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Manufacturing techniques

For the coating of gas turbine components, mainly the atmospheric plasma spray process (APS) is used. This results in lamellar structures which are typically interspersed with pores and microcracks. Such microstructures have a low modulus of elasticity which leads to correspondingly low stresses. Moreover, a good strain tolerance is made possible by opening the cracks. This is important since the difference in the thermal expansion between the substrate and the TBC causes thermal stresses during the heating and the cooling. The porosity also reduces the thermal conductivity and thus improves the thermal insulation effect of the coatings.

In addition to the thermally insulating ceramic coating, a thermal barrier coating system also consists of an intermediate metallic layer, the so-called bond coat. It improves the fusion adherence between the metallic substrate and the ceramic top coating. Moreover, it protects the substrates from the oxidation and corrosion caused by the hot fuel gases. The coatings are manufactured often using various thermal spraying processes such as low-pressure plasma spraying (LPPS), high-velocity oxy-fuel spraying (HVOF), or cold gas spraying.

In addition to the refinement of the mentioned processes, new techniques are also being tried out. One particularly interesting process in this respect is suspension plasma spraying (SPS) in which suspensions are introduced into the plasma flame instead of input feedstock materials in powder form. One process which is also at the development stage is the Plasma Spray-Physical Vapor process (PS-PVD). At a low pressure and a high power, this leads to the formation of a very large, highly laminar plasma jet. In this case, the input materials in powder form are vaporized, thus resulting in the deposition of coatings which have a columnar structure and exhibit an outstanding efficiency in the thermal cycle test.

WDS: Suspension Plasma Spraying: Injection of the Precursor into the plasma jet.Suspension Plasma Spraying: Injection of the Precursor into the plasma jet.

WDS: Columnar structured thermal barrier coating manufactured by PS-PVD.Columnar structured thermal barrier coating manufactured by PS-PVD.