As one of the high performance fibers, basalt fibers possess excellent performances such as a wide applicable temperature range from ‒269 °C to ‒700 °C[1], high specific strength and modulus [2], good chemical stability and corrosion resistance[3], so they have a wide application prospect in the fields of dielectric materials, structural materials, sound insulation materials, etc[4–5]. Particularly, in a non-vacuum environment of high temperature (≥600 °C), basalt fibers have the highest residual strength and the lowest mass loss rate compared with other high performance fibers like carbon fibers, ultra-high molecular weight polyethylene fibers and aramid fibers[3,6]. More importantly, basalt fibers are environment-friendly materials because of degradability and low manufacturing cost and energy consumption [7]. The application of basalt fibers on the composite strengthening and toughening of lightweight alloy can reduce manufacturing cost and broaden the application prospects in both civil and military fields of lightweight and high strength metal matrix composites [5, 8].
Currently, the researches about the performances of basalt fibers reinforced metals matrix composites have been repor-ted. The research of Karthigeyan R et al[9–10] indicated that the tribological behavior, coefficient of thermal expansion (CTE) and tensile properties of short basalt fibers (BF) reinforced aluminium matrix composites fabricated by stir casting process are obviously superior to those of matrix alloys, while relationship between interface and performance was not studied. Xie et al[11] studied the interface microstructure and mechanical properties of basalt fibers reinforced aluminum matrix composites (BF/Al) fabricated by vacuum pressure infiltration method, and the results showed that alumina oxide amorphous reaction layer is generated at the interface owing to the redox reaction between silicon dioxide of basalt fibers and Al matrix, and the bending properties of BF/Al com-posites were not improved. Also, the research about interface structure between basalt whiskers and A356 aluminium matrix and tensile strength of composites was reported by Onuh et al[12]. They found that metastable Al-Si-Ca intermetallic com-pounds and stable Al-Fe-Si-Mn intermetallic compounds are produced between basalt whiskers and A356 aluminium matrix, which can guarantee the load transfer from matrix to reinforcements, but the tensile strength of BF/Al only has a smaller improvement compared with that of A356 aluminium. Interestingly, the mechanical properties of BF/Al composites did not have an obvious improvement compared with that of alloy matrix, which were caused by the harmful interface reaction and fibers damage due to the phase transition of BFs in the preparation of composite materials. According to the principles of thermodynamics and dynamics[13–14], interfacial reaction and fibers phase transition can be effectively inhibited by the methods such as decreasing preparation temperature and shortening holding time. Akhlaghi et al[15] proposed a new method to fabricate BF/Al composites, which can significantly shorten holding time of composites and reduce the degradation of fiber properties. The process is as follows: firstly, aluminium coated BFs were prepared by the immersion of fibers in molten aluminium at 600±5 °C for 2 s, and then the BF/Al composites were fabricated by the sintering of aluminium coated BFs under the sintering pressure of 630 MPa at 300 °C for 7 min. However, the measured values of ultimate tensile strength were completely inconsistent with the theoretical value owing to the defects in composites, which were produced by the improper infiltration of fiber bundles with too short immersion time. Besides, the effects of infiltration temperature on the mechanical properties of BF/Al composites and tensile strength of basalt fibers were discussed[16]. Fully dense BF/Al composites with no obvious interface reaction can be obtained by vacuum pressure infiltration under 10 MPa at 660 °C for 10 min, while the mechanical properties have no increase compared with Ref.[12] owing to non-ideal distribution of reinforcement in matrix and decrease in inherent strength of fibers caused by the decomposition of surface treatment agents. Furthermore, Yang et al[17] studied the effects of reinforcement content on the mechanical properties of short basalt fibers reinforced Al matrix composites fabricated by hot-press sintering method and found that the mechanical properties of composites decrease when the fibers content exceeds 10wt%. Unfortunately, mechanical properties of BF/Al composites, having a large gap with theoretical value, were greatly inferior to those of carbon fibers reinforced aluminium matrix composites (CF/Al)[18–19], and it is of great significance to further improve the properties of BF/Al composites for the preparation of high performance and low cost Al matrix composites. Metallization modification of fibers surface not only inhibits the element interdiffusion between reinforcement and metal matrix but also improves the interface compatibility and wettability of composites, which has been widely used for the interface optimization of CF/Al composites[20–22]. Regret-tably, interface optimization by metallization modification of fibers in basalt fiber reinforced Al matrix composites was seldom reported.
In this work, the thickness-controlled nickel-cobalt-phosphorus (Ni-Co-P) complex alloy coatings were deposited on the surface of basalt fibers by electroless plating firstly, and then aluminium matrix composites reinforced by Ni-Co-P coated basalt fibers and uncoated basalt fibers were fabricated by vacuum hot pressing sintering under the pressure of 90 MPa at 490 °C for 10 min. The effects of Ni-Co-P intermediate layer on the microstructures and mechanical properties of BF/Al composites were investigated by the comparison of different composites. Moreover, the tensile failure mechanisms of the composites were analyzed.
The basalt fibers (BFs) as reinforcements were provided by Jiangsu Tianlong continuous basalt fiber Co., Ltd, China, and performance parameters are shown in Table 1. 2024Al powders provided by Hebei Qinbang New Material Technology Co., Ltd, China were used as the metal matrix.
Table 1
Performance parameters of basalt fibers
| Performance | Rm/×103MPa | E/GPa | δ/% | ρ/g·cm-3 | D/μm |
|---|
|
Value |
2.61 |
81 |
3.8 |
2.7 |
7 |
Before the deposition of Ni-Co-P alloy coatings, BFs were immersed in 50 g/L NaOH solution for 10 min to remove surface impurities and contaminants. The fabrication procedure of Ni-Co-P coated BFs is shown in Fig.1. Firstly, BFs were sensitized by the adsorption of Sn2+ ions on the surface through impregnation of HCl (40 ml/L) and SnCl2 (10 g/L) mixed solutions, and then washed several times by distilled water. Secondly, the sensitized BFs were activated by the Pd atoms adsorbed on surface caused by reduction of Sn2+ in the activating agent consisting of 0.5 g/L PdCl2 and 20 ml/L HCl. Then the active fibers were rinsed by distilled water. After sensitization and activation, the Ni-Co-P coatings with different thicknesses were deposited on BFs surface by ele-ctroless plating at the constant temperature of 70 °C and held by thermostat water bath cauldron for 0, 5, 10, 15 and 20 min, and the composition and pH of electroless plating solution are shown in Table 2. At the end of holding time, Ni-Co-P coated basalt fibers were cleaned by distilled water and dried at 60 °C for 8 h in a vacuum drying closet.
Fig.1 Electroless plating procedure of Ni-Co-P coatings on BFs
Table 2
Composition of Ni-Co-P electroless plating solution
| Component | Content/g·L-1 | Function |
|---|
|
Nickel sulfate |
12.08 |
Main salt |
|
Cobalt sulfate |
12.92 |
Main salt |
|
Sodium citrate |
40.00 |
Complexing agent |
|
Ammonium sulfate |
40.00 |
Buffer |
|
Sodium hypophosphite |
20.00 |
Reducing agent |
|
Sodium hydroxide |
- |
pH adjusting (8.9±0.05) |
Based on the comparison of thickness and integrity of Ni-Co-P coatings, the Ni-Co-P coated basalt fibers deposited for 10 min were used to reinforce 2024Al matrix composites. The fabrication process of BFs reinforced 2024Al matrix composites is as follows. Firstly, BFs were woven into preforms and filled by the 2024Al/alcohol suspension liquid with the volume ratio of 1:30 by vacuum filtration. After-wards, the filled preforms were placed in a graphite mould and dried at 45 °C by the vacuum drying closet. Lastly, the Al matrix composites were fabricated by the vacuum hot pressing sintering under 90 MPa at 490 °C for 10 min. The uncoated BF reinforced 2024Al matrix composites (BF/Al) and Ni-Co-P coated BFs reinforced 2024Al matrix composites (BF(Ni-Co-P)/Al) with fibers volume fraction of 10vol%, 20vol%, 30vol% and 40vol% were obtained by the regulation of volume ratio of reinforcement in matrix.
The phases of uncoated BFs and BFs modified with Ni-Co-P coatings were examined by X-ray diffractometer (D8 advance XRD, Germany). The scanning electron microscope (SEM, JSM-6700F, Japan) equipped with energy dispersive spectrometer (EDS) was used to investigate microstructure morphology and elemental composition of Ni-Co-P coatings and Al matrix composites. Interface microstructure and element distribution of BF(Ni-Co-P)/Al composites were observed by the transmission electron microscope (JEOL-200CX, Japan). The composites samples were processed into tensile specimen with gauge length of 10 mm using wire-electrode cutting, and then polished by 1000# waterproof sandpaper to remove the processing tool marks. The tensile properties of all the samples were tested using electronic universal testing machine (E44.304 MTS, USA) with the constant tensile speed of 0.2 mm/min. Lastly, the fractures of different composites were characterized by SEM.
2.1 XRD of basalt fibers and Ni-Co-P coated basalt fibers
The phases of uncoated BFs and Ni-Co-P coated BFs deposited for 10 min are shown in Fig.2. Clearly, there are no obvious sharp diffraction peaks in spectra of uncoated BFs, while the broad and weak diffuse reflection peak is observed at 20°‒30° (2θ), which may be caused by amorphous phase structure of basalt fibers. After Ni-Co-P coatings modification, some sharp diffraction peaks of Ni12P5 (38.5°), Ni-Co (44.7°), Co2P (65°) and Ni7P3 (78°) are observed in spectra of Ni-Co-P coated BFs. Diffuse reflection peaks of Ni and Co are difficult to distinguish because of their similar atomic radius and lattice constant. Interestingly, the element P exists in the form of nickel phosphoride and cobalt phosphate compounds instead of simple substance owing to the co-deposition of Ni, Co and P atoms.
Fig.2 XRD patterns of uncoated BFs and Ni-Co-P coated BFs
2.2 Effects of deposition time on microstructure of Ni-Co-P coatings
To investigate the variation of microstructure of Ni-Co-P coatings with the deposition time, the morphologies of Ni-Co-P coatings with different deposition time of 5, 10, 15 and 20 min are shown in Fig.3. As seen in Fig.3a, uncoated basalt fibers with uniform diameter of about 7 μm have a smooth surface, and no obvious defects exist. After deposition for 5 min, the Ni-Co-P coatings on BFs have a flat and compact surface (Fig.3b), while the local areas of fiber surface are exposed because of poor coatings integrality. When the deposition time increases to 10 min, the integrality and compactness of Ni-Co-P coatings (as shown in Fig.3c) improve obviously and a few Ni-Co-P cellular particles are generated. With further extending the deposition time, the amount of Ni-Co-P cellular particles are obviously increased and particle sizes become larger than those of coatings deposited for 10 min, as shown in Fig.3d and 3e. In summary, the microstructure of Ni-Co-P coatings with 10 min deposition time is superior to others, and the thickness is up to about 0.3 μm. Besides, the content of elements Ni, Co and P is 69.27wt%, 22.34wt% and 8.39wt%. The microstructure and composition of Ni-Co-P coatings are affected by the depo-sition mechanism of element Ni and Co. In the process of electroless plating, active hydrogen atoms (Had) are generated by hydrolysis reaction of hypophosphite under the catalysis of Pd, as shown in Eq.(1)[23]:
Fig.3 Morphologies of uncoated basalt fibers (a) and Ni-Co-P coated basalt fibers with deposition time of 5 min (b), 10 min (c), 15 min (d) and 20 min (e); EDS analysis results of the scanning area with deposition time of 10 min
Majority of Had react with nickel ions (Ni2+) and cobalt ions (Co2+) to produce Ni atoms and Co atoms (Eq.(2) and Eq.(3)), and then absorb on fiber surface in the form of atomic clusters[24], which is the reasons for the formation of Ni-Co-P cellular particles. Notably, the standard electrode potential (EΘ=‒0.25 V) of element Ni is greater than that of Co (EΘ= ‒0.277 V), so the producing rate of nickel atoms is signifi-cantly higher than that of cobalt atoms. Therefore, the Ni con-tent in Ni-Co-P coatings is higher than Co content. Besides, a small amount of Had atoms react with hypophosphite to induce the co-deposition of P and metal particles, which is the main reason for the formation of nickel phosphoride and cobalt phosphate compounds.
2.3 Interface microstructure of BF(Ni-Co-P)/Al compos-ites
The HAADF image and element distribution of interface of 2024Al matrix composites reinforced by Ni-Co-P coated basalt fibers prepared with 10 min deposition time are shown in Fig.4. As shown in HAADF image, highlight areas reflect that elements Ni, Co and P with high atomic numbers are distributed between Al matrix and basalt fibers, and an obvious intermediate layer with a thickness of about 160 nm forms, which is consistent with Ni-Co-P coating thickness. According to element mappings of Al and Mg, elements have no obvious diffusion to Ni-Co-P intermediate layer, indicating that alloy intermediate layer protects the basalt fiber from the reaction with 2024Al matrix. As can be seen from HAADF image and element distribution of O and Si, there is a remarkable interface formed between Ni-Co-P intermediate layer and BFs. The element distribution is affected by the mixing enthalpy of each element. According to Boltzmann's hypothesis [25–26], mixing enthalpy of Al-Co and Al-Ni (‒19 and ‒22 kJ/mol) is higher than that of Ni-Co (0 kJ/mol), which means that Ni and Co can form stable solid solution phase so that element Al cannot diffuse to the Ni-Co-P coatings and react with BFs. Similarly, the value of mixing enthalpy of Mg-Co is 3 kJ/mol, and diffusion and reaction of Mg with Ni-Co-P coatings are inhibited.
Fig.4 HAADF image and the element distribution of interface between basalt fibers and Al matrix (a), TEM image of interface of BF(Ni-Co-P)/Al composites (b), and HRTEM image of interface between basalt fibers and Ni-Co-P coatings (c)
The microstructure of interface of BF(Ni-Co-P)/Al composites is shown in Fig.4a. Obvious interfaces are formed between basalt fibers and Ni-Co-P intermediate layer, Ni-Co-P intermediate layer and Al matrix, consistent with HAADF image and element distribution. While dislocations are not generated near the interface, indicating that existence of Ni-Co-P intermediate layer improves the compatibility of thermal expansion coefficient between Al matrix and BFs. Interest-ingly, the intermediate zone with lattice mismatch with the thickness of 3 nm is formed between Ni-Co-P intermediate layer and BFs (Fig.4c), which is conducive to load transfer. In addition, amorphous structures of BFs indicate that BFs are not damaged by Ni-Co-P intermediate layer.
2.4 Density of BF(Ni-Co-P)/Al composites
The density of composites can reflect the lightweight effect of reinforcement. The theoretical density can be calculated according to Eq.(4):
where ρc, ρm and ρr mean density value of composites, matrix (2.81g/cm3) and reinforcement (2.63 g/cm3), respectively; Vr and Vm refer to the volume fraction of reinforcement and matrix, respectively. After calculation, the theoretical density of BF/Al composites and BF(Ni-Co-P)/Al composites with different BFs volume fractions is shown in Fig.5. For BF/Al composites, the values of measured density are lower than theoretical density, and they have the same variation from high to low with the increase in BF volume fraction, as shown in Fig.5a. Interestingly, the theoretical density of BF(Ni-Co-P)/Al composites increases with improvement of BF volume fraction, while the measured density increases first, then reduces, and reaches the maximum value when the BF volume fraction is up to 30vol%. The difference of density variation between BF/Al composites and BF(Ni-Co-P)/Al composites is mainly caused by the presence of Ni-Co-P coatings. Ni-Co-P coatings not only improve the wettability and compatibility between matrix and BFs but also fill the fiber bundles, which can efficiently inhibit the generation of cavity defects between BFs. Besides, the lightweight effects of BFs are restrained in BF(Ni-Co-P)/Al composites because the density of Ni-Co-P coatings is larger than that of 2024Al matrix and BFs. There-fore, the measured density of BF(Ni-Co-P)/Al composites is closer to the theoretical value. In other words, BF(Ni-Co-P)/Al composites is denser than BF/Al composites under the same BFs volume fraction conditions.
Fig.5 Theoretical density and measured density of BF/Al composites (a) and BF(Ni-Co-P)/Al composites (b)
2.5 Hardness of BF(Ni-Co-P)/Al composites
The variations in hardness of BF/Al composites and BF(Ni-Co-P)/Al composites with different BFs content are shown in Fig.6. With the increase in BFs, the hardness increases firstly and then decreases, and the maximum values of BF/Al and BF(Ni-Co-P)/Al composites are up to 60 HRB and 72 HRB, respectively, higher than the maximum value of 2024Al. The hardness of BF(Ni-Co-P)/Al composites is superior to that of BF/Al composites owing to the optimization of Ni-Co-P coatings. Ni-Co-P coatings inhibit the generation of cavity defect among fiber bundles while form Ni-Co solid solution so that the deformation resistance is improved. Besides, the excessive fiber content can cause the generation of cavity defects, so the hardness of composites with 40vol% BFs is lower than that of others.
Fig.6 Hardness of BF/Al composites and BF(Ni-Co-P)/Al composites
2.6 Tensile properties and failure behavior of BF(Ni-Co-P)/Al composites
The tensile properties of BF/Al composites and BF(Ni-Co-P)/Al composites with 0vol%, 10vol%, 20vol%, 30vol%, and 40vol% BF are shown in Fig.7a and 7b, respectively. Clearly, the yield strength (YS) and ultimate tensile strength (UTS) of composites increase first and then decrease with the improvement of BFs content, while the elongation (EL) decreases. The YS and UTS of BF/Al composites reach the maximum of 94 and 238 MPa, respectively when BFs content is up to 10vol%, while YS and UTS of BF(Ni-Co-P)/Al composites reach the maximum of 252 and 360 MPa, respectively when the BF content is up to 30vol%. Compared with those of 2024Al matrix, YS and UTS of BF(Ni-Co-P)/Al composites are increased by 240% and 76%, respectively, and EL is decreased by 40.9%.
Fig.7 Tensile properties (a‒b) and fracture morphologies (c‒d) of BF/Al composites (a, c) and BF(Ni-Co-P)/Al composites (b, d)
Compared with those of BF/Al composites, YS and UTS of BF(Ni-Co-P)/Al composites have higher improvement, which is attributed to the presence of Ni-Co-P intermediate layer. As observed from the fracture morphology of BF/Al composites (Fig.7c), the dimples on Al matrix indicate that the failure mechanism of Al matrix composites is not affected by the reinforcement, while basalt fibers fracture simultaneously with the matrix. Also, the micropores are formed between fibers due to inadequate filling of Al, and this may become the crack source. According to the Ref. [17], the reactions between basalt fibers and Al matrix occur, as shown in Eq.(5) and Eq.(6):
On the one hand, metallurgical bonding interface generated by the above reactions improves the load transfer from matrix to basalt fibers. On the other hand, interface reaction destroys the intrinsic structure of BFs so that the load-bearing effect of fibers is difficult to exert. The microcracks initiate at the interface and easily propagate into the fiber interior. Consequently, BF/Al composites is of brittle fracture and their tensile properties decrease with the increase in BF content, which is inconsistent with variation of the theoretical strength calculated by rule of mixture of composites. For BF(Ni-Co-P)/Al composites, Ni-Co-P intermediate layers not only restrain formation of cavity defects and interface reaction between basalt fibers and Al matrix, but also improve the load transfer by adjusting the interface bonding. In the tensile process, microcracks initiate in the matrix firstly, and then propagate to interface and deflect so that fracture and pull-out of fibers appear simultaneously. As shown in Fig.7d, some basalt fibers are pull-out, which are conducive to the EL of composites by the crack-bridging toughening. Other composites fail on the surface of fracture, and they have excellent load-bearing effect. Therefore, the tensile properties of BF(Ni-Co-P)/Al composites improve obviously and it exhibits progressive cumulative failure mode.
1) Ni-Co-P coatings can be deposited on the surface of basalt fibers and used to form stable Ni-Co-P intermediate layer to optimize the interface of basalt fibers reinforced 2024Al matrix composites by the inhibition of interface reaction and improvement of interface state between basalt fibers and Al alloy matrix.
2) The yield strength and ultimate tensile strength of BF(Ni-Co-P)/Al composites can reach to 252 and 360 MPa, respectively, which are higher than those of BF/Al composites and 2024Al matrix.
3) Owing to the presence of Ni-Co-P intermediate layer, the failure mode of basalt fibers reinforced 2024Al matrix composites changes from brittle fracture to progressive cumulative failure.
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