Zinc (Zn) is an important material for numerous applications since it has pre-eminent ductility and high ultimate tensile strain, as well high corrosion resistivity and good biocompatibility. However, since Zn suffers from low mechanical strengths, most of the applications would use Zn as a coating or alloying element. In this study, a new class of Zn-based material with a significantly enhanced mechanical property is developed. The zinc-10 vol % tungsten carbide (Zn-10WC) nanocomposite was fabricated by cold compaction followed by a melting process. The Zn-10WC nanocomposites offer a uniform nanoparticle dispersion with little agglomeration, exhibiting significantly enhanced mechanical properties by micropillar compression tests and microwire tensile testing. The nanocomposites offer an over 200% and 180% increase in yield strength and ultimate tensile strength (UTS), respectively. The strengthening effect could be attributed to Orowan strengthening and grain refinement induced by nanoparticles.

## Introduction

Zinc has been widely used in automotive, construction, and biomedical industries due to its high corrosion resistance and good biocompatibility [15]. More specifically, Zn-based alloys are commonly used in many applications such as an anticorrosion agent and galvanization due to its high corrosion resistance. Magnesium additions improve the corrosion resistance of zinc-aluminum alloy galvanized steel [6], in which the chemical resistivity was enhanced by fast cooling rate, such that small grain size of the primary Zn dendrites was obtained [7]. The good biocompatibility of Zn has broaden its applications to the biomedical field such as orthopedic implants and tissue generations [8,9]. The ideal corrosion rate of Zn in human body recently makes it a good candidate for potential applications in biodegradable implants [4,10]. Due to a relatively weak mechanical strength, Zn is widely used as an alloy addition [11] and coating material in galvanization [12] but not much for load-bearing structures [13]. It has been a long standing challenge to enhance the mechanical strength of zinc toward a prominent material with a combination of high strength, high corrosion resistivity and great biocompatibility. A great deal of studies have been conducted to improve the mechanical properties of Zn for applications such as load-bearing stents [4,14].

Since conventional manufacturing processes (e.g., alloying) have already reached their limits to improve mechanical properties of Zn [12], innovative methods have been applied to tackle this problem. For example, nanoparticles have been introduced to Zn to improve its properties [15]. According to the research on ceramic nanoparticle-reinforced metal matrix nanocomposites recently [1620], ceramic nanoparticles were able to improve the mechanical strength, not only by the intrinsic high mechanical strength but also by the grain refinement, which introduces more grain boundaries to impede the movement of dislocations. Additionally, the high-temperature thermal stability and chemical stability of ceramic nanoparticles can also avoid nanoparticle sintering and chemical reactions with the metal matrix during fabrication and processing [21,22]. One of the most important problems prohibiting nanocomposites from mass production is often the low wettability between nanoparticle and metal matrix, where high surface tension of metals hinders nanoparticles incorporation and homogeneous dispersion in scalable methods such as casting [2325]. Such nanoparticles agglomeration results in the formation of nanoparticle clusters, so that the advanced properties could not be achieved [26,27]. Innovative methods, such as ultrasonic cavitation-based process, molten salt-assisted incorporation, and stir casting [17,28,29], have been used to overcome the incorporation and dispersion problems with promising results to some extent.

In this study, high-density and uniformly dispersed tungsten carbide (WC) nanoparticles were used to enhance mechanical properties of Zn. More specifically, Zn-10 vol % WC (Zn-10WC thereafter) nanocomposites were fabricated by cold compaction followed by a melting process to obtain a more uniform dispersion of nanoparticles. This is a promising method for scalable manufacturing of Zn matrix nanocomposite with homogeneously dispersed nanoparticles. Furthermore, no significant acute toxicity of WC nanoparticles has been reported yet regarding to its biocompatibility [30]. Thus, Zn-10WC microwires, which have potential for weaving of biomedical stents, were also fabricated by thermal fiber drawing and mechanically tested.

## Methods and Experimental Results

### Fabrication of Zn–WC Nanocomposites.

Zn–WC nanocomposites were fabricated by cold compaction followed by a melting process. The schematic of the experimental setup is shown in Fig. 1. A 90% volume fraction of Zn micropowders (150 μm, Goodfellow) and a 10% volume fraction of WC nanoparticles (150 nm, U.S. Research Nanomaterials, Inc., Houston, TX) were weighted and mixed. The powder mixture were blended by a mechanical shaker (SK-O330-Pro) at 300 RPM for 30 min. The well-blended Zn–WC powder mixture was added to a cylindrical stainless steel mold (inner diameter: 19 mm) for cold compaction into a pellet using a hydraulic press under 85 kN at room temperature. The Zn–WC pellet was melted with manual stirring in an alumina crucible at the temperature of 450 °C by an electrical resistance furnace under a protection gas of Argon (Ar) for 30 min. This additional melting process aims at eliminating porosity and promoting the nanoparticle dispersion. The final product was cooled down under Ar gas protection. Pure Zn sample was also manufactured in the same conditions as reference.

### Microstructure Characterization and Nano-Indentation of Zn–WC Nanocomposites.

The Zn-10WC nanocomposite samples were characterized by scanning electron microscope (SEM) for microstructure analysis and by energy dispersive X-ray spectroscope (EDS) for quantitative element detection and dispersion analysis. The samples went through grinding and polishing (Allied M-Prep 5TM Grinder/Polisher) with a colloidal silica suspension of 0.5 μm and 0.02 μm, followed by an extra surface cleaning processing by a low-angle ion milling (4 deg, 3.25 keV with 10 μA) for 2 h. Figures 2(a)2(c) showed the uniformly distributed 10 vol % WC nanoparticles in Zn matrix. The relatively bright- and dark-phase areas corresponded to WC nanoparticles and Zn matrix, respectively. The microstructure of Zn–WC nanocomposite suggests that WC nanoparticles were separated by Zn of a few tens of nanometers. Energy dispersive X-ray spectroscope characterization indicates that the nanocomposite sample consists of zinc (77.6 wt %), tungsten (20.2 wt %), and oxygen (2.2 wt %), as shown in Figs. 2(d)2(g). Highly concentrated tungsten carbide was detected in such sample, with an equivalent to 11.2 vol %, which is within an acceptable error range due to the inaccuracy of the testing machine, implying that WC nanoparticles were fully incorporated into Zn. The average grain size of pure Zn and Zn–WC nanocomposites were also measured to be approximately 16.94.28 μm, respectively, as shown in Figs. 2(h) and 2(i).

Nano-indentation tests were performed to measure the elastic moduli of pure Zn and Zn-10WC nanocomposite using a nanoindenter (MTS Nano Indenter XP) with a Berkovich tip (20 nm radius, diamond). Table 1 presents that the elastic moduli of pure Zn and Zn-10WC nanocomposites are 64.4±8.8 GPa and 102.4±10.1 GPa, respectively. It is clear that the WC nanoparticles improved the elastic modulus of pure Zn significantly.

### Zn–WC Nanocomposite Micropillar Compression Test for Yield Strength Measurement.

In addition to study the mechanical strength, Zn-10WC nanocomposites were characterized by the microcompression tests using a nanoindenter (MTS Nano Indenter XP) with a 10 μm diameter-flat punch. Focus ion-beam (FEI Nova 600 Nanolab Dual-Beam FIB–SEM) was used to machine micropillars of 10 μm in height and 3.5 μm in diameter on nanocomposites samples and reference samples in Figs. 3(a) and 3(b), respectively. The results showed that nanocomposite micropillars have a uniform and dense nanoparticle dispersion on the surface. The compression data were shown in Fig. 3(c), where Zn–WC samples obtained significantly higher yield strength (118 MPa), more than five times higher than the pure Zn sample (22 MPa). Table 2 shows the yield and ultimate compressive strength of the pure Zn and Zn-10WC nanocomposites in the compression tests. The yield strength of the pure Zn and Zn-10WC nanocomposites are 23.4±3.6 MPa and 116.4±20 MPa, respectively. The average ultimate compressive strength of pure Zn and Zn-10WC are 51 MPa and 507 MPa. It is clear that WC nanoparticles significantly enhanced the mechanical strength of Zn.

### Tensile Testing Using Zn–WC Nanocomposites Microwires.

Zn-10WC nanocomposite microwires were fabricated by thermal fiber drawing method [31], while using borosilicate glass tubing (inner diameter: 1.0 mm, and outer diameter: 6.5 mm) as the cladding material. The nanocomposite wires could serve as the starting materials for stent fabrication. The nanocomposite preform was thermally drawn at 820 °C (feeding speed: 100 μm/s, and pulling speed: 2.5 mm/s) to obtain Zn–WC microwires of 200 μm in diameter with a draw-down ratio of 25. The glass cladding was etched out by 49% aqueous hydrofluoric acid to the cladding thickness of 0.1 mm, whereas the remaining glass shell was manually removed.

The Zn-10WC nanocomposite microwires were then tensile tested using a dynamic mechanical analyzer (Q 800 DMA, TA instruments). The result of the stress–strain curves were obtained for both nanocomposite and pure zinc microwires, as shown in Figs. 4(a) and 4(b). The pure Zn wires obtained a ultimate tensile strength (UTS) of 37 MPa, yield strength of 18 MPa, and ultimate tensile strain of 35.1%, while Zn-10WC nanocomposite microwires exhibited an UTS of 103 MPa, yield strength of 55 MPa, and ultimate tensile strain of 5.0%. Table 3 presents the yield and ultimate tensile strength of the pure Zn and Zn-10WC nanocomposites in the tensile test. The yield strength of the pure Zn and Zn-10WC nanocomposite are 13.4±4.6 MPa and 54.8±8.8 MPa, respectively. The average ultimate tensile strength of pure Zn and Zn-10WC nanocomposites are 27 MPa and 102 MPa. Further verification of WC nanoparticle dispersion in the microwires were obtained through SEM by inspecting the microwire surface and longitudinal cross section, as shown in Figs. 4(c)4(f).

## Discussion

Zn-10WC nanocomposites were successfully fabricated by cold compaction followed by a melting process. The method was able to efficiently incorporate WC nanoparticles into Zn while avoiding potential oxidation problem. During the melting process, molten Zn infiltrated into the nanoscale gaps among WC nanoparticles, preventing the nanoparticles agglomeration. WC nanoparticles in a molten Zn are then dispersed and stabilized by a thermally activated dispersion mechanism recently discovered [20]. Furthermore, microstructure characterization, nano-indentation tests, micropillar compression tests, and microwire tensile tests were performed.

The mechanical properties of the Zn–WC nanocomposites were substantially enhanced for two major reasons: Orowan strengthening and grain refinement. WC intrinsically offers a high hardness of 2600 HV and an ultimate compression strength of 2.7 GPa [32]. This Orowan strengthening by WC nanoparticles could be generally determined by
$ΔσOrowan=φGmbdp6Vpπ1/3$
(1)

where $φ$ is a constant equal to 2, $Gm$ is the shear modulus of Zn, $b$ is the Burgers vector, $Vp$ is the volume fraction, and $dp$ is the reinforcement size [33]. With a rough estimation assuming perfectly homogeneous dispersion, and $Gm=43GPa,Vp=10%,b=0.27nm,dp=200nm,φ=2$, the strengthening could be determined to be $ΔσOrowan=66.9MPa$.

Further strengthening could also be contributed to the grain refinement due to fact that nanoparticle impeded the solidification front. The average grain size of the Zn–WC nanocomposite was refined from 16.9 μm to 4.28 μm. The grain refinement could enhance the mechanical strength through the grain boundary strengthening, as known as Hall–Petch strengthening, corresponding to the equation
$Δσy=kd−1/2$
(2)

where $Δσy$ is the yield strength, $k$ is the strengthening coefficient ($kZn=0.22MPam1/2$) and $d$ is the grain size. Δσy is roughly calculated to be 52.8 MPa based on the equation. It should be noted that the theoretical predictions do not match the experimental results exactly, possibly due to defects and nonideal nanoparticle dispersion.

Although promising results have been shown in this work, a few factors needed to be considered to account for variation of results and error. Stress concentrations and crack propagations can occur at the micro/nanoscale porosities formed in material processing. Surface defects (notches and dimples) on the microwires that were created during the glass removal process could lead to a stress concentration. The residual stress or defects by the clamp in the DMA tensile machine might also induce a reduction in the strength of microwires under testing. In addition, the pillar size (3.5 μm in diameter) in the microcompression test is closed to 2 μm in the average grain size of Zn in the Zn-10WC nanocomposite, which affect the strength of the Zn-10WC nanocomposite.

## Conclusions

In summary, Zn–WC nanocomposite with a high volume fraction of WC nanoparticles has been successfully fabricated by cold compaction followed by a melting process. Zn–WC nanocomposite microwires were fabricated by thermal fiber drawing so that they could be used for stent weaving. WC nanoparticles were well dispersed and distributed in the Zn matrix. Zn–WC nanocomposites offer significantly enhanced mechanical properties, mostly due to Orowan strengthening and grain refinement. Due to surface defects after the glass removing process and some micro/nanoporosities in the Zn-10WC microwires, the tensile testing results do not meet the theoretical expectation well. Further study is needed. Zn–WC nanocomposite with high mechanical properties can open-up exciting new applications.

## Acknowledgment

This work was partially support by National Science Foundation. We thank A. Javadi at University of California, Los Angeles for his help with the discussion in nanocomposite fabrication. We also thank C. Linsley at University of California, Los Angeles, for his help with tensile testing.

## Funding Data

• Division of Civil, Mechanical and Manufacturing Innovation (1449395).

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