ELSEVIER Thin Solid Films 266 (1995) 58-61

X-ray photoelectron spectroscopy analysis of thermal and plasma-treated steel substrates and their interface formed with an aluminium layer

A. L&mar *, Y. Scudeller, F. Danes, J.P. Bardon Laboratoire de thermocin&ique, URA CNRS 869, ISITEM Vniversite’de Nantes, CP 3023, 44087 Nantes cs 03, France

Received 16 December 1994; accepted 20 April 1995

Abstract

X-ray photoelectron spectroscopy was used to investigate the interface formed between thermally and plasma-treated steel substrates and an aluminium layer deposited on them. Enhancement in adhesion of the metal on thermally treated steel substrates is explained by the structure change of the aluminium layer. In contrast, plasma treatments induce morphological modifications of the steel surface and favour deformation of compounds between the steel and the metal which in turn improve the adhesive strength.

Keywords: Adhesion; Aluminium; Steel; X-ray photoelectron spectroscopy

1. Introduction

Thermal and plasma treatments of the steel surface are

currently used to enhance the adhesive strength of the metal

layer deposited onto the steel substrates [ 11. Depending on the nature of the steel substrates and also on the nature of the

used gas, several explanations can be yielded for the modi- fication of the mechanical property of the metal-metal inter- face. The effects of substrate treatments before and after

deposition were investigated by X-ray photoelectron spec- troscopy (XPS) analysis. The results are discussed in relation to the adhesion property of the metal layer on the steel sub- strates.

2. Experimental procedure

The substrates studied in this work were hardened XC 70 steel (0.7% C, Vickers hardness 460 Hv). They were sub- mitted to some initial preparation which consisted of a mechanical polishing with emery paper (grades 320, 600, and 1200) and subsequently with diamond paste (6 and 1 pm). After this initial preparation, each substrate was intro- duced into the deposition chamber.

Thermal treatment of the samples was performed under a high vacuum condition (5 X lop4 Pa) at 300 “C for 3 h. The samples were then allowed to reach room temperature before

* Corresponding author.

0040-6090/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDIOO40-6090( 95)06646-2

aluminium deposition. The pure aluminium films were depos- ited by r.f. sputtering (non-magnetron) with a power density

of 9.5 X lo3 W rnw2 [2]. For plasma treatments, argon,

argon-nitrogen, or pure nitrogen were used with a pressure

of 5 Pa. The thickness of the Al films (0.2 pm) was kept constant in all samples. The adhesive strength of the Al layer

on the steel substrates was determined by measuring the mean critical load Lc from a scratch test. This load is defined as the load applied to the stylus at which the loss of adhesion occurs

[ 21. XPS measurements were performed on an ESCA ana- lyser with Mg Ka radiation (hv = 1253.6 eV). Spectra of

iron, carbon, oxygen (or nitrogen) and aluminium were recorded throughout the interfacial region of the samples and analyzed by computer program. This region is defined as one where Fe2p, C 1 s (or N 1 s) and A12p lines are simultaneously

present. It can be reached by successive removals of the layer using bombardment at a voltage of 4 kV. Binding energy data

were referenced to the Au 4f,,2 furnished by a gold plate fixed on the sample holder.

3. Results and discussion

The results of scratch test measurements performed on different samples are summarized in Table 1. We can see that the thermal treatment improves significantly the adhesion of the aluminium layer on steel substrates. In contrast, at a same temperature, there is no change of L, measured on ar-treated and untreated samples. The enhancement of the adhesive

A. Lahmar et al. /Thin Solid Films 266 (1995) 58-61 59

Table 1

Mean critical load L, measured in a steel-Al system with and without treat-

ment of the steel surface

A B C D E

Substrate temperature (“) 25 300 25 25 25

Mean crihcal load (g) 5 50 5 60 80

A, untreated; B, heated; C, Ar-treated; D, Ar + N,-treated, E. N,-treated

substrates

strength is obvious with Ar-N, and N, treatments, the critical

loads are 60 and 80 g respectively as compared with 5 g

measured on untreated samples. We also notice that the

treated samples have a brillant aspect denoting a high reflec- tivity of the metal layer.

Observation of the uncovered steel substrate by scanning

electron microscopy (SEM) reveals a smooth and uniform

surface for untreated and Ar-treated samples (Fig. 1 (a) ) .

The effect is even more spectacular in the case of a sample

heated at 300 “C in vacuum, for which the surface became

very rough (Fig. 1 (b) ) . The widening of the grain boundary

resulted from relief of surface stress. Subsequent changes

occur however in Ar-N,- and N,-treated substrates with the

formation of nitrided domain and microcavities in the sub-

strates (Fig. 1 (c) ) .

From this section, the principal conclusion about the ther-

mal treatment is that the strong enhancement of the mean

critical load of a sample arises from an increase in the rough-

ness of the substrate surface [ 31. However, the mean critical

load obtained for a coating deposited after ar-N, or N, treat-

ments is much stronger than that obtained after the best ther-

(a)

(b)

* b d =2.10-z m

* b

d = 2.1O-2 m

Fig. 1. Roughness profiles of the surface and related scanning electron micrographs. Untreated (or Ar-treated) (a), thermally treated (b ) and Ar-N2 (or N,) - treated (c) samples.

60 A. Lahmar et al. /Thin Solid Films 266 (1995) 58-61

_t_ Al (2~)

- c (Is)

Sputter time (min)

60 >. .z ul : c 40 .-

x m : A

20

7 Y : n

0 0 10 20 30 40 50

(b) Sputter time (min)

Fig. 2. XPS depth profiles obtained at different sputter times on an Al-coated

steel substrate: untreated (a) and thermally treated (b) substrates.

ma1 treatment. An explanation of these differences could be sought in a variation in the chemical nature of the interface. In order to clarify this, we proceeded to a series of physical- chemical analyses, the results of which are presented in the

Figs. 24. Fig. 3 shows the XPS spectra recorded on the surface of

Ar-N,-treated substrates. Both Fe2p,,, (Fig. 3(a) ) and 0 1s (Fig. 3 (b) ) lines indicate that the oxide formation is not accompanied by a variation in the binding energies of the 0 1s (or N 1s (Fig. 3(c) ) lines. The Fe2p,,, line has two components at 706.75 and 7 10 eV which correspond to metal- lic and oxidized (Fe,O,) forms respectively.

Upon Al deposition (Fig. 4), these lines are modified by diminution of the oxide layer present on the substrate surface which is compensated by a new line (N Is) at 397 eV. As for the A12p line, the deconvoluted curve shows two peaks at 75.6 (Al,O,) and 74.2 eV (A1203). The new features

observed in both nitrogen and oxygen spectra together with

the 74.2 eV component of the aluminium line strongly sug- gest that a metal-oxygen-nitrogen complex is formed in the

interface on the treated substrate. The bondings between the

different atoms are on the whole similar to those found in the untreated steel-Al interface [ 451, However, the thickness of the interfacial layer in the former case is about four times that

705

Bindlng energy (eV)

2 5 N 1s d

5 4

=” c” 2

z & 1

531 130 528 400 395

(b) BIndi,&, e”WOY (Sv) (cl Binding energy (eV)

Fig. 3. Narrow spectra of the iron line 2pj,, (a), the oxygen line 0 Is (b)

and the nitrogen line N Is (c), obtained on the surface of Ar-N,-treated

substrates. Sputter times: 0 min (curve 1). 2 min (curve 2). 4 min (curve

3) and 6 min (curve 4).

Y

Fe (2p 312)

O(lS)

Al (2~)

N (1s)

c (19

Sputter time (min)

Fig. 4. XPS depth profiles obtained at different sputter times on a sample

made of an Al coating on a steel substrate treated in an Ar-N, atmosphere.

A. Lahmar et al. /Thin Solid Films 266 (1995) 58-61 61

of the latter case indicating a deeper penetration of Al atoms

in the steel substrate. This corroborates the modifications of

the morphology of the substrate surface induced by ion bom-

bardment. In addition, we verify that the iron atoms react

preferentially with nitrogen to form the hard compound as

already suggested in some earlier studies [ 6,7].

Similar results are obtained with samples treated in a pure

N, atmosphere. It should be noticed that the penetration of

the nitrogen atoms is much more important than that of Ar-

N2 since traces of N2 are still found after complete removal

of the metal layer.

In conclusion, XPS analysis performed on steel-Al sys-

tems brings out some explanations for the improvement in

adhesion of an aluminium layer on the steel substrates. Both

the morphology change of the steel surface and the formation

of a complex in the interface seem to be responsible for the

adhesion enhancement in ar-N,- and N,-treated samples. As

for the thermal treatment, it is possible that the structural change of the Al film and of the steel substrate could be at the origin of the observed increase of the adhesive strength.

References

[I] G.H. Lee, M. Cailler and S.C. Kwon, Thin Solid Films, I85 ( 1990) 3% 55.

[2] A. Lahmar and M. Cailler, Thin Solid Films, 248 ( 1994) 204-211. [3] A. Lahmar, G.H. Lee, M. Cailler and C. Constantinescu, Thin Solid

Films, 198 (1991) 115-137. [4] J. Valli and U. M&e& Wear, 115 (1987) 215-221. [5] A. Lahmer and M. Cailler, Le Vide des Couches Minces. 272 (1994)

2 13-225. [6] A. Grill and D. Itzhak, Thin Solid Films, 101 (1983) 219-222. [7] G.H. Lee, S.C. Kwon, M. Cailler. S.R. Lee and W.S. Baek, Int. Con&

on Metallurgical Coatings and Thin Films, San Diego, 1991.