In the field of biodegradable metal materials, magnesium and its alloys are the most studied [1
]. Due to their suitable mechanical properties such as high specific strength, stiffness and damping ability, they are suitable materials for the preparation of bone implants [1
]. Because of its low corrosion resistance and therefore difficult degradation control, magnesium is not used in its pure state, but alloyed. Commercial magnesium alloys are not primarily designed for medical applications, but some studies on the biomedical purpose of these alloys were done before, showing good corrosion behavior and biocompatibility [8
]. For medical purposes, magnesium alloys with calcium, zinc, rare earth elements or manganese are being considered [10
]. Due to the low density of calcium, these alloys have a similar density as a natural bone. In addition, Ca2+
ions are beneficial for human bones, and Mg2+
ions support the function of Ca2+
ions and generally the treatment of injury [11
Zinc has been considered, in the field of biomedical applications, a suitable alloying element for magnesium alloys in terms of improving corrosion resistance and enhancement of mechanical properties [10
]. Considering zinc is a nobler metal than magnesium and zinc is also biocompatible, it can be a suitable biodegradable material [16
]. Zinc is a nutritionally-essential element in the human body; approximately 85% of Zn in the human body can be found in bone and muscle. It is an integral part of the structure of macromolecules and enzymes, and it participates in a large number of enzymatic reactions. The human zinc requirement for adult males is 10–15 mg/day (upper limit 40 mg/day) [19
]. The potential for systemic toxicity of metallic zinc should be nonexistent due to the rapid transport of ionic zinc in living tissue. Moreover, higher consumption (up to 100 mg/day) of zinc is considered non-toxic. However, more detailed cytotoxicity tests must be carried out in the future [15
Based on its positive effect on the human body, zinc can be considered a suitable base material for the preparation of biodegradable materials. There are only a few works dealing with the use of zinc alloys for biomedical applications. In the case of pure zinc, there is just one article [22
] dealing with the applicability of zinc as a bioabsorbable cardiac stent material, according to the author’s knowledge. The authors in [22
] demonstrated the biocorrosion behavior of zinc by a series of four wire samples implanted in the abdominal aorta of Sprague-Dawley rat for 1.5, 3, 4.5 and 6 months. Results show that zinc corrosion is slower than the corrosion of magnesium and its alloys [24
]. Zinc implants stay almost intact for approximately four months, and then, corrosion subsequently accelerates. Corrosion products of zinc are formed from zinc oxide and zinc carbonate. Due to the slow corrosion of zinc, there is sufficient time to exclude hydrogen resulting from the corrosion process. That is both the main difference and an advantage over magnesium [21
]. However, no mechanical tests results were provided in this work.
Generally, the mechanical properties of zinc are relatively low. The modulus of compressive elasticity of commercially pure zinc is not listed as an exact value because there is no region of strict proportionality in the compressive stress-strain curve. Therefore, this value was determined in the range from 70–140 GPa [25
]. The Vickers hardness of this material is 30 HV. The tensile strength of wrought pure zinc is in the range of 120–150 MPa, depending on the direction of rolling [26
]. The tensile strength of cast pure zinc is very low with the value approximately 25 MPa [17
]. Another article dealing with mechanical properties of zinc showed that applying hot extrusion (300 °C, extrusion ratio 10:1, 2 mm/min) can enhance the ultimate tensile strength to a value of approximately 100 MPa and a Vickers hardness to 44 HV5 [27
] and, in [28
], to approximately 110 MPa for hot extruded pure zinc material (250 °C, 14:1). Due to the poor mechanical properties, alloying of zinc is a suitable solution. As alloying elements, mostly magnesium [17
], aluminum [30
] and silver [28
] were studied.
Alloying zinc with aluminum provides substantial enhancement of the mechanical properties. That is due to the presence of Al-Zn solid solution and the volume fraction of the lamellar microconstituents from the monotectoid reaction acting as potential barriers for dislocation motion. With an addition of 5.5 wt % of aluminum, the tensile strength of material prepared by hot rolling at 350 °C reaches the value of approximately 308 MPa and a yield strength of approximately 240 MPa [30
]. However, the use of Zn-Al alloys in medicine is still limited, because of the uncertainty regarding the toxicity of aluminum. Although, the toxicity of aluminum has never been sufficiently proven [30
], its connection to neurological disorders is still discussed in the literature [32
]. Furthermore, the corrosion resistance of Zn-Al alloys tended to be lower compared to high purity zinc due to the intergranular corrosion. Moreover, volume expansion associated with the formation of corrosion products led to cracking and fragmentation of implants [30
]. Another alloying element with positive influence on mechanical properties and preserving biocompatibility is silver. With the addition of 7.0 wt % of Ag, the ultimate tensile strength of the cast zinc alloy increased up to 287 MPa [28
]. Moreover, silver has been used in medicine for healing wounds, and it is used, in the form of nanoparticles, for the prevention of the adherence of bacteria to the surface of implants [33
]. However, the presence of secondary phase particles in the Zn-Ag alloys led to micro-galvanic corrosion because they act as anodes. Consequently, Zn-Ag alloys have higher degradation rates in comparison with pure Zn [28
Probably the most discussed alloying element of zinc-based biodegradable materials is magnesium. Because of the existence of the hard Mg2
intermetallic phase in the alloy structure, the value of the hardness increases with a higher content of magnesium up to the value of 200 HV (cast alloy, 3 wt % of magnesium). With higher magnesium content (35–45 wt % of magnesium), the hardness of the cast alloys can reach values of approximately 285–300 HV1 for 35 wt % of magnesium (depending on the cast product cooling rate and subsequent microstructure). The hardness increases because of the presence of another strengthening phase in the alloy microstructure (MgZn2
). With the higher content of magnesium in the alloy, the content of the MgZn2
phase decreases and the content of the MgZn and Mg7
phases increases. The hardness falls again due to decreasing content of MgZn2
to 255–280 HV1 for the alloy with 45 wt % of magnesium (depending on the cast product cooling rate and subsequent microstructure) [29
]. However, the existence of brittle eutectic phases in alloys with the content of magnesium higher than 1 wt % has a negative effect on ultimate tensile strength. Ultimate tensile strength increases up to 150 MPa (1 wt % of Mg) and then decreases to the value of 30 MPa (3 wt % of Mg). That is the same value as for pure zinc prepared by the same method. Furthermore, elongation of Zn-Mg alloys reaches the highest value for the alloy with 1 wt % of magnesium [17
]. Another article deals with an enhancement of mechanical properties with hot extrusion processing. Hot extruded alloy with 0.8 wt % of magnesium reaches the ultimate tensile strength of approximately 300 MPa and a Vickers hardness of approximately 80 HV5. With the content of 1.6 wt % of magnesium, hot extruded materials reach the ultimate tensile strength of approximately 360 MPa and a Vickers hardness of approximately 97 HV5 [27
Most of the available articles deal with the influence of alloying elements and their content on the mechanical properties and corrosion resistance of zinc-based materials prepared by casting or mechanical treatment of cast products. The influence of the preparation method and its parameters on mechanical properties is studied only marginally [17
This work deals with three different methods of preparation of zinc materials by powder metallurgy: cold pressing, cold pressing followed by sintering and hot pressing. According to the author’s knowledge, there is no available publication related to the preparation of pure zinc by these methods. Besides the influence of the preparation method, this work also focuses on the influence of the particle size of the used powder materials on the resulting microstructure and mechanical properties.
2. Materials and Methods
For the experiment, two different zinc powders (99.8% purity, 7.5 μm and 150 μm mean particle size provided by Goodfellow Cambridge Limited Company (Huntingdon, UK) were used. The smaller, 7.5 µm particle size powder (Zn7.5) was prepared by the electrothermal process, and the larger, 150 µm particle size powder (Zn150) was prepared by air atomization. In order to prevent material oxidation, manipulation with Zn powder was carried out in an inert atmosphere (N2) in the glove box. The size and shape of metal powder particles were verified and analyzed by a Zeiss Evo LS 10 scanning electron microscope (SEM; Carl Zeiss Ltd., Cambridge, UK).
The particles of Zn powder with the declared particle size of 7.5 μm had spherical shapes, and their size ranged from 1 µm to a maximum value of 20 μm (Figure 1
a). The particles tended to clump. The smaller particles attached to the larger ones and created clusters. The particles of zinc powder with a declared particle size of 150 μm were irregularly rod-shaped with a minor amount of round-shaped particles (Figure 1
b). The SEM analysis of the powder showed that some particles in their largest dimension reached a size from 40–640 μm.
The purity of powders declared by the supplier was verified by EDS analysis. The larger content of oxygen, approximately 8.0 ± 0.5 wt %, was determined in Zn7.5 powder when compared to Zn150 powder, containing only 2.5 ± 0.5 wt %.
Experimental samples were processed in three ways: (i) By bidirectional cold pressing (CP), (ii) cold pressing followed by sintering (CP-S) and (iii) hot pressing (HP). A hollow cylindrical steel die with the inner diameter of 20 mm was used for zinc powders’ compaction. Before it was filled with zinc, the die surface was carburized in order to prevent adhesion of the base powder material to the surface of the die. The pressing of the powders was carried out by the Zwick Z250 Allround-Line universal testing machine (Zwick GmbH & Co.KG, Ulm, Germany) with a velocity of 2 mm/min. Pressures of 100, 200, 300, 400 and 500 MPa were used for prepared powders’ compaction.
For sintering, cold-pressed samples pressed under 100, 200, 300, 400 and 500 MPa were used. Cold-pressed samples were inserted into glass vials and then filled with argon of purity 4.6 and sealed. The sintering of pressed powder was done for 1 h at a temperature of 400 °C in the laboratory furnace preheated to the required temperature.
Hot pressing was carried out for 1 h at a temperature of 400 °C under 100, 200, 300, 400 and 500 MPa. Prepared bulk materials were in the form of cylindrical tablets with a diameter of 20 mm and a height of 5 mm.
Metallographic evaluation of prepared samples was performed in a conventional manner. Isopropanol was used as a lubricant and a rinse to prevent oxidation of the samples during grinding and polishing. For metallographic evaluation, the Zeiss Axio Z1M (Carl Zeiss AG, Oberkochen, Germany) inverted light optical microscope (OM) and Zeiss Evo LS 10 (Oxford Instruments, Abington, UK) were used. The microhardness of prepared samples was measured with the LM 248 at machine produced by LECO Company (Saint Joseph, MO, USA). The measurement was carried out in accordance with the ISO 6507-1 standard (Vickers method, applied load 25 g) on 10 positions on the sample.
The 3-point bend test was carried out on the Zwick Z020 (Zwick GmbH & Co.KG, Ulm, Germany) universal testing machine according to the ISO 7438 standard. One sample for each preparation condition was used for the 3-point bend test. Samples for testing were detracted from the central part of prepared tablets and ground up to 4 mm × 4 mm × 18 mm proportions with the support span of 16 mm. Fractographic evaluation of the fracture surfaces of broken samples was performed by SEM. Documentation was carried out in the area of applied tensile stress.
Microstructural analysis of materials prepared from Zn7.5 powder (Figure 2
) revealed lower deformability of powder particles when compared to Zn150 powder particles (Figure 3
). The considerably lower particle size and spherical shape of Zn7.5 powder may be the reason for this behavior.
Even though the mechanical properties of pure zinc powder particles are comparable, the size of the particles plays a similar role as in the case of grain size. The larger size of the particles was connected with their higher deformability during the processing, and the lower particle size resulted in grain boundaries’ (particles boundaries) strengthening of material. The strengthening of material was also connected with lower deformability of the material.
The deformability of individual powder particles influenced the material properties and content of porosity. Although the zinc particles were not significantly deformed in some of the cases (mostly Zn7.5, Figure 2
), the porosity of all the prepared materials decreased with increasing compacting pressure and even decreased with the application of temperature during the material processing (CP-S and HP). This fact can be attributed to a wide distribution of powder particle size, when the space between large particles was filled up with the smaller ones. However, the porosity of materials from Zn7.5 powder was substantially higher than in the case of material prepared from Zn150 powder, which can be also observed from the fractographic evaluation of broken samples (Figure 5
and Figure 6
). This can be explained by the larger range of the powder particle size measured for Zn150 powder (up to 640 μm) compared to Zn7.5 powder (up to 20 μm) and the adequate arrangement of the particles with different size in the compacted sample volume.
CP-S processing of Zn7.5 powder materials did not influence the material microstructure with as much intensity as the increasing compression pressure did. Only the minor effect of the following sintering of CP materials was observed by metallographic analysis (Figure 2
). In the case of Zn150 powder materials, the following sintering of CP samples led to the increase of the porosity of materials prepared at low pressures (Figure 3
c). Due to the high sintering temperature of 400 °C and high material porosity (of CP samples), the powder particles reduced the surface energy and changed their shape from irregular rod-shaped to spherical during sintering. At the same time, no dimensional changes of prepared samples were observed, which would generally correspond if a solid-state diffusion was the primary sintering mechanism [34
]. The shape changes of particles and no shrinkage could lead to pores growing between the particles.
The effect of HP processing is more evident on the Zn7.5 powder-based materials when compared to the Zn150 powder materials. Elevated temperature during the material processing resulted in improved plasticity and enhanced deformability of fine powder particles (Zn7.5 powder). The particles’ deformation is observable on materials’ microstructures documented in Figure 2
e,f. Elevated temperature applied during material processing resulted in activation of more slip systems in the hexagonal close packed (HCP) structure (also characteristic for zinc) [34
]. In the case of Zn150 powder materials, the elevated temperature of the processing had no evident influence on material microstructure. In the case of smaller powder particles, the improved deformability of the material due to the activation of more slip systems was much more significant compared to the larger powder particles, which were deformable even at room temperature. Quite good deformability of large powder particles was not significantly improved and remained the same as in the case of CP and CP-S materials from the microstructural point of view, Figure 3
There is only a small number of articles dealing with evaluation of the mechanical properties of pure zinc [17
]. From the mechanical properties point of view, materials in these studies were characterized mostly by Vickers hardness and tensile tests. The Vickers hardness of experimental materials prepared from Zn7.5 and Zn150 powders in the presented work was determined as 40–49 HV025 and 40–45 HV025, respectively. The values were higher in comparison to the cast pure zinc of 30 HV given in [17
]; however, the values were comparable with results of Vickers hardness testing of hot extruded pure zinc of 44 HV5 given in [27
]. Due to the obtained microstructure of materials prepared by powder metallurgy, the characteristic mechanical properties should correlate more with the properties of wrought materials than with cast materials. Considering the powder particles’ boundaries as grain boundaries, the microstructure of the materials processed from Zn7.5 powder (knowing the real particle size was up to 20 μm) is comparable with the fine recrystallized equi-axed grains with an average grain size of 20 μm characteristic for pure extruded zinc analyzed in [27
The resistance of prepared powder-based materials against the fracture during the three-point bend test was (besides particles deformability) influenced mainly by the mechanical interlocking of irregularities on the powder particle surfaces, which was promoted by plastic deformation during the pressing. The bonding of particles was responsible for the mechanism of the bulk material fracture. Because of the spherical shape of Zn7.5 powder particles, the flexural strength of compacted samples showed lower values when compared to materials prepared from Zn150 powder. Larger surface area and particles shape of Zn150 powder particles allowed easier contact of particles and interlocking of the surface irregularities, which was even enhanced by the particles’ larger deformability compared to the Zn7.5 powders, which was easily observed in the case of CP and CP-S materials, Figure 3
The surface layer of corrosion products on particles’ surface had also a negative influence on powder particles’ compaction during sintering and HP (processes performed at elevated temperature, enhancing diffusion processes). Only the diffusion mechanisms contributed to sintering of particles in the case of the absence of an external pressure [35
]. The oxide layer on the powder particles could affect the diffusion bonding of particles during sintering, and this can be seen in the case of CP-S Zn150 powder material compacted under 100 MPa (Figure 3
c). In this case, the observed porosity was higher than as in the case of CP materials. Due to the surface oxide layer on the powder particles, the sintering process could be affected by the diffusion delay. The role of the surface oxide layer in the sintering of metal powder particles was defined as follows: Shifting the mechanisms of the sintering process from a bulk-transport mechanism to that controlled by surface transport [34
]. Even though the content of oxygen was detected to be higher on the Zn7.5 powder particles, the powder particles’ low deformability seemed to have a larger influence on material compaction than the oxide layer on the powder particles. Due to the low particle deformability and spherical shape, only limited areas of individual particles were in direct contact, and the conditions for diffusion were worse compared to the larger and more plane particles of Zn150 powder material. However, according to the experimental observations, the influence of the oxide layer on the sinterability of zinc was minimal, and the presence of an oxide layer on powder particles makes essentially no difference in the kinetics of sintering [34
Even though the influence of processing parameters on Zn powder-based materials’ microstructure is minimal, the influence of the processing on the samples’ bending properties was observed. CP-S materials reached higher values of flexural strength for both powder particles sizes compared to the CP materials. The influence was, however, only minor compared to the influence of the HP.
In the case of Zn7.5 powder material, the CP samples prepared under 100, 200 and 300 MPa did not reach the required handling strength necessary for three-point bend testing, and only samples prepared under 400 and 500 MPa were tested. However, the values reached for the flexural strength and displacement before fracture were very low (Figure 4
a,c), which corresponds to low particle deformation and the subsequent low bonding and high porosity of the material (Figure 2
a,b). The transgranular fracture mechanism and porosity present on the fracture surfaces support this theory (Figure 5
a,b). Sintering of the Zn7.5 powder material resulted in higher material handling strength; however, the values of flexural strength reached were still low for materials prepared under 100–300 MPa (Figure 4
a). The fracture surface of the CP-S sample prepared under 200 MPa corresponded to the microstructural observation and low bending characteristics of the material reached (Figure 5
c). Sintering materials prepared under higher pressures resulted in flexural strength values higher than 40 MPa (41 and 76 MPa for 400 and 500 MPa pressure, respectively). However, the displacement before the fracture was still very low and comparable to the one of CP samples prepared under the same pressures (Figure 4
c). HP of Zn7.5 powder materials resulted in a significant increase of materials’ flexural strength, reaching maximal value of 322 MPa for material prepared under 400 MPa (Figure 4
a). The positive influence of compacting pressure of HP-processed Zn7.5 powder material was revealed by three-point bend test up to the compacting pressure of 400 MPa. Samples prepared under 500 MPa reached lower values of flexural strength than the maximum, which can be connected with the limited deformability of the material due to the HCP crystallographic structure of zinc [36
]. High compaction of the material during HP under 500 MPa resulted in strong particle bonding, limiting the transgranular fracture mechanism. With increasing compacting pressure, a larger amount of broken powder particles can be observed on the samples’ fracture surfaces (Figure 5
e,f). The cleavage mechanism playing a role in the material failure was responsible for the measured decrease of the flexural strength (HP material processed under 500 MPa); however, displacement before fracture still increased.
In the case of Zn150 powder materials, only the sample prepared by CP under 100 MPa did not reach the adequate handling strength necessary for the three-point bend test. Due to the higher deformability of larger powder particles compared to the Zn7.5 powder materials, CP and CP-S samples reached higher values of flexural strength and displacement before fracture when compared to the Zn7.5 samples. In the case of CP-S Zn150 powder materials, sintering had a significantly positive influence on material bending properties (improvement of flexural strength approximately twice compared to the CP materials). HP Zn150 powder materials reached even larger values of bending properties than the CP-S materials; however, the improvement was not as significant as in the case of Zn7.5 powder materials (Figure 4
). The differences between the CP-S and HP Zn150 powder materials are significant for materials prepared under 100–300 MPa; however, only a minor influence was observed in the case of materials prepared under 400 and 500 MPa (Figure 4
). This corresponds to the microstructural observations, where only a minor influence on the materials microstructure applying higher pressure (400–500 MPa) and temperature during HP was observed (Figure 3
). The bonding of particles of materials prepared under 300–500 MPa was comparable for CP-S and HP materials. HP Zn150 powder materials’ fracture surfaces were similar for all of the applied pressures during the preparation of materials, while differences in the CP-S materials’ fracture surfaces were observed (Small pressures during materials processing resulted in intergranular failure, while the combination of inter- and trans-granular fracture correlated with higher compacting pressures (Figure 6
Detailed fractographic analysis of the fracture surfaces of HP processed materials revealed the mechanism of crack propagation (Figure 5
f and Figure 6
f). Due to the tensile loading, microcracks through individual powder particles were observed, characteristic of the cleavage facets in Figure 5
f and Figure 6
f. The final crack responsible for the sample’s failure followed the powder particles’ boundaries connecting cleavage facets (cracked particles).