development and characterization of al based mmc by using...

Nano Trends: A Journal of Nanotechnology and Its Applications

Volume 12, Issue 2, Apr, 2012, Pages 01- 10.

__________________________________________________________________________________________

ISSN 0973-418X © NSTC 2011. All Rights Reserved

Development and Characterization of Al based MMC by Using RHA and

Metallurgical Grade SiO2 with Varying Percentage of Mg

Pallavi Deshmukh*, Jatin Bhatt, Dilip Peshwe, Shailkumar Pathak

Department of Metallurgical and Materials Engineering, V.N.I.T. Nagpur 440 010, Maharashtra, India

1. INTRODUCTION

Wear is an important phenomenon that occurs

at the interface of material. Al based MMCs

reinforced with ceramic particles exhibit better

mechanical properties in comparison to

monolithic Al alloys.

Consequently, they have been traditionally

employed as tribological parts in the

automotive industry for years due to their high

strength-to-density ratio and better resistance

to wear [1].

This quality is also the reason for use of

particulate reinforced Al alloy composites

which attracts increased usability in

tribological applications.

Metal matrix with particle reinforced

composites are gaining widespread acceptance

due to their higher specific strength, specific

modulus, and better wear resistance compared

to monolithic alloys [2-4].

While exploring the feasibility of using fly-ash

as a reinforcing material in the Al melt,

Rohatagi et al. [5, 6] observed that its low

density may aid the synthesis of a light weight

composite.

The particulate composite can be prepared by

injecting the SiO₂ particles into solid, semi-

solid or liquid matrix either through powder

metallurgy or liquid metallurgy route [7–9].

The later is preferred since it is fairly

economical and amenable to mass production.

ABSTRACT

Current investigation is focused on effective utilization of rice husk ash (RHA), an industrial waste available in

abundance by systematically dispersing it into an Al-Mg matrix resulting in production of composites by liquid

metallurgy route. The properties of two such products – rice husk silica and metallurgical grade silica are

evaluated and studied for comparison. Composites are produced with varying percentage of Mg. These

composites are synthesized by reinforcing amorphous nano sized (32-56 nm) rice husk SiO₂ particles and

metallurgical grade SiO₂ particles (10 µm) in Al-Mg alloy by liquid metallurgical route. These Al based MMCs

are characterized using X-ray Diffractometery, Scanning Electron Microscopy, micro hardness and wear tests.

Improved mechanical properties are observed in the composite synthesized by using rice husk silica in

comparison to metallurgical grade silica.

Keywords: SiO₂ nanoparticles, Metal Matrix Composite, Wear, Micro hardness

*Author for Correspondence

E-mail: [email protected]

Tel: +91-9881251100



__________________________________________________________________________________________


Stir casting fares are the simplest and most

economical of all liquid metallurgy routes.

However, the only drawback of this process is

the non-uniform distribution of the particulate

out of poor wettability and gravity regulated

segregation [10].

In present paper, SiO2 extracted from rice husk

is used as the reinforcing material while Mg is

added to enhance its wettability. Due to its

more open structure and a higher number of

dangling bonds, nano sized amorphous SiO2 is

comparatively more reactive than

metallurgical grade SiO2 [11].

2. MATERIALS AND METHODS

Appropriate quantity of Al-xMg (x=0.5, 1.0,

2.5 & 5 % w/w) master alloy is used in the

experiment. Muffle furnace (ELECTROHEAT

EN 120QT) was utilized for melting.

The alloy was placed inside the muffle furnace

at 750°C. For every batch, 100 gm of Al-Mg

alloy is melted in a graphite crucible. The SiO₂

particles are preheated to 500°C for 4 hours to

remove moisture. At the centre of the vortex,

SiO₂ particles (wrapped in an Al foil of

negligible weight), were added gently and the

melt was stirred vigorously.

This process was carried out until all the SiO₂

particles forms uniform dispersion in the melt.

Requisite stirring was maintained until

formation of vortex in the melt was achieved.

This procedure is repeated for rice-husk SiO₂

and metallurgical grade SiO₂. After intense

stirring, the melt was poured into a metallic

mould and analyzed for hardness, wear and

structural characterization studies.

A TEM (Phillips and Model number: CM200

with operating Voltage range of 20-200kv and

resolution of 2.4 Å) along with EDS was used

for the determination of the particle size and

elemental analysis of the reinforced particle. A

microhardness tester (Mitutoyo HM-112) is

used to study hardness.

The average of eight results was converted to

the standard Vickers hardness (50 gm & 100

gm load with pyramid diamond indenter).

DUCOM (TR-20 LE-M1) pin on disc machine

is used for dry slide wear testing. For every

specific sample, this test is repeated several

times (at least thrice) in order to ensure

repeatability.

The MMC samples are characterized by

Scanning Electron Microscopy (SEM) (JEOL

JSM – 6830A Analytical SEM) for wear track

study.

3. RESULTS AND DISCUSSIONS

Four samples of the Al-Mg alloy containing

varied percentages of Mg are used in the

synthesis of the composite (MMC).

The chemical compositions of the samples

used are shown in Table I.



__________________________________________________________________________________________


Table I: Composition of the Al-Mg Alloy Samples used in the Synthesis of the MMC.

Al-Mg Alloy (wt %) Mg (wt %) Fe (wt %) Si (wt %) Al

Al-Mg (0.5%) 0.5 0.19 0.095 Balance

Al-Mg (1.0%) 1.01 0.19 0.095 Balance

Al-Mg (2.5%) 2.42 0.19 0.095 Balance

Al-Mg (5.0%) 4.95 0.19 0.095 Balance

Amorphous SiO₂ (extracted from rice-husk) is

added in proportion of 5% w/w of the alloy

samples by liquid metallurgy route and the

composites are synthesized this procedure is

repeated for metallurgical grade SiO₂. The

SiO₂ extracted from rice-husk was in the range

of 32-56 nm while the metallurgical grade

SiO₂ was in the range of 10 µm (Figure 1c).

Figure 1 displays the EDS analysis along with

the TEM results of the SiO2 particles. EDS

profile (Figure 1b) of nanosilica particles

contained predominantly the elements of Si

and O. Both Si and O peaks correspond to the

SiO2. The inset SAD pattern in Figure 1 shows

the diffused rings indicating amorphous nature

of nanostructure SiO2.

(a) (b) (c)

Fig 1: (a) TEM Micrograph of Rice Husk SiO2 Particles in the range of 32-56 nm and their SAD

Pattern (inset). (b) EDS of Nanosilica Particles. (c) Metallurgical Grade SiO2 in the range of

10 µm at the Magnification of 2000X.

The micro hardness study is carried out at load

of 50 and 100 g to measure the hardness of the

composite. The resultant micro hardness alues

in form of bar diagram are shown in Figure 2.



__________________________________________________________________________________________


Fig. 2: Micro Hardness Values for Al-Mg-SiO2 Composite (Rice Husk) and for Al-Mg-SiO2

Composite(Metallurgical Grade) at Different Load Conditions (50 g, 100 g).

The above graph clearly indicates that

hardness values increase from Al-Mg (0.5%)-

SiO₂ (5%) to Al-Mg (2.5%)-SiO₂ (5%) and

then decreases. Addition of Mg to the system

improves the wettability between Al and SiO₂

which increases with the increase in Mg

content; resulting in increased amount of

reinforced particles (SiO₂) entering the matrix

(Al). Consequently, the hardness increases

with the formation of spinel structures which

are very hard in nature (Figure 3a).

Increase in the % of Mg beyond 2.5 results in

oxidation forming MgO which is brittle in

nature, causes reduction in hardness. Hence,

there is a decrease in micro hardness value for

Mg (5%). The micro hardness value is found

to be maximum for the Al-Mg (2.5%) - SiO₂

(5%) composite. Also from the bar diagram

(Figure 2) it is concluded that the micro

hardness value is higher in the case of

composite reinforced with rice husk silica than

metallurgical grade silica. The metallurgical

grade silica which is in micron size forms the

poor reactivity which results in formation of

porosity (Figure 3b). As the particle size is

larger than rice husk silica, the surface area is

less and hence, it forms less contact area with

the matrix. This results in the formation of

porosity. This porosity, coupled with

decreased percentage of spinel structure

formation, substantially decreases the hardness

and strength of the MMC.



__________________________________________________________________________________________


(a) (b)

Fig. 3: SEM Micrograph of Al Based Composite (a) Al-Mg (2.5%) AlloyRreinforced with Rice Husk

Silica Displaying Hard Spinel Structures (b) Al-Mg (2.5%) Alloy Reinforced with

Metallurgical Grade Silica Displaying Poor Reactivity due to Porosity.

Additionally, an increase in the hardness is

observed with the increase in the Mg content

(0.5-2.5%) because it forms a solid solution of

Mg in Al. Consequently, excessive lattice

distortion of Al occurs, resulting in formation

of finer grains having higher hardness (Figure

3a). The incremental trend in the hardness is

similar in case of rice-husk SiO₂ (Figure 2).

However, the same trend is not observed in

case of metallurgical grade SiO₂, since rice-

husk SiO₂ is in nano dimension (32-56 nm)

and more finely dispersed compared to

metallurgical grade SiO₂, resulting in lower

hardness values (Figure 2). As the % of Mg

goes beyond 2.5, the quantity of MgO also

increases reducing the hardness with increase

in wear loss (Table II). Uniform dispersion of

the fine reinforcements with fine grain size

(nm) in the matrix contributes to improve the

relative density and hardness of the composite

with rice husk silica. The constructive effects

of uniform dispersion of reinforcements in the

matrix on mechanical properties have been

proved by other workers [11, 12]. The nano

size rice husk silica particles forms the

uniform dispersion in the Al-Mg melt and Mg

present reduces the silica in the formation of

hard spinel structures as shown in the Figure

3a. The amount of porosities decreases

density, decreases hardness and increases wear

rate. Higher porosity causes an obvious

reduction in hardness. The wear rate of the

nanocomposite with nanosize SiO2 particulates

is the lowest due to its higher hardness. The

coarser particles of metallurgical grade silica

which are in micron dimensions, reduce the

specific surface area which in turn reduces the

density of the composite, causing reduction in

hardness (Figure 3b). The results of SEM

micrograph indicated the formation of spinel

structures with important phases such as

formation of Mg2Si and MgAl2O4 and their

distribution in main matrix, leading to increase

in the hardness of the composite [12].



__________________________________________________________________________________________


All the above eight samples were then

subjected to wear tests. To carry out the wear

test the RPM was set at 300, distance from

centre of disk to pin (track radius) was 50 mm,

pin diameter was 10 mm. The wear resistance

of the composite is measured in terms of the

wear loss at constant load conditions. The

values for weight loss for both types of

composites – Al-Mg-SiO₂ (SiO₂ from rice-

husk) and Al-Mg-SiO₂ (SiO₂ metallurgical

grade) for sliding distance of 1000 m under 10

and 20 N load are given in Table II.

Table II: Comparative Values of Weight Loss (mg) for the Composites Al-Mg-SiO2 (Rice Husk

SiO₂) and for Al-Mg-SiO2 (Metallurgical Grade SiO₂).

Composition

(wt %)

At 10N load At 20N load

RHA

Silica (nm)

Metallurgical grade

Silica(µm)

RHA Silica

(nm)

Metallurgical grade

Silica(µm)

Al-0.5%Mg-

5%SiO2

0.0105 gm 0.0112 gm 0.0115

gm(S1) 0.0125 gm(S5)

Al-1%Mg-

5%SiO2

0.0056 gm 0.0095 gm 0.0091

gm(S2) 0.0109 gm (S6)

Al-2.5%Mg-

5%SiO2

0.0036 gm 0.0064 gm 0.0043

gm(S3) 0.0062 gm(S7)

Al-5%Mg-

5%SiO2

0.0114 gm 0.0121 gm 0.0134

gm(S4) 0.0146 gm(S8)

From the above table, it is observed that the

wear loss is minimal in case of the sample

with 2.5% Mg and containing rice-husk SiO₂.

This was observed in both the cases where the

load was 10 N (Wt. loss=0.0036 gm) and 20 N

(Wt. loss=0.0043 gm). The relative density of

the composite decreases with increasing

particulate size of SiO2. A similar trend is also

observed from the graph plotted for the values

of hardness of these samples (Figure 2). The

wear rate of the nanocomposite, containing

nano sized SiO2 particulates, is the lowest.

Apart from higher hardness, minimal size of

SiO₂ particles and their refined grain size

contribute significantly to the low values of

wear. The microstructures under 20 N load of

all the eight samples (Al-Mg (0.5--5.0%)- SiO2

(Rice husk and metallurgical grade)) were

studied under SEM. The SEM images of the

wear track of the composite samples were

observed, and a comparative study was carried

out to analyze the effect of increase in the

percentage of Mg and the size the of

reinforced SiO₂ particles. From the Figure 3,



__________________________________________________________________________________________


Fig. 4: SEM Microstructure of Wear Track at 500X of Al-Mg- SiO2 (µm)(Metallurgical Grade) and

Al-Mg-SiO2 (nm) (Rice Husk Ash) with Varying Percentages of Mg.

Al-Mg-metallurgical Gr. SiO2( µm) Al-Mg-Rice Husk Ash SiO2 (nm)

S5: Al-Mg (0.5%)-RHA SiO2

(5%)

S6: Al-Mg (1.0%)-RHA SiO2

(5%)

S7: Al-Mg (2.5%)-RHA

SiO2

S8: Al-Mg (5.0%)-RHA

SiO2

S1: Al-Mg (0.5%)-Metallurgical grade SiO2

sssfghfhgfhgfhghsSvbvbvbghgfhgfhgfhSiO

2SiO2SiO2SiO2SiO2SiO2 (5%)


SiO2SiO2


SiO2


SiO2



__________________________________________________________________________________________


it is observed that the MMCs synthesized using metallurgical grade silica, with all the

four different composition of Mg ( 0.5, 1.5,

2.5, 5 wt%), shows lower hardness and higher

wear loss in comparison to composites

reinforced with RHA SiO2.. It is studied that

the wear resistance of a composite depends

mainly on the type of its reinforcement and

strength. Figure 3 (S1, S2, S3, S4, S5, S6, S7

and S8) illustrates the morphologies of worn

surface layers of the composites after their

abrasion. The micrograph S1, S2, S3 and S4

are of Al-Mg-SiO2 (Metallurgical Grade) with

the increasing % of Mg from 0.5 to 5.0,

respectively. And the micrograph S5, S6, S7

and S8 are of Al-Mg- SiO2 (Rice husk Silica)

with the increasing % of Mg from 0.5 to 5.0,

respectively .Worn surfaces of S1 and S5 were

rough, incurred a higher wear loss with deeper

wear grooves, and entrapped debris during

wear test. Some deep pits and continuous

scratches were observed on the microstructure

of the samples S1, S2, S4, S5, S6 and S8.

The wear particles and worn surface

morphology are closely related to the wear

mechanism. The sample number S3 and S7 are

of the composite Al-Mg (2.5%) metallurgical

grade SiO₂ and Al-Mg (2.5%) RHA SiO₂,

respectively. The hardness values from the

Table II displays that S3 and S7 have the

highest hardness as compared to the other

samples. As seen in the Figure 3, the wear loss

of specimens S3 and S7 is considerably lower

than that of the others. Similarly, highest wear

resistance is observed in specimens S3 and S7;

while the other samples have a lower wear

resistance. This can be attributed to the fact

that even after abrasion worn out particles

remained bound to the surface in specimens

S3 and S7; while the worn out particles were

released from the worn surfaces of specimens

(S1, S2, S4, S5, S6 and S8) during wear tests

processes. The corresponding worn traces of

the specimens (S1, S2, S4, S5, S6 and S8)

were found to be roughened, with deepened

grooves. The bond-strength of the

reinforcement in the MMCs greatly influences

their resistance to wear. Higher interfacial

bond strength generally results in higher

resistance to wear [11]. When the

reinforcement particles are coarse, the wearing

stress is large with fewer contact points

associated with larger stress concentrations.

Wear mainly results from the loss of matrix

when the reinforcing particles are small (nano

sized SiO2 particles) while it could be more

from micro-cracking (S6) and plough-off (S2)

of reinforcement when the particles are large

(metallurgical grade silica). If the added

particles are fine, the load carried by each

abrasive particle is small, so it penetrates only

into a shallow surface layer. A very miniscule

number of thin metal flakes were also

produced, as a result of mild wear (S7).

It is an established fact that uniform

distribution of reinforcement in a material

increases its resistance to wear [11]. The

literature also indicates that smaller (<10 µm)

and more equiaxed SiO2 reinforcement

particles in Al–Mg alloys exhibit



__________________________________________________________________________________________


better resistance to wear and inflict lesser

damage on counterpart surfaces compared to

larger (20–70 µm) particles [12]. This is

because larger particles are more prone to

fracture and pull out. It was also observed that

a small swelling and deep wide wear appeared

on the wear surfaces of S3 and S7. In addition,

scale-like feature characteristics of severe

adhesive wear appeared on the worn surfaces,

as shown in Figure 3 (S4 and S8). All these

factors interact simultaneously with one

another and synergistically influence the wear

performance of the composite. The micro

structural images of samples S3 and S7 in

Figure 3, illustrate the effect of reinforcement

size on wear of composites under identical

wear conditions. The size of reinforcement

alters the spacing between adjacent

reinforcements. An appropriate spacing

effectively reduces wear loss, which is also

influenced by the size of reinforcing particles.

Adding fine and uniformly dispersed

reinforcements to the matrix is an effective

approach to increase the wear resistance of a

composite (S7). The interfacial bond strength

was set to be sufficiently strong to prevent

interfacial debonding during wear. For a weak

reinforcement-matrix bond, adding

reinforcements simply reduced the wear

resistance due to interfacial failure (S2 and

S6).

If the reinforcements are small (rice husk

SiO2) and homogeneously dispersed, they can

increase the hardness of the matrix. This

results in a lower wear loss in comparison to

large silica particles in the present case of

metallurgical grade silica. Adding fine

dispersed reinforcement particles in the

matrix, facilitates reduction in the penetration

of abrasives into the matrix and wear damage

as well. More fine reinforcement particles can

be added, to decrease the mean free spacing

and to effectively resist the local wearing

force. Wear resistance of a composite is

influenced by the reinforcement-matrix

interfacial bond strength. Wear loss of the

composite is influenced simultaneously by the

spacing between reinforcements and their size.

To summarize, it was observed that a weak

reinforcement-matrix interfacial bond

generally caused continuous increase in wear

loss, while a strong interfacial bond resulted in

minimum wear loss (S7). Embedding fine

reinforcement particles in the matrix helps

reduce wear by strengthening the matrix and

blocks the penetration of abrasives more

effectively.

4. CONCLUSIONS

The micro hardness of the Al-Mg- SiO2 was

found to be maximum for 2.5% of Mg and by

using rice husk SiO2 of nano structure

dimension as reinforcement. The wear loss of

this composition was also found to be

minimum than other seven samples of

composites. The SEM micrograph of wear

track displays the surface morphology of the

worn out particles of all the composites with

varying % of Mg (0.5, 1.0, 2.5, 5.0) and with

two types of reinforcement (RHA SiO2 and



__________________________________________________________________________________________


metallurgical grade SiO2). From the SEM

micrographs of the worn out particles it was

concluded that for the composite with 2.5%Mg

and with rice husk silica as reinforcement, the

minimum worn out particles are seen which is

equivalent to maximum hardness and

minimum wear loss.

ACKNOWLEDGEMENT

I am thankful to the Institution of Engineers

(India) for the financial aid granted during this

project under the grant SCK/T-R&D/96/2009-

2010 and also to S. Chatterjee, S. Labhsetwar,

and V. Sharma who helped me in carrying out

the experimental work.

REFERENCES

1. Hosseini N., Karimzadeh F., Abbasi M. H.

et al. Materials and Design. 2010. 31.

4777-4785p.

2. Akbulut M., Durman M. and Yilmaz F.

Wear. 1998. 215. 170-179p.

3. Skolianos S. and Kattamis T. Z. Journal of

Materials Science and Engineering. 1993.

A163. 107-113p.

4. Jiang J. Q., Tan R. S. and Ma A. B.

Journal of Materials Science and

Technology. 1996. 12. 483-488p.

5. Rohatgi P. K., Weiss D. and Gupta N.

Journal of the Minerals, Metals and

Materials. 2006. 58. 71-76p.

6. Rohatgi P. K. and Guo R. Q. Proceedings

of the 59th Annual American Power

Conference.Chicago 1997. 828-833p.

7. Koczak M. J. and M. K. Prem Kumar.

Journal of the Minerals, Metals and

Materials. 1993. 45. 44-48p.

8. Maity P. C., Chakraborty P. N. and

Panigrahi S. C. Materials Letters. 1994.

20. 93-97p.

9. Kuruvilla A. K., Prasad K. S., Vanu

Prasad V. V. et al. SCR Metall Mater.

1990. 24. 873–878p.

10. Sarkar S., Sen S. and Mishra S. C. Journal

of Reinforced Plastics and Composites.

November, 20. 2008.

11. Sreekumar V. M., Pillai R. M., Pai B. et

al. Journal of Materials Processing

Technology. 2007. 192. 588-

594p.

12. Sun Y. and Ahlatc H. Materials and

Design. 2011. 32. 2983–2987p.

development and characterization of al based mmc by using...

Documents