Identification of Growth Promoter to Fabrication SiCp/Al<sub>2</sub>O<sub>3</sub> Ceramic Matrix Composites Prepared by Directed Metal Oxidation of An Al Alloy

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 1063-1068

Published Online November 2012 (http://www.SciRP.org/journal/jmmce)

Identifica ti on o f Growth Promoter to Fabrication

SiCp/Al2O3 Ceramic Matrix Composites Prepared by

Directed Metal Oxidation of an Al Alloy

Malkapuram Devaiah1,2, Thodeti Srihari3, Thankappan Pillai Rajasekharan1

1Ceramics & Composites Group, Defence Metallurgical Research Laboratory, Hyderabad, India.

2Department of Mechanical Engineering, Geethanjali College of Engineering & Technology,

Hyderabad, India

3Department of Mechanical Engineering, Sha dan College of Engineering & Technology, Hyderabad, India

Email: [email protected],

Received July 3, 2012; revised August 7, 2012; accepted August 14, 2012

ABSTRACT

SiC particulates reinforced alumina matrix composites were fabricated using Directed Metal Oxidation (DIMOX) pro-

cess. Continuous oxidation of an Al-Si-Mg-Zn alloy with different interlayers (dopents) as growth promoters, will en-

compasses the early heating of the alloy ingot, melting and continued heating to temperature in the narrow range of

950˚C to 980˚C in an atmosphere of oxygen. Varying interlayers (dopents) are incorporated to examine the growth con-

ditions of the composite materials and to identification of suitable growth promoter. The process is extremely difficult

because molten aluminum does not oxidize after prolonged duration at high temperatures due to the formation of a pas-

sivating oxide layer. It is known that the Lanxide Corporation had used a combination of dopents to cause the growth of

alumina from molten metal. This growth was directed, i.e. the growth is allowed only in the required direction and re-

stricted in the other directions. The react nature of the dopants was a trade secret. Though it is roughly known that Mg

and Si in the Al melt can aid growth, additional dopents used, the temperatures at which the process was carried out, the

experimental configurations that aided directed growth were not precisely known. In this paper we have evaluated the

conditions in which co mposites can be grown in large enough sizes for evaluation applicatio n and have arriv ed at a pro-

cedure that enables the fabrication of large composite samples by determining the suitable growth promoter (dopant).

Scanning electron microscopic, EDS analysis of the composite was found to contain a continuous network of Al2O3,

which was predominantly free of grain-boundary phases, a continuous network of Al alloy. Fabrication of large enough

samples was done only by the inventor company and the property measurements by the company were confirmed to

those needed to enable immediate applications. Since there are a large number of variable affecting robust grow th of the

composite, fabrication large sized samples for measurements is a difficult task. In the present work, to identify a suit-

able window of parameters that enables robust growth of the composite has been attempted.

Keywords: Ceramic-Matrix Composites; Scanning Electron Microscopy; Liquid Metal Infiltration; Al2O3, SiC

1. Introduction

The directed oxidation of molten metal has been utilized

to produce ceramic composites. This process offers the

ability to produce unreinforced [1 ], particulate-reinforced

[2] and fiber-reinforced [3,4] composites with a wide

range of composition and microstructures. The composite

formation process involves the rapid reaction of a molten

metal with a gaseous oxidant: for example, the reactions

of a molten aluminum alloy containing a growth dopant

such as a few wt% of Mg and a group IVA elements (Si,

Ge, Sn, or Pb) with air to form alpha aluminum oxide [2].

The reaction is sustained by the wicking of liquid me-

tal—along interconnected microscope channels in the re-

action pro duc t .

Numerous studies now exist of the oxidative grow th of

Al2O3/Al composites from Al alloys into free space [5-7],

experiments on the infiltration of preforms, which are

fewer, including the work of Manor et al. [8] into SiC

and of Nagelberg [9] into Al2O3. As an example, the re-

action of an aluminum alloy with oxygen produces an

Al2O3/metal composite containing both ceramic and me-

tal as three dimensionally interconnected networks [4].

The directed oxidation produced composite materials is

quite flexible and offers the ability to make near-net-

shape low-porosity in situ composites with a wide range

of composition and microstructures [10]. These compos-

ites can, in principle, be designated to have good tough-

M. DEVAIAH ET AL.

1064

ness and thermal shock resistances, as well as high stiff-

ness, wear resistance, and high temperature stability. The

reaction product may be formed by itself as a ceramic/

metal composite or as the matrix of a reinforced compos-

ite. In either case, the composite formation reaction is

sustained by the wicking of liquid metal along intercon-

nected microscopic channels within the alumina structure.

The microstructure of the α-Al2O3/metal composite in

the absence of a reinforcing preform has been described

by Breval et al. [11]. The composite in the absence of

preform was characterized by preferred orientation of in-

terconnected Al2O3 perpendicular to the plane of the

original alloy surface and a bond of MgAl2O4 at the in-

terface between the composite and original alloy surface.

In a previous paper, the microstructure development of

Al2O3/metal ceramic bodies grown from an Al-10 wt%

Si-3 wt% Mg alloy was discussed in detail [12]. Nagel-

berg et al. found the growth rate of Al-10 w t% Si-3 wt%

Mg alloy to exhibit activation energy of ~400 kj/mol.

Growth rates varied as the oxygen partial pressure to the

one quarter power. These authors proposed that the over-

all composite growth process involves the transport of

oxygen through the external MgO layer to the molten Al

alloy interface where it dissolves in the molten metal.

The dissolved oxygen diffuses through the molten metal

and then reacts with Al at the metal/Al2O3 interface to

form additional Al2O3. It was proposed that the growth

rate was controlled by the electronic conductivity of the

external MgO layer [13].

Characterizing the initial fabrication of the composite

microstructure in the absence of a reinforcing preform is

an essential component for the fabrication of a detailed

understanding of the growth dopants or promoter. This

paper describes the conversion of molten Al alloy into

Al2O3 by using different growth dopants with help of the

microstructure morphology (scanning electron micros-

copy) and fabrication of large enough sizes of ceramic

matrix composites with SiC particulates as reinforcing

preform.

2. Experimental Procedure

For growth of molten Al-9Zn-8.5Si-1.5Mg alloy in O2

atmosphere was studied by two experiments such as: 1)

The alloy is exposed in O2 atmosphere with different do-

pants to know exact growth promoter (dopant) without

preform. 2) Further experiments were carried with SiC

particulates as reinforcement (preform) to fabricate large

enough sizes SiC/Al2O3 ceramic matrix composite ma-

terials.

In initial experiments to know growth promoter studies,

a rectangular Al alloy ingot was machined into 15 mm ×

15 mm × 15 mm size piece (Total 9 Nos). The surface of

the Al alloy piece was evenly coated with a thin inter-

layer (dopants), namely SnO2, Bi2O3, CaCO3, MgO, ZnO,

TiO2, Y2O3, (SnO2 + Bi2O3), and (MgO + ZnO) and other

sides are coated with gypsum to prevent growth. The

coated Al alloy piece was preheated in an oven for 4 hrs.

The preheated alloy pieces are then placed in zircon sand

mold as shown in Figure 1. Then the samples were hea-

ted to a temperature in the narrow range of 950˚C to

980˚C in an atmosphere of oxygen for a dwell time of 62

hours followed by furnace cooling to room temperature.

Grown sample bodies were sectioned and metallo-

graphically polished. Microscopic observations were

made to determine the formatio n of Al2O3 at the top sur-

face of the sample with different interlayer (dopant) and

growth condition. Where appropriate, SEM and EDS

analysis was performed. Energy Dispersive Spectroscopy

(EDS) was especially helpful in identifying the chemical

composition and sequence of layers in the surface oxide

at the interface between the growing composite and the

air atmosphere. In the case of second experiment, to fab-

ricate large enough size of SiCp/Al2O3 composites with

different volume fractions were prepared by directed metal

oxidation process. This was comprised of two steps

namely preparation of SiC preforms with different vo-

lume fraction and appropriate heat treatment schedule to

aid formation of Al2O3 matrix. The volume fraction of

SiC was carried by using SiC particulates of different grit

sizes namely #100, #120 and #220. The corresponding

particle size, tapped packing density and volume fraction,

as calculated using density of α-SiC (3197 Kg/m3) are

displayed in Table 1. SiC powders were subjected to

artificial oxidation by heat treatment in air at 1100˚C for

4 hours. This would ensure that an adherent coating of

SiO2 would develop on the surface of SiC particles. Sub-

sequently, loose powder preforms (measuring 70 × 70 ×

20, in mm) of oxidized SiC with corresponding tapped

packing densities were contained in refractory crucibles.

Al alloy (Al-9Zn-8.5Si-1.5Mg) blocks were used as

source for formation of Al2O3. The Al alloy block was

ground and polished to good surface finish and cleaned

Figure 1. Zircon sand mold used to grow Al alloy pieces

with different interlayer in O2 at m o sphere.

M. DEVAIAH ET AL. 1065

Table 1. Details of volume fractions of composites studied in

this work.

Label Grit size

Average particle

size (µm) Tapped packing

density (Kg/m3) Volume

Fraction

B1 #100 125 1118 0.35

B2 #120 106 1286 0.40

B3 #220 53 1382 0.43

Figure 2. Schematic view of set-up used in DIMOX process

for fabrication of SiCp/Al2O3.

with acetone. Gypsum was coated on five sides of the

block to prevent growth into sides and the top, the bot-

tom side of the Al block is coated with growth at the in-

terface. A schematic of the experimental set-up is shown

in Figure 2. The samples were heated to a temperature in

the narrow range of 950˚C to 980˚C in an atmosphere of

oxygen for a dwell time of 62 hours followed by furnace

cooling to room temperature. Subsequent to cooling, the

composite block shall be subjected to machining in order

to obtain specimens with required dimensions necessary

for various measurements.

3. Results and Discussion

Experiments are performed by Al-9Zn-8.5Si-1.5Mg alloy

with different interlayer (dopants), namely SnO2, Bi2O3,

CaCO3, MgO, ZnO, TiO2, Y2O3, SnO2+ Bi2O3, and MgO+

ZnO to know the Al2O3 formation on the top surface [1].

Al alloy ingot without filler material was exposed into

oxygen; the exposed surface of the alloy ingot was

evenly coated with a thin layer of the selected dopant

(SnO2, Bi2O3, CaCO3, MgO, ZnO, TiO2, Y2O3, (SnO2+

Bi2O3), and (MgO+ ZnO) to promote uniform growth

initiation at temperatures varying from 950˚C to 980˚C in

O2 atmosphere. The details results of the as grown sam-

ples are shown in Table 2. As grown samples in oxygen

atmosphere are taken out from home made zircon sand

mold for examination. A small piece is cut from each

sample by low speed diamond saw and metallographi-

cally polished for scanning electron microscopy observa-

tions. Figure 3(a) shows the experimental sample of Al

alloy without filler material and exposed surface of the

alloy ingot was evenly coated with a thin layer of (SnO2

+ Bi2O3) interlayer (dopant) into air.

From the Figure 3(a) we can observe that the length

of the alloy as been changed and increase longitudinally.

The longitudinal cross section is shown in Figure 3(b). It

can be observed from Figure 3(a) the growth of the alloy

at the top surface of the sample and a hollow shape at the

top in cross section view (Figure 3(b)). Visual examina-

tion of the (SnO2+ Bi2O3) specimens showed that oxide

growth occurred exclusively on the exposed surface of

the metal, the microstructural features if the reaction pro-

duct were used to guide the selection of a growth pro-

moter. Figure 4 shows the macroscopic features of reac-

tion products grown from Al alloy at a process temperature

Table 2. Details of grown and non grown samples.

Label Interlayer Results

B1 SnO2 No growth

B2 Bi2O3 No growth

B3 CaCO3 No growth

B4 MgO No growth

B5 ZnO No growth

B6 TiO2 No growth

B7 Y2O3 No growth

B8 (SnO2 + Bi2O3) Composite grows Vigorously

B9 (MgO + ZnO) No growth

(a)

(b)

Figure 3. A schematic representation of as grown Al alloy

SnO2 + Bi2O3 as interlayer (a) Experimental sample (b)

cross section.

M. DEVAIAH ET AL.

1066

of 950˚C to 980˚C in O2 atmosphere using (SnO2+ Bi2O3)

as interlayer (growth promoter) system. Differences in

surface reflectivity reveal that the material is of columnar

character, consistent with the directed nature of the

growth process. Results from SEM observations show a

Al2O3 layer, below MgO and Al2O3 layer is formed and

followed by a metal. The Al2O3 columns in material pro-

cessed at 950˚C to 980˚C contain interpenetrant but fu lly

interconnected networks of Al2O3 and Al.

Typically; they are several millimeters wide and have

indistinct bound aries. Figure 5 show s the EDS line map-

ping observation shows the some microns of O and other

elements available in the parent metal and the dopants

(SnO2+ Bi2O3) also. Similar experiments were conducted

with different interlayer (dopants) which are listed in

Table 2. Figure 6 shows experimental sample and lon-

gitudinal cross section using CaCO3 as interlayer under

similar conditions. From the Figure the experiment sam-

ple clearly show that there in no change in the dimen-

sions of the sample and it retained its original dimension s

of the sample. The microscopic observations with CaCO3

as interlayer is not found any oxidation layer on the top

surface and any Al2O3 content at the top surface of the

grown sample as shown in Figure 7 and 8. Similarly

results are found in other experiments which are done by

other interlayer (dopants) which are listed in Table 2.

Figure 4. Representative grain boundaries between Al2O3

crystallites for as Al alloy with SnO2 + Bi2O3 as interlayer.

Figure 5. EDS line mapping representation of as grown Al

alloy SnO2 + Bi2O3 as interlayer.

Figure 9 shows as grown sample and longitudinal

cross section using Bi2O3 as interlayer under similar con-

ditions. The Figure 10 shows the scanning electron mi-

croscopic observations will not found any Al2O3 content

in the case Bi2O3 as interlayer same results can be ob-

served in EDS reorientation in Figure 11. The above ex-

periments reveal that the vigorous growth was found in

the case of (SnO2 + Bi2O3) as interlayer. Other dopents

which are listed in Table 2 will not found any growth.

From the observed results to fabricated SiC/Al2O3 ce-

ramic matrix composites (SnO2 + Bi2O3) can be used as

interlayer or growth promoter for the fabrication of the

large size SiC/Al2O3 ceramic matrix composites. Then

the experiments are done under similar conditions as a

function of SiC particulates as reinforcement with vol-

ume fractions in the range of 0.35 to 0.43 are used to pre-

pare the preforms to infiltrate into Al alloy to fabriccate

(a)

(b)

Figure 6. A schematic representation of as grown Al alloy

CaCO3 as interlayer (a) Experimental sample (b) cross sec-

tion.

Figure 7. SEM representation of as grown Al alloy CaCO3

as interlayer.

M. DEVAIAH ET AL. 1067

Figure 8. EDS of as grown Al alloy with CaCO3 as Inter-

layer.

(a)

(b)

Figure 9. A schematic representation of as grown Al alloy

Bi2O3 as interlayer (a) Experimental sample (b) cross sec-

tion.

Figure 10. SEM representation of as grown Al alloy with

Bi2O3 as interlayer.

Figure 11. EDS representation of as grown Al alloy with

Bi2O3 as interlayer.

large size SiCp/Al2O3 ceramic matrix composites. Figure

12 shows the different dimension of SiCp/Al2O3 ceramic

matrix composites are fabricated in the present work with

dimensions. Figure 12(b) with dimensions measuring 70

× 70 × 20, in mm.

(a)

(b)

Com

osite

Hollow

ion

Residua

l metal

(c)

Figure 12. Examples of composite growth obtained in the

present work (a) the aluminum alloy is allowed to oxidize

and grow into SiC particulate preform in a conical refrac-

tory mold. We note that the shape of the mould could be

replicated in the composite. (b) We also note the large size

(70 mm × 70 mm × 20 mm) of the composite samples. An

example of growth of composite from a rectangular mold. (c)

a example showing the way the composite grows-the growth

process was stopped midway, the outer surface was cleaned

and suitably machined to reveal the salient features. The

aluminum left after the process can be seen at the bottom.

The hollow region is the region from where the Al was used

up in the oxidation growth. The top is the composite that

grow. The composite is machined to reveal the absence of

macroscopic porosity. The edges of the alloy retain a box

shape because of the suppression of melting due to the

presence of gypsum coating.

M. DEVAIAH ET AL.

1068

4. Conclusion

The process details are reported for the specific example

of oxidation of molten Al in oxygen atmosphere to form

Al2O3. In the present work, the experiments are per-

formed with and without reinforcement to fabricate large

size SiCp/Al2O3 ceramic matrix composites. (SnO2 + Bi2O3)

was identified as suitable growth promoter for fabrication

SiCp/Al2O3 ceramic matrix composites. The scanning

electron observations r evealed that the conversion of me-

tal to oxide was po ssible on ly with (SnO2+ Bi2O3) dopant

as growth promoter. Other did not cause any growth of

oxide from molten metal. So, the (SnO2+ Bi2O3) dopant

was used as growth promoter for the fabrication of SiCp/

Al2O3 ceramic matrix composites. SiCp/Al2O3 ceramic

matrix composites are successfully fabricated with SiC

particulates as reinforcement with volume fractions in th e

range of 0.35 to 0.43 with dimensions measuring 70 × 70

× 20, in mm. the sample are used for different mechani-

cal and physical property measurements.

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