Valentino Rigato

The structure of tribologically improved MoS2–metal composite coatings and their industrial applications

Abstract The properties of MoS2 coatings deposited by closed field unbalanced magnetron sputter ion plating have been improved by the co-deposition of small amounts of metal [D.G. Teer et al., Surf. Coat. Technol. 94–95 (1997) 572]. These initial MoS2–metal composite (MoST) coatings were hard, adherent (critical load above 120 N) low friction (μ=0.02 at 40% humidity), wear resistant and less sensitive to water vapour than pure MoS2 coatings. The MoST coating has now been further developed and improved to give a coating with higher wear resistance than that originally developed and has been tested in a variety of industrial applications, showing excellent results for a wide range of cutting and forming applications. Industrial testing of coated tools has been performed and the results are presented. Laboratory test results using microhardness testing, scratch adhesion testing, pin-on-disc and reciprocating friction and wear tests are presented. The structure of the coating has been extensively studied by a variety of techniques, including optical microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy. The MoST coating is deposited by rotating the substrates between four targets: three MoS2 and one titanium. Because of the way in which the coating is deposited, it was initially assumed that the coating deposited could be a multilayer coating consisting of alternating layers of MoS2 and titanium. TEM and XRD analysis has been unable to detect the presence of any multilayers within the coating. Further analysis has been carried out to determine the detailed structure of the coating and the location of the titanium. TEM analysis also revealed that the MoST coating was quasi-amorphous and selected-area diffraction was unable to detect any crystalline structure. Further analysis on the amorphous nature of this coating and its stoichiometry is presented.

Properties of sputter-deposited MoS2/metal composite coatings deposited by closed field unbalanced magnetron sputter ion plating

It has been proved that the co-deposition of a proper amount of titanium can significantly improve the properties of sputter-deposited MoS2 coatings, leading to the deposition of coatings, which are harder, much more wear resistant and also less sensitive to atmospheric water vapour during tribological testing. The MoS2/Ti coatings, studied here by the authors, also give excellent industrial results in a wide range of applications. In this paper the mechanical and micro-structural properties of MoS2 coatings containing titanium with molar fractions up to 0.44 are studied. The mechanical properties are tested with pin-on-disk and reciprocating wear tests (multipass) in different experimental conditions. Frictional force and friction coefficient against a number of cycles are recorded. Coatings composition is determined by Rutherford and non-Rutherford backscattering (RBS) and coating thickness is measured by cross-section scanning electron microscopy (SEM). Structural analysis is carried out using X-ray diffraction (XRD). The mechanical properties are presented as a function of the Ti content and the optimal titanium concentration is determined. Other metals like Cr, W, Zr and Mo are also investigated. The composition of MoS2/metal composite coatings of the same quality of the MoS2/Ti films (‘optimized’ coatings) is studied by RBS.

A study of the structural and mechanical properties of Ti=MoS2 coatings deposited by closed field unbalanced magnetron sputter ion plating

Abstract The closed field unbalanced magnetron sputter ion plating system has been used to produce MoS2 coatings by co-deposition of Ti and MoS2. Previous results have shown that the coatings are harder than pure MoS2 coatings and that they show very low coefficient of friction and longer lifetimes. These Ti=MoS2 coatings show excellent wear resistance at high loads. The coefficient of friction is as low as 0.02 at a relative humidity of 40%, and conventional adhesion scratch tests indicate critical loads in excess of 120 N. This paper presents a study of the composition, structure and mechanical properties of these metal=MoS2 composite coatings using several analytical techniques (RBS, ERDA, XRD, scratch test, SEM and EDS). The analytical results are discussed and related to the mechanical properties of the coatings.

Multilayer nitride coatings by closed field unbalanced magnetron sputter ion plating

Abstract A range of ceramic multilayer nitride coatings, including NbxTiyN, NbxZryN, and MoxZryN, has been successfully deposited onto tool and die steels by closed field unbalanced magnetron sputter ion plating. The mechanical properties of the resulting coatings, including adhesion and hardness, have been evaluated by micro- and nano-indentation techniques, scratch testing etc. HPlast hardness up to 53 GPa has been achieved in the case of the NbxTiyN multilayers, 43 GPa for NbxZryN and 38 GPa for MoxZryN. The detailed microstructure of the coatings, including their respective nm-scale multilayer repeat distance, Λ, has been studied by advanced analytical techniques including high resolution transmission electron microscopy, ion backscattering (Rutherford, and non-Rutherford), and glancing angle X-ray diffraction, and focused ion beam machining and imaging. The coating properties are reviewed and correlated with the analytical data. Potential applications for, and some limitations of, the selected coatings are discussed.

Glow discharge vapour deposition polymerisation of polyimide thin coatings

Abstract The glow discharge vapour deposition polymerisation method for the preparation of polyimide coatings is presented. This method involves the deposition of the monomer vapours onto a solid substrate in a vacuum chamber. The evaporation of the monomers is obtained by using standard radio frequency magnetron sputtering equipment: the monomer powders are placed on a sputtering source and bombarded by low energy He ions at pressures in the range of 10–20 Pa. By a momentum-transfer mechanism, the ions dissipate their energy causing target heating and sublimation of the monomer molecules. The emission of volatile species from the monomers target surface and their interactions with the plasma components are studied by mass spectrometry and optical emission spectroscopy. Polyimide coatings with thickness ranging from 200 nm to 3 μm have been deposited onto silicon single crystals, sapphire and polyimide substrates. SEM micrographs of these coatings show a homogeneous and crack-free surface. The as-deposited films are mainly made of a mixture of polyamic acid and non-reacted monomers, as revealed by Fourier transform infrared spectroscopy. The imide peaks clearly appear only after thermal treatment (typically 250°C, 1 h, in air or argon). The electrical properties of the polyimide coatings (i.e. bulk and sheet resistivity and dielectric breakdown voltage) are those typical of an insulating material and comparable with the commercial polyimide. These films are characterised by good adhesion to the silicon, sapphire and other polyimide substrates. Chemical resistance to some reagents (acetone, hydrogen peroxide, nitric and hydrofluoric acid) has also been tested.

Study of the contaminant transport into granite microfractures using nuclear ion beam techniques.

Hydrated bentonite is a very plastic material and it is expected to enter in the rock microfractures at the granite/bentonite boundary of a deep geological high-level waste repository. This process is enhanced by the high swelling pressure of the clay. Since bentonite has a very good sorption capability for many radionuclides, the displacement of the clay might lead to a clay-mediated contaminant transport into the rock. The aim of this work is to study the contaminant transport into granite microfractures using nuclear ion beam techniques, and to determine to what extent the clay can favour it. To do so, bentonite previously doped with uranium, cesium and europium was put in contact with the surface of granite sheets. Granite sheets contacted with non-doped bentonite and with radionuclide solutions were also prepared as references. This allowed analysing the differences in the diffusion behaviour of the three systems: clay, radionuclides and clay plus radionuclides. A combination of Rutherford backscattering spectrometry (RBS) and other nuclear ion-beam techniques such as particle-induced X-ray emission (PIXE) and microPIXE was used to study the depth and lateral distribution of clay and contaminants inside granite. It was also tried to evaluate not only the diffusion depth and diffusion coefficients but also the different areas of the granite where the diffusants have a preferential access.

Design, deposition, and characterization of multilayer coatings for the Ultraviolet and Visible-Light Coronagraphic Imager

The Ultraviolet and Visible-Light Coronagraphic Imager is the baseline coronagraph for the European Space Agencys payload Solar Orbiter, a solar mission whose launch in 2011 is expected. To prove the feasibility of its innovative design, a sounding rocket version of the same instrument has been approved by NASA. One of the main technological challenges of the instrument is the achievement of multilayer optical coatings with suitable properties. We describe the design, fabrication, and characterization of such coatings.

RBS and μPIXE analysis of uranium diffusion from bentonite to the rock matrix in a deep geological waste repository

Abstract The possible diffusion mechanisms of uranium from bentonite to the rock matrix at the granite/bentonite interface of a deep geological radioactive waste repository were studied by means of Rutherford backscattering spectrometry and proton induced X-ray emission. The effects of the presence of clay on uranium diffusion into the granite were analysed. It has been shown that uranium diffuses into the granite either in the presence or in the absence of bentonite, but in presence of the clay, the diffusion coefficient is two orders of magnitude lower. Different diffusion paths were clearly observed in both cases: the uranium in solution penetrates in the rock specifically through certain minerals (Fe–Ti bearing ones), whereas in presence of the clay, the main uranium diffusion paths are defects of the rock (grain boundaries, micro-fractures).

Ion beam analysis and Raman characterisation of coatings deposited by cosputtering carbon and chromium in a closed field unbalanced magnetron sputter ion plating system

Abstract A novel non-hydrogenated a-C coating has been deposited by sputtering carbon and chromium targets in a closed field unbalanced magnetron sputter ion plating system, with a metallic adhesion layer between the substrate and the cosputtered layer. The properties of these new carbon coatings have been previously compared with those of more conventional hydrogenated metal-containing DLC. Reciprocating and pin-on-disk friction and wear tests have shown that the new carbon coatings have a lower friction coefficient and that their wear resistance is more than one order of magnitude higher than metal-containing hydrogenated DLC coatings. This paper presents an initial study of the composition and structure of these new coatings. Rutherford and Non-Rutherford backscattering of alpha particles and protons are used for studying the stoichiometry and for the determination of impurities. Elastic recoil detection analysis (ERDA) is used for depth profiling of hydrogen. Raman spectroscopy is used to investigate the structure of the different carbonaceous layers.

Surface treatment of HV electrodes for superconducting cyclotron beam extraction

At the Istituto Nazionale di Fisica Nucleare Laboratorio Nazionale del Sud of Catania, a K=800 superconducting cyclotron is in operation since 1994. The accelerator beam extraction is accomplished using electrostatic deflectors constituted by HV electrodes generating electric fields to 140 kV/cm in 6 to 8 mm gaps. The electrostatic performance is strongly dependent on the presence of high magnetic fields (up to 5 T) and of high ion energy and intensity of heavy ion beams. The behavior of these systems is mainly determined by the field electron emission effects on the cathode surface, by electron focalization processes due to the magnetic field and by heating effects due to the ion beams. A detailed study of these effects is under development starting with point, the characterization of the electrode surface vs. the material used (Ti, Cu, stainless steel, etc.) and of the best mechanical and chemical treatment available, Moreover, we are studying the electrostatic performance by using titanium surfaces and nitriding over a wide temperature range (300 to 800/spl deg/C). Preliminary results concerning the use of thermal processed polished surfaces are presented and discussed.