High Temperature Oxidation Behavior and Characterization of NiCrAlY Alloy Coatings Deposited by Cold Spray and Thermal Spray

With the demand for fuel economy and increased power, combustion temperatures in aero turbines are approaching the design limits of substrate alloys. Hence, thermal barrier coatings (TBC) applied on substrate alloys are under ever increasing demand to perform at higher temperatures. Thermal barrier coatings typically consist of a MCrAlY metallic bond coat and a zirconia-based ceramic topcoat. The primary function of TBCs is to provide increased oxidation resistance and reduced thermal conductivity.  Many studies revealed that oxidation behavior and formation of thermally grown oxide (TGO) on MCrAlY bondcoat plays a paramount role in failure modes and life limits of TBCs.  In this work, we will investigate the oxidation behavior of NiCoCrAlY alloy coatings deposited by cold gas dynamic spray, and compare the results with those deposited by thermal (APS and HVOF) spray.  In comparison to other thermal spray processes, cold spray utilizes high velocity rather than high temperature to produce coatings without melting feedstock, and thereby achieve free-oxide coatings. Theoretically, this would be beneficial for reserving oxidation protective elements such as Al and Cr in the subject coatings.  

Test specimens coated with the same chemistry of NiCoCrAlY alloy are fabricated using cold spray (CS), air plasma spray (APS) and high velocity oxygen fuel spray (HVOF).  The influences of the coating deposition processes on the isothermal oxidation behavior of the coatings at a high temperature will be investigated.  Oxidation kinetic, TGO growth and microstructure evolution of the coatings after long-term thermal exposure will be analyzed.  Meanwhile, the characterization of specimens is performed, including coating microstructures, morphologies and TGO phases before and after oxidation test.  Further, TBC specimens incorporated with those alloy coatings are tested in thermally cyclic test to examine the spallation resistance of the TBCs, with particular interest on the influence of TGO layer on TBC failure mechanisms.