C. Frantz

Mechanically reinforced and corrosion-resistant sputtered amorphous aluminium alloy coatings

Abstract Aluminium alloy coatings are potentially interesting candidates for the cathodic protection of construction steels. This paper reviews the main features of Al−TM−(N) metallurgical coatings (TM: transition metal = Cr, Ti) deposited by reactive magnetron sputtering of composite Al−TM targets in Ar−N 2 reactive gas mixtures. In particular, both binary and ternary amorphous coatings always present the best compromise between mechanical properties and corrosion resistance. Generally, they present favourable intrinsic compressive stress associated with good compactness, a microhardness around 6000 MPa, a Young modulus of at least 110 GPa, better ductility and wear resistance than crystallized alloys, while maintaining a corrosion potential around that of iron, and an excellent pitting resistance in a chlorine environment. The thermal stability of such coatings is guaranteed for applications up to 300 °C. Their crystallization at higher temperature yields stable (Al 4 Cr) or metastable (disordered AlTi) phases.

Structure—properties relationship of metastable Al-Cr and Al-Ti alloys deposited by r.f. magnetron sputtering: role of nitrogen

Abstract Aluminium-based alloy films are potential candidates for anodic corrosion protection of construction steels. However, the mechanical properties of such coatings must be reinforced, for example by addition of transition elements. In this paper, we will first present some results concerning Al-Cr and Al-Ti coatings synthesized by r.f. sputtering of composite targets. We will further discuss the influence of nitrogen incorporation into the coatings reactively sputter-deposited in different Ar-N 2 mixtures. After a description of the structural and microstructural features of Al-Cr-(N) and Al-Ti-(N) coatings, we will link the effect of their composition upon their mechanical characteristics such as intrinsic stress, microhardness or wear resistance as well as upon their corrosion resistance. The as-sputtered Al-Cr and Al-Ti coatings always remain microcrystalline or amorphous solid solutions, and their microhardness can reach about 10 000 MPa. Nitrogen incorporation favours the amorphization of the coatings. An extensive-to-compressive stress transition, which is clearly related to a densification of the coatings, is also observed as the Cr or Ti content increases and when nitrogen is incorporated into the reactor. Owing to the mechanical properties obtained for coatings remaining anodic compared with the construction steel substrate, future applications in mechanics are expected.

Attempted modelling of thickness and chemical heterogeneity in coatings prepared by d.c. reactive magnetron sputtering

Abstract A macroscopic d.c. reactive magnetron sputtering model, based on previous studies, is presented. This model takes into account the heterogeneous target erosion and the whole reactor geometry. We will show that, with the help of reduced basic metallurgical experiments, the main physical data required for computation can be estimated and included in the model. With this, both the composition and deposition rate of the coating can be computed for any point of the reactor in good agreement with experimental results.

Sputtering of Al-Cr and Al-Ti composite targets in pure Ar and in reactive Ar-N2 plasmas

Al-Cr-(N ) and Al-Ti-(N) coatings were deposited on glass slides by reactive magnetron sputtering of composite concentric Al-Cr or Al-Ti targets in different Ar-N 2 gaseous mixtures (N 2 mass flow <2 sccm). When sputtering occurs in pure argon, the chromium or titanium content increases with the insert diameter of chromium or titanium. For a given insert diameter, the introduction of nitrogen into the reactor yields changes in the Ti/Al or Cr/Al content ratio in the coatings. Both plasma diagnostics, performed by optical emission spectroscopy, and computer simulation allow description of the mechanisms of matter transfer. The composition changes observed can clearly be attributed to a combined effect of differential nitriding of the target with respect to its current density distribution and to the nitride-to-metal sputtering yield ratios involved. However, preferential nitriding of the outer zones of the targets was also observed when the crown was aluminium (with titanium insert) or titanium (with aluminium insert). This behaviour can be related to the sputtering wind (wind effect), which has also been simulated by considering a sinusoidal attenuation of the impinging flux of reactive species from the external zone to the centre of the target.

Influence of the target temperature on a reactive sputtering process

Abstract Reactive magnetron sputtering often presents an unstable sputtering mode preventing high rate deposition of stoichiometric ceramic films, the origin of which is mainly due to the avalanche-like target poisoning over a critical reactive gas partial pressure. In this paper, the effect of the target temperature is investigated for titanium targets sputtered in Ar–N 2 or Ar–O 2 reactive discharges. Paradoxically, raising the target temperature yields a stabilisation of the transition sputtering mode close to the elemental sputtering mode. With a discharge current modulated at low frequency, the stabilisation is complete for titanium sputtered in Ar–N 2 discharges.

An optical emission spectroscopy study of a reactive magnetron sputtering Ar-O2 discharge modulated at low frequency

Abstract Recent studies have shown that the target poisoning often encountered during sputter-deposition of oxides could be controlled by means of a low-frequency modulation of the discharge power. In order to understand the basic mechanisms resulting in improvement of the deposition rate of stoichiometric oxides such as TiO 2 , optical emission spectroscopy (OES) measurements have been carried out in both stable and unstable sputtering conditions. The current study shows that stoichiometric coatings can be synthesised even if they are periodically submitted to an under-stoichiometric flow of particles at the end of the step ‘high’ of the modulation. This suggests the existence of a characteristic time compatible with the diffusion kinetics of chemisorbed oxygen atoms m the under-stoichiometric layer of the deposit created.

Influence of oxygen flow rate on the structural and mechanical properties of reactively magnetron sputter-deposited Zr–B–O coatings

Abstract Zr–B–O coatings were deposited on both construction steel and stainless steel substrates by magnetron sputtering of ZrB 2 targets in reactive Ar–O 2 gaseous mixtures. In this paper, the deposition rate and the structural and mechanical properties of Zr–B–O films will be investigated as a function of the inlet oxygen flow rate. We will show that two domains are available owing to the sputtering phenomena. In the former, the coating enrichment is quite proportional to the oxygen flow rate. Even for a weak oxygen enrichment, amorphization of the Zr–B–O films is observed which also decreases both their intrinsic internal stress and microhardness, thus improving the physical quality of the coatings. As the oxygen flow rate increases, the amorphous coatings, initially composed of a single-phase ZrB 2− x O x solid solution, present an increasing fraction of amorphous ZrO 2 , and then, of amorphous B 2 O 3 simultaneously with the disappearance of the metallic solid solution. Higher oxygen flow rates allow the synthesis of optically transparent amorphous oxide mixture films, the composition and mechanical properties of which become independent of the inlet oxygen flow rate.

Basic properties of sputtered 310 stainless steel-nitrogen coatings

Abstract 310 stainless steel (SS)-nitrogen coatings with a nitrogen composition up to 40 at.% (14 wt.%) were deposited on steel substrates below 300 °C by d.c. reactive magnetron sputtering in mixed Ar-N 2 discharges using amagnetic austenitic 310 SS targets. The chemical and structural characterization of the coatings is presented in relation to the deposition parameters, and some of their properties (internal stress, adhesion, microhardness etc.) are evaluated. The as-deposited coating structure is usually a metastable disordered supersaturated f.c.c. solid solution but, depending on the substrate temperature and nitrogen concentration, an amorphous-like structure is seen to occur.

Reactive magnetron sputtering as a way to improve the knowledge of metastable f.c.c. nitrogen solid solutions formed during plasma assisted nitriding of Inconel 690

Low temperature plasma assisted nitriding (PAN) treatments of Inconel 690 (a nickel base alloy with a 30 wt.% chromium content) produce a complex layer constituted by two different metastable f.c.c. solid solutions denoted (γN1 and γN2). In addition, different layer thicknesses are observed for differently oriented grains. Reactive magnetron sputtering (RMS) of Inconel 690 in argon–nitrogen containing mixtures is used to produce homogeneous films constituted by a well defined γN phase. In reactive conditions, the as-deposited coatings (T<100 °C) containing up to 30 at.% of nitrogen are nanocrystalline supersaturated f.c.c. solid solution (γN) with a grain size lying between 3 and 7.5 nm. A comparison between the products synthesized by these two techniques (PAN and RMS) is presented in this study. The formation of γN phases in low temperature PAN is discussed in the light of electron backscatter diffraction measurements and by comparison with X-ray diffraction patterns of RMS coatings. By using the results obtained in situ by RMS on heated substrates and by tempering of as-deposited films, the decomposition products of the γN phase at high temperature (⩾450 °C) or long PAN treatments at 400 °C are identified to be f.c.c. CrN and γ(Ni,Fe) depleted in nitrogen.

Stable and unstable conditions of the sputtering mode by modulating at low frequency the current of a magnetron discharge

It is well known that target poisoning in reactive magnetron sputtering often causes a time-unstable sputtering mode which can shift toward the reactive mode and a low deposition rate of stoichiometric compounds. This paper presents a description of the relevant time-dependent phenomena, followed by the effect of square-wave modulation of the discharge current with regard to a titanium target sputtered in Ar-O2 reactive mixtures. A high magnitude of the discharge current and low duty are favourable parameters, along with an appropriate frequency compatible with the pumping speed. This paper also discusses the effect of the shape of the modulation.