IMPROVED INTERFACE COATING FOR CERAMIC FIBERS

FIELD OF THE INVENTION

The present invention relates to an improved interface

coating for ceramic fibers, particularly fiber-reinforced ceramic

composites, and to a process for forming an interface coating for

ceramic fibers to provide oxidation protection at elevated temperatures extending to 1500°C and hydrolysis protection.

BACKGROUND OF INVENTION

The high mechanical strength properties of ceramic fibers

and their resistance to chemical attack have made ceramic fibers

attractive for use in a variety of applications, such as reinforcement materials for aircraft engine parts and aircraft structures, as well as combustors and radiative burning parts. However, to retain their

desirable properties in oxidizing environments, it is necessary to coat

the fibers with an interface coating to provide a surface which will keep

the fibers distinct from the matrix with which the fibers form a composite.

Stated otherwise, the fibers are intended to reinforce the matrix and, therefore, cannot diffuse into the matrix during processing. It is the

ability of the fiber to be stressed independently of the matrix which

imparts desirable ductility and fracture toughness to ceramic fiber

composites. To maintain the fibers distinct from the matrix, the interface

coating should function as a lubricant and exhibit only a weak mechanical bond to the matrix.

Cu ently, pyrocarbon and boron nitride are the most commonly used interface coatings for ceramic fibers in fiber-reinforced

ceramic composites. Such coatings are chemically distinct from, and

react little with, the principal reinforcing fibers, which are typically

composed of silicon carbide, silicon nitride, aluminum oxides, and various metal silicates, and matrices, such as glass, metal oxide, silicon carbide, and silicon nitride. Pyrocarbon and boron nitride exhibit weak mechanical bonding to the matrix when applied to the fiber However,

both of these interface coatings are susceptible to relatively rapid

oxidation at temperatures exceeding 1000°C, so that their use is limited to ceramic composites designed to operate below 1000°C There is no commercially satisfactory interface coating suitable for sustained use

above 1000°C in oxidizing atmospheres, even though ceramic

composites are generally regarded as refractory materials. Boron nitride

coatings, which have a higher temperature capability than pyrocarbon

coatings in oxidizing atmospheres, are also subject to attack by

moisture. Although ceramic fibers and matrices exist which retain their mechanical properties in oxidizing environments at temperatures

exceeding 1000°C, fiber-reinforced composites which can withstand the

same conditions have not yet been developed for lack of a suitable

interface coating. Accordingly, there is a need for an interface coating which performs well under these conditions.

SUMMARY OF THE INVENTION

It has been discovered in accordance with the present invention that a silicon containing boron nitride coating having the

composition BSi,N _{1+1 33x} , hereinafter referred to as PB(Si)N, forms an

interface coating for ceramic fibers for use in fiber-reinforced ceramic

composites. The PB(Si)N interface coatings are particularly suitable for coating non-oxide fibers consisting mainly of silicon carbide or silicon nitride, in that they provide good deboπding characteristics. The silicon- containing boron nitride coatings of the present invention are better than

conventional boron nitride interface coatings in their resistance to

oxidation at elevated temperatures and in their resistance to moisture pick-up at room temperature. At the same time, the silicon-containing boron nitride coatings of the present invention mechanically debond

from the fiber, which promotes fracture toughness in the composite. In

accordance with the present invention, the preferred method for forming

the silicon-containing boron nitride coatings of the present invention is

to pyrolytically codeposit boron nitride and silicon nitride, under controlled deposition conditions, to form the general composition, if

stoichiometric, BSi _x N _{1+1 33)(} .

The interface coating of the present invention comprise a glassy or amorphous structure of a silicon-containing boron nitride having the general composition BSi _x N _1Λ3x , and containing from 2wt% to 42wt% of silicon, with substantially no free silicon present.

The process of the present invention for forming a silicon-

containing boron nitride coating on ceramic fibers comprises the steps

of: placing the fibrous material to be coated within a furnace chamber of a reactor vessel; heating the atmosphere within the furnace chamber to a uniform temperature of between 1300°C and 1750°C; reducing the chamber pressure to between 0.1 and 1.5 Torr; introducing reactant

vapors into said furnace chamber consisting essentially of ammonia and a gaseous source of both boron and silicon at a flow ratio to nitride the

deposit forming a coating complex of PB(Si)N on said fibrous material

having from 2wt% to 42wt% silicon, with substantially no free silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the present invention will become

apparent from the following detailed description of the present invention

when read in conjunction with the accompanying Figures:

Figures 1A to 1 D inclusive show at different magnifications that a

PB(Si)N coating having 20wt% silicon debonds readily from a silicon

carbide monofilament fiber formed by chemical vapor deposition.

DETAILED DESCRIPTION OF THE INVENTION

The interface coating of the present invention is

achieved by codepositing silicon with boron nitride under conditions to nitride the silicon with essentially no free silicon present to form a pyrolytic (boron and silicon) nitride interface coating having the

composition BSi _x N _{1+1 Z3xt} hereinafter referred to as PB(Si)N The coating

appears glassy or amorphous by x-ray diffraction and silicon and boron

are uniformly distributed throughout the deposit. This interface coating resists oxidation and hydrolysis better than conventional boron nitride coatings and debonds well from fiber substrates including non-oxide silicon carbide or silicon nitride fibers. The term "silicon carbide fiber"

includes any fiber which is used in continuous form (that is, the ratio of

its length in use to its diameter exceeds 1000), and which is composed

of at least 50% silicon and carbon by volume. These include, without limitation: Textron Specialty Materials Inc.'s SCS-senes monofilaments

made by a chemical vapor deposition process, in which SiC and other deposits are made on carbon or tungsten filament cores; Nippon

Carbon Company's "Nicalon" and "Hi-Nicalon" polyfiiameπt yarn

produced by pyrolyzing polymer precursors which contain some free

carbon, silicon oxide, and silicon oxycarbide phases, Ube Industπes's

"Tyranno Fiber" series of polyfiiament yarns which contain additions of

titanium and nitrogen to promote thermal stability; and Carborundum Company's SiC polyfilament yarn, which incorporates boron. Those skilled in the art will readily recognize that the term "silicon carbide fiber" tolerates other ceramic phases.

Conventional chemical vapor deposition may be used to deposit the interface coatings of the present invention using typical gas sources and deposition conditions for separately depositing boron nitride and silicon nitride, as known in the art, for example, pyrolytic

boron nitride may be separately formed on a free-standing structure by

the thermal decomposition of boron trichloride and ammonia vapors at a

reaction temperature of between 1450°C and 2300° C. In accordance with the present invention, pyrolytic boron nitride may also be codeposited with silicon under controlled conditions of gas flow rate and deposition temperature to form an interface codeposited coating

complex of PB(Si)N containing essentially no free silicon. The interface

coatings of the present invention do not show distinct x-ray diffraction

peaks, and so are considered to have a glassy or amorphous structure.

Silicon is distributed uniformly throughout the deposit. Useful interface

coatings have silicon contents in the range 5wt% to 42wt%. Coating thickness is required to be small in comparison with the diameter of the

uπcoated fiber, in order to minimize the volume of coating phase in the final composite, while protecting the filaments from oxidation, hydrolysis

and damage from textile-handling operations. Thus, for example, individual filaments of ceramic-fiber yarn, which have diameters 8μ to

15μ, require coatings 0.1 μ to 0.8μ thick, and more preferably 0.3μ to

0.6μ thick. Ceramic monofilaments having thicknesses =150μ thick,

however, receive coatings 1 μ to 12μ thick, and more preferably 4μ to 9μ thick.

The interface coatings are deposited in a furnace chamber

of a reactor vessel at chamber pressures of 0.1 to 1.5 Torr, with chamber pressures of 0.1 to 0.3 being preferred, for continuous coating

of yarns. The preferred deposition temperature range is between

1300°C and 1750°C, with the temperature range of 1300°C to 1450°C

preferred for coating yarn bundles. Coatings can be applied in a batch

process, as is well known, or to a moving array of filaments, as is described by A.W. Moore Mats. Res. Soc. Symp. Proc. 250, 269(1992). Batch CVD processing is used to coat yarns and monofilaments of

indefinite length.

Although the source gases for boron and silicon are not

critical, the preferred source gases are boron trichloride (BCI ₃ ), trichiorosilane (HSiCI ₃ ), and ammonia (NH ₃ ). Less desirable boron sources include boron trifluoride or diborane, and less desirable

sources of silicon include dichlorosilane and silicon tetrachloride.

Coatings are usually deposited with no diluent other than excess

ammonia, but argon or nitrogen can be used as diluents. Hydrogen can

also be used as a diluent. In order to nitride the deposit, one desires the ratio (NH ₃ )/(BCI ₃ + HSiCI ₃ ) to be in the range 1.3 to 3.0, and more preferably in the range 1.5 to 2.5, in which the formulas denote gas flow

rates. When these gases are used with or without inert diluents, the

desired range of interface coating compositions is attained if

trichlorosilane accounts for a mole fraction of the inlet gases in the

range 0.05 to 0.2, and if the ratio of gas flow rates 0.2 < (BCI ₃ )/(BCI ₃ + HSiCl ₃ ) < 0.5. Interface coatings produced by the present invention

contain silicon tied to the boron and nitride in a compositional relationship BSi _x N _{1+1 33x} , with essentially no free silicon present in the coating. The content of silicon may vary between 2wt% silicon and

42wt% silicon, with the preferred silicon content in excess of 5wt%, and

being based on the rate of oxidative weight loss for a given temperature.

The following Table 1 compares oxidative weight losses

for PBN and PB(Si)N coatings. When exposed to pure flowing oxygen at

1200°C for 12 hours, pure PBN coatings experience a weight loss =4.3

mg/cm -hour. PB(Si)N coatings, which contain 5wt% silicon, experience

weight losses =0.14 mg/cm ² -hour, and coatings which contain 42wt% silicon experience weight losses <0.003 mg/cm ² -hour. Rates of oxidative weight loss are thus decreased by factors of 30 to 1 ,430 by

incorporation of silicon in the coating, thereby extending the useful time at a given temperature.

TABLE 1

Oxidative Weight Losses of PBN and PB(Si)N

12-hour weight loss, 12-hour weight

Silicon 1200°C, Oxygen loss, 1510°C,

Sample Wt% mg/cm ² -hour Air mg/cm ² -hour

8926 0 1.460

9409-2 0 4.600

1 0 36.00

9409-1 0 4.000

2 2 6.00

8924 5 0.140

3 15 2.00

8933 16 0.007

4 18 0.40

A293 23 0.054

6 26 0.03

9001 33 0.044

9412-2 36 0.064

9412-1 36 0.057

9413-1 42 0.003

Since the thickness of interface coatings is less than 1 μ,

Table 1 shows that exposed pure BN coatings would be entirely

consumed in 7 minutes 12 1200°C in oxygen, and in less than one

minute at 1510°C in air. Or, if an array of fiber ends at the edge of a

composite were exposed to the same environments, pure PBN interface

coatings would recede at a rate of 1 cm/hour at 1200°C in oxygen and

at a rate of 20 cm/hour at 1510°C in air. They are not suitable for extended use in such environments. But PB(Si)N coatings containing

only 5wt% silicon experience much smaller weight losses at 1200°C, and coatings containing as little as 18wt% silicon experience similarly

small weight losses at 1510°C, providing a duration of hours for exposed fiber coatings or recession rates far less than 1 cm/hour for coatings on exposed fiber ends. The most preferred range of compositions, from this viewpoint, is 15wt% to 40wt% silicon.

PB(Si)N interface coatings of the present invention also

resist moisture better than conventional PBN coatings, which hydrolyze

to form borates by reactions such as:

BN + 3H ₂ 0 = H ₃ B0 ₃ + NH ₃ and 8BN + 19H ₂ 0 = (NH ₄ ) ₂ 0:4B ₂ 0 ₃ :6H ₂ 0 + 6NH ₃ .

This hydroiysis is undesirable for two reasons. First, it consumes the interface layer, which is typically about 0.5μ thick and is necessarily

much thinner than a ceramic fiber, so that it can no longer act as an

interface between the fiber and the matrix. When the interface layer is

consumed, the composite loses its fracture toughness and its ability to

fail gracefully Second, it introduces borates which can react with oxide

matrices or fibers at elevated temperatures, thereby changing their

structural properties. Table 2 compares the weight gains experienced

when three interface coating deposits were powdered, exposed to 95%

relative humidity at room temperature for 120 hours, and dried at 150°C for four hours in flowing dry nitrogen Whereas the pure PBN deposited

at 1080°C had an initial oxygen content of 14wt% and experienced a weight gain of 9% due to hydrolysis, and pure PBN deposited at 1400°C

had an initial oxygen content of 0 56wt% and experienced a weight gain

of 3 2% due to hydrolysis, PB(Sι)N containing 36wt% silicon deposited

at 1400°C had an initial oxygen content only 0 35wt% and experienced a weight gain of only 0.3% Thus, the decisively lower oxygen contents and hydrolysis rates of the PB(Sι)N interface coatings mark them as a

better choice than pure BN interface coatings in practical applications

such as combustors, radiative burning tubes and turbine parts, in which

fiber-reinforced ceramic composites are exposed to moisture and also to hot oxidizing atmospheres

Interfacial shear strengths of PB(Sι)N-coated silicon carDide fibers in a silicon carbide matrix are 20MPa to 30Mpa,

comparable to but somewhat lower than those of pure PBN coatings

Values in this range indicate good deboπding properties Scanning

electron micrographs of the interface, Figure 1 clearly show that a

PB(Si)N coating having =20wt% silicon debonds readily from a silicon carbide monofilament fiber formed by chemical vapor deposition (Textron Specialty Materials, Fiber SCS-0).

Desirable debonding properties of the present coatings

are attributable to a large difference in thermal expansion between

coating and fiber. Thermal expansions to 1500°C of SiC (the fiber substrate for much of this work), PB(Si)N, and PBN are respectively 0.72%, 025% to 0.35%, and 0.25%. Since the differences in thermal expansion exceed 0.20%, thermal strains can overcome the relatively

weak attraction between these coatings and the substrate. In fact, these coating always spalled from flat plates of polished silicon carbide and

from SiC tubes approximately one-inch in diameter, and intact coatings could be obtained only on the fibers, which were 150μ in diameter or smaller. When interface coatings are applied to fibers having a diameter

of 25μ or smaller, which generally are used as multifilament yarns or

tows, debonding is also promoted by differential thermal contraction

within the coating. As PBN and PB(Si)N, which are somewhat

anisotropic, are cooied from the deposition temperature, differences in radial and circumferential expansion produce internal stresses in the coating. Laminar cracks appear in the coating or at the coating-fiber

interface near a ratio of coating thicknesses to fiber radius

approximately 0.04 [L.F. Coffin, Jr., J. Am. Ceramic Soc, 47, 473

(1964)], thereby debonding the fiber from the matrix at the coating

interface. For a fiber of 15μ diameter, such as Nippon Carbon Company's "Nicalon" or "Hi-Nicalon," interface coatings have inherently weak interfaces when the thickness exceeds approximately 0.3μ.

Thicker coatings are generally preferred to protect the fiber from

damage during ceramic processing.

Oxidation and hydrolysis results are shown in the following table:

TABLE 2

Hydrolysis of PBN and PB(Si)N Compositions

Deposition Increases After

Temperature Siiico Oxygen Humidifϊcation

Sample °C n Wt% and Drying, Wt% Wt%

A 1080 0 14.0 9.0

9409 1400 0 0.56 3.2

9412 1400 36 0.36 0.3

The following Tables 3 and 4 show the process conditions

and oxidative weight loss rates at 1200°C in oxygen and the continuous

coating properties for a Hi-Nicalon yarn ceramic fiber at varying silicon concentrations, respectively.

TABLE 3

Process Conditions and Oxidative Weight Losses of PBN and PB(Si)N Coatings

Weight Loss Rates at 1200°C in Oxygen

Specimen Silicon Temperature P BCI NH HSiCI Diluent

Dilue nt Weight 10 Wt% Degrees C Torr Flow Flow Flow Flow

Lo ss Rate l/min l/min l/min l/min mg/ cm ₂ -hour I

20

TABLE 4 Continuous Coating of Hi-Nicalon Yarn

25

Run No Silicon Temperature BCI NH HSiCI Ar Coating

Deposition

Wt% Degrees C Torr flow Flow Flow Flow Thickness

Time l/min l/min l/min l/min um mm

30 9435A 1400 0 22 0 45 1 4 0 00 1 2 0 5-0.7

The following examples illustrate the invention:

1 Resistance of BN and PB(Si)N coatings to oxidation and hydrolysis at temperatures less than 1000°C was compared. PBN

specimen A containing no silicon was deposited at 1080°C at a pressure

of 0.25 Torr from a gas mixture consisting of BCI ₃ (0 5 l/min and NH ₃ (1 5

l/miπ) in a volume of 5 5 liters. A second PBN specimen 9409 containing no silicon was deposited at 1400°C at a pressure of 0 25 Torr from a gas

mixture consisting of BCI ₃ (0.52 l/min), NH ₃ (1 48 l/min) and Ar (4 l/min) in a volume of 5 5 liters Then a PB(Si)N specimen S412 containing 36wt%

silicon was deposited at 1400°C at a pressure of 0 115 Torr from a gas

mixture consisting of BCI ₃ (0 44 l/min, NH ₃ (1 51 l/min), and HSiCI ₃ (O 35 l/min) Oxidation and hydrolysis results are shown in Table 2 PBN specimen A deposited at 1080°C achieved an oxygen content 14 0wt% prior to hydrolysis, and gained an additional 9 0wt% after humidificatioπ

and drying. PBN specimen 9409 deposited at 1400°C achieved an oxygen

content 0.56wt% prior to hydrolysis, and gained an additional 3.2wt% after

humidification and drying. PB(Si)N specimen 9412, which contained 36 wt% silicon, achieved an oxygen content 0 36wt% prior to hydrolysis, and

gained an additional 0.3wt% after humidification and drying. Codepositioπ

of silicon thus reduces the oxygen uptake of the coating compared to that

of pure PBN deposited at the same temperature, and greatly reduces the

weight gain due to hydrolysis

2. Resistance of BN and PB(Si)N coatings to weight loss

in pure oxygen at 1200°C was compared using bulk specimens, weight losses and areas of which could readily be determined. Process conditions and results are shown in Table 3. PBN specimen 9409 los weight at an

average rate 4.3 mg/cm ² -hour. PB(Si)N specimen 8924, which contained

5wt% silicon, lost 0.14 mg/cm ² -hour. PB(Si)N specimen 9412, which

contained 35wt% silicon, lost 0.061 mg/cm ² -hour, and specimen 9413,

which contained 42wt% silicon, lost 0.003 mg/cm ² -hour. Dramatic decreases in thermal oxidation rate were thus obtained by using only 5wt% silicon, and the decreases continued until the composition was 42wt% silicon.

3. Silicon carbide monofilaments having a nominal diameter of 140μ (Textron Specialty Materials, Inc., Grade SCS-0) were

coated either with PBN or with PB(Si)N at 1400°C, using flow rates 0.5 IBCI- ₃ /min and 1.5 INhJ /min for 2.5 minutes. Pure PBN specimen 9436,

made at a pressure of 0.1 Torr, had a thickness of 0.7μ at a particular location in the reaction chamber. The pure PBN coatings of this run

exhibited good debonding from the fiber. PB(Si)N specimen 9437 was made by addition of 0.22 IHSiCI ₃ /min, which raised the pressure to 0.14 Torr, under otherwise identical conditions for the same deposition time of 2.5 minutes. The expected coating composition for the mole fraction HSiCI ₃

in the gas phase is 5wt%. The coating thickness at the same location in

the furnace was 0.9μ. Coating thickness, and debonding of the coating from the SiC substrate, are shown in Figure 1 for filaments coated at four different positions in the furnace. Another group of SiC filaments was

coated for three minutes at 1400°C at a pressure of 0.15 Torr using flow

rates 0.35 IBCI ₃ /miπ, 1.5 INH,/miπ, and 0.42 IHsiQI /min. The expected coating composition for this mole fraction HSiCI ₃ in the gas phase is 41wt%

silicon. Coating thickness at the same furnace location was 3.7μ to 4.3μ, and the coatings exhibited good debonding characteristics. These experiments show that SiC monofilaments can be coated with PB(Si)N

coatings at approximately the same deposition rate as PBN coatings

deposited under comparable conditions, and exhibit suitable debonding

characteristics. Measurements of interfacial shear strength by the fiber push-out method show that PB(Si)N coatings which have 5wt% to 40wt% silicon have shear strengths of 10MPa to 20MPa at room temperatures,

while pure PBN coatings have shear strengths of 10μ to 30μ. These low shear strengths indicate good debonding, and this range of values is

considered to be for all practical purposes identical for both groups.

4. Ceramic multifilameπt yarns can be coated with

PB(Si)N interface coatings in much the same way that they can be coated with pure PBN. Thus in run (435, "Hi-Nicalon," an 1800 denier, 500 filament silicon carbide yarn produced by Nippon Carbon Company was

passed continuously through a CVD furnace at 1400°C at a yarn speed of

2.75 feet/minute, so that the residence time for coating deposition was 66

seconds. Gas flows were adjusted as shown in Table 4, and 90-foot sections of yarn were produced in which the nominal silicon content of the coatings was Owt% (9435A), 20wt% (9435B), and 40wt% (9435C). Coating

thicknesses ranged from 0.4μ to 0.7μ; PBN coatings in this range of

thickness are considered to be optimal for most purposes by those skilled in the art.

5. Ceramic yarns and monofilaments can be coated with interface coatings consisting of multiple layers. Thus in Run 9434, several lengths of Textron monofilament SCS-0 and several lengths of Nippon Carbon "Hi-Nicalon" yarn were coated in the same batch CVD process first

with PBN at 1400°C for six minutes, then with PB(Si)N in which the silicon

content is approximately 30wt% based on HSiCI ₃ mole fraction in the inlet

gas. Throughout the run, the chamber pressure was 0.17 Torr, the BCI ₃ flow rate was 0.5 l/min and the NH ₃ flow rate was 1.5 l/min. Trichlorosilane

was supplied at 0.29 l/min to produce the PB(Si)N layer. Total coating thickness ranged from 3.3μ to 8.1 μ on SCS-0 monofilaments, and from

2.4μ to 3.6μ on individual filaments of "Hi-Nicalon" yarn. Scanning electron

micrographs of coated SCS-0 monofilaments showed that the PBN coating

debonded readily from the SCS-0 substrate and that the PB(Si)N overcoating debonded readily from PBN. The PB(Si)N overlayers were

approximately 2.5 times as thick as the PBN layer. This result shows that

coatings consisting of multiple layers can be deposited, retaining desirable debonding characteristics, and that PBN layers can be overcoated by

PB(Si)N to protect the PBN layer from hydrolysis and oxidation.