Surface Oxide Analysis of Water Atomised Al and AI-SI Povvders
Analiza oksidne plasti vodnoatomiziranih prahov Al in Al-Si
Hertl B1., B. Breskvar, IMT Ljubljana, L. Kosec, FNT, Odsek za metalurgijo in materiale, Ljubljana, A. Zalar, B. Praček, IEVT Ljubljana
A vvater atomisation technique has been used to produce rapidly solidified Al and Al-Si povvders. The thickness of the oxide layer was determined by Auger electron spectroscopy (AES). The type of the oxide was determined by x-ray diffraction.
Key vvords: water atomisation, Al-Si povvders, oxide layers
Po postopku vodne atomizacije smo izdelali prahove vrste Al in Al-Si. Debelino oksidne plasti na delcih smo ocenili z Augerjevo elektronsko spektroskopijo. Tip oksidne plasti smo določili z rentgensko difrakcijsko analizo.
Ključne besede: vodna atomizacija, Al-Si prahovi, oksidne plasti
1.	Introduction
The surface bound oxygen on atomised povvder is a very important factor in the processing of high per-formance PM materials. The surface condition of the povvder will have a significant impact on the amount and distribution of oxygen in the final compact and its mechanical properties. In this work results are presented on the investigation on the oxide layers in the base metal and the alloys AISi6, AISi12 and AISi191234.
2.	Experimental procedure
The morphology and surface topography of parti-cles were investigated in Scanning Electron Microscope. Depth profiles of the oxide layers were obtained by successive A~ ion etching and Auger analysis. The measured velocity of etching on the standard Cr203 sample was about 6nm/min. Ali ex-aminations vvere performed under the same condition. The oxide layer was identified by x-ray diffraction.
3.	Results
Povvders of aluminium and AISi6, AISi 12 and AlSil 9 alloys in two size ranges (0,09-0,125 mm and <0,045 mm) vvere investigated. The purpose was to establish the influence of the size range and the con-tent of silicon on the thickness of the surface oxide layer. The morphology of AISi 19 particles are presented on Fig 1a and Fig 2a, vvhile in Fig 1b and Fig 2b an example of their AES analysis is shovvn.
Bojan HERTL. dipl. inž.
Inšt. za kovinske materiale in tehnologije
L|ubljana. Lepi pot 11
Evaluations of the oxide layer thickness for the size range 0,09-0,125 mm are given in table 1 and for the size range <0,045 mm in table 2. Because of the particles morphology the topographic effect, during the ion etching, deteriorated the resolution of the analy-sis and a more clear distinction betvveen the oxide layer and the metal. A better resolution could be achieved only on planar surface. The evaluation of the AES analysis shovvs, that the particles in the size range 0,09-0,125 mm have almost the same average layer thickness (415nm) as the particles in the size range <0,045 mm (41 Onm).
Table 1: Oxide thicknesses for particles of size range 0,09-0,125 mm
Tabela 1: Debelina oksidnih plasti za velikostni razred delcev 0,09-0,125 mm
Sample	ion etching time(min)	oxide thickness(nm)
Al	65	390
AISi6	73,5	440
AlSil 2	71,5	430
AISi 19	67	400
Table 2: Oxide thicknesses for particles of size range <0,045mm
Tabela 2: Debelina oksidnih plasti za velikostni razred delcev <0,045mm
Sample	ion etching time(min)	oxide thickness(nm)
Al	75	450
AISi6	67	400
AlSil 2	61,5	370
AlSil 9	70	420
	AI(62eV)
-0-	AI(68eV)
	Si(92eV)
-*-	0(610 eV)
AI(S2eV) AI(63eV) Si(92eV) 0(510eV)
100
80
60
0
0 10 20 30 40 50 60 70 80 Sputter tlme [min!
Figure 2: VVater atomised AISi19 particles in the size range <0,045 mm: a) SEM morphology image, b) AES
depth profile of the oxide layer Slika 2: Vodnoatomizirani delci AISi 19 v velikostnem razredu <0,045 mm: a) SEM posnetek oblike delcev, b) AES globinski profil oksidne plasti
Figure 1: VVater atomised AISi19 particles in the size range 0,09-0,125 mm: a) SEM morphology image, b) AES depth profile of the oxide layer
Slika 1: Vodnoatomizirani delci AISi 19 v velikostnem razredu 0,09-0,125 mm: a) SEM posnetek oblike delcev, b) AES globinski profil oksidne plasti
0 10 20 30 40 50 60 Sputter tlme [mini
XRD spectra shovvn in Figure 3 were obtained for the oxide layer on 0,09-0,125 mm and on 0,045 mm povvder size. High intensity peaks indicates Al and Si presence in the sample. Inspite of the low intensity due to the small volume of analysed oxide, a few peaks (BrB4) were clearly recognized and the oxide present on the surface of the particles was identified as boehmite, aluminum oxide hydroxide (gamma-AIOOH).
Table 3: Measured parameters for B,-B4 Tabela 3: Izmerjeni parametri za B,-B4
Peak	2-theta(deg)	(h,k,i)
b,	14,458	(0,2,0)
b2	28,181	(1,2,0)
b3	38,337	(0,3,1)
b4	48,930	(0,5,1)
particles
Slika 3: Rentgenski difrakcijski spektri oksida na površini delcev prahu
4. Conclusions
The surfaces of the vvater atomised Al, AISi6, AlSil2 and AISi 19 povvders in tvvo different size ranges vvere examined. It is concluded that:
-	the oxide thickness, determined by AES depth analysis, is not significantly influenced by the Si content in Al matrix or by the size range of the particles. The determined average thickness of the oxide is approximately 0,4|xm.
-	The oxide in the layer on PM particles is boehmite (gamma-AlOOH).
5. References
1	G. Staniek, D. Shechtman, J. Remon, W. Bunk: VVater Atomisation of AI-5Cr-2Zr-1 Mn, pmi, 22, 1990, 4, 7-10
2	T. Tunberg, L. Nyborg: The Role of Povvder Surface State in High Alloy PM Materials, Povvder Metallurgy, 1994, 403-406
3	M. Torkar, B. Šuštaršič: Microstructural Characteristics of VVater Atomized Povvder of Ni-Superalloy, Prakt. Met. Sonderbd., 24, 1993
4	B. Hertl, F. Vodopivec, L. Kosec, B. Šušteršič, B. Breskvar: Vodna atomizacija zlitine Al-Si, 3. Slovenska konferenca o materialih in tehnologijah, Portorož, 1995