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1American Institute of Aeronautics and Astronautics

AIAA-2002-1686

INFLUENCE OF CONSTITUENTS ON THE PROPERTIES OF MELT-

INFILTRATED SiC/SiC COMPOSITES

Subodh K. Mital University of Toledo

Toledo, OH 43606, U.S.A.

Ramakrishna T. Bhatt Vehicle Technology Directorate U.S. Army Research Laboratory NASA Glenn Research Center Cleveland, OH 44135, U.S.A.

Pappu L.N. Murthy

NASA Glenn Research Center Cleveland, OH 44135, U.S.A.

ABSTRACTI

A micromechanics-based analysis and subsequent trade-off studies were performed to predict through-the-thickness thermal conductivity of 0/90 five-harness satin (5 HS) weave CVI-BN/CVI-SiC coated Sylramic (SiC) performs, and melt-infiltrated (MI) SiC/SiC composites. The predictions were made at room temperature (25 °C) and at 1300 °C in an inert environment. The primary objective of these trade-off studies was to evaluate the possibility of improving through-the-thickness thermal conductivity, as these composites are likely to be used primarily under thermal loading. BN and CVI-SiC volume fractions and different matrix compositions were evaluated to achieve this goal. In addition the predictions were compared with the available measurements obtained from a parallel experimental program. Preliminary results providing

I This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

comparison of predictions versus experimentally measured data are discussed in this paper.

INTRODUCTION

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composites are promising candidates for high temperature applications in the gas-turbine engines as well as airframe and propulsion structural components of space vehicles. These composites combine the excellent properties of silicon carbide, which include low density, good thermal shock resistance, low coefficient of thermal expansion, and good high temperature stiffness and strength properties, with a high toughness due to a variety of toughness mechanisms. An attractive characteristic of silicon carbide is that under oxidative conditions it forms a silica layer that could potentially protect the material from further oxidation. This potential that ceramic matrix composites (CMC) hold for predominantly high temperature applications have led to a

43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Con22-25 April 2002, Denver, Colorado

AIAA 2002-1686

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

American Institute of Aeronautics and Astronautics 2

multitude of research activities pertaining to fabrication, testing and modeling of these materials. In particular, SiC/SiC composites fabricated with MI method are candidate materials in many applications as they result in denser materials with desirable properties1,2. These advanced materials are candidates for structural components where the primary load is due to thermal gradients. It has also been observed that as the stresses on these structures are reduced, the useful life of these components can be substantially enhanced. Thermal stresses are function of the thermal conductivity of the material, in addition to the coefficient of thermal expansion and stiffness of the composite material. In general, as the thermal conductivity increases, the thermal gradients decrease, thus causing a decrease in the resulting thermal stresses. In this study, some key thermal and mechanical properties of the MI SiC/SiC composite with different fibers and matrix constituents will be predicted. These will also be compared with preliminary findings of the test results and recommendations will be made for most suitable composite systems for CMC components subjected to thermal loading. As a part of NASA’s HSR-EPM (High Speed Research Enabling Propulsion Materials) program, the authors had developed a micromechanics-based technique to predict the elastic mechanical and thermal properties of advanced woven preform (WPI) that stands for Woven fiber Preform construction and interphase/matrix Infiltration ceramic matrix composites. This technique is programmed into a computer code W-CEMCAN (Woven Ceramic Matrix Composites Analyzer)3. This computer code was used to create a calibrated set of temperature-dependent constituent properties for a Sylramic/CVI-BN/CVI-SiC/MI composite. As mentioned previously, these materials will likely be

used in applications where thermal loads are the primary loads. Resulting thermal stresses are function of thermal conductivity of the material, coefficient of thermal expansions and the stiffness of material. In particular, it has been observed that through-thickness thermal conductivity is crucial i.e. by improving the thermal conductivity of the material the through-thickness thermal gradient can be reduced, which in turn will result in lower stresses. However, the factors influencing the thermal conductivity in these material systems are not fully understood. The primary objective of this part of the study reported in this paper was to determine the influence of interface and matrix compositions on the through-the-thickness thermal conductivity. The discussion in this paper is therefore limited to this specific objective, even though other thermal and mechanical properties of composite systems can be predicted with W-CEMCAN computer code. The predictions are also compared with the limited measured data available. Major findings of this evaluation are then summarized.

MATERIAL FABRICATION AND TESTING

The 0/90 five-harness satin fiber architecture was used for various composites. Some measurements were made on the preforms i.e. before they were infiltrated with the matrix material. The performs were prepared by assembling 8 plies of the fabric mats into 152 x 229 mm stacks, and then infiltrating the assembly with the BN and SiC coatings by chemical vapor infiltration (CVI)4. Two types of performs were available – one with low vol% CVI-SiC coating and other with high (approximately twice the vol% of the low CVI) CVI-SiC coating. Unless otherwise noted, the volume fractions of Sylramic fibers, CVI-BN in the composites


were kept constant in both the preforms. The MI SiC/SiC composites were fabricated by Honeywell Advanced Composites, Inc. by a processing method similar to that discussed in ref. 5, while the SiC/Si were prepared at NASA Glenn Research Center by infiltrating molten silicon into the performs. For thermal diffusivity measurements, 9x9 mm. specimens were cut from a larger composite specimen. Thermophysics Laboratory, Blacksburg, VA, measured the transverse thermal diffusivity of the specimens in argon at 25 and 1300 0C. Thermal conductivity values were calculated from the knowledge of the density, specific heat and diffusivity values of the specimens.

WOVEN COMPOSITE MODELING: W-CEMCAN COMPUTER CODE

The modeling details will be explained briefly in the presentation with respect to a 5-harness satin weave. Figure 1(a) shows a cross section of the unit cell of a 5 HS weave and 1(b) shows a vertical slice taken from the cross section showing details of different constituents namely, the 0o fibers, 90o fibers and matrix rich area. These arise due to the geometry of the weave architecture. Also shown are the details of a typical unit cell from the interior of fiber tow with three distinct constituents’ fiber, matrix and BN coating (interphase). Depending upon the volume ratios of the constituent phases, there can be a thin layer of the CVI interphase material around the fiber tow so the matrix phase cannot enter the tow region. For modeling purposes, the parts where the fiber tow is straight, the construction is assumed to be like a [0/90] laminate. In other parts, where the fiber tow is wavy or has a “crimp,” the undulated shape of the fiber tow is assumed to be same as was assumed before by the authors3.Furthermore, it is assumed here that

laminate theory is applicable at each section of the model along the x-axis. One such section is shown in figure 1(b). For a slice in the straight region, the equivalent properties of the slice can be obtained by running a [0/90] laminate analysis. For slices where the fiber is undulated, the following technique is used – a typical slice in the undulated region looks like the one shown in figure 1(b). In general, it will have four regions -0o fiber region, 90o warp fiber region, a thin layer of matrix phase around the fiber tow region and a matrix rich area. The off-axis angle of the warp yarn in any slice is known because of the assumed geometrical shape of the warp yarn. Equivalent stiffnesses can be obtained using a laminate analysis of a [90/θ/0] laminate. From these one can obtain the longitudinal modulus at this section Exx. The 90 “ply” in this “laminate” represents a fill yarn; θ “ply” represents the warp yarn and the 0 “ply” represents the matrix rich area. The thickness of each “ply” is properly accounted for depending upon the location of a particular slice in the section. Equivalent through-the-thickness modulus Ezz can be obtained as the Eyy of the [0/θ/0] laminate. In a regular laminate analysis, the ply is oriented in the X-Y plane. In this situation, the 0o fiber tow has an inclination in X-Z plane. To account for that properly, the existing laminate analysis codes have to be used judiciously taking into account proper orientations. For that reason, the laminate analysis has to be run twice in order to get all equivalent properties of a slice where the fiber tow has an angle. This part of the stiffness averaging is performed using CEMCAN. Once the equivalent properties of the vertical slice are established the procedure is repeated for all other sections along the length of the representative volume element. In the next step these slices are stacked up as plies in a laminate and CEMCAN’s laminate analysis


capability is once again utilized to arrive at equivalent properties for the section. This now represents equivalent properties of a 5-harness [0/90] woven CMC material. The process is equally applicable to a N-harness [0/90] woven CMC in general.

RESULTS/DISCUSSIONS

Predicted through-the-thickness thermal conductivity and the available measurements are shown in figures 2-6. For modeling purposes, the volume fraction of constituents was assumed to be same as reported in ref. 3 and the thermal conductivity of various constituents was assumed to be as shown in Table I. It is also assumed that the composites contain a closed intra-tow porosity of about 6%. The standard melt-infiltration matrix used to fill the open porosity consists of approximately 50% SiC particles and 50% silicon. The baseline composite refers to Sylramic/BN/Low CVI-SiC/MI composite. Figure 2 shows the comparison between the predicted and measured through-thickness thermal conductivity of Sylramic/BN/CVI-SiC preforms at two use temperatures. There is a large difference between the two. Such a difference is possible partly because of physical characteristics of porosity. It could also be that a perform, with a total porosity of about 26%, can not be considered a typical case of porosity which are expected to be in 5-6% range. Preform is really an intermediate step in the overall fabrication process of the composite and the models in the W-CEMCAN computer code are not able/designed to simulate the fabrication process or the properties along the way. Figure 3 shows the influence of BN interfacial coating on the Sylramic/SiC composite (standard material with a low CVI-SiC content). Without the BN coating, through-the-thickness conductivity at both

use temperatures is considerably higher as expected. BN introduces a possible gap with the interfacial coating and CVI-SiC coating causing a considerable drop in the conductivity6.

Figure 4 shows the predicted and measured room-temperature through-thickness thermal conductivity of SiC/Si composite (with silicon as matrix). The predicted and measured values compare reasonably well. There appears to be some discrepancy in the measured value between the low and high CVI-SiC, because one would expect that in the case of high CVI-SiC composite, the CVI-SiC with lower thermal conductivity is replacing the silicon with higher thermal conductivity, so the composite the composite with a low CVI-SiC should have a higher conductivity as shown by the predictions. The measured data is showing differently, but since this is only one data point, it could also be attributed to the inherent scatter in the measurement as well as the composite property itself. Influences of CVI-SiC coating, BN coating and higher matrix porosity are shown in figure 5. For comparison, the baseline SiC/SiC composite (Sylramic/BN/SiC/MI) composite is also shown. From figure 5, one can infer that replacing the CVI-SiC with MI matrix or the presence of porosity up to 10% does not reduce the room temperature through-thickness thermal conductivity significantly. However, the through-thickness thermal conductivity of SiC/SiC composite containing BN interface is approximately half of that for SiC/SiC composites containing no BN interface. The effect of replacing standard MI matrix with pure silicon or replacing SiC particles by highly conductive particles such as AlN or TiB2 was also predicted. Those predictions are shown in figure 6. Even though the replacement particles (AlN and TiB2) have


significantly higher thermal conductivity as compared to that of SiC particles, overall improvement in composite conductivity is only about 10-15%, well within the expected scatter of the property. The reasons for this being that the particles constitute only about 10% of the total composite volume fraction. Replacing MI with pure silicon has a similar effect (i.e. only a marginal increase) on room-temperature through-thickness thermal conductivity.

SUMMARY OF RESULTS

The influence of constituents on through-the-thickness thermal conductivity was predicted using a micromechanics based woven composites analysis code. Composites are made of Sylramic fibers, coated with BN/CVI-SiC with various matrix compositions. Major finding are listed below:

1. In general, the preforms alone have very low through-the-thickness thermal conductivity as compared to a composite. Thermal conductivity increases rapidly as the matrix is deposited to fill in the open porosity.

2. The BN interface acts like an insulator

and significantly reduces through-the-thickness thermal conductivity of the composite.

3. Micromechanics based models can be

used to predict the trend in through-the-thickness thermal conductivity of SiC/SiC composites. However, the current W-CEMCAN computer code used in the analysis significantly over predicted through-the-thickness thermal conductivity of the fiber performs that typically contain a total porosity of anywhere from 25 to 30%.

4. The predictions show that neither the replacement of MI matrix with pure silicon nor the replacement of the SiC particles in MI matrix with highly conducting particles such as AlN or TiB2 appears to have a significant influence on the through-the-thickness thermal conductivity.

REFERENCES

1. M. Johnson, B. J. Bartlett, and W. A. Troha, “Material Challenges and Progress Toward Meeting the High Speed Civil Transport Propulsion Design Requirements,” Proceedings of the 13th International Symposium on Air Breathing Engines, Chattanooga, Tennessee, U.S.A., September 7-12, 1997, Vol. II, ISABE 97-7179, Billing. S., Ed, pp. 1321-1328.

2. D. Brewer, “HSR/EPM Combustor Materials Development Program,” Mat. Sci. and Eng., A261, 284 (1999).

3. P.L.N. Murthy, S. K. Mital, and J. A. DiCarlo, “Characterizing the Properties of a Woven SiC/SiC Composite Using W-CEMCAN Computer Code. NASA-TM 209173, June 1999.

4. J. Halada, Honeywell Advanced Composites Inc, Private communications.

5. S. K. Lau, S. J. Calandra, and R. W. Ohnsorg, “Process for Making Silicon Carbide Reinforced Silicon Carbide Composites” US Pat. #5,840,221, (1998).

6. R. T. Bhatt, and Y.L Chen, “Effects of High Temperature Argon Heat treatment on Tensile Strength and Microstructure of BN/SIC Coated SiC Fiber Preforms” in Proceedings of ICCM-12, Paper #758, (1999).


TableI. Thermal Conductivity of Constituent at 25 0C

CONSTITUENT CONDUCTIVITY, W/M.K Sylramic SiC fiber CVI-BN CVI-SiC SiC particles AlN particles TiB2 particles Silicon

3.1

65

43

48

126

94

92


X’ = A + B (epi)-1X’ X’/2

ht X

Z

00 fiber tow

900

fiber towA

Matrix richarea

0 fibertow

900fiber

tow

w

Fiber

BNcoating

CVI-SIC + porosity

0

Figure 1: Micromechanics model representation

Figure 2: Comparison between the predicted and measured through-the-thickness thermal conductivity of the Sylramic/BN/CVI-SiC performs

Con

duct

ivit

y,W

/m.K

Temperature, C

25 1300 25 13000

5

10

15

20

25

High CVI-SiC

Low CVI-SiC

Measured

Predicted


Figure 3: Through-the-thickness thermal conductivity of Sylramic/SiC composites with and without CVI-BN interface

Figure 4: Room-temperature through-the-thickness thermal conductivity of BN/SiC coated SiC/Si composite

Predicted

Measured

25 1300 25 13000

10

20

30

40

50

60

Temperature, C

Con

duct

ivit

y,W

/m.K with BN

without BN

0

5

10

15

20

25

30

Low CVI-SiC High CVI-SiC

Con

duct

ivit

y,W

/m.K

Predicted

Measured


Figure 5. Predicted influence of BN interface, matrix porosity and CVI-SiC coating on the room-temperature through-the-thickness thermal conductivity of SiC/SiC composite

Figure 6: Predicted influence of various matrix compositions on room temperature through-the-thickness thermal conductivity of BN/SiC coated Sylramic composites

0

10

20

30

40

50

60

Con

duct

ivit

y,W

/m.K

wit

hout

CV

I-Si

C

SiC

/SiC

(Bas

elin

e)

wit

h10

%po

rosi

ty

wit

hout

BN

/SiC

wit

h10

%P

oros

ity

0

10

20

30

40

Con

duct

ivit

y,W

/m.K

Bas

elin

eSi

:SiC

(50:

50)

Si:A

lN(5

0:50

)

Sion

ly

Si:T

iB2

(50:

50)

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