synthesis of reinforced magnesium embedded in

10
1 SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN CARBON MATRIX BY USING THERMIONIC VACUUM ARC (TVA) TECHNOLOGY R. VLADOIU, A. MANDES, V. DINCA, G. PRODAN, V. CIUPINA Department of Plasma Physics, Faculty of Applied Sciences and Engineering, Ovidius University, Mamaia 124, Constanţa, 900527, Romania Abstract. The aim of this work is to control the synthesis of nanocrystalline magnesium embedded in hydrogen-free amorphous carbon (a-C) matrix by Thermionic Vacuum Arc (TVA) method on glass, silicon and OLC 45 special substrate. Nanostructured coatings with homogenous surface without any faults (pinholes and cracks) were achieved at low temperatures to not affect the materials properties. The results on the morphology, composition and wettability of the coatings were investigated in terms of TEM, SEM with X-ray detection (SEM/EDX) and free surface energy (See System). Reports on optimized coatings by a graded structure and adjusting stress level were also discussed. Key words: C-Mg thin films, TVA method, TEM, SEM 1. INTRODUCTION Magnesium is a rather strong, silvery-white, light-weight metal, practically two-thirds the density of aluminum and one-fours of iron [1]. Moreover, magnesium is one of the lightest hydrogen storage material known [2 - 4]; it is also abundant and inexpensive, but some properties such high binding energy of hydrogen and the slow kinetics of uptake still could not satisfy the demands of application fields. With the expanding use of magnesium materials within the automotive and consumer goods sectors, there is currently growing interest in developing new applications [5, 6]. It is well-known that one way to alter the grain size of the material in the controlled way is by using the co-deposition of two materials. The desired material for combination is carbon which ensures both high lubrication of the composite layer and high resistance against chemical and corrosive environment attack [7]. Amorphous diamond-like carbons (DLC or a-C:H) have great potential in applications where tribologically stressed systems require low friction coefficients or wear resistance. In the case of carbon, the previous work proved that Thermionic Vacuum Arc method can be used to produce smooth DLC films with relatively low growth stress and high nanohardness with a controlled final percentage of sp 3 content, depending on plasma parameters [8 - 10]. It was found that the sp3:sp2

Upload: truongdiep

Post on 28-Jan-2017

222 views

Category:

Documents


2 download

TRANSCRIPT

Page 1: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

1

SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN CARBON MATRIX BY USING THERMIONIC VACUUM ARC (TVA) TECHNOLOGY

R. VLADOIU, A. MANDES, V. DINCA, G. PRODAN, V. CIUPINA

Department of Plasma Physics, Faculty of Applied Sciences and Engineering, Ovidius University, Mamaia 124, Constanţa, 900527, Romania

Abstract. The aim of this work is to control the synthesis of nanocrystalline magnesium embedded in hydrogen-free amorphous carbon (a-C) matrix by Thermionic Vacuum Arc (TVA) method on glass, silicon and OLC 45 special substrate. Nanostructured coatings with homogenous surface without any faults (pinholes and cracks) were achieved at low temperatures to not affect the materials properties. The results on the morphology, composition and wettability of the coatings were investigated in terms of TEM, SEM with X-ray detection (SEM/EDX) and free surface energy (See System). Reports on optimized coatings by a graded structure and adjusting stress level were also discussed.

Key words: C-Mg thin films, TVA method, TEM, SEM

1. INTRODUCTION

Magnesium is a rather strong, silvery-white, light-weight metal, practically two-thirds the density of aluminum and one-fours of iron [1]. Moreover, magnesium is one of the lightest hydrogen storage material known [2 - 4]; it is also abundant and inexpensive, but some properties such high binding energy of hydrogen and the slow kinetics of uptake still could not satisfy the demands of application fields. With the expanding use of magnesium materials within the automotive and consumer goods sectors, there is currently growing interest in developing new applications [5, 6].

It is well-known that one way to alter the grain size of the material in the controlled way is by using the co-deposition of two materials. The desired material for combination is carbon which ensures both high lubrication of the composite layer and high resistance against chemical and corrosive environment attack [7]. Amorphous diamond-like carbons (DLC or a-C:H) have great potential in applications where tribologically stressed systems require low friction coefficients or wear resistance. In the case of carbon, the previous work proved that Thermionic Vacuum Arc method can be used to produce smooth DLC films with relatively low growth stress and high nanohardness with a controlled final percentage of sp3 content, depending on plasma parameters [8 - 10]. It was found that the sp3:sp2

Page 2: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

2 ratio is higher at higher applied voltage, when the plasma potential and consequently the carbon ion energy are higher [11]. In previous studies, the co-deposition of metals and carbon using TVA technology was presented [12 - 14]. In order to minimize the internal stress and to get films with increased adherence, interlayer and graded layer with Fe, Cr, Al and Ni was deposited [15].

For this reason, there is a great request for joining of Mg in composites such as carbon containing nanostructured thin films, giving rise to unique combination of properties as wear and corrosion-resistant films on cutting tools and diffusion barrier in semiconductor technology. However, there are still many fundamentally and technologically important questions to be answered before find much wider applications, especially at the industrial level.

Concerning with this subject, increasing attention has been paid on the environmental friendly Thermionic Vacuum Arc (TVA) technology in the last years for at least two main reasons:

1) It is an efficient method to create advanced materials, due to its excellent versatility.

2) This technology allows coating of rather complex geometries of all kind of forming tools, such as ball bearings, gearwheels, chains and camshafts.

The effect of deposition conditions on the morphology, microstructure and phase composition of the coatings were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), topographical measurements, energy dispersive spectroscopy (EDS), and free surface energy (See System).

2. EXPERIMENTAL SET -UP

The TVA electrodes' arrangement inside of the vacuum chamber is schematically shown in Fig.1. The main elements were: the cathode filament - a tungsten wire of 1.5 mm diameter one time wound, settled inside of a Wehnelt cylinder just in its front plane. The anode is a round shape crucible of 15 mm diameter and 10 mm deep, made from tungsten, filled with the material to be deposited. The thermoelectrons generated from the heated cathode are focused toward the anode surface. In this way, the grains of magnesium are evaporated; therefore a steady state density of vapors is established in the interelectrodic space. An applied DC high voltage between the electrodes will accelerate the electrons coming from the filament and likewise the vapors from the anode are ionized.

The main working parameters with great interest in this one electron gun TVA configuration are described in Table 1. There are three groups of operation conditions: geometrical- stated before to close the vacuum chamber as indicated in Fig 1, for plasma ignition – in order to be maintained in stable conditions after the breakdown of the discharge and the last typical for deposition. In the second group

Page 3: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

3 it is important to pointed out that after the breakdown, the applied power has to sustain the electrical charges in order to compensate the loses due to ambipolar diffusion and inelastic collisions.

Fig. 1 Schematic diagram of the Plasma TVA experimental set-up

A major advantage of this technology is the facts that besides the evaporated neutral atoms, on the sample are coming also energetic ions provided by the material from the anode. Due to the energetic incident ions, the obtained thin film is compact, with a roughness under a few nanometers. The thin film is growing under the bombardment of those ions of the material to be deposited in vacuum. The ions have a random energy superposed on higher directed energy towards the wall achieving values of the order of 500 V. Another great advantage of this method is the fact that no post-processing such as high-temperature annealing is necessary.

Table 1 The summary of the main parameters of the C-Mg deposition in the TVA method- the geometrical parameters according to the diagram from Fig. 1.

Geometrical parameters C-Mg plasma ignition C-Mg deposition Anode - cathode

distance: da-c = 4 mm

Intensity of the current for heating filament:

I f = 43 A

Time t = 60 s

Anode – holder distance: The applied voltage for Film thinckness

Page 4: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

4

da-h = 170 mm ignition: Ua = 1kV

t f = 104 nm

Angle of the cathode from the vertical axis:

φ = 45o

The intensity of the current before breakdown:

I a = 11 mA

Work pressure during deposition

p = 2.4 ·10-3 torr Angle of the holder from

de vertical axis: α = 30o

The applied power during TVA plasma: P = 245 W

Rate of deposition r = 1.73 nm/s

Silicon wafers, glass and stainless steel/OLC discs were used as the film

substrates samples denominated as C-Mg/Si, C-Mg/Gl and C-Mg/OLC45. The special substrate OLC45 was used for specific industrial application interest. Each substrate was ultrasonically cleaned in acetone bath for 30 min and then dried. The clean and dry substrate was mounted on a holder in the vacuum chamber. The thickness of the film was measured during the deposition using a Cressington thickness monitor MTM 10.

Investigation of the film microstructure was carried out using Transmission Electron Microscopy (TEM) performed on a Phillips CM 120 ST (acceleration voltage of 120 kV) TEM with a resolution point of 1.4 Å and a magnification of 1.2 million times.

SEM images were performed using a Zeiss EVO 50 SEM having LaB6 cathode with Bruker EDX system. Based on the SEM images, the equipment can generate characteristic X-ray, secondary and back scattered electrons. EDX measurements were carried out with a Bruker accessory fitted on the Zeiss Evo 50 scanning electron microscope. The take-off angle is 35° and the detector’s resolution is 133 eV.

Quantitative topographical measurements was performed with the Olympus LEXT OLS4000 which is a confocal microscope capable of taking high resolution 3D images. The surface free energy as well as the wettability of the films was determined by means of SEE System.

3. RESULTS AND DISCUSSION

The particle size and morphology of the sample and the corresponding selected area electron diffraction (SAED) were examined by CM120ST transmission electron microscopy (TEM) with an accelerating voltage of 100�kV. In order to perform TEM investigations, the samples were prepared using a quick method described in [16, 17]. The method is inexpensive and provides a good working area with clean sample, without chemical or mechanical artifacts.

Page 5: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

5

Fig. 2 BFTEM image for C-Mg deposited on glass (left) and on silicon (right) – inset SEAD,

respectively

Morphology was determined from BF-TEM image (Fig 2). In the case of C-Mg/Gl, the structure of the film is not uniform. From the SAED the film can be indexed as hexagonal magnesium Mg (P 6(3)/mmc, a = 0.32095Å, c = 0.52104Å [18].

For C-Mg /Si bright-field TEM images show Mg nanoparticles with various orientations with respect to the substrate. The films are made of polycrystalline magnesium grains embedded in a carbon network, the size of which depends on the carbon content, but amorphous phases cannot be excluded. The measured interatomic distances revealed a nanostructured magnesium film with a d-spacing corresponding to hexagonal structure of Mg (P 6(3)/mmc, a = 3.2095Å, c = 5.2104Å) and cubic structure of MgO (Fm3m, a = 4.2170Å) [19]

As it is known, grain size plays a significant role in resultant coating surface, not only in terms of size but also in shape. We calculate the mean diameters assuming a lognormal distribution of the experimental data, where A is an arbitrary constant and xc is the maximum of the distribution.

+=

2

2

0 2

)/(lnexp

w

xxAyy c (1)

From the data obtained by TEM micrographs, the particle size histograms can

be drawn and the mean size of the particles can be determined. The crystallites’ dimension depends on the nature of the substrate. Fig. 3 shows the particle size distribution of C-Mg nanoparticles.

Page 6: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

6

Fig. 3 Grain size distribution of the C-Mg thin films deposited on different substrates: glass (left)

and silicon (right) - FERET diameters histogram

It can be seen that in the case of C-Mg/Gl the particle sizes possess a small and narrow size distribution in a range from 15 to 55 nm, and the mean diameter is about 30.8 nm.

The maximum frequency of appearance in the case of C-Mg/ Si (maximum value of the Log Normal fitting function) increased at following values: 55.64 nm (first peak), and 93.94 nm diameter (second peak) as can be derived from the histograms.

The technique of Scanning Electron Microscopy (SEM) was employed extensively throughout this study to examine images of both the surface morphology and 3D structure of corrosion deposits and films on samples of glass and special substrate OLC 45. The SEM images of the films show smooth and uniform surface without cracks and well covered to the OLC 45 substrate.

Page 7: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

7

Fig.4 SEM images of C-Mg samples in 2D and 3D: C-Mg/Gl (up) and C-Mg/OLC 45 (down)

Fig. 4 indicates that Mg particles embedded in the amorphous matrix are very fine, spherical and uniform, especially those deposited on glass, its presence being confirmed by the electrons diffraction (SAED). In the case of C-Mg deposited on OLC substrate the topography is according to the expectation, because these interesting samples were not polished before as glass or silicon. However, the industrial partner was satisfied with the properties of the films because the lifetime of the post-service of the coated pieces was drastically improved. Fig. 5 shows the surface morphology of the C-Mg nanocomposite’ films deposited on OLC substrate and the EDS mappings of the deposited elements.

Page 8: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

8

Fig. 5 Surface morphology of the C-Mg nanocomposites (magnification: 10000 x) Mg: red color and

C: green color

The contact angle measurements have shown reproducible results with an average value of 91.29 degrees for water and 88.40 degrees for ethylene glycol. The surface energy of the C-Mg film deposited on silicon substrate was approximately 19 mJ/m2.

4. CONCLUSIONS

The nanostructured films exhibited crystalline structure, more relevant in the case of C-Mg/Si, where the performed electron diffraction shows even polycrystalline structure with intensities of the diffracted beams higher than in the case of C-Mg/Gl. The measured interatomic distances on the C-Mg deposited on glass revealed a nanostructured magnesium film with a d-spacing corresponding to hexagonal structure of Mg. In the case of C-Mg films deposited on silicon beside hexagonal structure of Mg there is also cubic structure of MgO.

The mean values of the grain size evaluated from BFTEM images are of 30 nm for C-Mg/Gl and 55nm and 93nm for C-Mg/Si, respectively. The two peaks from the grain size histogram of C-Mg/Si are quite difficult to be attributed to the phases of the films for Mg or MgO, but in comparison with the SAED intensities of the diffraction pattern, we assumed 50 nm for MgO, and 93 nm for Mg respectively. Regarding the C-Mg/OLC samples, the industrial partner was very satisfied with the results of the films because the lifetime of the post-service of the coated pieces was drastically improved.

The surface free energy (SFE) determined by means of Surface Energy Evaluation System (See System) indicated a hydrophobic character for C-Mg thin films. TVA technology is a convenient, inexpensive process, has high productive capacity and the potential for further mass production in the industry.

Page 9: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

9 Acknowledgements: This work was supported by CNDI–UEFISCDI, project 78/2013, PN-II-ID-PCE-2012-4-0059

REFERENCES

1. Y. Kojima, Platform Science and Technology for Advanced Magnesium Alloys, Mater. Sci. Forum 350-351 3-18 (2000) 2. L. Pranevicius, D. Milcius, C. Templier, The effects of dynamic structural transformations on hydrogenation properties of Mg and MgNi thin films, Int. J. Hydrogen Energy 34 5131–5137 (2009) 3. G. Siviero, V. Bello, G. Mattei, P. Mazzoldi, Structural evolution of Pd-capped Mg thin films under H2 absorption and desorption cycles, Int. J. Hydrogen Energy 34 4817–4826 (2009) 4. Y. Zhang, J.A. Sharon, G.L. Hu, K.T. Ramesh, K.J. Hemker, Stress-driven grain growth in ultrafine grained Mg thin film, Scr. Mater. 68 424–427 (2013) 5. P. Fu, L. Peng, H. Jiang, Z. Zhang, C. Zhai, Fracture behavior and mechanical properties of Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) alloy at room temperature, Mater. Sci. Eng. A 486 572–579 (2008) 6. J. E. Gray, B. Luan, Protective coatings on magnesium and its alloys--a critical review, J. Alloys Compd. 336 88-113 (2002) 7. M. Rubio-Roy, C. Corbella, E. Bertran, S. Portal, M.C. Polo, E. Pascual, J.L. Andújar, Effects of environmental conditions on fluorinated diamond-like carbon tribology, Diamond and Relat. Mater. 18 923–926 (2009) 8. G. Musa, I. Mustata, M. Blideran V. Ciupina, R. Vlădoiu, G. Prodan, E. Vasile, H Ehrich, Thermionic Vacuum Arc (TVA) new technique for high purity carbon thin film deposition, Acta Physica Slovaka 55 417-421 (2005) 9. G. Musa, R. Vladoiu, V. Ciupina, J. Janik, Raman spectra of carbon thin films, J Optoelectron Adv M 8 621-623 (2006) 10. R.Vladoiu, V.Ciupina, C. Surdu-Bob, C. P. Lungu, J. Janik, J. D. Skalny, V. Bursikova, J. Bursik, G. Musa, Properties of the carbon thin films deposited by thermionic vacuum arc, J Optoelectron Adv M 9 862-866 (2007) 11. C. Surdu Bob, R. Vladoiu, M. Badulescu, G. Musa, Control over the sp2/sp3 ratio by tunning plasma parameters of the Thermionic Vacuum Arc, Diamond and Relat. Mater, 17 1625-1628 (2008) 12. R.Vladoiu, V. Ciupina, M. Contulov, A. Mandes, V. Dinca, G. Prodan, C. P Lungu, Structure and tribological properties of carbon based nanocomposites grown by TVA method, J Optoelectron Adv M 12 553-556 (2010) 13. V. Ciupina, R. Vladoiu, C.P. Lungu, V. Dinca, M. Contulov, A. Mandes, P. Popov, G. Prodan, Investigation of the SiC thin films synthetized by Thermionic Vacuum Arc method (TVA), EPJD 66 89 (2012)

Page 10: SYNTHESIS OF REINFORCED MAGNESIUM EMBEDDED IN

10 14. R. Vladoiu, V. Ciupina, A. Mandes, M. Contulov, V. Dinca, P. Popov, C. P. Lungu, Tribological properties of carbon-tungsten nanocomposites synthetized by thermionic vacuum arc (TVA) method, Romanian Reports of Physics, 64 1053-1060 (2011) 15. C.P. Lungu, I. Mustata, G. Musa, A.M. Lungu, O. Brinza, C. Moldovan, C. Rotaru, R. Iosub, F. Sava, M. Popescu, R. Vladoiu, V. Ciupina, G. Prodan, N. Apetroaei, Unstressed carbon-metal films deposited by Thermionic Vacuum Arc method, J Optoelectron Adv M 8 74-77 (2006) 16. V. S. Teodorescu, M-G. Blanchin - Microscopy and Microanalysis 15 15 (2009) 17. WWW-MINCRYST (2011). Crystallographic and Crystallochemical Database for minerals and their structural analogues. 18. WWW-MINCRYST, MAGNESIUM-2671. 19. WWW-MINCRYST, PERICLASE-3533.