Anali PAZU - Letnik 2, leto 2012, številka 1 
 
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The new approach of the production technique of discontinuous 
Cu-C composite  
Nov pristop v proizvodni tehniki diskontinurnega Cu-C 
kompozita 
Rebeka Rudolf
1,2
,
 
Ivan Anžel
1
 
1
University of Maribor, Faculty of Mechanical Engineering / Slovenia 
2
Zlatarna Celje d.d., Celje / Slovenia 
 
Abstract: The samples of the Cu-C composite were produced by two different techniques: (i) the sintering of 
powders, and (ii) the internal carbonisation of rapidly solidified pure copper. The aim of the present research 
was to produce a discontinuous Cu-C composite with the submicron dispersion of graphite particles. 
The results of our microstructural investigation show that the use of the mechanical alloying and sintering of 
consolidated powders is an inappropriate way to achieve a submicron dispersion of the graphite particles. On 
the other hand, the combination of rapid solidification and internal carbonisation was found to be an efficient 
technique to obtain the requested microstructure. 
Key words: Cu-C composite; sintering; internal carbonisation; microstructure 
Povzetek: Vzorci Cu-C kompozita so bili izdelani z dvema različnima tehnikama: (i) s sintranjem prahu in (ii) 
z notranjo karbonizacijo hitro strjenega čistega prahu. Namen raziskovalnega dela je bil izdelati diskontinuiran 
Cu-C kompozit s pod-mikrometrsko disperzijo grafitnih delcev. Rezultati mikrostrukturnih preiskav kažejo, da 
uporaba mehanskega legiranja ter sintranja konsolidiranih prahov ni učinkovita metoda za dosego pod-
mikrometrske disperzije grafitnih delcev. Po drugi strani pa se je kombinacija hitrega strjevanja in notranje 
karbonizacije pokazala kot učinkovita tehnika za dosego želene mikrostrukture. 
Ključne besede: Cu-C kompozit; sintranje; notranja oksidacija; mikrostruktura 
 
1. Introduction 
Copper is one of the most suitable engineering 
materials for thermal and electrical application. Like any 
other metal it softens at higher temperature, so the 
strengthening is one of the ways to overcome this problem. 
The copper matrix has been successfully strengthened with 
graphite reinforcements - particles and discontinuous or 
continuous fibre. The system Cu/C shows good thermal, 
electrical and stress properties in comparison to other Cu-
based metal matrix composites [1], on the other hand the 
Cu-C phase system is characterised by highly restricted 
mutual solubility of elements in solid state ( 
Cu
 with about 
0.02 at.% solubility of C and graphite with no reported 
solubility of Cu) [2]. Therefore the main problems in 
developing of Cu-C composite are the extremely weak 
interfacial bonding matrix-fibre and the attainment of a 
high volume percentage of finely dispersed, incoherent 
graphite particles of submicron size. Our research focuses 
on the achievement of the discontinuous Cu-C composite 
with the submicron dispersion of graphite particles.  
The Cu-C composite reinforced with C-particles can be 
produced by different techniques. In this paper two were 
presented: (i) sintering of powders, and (ii) internal 
carbonisation of rapidly solidified pure copper [3]. The 
sintering of Cu and C powders consists of the mechanical 
alloying and the annealing of the consolidated powders, 
while the combination of the rapid solidification (RS) and 
the internal carbonisation (IC) represents a new way in the 
conventional route production of the Cu-C composite. The 
rapid solidification serves as a process where the numerous 
defects in the microstructure were created, while the 
internal carbonisation enables the C-atoms segregate on 
these defects [4]. The increasing of C-concentration on 
these positions consequently produces the supersaturation 
and the thermodynamic conditions for the formation of C-
clusters, which in the further stage form the C-particles. 

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2. Experimental procedure 
2.1. Synthesis of sintered Cu-C composite 
The high purity copper powder (99.99%) with the 
particle size 140 m, and graphite powder with three 
different sizes of particles were separately mixed in an 
arthritic mixer for 1 hour. Figure 1 shows the typical shape 
of the initial Cu in C particles. Each mixture was 
subsequently cold pressed in a uniaxially press at 210 MPa. 
Obtained were greenbodies of about 99% of the bulk 
density having cylindrical shape with 70 mm in diameter 
and 3 mm in height. The compacted tablets were than 
annealed in a furnace at 1300 K and vacuum 10
-2
 mbar for 
3 hours. 
The described technology was used in preparing the 
following Cu-C specimens: 
- Composite A with 5 vol.% of spherical graphite (2r= 
1 m, 99,9995% purity), 
- Composite B with 5 vol.% of spherical graphite (2r= 2-15 
m, 99,9995% purity), 
- Composite C with 5 vol.% of nano-scaled graphite (2r= 
0.3 m, 99,6% purity) 
 
Figure 1. a) Cu particle, b) spherical graphite particles 
In the first step the microstructures of cold pressed 
(CP) and the sintered specimens were examined separately. 
In the second step the graphite volume fraction and also the 
porosity measurements of all specimens were performed: 
(i) in samples with the ideal geometry by calculation, and 
(ii) in samples with the irregular shape with the 
measurements of real density with pyknometer.    
2.2. Synthesis of Cu-C composite with combination of 
RS/IC 
The rapid solidification process of pure Cu (99.99%) 
was performed under an Ar atmosphere using a quartz 
crucible 48 mm in inner diameter and nozzle with an 
orifice diameter of 1.5 mm. The melt was ejected by Argon 
overpressure of 0.02-0.03 MPa onto the surface of a 
copper-beryllium wheel rotating at the speed of about 21 
m/s. Continuous ribbons 3 to 5 mm in width and 60 to 80 
m in thickness were produced. The microstructure of RS-

Cu
 ribbons consists of a zone with columnar 
Cu
 grains 
only a few m in size, extending from the wheel surface up 
to the free surface of the ribbon and is shown in Figure 2. 
The experimental specimens for IC were prepared from 
pieces of RS- 
Cu
 and nano-scaled powders of amorphous 
carbon. Short pieces of RS- 
Cu
 were packed in a powder of 
amorphous carbon, isostatically pressed into 3 cm long 
tablets and enclosed in a quartz ampoule. The heat 
treatments (HT) were carried out in a tube furnace at 
temperature of 1273 K, for various periods of time. 
Figure 2. Microstructure of RS- 
Cu
 matrix 
Microstructural characterisation of sintered and heat-
treated RS samples was carried out by optical microscopy 
(OM), scanning electron microscopy (SEM-Jeol JSM 840 
A) as well as EDX microanalysis (Link Analytical AN 
1000) and transmission electron microscopy (TEM). In 
 
 
b 

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addition Auger Electron Spectroscopy (AES-Microlab 310 
equipped with SEM and X-ray photoelectron Spectrometer 
-XPS) was used to determine the elemental composition of 
precipitated particles in the RS- 
Cu
 matrix. The surfaces of 
particles were cleaned and etched with Ar
+
 ions (E= 2keV, 
= 12 Acm
-2
, = 47°) to remove surface contaminants 
and to obtain compositional depth profiles of particles. 
The sintered and the heat-treated RS samples were 
etched in FeCl
3
-H
2
O-HCl-C
2
H
5
OH solution, while the thin 
foils of heat-treated RS ribbons were prepared by electro-
polishing in a solution CrO
3
 and CH
3
COOH. 
3. The results and discussion 
3.1. Analysis of sintered Cu-C composite 
EDX - analysis of cold pressed and sintered 
microstructure revealed the dark field inside grains 
consisting of fine graphite particles and the dark field on 
the grain boundaries presenting partially the pores and 
partially the great graphite agglomerates. The CP 
microstructure of samples A and B shows bimodal 
distribution of the graphite particles, which average size in 
the Cu-grains is roughly 0.5-0.7 m and on the grain 
boundaries about 13-14 m (Fig. 3a, 3b). During the 
mechanical mixing of the composite C the high degree of 
nano-scaled graphite agglomeration was observed, 
consequently in the CP microstructure only the large pores 
and the graphite agglomerates along the grain boundaries 
are identified (Fig. 3c). The average size of these 
agglomerates is about 18 m. The results of graphite 
volume fraction in the CP samples show that the fractions 
in all samples are slightly above ~ 3.5%, which is less than 
the theoretical value (5 %). 
The results of porosity measurement further show that 
the CP composites A and B have practically equal porosity 
(~0.5 vol.%), while the composite C has 6× higher value of 
porosity. It was also found that the volume fraction of 
porosity in the sintered A and B composites is higher than 
in the CP samples (~ 1.8%) and that the sintered composite 
C has an equal degree of porosity after sintering in 
comparison with the CP state.  
        
              a    b     c 
 
Figure 3.  Microstructure of A, B and C composites chemical etched with FeCl
3
-H
2
O-HCl-C
2
H
5
OH solution
The depth etching of cross and longitudinal section of 
A and B composites revealed that the smaller C-particles 
are manly oblong and of bar shape (Figure 4a in 4b). The 
stereometric analysis of graphite volume fraction by 
classical etched microstructure was based on assumption 
that the smaller C-particles are spherical; consequently the 
calculated values of graphite volume fraction were smaller 
than theoretical (~ 1.5%). Considering the real bar shape 
on the depth-etched microstructure the deviation of the 
quantitative determinate graphite volume fraction was 
minimised to 0.7%. The determinate values are a slightly 
greater than the theoretical value which is the result of a 
stronger Cu-matrix solving in the surrounding of C-
particles by the depth etching. 
 
a    b 

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35 
 
Figure 4. Depth-etched microstructure of 
composite A: a) cross and b) longitudinal 
section 
Based on the results of microstructural characterisation 
as well as on the results of quantitative microstructure 
analysis the following microstructural features and 
characteristic changes occurring during mechanical 
mixing, cold pressing and sintering been identified: 
- By mechanical mixing of powder an intensive 
agglomeration of C-particles occurred, whose tendency 
was inversely proportional to the initial size of graphite 
particles; 
- The degree of agglomeration determines a distribution 
of graphite on the volume of matrix. The greater initial size 
of the C-particles (lower degree of agglomeration) 
increases the fraction of the uniform distribution of the C-
particles, while at higher degree of agglomeration the 
priority of C-particles distributed along the boundary 
increases; 
- During cold pressing the microstructural evolution 
depends on two mechanisms, which consequently 
influence the bimodal distribution of graphite in the cold 
pressed samples. Graphite agglomerates were spread along 
the boundary, while by the dispersed C-particles imprinted 
in the surface of Cu-particles the sliding of C-layers took 
place. Consequently, the spherical morphology of graphite 
changes to bar morphology; 
- The presence of oxygen in the phase of sintering of 
the Cu-C composite caused an internal oxidation of 
graphite resulting in the increased porosity fraction. 
After each phase of sintering it was also found that in 
this manner the required fine sibmicron dispersion of 
graphite particles in the Cu-matrix could not be obtained. 
3.3. Analysis of internally carbonised RS Cu-ribbons  
Metallographic examination of the RS matrices of the 
heat-treated experimental samples after various time of HT 
revealed the microstructure consisting of 
Cu
 grains and 
very fine submicron particles homogeneously distributed 
through the volume of the matrix (Fig. 5). 
 
 
 
 
 
 
 
 
Figure 5. Micrographs of RS- matrix after 
HT at 1273 K for 9 min 
It was found that the volume fraction of precipitated 
particles depends on the temperature and time of HT. The 
higher degree of HT temperature and its duration causes 
the higher volume fraction of particles. The particles 
precipitate simultaneously throughout the entire volume of 

Cu
 matrix and no precipitation front appeared in the 
microstructure during the experiment. During the internal 
carbonisation C-atoms start with dissolution into the 
copper lattice, continue with diffusion in the matrix interior 
and concentrate in some places producing great super-
saturations of carbon. These places are most probably 
numerous defects in the crystal lattice of the RS- 
Cu
 matrix 
and at these positions was obtained enough high activity of 
carbon atoms needed for the formation of C-clusters, 
which in the further stage form the C-particles. 
In the "normal" internal oxidation processes the 
solubility product of compound at reaction front has to be 
exceed and the critical super-saturation for homogeneous 
precipitation is attained. Consequently fine particles of 
compounds start to precipitate from the solid solution. In 
our case, we cannot be deal with the solubility product 
because we do not have an alloy but the pure metal. So the 
critical supersaturation is needed for the formation of 
critical C-clusters, which in the further stage overgrow into 
the C-particles. On the basis of this theory and 
thermodynamic data [2] the following reaction of dissolved 
C on numerous defects in the crystal lattice of the RS- 
Cu
 
matrix was expected: 
[C]
Cu
 C
graphite
  (1) 
The results of the EDX analyses with the X-mapping 
confirm that the precipitated particles contain carbon 
corresponding to the distribution of elements in the region 
of the precipitated particles in the RS- 
Cu
 matrix as shown 
in Fig. 6, which confirms that these particles might be 
graphite particles. 

Anali PAZU - Letnik 2, leto 2012, številka 1 
 
36 
 
 
Figure 6. Distribution of elements in the region of precipitated particles in RS- 
Cu
 matrix
TEM examination and diffraction figure of the 
precipitated particles shown in the Fig.7 also revealed that 
the particles are pure polycrystalline graphite with the 
average size about 0.5 m. 
 
  a  b 
Figure 7. a) TEM image of polycrystalline graphite, b) diffraction figure of precipitated particles  
Additionally, the energy position and shape of an 
Auger peak in the direct energy spectrum (EN(E) versus 
the electron kinetic energy) as well as in the differentiated 
form (EdN(E)/dE- for display and peak identification) 
clearly show, that the particles consist of pure graphite 
(curve P1 and P2 in Figure 8). 
 
 
300 nm 
SEI 
C Ka Cu Ka 

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37 
 
Figure 8. The Auger spectra from the precipitated particles in the RS- 
Cu
 matrix
Based on these arguments we can conclude that the 
process of particles formation in the RS- 
Cu
 matrix during 
internal carbonisation consists of next stages: 
- in the first stage the carbon atoms dissolve at the 
Cu
/C 
interface into the lattice of 
Cu
 matrix and diffuse in 
their interior. While the critical concentration of carbon 
needed for graphite formation of clusters is higher than 
the maximum solubility of carbon in copper is no 
reaction in the RS- 
Cu
 matrix and the carbon 
concentration increases in the entire volume of the 
matrix up to the solubility limit. At numerous defects in 
the crystal lattice the carbon atoms are attracted and 
produce at these positions great supersaturations; 
- in the second stage, when the critical supersaturation 
for graphite formation is attained, fine particles of 
graphite start to precipitate from the copper matrix. The 
particles precipitate simultaneously throughout the 
entire volume of RS- 
Cu
 matrix, no precipitation front 
appears in the microstructure. These arguments confirm 
that the graphite precipitation primarily runs on the 
point defects (void clusters, voids, ..) and that the 
degree of supersaturation depends on the type of these 
defects and their position. The simultaneous 
precipitated C-particles are namely of different size.  
4. Conclusions 
On the basis of the received results and analyses the 
following conclusions can be summarised: 
1. The combination of rapid solidification and internal 
carbonisation makes the fabrication of discontinuous 
Cu-C composite with submicron size of graphite 
particles possible.  
2. The experiments confirmed the hypothesis, that during 
the internal carbonisation the C-atoms segregate on the 
defects in the rapidly solidified microstructure. The 
increasing of C-concentration on these positions 
consequently produces the supersaturation and the 
thermodynamic conditions for the formation of C-
clusters, which in the further stage form the C-particles. 
3. The volume fraction and size of the precipitated 
graphite particles are dependent on the initial rapidly 
solidified microstructure, on the temperature and the 
time of the internal carbonisation. 
P1 
P2 

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38 
 
4. Precipitation process of C-particles is run without 
precipitation of carbonisation front with a simultaneous 
precipitation of C-particles throughout the entire 
volume of 
Cu
- matrix. The C-concentration in the 
matrix reaches the maximum solubility, while the C-
supersaturation is achieved on the defects in the crystal 
lattice.  
5. The fabrication of the discontinuous Cu-C composite 
with the use of mechanical alloying and sintering was 
demonstrated as inappropriate for the achievement of 
submicron dispersion of graphite particles. 
6. During the mechanical mixing of powder an intensive 
agglomeration of C-particles occurred. The tendency 
was inversely proportional to the initial size of graphite 
particles. The degree of agglomeration determines a 
distribution of graphite on the volume of matrix. The 
greater initial size of the C-particles (lower degree of 
agglomeration) increases the fraction of the uniform 
distribution of the C-particles, while at higher degree of 
agglomeration the priority of C-particles distributed 
along the boundary increases. 
7. During the cold pressing the microstructural evolution 
depends on two mechanisms, which consequently 
influence the bimodal distribution of graphite in the 
cold pressed samples. Graphite agglomerates are spread 
along the boundary, while in the dispersed C-particles 
imprinted in the surface of Cu-particles the sliding of 
C-layers took place. Consequently, the spherical 
morphology of graphite changes to bar morphology. 
8. The presence of oxygen in the phase of sintering of the 
Cu-C composite caused an internal oxidation of 
graphite resulting in the increased porosity fraction. 
 References  
1. Dofman S., Fuks D.: " Formation and relative 
stability of interstitial solid solution at interfaces in 
metal matrix composites", Materials & Design 
Volume18, 1997, p. 4-6. 
2. Brandes E.A., Brook G.B. (eds.): "Smithell´s Metals 
Reference Handbook", Butterworth-Heinemann Ltd, 
London, 1992. 
3. Anzel I., R. Rudolf, A. Krizman, A.C. Kneissl: " 
Microstructural changes during internal carburisation 
of Cu-Al alloy", Praktischen Metallographie, 2002, p. 
401-413. 
4. RUDOLF, Rebeka, GILIĆ, M., ROMČEVIĆ, Maja 
J., ANŽEL, Ivan. Optical properties of plastically 
deformed copper : amorphous state with residual 
nanocrystals. Optoelectron. Adv. Mater. Rapid 
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