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135
Mo nanocrystallite effects on nanostructural properties
of stainless steel
A. Bahari and M. Roodbari Shahmiri
Department of Physics, University of Mazandaran, Babolsar, Iran
Mechanical properties of stainless steel can be improved by grain size refinement down to nanocrystalline structures. In the present work, the effects of Mo nanoparticles on stainless steel powders synthesized using the sol-gel method have been studied. The results show that decreasing of the size of Mo crystallites in stainless steel is accompanied by phase transformations as well as the mesoporous surface area and narrow pore size distribution. The structure of the synthesized sample was investigated by XRD (X-ray diffraction), AFM (atomic force microscopy) and SEM (scanning electron microscopy) technologies. As revealed in AFM, SEM images and XRD patterns, wormlike pores were formed and ordered to some extent. The influence of various conditions on the structure of ordered mesoporous stainless steel is also discussed.
Keywords: nanostructures, ball milling, stainless steel, sol-gel, XRD and SEM techniques
1. Introduction
New advances in growing pure ultrathin films are needed and advanced UHV chambers allow up to around 10-12... 10-14 Torr of pressure [1-3]. In order to grow nanoscale films for devices, however, we must have a better UHV condition and get lower pressure inside the UHV chambers [4-9]. This will be achieved through experiment and theory over the next decade. These materials have also important applications for space electronics. The development and fabrication of nanodevices requires special equipment (UHV chamber) as well as fast and pure growth facilities are needed.
For improving mechanical properties of stainless steel, sol-gel thin coatings of ball milled Mo were prepared from catalyzed sols and deposited by dip-coating technique on 316L stainless steel foils. The austenitic 316L stainless steel and the mixture of Mo component nanoparticles form polyhedron agglomerates of an average size of 300 |xm. Analysis of the data combined with SEM studies indicates that the films act as a geometric barrier against exposure to corrosive media.
Thin film hydroxyapatite deposits through dip-coating onto prefinished low carbon 316L stainless steel substrates were prepared using water based sol-gel technique [1013]. This technique provides several advantages in the control of microstructure and the thickness of the coatings. The coatings were annealed in air; sintering temperature rea-
ched 500 °C to enhance the crystallinity and purity of the films.
Intensive investigations are carried out to examine various aspects of material processing and characterization of ball-milled stainless steel with using these techniques. However no detailed report (to our knowledge) exists as yet on Mo arrangement in the stainless steel structures.
The aim of the present work is to study the phase transports to see if bulk, ball-milled stainless steel chamber can be used as a good UHV chamber material for growing pure ultrathin (down to 1 nm) films in the forthcoming nano-device fabrications.
2. Experimental procedure and details
The sol-gel process is a wet-chemical technique (chemical solution deposition) widely used recently in materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials starting from a chemical solution which acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are molybdenum oxides, which undergo various forms of hydrolysis and poly-condensation reactions.
Thus, the sol evolves towards the formation of a gellike diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. In this process,
© Bahari A., Roodbari Shahmiri M., 2013
Fig. 1. Flowchart of synthesis stainless steel nanoparticles
ammonium molybdate powder (1.16 g) was dissolved in deionized water to which was added citric acid crystals (0.38 g). The mixture was then stirred carefully using a
magnetic stirrer while ammonium hydroxide was added to obtain a pH of 7. The mixture was then heated in a furnace to a temperature of 100 °C for 20 h. Initially a zero gel and finally a powder was obtained. The most promising 316L stainless steel substrates were cut from large foils and degreased ultrasonically in acetone. This austenitic material was chosen because it has a low carbon content (<0.03 %) and is therefore less susceptible to sensitization during the heat treatment necessary for the densification of the coatings.
The powder was then heated to a temperature of 60 °C for 24 h to obtain a pale yellow powder. The characteristics of Mo in stainless steel synthesized by the sol-gel citration method was studied by using XRD, AFM and SEM techniques. It is worth noting that the samples are cleaned inside the ultrasonic bath after rinsing and washing in heated acetone then ethanol, the surface cleanliness is checked with SEM technique. Stainless steel powder is synthesized via simple sol-gel method as summarized in Fig. 1.
Figure 2 shows AFM images of stainless steel with and without Mo nanoparticles. As shown in these images and topography spectra in Fig. 3, Mo nanoparticles affect the stainless steel structures and stainless steel form polyhedron agglomerates and martensitic phase as confirmed with XRD pattern.
-44.2
X = 5 am
X = 10 am
Fig. 2. 2D (a, b) and 3D (c, d) AFM image of Mo nanoparticles in stainless steel content. Magnification: X2300 (a, c), x1000 (b, d)
Fig. 3. 2D topography spectra of stainless steel: without Mo (a) and with Mo nanoparticles (b)
The AFM images as well as SEM images (Figs. 4, 5) and topography spectra (Fig. 3) from the stainless steel specimens show mudflat cracking throughout an otherwise smooth and flat coherent surface layer covering the nano-bulk milled stainless steel structures with Mo nanoparticles. Higher magnification imaging in these figures reveals that stainless steel spans these cracks and appears to stabilize the surface layer against light atom penetration, leakage and tunneling currents through the UHV chambers.
XRD technique is also used for crystal phase identification and estimation of the crystallite size. XRD patterns were measured on a (GBC-MMA 007 (2000)) X-ray diffrac-tometer. The diffractograms were recorded with 0.02° step size in where the speed was 10°/min radiation over a 20 range of 10°.. .80° at a sampling width of 0.2 ° and a scann-
Fig. 4. SEM image of stainless steel content with (a) and without Mo nanoparticles (b)
m i
1° Mm ■
Fig. 5. SEM (a, b) and EDX (energy dispersive X-ray) image (c) of stainless steel content without (a) and with Mo nanoparticles (b, c)
60°
Fig. 6. XRD patterns of stainless steel with (a) and without Mo particles (b)
ing speed of 10°/min. The XRD patterns of Mo + stainless steel compose calcined at different temperatures. The crystalline peaks corresponded to Mo in the form of either highly pure austenite phase or mixed austenite-martensitic, except the as-prepared nanostructure showing amorphous phase. This point indicates that increase in austenite-to-martensi-tic transformation of stainless steel peak place when the amount of Mo nanoparticles (10 %) in stainless steel content is added. As shown in EDX in Fig. 5, c, a flow stress decreases very slowly. It consisted of columnar grains aligned parallel to the axis of rolling and equate axed grains with dislocation density.
3. Discussions
It is well-known that XRD sensitivity to detect crystalline phases depends on their concentration and their crystallite size when it is extremely small. A very small crystallite size produces diffraction peaks so wide that the diffraction pattern of its phase, when it appears in a low concentration, can be lost in background.
Data were acquired over the range of 26 from 10° to 80° indicate the change of crystalline structure. Figure 6, a illustrates the XRD patterns of stainless steel films. From the figure, it was found that all the films were polycrystal-line having austenite phase only. It is observed that the stainless steel exhibited characteristic peaks of austenite crystal plane (101), (200) and (211). It is found that austenite films are crystallized effectively for >100 °C. The reason may be that the kinetic energy of the impinging Mo nanoparticle is high enough to initiate crystallization. For the sample calcinated at 300 °C, other characteristic peaks of austenite crystal plane (204) and (220) appeared, but the intensity of these peaks is weak. It is observed that the intensities of the peaks of few austenite planes increased slightly with the more Mo nanoparticles (not shown here).
The peak intensity decreased with increasing milling time with broadening. It can be observed that after 20 h of milling, Mo peaks decreased drastically and disappeared gradually. The presence of Mo-Fe intermetallics have revealed to be present in a negligible amount from the diffraction pattern, which suggests only complete solubility. The significant broadening of the Mo peaks may be due to the reduction and refinement of the grain size and the increase in the internal strain induced from the repeated fracturing and welding process during mechanical alloying. Crystallite size, lattice microstrain and lattice parameter of each alloy powder has calculated from the XRD peak broadening. Following Scherrer equation, the crystallite sizes
have been determined for milled samples [14]: r= K1
P cos 6'
where r, K, A, P and 6 is mean crystallite dimension, X-ray wavelength, FWHM (in radians) and Bragg angle, respectively.
The crystallite size estimated from the XRD analysis of the powder was close to 30 nm. This suggests that major structural changes and dissolution of the alloying elements almost completed by 20 h and further milling refined the product by mechanochemical alloying.
This is because of the entrance of Mo atoms into the lattice of the stainless steel which changes distortion structure to amorphous structure in it.
Figure 2 shows changes in the morphology of stainless steel powders after mechanical alloying for 20 hours at x1000 and x2300 magnification. Initially at 0 h a coarse particle structure can be seen (not shown here). After 20 h of milling coarse-layered structure has appeared which may be due to repeated cold welding and fracturing of the powder particles. The coarse-layered structure gradually refined with the proceeding of milling as fracture predominates cold
1 — experimental
2 — calculated
-1--— i .'-T"
7 8 .•■'«, ,9.».- 10 -.< ,• ' '-
Difference
Fig. 7. The crystallite size determined with using X-powder technique: 6.2 (a), 3 (b), 6.8 (c) and 20 nm (d)
welding, because of strain hardening of materials. Then on further milling particle size decreases.
Finally, after 20 h of milling a finer refined structure has obtained. It shows a homogeneous chemical composition has obtained for sample milled for 20 h.
As shown in Fig. 7, the crystallite size estimated from the X-powder method is close to 3...20 nm. The permanent peak intensity in Fig. 7 is increased with adding Mo content as shown with broaden peak (Fig. 7, d). It can be observed that after adding Mo nanoparticles, Mo peak appeared which demonstrates the presence of the other crystal phase beside the other major crystal phases in the diffraction patterns and suggests only complete solubility. As shown in X-powder peaks (Fig. 7), the significant broadening of the Mo peaks may be due to the reduction and refinement of the grain size and the increase in the internal strain induced from the repeated fracturing and welding process during mechanical alloying. This suggests that major structural changes and dissolution of the clay elements almost shifted, due to the entrance of Mo atoms into
the lattice of the stainless steel which changes distortion structure to expanded structure in it.
Indeed, network porosity may have a key role in the nanoclustering size, because they are in the same time an efficient surface and interface for absorbing water and/or the other unwanted materials, and also a suitable morphological unit for the nucleation of nanoclay charges via trapping some Mo to replace Fe atoms [1, 2, 8, 9].
4. Conclusion
The main challenge in realizing the immense potential of nanoengineered stainless steel is to manufacture large components of bulk nanocrystalline stainless steel having superior properties and a reasonable cost. To meet this challenge, a number of innovative approaches are being developed to produce stable with lower cracks of stainless steel. Indeed, this is being aided by advanced characterization methods as presented here. The sol-gel method provides an excellent technique to prepare nanoparticle material. Experimental results indicated that the homogeneous hydrolysis of molybdenum via sol-gel route is a promising technique for preparing photosensitive material with uniform nanoparticles. In this study, nanocrystalline stainless steel particles have been successfully synthesised by chemical method and heat treatment process. In conclusion, nanosized MoO3 was synthesized using the citrate sol-gel method and the same was characterized using AFM, SEM and XRD techniques. The sample was then subjected to stainless steel stability again to leakage current and light atom penetration through the UHV chamber.
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Bahari Ali, Dr., Assoc. Prof., Department of Physics, University of Mazandaran, Babolsar, Iran, a.bahari@umz.ac.ir Roodbari Shahmiri M., Department of Physics, University of Mazandaran, Babolsar, Iran
