Oxidation Behaviour of NiCrAl and NiCoCrAl Bond Coatings Under Industrial Gas Turbine Conditions

Model bond coatings were deposited with a wide range of compositions and their oxidation behaviour investigated at 900 °C. These Ni–Co–Cr–Al coatings were deposited using magnetron sputtering (a physical vapour deposition technique) onto 10 mm diameter sapphire substrates. A range of compositions was generated by co-sputtering from a combination of two or three sources: Ni–10 wt% Cr, Ni–20Cr, Ni–50Cr, Ni–20Co–40Cr and/or Ni–40Co–20Cr combined with pure Al. The coatings were oxidised at 900 °C and the scales formed characterised. These data have been summarised into oxide predominance diagrams to show the relationships between coating compositions and the formation of protective Cr₂O₃ or Al₂O₃ scales, or the formation of other, less protective oxides (or mixed oxides). Both coating composition and exposure temperature (by comparison with earlier published research) were found to influence the oxide scale growth rate and oxide type, and thus the resulting degree of protection.     

Erosion, corrosion and erosion–corrosion of EB PVD thermal barrier coatings

Electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings (TBCs) have been used in gas turbine engines for a number of years. The primary mode of failure is attributed to oxidation of the bond coat and growth of the thermally grown oxide (TGO), the alumina scale that forms on the bond coat and to which the ceramic top coat adheres. Once the TGO reaches a critical thickness, the TBC tends to spall and expose the underlying substrate to the hot gases. Erosion is commonly accepted as a secondary failure mechanism, which thins the TBC thus reducing its insulation capability and increasing the TGO growth rate. In severe conditions, erosion can completely remove the TBC over time, again resulting in the exposure of the substrate, typically Ni-based superalloys. Since engine efficiency is related to turbine entry temperature (TET), there is a constant driving force to increase this temperature. With this drive for higher TETs comes corrosion problems for the yttria stabilised zirconia (YSZ) ceramic topcoat. YSZ is susceptible to attack from molten calcium–magnesium–alumina–silicates (CMAS) which degrades the YSZ both chemically and micro-structurally. CMAS has a melting point of around 1240 °C and since it is common in atmospheric dust it is easily deposited onto gas turbine blades. If the CMAS then melts and penetrates into the ceramic, the life of the TBC can be significantly reduced. This paper discusses the various failure mechanisms associated with the erosion, corrosion and erosion–corrosion of EB PVD TBCs. The concept of a dimensionless ratio D/d, where D is the contact footprint diameter and d is the column diameter, as a means of determining the erosion mechanism is introduced and discussed for EB PVD TBCs.     

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如何让你的照片充满活力

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