LthiS
FABRICATION AND PROPERTIES OF AL-MMC
REINFORCED WITH METALLIC COATED FINE
CERAMIC PARTICLES
Nahed.A.EI.Mahallawy**, Madiha Shoeib*, and Fouad. H.Mahmoud***
**Design and production Eng. Dept, Faculty of Engineering, Ain shams
University, Cairo, e-mail: mahalawy@aucegypt.edu
*Central Metallurgical Research & Development Institute, P.O.Box 87
El-Tabbing Helwan, Cairo
***Dept of Design and Production Eng, Shoubra Faculty of Engineering, Zagazig
University, e-mail: fn-egsPliotiail con.
ABSTRACT: Metal matrix composites (MMCs) reinforced with ceramic particles are among the
most promising new materials. However, they still suffer from limited use due to difficulties
encountered in their fabrication processes. These processes include mainly liquid metallurgy - such as
stir casting, and liquid metal infiltration - and powder metallurgy. The advantage of the liquid metal
process using ceramic particles is the possibility of using conventional technique which reduces the
cost of MMC products and the production of castings with large variety of sizes and complex shapes.
Liquid metal process includes first the incorporation o f the reinforcing particles, dispersing them
uniformly into the liquid metal then pouring into moulds. One of the difficulties encountered during
preparation of the composite, is the incorporation of the ceramic particles and dispersing them
uniformly in the liquid metal. The finer the particle size, the more difficult their uniform dispersion due to
the tendency to agglomerate and the poor wettability between the ceramic and the liquid metal.
In order to obtain a good quality composite, it is necessary to achieve a uniform distribution of the
ceramic reinforcement with minimization of defects such as particle agglomeration, porosity and
reaction products. The present work investigates the possibility of introducing fine ceramic particles (2 to
10 micron) into liquid aluminium. Improving wettability is achieved by applying a thin metal coat on the
ceramic particles- Al 2 03 and SiC- using electroless technique before introducing them into the liquid
aluminium. The ceramic particles were coated with nickel and copper under different conditions. The
coated particles arc then introduced into the liquid meta! while continuous stirring. The
microstructure of the MMC samples revealed that a limited amount of the particles between 1 and 2.5
vol % are introduced into the aluminium. However a uniform distribution with no agglomeration is
obtained. EDX analysis indicated that the Ni and Cu coat were dissolved in the Al forming solid
solutions. The hardness of the composites increased with particles additions; the rate of i ncrease is
much faster in case of SIC addition compared with that of Al 20 3 addition Future work is still needed in
order to incorporate higher volume fractions of the fine particles into the liquid metal.
KEYWORDS:
Metal matrix Composites , SiC and Al203 particle reinforcement, metal coating of
particles, stir casting, microstructure, hardness.
1. INTRODUCTION
Metal-ceramic particle composites have a large number of potential industrial applications, especially as
anti-friction and anti-abrasion materials. They are used in application such as automotive bearings, and
in machine tools , marine diesel engines , trucks , transmission lines , food processors, fans and
pumps (1). The production of MMCs using casting processes reduces their cost in comparison wi th
other production processes such as powder metallurgy in addition to the unlimited shapes of cast
products obtained by such processes.
The introduction of ceramic particles into the liquid metal is an important step before casting. Poor
wettability between the ceramic particles and liquid metal is usually encountered. Therefore, several
techniques have been developed for improving wettability. The most common , reviewed in ref [I) ,
includes, metal coating on ceramic particles (Ni- or Cu- coated, graphite shell, char, mica, Al203 in Al,
Al alloy, Ni or Cu alloy for Al203), addition of reactive elements to the melt (such as Mg to Ail or M
alloy), heat treatment of particles before dispersion (for Al 203, in Al or Al alloys) and ultrasonic
treatment (for Al203, graphite in Al).
For the introduction of particles in the molten metal, several techniques have been used including gas
injection in melt stream, pellet method, stir casting, ultrasonic dispersion, cotnpocasting, centrifugal
dispersion and chemical method. The most widely used among these techniques is the stir casting [ I].
However, most of these techniques were applied on coarse particles (20 to 200 pm). The difficulty of
introducing particles in molten metals increases as the particle size decreases to a few microns. On the
other hand, MMC incorporating fine ceramic particles are expected to present new attractive properties,
mainly mechanical, tribological, chemical and others. Previous work using powder metallurgy
technique for Cu-coated particles 23 and 7 tun revealed improved bonding between the copper-coated SiC
particles and the matrix leading to more efficient load transfer to the stronger particles, compared . to
decohesion between particles and matrix in case of uncoated particles. The smaller particles (7 pun)
composite exhibited superior strength and failure strain compared with those obtained with 23 gm
particles [2].
It is the purpose of this work to investigate the possibility of introducing fine (2-10 pm) ceramic
particles into an Al melt after being coated with Cu and Ni. The coat is applied using electroless
technique and the composites arc obtained using stir-casting. Previous work on electroless Ni coating of
ceramic particles gave encouraging results regarding improving wettability between coated particles
and liquid aluminium. [3].
The investigations include metallographic study for Vf and particle distribution, and EDX analysis.
Hardness measurements of the composites were also made.
2. EXPERIMENTAL WORK
2.1. MATERIALS
1
The materials used were commercial purity aluminium ingots and the reinforcements included A1,03
and SiC fine particles. Chemical analysis of the aluminium is given in Table I. The Al203 particles
were irregular platelets in shape 10.4 gm in length, 7.5pm in width and 3.5prn in thickness, as
measured using the SEM, Fig.la. The SiC particles were 2.5 to 3.5 pm in size and angular in shape,
Fig. lb.
Table 1: Chemical analysis of the Aluminium commercial purity
Fe
0.33
Si
0.137
Mg
0.016
Cu
0.13
Mn
0.011
Al
Bal.
2.2. ELECTROLESS COATING OF PARTICLES
The Al203 particles were coated with Ni (P) while the SiC particles were coated with copper.
In general, the clectroless coating is a chemical process relying on a sequence of etching sensitizing,
activating and plating, with important cleaning and rinsing stages. The steps and conditions for Ni
coating are found in ref [3]. While the steps for copper coating are found in ref [2]. For nickel coating.
different periods of time were applied, 5 min, 10 min and infinite time ( 2 hours) in the plating stage.
This was done in order to study the effect of plating time on the thickness of the Ni coat obtained.
For SiC particles an additional condition was applied by treating the particles in sodium tetraborate
solution for several hours to enhance wettability due to deposition of Na ions on the particle's surface. It
is to be noted that coating of the very fine SiC particles was difficult as the particles were scattered in the
aqueous solutions used and did not settle down easily.
:-/t. COMPOSITE PREPARATION
Crzed and uncoated particles were incorporated in aluminium by stir casting. The aluminium was first
meted in a ceramic crucible and superheated to about 750°C, while the particles were preheated to
35C-450°C. A stainless steel stirrer rotating at 800-900 rpm was used to create a vortex in the liquid
actinium, then the particles were added gradually at a rate of about 20 g/min while stirring. The axis t
the stirrer was parallel to the crucible and was moved up and down. After adding all the powder,
tring was maintained for about 25 minutes. The composites were then poured in a preheated steel die itt
faun rods about 15 mm diameter, 200 mm long. The conditions of composite samples prepar ed are Oen
in Tablet
at NIETALLOGRAPHIC OBSERVATION
A specimen was cut from each cast rod at both top and bottom positions to check particles distribution
aid uniformity. The specimens were ground with 1200 grit SiC paper, polished with Opm then torn
almond paste. A last finishing polish was achieved using vibration polishing equipment with colloidal
macs (OP-S suspension, Sinters). SEM , EDX and WDX analyses were made on CAM SCAN-series 4,
—ring Electron Microscope.
I. RESULTS AND DISCUSSION
IL CHEMICAL ANALYSIS
Sompies of each cast bar were cut for chemical analysis using spectrophotometer. The results are given a
Table 3.
leemical analysis was carried out on prepared composites in order to determine the effect of coating
- cents (Ni and Cu) on the overall alloy composition. Comparing the concentration of Cu and Ni
:-. in both Tables I and 3 indicates a slight enrichment in Cu and Ni in the aluminium. This is due to
:oat thickness on the particles which is of only a few microns. The particles incorporated in the
-:fnium was also a few percent, as will be shown later. There is also enrichment in Fe probably
the stirrer.
is therefore concluded that the matrix metal is only slightly affected by the coating of the
particles that impurities have also been added to the composites during the preparation steps.
Coating thickness of coated particles are determined from measurements of average size before and
after coating using SEM. Fig. 2a and b shows some coated particles. The results given in table 4
indicate that after 5 min a Ni coat of about 7 ttm is formed and that it does not further increase with
time. This result is the same for the length and width of Al 20 3 particles. However, along the thickness
of the particles only 2 pin of coat is deposited. For SiC copper coated particles Fig. 2b indicate a Cu
coat of about 1 pm thickness.
3.2. MICROSTRUCTURE ANALYSIS
All specimens were analysed metallographically in the unetched condition in order to study the particle
distribution and its uniformity and volume fraction.
In general, all specimens showed a dendritic structure with a phase (darker colour) present at the
dendrite boundaries Fig.3 . This phase was identified as Al-Fe intermetallic phase, using EDX analysis.
The particles were uniformly distributed in all cases and they were present inside the dendrites and at
the dendrite boundaries, Fig.3 and 4 .
No agglomerations were observed in all specimens, but only some areas with slightly higher particle
density, in case of uncoated particles. However, no agglomerations farming inter -particle voids were
observed.
Fig.5 a shows uniform particle distribution in case of SiC particles uncoated, while Fig.5b shows the
presence of particles inside the dendrites. The Al-Fe phase at the dendrite boundaries is also illustrated .
in Fig.5b
In the case where the particles were treated with sodium tetraborate, the particles were also uniformly distributed forming no agglomerations, Fig. 6a. Details of the particles are illustrated in Fig. 6b..
The volume fraction of particles was measured on at least 10 positions in each specimen using the
"Phase analysis" option in the " SIS computer program" installed on the optical microscope. The
results are given in table 5. Bottom and top denote the positions of the sample cut from the bottom and
top of the cast rod, respectively
•
The results in Table 5 indicate that the volume fraction in all specimens ranges between 0.22 and 2.45
which is less than that.expected (8 to 10 %) resulting from limited wetting of the very fine size particles
even in the metal coated condition.
It is also noticed from the results that the V I-is higher at the bottom of the cast rods compared with the
top of the rod. This means that particles sinking takes place in most cases.
In the case of N i coated Al203 , increasing the coating time affected only slightly VIcompared with the
uncoated condition. In case of SiC particulates the treatment used did not increase the volume fraction
incorporated in the matrix.
3.3. HARDNESS MEASUREMENTS
The hardness values were measured on all specimens top and bottom surfaces and the values obtained
were related to the type and Vf of the reinforcement. The results plotted in Fig.7 versus volume fraction
indicate that the hardness increases with V t for both SiC and Al 20 3. It is noticed from the results that
the rate of increase in hardness is higher in case of SIC particles compared with Al203 particles and that
smali Yf (less than 0.4 %) of SiC results in an increase in hardness. This could be due to the very small
size of the SiC particle (2.5 — 3.5 um) compared with that of Al203 (10 pet).
3.4 RESULTS OF EDX ANALYSIS
EDX analysis was carried out in order to analyse the phase at the dendrite boundary . It was found that it
contains Fe ranging between 6 and 1 I wt%, the iron resulting from the impurities in the aluminium and
during composite preparation. the analysis showed no particle/matrix reaction. The analysis made in the
matrix surrounding the coated particles indicate only a slight enrichment close to the particles on a
slibmieron scale. Fig.8 shows an example of the SEM micrograph where the points analysed arc indicated
4. CONCLUSIONS
0
I. The application of Ni coat on Al2 3 particles readied around 7 microns after 5 min immersion in
Ni solution while increasing the immersion time had no sensitive effect on coat thickness. Copper
coat was about I micron thick .
2.
3.
4.
The use of stir casting on coated ceramic particles resulted in incorporation of a limited
particle volume fraction , maximum 2.5 vol %, in the liquid aluminium
The hardness of the composite increases rapidly with very small amounts of reinforcing SiC
particles, (Vr= 0.4%) due to the very fine particle size.
The coat material dissolves in the aluminium forming dilute solid solutions
. 5. REFERENCES
I. P.K. Rohatgi, R. Asthana and S. Das "Solidification, Structures and Properties of
Cast Metal-Ceramic Particle Composites", International Metals Review, 1986, Vol
31, no. 3, pp 115-139.
2. A.M.Davidson and D.Regener, "A comparisn of Aluminum based metal-matrix
composites reinforced with coated and uncoated particulate silicon carbide",
Composites Science and Technology, vol 60, 2000, p:865469.
3. M.Shoeib, T.Y.Soror, M.A.Maamoun, M.S.EI- Basiouny, "Aluminum metal
–Ceramic Matrix composites reinforced by Electroless Nickel plating of ceramic
particles and fibers", to be published.