Vasco Teixeira (University of Minho)

Research interest:

Multilayered and nanocomposite coatings: nanostructured and gradient PVD coatings, TCOs-transparent conductor oxide films, optical and decorative coatings, selective nanograded coatings for high-efficiency solar thermal collectors, solar energy, smart and sensor thin films, thermal barrier coatings,residual stress analysis.

 

Nanostructured and gradient PVD coatings

Inroduction:

Conventional materials have grain sizes ranging from microns to several millimeters and contain several billion atoms each. Nanometer sized grains contain only about 900 atoms each. As the grain size decreases to the nanometer range, there is a significant increase in the volume fraction of grain boundaries or interfaces. A nanostructured crystalline material is one in which the spacing between lattice defects approaches inter-atomic distances. These characteristics strongly influence the chemical and physical properties of the material. For example, it has been found that nanostructured ceramics are sometimes tougher and stronger than the coarser grained ceramics and nanophase metals exhibit significant increases in yield strength and elastic modulus. It has also been shown that other properties (electrical, optical, magnetic, etc) are influenced by the fine grained structure of these materials. Physical Vapor Deposition (PVD) processes, such as magnetron sputtering have shown to be powerful methods for the synthesis of nanostructured ceramic thin coatings. Magnetron sputtering is a non-equilibrium process induced by collision processes of high-energy particles . This is a complex method to deposit thin films and coatings, because small changes in deposition parameters induce radical changes in physical properties of coatings. For the deposition of composite multiphase coatings this technique allows to control at an atomic level the addition of the elements to the coating matrix, and thus providing an improved uniformization of the phases, e.g. nanocrystalline phases embedded in an amorphous matrix such as ZrO2-Al2O3 or TiN-Si3N4 coatings.

Interfaces and grain boundaries are key parameters in designing nanolayered coatings. Internal interfaces in materials are extended defects including grain boundaries and interphase boundaries, found in almost every engineering materials. Interfaces play a crucial role for the performance of layered composite coatings. Extreme service conditions sometimes prevail such as high temperatures, aggressive surrounding media, interdiffusion, wear, etc. It follows that the structure of interfaces must be regarded from the point of view of the whole complexity of the thermodynamic conditions during fabrication and exploitation of coated components, frequently in thermodynamic non-equilibrium. Therefore, interface engineering is an important field of coating materials science and engineering. For many technological applications, the control of internal interfaces, including the orientation relationship, the interfacial misfit, dislocations, segregation, and of interface kinetics play a crucial role.

1-Ceramic coatings for energy systems

Besides the class of super-hard coatings based on nitride nanocomposite layers, oxide coatings like ZrO2 and Al2O3 are very interesting for hard and protective applications at high temperatures and optical applications, e.g. where high laser power is used. ZrO2 bulk ceramics and thin coatings are very interesting materials because of their outstanding mechanical, thermal, optical and electrical properties. Zirconia has a high melting point, high resistance to oxidation, low thermal conductivity, high hardness and high thermal expansion coefficient. These ceramic coatings are widely use in many technological applications such as heat resistant layers and TBC’s], optical coatings, for memory cells,, buffer layers for growing superconductors, chemical sensors for automotive applications , e.g. oxygen sensors and ion conductors, etc.

Zirconia based coatings, because of optical properties (high refractive index, low absorption over a broad spectral region from near-UV to mid-IR, high pulse laser damage threshold), thermal properties (low thermal conductivity and high thermal expansion coefficient), high dielectric constant, mechanical properties (high fracture toughness, thermal shock resistance) and corrosion-resistant properties at high temperatures, have many important scientific and technological applications. Physical Vapor Deposition (PVD) techniques, in particular magnetron sputtering are suitable technologies to deposit ceramic coatings with tailored structure (nanocomposite and multilayered coatings). Zirconia-Alumina (ZrO2-Al2O3) nanolayered thin coatings are a new method to stabilize the zirconia high temperature tetragonal phase at room temperature. ZrO2-Al2O3 transformation-toughening nanolaminates were prepared by reactive magnetron sputtering. In this project it is studied the structural properties of ZrO2-Al2O3 nanostructured coatings. A relationship between nanolayer concepts and nanoscaled coating architecture, properties of zirconia ceramics and structural stability study of zirconia coatings is established. Coatings of pure (undoped) zirconia presents a monoclinic phase with traces of tetragonal. The nanostructured coatings present a ZrO2 polycrystalline phase (monoclinic and tetragonal phases depending on the ratio of thickness in the nanolaminated structure) and an Al2O3 amorphous phase. The ZrO2 high temperature tetragonal phase content increases, as the nanolayers get thinner. After annealing in air at 1000ºC the alumina is preserved in amorphous state and the quasi-amorphous tetragonal zirconia nanosized grains crystallizes to tetragonal phase without any monoclinic transformation.

References:

[1] "Nanostructured ceramic coatings produced by magnetron sputtering", Vasco Teixeira (invited paper), Advanced Research Workshop on Nanomaterials and Coatings, Kiev, NATO series E-Appl Sci., Kluwer Acad. Publ., 2002 (in print)

[2]V. Teixeira, A. Monteiro, J. Duarte and A. Portinha ,"Deposition of composite and nanolaminate ceramic coatings by sputtering", Vacuum, Volume 67, Issues 3-4, (2002),p. 477-483

[3]Pengtao Gao, L. J. Meng, M. P. dos Santos, V. Teixeira and M. Andritschky ,"Study of ZrO2/Al2O3 multilayers", Vacuum, Volume 64, Issues 3-4, (2002) p. 267-273

 

2- Graded nanocomposite selective coatings based on chromium oxide and titanium oxynitride

In way to improve the performance of thermal solar collectors, is of greatest importance the development of good, durable and reproducible selective solar absorber surfaces, for photothermal conversion. Selective graded nanocomposite thin films were produced by D.C. magnetron sputtering of metallic Chromium and Titanium targets at constant target current, constant substrate bias and temperature. These films are graded ceramic-metallic nanocomposites, with a decreased gradient of metal concentration, from the substrate to the surface. Basically, these films, are formed with one cermet layer rich in metal, one or more cermet layers with less metal fraction than the first one, and the last is only ceramic layer (the antireflection layer). The metallic component, was deposited by D.C. non-reactive sputtering, while ceramic component, was deposited by D.C. reactive sputtering in an Argon and Nitrogen atmosphere or Argon an Oxygen.

The metallic and ceramic components of the cermets, were deposited in a sub-layer system, consisting in alternating metallic and nitride or oxide sub-layers.The sub-layers thickness was changed in a way, to change the metal volume fraction from the bottom to the top of the film. The effect in the optical properties of the metallic fraction in the layers and the number of sub-layers in the film is studied. Reactive DC magnetron sputtering allows to produce nanocomposite and graded coatings with a well defined portion of the metallic particles, whereby this project is concentrated on the systems Cr-Cr2O3 and Ti-TiNxOy. Furthermore, depending on deposition conditions of the sputtering processes, the solar absorber coatings deposited by sputter processes may be single layer, multilayers or gradient coatings where the metallic composition within the dielectric matrix varies across the coating thickness and in a controlled state of compressive stress at room temperature. For films based on Chromium, the optimum film selectivity achived was a solar absorptance of 94% and a thermal emitance oh 6%, at 82 degrees. For Titanium based films, the best selectivity achived was a solar absorptance of 91% and a thermal emitance of 4%. The microstructure and thickness were studied by scanning electron microscopy (SEM). The surface microtopography was analysed by atomic force microscopy (A.F.M.) and the metal concentration profile by Rutherford Backscattering Spectrometry (R.B.S.)

References:

[1]"Graded cermet selective coatings based on chromium and titatium", C. Nunes, V. Teixeira, m. L. Prates, N.P. Barradas, A. D. Sequeira,in "Advanced Coatings on Glass & Plastics for Large-Area or High-Volume Products", ed. C.P. Klages, H.J. Gläser, M. A. Aegerter, Braunschweig, Germany, p. 547-553,2002

[2]V. Teixeira, E. Sousa, M. F. Costa, C. Nunes, L. Rosa, M. J. Carvalho, M. Collares-Pereira, E. Roman and J. Gago ,"Chromium-based thin sputtered composite coatings for solar thermal collectors", Vacuum, Volume 64, Issues 3-4,(2002), p.299-305

 

Thermal barrier coatings

The major challenge facing the power generation industry moving into 21st century will be to achieve the targets of increased efficiencies brought about by stringent environmental regulations, whilst ensuring reliability, availability, maintainability and cost are not compromised.

The main objective of this project is to develop and study the thermo-mechanical behaviour of new advanced composite multilayered and nanostructured coatings for use at high temperature to protect metallic components in high temperature applications, such as in aeroengines and land-based gas turbine engines. Demands for increased energy conversion efficiency and performance in gas turbine engines are met in part by increasing combustion temperatures and reducing cooling systems. Temperature increases within the engine leads to the service limits of current superalloys, and thus an improved thermal and insulation protective coating system should be developed. Thermal Barrier Coatings (TBC's), which consist, traditionally, on a thick ceramic top coating (usually ZrO2Y2O3) deposited by Atmospheric Plasma Spraying (APS) on Ni superalloys pre-coated with a metallic bond coating (typically NiCrAlY) applied by Vacuum Plasma Spraying (VPS) are currently the state-of-the-art. New directions for advanced coatings in protective systems are being considered within this research project, taking in account the energy efficiency and the low emissions demands for european power plants. In the field of TBC’s, our group has produced and tested PVD ceramic coatings (stabilised zirconia with yttria and/or alumina) as diffusion gas barrier between the APS top coating and high temperature metallic substrates, such as cast Ni-based alloys and the new generation of single crystal superalloys. Furthermore, magnetron sputtered composites in the form of nanolaminates of ceramics and graded coatings will be developed by our research group to protect alloys used in advanced gas turbine components. The high temperature superalloys to be used will be the alloys that are currently used in real gas turbines applications and also the new generation of single crystal alloys. The composite multilayered PVD coatings will be used as diffusion barriers to protect at high temperature from oxidation and corrosion the gas turbine components. An innovation aspect of the project will be the application of this advanced surface coatings onto the new generation alloys that are being developed by european programmes in order to increase the corrosion/oxidation and interdiffusion resistance, with the possibility of combination with the thick ceramic plasma sprayed coating to be used as advanced thermal barrier coating. A key issue is the coating stress analysis and failure mechanisms at high temperature. Among the fundamental and applied material science studies to be carried out during this project, both for the advanced PVD coating systems and Plasma Sprayed TBC’s, envisages a coherent study of the following topics: coating microstructure, phase analysis, mechanical characterisation, elastic properties, residual stress measurements (X-ray diffraction, interferometry, substrate curvature by microdisplacement laser transducer and micro-Raman spectroscopy), stress modelling of multilayered and functionally gradient coatings (numerically and by Finite Element Programs), coating adherence determination, high temperature testing (furnace cycling, plasma torch rapid thermal cycling, thermalshock), interfacial decohesion and failure mechanisms analysis (experimental observations and life time models). Both the stress modelling and thermal cycling analysis will be an useful tool for evaluating the thermo-mechanical performance of the multilayered and graded protective coatings.

References:

[1]"Analysis of Residual Stresses in Thermal Barrier Coatings" ,V. Teixeira, M. Andritschky, W. Fischer, H.P. Buchkremer, D. Stöver, Journal of Materials Processing Technology, 92/93, p. 209-216, 1999

[2]"The effect of temperature of deposition and thermal cycling in residual stress state of zirconia based thermal barrier coatings".,V. Teixeira, M. Andritschky ,W. Fischer, H.P. Buchkremer, D. Stöver, Surface and Coatings Technology, 120/121, p.103-111, 1999

[3] V. Teixeira, M. Andritschky, H. Gruhn, W. Mallener, H.P. Buchkremer, D. Stöver , "Failure of Physically Vapor Deposition/Plasma-Sprayed Thermal Barrier Coatings During Thermal Cycling", Journal of Thermal Spray Technology, 9(2), p. 191-197, 20

[4]Gao, P., L.J. Meng, M. P. dos Santos, V. Teixeira, M. Andritschky, "Study of ZrO2 –Y2O3 films prepared by magnetron reactive sputtering", Thin Solid Films, 377, 32 (2000).

[5]Gao, P., L.J. Meng, M. P. dos Santos, V. Teixeira, M. Andritschky, "Characterisation of ZrO2 concentrations in the sputtering gases", Vacuum, 56, 143 (2000).

 

 

Optical and decorative coatings

1-Smart optical devices (electrochomic and thermochromic)

Thin films of many transition metal oxides such as tungsten oxide are well know to exhibit electrochromic properties. An electrochromic layered coating is characterised by its ability to show reversible and persistent changes of the optical properties under the action of electrical fields. The interest in such coatings has increased in the last few years because of their potential applications in a wide variety of optical modulation devices including energy efficient smart windows for solar control in architectural and automative applications.

Since last three decades, Electrochromic (EC) windows have been finding several applications like in anti-dazzling rear view mirrors, smart windows, for contrast-enhancement in some emissive display devices and in non-emissive large-area color display in information advertisement. The tungsten oxide (WO3) is one of the best choices for the primary working electrode in EC devices (ECDs). Multi-layer films based on WO3 , V2O5 and ITO (indium tin oxide) was mainly deposited by reactive dc magnetron sputtering onto glass substrates.

In this project we study the microstructure, the crystalline structure and the optical properties of tungsten oxide thin films deposited on glass and indium-tin oxide (ITO) coated glass. The films were produced by reactive DC magnetron sputtering of tungsten in plasma of argon and oxygen at different sputtering pressures and substrate temperatures for a constant target current, substrate bias and substrate-target distance. The film thickness and microstrucuture were studied by scanning electron microscopy. The surface microtopography was analysed by atomic force microscopy (AFM). X-ray diffraction (XRD) analysis was carried out to determine the degree of crystallinity and crystalline structure, grain size and lattice parameters of the produced thin films. Spectral transmittance in the visible and near infra-red of the deposited oxide films was measured using a double-beam spectrophotometer. Refractive index and absorption coefficients were calculated. From Bruggeman effective medium approximation we have calculated the packing coating density. The surface electrical resistivity of the tungsten oxide sputtered coatings was also determined by the four-point probe method. The device performance was studied by cyclic voltammetry using electrochemical Li intercalation/deintercalation, and ensuing optical data were recorded by spectrophotometry. The absorption mechanism in the electrochromic layered coating coloring under charge insertion is attributed to an excess charge localized on a transition metal site being transferred to a neighboring site upon photon absorption.

References:

[1]V. Teixeira, H. N. Cui, L. J. Meng, E. Fortunato and R. Martins, "Amorphous ITO thin films prepared by DC sputtering for electrochromic applications" ,Thin Solid Films, Vol.420-421,(2002), Pages 70-75

[2]Hai-Ning Cui, V. Teixeira, A.. Monteiro, "Microstructure study of indium tin oxide thin films by optical methods", Vacuum,Volume 67, Issues 3-4, (2002) p.589-594

[3]A. Monteiro, M. F. Costa, B. Almeida, V. Teixeira, J. Gago and E. Roman,"Structural and optical characterization of WO3 deposited on glass and ITO", Vacuum, Volume 64, Issues 3-4,( 2002) p. 287-291

2-Metallic Decorative environmentally friendly coatings based in Ni and Ni-Cr

Decorative coatings are generally applied on brass and plastic components with Ni and Cr normally by an electrolytic process (electrodeposition) Among the most wanted coatings are the hard decorative ceramic coatings ZrN, TiN, TiAlN, etc.. For the current production of a PVD decorative coatings are necessary this two distinct production processes (electrodeposition followed by PVD). Besides this aspect, the electrodeposition technique uses volatile organic compounds (VOC’s) and produces industrial wastes that are a very serious environmental problem. These facts will make the environmental regulation more restrictive, in a future not very far.

This project has the objective to substitute the electrodeposited coatings by applying PVD coatings which is an environmentally friendly deposition process.

Some potential applications for PVD decorative coating (metallic and coloured):

-Automobile parts

-Furniture, cutlery, bathroom fittings

-Household appliances and sports equipment

-Watch cases, luggage accessories, imitation jewellery

-Telecommunications devices and consumer electronics

Ni, NiCr and Cr thin coatings on brass, glass and ABS plastics substrates were deposited by magnetron sputtering technique. These thin coatings are interesting for optical, decorative and corrosion resistant layers. The main objective of this R&D project is to develop decorative metallic coatings and provide also an optimised interlayer to grow other hard coloured coatings such as TiN, ZrN, TiAlN and TiNxOy PVD coatings.

The influence of the sputtering gas pressure, the applied bias and substrate temperature during the deposition process on the microstructure and surface roughness were studied. We will use a phenomenological model of energetic bombardment of the growing film that explains the changes in the film microstructure and surface micro-roughness. AFM coating micro-roughness measurements were performed to get a more physical basis understanding of influence on the physical properties of the conditions of sputtering, energetic bombardment and surface mobility. The coating optical properties were also measured to correlate the changes of surface optical reflectivity of the layers with the observed changes of roughness. The topics discussed should provide some insights regarding the development of a methodology for mesoscopic modelling of the surface roughness evolution in sputtered coating systems where optical quality and surface characteristics such as wear resistant and corrosion resistant are a concern for technological applications.

References:

[1]"Residual stress mapping in PVD optical coatings", M. F. Costa, V. Teixeira, A. Monteiro, in "Advanced Coatings on Glass & Plastics for Large-Area or High-Volume Products", ed. C.P. Klages, H.J. Gläser, M. A. Aegerter, Braunschweig, Germany, p. 361-366, 2002

[2]"Revestimentos Decorativos a base de NiCr produzidos por processos PVD", Nuno Lima, Vasco Teixeira, in Proceedings TTES-2002, 3as Jornadas de Tratamentos Térmicos e Engenharia de Superfícies, ed. Vasco Teixeira, Teresa Vieira, H. Santos, P. Loureiro, SPM-Sociedade Portuguesa de Materiais, Univ. Coimbra, 2002

 

Sensor thin films

Semiconductor gas sensors have been the subject of extensive research and development with a view to their use as toxic and inflammable gas sensor devices for. However, their lack of reproducibility and selectivity necessitates improvements in the elaboration and characterisation of semiconductor materials.

The main objective of this project is the production and study of novel semiconductor composite materials, mainly SnO2-based thin nanocomposite films, in order to develop a more reliable, high performance sensor device for low-cost miniature systems of gas detection in the environment.

A common problem for gas sensor is the long-term instability of the gas response, gas selectivity and the time to reach steady-state of the response is often very long. These issues will be addressed in our work and the approach is to carefully investigate the processing of the novel sensor films, and to compare how different physical properties of the sensor affect the response. Emphasis is placed on those aspects of the science of materials that are concerned with the relationships between deposition, microstructure/composition, physical properties, gas sensing properties, and modelling of thin solid films behaviour.

The research program is aimed at carrying out basic research and gaining fundamental knowledge for the synthesis of novel thin nanocomposite films based in SnO2 from the vapor phase (using magnetron sputtering) that can enable the development of low cost, new solid state gas sensors devices with improved reproducibility, selectivity and long term-stability as well a gas sensor performance at moderate operating temperatures (i.e., low power consumption). Sensor parameters of SnO2 pure and doped materials will be investigated to detect the toxic gases, such as CO, CH4, and other test gases. The different films will be analysed by XRD, XPS, AES, AFM and SEM, Optical Spectroscopy and the gas sensitivity will determined in a prototype testing chamber. Changes in surface during gas-sensor interaction will be studied by in-situ spectroscopy techniques. It is expected a considerable improvement of sensor parameters for the new nanocrystalline SnO2 based nanocomposite films doped with different metals: SnO2-MO (M is: Mo, W, Cr, Fe, Ni, and Cu)

References:

[1]"Gas sensitive response of SnO2 thin film sensors produced by reactive DC magnetron sputtering", Key Engineering Materials 230-232 (2002) p.388-391

 

Selective coatings for high-efficiency solar thermal collectors

Problem Description

The use of renewable energy resources for heating domestic and industrial water has lead, for the development of solar thermal collectors, that a more concern on surface science aspects have been taken in account namely the development of the black absorber surface onto copper and aluminium tubes where the fluid runs. Traditionally the thermal collectors use black paints or, more often, thin films of electrodeposited chromium oxide (black chromium).

However these non-ideal selective surfaces show:

-an high solar absorptivity (aprox. 90-95%) and show an emmissivity too high (the ideal value should be <10%).

-the manufacture technologies used have a drastic environmental impact (resides and electrodeposited by-products rich in Cr, Ni, etc.)

-generally they are used for heating residential water

Technology Description:

The present technology has the objective of development of absorbers films for highly efficient solar collectors (emmissivity of the order of 4% and solar absorptivity of 95%), both for residential water heating (using Cr-Cr2O3 or TiNxOy) or industrial systems at high temperature (using Mo-Al2O3) using clean Physically Vapour Deposition (PVD) technologies.

The cathodic magnetron sputtering is a PVD deposition technology, being a clean technology to produce thin films and coatings, where no organic solvents neither products based in heavy metals are used as percursors or reaction products.

The deposition mechanism used vacuum where argon ions are accelerated towards a cathode (metal to be deposited) and by momentum transfer processes the metal atoms are ejected and sputtered onto the component that we want to coat.

The addition of oxygen or nitrogen allows the production of oxides or nitrides layers. This technique uses only a few amounts of material to produce a thin film (>300 nm in thickness) and allows an excellent control of adherence and physical properties, namely the emmissivity and absorptivity (key parameters to obtain spectrally selective thin films in the solar radiation spectrum).

Potential Users:

Thin surface coating on Cu and Al to be used by solar thermal collectors manufactures on the development of the absorber panel.

End users: residential heating, swimming-pool water heating, food and seed drying, industrial units that need hot water >250 ºC for steam turbines.

References:

[1]"Spectrally selective composite coatings of Cr-Cr2O3 and Mo-Al2O3 for solar energy applications",V. Teixeira, E. Sousa, M.F. Costa, C. Nunes, L. Rosa, M.J. Carvalho, M. Collares-Pereira, E. Roman, J. Gago, Thin Solid Films, 392, p. 320-326, 2001

[2] "Microtopographic Inspection of Cr-Cr2O3 Cermet Solar Absorbers", M. F. Costa, V. Teixeira, C. Nunes, Solar Opt. Materials XVI, SPIE vol. 3789, 140-148, 1999

[3]"Graded cermet selective coatings based on chromium and titatium", C. Nunes, V. Teixeira, m. L. Prates, N.P. Barradas, A. D. Sequeira,in "Advanced Coatings on Glass & Plastics for Large-Area or High-Volume Products", ed. C.P. Klages, H.J. Gläser, M. A. Aegerter, Braunschweig, Germany, p. 547-553,2002

 

Residual stress analysis

1-Numerical modelling of the hermo-mechnaical behaviour of layered and composite coatings

Residual stresses in functional coatings play an important role in the performance and lifetime of the coated component. The mechanical integrity of a protective coating is influenced by residual stresses which derive from four principal sources: growth stresses, geometric constraints, thermal gradients and service stresses.

Residual stress within the coatings occurs due to a mismatch between the coefficient of thermal expansion (CTE) of metallic substrate and ceramic coating, and also due to transient thermal gradients (e.g. during thermal cycling).

The origin of thermal residual stresses developed during the cooling down of a ceramic-metal structure (could be a joint or a coating) from an elevated temperature of fabrication. Imposition of a change in temperature differential in the expansion or contraction of the dissimilar layered materials results in a variation of the residual stress along the thickness direction of each layer. The resulting bending moment causes the layered composite to bend in order for it accommodate the thermal stresses. Away from the edges, the in-plane stress (parallel to the interface) is typically compressive in the ceramic layer (due to the smaller CTE of ceramic) and tensile in the metallic layer.

Tensile residual stresses in the ceramic coating cause perpendicular microcracking while compressive stresses tend to promote microcrack propagation at the interface. The mechanics of delamination and spallation by evaluation the crack driving force when an interface crack is present was analysed by Evans and Hutchinson [4]. It was determined that a driving force for crack propagation along the coating/substrate interface only arises when the coating buckles. Buckling can occur when there is a pre-existing flaw at the coating/substrate interface and when the in-plane compressive stress exceeds a critical value.Therefore, cracking in the ceramic or interfacial decohesion will affect the thermo-mechanical integrity of the functional component. In order to avoid failure the CTE mismatch stresses should be reduced. This can be done by replacing the sharp interface with an intermediate composite layer within which the composition and/or microstructure (and hence the thermal and mechanical properties) are smoothly varied from the ceramic to the metal material. This graded multilayer system is known as a Functionally Gradient Material (FGM) and has many of technological applications such as the functionally graded thermal barrier coatings for use in aero and land based turbine components. The gradients can be continuous or they can be micro-laminates or composites layers comprised of graded metals, ceramics, or gradients of porosity, etc. Sintering, diffusion bonding, thermal spraying, physical vapour deposition are examples of processing techniques which may be used to fabricate such compositionally graded multilayered systems (commonly referred to as functionally graded materials or coatings).

A basic FGM fabrication requirement is that the dissimilar materials (the metal and ceramic have large differences in density, thermal expansion, melting point, etc) should be homogeneously mixed in the desired proportions on the planes, normal to the thickness direction. Additionally, the optimal compositional pattern should be achieved with good reproducibility in the heat-flow direction. Three-dimensional gradient techniques can also be developed in order to produce practical FGM's with complex shapes.

References:

[1]"Mechanical integrity in PVD coatings due to the presence of residual stresses"
V. Teixeira, Thin Solid Films, 392, p. 276-281, 2001

[2]"Numerical analysis of the influence of coating porosity and substrate elastic properties on the residual stresses in high temperature graded coatings", Vasco Teixeira, Surface and Coatings Technology, vol. 146/147 p. 79-84, 2001

[3]"Modelling of Thermal Residual Stresses in Ceramic Coatings with a Graded Composite Interlayer",(invited paper), V. Teixeira, M. Andritschky, D. Stöver, NATO-ARW, Multilayered and Fibre-Reinforced Composites: Problems and Prospect, NATO series E-Appl Sci., Kluwer Acad. Publ., vol. 3/43, p. 393-408, 1998

[4]"Determination of biaxial modulus of chemical vapor-deposited diamond films", Qi Hua Fan, J.Grácio, E. Pereira, V.Teixeira, J.C.Tavares, Thin Solid Films, vol. 398/399, p. 265-269, 2001

2- Non Destructive Evaluation Technique for Micro-Defect and Residual Stress Mapping in Coatings

The non destructive evaluation (NDE) technique based in laser non-contact microtopographic inspection of the surface of physically vapour deposited (PVD), plasma enhanced chemical vapor deposited (PECVD) and and sol gel coatings deposited on large areas onto different engineering substrate materials, such as plastic, glass and metal components used automotive industry, allows the mapping of its surface defects, micro-roughness and residual stress [1-4]. The inspection is performed in a non-destructive, non-invasive way. The performance of our system allows a reliable and efficient inspection of the PVD hard, decorative and optical coatings produced in our thin coatings laboratory in order to detect flaws and surface defects, to evaluate the films roughness but also to determine residual stress distribution on the produced films. The inspection system we used (MICROTOP.06.MFC) , is an active optical triangulation sensor developed at University of Minho. It allows depth resolutions down to 4nm and lateral resolutions down to 1mm. The MICROTOP.03.MFC [1,4] is a robust and versatile system specially designed to accurately perform the microtopographic inspection of the rough surface of small samples It allows the inspection of a large variety of surfaces with resolutions that can be driven down to the submicron level with dynamic ranges up to 1:5000 (or 1:25000 with vertical scanning).

Our microtopographer is based on active optical triangulation and angle resolved scattering. The surface to be inspected is scanned by an oblique light beam. Two HeNe lasers at 632.8 and 534nm, and, one Xe white light sources are available and can be easily interchanged. The incident light is collimated and focused. A small, diffraction limited, bright spot is thus projected onto the sample. The bright spot is imaged both perpendicularly and specularly onto electronic photosensitive detection system in order to assess its lateral position. The area of the surface to be inspected is scanned point by point by the "sensor’s tip" (the light beam focused onto the surface). The highest system’s robustness was sought. Also a high lateral positioning resolution and accuracy is achieved. In order to perform the sample’s scanning it will be moved by means of a precision XY displacement table driven by precision step motors. At each scanning point the lateral spot’s position is obtained and registered. The horizontal spot's shift between scan positions is directly related with the height differences between those surface’ inspected points. The depth resolution depends of the system used and on its particular configuration.

The performance of our system allows a reliable and efficient inspection of the PVD optical and decorative coatings produced in our thin films’ laboratory in order to detect flaws and defects, to evaluate the films roughness but also to determine stress distribution on the produced films.

References

[1] Manuel F. M. Costa; "Triangulation Based Sensor for Non-Contact Micro and Nano Topographic Surface Inspection", "ICAPT’2000, Quebec City, Canada, July 2000", SPIE Vol. 4087, 1214-1221 (2000).

[2] V. Teixeira, "Mechanical integrity in PVD Coatings due to the presence of residual stresses", Thin Solid Films, 392, p. 276-281, 2001

[3] M. F. M. Costa, V. Teixeira, C. M. Nunes, A. Monteiro- "Spectrally selective coatings for glass windows used in the automative industry: Defects location and caracterization", Glass Processing Days, Proceedings, ed. Jorma Vitkala, , Tampere, Finland, p. 522-525, 2001

[4] M. F. Costa, V. Teixeira, C. Nunes, "Microtopographic Inspection of Cr-Cr2O3 Cermet Solar Absorbers", Solar Opt. Materials XVI, SPIE vol. 3789, 140-148, 1999

[5]L. Pereira, D. Brida, E. Fortunato, I. Ferreira, H. Águas, V. Silva, M. F. M. Costa, V. Teixeira and R. Martins ,"a-Si:H interface optimisation for thin film position sensitive detectors produced on polymeric substrates", Journal of Non-Crystalline Solids, Volumes 299-302, Part 2, (2002) p. 1289-1294

 

RECENT PROJECT SUMMARY:

 

 

NANOEFFECT: Nanocomposites with High Colouration Efficiency for Electrochromic Smart Plastic Devices

Abstract:

Inorganic-organic (hybrid) nanocomposites have become a new, versatile class of materials, exhibiting a vast application potential, due to their tailorable mechanical, optical and electrical properties. At the same time, there are still major drawbacks obstructing a broader exploitation of electrochromic technologies, e.g. high costs, poor colouration efficiency or lacking compatibility with plastic substrates. The proposed STRP NANOEFFECT will realize a major step to overcome these limitations by a new, integrated approach, that utilizes the potential of chemical nanotechnology to create a technology of horizontal economic interest. The main project outputs will be novel multifunctional hybrid nanocomposite coatings with high colouration efficiency, and, based thereupon, smart all-plastic devices, with outstanding performance in terms of cost-effectiveness, durability and electrochromic switching characteristics. The pilot application will be smart sunglasses, the feasibility of smart textiles will be evaluated. NANOEFFECT will explore the frontiers of knowledge in electrochromism and lead to a scientific breakthrough by creating nanomaterials with yet unknown property profiles. The project closely addresses main objectives of Thematic Priority NMP and important societal needs of the community (e.g. quality of life). A significant strategic impact (highly innovative products endowed with true competitive advantages, new avenues of research towards new potential applications) will result. The work will be performed by a well-balanced consortium (10 partners from 1 candidate, 1 third, and 5 member states; 2 of them SME, 2 of them from a less favoured region), which is highly complementary in terms of scientific expertise, personal skills and uniqueness of equipment. The integration of the participants' know-how and resources will provide significant added value and enable the consortium to carry out the project in an efficient and successful way.

 

-POCTI/CTM/61589/2004: "NANOCARBON-Nanocmposite coatings based in amorphous carbon and nanophases of metal-ceramic"

Abstract:

Diamond-like carbon (DLC) films can combine the properties of solid lubricating graphite structure and hard diamond crystal structure, i.e., extreme hardness approaching that of diamond, chemical inertness, high thermal conductivity and optical transparency without the crystalline structure of diamond. DLC films are characterised by mostly tetrahedral geometry of strong covalent carbon-carbon interatomic bonds that is caused by sp3 hybridisation state of valence electrons in carbon atom.<br>However these superhard DLC films lacked toughness and resistance to cross-sectional crack propagation. A possible method to improve film toughness would be through compositional modifications, e.g., incorporation of nanosized metal clusters in the DLC matrix to produce composite films or hydrogenation of amorphous carbon films.<br>In this project novel nanocomposite coatings with improved toughness based in nanosized crystalline phases of metals and ceramics embedded in amorphous carbon matrix will be developed: nc-Me/a-C and nc-MeNxCy/a-C(Me) with Me =Cu, Al, Ti, Zr, Y, Si etc.. These innovative coating architectures will be adopted in order to decrease substantially residual stress, improve adherence and fracture toughness, obtain low friction coefficient and high wear-resistance. Some of the selected coatings could also present an enhanced biocompatibility thus their potential applications in biomedical applications (DLC promotes additional resistance to bacterial adhesion).<br>A detailed characterisation of the coating’s physical and mechanical properties will be carried out in order to understand the fundamental aspects of thermo-mechanical behaviour as a complex coating system. Microstructure will be characterized by TEM, XRD and Raman Spectroscopy. In order to characterize morphology SEM and AFM will be used. Residual stresses will be determined from the substrate deflection. Bending and vibration tests will be used to obtain the coating’s mechanical performances such as the elastic modulus, coating fracture toughness and ultimate load. Finally, the material coating biocompatibility will be studied through the evaluation of the in vitro cytotoxicity based on the morphological examination of cell damage and growth when in direct contact with the materials. <br>The research team has already shown his scientific level in the field of science and technology of thin films and in the field of nanostructured coatings. Therefore, we use firstly our previous concepts and results obtained by the research team within the frame of projects on nanocomposite coatings. This will allow us to make a substantial progress in R&D and to reach our final objectives. The project will also use the available research infrastructures and will contribute to the scientific formation of one PhD student and the formation of a young post-graduated student

 

 

-POCTI/EME/39316/2001: "PVDCOAT-Composite and multilayered protective coatings for efficient energy systems"

-POCTI/P/CTM/11235/2001: "Spectrally Selective Coatings for Solar Energy Applications"

 

-EC-5FWP-PSA-Industrial Applications of Solar Thermal Energy- Improving Human Potential Programme/2001. "Solar furnace for high temperature treatment of advanced composite ceramic coatings"

 

HarDeCoat : Development of novel hard decorative coatings based on transition metal oxynitrides

Abstract:

The project aims at the development of new functional coatings for decorative and micro-optoelectronic applications. The coatings based on transition metal oxynitrides will be deposited by different Physical Vapour Deposition techniques. The idea is to combine the excellent mechanical properties of the metal nitrides with specific optical characteristics (colour) of the oxides. By tuning the deposition conditions, one can obtain coatings of compositions varying from oxides to nitrides, thus exploring the vast spectrum of their optical and mechanical properties.

The main technical deliverable will be a clean coating deposition process that is cost-competitive as compared to traditional electroplating methods. The use of plasma-based technology is resource efficient and reduces the environmental impact, resulting in the "greening" of industry.

During this project, new materials are elaborated, characterised and their applications will be tested on an industrial scale. The oxynitride coatings will meet consumer demands because of their safety and extended lifetime.

The project also contributes to the development of knowledge about a novel class of materials. A better understanding of the fundamental properties of transition metal oxynitrides is required to get insight into the materials’ functionality.

The consortium of 8 partners (6 research institutes and 2 SMEs) from four European countries will bring together the critical mass of resources and competencies for this project. The role of the industrial partners is to approve and implement the main project deliverables.

The following technology widespread aims to reinforce the leading position of Europe in the decorative market and to improve its global competitiveness.

 


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