This result only applies once the TGO is thick enough to assure that the bond coat yields on cooling.

At instant B, at the end of the reheat step, coincident with Tmax since there is no reverse yielding, the stress in the convex TGO becomes:

The difference between the stress in the convex TGO at instant B and its yield strength dictates the increment in elongation strain as

Et tgo

This result applies when Aeg < Aeg. This is the strain that dictates the change in radius of curvature on a cycle-by-cycle basis, discussed next.

The elongation strain introduced into the convex TGO during each cycle dictates the change in curvature. The stresses are essentially the same at the beginning and the end of each cycle. The only difference is that attributed to the small change in TGO thickness. During growth, the elasticity of the bond coat restricts the strain that can be realized. Then, upon cooling, once the bond coat yields, the associated plastic strains accommodate the TGO elongation. This is why the change in curvature develops primarily upon cooling. The change in elongation strain in the convex TGO layer at this stage relates to the change in curvature, Dk, by:

Since it is elastic, the change in the strain in the concave TGO layer, Det11 » 0 . Accordingly, the change in curvature per cycle becomes:

The basic implication is that a change in curvature upon thermal cycling is enabled by a high TGO yield strength, rg and relatively low bond coat strength, rf. An inversion occurs when r^ becomes too small (<10MPa) because reverse yielding upon reheating negates the plastic displacements on cooling.

Alternate Stabilizers.

Phase Equilibria.

The additions of interest are rare earth oxides to conventional YSZ, and the initial emphasis is on Gd. The fundamental thermodynamic question is the position of the To surfaces for the cubic/tetragonal and tetragonal/monoclinic transformation as these determine the viable composition range for the metastable t' phase. Little is known, however, about the relevant phase equilibria and associated free energy functions for the relevant phases. The ZrO2-YO1.5 system has been extensively studied and assessed but some regions are still under debate, particularly the lower temperature equilibrium on the ZrO2-rich end. There are also partial and/or tentative diagrams derived experimentally for many of the ZrO2-REO 1.5 systems, but the uncertainty on the ZrO2-rich end tends to be generally greater than for the ZrO2-YO 1.5. It has been suggested, however, that the lanthanide systems exhibit regular trends with ionic size in their thermodynamic behavior as solutes within ZrO2. The calculated stability range of the fluorite solid solution is reported to increase systematically in composition and temperature with decreasing size of the dopant cation (La ®Lu). Concomitantly, the stability of the equilibrium pyrochlore zirconate decreases with decreasing ionic size from La to Gd, beyond which is replaced by the 5 structure whose stability increases with further reduction in the ionic size (Ho ®Lu). The calculated phase diagrams in the literature also suggest an increase in the stability of the tetragonal phase with decreasing ionic size, but no significant discussion of this trend is provided.

In view of the limited information on the ZrO2-GdO1.5 system, an effort was initiated to develop an understanding of the general features of the ZrO2-rich end of the phase diagram, as well as the extension of the phase fields into the ternary system ZrO2-YO1.5- GdO1.5. First, binary samples with different concentrations of GdO1.5 or YO1.5 were prepared from mixed precursor solutions. (The YO1.5 samples were intended to ascertain the reliability of the method and provide comparison with GdO1.5 materials of similar composition.) The solutions were flash-dried by spraying them onto a hot (300°C) Teflon-coated surface to minimize segregation. The resulting powders were pyrolyzed at 900°C, pelletized, heated to 1600°C for 24 h, and then to 1200°C for an additional 168 h (one week), and in selected cases for up to 504 h (three weeks). Phase composition after these treatments was undertaken by X-ray diffractometry. Some samples were also examined by Raman spectroscopy, which is more sensitive than XRD for detecting small amounts of tetragonal or monoclinic phases.

The results are summarized in Figure 4, which depicts a tentative 1200°C isothermal section in the ZrO2-YO1.5-GdO1.5 ternary and the corresponding ZrO2-REO binaries. The results for the ZrO2-YO1.5 are in remarkable agreement with the thermodynamic assessment in the literature, except for the 5-zirconate which retained the disordered fluorite structure even after the relatively lengthy heat treatment. The Gd samples, however, revealed some significant differences relative to the calculated phase diagram. Notably, the fluorite field at 1200°C is substantially wider than that predicted by the thermodynamic model, and hence the eutectoid reaction F ® Py + m is probably located at lower temperatures. These changes have been incorporated into Figure 4. The limited results so far suggest that the ZrO2-rich end of the ZrO2-GdO1.5 system is probably closer to the ZrO2-YO1.5 than initially anticipated. Further work, however, is necessary to confirm the proposed diagram and outline the Gd-rich end, but the present results provide sufficient insight to proceed with the exploration of the ternary, which is now ongoing.

Relative stabilizing efficiency

An investigation has been undertaken on the relative stabilizing efficiency of candidate cations to be added to YSZ. Compositions of initial interest are shown on Figure 4. They include different combinations of Y and Gd at a constant level of total stabilizer addition (7.6 mole% MO 1.5, equivalent to 7 wt.% Y2O3), as well as additions of GdO1.5 at a fixed level of YO1.5. Preliminary results for the 7.6% YO 1.5 and GdO1.5 are shown by the X-ray diffraction data in Figure 5, together with results for LaO1.5, which is also of interest as an alternative stabilizer. Compositions are prepared by precursor methods, which allow the low temperature synthesis of chemically homogeneous t' without the need for long heat treatments in the fluorite field, which obviously involves very high temperatures. The compositions are heat treated as powders, to avoid possible constraints to the transformations introduced in a compact. The initial condition corresponded to the product of the 900°C pyrolysis treatment. The powders were subsequently heated for 24 h periods and characterized after each stage. The treatment consisted of 4 cycles at 1200°C, followed by single cycles at 1250, 1300, 1350, 1400 and 1450°C.

All samples were found to be single phase tetragonal (t') after pyrolysis. The least stable material was that containing La, which exhibited partitioning into t + pyrochlore after 24 h at 1200°C, and subsequent transformation of the t phase into monoclinic. The material stabilized with Gd was still t' after the treatment at 1300°C, but exhibited detectable monoclinic phase after 1350°C. In contrast, the Y bearing material showed only a trace of monoclinic after the 1400°C cycle and clear presence of monoclinic after 1450°C. The above results, albeit preliminary, suggest that direct substitution of Y in

YSZ may be detrimental to the phase stability of the coating. The preferred strategy is then to retain Y as the main stabilizer, with the additional dopant addition tailored for optimum effect on the optical properties.

Synthesis of Compositions with Mixed Rare Earth Oxide

A methodology has been developed to deposit a TBC with simultaneous addition of Y and Gd. In principle, the compositions could simply be ordered from a commercial supplier, but that approach is expensive and limits the flexibility to try multiple new compositions. The strategy selected involved using standard YSZ ingots, which are then infiltrated with a solution of Gd nitrate precursor, dried and pyro-lyzed at 1200°C to incorporate the Gd2O3 into the structure. To prevent segregation during drying the precursor is gelled immediately after impregnation. The amount of Gd added can be tailored by controlling the concentration of the solution and the number of impregnation cycles.

An ingot produced in this manner was used to deposit (by electron beam deposition at UCSB) a ~120 |im thick coating of ZrO2-7.6YO 1.5-7.6GdO 1.5 on a FeCrAlY substrate, previously polished and pre-oxidized. The resulting TBC is shown in cross-section in Figure 6. Energy dispersive spectral (EDS) measurements revealed the Gd composition to be uniform throughout the thickness. The columns are well developed, with the typical appearance of YSZ deposited in the same manner. A cursory examination suggests that the feathery structure within the columns is more pronounced than in YSZ, but this issue needs to be studied more carefully. Samples of this type will be used for subsequent studies of morphological evolution during heat treatment, as well as conductivity measurements.

Figure 4. Tentative ternary section for the ZrO2-GdOi.5-YOi.5 at 1200°C, and the corresponding binaries. The circles represent experimental compositions and the heat treatments to which they were subjected. Note that the 7.6% composition exhibits partitioning because of a previous 24 h exposure at 1600°C. The empty circles in the ternary represent the compositions involved in the phase stability study.

Figure 5. Stability of the t' phase for three different cations at the same level of addition. The asterisks indicate the position of the monoclinic peaks, and the legends on the left the heat treatment corresponding to the XRD scan.
Figure 6. ZrO2-7.6YO1.5-7.6GdO1.5 thermal barrier coating deposited by EB-PVD from an ingot prepared by impregnation of a Gd precursor into a YSZ matrix.

AGTSR Subcontract Number 01-01-SR091 First Semi-Annual Report August 1, 2001 to February 1, 2002

Ridges on the surface of Pt-Al bond coat

Maurice Gell and Eric Jordan, Principal Investigators School of Engineering University of Connecticut, Storrs, CT

In collaboration with

Gerry Meier and Fred Pettit Materials Science & Engineering Department University of Pittsburgh, Pittsburgh, PA

And Yongho Sohn Adv. Materials Processing & Analysis Center University of Central Florida, Orlando, FL.

Executive Summary

Advanced industrial gas turbine engines require the use of reliable and highly durable thermal barrier coatings (TBCs) and metallic, stand-alone coatings to meet performance and durability goals. Current TBCs and stand-alone metallic coatings lack the necessary durability and reliability. Much progress has been made in understanding the mechanisms of damage initiation and progression in current TBCs and metallic coatings that ultimately leads to spallation. This understanding indicates that significantly improvements in TBC and metallic coating life and reliability can be achieved by focusing on coating composition and processing. The University of Connecticut (UConn), the University of Pittsburgh (UPitt), and the University of Central Florida (UCF) are partnering in this research program that has the potential of improving electron beam physical vapor deposition (EB-PVD) TBC and metallic coating life by more than a factor of 3X.

Research conducted by the UCONN and Pitt, including previous AGTSR programs, indicates that spallation in electron beam physical vapor deposited TBCs depends strongly on (1) the perfection of the initial, thermally grown oxide (TGO), (2) the magnitude of the localized out-of-plane tensile stress at the TGO to bond coat interface, and (3) the adherence of the TGO during thermal cycles. In this proposed program, the salient bond coat composition and processing features will be systematically investigated in order to demonstrate the optimum combination of features that will provide at least a 3X durability improvement compared to current TBCs and stand-alone metallic coatings. The following features will be assessed individually and in combination for platinum aluminide (Pt-Al) and MCrAlY coatings used as both bond coats for TBCs and as stand-alone coatings:

I. TGO Perfection: (a) presence or absence of metastable/transient oxides versus the desirable stable alpha alumina oxide.

II. TGO Stress: (a) surface roughness, (b) presence or absence of bond coat surface defects.

III. TGO Adherence: (a) presence or absence of active elements (silicon and hafnium) contributing to improved TGO adherence.

Most of the first reporting period has been devoted to (1) obtaining coated specimens, (2) evaluating commercially viable surface finishing treatments and (3) conducting initial oxidation trials.

All metallic and TBC coatings are being provided by Howmet's Technology Center and Thermatech Division. Initial Pt-Al bond coated CMSX-4 specimens have been provided and NiCoCrAlY bond coats with and without silicon and hafnium are on order.

Toward the end of the current reporting period the remaining 130 of 150 samples of three different types were received. Experiments on the effectiveness of media finishing as a method of improving the surface finish of samples were carried out in the previous reporting period on the PtAl bond coated samples and were promising enough to select this method as a surface improvement method for all sample types. In addition further preoxidation have been carried out on the PtAl bondcoats. Unexpected void formation under the oxide was found under the oxide for these samples in the previous reporting period. Through a series of experiments a heat treatment that eliminated the unwanted void formation was found. As a result the preoxidation heat treatment for PtAl samples has be initiated which will be followed by EB-PVD application of the TBC. For the VPS MCrAlY samples shot peening has been done and was found to yield an unexpectedly rough surface. Surface smoothing experiments are under way to remedy the surface finish problems. It is expected that media finishing will be chosen as one of the surface conditions for all MCrAlY samples and laboratory metallographic finishing will be chosen as the second contrasting surface finish. All samples will be sent for application of the TBC by April 15. Thus the first phase of the program is rapidly coming to an end having received all samples, selected surface finish and preoxidation treatments. The next phase will involve comparison of the performance of the different samples in cyclic furnace tests.

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