Thermal fatigue testing of coatings for fusion reactor applications

Thin Solid Films, 83 ( 1981) 79-85 METALLURGICAL AND PROTECTIVE COATINGS

79

T H E R M A L F A T I G U E T E S T I N G OF C O A T I N G S FOR FUSION REACTOR APPLICATIONS* A. W. MUL[.ENDORE, J. B. WHITLEY AND D. M. MATTOX

Sandia National Laboratories, Albuquerq,w, N M 87185 ( U.S.A. ) [Received March 23, 1981 ; accepted May 18, 1981)

Thermal fatigue testing was performed on eight coated or clad materials which have potential application as limiters in pulsed tokamak fusion devices. They are ( 1) chemically vapor-deposited coatings of TiC, TiB 2 and boron on graphite, (2) plasma-sprayed TiB z on copper, (3) a chemical conversion coating of VB 2 on vanadium-clad copper, (4) titanium-clad copper and (5) vanadium-clad copper. Testing consisted of up to 1000 cycles of electron beam heating for 1.5 s at beam power densities of 1 and 2 kW cm- 2. Three materials, chemically vapor-deposited TiC and TiB 2 on graphite, and plasma-sprayed TiB 2 on copper, survived the 1000 cycle 2 kW cm -2 test with slight but acceptable damage. The most notable test failurc was VB 2 on vanadium-clad copper which deformed severely by a thermal ratcheting mechanism and displayed subsurface melting.

1. INTRODUCTION

A variety of coated materials have been cvaluated for application as limiter and armor components of pulsed tokamak fusion reactors ~-4. The coatings offer a means of tailoring the surface to obtain favorable ion and arc erosion resistance, appropriate hydrogen isotope recycling characteristics and a low to moderatc atomic number to minimize radiation losses from atoms which enter the plasma. The in-service exposure of these coatings to many thousands of cycles of high encrgy density pulses imposes severe requirements on their resistance to thermal fatigue. Thus an important part of the screening and testing program consisted in subjecting the materials to a simulation of this thermal fatigue environment. The testing utilized pulsed electron beam heating of the coated side of the candidate materials for as many as 1000 cycles. It was carried out as a part of a joint coating evaluation program 5 with Princeton Plasma Physics Laboratory, Argonne National Laboratory and Sandia National Laboratories. Additional thermal shock and thermal fatigue testing of the same materials has been reported by Ulrickson and Cecchi 6. * Paper presented at the International Conference on Metallurgical Coatings. San Francisco, CA, U.S.A.. April 6-10, 1981. Elsevier Sequoia:Printed in The Netherlands

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2. EXPERIMENTAI. I)EIAII.S

The materials investigated included 11} chemically vapor-deposited TiC, TiB= and boron on graphite, (2} plasma-sprayed TiB, on copper, t3l a chemical conversion VBe coating on vanadium-clad copper, I4) titanium-clad copper and {5J vanadium-clad copper. Table l identifies the coatings, and their fabrication is described in the indicated references. T,,\BI.E I [ ().~.II I) \ \ I )

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The samples, 2.54 cm x 2.54 cm in area by' 1.27 cm thick, were tested in multiples of four to eight in vacuum ll() -5 T o m in the electron beam thernaal fatigue apparatus described in rcf. 2. The samples, on a 32 position motorized computercontrolled table, were sequentially indexed under the electron gun and heated over its entire area with 1.5 s pulses from a rastered 1400 Hz) electron beam {with a 40 ms rise timel. The repeat time (for a given sample) was 300 s and the total number of cycles was either 200 or 1000. The 2(X)cycle tests of the eight candidate materials were conducted both in the water-cooled mode (samples resting on a water-cooled copper stage) at a beam power density of 2 kW cm -' and in a radiation-cooled mode (samples elevated 6 rnm above the stagej at 1 kW cm -' The I(R)0 cycle tests were made on four selected materials and conducted only in the water-cooled mode. The front and rear surface temperatures were measured as functions of timc with an I R radiation thermometer and thcrmocouples respectively. The surface condition was monitored during testing with a color television monitor and an optical telescope, and the electron beam was cancelled when a coating failure was observed. Post-test examination of samples included surface observation by optical and scanning electron microscopy. X-ray diffraction and metallographic sectioning. To assess the material's ability to withstand individual exposures to higher energy density as might occur in fault conditions, additional electron beam tests

THERMAl. FATIGUE TESTING OF COATINGS FOR FUSION REACTOR APPLICATIONS

81

were performed. Pulses of beam power density up to 22 kW cm - 2 of duration 0.020.5 s were utilized. These results have been reported previously in ref. 13. 3. RESUt.TS A summary of the results of all tests is given in Table II. With only a few exceptions, all samples of the eight materials survived the 200 cycle exposures to 1 and 2 kW cm - z in the radiation-cooled and water-cooled testing mode respectively. Two of the four vanadium-clad copper samples and one of the titanium-clad samples suffered partial or complete cladding separation from the substrate and consequent melting of the cladding early in the test. These samples were thought to have come from the outer region of the explosively bonded plate where bonding is unreliable and thus the failures do not represent an inherent fault of the material type ~4. In view of the successful performance of the materials at 200 cycles, the 1000 cycle tests were performed only in the more severe mode: 2 kW cm-2 and water cooled. Eight samples of four selected materials were tested for an additional 800 cycles, three of each material having had the previous 200 cycle exposure. Figure 1 shows the overall appearance of the samples after test. The samples labeled with a white asterisk are those which had been exposed to the previous 200 cycles. A severe deterioration of the VB,/V/Cu samples is seen. Seven of the eight were judged to have failed between 138 and 610 cycles and testing was stopped. All other materials ran the full course of testing. Some edge damage can be seen on the plasma-sprayed TiB z on copper, only surface staining can be observed on the chemically vapordeposited TiC on graphite and, on the chemically vapor-deposited TiB z on graphite, edge staining and a central feature (on four samples) which indicates surface melting should be noted.

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Fig. 1. Post-test appearance of 1000 cycle thermal fatigue samples.

The peak temperatures TMgiven in Table II were measured on a 3 mm spot near the center of the samples and were corrected for window losses and emissivities. The emissivities were obtained from reflectivity r e r s u s wavelength data taken at room

82

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THERMAL FATIGUE TESTING OF COATINGS FOR FUSION REACTOR APPI.ICATIONS

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temperature for each material I s. Two sources of error which have not been assessed adequately are (1) the change in emissivity of the sample as a result of surface alterations during the test and (2) the effect of surface temperature gradients (revealed by the stain patterns in Fig. I). These could result in a departure of the measured value of perhaps 1130K or more from the peak surface temperature of the sample. The rest temperature Ta (the minimum temperature at the base of the samples) is a thermocouple measurement and hence quite reliable. The central surface melt region of the TiB 2 on graphite occurred very late in the testing as the apparent result of carbon and boron exchange between coating and substrate. The melt thus apparently represents a ternary (Ti-B-C) cutectic melt which according to Rudy 16 occurs at 2670 K. X-ray diffraction of the melt area reveals the presence of TiB z, TiC and graphite, and scanning electron microscopy of the melted area also shows a multiphase structure. Other changes in the TiB z surface appearance were t 1}a smoothing of the original facetted (5 lam facets) surface, (2) the development of a new well-defined grain size of approximately 10 lam diameter, (3} the appearance of some grain boundary voids and (4t the development of a network of coating cracks - b o t h intergranular and transgranular. No separation of the coating from the substrate could be detected. The stained appearance of the TiC/graphite (and TiB2/graphite ) samples is attributed to the volatilization and redeposition of a titanium species (probably with chlorine, oxygen and/or nitrogen) from the TiB 2 on graphite samples. A similar deposit was observed on annealing of the TiB 2 samples at 1470 K. The only other changes in the TiC/graphite samples were (1) the development of a fine crack network (primarily intergranular) in the coating which does not penetrate into the substrate and (2) the appearance of thermal etching of the surface which is most pronounced at the center of the samples. The random crack network in the coating is interpreted as resulting from a small amount of compressive plastic strain in the TiC 17 during the temperature rise period followed by tensile cracking in the cooling portion of the cycle. The same interpretation applies to the central region of the TiB2/graphite surface, but near the outside the cracks align themselves perpendicularly to the edge. This indicates an origin related to the radial surface temperature gradient and the fact that the ductility ofTiB 2 is tower than that of TiC. Three of the plasma-sprayed samples of TiB 2 on copper showed an edge delamination within the coating and a resultant exfoliation. X-ray diffraction revealed the conversion of some of the TiB 2 to Ti20 3 and the coating deterioration is attributed in part to this effect. A darkening of the color of the coating was also observed as the result of testing. The only material which exhibited severe damage in the 1000 cycle testing was the VB2/V/Cu. A dramatic expansion of surface regions occurred during cycling which consumed the clearance in the sample cavities of the graphite platen and then caused rumpling of the surface, distortion of the entire sample and the development of internal fissures. The polished and etched cross section of a sample is shown in Fig. 2. The sample distortions drastically limited contact with the water-cooled table and, together with internal fissures, caused a temperature rise to the point of subsurface m'elting. The failure is attributed to a thermal ratcheting mechanism ~, i.e. to a non-recoverable stepwise tensile plastic deformation of surface regions caused by a complex interaction of temperature gradients, differential expansion

84

A. W . MUI.I.I-NI)ORt-, J. B. WIII'II.I-Y, I). M. MAI I()X

and the temperature dependence of the yield points of the components. It is clear that this particular clad material exhibits unacceptable behavior and that similar clad materials must be analyzed and tested for a similar behavior. Ulrickson and Cecchi" did not observe a similar behavior of the vanadium-clad copper {not borided) which they tested to 1000 cycles in a pulsed electron beam apparatus. However, the irradiated area in their test was only carried out on a fl'action of the sample area. Thus it appears that the additional constraint of the unheated periphery of the sample may have precluded the operation of the ratcheling mechanism.

I"ig 2. A c r o s s r,ection o f a \.'B~ V C u s a m p l e :after a I(R)0 c vclc t h e r m a l I~Higtl¢ lest.

In several respects the thermal filtigue results of Ulrickson and Cecchi arc similar to those reported here. (1) The TiC-coated graphite was found to have very good characteristics. (2) Their plasma-sprayed TiB 2 on copper developed a pit to a depth of approximately half the coating thickness which may be similar to the edge exfoliation which we observed. (3) Their test of TiB2 on graphite produced a coating spall which illustrates the more brittle character of thc Ti B 2 {compared with Ti('l and probably a more severc thermal stress environment associated with the local heating in their test. 4. SUMMARIZIN(I REMARKS

The thermal filtigue testing of a wu'iety of potential candidate materials for pulsed tokamak fusion device applications was performed under conditions which simulate the in-service temperature cycle. Several materials, notably chemically vapor-deposited TiC and TiB e coatings o11 graphite substrates and plasma-sprayed TiB 2 on copper exhibited acceptable performances for a 10(X)cycle exposure to 1.5 s pulses of electron beam heating at a power density of 2 kW c m - e. Severe damage under these conditions was sustained by horided vanadium-clad copper as the resuh of a thermal ratcheting deformation characteristic. Minor damage sustained by the successful materials was evalualed by. post-test analyses of the samples.

THERMAL FATIGUE TESTING OF COATINGS FOR FUSION REACTOR APPLICATIONS

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ACKNOWLEDGMENT T h i s w o r k w a s s u p p o r t e d b y t h e U.S. D e p a r t m e n t DE-AC04-76-DP00789.

of Energy under Contract

REFERENCES 1 D.M. Mattox, A. W. Mullendore, J. B. Whitley and H. O. Pierson, Thin Solid Fihns, 73 (1) (1980) 101. 2 A.W. Mullendore, J. B. Whitley and D. M. Mattox, J. Nucl. Mater., 93 94 (1980) 486 492. 3 A . W . Mullendore, J. B. Whitley, D. M. Mattox and R. K. Thomas, Proc. 8th Symp. on the Engineering Problems of Fusion Research, San Francisco, CA, November 1 3 16, 1979. in IEEE Puhl. 79-CH1441-5 NPS, Vol. Ill, p. 1605. 4 D.M. Mattox, Coatings for fusion reactor environments, 1bin Solid Films, 63 (I 979) 213. 5 J. Cecchi, J. Nucl. Mater., 93- 94 (1980) 28. 6 M. UIrickson and J. Cecchi. Thin Solid Films, 73 (I 980) 133. 7 H.O. Pierson, E. Randich and D. M. Mattox. J. Less-Common Met., 67 (1979) 381. 8 L. Aggour, E. Fitzer and J, Schlichting, TiC coatings on graphite by CVD. In J. M. Blocher, Jr., H. E. Hintermann and L. H. Hall (eds.), Proc. 5th Int. C'on~. on Chemical Vapor Deposition. Slough. 1975, Electrochemical Society, Princeton, N J, 1975, p. 600. 9 A.W. Mullendore, D. M. Mattox, J. B. Whitley and D. J. Sharp, Thin Solid Films, 63 (1979) 243. 10 A.A. Popoff, Mech. Eng., 100 (5) (1978) 28. 11 H.O. Pierson and A. W. Mullendore, Thin Solid Fibns, 63 (1979) 257. 12 M. Kaminsky, 7bin Solid Fihns, 73 (1980) 117. 13 J.B. Whitley, A. W. Mullendore and D. M. Mattox, The status of low Z coatings for fusion reactor applications, 4th Ibp. Meet. on the Technology of Controlled Nuch'ar Fushm, American Nuch'ar Society, King q/Prussia, PA, October 14- 17, 1980, to be published. 14 M. Kaminsky, personalcommunication. 15 A . W . Mullendore, Characterization of low Z coated materials for fusion reactor applications. SAND Rep. 80-0809, 1981 (Sandia National Laboratories). 16 E. Rudy, AFML Tech. Rep. 65-2, Part V, May 1969 (U.S. Air Force Systems Command, Air Force Materials Laboratory). 17 A . W . Mullendore, J. B. Whitley, H. O. Pierson and D. M. Mattox, Mechanical properties of chemical vapor-deposited coatings tor fusion reactor applications, 27th Natl. Syrup. ~fthe .4merican Vacuum Society, Detroit, MI, October 13 17, 1980, in J. Vac. Sci. Technol., 18 (3) (1981) 1049. 18 D.R. Miller, J. Basic Eng., 81 (1959) 190.