Ductile-to-brittle transition temperature behavior of platinum-modified coatings

Materiais Science and Engineering, 88 (1987) 227-231

227

Ductile-to-Brittle Transition Temperature Behavior of Platinum-modified Coatings* D. VOGEL, L. NEWMAN, P. DEB and D. H. BOONE

Naval Postgraduate School, Monterey, CA (U.S.A.) (Received April 28, 1986)

ABSTRACT

It has been reported that the addition of platinum to diffusion aluminide coatings can significantly increase the ductile-to-brittle transition temperature (DBTT) but the current database is small. Comparison of results is difficult because o f the number o f variables in the coating systems, their processing, the resulting coating morphologies and the testing techniques used. Following initial investigations o f the structural variations possible for the platinum-modified aluminides on an IN738 substrate, studies were initiated on the structural and compositional dependences o f the DBTT. Results confirmed that the addition of platinum to an aluminide coating had the effect o f increasing the DBTT, but a range of transition temperatures exists for the various structural forms which can overlap those for the higher aluminum content unmodified coatings. An additional observation was the presence o f high residual compressive stress levels in the modified coatings.

1. INTRODUCTION

The aluminide coatings are well understood and have been the basis for the protective coating systems used in gas turbines for over 30 years. However, further improvements in oxide adherence and resistance to hot corrosion attack to meet the expanding needs of new power plants and advanced alloy substrates could lead to even broader utility for this class of coatings. The use of platinum as

*Paper presented at the International Symposium on High Temperature Corrosion, Universit6 de Provence, Marseille 13331, France, July 7-11, 1986. 0025-5416/87/$3.50

a modifying element has shown promise in these areas, with encouraging results in oxidation and hot corrosion testing being reported

[1]. Although the importance of the mechanical behavior of coating-substrate systems is well documented and has been thoroughly reviewed [ 2 ], very few data are available for the platinum-modified coatings and the correlation between the actual structures tested and the testresults obtained is unavailable and/or confusing in many cases. This study was initiated to determine the ductile-to-brittle transition temperature (DBTT) of some of the structural variations of the platinum aluminides. As with most other diffusion coatings, the Pt-A1 structure does not exhibit a single variant, but rather a fairly wide spectrum of structures can be achieved depending on processing parameters. The characteristic structures produced by the various coating processes have been discussed in another research paper [1]. A brief outline of the three possible variations follows. Type 1 is a continuous surface layer of PtA12. This results from minimum platinum diffusion prior to aluminizing and can be produced by either the high temperature, low activity (HTLA) outward-type or the low temperature, high activity (LTHA) inwardtype aluminizing process. Type 2 is a two-phase PtA12-NiA1 structure. This occurs with a lower level of platinum distributed over a wider coating fraction and can be obtained with either HTLA or LTHA aluminizing processes. However, intermediate coating structures vary depending on the specific process used. The distribution and volume percentage of the PtA12 are dependent on pre- and post-aluminizing heat treatments. © Elsevier SeQuoia/Printed in The Netherlands

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Type 3 is a single-phase NiA1 structure with platinum in solution. This generally requires greater platinum substrate diffusion with a lower coating aluminum level established either during aluminizing or with post-coating heat treatment.

2. E X P E R I M E N T A L D E T A I L S

2.1. Test procedure Standard IN-738 tensile specimens were ground to a 12 pin r.m.s, finish and electroplated with 10 #m of platinum. The platinumcoated specimens were then given selected diffusion and aluminizing treatments (Table 1). A substrate required post-coating solution and an aging treatment of 2 h at 1120 °C in an inert atmosphere plus 24 h at 850 °C was then applied. Tensile testing was conducted in a standard test unit with an appropriate furnace. A specimen-mounted extensometer with a dial gauge was used to measure elongation. The test specimens were strained at a rate of 1.2 × 10 -4 s-1 to coating failure. Coating cracking was detected by using an audio measuring system adapted from a procedure developed by Lehnert and Schmidt [3] and others. An acoustic transducer was employed in conjunction with an oscilloscope to produce a visual signature of the coating failure. Sufficient calibration runs were made to establish the accuracy, sensitivity and repeatability of the procedure. For comparison of test results a transition temperature was defined as that temperature

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which corresponds to a fracture strain of 0.6%, as previously suggested by Lowrie and Boone [4].

2.2. Specimen structures With the coating structural types selected, there are a number of variables in the coating process which can result in structural variations. These variables include aluminum level, platinum level and coating thickness. Several of these variables are interrelated (e.g. aluminum level and coating thickness). The coating process parameters were selected to produce a nominal coating thickness of 75 gm, but for a constant aluminum pick-up of 10-12 mg c m -2 a variation in resultant thickness results for the various heat treatments. To determine precisely the structure of the coatings tested, scanning electron microscopy and a calibrated electron beam microprobe analysis were employed. Figures 1 and 2 show the structural details.

3. R E S U L T S A N D D I S C U S S I O N

The results of the DBTT testing of these structures are presented in Figs. 3 and 4. Testing of additional platinum-modified structures and the baseline aluminide structures with no platinum is in progress, but the data are not available. Previously reported data for the standard aluminide coatings reported by Goward are included [ 5 ], although detailed structural analyses are not available. As seen in Figs. 1 and 2, the aluminum level and the coating morphology can be varied and controlled by the processing parameters. The result is that an envelope of transition temperatures is obtained based on these variables and the coating thickness. Coatings with higher surface aluminum and platinum levels exhibited a higher DBTT. For the same platinum pre
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Vogel [6] noted, quite surprisingly, that the structures which have a higher DBTT exhibit a higher apparent room temperature ductility. Closer examination has indicated that this phenomenon may be a manifestation of a high level of residual compressive stresses in the coating. Several other researchers have noted similar behavior with detrimental effects on coating behavior [7]. Additional testing at temperatures above 800 °C is planned, together with measure-

ments of rOom temperature residual stress state.

4. C O N C L U S I O N S

(1) The presence of a continuous PtA12 layer at the surface of an aluminide coating significantly increases the DBTT. PtA12 distributed as a second phase in the NiA1 matrix does not have as severe an effect. The varia-

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determines the coating morphology dictates that mechanical testing be closely allied with structural characterization. (4) Further studies are warranted to establish baseline structures and DBTTs, to determine the effect of single-phase structures and to measure the magnitudes of the observed residual stresses and are currently under way at the Naval Postgraduate School.

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ACKNOWLEDGMENTS

The assistance of the technical staff of the coating divisions of H o w m e t Corporation and Turbine Components Corporation in coating preparation and electron probe microanalysis is gratefully acknowledged.

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REFERENCES

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tions in coating structure and composition have a strong effect on the DBTT. Lower surface platinum levels and a greater distribution of the PtA12 and platinum in solution over a larger coating zone significantly reduce the DBTT to levels comparable with the higher aluminum level unmodified aluminides. (2) A high and apparently structure- and composition-dependent level of residual compressive stresses is present in the coating at r o o m temperature. (3) The coating thickness and elemental distribution are not independent variables. The complex interplay of factors which

1 P. Deb, D. H. Boone and R. Streiff, Effects of microstructural morphology on the performance of platinum aluminide coatings, Proc. ASM Syrup. on Coatings for High Temperature Oxidation Resistance, Toronto, October 1985, American Society for Metals, Metals Park, OH, to be published. 2 A. Strang and E. Lang, Effect of coatings on the mechanical properties of superalloys, in R. Brunetaud, D. Coutsouradis, T. B. Gibbons, Y. Lindblom, D. B. Meadowcroft and R. Stickler (eds.), Proc. Symp. on High Temperature Alloys for Gas Turbines, Liege, October 4-6, 1982, Reidel, Dordrecht, 1982, p. 469. 3 G. Lehnert and W. Schmidt, Ductility of metallic diffusion type coatings on nickel-based alloys, Project 01-ZB-157-D20 Rep., December 1979 (Thyssen Edelstahlwerke Ag. Forschungsinstitut). 4 R. Lowrie and D. H. Boone, Composite coatings of Co-Cr-A1-Y plus platinum, Thin Solid Films, 45 (1977) 491. 5 G. Goward, Protective coatings for high temperature alloys state of technology, Syrup. on Properties of High Temperature Alloys with Emphasis on Environmental Effects, Las Vegas, NV, 1976. 6 D. Vogel, Determination of the ductile-to-brittle transition temperature of platinum aluminide gas turbine blade coatings, NPS Thesis, Monterey, CA, September 1985. 7 T . F . Manley, Plastic instability of aluminide and platinum modified diffusion coatings during 1100 °C cyclic testing, NPS Thesis, Monterey, CA, December 1985.

Fig. 2. (a), (c), (e), (g) Scanning electron micrographs and (b), (d), (f), (h) electron beam microprobe analyses of HTLA coatings ( . . . . nickel; - - -, a l u m i n i u m ; - - . - - , platinum): (a), (b) no platinum, a two-zone outward-type coating (this coating was not tested and is included for structure comparison only); (c), (d) coating 2 obtained by platinum diffusion at 980 °C for 2 h, a single-phase NiAl(Pt) type of coating; (e), (f) coating 4 obtained by platin u m diffusion at 1052 °C for 1 h, a single-l~hase NiAI(Pt) type of coating; (g), (h) coating 5 obtained by platinum diffusion at 1080 °C for 4 h, a two-phase PtA12-NiA1 coating.