Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Models for TBC erosion and FOD

Thermal barrier systems used in gas turbines exhibit three major categories of failure: one based on oxidation and the second on impact by projectiles ingested into the gas stream and a third based on chemical attack by molten injected deposits (CMAS). Each category has been subject to a combination of experimental assessment and/or modelling. The models of oxidation-induced failure have reached a maturity that allows trends with constituent properties to be ascertained. The situation is much less mature for failure mechanisms caused by impact or CMAS. The second mode, particle impact giving rises to erosion or foreign object damage (FOD) has been extensively studied and a protocol has been developed to establish connections between material removal rates caused by particle impact and the properties of the thermal barrier material. The assessment is confined to materials deposited using electron beam physical vapour deposition (EB-PVD), which have a columnar microstructure.

The output of this research is organized as follows. Basic penetration mechanics are summarized and used to establish formulae that relate the forces, stresses and penetrations to the kinetic energy of the impact. A mapping scheme is devised that provides a basis for further assessments. The results are combined in a manner that enables the derivation of scaling relations that characterize: (a) the thresholds for material removal, (b) the transitions between major mechanisms (expressed in terms of a mechanism map) and (c) some aspects of material removal at kinetic energies above the thresholds.

A combinations of large kinetic energy and high temperature, causes the material plastically deformation and density around the contact site. The deformation zones develop over a millisecond timescales, as the impacting particle decelerates. Outside the densified zone, kink bands form and extend diagonally downward, toward the interface with the thermally grown oxide (TGO). Within the bands, the columns are plastically bent, causing the boundaries of the kink band to crack, and thereby weakening the material. The similarity between impact and indentation indicates that the plasticity-based mechanisms governing material removal are not strongly affected by strain-rate. In some cases, the bands reach the interface with the thermally grown oxide (TGO). When this happens, they nucleate a delamination that extends outward from the impact site, along a trajectory within the TBC, just above the TGO. Such delaminations provide a mechanism for creating large-scale spalls, known as Foreign Object Damage (FOD).

During initial impact, elastic waves are induced that interact with flaws in the columns. Within nanoseconds, bending waves propagate at the tops of the columns to accommodate the projectile as it penetrates. The localized bending causes trans-columnar cracks beneath the surface. The ensuing array, upon linkage, causes small amounts of material to be removed. Elastic waves also reflect off the bottom of the columns, becoming tensile waves that propagate back to the surface. The timescale is on the order of 60ns. These waves may also cause cracks to form and extend across the columns. This mechanism leads to small-scale loss of the outer part of the thermal barrier coating, known as Erosion.

The scaling analysis provides some basic insight about the relative importance of the properties of the TBC having the greatest influence on erosion. As expected, elevating the TBC toughness has the most pervasive influence, especially through its role in elevating the cracking threshold. The corresponding role of the TBC yield strength (or hot hardness) is not transparent without guidance from models. The implication from the models is that softer materials (at high temperature) should have a substantially higher cracking threshold. This prediction has been tested by comparing the erosion trends among TBCs with different high temperature penetration resistance. Note, however, that for softer TBCs, the craters would be deeper. This would not be a problem for normal impacts, since there is no material removal below the cracking threshold. But cratering could adversely affect the material loss if a plastic ploughing mechanism were to operate when the particles arrive at high obliquity.

Progress toward a mechanistic understanding at ultra-high temperatures (1000-1500C) has been limited by the absence of well-controlled experiments capable of duplicating turbine engine conditions. The challenges are the high temperatures (typically 1100C), the high impact velocities (490m/s), the size and composition of the particles (usually calcium-magnesium-alumino-silicate: CMAS). This requires a strategy, involving tests conducted within a temperature gradient, to achieve the correct surface temperatures and CMAS penetration into the ceramic to provide insight and understanding about damage mechanisms involving CMAS.

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University of Cambridge
Trumpington Street
CB2 1PZ Cambridge
United Kingdom
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