July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 8 4 0 iTSSe TSS iTSSe TSS T he characteristics of M(Ni and/or Co)CrAlYX(Ta, Hf, Si, Zr, others) bond coatings (or overlay coatings) used in gas turbines are influenced by major and minor alloying elements as well as process-related specifications such as thickness, porosity, oxygen content, and constituent phases of the coating. The manufacturing characteristics of the bond coatings are substantially affected by the spray facilities and process parameters used, which are employed to produce metallic bond coatings with different oxidation kinetics during thermal exposure in air (service). Low pressure plasma spray, or LPPS (a derivative of vacuum plasma spray, or VPS) and high velocity oxygen fuel (HVOF) methods produce high qual- ity bond coatings for plasma spray thermal barrier coatings (TBCs) compared with air plasma spray (APS) bond coatings. Further development of new coating technologies such as high velocity air fuel (HVAF), high velocity APS, and high pres- sure cold spray is aimed at increasing the lifespan of TBC sys- tems at elevated temperatures. TBC SYSTEMS AND SERVICE LIFE TBCs were developed for application to turbine engine hot components to meet industry demand for higher gas tur- bine engine performance. A conventional TBC system typi- cally consists of a metallic substrate, MCrAlY (M = Ni or Co, or MODIFIED BOND COATINGS IMPROVE SERVICE LIFE OF PLASMA SPRAYED THERMAL BARRIER COATINGS AND PROTECTIVE PERFORMANCE OF OVERLAY COATINGS New thermal and cold spray technologies are being developed to extend the lifespan of thermal barrier coating systems at elevated temperatures. Reza Daroonparvar,* Charles M. Kay,* and J. Karthikeyan* ASB Industries Inc., Barberton, Ohio *Member of ASM International 6 FEATURE both) bond coating, and a conventional yttria-stabilized zir- conia (YSZ, ZrO 2 -8 wt% Y 2 O 3 ) top coating—the thermal barrier coating [1-3] . A thermally grown oxide (TGO) layer forms and grows thicker at the bond coating-top coating interface during high temperature oxidation. Fast, inhomogeneous growth of a TGO bilayer (Al 2 O 3 /fast-growing oxides, particularly spinels), which has higher maximum radial and axial stresses compared with the Al 2 O 3 TGO monolayer, leads to the formation and propa- gation of horizontal microcracks at the TGO-YSZ interface [1,4-8] . Formation and growth of the Al 2 O 3 layer cannot continue on the bond coating surface when the aluminum reservoir in the coating is reduced to a certain level (less than a critical value). This phenomenon (breakaway oxidation) can lead to chemi- cal failure of the bond coating and formation of non-alumi- na oxides (bilayer TGO in Fig. 1) on the bond coating during oxidation [8] . Incremental TGO growth rate and rapid dissolution of β phase (the aluminum reservoir in the coating) can result froman over-doping effect (bond coatings with higher reactive COATING ACRONYMS 101 APS – air plasma spray CGDS – cold gas dynamic spray HVAF – high velocity air fuel HV-APS – high velocity air plasma spray HVOF – high velocity oxygen fuel LPPS – low pressure plasma spray RE – reactive element TBC – thermal barrier coating TGO – thermally grown oxide VPS – vacuumplasma spray Fig. 1 — Undesirable thermally grown oxide bilayer formation on NiCrAlY bond coating in a thermal barrier coating system after oxidation at 1000°C.