C. Louro

Hardness versus structure in W-Si-N sputtered coatings

In this research work the influence of the addition of Si on the structure, morphology and hardness of W‐(N ) coatings was studied. The films were deposited by reactive sputtering from a W target superimposed with increasing number of Si small plates. The partial pressure ratio between nitrogen and argon was varied in the range 0‐2. Thus, nitrogen content was in the range 0‐60 at.%, and the silicon-to-tungsten contents ratio reached, for the greatest number of silicon plates, 0.5. Depending on the nitrogen content, the structure of the films varies from the single b.c.c. tungsten phase to the f.c.c. NaCltype W 2 N. The synergistic action of both Si and N can originate the formation of amorphous structures. The hardness of the films was determined by ultramicroindentation technique by using either low loads (70 mN ) and/or an empirical model which allows to eliminate the influence of the substrate on the measured values. Hardness values as high as 50 GPa was obtained. Generally, the films containing amorphous phases present lower hardness values than crystalline ones.

How is the chemical bonding of W-Si-N sputtered coatings?

Thin films of WSiN, deposited by reactive magnetron sputtering, were investigated using X-ray photoelectron spectroscopy Ž. XPS . The objective of this research work was to study the chemical binding state of these coatings. During the sputtering process, the established atomic bonds may possibly not agree to the elemental bonding preview using the values of chemical affinity, which decrease for SiN, Si Wt o WN bonds. XPS data show that in W-based films which have simultaneous additions of Si and N, a SiN type is the preferential bond established. This behaviour is confirmed either by the evolution of the SiW atomic ratio of the as-deposited coatings, which increase with the N content, or the variation of the lattice parameters of the b.c.c. -W phase for WN, WSi and WSiN systems. However, since no compound of silicon nitride was detected, it was concluded that this phase, formed by specific contents of Si and N must have been present in the WSiN films in the amorphous state. 2001 Elsevier Science B.V. All rights reserved. Ž.

Thermal Oxidation of Tungsten‐Based Sputtered Coatings

The effect of the addition of nickel, titanium, and nitrogen on the air oxidation behavior of W-based sputtered coatings in the temperature range 600 to 800 C was studied. In some cases these additions significantly improved the oxidation resistance of the tungsten coatings. As reported for bulk tungsten, all the coatings studied were oxidized by layers following a parabolic law. Besides WO{sub 3} and WO{sub x} phases detected in all the oxidized coatings, TiO{sub 2} and NiWO{sub 4} were also detected for W-Ti and W-Ni films, respectively. WO{sub x} was present as an inner protective compact layer covered by the porous WO{sub 3} oxide. The best oxidation resistance was found for W-Ti and W-N-Ni coatings which also presented the highest activation energies (E{sub a} = 234 and 218 kJ/mol, respectively, as opposed to E{sub a} {approx} 188 kJ/mol for the other coatings). These lower oxidation weight gains were attributed to the greater difficulty of the inward diffusion of oxygen ions for W-Ti films, owing to the formation of fine particles of TiO{sub 2}, and the formation of the external, more protective layer of NiWO{sub 4} for W-N-Ni coatings.

The depth profile analysis of W-Si-N coatings after thermal annealing

Abstract The aim of this research was to examine in detail the effect of the thermal annealing process with temperatures up to 1200 °C on W-Si-N amorphous films deposited onto refractory steel substrates. Particular attention was paid to the analysis of the chemical composition and the mechanical properties through the coating thickness. The results obtained indicate that the crystallisation process leads to an increase in the hardness of the coating, while for annealing temperatures higher than 1000 °C a decrease was observed. Up to 1000 °C, no significant changes in properties were detected across the thickness of W-Si-N films. However, for higher annealing temperatures, particularly at 1200 °C, an important loss of nitrogen was detected at the surface layer, accompanied by elemental inter-diffusion between the substrate and the coating. As a consequence, a thin layer was formed at the interface, composed of a mixture of phases integrating elements from both the film and the substrate

The oxidation behaviour of mixed tungsten silicon sputtered coatings

Abstract W-Si-N coatings were deposited by sputtering and their chemical composition, structure, thermal and oxidation behaviour were characterised. Si-containing films are essentially amorphous. W 69 Si 31 film crystallises at 750 °C as α-W and W 5 Si 3 phases whereas no significant structural transformations were observed for W 24 Si 21 N 55 film up to 1000 °C, In both cases elemental diffusion (Si and N) for the substrate was detected after thermal annealing. These coatings present much better oxidation resistance than W and W 45 N 55 coatings.

The role of nickel in the oxidation resistance of tungsten-based alloys

Abstract In this work, W–N–Ni coatings were produced with different contents of nickel and nitrogen. The films were oxidised in pure air at increasing temperatures up to 800°C. An increase in the oxidation resistance upon the addition of an increasing amount of nickel was observed. The oxidation mechanism involves the formation of two nickel-rich, external oxide layers of NiO and NiWO 4 . The limiting step seems to be the outward diffusion of either Ni 2+ or W 6+ ions through the oxide layers. To retain the oxidation resistance of the amorphous W–N–Ni coatings at 800°C, it is necessary that crystallisation does not induce a dimensional stress capable of destroying the protective oxide layers.

Oxidation of sputtered W-based coatings

Abstract In this paper, a review of the influence of the addition of different chemical elements to some transition metal nitrides and carbides on their oxidation behaviour will be presented. The role of the addition of ‘reactive elements’ (RE) on the type of oxide phases formed, on the morphology of the oxide layers, on the oxidation kinetics and on the oxidation rate is emphasized. Examples of the system W–N/C when Ti, Ni and Si are added, will be shown. The beneficial action of the additional element on oxidation resistance can be due either to the formation of some type of protective oxide layer, apart from the typical oxides formed for those metal compounds, or to the blocking effect to the elemental diffusion, which is due to some type of compound precipitation in the diffusion paths.

USE OF ULTRAMICROINDENTATION TO EVALUATE THE DEGRADATION OF SPUTTERED COATINGS

Abstract The aim of this research work was to study the influence of chemical degradation on the mechanical properties of the non-degraded zones of sputtered W–N–Ti coatings. For this, an ultramicroindentation technique has been used before and after degradation, either on the zones between the pits in the corroded samples, or on the non-oxidised layers after removal of the top degraded layers. By using different applied loads it was possible to obtain results with and without the influence of the substrate. Thus, two types of results were obtained, those determined by using an empirical model, which eliminates the influence of the substrate, and those measured directly.Very small differences of less than 20% were obtained in the measured values before and after degradation, indicating that the non-degraded zones maintain their as-deposited mechanical properties.

Oxidation behaviour of W-N-M (M = Ni, Ti) sputtered films

Abstract In this research work we studied the influence of metal (Ni or Ti) addition on the oxidation behaviour of W-N coatings at different temperatures. For this purpose steel substrates were coated by reactive (N 2 ) sputtering composite W-10 wt.%Ti or Ni targets. The highest oxidation resistance was found for W-N-Ni, followed by W-N-Ti and finally by W-N. This behaviour is explained as a function of the as-deposited structure and morphologies of the coatings and of the compactness and type of the oxide formed. W-N-Ni films give rise to denser oxide scales than W-N films and to different types of oxide in comparison with W-N-Ti films. This behaviour is verified only up to 750 °C. Afterwards, the oxidation behaviour of W-N-Ni coatings is similar to the other films. This temperature corresponds to the crystallization onset of the as-deposited amorphous structure of W-N-Ni.

Influence of Heat Treatment on the Structure of W-Si-N Sputtered Films

W-Si-N sputtered coatings with different chemical compositions were deposited by magnetron sputtering and annealed in N2(H2) and Ar(H2) atmospheres up to 1400oC. The results show that the structure of the as-deposited films depends on their chemical composition. Films with (W+Si)/N ratios close to 1 and low silicon contents are crystalline and formed by either a �-WN or a �-W2N structure. High silicon contents induce amorphicity of these films. This type of structure was also observed in the as-deposited films with low nitrogen contents. The W46N54 and W36Si15N49 crystalline films maintain their initial structure up to temperatures of 1200oC. The amorphous W27Si20N53 coating crystallise into �-W + �-W2N at 1050oC. The �-W phase was detected in all the coatings with (W+Si)/N ratio close to 1 annealed at temperatures equal or higher than 1050oC. Concerning the coatings with lower nitrogen contents (W69Si23N8 and W64Si9N27) annealed in an argon atmosphere the results showed that they crystallize at relatively low temperatures (� 700oC). Simultaneously, the films lose nitrogen. The final Si/N atomic ratio of these annealed films corresponds to the stoichiometry of the Si3N4 phase. The higher is the silicon content, the lower is the nitrogen loss during annealing.