Infiltration of SiC preforms with iron silicide melts: microstructuresand properties

Yi Pan a,*, Ming Xia Gao a, Filipe J. Oliveira b, Joaquim Manuel Vieira b,Joao Lopes Baptista b

a Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, Chinab Department of Ceramics and Glass Engineering, UIMC, University of Aveiro, 3810-193 Aveiro, Portugal

Received 6 December 2002; received in revised form 28 April 2003

Materials and Engineering A359 (2003) 343�/349

www.elsevier.com/locate/msea

Abstract

Spontaneous infiltration of SiC preforms with FeX SiY (�/Fe3Si, Fe5Si3 and FeSi) melts produced SiC/FeX SiY composites with

nearly full density (]/96.5% theoretical density). The phases, microstructures and mechanical properties have been studied with

conventional materials characterization techniques including X-ray diffraction, optical microscopy, scanning electron microscopy

etc. A key issue behind these studies is dissolution of SiC in FeX SiY melts during infiltration. The dissolution of SiC in Fe3Si melt led

to carbon precipitation and phase changes (Fe3Si disappeared and Fe5Si3 and FeSi formed). However, Fe5Si3 and FeSi infiltration

gave no carbon precipitation, but SiC sintering, particle coalesce and grain growth instead. Extra-large SiC grains and SiC single

crystals were found in fine SiC (0.5 mm) infiltrated by Fe5Si3. Phase equilibrium calculations for Fe�/Si�/C system were performed at

1873 K using CHEMSAGE in order to study SiC solubility and the condition for the precipitation of SiC rather than C in Fe�/Si melt.

Mechanical properties including micro-hardness, bending strength and Weibull modulus of all infiltrated samples were tested and

analyzed on the basis of phase and microstructure observations.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Melt infiltration; Silicon carbide; Iron silicides; Silicon carbide composites

1. Introduction

High technology developments demand high perfor-

mance materials in terms of mechanical, thermal and

environmentally tolerance. These requirements are not

fulfilled at the same time by a single material such as

ceramics or metals alone. Composites with ceramic

particulate reinforced alloys and intermetallics have

thus been envisaged and developed. A preferable route

for fabrication of these composites is through infiltra-

tion of porous ceramic preforms with molten alloys. For

example, Al-alloys have been successfully infiltrated

with and without hydraulic or gas pressure into SiC,

Al2O3 and AlN performs [1�/3]. Among all infiltration

methods, pressureless or spontaneous infiltration is the

most attractive because of its spontaneousness. How-

ever, spontaneous infiltration requires an excellent

wetting of solid particles by molten metals, which has

been scarce. The specific surface energy of most metallic

melts is around 1 J m�2. A porous preform made from a

powder with the particle size of 1 mm usually has an

average pore size of 0.1 mm. In the case of wetting as

mentioned above, the naturally developed capillary

pressure driving the melt to penetrate through the

porous preform would be 10 MPa, one hundred atmo-

spheres, about the same as the hydraulic pressure needed

to infiltrate Al-alloys into SiC performs [1,2]. Under

such a high capillary pressure the melt is able to

spontaneously move inwards and to fill all the pores

inside the perform, from which densified metal matrix

composites with ceramic particulate reinforcements can

thus be prepared. Some previous researchers have made

use of spontaneous infiltration technique to make

cermets and cemented composites containing TiC and

aluminides of nickel and iron [4�/6].* Corresponding author. Tel.: �/86-571-8795-3008.

E-mail address: yipan@zju.edu.cn (Y. Pan).

0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0921-5093(03)00373-3

It has been found by the authors that molten silicides

of Co and Fe wet extremely well and are able to

spontaneously infiltrate into SiC powder performs [7�/

9]. In this paper the infiltration of SiC preforms withvarious iron silicide melts, formulated by Fe3Si, Fe5Si3and FeSi are further studied in terms of the chemical

reaction, sintering and microstructure evolution accom-

panying the infiltration and their effects on the mechan-

ical properties. According to Eustathopoulos et al. [10]

high temperature wetting of SiC by metals and alloys

always involves the reactions of SiC with these metals,

particularly transition metals. In this study the infiltra-tion of SiC with Fe3Si, Fe5Si3 and FeSi proved the

accompanying reactions including dissolution, precipi-

tation and new phase formation in the infiltration

process. SiC dissolved in the silicide melts leading to

liquid phase sintering of SiC. The dissolution of fine SiC

particles in the melts and subsequent precipitation onto

large SiC particles produced extra-large grain growth.

Single SiC crystal with a size of about 200 mm, muchlarger than the particle size of the starting powder, were

found. Besides original phases new phases including

carbon precipitation were also examined. The measure-

ments of the mechanical properties revealed strong links

to the microstructures, phases and infiltration condi-

tions. Thermodynamic analyses are carried out on the

reactions, dissolutions and precipitation. The wetting,

sintering and grain growths are successfully explained bythe thermodynamic work.

2. Experimental

Three iron silicides, Fe3Si, Fe5Si3 and FeSi, were arc-

melted using 99.97% Fe and 99.9999% Si. All ingots

were remelted four to six times to achieve chemical

homogeneity. The ingots were finally cut into smallpieces for the infiltration experiments. X-ray diffraction

(XRD) showed that they were single phase, Fe3Si

(JCPDS 35-519), Fe5Si3 (JCPDS 34-483) and FeSi

(JCPDS 38-1397), respectively.

Two kinds of a-silicon carbide powders, NF0/860,

Kempten GmbH D50�/1.6 mm and UF-15, H.C. Starck,

D50�/0.5 mm were used in this study. Disc shaped

samples having a diameter of 45 mm and a thickness of5�/6 mm were pressed in a steel die under 120 MPa from

the powders. The relative packing densities of the discs

were 53�/55% and 50�/52%, for samples from the NFO/

860 and UF 15 powders, respectively.

The infiltration of a SiC preform by each of the three

iron silicides was conducted in an Astro furnace in a so-

called indirect infiltration mode, which is illustrated in

Fig. 1. Between the preform and iron silicide fragments,there was a small column (8 mm in diameter and 12�/14

mm high) made from same SiC as in the perform. The

melt wetted the column first and entered the preform

second so that uncontrollable quickly covering and

sealing of the preform surface was avoided. The heating

program was set for all infiltration experiments to be

fast heating at 50 8C min�1 up to 1600 8C and main-taining at this temperature for 60 min under flowing

argon followed by fast cooling by turning power off and

leaving cooling water on. The indirect infiltration has

proved a successful infiltration method in such cases

where the melt wets the preform too well. The infiltrated

samples were dense and free of visible defects.

After infiltration the samples were first characterized

using the relative volume fractions of SiC, silicide andporosity existing inside the sample, which involved

dimension and density measurements before and after

infiltration and a simple calculation. The crystalline

phases of the samples after infiltration were analyzed

using an X-ray diffractometer (Rigaku, Geigerflex/D

and Cu-Ka radiation). Infiltrated samples were cut,

polished and examined using optical microscopy (OM)

and scanning electron microscopy (SEM, S-4100, Hita-chi, Japan). Room temperature bending tests were

performed in the 3-point bending mode in an Instron

4510 testing machine. The bar sample size was 4�/5�/

30 mm and the span was 25 mm. The flexural strength of

20 bars cut from the discs prepared at exactly the same

condition was statistically analyzed using the Weibull

distribution. Micro-hardness was determined in a Vicker

hardness tester at 9.8 N.

3. Results and discussion

The two kinds of SiC powers were observed under

SEM and their morphologies are shown in Fig. 2(a) and

(b), respectively. The observed particle sizes of SiC are in

agreement with 1.6 mm (NF0/850) and 0.5 mm (UF15),

which were provided by the producer.In the infiltration experiments all the three silicides

exhibited excellent infiltrations into the preforms made

from the two SiC powders. The infiltrated samples were

first characterized and the results are given by Table 1.

Each set of values in Table 1 is the average of six

samples processed under exactly the same conditions.

The diameter shrinkage of 7% found after Fe5Si3infiltration indicates an effective sintering of SiC bythe aid of liquid Fe5Si3 accompanying the infiltration.

Such a sintering was also detected in the infiltration of

Fe3Si, but much less than that in Fe5Si3 (2% linear

shrinkage), and it was not found in FeSi infiltration. The

XRD patterns of the infiltrated samples are shown in

Fig. 3, and the phases identified are summarized also in

Table 1.

Because of covalent bonding SiC sintering needs veryhigh temperature, much higher than the infiltration

temperature in this study. Binding of SiC particles using

metallic binders is also scarcely possible because of

Y. Pan et al. / Materials and Engineering A359 (2003) 343�/349344

strong reactions of SiC with metals or otherwise

extremely poor wetting [11]. The sintering of SiC during

infiltration of iron silicides found in this study is

significant. The phase analyses on the samples after

infiltration (Table 1 and Fig. 3) suggest that dissolution/

precipitation took place as the SiC particles came into

contact with liquid silicides through infiltration.

Solubility limits of SiC in liquid Fe�/Si alloys with

continually changing Si contents at 1873 K were

obtained by performing calculations using GTT CHEM-

SAGE [12], the application software for thermodynamic

calculations developed by Gunnar Eriksson with assis-

tance from Klaus Hack, Marianne Philipps and Stephan

Petersen. The calculation proceeded in the following

steps: read a thermodynamic data-file; enter the phase

equilibrium menu; enter the contents of Fe, Si, C and

SiC (corresponding to 484 points in the composition

triangle of the Fe�/Si�/C ternary system shown in Fig. 4);

enter temperature (1873 K) and pressure (0.1 MPa) for

incoming phases and compounds (binary and ternary

liquid, C and SiC); start calculations to obtain relative

Gibb’s free energy and then activities of the three

components, ASi, AFe and AC, in the liquid solution

corresponding to each composition point; display the

calculation results, mainly the curves, OB and OD in

Fig. 4 on which RT (ln ASi�/ln AC)�/DG0SiC (DG0

SiC,

standard Gibb’s energy of formation of SiC with respect

to solid Si and C) and ln AC�/0 (standard state: solid C)

hold for stoichiometric SiC precipitation and carbon

precipitation, respectively. In other words, the equili-

Fig. 1. Scheme of indirect infiltration.

Fig. 2. SiC powders (a) NF0/860 1.6 mm; (b) UF15 0.5 mm.

Y. Pan et al. / Materials and Engineering A359 (2003) 343�/349 345

brium between dissolution and precipitation of solid SiC

in the Fe�/Si liquid phase described by SiC(s)U//Si/�/C¯

,

where the underlined species are in solution, is repre-

sented by OB. The precipitation of graphite at the

interface is represented by the curve OD, and is

described by C¯U/C(s). The whole reaction for C

precipitation can thus be written as SiC(s)U//Si/�/C(s). A

critical point O is determined to be XC�/0.11, Xsi�/0.32

and XFe�/0.57. The solid line SiC�/O hits the Si�/Fe line

at point A, where XSi�/0.27 and XFe�/0.73. Fe3Si is to

the right (Fe richer) side and Fe5Si3 and FeSi are to the

left side of point A. Saturated SiC dissolution in Fe3Si

melt leads to a precipitation of carbon rather than SiC,

and that in Fe5Si3 and FeSi leads only SiC to precipita-

tion, which is in agreement with the phases after

infiltration shown in Table 1 and Fig. 2. This can also

explain why Fe3Si disappeared and Si richer phases,

Fe5Si3 and FeSi, were generated with the carbon

precipitation. Besides original phases, infiltration with

Fe5Si3 created a new phase FeSi, but infiltration with

FeSi did not. It can be reasonably assumed that the final

phases shown in Table 1 resulted from the solidification

of equilibrated phases (liquid and SiC or C) at 1873 K in

the fast furnace cooling.The microstructures of the composites by the infiltra-

tion of the three silicides were examined using OM and

SEM. The microstructure of the SiC (NFO/860) infil-

trated with Fe3Si shown in Fig. 5 is somehow discontin-

uous and dark precipitated carbon particles can be

clearly seen. However, those infiltrated with Fe5Si3 and

FeSi, having relative densities above 96%, are contin-

uous with SiC particles well distributed in matrices

(Figs. 6 and 7, respectively). One point worth noting is

that after infiltration the SiC particles observed in Figs.

5 and 6 are several times larger than the starting SiC

(NF0/860, 1.6 mm), indicating that each particle is a hard

agglomerate due to coalescing of several original

particles. The SiC sintering evidenced by a large

shrinkage is also due to the SiC dissolution in liquid

silicides. For SiC particles surrounded by the first

infiltrated melt, the outer layer of each SiC particle

dissolved in the melt, and then deposited onto inter-

particle contacts between neighboring particles, leading

to necking and coalescence. The shrinkage of Fe3Si

infiltrates is less than that of infiltrates of Fe5Si3, which

may be due to the volume increase during the carbon

precipitation. However, FeSi infiltration had no carbon

precipitation, and the negligible sintering may be

attributed to the relatively low solubility of SiC in liquid

FeSi as seen in the phase diagram (Fig. 4). The SiC

particle size after the infiltration of FeSi (Fig. 7) was

nearly the same as that of the original powder,

confirming that the sintering involving particle coales-

cence was not operative during FeSi infiltration.

The above argument is further supported by the

infiltration of Fe5Si3 into the preform of fine SiC

(UF15, D0.5�/0.5 mm). Figs. 8�/10 are the micrographs

taken under OM and SEM in three different areas of

such an infiltrated samples, in which large regular SiC

particles, and even well developed single crystal SiC

grains (hexagons of 200 mm large) are observed. The

explanation is that the SiC particle size was so small that

it was completely miscible in the Fe5Si3 melt at the

infiltrating temperature (1873 K). The regularly shaped

SiC particles and single SiC crystals were produced by

SiC precipitation out of the SiC saturated melt during

cooling. In Fig. 10 the largest SiC single crystal has some

cracks associated. They were produced by the thermal

mismatch between the large SiC grain and the matrix. It

needs to be mentioned again that this is only found in

SiC(UF15)�/Fe5Si3 system.

The mechanical properties including Vickers hard-

ness, flexure strength and Weibull moduli of SiC/Fe3Si,

SiC/Fe5Si3 and SiC/FeSi composites prepared by spon-

taneous infiltration are reported in Table 2. For

Table 1

Shrinkage, densification and crystalline phases after infiltration of SiC (NF0/860) preforms

Infiltrate Shrinkage (%) (Vol.%) (SiC/silicide/porosity) Phases after infiltration

Fe3Si 2 59.5/37.0/3.5 SiC, Fe5Si3, FeSi, C

Fe5Si3 7 67.3/31.0/1.7 SiC, Fe5Si3, FeSi (trace)

FeSi 0 53.7/42.2/4.1 SiC, FeSi

Fig. 3. XRD patterns for SiC specimens infiltrated with (A) Fe3Si; (B)

Fe5Si3 and (C) FeSi.

Y. Pan et al. / Materials and Engineering A359 (2003) 343�/349346

comparison, the values of monolithic iron silicides

solidified and tested under the same condition as the

infiltrated samples are also shown in Table 2.

SiC(NF0/860)/Fe3Si has the lowest bending strength

and lowest Weibull modulus. This can be simply

attributed to that the precipitated free carbon acting as

defects and flaws (Fig. 5). SiC(NF0/860)/Fe5Si3 and

SiC(NF0/860)/FeSi give higher microhardness, higher

bending strength and similar Weibull moduli than

monolithic silicides, indicating SiC particles strengthen-

ing effects on Fe5Si3 and FeSi without sacrificing the

reliability of the silicide matrices. SiC(UF-15)/Fe5Si3 is

the strongest material, but its Weibull modulus is

relatively low. The extra-large SiC grains observed in

SiC(UF-15)/Fe5Si3 (Figs. 8�/10) and the cracks created

due to thermal expansion mismatch decreased the

reliability of the materials even though the fine SiC

particles tended to generate a marked strengthening

effect. The microhardness values of SiC(NF0/860)/

Fe5Si3, SiC(NF0/860)/FeSi and SiC(UF-15)/Fe5Si3 are

much higher than those of the respective monolithic

silicides. Indentation tests was not performable to

Fig. 5. Micrograph of the SiC (1.6 mm) sample infiltrated with Fe3Si. Fig. 6. Micrograph of the SiC (1.6 mm) sample infiltrated with Fe5Si3.

Fig. 4. The isothermal section of Fe�/Si�/C ternary diagram at 1873 K.

Y. Pan et al. / Materials and Engineering A359 (2003) 343�/349 347

SiC(NF0/860)/Fe3Si because of the considerable amountof loose carbon particles precipitated in the composite

(Fig. 4).

The study of mechanical properties of the infiltrated

composites suggests that particulate reinforced metallic

composites can be fabricated by spontaneous infiltra-

tion. The method is simple and economic and the

reinforcing effect is positive if no harmful reaction

occurs between the components and if the microstruc-ture is developed in a controllable way.

4. Summary

Fe3Si, Fe5Si3 and FeSi all have very good wettability

on SiC and can be spontaneously infiltrated into SiC

powder preforms. The melts can almost fully fill in the

porous volume inside the preforms by infiltration, and

SiC particulate reinforced iron silicide matrix compo-

sites can be obtained. Accompanying the infiltration, the

dissolution of SiC in these silicide melts has been provedto be responsible for the microstructure evolution and

mechanical property differences among the composites.

SiC partial sintering and particle coarsening by coales-

cence are due to dissolutions, diffusion and precipitation

in the melt. Free carbon precipitates following the

dissolution of SiC in Fe3Si at 1873 K and it deteriorates

the mechanical properties. The dissolution of SiC in

Fe5Si3 and FeSi yields no carbon precipitation and,

therefore, there are net reinforcing effects. However, the

dissolution of very fine particulate SiC (UF-15) results

in anomalous grain growth and single SiC crystal

growth, thus reducing the reliability of the composite.

The dissolution of SiC has been analyzed using CHEM-

SAGE software and SGTE thermodynamic data to

construct isothermal sections of the Fe�/Si�/C phase

diagram at the infiltration temperature and at 1873 K. A

critical Si content in liquid Fe�/Si system has been found

to be Xsi�/0.27. SiC saturated liquid in which Si is lower

than Xsi�/0.27 leads to carbon precipitation, otherwise

SiC is the only solid stable phase, which is in agreement

with the experimental results. The microstructures are a

result of fast cooling, that freezes the composition of the

liquid and determines the final phase equilibrium.

Fig. 9. Large SiC particles in the SiC (0.5 mm) sample infiltrated with

Fe5Si3.

Fig. 10. SiC single crystals in the SiC (0.5 mm) sample infiltrated with

Fe5Si3.Fig. 8. Micrograph of the SiC (0.5 mm) sample infiltrated with Fe5Si3.

Fig. 7. Micrograph of the SiC (1.6 mm) sample infiltrated with FeSi.

Y. Pan et al. / Materials and Engineering A359 (2003) 343�/349348

Acknowledgements

Supported by the National Science Foundation of

China (Grant No. 59672031 and Grant No. 50272062),

by the Foundation of Science and Technology of

Portugal, Filipe Oliveira acknowledges a post-doctoralscholarship from the 28 Quadro Comunitario de Apoio.

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Table 2

Selected mechanical properties of SiC infiltrated with silicides

Infiltrate SiC Vickers microhardness Flexure strength (MPa) Weibull modules

Fe3Si NF0/860 �/ 375 10

Fe5Si3 NF0/860 1380 635 27

FeSi NF0/860 1400 420 28

Fe5Si3 UF-15 1550 610 14

Fe3Si, Monolithic 700 410 25

Fe5Si3 Monolithic 750 485 28

FeSi Monolithic 850 280 20

Y. Pan et al. / Materials and Engineering A359 (2003) 343�/349 349