Klaus Fritscher

EB-PVD Y2O3 and CeO2/Y2O3 Stabilized Zirconia Thermal Barrier Coatings- Crystal Habit and Phase Composition.

ZrO2-based ingot sources with stabilizing oxides of 6.5 and 20 wt.% Y2O3 and 252.5 wt.% CeO2Y2O3 respectively were used to deposit thermal barrier coatings (TBCs) on rotating cylindrical electron beam physical vapour deposition (EB-PVD) NiCoCrAlY-coated IN 100 substrates by reactive high rate EB-PVD. The TBCs were investigated by scanning electron microscopy, X-ray fluorescence and X-ray diffraction. The phases within the TBCs were t′ tetragonal for 6.5 wt.% Y2O3ZrO2 and cubic for 20 wt.% Y2O3ZrO2 TBCs. The phases found in ceria-stabilized TBCs were t′, cubic and monoclinic depending on the compositional fluctuations within the layers. Compositional variations were due to wide differences in the respective vapour pressures of the various oxides in ceria-stabilized ingot sources which become effective in single-source EB-PVD processing. The apparent crystal habits in the TBCs were correlated with process parameters, phases and chemistry and with regard to structural growth models.

Oxide scale formation on an MCrAlY coating in various H2-H2O atmospheres

Abstract Virgin electron beam physically vapor deposited NiCoCrAlY coatings on Nimonic 75 sheets were glass bead peened, spark machined to coupons and variously heat treated in vacuum and in H 2 -H 2 O mixtures of different oxygen partial pressures at 1080 °C for 4 h and 1100 °C for 1 h and 28 h. Oxide scale formation was investigated by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, X-ray fluorescence and Auger electron spectroscopy. Continuous alumina layers were obtained by annealing in H 2 -H 2 O atmospheres whereas discontinuous oxides formed in a “technical” vacuum environment. The growth rates of the oxide films are compared with literature data on alumina forming alloys, and the distinctive changes are related to the influences of the specific atmosphere and to the interference of foreign elements with alumina. Mechanical properties of the scales also depend on growth conditions. Post-coating procedures such as glass bead peening and spark-erosion cutting have an effect on cleanness and scaling behavior.

Two-source jumping beam evaporation for advanced EB-PVD TBC systems

Abstract The continuous increase of the turbine inlet temperature in gas turbines necessitates new TBCs with a temperature capability beyond the current partially yttria stabilized material coatings. The present paper focuses on two-source jumping EB-PVD processed novel candidate layers for future TBC applications. It is shown that mixtures of oxides with widely different vapor pressures can be manufactured by this technique. The microstructure of the layers depends strongly on deposition conditions and on materials properties as well. Partial yttria stabilized zirconia coatings show no differences in microstructure and phase formation if deposited either by two-source jumping-beam or by one-source one-beam evaporation, while for ceria stabilized zirconia coatings large differences mainly in chemistry are found; depending on the jumping frequency multilayers are formed. In mixed silica zirconia coatings, crystalline zircon (ZrSiO 4 ) was formed neither in the as coated condition nor after annealing. Finally, jumping beam experiments allowed a detailed understanding of the growth kinetics of EB-PVD TBCs and the formation of a feather-like structure within the columns.

Development of a low-expansion bond coating for Ni-base superalloys

Abstract Ternary NiCrAl alloys were modified by the addition of Ti and Si in order to adjust their coefficients of thermal expansion (CTE) to less than 15 × 10 −6 K −1 (from room temperature to 1000 °C) without sacrificing essential high temperature oxidation resistance. Vacuum induction melted cast alloy samples were investigated by dilatometry and thermogravimetry (TG). Oxidative TG was conducted isothermally at 900 and 1000 °C and cyclically between 600 and 1100 °C for 100 h respectively. The CTE is reduced mainly by Ti and secondly by Cr additions. Quinary alloys which showed optimal oxidation resistance essentially exhibit phase stability in the solid state at all temperatures. Approximately 4 at.% Si is needed to attain low oxidation rates and to prevent oxide spallation as well. The beneficial effect of Si on oxidation behavior is attributed hypothetically to its ability to initiate the formation of protective alumina and subsequently silica. High Cr contents lessen the beneficial effect of Si owing to the concurrent formation of chromia and/or titania.

Nucleation and Growth of Oxide Constituents on NiCoCrAlY Bond Coats During the Different Stages of EB-PVD TBC Deposition and Upon Thermal Loading

Progression of TGO scale in a NiCoCrAlY/EB-PVD TBC system is monitored via analytical SEM and TEM from the first vacuum anneal of the NiCoCrAlY bond coat through the different processing stages of the EB-PVD Y-PSZ coating (preheating; initial and advanced stages of TBC deposition) and upon cyclic fatigue at T >1100°C. During the early stages segregation of oxide constituents to the TGO scale is dominated by yttria giving rise to a transient “off-plane” mixed zone (Al2O3, ZrO2) microstructure textured perpendicular to the TGO/TBC interface. Subsequently Y-aluminates and spinel are formed. Local phase incompatibility of coexisting oxides is considered critical for the formation of failure-relevant clustered pores, as verified from the bond coat/TGO interface and the TGO mixed zone adjacent to the TBC top layer.

Analytical electron microscopy of the mixed zone in NiCoCrAlY-based EB-PVD thermal barrier coatings: as-coated condition versus late stages of TBC lifetime

Abstract The microstructural evolution of the alumina-zirconia mixed zone in a NiCoCrAlY-based electron beam physical vapor deposited (EB-PVD) yttria partially stabilized zirconia (Y-PSZ) thermal barrier coating (TBC) system from the as-coated condition into the advanced stages of TBC lifetime is monitored by analytical transmission electron microscopy (TEM). In the as-coated condition yttria-rich islands at the thermally-grown oxide (TGO)/TBC interface locally impede zirconia uptake of the scale during TBC deposition and give rise to the formation of an “off-plane” alumina-zirconia mixed zone textured perpendicular to the TGO/TBC interface. During prolonged isothermal/cyclic oxidation an increased chromium diffusion through the TGO scale turns the mixed zone into a reaction zone introducing a morphological instability of the mixed zone/TBC interface due to solutioning of the bottom TBC layer. This microstructural pattern is corroborated by a triple-stage growth model for the mixed zone during three successive stages in TBC lifetime: (i) during TBC deposition, the thickness of the mixed zone increases due to predominant outward aluminum diffusion and uptake of zirconia. No columnar alumina zone (CAZ) has formed at this stage, (ii) upon completion of the transition alumina-to-corundum phase transformation the thickness of the mixed zone remains constant while the change in diffusion mechanism for an inward oxygen diffusion process now initiates parabolic growth of the columnar alumina sublayer of the TGO scale, (iii) in the late stage of TBC lifetime an marked outward chromium diffusion from the bond coat causes the mixed zone to resume growth due to TBC destabilization and the formation of a (Al, Cr)2O3 mixed oxide matrix phase. A transient YCrO3 phase is proposed for driving the destabilization of yttria-rich sections of the bottom TBC layer.

Transformation and oxidation of a sputtered low-expansion Ni-Cr-Al-Ti-Si bond coating for thermal barrier systems

Abstract A model low-expansion Ni-Cr-Al-Ti-Si bond coating was applied to nickel-base IN 100 and CMSX-4 substrates by magnetron sputtering. The single-phase coating consisted of Ni(Cr, Al, Ti, Si) phase with cubic B2 structure. Transformation of the as-sputtered coating at increasing temperatures was investigated by XRD and DSC. Essentially two-step transformation occurs during heating. In the first step γ-Ni forms due to diffusional loss of Ti and Si from the B2 phase which subsequently forms cubic Ni 16 Ti 6 Si 7 . After long-term exposure at high temperatures α-Cr precipitates within the coating. Oxidation of the Ni-Cr-Al-Ti-Si coating was investigated by isothermal exposure to air between 900 and 1100 °C up to 4000 h. Marked interdiffusion with the IN 100 substrate leads to higher oxidation rates of the coating compared with the CMSX-4 substrate. Oxide scale morphology and composition strongly depends on the temperature as shown for Ni-Cr- Al-Ti-Si coated IN 100. Mixed oxide scales were found containing α-Al 2 O 3 Θ-Al 2 O 3 at 900 °C and α-Al 2 O 3 TiO 2 at 1000 and 1100 °C. However, on Ni-Cr-Al-Ti-Si coated CMSX-4 α-Al 2 O 3 is formed exclusively even after 4000 h of exposure at 1000 °C, which imparts considerably reduced mass gain. Oxide scales remain adherent to the bond coating even during cyclic thermal loading.

Oxidation and lifetime of PYSZ and CeSZ coated Ni-base substrates with MCrAlY bond layers

Abstract IN100 blades and CMSX-4 samples with NiCoCrAlY bond coats manufactured via LPPS and EB-PVD were cyclically tested in a burner rig at 1120°C and a furnace at 1100°C. Two different TBC chemistries were deposited by EB-PVD techniques: PYSZ and CeSZ. In all cases the performance of the CeSZ coated variances was superior to the PYSZ coated counterparts. TGO growth kinetics were slightly slower for the CeSZ coated samples. The benefits in lifetime are mainly attributed to the mixed zone oxides that tolerate spinel phase formation due to their ability to “wet” each other. Pore formation in the mixed zone is hereby suppressed favoring crack blunting mechanisms.

Phase stability, oxidation, and interdiffusion of a novel Ni−Cr−Al−Ti−Si bond-coating alloy between 900 and 1100°C

A novel, low-expansion experimental Ni−Cr−Al−Ti−Si bond-coating alloy was investigated in the as-cast state concerning its phase stability, oxidation resistance in air, and interdiffusion with single-crystal IN-100 at 900, 1000, and 1100°C. Isothermal oxidative thermogravimetry was employed up to 500 hr. Interdiffusion was compared to a commercial Ni−Co−Cr−Al−Y alloy on IN-100. Oxidized Ni−Cr−Al−Ti−Si specimens and diffusion couples were characterized by metallography, SEM, EDX, XRD, and XRF. The Ni−Cr−Al−Ti−Si alloy provides good oxidation resistance in air at least up to 1000°C. The alloy is an alumina former. Due to its coarse microstructure, other oxides (e.g., rutile) may form and considerably dominate the oxidation behavior. The kinetics of oxidation were correlated with temperature, formation of phases, and morphology of oxides. Interdiffusion fluxes between Ni−Cr−Al−Ti−Si and IN-100 were mainly directed to the superalloy. They were faster than in Ni−Co−Cr−Al−Y/IN-100 diffusion couples.

Fabrication of TBC-armored rocket combustion chambers by EB-PVD methods and TLP assembling

Abstract A thermal barrier coating (TBC) system for rocket chambers made of Cu-based high strength alloys has been developed in a pilot project in line with EB-PVD (electron-beam physical vapor deposition) technology aiming at TBC application on Cu-based walls of real rocket combustion chambers. The TBC system consists of a metallic bond coating compatible with Cu-based material and an yttria partially stabilized zirconia TBC. The TBC overlayer is a distinctive ceramic structure designed for an exceptionally low Young’s modulus to withstand the extreme mismatch stresses between the internally LN-cooled high thermal expansion Cu metal base and the low thermal expansion hot ceramic shell. The TBC system has been qualified under close-to-service conditions on cylindrical LH2-cooled combustion chamber segments, where they have performed superior. As EB-PVD technology is a line-of-sight process that is rather able to coat internal cavities, a transient liquid phase (TLP) joining technique for fully coated parts has been developed, that allows to assemble complete components out of vapor-accessible fully coated parts. It is capable, e.g. to incorporate sinuous cooling passages in the throat areas of combustion chambers, and/or to assemble oversized parts out of smaller components by maintaining parent metal properties. A manufacturing process is outlined for making internal TBC armored combustion chambers.