Science of Sintering, 46 (2014) 15-21
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doi: 10.2298/SOS1401015I
UDK 665.7.035.8; 53.086
Effect of Sintering Temperature on Electrical and
Microstructure Properties of Hot Pressed Cu-TiC Composites
S. Islak1*), D. Kır2, S. Buytoz3
1
Kastamonu University, Faculty of Engineering and Architecture, Department of
Materials Science and Nanotechnology Engineering, 37000, Kastamonu, Turkey
2
Kocaeli University, Hereke Vocational High School, 41400 Kocaeli, Turkey
3
Firat University, Faculty of Technology, Department of Metallurgy and Materials
Engineering, 23100 Elazig, Turkey
Abstract:
In this study, Cu-TiC composites were successfully produced using hot pressing
method. Cu-TiC powder mixtures were hot-pressed for 4 min at 600, 700 and 800 °C under
an applied pressure of 50 MPa. Phase composition and microstructure of the composites hot
pressed at different temperatures were characterized by X-ray diffraction, scanning electron
microscope, and optic microscope techniques. Microstructure studies revealed that TiC
particles were distributed uniformly in the Cu matrix. With the increasing sintering
temperature, hardness of composites changed between 64.5 HV0.1 and 85.2 HV0.1. The highest
electrical conductivity for Cu-10 wt.% TiC composites was obtained for the sintering
temperature of 800 °C, with approximately 68.1 % IACS.
Keywords: Cu-TiC composites, Electrical conductivity, Microstructure,Sintering temperature.
1. Introduction
Copper is commonly used as electrical contact material today due to its high electrical
and thermal conductivity, corrosion resistance, low cost and easy production [1, 2]. Pure
copper parts produced by powder metallurgy are used in electrical and electronic applications
owing to high electrical conductivity. However, low hardness, strength and low wear
resistance restrict usage area of pure copper. Mechanical characteristics and wear resistance of
the copper are generally enhanced with two ways; age hardening mechanism or addition of
hard secondary phases [3, 4]. During age hardening; addition of small quantities of chromium
and zirconium into copper causes precipitation of secondary hard phases, which could not be
dissolved in the copper at low temperatures. Alloys hardened by this ageing process lose their
strengths at temperatures above 500 °C with grain coarsening of precipitated hard phases due
to structural instability [5]. On the other hand, in the second method, Cu matrix composite
materials are produced by adding carbide, oxide, and boride into copper. Characteristics of the
copper could be improved by producing particle reinforced Cu matrix composite materials[6].
Ceramic reinforced metal matrix composite materials are used for structural applications in
wear industry due to super toughness and wear resistance. Especially alumina and silicon
carbide based composite materials do not lose their hardness and wear resistances during high
temperature applications [7-9]. Cu matrix composite materials are alternative materials when
_____________________________
*)
Corresponding author: serkanislak@gmail.com
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high electrical/thermal conductivity and good wear resistance are required. Considering
carbide reinforced copper composite materials; studies conducted with SiC in the literature
are remarkable. Celebi Efe et al. fabricated Cu-SiC composites via cold pressing technique by
making addition of SiC at various ratios into copper and subsequently sintering the mixture at
different temperatures. It was determined that as a result of increasing SiC addition, a great
quantity of porosity appeared in composites produced via cold pressing and consequently
values of electrical conductivity decreased [10]. Zhan and Zhang coated SiC particles with
nickel using electroplating method in order to obtain a stronger bonding on Cu and SiC
interface in Cu-SiC composites. While relative densities of coated composites were higher
compared to uncoated composites, the electrical conductivity remained almost the same.
However, mechanical features of coated composites came out good due to strong interface
bonding [11]. In this study, TiC between 0-15 wt.% was added into copper. Microstructure
and electrical features of composites produced using hot pressing technique were investigated.
2. Experimental Studies
Pure copper powder (in averagely 20 µm grain size) and titanium carbide powder (in
averagely 10 µm grain size) were used in the experiments. TiC was added at 10 wt.% rate into
Cu matrix. Fig. 1 illustrates SEM images of copper and titanium carbide powders. While
copper powder had a dendritic structure, titanium carbide powder had a sharp-edged structure.
Fig. 1. SEM images of (a) copper and (b) titanium carbide powders.
Copper and titanium carbide powders were mixed using powder mixing machine at
20 rpm for 30 minutes in such a way that the mixture would become homogenous. The
powder mixture was put into graphite moulds and pressed using an automatic hot pressing
machine for 4 minutes at 600, 700 and 800 °C sintering temperature under pressure of 50
MPa. Relative densities of samples were measured according to Archimedes' principle.
Micro-hardness measurements of pure copper and Cu-TiC composites were performed using
a Vickers hardness instrument under a load of 100 gf. The electrical conductivity of Cu-TiC
composites was evaluated with eddy current instruments in accordance with ASTM standard
E1004-02 [12]. The electrical conductivity measured by this equipment is usually expressed
as a percentage of the conductivity of the International Annealed Copper Standard (% IACS).
For metallographic study, the samples were prepared according to standard metallographic
procedure. An optical microscopy, scanning electron microscope, and X-ray diffractometer
were used to investigate microstructure and phase composition of composites.
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3. Results and discussion
The Cu-TiC composites were successfully produced using the hot pressing method
together with a sintering time of 4 minutes at 600, 700 and 800 °C, under a pressure of 50
MPa. Fig. 2 illustrates optical images of Cu and Cu-TiC composites produced via hot
pressing. When the microstructure is examined, three different structures draw the attention.
Gray and sharp-edged grains represent TiC, dark gray areas represent porosities, and finally
light gray areas represent Cu matrix. TiC particles distributed uniformly in the Cu matrix. Lee
et al stated that if reinforced particles do not distribute homogeneously, this situation would
affect mechanical and electrical features of the composite negatively [13]. TiC particles were
positioned as embedded since they were within grain borders of ductile Cu matrix. It can be
easily seen from the microstructure images that there is decrease in the quantity of the pores
which are found in the microstructure along with the increase in the sintering temperature.
Temperature,
°C
wt.% TiC
0
10
600
700
800
Fig. 2. Optic micrographs of Cu–TiC composites.
The XRD analysis was performed for each composite in order to determine whether a
phase formed to provide bonding in the interface of copper (matrix) and titanium carbide
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(reinforcement) particles or not. Fig. 3 illustrates XRD graphics of the composites. It is seen
from graphics that Cu and TiC phases formed. Any phase did not form between Cu and TiC.
This situation demonstrated that there was no chemical reaction between Cu and TiC.
Furthermore, XRD graphics obviously show that no oxide phase formed in Cu-TiC
composites.
1500
Δ Cu
♦ TiC
Δ
1000
Lin (Counts)
Δ
♦
♦
Δ
♦
Cu-10 % TiC
500
Cu
0
20
30
40
50
60
70
80
90
2-Theta-Scale
Fig. 3. XRD graphics of the Cu-TiC composites.
Fig. 4 illustrates relative density of Cu-TiC composites produced via hot pressing.
Relative densities of the composites were determined according to Archimedes' principle.
With the addition of TiC, relative densities of composites decreased. Relative density of hot
pressed pure copper was measured between 97.9 % and 99.1 %. When TiC was added at 10
wt.% rate into Cu matrix, the relative densities of composites changed between 79.9 % and
86.4 %. This decrease in the relative densities could be associated with the fact that increasing
rate of TiC affected the sintering adversely [14]. Another reason is that huge difference
between melting points of matrix and reinforcing member, namely Cu and TiC was an
inhibiting factor in the rearrangement of particles during sintering. Moreover, the fact that
density of TiC is lower than density of copper is another reason for the decrease in the relative
densities. There was an increase in the relative densities as sintering temperature increased
from 600 °C to 800 °C. Reason of this increase is that at higher sintering temperatures, a
denser structure is formed due to higher diffusion rates [15, 16].
Relative density (%)
100
90
80
70
Cu
Cu-10 wt.% TiC
60
600
700
Sintering temperature (°C)
Fig. 4. Relative density of Cu-TiC composites.
800
S.Islak et al./Science of Sintering, 46 (2014) 15-21
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The effect of sintering temperature and TiC addition on the hardness of composites is
depicted in Fig. 5. Hardness of Cu-TiC composites produced with addition of TiC
significantly increased from 38.6 HV0.1 to 85.2 HV0.1. This hardness increase was caused by
dispersion strengthening effect of TiC. Additionally, the increase in addition of TiC caused an
excessive increase in dislocation density in the Cu matrix and consequently it is thought that
hardness of composites increased. As understood from the hardness graph, there were
significant increases in the hardness values of the alloys along with the increase in sintering
temperature. This may be due to the activated sintering mechanisms leading to a reduced
porosity due to greater mass transportation at high sintering temperatures [17, 18].
100
90
80
Hardness (HV0.1)
70
60
50
40
30
20
Cu
Cu-10 wt.% TiC
10
0
600
700
800
Sintering temperature (°C)
Fig. 5. Hardness of Cu-TiC composites.
Tab. I illustrates results of electrical conductivity test of Cu-TiC composites. With
addition of titanium carbide, the electrical conductivity of composites decreased. While the
electrical conductivity of Cu with no addition was measured as 84.9 – 90.2 % IACS, the
electrical conductivity of composites with 10 % TiC addition to Cu matrix was measured as
63.5 – 68.1 % IACS. Porosity and oxidation could be asserted as the reason for low electrical
conductivity of pure copper [19]. Interaction between free electrons and nucleus is weak in
the metals. Therefore, electrons easily move and accordingly electrical conductivity of metals
is good. However, electrons are firmly bonding to the nucleus in the carbides and electrons do
not move.
Tab. I. Electrical conductivity values of Cu-TiC composites.
Electrical conductivity (% IACS)
Samples
600 °C
700 °C
800 °C
Cu
84.9
88.7
90.2
Cu-10 wt.% TiC
63.5
64.9
68.1
For this reason, electrical conductivity of carbides is weak [20]. Since rate of Cu
matrix in the addition of TiC decreased, it is an expected result that electrical conductivities of
the Cu-TiC composites would decrease. Reinforced TiC particles in the composite exhibited
an effect inhibiting the movement of Cu electrons. With the sintering temperature increased,
the electrical conductivity of composites increased. The electrical conductivity of Cu–TiC
composites were 63.5 % IACS, 64.9 % IACS and 68.1 % IACS for the samples sintered at
600 °C, 700 °C and 800 °C respectively. Our results are, however, in contrast to the findings
of Celebi Efe et al [10]. They reported that the electrical conductivity of Cu-5% SiC
S. Islak et al. /Science of Sintering, 46 (2014) 15-21
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composites decreased from 59.9% IACS to 53.3% IACS with increasing the from 900 ° C to
950 ° C of sintering temperature. They explained this state with the formation of oxide at high
temperatures. However, in our study, oxide formation did not occur because the maximum
sintering temperature was selected 800 °C.
4. Conclusions
‫־‬
‫־‬
‫־‬
‫־‬
‫־‬
The Cu-TiC composites were successfully produced using the hot pressing method
together with a sintering time of 4 minutes at 600, 700 and 800 °C, under a pressure of 50
MPa.
Microstructure studies revealed that TiC particles were distributed uniformly in the Cu
matrix.
The presence of Cu and TiC was confirmed by X-ray diffraction analysis. XRD analysis
showed that there was no formation of copper oxide which affected electrical properties
of composites negatively.
With the introduction of TiC, the hardness of composites increased while relative density
decreased. Both relative densities and hardness increased as sintering temperature
increased.
The electrical conductivity of Cu–TiC composites decreased with the introduction of TiC
as expected. With the sintering temperature increased, the electrical conductivity of
composites increased. The highest electrical conductivity value of 68.1 % IACS was
obtained for Cu–10 % TiC composite sintered at 800 °C.
Acknowledgments
This research was financially supported by Kastamonu University Scientific Research
Projects Coordination Unit (Project no: KUBAP-01/2012-15).
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Tables
Садржај: У овом раду, успешно су синтетисани Cu-TiC композити применом методе
топлог пресовања. Смеше Cu-TiC прахова су топло пресовани 4 min на 600, 700 и 800
°C под притиском од 50 MPa. Фазни састав и микроструктура топло-пресованих
композита одређени су рендгенском дифракцијом, скенирајућом електронском
микроскопијом и оптичким микроскопом. Микроструктурна анализа је открила да су
честице TiC униформно распоређене унутар матрикса Cu. Са порастом температуре
синтеровања, чврстоћа композита се мења од 64,5 HV0.1 до 85,2 HV0.1. Највиша
вредност електричне проводности за композит Cu-10 wt.% TiC је постигнута на
температури синтеровања од 800 °C, и износила је апроксимативно 68,1 % IACS.
Кључне речи: Cu-TiC композити, електрична проводност, микроструктура,
температура синтеровања.