Properties of copper based composite materials reinforced with alumina particles and fibers

JACEK W. KACZMAR, KRZYSZTOF NAPLOCHA, ADAM KURZAWA

Institute of Production Engineering and Automation Wroclaw University of Technology

ul. Lukasiewicza 3/5, 50-371 Wroclaw POLAND

jacek.kaczmar@pwr.wroc.pl http://www.itma.pwr.wroc.pl

Abstract: In order to manufacture the copper based composite materials reinforced with alumina particles or fibres the squeeze casting method was developed. Porous preforms prepared by wet forming process were infiltrated in special device applying the pressure of 90MPa. At elaborated process parameters composite materials exhibited dense microstructure and proper matrix-reinforcement interface. Copper oxide Cu2O existing in molten copper, which can improve poor wetting, reacted with alumina forming the CuAl2O4 spinels. Composite materials designed for brake disk application were subjected to strength, wear and thermal conductivity examinations. Particle reinforcing significantly improve offset yield strength R0,2, which was almost constant in entire temperature range of 20-300°C. Increase of bending strength was obtained by reinforcing with fibres whereas particles improve strength only at higher temperature. During dry sliding tests performed under pressures of 0.2 and 1.0 MPa, wear rate of unreinforced Cu was significantly larger than correspondingly composite materials reinforced with 20% of fibres and 50-60% of particles. During friction wear products from counterpart were embedded into the copper based composite subsurface resulting in decreasing of friction coefficient from 0.72 for unreinforced copper to about 0.6 for reinforced composite material. Particles significantly reduced thermal conductivity of Cu though it was still acceptable what provided large heat dissipation from the friction area. Key-Words: - Cu matrix composite, ceramic fibres, ceramic particles, alumina, preform, infiltration 1 Introduction

The properties of copper matrix composite materials have been widely examined, since they can provide several advantages over monolithic alloy like excellent thermal and electrical conductivities, good toughness, formability, high wear and oxidation resistance [1,2]. As advanced materials they are used in many application, such as radiators, electronic contact devices, casing in jet engines and in recent years as substitute materials for cylinder heads, liners and brake disc in automotive industry [3-5].

Sliding against metals or ceramics under dry condition proceeds with high contact temperature, microcutting and even seizure. Therefore, reinforcing ductile Cu with hard alumina should result in dissipation of heat from the wear area, stable friction coefficient and good strength in the wide temperature range. These properties are generally affected by microstructure of composite materials created during manufacturing.

Cu based composite materials reinforced with alumina reinforcing elements have been fabricated by reaction sintering, pressure infiltration, stir casting or internal oxidation [6-8]. All these methods tend to produce sound microstructure and proper interface. Taking into account the poor wetting of ceramic elements by copper, coating of ceramic elements or very high infiltration temperature are needed to improve adhesion at the interface reinforcement-matrix and then improve the load transfer from the matrix to strengthening elements or other physical properties like heat transfer. Moreover, high infiltration pressure typically applied in squeeze casting operation can enhance reinforcement-matrix contact and reduce porosity. As the industry method provide fast process, near net shape product at reasonable cost.

In the present study, significant effort to adopt squeeze casting method to produce Cu composite materials reinforced with alumina particles or fibres was made. Investigations of mechanical and physical properties aimed to develop new materials

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for machine parts subjected to wearing and high load. Designed materials meet the expectations of cooperating research centum CRF Torino Fiat for production of brake disk.

2 Experimental details Producing of composite materials included two stages: manufacturing of alumina preforms and infiltration of preforms with molten Cu by squeeze casting method. Preforms characterized by the open porosities were made from Saffi ceramic fibres or alumina particles, see Table 1. Nanocrystalline structure of fibres was stabilized by silica which inhibits grain growth at higher temperatures. Edgy shaped particles contain low volume of porosity, typical for α−Al2O3. To produce preforms and create stable structure for infiltration, wet forming process incorporating mixing of fibres in water solution of silica binder, drying and firing at 950°C was applied. Preforms with skeleton structure, see Fig.1. were preheated to 1100°C, mounted in a special designed die and immediately infiltrated with molten Cu under the pressure of 90MPa. Table 1. Specification of alumina particles and Saffil fibres. Material Composition

(wt.%) Density (g/cm3)

Diameter (μm)

Al2O3 fibre

Al2O3-δ: 96-97, SiO2: 2-4,

3.3 3-5

Al2O3 particle

Al2O3-α: 96, Al2O3-β: 3,

residiues: Na2O, Fe2O3, SiO2

3,95 5-6

Fig.1 Fracture of preform characterized by 50% porosity made from alumina particles joined with silica binder

Composite materials and unreinforced Cu specimens were examined at Instron 1126 strength tester at the temperature range of 20-300 °C. During wear tests, (pin-on-disc method) specimens were pressed against the gray cast iron disc under the pressures of 0.2 and 1MPa. Microstructure and chemical compositions were examined with an optical microscope and the scanning microscope Hitachi S-3400N. 3 Results and discussion Compression strength was investigated at the temperature range of 20-300°C on copper based composite materials and unreinforced Cu. Composite materials reinforced with larger volume of alumina reinforcing elements can be classified as a brittle materials and they fractured with 45° slip plane. On the other hand, the ductile Cu specimens were compressed with no single clear point of failure and determination of ultimate compression strength was impossible. Also composites containing 10% of fibres at high temperature were deformed into barrel shape and should be treated as a ductile. Therefore to compare materials and evaluate strength increase as the effect of introduction of ceramic elements (fibers, particles) into the copper matrix only apparent elastic limit or offset yield strength R0,2 can be determined. Figure 2 shows the yield strength improvement R0,2 of Cu casting reinforced with two types of alumina. Strengthening with fibers clearly improve yield strength, especially at higher temperature. Composite materials with 20% fibres revealed the yield strength of ca. 300MPa, almost 3 times higher strength than of pure Cu. But distinct increase was observed in the case of composite reinforced with 50% of particles. Strength exceeded 600MPa and was stable in entire temperature range.

Investigations of bending strength were performed at the same temperature ranges and on similar material groups. At elevated temperatures composite materials, in relation to Cu casting, exhibited higher strength by 20-25%, when reinforced with fibres or slightly lower in the case of particle reinforcing, see Fig.3. With increase of temperature of investigations, strength of Cu significantly decreased, whereas composite materials strengthened with alumina particles and fibres showed relatively good yield strength. Thus, at 300°C bending strength of composite materials reinforced with particles was 250-300MPa, about two times higher then of Cu unreinforced materials,

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50

250

450

650

0 100 200 300

Temperature [°C]

R0,

2 [M

Pa]

50% particles 20% fibers10% fibers Cu

100

200

300

400

500

0 100 200 300 400

Temperature [C]

Bend

ing

stre

ngth

[M

Pa]

Cu10% fibers20% fibers50% particles60% particles

while reinforcing with fibers resulted in even better strength of 350-400MPa.

Fig.2. Offset yield strength R0,2 of unreinforced Cu and copper based composite materials reinforced with 10, 20 vol.% of fibers or 50 vol.% of particles. Fig.3. Bending strength in 20-300°C of unreinforced Cu and composite materials reinforced with 10, 20vol.% of fibers or 50 vol.%, 60vol.% of particles.

Microscopic observations included analysis of fracture development, phase composition at the reinforcement-matrix interface and its ability to transfer the stresses from the matrix to reinforcing elemnts. Taking into account the three-point-bending tests, the fracture was initiated at the side of the sample where the tensile stresses were dominant and then crack propagated to the compression stress side. Microstructure observations of unreinforced Cu have shown that deformation and fracture proceeded through interdendritic regions. Though, melting in induction furnace was performed under the protective nitrogen atmosphere and during transfer and pouring of molten metal into the die Cu was subjected to

intensive oxidation. Therefore, the resulting copper oxides accumulated and formed Cu-Cu2O eutectic between dendrite grains. As areas of lower strength were significantly deformed under load and cracked first. Intertwined dendrite arms were disrupted and their fragments fixed in fractured structure, see Fig.4. It could be ascertained that stresses were transferred by dendrites whereas eutectic weakened microstructure and reduced casting strength.

In composite materials, subjected to high tension (at the one side of specimen), hard reinforcement transferring load were broken and in the case of particles they detached from the matrix. Usually, fibres arranged in the direction of load cracked in places where eutectic accumulated and high plastic strain occurred, see Fig.5. Thus, high plastic deformation resulted in high shear stress at the interface which put fibres in tension exceeded their strength. Number of fibrer fragments increases at higher temperature especially in materials with lower fibre volume. In the case of particle reinforced composite materials the cracks propagated through some structure defect and brittle fracture developed abruptly.

Rys.4. Fracture of unreinforced Cu with Cu2O oxides originated from eutectic and torn dendrite arms. Observations of composite materials fractures with microanalysis of phases at the interface shown limited reaction of alumina with CuO2 originated from eutectic. As a result, on the fibers surfaces CuAl2O4 spinels were formed, see Fig.6a, 6b, which in equilibrium existed between 800-900°C [9]. In the case of α-Al2O3 particles at the interface was also observed more favorable CuAlO2 compound. Moreover, TEM investigations confirmed the presence of thin (60-80nm thick) SiO2 coatings on fibers created from binder used to produce preform.

Dendrite arm

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` Fig.5 Microstructure of Cu-10 vol.%-Al2O3 fibres composite materials in extended side of bending test specimen with visible crack of fibre in areas of Cu-CuO2 eutectic.

During infiltration Cu can quickly diffuse through SiO2 which was decomposed and released oxygen reacted at the interface. Thus, spinel formation can be enhanced by SiO2 layer, which, on the other hand, reduces wetting angle [10] and improves interface strength. In fracture surface no pool-out fibres or detached particles were observed, see Fig.6 Fibres aligned perpendicular to the stresses can crack but their fragments were still attached to the matrix.

Examination of wear resistance performed at dry condition shown relatively small effect of load on wear rate of Cu matrix, see Fig.7. Reinforcing of copper matrix with fibres improved resistance, which under pressure of 0.2MPa for Cu-20%Al2O3 composite materials was 6-7 times better. However, the best effect was achieved when the matrix was strengthened with 50-60% of particles. Wear rate was independent on the pressure and under pressure of 1MPa resistance to wear of composite materials reinforced with particles was almost similar to resistance to wear of fibre reinforced composite material at low pressure. Composites exhibited smaller friction coefficient than unreinforced Cu, see Table 2, though tested after long sliding distance, against preheated to ca. 100°C counterpart slightly increased and finally stabilized. Thermal conductivity of Cu matrix provide heat dissipation and decreasing of temperature at friction area. Examined thermal diffusivity of composite materials, see Fig.8, was slightly reduced by fibre reinforcing, whereas high volume of particles decreased diffusivity almost three times in relation to unreinforced Cu. However, it was still much higher than thermal diffusivity of cast iron brake disk or aluminium based composite materials [11].

a) b) c) Fig. 6. Fracture of Cu-20%Al2O3 composite (a), EDS analysis of spinel on fibre (b), fracture of Cu+50 vol.% of particle after bending test at 300°C (c)

Generally, obtained properties meet the requirements of the design for friction pairs in automotive applications and are recommended by the CRF Fiat target. Further investigations will be performed in order to take decision on applications of particles or fibres as the strengthening elements.

Al2O3 particle

CuAl2O4 spinel

Crack transferred between fibres

cracks

40µm

Cu-CuO2 eutectic

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0

2

4

6

8

10

0% Cu 10%fibers

20%fibers

50%particles

60%particles

Wea

r rat

e [m

3 /m*1

0-12 ] 0.2 MPa

1 MPa

0

50

100

150

0 200 400 600 800

Temperature [C]

Ther

mal

diff

usiv

ity [W

/m*K

].

CuCu-10% fibersCu-20% fibersCu-50% particlesCu-60% particles

Fig.7. Effect of load and reinforcement volume on the wear rate of unreinforced Cu and composite materials

Table.2. Friction coefficients µ and groove depth in cast iron counterpart after test with unreinforced Cu

and composite materials Material µ depth

Cu 0.72±3 0±5µm 10% fiber 0.65±2 10±5µm 20% fiber 0.69±4 25±5µm

50% particles 0.59±2 20±5µm 60% particles 0.58±2 25±5µm

Fig.8. Thermal diffusivity of unreinforced Cu and composite materials in 20-600°C range

In order to determine the effect of reinforcement on friction process, microscopic observations of subsurface were performed. Under lowpressure of 0.2MPa, in plastically deformed Cu matrix the fibres were broken and then like a particles moved according to friction direction. Subsequently, fragmented fibres or particles were mixed with deformed matrix producing layer with new structure, see Fig.9. Additionally debris were embedded into composite materials structure. Then, especially in particle reinforced composite materials, new external layer protecting surface and

reducing wear rate arose. With progress of friction, at crushed reinforcement and weakened interface, crack can be initiated and small pieces of material in form of scale were separated. Increasing of load accelerated wear which proceeded with abrasion, microcutting and grooving. Therefore, composite subsurface was constantly removed and then Cu matrix was slightly deformed with almost intact reinforcement. Rys.9 Wear subsurface of Cu+20 vol. % Al2O3 fiber composite materials with crack propagating between fragments of crushed fibres

Serious problems occurred when examination of counterpart profiles shown their grooving, see Table.2 and Fig.10. During friction of unreinforced Cu, worn product (FeO, Fe2O3, C, Cu-oxides) adhered to counterpart surface and then protected it. Thus, no groove was observed. In the case of composite hard fragments of alumina reinforcement, as a loose materials, abraded counterpart and accelerate wear rate. Therefore at higher volume of fibers or particles groove depth reached about 25µm. Only after friction against composite reinforced with 10 vol.% of fiber wear of counterpart was relatively low and satisfactory.

Fig.10. Groove profile in the counterpart made from cast iron after test with Cu+50 vol.% of particle strengthend copper based composite material composite under pressure of 1MPa.

40µm 0.2MPa

crack propagation

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4.Conclusions The effort for developing of squeeze casting

parameters for copper based composite materials resulted in dense microstructure with proper interface quality between matrix and alumina reinforcement. Copper strengthened with particles or fibres were forseen for brake disc materials and exhibited following properties:

1. The offset yield strength R0,2 of Cu increased

almost 3-times when was reinforced with 20 vol.% of fiber and 6-times when reinforced with 50vol.% of particles.

2. Reinforcing with particles slightly reduces bending strength at ambient temperature, but at high temperature improves it significantly. Composite materials reinforced with fibres provide 30-300% increase of bending strength, in relation to unreinforced Cu, respectively at 20 and 300°C.

3. During infiltration molten metal containing Cu-CuO2 eutectic reacts with Al2O3 producing CuAl2O4 spinel at the interface. Reaction can be enhanced by silica used as a binder in preform production, which decomposed and released oxygen.

4. Reinforcing with alumina fibres clearly decreased wear rate especially under smaller 0.2MPa pressure. The highest improvement provide particles though wear of counterpart needs further optimization of alumina content and friction condition.

5. Wear resistance of composite materials combined with good thermal conductivity, high and stable friction coefficient meet the requirements for brake disk materials.

Acknowledgement This work was supported by the EU FP7 Project “Micro and Nanocrystalline Functionally Graded Materials for Transport Applications“ (MATRANS) under Grant Agreement no. 228869” References: [1] Ke Chu, Qingying Wu, Chengchang Jia,

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[9] Ramón David Díaz Martínez: Synthesis of CuAlO2 thin films by pulsed laser deposition, A thesis of master of science in physics, University of Puerto Rico, 2009

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