Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Microstructure design guidelines for erosion

Electron Beam Physical Vapour Deposited (EB-PVD) Thermal Barrier Coatings (TBCs) used in gas turbines may fail in service due to one of three mechanisms; oxidation induced failure of the bondcoat, damage induced by erosion or foreign object impact (FOD) with the columnar EB-PVD ceramic, and/or chemical attack of the ceramic part of the TBC by injested deposits (calcium magnesium alumino-silicates, CMAS).
This study reports microstructural design guidelines to limit damage due to Erosion and Foreign Object Impact (FOD).

Erosion is associated with small particle impact, where the near surface region - the top 20um - of individual columns is cracked due to particle impact. Damage is associated with elastic stress waves, which are generated during impact, propagate down the columns, and interact with flaws within each column of the TBC.

Monte-Carlo modelling of this damage process has shown that column diameter has a major influence on material removal rates. This is associated with the impact dynamics and the likelihood and depth of cracks in individual columns. Only when a network of near surface cracks in adjacent columns is formed is material lost on subsequent impact. Thus smaller diameter columns means less material lost per impact and this has been validated by experiment, both a room temperature and elevated temperatures. The rate of small particle erosion appears to change little with temperature but does depend on the impacting particle size (specifically the ratio between the impacting particle size and the column diameter) and the impact velocity. Up to impacting particle size that is x2-x3 of the column diameter small particle erosion results. For particles greater than this one may observe ceramic compaction - densification - without material loss, under these conditions the energy density transmitted to the columns is insufficient to initiate column fracture, but sufficient to induce some local plasticity, hence densification. This mode of compaction damage is more likely to occur at increase temperatures, for small particle impaction. Glancing angle impact is also less damaging than normal impact, with material loss a function of the normal component of the impact velocity.

Column inclination - the angle of growth of the EB-PVD morphology - is also a significant factor. The lowest erosion rates are associated with a 'normal' columnar microstructure. When the columnar structure is inclined, material loss due to erosion increases. This is significant for inclinations to the component surface shallower than 58deg. and most severe for inclinations below 20deg.

Erosion rates increase with high temperature aging and are also dependent on ceramic composition. Small additions of ternary and quaternary additions at the 1-2 mole% level have been observed to modify the erosion performance of the TBC, with the behaviour dependent on the level and specific addition added.

Foreign object impact produces severe damage and significant material loss down to the ceramic bond coat interface. Delamination cracks along the interface may also result giving significant ceramic material loss and therefore loss of thermal protection. The study and modelling of FOD has been a significant part of this work. Foreign object impact - damage from large particles in the gas stream - results in gross plastic damage to the columnar microstructure, densification, and the generation of shear cracks through the TBC to the ceramic/ bondcoat interface. Thus FOD develops from compaction damage when the plastic damage is excessive and shear cracks can for and propagate through the ceramic part of the TBC. Depending on test temperature, either shear cracking or column buckling may be observed. Shear cracking is observed at all test temperatures, while column buckling is only observed at elevated temperatures.

Foreign Object Damage is associated with large particle (generally greater than 200um), high velocity (in excess of 100m/s) impacts. Small changes to the ceramic composition, through the addition of ternary and quaternary additions at the 1-2 mole% level do not appear to influence the TBCs damage tolerance to FOD, although this level of addition has an influence on small particle erosion.

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Reported by

Cranfield University
United Kingdom
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