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Showing papers in "Surface & Coatings Technology in 2004"


Journal ArticleDOI
TL;DR: In this article, the authors present a recent understanding of the lubrication mechanisms of both traditional and new solid lubricants, with particular emphasis on solid lubricant methods and practices, as well as their applications.
Abstract: In recent years, several new solid lubricant and modern lubrication concepts have been developed to achieve better lubricity and longer wear life in demanding tribological applications. Most of the traditional solid lubricants were prepared in the form of metal, ceramic and polymer-matrix composites. They have been used successfully in various engineering applications. Recent progress in thin-film deposition technologies has led to the synthesis of new generations of adaptative, self-lubricating coatings with composite or multilayered architectures, by using duplex/multiplex surface treatments. These modern self-lubricating coatings progressively make their way into the commercial marketplace and meet the ever-increasing performance demands of more severe applications. The present paper reviews our recent understanding of the lubrication mechanisms of both traditional and new solid lubricants, with particular emphasis on solid lubricant methods and practices.

481 citations


Journal ArticleDOI
TL;DR: In this paper, the hysteresis curves of the two high-entropy alloys in reactive sputtering are quite different in comparison to those of elements or simple alloys.
Abstract: Multi-element high-entropy alloys are alloy systems with n (5≤n≤13) principal elements each having an atomic percentage no more than 35%. Using the alloys of Fe-Co-Ni-Cr-Cu-Al-Mn and Fe-Co-Ni-Cr-Cu-Al0.5 as target material in reactive sputtering, nitride films were deposited. The hysteresis curves of the two high-entropy alloys in reactive sputtering are quite different in comparison to those of elements or simple alloys. The film deposition rate decreased with increasing nitrogen gas flow and the highest film thickness was in excess of 2.5 μm. The alloy films are crystalline with structures of a mixed FCC and BCC or simple FCC solid solution, while the crystallinity of the nitride films decreased and approached amorphous with increasing nitrogen gas flow. The composition of the alloy films was similar to their original targets, and the nitrogen content of the nitride films increased with increasing nitrogen flow, to a maximum of 41.1 at.% nitrogen. The values of resistivity of the two alloy films were 108 and 135 μΩ cm, respectively, and those of their nitride films increased with nitrogen flow, to a factor of 3 of the alloy film. The rms surface roughness measured by AFM decreased significantly from 9 to 13 nm for the alloy films to only 1–3 nm for the nitride films. Values of hardness are about 4 GPa for alloy films and about 11 GPa for nitride films. The growth rate, the resistivity, and the hardness of the resulting nitride films were not affected too much by substrate bias due to the amorphous nature of the nitride films.

412 citations


Journal ArticleDOI
TL;DR: In this paper, preliminary corrosion tests on magnesium specimens in simulated body fluid (SBF) with and without Cl − ions have been investigated, and the corrosion results showed that alkali and heat-treated magnesium has relatively high corrosion resistance in SBF, compared to untreated samples.
Abstract: Compared to other popular metallic biomaterials, magnesium has many advantages, which include high specific strength-to-mass ratio, non-toxicity and similar elastic modulus to that of human bone. However, the knowledge gap in corrosion resistance in physiological environment has prevented it from being a substitute for human hard tissues. In this paper, preliminary corrosion tests on magnesium specimens in Simulated Body Fluid (SBF) with and without Cl − ions have been investigated. Cytotoxicity tests were then carried out for developing a new biomaterial. The corrosion results showed that alkali and heat-treated magnesium has relatively high corrosion resistance in SBF, compared to untreated samples. Calcium-phosphate apatites were detected on the treated samples after they had been soaked in SBF for 14 days. In cytotoxicity tests, no signs of morphological changes on cells or inhibitory effect on cell growth were detected.

412 citations


Journal ArticleDOI
TL;DR: In this article, the authors discuss the benefits of using the ratio of hardness to elastic modulus (H / E ) as an indicator of coating durability since this parameter essentially describes the elastic strain to failure capability (and resilience ) of a candidate material.
Abstract: There is increasing scientific and commercial interest in the development of nanostructured coatings, particularly those based on low-miscibility ‘ceramic–ceramic’ or ‘ceramic–metal’ crystalline/amorphous nanocomposite phase mixtures deposited by plasma-assisted PVD or CVD. In laboratory mechanical testing, extreme values of hardness (which may be in excess of 70 GPa) are often found for such films, similar to those claimed for intrinsically hard materials such as c-BN and diamond. High hardness is, however, often accompanied by an associated high elastic modulus, which although desirable in principle for cutting tool materials and/or coatings, may in practice limit coating durability, on low-strength, low-modulus substrates (e.g. low-alloy steels and the light alloys) and in many wear applications other than metal cutting. In this paper, we discuss the benefits of using the ratio of hardness to elastic modulus ( H / E ) as an indicator of coating durability since this parameter essentially describes the elastic strain to failure capability (and resilience ) of a candidate material. Furthermore, we consider the likely need for tribological coatings to accommodate some degree of substrate deformation; in this respect film toughness , i.e. ‘engineering toughness’ in the sense of an ability to absorb deformation energy (both elastic and plastic) needs to be considered. The concept of predominantly metallic films with a nanograined and/or glassy microstructure (containing little or no high-modulus ceramic constituents) is introduced, through which we point to the importance of retaining ‘sufficient’ coating hardness, whilst reducing coating elastic moduli to more closely match those of candidate substrate materials. With regard to the implications of H / E for practical tribological coating applications, we propose that closer matching of the coating/substrate interfacial elastic properties and thus an improved ability for the coating to accommodate substrate strain, where necessary, is often a more important factor in wear resistance than is extremely high hardness.

390 citations


Journal ArticleDOI
TL;DR: In this article, the effect of coating time on phosphorus content, thickness, structure and hardness of the deposits were analyzed, and the X-ray diffraction patterns showed that by changing the coating times, amorphous or crystalline structure could be obtained.
Abstract: Mild steel was electroless coated with nickel–phosphorus alloy from a bath containing sodium hypophosphite and glycin–citrate complexing agents. The effect of coating time on phosphorus content, thickness, structure and hardness of the deposits were analyzed. Corrosion parameters such as current density, corrosion potential and corrosion rate were obtained from tafel polarization curves and immersion corrosion tests in aerated 3.5% NaCl solution. The X-ray diffraction (XRD) patterns, anodic polarization curves and scanning electron microscopy (SEM) of heat-treated specimens obtained in various coating times were also studied. The X-ray diffraction patterns showed that by changing the coating times, amorphous or crystalline structure could be obtained. The coatings containing 11.1–13.1% phosphorous were amorphous, and showed better corrosion resistance than microcrystalline structure. The heat-treated specimens obtained in different coating time had more hardness and corrosion resistance than unheated samples. According to the polarization studies and scanning electron microscopy images, the heat-treated samples with 10.8 and 10.1% phosphorous show the intergranular corrosion, while the coatings with 11.7 and 12.2% of phosphorus have the least corrosion rate. Immersion corrosion test data at the 4-month interval had good agreement with electrochemical polarization results.

282 citations


Journal ArticleDOI
TL;DR: In this article, the effect of microstructure and plating parameters on AZ91D alloy was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis.
Abstract: Electroless nickel-plating on AZ91D magnesium alloy has been investigated to understand the effect of substrate microstructure and plating parameters. The initial stage of the deposition was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis on substrates plated for a very short interval of time. The early stage of growth was strongly influenced by the substrate microstructure. Plating was initiated on b-phase grains probably due to the galvanic coupling of b and eutectic a-phase. Once the b-phase was covered with the coating, it then spread onto eutectic a and primary a-phase. The coating produced with the optimised bath showed 7 wt.% phosphorus with a hardness of approximately 600–700 VHN. The optimum ligand to metal ion ratio was found to be 1:1.5, while the safe domain for thiourea (TU) was in the range of 0.5–1 mgyl. Fluoride was found to be an essential component of the bath to plate AZ91D alloy with an optimum value of 7.5 gyl. The presence of 0.25–0.5 mgyl mercapto-benzo-thiosole (MBT) found to accelerate the plating process. 2003 Elsevier B.V. All rights reserved.

268 citations


Journal ArticleDOI
TL;DR: In this paper, an atmospheric plasma sprayed thick thermal barrier coatings (TBCs), comprising of 1.5 mm thickness yttria stabilized zirconia (YSZ) coating, have been developed for increasing thermal protection of combustor applications.
Abstract: Atmospheric plasma sprayed thick thermal barrier coatings (TBCs), comprising of 1.5 mm thickness yttria stabilized zirconia (YSZ) coating, have been developed for increasing thermal protection of combustor applications. Different segmentation crack densities of the YSZ coating were created by controlling the deposition conditions. It was found that the substrate temperature played a dominant role in determining the segmentation crack density. The density was found to increase with the increase of substrate temperature and liquid splat temperature. High passage thickness, if achieved by low plasma gun speed, contributed to improve the segmentation crack density. The density could not be effectively improved by increasing the powder feed rate, although a high passage thickness was obtained in this case. The pores, mainly consisting of delaminations with the radii range between 0.02 and 1 μm, prevent the segmentation cracks from propagation. The coatings sprayed at high substrate temperature show an excellent intersplat bonding and often even continuous columnar grains through several splats. Additionally, horizontally branching cracks were also formed together with the segmentation cracks when a high lamellar thickness was used during the spraying. Thermal cycling tests showed that the coating with a segmentation crack density of approximately 3.6 mm−1 had a lifetime of more than 1700 cycles at 1238 °C (surface)/938 °C (bond coat), indicative of excellent thermal shock resistance. Failure of the TBCs occurred by chipping in the surface layer of YSZ coating, which is different from the traditional interface delamination failure occurred with thinner TBCs.

261 citations


Journal ArticleDOI
TL;DR: In this paper, an image analysis (IA) method was used for characterization of porosity in thermally sprayed coatings and the results were complemented with the microstructural information obtained using advanced characterization techniques, such as small-angle neutron scattering (SANS) and computer micro-tomography (CMT) carried out on similar coatings.
Abstract: The microstructures of thermally sprayed coatings are very complex and incorporate process-dependent defects such as globular pores, interlamellar pores, cracks (in case of ceramics), etc. Porosity is a prevalent feature in the microstructure and affects a wide range of coating properties such as elastic modulus, thermal conductivity and dielectric behavior. Various methods are employed for quantitative measurement of porosity, which forms an important and integral part of microstructural characterization of thermal spray coatings. Studies have been conducted here to establish image analysis (IA) as a reliable method for characterization of porosity in thermally sprayed coatings. The versatility of IA methods for microstructural quantification has been investigated for diverse coatings deposited with distinct spray processes and material feedstock characteristics. The IA methods put forth have been successfully applied to a variety of thermally sprayed coatings; materials of interest in this study being produced from partially stabilized zirconia (PSZ, ZrO2+8% Y2O3) and alumina ceramics. The results have been complemented with the microstructural information obtained using advanced characterization techniques, such as small-angle neutron scattering (SANS) and computer micro-tomography (CMT) carried out on similar coatings. The trends shown by these different methods for variation in porosity with respect to feedstock characteristics were in agreement. This study confirms the applicability of image analysis as a straightforward, versatile, reliable and inexpensive method for the characterization of porosity.

256 citations


Journal ArticleDOI
TL;DR: In this article, the nucleation and growth of the cerium-based conversion coating on AA2024 was studied using complementary surface analysis techniques, as atomic force microscopy in the Kelvin probe mode (SKPFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).
Abstract: The chemical conversion treatment for aluminium alloys based on the immersion in cerium chloride/hydrogen peroxide solutions is one of the possible alternatives to the chromate conversion process for the corrosion protection of aluminium alloys. The nucleation and growth of the cerium-based conversion coating on AA2024 was studied using complementary surface analysis techniques, as atomic force microscopy in the Kelvin probe mode (SKPFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of both the intermetallic particles and the copper in solid solution was considered. According to this aim, different surface preparation procedures prior to the conversion process were investigated. Acid pickling and Ce-based deoxidising, as well, were found to cause the formation of copper or copper oxide deposits, i.e. copper smut, on the AA2024 surface. While the intermetallics do not act as preferential nucleation sites, the deposition of copper on the surface is a necessary condition in order to form a thick cerium oxide film. On the other hand, the copper smut strongly decreases the adherence of the conversion layer to the aluminium substrate.

243 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of dopant concentration, heating treatment and annealing in reducing atmosphere on the microstructure as well as on the electrical and optical properties of the thin films is discussed.
Abstract: Transparent and conductive high preferential c-axis oriented ZnO thin films doped with Al have been prepared by sol–gel method using zinc acetate and aluminium chloride as cations source, 2-methoxiethanol as solvent and monoethanolamine as sol stabilizer. Film deposition was performed by dip-coating technique at a withdrawal rate of 1.5 cm min−1 on Corning 1737 glass substrate. The effect of dopant concentration, heating treatment and annealing in reducing atmosphere on the microstructure as well as on the electrical and optical properties of the thin films is discussed. The optical transmittance spectra of the films showed a very good transmittance, between 85 and 95%, within the visible wavelength region. The minimum resistivity of 1.3×10−3 Ω cm was obtained for the film doped with 2 wt.% Al, preheated at 400 °C and post-heated at 600 °C, after annealing under a reduced atmosphere of forming gas.

236 citations


Journal ArticleDOI
TL;DR: In this article, a critical review of the properties of the main classes of carbon films used for magnetic storage disks is presented, and the main approaches to assess the structural and morphological properties of ultra-thin carbon layers are reviewed.
Abstract: Diamond-like carbon films form a critical protective layer on magnetic hard disks and their reading heads. The ultimate limit to storage density is the super-paramagnetic limit, where the thermal energy is able to overcome the coercive energy of the magnetic bit. Perpendicular recording should allow storage densities up to ;1 Tbityinch . This requires the read head to approach 2 closer to the magnetic layer and ever-thinner layers of carbon 1–2 nm thick. A critical review of the properties of the main classes of carbon films used for magnetic storage disks is presented. Tetrahedral amorphous carbon can provide the atomic smoothness, continuity and density required for magnetic storage applications down to a few atomic layers thickness. The main approaches to assess the structural and morphological properties of ultra-thin carbon layers are reviewed. Raman spectroscopy, Xray reflectivity, atomic force microscopy and surface acoustic waves based methods allow a full non-destructive characterization of ultra-thin carbon layers. 2003 Elsevier B.V. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, the role of the phosphating bath components on the zinc conversion process and the possibility of obtaining phosphate layers on magnesium AM60 alloy by immersion in the various phosphatation solutions was investigated.
Abstract: The corrosion protection by zinc phosphate conversion coating on magnesium alloy AM60 is studied. Three phosphatation solutions containing phosphoric acid, phosphate ions, nitrates and nitrites added with zinc and fluorides were used. Therefore, the present investigation aims to study the role of the phosphating bath components on the phosphating process and to enhance the possibility of obtaining phosphate layers on magnesium AM60 alloy by immersion in the various phosphatation solutions. The morphology and the coating composition on the electrode surface were analysed (SEM, EDX, X-ray diffraction and Raman spectroscopy). The phosphate films formed are mainly composed of tetra-hydrated zinc phosphate, otherwise known as hopeite. A mechanism is proposed to explain the germination and the growth of the phosphate crystals on AM60 alloy.

Journal ArticleDOI
TL;DR: In this article, the authors used a ball-on-disk tribometer with stainless steel and cemented carbide balls as a counter material, with and without lubricant, to evaluate the effect of vanadium and aluminum in CrN films.
Abstract: Chromium nitride (CrN) films are superior to the titanium nitride film in corrosion and wear resistances, and friction behavior. CrN has been widely applied to the molding dies, machine parts and sliding part. In the present paper, additions of aluminum and vanadium into CrN films were performed with an expectation of improvement in tribological properties. CrN, CrAlN and CrVN were deposited by a cathodic arc ion plating. Deposited films were characterized by X-ray diffraction for crystal structure identification and energy-dispersive X-ray spectroscopy for chemical composition analysis. Diffraction peaks that appeared were similar in position and orientation in all films because the crystal structure and the lattice constant for CrN, VN and AlN are close to each other. The composition of the film deposited with Al and V was estimated to be Cr70Al30N and Cr50V50N, respectively. Knoop hardness test showed that CrAlN was harder than CrN and CrVN. Friction and wear tests were carried out by a ball-on-disk tribometer with stainless steel and cemented carbide balls as a counter material, with and without lubricant. Flaking occurred on CrN with stainless steel ball in wear tests without lubrication by ball-on-disk tribometer, but that did not occur on CrAlN and CrVN films. In the case of the wear test with cemented carbide ball, depth of wear track on CrN film reached to the substrate. The friction coefficient was almost the same for CrN and CrAlN films; however, that for CrVN film was lower than other films, in motor oil. V addition into CrN film successfully improved its tribological properties.

Journal ArticleDOI
TL;DR: In this article, X-Ray diffraction and pole-figure analysis were used to study the crystallinity and crystal orientation of polycrystalline ZnO films with a c-axis preferential orientation.
Abstract: In this study, the ZnO films were deposited to different thicknesses by r.f. magnetron sputtering. X-Ray diffraction and pole-figure analysis were used to study the crystallinity and crystal orientation. The results showed that ZnO films deposited to a thickness below 500 nm were polycrystalline with a c-axis preferential orientation. However, ZnO films, 500 or 600 nm in thickness, exhibited good self-texture. The optical properties of ZnO films did not depend significantly on the crystallographic orientation or degree of texturing. They were mainly affected by the grain size and carrier concentration.

Journal ArticleDOI
TL;DR: In this article, the feasibility of using the Keronite® plasma electrolytic oxidation process to overcome the problem of fatigue performance reduction caused by anodising treatments in a Mg alloy is studied.
Abstract: In the paper, the feasibility of using the Keronite® plasma electrolytic oxidation process to overcome the problem of fatigue performance reduction caused by anodising treatments in a Mg alloy is studied. Two types of coatings produced using different current regimes, and having two thicknesses of ∼7 and ∼15 μm, were tested using a rotating bending fatigue tester. SEM, XRD and optical microscopy techniques were used to evaluate possible fracture mechanisms involved in the initiation and propagation of the fatigue cracks. The results of the investigation demonstrate that Keronite® coatings may cause no more than a 10% reduction in the endurance limit of the Mg alloy, which is substantially lower than the effect from conventional anodising. A probable cause of that reduction seems to be distortion of the metal subsurface layer rather than structural defects introduced by the oxide film.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the fabrication and microstructure of two La2Zr2O7-coating, deposited by single-source EB-PVD processing.
Abstract: New ceramic thermal barrier coating (TBC) compositions superior to state-of-the-art PYSZ material are considered to overcome the problems related to phase stability and service-induced sintering within the columnar feather-like structure of current EB-PVD TBCs. Among those candidates for gas turbine applications, the pyrochlore-structure based TBCs, e.g. undoped and RE-oxide doped La2Zr2O7 offer very attractive properties. This work describes the fabrication and refers to the relations regarding manufacture and microstructure of two La2Zr2O7-coatings, deposited by single source EB-PVD processing. The compositions contain an undoped La2Zr2O7-coating and an Y2O3-doped La2Zr2O7-coating. The aspects leading to the successful manufacture of a homogeneously aligned microstructure in EB-PVD TBCs rely on the careful control of deposition parameters as well as ingot (evaporation source material) quality. Hence, the microstructure of the EB-PVD La2Zr2O7–coating analyzed by SEM/EDX and XRD have been correlated with the processing conditions in order to understand the controlling parameters for achievement of these coatings.

Journal ArticleDOI
TL;DR: In this paper, the impact of tribological properties of DLC coatings on cutting performance was examined in a pin-on-disk test as well as in aluminum alloy machining, and the results showed that DLC coated tools can be used to reduce adhesion between a tool and a work material and the formation of a built-up edge.
Abstract: The purpose of this study is to examine the impact of tribological properties of DLC coatings on cutting performance. In this work, DLC was prepared on a cemented carbide substrate by using a vacuum arc discharge with a graphite cathode. DLC was identified as a non-hydrogenated amorphous carbon film with the thickness of 0.1 μm. DLC coatings were evaluated in a pin-on-disk test as well as in aluminum alloy machining. DLC had a low friction coefficient (0.1) against aluminum alloy and an excellent anti-adhering property. The results showed that DLC coated tools can be used to reduce adhesion between a tool and a work material, and the formation of a built-up edge. In order to analyze the cutting process, a finite element method (FEM) simulation at the tool–chip interface was employed, to compare the experimental evidence. In particular, the comparison was carried out taking into account the chip shapes and cutting force. The FEM simulation of a chip formation process was in qualitative agreement with the experiments, and this enabled an explanation of the phenomenon of the cutting process. In the case of dry machining of the aluminum alloys, DLC coatings enhance the tool performance, the machined surface integrity and the tool life compared to an uncoated tool.

Journal ArticleDOI
TL;DR: In this article, the role of the binder in an intumescent paint was investigated using thermogravimetric analysis, solid state NMR and FTIR analysis, and it was found that the thermal stability increases when the copolymer is based on substituted styrene.
Abstract: This study investigates the role of the binder in an intumescent paint. In fact, it is generally known that acid source, carbon source and blowing agent are the main ingredients of such a paint. However, since the binder may react with these ingredients, it is also a very important component of an intumescent paint. To begin with, the effect of the nature of the monomers composing the polymeric binder, on the chemical reactivity between the binder and the intumescent additives is investigated using thermogravimetric analysis, solid state NMR and FTIR analysis. It is found that the thermal stability increases when the copolymer is based on substituted styrene. Subsequently, the efficiency of protective behaviour of the intumescent coatings is evaluated varying the nature of the binder resin. It is found that the thermal insulation is greatly improved when using a mixture of a linear copolymer presenting a good reactivity with the acid source and a cross-linked copolymer as binder in the intumescent paint.

Journal ArticleDOI
TL;DR: In this paper, the influence of plating parameters on the content of the co-deposited Al 2 O 3 particles in Co-Ni alloys was investigated, and it was shown that the presence of Al 2O 3 particles greatly improves the hardness and the wear resistance of composite coatings.
Abstract: Composite coatings of Co–Ni–Al 2 O 3 were studied by electrolytic codeposition of Co–Ni alloys and Al 2 O 3 from a sulfamate electrolyte containing Al 2 O 3 particles. The influence of plating parameters on the content of the co-deposited Al 2 O 3 particles in Co–Ni alloys was investigated. The maximum value of co-deposited Al 2 O 3 can be achieved at a particle content of 80 g l −1 in bath, a current density of 3 A dm −2 , a pH of 4.5, and a stirring rate of 100 rpm. In the process of codeposition, cathodic polarization increases with the increase of Al 2 O 3 concentration in bath and cobalt ions in electrolyte caused the reduction of polarization while nickel ions do not change the polarization behavior. Surface morphology and microstructure of Co–Ni–Al 2 O 3 coatings were determined by means of scanning electron microscopy, atomic force microscopy and X-ray diffraction. It was found that the phase structure of solid solution cannot be varied by codeposition of Al 2 O 3 particles in Co–Ni alloys, and it only influences the growth and orientation of crystal planes. It was shown that the presence of Al 2 O 3 particles in deposit greatly improves the hardness and the wear resistance of composite coatings. However, the codeposition of Al 2 O 3 increases the tensile internal stress of Co–Ni–Al 2 O 3 deposit. The coefficient of thermal expansion and the thermal conductivity of Co–Ni–Al 2 O 3 composite coatings are varied with the increase of Co contents and the temperature.

Journal ArticleDOI
TL;DR: In this article, the effects of vanadium oxides on tribological properties of TiAlN/VN superlattices at high temperatures were investigated and the results showed that addition of VN to hard coatings has high potential to achieve a low friction effect due to the formation and melting of a V2O5 containing oxide, in addition to its conversion into easily shearable lower-oxidized vanadium phases.
Abstract: TiAlN/VN superlattices are potential candidates for dry machining due to their high hardness and excellent tribological properties. It has been reported that VN easily oxidizes at relatively low temperatures and forms V2O5 having lubricious properties. The aim of this work is to investigate the effects of vanadium oxides on tribological properties of TiAlN/VN superlattices at high temperatures. During differential scanning calorimetry (DSC) in an argon/oxygen atmosphere it was found that the coatings oxidize at approximately 450 degreesC. Melting and boiling point of the formed oxides could be determined by DSC to approximately 635 degreesC and 1400 degreesC, respectively. It was observed by DSC and X-ray diffraction that the melting phase, a V2O5, containing oxide, transforms into a VO2 containing oxide causing the loss of the liquid phase. Dry sliding tests showed that up to 500 degreesC the friction coefficient increases from 0.55 to 0.95. It drops to approximately 0.18 at 700 degreesC and remains there as long as a liquid surface oxide is present. If most of V2O5 is converted into lower-oxidized vanadium the friction coefficient increases to a steady state value of approximately 0.55 at 700 degreesC. The results obtained show that addition of VN to hard coatings has high potential to achieve a low friction effect due to the formation and melting of a V2O5 containing oxide, in addition to its conversion into easily shearable lower-oxidized vanadium phases. (C) 2003 Elsevier B.V. All rights reserved.

Journal ArticleDOI
TL;DR: In this article, the specific permeability of oxygen in plasma sprayed zirconia has been measured to be approximately 10 −16 m 2, with some dependence on microstructure, and hence on spraying conditions.
Abstract: The specific permeability of oxygen in plasma sprayed zirconia has been measured to be approximately 10 −16 m 2 , with some dependence on microstructure, and hence on spraying conditions. Permeability rises with increasing temperature, as expected, and differences of a factor of approximately 4 were observed between room temperature and 600 °C. Maximum oxygen fluxes through the top coat via gas permeation, estimated for typical operating conditions of a thermal barrier coating (TBC) in a gas turbine, have been estimated and compared with those expected via ionic diffusion, obtained using published data. It is shown that gas permeation is normally expected to dominate. However, the rate of oxide growth at the bond coat/top coat interface of a TBC is controlled by diffusion through the oxide layer and either mechanism of oxygen transport through the top coat could generate fluxes sufficient to maintain this condition. This explains why oxide growth rates are similar whether or not the top coat is present. The permeability data, and the dependence of permeability on spraying conditions, may be of interest for other applications, such as permeable fuel cell membranes.

Journal ArticleDOI
TL;DR: In this article, a solution precursor plasma spray (SPPSP) process was used to obtain high spallation life for thermal barrier coatings. But, the failure of SPPS TBCs occurs within the ceramic top coat, near the ceramic/bond coat interface.
Abstract: The solution precursor plasma spray (SPPS) process offers the prospect of depositing highly durable thermal barrier coatings (TBCs) of low thermal conductivity. In this study, a Taguchi design of experiments was employed to optimize the SPPS process. The spallation life of SPPS TBCs on a MCrAlY bond coated Ni-base superalloy substrate deposited under the optimized processing conditions was demonstrated to be more than 2.5 times of that of a conventional plasma sprayed TBC with the same substrate and bond coat. The superior durability of SPPS TBCs is associated with their novel microstructures, which include: (i) a ceramic matrix containing micrometer and nanometer porosity, (ii) the presence of very fine splats (0.5 to 5-μm diameters), (iii) through-thickness cracks, and (iv) improved ceramic to bond coat adhesion. The failure of SPPS TBCs occurs within the ceramic top coat, near the ceramic/bond coat interface. Buckling spallation is the failure mode observed for all tested samples. It was also demonstrated that the SPPS process is capable of depositing thick (>2 mm) and durable TBCs.

Journal ArticleDOI
TL;DR: In this article, the effects of a dielectric barrier discharge (DBD) plasma surface treatment on polytetrafluoroethylene (PTFE), polyimide (PI) and poly (lactic acid) (PLA) films in terms of changes in surface wettability and surface chemistry were investigated.
Abstract: This paper, based on an orthogonal experimental design and analysis method, reports the effects of a dielectric barrier discharge (DBD) plasma surface treatment on polytetrafluoroethylene (PTFE), polyimide (PI) and poly (lactic acid) (PLA) films in terms of changes in surface wettability and surface chemistry. The purpose was to study the influence of the main operating parameters, i.e. plasma power, treatment period duration (treatment cycles) and electrode gap on the resultant surface properties. Statistical analysis was carried out to develop an equation which expresses surface properties (water contact angle and oxygen enrichment, as observed by XPS analysis) in terms of these operational parameters. It was observed that the plasma parameters have a selective effect on the changes observed for the polymers processed. In particular, plasma processing time (treatment cycles), plays an important role in the treatment of PTFE and PI in this study, whereas the size of the electrode gap plays the dominant role in the treatment of PLA. Fast surface activation can be achieved in all cases after only a few seconds of treatment duration. The wettability improvement observed in all cases was attributed to changes in both surface chemistry and surface micro-structure.

Journal ArticleDOI
TL;DR: In this article, the influence of surface roughness on the galling properties of forming tool steel was investigated and the main observation was that the tool surface dramatically improved by reducing the surface topography.
Abstract: The aim of the present work is to elucidate the influence of surface roughness on the galling properties of coated forming tool steel. The tribological evaluation included TiN, TiB 2 , TaC and WC/C coatings deposited on cold work tool steel. Representing a material difficult to form, austenitic stainless steel was used as a counter-material. A special test configuration made it possible to gradually increase the normal load during forward sliding strokes, and to correspondingly decrease the load during reversed ones. In this investigation, the load was varied between 100 and 1300 N, corresponding to a contact pressure between 2 and 5 GPa. The main observation is that the galling and anti-sticking properties of the tool surface dramatically improve by reducing the surface topography. Consequently, reduced substrate roughness or polishing of the contact surface after coating is highly recommended. However, selection of a carbon-based low-friction coating leads to reduced probability of worked material adhesion even at high surface roughness values and under starved lubrication.

Journal ArticleDOI
TL;DR: In this paper, the formation of a Ti-Zr based conversion layer on AA6060 aluminium has been studied electrochemically and by use of various surface analytical methods, and it has been shown that the α-Al(Fe,Mn)Si particles present in AA 6060 are cathodic to the aluminium matrix in a fluorotitanate-zirconate (H 2 TiF 6 −H 2 ZrF 6 ) based solution, and reduction reactions occur predominantly on the particles.
Abstract: Formation of a Ti–Zr based conversion layer on AA6060 aluminium has been studied electrochemically and by use of various surface analytical methods. Measurements show that the α-Al(Fe,Mn)Si particles present in AA6060 are cathodic to the aluminium matrix in a fluorotitanate–zirconate (H 2 TiF 6 –H 2 ZrF 6 ) based solution, and reduction reactions occur predominantly on the particles. The alkaline diffusion layer thus formed causes preferential deposition of a hydrated Ti and Zr oxide covering the particles and surrounding areas, eventually leading to a conversion layer with significant variations in thickness. In addition to substrate metallurgy, agitation and pH of the conversion bath are factors that significantly control the conversion layer deposition. The conversion layers formed cause only a small reduction in the cathodic activity of the α-Al(Fe,Mn)Si particles and do not impart improved corrosion resistance of the AA6060 alloy in chloride solution.

Journal ArticleDOI
TL;DR: In this paper, the authors apply an analytical technique to calculate residual stress in thermally sprayed deposits based on geometric properties, which is a combination of quenching stresses and cooling stresses, post-deposition.
Abstract: Due to the nature of the high velocity oxy-fuel thermal spray process, residual stress build up in thick deposits is a significant and a limiting problem. The residual stress-state that evolves in a deposit is largely dependent on the thermal conditions to which the system has been subjected, and is a combination of quenching stresses, which arise during deposition, and cooling stresses, post-deposition. It follows that precise control of these phenomena is essential, if a thick deposit is to be thermally sprayed. This paper applies an analytical technique to calculate residual stress in thermally sprayed deposits based on geometric properties. Residual stress results for WC–Co (tungsten carbide–cobalt) samples are compared to experimental results (X-ray diffraction and hole-drilling method). A change in deposit stress-state from tensile to compressive stress with deposit thickness is analysed in terms of quenching and cooling stresses.

Journal ArticleDOI
TL;DR: The structure of undoped, Al-doped ZnO (AZO) and Ga-Doped (GZO) thin films grown on sapphire and NaCl substrates by 193 nm pulsed laser ablation of a ZnOs target in a low background pressure of oxygen was investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD) as discussed by the authors.
Abstract: The structure of undoped, Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films grown on sapphire and NaCl substrates by 193 nm pulsed laser ablation of a ZnO target in a low background pressure of oxygen was investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The films on sapphire grew with the polar (0002) orientation. The samples deposited on NaCl, at substrate temperatures above 570 K, presented a mixture of polar and non-polar orientations. All samples demonstrated improved crystalline quality, as measured by the FWHM of the ZnO (0002) rocking curve, with increasing substrate temperature. The best crystalline quality was observed for the undoped films. The inclusion of Al or Ga into the lattice degraded the crystallinity of the films, but allowed production of highly conductive films. AZO and GZO film resistivities were measured using a four-point probe method and were found to decrease with increasing deposition temperature. Film thickness was determined using variable angle spectroscopic ellipsometry.

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TL;DR: In this article, the effect of surface adsorbates on splat formation during thermal spraying is examined by controlling substrate adsorption, and it is shown that the existence of evaporable adsorates on the substrate surface influences significantly the splatting and consequently splat morphology.
Abstract: The effect of surface adsorbates on splat formation during thermal spraying is examined by controlling substrate adsorption. Splats are formed on a polished flat stainless steel substrate surface by plasma spraying. The adsorption state of the substrate is controlled with different organic substances of different boiling points and different preheating temperatures. The droplet materials used are aluminum, nickel, and Al 2 O 3 . Three kinds of organic substances used are xylene, glycol and glycerol, which are brushed on the surface of the substrate before spraying. It is revealed that when preheating temperature of the substrate is lower than the boiling point of the organic adsorbates on the substrate surface, the splashing occurs during droplet flattening which results in the formation of the splats with an irregular morphology. However, when the preheating temperature exceeds approximately 50 °C over the boiling point of the organic adsorbates and evaporates all organic substance adsorbed on the surface, the regular disk type splats are formed which corresponds to no occurrence of the splashing during splatting. It is clearly shown that the existence of the evaporable adsorbates on the substrate surface influences significantly the splatting and consequently the splat morphology. On the basis of the result, the evaporated-gas-induced splashing model is proposed to explain the occurrence of splashing during splatting of a spray molten droplet and the formation of the reduced splat. The effect of preheating temperature of a substrtae surface on water adsorption and desorption characteristics in ambient atmosphere is disscussed from the point of view to create a clean surface and regular disc splat.

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TL;DR: In this article, it is argued that the often-considered ion matrix sheath is only an educational case, whereas most realistic transient sheaths obey a time-dependent Child law.
Abstract: Pulsed plasmas use much higher power during each pulse compared to continuously operated plasmas. This feature and the appearance of additional new variables such as pulse duty cycle can lead to new, flexible, sometimes enabling processes for surface modification and thin film deposition, as discussed for plasma immersion ion processing, high power pulsed sputtering, thin film deposition with pulsed cathodic arcs and metal plasma immersion ion implantation and deposition. In all of these processes, transient sheaths are of great importance. The fundamentals of plasma sheaths are briefly reviewed. It is argued that the often-considered ion matrix sheath is only an educational case, whereas most realistic transient sheaths obey a time-dependent Child law. The recovery of pulsed sheaths is usually less considered than the formation of sheaths. If the pulse fall time normalized by the inverse ion plasma frequency is much greater than unity, sheath collapse can be simply described by a retreating Child law sheath. In the opposite case, electrons refill the ion space charge and complete recovery of the boundary layer is determined by ambipolar diffusion from the bulk plasma. The case of high power pulsed sputtering is discussed in greater detail, especially the appearance of a kink in the current–voltage characteristic, which changes its slope for current densities exceeding approximately 600 mA/cm 2 . It is suggested that high pulse power opens the possibility that self-sputtering can occur during each pulse, especially for target materials of high sputter yield. The necessary condition for self-sputtering is αβγ >1, where α , β and γ are the ionization probability, ion return probability and sputter yield, respectively.

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TL;DR: In this article, four different coatings were deposited onto an AISI 1020 steel substrate: (i) WC-12%Co, (ii) as-sprayed (AS) 50%(WC 12%Co)+50%(NiCr), (iii) post-melted (PM) and (iv) a duplex system comprising a WC 12% Co top layer and a NiCrAl interlayer.
Abstract: The deficient performance of thermally sprayed coatings in cavitation erosion tests is often attributed to the nature of their lamellar microstructure. Poor coating/substrate adhesion, low toughness and tensile residual stresses, which are introduced during the deposition process, can also adversely affect their cavitation resistance. In order to improve the cavitation performance of such coatings, it is also important to control some of the coating properties such as elastic modulus (E) and hardness (H). By reducing E and increasing H (i.e. to ensure a higher H/E ratio), a better tribological performance is usually achieved. The erosive environment also plays a decisive role in determining the final chemical composition of the coating. Thermally sprayed WC–Co coatings are well known because of their high hardness and, in such cemented carbides, the corrosion resistance is frequently affected by the susceptibility of the cobalt binder to chemical attack. In this work, an attempt to improve the cavitation resistance of WC–Co coatings was made by either modifying coating composition (and therefore modifying some coating properties such as hardness, elastic modulus and toughness) or by carrying out a ‘melt’ post-deposition treatment in order to disrupt the intrinsic lamellar microstructure of the coating. Four different coatings were deposited onto an AISI 1020 steel substrate: (i) WC–12%Co; (ii) as-sprayed (AS) 50%(WC–12%Co)+50%(NiCr); (iii) post-melted (PM) 50%(WC–12%Co)+50%(NiCr) and (iv) a duplex system comprising a WC–12%Co top layer and a NiCrAl interlayer. The ‘PM’ coating produced from the pre-alloyed powder 50%(WC–12%Co)+50%(NiCr) displayed a higher elastic modulus (measured by Knoop indentation) and a lower hardness (and thus a lower H/E ratio) than the WC–12%Co. Also, the fracture toughness of the latter (measured by Vickers indentation tests) was increased from 1.6±0.9 to 32±12 MPa m1/2. The worst performance in cavitation erosion tests was achieved by the WC–12%Co coating, which showed the highest mass loss throughout the test. Conversely, the ‘PM’ 50%(WC–12%Co)+50%(NiCr) coating exhibited the best cavitation resistance and a correlation between coating toughness and cavitation resistance could be established.