Nanostructured coatings for engineering tribological applications

The use of a nano-phased powder (specifically WC/Co) for thermal spray application produces a coating which has the potential for a dry sliding friction coefficient which is lower than that obtained when using conventional powders. Hence, the coatings have the potential for application in many adhesive sliding situations where reduced friction would be desirable. The current status is that the findings will be exploited through commercial interaction with MBN and the first applications to be explored by Poeton will be engine cylinder bores and anti-scuffing coatings, where powders is blended with a solid lubricant.

The present result has been reached as a result of the execution of the WP3 of the Project. The main objectives of the WP3 were: - Development of technology prototype of thermo-chemical diffusion synthesis of solid lubricant compositions based on MeS2 directly in surface layers of friction parts; - Correlation between solid lubricant layers properties and process parameters; - Fabrication of samples for wear tests by the developed method of thermo-chemical diffusion synthesis. Great interest to technologies of deposition of molybdenum disulphide (MoS2) is caused by outstanding tribological properties of coatings and films. The good tribological properties of MoS2 are determined by its structure and weak bonds between S-Mo-S layers (perfect cleavage). Lubricating properties of molybdenum disulfide are especially effective in vacuum and inert gas environments over a wide range of temperature and load. Nowadays, PVD methods are successfully used in production of high-performance MoS2 coatings. However, high production cost of the methods is the strong factor limiting their wide industrial application. The alternative method of deposition of MoS2 coatings (thermal-diffusion synthesis) is presented. This method of synthesis of MoS2 coatings comprises two stages: (a) formation of the molybdate and molybdenum oxide layer on the surface of the steel substrate; (b) treatment of the substrate in the vaporous sulphur environment to form MoS2. Thickness of the MoS2 coatings varies within 5-60 microns, transition zone on the coating/substrate boundary is observed. Friction tests carried out under ambient conditions at 625-1250 MPa contact pressure showed values of the friction coefficient 0.17-0.07 respectively. Diffusion methods of applying of synthetic molybdenum disulfide solid lubricant coatings provide the great possibilities for operational reliability of tribological assemblies operating in hostile environments (vacuum, radiation and oxygen environment, etc.) as compared with the other methods (PPS-powder plasma spraying, DPD-powder detonation deposition, PVD-physical vapour deposition, CVD-chemical vapour deposition, LCVD-laser induced chemical vapour deposition, etc.). Current status of the results: the technology prototype was developed, the equipment prototype was designed, fabricated and installed at ENISE, numerous thermo-diffusion coatings fabricated and examined.

CSGI has acquired a considerable understanding and knowledge of Nanopowders behaviour in thermal spraying as correlated to manufacturing by high-energy milling. Materials design approaches experimented allowed to conceive the composite systems. A number of characterization techniques were established for powders, coatings and consolidated materials, to assess the structure and properties. Consolidation work, allowed learning the kinetics of densification under HIP conditions. The knowledge acquired will serve to CSGI in further developmental activities in the field of nano-materials for wider use even beyond thermal spraying and cladding.

Laser Cladding Technology of laser cladding through coaxial nozzle to fabricate coatings with tailored properties is developed. Two methods of wear-resistant coatings deposition were employed: injection of powder into the melting pool, created by laser beam and laser cladding from pre-deposited layers. Special coaxial nozzle has been developed to realise deposition process. The following powder blends were tested: FeCu (60/40 %Wgt) nano-composite alloy powder; FeCu+WC/Ni(802/0 %Vol); CuSn (50/50 %Vol) alloy; CuSn+WC/Ni (80/20 %Vol); FeCu+WC/Ni (80/20%Vol); FeCu (60/40 % Vol); WC/Ni (83/17 %Vol) and several others. The coating Stellite/ CuSn/ nanoWC/ Co with Solid Lubricant matrix reinforced both by internal structure (Stellite lines) and dispersed ceramic phase have shown the best performance with stable dry friction coefficient 0.12. The main components distribution for that coating shows gradual change in their concentrations: decrease of Cu and Sn and increase of WC from the surface to the depth 100mm. It is proved that radiation mode is one of the critical parameters defining the coating¿s microstructure (and thus, wear properties). Application of pulsed-periodic mode provides much better component remixing (e.g. in a case of coatings producing from Metal Matrix Composites) in comparison with continuous wave mode. It is proved that the different modes of laser action and affect coating microstructure (for the same composition) and result in different friction coefficients - 0.45 and 0.25 for continuous wave and pulsed-periodic mode, correspondingly. Laser Alloying Different methods of alloying of aluminium and iron were tested. Original techniques (in particular, multi-layer coatings deposition) were employed in order to provide uniform distribution of doping elements, micro-hardness, tribologic properties, etc of the modified layers. Laser alloying of systems with great differences in physico-chemical properties such as: Al-Sn(In, Pb, Fe), Fe-Sn(Pb), Al-Fe-Sn was performed. Various types of laser surface treatment (continuous wave and pulse-periodic Nd:YAG lasers) were utilised and related to modified zone properties. Present research employed different original methods of samples preparation: (a) the collage of a sheet of the metal to be deposited; (b) the insertion by pressure or by melting of a wire containing the alloying elements inside a triangular groove on the substrate; (c) the pre-deposition of multi-layer coatings: pressing on the base material surface, several sanded metallic films (50µm thick);and (d) the application of powder mixture instead of metallic films. Current status of the results: the most promising coatings are under detail examination of structure and optimisation of laser cladding parameters.

IfU has developed a software application of modern simulation methods for solution of various problems related to tribology and durability analysis. Among them there are the following problems, which were solved: - Strength Analysis for Turbine Blades After Building Up Restoration (coating deposition, coating destruction); - Stress Analysis of the Glue Layer between friction parts in Knee Joint Model Under different boundary conditions and loads; - Dynamical Analysis of the bearings with Respect to their Wear; - Wear of cutting tools for machining. This software application is based on finite element analysis method. The main approach applied for simulation of tribological problem of LTS (wear behaviour and durability analysis of butterfly valve) is application of finite element analysis method to simulate dynamic behaviour of real valve (contact stresses, friction coefficient, contact temperature, etc.). The main goal is to replace expensive tribological trials of butterfly valve by virtual experiments allowing choosing correctly the coatings to be applied under specific exploitation conditions.

A fast and robust diagnostic system was developed for real-time monitoring optical emission of thermal spraying processes. The working principle is an acousto-optic interaction; therefore the AOS has no moving components inside. In contrast to conventional spectrometers, in which diffraction gratings, prisms or Fourier transforming methods performs the spectral dispersion of a broadband optical signal, an acousto-optic tuneable filter (AOTF) is used as a narrow band wavelength filter. That's why it is not necessary to have an entrance slit (one dimensional) and it is possible to use a two dimensional aperture. That means the AOS allows detecting extreme small light appearances. The wavelength selection is realised by an acousto-optic interaction inside of an anisotropic SiO2-crystal. An ultrasound wave is applied to the crystal, inducing a periodical oscillation of the refractive index. This resonant acousto-optic interaction at such a phase grating results in a refracted light wave that is turned in the polarisation direction. The AOS supports 3 modes of operation for process control: Spectral scanning mode (called spectrum mode): The spectrum mode delivers complete intensity information from a wavelength range. The wavelength range is scanned by stepping through with acquisition time of minimum 5 milliseconds per step. Characteristic emission lines can be found in the spectrum using an automatic peak search tool. An advanced emission library tool based on data from the library supports the identification of the lines found in the spectrum. Chronogram / time tracking mode: The chronogram mode uses the advantage of fast random wavelength access. The AOS spectrometer control software supports the quasi-simultaneous watching of up to 16 spectral emission lines with an acquisition time of down to 5 milliseconds each. The total time resolution depends from the number of selected emission lines. This mode is connected with an analogue output, which can be set to values calculated by a user-defined algorithm from the measured intensities. The chronogram mode can be used to monitor or to control thermal spraying processes.

Innovative technology for nano-structured mini-parts (power chain bushings and washers) fabrication by high-density powder metallurgy was developed. The technology ensures to achieve density of 7.7-7.75g/cc for powdered steels. Surface nano-structuring was performed by novel impregnation and surface deformation methods. Both high tribological properties (low coefficient of friction and high wear resistance) and high accuracy of components produced is achieved. Large scale manufacturing technology was elaborated. The nano-structuring operations and whole high dense Powder Metallurgy Technology route were elaborated in detail. The new design of nano-structured self-lubricating chain bushings with edge cones was developed and impregnation technology was elaborated. The solid lubricant surface modification of high dense washers was applied. The exploitation tests of new nano-structured mini-components were performed. The application potential of two nano-structured mini-components in Europe is about 2-5mln pieces per year

Wear and friction tests have been applied to materials developed in the frame of the project with a wide range of conditions covering: - typical working conditions of components of end users of the project. This implies the choice of counter-face materials, contact pressure, speed, and temperature - other generic conditions to be able to widespread the results to other applications. Tribological tests included: - Friction behaviour test. - Erosion resistance test. Friction behaviour of the various coatings developed was assessed; the assessment was made on 2 genuine tribometer allowing performing test with different loads and with different working temperature. An adapted test bench was used to assess the behaviour of new coatings when submitted to solid particles erosion with different impingement angle - it was shown that there were discrepancies between the different powders tested, and that some of the powders developed for the project conducted to improved friction behaviour - in the particular case of erosion tests that cannot be performed with a standardised apparatus benchmarks were chosen among already existing materials. The first results obtained are encouraging: - among materials tested, some of them match the objectives defined in the project in term of low friction or in improved erosion resistance. Results have now to be confirmed on optimised materials and also real parts

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