CA2066460A1

CA2066460A1 - Coating for ceramic composites

Info

Publication number: CA2066460A1
Application number: CA002066460A
Authority: CA
Inventors: Jane S. Copes
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1991-04-12
Filing date: 1992-04-21
Publication date: 1994-12-09
Also published as: DE4215114A1; JPH0733564A; FR2709124B1; FR2709124A1

Abstract

ABSTRACT

A coating composition for use with ceramic composites to reduce gas permeability of the, composites as well as provide an adhesive force to the composites. The coating composition comprises an aqueous dispersion of an aluminum phosphate precursor, silicon carbide, and alumino-borosilicate.

Description

206~60 ` I

PA~ENT
COATI~ ~oa C~A~TC CQ~PO~ITB~

FIEL~_P~ Llay~ ON
This invention relate~ to a ceramic coating ~ox high-tempera~ure 6ilicon carbide ceramic composites used in gae-~ired radiant burner tubes, gas burnex -~
nozzle liners, heat exchanger6, and other furnace components. The ceramic coating of the present 10 invention ~ubstantially matches the thermal expansion coefficient o~ the compositQ6, thus re~ulting in a gas-impermeable composite that still maintains its toughnes~. The invention also relates to using the ceramic coating to bond ceramic composites together.
BACXa~O~N~ QF ~ INVENTION
Furnace components such as radiant burner tubes must be able to withstand high temperatures and corrosive environmentg in industrial heat-treating and 20 in aluminum melting furnace~. Commercially-availabla burner tubes operate in the range from about 900COC to about 1250C and are generally metal alloy tubes, - ~ -ceramic monolith tubes, and ceramic composite tubes.
Of the first type, niakel-based superalloy metal tubes 25 are commonly used, but are limited to the lower temperature range of ~00-1100aC. of the second type, -~
monolithic silicon carbida radiant burner tubes are commonly used and generally have temperature capabilities up to about 1250C but suffer from the 30l brittle failure problems typical of monolithic ceramic shapes. Furnace components, used in very high temperatureE and in corrosive environments, require a special ~election of ~aterials to avoid chemical and mechanical di~integratlon o~ the ceramic.
35 Ceramic-ceram~c compoeites, using ceramic fibers and cloths as rein~orcements in a oeramic matrix, are the ~ --third type o~ tube and are frequently the most ~

~0~6460 desirable choice ~or u8e in high temperature, chemically-corroslv~ environments.
OnQ type of commerci~lly-available radiant burner tube i~ produced under the trade de~ignation "SICONEX
5 FIBER-REINFORCED CERAMIC,~ and is commercially available from the 3M Company of St. Paul, Minnesota.
"SICONEX FIBER-REINFORCED CERAMIC" radiant burner tube i8 a ceramic-ceramic compo~ite comprising aluminoborosilicate fiber~ in a silicon carbide matrix.
10 "SICONEX FIBER-REINFORCED CERAMIC" radiant burner tube is prepared by first forming a tube or other shape of alu~inoboro3ilicate ceramic fibers (e.g., a tube or other shape made of alu~inoboro~ilicate fibers marketed under the trad~ d~signation "NEXTEL CERAMIC FIBERS" by 15 the 3M Company) by braiding, weaving, or filament-winding the ceramic fibers. The ceramie fiber shape is treated with a phenolic resin to rigidize it, and then coated via chemical vapor deposition at temperatures ranging from 900 to 1200C to produce a 20 relatively impermeable, chemically-re~istant matrix of a refractory material such a~ beta-silicon carbide.
The resultant rigid ceramic compo-~ite is then useful at high temperature~ and in corro~ive environments.
However, the utility of these materials as furnace 25 compon~nts can, depending on the degree of thelr permeabllity to gases, b~ ~omewhat limited.
Ceramic-ceramic composites ~uch as those marketed under the trade designation "SICONEX" by the 3~ Company are comprised o~ relatively open networks of ~ibers and can 30 remain permeable to gases, even after ex~en~ive overcoating with a ceramic (e.g., silicon carbide) layer.
Whlle there have been many approache~ to sealing cera~ic comp~site sur~aces, the~s attempts have not 35 been coupled with ~ufficient matching o~ chemical, thermal, and mechanical properties of the coatin~ to achieve adequate thermal and chemical behavior at 2066 16~
~ 3 extreme temperatures and rQaction conditions. Thermal expansion coefficient matching i8 e~pecially critical due to th~ alevated temperature~ of use and repeated thermal cycling in typical ~urnace application Previou~ work in this field generally i8 direct~d to coating, sealing, or adherin~ refractory materials. ~ ;
U.S. Patent No. 4,358,500 and related U.S. Patent No.
4,563,219 dQscribe a composition for bonding refractory materials to a porous base fabric such as fiberglass, 10 using a ~oating compri~ed o~ colloidal silica, monoaluminum phosphate, and aluminum chlorohydrate.
The coating provides heat and flama protection to the fibergla~6 fabric.
U.S. Patent No. 4,507,355 describes an inorganic 15 bind~r prepared from colloidal silica, monoaluminum - -phosphate, aluminum chlorohydrate and a catalyst of alkyl tin halide. Thi~ mixture is applied to the preferred substrate f~bergla6s to ~orm a heat-resistant ~abric. -~
U.S Patent 4,5g2,966 teaches a method of strengthening a substrate (fiberglass or fiberglass composites) by impregnating the substrate with, for example, aluminum or magnesium phosphat~, magnesium ~ ~
oxide, or wollastonite, and a non-reactive phosphate. ~-25 This i~ deRcribed as a cement which lends ~trength to the fiber substrat0.
U.S. Patent 4,650,775 describe~ a thermally-bonded fibrous product wherQin aluminosilicate fibers are ~onded together with silica powder and boron nitride 30 powder. These mixtures can bs formed into different shapes and us~d as diesel 500t filters, kiln furniture, combu~tor liners, and burner tube~.
U.S. Patent No. 4,711,666 and related U.S. Patent No. 4,769,074 describe an oxidation prevention coating 35 ~or graphite. A bind~r/0usp~nsion of colloidal silica, mono-aluminum phosphatQ and ethyl alcohol is applied to a graphite surfac~ and prevente oxidation during heat cycli~g.
U.~. Patent No. 4,861,410 de~cribes a mathod of ~oining a metal oxide ceramic body such a~ alumina with a paste of a 501 of a metal oxide, aluminum nitrate,and 5 silicon carbide. This method is used to repair cracks in ceramic materials and to permanently join ceramic structure~ togeth~r.
Silicon carbide-cerAmic fiber composites would benefit greatly from a aoating that would protect the 10 composite~ in high temp~rature and corrosivQ
environment~. To be most effective for high temperature u3e3, the coating needs to match the thermal expansion coe~ficient of the composite.
In US~8 which requir~ minimal trans~er of gases 15 through the walls, the coating needs to reduce the permeability of the silicon carbide-ceramic fiber composite. A further nead in this field is the ability to ad~oin ceramic composite pieces together or to patch holes in the composite article To date, there has not been a coating composition which matches th~ thermal expan6ion coefficient of an aluminoborosilicate fiber-silicon carbide-coated composlte under high temperature conditions, limits gas permeability and can be u~ed to adjoin the 25 aforementioned composites together. The increased use of ¢eramic compo~ite6 in high temperature and corrosive environments creates a need for a coating composition with the above attributes.

30 SUMMARY OF THE INV~NTION
The preBent invention provides a f ired ceramic composite comprising: (a) a base fabric o~
aluminoboro~ilicate fibers; (b) a carbonaceous layer coated on tha base fabr~c; (c) a silicon carbide layer 3S coated over the carbonaceous layex; and (d) a mixture comprisinq ~ilicon carbide and aluminum phosphate having a molar ratio o~ ~ilicon carbide to aluminum .. ~,,, ..... ~ .. . ~ . .

206~l~a pho~phata in the range o~ about 1:1 to 50:1 and aluminoborosilicate partiole~ in the weight range o~ -about 0.5 to 70 weight percent of the total mixture, coated over the Bilicon carbide layer.
The ceramic composit~ iB formed by coating a silicon carbide coated composite of aluminoborosilicate fibers with a ceramic precursor coating comprised of an aqueous su6pengion of an aluminum phosphate precursor, flakes or chopped fibers of aluminoborosilicate and ~ -10 silicon carbide powder, flakes, or fibers. Typically, the composite article according to the present invention i5 impermeabl~. The term "impermeable" is meant to denote a coating which is substantially impermeable to gases passing through the coating. The 15 coating can be applied by spraying, dipplng, or brushing. ~he coating i~ dried in air and then fired to form a hard and durable coating. -By application of this coating, the strength of the ceram~c composite, as ~easured by internal 20 pressurization to failure, i6 equal to or slightly higher than that of an uncoated composite tube. Thi~ ;
behavior is an important 5 indicator of the composite ~-character of the final coated structure. It is particularly desirable to avoid ~iring or reacting 25 compo ite material~ to a point at which the compo~ite actually takes on the characteristic~ of a monolithic ~tructurQ. In a practical enBe ~ the re~ult of monolithic bQhavior i~ a dramatically increased brittlene~s of the material; hence, monolithic 30 structures are dramatically less effective for uses which sub~ect the material to mechanical stress. A
ceramic composite having th~ coating of the present invention result~ in a tough ~tructure and not a ~- ;
monolithic ~tructure. The coating may also ~e used as 35 a bond coating which ~ecures two ceramic substrates, particularly tubes, together.

2~6~ -~6~

In accordancs with this invention, ~ilicon carbide ceramic compo~ites are coated with an aqueous Buspension of monoaluminum pho~phate (Al(H2PO4) 3 flakes 5 or chopped fibers of aluminoboro~ilicate, and silicon carbide powder. The coating iB most easily applied by brushing it onto the composite surface, although other application method~l such as dip coating or spraying, could be used. once the coating is applied to the 10 composite, it i8 allowed to air dry, and then fired to about 1000C to form a hard and durable ceramic coating.
There ar~ many sil~con carbide ceramic composites which could be used in con~unction with the coating 15 compositions o~ the present invention. One brsnd of composite i~ the afore-mentioned "SICONEX
FIBER-REINFORCED CERAMIC," commercially available from the 3M Company, St. Paul, Minnesota. These composites are formed by first braiding, weaving, or filament-20 winding ~ibers of aluminoborosilicate (sold under thetrade designation "NEXTEL 312 CERAMIC FIBER," having an alumina:boria mole ratio of from 9:2 to 3:1.5 and containing up to 65 weight percent silica, as described in U.S. Patent 3~795,524, assigned to the 3M Company) 25 to form a de~ired shape, such a& a tube. The tube is coated with a phenolic resin in an organic solvent to rigidiz~ ~t and thereaftsr coated with silicon carbide via chemical vapor deposition.
The coating of the present invention is comprised 30 of silicon c~rbide, aluminum phosphate and :
aluminoborosilicate. An available source of silicon carbide i8 available as fine abrasive powder, commercially available from Fu~imi Kenzamaki Kogyo Co., ::
Inc., Nagoya, Japan. Other forms of silicon carbide 35 include flakes or fiber~. In the preferred embodiment, 1-50 micrometer diameter silicon carbide powder is used.

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2066~6~
.

The precursor aluminum phosphate present in the coating i~ prepared by discolving aluminu~ metal in pho~phoric acid. A solution, 50 weight percent of Al(H2P0~)3 in water, iB available from Stouffer Chemical 5 Company, Westport, Connecticut. A~ the coating is fired, water and a portion of phosphate are released from the aluminum phosphate solution. Thus, aluminum phosphate is le~t after firi~g. The mole ratio of silicon carbide to aluminum pho~phate (SiC:AlP04) in the 10 fired coating i8 preferably in the range of about 1:1 to 50:1. Most preferably, the mole ratio o~ SiC:AlP04 in a fired coating is in the range of about 5:1 to 30:1.
Aluminoborosilicat~ i~ al60 added to the coating 15 composition. This may be in ~he form of powder, flakes or fibers. Preferably, aluminoborosilicate in the ~orm of fibers iB used. Such ~ibers are commercially available under the trade deeignation "NFXTEL CERAMIC
FIBER" from the 3M Company. The ceramic ~iber yarn 20 ranges in diam~ter from 11 to 15 micrometers and is chopped by passing the yarn between two steel roller~
with knurled surfaces.- other methods of chopping include ball milling or other methods known in the art.
The yarn i~ chopped to an average fiber length of about .02 to .05 mm. The weight percent of the aluminoborosilicate of the total coating composition is in the range of about 0.5 to 70% and, preferably, in the range o~ about 30 to 70%.
To fashion the ceramic composites for testing the ~ ~-30 di~ferent coating compositions of the present invention, caramic fiber braid (co~mercially available under the trade deRignation "NEXTEL CERAMIC FIBER
BRAID" from the 3M Company) was fit onto a 5 cm di~meter aluminum mandrel, and a solution of 10 ml of - ~-35 phenolic reYin ~60-64% solids, commercially available --under the trade designation "D~RITE SC-1008 PHENOLIC : .

206G -~6a RESINI' from Borden Chemical, Columbu~, Ohio~ in g0 ml of methanol was prepared. A small amount of the resin solution was poured over the ceramic fiber tube while rotating tha mandrel, to a~ure uniform coverage by the 5 resin. The tube was then dried in air until ~olvent odor could no longer be detected, and then cured in air at 200OC for 20 minute3. Thi6 process resulted in a rigid tu~e having a golden color due to the cured polymer layer.
The rigid preform was placed in a chemical vapor deposition chamber, as i8 well known in the art, wherein vacuum is applied and the chamber is heated.
Hydrogen gas was bubbled through dimethyldichlorosilane (DDS) and passed through the CVD furnace chamber, 15 thermally decomposing the DDS which thereby deposited a layer of silicon carbide on the preform. By-product and unreacted gase~ exited the opposite end of the tube to the vacuu~ pumping and ~crubbing system. Typical process conditions for the~e sample~ were pressure~ of 20 5 to 50 torr, flow rate~ of 0.15 liters per minute(lpm) of DDS, and 1.0 lpm of hydrogen ga~ at temperatures of ~ -900 to 1000C. Coating ti~e6 ranged from 4 to 8 hour Under these proceqs condition6 and times, the samplea received fro~ about 100 to about 200 weight 25 percent increase due to silicon carbide deposition. In this process, SiC coats and infiltrates the fibers and the re~in coat i8 also decomposed to ~orm a carbonaceous layer on the surface of the prefor~. It is use~ul to examine the fractured surfaces of broken 3q composites made in the abov~ manner. The fractured surfaces re~ulted in a "bru~hy" fracture surface which ~ndicates that the coated material has composite rather than monolithic properties, and that heating and proces~ing 8~ep8 have not destroyed the desired 35 composite prop~rti~s.
Coupons of a fib~r-rainforced ceramic (commercially available under the trade designation , . .
;

~ 2~66~160 g :. .
"SICONEX FIBER-REINFORCED CERAMIC" from the 3M company~
were prepared in a manner ~imilar to the tubes, using woven ceramic fiber (commercially available under the trade designation "NEX~EL 312 CERAMIC FI~ER'I ~rom the 5 3M Company) fabric. Coupons were convenient for carrying out initial studias of coating fea~ibility and were more convenient to u8e in order to examine the adhesion and hardne~s of ths coating. Adhesion of the coating on an exposed edge and the performance of the 10 coated edge are also importan~ indicators of the coating performance.
Many sizes of tubes of the ceramic-ceramic composite were coated and tested. Permeability of the final fired tubes was te~ted by a differential flow . ~:
15 test using a flow meter.
Though not being bound by theory, it is believed that the coating work~ to maintain the composite characteri6tic6 o~ its composite substrate as well a~
to match the thermal expansion coefficient of the : -20 substrate (which is important in furnace and high temperature application~) because th~ coating itself is a composite material, being compri6ed of flake or ~ibers and particles in a matrix. The flake~, fibers, and particl2s act to fill the porous site~ in the 25 matrix, thereby blocking the ~low of gas through the porous sit~s. Further, this discontinuous phase also deflects cracks that may initiate in the coating ~rom mechanical or thermal 6tress~s.
' 30 Ex~mpl~ 1 Aluminoborosllicate ceramic fiber (commercially available under the trade designation "NEX~EL CERAMIC
FIBER" ~rom the 3~ Company, St. Paul, Minnesota) ranging ln diameter from 11 to 15 micrometers was 35 chopped by passing the ~eramic fiber yarn between ~wo ste~l rollers with knurled surface~. This resulted in ~hopped ~iber~ with an average length of about 50 .J~"' 2 ~ 6 ~

micrometer3.
To a 50 percent by weight 801ut~0n 0~ monoaluminum phosphate, (Al(H2POj)3 oommercially available from Stauffer, We~tport, Connecticut) was added ~licon 5 carbide powder (#1500, 8 micron, commercially available from Pu~imi Kenmazai Kogyo Co., ~td., Nagoya, Japan) and chopped aluminoborosilicate ceramic fiber (commercially available under the trade de~ignation "NEXTEL 312 CERAMIC FIBER" from the 3M Company).
10 Deio~ized water wa6 added to some mixtures to adjust the consistency for coatability. Table I shows compositions representing approximately 40-70% fired solids and mole ratios of SiC to AlPO4 in the fired product o~ from about 5 to about 20 TAB~ I COATING ~OMPOSITIONS
c~mponent ma~s.g moles SiC:AlPO1 ~ fired solids a. Al(H2PO4)32.9 6 40 :
SiC powder1.1 alumino-boro~ilicate fibar ("N~XTEL
CERANIC :
FIBER") 1.3 deionized water 2.0 b. Al(H2PO4)345.0 6 50 SiC powder 16.3 alumino-boro~ilicat~
~iber ("NEXTE~
CERAMIC
FIBER") 11.4 deionized water ---2~f~6~

.

c. Al(H2PO4~3 50-0 6 55 SiC powder 1~.
alumino-boro~ilicate fi~er ("NEXTEL
CERAMIC
FIBER") 21.7 deion~zed water 15 d. Al(HlPO4)3 5.0 16 69 SiC powder 5.Q
alumino-borosilicate fiber ("NEXTEL
CERAMIC
FIBER") 5.0 deionized watQr 1.0 :
e. Al(H2PO4)3 1.5 20 47 :
SiC powder 1.9 alumino-boro~ilic~te fiber (~NEXTEL ~ :
CERAMIC
FIBER") 1.9 deionized water 4.0 Tube-shaped ~iber-reinforced ceramic (co~mercially available under the trade de~ignation "SICONEX FIBER-45 REINFORCED CERAMIC" from the 3M Company) samples weredipped in, or painted with, each coating formulation, typically ~n only one pa~s. Coated parts typically ::
weighed 10 to 20% more than the weight of the original -~
part and had a coating thickne3s of about I mm. The 50 coated parts were allowed to dry at ambient temperature -~

2~6~6a ,,~ . .

and humidity for 24 hours and then were gired in air by ramping th~ temperature at 250C per hour to 1000C, and holding for 1 hour. Th~ coatings were hard and durable as indicated by attempting to remove or crack 5 the coating by scratching the surface with a steel needle. Intact ceramic fibers and particIes o~ SiC
could be seen by examination under a microscope at 50X
magnification. X-ray diffraction powder patterns of the fired coatings showed beta-SiC, mullite, and ALP0 4 10 as crystalline phases.

Bxa~ple 2 In order to test the per~eability of a sample before and a~ter coating, tube-shaped samples were 15 used. Through-wall permeability of two tubes (5.0 cm outer diameter x 20.0 cm long) was measured by closing each end o~ the tube with a one-hole stopper, and flowing air through the tube. Air at a regulated pre~sure of 1 atmosphere (1.03 Kg/cm2) was admitted 20 through a needle valve and monitored by a flow meter at the inlet end of the tube. A ma~ometer at the exit end of the tube measured the difference in pressure between the inside of the tube (pressurized air flowing through it) and the outside of the tube (room pressure). For a 25 particular pre6sure drop, the air flow in cm3/min is read from the ~low meter. This flow rate, divided by the sur~ace area of the tub~, is permeability (cubic centimeters per minute per ~quare centimeter).
A coating of 55 weight percent fired solids and a 30 6:1 SiC:AlP04 mole rat~o (as per Example lc) was applied to the outside surface of the tubes. The wet coating wa6 12 to 15% of the original part weight. After air drying, the tubes wera fired to 1000C. ~he tubes were weighed and perme~bility checked again. Table II shows ~ -35 weight and permeability changes: ~

20~6 ~
,~ ~

TABLE II PERMEABILITY DATA
. permeabil~ty tubewei~ht(om~ %wt.aain(cm3minlcm-coat~d coated uncoated & fired uncoated ~_~iEÇ~
177.62 85.19 9.8%132.0 1.2 295.80 103.86 8.610.2 ~.02 ~ .
10 Gas permea~ility wa5 reduced by a factor of approximately 100 for tube 1 and a factor o~ 500 ~or tube 2.

~x~mple 3 15Two 5.0 x 20.3 cm fiber-rein~orced ceramic . :
compo~lte tube~ ("SICONEX FIBER-REINFORCED CERAMIC'I) ~-:
were coated as described in Example 1 with the coating formulation of Example lc (designated A in Ta~le III, below), and two tube~ with no coating (designated B in 20 Tabl~ III) wer~ ~ired together to 1000C for 1 hour.
All tubes were cut into 2.5 c~ long ring~ in order to do strength te~ting.
Additional &ample~ were prepared to evaluate-the ~:
coating a~ an edge protector for the fiber-reinforced 25 ceramic. Three 15.2 cm (6'l) #amples were cut from one 5.1 by 45.7 cm (2"x18") tube and treated a~ follows~
Sample C (end~ of 15.2 cm piece coated, heat treated to 1250C for 10 hour~), Sample D (cut into 2.5 (1 inch) samples, cut edge coated, heat treated at 1250C for 10 :~
30 hour~), and Sample E (cut into 2.5 cm (1 inch) samples, .
heat treated at 1250C for 10 hours).
Burst ~trength was ~easured on 2.5 cm (1 inch) : :
rings from all tubes by internal pres6urization to ~ailure (burst test); average 5 re6ults of the samples 35 are shown in Table III.
,: .

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2066~6~

TA~LE III
STRENGTH DATA
bur~t str~n~th trea~ t aver~ge. MPa (p~ t. dev.
5 1000C, 1 hr.
A coated 65.9 (9420) 3.8 ~540) B uncoated 64.6 (9230) 10.3 (1470) 1250C, 10 hr.
C HT. as pi~c~, cut 56 ~8000) 6.4 (920~
10 D cut, edge coated, HT. 47.9 (6840) 7.6 (1080) E cut, no coating, HT. 37.9 (5420) 4.7 (670) In comparing ~ample~ A and B, the burst strength of the samples ~hows some improvement after coating.
In the data ~or Sample C (15 cm-long sample, heated, sectioned, and tested) and E (8iX 2.5 cm ring samples, heated, and teated), it appeared that cutting samples before heat treating resulted in a loss of strength o~ about 33% with uncut sample~. Cut ~ampls~
20 which were al~o edge-coated (Sample D) suffered only about a 15% strength loss. Fracture surface of Sample~ C and D are "brushy" (meaning individual fibers are visible and have not fused to~ether during heat treatment) and composite-l$ke, while fractured samples 25 of E were quite brittle with le6s evidence of fiber pull-out. Although not intending to be held to any theory, it is speculated that unprotected edges allow oxygen to penetrata into the interface between fibers and the matrix. oxidation within the matrix is 30 suspected to result in bonding between the fibers and ~
the matrix and; thus, brittle fracture behavior -results. ~-: ~ ' Ex~pl~ 4 -Thrse coating formulation~ were prepared a~ ~ -de~cribed in Example 1 with the formulation of Example ld, except that the particle size of the SiC wa~
' '' ~ - ; , . ... , .... - .. ~ . .. .. . . . .

2066ll6~ ' :

varied. Th~ particle size~ were one micron, 8 mlcron, and 50 ~icron SiC powd~rs, commercially available ~rom Fujimi Kenmazai Kogyo Co. Ltd., Nagoya, Japan. Small fiber-reinforced ceramic (I~SICONEX FIBER-REINFORCED
5 CERAMICI~) composit~ samples werQ painted with the coatings and fired first to 1onooc for a period of one hour at a heat-up rate of 250C/hour and then to 1200C
for a period of one hour. Each 6ample was hard and durable as indica~ed by visual inspection after 10 attempting to remove or crack the coating by scratching the surface with a ~teel needle. Thus, a wide range of silicon carbide particle ~ize~ and a wide firing temperature range produce acceptable coatings.

15 Ex~ple 5 This Qxample shows how the coating compo~ition3 can be used as adhesive~ to join two samples together.
To te~t for shear strength of the coating when used as a bonding agent, 2.5 cm-long fiber-reinforced ceramic 20 tubes ("SICONEX FIBER-REINFORCED CERAMIC") of two different diameters were used t5 cm and 4.4 cm in outer d~ameter).
The tubes were ~oined together by fitting the smaller diameter tub~ part-way into the larger tube, 25 6uch that th~ smaller diameter tube pro~ected 1.25 cm out of the larger diameter tube. A 1.25 cm band of coating (70 weight% solids) was placed on the outer surface o~ the æmaller tube, and then a 1.25 cm wide piece of aluminoborosilicate ceramic fiber tape 30 (commercially available under the trade designation -"NEXTEL 312 CERAMIC FIBER TAPE" from the 3~ Company) was placed on the coating. Additional coating was added to the tape, and then the tube with the coating and the ceramic fiber tape was fitted into the larger 35 tube.
Addi~ional coatlng was added to ~ill the gap between the two tubes. ~his bonded piece was dried for : i .. ,, ~ ~ .- , : : : . : ~ :.;; -.~ . ~ - ~ : - -.,, , ~ . . ~ ~
, ~ , , 2066~60 24 hours at ambient temperature and humidity, heated for lo hours at 110C, and ~ired for 2 hours at 1000C.
An axial co~pression te~t of the joined tubes was carried out. In thi~ test, pressure was applied to the 5 long axis of the ~oinad tubes to try to break the adhesive bond formed by the dried and fired coatlng betweQn the two tube6. Axial compression tests of fired tubes were carried out at .051 cm/min (.02"/min) crosshead speed with an In~tron Model 1125 load frame.
10 Joints tested in this way did not fail under a 1000 lb.
(455 Kg) load at room temperature. This indicates that the coating can be usQd e~fectively to join -fiber-reinforced ceramic composite tubes ("SICONEX
FIBER-REINFORCED CERAMIC COMPOSITE TUBES") together.
15 This is u~eful for making T- or U-shaped ~ubes, or for cases in which the tube diameter must change in order to fit another piece.
A further test of the bonding ~trength o~ the coat~ng was to rapidly cycle ~oined piece~ through a 20 heating and cooling sequenc~. Two 5-cm long by 4.4 cm ~ ~-diameter ~ib~r-rein~orced ceramic composite tubes ~"SICON~X FIBER-REINFORCED CERAMIC COMPOSI~E TUBES") were butt-~oined u~ing the coating composition prepared as de~cribed above. An outer sleeve of 5 cm diameter 25 and 2.5 cm long was addQd at the joint to further reinforce the butt-~oint. The assembled tube was dried and fired as described abova. The ~oined tubes were flame-cycle te~ted by heating the in~ide of the ~oined ;-tubes with the gas fla~e of a Meeker burner to a 30 temperature of approxi~ately 800C while cooling the -~
outside of the tube with a flow of compressed air.
ThQse heat cycle~ did not cause failure of the bonds.
Further heating of thi~ heat-cycled joint for 100 hours at 1000C in a~r caused no detectable strength change. --~` 2066`~6~

æxampl~ C
In order to show utility of ths coating ~ormulations as an adhe~$ve ~or patch~ng ~iber-rein~orced ceramic compo~it~ tubes (IlSICONEX FIBER- :
5 REINFORCED CERAMIC COMPOSI~E") composite part~
together, a coating with 70 waight% 601ids wa~ applied by brushing ~t onto a ~iber-reinforced ceramic composite tube ("SICONEX FIBER-REINFORCED CERAMIC
COMPOSITEII), drying in air ~or several hours, and 10 firing with a ga~-air torch of the kind typically uæed ~ ~:
for gla88 wor~ing. co~pon~ntB of the coating melted slightly, lightened in color, and then hardened.
Th~ coating i~, thu~, effective in attaching a :~
patch ts another fibQr-rein~orced ceramic composite ~ :
15 tube ("SICONEX FIBER-REINFORCED C~SRAMIC COMPOSITE" ) with a hole in it or in bridging small gaps or crack~
in fiber-reinforced ceramic compo~ite tubes (~SICONEX
FI8E~-REINFORCED CERAMIC COMPoSITE'I) in situations where the tubos are in need of repair and require spot 20 heat-treating. :

As will be apparent to those killed in the art, : :
variou~ other ~odif~c~tion~ can be carried out ~or the abov~ di~closure without departing from the spirit and 25 scope o~ the lnv~ntion.

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Claims

1. A fired ceramic composite comprising:
(a) a base fabric of aluminoborosilicate fibers;
(b) a carbonaceous layer coated on said base fabric;
(c) a silicon carbide layer coated over said carbonaceous layer; and (d) a mixture comprising silicon carbide and aluminum phosphate having a molar ratio of silicon carbide to aluminum phosphate in the range of about 1:1 to 50:1 and aluminoborosilicate particles in the weight range of about 0.5 to 70 weight percent of the total mixture, coated over said silicon carbide layer.

2. The ceramic composite of claim 1 wherein said aluminoborosilicate particles are chopped fibers.

3. The ceramic composite of claim 1 wherein said aluminoborosilicate particles are flakes.

4. The ceramic composite of claim 1 wherein said silicon carbide and aluminum phosphate molar ratio is in the range of about 5:1 to 30:1.

5. The ceramic composite of claim 1 wherein said weight range of aluminoborosilicate particles is about 30 to 70 weight percent of the total mixture.

6. An unfired coating composition useful for reducing the gas permeability of a silicon carbide coated aluminoborosilicate ceramic composite, said unfired coating comprising:
(a) a dispersion of aluminum metal dissolved in phosphoric acid;
(b) silicon carbide and aluminum phosphate in an amount sufficient to create a fired molar ratio range of silicon carbide to aluminum phosphate of about 1:1 to 50:1; and (c) particles of aluminoborosilicate dispered therein in the weight range of about 0.5 to 70 weight percent of the total mixture.

7. The coating composition of claim 6 wherein said aluminoborosilicate particles are fibers.

8. The coating composition of claim 6 wherein said aluminoborosilicate particles are flakes.

9. The coating composition of claim 6 wherein said mixture comprises silicon carbide and aluminum phosphate having a molar ratio in the range of about 5:1 to 30:1.

10. The coating composition of claim 6 wherein said weight range of aluminoborosilicate particles is about 30 to 70 weight percent of the total mixture.