Showing papers in "Surface & Coatings Technology in 2001"

The role of hydrogen in tribological properties of diamond-like carbon films☆

Abstract: Extensive research on diamond and diamondlike carbon (DLC) films in our laboratory has further confirmed that hydrogen plays an important role in the tribological properties of these films. Specifically, model experiments in inert gas environments revealed a very close relationship between the hydrogen-to-carbon (H/C) ratios of source gases and the friction and wear coefficients of the resultant DLC films. The friction coefficient of films grown in source gases with very high H/C ratios (e.g. 10) was superlow (0.003), whereas that of hydrogen-free DLC films (with essentially zero H/C ratio) was very high (0.65). The friction coefficients of films grown in source gases with intermediate H/C ratios were between 0.003 and 0.65. Experiments also revealed that the frictional properties of these films were very sensitive to test environments. Specifically, when tested in open air, the friction coefficient of hydrogen-free DLC dropped to 0.25, whereas that of highly-hydrogenated DLC increased to 0.06. Fundamental knowledge combined with surface analytical and tribological studies have led to the conclusion that the type and extent of chemical interactions between carbon–carbon, carbon–hydrogen, and carbon–adsorbate atoms at the sliding-contact interfaces determine the friction and wear properties of DLC films.

Structure-performance relations in nanocomposite coatings

Abstract: Properties of hard nanocomposite coatings, especially hardness, can be explained by their nanostructure. Hardness maxima are found for different nanocrystalline/amorphous materials deposited by different techniques at typically 20% of the amorphous phase. A zone model is proposed which correlates the hardness to the relative phase content. The hardness of nanocomposite coatings peaks at the common minimum of the grain size of the crystalline phase and the grain separation. For an adequate description of the performance of a coating, the thermal stability, oxidation behavior and frictional behavior should be included in addition to hardness. In a friction situation involving at least two friction partners, the overall behavior of the system is determined by many-body interactions. While thermal stability and oxidation properties as inherent material properties can be directly linked to the nanostructure of the coating, the frictional behavior of a coating cannot be generalized independent of the friction conditions.

Electrodeposition and sliding wear resistance of nickel composite coatings containing micron and submicron SiC particles

Abstract: SiC particles of three different sizes, namely 5, 0.7 and 0.3 μm, were codeposited with nickel from Watts’ solutions. It was found that for a given number density of particles in the plating solution, the number density of particles in the coating increases with decreasing particle size. The friction and wear behavior of these composite coatings was evaluated in uni- and bi-directional sliding tests against corundum balls. The best sliding wear resistance was obtained with Ni–SiC composite coatings containing 4–5 vol.% submicron SiC particles.

Development and implementation of plasma sprayed nanostructured ceramic coatings

Abstract: A broad overview of the science and technology leading to the development and implementation of the first plasma sprayed nanostructured coating is described in this paper. Nanostructured alumina and titania powders were blended and reconstituted to a sprayable size. Thermal spray process diagnostics, modeling and Taguchi design of experiments were used to define the optimum plasma spray conditions to produce nanostructured alumina–titania coatings. It was found that the microstructure and properties of these coatings could be related to a critical process spray parameter (CPSP), defined as the gun power divided by the primary gas flow rate. Optimum properties were determined at intermediate values of CPSP. These conditions produce limited melting of the powder and retained nanostructure in the coatings. A broad range of mechanical properties of the nanostructured alumina–titania coatings was evaluated and compared to the Metco 130 commercial baseline. It was found that the nanostructured alumina–titania coatings exhibited superior wear resistance, adhesion, toughness and spallation resistance. The technology for plasma spraying these nanostructured coatings was transferred to the US Navy and one of their approved coating suppliers. They confirmed the superior properties of the nanostructured alumina–titania coatings and qualified them for use in a number of shipboard and submarine applications.

Investigation of creep behaviour under load during indentation experiments and its influence on hardness and modulus results

Abstract: To improve the accuracy and comparability of hardness and modulus results from nanoindentation experiments an evaluation of the creep behaviour is required. Creep depends on the material and normally diminishes to very low values within some seconds. Nevertheless, it influences the maximum depth and the upper part of the unloading curve in a way that measurement errors of more than 20% may occur. In this work, a detailed analysis of the creep behaviour for different film and substrate materials is done. In addition, the influence of loading time and hold period at maximum load on the hardness and modulus results is investigated. The results show that especially for materials with low hardness-to-modulus ratio (mostly metals), the modulus results are not reliable if the hold period is chosen too low. Hold periods are proposed in dependence on the material type that should be kept for high accuracy measurements.

Surface roughness of diamond-like carbon films prepared using various techniques

Abstract: Diamond-like carbon (DLC) films have been deposited onto mirror-finish silicon surfaces using three different techniques: (a) RF glow discharge from methane, (b) DC magnetron sputtering from a graphite target, with substrate RF bias; and (c) an ion beam generated from a cathodic arc discharge. Nanoscale atomic force microscopy (AFM) (tapping mode) surface roughness analysis has been carried out on as-deposited and post-deposition treated DLC films. It was found that, in terms of the effect on surface roughness, there is a threshold value of the ion impingement energy. Below this ion energy, which has a value of approximately 50 eV for each of the three techniques studied, the DLC surface is very rough on a nanoscale, while above it the roughness drops sharply. This threshold is close to previously reported atomic displacement threshold energies for graphite and diamond. These observations have been explained in terms of the tendency for surface diffusion to generate sp 2 -rich surface clusters which roughen the surface, whereas high impingement energies lead to ion implantation, and hence less efficient surface diffusion, and to preferential sputter removal of protruding regions, which also favours low roughness. Observed effects of substrate temperature and exposure to atomic hydrogen have also been explained in terms of these mechanisms of roughening and smoothing.

Wear and corrosion behaviour of thermally sprayed cermet coatings

Abstract: In order to protect machining parts against wear and corrosion, they are coated by cermet coatings. The coatings consist of WC or Cr3C2 particles in a metal binder, which can be a pure metal or a mixture consisting of Ni, Cr, Co. The examined coatings were produced by high velocity oxy-fuel (HVOF) spraying and were investigated with regard to erosion and corrosion resistance. The combined erosion corrosion tests were carried out at ambient temperature in 0.1 M NaOH and 0.1 M H2SO4 solutions containing sand. Information about the corrosion resistance was gained from electrochemical polarization measurements and salt spray test (NaCl solution). The results of combined wear and corrosion tests showed that the corrosion properties of the sprayed coatings strongly affect the materials loss rate under wear corrosion conditions. Coatings with a less corrosion resistant matrix present enhanced erosion, also. The erosion mechanism of the carbide coatings seams to be controlled by the skeletal network of the carbides. For comparison, flame and plasma sprayed hard Cr2O3 coatings were examined. Due to the low electrical conductivity the corrosion rate of these coatings was very low. Under erosion conditions the hard Cr2O3 coatings exhibited a high erosion rate and the erosion mechanism seems to be that of the brittle erosion due to grain-by-grain removal of oxide grains during impact.

Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3

Abstract: Various effects of chromate conversion coatings (CCCs) and chromate in solution on the corrosion of AA2024-T3 and pure Al are studied in this work. Raman spectroscopy was used to investigate the nature of chromate in CCCs through a comparison with the spectra of known standards and artificial Cr(III)/Cr(VI) mixed oxides. Chromate was shown to be released from CCCs and to migrate to and protect a nearby, uncoated area in the artificial scratch cell. However, experiments investigating the effect of chromate in solution on anodic dissolution kinetics under potentiostatic control indicated that large chromate concentrations were needed to have an effect.

Magnetron sputtering of hard nanocomposite coatings and their properties

Abstract: The article reports on the present status of knowledge in the field of hard and superhard nanocomposite coatings prepared by magnetron sputtering. Special attention is devoted to the two-phase nanocomposites composed of one hard and one soft phase. Trends of the next development are outlined.

Microstructure and mechanical/thermal properties of Cr–N coatings deposited by reactive unbalanced magnetron sputtering

Abstract: Chromium nitride (CrN) is a hard material and a well-established coating for applications where severe corrosion and friction conditions are present. In this work, we report on the influence of nitrogen/argon flow rate ratio, ion energy and ion/atom flux ratio on the microstructure, hardness, residual stresses and thermal stability of magnetron sputtered chromium nitride coatings. The coatings were characterized with respect to thickness, morphology, chemical composition, microstructure and hardness. Hardness values up to 38.4 GPa could be obtained for stoichiometric CrN, which strongly depend on the grain size and residual stress. Thermal coating properties were evaluated using stress measurements during thermal cycling and XRD analyses after annealing at 500 and 700°C. Film stresses up to 700°C were measured from the bending of coated silicon specimens using the Stoney formula. Stress relaxation occurring during this temperature treatment strongly depends on the biaxial stresses in the as-deposited state. The interrelationships between growth conditions, microstructure, mechanical and thermal properties will be presented and discussed.

Advantages of using self-lubricating, hard, wear-resistant mos2-based coatings

Abstract: The paper reports a summary of MoS2 development through years. The properties of MoS2 coatings can be improved by the co-deposition of small amounts of titanium. These MoS2/Ti composite coatings known as MoST produced by closed-field unbalanced magnetron sputtering, are harder (1000–2000 compared to 400 HV for MoS2), much more wear-resistant (by a factor of 100) and also less sensitive to atmospheric water vapour (improvement by a factor of 2800 compared to MoS2) during tribological testing. They retain a low specific wear rate of 4×10−17 m3/N m, a low friction coefficient of 0.02–0.1, and a high load-bearing capacity up to 5 GPa. Alternative MoS2/material composite coatings have been developed with similar properties. These coatings have given excellent industrial results for a wide range of cutting and forming applications. Recent industrial performance data related to the characteristics of these MoST auto-lubricating coatings, which are utilised today in large-scale production, are presented.

Corrosion resistance of multi-layered plasma-assisted physical vapour deposition TiN and CrN coatings

Abstract: The corrosion resistance of mild steel coated with single layered and multi-layered TiN and CrN coatings has been studied. The base material was coated with TiN and CrN by an electron-beam plasma-assisted physical vapour deposition (PAPVD) technique. ‘Single’ layers of TiN or CrN, which normally include an interlayer (approx. 100–200 nm) of Ti or Cr under the main TiN or CrN film, were prepared; four layers of TiN (or CrN) were produced by four sequential repetitions of the single-layer process. The microstructural features and corrosion performance were then compared. It is shown that applying four layers of coating can enhance the corrosion performance of PVD TiN and CrN coated mild steel. The corrosion resistance improvement is not only attributed to the increase in thickness, but also to the internal microstructure and phase composition. CrN coatings produced in this work proved to be particularly promising in terms of corrosion resistance, owing to their dense non-columnar structure which contained a mixture of three phases: Cr (b.c.c.), Cr2N (hexagonal) and CrN (f.c.c.). Mild corrosion reactions were observed in CrN coated steel during various electrochemical tests in aqueous salt solution, indicating that inter-phase corrosion had caused a redistribution of the current flow, so as to eliminate current concentration at small through-coating pinholes. This prevented rapid galvanic attack at the coating/substrate interface. More importantly however, in comparison with the columnar structure of TiN coatings, the dense structure with fine equiaxed crystallites made the CrN coatings less permeable to the corrosive medium.

Tribochemistry between hydrogen and diamond-like carbon films

Abstract: The objective of the present work is to propose a model related to the role of hydrogen on the friction mechanism of DLC films. An up-to-date review of the effect of hydrogen on the tribology of DLC films is presented first. Selected experiments performed on two model hydrogenated DLC films are then presented to demonstrate how hydrogen, both as a constituent of the carbonaceous film or as a gaseous species introduced in the surrounding environment during the friction process can influence the intermediate and steady-state friction regimes, in the absence of any oxidating species. For the film with the highest hydrogen content, superlow friction (10−3 range) is reached rapidly in an ultrahigh vacuum. For the film containing the lowest hydrogen content, the combination of a controlled temperature during friction (150°C) with hydrogen diffusion from the bulk of the film towards the sliding activated surfaces of the hydrogen carbon-to-carbon is responsible for an intermediate period with friction in the 10−3 to 10−2 range. Then the steady-state friction coefficient rises up to 0.6, typical for low hydrogenated a-C:H films in vacuum or inert atmospheres. A superlow friction steady-state regime may be controlled over longer periods by introducing a significant pressure of pure hydrogen surrounding the contact during the friction process. Argon at the same pressure does not have any similar lubricating effects. Tribochemistry between hydrogen and the carbonaceous network is thus responsible for the control of the superlow friction regime observed with a-C:H coatings in selected conditions of film composition and atmosphere.

Role of hexavalent chromium in the inhibition of corrosion of aluminum alloys

Abstract: Selection of alternatives to hexavalent chromium for inhibitors and inhibiting films for Al and its alloys requires a clear understanding of how hexavalent chromium inhibits corrosion. To this end, the following hypotheses have been proposed: residual chromate in conversion coatings provides self healing; hydrated Cr(III) oxide films are uniquely inert and hydrophobic; oxyanions of Cr(VI) rapidly passivate secondary phases in the highly unstable Cu-rich aluminum alloys; and adsorption of hexavalent chromium on passive aluminum oxide makes it less susceptible to chloride attack. Evidence supporting these hypotheses come from X-ray spectroscopic analysis (XANES) of conversion coatings on aluminum, characterization of Cr(VI) oxyanion adsorption on anodized aluminum via a piezo–electrokinetic (PEK) method, and the electrochemical current vs. voltage response model intermetallic compounds.

Characterization of TiNi shape-memory alloy thin films for MEMS applications

Abstract: Thin film shape-memory alloys (SMAs) have been recognized as promising and high performance materials in the field of microelectromechanical systems (MEMS) applications. In this investigation, chemical composition, microstructure and phase transformation behaviors of sputter deposited TiNi films were studied. The surface and cross-section morphology of the deposited coating was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results from the differential scanning calorimeter (DSC) showed clearly the martensitic transformation upon heating and cooling. X-Ray diffraction analysis (XRD) also revealed the crystalline structure changing with temperature. By depositing TiNi films on the bulk micromachined Si cantilever structures, micro-beams exhibiting a good shape-memory effect were obtained. Finite element simulation results of the deformation of micro-beam (using the measured NiTi thin film parameters) agree quite well with the measured behavior.

Evaluation of microhardness and elastic modulus of thermally sprayed nanostructured zirconia coatings

Abstract: Results concerning microhardness and roughness ( R a ) of plasma sprayed coatings fabricated from nanostructured partially stabilized zirconia (PSZ) feedstock are presented. Nanostructured zirconia particles were plasma sprayed (Ar/H 2 ) at three power levels, with two argon flow rates at two spray distances. The results indicate that the microhardness, elastic modulus and roughness of the nanostructured zirconia coatings exhibit the following trends: the smoother the roughness, the higher the microhardness and elastic modulus. It was found that roughness is an indicator of the coating state that reflects the intrinsic microstructure of the coatings. It was ascertained that a surface profilometer could be used to determine the level of microhardness and elastic modulus as a non-destructive and in situ test by simple comparison with standard samples.

Application of atmospheric pressure dielectric barrier discharges in deposition, cleaning and activation.

Abstract: Dielectric barrier discharges (DBDs) at atmospheric pressure are obtained using mixtures of He and Ar as carrier gasses and various reactive additives such as hydrocarbons, hydrogen and nitrogen. These DBDs are used in three applications: deposition of polymer films; cleaning of Ag and Cu substrates; and activation of polyurethane and steel surfaces. In the case of the film deposition, several process conditions are investigated and the resulting films are analysed by scanning electron microscope, Fourier transform infrared spectroscopy and NMR. In another series of experiments Ag and Cu surfaces, covered with sulfide and oxide layers, are treated by means of a DBD in helium or argon with hydrogen added. The surfaces are analysed with X-ray photoelectron spectroscopy. Finally, a He–N2 plasma is used as an activator of polyurethane.

Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems

Abstract: EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline (SX) substrate alloys were thermally cycled at 1100°C. TBC spallation does not correlate solely to TGO thickness, but depends also very much on the substrate alloy. The longest lifetimes are achieved on Hf-containing alloys while SX alloys suffer from early TBC spallation. The formation of the thermally grown oxide was investigated in detail by TEM. A mixed layer of alumina and zirconia exists in the as-coated condition. After initial slight thickening, the thickness of this mixed layer remains constant over a long period of time. During thermal exposure, a continuous layer of pure α-alumina forms and grows underneath the mixed zone by oxygen inward diffusion.

Penetration of plasma effects into textile structures

Abstract: The penetration of plasma into textile materials turns out to be a crucial point for plasma modification of fabrics. Throughout the process where plasma treatment can be applied approximately 1–100 mbar is evaluated to be the optimum. This is due to the correlation between characteristic geometrical distances in fabrics and the mean free path of modifying particles in the gas phase as well as the energy transfer from activated plasma particles to surrounding inactive gas particles and to surface sites of textile fibres. Theoretical calculations and experimental results for the hydrophilisation of fabrics prove this behaviour.

A new class of high performance PVD coatings for carbide cutting tools

Abstract: A number of advanced PVD coating designs based on Ti–Al–N–C–B were evaluated in metalcutting Monolayer PVD TiN, TiAlN, TiB 2 and different variants of TiAlN multilayer coatings were deposited on WC-6 wt% Co hardmetal inserts The coatings were applied either by cathodic arc processes or a high-ionization magnetron sputtering process The coated tools were evaluated in milling of ductile and gray cast irons with and without coolant, and in turning of Inconel 718 and a hypereutectic AlSi alloy The TiAlN-multilayer coated tools showed the best performance in dry milling applications; the TiAlN-monolayer coated tools performed better under wet milling The observed results are consistent with a model that takes into consideration the inherent residual stresses within the coating, the stresses during machining, and the bonding strength of the coating layers to the substrate In Inconel 718 turning, the TiAlN-multilayer coating showed some performance advantage over the TiAlN-monolayer and the TiN/TiCN/TiAlN-multilayer coating particularly at higher speed In the turning of the aluminum alloy, PVD TiB 2 had performance advantage over PVD TiAlN and PVD TiN, which could be correlated with their relative hardness values

Hardness, friction and wear characteristics of nickel-SiC electroless composite deposits

Abstract: Composite coatings constitute a new class of materials which are mostly used for mechanical and tribological applications. Among these materials, nickel deposits with incorporation of hard ceramic particles such as silicon carbide (SiC), combine anti-corrosion properties (due to the presence of nickel), with mechanical and tribological performances (due to the presence of particles of SiC). In the present study, electroless Ni–SiC composite deposits with various amounts of SiC have been prepared and characterized. The first part of this paper considers the materials, the experimental conditions and the characterization of the coatings. The second part is dedicated to the study of the mechanical (hardness) and tribological (friction resistance and wear) properties of the co-deposits. Friction tests were carried out using a ball on disk tribometer. The mechanisms of wear were analysed from observations of the wear scars and their characterization using two-dimensional and three-dimensional profilometry. The results show that increasing the size or the rate of SiC particles incorporated lead to an increase in both the hardness of the films and friction coefficient when sliding against a steel ball.

Influence of particle in-flight characteristics on the microstructure of atmospheric plasma sprayed yttria stabilized ZrO2

Abstract: The atmospheric plasma spraying of a yttria stabilized ZrO 2 top-layer of a thermal barrier coating (TBC) produces a complex microstructure consisting of a wide variety of cracks and pores. These voids are known to influence the thermal conductivity and mechanical properties of the TBC. In this study, the influence of the plasma spray process on the microstructure of the coating and deposition efficiency was investigated with the aim of achieving better knowledge and control of the process. Eight process parameters to control the plasma process were employed in a fractional factorial designed experiment involving 16 different thermal barrier coatings. The microstructure of the coatings, characterized by seven features, in particular those of cracks and pores, was studied by means of scanning electron microscopy (SEM) and image analysis (IA) and the extent of the different features were quantified. For each sprayed coating, the particle velocity and particle temperature were measured prior to impact, using the optical measure system DPV 2000. The four spray gun parameters controlling the plasma plume were found to each have a significant influence on the particle properties. The remaining four parameters did not affect the particle properties, but instead influenced the coating microstructure directly. Multiple linear regression was used to find models describing how the particle properties and the other process parameters were related to the coating microstructure. The results showed particle velocity, particle temperature, spraying angle and substrate temperature to be the most important parameters influencing the coating microstructure. The influence of the different parameters and particle properties on the microstructure features varied, however.

Nitrogen and carbon expanded austenite produced by PI3

Abstract: Expanded austenite can be formed either by nitrogen or carbon plasma immersion ion implantation (PI3 ™) from a nitrogen or methane plasma at elevated temperatures. The structure and properties of nitrogen and carbon expanded austenite layers produced on austenitic stainless steel X5CrNi189 are compared. A new structural model of expanded austenite based on a defect rich face centred cubic (fcc) lattice is proposed. Although the structure of the two expanded austenite layers is similar, there is a remarkable difference in the uptake of nitrogen or carbon, despite the use of similar treatment conditions. The modified surfaces have different hardness, corrosion and wear properties.

An XPS study of cerium dopants in sol–gel coatings for aluminum 2024-T3

Abstract: Corrosion protection is a key requirement for coatings on aircraft as the US Air Force extends the lifetime of its fleet. Coating systems for aluminum have long incorporated chromates in conversion coatings to protect against corrosion, but environmental restrictions require that alternative coating systems be developed. Cerium has been proposed as an alternative to chromate inhibitors, as have several other rare earth elements, because the rare earths behave as cathodic inhibitors in aluminum. Epoxy silicate sol–gels, containing a few wt.% of cerium salts, were investigated as coatings on aluminum alloy 2024-T3. The salts used were cerium(III) chloride, cerium(III) nitrate hexahydrate, and ammonium cerium(IV) nitrate. X-Ray photoelectron spectroscopy (XPS/ESCA) was used to study both the doped sol–gels and reference cerium compounds in order to determine the oxidation state of the cerium at the surface of the sol–gel coatings. No change in the oxidation state of the cerium in the sol–gels was found. Coupled with electrical impedance spectroscopy measurements, the incorporation of cerium into sol–gels seems promising for future corrosion protection of aluminum 2024-T3.

Transformation of monetite to hydroxyapatite in bioactive coatings on titanium

Abstract: Calcium phosphates have a wide range of pH stability, depending on their Ca/P ratio. Under physiological conditions (pH ≈7), the most stable calcium phosphate is hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 . Acidic calcium phosphates, like dicalcium phosphate, CaHPO 4 (monetite) and dicalcium phosphate dihydrate, CaHPO 4 ·2H 2 O (brushite), are thermodynamically unstable under pH values greater than 6–7 and undergo transformation into more stable calcium phosphates. It means that, when placed in vivo (pH ≈7), acidic calcium phosphates convert to hydroxyapatite. In the present study, a coating of crystalline monetite oriented along the [112] axis was electrochemically deposited on titanium substrates. This monetite coating was subsequently converted to hydroxyapatite by immersion in alkaline solutions. The result was a crystalline hydroxyapatite coating oriented along the [002] axis. Different alkaline solutions produced the same result. Studying the effect of immersion time on the transformation indicated that 4 h were required to complete the conversion from monetite to hydroxyapatite. The transformation occurred by a dissolution–reprecipitation mechanism, i.e. the monetite coating was continuously dissolved and reprecipitated as hydroxyapatite. This combined electrochemical deposition and chemical conversion process produced hydroxyapatite coatings with satisfactory adhesion to the substrate and a thickness between 10 and 30 μm.

Oxidation-induced failure of EB-PVD thermal barrier coatings

Abstract: Oxidation-induced failure of EB-PVD thermal barrier coatings (TBC) deposited on a single-crystal superalloy with a platinum aluminide bond coat has been studied in order to determine the specific mechanisms leading to TBC spallation. Cyclic oxidation tests performed at 1150°C show that failure of the TBC occurs when the alumina scale, growing at the bond coat–TBC interface, attains its critical thickness. Therefore, the oxidation rate of the bond coat is considered the major factor affecting TBC durability. The growth rate, morphology and residual stress in the alumina scale depend on the method of the bond coat surface treatment prior to TBC deposition. While the TBC failure is ultimately driven by the elastic strain energy stored in the scale on cooling, the magnitude of the compressive stress in the scale has no apparent effect on the TBC lifetime. Except for the sample edges, no significant damage in the scale under the TBC has been detected by luminescence spectroscopy prior to failure.

Mechanical properties of superhard nanocomposites

Abstract: Novel superhard nanocomposites prepared according to the generic design concept [Thin Solid Films 268, (1995) 64], which is based on the formation of the appropriate nanostructure due to strong segregation and spinodal decomposition, show an unusual combination of mechanical properties, such as high intrinsic (i.e. not falsified by a large compressive stress) Vickers microhardness from 40 to ≥100 GPa, high elastic recovery (up to ≥90%), high resistance against crack formation even at a large strain of ≥10% and high thermal stability. We shall show that these properties can be relatively easily understood on the basis of conventional fracture mechanics scaled down to dimensions of a few nanometers small nanocrystals and nanocracks, in combination with a low concentration of possible flaws introduced into the material during its preparation. The latter is a consequence of the ‘self-organization’ of the system due to the thermodynamically driven formation of the stable nanostructure.

Surface processes and diffusion mechanisms of ion nitriding of stainless steel and aluminium

Abstract: Model experiments with low-energy ions under controlled vacuum conditions have been performed in order to identify the surface processes and diffusion mechanism associated with nitrogen transport during ion nitriding of stainless steel 316 and pure aluminium. A necessary condition for efficient nitriding is the transmission of the implanted ions through a surface oxide layer, which results from a balance of ion sputtering and re-oxidation from the residual gas. For ion energies of approximately 1 keV and ion current densities of approximately 0.2 mA/cm 2 , oxygen partial pressures of less than approximately 3×10 −6 and 3×10 −7 mbar are required for stainless steel and aluminium, respectively. Diffusion under the influence of traps controls the transport of nitrogen in stainless steel, with dynamic trapping at the Cr atoms of the alloy. In contrast, stoichiometric AlN grows on aluminium, due to Al diffusion from the underlying bulk. From the surface and diffusion mechanisms, limitations of plasma nitriding and plasma immersion ion nitriding are derived, and the implications for industrial applications are discussed.

Potentiodynamic evaluation of sol–gel coatings with inorganic inhibitors

Abstract: Sol–gel coatings were investigated as potential replacements for chromate-based surface treatments on aircraft aluminum alloys. Unlike chromate based treatments current sol–gel coatings do not have the ability to leach corrosion inhibitors upon coating damage and minimize corrosion of the unprotected area. As an alternative, environmentally compliant non-chromate inhibitors of Ce(NO3)3, NaVO3 and Na2MoO4 were incorporated into a Zr-epoxy sol–gel. Results are reported on the coatings chemical analysis and their corrosion protection performance based on electrochemical studies. Aluminum alloy 2024-T3 test coupons coated with protective sol–gel films were found to provide considerable corrosion protection. The improved performance characteristics were derived from the sol–gel ability to form a uniform, low defect, barrier coating. Coatings doped with Ce(NO3)3 had barrier properties at least as good as the standard sol–gel coatings. Coatings with Na2MoO4 and NaVO3 did not provide adequate corrosion protection. The use of corrosion inhibitors within organically modified sol–gel coatings is discussed.

Thermal cycling of EB-PVD/MCrAlY thermal barrier coatings: I. Microstructural development and spallation mechanisms

Abstract: Microstructural changes, damage initiation and spallation of a production TBC, which consists of: an electron beam physical vapor deposited ZrO2–7 wt.% Y2O3 (YSZ); thermally grown oxide (TGO); MCrAlY bond coat; and a polycrystalline IN-738 superalloy substrate, were examined as a function of thermal cycles at 1121°C. Thermal cycling for TBC specimens in air consisted of a 10-min heat-up, a 40-min hold at 1121°C and a 10-min quench. Microstructural characterization was carried out by: photo-stimulated luminescence piezo-spectroscopy (PLPS); X-ray diffraction (XRD); scanning electron microscopy (SEM); and energy dispersive spectroscopy (EDS). Development of microstructure in the YSZ coatings, growth of the thermally grown oxide (TGO) and its constituents, and phase transformations in the MCrAlY bond coat were examined as a function of thermal cycles. Instability of the TGO/bond coat interface (i.e. rumpling), leading to localized cracking at the YSZ/TGO interface and within the YSZ, was observed after as few as five thermal cycles. Spallation of YSZ coatings occurred after approximately 400 cycles. Significant void formation at the TGO/bond coat interface and formation of a Ni/Co rich oxides at the TGO/bond coat interface due to rapid internal oxidation of Al-depleted MCrAlY bond coat were strong contributors to spallation failure of the TBC. The final spallation of TBC results from a ‘link-up’ of damage at the TGO/bond coat interface with the rumpling-induced micro-cracking at the YSZ/TGO interface.