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Fabrication of Zinc–Tungsten Carbide Nanocomposite Using Cold Fabrication of Zinc–Tungsten Carbide candidate for potential applications in biodegradable implants [4,10]. Due to a relatively weak mechanical strength, Zn is widely Nanocomposite Using Cold Compaction used as an alloy addition [11] and coating material in galvaniza- tion [12] but not much for load-bearing structures [13]. It has been Followed by Melting a long standing challenge to enhance the mechanical strength of zinc toward a prominent material with a combination of high Injoo Hwang strength, high corrosion resistivity and great biocompatibility. A great deal of studies have been conducted to improve the mechan- Department of Mechanical and Aerospace Engineering, ical properties of Zn for applications such as load-bearing stents University of California, Los Angeles, [4,14]. Los Angeles, CA 90095 Since conventional manufacturing processes (e.g., alloying) e-mail: [email protected] have already reached their limits to improve mechanical proper- ties of Zn [12], innovative methods have been applied to tackle Zeyi Guan this problem. For example, nanoparticles have been introduced to Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/140/8/084503/6406225/manu_140_08_084503.pdf by guest on 02 October 2021 Department of Mechanical and Aerospace Engineering, Zn to improve its properties [15]. According to the research on ceramic nanoparticle-reinforced metal matrix nanocomposites University of California, Los Angeles, recently [16–20], ceramic nanoparticles were able to improve the Los Angeles, CA 90095 mechanical strength, not only by the intrinsic high mechanical e-mail: [email protected] strength but also by the grain refinement, which introduces more grain boundaries to impede the movement of dislocations. Addi- Xiaochun Li1 tionally, the high-temperature thermal stability and chemical Fellow ASME stability of ceramic nanoparticles can also avoid nanoparticle sin- Department of Mechanical and Aerospace Engineering, tering and chemical reactions with the metal matrix during fabri- cation and processing [21,22]. One of the most important University of California, Los Angeles, problems prohibiting nanocomposites from mass production is Los Angeles, CA 90095 often the low wettability between nanoparticle and metal matrix, e-mail: [email protected] where high surface tension of metals hinders nanoparticles incor- poration and homogeneous dispersion in scalable methods such as casting [23–25]. Such nanoparticles agglomeration results in the formation of nanoparticle clusters, so that the advanced properties Zinc (Zn) is an important material for numerous applications could not be achieved [26,27]. Innovative methods, such as ultra- since it has pre-eminent ductility and high ultimate tensile strain, sonic cavitation-based process, molten salt-assisted incorporation, as well high corrosion resistivity and good biocompatibility. How- and stir casting [17,28,29], have been used to overcome the incor- ever, since Zn suffers from low mechanical strengths, most of the poration and dispersion problems with promising results to some applications would use Zn as a coating or alloying element. In extent. this study, a new class of Zn-based material with a significantly In this study, high-density and uniformly dispersed tungsten enhanced mechanical property is developed. The zinc-10 vol % carbide (WC) nanoparticles were used to enhance mechanical tungsten carbide (Zn-10WC) nanocomposite was fabricated by properties of Zn. More specifically, Zn-10 vol % WC (Zn-10WC cold compaction followed by a melting process. The Zn-10WC thereafter) nanocomposites were fabricated by cold compaction nanocomposites offer a uniform nanoparticle dispersion with little followed by a melting process to obtain a more uniform dispersion agglomeration, exhibiting significantly enhanced mechanical of nanoparticles. This is a promising method for scalable manu- properties by micropillar compression tests and microwire tensile facturing of Zn matrix nanocomposite with homogeneously dis- testing. The nanocomposites offer an over 200% and 180% persed nanoparticles. Furthermore, no significant acute toxicity of increase in yield strength and ultimate tensile strength (UTS), WC nanoparticles has been reported yet regarding to its biocom- respectively. The strengthening effect could be attributed to Oro- patibility [30]. Thus, Zn-10WC microwires, which have potential wan strengthening and grain refinement induced by nanoparticles. for weaving of biomedical stents, were also fabricated by thermal [DOI: 10.1115/1.4040026] fiber drawing and mechanically tested. 2 Methods and Experimental Results 1 Introduction 2.1 Fabrication of Zn–WC Nanocomposites. Zn–WC nano- compositeswerefabricatedbycoldcompactionfollowedbya Zinc has been widely used in automotive, construction, and bio- melting process. The schematic of the experimental setup is medical industries due to its high corrosion resistance and good shown in Fig. 1. A 90% volume fraction of Zn micropowders biocompatibility [1–5]. More specifically, Zn-based alloys are (150 lm, Goodfellow) and a 10% volume fraction of WC nano- commonly used in many applications such as an anticorrosion particles (150 nm, U.S. Research Nanomaterials, Inc., Houston, agent and galvanization due to its high corrosion resistance. TX) were weighted and mixed. The powder mixture were Magnesium additions improve the corrosion resistance of zinc- blended by a mechanical shaker (SK-O330-Pro) at 300 RPM aluminum alloy galvanized steel [6], in which the chemical resis- for 30 min. The well-blended Zn–WC powder mixture was tivity was enhanced by fast cooling rate, such that small grain size added to a cylindrical stainless steel mold (inner diameter: 19 of the primary Zn dendrites was obtained [7]. The good biocom- mm) for cold compaction into a pellet using a hydraulic press patibility of Zn has broaden its applications to the biomedical field under 85 kN at room temperature. The Zn–WC pellet was such as orthopedic implants and tissue generations [8,9]. The ideal melted with manual stirring in an alumina crucible at the tem- corrosion rate of Zn in human body recently makes it a good perature 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 nano- particle dispersion. The final product was cooled down under 1Corresponding author. Manuscript received January 4, 2018; final manuscript received April 13, 2018; Ar gas protection. Pure Zn sample was also manufactured in published online May 21, 2018. Assoc. Editor: Donggang Yao. the same conditions as reference. Journal of Manufacturing Science and Engineering AUGUST 2018, Vol. 140 / 084503-1 Copyright VC 2018 by ASME Fig. 1 Schematic of experimental method Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/140/8/084503/6406225/manu_140_08_084503.pdf by guest on 02 October 2021 2.2 Microstructure Characterization and Nano-Indentation analysis. The samples went through grinding and polishing (Allied of Zn–WC Nanocomposites. The Zn-10WC nanocomposite sam- M-Prep 5TM Grinder/Polisher) with a colloidal silica suspension ples were characterized by scanning electron microscope (SEM) of 0.5 lm and 0.02 lm, followed by an extra surface cleaning for microstructure analysis and by energy dispersive X-ray spec- processing by a low-angle ion milling (4 deg, 3.25 keV with troscope (EDS) for quantitative element detection and dispersion 10 lA) for 2 h. Figures 2(a)–2(c) showed the uniformly Fig. 2 (a)–(c) Microstructure of Zn-10WC nanocomposite by SEM with different magnifica- tion. (d)–(g) EDS detection of elements Zn, W, and O, indication Zn matrix, WC nanoparticles, and oxidations. (h) and (i) Grain size of Zn and Zn-10 vol % WC microstructure by SEM. 084503-2 / Vol. 140, AUGUST 2018 Transactions of the ASME Table 1 Elastic moduli of pure Zn and Zn-10WC nano- 2.3 Zn–WC Nanocomposite Micropillar Compression Test composites for Yield Strength Measurement. In addition to study the mechanical strength, Zn-10WC nanocomposites were character- Samples Pure Zn (GPa) Zn-10WC (GPa) ized by the microcompression tests using a nanoindenter (MTS Nano Indenter XP) with a 10 lm diameter-flat punch. Focus ion- 1 62.2 102.9 2 55.6 100.6 beam (FEI Nova 600 Nanolab Dual-Beam FIB–SEM) was used to 3 67.5 112.5 machine micropillars of 10 lm in height and 3.5 lm in diameter 4 66.4 94.5 on nanocomposites samples and reference samples in Figs. 3(a) 5 70.4 101.6 and 3(b), respectively. The results showed that nanocomposite Average 64.4 102.4 micropillars have a uniform and dense nanoparticle dispersion on Standard deviation 5.1 5.8 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 distributed 10 vol % WC nanoparticles in Zn matrix. The rela- (22 MPa). Table 2 shows the yield and ultimate compressive Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/140/8/084503/6406225/manu_140_08_084503.pdf by guest on 02 October 2021 tively bright- and dark-phase areas corresponded to WC nanopar- strength of the pure Zn and Zn-10WC nanocomposites in the com- ticles and Zn matrix, respectively. The microstructure of Zn–WC pression tests. The yield strength of the pure Zn and Zn-10WC nanocomposite suggests that WC nanoparticles were separated by nanocomposites are 23.463.6 MPa and 116.4620 MPa, respec- Zn of a few tens of nanometers. Energy dispersive X-ray spectro- tively. The average ultimate compressive strength of pure Zn and scope characterization indicates that the nanocomposite sample Zn-10WC are 51 MPa and 507 MPa. It is clear that WC nanopar- consists of zinc (77.6 wt %), tungsten (20.2 wt %), and oxygen ticles significantly enhanced the mechanical strength of Zn. (2.2 wt %), as shown in Figs. 2(d)–2(g). Highly concentrated tung- sten carbide was detected in such sample, with an equivalent to 2.4 Tensile Testing Using Zn–WC Nanocomposites Micro- 11.2 vol %, which is within an acceptable error range due to the wires.
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