C.H. Henager Jr. et al Journal of Nuclear Materials 367-370 (2007)1139-114 coating and joining technologies for fusion. There is with approximate viscosities of less than 10 cP using as either a coating or joining technology for Sic/ Joining was accomplished by slurry application Sic composites and a diverse set of technologies using a dropper with a nominal weight applied may be advantageous at this stage of our efforts in during curing at 423 K in moist air. This was fol- design of fusion power plants. However, a diverse lowed by pyrolysis with a nominal pressure of about set already exists in the form of preceramic poly- 1 MPa in air at 1473 K. One joint was also processed mers,reaction bonding, glass-ceramic seals, and at 1073 K in nitrogen without any applied pressure brazing methods [3]. In this paper we present Coatings were also synthesized using the same slur tional coating and joining methods and technol ries by dip coating onto SiC/SiC composite coupons that have advantages, as well as disadvantages, as well as on 316 stainless steel coupons. Coatings compared to previously developed methods. were pyrolyzed at 1473K for SiC/SiC and at 1073 K for the 316 steel. Joint strengths were tested 2. Coating and joining methods in single-lap shear geometry at ambient temperature Some of the coated steel coupons were exposed up to 2.1. Polysiloxane preceramic polymers 1000 h at 1073 K in air to determine coating integrity and oxidation protection Coated coupons were sec Preceramic polymers, such as polycarbosilanes tioned and examined using an SEM for microstruc and polysiloxanes, with inert and reactive fillers are tural information. Both Hexaloy SiC and SiC/Sic used for SiC/SiC joining technologies [7, 16-18]and coupons were joined and tested in this study have performed adequately as strong joints. Polycar bosilane, which converts to SiC, requires high 2.2. Solid-state displacement reactions temperature processing and inert handling, while polysiloxanes, which convert to Si-O-C, can be Previous research at Pnnl on solid-state dis- pyrolyzed at lower temperatures and can be handled placement reactions demonstrated that the reaction in air. Joints made from such materials exhibit between TiC and Si produced an interwoven struc strengths that range from 5 to 30 MPa when tested in shear. a known difficulty with preceramic poly ture of Ti3 SiC2, TiSi2, and SiC, with the majority phases being the ternary Ti3SiC2 and SiC [24-27 mers is the mass loss, which can exceed 50%, on This reaction was used to make joints from tape cast conversion to a ceramic phase. A slightly different approach uses a linear chain of polyhydrido. powder mixtures of TiC and Si powders, which were methylsiloxane(PHMS)as a precursor to a highly 99.99% purity having average diameters less than 45 TiC.Si crosslinked polysiloxane, by applying a catalytic 200 um thick and were cut to shape and applied chemistry approach developed at SRI International, between either Hexaloy Sic coupons or CVI SiC that has the advantage of much lower mass loss on ceramic conversion compared to other systems [19 composite coupons. Joints were formed by heating in argon to 573 K at 5 K/min and holding for 2 h 23] and pyrolysis occurs at temperatures as low as for binder burnout with a nominal applie&? 10 K 873K ed by heating to 1473k or 1673K PHMS, which is a low molecular weight low vis- min and holding for I h at 30 MPa applied pressure cosity liquid, is catalytically cured in-situ after its a Joints were tested in shear using a double-notch application and converts on heating in inert envi- shear and sectioned for SEM examination ronments to a silicon oxycarbide phase but can be modified by side group additions to produce a more carbon-rich oxycarbide phase [23]. For the work 3. Results and discussion here, however, PHMS with no side group additions is used and filled with SiC, Al, and Al2O3 powders, 3.1.Joining singly and in combination. The Sic powders are 0.7 um average diameter pure SiC, the Al powders Polymer slurry joints between Hexaloy Sic cou- were in the form of flakes 1-2 um in size, and the ons made with PHMS filled with SiC. Al/Sic Al2O3 powders were submicron diameter. Powder loadings were in the range of 40-60% by volume I Hi-Nicalon Type-S fibers from GE Power Systems with a 2D and were processed in the form of liquid slurries 5-harness satin weave architecturecoating and joining technologies for fusion. There is no single technology that can be applied uniformly as either a coating or joining technology for SiC/ SiC composites and a diverse set of technologies may be advantageous at this stage of our efforts in design of fusion power plants. However, a diverse set already exists in the form of preceramic poly￾mers, reaction bonding, glass–ceramic seals, and brazing methods [3]. In this paper we present addi￾tional coating and joining methods and technologies that have advantages, as well as disadvantages, compared to previously developed methods. 2. Coating and joining methods 2.1. Polysiloxane preceramic polymers Preceramic polymers, such as polycarbosilanes and polysiloxanes, with inert and reactive fillers are used for SiC/SiC joining technologies [7,16–18] and have performed adequately as strong joints. Polycar￾bosilane, which converts to SiC, requires high￾temperature processing and inert handling, while polysiloxanes, which convert to Si–O–C, can be pyrolyzed at lower temperatures and can be handled in air. Joints made from such materials exhibit strengths that range from 5 to 30 MPa when tested in shear. A known difficulty with preceramic poly￾mers is the mass loss, which can exceed 50%, on conversion to a ceramic phase. A slightly different approach uses a linear chain of polyhydrido￾methylsiloxane (PHMS) as a precursor to a highly crosslinked polysiloxane, by applying a catalytic chemistry approach developed at SRI International, that has the advantage of much lower mass loss on ceramic conversion compared to other systems [19– 23] and pyrolysis occurs at temperatures as low as 873 K. PHMS, which is a low molecular weight low vis￾cosity liquid, is catalytically cured in-situ after its a application and converts on heating in inert envi￾ronments to a silicon oxycarbide phase but can be modified by side group additions to produce a more carbon-rich oxycarbide phase [23]. For the work here, however, PHMS with no side group additions is used and filled with SiC, Al, and Al2O3 powders, singly and in combination. The SiC powders are 0.7 lm average diameter pure SiC, the Al powders were in the form of flakes 1–2 lm in size, and the Al2O3 powders were submicron diameter. Powder loadings were in the range of 40–60% by volume and were processed in the form of liquid slurries with approximate viscosities of less than 10 cP using cyclohexane as a solvent. Joining was accomplished by slurry application using a dropper with a nominal weight applied during curing at 423 K in moist air. This was fol￾lowed by pyrolysis with a nominal pressure of about 1 MPa in air at 1473 K. One joint was also processed at 1073 K in nitrogen without any applied pressure. Coatings were also synthesized using the same slur￾ries by dip coating onto SiC/SiC composite coupons, as well as on 316 stainless steel coupons. Coatings were pyrolyzed at 1473 K for SiC/SiC and at 1073 K for the 316 steel. Joint strengths were tested in single-lap shear geometry at ambient temperature. Some of the coated steel coupons were exposed up to 1000 h at 1073 K in air to determine coating integrity and oxidation protection. Coated coupons were sec￾tioned and examined using an SEM for microstruc￾tural information. Both Hexaloy SiC and SiC/SiC coupons were joined and tested in this study. 2.2. Solid-state displacement reactions Previous research at PNNL on solid-state dis￾placement reactions demonstrated that the reaction between TiC and Si produced an interwoven struc￾ture of Ti3SiC2, TiSi2, and SiC, with the majority phases being the ternary Ti3SiC2 and SiC [24–27]. This reaction was used to make joints from tape cast powder mixtures of TiC and Si powders, which were 99.99% purity having average diameters less than 45 lm with a TiC:Si ratio of 3:2. Tapes were about 200 lm thick and were cut to shape and applied between either Hexaloy SiC coupons or CVI SiC composite coupons.1 Joints were formed by heating in argon to 573 K at 5 K/min and holding for 2 h for binder burnout with a nominal applied pressure followed by heating to 1473 K or 1673 K at 10 K/ min and holding for 1 h at 30 MPa applied pressure. Joints were tested in shear using a double-notch shear and sectioned for SEM examination. 3. Results and discussion 3.1. Joining Polymer slurry joints between Hexaloy SiC cou￾pons made with PHMS filled with SiC, Al/SiC, 1 Hi-Nicalon Type-S fibers from GE Power Systems with a 2D 5-harness satin weave architecture. 1140 C.H. Henager Jr. et al. / Journal of Nuclear Materials 367–370 (2007) 1139–1143