Effet de Silicium sur les propriétés de Ti déposé par évaporation thermique sous vide
D.Dergham, L.Chekour,N.Madaoui, S.Hassani, M.Ouchabane, F.Lakoui
1Advanced Technology Development Center CDTA
“City 20th August 1956 Baba Hassen 5 July 1962 ،16303 »
2Laboratory of Microstructures and Defects in Materials Constantine 1University, Algeria Road of Ain El Bey Constantine
Email:ddergham1@gmail.com tel: 213(0)21 35 10 18
Abstract.
In this work, structural, morphological, and mechanical properties of Ti-Si thin films grown by Vacuum Thermal Evaporation were investigated. A series of Ti-Sicoatings have been deposited byvacuum thermal evaporation technique, on Z200 steel and Si (100) substrates.
180 mg of titanium powder and,4at.%, 7at.%, 11at.%, 17 at.%of Silicon grains were used as deposition source.X-ray diffraction, Scanning Electronic Microscopy were employed for structural and morphological study of the films, nano-indentation hardness testing test was used to evaluate the mechanical properties, the corrosionbehaviors of the coatings were studied in aerated 3.5 wt. % NaCl aqueous solutions by interpretation of theelectrochemical anodic polarization curves.The X-Ray Diffraction patterns reveal that all films are polycrystalline and matched those of α-Ti , Ti5Si3, Ti-Si, and Ti4Si3,the hardness and the Young’s Modulus increase firstly to achieve a maximum value 33 GPa, and 795 GPa then decrease smoothly with a further addition of Si to a weak value
Keys Words:Ti-Si, Vacuum Thermal Evaporation
1. Introduction.
In the last decades a great deal of interest was focused to Titanium alloy due to a lot of practical desirable properties as high strength, specific modulus and good corrosion resistance [1, 2]
Ti–Si coatings have become moreand more attractive for their outstanding performance (e.g., highhardness, excellent wear resistance, good thermal and chemicalstability) [1–2]
A series of Ti–Si thin films with different Si content were deposited on steel Z 200 substrate using a powder of titanium, and silicon grains, by vacuum thermal evaporation, the silicon effect on the microstructure, mechanical properties of the films were investigated. X-ray diffraction and nanoindentation technique were employed tocharacterize the as deposited films.
The coating was found to be polycrystalline; Fig. 1 shows the XRD patterns of Ti-Si films deposited on Steel Z200 with various Si contents, and exhibit a high hardness of 33 GPa when the Si content was 11 at. %.A further adding of Silicon decreases the hardness,The corrosion behaviors of the coatings were studied in aerated 3.5 wt.% NaCl aqueous solutions by interpretation of the electrochemical anodic polarization curves
2. Experimental details
Ti–Si coatings were deposited onto Z200 tool steel and Si (100) wafer by vacuum thermal evaporation technique; the materials used in this study were powder Titanium with the purity of 99%, and grains polycrystalline Silicon.The samples were machined into cylinderwith a dimension of 20 mm × 3 mm. Then they were polished to reduce their surface roughness.
All substrates steel and Silicon were cleaned with ethanol and acetone, and then rinsed in ionizedwater, before placing in inside the vacuum chamber, this one was cleaned, the powder Titanium and the grains Silicon were placed in a Tungsten crucible.
The substrates were kept at a distance of 20 cm from the source. Thebase vacuum level was 2*10-5 Torr and the deposition pressure was around 2.2*10-4Torr. The evaporation current was 300 A and a bias of 90 V. The time of evaporation is about 40S
The crystal structure of the films was identified by X-ray diffraction (XRD), using diffractometer Philips X’PertMPD, with Cu-K radiation source. The scanning (2) was performed from 15° to 85°.
The hardness and elastic modulus of the films were measured by a nanoindenter Bruker AXS BW/508/98/RÖ CSM tester equipped with a Berkovich diamond tip. Nanoindentation tests were carried out under load control with a load of 10 mN,the maximum penetration depth during the tests was found to be less than10 % of the Ti-Si coating thickness, wich avoids the substrate effect
The corrosion behaviors of Ti-Si coatings were evaluated using a PARSTAT4000 potentiostat/Galvanostat interfaced with a computer. Test specimens connected to a working electrode, and two high-purity graphite rods and the saturated calomel electrode (SCE) were
working electrode whose surface area exposed to the corrosive medium was of approximately 1 cm2. Potentiodynamic polarization curves were obtained with a scan rate of 0.166 mV/s.
Corrosion potentials (Ecorr) and the corrosion current densities (icorr) were deducted by using instantaneous VersaStudio corrosion analysis software.
The immersion tests were carried out by suspending the discshaped samples in a still of (3, 5
% weight) NaCl solution open to air, stirred,at room temperature to ensure steady-state conditions. The specimens were exposed to the test solution for 30 min.
Table.1. Parameters of Ti-Si coatings vacuum vapor evaporation technique
Parameters Value Value
Target Material Ti, Si(4 % , 7%, 11%, 17% )
Substrate temperature 290°C
Working pressure 2.2 E-4mbar
3. Results and discussion.
Figure.1. Shows the XRD patterns of the Ti-Si thin films deposited on Z200 steel substrate, it can be seen there is no peak occurring for Ti thin film, with 4 at % Si contents wish indicating that Silicon may be in the amorphous state, when the Si contents increase in the Ti the film becomes polycrystalline and four phases appeared (Ti-Si, TiSi2, Ti5Si3, Ti5Si4).According to Salpadoru N H, et all [7] at the equilibrium state Ti-Si should be Ti3Si however (TiSi2, Ti5Si3, Ti5Si4) are dominate this is due to the fact that the formation of Ti3Si is a very slow procedure
0 20 40 60 80 100
1500 2000 2500 3000 3500 4000 4500
TiSi2Ti5Si3 Ti5Si3Ti5Si3
TiSi2TiSi2Ti Ti5Si3
Ti5Si3 TiSi(102) Ti(200) Steel
Steel
Intensity(u.a)
2 theta (°)
Ti 4 i 7 % Si 11 % Si 17 % Si
Ti5Si4 Steel
Figure.1. XRD Patterns of T i-Si alloys with different Si amount
In order to investigate the effect of Silicon processing on the mechanical properties of Ti-Si Coating nanoindentation tests were carried out on the samples. Figure.2. presents the variation of the hardness and the Young Modulus of the Ti-Si thin films versus the Silicon quantities, the hardness and the Young Modulus have the same trends, it can be seen that the hardness and the Young’s Modulus of Ti-Si is about 17 GPa, and 467 GPa respectively,The high hardness of the deposited Ti-Si withlow Si content may be owing to the lattice distortion[6]
by adding a small amount of silicon they increase firstly to achieve a maximum values of 33 GPa, and 795 GPa respectively when the silicon content is 11 Si at %, then decrease with a further increase in the Silicon content to achieve 9 GPa, and 123 GPa respectively when the silicon content is 17 at %, this evolution is observed in all deposition of the Ti-Si a small quantities of Si increase the hardness and Young’s Modulus, than decrease.
In order to obtain superior resistance to plastic deformation, it isdesirable to obtain coatings that possess high hardness and lowerelastic modulus. This behavior is well expressed by the H3/E*2ratio, where H and E* are the hardness and effective modulus ofthe coating. E* is expressed as E*=E/(1−ν2), where E is theYoung’s modulus, and ν is the Poisson ratiothe H3/E*2 ratio of the deposited Ti−Si film with 17at.%Si showed the highest value of 0.323GPa among the studiedfilms. The H3/E*2ratio was enhanced by the introduction of Si into the Ti structure. This provides higher resistance to plastic deformation of the as deposited Ti−Si coatings.
-5 0 5 10 15 20 25 30 35
0 20 40
Si (at.%)
Hardness (GPa)
0 500 1000
Young's Modulus (GPa)
Figure.2.Evolution of Hardness and Young's Modulus of Ti-Si versus Si
The polarization curves of Ti, Ti-Si (7.5mg/100mg and 20mg/100mg) coatings in 3.5% NaCl
The Ecorr and Icorr values have been calculated using the Tafel extrapolation method and are given in Table.1.1.
The corrosion potential of the Ti-Si is about -0.21 V, adding Silicon to the Ti coating enhances the corrosion resistance; it passes to -0.15 V for Ti with 7.5 at % of Silicon then to - 0.13 V for Coating with 20 at %, this results confirms that coating with Silicon content exhibit a good corrosion resistance with the increasing the Si content, the Icorr of coatings were gradually decreased and thecoatings with 7.5 at% has the lowest Icorr of all the coated samples, meaning the lowest corrosion rate.
From the polarization test results, the protective efficiency, Pi (%) of the films can be calculated by Eq. (1)
(%) = 1 − 100 … … … (2)
Where Icorr and indicate the corrosion current density of the film and substrate, respectively.
For Ti-Si with 7.5at % of Silicon Pi= 54.67 %
For Ti-Si with 20 at % of Silicon Pi= 5.08 %
It’s clear that the coating with 7.5 at % has the highest protective efficiency of 54.67 % caused by the lowest corrosion current
-10 -9 -8 -7 -6 -5 -4
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
Potentiel,(V) (mV/ECS)
Courant density, (I) Logi(nA.cm-2) Ti
7.5 mg 20 mg
Figure.3.The potentiodynamic Polarizationcurve of different coatings of Ti-Si (7.5mg/100mg and 20mg/100mg) and Ti substrat in 3.5% NaCl solution at 25°C.
substrats Ecorr(I=0) (mV/ECS)
Icorr
(nA/cm2)
Rp (kΩ.cm2)
CR
(mpy)
Titane -207.652 622.189 53.497 0.91314
Ti-Si (7.5mg/100mg) -154.185 282.032 85.533 0.57113
Ti-Si (20mg/100mg) -132.252 590.553 80.15 0.49269
Table 1.1:Results of potentiodynamic polarization tests
4. Conclusion:
We investigated the Si effect on the Ti-Si coating by depositing Ti-Si coatings on Z200 Steel using vacuum thermal evaporation technique
DRX characterization of the samples, revealed a formation of Ti-Si, Ti5Si3, and Ti5Si4
The hardness and the Young’s Modulus have the same behavior; they increase firstly by increasing Silicon contents, and then decrease smoothly
The results obtained from the electrochemical corrosion test shows Ti-Si coating with 7.5at%
has better corrosion resistance property compared to the Ti
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