V. Teixeira

Mechanical integrity in PVD coatings due to the presence of residual stresses

Abstract The functionality and reliability of coated devices are strongly related to residual stresses of thin films and coatings. A methodology for thermo-mechanical design in layered coatings is needed that complements experimental procedures for evaluation of coatings. This paper discusses the role of residual stresses on the mechanical integrity of physical vapour deposition (PVD) functional coatings. A numerical model of the residual stress distribution within a layered metal–ceramic composite coating is presented. The stress-induced failure modes are analysed. The topics discussed should provide some insights regarding the development of a methodology for designing fail-safe layered coating systems used in optical devices.

Effects of deposition temperature and thermal cycling on residual stress state in zirconia-based thermal barrier coatings

Abstract Advanced ceramic multilayered coatings are commonly used as protective coatings for engine metal components to improve performance, e.g. thermal barrier coatings (TBCs). Zirconia-based TBCs were produced by plasma spraying process and characterized in terms of microstructure, porosity, elastic modulus, adherence and residual stresses. In this contribution the residual stresses in multilayered coatings applied on Ni based superalloys for use as thermal barrier coatings were studied both by numerical modelling and experimental stress measurement. The thermal residual stresses generated during the spraying process of duplex TBCs were simulated by using an heat transfer finite element program and an elasto-plastic biaxial stress model. The TBC system was subjected to different thermal cycling conditions (maximum temperature, heating up and cooling down rates, dwell time at maximum temperature, etc.). The stress distribution within the TBC was also modelled after thermal cycling. The stress state in the as-deposited and in thermally cycled coatings was verified using an X-ray diffraction technique. The measurements were in good agreement with the residual stress modelled calculations. It was observed that the residual stresses were dependent on the thermal history of the TBC (as-deposited and thermally cycled). It is proposed that thermal cycling allowed the stresses to relax by microcracking and creep mechanisms at high temperature such that on cooling down to room temperature, an in-plane biaxial compressive stress will arise on the zirconia top coating due to the difference on the coefficients of thermal expansion between the metallic substrate and ceramic coating material.

Spectrally selective composite coatings of Cr–Cr2O3 and Mo–Al2O3 for solar energy applications

Abstract Efficient solar photothermal conversion benefits from spectrally selective absorber surfaces. In this paper, a numerical model that allows correlation of the selectivity of the absorbers produced to the collector efficiency is presented. Since magnetron sputtering is a promising method to produce thin, solar selective films, a study of cermet Cr–Cr 2 O 3 and Mo–Al 2 O 3 coatings obtained by this technique in a reactive atmosphere is presented. The multilayered cermets produced have a thickness of approximately 300 nm and were based on metallic chromium (molybdenum) in a matrix of a chromium oxide (aluminium oxide) with a gradient in oxygen composition. The selective cermet graded films were produced by a reactive DC magnetron sputtering of pure chromium (aluminium with molybdenum) target in a plasma of argon–oxygen at different sputtering pressures (ranging from 5×10 −3 to 1.2×10 −2 mbar) and substrate temperatures (150 and 250°C). The microstructure, surface roughness, crystallographic phases, composition and chemical analysis were determined by X-ray photoelectron spectroscopy, reflectivity spectra in the vis/NIR region were analysed, and thermal emissivity was measured with an emissometer. The coatings have high spectral selectivity, with solar absorption ranging from 0.88 to 0.94 and thermal emissivity ranging from 0.15 to 0.04, depending on the coating materials and sputtering conditions.

Amorphous ITO thin films prepared by DC sputtering for electrochromic applications

Indium-Tin-Oxide (ITO) thin films were deposited on glass substrates using DC magnetron reactive sputtering at different bias voltages and substrate temperatures. Some improvements were obtained on film properties, microstructure and other physical characteristics for different conditions. Amorphous and polycrystalline films can be obtained for various deposition conditions. The transmission, absorption, spectral and diffuse reflection of ITO films were measured in some ranges of UV-Vis–NIR. The refractive index (n), Energy band gap Eg and the surface roughness of the film were derived from the measured spectra data. The carrier density (nc) and the carrier mobility (μ) of the film micro conductive properties were discussed. The films exhibited suitable optical transmittance and conductivity for electrochromic applications.

Residual stress and cracking in thin PVD coatings

Abstract High temperature processing is currently present in the industrial production of advanced functional coatings. This paper discusses the role of residual stress on the mechanical integrity of PVD protective coatings. A numerical model of the residual stress distribution within a layered metal-ceramic composite coating is presented. The stress-induced failure modes are analysed. Residual stress distribution near the edges and within microcracked ceramic coatings is also analysed. The topics discussed should provide some insights into the development of a methodology for designing fail-safe coating systems which should complement experimental procedures for evaluation of protective coatings.

Characterisation of ZrO2 films prepared by rf reactive sputtering at different O2 concentrations in the sputtering gases

Abstract Zirconium oxide (ZrO 2 ) films have been prepared by rf reactive magnetron sputtering at different O 2 concentrations in the mixture sputtering gases. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and optical spectroscopies. The influence of O 2 concentration in the sputtering gases on the microstructure, residual stress and optical properties of the films has been studied. Also, the effect of loose packing structure caused by the high O 2 gas concentration on the deposition rate has been discussed.

Numerical analysis of the influence of coating porosity and substrate elastic properties on the residual stresses in high temperature graded coatings

Abstract The functionality and reliability of coated devices are strongly related to residual stresses of coatings. The major problem in thermal barrier coatings (TBCs) applied to gas turbine components is the failure by spallation of ceramic coating under thermal cycling processes. In order to prevent spallation and to improve the thermo-mechanical behaviour of the TBC the interfacial stresses in the coating system should be reduced. To overcome this problem it is desirable to introduce a graded layer between the metallic bond coat and the zirconia top coating. Therefore, a detailed study of the optimisation of the gradient profile is necessary in respect to thermal stress relief. In this paper a numerical model of thermal stress distribution within a multilayered system which consists of a functionally gradient material (FGM) is presented. The structure of the graded coating system is made of a ceramic layer and a metallic layer, where between them there is an interlayer which is a graded composite made of the metal (NiCr-alloy) and the ceramic (ZrO 2 Y 2 O 3 ). The effects on residual stress distribution of elastic properties of the alloy substrate, the graded interlayer thickness and ceramic layer porosity were analysed for the case of a fully graded TBC using a linear compositional profile for the FGM. This model will provide some insights regarding the development of a methodology for designing fail-safe graded coating systems used in high temperature applications.

Synthesis and characterization of VO2-based thermochromic thin films for energy-efficient windows

Thermochromic VO2 thin films have successfully been grown on SiO2-coated float glass by reactive DC and pulsed-DC magnetron sputtering. The influence of substitutional doping of V by higher valence cations, such as W, Mo, and Nb, and respective contents on the crystal structure of VO2 is evaluated. Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO2 is discussed. In summary, all the dopant elements--regardless of the concentration, within the studied range-- formed a solid solution with VO2, which was the only compound observed by X-ray diffractometry. Nb showed a clear detrimental effect on the crystal structure of VO2. The undoped films presented a marked thermochromic behavior, specially the one prepared by pulsed-DC sputtering. The dopants effectively decreased the transition of VO2 to the proximity of room temperature. However, the IR modulation efficiency is markedly affected as a consequence of the increased metallic character of the semiconducting phase. Tungsten proved to be the most effective element on the reduction of the semiconducting-metal transition temperature, while Mo and Nb showed similar results with the latter being detrimental to the thermochromism.

Graded selective coatings based on chromium and titanium oxynitride

Abstract In order to improve the performance of thermal solar collectors, the development of good, durable and reproducible spectrally selective solar absorber surfaces for photothermal conversion is of greatest importance. Spectrally selective cermet coatings were produced by DC magnetron sputtering using metallic chromium and titanium targets at constant target current, substrate bias and substrate temperature. These coatings are graded cermets with a metal concentration decreasing from the substrate to the coating surface. Basically, these coatings are formed with a layer structure consisting of a cermet layer enriched in metal, one or more cermet layers with less metal than the first one, and on top a pure ceramic layer as an antireflection layer. The metal layer was deposited by non-reactive DC sputtering, while the ceramic layers were deposited by DC reactive sputtering in argon–nitrogen or argon–oxygen atmosphere. The metallic and ceramic layers were deposited as a sub-layer system, consisting of alternating metallic and nitride or oxide sub-layers. The effect of the optical properties in dependence on the metallic fraction in the layers and the number of sub-layers in the film is discussed. For coatings based on chromium, the optimum selectivity achieved was a solar absorptance of 94% and a thermal emittance of 6% at 82 °C. For titanium oxynitride based coatings, the best selectivity achieved was a solar absorptance of 91% and a thermal emittance of 4%. The microstructure and thickness were studied by scanning electron microscopy (SEM). The surface microtopography was analysed by atomic force microscopy (AFM) and the metal concentration profile by Rutherford backscattering spectrometry (RBS).

Microwave plasma chemical vapour deposition diamond nucleation on ferrous substrates with Ti and Cr interlayers

Abstract Diamond-coated steel is considered an important issue in synthetic diamond technology due to the great economical importance of enhancing the wear resistance and surface hardness of commercial Fe-based alloys. However, direct diamond coating by chemical vapour deposition (CVD) is rather problematic—adhesion and growth are seriously affected. The use of interlayers is a common approach to minimise these problems. This work reports an investigation on the establishment of good nucleation and growth conditions of diamond films by microwave plasma CVD (MPCVD) on ferrous substrates coated with Ti and Cr interlayers. Commercial grade ferrous substrates were pre-coated with commercial interlayers by sputtering (Ti, Cr) and electroplating (Cr) techniques. Steel substrates led to better results than iron cast substrates. The best films were obtained on Ti pre-coated steel substrate. The results on Cr interlayers pointed to the advantage of electroplating over the physical vapour deposition (PVD) sputtering. From the two selected parameter sets for diamond deposition, the one using lower power level conducted to the best results. Initial roughness and growth parameters were found to counteract on the uniformity of the diamond films. The morphology was studied by scanning electron microscopy (SEM), the roughness was estimated by profilometry, while diamond quality and stress state were evaluated by μ-Raman spectroscopy.