Thermal barrier coating experience in gas turbine engine at Pratt & Whitney

摘要

Pratt and Whitney has accumulated over three decades of experience with thermal barrier coatings (TBC). TBC's were originally developed to reduce surface temperatures of combustors of JT8D gas turbine engines to increase the thermal fatigue life of the components. Continual improvements in design, processing and properties of TBC's have extended their applications to other turbine components such as vanes, vane platforms and blades with attendant increases in performance and component durability. Plasma spray based generation 1 (Gen 1) combustor TBC with 7 w/o yttria partially stabilized zirconia deposited by air plasma spray (APS) on an APS NiCoCrAlY bond coat continues to perform extremely well in all product line engines. Durability of this TBC has been further improved in Gen 2 TBC for vanes by incorporating low pressure chamber plasma sprayed (LPPS) NiCoCrAlY as a bond coat. The modification has improved TBC durability by a factor of 2.5 and altered the failure mode from a 'black failure' within the bond coat to a 'white failure' within the ceramic. Further improvements have been accomplished by instituting a more strain tolerant ceramic top layer with electron beam physical vapor deposition (EB-PVD) processing. This Gen 3 TBC has demonstrated exceptional performance on rotating airfoils in high thrust rated engines by improving blade durability by 3x, by eliminating blade creep, fracture and rumpling of metallic coatings used for oxidation protection of the airfoil surfaces. A TBC durability model for plasma sprayed as well as EB-PVD systems is proposed which involves the accumulation of compressive stresses during cyclic thermal exposure. The model attempts to correlate failure of the various TBCs with elements of their structure and its degradation with thermocyclic exposure.