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A new approach in the preparation of biodegradable Mg-MgF2 composites with tailored corrosion and mechanical properties by powder metallurgy
Materials Letters 227 (2018) 78–81
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
A new approach in the preparation of biodegradable Mg-MgF2 composites with tailored corrosion and mechanical properties by powder metallurgy Drahomir Dvorsky ⇑, Jiri Kubasek, Dalibor Vojtech Faculty of Chemical Technology, Department of Metals and Corrosion Engineering, University of Chemistry and Technology Prague, Technická 5 166 28 Praha 6 – Dejvice, Czech Republic
a r t i c l e
i n f o
Article history: Received 13 March 2018 Received in revised form 17 April 2018 Accepted 11 May 2018
Keywords: Biomaterials Composite material Corrosion Powder technology Sintering
a b s t r a c t This paper presents an innovative way of preparation of the magnesium-fluoride composite material with improved corrosion resistance via powder metallurgy. The preparation consists of immersion of the magnesium powder in hydrofluoric acid followed by compacting by spark plasma sintering. Therefore, the material with the continuous network of MgF2 is prepared. Such material is characterized by highly decreased corrosion rate and improved mechanical properties compared to the sintered uncoated powder. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The high corrosion rate of magnesium and its alloys limits their use as materials for biodegradable implants, however, corrosion resistance and also mechanical properties are highly affected by the manufacturing process. Powder metallurgy route is considered as a powerful processing technique which can be used to improve mechanical properties and corrosion resistance of final products [1,2]. Various methods of compacting are known, however, extrusion is used frequently. During this process, individual particles of the powder are strongly deformed and surface cover is destroyed. On the contrary, spark plasma sintering (SPS) preserve individual particles with the original surface of the powder particles [3]. In addition, SPS is a very fast compacting process which takes several minutes at medium temperatures compared to the conventional methods of sintering. Corrosion resistance of magnesium alloys can be improved by protective coatings [4–6]. Fluoride coatings are well known thin layers (0.1–4 mm) [7–9], with excellent bonding strength in the range from 33 to 43 MPa [7,10]. The cytotoxicity tests were successfully applied on magnesium fluoride coatings with promising results [10,11]. Moreover, the antibacterial properties of fluoride
⇑ Corresponding author. E-mail address: [email protected] (D. Dvorsky). https://doi.org/10.1016/j.matlet.2018.05.052 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
coating were discovered [10] and magnesium fluoride coatings were also successfully tested on animals [8,12,13]. Present research offers a unique combination of coating technology and powder metallurgy. After preparation of MgF2 protective layer on powder particles, the continuous network of the MgF2 layer will remain in the microstructure after compaction. Based on the characteristics of the MgF2 network and also conditions of SPS, mechanical properties and corrosion rate of final products can be controlled.
2. Materials and methods Atomized powder of commercial magnesium (50 g) with round shaped particles with a diameter ranging between 50 and 300 mm (Fig. 1a) was surrounded by a very thin oxide film. The dangerous impurities of the powder were investigated by the ICP-MS (Elan DRC-e) (90 ppm Fe, 10 ppm Cu, 20 ppm Ni). The powder was immersed and stirred in 300 ml of 40% HF for 96 h. The HF was then poured out and the powder was rinsed with distilled water multiple times and filtered through filter paper and rinsed with ethanol multiple times. The powder was then desiccated at 50 °C. The immersed powder is pictured in Fig. 1b. Thin coating (1–2 lm) of MgF2 was created on the surface. Coated and uncoated atomized powder was processed by SPS method at 500 °C with heat rate 100 °C/min and 7 kN pressure
D. Dvorsky et al. / Materials Letters 227 (2018) 78–81
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Fig. 1. The microstructure (SEM and EDS) of A) Mg powder, B) Mg powder immersed in HF (with EDS analysis of F), C) Mg, D) Mg-MgF2 composite (with EDS analysis of F).
level and with operation time 10 min. The SPS machine HP D 10 FCT system GmbH was used. The final cylindrical rods had a diameter of 20 mm and height of approximately 10 mm. The microstructures of the compact materials were characterized by electron scanning microscope (SEM – TescanVEGA3) with energy dispersion spectrometry (EDS, AZtec). Porosity was evaluated by image analysis (ImageJ) of 10 cuts. Samples were ground on SiC grinding papers (P80-P2500) and polished on diamond paste D3, D2, and D0.7. The final polishing was done on Etosil E. Compressive and tensile tests were performed on LabTest 5.250SP1-VM at room temperature. The specimens for compressive tests were rectangular (5 5 7 mm), while the specimens for the tensile tests were ‘‘dog-bone” shaped samples with 10 mm in length, 2 mm in thickness and 4 mm in width in the constricted area. The strain rate of 0.001 s 1 was used. Basic mechanical data were evaluated. Immersion tests were performed in simulated body fluid (SBF) at 37 °C for 14 days. The ratio of solution volume to the surface area was 100 mlcm 2. After 14 days, samples were removed from the immersion solution and were rinsed in distilled water and dried. The corrosion products were removed by the solution of 200 gl 1 CrO3, 10 gl 1 AgNO3, 20 gl 1 Ba(NO3)2 at room temperature. Samples were then dried and weighted. The corrosion rate was calculated from weight changes.
3. Results and discussion 3.1. Microstructure Both as-received magnesium powder and magnesium powder processed in hydrofluoric acid were sintered by SPS. The powder processed in hydrofluoric is covered by MgF2. The coating adapted to the new shape of particles and it was preserved after sintering. Therefore, individual powder particles and the network of oxide or fluoride are distinguishable in both specimens after SPS (Fig. 1c and d). Moreover, the fluoride layer may influence the Joule’s heat during the sintering process as it creates non-conductive layer [14]. The porosity of Mg was evaluated by image analysis and reached only 0.3%, while the composite material had the porosity below 0.1%. The thickness of the MgF2 layer was identified by line scan analysis and ranged between 3.4 and 3.8 mm. The total amount of F on the surface of sintered samples was 2 wt%. The average grain size of pure Mg was approximately 19 ± 3 mm after sintering, which was the similar value compared to the atomized powder. 3.2. Mechanical properties The compressive and tensile curves are summarized in Fig. 2a. Mechanical properties of Mg-MgF2 composite were superior com-
80
D. Dvorsky et al. / Materials Letters 227 (2018) 78–81
Fig. 2. A) Compressive properties (dashed line) and tensile properties (solid line), B) Fracture surface of Mg, C) Fracture surface of Mg-MgF2 composite.
Fig. 3. The cuts (A and B) and the surfaces (C and D) of the material after corrosion (SEM and EDS) A and C) Mg; B and D) Mg-MgF2 composite.
D. Dvorsky et al. / Materials Letters 227 (2018) 78–81
pared to Mg, and such properties are comparable to that of as-casted magnesium ingot [15]. In the present case, the ultimate tensile strength of the Mg/MgF2 corresponds to the average bonding strength of the fluoride coatings (33–45 MPa) [7,10], and therefore, this is considered as the main factor affecting material strength. Hence, cohesion between Mg/MgF2 is considered as the main reason for slightly improved values of ultimate tensile strength compared to Mg. The connection of particles through the fluoride layer is also confirmed by the EDS analysis of the fracture surface (Fig. 2c). The fracture spreads through oxide and fluoride layers on the original particle surface. Improvement of mechanical properties can be easily obtained by variation of the thickness of surface layers on particles which can be achieved by proper chemical treatment of powders. 3.3. Corrosion properties The corrosion rate of sintered Mg after 14 days of immersion in SBF was 2.2 ± 0.2 mmy 1. The Mg-MgF2 composite showed the reduction of the corrosion rate to just 0.7 ± 0.1 mmy 1. In Fig. 3a one can see that the corrosion of Mg proceeds around individual particles at the interface of oxide and magnesium matrix. Such behavior causes fast moving of the corrosion front inside the material and releasing of whole powder particles of magnesium. On the contrary, Fig. 3b shows that the corrosion spreading is slowed down on the fluoride layer in the Mg-MgF2 composite material. Such behavior is illustrated by the amount of F on the surface of the corroded sample as it raised from 2 wt% to 12 wt%. Mg-MgF2 composite material degraded differently compared to pure Mg as can be seen in Fig. 3. The shells of the fluoride layers are preserved even after removing the corrosion products. Although MgF2 protective network significantly slows down the degradation process, the corrosion is not stopped completely. During the dissolution of particle that is in contact with corrosion environment, corrosion can be spread to the neighboring particle through the weakest part of the MgF2 layer. Although corrosion can spread between particles, the material is generally corroded homogenously because many corrosion barriers in the form of MgF2 are presented in the material. Presented elementary results document huge potential in developing new biodegradable materials with tailored mechanical and corrosion properties by the combination of chemical processing of metallic powders and powder metallurgy techniques. However, a lot of scientific work is still necessary to tune conditions of chemical pre-treatment, compaction by different techniques or simply to determine the effect of powder size on resulting properties. 4. Conclusion Powders of pure magnesium were successfully immersed in hydrofluoric acid to form a protective MgF2 coating on the surface of particles. Such chemically treated powders were compacted by spark plasma sintering to form the composite material with Mg matrix and continuous network of MgF2 at particle interfaces with
81
about 3.5 mm in thickness. Both tensile and compressive mechanical properties were increased compared to the Mg, moreover, corrosion tests performed in SBF revealed that MgF2 network caused almost 70% reduction in corrosion rate. The obtained corrosion rate of 0.7 mmy 1 is a surprisingly low value for magnesium. Considered material processing can be easily applied also for other magnesium alloys and even better mechanical and corrosion properties may be obtained by variations in procedures. Nevertheless, this method opens doors for preparation of various magnesium composite materials with tailored corrosion and mechanical properties for applications in medicine. Acknowledgment Authors wish to thank the Czech Science Foundation project no. P108/12/G043 for the financial support of this research. References [1] G.S. Upadhyaya, Powder Metallurgy Technology, Cambridge International Science Publishing, 1998. ˇ avojsky´, D. Vojteˇch, N. Beronská, M. Fousová, [2] J. Kubásek, D. Dvorsky´, M. C Superior properties of Mg–4Y–3RE–Zr alloy prepared by powder metallurgy, J. Mater. Sci. Technol. 33 (2017) 652–660. [3] D. Dvorsky, J. Kubasek, D. Vojtech, M. Cavojsky, Structure and mechanical properties of WE43 prepared by powder metallurgy route, Manuf. Technol. 16 (2016) 896–902. [4] L.-Y. Cui, S.-D. Gao, P.-P. Li, R.-C. Zeng, F. Zhang, S.-Q. Li, E.-H. Han, Corrosion resistance of a self-healing micro-arc oxidation/polymethyltrimethoxysilane composite coating on magnesium alloy AZ31, Corros. Sci. 118 (2017) 84–95. [5] L.-Y. Cui, H.-P. Liu, W.-L. Zhang, Z.-Z. Han, M.-X. Deng, R.-C. Zeng, S.-Q. Li, Z.-L. Wang, Corrosion resistance of a superhydrophobic micro-arc oxidation coating on Mg-4Li-1Ca alloy, J. Mater. Sci. Technol. 33 (2017) 1263–1271. [6] D. Zhang, Z. Qi, H. Shen, B. Wei, Y. Zhang, Z. Wang, In vitro degradation and cytocompatibility of magnesium alloy coated with Hf/PLLA duplex coating, Mater. Lett. 213 (2018) 249–252. [7] M. Ren, S. Cai, T. Liu, K. Huang, X. Wang, H. Zhao, S. Niu, R. Zhang, X. Wu, Calcium phosphate glass/MgF2 double layered composite coating for improving the corrosion resistance of magnesium alloy, J. Alloy. Compd. 591 (2014) 34–40. [8] J.-H. Jo, B.-G. Kang, K.-S. Shin, H.-E. Kim, B.-D. Hahn, D.-S. Park, Y.-H. Koh, Hydroxyapatite coating on magnesium with MgF2 interlayer for enhanced corrosion resistance and biocompatibility, J. Mater. Sci. -Mater. Med. 22 (2011) 2437–2447. [9] D. Dvorsky, J. Kubasek, D. Vojtech, Corrosion protection of WE43 magnesium alloy by fluoride conversion coating, Manuf. Technol. 17 (2017) 440–446. [10] T. Yan, L. Tan, B. Zhang, K. Yang, Fluoride conversion coating on biodegradable AZ31B magnesium alloy, J. Mater. Sci. Technol. 30 (2014) 666–674. [11] X. Liu, Z. Zhen, J. Liu, T. Xi, Y. Zheng, S. Guan, Y. Zheng, Y. Cheng, Multifunctional MgF2/polydopamine coating on Mg alloy for vascular stent application, J. Mater. Sci. Technol. 31 (2015) 733–743. [12] J.E. Sun, J. Wang, H. Jiang, M. Chen, Y. Bi, D. Liu, In vivo comparative property study of the bioactivity of coated Mg–3Zn–0.8Zr alloy, Mater. Eng.: C 33 (2013) 3263–3272. [13] C.M. Weber, R. Eifler, J.-M. Seitz, H.J. Maier, J. Reifenrath, T. Lenarz, M. Durisin, Biocompatibility of MgF2-coated MgNd2 specimens in contact with mucosa of the nasal sinus – a long term study, Acta Biomater. 18 (2015) 249–261. [14] Z.A. Munir, U. Anselmi-Tamburini, M. Ohyanagi, The effect of electric field and pressure on the synthesis and consolidation of materials: a review of the spark plasma sintering method, J. Mater. Sci. 41 (3) (2006) 763–777. [15] D. Ahmadkhaniha, A. Järvenpää, M. Jaskari, M.H. Sohi, A. Zarei-Hanzaki, M. Fedel, F. Deflorian, L.P. Karjalainen, Microstructural modification of pure Mg for improving mechanical and biocorrosion properties, J. Mech. Behav. Biomed. Mater. 61 (2016) 360–370.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Featured Letter
A new approach in the preparation of biodegradable Mg-MgF2 composites with tailored corrosion and mechanical properties by powder metallurgy Drahomir Dvorsky ⇑, Jiri Kubasek, Dalibor Vojtech Faculty of Chemical Technology, Department of Metals and Corrosion Engineering, University of Chemistry and Technology Prague, Technická 5 166 28 Praha 6 – Dejvice, Czech Republic
a r t i c l e
i n f o
Article history: Received 13 March 2018 Received in revised form 17 April 2018 Accepted 11 May 2018
Keywords: Biomaterials Composite material Corrosion Powder technology Sintering
a b s t r a c t This paper presents an innovative way of preparation of the magnesium-fluoride composite material with improved corrosion resistance via powder metallurgy. The preparation consists of immersion of the magnesium powder in hydrofluoric acid followed by compacting by spark plasma sintering. Therefore, the material with the continuous network of MgF2 is prepared. Such material is characterized by highly decreased corrosion rate and improved mechanical properties compared to the sintered uncoated powder. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The high corrosion rate of magnesium and its alloys limits their use as materials for biodegradable implants, however, corrosion resistance and also mechanical properties are highly affected by the manufacturing process. Powder metallurgy route is considered as a powerful processing technique which can be used to improve mechanical properties and corrosion resistance of final products [1,2]. Various methods of compacting are known, however, extrusion is used frequently. During this process, individual particles of the powder are strongly deformed and surface cover is destroyed. On the contrary, spark plasma sintering (SPS) preserve individual particles with the original surface of the powder particles [3]. In addition, SPS is a very fast compacting process which takes several minutes at medium temperatures compared to the conventional methods of sintering. Corrosion resistance of magnesium alloys can be improved by protective coatings [4–6]. Fluoride coatings are well known thin layers (0.1–4 mm) [7–9], with excellent bonding strength in the range from 33 to 43 MPa [7,10]. The cytotoxicity tests were successfully applied on magnesium fluoride coatings with promising results [10,11]. Moreover, the antibacterial properties of fluoride
⇑ Corresponding author. E-mail address: [email protected] (D. Dvorsky). https://doi.org/10.1016/j.matlet.2018.05.052 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
coating were discovered [10] and magnesium fluoride coatings were also successfully tested on animals [8,12,13]. Present research offers a unique combination of coating technology and powder metallurgy. After preparation of MgF2 protective layer on powder particles, the continuous network of the MgF2 layer will remain in the microstructure after compaction. Based on the characteristics of the MgF2 network and also conditions of SPS, mechanical properties and corrosion rate of final products can be controlled.
2. Materials and methods Atomized powder of commercial magnesium (50 g) with round shaped particles with a diameter ranging between 50 and 300 mm (Fig. 1a) was surrounded by a very thin oxide film. The dangerous impurities of the powder were investigated by the ICP-MS (Elan DRC-e) (90 ppm Fe, 10 ppm Cu, 20 ppm Ni). The powder was immersed and stirred in 300 ml of 40% HF for 96 h. The HF was then poured out and the powder was rinsed with distilled water multiple times and filtered through filter paper and rinsed with ethanol multiple times. The powder was then desiccated at 50 °C. The immersed powder is pictured in Fig. 1b. Thin coating (1–2 lm) of MgF2 was created on the surface. Coated and uncoated atomized powder was processed by SPS method at 500 °C with heat rate 100 °C/min and 7 kN pressure
D. Dvorsky et al. / Materials Letters 227 (2018) 78–81
79
Fig. 1. The microstructure (SEM and EDS) of A) Mg powder, B) Mg powder immersed in HF (with EDS analysis of F), C) Mg, D) Mg-MgF2 composite (with EDS analysis of F).
level and with operation time 10 min. The SPS machine HP D 10 FCT system GmbH was used. The final cylindrical rods had a diameter of 20 mm and height of approximately 10 mm. The microstructures of the compact materials were characterized by electron scanning microscope (SEM – TescanVEGA3) with energy dispersion spectrometry (EDS, AZtec). Porosity was evaluated by image analysis (ImageJ) of 10 cuts. Samples were ground on SiC grinding papers (P80-P2500) and polished on diamond paste D3, D2, and D0.7. The final polishing was done on Etosil E. Compressive and tensile tests were performed on LabTest 5.250SP1-VM at room temperature. The specimens for compressive tests were rectangular (5 5 7 mm), while the specimens for the tensile tests were ‘‘dog-bone” shaped samples with 10 mm in length, 2 mm in thickness and 4 mm in width in the constricted area. The strain rate of 0.001 s 1 was used. Basic mechanical data were evaluated. Immersion tests were performed in simulated body fluid (SBF) at 37 °C for 14 days. The ratio of solution volume to the surface area was 100 mlcm 2. After 14 days, samples were removed from the immersion solution and were rinsed in distilled water and dried. The corrosion products were removed by the solution of 200 gl 1 CrO3, 10 gl 1 AgNO3, 20 gl 1 Ba(NO3)2 at room temperature. Samples were then dried and weighted. The corrosion rate was calculated from weight changes.
3. Results and discussion 3.1. Microstructure Both as-received magnesium powder and magnesium powder processed in hydrofluoric acid were sintered by SPS. The powder processed in hydrofluoric is covered by MgF2. The coating adapted to the new shape of particles and it was preserved after sintering. Therefore, individual powder particles and the network of oxide or fluoride are distinguishable in both specimens after SPS (Fig. 1c and d). Moreover, the fluoride layer may influence the Joule’s heat during the sintering process as it creates non-conductive layer [14]. The porosity of Mg was evaluated by image analysis and reached only 0.3%, while the composite material had the porosity below 0.1%. The thickness of the MgF2 layer was identified by line scan analysis and ranged between 3.4 and 3.8 mm. The total amount of F on the surface of sintered samples was 2 wt%. The average grain size of pure Mg was approximately 19 ± 3 mm after sintering, which was the similar value compared to the atomized powder. 3.2. Mechanical properties The compressive and tensile curves are summarized in Fig. 2a. Mechanical properties of Mg-MgF2 composite were superior com-
80
D. Dvorsky et al. / Materials Letters 227 (2018) 78–81
Fig. 2. A) Compressive properties (dashed line) and tensile properties (solid line), B) Fracture surface of Mg, C) Fracture surface of Mg-MgF2 composite.
Fig. 3. The cuts (A and B) and the surfaces (C and D) of the material after corrosion (SEM and EDS) A and C) Mg; B and D) Mg-MgF2 composite.
D. Dvorsky et al. / Materials Letters 227 (2018) 78–81
pared to Mg, and such properties are comparable to that of as-casted magnesium ingot [15]. In the present case, the ultimate tensile strength of the Mg/MgF2 corresponds to the average bonding strength of the fluoride coatings (33–45 MPa) [7,10], and therefore, this is considered as the main factor affecting material strength. Hence, cohesion between Mg/MgF2 is considered as the main reason for slightly improved values of ultimate tensile strength compared to Mg. The connection of particles through the fluoride layer is also confirmed by the EDS analysis of the fracture surface (Fig. 2c). The fracture spreads through oxide and fluoride layers on the original particle surface. Improvement of mechanical properties can be easily obtained by variation of the thickness of surface layers on particles which can be achieved by proper chemical treatment of powders. 3.3. Corrosion properties The corrosion rate of sintered Mg after 14 days of immersion in SBF was 2.2 ± 0.2 mmy 1. The Mg-MgF2 composite showed the reduction of the corrosion rate to just 0.7 ± 0.1 mmy 1. In Fig. 3a one can see that the corrosion of Mg proceeds around individual particles at the interface of oxide and magnesium matrix. Such behavior causes fast moving of the corrosion front inside the material and releasing of whole powder particles of magnesium. On the contrary, Fig. 3b shows that the corrosion spreading is slowed down on the fluoride layer in the Mg-MgF2 composite material. Such behavior is illustrated by the amount of F on the surface of the corroded sample as it raised from 2 wt% to 12 wt%. Mg-MgF2 composite material degraded differently compared to pure Mg as can be seen in Fig. 3. The shells of the fluoride layers are preserved even after removing the corrosion products. Although MgF2 protective network significantly slows down the degradation process, the corrosion is not stopped completely. During the dissolution of particle that is in contact with corrosion environment, corrosion can be spread to the neighboring particle through the weakest part of the MgF2 layer. Although corrosion can spread between particles, the material is generally corroded homogenously because many corrosion barriers in the form of MgF2 are presented in the material. Presented elementary results document huge potential in developing new biodegradable materials with tailored mechanical and corrosion properties by the combination of chemical processing of metallic powders and powder metallurgy techniques. However, a lot of scientific work is still necessary to tune conditions of chemical pre-treatment, compaction by different techniques or simply to determine the effect of powder size on resulting properties. 4. Conclusion Powders of pure magnesium were successfully immersed in hydrofluoric acid to form a protective MgF2 coating on the surface of particles. Such chemically treated powders were compacted by spark plasma sintering to form the composite material with Mg matrix and continuous network of MgF2 at particle interfaces with
81
about 3.5 mm in thickness. Both tensile and compressive mechanical properties were increased compared to the Mg, moreover, corrosion tests performed in SBF revealed that MgF2 network caused almost 70% reduction in corrosion rate. The obtained corrosion rate of 0.7 mmy 1 is a surprisingly low value for magnesium. Considered material processing can be easily applied also for other magnesium alloys and even better mechanical and corrosion properties may be obtained by variations in procedures. Nevertheless, this method opens doors for preparation of various magnesium composite materials with tailored corrosion and mechanical properties for applications in medicine. Acknowledgment Authors wish to thank the Czech Science Foundation project no. P108/12/G043 for the financial support of this research. References [1] G.S. Upadhyaya, Powder Metallurgy Technology, Cambridge International Science Publishing, 1998. ˇ avojsky´, D. Vojteˇch, N. Beronská, M. Fousová, [2] J. Kubásek, D. Dvorsky´, M. C Superior properties of Mg–4Y–3RE–Zr alloy prepared by powder metallurgy, J. Mater. Sci. Technol. 33 (2017) 652–660. [3] D. Dvorsky, J. Kubasek, D. Vojtech, M. Cavojsky, Structure and mechanical properties of WE43 prepared by powder metallurgy route, Manuf. Technol. 16 (2016) 896–902. [4] L.-Y. Cui, S.-D. Gao, P.-P. Li, R.-C. Zeng, F. Zhang, S.-Q. Li, E.-H. Han, Corrosion resistance of a self-healing micro-arc oxidation/polymethyltrimethoxysilane composite coating on magnesium alloy AZ31, Corros. Sci. 118 (2017) 84–95. [5] L.-Y. Cui, H.-P. Liu, W.-L. Zhang, Z.-Z. Han, M.-X. Deng, R.-C. Zeng, S.-Q. Li, Z.-L. Wang, Corrosion resistance of a superhydrophobic micro-arc oxidation coating on Mg-4Li-1Ca alloy, J. Mater. Sci. Technol. 33 (2017) 1263–1271. [6] D. Zhang, Z. Qi, H. Shen, B. Wei, Y. Zhang, Z. Wang, In vitro degradation and cytocompatibility of magnesium alloy coated with Hf/PLLA duplex coating, Mater. Lett. 213 (2018) 249–252. [7] M. Ren, S. Cai, T. Liu, K. Huang, X. Wang, H. Zhao, S. Niu, R. Zhang, X. Wu, Calcium phosphate glass/MgF2 double layered composite coating for improving the corrosion resistance of magnesium alloy, J. Alloy. Compd. 591 (2014) 34–40. [8] J.-H. Jo, B.-G. Kang, K.-S. Shin, H.-E. Kim, B.-D. Hahn, D.-S. Park, Y.-H. Koh, Hydroxyapatite coating on magnesium with MgF2 interlayer for enhanced corrosion resistance and biocompatibility, J. Mater. Sci. -Mater. Med. 22 (2011) 2437–2447. [9] D. Dvorsky, J. Kubasek, D. Vojtech, Corrosion protection of WE43 magnesium alloy by fluoride conversion coating, Manuf. Technol. 17 (2017) 440–446. [10] T. Yan, L. Tan, B. Zhang, K. Yang, Fluoride conversion coating on biodegradable AZ31B magnesium alloy, J. Mater. Sci. Technol. 30 (2014) 666–674. [11] X. Liu, Z. Zhen, J. Liu, T. Xi, Y. Zheng, S. Guan, Y. Zheng, Y. Cheng, Multifunctional MgF2/polydopamine coating on Mg alloy for vascular stent application, J. Mater. Sci. Technol. 31 (2015) 733–743. [12] J.E. Sun, J. Wang, H. Jiang, M. Chen, Y. Bi, D. Liu, In vivo comparative property study of the bioactivity of coated Mg–3Zn–0.8Zr alloy, Mater. Eng.: C 33 (2013) 3263–3272. [13] C.M. Weber, R. Eifler, J.-M. Seitz, H.J. Maier, J. Reifenrath, T. Lenarz, M. Durisin, Biocompatibility of MgF2-coated MgNd2 specimens in contact with mucosa of the nasal sinus – a long term study, Acta Biomater. 18 (2015) 249–261. [14] Z.A. Munir, U. Anselmi-Tamburini, M. Ohyanagi, The effect of electric field and pressure on the synthesis and consolidation of materials: a review of the spark plasma sintering method, J. Mater. Sci. 41 (3) (2006) 763–777. [15] D. Ahmadkhaniha, A. Järvenpää, M. Jaskari, M.H. Sohi, A. Zarei-Hanzaki, M. Fedel, F. Deflorian, L.P. Karjalainen, Microstructural modification of pure Mg for improving mechanical and biocorrosion properties, J. Mech. Behav. Biomed. Mater. 61 (2016) 360–370.