J. Cent. South Univ. (2020) 27: 2567-2577

DOI: https://doi.org/10.1007/s11771-020-4482-z

Preparation and characterization of different surface modified SiCp reinforced Al-matrix composites

LU Pin-hui(吕品回), WANG Xiao-feng(王小锋), DONG Cui-ge(董翠鸽),PENG Chao-qun(彭超群), WANG Ri-chu(王日初)

School of Materials Science and Engineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: The effects of SiCp surface modifications (Cu coating, Ni coating and Ni/Cu coating) on the microstructures and mechanical properties of Al matrix composites were investigated. Surface modification of SiC particles with Cu, Ni and Cu/Ni, respectively, was carried out by electroless plating method. SiCp/Al composites were prepared by hot pressed sintering followed by hot extrusion. The results show that the surface modification of SiC particles plays an effective role, which is relative to the type of surface coating, and the interfacial bonding become stronger in the following order: untreated SiCpUTS) and fracture strain (ε_f) of 389 MPa and 6.3%, respectively. Compared with that of untreated-SiCp/Al composites, the σ_UTS and ε_f are enhanced by 19.3% and 57.5%.

Key words: SiCp/Al composite; surface modification; electroless plating; mechanical properties; interfacial bonding

Cite this article as: LU Pin-hui, WANG Xiao-feng, DONG Cui-ge, PENG Chao-qun, WANG Ri-chu. Preparation and characterization of different surface modified SiCp reinforced Al composites [J]. Journal of Central South University, 2020, 27(9): 2567-2577. DOI: https://doi.org/10.1007/s11771-020-4482-z.

1 Introduction

SiCp/Al composites are good engineering materials with excellent modulus and strength, good thermal physical properties and low cost [1-4]. These excellent characteristics open up a lot of applications for SiCp/Al composites in modern technologies. Powder metallurgy, stir casting and pressure infiltration are the common methods to prepare SiCp/Al composites while the main challenge lies in obtaining the composites with a good interface condition [5, 6]. However, SiC particles tend to agglomerate, resulting in a plenty of micro-pores. These micro-pores would cause cracks under external stress, leading to interface de-bonding and weaken the mechanical properties of composites severely, increasing the risk of failure during service period and restricting its applications [7-11].

Improving the interface strength is the key point and hotspot of SiCp/Al composites [12]. These research mainly focuses on matrix alloying, optimization of processing technique and surface modification of reinforcement [13-15]. Previous researches show that Mg/Si could reduce the melting point of aluminum matrix and achieve the densification of Al-SiC system at lower temperature. Moreover, the addition of elemental Si can restrain the generation of Al₄C₃ phases effectively [16, 17]. However, the amount of alloying elements is difficult to control and the inappropriate addition of those elements can damage the strength and ductility of the composites [18]. By optimizing the preparation process and parameters, the main problems faced by the composites can be improved to some extent, but it is impossible to solve the problem completely. Coating metal layer such as Cu, Ni on SiC particles could availably improve the properties of SiCp/Al composite materials [19-21]. It is confirmed that Cu coating could react with Al matrix to form Al-Cu phase during the preparation process, which greatly facilitates the densification process and improves the mechanical behavior of SiCp/Al composites. As for Ni coating, it remains stable and acts as a transition layer between Al matrix and SiC particles during the densification and sintering process. The presence of Ni coating enhances the interfacial strength by changing the metal-ceramic interface into metal-metal interface [22, 23].

However, a good interfacial bonding is based on the premise of sufficient densification of the composites and excellent interfacial strength between the SiCp and Al matrix [24]. Therefore, it is significant to investigate the mechanism and develop a new metal coating which is benefit to the wettability, densification as well as interfacial strength.

In this work, Cu layer, Ni layer and Ni/Cu layer were plated on the surface of SiCp, respectively. And then Cu-coated, Ni-coated and Ni/Cu-coated SiCp/Al composites were prepared by hot pressed sintering and hot extrusion. The effects of Cu layer, Ni layer and Ni/Cu layer on SiCp/Al composites were compared and analyzed.

2 Experimental

2.1 Materials

The Al alloy powders (Al-12.0Si-1.0Mg, purity 99.9%, average particle size 70 μm, supplied by Brightstar) were used as matrix. SiC particles with an average size of 14 μm were used as reinforcement of the SiCp/Al composites.

2.2 Surface modification of SiCp

In this part, surface metallization of SiC particles with nickel (Ni-coated SiC), copper (Cu-coated SiC) and nickel/copper (Ni/Cu-coated SiC) was carried out by electroless plating. The details are as follows.

SiC particles were cleaned with 10 vol% HNO₃. SiC particles’ were sensitized in the dilute HCl solution containing SnCl₂, followed by activation in solution containing PdCl₂. Finally, Cu layer, Ni layer and Ni/Cu layer of SiC were obtain by wet-precipitation method using aqueous solution containing NiSO₄ (25 g/L) and CuSO₄ (40 g/L).

2.3 Fabrication of composites

The specimens were prepared by vacuum hot pressing sintering and the processes were as follows. First, the matrix and reinforcement powder were ball-mixed at 300 r/min for 4 h in a powder rotator mixer. The mixed powders were hot-press sintered at 550 °C for 30 min, with a pressure of 25 MPa and a heating rate of 25 °C/min. The SiCp/Al composite specimens were extruded with an extrusion ratio of 10:1 under 400 °C to obtain the SiCp/Al composite extruded bars with a diameter of 10 mm, followed by the solution treatment (510 °C, 2 h), water quenching and artificially aging (190 °C, 18 h).

2.4 Materials characterization

The morphologies of different surface modified SiCp and the microstructures of the SiCp/Al composite were observed by Quanta-200 SEM. The different surface modified SiCp was identified by D/max-vb 2500 X-ray diffraction (XRD). The density and hardness of SiCp/Al composite were measured by Archimedes drainage method (HBS-62.5-type Brinell hardness tester). Detailed microstructures were investigated by XJP-6A optical microscope. Element distribution of SiCp/Al composite was demonstrated by JXA8230 electron probe microanalyzer (EPMA). The tensile properties of SiCp/Al composite were tested by an Instron MTS 810 mechanical testing machine (0.2 mm/min) at room temperature. The average tensile properties were obtained from three tests and the hardness was obtained as the average of five measurements.

3 Results and discussion

3.1 Morphologies and phases of different surface modified SiCp

Figure 1 shows the surface morphology, element and phase composition of different surface modified SiCp. It can be seen that the surface of modified SiCp became clean with sharp edges and angles (Figures 1(a), (e)). Uniform and intact Cu, Ni and Ni/Cu coatings were successfully deposited on the surface of reinforcements. The Cu coating (Figures 1(b), (f), (i)) consisted of pure Cu. The Ni coating (Figures 1(c), (g), (i)) coated on the surfaces of the SiCp consisted of Ni and Ni₃P. Coating composition of Ni/Cu-coated SiCp was Ni, Ni₃P and Cu (Figures 1(d), (h), (i)).

Figure 1 SEM image and EDS spectra of:

3.2 Microstructures of SiCp/Al composites

Figure 2 shows the OM micrographs of different surface modified 10 vol% SiCp/Al composites and the black particles are SiCp and the white area is Al matrix. The distribution of SiCp is homogeneous. The reasons are as follows: 1) the pretreatment subdues the agglomeration of SiCp itself [25]; 2) the metal coatings improve the wettability between reinforcement and matrix [26], and the metal coating can prevent direct contact between different SiCp; 3) the predisposed prior to preparation of SiCp/Al composites improved the distribution of reinforcement in the matrix.

Figure 3 shows the microstructures of composites reinforced with 10 vol% of surface modified SiCp. It can be observed from Figure 3(a) a large number of micro-pores in the Al matrix and obvious defects around the untreated SiCp. It reveals that it’s difficult for the composites to reach densification even after hot extrusion, and the interfacial bonding was weak because of the poor wettability between SiCp and Al matrix [26].

Figure 3(b) shows that little micro-pore or obvious defects in the microstructures of Cu-coated SiCp/Al composites. In addition, some Al₂Cu phases appeared in the Al matrix. Since Cu is easily diffused into Al matrix at high temperatures, the interface of Cu-coated SiCp/Al turns to be the direct contact between SiCp and Al matrix after the sintering and hot extrusion process. As seen in Figure 3(c), the SiCp was encapsulated tightly by the Ni coating in the microstructures of Ni-coated SiCp/Al composite. But it can’t enhance the densification process of composites. As shown in Figure 3(d), we can find that there were few micro- pores and defects in the microstructures of Ni/Cu-coated SiCp/Al composites, and SiCp was surrounded by coatings which are thinner than Ni coatings in Figure 3(c).

Figure 2 OM images of 10 vol% SiCp/Al composites reinforced with:

Figure 3 SEM micrographs of 10 vol% SiCp/Al composites reinforced with:

In the sintering process of SiCp/Al composites, hard SiCp hinders the compression deformation of Al alloy powder and the appearance of sintering neck, leading to the appearance of some tiny holes. Simultaneously, due to the poor wettability between SiCp and Al matrix, the molten aluminum can not spread completely on the surface of SiCp, resulting in cracks and other defects at the interface between the reinforcement and the matrix. Therefore, some flaws such as micro-pores and cracks existed in the microstructures of untreated SiCp/Al composites as observed in Figure 3(a). This was consistent with the report. Even if the hot pressing temperature was raised to more than 783 K, it was difficult for Al alloy to achieve absolute dense [26]. The high temperature in hot pressing sintering and hot extrusion caused the diffusion and dissolution of Cu coating coated on SiCp into the Al matrix. The liquid phases filled most of the micro-pores and cracks in the SiCp /Al composites as observed in Figure 3(b). However, ultimately the SiCp contacted directly with the Al matrix, and the interfacial strength increased little. According to the phase diagram of Al-Ni, the solid solubility of Ni in Al is only 5% at 973 K. The Ni coating coated on SiCp rarely diffuses and dissolves into Al matrix. The Ni coating improved the wettability of SiCp and Al matrix. In addition, the Al-Ni compounds may be formed at the contact position of Ni coating and Al matrix, which further enhanced the interfacial strength. However, it was inevitable that there were some micro-pores and cracks in the microstructures of Ni-coated SiCp/Al composites as observed in Figure 3(c). The Cu of Ni/Cu coating diffuses and dissolves into Al matrix to fill the micro-pores and cracks while the thinning coating continued to encapsulate the SiCp and offered a transition layer to enhance the interfacial bonding as observed in Figure 3(d).

Figure 4 shows the EPMA element distribution of different surface modified 10 vol% SiCp/Al composites. Figure 4(a) presents the distribution of Cu in Cu-coated SiCp/Al composites. It can be seen that the edges of SiCp were not coated by Cu coatings. The white debris in Figure 4(a) are the Al-Cu phase and simple substances formed by diffusion of Cu coating. Figure 4(b) shows the distribution of Ni in Ni-coated SiCp/Al composites. It can be seen that Ni elements were concentrated near SiC particles, which shows that the Ni coating was still intact after the whole preparation process including hot press sintering and hot extrution, the white debris in Figure 4(b) are tiny Ni particles dropped by mixing raw materials.

Figure 5 presents the distribution of Ni and Cu elements in Ni/Cu-coated 10 vol% SiCp/Al composites. It can be seen that the Cu element was mainly distributed in Al matrix while the Ni element was concentrated in the vicinity of SiCp. At the initial stage of semi-solid sintering at lower temperature, the Cu in the Ni/Cu coating layer and Al matrix would dissolve and diffuse, finally forming Al-Cu phase [27]. The solubility of Cu in Al decreases with the increase of temperature, and the relatively stable liquid phase enwraps the SiCp. The liquid phase filled most of the micro-pores and cracks in the composites, and improved the density of the composites. Then the liquids phases were converted into compounds of α-Al and Al-Cu phase while the temperature of composites drops. Those were consistent with the analysis above.

3.3 Properties of SiCp/Al composites

Figure 6 shows the physical property of different surface modified 10 vol% SiCp/Al composites. The actual density of SiCp/Al composites exceeds 98% of the theoretical density observed in Figure 6(a), which indicates that the SiCp/Al composites have good compactness. The relative density of the matrix Al-Si-Mg alloy was 99.56%, while the relative densities of the four kinds of SiCp/Al composites were 98.00%, 99.17%, 98.05% and 98.48%, respectively, lower than that of matrix Al alloy. The addition of hard SiCp hindered the compression deformation of Al alloy powder and increased the difficulty of forming the sintered neck, and decreased the shrinkage and density of the composites [28]. The Cu-coated SiCp/Al composites had the largest relative density, followed by the Ni/Cu-coated SiCp/Al composites. The Cu content in the Ni/Cu coating was less, so that the density of Cu-coated SiCp/Al composites was higher than that of Ni/Cu-coated SiCp/Al composites. This was consistent with the microscopic structure described in Figure 3. According to Figure 6(b), the addition of SiCp greatly improved the hardness of Al matrix. Due to the formation of hard phases such as Al₂Cu, Cu-coated SiCp/Al composites had the highest hardness, which has reached HB 160.9, while the Ni/Cu-coated SiCp/Al composites reached HB 151.6. The hardness of Ni-coated SiCp/Al composites was basically the same as that of untreated SiCp/Al composite, which means that the effect of Ni coating is not obvions.

Figure 4 EPMA images of 10 vol% SiCp/Al composites reinforced with Cu-coated (a) and Ni-coated SiCp (b)

Figure 5 EPMA images of 10 vol% SiCp/Al composites reinforced with Ni/Cu-coated SiCp:

Figure 6 Density (a) and hardness (b) of different surface modified 10 vol% SiCp/Al composites

The room engineering tensile stress-strain curves and tensile properties of the Al alloy and different surface modified 10 vol% SiCp/Al composites are shown in Figure 7 and Table 1, respectively. The 10 vol% untreated-SiCp/Al composites exhibited small different strength (σ_0.2), ultimate strength (σ_UTS) and decreased ε_f compared with the Al alloy. According to the mixing low in strengthening mechanism of metal matrix composites [1], the tensile strength of 10 vol% untreated-SiCp/Al composites should be much higher than that of the Al matrix in theory. However, due to the existence of micro-pores and weak bonding interface, mechanical properties of 10 vol% untreated-SiCp/Al composites were greatly weakened. It can be seen that the σ_0.2, σ_UTS and ε_f of the surface modified 10 vol% SiCp/Al composites increased compared with the untreated ones. Among them, the Cu-coated SiCp/Al composites had the highest σ_UTS of 405 MPa while the ε_f is 4.7%. Those could be attributed to higher density and a large number Cu₂Al phases. The Ni-coated SiCp/Al composites had σ_UTS of 352 MPa and ε_f of 5.4%. The improved interface improved the ε_f of composites, but the existence of micro-pores damaged the strength of the composites. The Ni/Cu-coated SiCp/Al composites had σ_UTS of 389 MPa and the ε_f of 5.3%. Ni/Cu coating not only decreased the micro-pores in the composite structure, but also improved the bonding strength of the interface.

Figure 7 Room engineering tensile stress-strain curves of Al alloy and different surface modified 10 vol% SiCp/Al composites

Table 1 Room tensile properties of Al alloy and different surface modified 10 vol% SiCp/Al composites

Particle strengthening, load transfer and grain refinement generated by the reinforcement phase are the main strengthening mechanisms for SiCp/Al composites [29]. Metal coatings have obvious enhancement to SiCp/Al composites, and the enhancement of Cu coating is the maximum. Surface modification of reinforcement could increase densification of composite and improve interface integration, thus exhibiting enhanced hardness and tensile strength.

The interface condition between Al and SiCp determines the mechanical properties of SiCp/Al composites [30]. The wetting angle between original SiCp and Al matrix reaches 155°, which shows the poor wettability at interfaces. This weak interface cannot effectively transfer external loads from Al matrix to SiCp. Therefore, the mechanical properties of untreated-SiCp/Al composites were not much better than that of Al matrix [31, 32]. Figure 8 shows the tensile fracture morphologies of SiCp/Al composites. Figures 8(b), (d), (f), (h) are enlargement of the red circle part of Figures 8(a), (c), (e), (g), respectively. Due to the weak interfacial bonding, most SiCp was separated from the Al matrix during the fracture process, as shown in Figure 8(a). It could be found by observing the fracture of Cu-coated SiCp/Al composites that some SiCp remained in the fracture surface, and these SiCp were closely bounded to the matrix, indicating that composites exhibit improved resistance to crack extension, as shown in Figure 8(b). In the tensile fracture of Ni-coated SiCp/Al composites, a few gaps still existed at the interface as shown in Figure 8(c). Therefore, its tensile strength was lower than Cu-coated SiCp/Al composites and this was consistent with the above analysis on the structure of composites. In addition, the elongation of composites was improved due to the improvement of interfacial wettability. Some dimples were presented at the fracture of Ni/Cu- coated SiCp/Al composites observed in the Figure 8(d), which refers to elevated toughness. Furthermore, SiCp was closely bounded to Al matrix and no obvious cracks were found, which was consistent with microstructures’ analysis of Figure 3(d). Good interfacial could enhance bonding strength and good wettability could effectively transfer external loads from matrix to reinforcement and enhance the mechanical properties of composites.

Figure 8 Micrographs of fracture surface 10 vol% SiCp/Al composites reinforced with untreated SiCp (a, b), Cu-coated SiCp (c, d), Ni-coated (e, f), Ni/Cu-coated SiCp (g, h)

4 Conclusions

1) Uniform and complete Cu, Ni and Ni/Cu coatings were successfully prepared on the surface of SiCp by electroless plating. The composition of Cu coating is pure Cu; the Ni coating consists of Ni and Ni₃P; the composition of Ni/Cu-coated SiCp is Ni, Ni₃P and Cu.

2) The SiCp in the composites was distributed homogenously. The Cu in the coating layer diffused into the Al alloy matrix and filled defects such as micro-pores and cracks in matrix and interface during the fabrication of composites.

3) Among those four different kinds of surface modified SiCp/Al composites, Cu-coated SiCp/Al composites presented the highest density (2.6775 g/cm³), hardness (HB 160.9) and tensile strength (405 MPa), Ni/Cu-coated SiCp/Al composites had the best comprehensive mechanical properties. The σ_UTS (389 MPa) and ε_f (6.3%) of Ni/Cu-coated SiCp/Al composites were 19.3% and 57.5% higher than those of the untreated SiCp/Al composites (326 MPa and 4.0%), respectively.

Contributors

WANG Xiao-feng and DONG Cui-ge have made substantial contributions to the conception or design of the work. LU Pin-hui conducted the literature review and wrote the first draft of the manuscript. PENG Chao-qun and WANG Ri-chu revised it critically for important intellectual content.

Conflict of interest

LU Pin-hui, WANG Xiao-feng, DONG Cui-ge, PENG Chao-qun and WANG Ri-chu declare that they have no conflict of interest.

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(Edited by ZHENG Yu-tong)

中文导读

表面改性SiC颗粒增强Al基复合材料的制备与表征

摘要：本文研究了SiCp表面改性(Cu涂层、Ni涂层和Ni/Cu涂层)对Al基复合材料组织和力学性能的影响。采用化学镀方法对SiC颗粒分别进行了Cu、Ni和Cu/Ni涂层表面改性。采用热压烧结、热挤压法制备了SiCp/Al复合材料，并用扫描电镜(SEM)、能谱仪(EDS)、电子探针(EPMA)和X射线衍射仪(XRD)对其进行了表征。结果表明，SiC颗粒的表面改性与表面涂层类型有关，其界面结合强度依次为：未处理SiCpUTS)为389 MPa，断裂应变(ε_f)为6.3%。与未处理SiCp/Al复合材料相比，σ_UTS和ε_f分别提高了19.3%和57.5%。

关键词：SiCp/Al复合材料；表面改性；化学镀；力学性能；界面结合

Foundation item: Project(2017zzts111) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2019-10-06; Accepted date: 2020-05-11

Corresponding author: WANG Xiao-feng, PhD, Associate Professor; Tel: +86-13467516329; E-mail: wangxianfeng@csu.edu.cn; ORCID: https://orcid.org/0000-0002-9913-1064