On the estimation of threshold pressures in infiltration of liquid metals into particle preforms

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Scripta Materialia 59 (2008) 243–246 www.elsevier.com/locate/scriptamat

On the estimation of threshold pressures in infiltration of liquid metals into particle preforms J.M. Molina,a,b,c,* R. Prieto,a,b M. Duarte,a,b J. Narcisoa,c and E. Louisa,b,d a

Instituto Universitario de Materiales de Alicante, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain b Departamento de Fı´sica Aplicada, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain c Departamento de Quı´mica Inorga´nica, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain d Unidad Asociada del Consejo Superior de Investigaciones Cientı´ficas, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain Received 8 February 2008; revised 14 March 2008; accepted 15 March 2008 Available online 26 March 2008

Threshold pressures for infiltration of different metals into preforms of ceramic particles of various nature and morphology were experimentally determined and the results compared with those estimated by using the specific particle surface areas derived from laser diffraction and gas adsorption. Whilst laser diffraction provides an under estimation of the areas involved in the infiltration experiments, and thus of threshold pressures, gas adsorption offers reasonable values for particles that are regular and free of nanostructured surfaces. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Infiltration; Particle surface areas; Metal matrix composites; Capillary phenomena; Wetting

Pressure infiltration has become one of the most commonly used manufacturing processes to produce metal matrix composites [1–4]. The threshold pressure, or minimum pressure to promote infiltration, is a very important parameter, and of great interest from both the scientific and industrial perspectives [5–9]. Its prediction is now an important issue in the context of fabrication of composite materials. Molina et al. [8] have suggested that the specific surface areas of SiC particles derived from gas adsorption could be used to estimate the threshold pressure for infiltration of SiC beds with liquid aluminium, irrespective of the particle distribution, be it mono- or bimodal. The analysis is based on the assumption that the whole measured surface area is wetted by the metal at the pressure at which the metal starts penetration into the preform (slug-flow assumption). From engineering soil science [10] it is well known that, when a liquid displaces another liquid (or gas) that is wetting a porous preform, the porous body is not filled at once but, rather, is filled progressively. The validity of the assumption in Ref. [8]

* Corresponding author. Address: Instituto Universitario de Materiales de Alicante, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain. Tel.: +34 965 90 3400x2055; fax: +34 965 90 3464.; e-mail: [email protected]

is then determined by the validity of the slug-flow assumption in infiltration [2,10–12]. The slug-flow hypothesis is used extensively as it represents a useful and simple idealization of the infiltration process and in some cases (especially for simple powder geometries and low span of the size distribution) is a sufficiently good approximation. A detailed discussion of this issue can be found in Refs. [11,12]. In the current paper we analyse the influence of the particle morphology and surface topology on the estimation of the threshold pressure of infiltration of different powder compacts with various liquid metals by assuming slug-flow infiltration. The specific surface areas of reinforcements have been determined by means of two largely different and widely used techniques, namely, laser diffraction and gas adsorption. The analysis corroborates that the specific surface areas obtained with the gas adsorption technique offer a good estimation of threshold pressure. However, irregularities in some particles (like asperities, surface roughness and micro-porosity), which are clearly accounted for by gas adsorption, do not contribute to the area wetted by the metal during infiltration and hence the values of threshold pressure are overestimated. The values provided by laser diffraction are, in general, not helpful since they are derived from a geometrical simplification of considering the particles smooth and spherical.

1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.03.019

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J. M. Molina et al. / Scripta Materialia 59 (2008) 243–246

Three metals were used in the infiltration experiments: Sn (99.9% of nominal purity), pure Al (99.7%) and Al–12 wt.% Si (the Al–Si eutectic). The Sn was supplied by Merck KGaA (Darmstadt, Germany), while the pure Al and the eutectic alloy were supplied by Leichtmetallkompetenzzentrum Ranshofen GmbH (Ranshofen, Austria). (Chemical analysis revealed 0.1% Fe as the major impurity in both the alloy and the pure Al). Seven particulates were used in this work, namely, spherical glassy carbon particles of two sizes (GC1 and GC2), angular graphite (FU4616), regular alumina (AA10), angular alumina (F120) and angular SiC of two different sizes (SiCF100 and SiCF500). Figure 1 illustrates how different their morphologies are. The main characteristics of the particulates are gathered in Table 1. The density was determined by means of the picnometric method. The average diameter, span of the size distribution and specific surface area S LD were measured by means of laser diffraction using a particle size distribution analyzer (Malvern Instruments GmbH, Herrenberg, Germany). BET surface areas were derived

Figure 1. SEM images of the main types of particle morphologies used in this work: (a) GC1, (b) GC2, (c) graphite FU4616, (d) regular alumina AA10, (e) angular alumina F120 and (f) angular SiC F100.

Table 1. Density q, average diameter D and span of the size distribution of the particulate used in this work

from nitrogen adsorption isotherms, measured in an Autosorb 6-b apparatus from Quantachrome Instruments (Florida, USA). The volumetric method was used to do this, and the isotherms were analysed with the help of the standard BET theory [13]. Spherical carbon particles were supplied by Alfa Aesar, under the commercial denomination of ‘glassy carbon’. Actually, glassy carbon is a brittle form of carbon with a randomized structure, for which the manufacturer specifies zero open porosity at its surface. However, GC1 and GC2 present an important difference in their specific surface areas that cannot be rationalized by their difference in size (S BET is extremely high for GC1 particles). Standard graphite particles (FU4616) are rather angular and have a high surface porosity, which leads to a high BET surface area S BET (see Table 1). Alumina and SiC particles have all relatively low BET surface areas. Particles were packed into quartz tubes of 4.5 mm inner diameter by repeatedly adding a small amount of powder that was compacted by tapping with well-defined impacts with a weight falling freely over a distance of 35 mm (see Refs. [7–9] for details). The particle volume fraction was calculated by measuring the preform weight and dimensions (height and diameter). The composites were manufactured by gas pressure assisted liquid metal infiltration. The infiltration chamber and the experimental procedures used in this work are described in Refs. [4,7,14]. The metal was melted in alumina crucibles of 45 mm inner diameter and 72 mm height, and heated up to the infiltration temperature. The tube containing the packed powder was attached at the top of the pressure chamber and preheated by holding it just above the melt for about 80 s. Just before immersion, the metal surface was thoroughly cleaned. The chamber was closed and pressure was raised (introducing nitrogen gas) up to the chosen pressure at a rate of 50–60 kPa s1 . After 110 s at pressure, the chamber was vented at 30–70 kPa s1 . The sample was taken out of the melt and air cooled, and the infiltration height measured. Some samples were sectioned for metallographic observation. As reported in Table 2, the volume fraction V p reaches a value of 0.66 in GC1 compacts, which is larger than the one expected in compacts of identical spheres, namely 0.59–0.64 [15]. This is surely due to the span of Table 2. Experimental (P 0 exp) and calculated (P 10 and P 20 ) threshold pressures of infiltration for the composites manufactured with Sn, Al and Al–Si

Particle

q ðg=cm3 Þ

D (lm)

Span

S LD ðm2 kg1 Þ

S BET ðm2 kg1 Þ

Particle

Vp

Metal

Ti (K)

P 0 exp (kPa)

P 10 (kPa)

P 20 (kPa)2

GC1 GC2 FU4616 Al2 O3 –F120 Al2 O3 –AA10 SiC–F100 SiC–F500

1.42 1.42 2.04 3.90 3.90 3.21 3.21

16.1 24.0 27.2 145 12 167.6 16.92

0.61 0.45 0.99 0.56 0.59 0.71 0.82

280 200 130 57 134 13 128

24,000 325 3620 80 490 91 338

GC1 GC2 FU4616 Al2 O3 –F120 Al2 O3 –AA10 SiC-F100 SiC-F500

0.66 0.60 0.56 0.56 0.59 0.58 0.55

Sn

573

Al–12Si

963

Al

1023

379 215 622* 162 972 190 778

378 188 143 109 1044 31 262

32,400 339 3995 169 978 211 797

The span is defined as (D(90)–D(10))/D(50), where D(x) is the diameter below which x% of the particulates are found. The particle specific surface areas, as derived from laser diffraction S LD or gas adsorption technique S BET , are also shown.

P 10 and P 20 are calculated from specific surface areas obtained by laser diffraction and gas adsorption, respectively. T i indicates the infiltration temperature. V p is the volume fraction of the particle compacts. *Value from Ref. [9].

J. M. Molina et al. / Scripta Materialia 59 (2008) 243–246

P 0 ¼ clv cos h

Vp qS p ð1  V p Þ

ð1Þ

where clv is the liquid–vapor surface tension, h is the contact angle at the liquid/solid interface, V p is the particle volume fraction, and q and S p are the particle density and specific surface area (m2 kg1 ) of reinforcement, respectively. S p is a quantity that depends not only on the average diameter of the particles but also on their surface topology. Among the main techniques currently used to measure the specific surface area of a finely divided solid, two are of easy and common use, namely laser diffraction and gas adsorption. In laser scattering the specific surface area is calculated by averaging 1/d, where d is the diameter of a given particle (see Ref. [8] for a detailed discussion). This averaging procedure masks the details of the surface, which is taken to be smooth and equal to the surface that spheres of d diameter would have. On the other hand, in the gas adsorption technique the gas phase (commonly nitrogen) has access to the narrowest pores, given its small molecular dimension. As a result, the specific surface area S p provided by the gas adsorption technique can be considered as fully informative, since it provides detailed information

on irregularities, protuberances and ridges of the particles surface. For this reason, it can be taken as an upper-bound for the prediction of the threshold pressure for infiltration. Figure 3 shows the comparison of the predicted results of the threshold pressure of infiltration (by means of Eq. (1) using S p obtained from the two techniques, laser diffraction and gas adsorption) vs. the experimental results. The parameters used in the calculations are reported in Table 3. As Figure 3 shows, threshold pressures calculated by using the areas obtained with laser diffraction are all underestimated with an error greater than 10%. The gas adsorption technique, however, provides values of threshold pressure within less than 10% above the experimental values. A remarkable case is that of particles GC2, for which both techniques give accurate results due to their spherical shape and absence of nanofeatures. GC1 and FU powders, however, present estimated values well above the experiments. The reason for this may be found in the information contained in the isotherms of the different powders. Figure 4 shows the N 2 adsorption isotherms for GC1 and SiCF500 powders. The FU4616 powder presents an isotherm with intermediate shape; for the rest of the powders the isotherms are like the one for SiCF500. In terms of the classification given by Brunnauer et al. [13], Figure 4a corresponds to a Type I isotherm, while Figure 4b shows the shape of a typical Type II isotherm. Type I isotherms are observed for microporous solids having relatively small external surfaces (in terms of the convection recommended by IUPAC, a micropore corresponds to a pore with an opening of less than 5

log Po calculated (kPa)

the size distribution that, although not large, is significant (0.66). The V p in GC2 is smaller due to the lower span and lies within the range reported in Ref. [15]. The threshold pressures in Table 2 were derived from measurements of infiltrated height at different applied pressures, as described in previous publications [4,7,9,14]. Figure 2 shows optical micrographs of composites fabricated by infiltration of different particles and metals. A general salient feature is that particles appear to be homogeneously distributed in the metallic matrix. Moreover, the pictures do not show any evidence of particle breaking, which is especially important for the graphite materials given their low mechanical properties. For a porous preform with a monomodal particle size distribution, the threshold pressure for infiltration with a liquid metal is given by [8]

245

4 3 2 1 1

2 3 log Po experimental (kPa)

4

Figure 3. Logarithmic plot of the threshold pressure calculated with Eq. (1) vs. that determined experimentally. Symbols: big squares, GC1; rhombuses, GC2; little squares, FU4616; triangles, SiC; and circles, Al2 O3 . Filled symbols correspond to gas adsorption, and empty symbols to laser diffraction. The line represents the identity function (P calc ¼ P exp ).

Table 3. Surface energies of Sn, pure Al and Al–12Si and contact angles of these metals on graphite, SiC and alumina respectively at the infiltration temperatures for each metal (573 K for Sn, 963 K for Al– 12Si and 1023 K for pure Al)

Figure 2. Optical microscopy images of some of the composites: GC1/ Sn (a), FU4616/Al–12Si (b), Al2 O3 –F120/Al–12Si (c) and SiCF100/Al (d), infiltrated at 1 MPa at the temperatures given in Table 2.

a

Metal

rLV (mJ m2)

Contact angle h – (system)

Sn Al–12Si

560 [16] 847 [17]

Al

870 [18]

151° (Sn/glassy carbon) [19] 114° (Al–12Si/Al2 O3 ) [20]a 120° (Al/graphite) [21] 127° (Al/SiC) [22]

Value derived from infiltration experiments.

J. M. Molina et al. / Scripta Materialia 59 (2008) 243–246

0.5

0.1

0.4

0.08 n (mmol/g)

n (mmol/g)

246

0.3 0.2

0.06 0.04

0.1

0.02

0

0 0

0.2

0.4

0.6 P/Pc

0.8

1

1.2

data. Laser diffraction, in its turn, is found not to be useful, as the averaging procedure underestimates the specific surface area in many cases (spherical particles excluded).

0

0.2

0.4

0.6 0.8 P/Pc

1

1.2

Figure 4. Nitrogen adsorption isotherms (moles per unit of mass vs. relative pressure P =P c , P c being the saturation pressure) for GC1 (a) and SiCF500 (b) particles. Note that while (a) is a Type I isotherm, (b) is clearly of Type II.

2 nm). Their limiting uptake is governed by the accessible micropore volume rather than by the internal surface area. Micropores are readily filled at very low relative pressures. The filling of a significant portion of micropores is indicated by a large and steep increase of the isotherm near its origin and subsequent bending to a flat level, this plateau corresponding to the micropore contribution. Type II isotherms present the normal form obtained with a non-porous or macroporous adsorbent. The type II isotherms represent unrestricted monolayer– multilayer adsorption. The micropore volume in Type I isotherms can be calculated by making use of the standards DIN 66135 and ISO 15901-3. These methods are based on the measurement of the adsorption of gases at constant low temperature and the evaluation of the initial part of the isotherm. For particles like GC1, geometrically spherical but with micropores in their surface, the contribution of the micropores to the specific surface area can be extracted for the estimation of threshold pressure (as the pressures needed to infiltrate pores of some nanometers are in the range of several hundreds of megapascals). If we do this, the spherical glassy carbon GC1 has then an apparent specific surface area of 410 m2 kg1 , a value 98% lower than that reported in Table 1. The corresponding threshold pressure is then 0.41 MPa, which is 8% close to the value experimentally measured. This procedure, however, is not applicable to the particles FU4616 because the amount of gas filling in the micropores cannot be easily estimated, given the wide pore distribution of these particles. Neither laser diffraction nor gas adsorption can offer for these particles a reasonable estimation of the area wetted by the liquid metal during infiltration. In conclusion, threshold pressures of infiltration can be estimated by using the specific surface areas derived from gas adsorption for reinforcements which are fairly regular and do not expose a nanostructured surface. For the other cases, the gas adsorption technique provides only an upper-bound. The validity of the method can be checked by looking at the specific isotherm in each case: only type II isotherms seem to guarantee reliable

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