Effect of Spark Plasma Sintering Temperature on Phase Evaluation and Mechanical Behaviour of cu- 4 Wt% SiC Composite

Spark plasma sintering (SPS) is a novel approach to fabricate Cu- SiC composites which have a relatively broad range of potential uses in space applications. The Cu- 4 wt% SiC composite with homogeneously dispersed SiC particles has been successfully synthesized at various SPS temperatures. In this study, the effect of SPS temperatures on the phase evaluation and mechanical characteristics of the Cu- 4 wt% SiC composite was investigated. From the results, it was confirmed that the optimum sintering temperature for Cu- 4 wt% SiC composite is 950°C. Raising the spark plasma sintering temperature from 850°C to 950°C led to a higher concentration of copper-liquid phase which accelerates the SiC particle rearrangement and fills the interstitial voids present in the interfaces of matrix and reinforcements which improves the mechanical properties of the Cu- 4 wt% SiC composite. However, increasing the SPS temperature by more than 950°C prone to the generation of the copper net and inhomogeneous SiC particle dispersion in the copper phases and declines the performance characteristics of the synthesized composite. The Cu- 4 wt% SiC composite sintered at 950°C exhibits superior mechanical characteristics than the composite sintered at 850°C, 900°C and 1000°C.


Introduction
The researchers have gained a keen interest to synthesize copper-based metal matrix composites due to the inherent mechanical, physical properties and low cost of composite synthesis [1].The copper composites reinforced with SiC particles are utilized to fabricate electrical relays, circuit breakers, heat sinks etc., [2,3].Copper is considered a functional and strategic element due to its exceptional properties like higher fatigue resistance, good ductility and lesser electrical resistivity (1.75 × 10 −8 Ωm), higher electrical conductivity (4.3 W/m°C) and good resistance to corrosion and oxidation [4].However, the presence of inferior mechanical characteristics such as lower resistance to wear and minimum elastic modulus (145 GPa) limits the usage of pure copper in automobile and industrial applications [5].The properties of the copper composite are enhanced by reinforcing the ceramic particles such as TiC [6], Y 2 O 3 [7], SiC [8], WC [9] and Al 2 O 3 [10] with optimal wt% of particles.Among these reinforcements, SiC reinforced Cu matrix exhibits improved hardness, better corrosion and wear resistance, higher elastic modulus (upto 414 MPa), the withstanding capability of higher temperature without deteriorating the strength and excellent resistance to fracture (4.5 MPa m 1/2 ) [11,12].
The presence of low wettability between the particles in Cu-Nb [9], Cu-W [13], Cu-WC [9] and Cu-Ta [14] composites made it difficult to powder consolidation and create weak bonding among adjacent particles which lead to the lesser mechanical strength of the composite [15].Numerous fabrication techniques have been available to synthesize the Cu-SiC composite such as chemical reduction [16], electro-forming [17], powder metallurgy [18] and stir casting process [19].However, the powder metallurgy route of composite synthesis produces a nearly net-shaped composite with enhanced mechanical properties with fewer defects, as well as initiates the wettability among the particles during the sintering process.The sintering technique has an obvious impact on the characteristics of alloys.Hossein Torabi et al., [20] investigated the microhardness of the spark plasma sintered Cu-10Sn/SiC composite and found that the maximum microhardness of 240 HV at 10% SiC reinforcement due to the reinforced SiC particles offers resistance to the dislocation movement up to the 10% SiC particles.Hailong Wang et al., [21] examined the mechanical behaviour of the spark plasma sintered SiC/Cu-Al composite at different sintering temperatures varying from 600°C to 800°C and found that the optimum microhardness of 70 Hv at 700°C due to the formation of thin grain boundary and uniform SiC grain dispersed in the Cu-Al matrix.Y.C Lin et al., [22] reveal that the Cu/ SiC composite fabricated via SPS technique attains a maximum hardness of 80 HRN for 15 wt% SiC reinforcement due to the developed strong interface bond among the SiC and Cu particles.
From the currently available literature, no significant work reported on the spark plasma sintering of the Cu-SiC composites.Additionally, no comprehensive study has been reported on the SPS temperature effects on the phase evaluation and mechanical properties of the SiC reinforced Cu composites.Hence, in this work an attempt has been made to explore the SPS temperature effect on the microstructural changes and mechanical characteristics of the Cu-SiC composite to achieve the optimum mechanical properties.

Materials and Fabrication Method
In this present study, the copper powder (99.9% purity) with An average grain size of <30 μm and Silicon carbide particles with a grain size of <40 μm were procured from Nanoshell Pvt. Ltd., Delhi, utilized as matrix and reinforcement materials to fabricate the Cu-SiC composite.The properties of the matrix and reinforcements were shown in Table 1.To prevent the powders from moisture absorption, preheating was performed at 100°C in a hot air oven [23].The Cu and SiC powders were placed in a RETSCH PM400 ball mill with a charge ratio (Ball: Powder) of 10:1 to achieve proper dispersion of SiC particles in the Cu matrix and to initiate the strain hardening phenomenon among the composite powders [24].1.5 wt% stearic acid solution was mixed to the composite powders to minimize the cold welding among the tungsten carbide balls and composite powders [25].The complete milling process was performed under an inert gas blanket to avoid the contamination of the powder mixture.The Cu-4 wt% SiC composite was synthesized through a SPS technique by varying the sintering temperature from 850°C to 1000°C in steps of 50°C.A compaction pressure of 30 MPa and 5 min holding time was applied to obtain the composites through the SPS technique [26].

Characterization of Composites
The morphology and elemental phases of the composite powders were analyzed by performing scanning electron microscopy (Carl Zeiss EVO 50), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD).The synthesized Cu-4 wt% SiC composites were subjected to a Vickers hardness (HV) test on a Micro-hardness machine by applying a 200 g load with 50 s holding time [27,28].The electrical conductivity (EC) of the synthesized composites was determined by using the Sigma 2008B digital electrical conductivity meter [26,29].The porosity was calculated as per Archimedes' method and the theoretical density was determined by using the law of mixture as shown in Eq. ( 1).
Here, ρ SiC, and ρ Cu are the theoretical densities of the SiC and Cu.
The Cu-4 wt% SiC composite was subjected to compression test on a micro UTM M30 model at room temperature according to ASTM E9 specifications having specimen dimensions of 26 mm length and 13 mm diameter with a strain rate of 0.05 mm/s.The synthesized composites for hardness test and compression strength was depicted in Fig. 1.  cold welding in the ball milling process [30].Furthermore, the composite powders comprise of a layered structure with a highly densified core structure (marked as "A" with a "+" sign) and a thick layer of nanoparticles surrounding the core structure (indicated as "B" with a red "+" sign).

Analysis of cu-4 Wt% SiC Composite Powder
The layered structure enhances grain refinement during the sintering stage [30,31].The presence of sharp diffraction peaks of Cu and SiC was observed in the XRD pattern (Refer to Fig. 2c), which is evident that the higher crystallinity in the milled powders.The obtained XRD peaks matched with SiC JCPDS card number 00-1129-23 [32,33] and Copper JCPDS card number 00-003-1018 [34].No diffraction peaks were observed in the XRD except Cu and SiC which indicates that the composite powders were not undergone oxidation during ball milling.
The XRD pattern for the synthesized Cu-SiC composite sintered at various sintering temperatures was depicted in Fig. 2d which confirms the presence of CuO intermrtallic compound in the Cu-SiC composite sintered at 1000°C.The structure of the ball-milled Cu-4 wt% SiC composite powder was analyzed through transmission electron microscopy and the obtained TEM images were depicted in Fig. 3.It was noticed that the sintering necks among the neighbouring powder particles are distinct and several smaller powder particles have adhered on the bigger dark particle surface.Further, the selected area diffraction pattern (SADP) in Fig. 3b confirms that the dark particles are related to copper and the layered grey-coloured particles present on the surface are SiC reinforcements.Moreover, spherical particles having core-shell structures were present in Fig. 3c which are identical to that found by Wang et al., [35].The fast fourier transform (FFT) micrograph of the core is indexed by Cu in the direction of [444 ] and its corresponding inverse fourier transform (IFFT) micrograph reveals a 0.132 nm crystal face distance related to Cu(220).The FFT image of the shell is indexed by SiC in [002 ] direction and the IFFT image reveals a 0.024 nm which matches with the ( 110) SiC lattice spacing [36].

Microstructural Study of cu-4 Wt% SiC Composite
Figure 4 depicts the obtained artificial fractures on the Cu-4 wt% SiC composite surface sintering at different sintering temperatures varying from 850°C to 1000°C.The sintering necks are developed among the adjacent particles and the necking zone fracture was marked as the yellow circle in Fig. 4a.Song et al., [37] observed that the properties of the sintering phenomenon in the SPS process are significantly distinct in comparison to the conventional sintering technique.In the SPS method, the enhanced uniform dispersion of temperature from the core part of the powder particles to the surface initiates the generation of the sintering necks at the adjacent particle contact surfaces.As the sintering was raised to 900°C, enlargement of sintering necks and tearing in certain areas as shown in Fig. 4b was identified due to the increase in the rapid propagation of heat among the contact composite powders at higher sintering temperatures [27,38].With further increase in the sintering temperature to 950°C, numerous dimples were identified and the corresponding EDS image depicts the eventual dispersion of the SiC reinforcements and Cu particles as shown in Fig. 4d.Furthermore, when the sintering temperature was further raised to 1000°C, the SiC reinforcements become coarser and the matrix Cu particles are agglomerated in the neighboured of the copper as depicted clearly in Fig. 4e and f.
The microstructure of the synthesized Cu-4 wt% SiC specimen sintered at various sintering temperatures is analysed to study the elemental phase dispersion in the composite.At 850°C SPS temperature, the SiC particles are agglomerated and the Cu matrix is surrounded by the SiC reinforcements due to the improper sintering of the composite powders at 850°C sintering temperature as depicted in Fig. 5a and the corresponding EDS mapping was shown in Fig. 5b.With increasing the sintering temperature from 850°C to 900°C, the SiC reinforcements are sintered effectively and distributed uniformly throughout the Cu matrix   5c).The EDS spectra confirm that the existing elements in the synthesized composites are Cu, SiC and a minor proportion of the added magnesium element during the milling process to improve the wettability among the copper and SiC composite powders [24,39,40].During the spark plasma sintering process, the presence of chemical concentration and temperature gradients among the composite powders creates the tension gradient on the specimen surface.The combined effect of the irregularity in the SiC powders and tension gradient aided in more uniform dispersion of SiC particles sintering at 950°C [37,41].
Moreover, the Composite sintered at 1000°C sintering temperature reveals the formation of SiC agglomerations in the Cu matrix due to the increase in the energy absorption capability of the composite powder at higher temperatures.In addition to this, the presence of temperature gradients in the powders and the maximum generated temperature at the powder junction accelerates the SiC agglomerations along the grain boundaries.Specifically, when the sintering temperature was increased to 1000°C, the generated voltage between the composite powder is significantly high to make the sintering at the interface between the SiC particles and leads to SiC agglomerations as shown in Fig. 5g and EDS was shown in Fig. 5h.
The TEM micrograph for the diffused layer among the Cu and SiC reinforcement in the Cu-4 wt% SiC composite was depicted in Fig. 6.From Fig. 6, it was clear that the diffusion layer thickness is near about 6 nm.As the sintering temperature is raised to the melting point of copper (1000°C), The matrix copper converts into a semi-molten phase and starts diffusion among the Cu and SiC particles [42].The mechanical properties of the Cu-SiC composite are enhanced due to the established diffusion layer.In addition to this, the presence of core-shell microstructure of the Cu-SiC composite powder (Refer Fig. 3c) and plasma discharge among the adjacent particles accelerates the diffusion layer formation among the Cu matrix and SiC reinforcements.

Mechanical Properties Investigation
Figure 7 illustrates the relative density (RD), Vickers hardness (HV), Electrical conductivity (EC) and compression strength for the synthesized Cu-4 wt% SiC composite sintered at various sintering temperatures.The compressive strength and stress-strain curve under the compressive load at various sintering temperatures is depicted in Fig. 8. Due to the improper sintering of the powder particles at 850°C, the Cu-4 wt% SiC composite exhibits poor performance than the composite sintered at 900°C, 950°C and 1000°C.The relative density (RD) of 90% and 1.2 GPa Vickers hardness was noticed for the Cu-4 wt% SiC composite at 850°C sintering temperature due to the presence of SiC agglomerations and the blow holes which resulted from improper composite powder sintering.In addition to this, the formation of sintering necks at 850°C (refer Fig. 4a) leads to the minimum EC of 70% IACS and the lowest compressive yield strength of 250 MPa due to the incomplete network (sintering necks) which can deteriorate the strength and impedes the current passage during the sintering.An improved copper network is obtained by raising the sintering temperature, which promotes the formation of sintering necks among the particles.At higher sintering temperatures, the semi-molten copper content increases and enhances the wettability between the SiC and Cu particles.The RD of 98%, hardness of 1.7 GPa, EC of 86 IACS and compressive strength of 349 MPa were obtained for the composite sintered at 950°C, which are 8.88%, 41.6%, 22.8% and 39.6% more than the composite sintered at 850°C.However, the composite sintered at 1000°C improves the EC to 87% IACS, whereas the HV, RD and compression strength reduced to 1.3 GPa, 91% and 320 MPa respectively.
The thickness of the diffusion layer increases with increasing the sintering temperature, which significantly improves the compression strength and densification of the composite [43].Moreover, the melting point temperature of silicon carbide (2730°C) is substantially higher Fig. 6 TEM micrographs for diffusion layer between the Cu and SiC particles than the composite sintered at 1000°C, which makes the improper sintering of the SiC particles present on the inter-connected regions [44].The existing copper vapour at 1000°C creates blow holes and surface defects during the composite colling process.Hence the lower RD attained at 1000°C than the composite sintered at 950°C.This phenomenon considerably affects the Vickers hardness, compression strength and EC at 1000°C.The obtained values of the EC, hardness and compressive strength in this present work were higher than the composite synthesized via vacuum sintering and extrusion process as reported by Wang et al., [35].
The SPS process consists of four distinct stages which include activation of the powder, generation of sintering necks, broadening of the sintering neck and rapid powder densification [35,45].The applied high intensity pulsed current produces a spark among the composite powder particles which can remove the oxide films and entrapped gas on the powder surface.Finally, the powder particles are energized and the interface contact temperature raises sharply to cause the partial melting of the copper core.The melted copper enhances the SiC particle rearrangement and causes copper neck and SiC particle segregation on the sintering neck surface as depicted in Fig. 4a and b.As the sintering temperature raises, the highly intense spark enlarges the melting zone which creates a thin layer of liquid at the interface particle zone during the SPS process [46,47].The SiC particle dispersion depends on the generated liquid copper layer.Hence, the volume of the liquid copper phase and the viscosity of the molten copper is responsible for the SiC particle rearrangement.The dynamic viscosity ( in mPa-s) of the liquid is calculated as per Eq. ( 1) [26].The surface tension is calculated by using the linear relationship as shown in Eq. ( 2) From Eqs. ( 1) and (2), Higher sintering temperature leads to minimum surface tension (σ) and η.The generated low viscosity and thicker copper liquid occupies the surface of the SiC particles and fills the voids present in the synthesized composite to enhance its density and make the composite have minimum porosity levels.
From experimental results it was identified that there was an enhancement of 8.88% RD, 4.16% VHN, 39.6% Compression strength and 22.85% EC for the Cu-4 wt% SiC composite sintered at 950°C when compared to composite sintered at 800°C.Increasing the sintering temperature beyond 950°C leads to violent spark discharge that increases the molten copper content and is prone to the gravity segregation of SiC particles which further accelerates the SiC agglomerations in the melted copper core.Though the indicated temperature by the double-wave infrared indicator is about 1000°C, the powder contact surface melts and enhances the generation of copper vapour, eventually leading to the development of pores in the sample.The generated agglomerations and the presence of pores prone to reduction in the compression strength, relative density and Vickers hardness of the Cu-4 wt%SiC composite sintered at 1000°C.

Conclusions
1.The Cu-SiC composite powders were successfully ball milled and no other element than the Cu and SiC was identified in the XRD graph of the milled composite powders.The Cu-4 wt% SiC composite with uniform SiC dispersion was successfully fabricated through SPS technology.
2. An increase in sintering temperature assisted in the rearrangement of SiC particles.However, SiC agglomerations in the copper matrix were observed at 1000°C sintering temperature due to the induced gravitational force in the molten copper matrix.tially melted copper support the strengthening of the composite.Despite this, the gravity separation of SiC together with copper vapour leads to deterioration of the RD, Vickers hardness and compression strength of the composite sintered at 1000°C SPS temperature.4. The optimum sintering temperature for synthesized Cu-4 wt%SiC composite was found to be 950°C and the corresponding RD, EC, VH and compression strength was found to be 98%, 86 IACS, 1.7 GPa and 349 MPa respectively, which were 8.88%, 22.85%, 29.41% and 39.6% more than the composite sintered at 850°C.

Figure
Figure 2a, b, c depicts the SEM and XRD analysis of the ball-milled Cu-SiC composite powders.The composite powders are in uneven shape as shown in Fig. 2a which is mainly due to the formation of clusters among the composite powders.The presence of a high surface area of the fine copper particles tends to agglomeration during the partial

Fig. 3 a
Fig. 3 a TEM image of the Cu-4 wt% SiC composite powder at low magnification, b High magnification TEM image of sintering necks, c FFT and IFFT images of the core-shell

Fig. 5 a
Fig. 5 a and b SEM and EDS mapping of the composite sintered at 850°C, c and d at 900°C, e and f at 950°C, g and h at 1000°C

Fig. 7 aFig. 8
Fig. 7 a Relative density, b Electrical conductivity, c Vickers Hardness and d Compression strength of the Cu-SiC composite

3 .
With increasing the SPS temperature, the mechanical properties of the Cu-4 wt%SiC composite reaches a maximum of up to 950°C.The uniform SiC dispersion in the copper, the improved diffusion among the SiC reinforcement and Cu matrix and the presence of par-σ = 1330 − 0.23(T − 1085)

𝜌 Cu Table 1
Properties of the matrix and reinforcement powders