ROM J. Leistensnider

Y N TO SATISFY P R O J E a COMMITMENT? NUMBER J. Bond

- I HOW? NFORMATION REQUESnRELEASE

>ATE SENT DATE INFO. REQUIRED PROJECT AND REQ. NO. 3/27/92 WBS H.5.1.1

UBJECT DEVELOPMENT OF TUNGSTEN FOIL TO GRAPHITE BOND

1.0 SUMMARY

REFERENCE EDJDIRECTNE NO.

APPLICABLE UNTIL (DATE)

This PIR presents the results of the effort to develop a bond between tungsten foil and graphite. Two braze designs using ZrVTi were investigated; bare, and with a sputtered Mo barrier on the graphite. Three diffusion bonds were investigated; bare, with a sputtered Co barrier, and with a sputtered Mo barrier. Of all of the bonds, the sputtered Mo barrier diffusion bond gave the best results, meeting the bond strength and ECR requirements.

URPOSE

APPLIED TECHNOLOGY

Any further distribution by any holder of this document or data therein to thirdparties mpresenting foreign interests, foreigngovemments, foreign companies and foreign subsidkvies or foreign divisions of US. companies shall be appmved b the Associate Deputy Assistant SecretaJy for Space and Defense Power Systems, US. Department of Enew. Furthec fireign p a q releare may requite DOE approval pursuant to Federal Regulation 10 CFR part 810, andlor may be subject to Section 127 of the Atomic Energy Act.

ENGINEERING WORK RECORD y N LOCATIONOFSUPPOKllNG LOCATION I.D. OF COMPUTER RUNS'

FILE NUMBER'

FILE ENTRY' DATNCALCULATIONS'

FUNCTIONAL MANAGER APPROVING MANAGER # R ; w & APPROVALS" i T , ~ Y & RETENTION REQUIREMENTS PAGE NO. Distribution:

See attached list MASTERS FOR COPIES FOR

1 3 MOS

1 6 .UOS

L O F A 1 1 12 UOS

1 UOS

1 MO.

3 MOS

6 MOS

MOS

n n DO NOT DESTROY

7 ~ 1 1 6 9 2 . 4 . . APPLICABLE TO S A N JOSE OPERATIONS ONLY * * AT LEAST ONE OF THE 'IWO APPROVAL SIGNATURES IS NECESSARY

I Y I N DATA VERIFIED

2.0 QBJFCTIVE The objective of the bond development was to develop a bond between tungsten foil and graphite which was stronger than the minimum of the graphite strength or 2000 psi in tension and had an ECR (electrical contact resistance) less than 25 Fa-cm2. Two approaches were investigated, a braze which could be used on the side of the module (Figure 1 a) and a diffusion bond which could be used in the interior of the module (Figure 1 b). The major roadblock which needed to be over- come was the fact that all of the materials in contact with the graphite, zirconium, vanadium, tita- nium, and tungsten, form carbides which are weak. The carbides are weak because the carbide has a lower volume than the original carbon and metal, so high stresses are induced in the carbide layer which lead to low strengths when additional loads are applied.

Figure 1. Interconnect Designs

a. Brazed Design b. Diffusion Bonded Design

Diffusion bond

3.0 MATERIALS The bulk materials used in this investigation were three different graphites and three different forms of tungsten foil. Two different types of Z r m braze were also used. SRRL GRAPHITE

SRRL is the graphite which will be used in theTA Cell electrodes. The material used was all from the free samples sent in 1992 by the manufacturer.

8826 was used in the first tests performed, as SRRL was not yet available. 8826 and SRRL should have nearly identical properties, so development performed on one should be applica- ble to the other.

At one time, UT-87 was going to be used in the TA Cell in the electrode between the tungsten and the SiGe. Therefore it was included in some of the tests.

Although the interconnect design uses 5 mil foil, available 3 mil foil was entirely adequate for bond development. The foil was purchased from The Rembar Company, and was identified as lot #1764.

8826 GRAPHITE

UT-87 GRAPHITE

3 MIL TUNGSTEN FOIL

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I ,

1 MILTUNGSTEN FOIL 1 mil foil was used in addition to the 3 mil foil to determine if there was any sensitivity to foil thick- ness. The foil was also purchased from Rembar, and was identified as lot #I32431 6.

The TA Cell design uses 5 mil perforated tungsten inside the module. Since none was available at the time, 1 mil perforated foil was used to give an early indication whether perforating the foil would create any problems with the bonds. The foil was purchased from Rembar, and was iden- tified as PO #HGM172757. The foil was sent to Buckbee-Mears and was perforated using a photo maskkhemical etch process to create 0.002 to 0.003 inch through holes in a hexagonal pattern with a 0.0032 inch center to center distance. The resulting foil was 55% porous.

This version of the braze material was made by METGLAS, a division of Allied Signal, using a rapid solidification technique. The raw material was ingots of 56 wt% Zr, 28 wt% V, 16 wt% Ti which had been made by CR&D. The product form was thin strips which were 1 to 4 mils thick, about 1/8 inch wide, and many inches long.

This version of the braze material was made by Microfoils Inc. using a rolling technique, starting with the same raw material. The product form was approximately 2" by 4" sheets with a uniform thickness of 3 mils.

1 MIL PERFORATED TUNGSTEN FOIL

STRIP ZrVTi BRAZE

ROLLED ZrVll BRAZE

4.0 EXPER IMENT D ESIGN

The experiment proceeded in two fronts simultaneously, the first being the development of the brazed bond and the second being the development of the diffusion bond. 4.1 ZrVTi BRAZE

The ZrVll braze material was chosen to braze tungsten foil to graphite because it meets the tem- perature requirements, it is compatible with tungsten, it has appropriate vapor pressure for use at 12OO0C in a 1 0-6 torr vacuum, we have significant experience with the material, and it is readily available. It had been established by TECO that all of the components of the braze reacted with graphite, forming carbides. Therefore, the first experiment was designed to establish whether this carbide formation would interfere with the ability of the braze to adhere to the graphite, and assum- ing that it did not, to develop the bonding cycle. Table 1 shows the design for these specimens. Note that all of the brazes described in this report were made with 6 psi of bonding pressure. After bonding, the final two specimens were exposed to an aging cycle of 1 hour at 1200°C to simulate the additional thermal cycling (brazing of the cells into the test fixture or the TCA) that the braze would have to withstand. After the bonding and aging cycle was established four additional specimens were fabricated for tensile and ECR testing. Their design is shown in Table 2.

After the results from the second set of coupons were known athird set of coupons was made with sputtered Mo on the graphite bonding surface. Their design is shown in Table 3.

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Table 1. Design For First Set of ZrVTi Braze Specimens

Materials

8826 graphite

Bonding/Aging Cycle

1) Heat to 700°C at 20"C/min Hold at 700°C overnight Heat to 1150°C at 20 C/min Hold at 1150°C for 30 min Heat to 1204°C at 12"C/min Hold at 1204°C for .5 min Step to 1210°C, hold for 3 min Step to 1230°C, hold for 6 min Cool to 1120°C at 9"C/min Hold at 1120°C for 10 min Cool to 20°C at 20"C/min No aging

3)

A in cycle incorporated in 8 '8 o onding cycle

Heat Hold Heat Hold Step Step Cool Hold Cool Age

to 1150°C at 20"C/min at 1 150°C for 30 min to 1204°C at 12"C/min at 1204OC for .5 min to 121OoC, hold for 3 min to 123OoC, hold for 6 min to 1120°C at 9"C/min at 1120°C for 10 min to 20°C at 20"C/min 1 hour at 1200°C

Heat to Hold at Heat to Hold at Heat to Hold at Heat to Hold at Heat to Hold at Heat to Hold at Heat to Hold at Heat to Hold at Cool to Cool to Hold at Heat to Hold at Cool to

250°C at 5"C/min 250°C for 15 min 350°C at 5"C/min 350°C for 15 min 450°C at 5"C/min 450°C for 15 min 550°C at 5"C/min 550°C for 15 min 750°C at S"C/min 750°C for 15 min 950°C at 5"C/min 950°C for 15 min 1150°C at 5"C/min 1150°C for 20 min 1210°C at 12"C/min 1210°C for 6 min 1125°C at 8.5"C/min 400°C at 20"C/min 400°C for 60 min 1200°C at 5"C/min 1200°C for 60 min 20°C at 20"C/min

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. .

Table 2. Design For Second Set of ZrVTi Braze Specimens

Materials Bonding/Aging Cycle

Heat to 250°C at 5"Clmin Hold at 250°C for 15 min Heat to 350°C at 5"C/min Hold at 350°C for 15 min Heat to 450°C at 5"C/min Hold at 450°C for 15 min Heat to 550°C at 5"Clmin Hold at 550°C for 15 min Heat to 750°C at 5"C/min Hold at 750°C for 15 min Heat to 950°C at 5"C/min Hold at 950°C for 15 min Heat to 1150°C at 5"C/min Hold at 1150°C for 20 min Heat to 1210°C at 12"C/min Hold at 1210°C for 6 min Cool to 1125°C at 8.5"C/min Cool to 400°C at 20"C/min Hold at 400°C for 60 min Heat to 1200°C at 5"C/min Hold at 1200°C for 60 min Cool to 20°C at 20"C/min

c. Table 3. Design For Third Set of ZrWi Braze Specimens

Materials Bonding/Aging Cycle

Heat to 250°C at 5"C/min Hold at 250°C for 15 min Heat to 350°C at 5"C/min Hold at 350°C for 15 min Heat to 450°C at 5"C/min Hold at 450°C for 15 min Heat to 550°C at 5"C/min Hold at 550°C for 15 min Heat to 750°C at 5"C/min Hold at 750°C for 15 min Heat to 950°C at 5"C/min Hold at 950°C for 15 min Heat to 1150°C at 5"C/min Hold at 11 50°C for 20 min Heat to 1210°C at 12"C/min Hold at 1210°C for 6 min Cool to 1125°C at 8.5"C/min Cool to 400°C at 20"C/min Hold at 400°C for 60 min Heat to 1200°C at 5"C/min Hold at 1200°C for 60 min Cool to 20°C at 20"C/min

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c

4.2 plFFUSlON BONQ Tungsten carbide forms if tungsten is bonded directly onto graphite. To interfere with the formation of weak tungsten carbide two sputtered materials were used as barriers, Co and Mo. Co had PO- tential because a number of W-CO-C cermets are available with high strength, and it was hypothe- sized that the materials in the bond could form such a cermet. Also, since Co has a low melting temperature, 1495OC, it had potential as a braze material. Mo had potential because it formed a carbide with little volume loss, and thus low induced stresses. The appropriate time/temperature/pressure cycle for the diffusion bonds was not known, so a vari- ety of conditions were tested with the first set of coupons. Bare specimens were bonded simulta- neously in the same fixture as sputtered specimens to investigate the formation and behavior of tungsten carbide. The design of the bare specimens is shown in Table 4, the design of the Co sput- tered specimens is shown in Table 5, and the design of the Mo sputtered specimens is shown in Table 6. All of the coupons were made with 8826 graphite. After bonding, some of the coupons were exposed to an aging cycle of 8 hours at 12OO0C to simulate the additional thermal cycling (perimeter glassing, lateral glassing, bonding of the pad facesheet to the lateral glass, brazing of the cells into the test fixture or the TCA) that the bond would have to withstand. After the results from the first set of test were known three more sets of specimens were made, all with sputtered Mo as the barrier. The second set was designed to confirm the results from the first set of coupons, and was made with QRRL graphite, 3 mil tungstenfoil, and sputtered Mo (Table 7). The next set was designed to verify that perforating the tungsten foil would not affect the strength of the bond, so 1 mil perforated tungsten was used in place of the solid foil (Table 8). At the time that this development was occurring both QRRL and UT-87 were being used in the elec- trode design of the TA Cell, so the last set was made with UT-87 graphite (Table 9).

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Table 4. Design for Bare Diffusion Bonds

Materials Bonding Cycle

I

I 1 or 3 mil W foil

1) 8826 Graphite

B 8826 Graphite

2)

3)

Apply2000 si Heat to 1 5Og"C at ZO"C/min Increase to 4000 DSI Hold at 1500°C for 30 min Decrease to 2000 si cool to zooc at 28"Clmin

Apply 2000 si Heat to 12OB"C at 5"C/min Hold at 1200°C for 8 hours Cool to 20°C at 5"C/min

Heafto 140b"C at 2O"C/min Increase to 4000 psi Hold at 1400°C for 30 min Decrease to 2000 osi cool to 2 0 0 ~ at 26"C/min

Apply2000 si Heat to 144l"C at 20"C/min Step to 1545°C Hold at 1545°C for 5 min Step to 1445°C Cool to 20°C at 20"C/min

5) Apply2000 si Heat to 120g"C at 5"C/min Hold at 1200°C for 8 hours Cool to 20°C at 5"C/min

6) Apply2000 si Heat to 144!!"C at 20"C/min Hold at 1445°C for 60 min Cool to 20°C at 20"C/min

- 7 -

Table 5. Design for Co Sputtered Diffusion Bonds Materials Bonding Cycle

Apply 2000 si Heat to 144g"C at 20"C/min Step to 1545°C Hold at 1545°C for 5 min Step to 1445°C Cool to 20°C at 20"C/min

Apply 2000 si Heat to 1201°C at 5"C/min Hold at 1200°C for 8 hours cool to 20°C at 5"C/min

3) Apply 2000 si Heat to 1 44g" C at 20 " C/.min Hold at 1445°C for 60 m y Cool to 20°C at 20"C/min

Table 6. Design for Mo Sputtered Diffusion Bonds Materials Bonding Cycle

I I 8826 Graphite I I 3)

Apply 2000 si Heat to 150 g "C at 2O"C/min Increase to 4000 PSI Hold at 1500°C for 30 min Decrease to 2000 SI Cool to 20 O C at 2 8 " C/min

Apply2000 si

Hold at 1200°C for 8 hours Cool to 20°C at 5"C/min

Heat to 120 8 "C at 5"Clmin

Apply2000 si Heat to 1401°C at 2O"C/min Increase to 4000 PSI Hold at 1400 " C for 30 min Decrease to 2000 SI cool to 2 0 0 ~ at 28"~ lmin

- 8 -

Table 7. Design for Second Set of Mo Sputtered Diffusion Bonds Materials Bonding/Aging Cycle

Apply 2000 si Heat to 1208°C at 5"C/min Hold at 1200" C for 4 hours Cool to 20°C at 5"C/min Age 8 hours at 1200 "C

SRRL Graphite

Table 8. Design for Mo Sputtered Diffusion Bonds With Perforated Tungsten Materials Bonding/Aging Cycle

SRRL Graphite 1 Table 9. Design for Mo Sputter

Materials I I I t

,ed

Apply2000 si Heat to 12OB"C at 5"C/min Hold at 1200°C for 4 hours Cool to 20°C at 5"C/min Age 8 hours at 1200°C

Diffusion Bonds With UT-87

Bonding/Aging Cycle

Apply 2000 si

Hold at 1200°C for 4 hours Cool to 20°C at 5"C/min Age 8 hours at 1200°C

Heat to 120 8 "C at 5"C/min

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5.0 EXPEWENT PROCEDURE

This section describes in some detail the procedures which were followed in performing the experi- ments described in Section 4. Discrepancies between the plan and what actually happened are also included in this section. For more detail the ETs (Engineering Test Tickets) which accompa- nied the hardware would have to be read. 5.1 7rVT BRAZE The first set of 3 coupons (Table 1) was fabricated using the procedures defined in ETs 3360 and 3499. In short the following was done.

Fabricate tungsten pieces.

Fabricate graphite pieces.

Shear 3 mil tungsten in building 100 into 0.5” by 0.25” rectangles, lap the edges. Ultrasonic clean in acetone/methanol, ultrasonic rinse in methanol.

Machine 8826 graphite into 0.5” by 0.25” by 0.1” blocks. Ultrasonic clean in acetone. Vacuum fire 3 hours at 1500°C.

Cut strip braze to size with scissors.

Due to the curvature of the braze, M150A1 Binder was used on the bonding surfaces to hold the braze in place on the graphite and the tungsten foil in place on the braze. The coupon was placed in the center of the bottom of the cylindrical brazing fixture and covered with an alumina block, upon which a 3/4 pound weight was placed.

Braze coupon in the Vacuum Industries Inc. (subsidiary of GCA Corp.) Series 3600 Model 2430-2 furnace in 25817 using cycle 1 on Table 1.

Fixture second coupon using the same procedure as the first. Braze coupon using cycle 2. Age second coupon 1 hour at 1200°C in the GCA furnace in M9119. Fixture third coupon using the same procedure as the first. Braze coupon using cycle 3.

Fabricate braze pieces.

Fixture first coupon.

All three coupons were potted and polished for microscopic evaluation. The first and third coupons were also prepared for SEM analysis by grinding to the appropriate dimension and sputtering a thin layer of AuPd on part of their surfaces to prevent the buildup of electrical charge on the speci- men. The second set of four coupons (Table 2) was fabricated using the procedures defined in ETs 3485, 3486, and 3562. Rolled braze was substituted for the strip braze in the final coupon of this set since it had just become available and it significantly simplified the fixture procedure. In short the follow- ing was done. Details which are the same as in the previous procedure are summarized, with just the differences noted.

Fabricate 0.5” by 0.5” square tungsten pieces. Fabricate 0.5” by 0.5” by 0.1” 9RRL graphite pieces, vacuum firing 3 hours at 1500°C. Fabricate braze pieces.

Fixture first two coupons. Cut strip and rolled braze to size with scissors.

Again, binder was used on the bonding surfaces to hold the materials together. The coupons were placed in the brazing fixture, but 1.5 pounds was used due to the larger coupons size.

Braze coupons in the GCA furnace in 25B17 using the cycle on Table 2. Fixture second two coupons.

c

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These were fixtured the same as the first two, but an error was made and one of the graphite blocks was not included. The result was one double coupon which had graphite / braze / tungsten / braze / graphite / braze / tungsten / braze / graphite.

Braze coupon in the GCA furnace in 2581 7 using the cycle on Table 2. The double coupon was half strip braze and half rolled braze. The first coupon was ground to 0.4” by 0.4”, the second ground to 0.45” by 0.155”, and the double coupon ground to 0.4” by 0.4”. The 0.4” by 0.4” coupons were bonded to 0.38” square tensile studs with epoxy (appendix A). They were tensile tested at 0.002 in/min in theTinius Olsen 10000 test machine in 27B39. The ECR test for the 0.45” by 0.155” coupon was cancelled due to the results from the tensile tests. The third set of two coupons (Table 3) was fabricated using the procedures defined in ETs 3485, 3486, and 3622. In short the following was done.

Fabricate 0.5” by 0.5” square tungsten pieces. Fabricate 9RRL graphite pieces, vacuum firing 1 hour at 1500°C. Sputter barriers.

Fabricate braze pieces. Fixture coupons.

Sputter approximately 2 bm Mo onto one side of each graphite block.

Since the braze was flat, binder was only added to the edges of the coupons alter they had been stacked to ease transferring them into the fixture. After the parts had been tested it was discovered that an error had been made in the fixturing and one of the graphite blocks had been placed Mo side away from the bond, leaving the bare side exposed to the braze.

Braze in the GCA furnace in 2561 7 using the cycle on Table 3. Both coupons were ground to 0.4” by 0.4”, bonded to tensile studs, and tensile tested. 5.2 DIFFUSION BOND The set of bare bonds (Table 4), the first set of sputtered Mo bonds (Table 5), and the set of sput- tered Co bonds (Table 6) were fabricated simultaneously using the procedures defined in ETs 3274, 3359 and 3533. In short the following was done.

Fabricate tungsten pieces.

Fabricate graphite pieces.

Shear 3 mil tungsten into 0.5” by 0.5” squares, lap the edges. Ultrasonic clean in acetone/methanol, ultrasonic rinse in methanol.

Machine 8826 graphite into 0.5” by 0.5” by 0.1 ” blocks. Ultrasonic clean in acetone. Vacuum fire 3 hours at 1500°C.

Sputter approximately 2 Fm Mo or Co onto one side of the graphite blocks and both sides of the tungsten foils.

The coupons were fixtured in stacks of four, with two bare and two sputtered coupons. Within each type of coupon one had 1 mil tungsten foil and one had 3 mil foil, making each coupon different. The bonding fixtures were the standard graphite ones for use without SiGe in the clean room, 27828. The stack was free-standing in the center of the fixture. No DAG or alumina wafers were used inside the fixture since all contact points were graphite to graphite.

Bond in either Thermal Technology Inc. Brew Precision Bonding Furnace, Model 914-M in 27628 using the cycles on Tables 4, 5 and 6. For the 1200°C cycles alumina wafers were used between the fixture and the TZM

Sputter barriers.

Fixture coupons.

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platens inside the furnace. For the higher temperature cycles, between the fixture and the platens were GRAFOIL, Zr02 wafers, and alumina wafers to prevent reactions.

Half of the coupons, those with 1 mil tungsten foil, were ground and polished for microscopic evalu- ation. Four of these were further prepared for SEM analysis. Of the six remaining bare coupons, two were ground to 0.4” by 0.4”, bonded to studs, and tensile tested, three were aged 8 hours at 1200°C in the GCA furnace in M9119, and the last one was aged 8 hours at 1200OC in M9119 and ground to 0.45” by 0.155”. Of the three remaining sputtered Mo coupons, one was ground, bonded and tensile tested, one was aged 8 hours at 12OO0C in M9119, ground, bonded and tensile tested, and the last was aged 8 hours at 1200°C in M9119, ground to 0.45” by 0.1 55”, bonded to graphite studs and ECR tested. Of the three remaining Co coupons, one was ground, bonded, and tensile tested, and the other two were held.

The second set of sputtered Mo bonds (Table 7) was fabricated using the procedures defined in ETs 3485, 3486, and 3578. In short the following was done.

Fabricate 0.5” by 0.5” square tungsten pieces. Fabricate QRRL graphite pieces, vacuum firing 1 hour at 1500°C. Sputter Mo barriers. Fixture coupons into a two-high stack. Bond in either Precision Brew furnace in 27828 using the cycle on Table 7. Age 8 hours at 1200°C in the GCA furnace in M9119.

Both coupons were ground to 0.4” by 0.4”, bonded to studs, and tensile tested.

The set of sputtered Mo bonds with perforated tungsten (Table 8) was fabricated using the proce- dures defined in ETs 3486 and 3578. In short the following was done.

Fabricate tungsten pieces. EDM perforated tungsten into 0.5” by 0.5” squares. Clean in 50/50 hydrofluoric acid/DI water, rinse in DI water.

Fabricate 9RRL graphite pieces, vacuum firing 1 hour at 1500°C. Sputter Mo barriers. Fixture coupons into a two-high stack. Bond in either Precision Brew furnace in 27628 using the cycles on Table 8.

Since the perforations remove roughly half of the surface area of the tungsten, the applied load was one half the load applied to the solid tungsten coupons in order to achieve the same pressure on the bonding surfaces.

Age 8 hours at 12OO0C in the GCA furnace in M9119.

Both coupons were ground to 0.4” by 0.4”, bonded to studs, and tensile tested.

The set of sputtered Mo bonds with UT-87 (Table 9) was fabricated using the procedures defined in ETs 3274, 3380, and 3596. In short the following was done.

Fabricate 0.5” by 0.25” rectangular tungsten pieces. Fabricate UT-87 graphite pieces, vacuum firing 1 hour at 1500°C. Sputter barriers. Fixture coupons into a four-high stack.

Bond in either Precision Brew furnace in 27628 using the cycles on Table 9. Age 8 hours at 1200°C in the GCA furnace in MQ119.

Two foil rectangles were used side by side at each bond to use residual tungsten.

Three coupons were ground to 0.4” by 0.4”. An error was made and the other coupon was ground to 0.391” by 0.4”. All were bonded to studs and tensile tested.

- IL-

6.0 RESULTS

6.1 ZrVTi BRAZE The results are summarized in Table 10. The column titled “Design Table” is a reference to the tables in Section 4 which contain more complete descriptions of the coupons.

Table 10. Summarized Results For ZrWi Braze Coupons

Desi n Graphite S uttered Aged Assessment Results Tabe P Type farrier

1 8826 None No SEM No porosity, braze segregates 1 8826 None Yes Micrograph Same appearance 1 8826 None Yes SEM No porosity, braze segregates

2 SRRL None Yes Tensile 1147 psi1 2 SRRL None Yes ECR Cancelled 2 SRRL None Yes Tensile 479 psi1 2 SRRL None Yes Tensile Cancelled

SRRL Mo Yes Tensile 1137 psi2 SRRL Mo Yes Tensile 1847 m i 2

Failed along bond line Failed partly along bond line

The first three coupons which were made (Table 1) showed good wetting and no porosity. During the braze cycle the braze separated into distinct regions with very different compositions, with the zirconium being attracted to the graphite, and the vanadium collecting in balls to the exclusion of zirconium (Figure 2). This type of behavior, separating into different compositions, also occurs in the other studies of the braze. In addition, the braze separated the grains of the tungsten foil to a depth of about 0.5 mils. After aging for 1 hour at 1200°C the distribution is about the same. The two specimens from the second set of coupons (Table 2) which were tensile tested failed along the bond lines at relatively low loads, 183.5 Ibs and 76.7 Ibs. Conservatively assuming that the entire 0.4” by 0.4” bond area carries the load, these coupons failed at stresses of 1147 psi and 479 psi. An XRD (X-ray diffraction) analysis was performed on the failure surfaces of the stronger coupon, showing a significant amount of ZrC formation. The weaker coupon was the double cou- pon, which was a sandwich with three blocks of graphite separated by two independent brazes. Since only one of the brazes was affected by the tensile test the second braze could be tested after regrinding the coupon flat and parallel. But since the other specimens had such low strengths the test was cancelled. Since the tensile tests showed that the bond was not acceptable the ECR mea- surement was also cancelled. One of the specimens from the third set of coupons (Table 3) failed at 1 137 psi (1 81.9 Ibs) and the other failed at 1847 psi (295.5 Ibs). Both of the failures were mostly in the graphite but were also partly along the bond line. Unfortunately the coupons were not tracked separately, and it is not known which of the two failed at the higher load. It is important because microscopy showed, and SEM/EDS (Scanning Electron Microscope/Energy Dispersive Spectroscopy) verified, that one of the coupons had been fixtured incorrectly and the sputtered Mo on one of the graphite blocks faced away from the braze, leaving bare graphite against the braze. Curiously, this is not the side of the

- 13 -

Dark area to lower right is graphite, light area to upper left is tungsten

Figure 2. ZrVTi Braze Composition After Bonding

- 14-

c

bond that failed, the side which had the sputtered Mo barrier failed. This may indicate that the Mo barrier is not needed for the rolled ZrWi braze, and that ZrC formation may be slower for the rolled braze than for the strip braze. Another difference between the strip braze coupons (Table 2) and the rolled braze coupons is that the M150A1 binder was used on the bonding surfaces of the strip braze, but only around the edges of the rolled braze. Additional testing to resolve these questions was suspended, as the TA Cell design was finalized to a configuration which did not require a braze between tungsten and graphite.

6.2 DIFFUSION BO ND

The bare coupons performed as expected, forming weak tungsten carbide. Before aging, all of the coupons showed no porosity. Thetwo tensiletests which were performed failed at 581 psi (92.9 Ibs) and 894 psi (143.1 Ibs). Both of them failed at the bond line. Of the four coupons which were aged 8 hours at 1 2OO0C, one fell apart at the bond line in the furnace, one fell apart in the box during transportation from the furnace, one fell apart during grinding, and one was successfully ground into an ECR coupon. An X-ray diffraction analysis was performed on the coupon which failed in the furnace to determine the composition of the failure surface. The tungsten side was predomi- nantly WC with some W&, and the graphite side was predominantly WC. Due to the poor strengths exhibited the ECR test was not performed. The results are summarized in Table 11.

Table 11. Summarized Results For Bare Diffusion Bond Coupons

Desi n Graphite Bondins Tungsten Aged Assessment Tabk Type Cycle Thickness

(mils)

Results

4 8826 1 1 No SEM No porosity 4 8826 1 3 No Tensile 581 psi 4 8826 2 1 No Micrograph No porosity 4 8826 2 3 Yes Tensile Failed in machining 4 8826 3 1 No Micrograph No porosity 4 8826 3 3 Yes ECR Cancelled 4 8826 4 1 No SEM No porosity 4 8826 4 3 Yes Tensile Failed in box 4 8826 5 1 No Micrograph No porosity 4 8826 5 3 No Tensile 894 psi 4 8826 6 1 No Micrograph No porosity 4 8826 6 3 Yes Tensile/XRD Failed in furnace,

WC and W& formec * See Table 4 for an explanation of bonding cycles

The sputtered Co coupons which had been bonded above the melting point of the Co, 1495OC, showed aflowing of Co with dissolved tungsten grains in the flow (Figure3). The lowertemperature bond made at 120OOC showed an unusual alloy of W-Co-C. The 1 mil tungsten foil expanded to 1.4 mils, as the Co and the C combined with it. Pools of almost pure Co formed at the midline of the foil for no obvious reason, since based on diffusion the Co should have higher concentrations at the edges of the tungsten, not the middle. Figure 4 shows a SEM photo of the bond. The one coupon which was tensile tested in the as fabricated condition failed in the graphite at 2221 psi

- 15-

Figure 3. Co Bond Made Above Melting Temperature

\ --

v L - c ,

L -7 IC

- t

Figure 4. Co Bond Made At 12OO0C

- 16-

(355.4 Ibs). The results are summarized in Table 12. Additional study on sputtered Co was sus- pended because the sputtered Mo results were far more promising.

Table 12. Summarized Results For Sputtered Co Diffusion Bond Coupons

Desi n Graphite Bondin@ Tun sten Aged Assessment

fmilsl a Tabe B Type Cycle Thic ness

Results

5 8826 1 1 No Micrograph Melted Co 5 8826 1 3 No None --- 5 8826 2 1 No SEM W-Co-C alloy 5 8826 2 3 No Tensile 2221 psi 5 8826 3 1 No Micrograph Melted Co 5 8826 3 3 No None ---

* See Table 5 for an exdanation of bondina cvcles

The sputtered Mo coupons had high strength and low ECR, even after aging. The results are sum- marized in Table 13.

The coupons which were made with 8826 graphite (Table 6) all showed no porosity. The coupon which had been bonded at 12OO0C was tensile tested in the as fabricated condition, and failed in the graphite at 2334 psi (373.4 Ibs). The other two coupons with 3 mil tungsten foil were aged 8 hours at 1 2OO0C, and one was tensile tested and the other was ECR tested. The tensile test failed at 2518 psi (402.8 Ibs) in the graphite, and the ECR test measured resistances of 3.5 pa-cm* at 6OO0C, and 6.5 pa-cm2 at 1 OOO°C. The optical analysis of the coupons showed that all of the bond cycles resulted in bonds which were essentially the same, so the tensile and ECR results from the cycles other than the 12OO0C one were taken to apply to it also.

The coupons which were made with 9RRL graphite (Tables 7 and 8) were all tensile tested after aging, and all of them failed in the graphite. The failure load was somewhat lower than the failure load for the 8826 coupons, showing a difference in the strengths of the graphites but not necessari- ly a difference in the strengths of the bonds. The results were similar with solid 3 mil tungsten or perforated 1 mil tungsten, showing that the perforations do not harm the strength of the bond suffi- ciently to have any significance. This set of coupons verified the strength results from the 8826 coupons.

The coupons which were made with UT-87 graphite (Table 9) were all tensile tested after aging, and all of them failed partly in the epoxy, partly in the graphite. The failure loads were all over 2000 psi, so they met the strength requirements even though the bonds were not tested to their actual ultimate strengths. After this happened the procedure to bond the coupons to the tensile studs was improved to insure that the epoxy would not fail at such low loads. Although the coupons could have been reground and retested they were cancelled, because UT-87 had been removed from the TA Cell design and the information was no longer needed.

c

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Table 13. Summarized Results For Sputtered Mo Diffusion Bond Coupons 1

Desi n Graphite Bondint Tun sten Aged Assessment Tab8 Type Cycle Thic 9( ness

(mils)

Results

6 8826 1 1 6 8826 1 3 6 8826 2 1 6 8826 2 3 6 8826 3 1 6 8826 3 3

No SEM No porosity Yes Tensile 2518 psi No Micrograph No porosity No Tensile 2334 psi No Micrograph No porosity

3.5 pn-cm2 6.5 hn-cm2 8 p8&% Yes ECR

7 SRRL 3 Yes Tensile 1944 psi’ 7 SRRL 3 Yes Tensile 1671 psi1 8 SRRL 1 (perforated) Yes Tensile 1705 psi1 8 SRRL 1 (Derforated) Yes Tensile 1903 psi1

9 UT-87 3 Yes Tensile 2341 psi2 9 UT-87 3 Yes Tensile 3659 psi2 9 UT-87 3 Yes Tensile 281 7 psi2p3 9 UT-87 3 Yes Tensile 3843 psi2

* See Table 6 for an explanation of bonding cycles Failed in graphite

2 Failed partly in epoxy 0.391 ” by 0.4” coupon

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7.0 CONCLUSIONS

The ZrVri brazed bond would require additional development to determine whether the sputtered Mo barrier is needed with the rolled braze material, and what the actual strength and ECR behavior would be. The sputtered Co barrier diffusion bond produces an alloy of W-Co-C which, while possibly meeting the bond requirements, would require additional development to understand it. Both of these development programs have been suspended due to the success of the sputtered Mo barrier diffusion bond. The best method for bonding tungsten foil to graphite is to sputter Mo on the bonding surfaces, then to perform a diffusion bond at 12OO0C at 2000 psi for 4 hours. Such a bond meets both the strength and ECR requirements after aging to simulate the cell fabrication process. The bond may also be used on an already partially assembled module sincethetemperature is only 12000C. The experiments described in this PIR established the feasibility of the bond and produced preliminary strength and ECR values. More vigorous testing should now be performed, with a much larger number of coupons, to give more reliable strength and ECR values and information about the op- erational life of the bonds.

8.0 ACKNOWLEDGEMENTS

I would like to thank Tom Peters for metallography, Earl Feingold for SEM/EDS/XRD, Ed Burten- shaw for tensile test, Jack Hanson for ECR test, and Jim Cooper and Dominick Centurioni for fixtur- ing. Special thanks to Jaime Reyes, Mike O'Leary, Don Bloyer, Paul Gorsuch, Larry DeFillipo, Bob Schreiber and Wanda McGlinchey for answering my many questions.

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Appendix A PROCEDURE FO R BOND ING/DEBOND ING COUP0 NS TO TENS ILE STUDS

1. Prepare studs.

Screw a bolt into the hole to keep grit out of the threads. Grit blast clean the bonding surface of two studs (figure A l ) using the Empire Equipment Abrasive Corp. Model MH 3040 S grit blast-

Figure A1 . Tensile Studs

Threaded .,2-18-2B Hole I

er in 23848 (preferred), or the S.S. White Airbrasive 6500 System grit blaster in 28B32. Wipe surface with acetone on a cotton swab to remove residual dust.

2. Apply epoxy to bonding surfaces

Spread as thin a continuous layer as possible of epoxy (Hysol Aerospace Products, Dexter Adhesives and Structural Materials Division, EA 9432 NA, Lot 0290-1) on all bonding surfaces with a small spatula.

3. Place studs and coupon in fixture.

Stack one stud, the coupon, then the other stud vertically against the V groove in one of the halves of the graphite bonding fixture (figure A2). Center the coupon in line with the studs as well as is reasonable visually. Install four graphite screws to hold the fixture closed, with the bottom two screws snugged finger tight, to keep the studs from sliding out the bottom, and the top two screws slightly loose, to allow the top stud to slide to apply pressure to the bonds.

4. Cure the epoxy.

Place the bonding fixture vertically in either the GCA Corp., Precision Scientific Group Mechani- cal Convection Oven, Model 18 EM, in 22B35 or the Grieve Laboratory Oven, Model LW-201 c, in 28632. Cure for 2 hours at 125°C. After fixture cools remove fixture.

5. Perform tensile test.

6. Carburize the epoxy.

Place the studs with epoxy and residual attached pieces of coupon in the Lindberg Laboratory Box Furnace, Model 51 894 in 22835. Bake for 1 hour at 350°C (set temperature control to 310°C to get the correct temperature).

7. Remove coupon.

Coupon may be twisted, pried, of scraped off.

- 2 0 -

c

~~~

I

0 0

0 0

Figure A2. Graphite Bonding Fixture

.05" 1.04"

4 ------------- - 1 SO"---

* * .261" -D .50" 3 (0 .an 4

U @ Graphite screws

Side 2

Side 1

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\ . ,

DISTRIBUTION

D. Bloyer

J. Bond

D. Centurioni

R. Cockfield

J. Cooper

L. DeFillipo

D. Elliott

E. Feingold

I? Gorsuch

V. Haley

J. Hanson

D. Hill

T. ffill

J. Leistensnider

D. Matteo

W. McGlinchey

M. O’Leary

T. Peters

J. Reyes

R. Schreiber

1 RS1 Library (2)

SP-100 Library (2)