Fabrication and Fibre Matrix Interface Characteristics of Cu/C(Fe) Composite

Cu/C composites were successfully fabricated by three step electro-deposition. The effects of hot pressure temperature and alloy element Fe on the interface characteristic of Cu/C composite were investigated. The results showed that the hot pressure temperature and interface characteristic have a great effect on mechanical properties of the composite. The tensile strength and hardness increase firstly and then decrease with the increasing of hot pressure temperature, and a tensile rupture appears when the hot pressure temperature is 650, 700 and 900C. The addition of alloy element Fe not only improves the tensile strength and the lateral shear strength of the Cu/C composite, but also changes the interface bond type from the physical bond type to the chemical bond type.


Introduction
Carbon fibers possess a high specific strength, specific modulus, high thermal and electric conductivity, low expansion coefficient and good self-lubricancy, which were widely used into resins and metals as reinforcements to fabricate high performance composites [1,2].Cu/C composites are promising functional materials used as electrical packing, electrical devices, heat exchangers, etc., due to their high strength, lower expansion coefficient, lower frictional coefficient, good wear resistance, high electrical conductivity and heat conductivity [3][4][5][6][7][8].However, neither does liquid copper wet carbon nor does chemical reaction occur at the interface between copper and C. Therefore, it was very difficult to prepare the Cu/C composites with good interface bonding, which limited their further application [9,10].
Copper coatings on the surface of C by electroplating or electroless plating are good and simple methods to solve the problem of poor wettability, and the common fabrication of Cu/C composites involves the process of copperization plus powder metallurgy [5,11,12].To overcome the limitations, a new technique, electro-deposition-vacuum hot pressure [13], has been developed, in which the carbon fiber-reinforced copper-matrix composites are synthesized.
In the present study, the effects of hot pressure temperature and alloy element Fe on the interface characteristic of Cu/C composite were investigated.The main objective of the present study was to improve the interface bonding characteristic, and to synthesize a high strength Cu/C composite.

Experimental Methods
In the present experiment, composite silks and composite blanks were fabricated by the three steps electrodeposition method [13].In order to control the fiber ratio (V f ) of Cu/C composites, processing parameters (time and current density) of electrodeposition could be regulated individually.Furthermore, carbon fiber needed pretreatment before copper was deposited, and the whole process included four steps: degumming, coarsening, bathing and drying.A technology flowing chart is shown in Fig. 1.Then, in order to improve properties of Cu/C composite and investigate the effect of alloy element Fe on interface bond characteristic of Cu/C composite.Cu/C composites were coated with Fe (the thickness of this coating was 20μm).Composite silks were cut, and blanks fabricated, subsequently the blanks were sintered to carry out hot-pressing diffusion at 700~800 o C in a high vacuum (the degree of vacuum: 2×10 -2 Pa) furnace, after cooling, Cu/C(Fe) composite blanks were cleaned ultrasonically before and after mechanical properties measurements.The strain rate was measured by a Constant-Strain Rate Tensile Testing Machine; the tension strength was measured by a MTS electron tensile testing machine (America); the macrohardness was measured by a HBM-3000 Brinell Hardness Tester; the microstructures were investigated using scanning electron microscopy (SEM) (Model JSM-5310, Japan) and a transmission electron microscope (TEM) (Model H-800, Japan), and the transmission sample preparation process included three steps: preparation of sample cross sections → ion thinning→ the transmission sample.

Results and discussion 3.1 The effect of hot pressure temperature on tension strength and hardness of Cu/C (Fe) composite
According to a previous study [11], the ideal time and pressure under hot-pressing conditions are 40 min and 10MPa, respectively.Therefore, the same parameters were used in this experiment.Fig. 2 and Fig. 3 show the tensile strength and hardness at different temperatures.It can be seen that the tensile strength and hardness increase first and then decrease with the increase of hot pressure temperature.The maximum value is up 700 MPa and 92 HB when the hot pressure temperature is 700 o C. The increase of hardness is attributed to the increase of the densification of the Cu matrix because the hardness of the matrix is determined by the densification of the matrix [13].In addition, diffusion of the Fe and Cu matrix reaches gradually an equilibrium state with the increasing of the hot pressure temperature, which leads to solution strengthening of the Cu matrix, so hardness increases.The reason for the decrease in tension strength for a high hot pressure temperature may be as follows: at low temperature, the Fe diffusivity is not high enough to promote the formation of a good interface bond by diffusion or by the formation of carbides, while at high temperature, the solubility of Fe in Cu is higher and most of Fe probably diffuses in the Cu matrix.Then, depending on the cooling rate, some Fe clusters in Cu may appear and increase the tensile strength.The reason for the decrease in tension strength for a high hot pressure temperature may be as follows: the reaction between alloy element Fe and carbon fiber gets more and more violent with the increase of the hot pressure temperature, when the interface thickness reaches a certain extent, the interface structure is destroyed by the resident stress in the interface, this leads to interface strength decreasing [14].On the other hand, the formation of excessive compounds decreases the effective loading area of carbon fiber, so the tension strength decreases when hot pressure temperature further increases.Added to this, it was related to the composite with Fe.

Relation between heat pressure temperature and tensile rupture micrograph of Cu/C (Fe) composite
Fig. 4 shows a tensile rupture micrograph of the Cu/C (Fe) composite for different hot pressure temperatures.It can be seen that the tensile rupture appears when the hot pressure temperature is 650, 700 and 900 o C, respectively.At 650 o C, the fiber pulled out is longer (see Fig. 4(a)), this indicates that the interface bond strength is lower, and the fiber is adverse to toughening, therefore, the tensile strength is lower.At 900 o C, the fiber pulled out is shorter (see Fig. 4(c)), this indicates that the interface bond strength is higher, and the fiber pull-out mechanism does not contribute to the toughening, therefore, the strength is lower.At 700 o C, the fiber pulled out is moderate (see Fig. 4(c)), this indicates that the interface bond strength is proper, and the fiber fully played a toughening effect [15], therefore, the strength is the highest.

Influence of alloy element Fe on interface bond characteristics of Cu/C composites
In order to improve properties of the Cu/C composite, the effect of alloy element Fe on the interface bond characteristic of Cu/C composite was investigated.
According to [16], it can be found that the interface bond strength (T f ) is proportional to the shear strength (T c ). Namely, the shear strength (T c ) and interface bond strength (T f ) significantly increase when an alloy element (Fe) is deposited on the surface of Cu/C composites.Fig. 5(a) shows a shear fracture micrograph of the Cu/C composite for a physical bond type.It can be seen that the uncovered carbon fiber is found on the whole shear fracture surface, and the matrix Cu can barely be seen.This indicates that the matrix does not participate in shear fracture and shear stress is undertaken by interface shear stress.Fig. 5(b) shows a shear fracture micrograph of the Cu/C (Fe) composite for a chemical bond type.It can be seen that shear fracture occurred in the internal matrix, the matrix participates in the shear fracture, compared to the physical bond type (Fig. 5(a)), and the chemical bond type has a higher interface bond strength [17].

Fig. 6 TEM interface micrograph of Cu/C (Fe) composites
Further microstructural analysis was carried out by TEM.Fig. 6 shows TEM images of the Cu/C (Fe) composite interface.It can be seen from Fig. 6 that the interface is divided into three regions.The result shows that region I mainly consists of carbon fiber; region II is the interface diffusive layer; region III is the solid solution layer, namely, matrix.
To sum up, because of the addition of alloy element Fe in Cu/C composite, the interface bond type of the Cu/C composite changes from a physical bond type into a chemical bond type, the strength of the Cu/C composite improves under reasonable processing parameter conditions by improving the interface bond strength [18].

Conclusions
(1) The tensile strength and hardness increase firstly and then decrease with the increasing of hot pressure temperature.
(2) A tensile rupture appears when the hot-pressing temperature is 650, 700 and 900 o C, respectively, but the interface bond strength is higher at 700 o C.
(3) The addition of alloy element Fe not only improves the tensile strength and the lateral shear strength of the Cu/C composite, but also changes the interface bond type from the physical bond type to the chemical bond type.

Fig. 1
Fig. 1 Technology flowing chart in the experiment.

Fig. 2
Fig. 2 Change of tension strength with temperature.

Fig. 3
Fig. 3 Change of hardness with temperature.