CA2131016A1 - Process for increasing strength of structural ceramic materials by forming ceramic coating on surface and product formed thereby - Google Patents
Process for increasing strength of structural ceramic materials by forming ceramic coating on surface and product formed therebyInfo
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- CA2131016A1 CA2131016A1 CA 2131016 CA2131016A CA2131016A1 CA 2131016 A1 CA2131016 A1 CA 2131016A1 CA 2131016 CA2131016 CA 2131016 CA 2131016 A CA2131016 A CA 2131016A CA 2131016 A1 CA2131016 A1 CA 2131016A1
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- ceramic material
- coating
- structural ceramic
- coating solution
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
- C04B41/009—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone characterised by the material treated
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
- C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
- C04B41/4505—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements characterised by the method of application
- C04B41/455—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements characterised by the method of application the coating or impregnating process including a chemical conversion or reaction
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
- C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
- C04B41/52—Multiple coating or impregnating multiple coating or impregnating with the same composition or with compositions only differing in the concentration of the constituents, is classified as single coating or impregnation
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
- C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
- C04B41/81—Coating or impregnation
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
- C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
- C04B41/81—Coating or impregnation
- C04B41/89—Coating or impregnation for obtaining at least two superposed coatings having different compositions
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- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
- C04B2111/72—Repairing or restoring existing buildings or building materials
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Ceramic Engineering (AREA)
- Materials Engineering (AREA)
- Structural Engineering (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Ceramic Products (AREA)
- Silicon Polymers (AREA)
- Application Of Or Painting With Fluid Materials (AREA)
Abstract
A method is disclosed for increasing the strength of and repairing surface defects in a structural ceramic material which comprises coating the structural ceramic material with a tractable preceramic coating material capable of pyrolyzing to form a ceramic coating and heating the coated structural ceramic material to a temperature sufficiently high to permit pyrolysis of the preceramic polymer for a period of time sufficient to form a ceramic coating on the surface of the structural ceramic material.
Description
WO 93/21131 Pcr/lJs93/o329o ,........................................................................... :
213lol~
pR~OCF.SS FOR ll~lCR~lN('. STRF.NGTH OF STRUCTURAl.
CF-RA~I~C l~lATF.RlALS BY FORMlN(~. CERAM~C C(~ATlN(~. ON
SURFACF ANn PROI)UCT FORMF.D TH~FR~Y
E;ield of the lnvention The invention relates to the strengthening of structural ceramic materials by fonning a polymer-derived ceramic coating on the surface of the structural ceramic material. The invention additionally relates to a process for improving the strength of and repairing surface defects in struc~al ceran~c materials by coating thern with a polymer-denved preceramic layer and then conver~ng the preceramic layer to a ceramic coating. More particularly, the invention relates to a process for strengthening structural silicon nitride and alumina materials by coating them with a polymer-derived prece~mic layer and then conver~ng the pecaan~ic layer to a c~amic coating In addi~ion to strengthening stNctural cerarnic materials and repairing surface defects, the invention can replace elaborate and costly fine machining of ceramic materials.
Background of the Invention The use of manufactured struct-ural cerarnic products is compromised by the great rcduc~on in strength of the struc~l material due to surface and near-surface defects induced by p~ocessing and machining operations or inherent in the material itself.
Attempts to preserve the integrity of manufactured products, fo~ exampk, glass materials, have been made by coating the product with various materials. Yoshida et al., U.S. Patent No.
4,370,385 describes coating a glass vessel with an organopolysiloxane composition to provide a scuff-masking coating over the glass.
Wieczoqrek et al., U.S. Patent No. 4,409,266, provides a shatteIproof coating on glass by applying to the glass surface a silane adhesion promoter and a two-component system which rea~s to f~n a polyurethane binder.
Kurita et al., U.S. Patent No. 4,656,221, conceals graze marks on a glass bottle by coa~ng the glass with a composition formed from polydiorganosiloxane components and a surfactant. The coating is applied to the glass surface as an emulsion and allowed to air dry.
ln general, the formation of ceramic coatings on substrates through the pyrolysis of preceramic polymer coatings is known. For example, Gaul, in U.S. Patent Nos. 4,395,460 and 4,4()4,153, fonns a silicon carbide coating on a substrate by coating the substrate with a polysilazane polyrner and then heating the coated substrate in an inert atrnosphere or in a vacuum to an devated ternperature of at least 750 C.
wo 93/21131 2i~0i`6 Pcr/usg3/o3290 Seyferth et al., U.S. Patent Nos. 4,482,669; 4,720,532; 4,705,837; and 4,645,807, ~
teaeh fon~ing an oxidation-resistant eoating on otherwise oxidizable rnaterials such as pyroly~c graphite by applieation of a preeeramic poly~ner coating over the rnaterials followed by pyrolysis of the preee~amic eoating to form a eerarnic eoating. The pr~c~e polymer materials respoelively used in the Seyferth a al. patents inelude: (1) polysilazanes that are synthesized by strong base-eatalyzed polymerization of eyclomethylsilazane, whieh is the ammonolysis produet fonned by reaeting anhydrous ammonia with a mixnlre of dihalohydridosilanes and trihalosilanes; (2) polymers formed by reaeting polysiloxane with a polysilylamide, whieh is an intermediate potassium salt of the polymer in (l); and (3) polymers fanned by ~eacdng an organopolysilane of the formula [(RSiH)X(RSi)~]N with aL~ali metal amides or silyb~des. See also Cramer, Ceramic Bulle~in, Vol. 68, No. 2, 1989, pp. 415-419.
Coblenz et al., in an anicle entided "Fonnation of Ceramic Composites and Coatings U~ng Polymer Pyrolysis" on pp. 271-28S of a publication e)ltitled "EmGrgent Process Methods for High-Teehnology Ceramics", edited by Davis et al. and published in 1984 by Plenum PubLishing Corporaion, describe the coating of carbon and silieon nitride materials with a dimethylsiloxydiphenylsiloxycarborane polymer and with a silazane oligomer. They ~port ~at dle resulting eoadngs were of poor quality with shrinking and cracking of the coadngs no~d.
Winter a al., U.S. Patent No. 3,892,583, and Verbeck, U.S. Patent No. 3,853,567,descnbc the farmation of shaped articles of homogeneous mixtures of silicon carbide and silicon nitridc. The homogcneous mixtures are also said to be useful in forming films, flakes, and coa~ngs.
Bancy et al., U.S. Patent No. 4,666,872, describes coating substrates with R3SiNH-containing silazane polymers to which have been added certain precious metals, followed by l~ing the substrate to an elevated temperature of at least 7S0C in an inert atmosphere or vacuum to form a ceran~c-coated article.
Bujalski, U.S. Patent No. 4,668,642, discloses coating substrates with R3SiNH-containing silazane polyrners to which have been added certain boron compounds, followed by l~ng the substrate to an elevated temperature of at least 750 C in an inert atmosphere or vacuutn to foQm a ceramic-coated article.
While the use of such ceramic materials may generally protect the surface of a substrate against scratching or other abrasive action, as well as impar~ing some oxidation protection for oxidizable substrates such as the above-described carbonaceous materials, such coatings are not 3~ generàlly known to impart any physical s~rength to the substrate itself when applied as thin layGrs, e.g., l~ss than 10 microns in thickness.
Strucn~al cc~arnic materials, however, in addition to needing surface protection such as affordcd by the above discussed organic coating materials, need physical strengthening as well.
Barbee et al., U.S. Patent No. 4,781,970 describes strengthening of ceramics and glass-ceramics such as spumodumene (Li20 A1203 4SiO2) and cordierite (2MgO 2A1203 5SiO2) WO93/21131 Z~,3~.,0;~6 PCr/USg3/032gO
~y chemical vapor deposition or sputtering deposition of SiO2. The strengthening of the ceramic substrates by the invention of Barbee et al. is a result of the differential thermal coeffficient of expansion (TOE) between the coaing material and the substrate, with the coating material having a roquisite lower TOE than the substrate, rather than being the result of filling in S fine surface flaws. Furthennore, the deposition technique is time-consuming and expensive.
It should be noted that certain aspccts of the present invention are discussed in the following patents of con~non assignment herewith. Laine et al., U.S. Patent No. 4,612,383, describe a method of preparing polysilazanes by reaction of a starting rnaterial containing Si-H, Si-N, oq Si-Si groups with hydrogen or an amine in the presence of a catalyst. Laine et al., 10 U.S. Patent No. 4,788,309, detail a method of carefully controlling the reaction products ob~ained by rcaction of a st~ting matenal containing Si-H, Si-N, or Si-Si groups with hydrogen or an amine in the presence of a catalyst and enabling further reaction of the products to give an additional set of compounds. This patent provides methods of making sila?ane ~Is that are not necessarily polymeric. Blum et al., U.S. Patent No. 5,008,422, 15 describe new ~s to high molecular wdght polysilazanes and polysiloxazanes, the new high molecular weightpolymers themselves, and methods of formation of the prcce~rnic coating mate~ials when such ooating materials comprise polymers, for example, of polysilazane, polysibxa~e, and polysiloxanc suitablc for usc as coadng materials in dle plactioc of ~c present invendon. Scc also Blum at al., U. S. Palents 5,017,529 and 20 S,162,136. U. S. Patent 5,162,136 providcs a proccss for strcngthcning glass by coating it widl a p~nnc layer and then ooverting the p~nic layer to a ccr~mc coating. Blum ct ;L U.S. Patent No. 4,952,715, dclineate a novcl class of silazancs including cyclomenc sila~ne units within an oligomeric or polymeric struc~re. Blum ct aL. U.S. Patent No.
5,055,431, dcscribc thc synthesis of silazane and polysilazane compounds which have more 2~ than one cyclomic silazane in their structure, the pyrolysis of these compounds upon fabrication to give ce~nic coatings, fibers and arhcles as well as the use of the compounds as binders. The disclosures of these related patents are hereby incorporated by reference in their entirety.
Additional aspects of the present invention are discussed in the following co-pending 3~ U S patent applications of common æsignment hcrewith, the discloswcs of which are also incoqporated by refercnce herein. U.S. Application Serial No. 541,331, filed 19 June 1990, sets forth certain details of the present coating procedures and identifies specific preceramic materials useful thcrewith.
35 Disclosure of the Invention It is an object of this invention to provide a process for treating the surface of a structural ceIamic matedal by wct-coadng with a ce~amic precursor solution and convcrting the precursor at ~c surfacc to a ceranuc matelial by heat process and thereby, to increase the strength of the struc~ral c~amic material. The idea behind this invention is that deposition of ccramic 40 precursor solutions at surface and subsurface defects, followed by pyrolysis, can restore or W093/21131 2~31016 PCr/USg3/032go even improve the odginal s~rength of the material and repair damage caused by machining. 1 invention can replaee elaborate and costly fine machining of ceramic materials.
It is another object of the invention to provide a process for treating the surface of a struetural ceramie material to inerease the strength of the structural eeramie material wherein a S ceramie eoadng is fonned on the struetural ee~ne material surface.
It is yet another object of this invention to provide a proeess for treating the surface of a struetural eeramie material to repair surfaee defeets and thereby, inerease the strength of the strucn~al eeramie material by pyrolyzing or heating a ceramie I = rcoating material applied to the surfaee of the struetural eeramie material by wet teehniques, for example, dipping, ~awing, spraying, painting and the like, to fonn a eeramic coating on the structural ceramic material surfaee.
It is a further objec~ of this invention to provide a process for treating the surface of a stmcn~al ceramie material to increase the strength of the structural ceramic material which has been preselec~d to provide optimal viseosity, wetting and adhering properties, and ceramic yield, and whieh is applied to the surface of the struetural ceramic material to form a co~n~ic coaing on the slruc~ral eeramie material surface upon pyrolysis.
It is yet a further objeet of this invenion to provide a st1uctural ee~nic material having a ~a~me eoaing fonned thereon to inerease the strength of the structural eeramic material by pyrolyzing or hea~ng a eeramie preeursor eoating material, which has been preselected to p~vide opdmal ~iscosity, weting and adhering propemes and ceran~ie yield, and applied to the su~facc of thc s~uc~l ceramic matelial to forrn a ccramic coating on thc strucnnal ce~mic mataial s~face upon pyrolysis.
It is yct a fur~cr ob~ect of this invcntion to p~vidc a structural silicon nitridc matc~al having a ce~amic coating f~med tnereon to increase thc strength of thc structural silicon nitride material by pyrolyzing or heating a ceramic precursor coating matcrial which has been Fcselected to providc opimal viscosity, wetting and adhering properties and ceramic yield, and applied to thc surface of thc stluctural silicon nitride material to form a ceramic coating on the slrucn~ silicon nitride material surface upon pyrolysis.
These and other objects of the invention will be apparent from the following descnption.
In accordance with the invention, a method is provided for improving the strength of a struc~al ccramic material which comprises: coating the stn~ctural ceramic material with a tractablc ce~amic precursor coating capable of pyrolyzing to fonn a ceramic coating; and heating the coated struct~al ceramic material to a temperature suf~lciently high to perrnit conversion of the ceramic pr~ for a period of time sufficient to form the cerarnic coating on the surface 3~ of the structural ceramic material.
~ ~ t ~
WOg3/21131 Z1310~6 Pcr/usg3/o32go "~etail~d Descripdon of the Invention Before the present methods are diselosed and described, it is to be understood that this invendon is not limited to the speeific eoating proeess and subsequent pyrolysis of coated strucn~al ceramic rnaterhls, or to the speeific structu~l cerarnic rnaterials or eeramic coated 5 struetural eeramic materials, i.e., polyeyelornethylsilazane-eoated silicon nitride, or the like, as sueh may, of course, vary. It is also to be understood that the terrninology used herein is for the purpose of deseribing partieuhr embodiments only and is not intended to be limiting.
It must be noted that, as used in the speeifieation and the appended elaims, the singular faqms "a", "an" and "the" inelude plural referenees unless the eontext elearly dictates othe~wise.
10 Thus, for exampb, reference to "a lay" ineludes multiple layers of eeramic material, reference to "a eoating" ineludes multiple ooatings with a eeramie coating material, and the like.
In the specifieation and in the elaims whieh follow reference will be made to a number of tem~s whieh shall be defined to have the fol10wing meanings:
The tenn "traetable", as used herein with respeet to the preceramie eoating material used 15 h the pIaetiee of the invention, is intended to define a material which is soluble in organic or inor~nie solvents, meltable, or malleable, or which ean be processed like an organic polymer to form a tesired shape, i.e., in this ease, a coating on a eeramic substrate.
By use of the tenns "pyrolysis" o~ "pyrolyze" is meant the transformation of the~ca~c ooadng material 0~sor, to a eeramie produet by a heating proccss in which a 20 multipk tha~mal ~acdviq occurs, or thc reaction of such materials with oxygen or nitrogenous gas o~er gascs yresent during the pyrolysis to foqm materials separablc from the resulting c~mc coa~ing on the strucn~al caamic material. lt also may be definet as the minimum en~e at whicb farmation of a ceramic coating occurs for any given preceramic coating matcrial, c.g., polymer or precursor, by losing its functional groups or nature coincidentally 25 ~vith thc cxtcnsivc formation of crosslinking.
The "ccramic" which is foqmed by the pyrolysis rnay be defined as an inorganic material ~at forms a highly crosslinked network of covalent (sigma) bonds which may contain addi80nal coordina80n bonds. In most cases, except carbon, the ceramic material contains at least two chcmical elements. In some cases both are metals or metalloids, e.g., borides and 30 silicides, or non-metals, e.g., PN, AsS3, or SiP04. The ceramic material can be in an amoTphous, crystalline, glass-ceramic, or solid solution form and usually is stable at high tempaa~e.
The terms "structural ceramic material" or "ceramic subs~ate" are used interchangeably herein to define the material or substrate onto the surface of which the polymer-derived ceramic 35 coating is fom~d by pyrolyzing a preceramic coating material applied to said surface as herein discloscd. TJle tenn "ceranuc" to describe the substrate is as defined above. By way of cxample, and not of limitation, the structural ceramic materials may include Si3N4, SiC, A1203, ~2~ alun~inum silicates, aluminum titanates, mullite, aluminum borosilicates, AlN, and TiN, or mixtures thereof.
WO93/2113l 2l~m6,~ PCl/US93/03290 The temn "ceramic yield" of the eeramic precursor coating material, such as a precursor preeerarnie polymer, upon pyrolysis, as used herein, is intended to define the ratio of the wdght of the eeram~ie eoating after pyrolysis to the weight of the eoating before pyrolysis.
The te~m "eeramie preeursor" as used herein is intended to inelude inorganie and5 ~Irganometallie eompounds, inorganie polymers, and organometallie polymers, while the terrn ~preceramie polymer" is intended IO define a tractable eompound with any number of monomerie units which is sufficient to be deposited as a coating on substrates and to form cunie co~osidons upon pyrolysis or heat treatment. Both terms are iintcndcd to be included in the teIm "eeramie preeu~soreoating material".
The invention provides a proeess for improving the strength of a struetural eeramic matcrial by foiming a eelunie eoating on the surfaee of the struetural eeramie material. The eeramie eoating is fonned by applying a plecan~e coating material to the surfaee of the struct~al ceramie material and then pyrolyzing or heating the preceramie coating material at a en~aa~e suffieiently high to permit the eonversion of the precursor eoating to a ceramic 15 eoating.
When stn~ al ceramic substrates are coated with ceramic, in aeeordance witn the praetice of the invention, the streng~th of the struetural ceramic material rnay be increased. on the avclage, by 15%. A few examples have shown inereases of up to 20% to 30%. While not wishing to be bow~d by any pardcular theory as to why such strength ine~ases oceur, we have 20 tboizod that the ccramie coaling maberial may aet to increase the strength of the substrate ma~ial by healing surfaeedefects in the substrate material. The present inventors further ~te that chemical orphysical intc~ions benvcen the coating and thc substratc play a rolc in dle, s~engd~nging mcchanism. Strcss~n~sion cffect may be anothermechanism to m~rvc thc strength. Chcmical intcractions between the substrate and thc coating may be 25 advant~geous but migration of material from thc subs~rate surface arca to thc coating should be Fcvcntcd if it will causc the formation of undercoating holes and pinholes and/or cracks in the coatings.
Thc prcceramic coating material, e.g., a polymer or precursor, from which the ccramic coating will be foTmed may comprise any tractablc inorganic or organometallic polymcr or 30 compound capable of be~ng in a liquid forrn or in solution and of wetting and adhering to the surfacc of the structural ceramic material and capable of conver~ing to a ce~amic coating on a c~amic subs¢ate by heat treatment. By way of exarnple, and not of limitation, the cerarnic coatings which may be forrned from such precursors may contain Si3N4, SiCN, SiC, C, SiON2, SiO2, ZrO2, A12O3, Y2O3, AlN, B4C, BN, TiC, WC, W2C, Mo2C, TiN, TiO2, 35 CaP2O8, and others, metal phosphates, metal silicates, metal borates, as well as other ceramics l~own for dleir mength and hardness, including other silicides, borides, nitrides, carbides, or 03cides, or mixtures thereof.
WO93/21131 2~ PCr/US93/032go Exampbs of preceramic coating materials such as polymers or precursors from which such ceramic coatings may be folmed compnse polysilazanes, such as poly-N-methylsilazane and/or polycyclomethylsilazane, from which ceramic compositions of SiN, SiCN, SiO2, SiOC, SiON and SiOCN (depending upon the atmosphere used during pyrolysis) may be 5 fonned; pdysiloxazane; polysiloxane, including polymethyl-silsesquioxane and polyhydridomethylsiloxane from which ceramic coating containing Si and O and potentially other elements such as C and N can be formed; polysilanes; polycarbosilanes; polyboranes;
poly~tmnes; polyaminoboranes; polyaminotitanium; and combinations thereof. Examples may also includc precu~sors to rlo2, AIN, A1203, ZrO2, Y203, metal phosphates, and 10 others. Thc above dcscribcd p~ic coating materials either may be obtained commercially may be readily synthesized using methods know to those skilled in the art. Reference may also be had to U.S. Patent Nos. 4,612,383, 4,788,309, 4,952,715, S,008,422 and 5,055,431, citcd and i~polated by referencc above. ln addition, ceramic precursor coating Jlutc~ials may includc compounds and polymers which arc totally inorganic and solublc in 15 water. By way of cxamplc, and not of limitation, inorganic ceramic precursor coating mate~als may includc Ca(H2P04)2 and Al(H2P04)3. Precursors made by sol-gel technology may also bc uscd. Howcver, thcy arc expectcd to bc less favorable due to short term stability in solution or on dlc shclf, and low ccramic yields and high shrinkage are obtained upon their pyrolysis.
It is p~efcrablc that thc ccramic yield of the precursor be at least S0 wt.%. More 20 ~efc~ably, it dlould bc a~ove 70 wt.% and most prcfcrably, it should be above 85 wt %.
Thc ~c~ ca~ic ma~ial which rnay bc strengthened by fosmation of thc cerarnic coadng ~on, in accordancc with thc invention, may comprise any particular shape,includilg flat shects, and shaped objccts, such as bars, rods, fibers or the like.
Thc struc~al ccramic material which may be strengthened by fo~nation of the cerarnic 25 coating ~c~on, in accordance with the invention, may include, by way of example and not of limitation, silicon nitride and A1203.
It may some~imes be prefeITed or necessary to pretreat the structural ceramic matenal with HF or other acids or tetrachlorosilane in order to hydroxylate the surface and thereby to op~ze the coating of the ceramic substrate with the preceramic coaang material. Whether or 30 not it is necessary to pretreat the structural ceramic material and the methods whereby such pretreatmcnt is to be effected may be dete~mined using routine experimental techniques.
The choice of the preceramic coating matelial and the pyrolysis schedule may vary depending on the substrate matelial. It is not necessary that a type of polymer that performed well for one kind of material will strengthen another type of material. The chemical interactions 35 at the intærface between the coating and the substrate is assumed tO play a role in the s~engtl~ng mechanism. Therefore, the development of a strengthening process for various matcria~s roquires screcning of polymer and process conditions individually for each material (sec Examplcs 1-3).
W093/21131 213~E6 Pcr/US93/03290 The preceramic coating rnaterial rnay be applied to the cerarnic substrate by any convenient method such as by dipping in a selec~ed coating solution or by spraying, paindng, spin~ing, or the like, with such coadng solution, the solution having a predetermined concentraion preferably between 0.1 and lOû wt.%, rnore preferably between about S and 30 5 wt.% for most applieadons. The pr~cerarnie coating rnaterial is applied to the cerarnic substrate in an amount sufficient to provide, upon subsequent pyrolysis, a ceramic coating of from about 0.01 to about 20 microns and preferably from about 0.05 to about 4 microns. Typically, the desired eeramic eoadng thiekness is aehieved in a single coating layer. i.e., without the use of several built-up layers. However, if neeessary, the coating process may be ~peated to build up 10 the desirvd thiekness without the fonnation of a substantial level of craeks in the developed eamie eoating. In addition, eraeked eoatings ean be healed by additional coating procedure.
Aehie~nng the desired thiekness of the eventual eeramic coating layer on the structural caamic mate~al is related to both the initial eoating thickness of the pecnan~ic coating material and the eeramie yield of the ~nuc coating material, e.g., polyrner or preeursor. The 15 coating thickness of the lnecewnic coating material is, in tUIn, related to the viscosity of the ooa~ng rnaterial solution and the arnount of sohent added to it to permit it to be applied to the surfaee of the ceramic substrate. The eeramic yield is related to: (1) the rnolecular strueture with referenee to whether the coating material eomprises a branched, erosslinked, or ring-type polymer, (2) ~e leeular wdght of the eoating material with higher leeular wdght 20 polymers favorcd because of thehigher ceramie yield of such mate~ials; (3) the latent reaetivity of the coa~ng ma~rial that allows theirrnosetting propcrties and furth crosslinking as desired (2); and (4) small amounts of ext~aneous organic groups if needed to p~vide tractability and shelf stability.
Gcnc~lly speaking, to achieve the desired ceramic coating thickness unifo~nly across t~he 25 surfacc of thc ceramic subs~rate, the pre~an~c coating material should have a sufficient lecular weight, e.g., be sufficiently polymerized, to provide a minimum viscosity, after rcmoval of solvent, of at least 1-3 poise so that the coating will remain in a uniform thickness on the structural ceramic material as the coating is heated to the pyrolysis temperature.
Altematively, ~e precursor can be a monomeric cornpound capable of condcnsation or cross-30 linlcing at the substrate surface prior to evaporation.
In some instances the preceramic coating material, although sufficiently polymeAzed toachieve the desired ceramic yield, will have a viscosity sufficiently low to permit it to be app lied to the structural ce~amic material as a uniforrn and hogeneous coating without the necessi o~diluting the coating material in a solvent, i.e., the "tractable" coating material, such as a 35 polymer, will be liquid o~, by application of heat, meltable.
Howcver, in othcr instanccs, the preceramic coating rnaterial will need to be diluted to bwer thc viscosity sufficicntly to facilitate application of the preceramic coating material as a coating on thc surfacc of the ce~amic substrate and to control the thickness of the coating In such cases, the p~c coating material must be capable of being dissolved in a solvent WO 93/21131 Z13~,,~, z~ ~. PCI`/US93/03290 ~ hich may later be removed from the coating without negative impact on the desired formation of the ceramic coating layer, i.e., the "tractable" coating material must be soluble.
Thus, in any event, the preceramic coating material, such as a polymer or precursor, must not only be of sufficiently high molecular wdght or non-volatile, or cross-linkable, to achieve 5 the desi~d Imnimum ceramic yield, but it must be "tractable" as well, as previously defined.
The p~mic coating material may be applied to the structural ceTannc material at ambien~ temperature, i.e., about 2~25 C, or either the coating material or the structural ceramic material may be at an elevated temperature.
In cases where the precerarnic polymer is sensitive to moisture and/or oxygen, it is i0 preferable to deposit the solution coating under a dry and/or oxygen- free environment. This is especially prefable in cases where it is desircable to exclude oxygen frorn the final coating composidon. However, from a practical processing point of view, it is preferable to use a polymer that is not air sensitive or only slighdy air sensitive as this will not reguire spccial costly oquipment and more expensive operation. In cases where oxygen is desired to be in the fin~ coating ce~amic cornposition, polymers sensitive to air may be preferably coated in air or exposed to air in~di~cly after the coating procedure and prior to heat treatrnent to allow curing alt low temperature.
The mimmum tempaatune to which the coated structural cerarnic material must be heated will be daa~d by the p~icularpreceramic coating material used and the minimurn Ico_eatwl~ch acamicmatesial may bef~rmcd from such aprecanuccoating material. Thus, the minimum pyrolysis temperature must be at least at, and preferably above, the ~e at which dle pec~m~c coating material converts to a ceramic material. This dctcmlinaion of thc convcrsion of the p~c ooating material to a ccra~c ma~erial may be TGA, IR, XRD, NMR, and elemental analysis.
The mimmum temperature, however, must be high enough to permit organic groups, if containcd in thc preccra~c coating material, e.g., polymer or organometallic cornpound, to be pgrolyzed, leaving only an inorganic ceramic network. For example, when a poly-N-methylsilazane polymer having a molecular weight of 800-3000 Daltons is used, the minimum ~an~e needed to remove extraneous methyl groups from the coating and to forrn the ccramic material is about 300 C to SSO C. However, higher tempcratures may be needed to aUow desired chernical orphysical interactions between the coating and thc substrate or to densify or crystalizc the cerarnic coating materials.
The preceramic coating material may be pyrolyzed to convert it into a ceramic coating using a variety of heating schcdules, including different heating rates, dwell times at inteImediate and tnaximum temperatures, and cooling rates, depending upon the specific material used.
In any of thc embodiments used for heating the coated ceramic substrate to the pyrolysis ~n~uue, ~c ooatcd ccramic substrate is held at the pyrolysis temperature for a period of dmc sufficicnt to pcm~it fonnation of the ceramic coating and to develop the desired strength. It will be und~stood that if less than the optimal dwell time is used for the particular preceramic W093/21131 21~6 ' Pcr/usg3/o3290 coating and structural ceramic material, a cc~ic coating may be formed over the ceramic substrate in accordance with the invendon, but the coated cerarnic substrate may not have as much strength or hardneæ as is possible if the dwell time is extended. There will then be a tradc~f between desired strength and process economics.
The holding or dwell time of the coated ceramic substrate at the pyrolysis temperature may, therefore, vary from a minimum of O minutes, preferably at least about S minutes, and most preferably at least about 90 minutes. up to a maximum time of about 2 hours. Longer time periods may be used, but are usually unnecessary and. therefore, not economically justifiable. In some instances shorter dwell times of, for example, 30 60 minutes may be used, even though rnaximum strength hæ not been devdoped, if the economics of the process justifies the trade-off of reduced dwell time with reduced strength.
It should be further noted, with respect to the period of time used to initially heat the coated structural ceramic material up to the pyrolysis temperature, that it is desirable, from a standpdnt of econon~ics, to conduct the wann up period, dwell period at the pyrolysis ~, and cooling-off period in as shart a period of time as possible. Preferably. the er~re pyrolysis period is carried out in a period of about 2 hours. By "the entire pyrolysis period" is meant f~n the time the coated ceramic substrate is placed in the furnace at room ~e until the ternpcrature during cooling again reaches 200 C.
The coated-stn~c~al cerarnic material, duling both the temp~e ramp-up peri-ld and tbe dwell per;od, may be rnaintained in an inert atmosphere such as argon, a nitrogen-contai~ng atmosphere such as N2 ~ ammonia, a hydrogen atrnosphere, oq mixtures of the aforementioned atmospheres, and a dly- or wet-airatmosphere. depending upon the chernical oonstitucnts o~f the p~mic polynxr and the desired ce~amic-coating composition. For ac;unple, the foqmation of a ceramic with Si-O bonds is favored when using an air atmosphere wbe~eas the foTmation of a ceramic with Si-N bonds is favored when thc p~amic polymer contains both silicon and nitrogen and either an inert or a nitrogen-containing a~nosphere is used during the entire pyrolysis period.
The choice of the pyrolysis environment in conjunction with the preceramic polymer structure will deteImine the final ceramic composition. Any precursor that is pyrolyzed in air will provide a final composition consisting mainly of oxide material. If the pyrolysis is conducted in an inert atmosphere such as nitrogen or argon, the final composition will contain substandal amounts of carbon if the precursor contained organic pendant groups or carbon was an elernent in the polymer skeleton (e.g., polycarbosilane). If nitride coating is desired then the polyrners should be preferably pyrolyzed in arr~nonia. Oxyni~ide and oxycarbide coatings can also be fo~rned if oxygen is incorporated in the skeleton of the original polymer or during the pyrolysis pcIiod. The choice of the final coating material might vary based on the application and thc physical and/or chcmical conditions at which the product is going to perforrn.
Howcver, for cconomic rcasons, it is preferable to conduct the pyrolysis in air.
wo 93/21 131 21 3,~ PCr/USg3~032so The following exarnplesq~ill serve to further iUustratc the invendon, including some of thc paramcters of the process.
It is to bc undcrstood that whilc the invcndon has been described in conjunction with the prefcrred embodimcnts thcrcof. that the foregoing dcscription and the examplcs which follow 5 are intended to illustrate and not lirnit the scope of the invention. Other aspects, advantages and rnodifications within the scope of the invention will be apparent to those skilled in the art to which thc invention pertains.
Five ground, highly polished beveled silicon nitride bars, the dirnensions of which were approximatcly 50 mm x 4 mm x 3 mm, wcre ultrasonically cleaned and then dehydrated by heating at S00 C undcr nitrogen for 1 hour. Bars were then dipped into a 10 wt.% polymer s~lution in tc~ahydrofuran (udess otherwise indicated) under a selected atmosphere and pyrolyzed as dcscribed below. (Pyrolysis conditions are set forth in Table 1 below.) The pyrolysis proccss was carried out under the selected atmosphere (1 atm) according to thc following schedule: (a) heatingrate of 5-C or 10-C/minute (see Table 1), (b) dwell period at the maximum temperature for 1 hour, and (c) cooling rate of 10 C/min. The entire coating and pyrolysis plwed~e were repeated 3 tirnes. Thc bars were tested for strength in accordance widl the fo~-point be~d tcst of ASTM C158. Results are set forlh in Table 1. These results 20 dn~_ that the s1rcngth of ground and lapped silicon nitride bars can be significandy m~ved by f~ng a caamic coaing on tlu~ surfacc by the process dcscribed above. The ~ts a1so show tbat similar or grcater s~ength can be achicved when thc precu~or coating rn;u~i~ is c~d to a ce~ic by hea~ng in air or a nitrogcnous gas.
WO 93/21131 Z13iQ16 PCr/US93/03290 Ta~
Results of Four-Point-Strength Measurements of Coated Ground and Lapped Si3N4 Bars ' Example Polymer Atmosphere Heating Strength 10% Coat Heat ,~ched~lea ((Sav MPa!
UNTREATE~b -- AIR 10 /900 /2h 827 1 PCSC A~d AIR 5-/150-/4h 10 /~00 /lh 827 2 PCS N2e N2 10 /150-/4h 10-/900-/lh 895 3 PCMSf N2g NH3 5-/150-/2h N2 10 /900 /lh 860 4 Ca(H2P04)2h A~i AIR 5-/150-/4h 10 /800 /lh 916 PCMS N2 AlR 10-1900-/lh 990 6 PCMS N2 A~ 10 /900-/lh 817i 2~
7 PNMSk N2 AIR 10 /~OO-/2h 981 8 PNMS N2 NH3 1-/150-/4h 10 1~00-/2h 893 a Rate of tempe~ature increase in C per min/dwell temperatur~,, in C/duration at dwell ~a~ in hours. Each line entry represents a sequenaal step in the heating schedule 35 b Heat treated 3 times to ~O C at a rate of 10 ~ per min with a dwell period of 2 hours each time.
c Polycarbosilane in tetrahydrofuran solution The polycarbosilane was trea~ed with 50 ppm Ru3(CO)2 as a crosslin~ng catalyst found to e~fectively increase the ceramic yields d Coated and dried twice in air e Coated and ~ied three times in N2 45 f Polycyclor,nethylsilazane in tetrahydrofuran solution g Coat~d and dried twice in N2 h Soluion in H2O.
i Coated and dried three times in air WO93/21131 ~ "J ~'7~5~ ` PCI/US93/03290 Two extremdy wcak bars wcre included in this batch of five bars indicating majorinternal flaws. the othcr three bars obtained strength values of 887, 941, and 978 MPa.
k Poly-N-methylsila2ane in tetrahydrofuran soluion.
ExamDIe 2 The p~ocedures of Exan~ple I can be followed using as-ground silicon nitride bars and 10 thereby achieve essen~ally identical results (see Table 2).
Table 2 Results of Four-Point-Strength Measurements of Coated As-ground Si3N4 Bars Example Polymer Atmosphere Heating Strength ~Umber 10% Coat Heat Sc~ulea (av MPa) 9 PCMSb N2 AIR 10-~gO0 /;!h 954 -- -- 10-/900 /lh 827 2~
Ca(H2PO4)2 AIR 10 /gO0 /4h 837 30 a Ratc of te~ature increase in C per min/dwell temperature in C/duration at dwell tempe~ature in hows. Each line entry represents a sequential step in the heating schedule.
b Polycycl~ethylsilazane in tetrahydrofuran solu~on.
WO93/21131 2~3ldi~ Pcr/US93/03290 ~am~.le...~
T~le procedures of Example 1 can be followed using A12O3 bars and thereby achieve essentially identical results (see Table 3).
.
Table 3 Results of Four-Point-Strength Measurements of Coated A12O3 Bars ~ple Polymer Atmosphere Heating Strength ~umber 10% Coat Heat ~h~a (o ~a~
UNlREATE~b -- A~ lO ~900 /2h 223 11 PNMSC N2 AIR 10 /gO0 /2h 332 12 Ca(H2Po4)2 AlR AIR 10 /900/2h 208 20 13 Ca(H2PO4~2 AIR AIR 10 /900 /2h 247 14 PCSd N2 AIR 10 /900 /2h 265 25 a Ratc of ~npcraturc inc~easc in C pcr min/dwell tcmperature in C/duration at dwell empa~e in hours. Each linc entry rcprcscnts a scquential step in the hea~ing schedule.
b Heat ~ed 3 ~mes to 900 C at a rate of lO-C per min with a dwell period of 2 hours each ~me.
c Poly-N-methylsilazane in te~ahydrofuran solution.
d P~lycar~osilane in ~trahydrofuran solution.
213lol~
pR~OCF.SS FOR ll~lCR~lN('. STRF.NGTH OF STRUCTURAl.
CF-RA~I~C l~lATF.RlALS BY FORMlN(~. CERAM~C C(~ATlN(~. ON
SURFACF ANn PROI)UCT FORMF.D TH~FR~Y
E;ield of the lnvention The invention relates to the strengthening of structural ceramic materials by fonning a polymer-derived ceramic coating on the surface of the structural ceramic material. The invention additionally relates to a process for improving the strength of and repairing surface defects in struc~al ceran~c materials by coating thern with a polymer-denved preceramic layer and then conver~ng the preceramic layer to a ceramic coating. More particularly, the invention relates to a process for strengthening structural silicon nitride and alumina materials by coating them with a polymer-derived prece~mic layer and then conver~ng the pecaan~ic layer to a c~amic coating In addi~ion to strengthening stNctural cerarnic materials and repairing surface defects, the invention can replace elaborate and costly fine machining of ceramic materials.
Background of the Invention The use of manufactured struct-ural cerarnic products is compromised by the great rcduc~on in strength of the struc~l material due to surface and near-surface defects induced by p~ocessing and machining operations or inherent in the material itself.
Attempts to preserve the integrity of manufactured products, fo~ exampk, glass materials, have been made by coating the product with various materials. Yoshida et al., U.S. Patent No.
4,370,385 describes coating a glass vessel with an organopolysiloxane composition to provide a scuff-masking coating over the glass.
Wieczoqrek et al., U.S. Patent No. 4,409,266, provides a shatteIproof coating on glass by applying to the glass surface a silane adhesion promoter and a two-component system which rea~s to f~n a polyurethane binder.
Kurita et al., U.S. Patent No. 4,656,221, conceals graze marks on a glass bottle by coa~ng the glass with a composition formed from polydiorganosiloxane components and a surfactant. The coating is applied to the glass surface as an emulsion and allowed to air dry.
ln general, the formation of ceramic coatings on substrates through the pyrolysis of preceramic polymer coatings is known. For example, Gaul, in U.S. Patent Nos. 4,395,460 and 4,4()4,153, fonns a silicon carbide coating on a substrate by coating the substrate with a polysilazane polyrner and then heating the coated substrate in an inert atrnosphere or in a vacuum to an devated ternperature of at least 750 C.
wo 93/21131 2i~0i`6 Pcr/usg3/o3290 Seyferth et al., U.S. Patent Nos. 4,482,669; 4,720,532; 4,705,837; and 4,645,807, ~
teaeh fon~ing an oxidation-resistant eoating on otherwise oxidizable rnaterials such as pyroly~c graphite by applieation of a preeeramic poly~ner coating over the rnaterials followed by pyrolysis of the preee~amic eoating to form a eerarnic eoating. The pr~c~e polymer materials respoelively used in the Seyferth a al. patents inelude: (1) polysilazanes that are synthesized by strong base-eatalyzed polymerization of eyclomethylsilazane, whieh is the ammonolysis produet fonned by reaeting anhydrous ammonia with a mixnlre of dihalohydridosilanes and trihalosilanes; (2) polymers formed by reaeting polysiloxane with a polysilylamide, whieh is an intermediate potassium salt of the polymer in (l); and (3) polymers fanned by ~eacdng an organopolysilane of the formula [(RSiH)X(RSi)~]N with aL~ali metal amides or silyb~des. See also Cramer, Ceramic Bulle~in, Vol. 68, No. 2, 1989, pp. 415-419.
Coblenz et al., in an anicle entided "Fonnation of Ceramic Composites and Coatings U~ng Polymer Pyrolysis" on pp. 271-28S of a publication e)ltitled "EmGrgent Process Methods for High-Teehnology Ceramics", edited by Davis et al. and published in 1984 by Plenum PubLishing Corporaion, describe the coating of carbon and silieon nitride materials with a dimethylsiloxydiphenylsiloxycarborane polymer and with a silazane oligomer. They ~port ~at dle resulting eoadngs were of poor quality with shrinking and cracking of the coadngs no~d.
Winter a al., U.S. Patent No. 3,892,583, and Verbeck, U.S. Patent No. 3,853,567,descnbc the farmation of shaped articles of homogeneous mixtures of silicon carbide and silicon nitridc. The homogcneous mixtures are also said to be useful in forming films, flakes, and coa~ngs.
Bancy et al., U.S. Patent No. 4,666,872, describes coating substrates with R3SiNH-containing silazane polymers to which have been added certain precious metals, followed by l~ing the substrate to an elevated temperature of at least 7S0C in an inert atmosphere or vacuum to form a ceran~c-coated article.
Bujalski, U.S. Patent No. 4,668,642, discloses coating substrates with R3SiNH-containing silazane polyrners to which have been added certain boron compounds, followed by l~ng the substrate to an elevated temperature of at least 750 C in an inert atmosphere or vacuutn to foQm a ceramic-coated article.
While the use of such ceramic materials may generally protect the surface of a substrate against scratching or other abrasive action, as well as impar~ing some oxidation protection for oxidizable substrates such as the above-described carbonaceous materials, such coatings are not 3~ generàlly known to impart any physical s~rength to the substrate itself when applied as thin layGrs, e.g., l~ss than 10 microns in thickness.
Strucn~al cc~arnic materials, however, in addition to needing surface protection such as affordcd by the above discussed organic coating materials, need physical strengthening as well.
Barbee et al., U.S. Patent No. 4,781,970 describes strengthening of ceramics and glass-ceramics such as spumodumene (Li20 A1203 4SiO2) and cordierite (2MgO 2A1203 5SiO2) WO93/21131 Z~,3~.,0;~6 PCr/USg3/032gO
~y chemical vapor deposition or sputtering deposition of SiO2. The strengthening of the ceramic substrates by the invention of Barbee et al. is a result of the differential thermal coeffficient of expansion (TOE) between the coaing material and the substrate, with the coating material having a roquisite lower TOE than the substrate, rather than being the result of filling in S fine surface flaws. Furthennore, the deposition technique is time-consuming and expensive.
It should be noted that certain aspccts of the present invention are discussed in the following patents of con~non assignment herewith. Laine et al., U.S. Patent No. 4,612,383, describe a method of preparing polysilazanes by reaction of a starting rnaterial containing Si-H, Si-N, oq Si-Si groups with hydrogen or an amine in the presence of a catalyst. Laine et al., 10 U.S. Patent No. 4,788,309, detail a method of carefully controlling the reaction products ob~ained by rcaction of a st~ting matenal containing Si-H, Si-N, or Si-Si groups with hydrogen or an amine in the presence of a catalyst and enabling further reaction of the products to give an additional set of compounds. This patent provides methods of making sila?ane ~Is that are not necessarily polymeric. Blum et al., U.S. Patent No. 5,008,422, 15 describe new ~s to high molecular wdght polysilazanes and polysiloxazanes, the new high molecular weightpolymers themselves, and methods of formation of the prcce~rnic coating mate~ials when such ooating materials comprise polymers, for example, of polysilazane, polysibxa~e, and polysiloxanc suitablc for usc as coadng materials in dle plactioc of ~c present invendon. Scc also Blum at al., U. S. Palents 5,017,529 and 20 S,162,136. U. S. Patent 5,162,136 providcs a proccss for strcngthcning glass by coating it widl a p~nnc layer and then ooverting the p~nic layer to a ccr~mc coating. Blum ct ;L U.S. Patent No. 4,952,715, dclineate a novcl class of silazancs including cyclomenc sila~ne units within an oligomeric or polymeric struc~re. Blum ct aL. U.S. Patent No.
5,055,431, dcscribc thc synthesis of silazane and polysilazane compounds which have more 2~ than one cyclomic silazane in their structure, the pyrolysis of these compounds upon fabrication to give ce~nic coatings, fibers and arhcles as well as the use of the compounds as binders. The disclosures of these related patents are hereby incorporated by reference in their entirety.
Additional aspects of the present invention are discussed in the following co-pending 3~ U S patent applications of common æsignment hcrewith, the discloswcs of which are also incoqporated by refercnce herein. U.S. Application Serial No. 541,331, filed 19 June 1990, sets forth certain details of the present coating procedures and identifies specific preceramic materials useful thcrewith.
35 Disclosure of the Invention It is an object of this invention to provide a process for treating the surface of a structural ceIamic matedal by wct-coadng with a ce~amic precursor solution and convcrting the precursor at ~c surfacc to a ceranuc matelial by heat process and thereby, to increase the strength of the struc~ral c~amic material. The idea behind this invention is that deposition of ccramic 40 precursor solutions at surface and subsurface defects, followed by pyrolysis, can restore or W093/21131 2~31016 PCr/USg3/032go even improve the odginal s~rength of the material and repair damage caused by machining. 1 invention can replaee elaborate and costly fine machining of ceramic materials.
It is another object of the invention to provide a process for treating the surface of a struetural ceramie material to inerease the strength of the structural eeramie material wherein a S ceramie eoadng is fonned on the struetural ee~ne material surface.
It is yet another object of this invention to provide a proeess for treating the surface of a struetural eeramie material to repair surfaee defeets and thereby, inerease the strength of the strucn~al eeramie material by pyrolyzing or heating a ceramie I = rcoating material applied to the surfaee of the struetural eeramie material by wet teehniques, for example, dipping, ~awing, spraying, painting and the like, to fonn a eeramic coating on the structural ceramic material surfaee.
It is a further objec~ of this invention to provide a process for treating the surface of a stmcn~al ceramie material to increase the strength of the structural ceramic material which has been preselec~d to provide optimal viseosity, wetting and adhering properties, and ceramic yield, and whieh is applied to the surface of the struetural ceramic material to form a co~n~ic coaing on the slruc~ral eeramie material surface upon pyrolysis.
It is yet a further objeet of this invenion to provide a st1uctural ee~nic material having a ~a~me eoaing fonned thereon to inerease the strength of the structural eeramic material by pyrolyzing or hea~ng a eeramie preeursor eoating material, which has been preselected to p~vide opdmal ~iscosity, weting and adhering propemes and ceran~ie yield, and applied to the su~facc of thc s~uc~l ceramic matelial to forrn a ccramic coating on thc strucnnal ce~mic mataial s~face upon pyrolysis.
It is yct a fur~cr ob~ect of this invcntion to p~vidc a structural silicon nitridc matc~al having a ce~amic coating f~med tnereon to increase thc strength of thc structural silicon nitride material by pyrolyzing or heating a ceramic precursor coating matcrial which has been Fcselected to providc opimal viscosity, wetting and adhering properties and ceramic yield, and applied to thc surface of thc stluctural silicon nitride material to form a ceramic coating on the slrucn~ silicon nitride material surface upon pyrolysis.
These and other objects of the invention will be apparent from the following descnption.
In accordance with the invention, a method is provided for improving the strength of a struc~al ccramic material which comprises: coating the stn~ctural ceramic material with a tractablc ce~amic precursor coating capable of pyrolyzing to fonn a ceramic coating; and heating the coated struct~al ceramic material to a temperature suf~lciently high to perrnit conversion of the ceramic pr~ for a period of time sufficient to form the cerarnic coating on the surface 3~ of the structural ceramic material.
~ ~ t ~
WOg3/21131 Z1310~6 Pcr/usg3/o32go "~etail~d Descripdon of the Invention Before the present methods are diselosed and described, it is to be understood that this invendon is not limited to the speeific eoating proeess and subsequent pyrolysis of coated strucn~al ceramic rnaterhls, or to the speeific structu~l cerarnic rnaterials or eeramic coated 5 struetural eeramic materials, i.e., polyeyelornethylsilazane-eoated silicon nitride, or the like, as sueh may, of course, vary. It is also to be understood that the terrninology used herein is for the purpose of deseribing partieuhr embodiments only and is not intended to be limiting.
It must be noted that, as used in the speeifieation and the appended elaims, the singular faqms "a", "an" and "the" inelude plural referenees unless the eontext elearly dictates othe~wise.
10 Thus, for exampb, reference to "a lay" ineludes multiple layers of eeramic material, reference to "a eoating" ineludes multiple ooatings with a eeramie coating material, and the like.
In the specifieation and in the elaims whieh follow reference will be made to a number of tem~s whieh shall be defined to have the fol10wing meanings:
The tenn "traetable", as used herein with respeet to the preceramie eoating material used 15 h the pIaetiee of the invention, is intended to define a material which is soluble in organic or inor~nie solvents, meltable, or malleable, or which ean be processed like an organic polymer to form a tesired shape, i.e., in this ease, a coating on a eeramic substrate.
By use of the tenns "pyrolysis" o~ "pyrolyze" is meant the transformation of the~ca~c ooadng material 0~sor, to a eeramie produet by a heating proccss in which a 20 multipk tha~mal ~acdviq occurs, or thc reaction of such materials with oxygen or nitrogenous gas o~er gascs yresent during the pyrolysis to foqm materials separablc from the resulting c~mc coa~ing on the strucn~al caamic material. lt also may be definet as the minimum en~e at whicb farmation of a ceramic coating occurs for any given preceramic coating matcrial, c.g., polymer or precursor, by losing its functional groups or nature coincidentally 25 ~vith thc cxtcnsivc formation of crosslinking.
The "ccramic" which is foqmed by the pyrolysis rnay be defined as an inorganic material ~at forms a highly crosslinked network of covalent (sigma) bonds which may contain addi80nal coordina80n bonds. In most cases, except carbon, the ceramic material contains at least two chcmical elements. In some cases both are metals or metalloids, e.g., borides and 30 silicides, or non-metals, e.g., PN, AsS3, or SiP04. The ceramic material can be in an amoTphous, crystalline, glass-ceramic, or solid solution form and usually is stable at high tempaa~e.
The terms "structural ceramic material" or "ceramic subs~ate" are used interchangeably herein to define the material or substrate onto the surface of which the polymer-derived ceramic 35 coating is fom~d by pyrolyzing a preceramic coating material applied to said surface as herein discloscd. TJle tenn "ceranuc" to describe the substrate is as defined above. By way of cxample, and not of limitation, the structural ceramic materials may include Si3N4, SiC, A1203, ~2~ alun~inum silicates, aluminum titanates, mullite, aluminum borosilicates, AlN, and TiN, or mixtures thereof.
WO93/2113l 2l~m6,~ PCl/US93/03290 The temn "ceramic yield" of the eeramic precursor coating material, such as a precursor preeerarnie polymer, upon pyrolysis, as used herein, is intended to define the ratio of the wdght of the eeram~ie eoating after pyrolysis to the weight of the eoating before pyrolysis.
The te~m "eeramie preeursor" as used herein is intended to inelude inorganie and5 ~Irganometallie eompounds, inorganie polymers, and organometallie polymers, while the terrn ~preceramie polymer" is intended IO define a tractable eompound with any number of monomerie units which is sufficient to be deposited as a coating on substrates and to form cunie co~osidons upon pyrolysis or heat treatment. Both terms are iintcndcd to be included in the teIm "eeramie preeu~soreoating material".
The invention provides a proeess for improving the strength of a struetural eeramic matcrial by foiming a eelunie eoating on the surfaee of the struetural eeramie material. The eeramie eoating is fonned by applying a plecan~e coating material to the surfaee of the struct~al ceramie material and then pyrolyzing or heating the preceramie coating material at a en~aa~e suffieiently high to permit the eonversion of the precursor eoating to a ceramic 15 eoating.
When stn~ al ceramic substrates are coated with ceramic, in aeeordance witn the praetice of the invention, the streng~th of the struetural ceramic material rnay be increased. on the avclage, by 15%. A few examples have shown inereases of up to 20% to 30%. While not wishing to be bow~d by any pardcular theory as to why such strength ine~ases oceur, we have 20 tboizod that the ccramie coaling maberial may aet to increase the strength of the substrate ma~ial by healing surfaeedefects in the substrate material. The present inventors further ~te that chemical orphysical intc~ions benvcen the coating and thc substratc play a rolc in dle, s~engd~nging mcchanism. Strcss~n~sion cffect may be anothermechanism to m~rvc thc strength. Chcmical intcractions between the substrate and thc coating may be 25 advant~geous but migration of material from thc subs~rate surface arca to thc coating should be Fcvcntcd if it will causc the formation of undercoating holes and pinholes and/or cracks in the coatings.
Thc prcceramic coating material, e.g., a polymer or precursor, from which the ccramic coating will be foTmed may comprise any tractablc inorganic or organometallic polymcr or 30 compound capable of be~ng in a liquid forrn or in solution and of wetting and adhering to the surfacc of the structural ceramic material and capable of conver~ing to a ce~amic coating on a c~amic subs¢ate by heat treatment. By way of exarnple, and not of limitation, the cerarnic coatings which may be forrned from such precursors may contain Si3N4, SiCN, SiC, C, SiON2, SiO2, ZrO2, A12O3, Y2O3, AlN, B4C, BN, TiC, WC, W2C, Mo2C, TiN, TiO2, 35 CaP2O8, and others, metal phosphates, metal silicates, metal borates, as well as other ceramics l~own for dleir mength and hardness, including other silicides, borides, nitrides, carbides, or 03cides, or mixtures thereof.
WO93/21131 2~ PCr/US93/032go Exampbs of preceramic coating materials such as polymers or precursors from which such ceramic coatings may be folmed compnse polysilazanes, such as poly-N-methylsilazane and/or polycyclomethylsilazane, from which ceramic compositions of SiN, SiCN, SiO2, SiOC, SiON and SiOCN (depending upon the atmosphere used during pyrolysis) may be 5 fonned; pdysiloxazane; polysiloxane, including polymethyl-silsesquioxane and polyhydridomethylsiloxane from which ceramic coating containing Si and O and potentially other elements such as C and N can be formed; polysilanes; polycarbosilanes; polyboranes;
poly~tmnes; polyaminoboranes; polyaminotitanium; and combinations thereof. Examples may also includc precu~sors to rlo2, AIN, A1203, ZrO2, Y203, metal phosphates, and 10 others. Thc above dcscribcd p~ic coating materials either may be obtained commercially may be readily synthesized using methods know to those skilled in the art. Reference may also be had to U.S. Patent Nos. 4,612,383, 4,788,309, 4,952,715, S,008,422 and 5,055,431, citcd and i~polated by referencc above. ln addition, ceramic precursor coating Jlutc~ials may includc compounds and polymers which arc totally inorganic and solublc in 15 water. By way of cxamplc, and not of limitation, inorganic ceramic precursor coating mate~als may includc Ca(H2P04)2 and Al(H2P04)3. Precursors made by sol-gel technology may also bc uscd. Howcver, thcy arc expectcd to bc less favorable due to short term stability in solution or on dlc shclf, and low ccramic yields and high shrinkage are obtained upon their pyrolysis.
It is p~efcrablc that thc ccramic yield of the precursor be at least S0 wt.%. More 20 ~efc~ably, it dlould bc a~ove 70 wt.% and most prcfcrably, it should be above 85 wt %.
Thc ~c~ ca~ic ma~ial which rnay bc strengthened by fosmation of thc cerarnic coadng ~on, in accordancc with thc invention, may comprise any particular shape,includilg flat shects, and shaped objccts, such as bars, rods, fibers or the like.
Thc struc~al ccramic material which may be strengthened by fo~nation of the cerarnic 25 coating ~c~on, in accordance with the invention, may include, by way of example and not of limitation, silicon nitride and A1203.
It may some~imes be prefeITed or necessary to pretreat the structural ceramic matenal with HF or other acids or tetrachlorosilane in order to hydroxylate the surface and thereby to op~ze the coating of the ceramic substrate with the preceramic coaang material. Whether or 30 not it is necessary to pretreat the structural ceramic material and the methods whereby such pretreatmcnt is to be effected may be dete~mined using routine experimental techniques.
The choice of the preceramic coating matelial and the pyrolysis schedule may vary depending on the substrate matelial. It is not necessary that a type of polymer that performed well for one kind of material will strengthen another type of material. The chemical interactions 35 at the intærface between the coating and the substrate is assumed tO play a role in the s~engtl~ng mechanism. Therefore, the development of a strengthening process for various matcria~s roquires screcning of polymer and process conditions individually for each material (sec Examplcs 1-3).
W093/21131 213~E6 Pcr/US93/03290 The preceramic coating rnaterial rnay be applied to the cerarnic substrate by any convenient method such as by dipping in a selec~ed coating solution or by spraying, paindng, spin~ing, or the like, with such coadng solution, the solution having a predetermined concentraion preferably between 0.1 and lOû wt.%, rnore preferably between about S and 30 5 wt.% for most applieadons. The pr~cerarnie coating rnaterial is applied to the cerarnic substrate in an amount sufficient to provide, upon subsequent pyrolysis, a ceramic coating of from about 0.01 to about 20 microns and preferably from about 0.05 to about 4 microns. Typically, the desired eeramic eoadng thiekness is aehieved in a single coating layer. i.e., without the use of several built-up layers. However, if neeessary, the coating process may be ~peated to build up 10 the desirvd thiekness without the fonnation of a substantial level of craeks in the developed eamie eoating. In addition, eraeked eoatings ean be healed by additional coating procedure.
Aehie~nng the desired thiekness of the eventual eeramic coating layer on the structural caamic mate~al is related to both the initial eoating thickness of the pecnan~ic coating material and the eeramie yield of the ~nuc coating material, e.g., polyrner or preeursor. The 15 coating thickness of the lnecewnic coating material is, in tUIn, related to the viscosity of the ooa~ng rnaterial solution and the arnount of sohent added to it to permit it to be applied to the surfaee of the ceramic substrate. The eeramic yield is related to: (1) the rnolecular strueture with referenee to whether the coating material eomprises a branched, erosslinked, or ring-type polymer, (2) ~e leeular wdght of the eoating material with higher leeular wdght 20 polymers favorcd because of thehigher ceramie yield of such mate~ials; (3) the latent reaetivity of the coa~ng ma~rial that allows theirrnosetting propcrties and furth crosslinking as desired (2); and (4) small amounts of ext~aneous organic groups if needed to p~vide tractability and shelf stability.
Gcnc~lly speaking, to achieve the desired ceramic coating thickness unifo~nly across t~he 25 surfacc of thc ceramic subs~rate, the pre~an~c coating material should have a sufficient lecular weight, e.g., be sufficiently polymerized, to provide a minimum viscosity, after rcmoval of solvent, of at least 1-3 poise so that the coating will remain in a uniform thickness on the structural ceramic material as the coating is heated to the pyrolysis temperature.
Altematively, ~e precursor can be a monomeric cornpound capable of condcnsation or cross-30 linlcing at the substrate surface prior to evaporation.
In some instances the preceramic coating material, although sufficiently polymeAzed toachieve the desired ceramic yield, will have a viscosity sufficiently low to permit it to be app lied to the structural ce~amic material as a uniforrn and hogeneous coating without the necessi o~diluting the coating material in a solvent, i.e., the "tractable" coating material, such as a 35 polymer, will be liquid o~, by application of heat, meltable.
Howcver, in othcr instanccs, the preceramic coating rnaterial will need to be diluted to bwer thc viscosity sufficicntly to facilitate application of the preceramic coating material as a coating on thc surfacc of the ce~amic substrate and to control the thickness of the coating In such cases, the p~c coating material must be capable of being dissolved in a solvent WO 93/21131 Z13~,,~, z~ ~. PCI`/US93/03290 ~ hich may later be removed from the coating without negative impact on the desired formation of the ceramic coating layer, i.e., the "tractable" coating material must be soluble.
Thus, in any event, the preceramic coating material, such as a polymer or precursor, must not only be of sufficiently high molecular wdght or non-volatile, or cross-linkable, to achieve 5 the desi~d Imnimum ceramic yield, but it must be "tractable" as well, as previously defined.
The p~mic coating material may be applied to the structural ceTannc material at ambien~ temperature, i.e., about 2~25 C, or either the coating material or the structural ceramic material may be at an elevated temperature.
In cases where the precerarnic polymer is sensitive to moisture and/or oxygen, it is i0 preferable to deposit the solution coating under a dry and/or oxygen- free environment. This is especially prefable in cases where it is desircable to exclude oxygen frorn the final coating composidon. However, from a practical processing point of view, it is preferable to use a polymer that is not air sensitive or only slighdy air sensitive as this will not reguire spccial costly oquipment and more expensive operation. In cases where oxygen is desired to be in the fin~ coating ce~amic cornposition, polymers sensitive to air may be preferably coated in air or exposed to air in~di~cly after the coating procedure and prior to heat treatrnent to allow curing alt low temperature.
The mimmum tempaatune to which the coated structural cerarnic material must be heated will be daa~d by the p~icularpreceramic coating material used and the minimurn Ico_eatwl~ch acamicmatesial may bef~rmcd from such aprecanuccoating material. Thus, the minimum pyrolysis temperature must be at least at, and preferably above, the ~e at which dle pec~m~c coating material converts to a ceramic material. This dctcmlinaion of thc convcrsion of the p~c ooating material to a ccra~c ma~erial may be TGA, IR, XRD, NMR, and elemental analysis.
The mimmum temperature, however, must be high enough to permit organic groups, if containcd in thc preccra~c coating material, e.g., polymer or organometallic cornpound, to be pgrolyzed, leaving only an inorganic ceramic network. For example, when a poly-N-methylsilazane polymer having a molecular weight of 800-3000 Daltons is used, the minimum ~an~e needed to remove extraneous methyl groups from the coating and to forrn the ccramic material is about 300 C to SSO C. However, higher tempcratures may be needed to aUow desired chernical orphysical interactions between the coating and thc substrate or to densify or crystalizc the cerarnic coating materials.
The preceramic coating material may be pyrolyzed to convert it into a ceramic coating using a variety of heating schcdules, including different heating rates, dwell times at inteImediate and tnaximum temperatures, and cooling rates, depending upon the specific material used.
In any of thc embodiments used for heating the coated ceramic substrate to the pyrolysis ~n~uue, ~c ooatcd ccramic substrate is held at the pyrolysis temperature for a period of dmc sufficicnt to pcm~it fonnation of the ceramic coating and to develop the desired strength. It will be und~stood that if less than the optimal dwell time is used for the particular preceramic W093/21131 21~6 ' Pcr/usg3/o3290 coating and structural ceramic material, a cc~ic coating may be formed over the ceramic substrate in accordance with the invendon, but the coated cerarnic substrate may not have as much strength or hardneæ as is possible if the dwell time is extended. There will then be a tradc~f between desired strength and process economics.
The holding or dwell time of the coated ceramic substrate at the pyrolysis temperature may, therefore, vary from a minimum of O minutes, preferably at least about S minutes, and most preferably at least about 90 minutes. up to a maximum time of about 2 hours. Longer time periods may be used, but are usually unnecessary and. therefore, not economically justifiable. In some instances shorter dwell times of, for example, 30 60 minutes may be used, even though rnaximum strength hæ not been devdoped, if the economics of the process justifies the trade-off of reduced dwell time with reduced strength.
It should be further noted, with respect to the period of time used to initially heat the coated structural ceramic material up to the pyrolysis temperature, that it is desirable, from a standpdnt of econon~ics, to conduct the wann up period, dwell period at the pyrolysis ~, and cooling-off period in as shart a period of time as possible. Preferably. the er~re pyrolysis period is carried out in a period of about 2 hours. By "the entire pyrolysis period" is meant f~n the time the coated ceramic substrate is placed in the furnace at room ~e until the ternpcrature during cooling again reaches 200 C.
The coated-stn~c~al cerarnic material, duling both the temp~e ramp-up peri-ld and tbe dwell per;od, may be rnaintained in an inert atmosphere such as argon, a nitrogen-contai~ng atmosphere such as N2 ~ ammonia, a hydrogen atrnosphere, oq mixtures of the aforementioned atmospheres, and a dly- or wet-airatmosphere. depending upon the chernical oonstitucnts o~f the p~mic polynxr and the desired ce~amic-coating composition. For ac;unple, the foqmation of a ceramic with Si-O bonds is favored when using an air atmosphere wbe~eas the foTmation of a ceramic with Si-N bonds is favored when thc p~amic polymer contains both silicon and nitrogen and either an inert or a nitrogen-containing a~nosphere is used during the entire pyrolysis period.
The choice of the pyrolysis environment in conjunction with the preceramic polymer structure will deteImine the final ceramic composition. Any precursor that is pyrolyzed in air will provide a final composition consisting mainly of oxide material. If the pyrolysis is conducted in an inert atmosphere such as nitrogen or argon, the final composition will contain substandal amounts of carbon if the precursor contained organic pendant groups or carbon was an elernent in the polymer skeleton (e.g., polycarbosilane). If nitride coating is desired then the polyrners should be preferably pyrolyzed in arr~nonia. Oxyni~ide and oxycarbide coatings can also be fo~rned if oxygen is incorporated in the skeleton of the original polymer or during the pyrolysis pcIiod. The choice of the final coating material might vary based on the application and thc physical and/or chcmical conditions at which the product is going to perforrn.
Howcver, for cconomic rcasons, it is preferable to conduct the pyrolysis in air.
wo 93/21 131 21 3,~ PCr/USg3~032so The following exarnplesq~ill serve to further iUustratc the invendon, including some of thc paramcters of the process.
It is to bc undcrstood that whilc the invcndon has been described in conjunction with the prefcrred embodimcnts thcrcof. that the foregoing dcscription and the examplcs which follow 5 are intended to illustrate and not lirnit the scope of the invention. Other aspects, advantages and rnodifications within the scope of the invention will be apparent to those skilled in the art to which thc invention pertains.
Five ground, highly polished beveled silicon nitride bars, the dirnensions of which were approximatcly 50 mm x 4 mm x 3 mm, wcre ultrasonically cleaned and then dehydrated by heating at S00 C undcr nitrogen for 1 hour. Bars were then dipped into a 10 wt.% polymer s~lution in tc~ahydrofuran (udess otherwise indicated) under a selected atmosphere and pyrolyzed as dcscribed below. (Pyrolysis conditions are set forth in Table 1 below.) The pyrolysis proccss was carried out under the selected atmosphere (1 atm) according to thc following schedule: (a) heatingrate of 5-C or 10-C/minute (see Table 1), (b) dwell period at the maximum temperature for 1 hour, and (c) cooling rate of 10 C/min. The entire coating and pyrolysis plwed~e were repeated 3 tirnes. Thc bars were tested for strength in accordance widl the fo~-point be~d tcst of ASTM C158. Results are set forlh in Table 1. These results 20 dn~_ that the s1rcngth of ground and lapped silicon nitride bars can be significandy m~ved by f~ng a caamic coaing on tlu~ surfacc by the process dcscribed above. The ~ts a1so show tbat similar or grcater s~ength can be achicved when thc precu~or coating rn;u~i~ is c~d to a ce~ic by hea~ng in air or a nitrogcnous gas.
WO 93/21131 Z13iQ16 PCr/US93/03290 Ta~
Results of Four-Point-Strength Measurements of Coated Ground and Lapped Si3N4 Bars ' Example Polymer Atmosphere Heating Strength 10% Coat Heat ,~ched~lea ((Sav MPa!
UNTREATE~b -- AIR 10 /900 /2h 827 1 PCSC A~d AIR 5-/150-/4h 10 /~00 /lh 827 2 PCS N2e N2 10 /150-/4h 10-/900-/lh 895 3 PCMSf N2g NH3 5-/150-/2h N2 10 /900 /lh 860 4 Ca(H2P04)2h A~i AIR 5-/150-/4h 10 /800 /lh 916 PCMS N2 AlR 10-1900-/lh 990 6 PCMS N2 A~ 10 /900-/lh 817i 2~
7 PNMSk N2 AIR 10 /~OO-/2h 981 8 PNMS N2 NH3 1-/150-/4h 10 1~00-/2h 893 a Rate of tempe~ature increase in C per min/dwell temperatur~,, in C/duration at dwell ~a~ in hours. Each line entry represents a sequenaal step in the heating schedule 35 b Heat treated 3 times to ~O C at a rate of 10 ~ per min with a dwell period of 2 hours each time.
c Polycarbosilane in tetrahydrofuran solution The polycarbosilane was trea~ed with 50 ppm Ru3(CO)2 as a crosslin~ng catalyst found to e~fectively increase the ceramic yields d Coated and dried twice in air e Coated and ~ied three times in N2 45 f Polycyclor,nethylsilazane in tetrahydrofuran solution g Coat~d and dried twice in N2 h Soluion in H2O.
i Coated and dried three times in air WO93/21131 ~ "J ~'7~5~ ` PCI/US93/03290 Two extremdy wcak bars wcre included in this batch of five bars indicating majorinternal flaws. the othcr three bars obtained strength values of 887, 941, and 978 MPa.
k Poly-N-methylsila2ane in tetrahydrofuran soluion.
ExamDIe 2 The p~ocedures of Exan~ple I can be followed using as-ground silicon nitride bars and 10 thereby achieve essen~ally identical results (see Table 2).
Table 2 Results of Four-Point-Strength Measurements of Coated As-ground Si3N4 Bars Example Polymer Atmosphere Heating Strength ~Umber 10% Coat Heat Sc~ulea (av MPa) 9 PCMSb N2 AIR 10-~gO0 /;!h 954 -- -- 10-/900 /lh 827 2~
Ca(H2PO4)2 AIR 10 /gO0 /4h 837 30 a Ratc of te~ature increase in C per min/dwell temperature in C/duration at dwell tempe~ature in hows. Each line entry represents a sequential step in the heating schedule.
b Polycycl~ethylsilazane in tetrahydrofuran solu~on.
WO93/21131 2~3ldi~ Pcr/US93/03290 ~am~.le...~
T~le procedures of Example 1 can be followed using A12O3 bars and thereby achieve essentially identical results (see Table 3).
.
Table 3 Results of Four-Point-Strength Measurements of Coated A12O3 Bars ~ple Polymer Atmosphere Heating Strength ~umber 10% Coat Heat ~h~a (o ~a~
UNlREATE~b -- A~ lO ~900 /2h 223 11 PNMSC N2 AIR 10 /gO0 /2h 332 12 Ca(H2Po4)2 AlR AIR 10 /900/2h 208 20 13 Ca(H2PO4~2 AIR AIR 10 /900 /2h 247 14 PCSd N2 AIR 10 /900 /2h 265 25 a Ratc of ~npcraturc inc~easc in C pcr min/dwell tcmperature in C/duration at dwell empa~e in hours. Each linc entry rcprcscnts a scquential step in the hea~ing schedule.
b Heat ~ed 3 ~mes to 900 C at a rate of lO-C per min with a dwell period of 2 hours each ~me.
c Poly-N-methylsilazane in te~ahydrofuran solution.
d P~lycar~osilane in ~trahydrofuran solution.
Claims (37)
1. A method for improving the strength of and repairing surface defects in a structural ceramic material which comprises:
a) forming a tractable ceramic precursor coating solution by dissolving in an organic solvent or in water an inorganic or organometallic compound or an inorganic or organometallic polymeric preceramic coating material to provide a liquid preceramic coating solution:
1) capable of converting by heat treatment after solvent evaporation to a ceramic coating selected from the group consisting of nitrides, carbides, oxides, silicides, and borides;
2) capable of providing a ceramic yield of at least 50 wt.%;
3) being a solid or having a viscosity, after application to the structural ceramic material and removal of solvent, of at least 1 poise; and 4) capable of wetting and adhering to the structural ceramic material to form a uniform coating on the surface of the structural ceramic material;
b) coating said structural ceramic material with said preceramic liquid coating solution;
c) heating said coated structural ceramic material in a gaseous environment selected from the class consisting of an inert atmosphere, a nitrogen-containing atmosphere, a hydrogen atmosphere, an oxygen atmosphere, and a dry- or wet-air atmosphere, or mixtures of the aforementioned atmospheres, to a pyrolysis temperature at least capable of converting said preceramic liquid coating solution to a ceramic coating; and d) maintaining said coated structural ceramic material at said pyrolysis temperature for a period of time sufficient to form the ceramic coating and remove those portions of said coating material which do not participate in forming said ceramic coating.
a) forming a tractable ceramic precursor coating solution by dissolving in an organic solvent or in water an inorganic or organometallic compound or an inorganic or organometallic polymeric preceramic coating material to provide a liquid preceramic coating solution:
1) capable of converting by heat treatment after solvent evaporation to a ceramic coating selected from the group consisting of nitrides, carbides, oxides, silicides, and borides;
2) capable of providing a ceramic yield of at least 50 wt.%;
3) being a solid or having a viscosity, after application to the structural ceramic material and removal of solvent, of at least 1 poise; and 4) capable of wetting and adhering to the structural ceramic material to form a uniform coating on the surface of the structural ceramic material;
b) coating said structural ceramic material with said preceramic liquid coating solution;
c) heating said coated structural ceramic material in a gaseous environment selected from the class consisting of an inert atmosphere, a nitrogen-containing atmosphere, a hydrogen atmosphere, an oxygen atmosphere, and a dry- or wet-air atmosphere, or mixtures of the aforementioned atmospheres, to a pyrolysis temperature at least capable of converting said preceramic liquid coating solution to a ceramic coating; and d) maintaining said coated structural ceramic material at said pyrolysis temperature for a period of time sufficient to form the ceramic coating and remove those portions of said coating material which do not participate in forming said ceramic coating.
2. The method of claim 1 wherein said step of coating said structural ceramic material with said preceramic liquid coating solution further comprises coating said structural ceramic material with an amount of said preceramic liquid coating solution sufficient to form a ceramic coating, after said pyrolysis step, having a thickness of from about 0.01 to about 10.0 microns.
3. The method of claim 2 wherein said thickness is from about 0.05 to about 5.0 microns.
4. The method of claim 1 wherein said preceramic liquid coating solution is comprised of an organic based polymeric preceramic liquid coating solution.
5. The method of claim 1 wherein said preceramic liquid coating solution is comprised of an inorganic based polymeric preceramic liquid coating solution.
6. The method of claim 1 wherein said inorganic compound is comprised of an inorganic salt.
7. The method of claim 6 wherein said inorganic salt is comprised of a metal hydridophosphate salt.
8. The method of claim 1 wherein said tractable ceramic precursor coating solution is comprised of an organometallic polymeric preceramic liquid coating solution.
9. The method of claim 8 wherein said organometallic polymeric preceramic liquid coating solution is comprised of a solution of an organometallic compound capable of crosslinking or decomposition prior to evaporation.
10. The method of claim 1 wherein the concentration of said liquid preceramic coating solution is between 0.1 and 100 wt.%.
11. The method of claim 10 wherein the concentration of said liquid preceramic coating solution is between 5 and 30 wt.%.
12. The method of claim 1 wherein the ceramic yield is at least 70 wt.%.
13. The method of claim 12 wherein the ceramic yield is at least 85 wt.%.
14. The method of claim 1 wherein said step of coating said structural ceramic material with said preceramic liquid coating solution is repeated at least one additional time.
15. The method of claim 14 wherein said step of coating said structural ceramic material with said polymeric preceramic liquid coating solution is repeated 2 to 5 times.
16. The method of claim 1 wherein said gaseous environment comprises a hydrogen atmosphere.
17. The method of claim 1 wherein said gaseous environment comprises an oxygen atmosphere.
18. The method of claim 1 wherein said pyrolysis step further comprises heati?
said coated structural ceramic material at a rate of from about 30°C per hour up to a rate of about 3300°C per hour up to at least said pyrolysis temperature.
said coated structural ceramic material at a rate of from about 30°C per hour up to a rate of about 3300°C per hour up to at least said pyrolysis temperature.
19. The method of claim 18 wherein said pyrolysis step comprises heating said coated structural ceramic material at a rate of from 100°C per hour to about 600°C per hour.
20. The method of claim 1 wherein said pyrolysis step further comprises heating said coated structural ceramic material up to said pyrolysis temperature by inserting said coated structural ceramic material into a pyrolysis zone already preheated to said pyrolysis temperature.
21. The method of claim 1 wherein said maintaining step further comprises maintaining said coated structural ceramic material at said pyrolysis temperature for a period of time of at least about 10 minutes.
22. The method of claim 21 wherein said maintaining step further comprises maintaining said coated structural ceramic material at said pyrolysis temperature for a period of time of at least about 60 minutes.
23. The method of claim 1 including the further step of cooling said coated structural ceramic material from said pyrolysis temperature down to a temperature of about 200°C at a rate not exceeding about 60°C per minute.
24. The method of claim 1 including the further step of cooling said coated structural ceramic material from said pyrolysis temperature down to a temperature of about 200°C at a rate not exceeding about 30°C per minute.
25. The method of claim 1 including the further step of cooling said coated structural ceramic material from said pyrolysis temperature down to a temperature of about 200°C at a rate not exceeding about 10°C per minute.
26. The method of claim 1 wherein said structural ceramic material is silicon nitride.
27. The method of claim 1 wherein said structural ceramic material is Al2O3.
28. The method of claim 1 further comprising pretreating said structural ceramicmaterial with acid prior to coating with said preceramic liquid coating solution.
29. The method of claim 28 wherein said acid is comprised of HF.
30. The method of claim 1 further comprising pretreating said structural ceramicmaterial with tetrachlorosilane prior to coating with said preceramic liquid coating solution.
31. A structural ceramic material coated with a layer of ceramic material to increase the strength of said structural ceramic material and formed by coating said structural ceramic material and heating said coated structural ceramic material at a temperature sufficient to form said ceramic coating on said structural ceramic material.
32. The ceramic coated structural ceramic material of claim 31 wherein said ceramic-coated structural ceramic material comprises silicon nitride coated with ceramic.
33. The method of claim 1 wherein said gaseous environment comprises an inert atmosphere.
34. The method of claim 1 wherein said gaseous environment comprises a nitrogen-containing atmosphere.
35. The method of claim 33 wherein said inert atmosphere is comprised of Ar.
36. The method of claim 34 wherein said nitrogen-containing atmosphere is comprised of N2.
37. The method of claim 34 wherein said nitrogen-containing atmosphere is comprised of NH3.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86860892A | 1992-04-14 | 1992-04-14 | |
| US07/868,608 | 1992-04-14 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2131016A1 true CA2131016A1 (en) | 1993-10-28 |
Family
ID=25352001
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2131016 Abandoned CA2131016A1 (en) | 1992-04-14 | 1993-04-08 | Process for increasing strength of structural ceramic materials by forming ceramic coating on surface and product formed thereby |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0636113A1 (en) |
| JP (1) | JPH07505855A (en) |
| CA (1) | CA2131016A1 (en) |
| WO (1) | WO1993021131A1 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE19500832C2 (en) * | 1995-01-13 | 1998-09-17 | Fraunhofer Ges Forschung | Dense silicon nitride composite and process for its manufacture |
| US6413578B1 (en) * | 2000-10-12 | 2002-07-02 | General Electric Company | Method for repairing a thermal barrier coating and repaired coating formed thereby |
| JP5500671B2 (en) * | 2009-09-01 | 2014-05-21 | 一般財団法人ファインセラミックスセンター | Molding material, method for producing the same, and high-temperature superheated steam generation system including the same |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5128494A (en) * | 1985-04-26 | 1992-07-07 | Sri International | Hydridosiloxanes as precursors to ceramic products |
| US4617232A (en) * | 1982-04-15 | 1986-10-14 | Kennecott Corporation | Corrosion and wear resistant graphite material |
| JP3059453B2 (en) * | 1988-08-01 | 2000-07-04 | エス・アール・アイ・インターナシヨナル | Method for increasing the strength of glass by forming a ceramic coating on the surface and products made thereby |
| DE3904118A1 (en) * | 1989-02-11 | 1990-08-16 | Hoechst Ag | HIGHLY FIXED COMPOSITE CERAMICS, METHOD FOR THE PRODUCTION THEREOF AND THEIR USE |
-
1993
- 1993-04-08 WO PCT/US1993/003290 patent/WO1993021131A1/en not_active Application Discontinuation
- 1993-04-08 CA CA 2131016 patent/CA2131016A1/en not_active Abandoned
- 1993-04-08 JP JP5518463A patent/JPH07505855A/en active Pending
- 1993-04-08 EP EP93909284A patent/EP0636113A1/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| JPH07505855A (en) | 1995-06-29 |
| EP0636113A1 (en) | 1995-02-01 |
| WO1993021131A1 (en) | 1993-10-28 |
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