103

XV Międzynarodowa Studencka Sesja Naukowa

Katowice, 15 Maj 2014

Production and properties of siluminum based composites

with Si3N4 reinforcement and carbon nanotubes additive.

B. ADAMCZYK

*a, B. HEKNER

*

* Silesian University of Technology, Department of Materials Science,

Krasinskiego 8, 40-019 Katowice, Poland

abarb.adamczyk@yahoo.com

Introduction

Aluminum matrix composites (AMCs) are widely used because of their low weight and corrosion

resistance. The problem of low mechanical properties of aluminum alloys [1] is usually solved by using ceramic

reinforcement. High temperature during process of joining components are the reasons of chemophysical

reactions on the matrix/reinforcement interface [2]. Due to that we get higher wear resistance, creep resistance

and better dimensional stability. However the problem might be toughness. Because of all this reason AMCs

are widely applied as brake pads and discs, satellites or structural components for aircrafts [3].

Mentioned properties depend on the type of the reinforcement, the size of particles and their dispersion.

This is why researchers put great attention to proper selection of types of reinforcements [4], with the most

popular SiC and Si3N4 particles.

Wishing to apply materials for e.g. brake pads or brake discs their high tribological properties

are important. Carbon addictions like glassy carbon or carbon nanotubes enhance desired properties regardless

of ceramic reinforcement [5]. Problems regarding dispersion of the carbon nanotubes within the metal matrix

often could cause the opposite effect and lower mechanical properties are observed in the processed materials.

This work examined preparation of siluminum-Si3N4 composite particles with various carbon additives

processed by mechanical alloying followed by hot pressing or free sintering. The objective of this study was

to obtain the bulk composite samples with 2% and 5 wt% carbon nanotubes content. There was also important

to reduce the diameter of reinforcing silicon nitride particles with simultaneous manufacturing the composite Al-

Si-Si3N4 particles by the mechanical alloying process.

Experimental

The composite powders were processed by the planetary high energy ball milling of the initial powders

followed by densification of the resultant composite powders. Silumin powder (Sulzer Metco), silicon nitride

powder (abcr GmbH&Co. KG) and multiwalled carbon nanotubes (Helix Material Solutions) were

the commercial products while glassy carbon was synthesized in our laboratory via pyrolysis of phenol –

formaldehyde resins in an inert atmosphere at temperature about 2000°C [6]. Composites of the examined

batches and characterization of the initial powders is given in Table 1.

The materials have been milled using the planetary ball mill (Fritsch Pulverisette 7) in silicon nitride

jars together with silicon nitride milling balls, for 1 h, using ø 5 mm balls, and a 8:1 ball to powder weight ratio.

Stearic acid in an amount of 5 wt% was added to the mixture before milling in order to prevent welding of metal

particles. The quality of the milled powders was checked visually and in the Scanning Electron Microscope

Hitachi S4200.

104

Table 1. Composition of the examined batches and characterization of the initial powders.

Components wt [%] Characterization

2% CNT 5% CNT Density

[g/cm3]

Powder bulk density

[g/cm3]

Particle size

[µm]

Siluminum 80 80 2,7 1,24 40-106

CNT 2 5 1,4 - 1,2

Si3N4 13 10 3,2 0,25 <15

Glassy carbon 5 5 1,5 - 5-200

To avoid breaking nanotubes and at the same time to have a good dispersion the process of milling was

performed in two steps (Table 2). To avoid the contact of the powders with air between the both steps (after

milling the mixture Si3N4, siliminum and glassy carbon and before adding CNTs) the chambers were moved

to the glove-box with nitrogen atmosphere. Next the CNT were added. To every step of process 5% stearic acid

was added to powders. After completing the process of milling the powders were checked in SEM if the CNT

are still able to be observed.

Table 2. The milling procedure and parameters for the composite powder formation.

Two options of sintering were tried – hot pressing and free sintering. The parameters of the processes

are shown in Table 3. Because of the very experimental stage of investigations only the sample with 2% CNTs

content was sintered. In hot pressing the loose composite powder was poured into a graphite dye and vacuum

atmosphere was used.

The free sintering process was preceded by the uni-axial compaction of powders using hydraulic press

and oxidation of organic additives in air atmosphere at 240oC for 2 h. The final densification was conducted

in a graphite furnace (Thermal Technology GMBH) with argon flow. Microstructure and chemical composition

of the hot pressed samples was checked using SEM in longitudinal section.

Table 3. Parameters of sintering.

Stage

Operating

temperature

[°C]

Holding

time

[min]

Punch

pressure

[MPa]

Chamber

pressure

Hot pressing

I 480 20 10 10

-3 torr

II 670 15

I 480 20 10 10

-3 torr

II 655 15

Free sintering - 655 35 0

Argon,

atmospheric

pressure

Results and discussion

The milled composite powders as well as sintered specimens were checked using Scanning Electron

Microscope. SEM images revealed well embedded CNTs in the siluminum particles (Figure 1). CNTs were

easily found in a sample that contained 2 wt% of CNTs and in the sample with 5 wt% content the clusters of

CNTs were also found. It is believed that proper results of the process were obtained as a result of glassy carbon

Step Chemical composition Milling

time [min]

B/P rpm

I Si3N4 + siluminum + glassy carbon + stearic acid 30 8:1 1000

II (Si3N4 + siluminum + glassy carbon) + CNT + stearic acid 30 8:1 500

105

presence – it helps in proper milling of powders and it avoided welding of metal particles. Moreover, lower

energy milling in the second step protects CNTs from breaking.

Figure 1. SEM images of the milled powders with 2wt% of CNTs (a) and with 5wt % of CNTs (b);

a – CNTs are observed; b – many CNTs are observed as well as clusters of nanotubes.

Sintering the sample with 2% CNTs content was the next step in process of composites manufacturing.

Unfortunately, the metal outflow of both samples was observed after hot-pressing at 670 and 655oC. That effect

was caused by too high temperature of the process and it resulted in the deviation of the initial chemical

composition. To avoid the metal outflow, free sintering at lower temperature of 655°C was chosen. There was

no metal outflow observed after process. Eutectic temperature for silumins is 577oC below both, the applied

hot-pressing and free-sintering temperature. Thus the reason of the metal outflow during hot-pressing is because

the metal’s melting point is lower in vacuum atmosphere.

Sintered samples were checked in the Scanning Electron Microscope. Those studies showed that

in both sintered by hot pressing samples the reinforcement was uniformly distributed over the observed section

(Figure 2a, 3a).

Figure 2. SEM images of the hot pressed samples in 670°C with 2wt% of CNTs;

a – uniformly distributed reinforcement; b – glassy carbon particles and aluminum rich areas.

There were also glassy carbon particles with their typical shape and sharp edges (Figure 2b, 3b), what was

confirmed by the point chemical analysis. Chemical analysis revealed also some aluminum rich areas, without

particles of reinforcement. It has also been found that brighter points are the particles of silicon nitride which are

mainly distributed at the grain boundary.

a) b)

a) b)

106

Figure 3. SEM images of the hot pressed samples in 655°C with 2wt% of CNTs;

a – uniformly distributed reinforcement; b – glassy carbon particles and aluminum rich areas.

Summary

The presented investigations are the first part of the project on manufacturing aluminum matrix

composites with ceramic and carbon nanotubes reinforcement.

The resultant composites were obtained by mixing powders in the planetary ball mill and sintering.

It was found that an addition of glassy carbon helps in manufacturing good quality composite powder by altering

the milling involved reactions.

Hot pressing and free sintering techniques were tried for densification of composite powders. First

option resulted in samples that were affected by the metal outflow what produced a disorder of the initial

chemical composition. However, in both hot pressed samples chemical analysis revealed aluminum-rich areas,

without particles of reinforcement.

Free sintered sample do not reveal metal outflow but densification process must be research further.

Acknowledgements

The research results were obtained in the framework of the Matera project ”SiNACERDI” (MATERA/HPE-

2217) funded by NCBiR in Poland.

References

[1] „Konstrukcyjne materiały metalowe, ceramiczne i kompozytowe” M. Kaczorowski, A. Krzyńska

[2] A. Boczkowska, J. Kapuściński, K. Puciłowski, S. Wojciechowski “Kompozyty”

[3] A. Wojciechowski, J. Sobczak „Kompozytowe tarcze hamulcowe pojazdów drogowych”

[4] “Wear mechanism in hybrid composites of graphite-20 pct SiC in A356 aluminium alloy” W. Ames, A.T.

Alpas

[5] “Tribological properties of heterophase composites with an aluminium matrix” J. Myalski, J. Wieczorek, A.

Dolata-Grosz

[6] J. Myalski „Kształtowanie właściwości tribologicznych kompozytów zawierających węgiel szklisty”

a) b)