A. Billard

Structure/mechanical properties relationship of titanium–oxygen coatings reactively sputter-deposited

Abstract Ti–O coatings are DC sputter-deposited on HSS substrates from titanium targets in various Ar–O 2 reactive mixtures leading to different structural states, from pure titanium to TiO 2 films. These coatings are structurally (X-ray diffraction measurements) and mechanically (micro hardness and intrinsic stress measurements) characterised as a function of the inlet oxygen flow rate. Special attention is paid to the case of FCC TiO x (0.9 x x layers composition and internal stress are found to depend on deposition parameters like oxygen flow rate, substrate bias and its position with respect to magnetron axis. The intrinsic stress of TiO films is shown to control the cracking damage under high-load Vickers indentation as well as in single pass and multi-pass scratch testing operation. Finally, coating delamination is likely to occur by interface cracking triggered by ‘coalescence along the interface’ from two neighbouring through-thickness crack tips.

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.

Main features of magnetron sputtered aluminium–transition metal alloy coatings

Abstract Aluminium-based alloy coatings are potential candidates for galvanic corrosion protection of mechanical steel parts. This paper presents the microstructure and morphological characteristics as well as mechanical and corrosion properties for Al–TM systems (TM=Ti, Cr, or Fe) sputtered from single composite targets. The low temperature, as-deposited aluminium rich coatings are metastable supersaturated single or two-phase fcc, or amorphous solid solutions. It is shown that transition metal solubility in aluminium depends on TM to Al atomic size ratios. A good compromise between microhardness (7000 MPa) and low open-circuit potential can be achieved in Al–Cr and Al–Ti systems, whereas Al–Fe alloys cannot be used as sacrificial coatings above 10 at.% Fe for which the microhardness is only approximately 3500 MPa. The structure–properties relationship of amorphous or microcrystalline aluminium-rich, as-deposited films are discussed in detail.

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.

A way to decrease the nitriding temperature of aluminium : the low-pressure arc-assisted nitriding process

Abstract For the purpose of applications in mechanics, nitriding of aluminium has been performed in a high-current (300 A), low-voltage (20–45 V) and low-pressure (0.8 Pa) thermionic arc. Although nitriding of ferrous materials is efficient in this arc-assisted nitriding process even for unbiased workpieces, an additional negative substrate bias voltage is necessary to process aluminium. Ion bombardment is necessary not only for ion cleaning in an Ar–H 2 gas mixture but also for the nitriding treatment in Ar–N 2 . Under these conditions, a compact and continuous aluminium nitride layer with hexagonal AlN phase is formed on pure aluminium at 450°C. The kinetics of aluminium nitride formation at low temperature (between 340 and 460 °C) is characterized by a two-stage mechanism comprising first the nucleation and growth of nodular AlN grains, followed by the formation of a continuous AlN layer. The growth rate of the aluminium nitride layer seems to be controlled by the rate of the chemical reaction to form AlN, rather than the rate of nitrogen diffusion. Some tribological tests performed on the aluminium nitride layers are also reported in order to evaluate the improvement in friction and wear behaviour.

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.