Cooretec – Project
Partner RWTH Aachen: Lehrstuhl für Lasertechnik LLT
Institut für Allgemeine Mechanik IAM
Forschungszentrum Jülich: Institut für Energie- und Klimaforschung; IEK-2
Companies: MAN Diesel und Turbo, Oberhausen
Siemens AG, Mülheim / Ruhr
General Electric, Mannheim
Front stage blades and vanes of industrial gas turbines and aero engines have to be cooled intensively due to the high temperature of combustion gases. To reduce temperatures of the metallic blades further, ceramic thermal barrier coatings (TBCs) based on yttria stabilized zirconia with 300 to 500 mm thickness have to be applied, which limit heat flow to the metal due to their low heat conductivity. Cooling holes are usually drilled by electro discharge machining or electrochemical machining as a last manufacturing step before applying the ceramic TBC system which requires additional activities to prevent closure of cooling holes as a last manufacturing step.
Laser drilling offers the advantage, that cooling holes can be drilled into TBC coated materials as the last manufacturing process. Using established laser drilling techniques, part of the melt in the cooling holes becomes redeposited as recast layers on the cooling hole walls and form small cracks as a consequence of shrinking during cooling.
These small cracks can act as crack initiation sites in rotating blades of a turbine, resulting in loss of load bearing cross section as a consequence of crack growth during operation. To avoid the recast layers, a combination of high productive laser beam drilling with subsequent ultra-short laser beam processing results in recast layer free cooling holes of TBC coated components. These activities are performed at LLT, RWTH Aachen. As a first step cooling holes with a smaller diameter are drilled using pulsed laser radiation with high pulse energy up to 50 J. In a second drilling step, the recast layer will be removed using ultra-short laser ablation. Due to ultra-short pulses with durations of approx. 10 picoseconds, a “cold” ablation is established so that no heat is transferred into the components. The base material consists of directionally cast (DS) Nickel base alloys of the 2nd generation, which are used in advanced gas turbines.
Laser drilled samples will be investigated at IEK- 2 at Forschungszentrum Jülich under thermos-mechanical fatigue (TMF) conditions in the temperature between room temperature and 950 °C up to 1050 °C. Goal of these investigations is the influence of cooling holes on the TMF – life of TBC coated DS Alloys. Mechanical stresses occurring during TMF testing will be compared with FEM calculations which are performed at IEK-2 an IAM at RWTH.
Fig.1: Cooling holes in a TBC coated blade of an industrial gas turbine (Copyright YXLON)
Fig. 2: TMF test facility with radiation furnace for investigation of TBC coated nickel base superalloys
Fig. 4: Cooling hole with and without recast layaer - With recast layer and crack in base material
Fig. 5: Cooling hole with and without recast layaer - without recast layer