Analysis of Microstructure and Mechanical Properties of Bi-Modal Nanoparticle-Reinforced Cu-Matrix

Abstract

Bi-modal particles are used as reinforcements for Cu-matrix. Nano TiC and/or Al₂O₃ were mechanically mixed with Cu particles for 24 h. The Cu-TiC/Al₂O₃ composites were successfully produced using spark plasma sintering (SPS). To investigate the effect of TiC and Al₂O₃ nanoparticles on the microstructure and mechanical properties of Cu-TiC/Al₂O₃ nanocomposites, they were added, whether individually or combined, to the copper (Cu) matrix at 3, 6, and 9 wt.%. The results showed that titanium carbide was homogeneously distributed in the copper matrix, whereas alumina nanoparticles showed some agglomeration at Cu grain boundaries. The crystallite size exhibited a clear reduction as a reaction to the increase of the reinforcement ratio. Furthermore, increasing the TiC and Al₂O₃ nanoparticle content in the Cu-TiC/Al₂O₃ composites reduced the relative density from 95% for Cu-1.5 wt.% TiC and 1.5 wt.% Al₂O₃ to 89% for Cu-4.5 wt.% TiC and 4.5 wt.% Al₂O₃. Cu-9 wt.% TiC achieved a maximum compressive strength of 851.99 N/mm². Hardness values increased with increasing ceramic content.

1. Introduction

Copper strengthening is a current priority due to the pressing need to use it in various applications requiring a balance of properties [1,2,3]. Metal-matrix composites are most promising in achieving balanced mechanical properties between nano and microstructure materials [4,5,6,7,8]. Copper is used in many industries owing to its low cost, ease of manufacturing, and good corrosion resistance [9]. The main drawbacks of pure copper are its substantial low strength, high coefficient of thermal expansion (CTE), and generally poor mechanical properties [10]. One effective way to overcome these limitations is to reinforce copper with ceramic particles to obtain composites with superior properties. The effectiveness of dispersed particles in matrix strengthening depends primarily on particle characteristics: size, distribution, spacing, thermodynamic stability, and low solubility and diffusivity of its constituent elements in the matrix. Among ceramic particles, alumina nanoparticles have shown outstanding mechanical properties even at high temperatures, as well as low production costs [11,12]. In addition, TiC is an attractive candidate for metallic matrices such as copper (Cu), iron (Fe), aluminum (Al), titanium (Ti), and nickel (Ni) because of its high hardness, high melting point, and abrasion resistance with good electrical conductivity [12,13,14]. Due to the aforementioned factors, Cu reinforced with (TiC-Al₂O₃) composites led to a more viable material.

Numerous techniques have been used to fabricate reinforced copper matrix composites (CMCs), including molecular-level mixing (MLM) [15], in situ metallurgy [12,13], flake powder metallurgy [16,17], high-energy ball milling (HEBM) [7,18,19,20], friction stir processing [21,22,23,24,25,26], high-pressure torsion [26], and rolling [27,28,29,30]. Although these techniques enhanced the mechanical properties of processed composites, they resulted in an inhomogeneous distribution of particle reinforcements within the matrix. Additionally, they have the potential to cause morphological and structural damage, as demonstrated through carbon nanotubes (CNTs) within a copper matrix [31].

The spark plasma sintering (SPS) method, developed recently, is a new technique for synthesizing metal matrix composites. The SPS technique has piqued researchers’ interest due to its advantages of sintering at relatively low temperatures, higher heating speeds, shorter processing times, and the absence of pre-compression as in conventional sintering. Thus, the SPS technique enables the fabrication of nanostructured composites without the high grain growth rate associated with traditional sintering methods. As a result, SPS composites exhibit exceptional mechanical properties at room temperature, even at elevated temperatures [32].

To the authors’ knowledge, few papers discuss the solid-state spark plasma sintered Cu-Al₂O₃ [33] and Cu-TiC [34,35,36], respectively. However, no information on the synthesis and mechanical investigation of hybrid Cu-Al₂O₃-TiC through mechanical alloying and SPS techniques have been released. Thus, this work fabricated three separate nanocomposites of Cu-TiC, Cu-Al₂O₃, and hybrid Cu-TiC-Al₂O₃, using mechanical alloying and SPS processing. The influence of the TiC and Al₂O₃ nanoparticles content on the microstructure and mechanical properties of the prepared nanocomposites was also investigated.

2. Materials and Methods

2.1. Materials

Copper (Cu) powder with 99.9% purity (supplied by AlphaChemical, MA, USA) with an average particle size of 10 μm was used as a metal matrix. Alumina (Al₂O₃) nanopowder with 99.7% purity (supplied by Alpha Chemicals, MA, USA) with an average particle size of 50 nm and titanium carbide (TiC) nanopowder with 99.7% purity (supplied by Inframat Advanced Materials, L.L.C., CT, USA) with an average size of 100 nm were used. Both TiC and Al₂O₃ were used as individual/hybrid reinforcement. The Cu powder was mixed with 3, 6, and 9 wt.% of individual/hybrid reinforcement of TiC and Al₂O₃ using a ball milling technique for 24 h. The powders were mixed in a stainless-steel vial and protected from oxidation using highly pure argon gas using a 25:1 ball to powder ratio (BPR), 110 rpm, and a ball diameter of 5 mm. Stearic acid (1.5 wt.%) was used as a process controlling agent (PCA). Figure 1 and

Table 1. Composition of the prepared specimens and their contents in the Cu matrix.

	Materials after Sintering by (SPS)	Composition
		Matrix	Reinforcement
		Cu [wt.%]	TiC [wt.%]	Al2O3 [wt.%]
0	Pure copper	100	-------	-------
I	Cu-3 wt.% TiC	97	3	-------
	Cu-6 wt.% TiC	94	6	-------
	Cu-9 wt.% TiC	91	9	-------
II	Cu-3 wt.% Al2O3	97	-------	3
	Cu-6 wt.% Al2O3	94	-------	6
	Cu-9 wt.% Al2O3	91	-------	9
III	Cu-1.5 wt.% TiC and 1.5 wt.% Al2O3	97	1.5	1.5
	Cu-3 wt.% TiC and 3 wt.% Al2O3	94	3	3
	Cu-4.5 wt.% TiC and 4.5 wt.% Al2O3	91	4.5	4.5

show the composition of fabricated samples.

2.2. Spark Plasma Sintering (SPS)

The sintering process was performed using a spark plasma sintering technique (DR. SINTER LAB Model: SPS-1030, Syntex, Osaka, Japan). In all experiments, the powder was loaded into a graphite die with an inner diameter of 15 mm with graphite foil and enclitic by 0.5 mm thick graphite cover to prevent the friction of the sample with the die during the compaction process and to minimize heat loss. Before sintering, the SPS chamber was evacuated to a pressure below 5 Pa. The samples were heated from room temperature up to 950 °C by pulsed D.C. current using the heating rate of 20 °C/min. The samples were then held at the maximum temperature for 45 min under a uniaxial pressure of 30 MPa applied since the first minute of heating. This processing route was used to fabricate the Cu-TiC/Al₂O₃ nanocomposites.

2.3. Mechanical Properties

The hardness was measured along the polished surface of the specimen using a Vickers hardness tester (HMV-2T Model SHIMADZU, Kyoto, Japan). The test was carried out under 100 g load for 15 s dwell time.

The microhardness values were evaluated for an average of twelve readings on the surface of each sample. The compression test for the investigated specimens was carried out using a universal testing machine. In the compression test, three samples were investigated, and average results were obtained. The dimensions of the specimens for compression tests were 6 mm in diameter and 15 mm in length. The applied crosshead speed was 0.05 mm/s, and the test was performed at room temperature.

3. Results

3.1. XRD Analysis

Figure 2 shows the XRD patterns of the prepared ten samples, pure Cu and 3, 6, 9 wt.% TiC/Al₂O₃ (individual and hybrid) nanocomposites. Only peaks corresponding to Cu, TiC, and Al₂O₃ appeared, whereas pattern-like Cu was observed in the case of 3 wt.% Cu/TiC/Al₂O₃ samples; this may be due to the lower percentage of both TiC and Al₂O₃ that are below the limits of the XRD device. This may be attributed to the controlled milling and sintering process in an argon atmosphere which shows that no other peaks for any new phases or intermetallic compounds were formed due to the rapid consolidation process (45 min) during the SPS technique.

The crystallite size was assessed by the classical Williamson–Hall method (FWHM) from the broadening of XRD peaks and using the following formula [37,38]:

$$\frac{\mathsf{\beta }\cos \mathsf{\theta }}{\mathsf{\lambda }}=\frac{\mathrm{k}}{\mathrm{d}}+2\mathsf{\epsilon }\left(\frac{2\sin \mathsf{\theta }}{\mathsf{\lambda }}\right)$$

(1)

where β is the full width at half maximum height (FWHM), θ is the Bragg’s angle of the peak, λ is the wavelength of X-ray (0.15406 nm), K is a dimensionless shape factor (0.9), which depends on the material, d is the crystallite size, and ε is the microstrain.

Figure 3 shows the effect of ceramic ratio on the crystallite size. A clear reduction of the crystallite size with increasing wt.% of ceramic additives was observed. Al₂O₃ and TiC are ceramic materials that act as internal balls that reduce the particle size [39,40]. In addition, the SPS technique achieved the consolidation process, which is a rapid method for the sintering in which no chance for the grain growth of the particles occurs [41,42]. The crystallite size of pure copper was ~105 nm, whereas the crystallite size for the produced composites was in the range of 5–25 nm.

3.2. Densification

The density of composite material is the most important parameter, which significantly affects both physical and mechanical properties. The relative density is calculated and plotted in Figure 4. Relative density is the ratio of the measured and theoretical density of the sample. Measured density was determined by the Archimedes method, and the theoretical density was calculated from the simple rule of mixtures. Each percent of pure Cu and 3, 6, 9 wt.% TiC/Al₂O₃ (individual and hybrid) nanocomposite were tested by three samples, and the average results were obtained. It was observed that the relative density decreased with increasing reinforcement content for all composites, as shown in Figure 4. The maximum relative density (~96%) was achieved by adding 3 wt.% TiC to copper, whereas the minimum relative density (89%) was obtained for 9 wt.% Al₂O₃/copper composite. TiC (4.91 g/cm³) and Al₂O₃ (3.987 g/cm³) also have lower densities than Cu [7,35]. So, the addition of a light material to a denser one decreased the overall density of the prepared composites. This may be attributed to the presence of hard ceramic material with a high melting point into a ductile metal such as Cu that may hinder the high densification and increase the porosity content accompanied by the high fraction of reinforcement [7,43].

Ayman Elsayed et al. [11] studied experimental investigations for the synthesis of W–Cu nanocomposite through spark plasma sintering, and they concluded that using the SPS technique led to reaching a maximum of 90% relative density. On the other hand, a relative density of 98.1% was reached for Cu-Fe-Al₂O₃-MoS₂ composite sintered using the SPS route [33]. Moreover, Babapoor et al. [44] investigated the effects of spark plasma sintering temperature on the densification of TiC. They reached a relative density of 99.4% at 1900 °C for 7 min under 40 MPa using the TiC powder with a mean particle size of 7 μm. They also suggested that there is an optimum temperature for reaching the maximum density.

3.3. Microstructure Analysis

Figure 5 shows the FE-SEM micrograph of TiC-reinforced copper composite using 3%, 6%, and 9% TiC addition to Cu. Two phases are observed; the dark-gray phase represents the Cu matrix, and the black phase is the TiC particles. For 3 wt.% samples, TiC and Al₂O₃ particles are concentrated along the grain boundaries in a chain form, while 6 and 9 wt.% samples were homogeneously distributed all over the Cu matrix. This may be attributed to the suitable mechanical milling parameters and good SPS technique applied. The SPS technique leads to a finer structure compared with traditional routes [18,44].

The SEM investigation of Cu/Al₂O₃ microstructure is shown in Figure 6. Two phases are observed; the dark-gray phase represents the Cu matrix while the white phase represents the Al₂O₃ particles. The dispersion of Al₂O₃ inside the copper matrix is observed for Cu/3% Al₂O₃ with a little agglomeration of Al₂O₃ reinforcement. On the other hand, white areas of agglomerated alumina reinforcement are revealed within the Cu/6% Al₂O₃ matrix grain boundaries, whereas very fine particles are dispersed within the grain interior. Moreover, the SEM of Cu/9% Al₂O₃ composite shows that most of the Al₂O₃ nanoparticles are agglomerated along grain boundaries, and a small percentage are dispersed with the grains. Some authors have also concluded that increasing agglomeration steadily occurs, along with increasing the weight percentage of reinforcement [9,15].

The combination of both TiC and Al₂O₃ for reinforcing copper (Cu/hybrid nanocomposite) with point analysis EDS is shown in Figure 7a,b. The homogeneous distribution of TiC nanoparticles is predominant, while the agglomeration of some Al₂O₃ is observed along grain boundaries. More agglomeration of alumina particles along grain boundaries is observed with increasing hybrid percentage (TiC and Al₂O₃).

3.4. Mechanical Strength

Figure 8 represents the stress–strain curves for pure copper and TiC/Al₂O₃-reinforced copper matrix composites, while the key mechanical properties obtained from the compression test are plotted in Figure 9. The compressed samples are photographed in Figure 10. The addition of TiC enhanced the compression strength of copper and reached its maximum compression strength of ~852 N/mm² at 9 wt.% TiC, whereas Al₂O₃ additions exhibited a dramatic effect on the Cu strength. Increasing Al₂O₃ from 3 to 6 wt.% increased the Cu strength, but a clear failure of strength is noticed at 9 wt.% ratios at which a minimum value of compression strength 367.8 N/mm² resulted. After the compression test, the Cu-6% Al₂O₃ and 9% Al₂O₃ samples were destroyed (see Figure 10).

Moreover, the strength of hybrid composites increased firstly with increasing the percentage of reinforcement up to 6% and then slightly decreased. The extreme drop in the compression strength of the Cu/9% Al₂O₃ composite may be attributed to particle-to-particle contact resulting from ceramic particle agglomeration (see Figure 6). The high compression strength of TiC-strengthened Cu prepared by the SPS route compared with Cu/Al₂O₃ composite with the same wt.% is attributed to a combined effect of ultrafine grain (UFG) structure by the Hall–Petch mechanism and the obstruction of dislocation movement by nanoscale ceramic particles in the grain interior by the Orowan mechanism [8,43].

The effect of ceramic additions on the hardness of copper is illustrated in Figure 11. The hardness steadily increases with increasing the wt.% of reinforcement for synthesized composites. Cu/9% Al₂O₃ obtained the maximum hardness (211 HV), whereas two composites that obtained the minimum hardness value of 112 HV are Cu/3% TiC and Cu/3% hybrid. Many reasons could explain this. The first is that adding high-hardness and high-strength ceramic materials such as TiC and Al₂O₃ on the ductile Cu matrix increases the overall hardness. The second is that the addition of nanomaterials with the incorporation of nanoparticles between the Cu particles improves the hardness as a grain reinforcement takes place accordingly.

3.5. Strengthening Criteria

Increasing the strength of metallic materials is based on two competing factors. The first is work hardening, and the second is dynamic softening. Work hardening is caused by dislocations, multiplication, pileup, and tangle. Dynamic softening is caused by dislocations, rearrangement, and interactions. In the present work, the compression test and hardness measurement are carried out at room temperature, which is why the dynamic softening factor would not be probable, and the work hardening mechanism would affect the enhancement of strength and hardness. This is true for pure metal and alloys, unlike the composite materials where the contribution of ceramic additions to the matrix to enhance the properties should be considered.

Moreover, it is worth mentioning that some authors have considered the strengthening mechanisms in ceramic-reinforced composites [6,7,15,40,42,43,44]. The addition of TiC nanoparticles to the Cu matrix retained grain growth during sintering due to the peening effect of TiC for grain boundary movement and the strengthening effect of dispersed TiC in the Cu matrix grains where a mismatch of coefficient of thermal expansion is present [45] (see Figure 12). Furthermore, increasing the TiC fraction increased the strength and hardness of Cu-TiC composites in the present work. Another strengthening mechanism of TiC dispersion is the Orowan mechanism, especially at low fractions of TiC [6]. Compared to the other two cases, the composites with 3% and 6% Al₂O₃ gave the highest yield strength. This is a signal of increasing material strength with decreasing ductility. This may be attributed to the good adhesion between Cu matrix and TiC nanoparticles than between Cu and Al₂O₃ [46]. An extreme drop in the Cu-Al₂O₃ composite strength is noticed at 9 wt.% of Al₂O₃. This unexpected behavior may be attributed to the agglomeration of some alumina particles in the Cu matrix, increasing the chance for particle-to-particle contact (see Figure 13). A balanced behavior was observed with the hybrid (TiC/Al₂O₃) additions to the Cu matrix in which the combined effects of both ceramics are clear.

Reinforcing the Cu matrix, which is ductile in nature with two types of ceramic materials—ceramic carbide (TiC) and ceramic oxide (Al₂O₃)—helps to improve the mechanical properties of the Cu matrix. Both TiC and Al₂O₃ are at the nanoscale; therefore, by high ball milling, they filled the interstitial voids between Cu particles. As a consequence, the strengthening effect of both of them is distributed all over the Cu matrix. The hardness estimation test increased as the nano-ceramic hard particles were increased. This can also be explained by the resistance of the hard ceramic particles to the indenter from greater depth in the Cu-composite surface.

Consequently, the hardness is enhanced [46,47]. For the compression test, the presence of the nano-ceramic particles dispersed formally in the Cu matrix prevents the dislocation of the particles. In addition, as these hybrid reinforcements are at the nanoscale, they fill the voids; consequently, the strength of samples is increased [47,48,49].

Table 2. Comparing the present study with literature data of previous investigations.

Composite	Method	Density, (g/cm3)	Ultimate Stress, (MPa)	Yield Stress, (MPa)	Elongation, (%)	Hardness, (HV)	Ref. No
Pure copper	SPS at 950 °C	97	N/A	127.15	1.69	81	[Present study]
Cu-3 wt.% TiC		96	741.47	313	3.97	111.9
Cu-3 wt.% Al2O3		95	587.43	500	6.17	149
Cu-1.5 wt.% TiC and 1.5 wt.% Al2O3		95	414.86	317.46	5.25	112
Cu-5 wt.% TiC	Hot Press at 700 °C	93.3	N/A	N/A	N/A	67.3	[50]
Cu-5 vol.%TiC	Hot extrusion	N/A	N/A	N/A	N/A	112	[51]
Cu-5 vol.% TiC	SPS	N/A	712	661	N/A	221	[14]
Cu-77 vol.% TiC	Sintering in Vacuum Furnace at 900 °C	93.4	N/A	N/A	N/A	544	[52]
Cu-5.3 vol.%TiC	SPS	N/A	602	572	N/A	194	[53]
Cu-3 wt.% Al2O3 with Coating Ag	Sintered at 950 °C	95.9	N/A	N/A	N/A	85	[7]
Cu-10 vol.% Al2O3	Sintered at 880 °C	83.49	N/A	N/A	N/A	71	[54]
Cu-3 vol.% Al2O3	Sintered at 850 °C	91.5	350	N/A	0.51	77	[55]
Cu-5 vol.% Al2O3	Sintered at 850 °C	88	550	N/A	0.46	100
Cu-5 vol.% Al2O3	Sintering H2 at 850 °C		530	450	2.5	155	[56]
Cu-2.7 wt.% Al2O3	Sintered at 950 °C	92.53	460	350	N/A	54.83	[57]
Cu-5 vol.% Al2O3	Conventional Sintering N₂	84.3	N/A	N/A	N/A	49	[58]
Cu-5 vol.% Al2O3	Conventional Sintering Ar	84.3	N/A	N/A	N/A	48
Cu-5 vol.% Al2O3	Conventional Sintering H₂	94.4	N/A	N/A	N/A	79
Cu-5 vol.% Al2O3	SPS at 700 °C	92.2	N/A	N/A	N/A	125
Cu-2.75 wt.% Al2O3	Pulsed Electric Current Sintered (PECS)	99.6	N/A	N/A	N/A	94.83	[59]

shows a comparison between the present study and previous work used to fabricate copper composites reinforced with alumina and titanium carbide nanoparticles. In this work, different concentrations of nano alumina and/or nano titanium carbide particles were used as a reinforcement material to the Cu matrix manufactured by the SPS technique. This work is compared with the same composites prepared by traditional sintering, vacuum sintering, hot pressing, and hot extrusion. The table shows that the composites produced by the SPS technique have the best mechanical properties compared with the other consolidation techniques.

4. Conclusions

Cu-(TiC and/or Al₂O₃) nanocomposites were synthesized successfully using mechanical milling followed by the spark plasma sintering (SPS) technique.

The density decreased with increasing percentages of the nano reinforcements (TiC and/or Al₂O₃). A maximum relative density of ~96% was achieved with the addition of 3 wt.% TiC to copper, whereas the minimum relative density (89%) was obtained by adding 9 wt.% Al₂O₃ to copper.

Agglomerated areas of Al₂O₃ nanoparticles around grain boundaries were observed, and increased with increasing Al₂O₃ fractions that, in turn, adversely affect the mechanical properties.

The compression strength of Cu/TiC increased with increasing the TiC fraction, and a maximum value of 851.99 N/mm² was obtained by Cu/9% TiC. A dramatic behavior was observed for Cu/Al₂O₃ composites that gave the minimum compressive strength of 367.8 N/mm² resulted at 9% Al₂O₃.

The maximum hardness of 211 HV was obtained by Cu/9% Al₂O₃, whereas two types of composites obtained the minimum hardness value of 112 HV: the Cu/3% TiC and Cu/3% hybrid.