Platinum Metals Rev., 1999, 43, (2), 59
Noble Metal Aluminide Coatings for Gas Turbines
Platinum aluminide diffusion coatings act as a remedy against the aggressive environments in which modern nickel-based gas turbine blades operate. Whether as a coating for environmental protection (1) or as a bondcoat for a thermal barrier coating (2), platinum aluminides are used to provide protection for turbine components against the oxidation and hot corrosion conditions generated by a combustion environment. The coating achieves this by promoting the formation of an oxide scale which acts as a barrier between the environment and the component. Ideally, the scale should be alumina-based and be adherent, complete, compact and have a slow growth rate (3). However, these properties, which are central to the performance of the coating, can be undermined by attack from the environment and from damaging elements within both the coating itself and the substrate material, such as sulfur or titanium.
Studies conducted by the Surface Engineering Research Group (SERG) at the University of Northumbria at Newcastle have highlighted potential improvements for platinum aluminides which can be achieved by such means as the optimisation of the coating production process, the addition of noble metals and the addition of active elements.
Effects of the Coating Production Process
Platinum aluminides, produced using a ‘high aluminium activity pack’ process, provide excellent protection for nickel-based turbine blades. However, as these coatings are formed by an inward diffusion mechanism, the integrity of the alumina scale is particularly susceptible to degradation, as damaging substrate elements are present in the as-processed coating. During service, typically in a rotating turbine blade, these elements will diffuse in an outward direction to the oxide/coating interface where they will act to lower the adhesion of the scale, thus decreasing the effective life of the coating.
However, in contrast, SERG has investigated the potential improvements offered by producing the coating using a ‘low aluminium activity, out-of-pack’ process (4). A low activity process forms the aluminide by the outward diffusion of nickel from the substrate (5), while the out-of-pack method is similar to a chemical vapour deposition (CVD) process, where the components are suspended above an aluminising pack, the aluminium halide gas being transported over their surface by a carrier gas (6).
The combination of these processes yields a ‘cleaner’ platinum aluminide, where the presence of damaging substrate elements within the coating is reduced. Upon exposure to a high temperature oxidising atmosphere, the growth of the alumina scale is more controlled, see Figure 1, and the adherence of the oxide scale is improved, due to the reduction of the outward diffusion of elements, such as titanium. Therefore, by designing the coating so that the outward diffusion of damaging elements is hindered, the effective life of the coating can be extended.
Addition of Active Elements
The benefits of adding active elements to overlay coatings are well documented in the literature (4). The inclusion of elements, such as yttrium and hafnium, has a number of beneficial effects, for example, decreasing the oxide growth rate, enhancing the mechanical properties of the oxide and acting as sulfur getters (7). Equally, investigations of the addition of active elements to β-NiAl intermetallics have revealed similar benefits (8). Together with the Centro de Fisca Nuclear, Portugal, SERG has investigated the effects of the ion implantation of yttrium and hafnium into a platinum aluminide at a dosage of between 1 × 1015 and 9 × 1015 ions cm-2 (9). The effects of the additions on coating performance were inconclusive, but the work highlighted the importance of coating and substrate composition with respect to coating life.
The use of other noble metals as an addition with, or as a replacement for, platinum can lead to improved coating performance. For example, rhodium additions have been found to increase the lifetime of coating systems (10). The oxidation and hot corrosion performance of a commercially available platinum-rhodium modified aluminide coating has also been investigated. In a hot corrosive environment, the coating behaved at least as well as a platinum aluminide coating, see Figure 2, while in oxidising (pO2 ∼ 0.2 × 105 Pa) conditions, the superior stability of the β-type phase within the platinum/rhodium coating led to enhanced durability of the coating.
Effects of Iridium Additions
The performance of iridium-modified aluminides has rarely been reported in the open literature. Iridium has been reported to exhibit low oxidation rates compared to other refractory metals, and is known to have a low oxygen diffusivity (11). Investigations have also shown that IrAl intermetallics have potential as alumina-formers (12). Therefore, an assessment of the potential benefits of iridium and iridium/platinum additions to a low activity aluminide coating was undertaken (13). It was revealed that, compared to platinum aluminides, the iridium-based coatings were relatively thin (a distinct advantage for turbine applications), they promoted alumina-based scales and also formed effective barriers against the outward diffusion of certain damaging elements, such as hafnium. However, the stability of the coating and the adherence of the oxide were lower than those usually exhibited by a platinum aluminide coating. Therefore, further development of the coating systems is required, so as to fully exploit the benefits of iridium.
Overall, it has been demonstrated that by close control of the composition of the coating and by the addition of other noble metals, the effectiveness of platinum aluminide coatings can be greatly increased.
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