity formation) to their final shape. Due to this, the blades are expensive, the total cost of a rotor of 102 blades being €200 000. To stave off failure, the alloys are directionally solidified (DS) to give long grains with grain boundaries parallel to the applied stress. DS alloys are standard in high performance engines and are now also in use in civil aircrafts. The improved creep properties of the DS alloy will allow the engine to run at a flame temperature approximately 50 °C higher than before, for a doubling in production cost. Figure 20.1 shows how this evolutionary process has resulted in a continual improvement of creep properties of nickel alloys over time and shows how the amounts of the major foreign elements have been juggled to obtain these improvements – keeping a watchful eye on the remaining necessary properties. The figure also shows how improvements in alloy manufacture – in this case the use of directional solidification – have helped to increase the operating temperature. Nevertheless, it is clear from the graph that improvements in nickel alloys are now nearing the point of diminishing returns.

Internal cooling of the blade enabled the inlet temperature of the turbine to be increased immediately by 100 °C with no change in alloy composition. Later, film cooling constituted a continuous improvement in turbine blades (Figure 20.2). But by further ducting cold air through the blades will begin to reduce the thermal efficiency, and thus a unique characteristic of some alloys, called eutectics, was explored. These eutectics form spontaneously on aligned reinforced structure, which is usually a compound with high melting temperature, whose fibers improve the creep properties of the resulting composite alloy.

Figure 20.3 shows how eutectics are made, and Table 20.5 lists some typical high temperature composites under study for turbine blade applications.

As shown in Figure 20.2, if directionally solidified eutectics (DSEs) prove successful, they will allow the metal temperature to be increased by ≈100 °C above conventional DS nickel alloys and the inlet temperature by ≈200 °C (because of a temperature scaling effect caused by the blade cooling). Further improvements in alloy design are underway in which existing nickel alloys and DSEs are being blended to give a fiber‐reinforced structure with precipitates in the matrix.

The ceramics best suited for structural use at high temperatures (≥1000 °C) are listed in Table 20.6 and compared with nickel‐based superalloys. The comparison shows that all the ceramics have attractively low densities, high moduli, and high melting points (and thus excellent creep strength at 1000 °C); and many are completely resistant to oxidation – they are oxides already. But several have poor thermal conductivity (leading to high thermal stresses), and all have very low toughness.

To overcome their major problems, ceramic matrix components are under development: they combine strong, exceptionally perfect, fibers (such as silicon carbide or alumina, grown by special techniques) in a matrix of ceramics (silicon carbide or alumina again), made by conventional means to give a material that you might think of as “high temperature wood.”

Any major materials development program, such as that on the DSEs, can only be undertaken if a successful outcome would be cost effective. The cost of the materials themselves must also be considered. For example, the use of greater quantities of exotic materials, such as hafnium, will drive the cost of blades up. But if new alloys offer improved life or inlet temperature, there is a strong incentive to pursue them.

As we saw above, the alloys used for turbine blades contain large amounts of chromium, dissolved in solid solution in the nickel matrix. Now, if we look at Table 20.1, we see that the formation of Cr₂O₃ releases much more energy (701 kJ mol⁻¹ of O₂) than NiO (439 kJ mol⁻¹ of O₂). This means that Cr₂O₃ will form in preference to NiO on the surface of the alloy. Obviously, the more Cr in the alloy, the greater the preference for Cr₂O₃. At the 20% level, enough Cr₂O₃ forms on the surface of the turbine blade to make the material act a bit as if it were chromium. Suppose for one moment that our material is chromium. Table 20.2 shows that Cr would lose 0.1 mm in 1600 hours at 0.7 T_M. Of course, we have forgotten one thing, 0.7 T_M for Cr is 1504 K (1231 °C), whereas, as already mentioned, for Ni it is 1208 K (935 °C). We should, therefore, consider how Cr₂O₃ would act as a barrier to oxidation at 1208 K rather than at 1504 K (Figure 20.4). The oxidation of Cr follows parabolic kinetics with an activation energy of 330 kJ mol⁻¹. Then the ratio of the times required to remove 0.1 mm (from equation , where A_p and Q_p are constants) is

20.1

Thus, the time at 1208 K is t₂ = 0.65 × 1.6 × 10⁶ hours = 1.04 × 10⁶ hours. Note that in Eq. 20.1, t₁ and t₂ are the oxidation times with t₂ > t₁.

Now, as we have said, there is only at most 20% Cr in the alloy, and the alloy behaves only partly as if it were protected by Cr₂O₃. In fact, experimentally, we find that 20% Cr increases the time for a given metal loss by only about 10 times, i.e. the time taken to lose 0.1 mm at blade working temperature becomes 600 × 10 hours = 6000 hours rather than 10⁶ hours.

Why this large difference? Well, whenever you consider an alloy rather than a pure material, the oxide layer – whatever its nature (NiO, Cr₂O₃, etc.) – has foreign elements contained in it, too. Some of these will greatly increase either the diffusion coefficients in, or electrical conductivity of, the layer and make the rate of oxidation through the layer much more than it would be in the absence of foreign elements contamination.

This 0.1 mm loss in 6000 hours from a 20% Cr alloy at 950 °C, though better than pure nickel, is still not good enough. The obvious way out of this problem is to coat the blades with a protective layer (Figure 20.5). This is usually carried out by spraying molten droplets of aluminum onto the blade surface to form a layer, some microns thick. The blade is then heated in a furnace to allow the Al to diffuse into the surface of the Ni. During this process, some of the Al forms compounds such as AlNi with the nickel – which are themselves good barriers to oxidation of the Ni – while the rest of the Al becomes oxidized up to give Al₂O₃.

Because oxides are usually quite brittle at the temperatures encountered on a turbine blade surface, they can crack, especially when the temperature of the blade changes and differential thermal contraction and expansion stresses are set up between alloy and oxide. These can act as ideal nucleation centers for thermal fatigue cracks, and because oxide layers in nickel alloys are well stuck to the underlying alloy (they would be useless if they were not), the crack can spread into the alloy itself.

The properties of the oxide film are thus very important in affecting the fatigue properties of the whole component. The pure refractory metals Nb, Ta, Mo, and W have high melting temperatures (2740, 3250, 2880, and 3680 K, respectively) and should therefore have very good creep properties. But they oxidize very rapidly indeed (see Table 20.2) and are utterly useless without coatings. The problem with coated refractory metals is that if a break occurs in the coating (e.g. by thermal fatigue, erosion by dust particles, etc.), catastrophic oxidation of the underlying metal will take place, leading to rapid failure. The “unsafeness” of this situation is a major problem that has to be solved before we can use these on‐other‐counts potentially excellent materials.

The ceramics SiC and Si₃N₄ do not share this problem. They oxidize readily (Table 20.1); but, in doing so, a surface film of SiO₂ forms that gives adequate protection up to 1300 °C. And because the film forms by oxidation of the material itself, it is self‐healing.

20.3 Oxidation of Silicides for VLSI Applications

Electronic materials are those used in electrical industries, electronics and microelectronics, and the substances for the building up of integrated circuits (ICs), circuit boards, packaging materials, communication cables, optical fibers, displays, and various controlling and monitoring devices. Discovery, development, and application of new materials are the robust power for the development of human society.

Processing of electronic materials mainly includes crystal growth, epitaxy, lithography, deposition, annealing, doping (diffusion or ion implantation), etching (dry or wet), and metallization. In ICs, the fundamental electronic switching device is the (complementary) metal oxide semiconductor field effect transistor (MOSFET), which has at least three terminals – the gate, source, and drain. The gate electrode is separated electronically from the source and drain by a thin dielectric film, which is commonly SiO₂. Formation of this film could be accomplished by using sputtering, evaporation, chemical vapor deposition, or atomic layer deposition. Thermal oxidation is also used to grow the gate dielectric films due to its ease in processing control, but it results in many problems in association with material degradation at high temperature; these problems lead to the formation of oxides, carbides, sulfides, nitrides, etc., but when these oxidation processes are well controlled, they have been finding many applications in the modern electronics industry for surface cleaning, film growth, preparation of new materials, and building up of novel structures for devices. To date, Si is still the backbone of the modern semiconductor industry due to its (i) availability in a wide variety of sizes and shapes, (ii) mature material preparation and property control, (iii) native oxide films on its surface, and (iv) compatibility to planar IC technology.

Silicides of transition metals are used in two distinct ways in very large scale integrated (VLSI) circuits: (i) in the contact pads of individual devices on a chip and (ii) in the interconnection lines. For the latter applications, two particular properties of the silicides make them desirable: their metallic electrical conductivity and the possibility to produce a protective coating of SiO₂ under heat treatment in an oxidizing ambient. Of those two attributes, the second one is particularly attractive because it offers inherent advantages in multilevel metallization schemes. To use this feature of silicides successfully, an understanding of the oxidation process is needed. It is convenient to subdivide the subject into two parts, depending on whether the silicide rests on a substrate of excess silicon or whether the substrate is inert, that is, the substrate is in a fully oxidized state, such as SiO₂. Other reasons for the use of silicides in VLSI applications are their very low resistance; good process compatibility with Si, e.g. ability to withstand high temperatures; oxidizing ambients; various chemical cleans used during processing; little or no electronization; easy to dry etch; and good contacts to other materials. But these benefits rise many problems in integrating silicides in an IC, and under this point of view, the actual preferred silicides are WSi₂, TiSi₂, NiSi, and CoSi₂. Table 20.7 indicates preferred silicides for VLSI application. The main techniques used for the formation of these silicides are (i) metal deposition on Si and formation by thermal heating, laser irradiation, or ion beam mixing (these are sensitive to interface cleanliness and heavy doping, selective silicidation on Si possible, and widely used for silicides of Pt, Pd, Co, and Ti), (ii) co‐evaporation (E‐gun) of metal and Si (poor process control, poor step coverage, and good tool for research but not used in manufacturing), and (iii) sputtering from a composite target (possibility of high level of contaminants like C, O, Na, and Ar; poor step coverage; used for MoSi₂ and WSi₂).

20.3.1 Oxidation of Silicides on Silicon

Typically, silicides are not used alone as interconnections, but with an underlayer of polycrystalline Si. Upon thermal treatment in an oxidizing ambient, most silicides form a SiO₂ overcoat while the silicide layer below is morphologically preserved. The rate of growth of the SiO₂ layer is different for the different silicides, and these rates also differ from that of pure Si. They can be represented by the linear–parabolic growth law

20.2

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