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SURFACE CHARACTERIZATION OF WATER ATOMIZED MOLYBDENUM ALLOYED STEEL POWDER Programme Director: Janusz Kanski Submitted By: Yawar Abbas Khan(791015-7051) MATERIALS SCIENCE: CHARACTERIZATION (FTF-155) 2004-Göteborg Sweden.

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Content

1 Introduction…………………………………………………………4 2 Techniques used…………………………………………………….5

2.1 X-ray photo electron spectroscopy (XPS)……………………...5 2.2 Auger electron spectroscopy (AES)…………………………….5

2.2.1 Instrument…………………………………………………..6 2.3 Scanning electron spectroscopy (SEM)………………………...7

2.3.1 Inlens Detector………………………………………………9 3 Comparison. ………………………………………………………..9 4 Introduction to other techniques…………………………………….10 4.1 scanning ion mass spectroscopy………………………………..10 4.2 atom probe field ion microscopy……………………………….11 4.3 scanning tunneling microscopy………………………………...11 4.4 atom force microscopy………………………………………....11 5 Experimental…………………………………………………….....11 5.1 sample……………………………………………………….....11 5.2 run……………………………………………………………...12 6 Results…………………………………………………...................12 6.1 SEM…………………………………………………………...12 6.2 EDX…………………………………………………………...14 7 Discussion…………………………………………………………..18 8 Conclusion……………………………………………………….....18 9 Further studies………………………………………………………19

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Abstract:

In this report mainly two characterization techniques are involved to characterize the surface of water atomized Cr-alloy steel powder. Auger electron spectroscopy (AES) and Scanning electron microscopy (SEM).

The powder is heat-treated, in a furnace connected to the AES/XPS-instruments, at 800 C° for 1 hour in vacuum. The chemical analysis and surface morphology of the oxide film after heat treatment will be studied using the techniques. The main focus will be on the comparison between AES and SEM, which both use an electron beam as source for gaining information of the surface characteristic of the material.

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1- Introduction: Metallic powders are made by different atomisation techniques (water, oil or inert gas). Solid components are made by pressing the powder to get the desired shape. The surface bound oxides present on the powder particles are desired to be reduced, obtaining high densities at the same time. Therefore the investigation of surface oxides is of scientific interest and important from the technical point of view. In this report I have studied water atomized molybdenum alloyed Steel powder Astaloy Mo (1.5 wt %). Initially the sample was heat treated at 800 °C under vacuum in XPS (x-ray photoelectron spectroscopy). The sample was transferred to auger electron spectroscopy. In AES the survey spectra are taken to check which elements are in surface oxide layer. SEM\EDX gives information of the same sample at better resolution and magnification. Finally comparison of the results taken from AES and SEM\EDX has been done.

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2- TECHNIQUES USED:

2.1 X-ray photoelectron spectroscopy: In X-ray photoelectron spectroscopy (XPS) - also called electron spectroscopy for chemical analysis (ESCA)- X-rays excite photoelectrons, and the emitted electron signal is plotted as a spectrum of binding energies. Differing chemical states resulting from compound formation are reflected in the photoelectron peak positions and shapes. Spectral information is collected from a depth of 2-20 atomic layers, depending on the material studied. The binding energy of the photo electron leaving the sample is given by the equation

E = hυ - Ek - Φ Where hυ is the energy of X-ray, Ek is the kinetic energy of the photo electrons and Φ is the work function of spectrometer. XPS gives information about the composition and chemical analysis of the outermost atomic or molecular layers of the sample.

2.2 Auger electron spectroscopy (AES): This surface analytical technique, which is similar to SEM/EDX, provides the concentration and distribution of the major elements, except H and He, on conductive surfaces. A focused beam of electrons (1 to 25 KeV in energy) excites atoms on a solid surface by generating core holes, instead of x-rays, the detected particles are auger electrons emitted when the excited atoms relax by a two electron "Auger" process. When combined with an ion beam for sputter etching, AES can also provide in depth concentration profiles of the major elements. In auger process incident electron creates a shell vacancy which can be filled by outer electron releasing extra energy in the form of auger electron. The energy of the auger depends upon the electron levels involved not on the incident beam energy.

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E K, L1, L23 = EK - (E L1+E L23) (Energy of auger electron) Auger electron

Where EK, EL1, E L23 are the binding energies for the levels involved in the Auger process. With AES the elements present on the outermost surfaces can be detected. The typical analysis depth is in the range of 3-10nm.

2.2.1 Instrument: The instrument (PHI 660) used in this technique has ion gun providing the possibility of depth profiling as well. The etch rate is calibrated using an oxidised tantalum foil with well known Ta2O5 thickness.

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2.3 Scanning Electron Microscope (SEM): By use of high resolution low voltage SEM equipped with inlens detector, the surface morphology of the conductive specimens can be studied. The 3-d view of the specimen surface can be seen due to better depth of focus, on the other hand it has better spatial resolution (5-10 nm spot of electron probe). The image formation is possible by simultaneously detecting the secondary electrons coming from near surface (50-500A) of specimen. Field-emission SEM. Enables high resolution (~1 nm at a magnification of ~ 650,000x) and low beam-voltage imaging, in addition to high-resolution EBIC characterization. Chemical analysis has been done using x-ray energy dispersive spectrometer (XEDS) with integrated INCA software.

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Secondary electrons as the source of information in SEM.

The use of SEM-imaging and x-ray micro analysis has mainly been addressed to get selective information from the particulate compounds at better magnification on the surface of the powder after transferred from AES. The inlens detector used during study gives better topographical contrast than standard secondary electron detector. The quality of the image is improved in case of the inlens detector.

.. (a) (b)

Comparison of image, small oxide particles on surface of powder a)-from inlens detector SEM b)-from standard detector SEM

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2.3.1 Inlens Detector. At high vacuum mode and annular inlens detector as well as a lateral SE-detect cab be used.Inlens detector provides high resolution data, the lateral detector is used to provide optimum topographical information. Signals from both detectors are mixed electronically in any ratio to optimize image quality. Below the figure shows the position of an inlens SE detector.

3 Comparison: Working principle of AES and SEM is same, both are using electron beam as source for information of specimen surface. But both have some technical limits of information. Therefore in the study of small oxide compounds on the surface of the powder particles information from both techniques have been combined. From AES it is easy, to take information from a spot (like compound particulate of few nm on surface of powder). And one is surer about survey spectra that it gives information about elements present and almost no information from bulk. This is because AES is more surface sensitive (auger electron). But it is not easy to

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get good images (from AES) in order to trace spot those particulate compounds due to limit of resolution. However using SEM gives a better image of powder particles and resolution is good enough that one can clearly see tiny particulate compounds sitting on powder. Now targeting these particulate compounds, EDX spectrum shows the elements present. Since the information source (X-rays) in EDX comes from 1µm deep that is deeper than auger, one gets mixed information from compounds and bulk. The information depth for Auger analysis is the top 2-20 atomic layers. Therefore data has been collected from both techniques to get an idea about the chemical composition and physical status of the oxides.

4 Introduction to other techniques:

4.1 Secondary ion mass spectroscopy: The surface of the specimen is bombarded by primary beam of ions O2+, Ar+ (by plasma discharge) or Cs+ (thermionic emission) or Ga+ (liquid metal field emission). This results results in collision cascade, giving secondary particles both positive and negative (monatomic, clusters, molecular). These particles are directed (system of optics) so that they could be detected by mass spectrometer. Practically SIMS gives quantitative analysis of the specimens with a standard, with known composition for concentration measurement and sputtering rate for depth profiling. Benefits include sensitivity on the ppb level, good depth resolution, good dynamic range, and rapid analysis of depth profiles.

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4.2 Atom probe field ion microscopy. The specimen with a tip radius of less than 50nm is given high positive charge and electric field is created on the tip. The field strength depends on the geometry of the specimen. The atoms are removed from the surface of the tip due to field evaporation process and become positively charged ions. These ions are repelled and there mass are detected by the time of flight spectrometry. Though other microscopes are able to reach same resolution but some experiments are unique to APFIM for example to study behavior of single atoms or cluster on the surface of solids.

4.3 Scanning tunneling microscopy: The scanning tunneling microscope (STM) provides a picture of the atomic arrangement of a surface. The principle of STM is that it uses a sharpened, conducting tip, with a bias voltage applied b\w sample and tip. When this tip is brought near (10 A°) of the sample, the electrons within the sample\tip (depends on bias direction) “start tunneling”. The tunneling current is measured and it varies with the distance b\w the tip and sample, giving signals for the STM imaging. STM imaging uses two modes a)-constant height mode b) - constant current mode.

4.4 Atomic force microscopy: To acquire an image the microscope raster-scans the probe over the sample while measuring the local property in question. It measures topography with a force probe. AFM operates by measuring attractive or repulsive forces between a tip and the sample. In its repulsive "contact" mode, the instrument lightly touches a tip at the end of a leaf spring or "cantilever" to the sample. As a raster-scan drags the tip over the sample, some sort of detection apparatus measures the vertical deflection of the cantilever, which indicates the local sample height. Thus, in contact mode the AFM measures hard-sphere repulsion forces between the tip and sample. AFM can achieve a resolution of 10 pm, and unlike electron microscopes, can image samples in air and under liquids.

5 Experimental:

5.1 Sample: The conductive sample was heat treated in the XPS for one hour at 800C° under vacuum. Heat treatment was given so that the entire iron oxide layer present on the surface of the powder could be removed. Then it was transferred to AES under vacuum.

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5.2 Run:

AES: In AES secondary electron image mode was set. Then six area of interest were spotted and survey spectra of each area were taken one by one. Analysis time for each spectrum was approximately six minutes. The peaks in the spectra were matched with the standard spectra to know which peak represents which element. SEM: Sample was then shifted to SEM room, where secondary electron images were taken at higher resolution using inlens detector. To make chemical analysis of the oxide particles and layer EDX detector was introduced. Different spectrums were taken by selecting interesting areas from clear image. The data collected from both techniques was discussed and final conclusion about characterization of steel powder has been made.

6 Results: 6.1 SEM: Image of the powder particles, better resolution than auger.

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Note oxide particles that cannot be seen clearly in auger image.

SEI, taken from inlens SEI taken from standard detector. detector.

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6.2 EDX:

1/7/04 12:55:19 PM

Comment: Beam focused on oxide particulate, spectrum shows more amount of Cr , O, Fe.

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1/7/04 12:56:01 PM

Comment: In image beam spotted at layer and interesting features in spectrum are Fe, O, (iron oxide), C. The higher amount of “O” (just before iron peak) is because the sample was transferred from AES to SEM in air.

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Comment: Spectrum shows that the particulate may be an oxide of Cr, Fe,

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1/7/04 12:55:19 PM

Comment: Beam focused on oxide particulate which due to low resolution cannot be seen in auger, spectrum shows more amount of Cr , O, Fe

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7 Discussion: The spectra taken from the AES, shows the peaks of Fe, S, C, O, N. Elements like Mo, Cr, that were expected to be present along with oxygen (oxides) were not found. One reason was noise in the spectra, may be longer analysis time give better results. However in some spectra there is slight variation in the peak of oxygen. This presence of oxygen may be because of some air though the sample was transferred from XPS to AES in the presence of vacuum. Since auger is much sensitive instrument so the spectra shows the presence of oxygen. High resolution images taken from SEM clearly show the presence of oxide particle (mostly white) on the surface of the powder. And there spectrum from EDX show that these particles are either of Cr-oxide, Mn-oxide, or Fe-oxide. The presence of iron in the EDX peak may be from the bulk layer of the powder and not from those particulates because information source of EDX is x-rays. However a layer of Fe-oxide may cover particles during transfer from AES to SEM. Keeping all results of AES, SEM\EDX together, a good quantitative analysis of oxide layer and particulates was done. If AES can be combined with the facility of looking image at same resolution and magnification as in SEM, then all analysis can be done in less time and better accuracy.

8 Conclusion:

• Iron and the alloying elements present on the surface were found to be oxidized. • Presence of oxygen and iron in all spectra of auger and edx, shows that there

seems to be continuous layer of iron oxide. • However the oxide particles of Cr, Mn, Mo exist in small particulates b\w iron

oxide layer. • Presence of sulfur peak in auger results needs to be investigated.

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9 Further studies:

• Investigation at auger electron spectroscopy with the facility of SEM image (high resolution) can give better and quick results.

• More samples, heat treated at different temperatures, (1100, 1200 C) can be studied to get better idea about the surface oxides.

• If possible in XPS the sample can be sintered under working conditions, and then characterized