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Thermal loading induces modifications of the precipitation microstructure of Al–Si–Cu–Mg alloys. This study focuses on the effect of deformation on precipitation microstructure during thermomechanical loadings. Several specimens were thermomechanically cycled while others were exposed to the same thermal cycles without any mechanical loading. The nature and morphological characteristics of the precipitation microstructure of the thermomechanically cycled specimens are compared to those of the thermally aged ones, using transmission electron microscopy (TEM), in order to assess the effect of deformation on the precipitation microstructure and especially on the kinetics of precipitate growth. The absence of any significant effect of superimposed straining during thermal cycling is discussed. Implications for the prevision of yield strength degradation during service operation are briefly presented.

Micrographs of metallographic sections show that additive manufacturing, with its local heat input and rapid cooling rates, brings about microstructures in metal components that differ from those generated in conventional manufacturing processes, such as casting or forging. The example of samples/components made from the material Alloy 718 and manufactured using a laser (Laser Powder Bed Fusion, abbr.: LPBF, designation according to DIN EN ISO ASTM 52900 [1]) shows a very low coarsely distributed porosity. The porosity is limited to the gas porosity already introduced during the atomization step of the powder manufacturing process. As opposed to the microstructure of conventionally produced Alloy 718, the precipitates formed are very small and can only be revealed in the light microscope when high magnifications are applied. Evenly distributed, cruciform γ” precipitates are formed. The grain structure is formed independent of the LPBF-typical melting zones. Isolated dendritic structures can be observed at the interfaces of the melting zones. In the samples manufactured by LPBF, grain structures in the longitudinal section differ from those in the cross section: It can be observed that the grains are elongated in the build direction, which explains the anisotropic behavior of the materials in the tensile test. The width of the melting zones and the laser track spacing can easily be measured in the cross section, while the depth of the melting zones is well recognizable in the longitudinal section. Mechanical properties such as those found in cast or forged Alloy 718 are already present in the stress relief annealed condition.

Due to the health hazard, emanating from chromium trioxide based coating processes, the longtime approved hard chromium platings have to be replaced by alternative coating systems in future. Therefore, the development of PVD coatings, which protect mild steel substrates from both, corrosive and tribological stresses, is a key demand for the PVD coating industry. These coatings have to achieve comparable properties for wear protection and corrosion resistance like chromium platings and also must fulfill the current sustainability criteria in terms of environment protection and health. Recently, the authors developed a TiMgGdN coating system with improved corrosion properties for steel substrates. The superior corrosion properties of TiMgGdN was initially discovered for surface protection of magnesium alloys, which is the most corrosive construction metal (Hoche et al., 2014). In the present work, the TiMgGdN coating concept was applied to mild steel substrates. The coating composition and the deposition parameters were varied to optimize the coatings for steel substrates. The coatings were deposited by using both, DC and HiPIMS sputtering. The influences of the chemical composition, the deposition parameters, the substrate roughness and the sputter mode on the corrosion properties were examined. The coatings were characterized concerning their microstructural, mechanical and chemical properties. Salt spray tests as well as electrochemical measurements were carried out to examine the corrosion properties of the coated specimens. Furthermore, the electrochemical properties as well as the open circuit potentials of the coating materials themselves were determined.

Precise description of the hydrogen charged state of mechanical test specimens including the spatial and energetic hydrogen distribution is considered to be essential for understanding the mechanisms of hydrogen embrittlement. Here, a simple hydrogen absorption model for hydrogen charging in liquids was developed for cylindrical specimens. The absorption model is based on an approximate solution of Fick's laws of diffusion and allows determining hydrogen diffusion coefficients and surface concentrations by measuring the total amount of absorbed hydrogen as a function of the cylinder height. Total hydrogen contents after charging of several cylinders consisting of quenched and tempered steel 42CrMo4 (1.7225) were measured by carrier gas hot extraction and the applicability of the absorption model on the measurement was demonstrated by mathematical fitting. The diffusion coefficient for this material was determined to be D = 3.8 10 − 11 m 2 / s and the hydrogens surface concentration increases with increasing charging time.

PVD-TiMgGdN coatings have been successfully developed by the authors using an industrial DC-magnetron PVD unit. The coatings reveal excellent corrosion protection capabilities for mild steel substrates for at least 800 h in the neutral salt spray test (NSS). MgGd was added to the TiN matrix by using segmented (Seg.) multi-component targets, consisting of segments of pure Ti and of a Mg- 30 wt.-% Gd alloy. The Seg. targets caused an increased defect density due to droplets forming at the gaps between the segments which leads to an unsatisfactory wear performance. To overcome the low mechanical and wear properties by keeping the coatings excellent corrosion performances stable, a TiMgGd target with a specific Ti:Mg:Gd ratio was manufactured by powder metallurgy (PM) technology processed out of pure metal powders. In the present work, the properties of the corrosion and wear performance of DC-PVD-TiMgGdN coatings, which were synthesized onto corrosive mild steel substrates by using a Seg. target and a PM target of comparable Ti:Mg:Gd ratios together with identical deposition parameters, are compared with each other. The coatings were characterized regarding their microstructural, mechanical, physical and corrosion properties. Furthermore, ball on disk wear tests were conducted by reciprocal dry sling SRV III tests. It is shown that the excellent corrosion performance, could significantly be boosted by using the PM TiMgGd sputter target. Moreover, a remarkable increase in wear resistance of the TiMgGdN coatings can be examined by the use of a PM sputter target. It was found that the chemical properties of the TiMgGdN coatings as well as the nitrogen gas flow have a major influence on the mechanical coating properties and on the resulting microstructure, which directly correlates with the corrosion and wear properties.

AISI 304L and AISI 316L are metastable austenitic stainless steels, which exhibit a good corrosion resistance due to their chromium contents but suffer from poor mechanical properties and therefore exhibit poor wear resistance. A slight hardening effect is gained by the formation of deformation-induced martensite either by cold-rolling or shot-peening. A considerable surface hardness could be achieved by plasma nitriding processes due to the formation of the so-called S-phase, the nitrogen diffusion zone. In industrial manufacturing processes cold-deformation could not be avoided and no final annealing is carried out to eliminate the deformation-induced martensite. Therefore, the knowledge about the behavior of the microstructure regarding the nitriding process and the resulting properties is fundamental. The S-phase is also assumed to improve the corrosion resistance if the plasma nitriding is performed below a critical temperature. Above this temperature chromium nitrides precipitate in the matrix and decrease the corrosion resistance. In this study, AISI 304L and AISI 316L were solution annealed and afterwards work hardened with a cold rolling degree of 38%. The surfaces were polished and shot-peened, respectively. The amount of deformation induced martensite depends on the material and this, in turn influences the nitriding behavior. It will be shown, that a highly deformed microstructure promotes the growth of the S-phase. Furthermore, the resulting surface hardness and corrosion resistances in saline solution were examined. The corrosion properties are highly dependent on the surface topography. Shot-peening increases the roughness and also induces crack like artifacts which accelerate corrosion kinetics. The results were evaluated statistically to derive correlations between the microstructure, the plasma nitriding behavior and the resulting properties.

Austenitic stainless steels exhibit good corrosion resistance due to their high chromium contents. Metastable austenitic stainless steels like 304L or 316L undergo a phase transformation to deformation-induced martensite by cold working, e.g. shot-peening. Furthermore, austenitic stainless steels suffer from quite poor mechanical properties which make them susceptible to wear. Therefore, plasma nitriding processes enhance the surface hardness up to the eight times of the original hardness value, thus improving the wear resistance. Additional, the corrosion resistance can be improved, too, if plasma nitriding is carried out below a critical temperature to avoid the precipitation of chromium nitrides. In this study, two austenitic steels – 304L and 316L – were used in a solution annealed condition with either polished or shot-peened surfaces. Furthermore, these samples were tested without plasma nitriding, with plasma nitriding or annealing and a combined nitriding and post annealing process. The microstructural properties were examined by optical microscopy (OM) and scanning electron microscopy (SEM), phases were analyzed by X-ray diffraction (XRD) and the corrosion properties were tested by means of potentiodynamic polarization testing in 5% NaCl solution. Statistical evaluations were carried out to identify significant effects between the microstructure and nitriding conditions and the corrosion properties. The results indicate, that plasma nitriding improves the corrosion resistance of both steels with polished surfaces, while shot-peening increases the corrosion rates. Surface hardness measurements show the positive influence of plasma nitriding to surface hardness. Furthermore, a correlation between specimen treatment and the corresponding microstructures, the nitriding and the annealing process and the corrosion resistance will be presented.

Delayed brittle fractures of high-strength bolts of the strength class 10.9 are presented, taking the example of three damage cases. The respective damage mechanisms could be attributed to hydrogen induced stress corrosion which was caused, in turn, by hydrogen absorption during operation. The examples were chosen with a particular focus on the material condition‘s susceptibility which explains the cause for the occurrence of the damage mechanism. However, in only one of the three cases the susceptibility was evident and could be explained by violations of normative specifications and an unfavorable material choice. Whereas in the two other examples, only slight or no deviations from the standards and/or regulations could be found. The influencing parameters that caused the damage, those that further promoted the damage, as well as possible corrective actions are discussed taking into account the three exemplary damage cases.

In the past, liquid metal induced galvanizing cracks were either analyzed on the metallographic section in the respective partially ground plane of view, or the exposed main fracture surface was examined in the scanning electron microscope (SEM). Thus, possibly existing secondary cracks remained undetected. Three-dimensional computed tomography (CT) allows for a representation of all cracks including their branchings and ramifications and makes possible an analysis of an overall picture of the network of cracks. Differences in the effect of different zinc melts can be further identified by means of 3D analysis and the load situations that promote the crack initiation during hot-dip galvanizing can be differentiated more clearly using laboratory samples. Key results are the representation of cracks parallel to the direction of the main load (B-cracks) which are oriented with the pearlite bands and apparently penetrate significantly deeper into the base material than the macroscopically visible crack which is orientated perpendicular to the direction of the main load (A-crack). A relationship between the occurrence of B-cracks and the used zinc alloys could only conditionally be shown. Examinations on flat samples, which had been dipped into various metal melts under load-free condition, yielded more or less comparable results, in particular for molten pure tin and molten pure lead. In this case, the pearlite phase is preferentially damaged even without introducing an external load into the samples.

Today, innovative coatings must ensure wear protection and corrosion resistance for engineering materials with high corrosion susceptibility, e.g. mild steels, as well as the current sustainability criteria in terms of environment protection and health. PVD technology fulfils most of these demands, but unfortunately, the corrosion protection performance is unsatisfactory due to the typically low coating thicknesses of only few micrometres. One solution to overcome the corrosion problem are TiMgREN (RE = Rare Earth) based coatings, which were developed by the authors. Even with 5 μm thick monolithic TiMgGdN coatings a corrosion resistance of at least 800 h could be achieved in the salt spray test without any macroscopic corrosion attack. In the present work, the influence of the coating composition, the substrate material and the surface roughness on the corrosion properties was examined. Besides, the application of Gd as alloying element, the influence and the effect of other rare earth metals (La, Ce) on the corrosion performance were investigated. As benchmark, the TiMgGdN coatings were compared with industrial standard PVD coatings, DLC and electroplated Cr regarding the corrosion and the wear performance. The coatings were characterized concerning their microstructural, mechanical and chemical properties. Salt spray tests as well as electrochemical measurements were carried out to examine the corrosion properties of the coated specimens. Furthermore, ball on disk wear tests were conducted by reciprocal dry sling tests by using a SRV 3 tribometer. It was shown that by variation of the deposition parameters, the coating properties could be improved concerning both, corrosion and wear performance. Furthermore, the effect of the used rare earth metal type and the rare earth metal content on the coating properties was investigated. The TiMgGdN coatings exhibit superior corrosion properties in comparison to the industrial standard coatings.

The metastable austenitic steels 1.4307 and 1.4404 are characterized by a fcc lattice with paramagnetic properties which can be transformed by cold forming to ferromagnetic bcc martensite with acicular structures via a defect-rich intermediate stage. However, using classical wetchemical etching methods, defects and bcc martensite can visually not be clearly distinguished. Usually, magnetic or X-ray diffraction methods are used to quantify the ferromagnetic fractions. However, they do not allow for imaging the bcc phase distribution in the microstructure. Even with EBSD measurements in defect-rich phases, which, for process-related reasons, involve high investigation costs and long measurement durations, this is often only possible to a limited extent. The present study presents fast and simple contrasting by means of ferrofluids. It allows for a precise and spatially resolved differentiation between (ferromagnetic) martensite and (paramagnetic) defect structures on metallographic cross-section by optical microscopy. Furthermore, it is shown that, after a ferrofluid treatment of metallographic cross-sections made of duplex steel, ferromagnetic ferrite areas already show evidence of grain orientations.

Zink und seine Legierungen zeichnen sich unter anderem durch ausgezeichnete Gieseigenschaften und sehr gute Bestandigkeit gegen atmospharische Korrosion aus. Etwa 50 % der gesamten Druckgusserzeugung entfallt auf Zinklegierungen, insbesondere zur Herstellung von Kleinteilen und filigranen Bauteilen. Aufgrund seiner hexagonalen Struktur in Verbindung mit dem ungunstigen c/a-Verhaltnis besitzt es anisotrope Eigenschaften, so dass die Basalebene der hexagonalen Elementarzelle sprodbruchanfallig ist [1].

The use of magnesium alloys has enormous potential in terms of weight saving, resource availability and sustainability. This contrasts with the need to protect the surface from wear and corrosion, which is necessary to increase the depth of application of this promising material in industrial applications. The general requirements for the surface protection and surface functionalization can be fulfilled in particular by PVD coatings. However, so far there are no industrial PVD coatings available that meet the requirements for corrosion protection of the magnesium substrate. Common PVD coatings cannot be applied to magnesium due to the risk of severe galvanic corrosion between substrate and coating. In previous studies, TiMgN was identified as a prospective layer material for the corrosion protection of magnesium alloys as well as for the functionalization of the surface with regard to wear resistance and decoration. In the present work, the TiMgN coating concept has been applied to an industrial 4-cathode magnetron sputter unit of the type Cemecon CC800/9-HiPIMS. As substrate material the magnesium alloy AZ31hp was used. In addition to pure TiMgN, the rare earth elements Y and Gd were alloyed to the TiMgN coating in order to improve the corrosion properties. The influences of coating composition, target design and sputtering mode ( D irect C urrent — DC and Hi gh P ower I mpulse M agnetron S puttering — HiPIMS) were examined with regard to the chemical, mechanical and structural properties as well as the corrosion properties. It will be shown, that the corrosion properties of the TiMgN coated magnesium alloys depend significantly on the deposition conditions and on the coating microstructure, while the defect density seems to be of minor influence. Alloying of Gd to the coating leads to a further increase in corrosion resistance to more than 300 h in the salt spray test, while alloying of Y shows no positive effect.

Austenitic stainless steels are widely used in medical and food industries because of their excellent corrosion resistance. However, they suffer from weak wear resistance due to their low hardness. To improve this, plasma nitriding processes have been successfully applied to austenitic stainless steels, thereby forming a thin and very hard diffusion layer, the so-called S-phase. In the present study, the austenitic stainless steels AISI 304L and AISI 316L with different microstructures and surface modifications were used to examine the influence of the steel microstructure on the plasma nitriding behavior and corrosion properties. In a first step, solution annealed steel plates were cold-rolled with 38% deformation degree. Then, the samples were prepared with three kinds of mechanical surface treatments. The specimens were plasma nitrided for 360 min in a H2–N2 atmosphere at 420 °C. X-ray diffraction measurements confirmed the presence of the S-phase at the sample surface, austenite and body centered cubic (bcc)-iron. The specimens were comprehensively characterized by means of optical microscopy, scanning electron microscopy, glow discharge optical emission spectroscopy, X-ray diffraction, surface roughness and nano-indentation measurements to provide the formulation of dependencies between microstructure and nitriding behavior. The corrosion behavior was examined by potentio-dynamic polarization measurements in 0.05 M and 0.5 M sulfuric acid and by salt spray testing.

Due to their low density and high specific strength, magnesium alloys exhibit high economic and ecological potential to save weight and fuel. Nevertheless magnesium alloys are known for their high susceptibility to corrosion attack and their low wear resistance. So the surface has to be protected. PVD coatings are known for their good wear resistance and decorative properties. Nevertheless, common PVD coatings cannot be applied to magnesium due to the risk of severe galvanic corrosion between substrate and coating. Therefore, coating concepts have been tailor‐made for magnesium alloys which ensure both good corrosion resistance and high hardness. The coatings are based on TiMgAlN. Besides this, the coating color can be varied in a wide range by adjusting the magnesium content to meet decorative demands. TiMgAlN coatings were deposited by a commercial PVD deposition unit type CemeCon CC800/9 using both DC and HiPIMS technology. The coatings were examined concerning their chemical, structural and electrochemical properties. TiMgAlN coatings were able to withstand a 48 h salt spray test without corrosion attack. Additionally electrical impedance spectroscopy was carried out to examine electrochemical properties more closely. It was shown that XRD phase structure is crucially influenced by the substrate material (HSS and AZ31) although coatings were deposited in the same batch and chemical composition is identical.

Austenitic stainless steel is equipped with an excellent corrosion resistance, whereby plasma nitriding of austenitic stainless steel principally results in a high hardness associated with an excellent wear resistance. Whenever a high wear resistance accompanied by the existing corrosion resistance of austenitic stainless steel is required, the application of plasma nitrided stainless steel is restricted. The research goal is maintenance of the wear resistance with the least possible loss of corrosion resistance. In the present study AISI 304 austenitic stainless steel was plasma nitrided in a range of temperature from 325 °C to 550 °C. Examinations revealed that in addition to treatment temperature, time, pulse ratio and the ratio of gas mixture affected the corrosion resistance of plasma nitrided austenitic steel. Results of microhardness tests and potentiodynamic polarization tests showed that an enhancement of corrosion resistance in addition to preservation of hardness is possible. A reduction of plasma power as well as the addition of argon to the process gas induced an increase of the corrosions resistance with just a slight drop in hardness. Characterization of the chemical composition was determined by glow discharge optical emission spectroscopy (GDOS). Phases of expanded austenite were detected by XRD analysis.

Nowadays, fossil power plants are increasingly required to start up and shut down frequently due to the flexibility of electrical power demand. Thermomechanical fatigue (TMF) induced by temperature transients with superimposed creep on the heated surfaces of components leads to a significant reduction of lifetime. In this paper, the influence of temperature transients on the crack initiation behavior of high-chromium rotor steel of the type X12CrMoWVNbN10-1-1 was studied by performing uniaxial and biaxial service-type TMF experiments. The experiments represent a range of steam turbine cycles with a maximum temperature of 600° C. A significant lifetime reduction was observed on TMF loading compared to isothermal loading under the same mechanical strain cycle. Metallographic examinations have been employed to characterize the associated thermal fatigue damage mechanisms for comparison of the damage evolution under isothermal loading. In particular, the evolution of damage was investigated by systematic metallographic examinations to study the temperature influence on the crack initiation behavior.

Tests have been performed on hot-dip galvanized four point bend specimens to investigate how different levels of compressive stresses and tensile loads under simultaneous thermal activation below the melting temperature of the zinc coatings affect liquid metal assisted cracking (LMAC). While the material volumes under tensile stress developed intergranular cracking, areas under compressive stress – even using a tin and lead alloyed zinc melt – did show no signs of damage. The flat specimens tested without superimposed loading did only show intergranular edge defects in the order of several grain layers when ageing during several hours in differently alloyed zinc melts. Moreover, in the context of the tests, the dominating effect of the alloying elements lead and tin could be confirmed for the intergranular crack formation. Furthermore, the particular importance of the material condition for the occurrence of the damage by LMAC could be shown by means of the progression of the crack formation in the microstructure.

Magnesium is a prospective material to save weight and fuel due to its low density and its high specific strength. Nevertheless corrosion and wear resistance are weak so that the surface has to be protected. Previous studies of the authors pointed out the potential of TiMgAlN PVD-coatings for surface protection of Mg-alloys. Within the presented study, the chemical, structural and electrochemical properties of TiMgAlN based PVD coating materials were examined entirely. Therefore, coatings of different compositions were deposited onto the magnesium alloy AZ31 and on glass substrates. Glass substrates were used to examine the electrochemical properties of the coating materials without any substrate influence. The magnesium content in the coatings varied between 1 at.% and 60 at.%. Increasing Mg content in the coatings improves mechanical and electrochemical properties, thus making TiMgAlN prospective candidates to protect magnesium against wear and corrosion. Beyond this, addition of magnesium leads to a densification of the coating microstructure.

Si–C–N based materials exhibit promising properties such as excellent oxidation resistance and thermal stability even at temperatures above 1300 °C, thus making them promising candidates for high temperature applications. The properties of SiCN based coatings strongly depend upon the synthesis method and the synthesis parameters. In the present study, SiCN coatings were deposited by RF, DC and HiPIMS sputtering. Beyond this, the target configuration was varied for DC and HiPIMS sputtering. The coatings were characterized regarding their chemical, structural, mechanical and thermal properties. Since the SiCN coatings are amorphous, FT-IR and Raman spectroscopy as well as XPS were used to gather structural information. The structural and the thermal properties show a significant dependency on the sputtering method and target configuration, while the mechanical and chemical properties are only slightly affected.

Since most conventional coatings are not stable at temperatures above 1000 °C, new coating concepts have to be introduced. Si–C–N based materials exhibit promising properties such as excellent oxidation resistance and thermal stability even at temperatures above 1300 °C. In the present study amorphous PVD–SiCN coatings were synthesized by conventional DC- and RF-magnetron sputtering as well as with a combined sputtering process using one target in the DC mode and one target in the HPPMS (High Power Pulse Magnetron Sputtering) mode. The coatings were characterized regarding their chemical, structural and mechanical properties. The coating properties strongly depend on the chemical composition and the structure of the amorphous network. Beyond that, the study includes thermal analysis of the coating materials, synthesized with the different sputtering techniques.

The authors developed a coating concept based on amorphous Si/C/N coatings, with promising high temperature properties. Due to a graded coating design, the adhesion, the coating hardness as well as the coefficient of friction were optimized. The coatings showed excellent high temperature stability against chemical decomposition, structural changes and surface oxidation up to 1 350 °C. To allow the deposition of coatings with a certain chemical composition, a numerical relationship between the composition of the coatings and the deposition parameters was established.

This paper was amended in June 2013 because of a change of the author's names. The corrected author byline is presented on the last page of the pdf-file. Within the presented study, the economic potential of PVD-coatings for the surface refinement and protection of Mg-alloys is fully examined. The research includes the introduction of new pre-treatment methods by plasma and chemical polishing, followed by corrosion-inhibiting PVD-coating based on TiMgAlN and thin decorative PVD-finish-film consisting of Nb. Anodic oxidation of the PVD-finish film guarantees the full colour palette by interference effects. It will be shown that chemical and plasma polishing can be applied for the surface pre-treatment of magnesium alloys. The TiMgAlN coated magnesium alloys withstand the 96 h salt spray test without any significant corrosion damage.

SiCN films have been produced along the tie line SiC–Si3N4, SiC–C3N4 and Si3N4–C3N4 by means of reactive magnetron sputtering of a silicon target in an Argon/Nitrogen/Acetylene atmosphere. The mechanical, chemical and structural properties have been thoroughly investigated by means of indentation hardness testing, pin on disk wear testing in reciprocating sliding motion, GDOES, FT-IR, Raman spectroscopy, XPS and STA. The main aim of this investigation was to establish the relationship between deposition conditions, resulting mechanical, chemical and structural properties and the respective wear properties. Depending on their position in the Si–C–N phase diagram, the hardness of the films varies over a broad range, with maximum values of around 30 GPa, while Young's modulus remains in a narrow range around 200 GPa. XPS spectra showed the main component to be Si–C but Si–N and to a lesser extent C–C bonds were also detected. Further, IR spectra suggested the presence of the carbodiimide group. Raman Spectra show a varying ratio of sp3 to sp2 carbon, depending on deposition condition. The hardest films were found along the SiC–Si3N4 tie line. These films exhibited the highest load carrying capacity in lubricated sliding as well as the best adhesion. In dry sliding their brittleness coupled with a high coefficient of friction led to premature coating failure. Carbon rich films have a very low coefficient of friction leading to good wear behaviour in dry conditions, but their ability to withstand high Hertzian pressures is reduced. The low coefficient of friction is attributed the more graphitic structure of the free carbon in the films. Good wear properties in dry reciprocating sliding as well as high load carrying capacity in lubricated sliding were achieved by producing gradient films from silicon rich to carbon rich at the surface.

In der spanenden Fertigung wird aus vielfaltigen Grunden zunehmend auf den Einsatz von Kuhl- und Schmieremulsion zugunsten der Trockenbearbeitung verzichtet. Die sich dadurch ergebenden hoheren Belastungen der Werkzeugschneiden erfordern eine stete Weiterentwicklung der Beschichtungen. Am Institut fur Werkstoffkunde (IfW) in Darmstadt entwickelte CrAlYN-Beschichtungen zeigen besonders unter hoher Last und hoher Temperatur gunstige tribologische Eigenschaften. Es wurden Proben unterschiedlichen Yttriumgehaltes eingehend charakterisiert und einer industriellen TiAlN-Beschichtung gegenubergestellt. Obwohl die CrAlYN-Schichten in ihrer Zusammensetzung noch nicht ausgereift sind und somit ein hohes Optimierungspotential bieten, zeigen sie vielfach vergleichbare und sogar bessere Eigenschaften als die bereits hochgradig optimierten TiAlN-Schichten. Das mit zunehmendem Yttriumgehalt amorphere Gefuge der CrAlYN-Schichten fuhrt zu einer feinen und dichten Schichtmorphologie mit ebener und glatter Oberflache, die gegenuber der porosen Stengelstruktur des TiAlN Vorteile sowohl beim Verschleis durch Werkstoffzerruttung als auch beim Oxidationswiderstand bietet. Auserdem ist eine deutlich verbesserte Adhasion der Schicht auf dem Tragermaterial festzustellen, die sich in hoheren ertragbaren Lasten beim Ritztest ausert.

Although magnesium alloys became popular in the first half of the 20th century, the bad corrosion properties prevented their breakthrough in industrial mass production. Since the technology for the production of high purity alloys was introduced in the 1970s, magnesium alloys became more and more in the focus of industrial attention. Today magnesium alloys are state of the art in structural parts in automotive industry. Despite its outstanding properties like good castability, low density and nearly unlimited availability the negative aspects like weak corrosion and wear behaviour still limit the application of magnesium in industry [1]. So, the only economic solution is the deposition of a coating or a suitable surface treatment which provides both, wear and corrosion resistance. Today, plasma electrolytic anodisations are state of the art [2–5]. They provide acceptable corrosion resistance and protect the magnesium from mechanical damage due to their high hardness. On the other hand, their high porosity limits their use in combination with electrochemically noble materials, leading to galvanic corrosion [6]. In addition, the high surface roughness of the plasma electrolytic anodisations restricts their use in tribological applications, particularly under dry sliding conditions [7]. On the other hand, due to the high life time recommendations the application of magnesium in the automotive industries motion component field is a long term process. Nevertheless, there is a quite high industrial interest to apply magnesium in the motion component field in consumer applications like do-it-yourself or gardenig. Some examples are motor components of lawnmovers, motor saws or drills. Especially for these fields of application there are quite high demands on the corrosion properties due to undefined storage and the conditions during usage. In order to achieve smooth surfaces with high quality, the PVD technology moves into the centre of interest. Since the 1980s PVD coatings are well established and widely used for different industrial applications, mainly for steel and tool coatings. The authors were the first who carried out serious studies on the development of PVD coatings for magnesium alloys since 1999 [6, 7]. The extensive research activities lead to the recent development of a coating system, which provides both, good wear properties as well as good corrosion behaviour. Entwicklung von verschleis- und korrosionsbestandigen PVD-Schichten fur Magnesiumlegierungen Mit der industriellen Beherrschung der Herstellung hochreiner Magnesium-Legierungen ruckt der Konstruktionswerkstoff Magnesium seit etwa 30 Jahren mit steigender Tendenz ins Blickfeld der Fertigungs- und Leichtbauindustrie und ist heute im Bereich statischer Bauteile in der Automobilindustrie nicht mehr wegzudenken. Trotz positiver Eigenschaften wie geringe Dichte, einfache Giessbarkeit und praktisch unbegrenzte Verfugbarkeit stehen der Ausschopfung wirtschaftlich relevanter Anwendungsfelder bis heute negative Eigenschaften wie mangelnde Verschleis- und Korrosionsbestandigkeit – insbesondere im Verbund mit anderen Metallen – und zu teure bzw. umweltfeindliche Verfahren zur Oberflachenvergutung im Wege [1]. Zur Losung dieser Probleme sind Oberflachenbehandlungen von Magnesium unerlasslich, bei denen gegenwartig vorwiegend Verfahren zur anodischen Oxidationen unter Funkenentladung zum Einsatz kommen [2–5]. Diese Oberflachenbehandlungen weisen eine gute Korrosionsbestandigkeit auf und schutzen den Grundwerkstoff durch ihre hohe Harte vor mechanischen Beschadigungen. Die ausgepragte Porositat der Oxidschichten schrankt die Anwendung unter Kontaktkorrosionsbedingungen allerdings stark ein bzw. macht diese unmoglich [6]. Daruber hinaus wirkt sich die hohe Rauheit der Oberflachen auf tribologische Anwendungen unter Gleitbeanspruchung negativ aus [7]. Obwohl das Potenzial von Magnesiumlegierungen aufgrund des gunstigen Massentragheitsmoments gerade im bewegten Bauteilbereich ideal ausgeschopft werden kann, ist ein Einsatz in der Automobilindustrie aufgrund der hohen Lebensdaueranforderungen ein langfristiger Prozess. Dennoch besteht ein hohes Interesse der Industrie, Magnesium-Legierungen im bewegten Bauteilbereich einzusetzen. Als Beispiele sind Heimwerker- oder Gartengerate zu nennen, zum Beispiel Rasenmaher, Motorsagen oder Bohrer. Gerade im Heimwerker- und Gartenbereich sind die Anforderungen an die Korrosionsbestandigkeit auf Grund undefinierter Lager- und Betriebsbedingungen jedoch sehr hoch. Zur Erzielung sehr glatter, qualitativ hochwertiger Oberflachen fur Magnesium-Legierungen ruckte in jungster Vergangenheit die PVD-Technologie immer mehr in den Vordergrund. Bereits seit den achtziger Jahren des 20. Jahrhunderts kommen PVD-Schichten zum Einsatz und finden heute breite Anwendung in zahlreichen Industriezweigen. Seit Ende der neunziger Jahre des 20. Jahrhunderts befassen sich die Autoren mit der Entwicklung von PVD Schichten fur Magnesiumlegierungen [6, 7]. Umfassende Forschungsaktivitaten fuhrten in jungster Zeit zu der Entwicklung eines Schichtsystems, dass sowohl gute Verschleis- als auch gute Korrosionseigenschaften aufweist und im Folgenden beschrieben wird.

Although magnesium alloys were popular in the first half of the 20th century, the bad corrosion properties prevented their breakthrough in industrial mass production. Since the technology for the production of high purity alloys was introduced in the 1970s, magnesium alloys became more and more in the focus of industrial attention. Today magnesium alloys are state-of-the-art in construction parts in automotive industry. Despite its outstanding properties like good castability, low density and nearly unlimited availability the negative aspects like weak corrosion and wear behaviour still limit the application of magnesiums in industry, due to the need of sufficient surface protection (E. Aghion, B. Bronfin, Mater. Sci. Forum2000, 350–351, 19). Today, plasma electrolytic anodisations are state-of-the-art (H. Haferkamp, “Magnesiumkorrosion—Prozesse, Schutz von Anode und Kathode”, in: Moderne Beschichtungsverfahren, F.-W. Bach, T. Duda, Eds., Wiley-VCH, Weinheim 2000, ISBN 3-527-30117-8, 242; M. Thoma, Metalloberflache1984, 38, 393; T. W. Jelinek, Galvanotechnik2003, 94, 46; A. Kuhn, Galvanotechnik2003, 94, 1114). They provide acceptable corrosion resistance and protect the magnesium from mechanical damage due to their high hardness. On the other hand, their high porosity limits their use in combination with electrochemically noble materials, leading to galvanic corrosion (J. Senf, “Untersuchung und Beschreibung von Magnesiumdruckgusslegierungen unter tribologischer, korrosiver und mechanisch-korrosiver Beanspruchung, Berichte aus der Werkstofftechnik”, Shaker Verlag, Germany 2001, ISBN 3-8265-8428-7). In addition, the high surface roughness of the plasma electrolytic anodisations restricts their use in tribological applications, particularly under sliding conditions (H. Hoche, “Grundlegende Untersuchungen zur Entwicklung von PVD-Beschichtungen auf Magnesiumlegierungen im Hinblick auf die Erhohung der Verschleisbestandigkeit und unter Berucksichtigung des Korrosionsverhaltens”, Dissertation, TU-Darmstadt D17, Shaker Verlag, Germany 2004). In order to achieve smooth surfaces with high quality, the PVD technology moves into the centre of interest. Since the 1980s PVD coatings are well established and widely used for different industrial applications, mainly for steel and tool coatings. The authors were the first who carried out serious studies on the development of PVD coatings for magnesium alloys in 1999 (J. Senf, “Untersuchung und Beschreibung von Magnesiumdruckgusslegierungen unter tribologischer, korrosiver und mechanisch-korrosiver Beanspruchung, Berichte aus der Werkstofftechnik”, Shaker Verlag, Germany 2001, ISBN 3-8265-8428-7; H. Hoche, “Grundlegende Untersuchungen zur Entwicklung von PVD-Beschichtungen auf Magnesiumlegierungen im Hinblick auf die Erhohung der Verschleisbestandigkeit und unter Berucksichtigung des Korrosionsverhaltens”, Dissertation, TU-Darmstadt D17, Shaker Verlag, Germany 2004). The extensive research activities lead to the recent development of a coating system, which provides both, good wear properties as well as good corrosion behaviour.

Bei der Trockenbearbeitung wirkt an der Werkzeugschneide ein Belastungskollektiv aus mechanischen, thermischen und chemischen Einflussgrosen. Im Vergleich zur konventionellen Bearbeitung unter Verwendung von Vollstrahl-Emulsionskuhlung erhoht die Trockenbearbeitung die auf den Schneidkeil wirkenden Belastungen. Eine sehr gute Moglichkeit die Schneide vor thermischen, abrasiven und tribo-oxidativem Verschleisangriff zu schutzen stellt die Verwendung von PVD Beschichtungen dar. Neu entwickelte PVD Beschichtungen aus CrxAlyYzN, CrxAlyBzN und CrxAlySizN- wurden sowohl im tribologischen Modelltest als auch im realen Zerspanungstest auf ihre Eignung zur Trockenbearbeitung untersucht. In diesem Paper wird neben der verwendeten Beschichtungstechnologie detailliert auf die Schichteigenschaften eingegangen. Im Zusammenhang mit dem im Zerspanungstest gemessenen Verschleisverhalten und der Prozesskrafte werden anschliesend Ruckschlusse auf das weitere Optimierungspotential dieser Schichtsysteme gezogen. Tool coatings for dry machining During dry machining a strain collective consisting of mechanical, thermal, and chemical loads is imposed upon the cutting edge. Compared to conventional machining using cooling lubrication fluids, the loads are increased in dry cutting. A feasible solution to protect the cutting edge from thermal wear, abrasion, and tribo-oxidation is the application of hard coatings. Newly developed CrxAlyYzN, CrxAlyBzN and CrxAlySizN PVD coatings were both evaluated in tribological model tests and machining tests concerning their suitability for dry cutting applications. Herein, the used coating technology and the coating properties are described in detail. The measured tool wear and the process forces give further hints for the optimization of the coating system.

The present study investigates the potentials of newly developed (Cr,Al)N coatings compared to uncoated tools. The aim of using chromium-based coatings is to create a thermal barrier and thus redirecting the heat from work piece and tool into the chip. Due to their high resistance for oxidation, CrxAlyN coatings with different chromium to aluminum contents were deposited onto indexed carbide inserts. In order to find improved coatings for dry machining operations both tribological and wear tests were performed. These tests include fundamental properties such as thickness, hardness, and critical load. With Glow Discharge Optical Emission Spectroscopy (GDOES) depth profile analyses were conducted. In reciprocating sliding tests, using an SRV III apparatus from Optimol, friction and wear has been studied in a ball-on-disc arrangement. Longitudinal turning tests were performed to characterize the dry machining characteristics. As described above, the coatings' characteristics such as hardness and critical load as well as the results from the reciprocating sliding test correlate with the dry turning tests. The best coatings in the tests have similar chromium and aluminum contents in the region of 15 at.%, the nitrogen content was about 60 at.%, respectively. An increase of the Cr content in the coating deteriorates the performance of the coated carbide beneath the level of the uncoated tool. An adhesion phenomenon that leads to a formation of an adherent layer on the cutting edge is attributed to the completely unlimited mutual solid solubility of Cr and Fe at a temperature beyond 800 °C.

Amorphous Si/C/N:H where synthesized by plasma enhanced chemical vapor deposition (PE-CVD) using the single-source precursors hexamethyldisilazane (HMDS) and bis(trimethylsilyl)carbodiimide (BTSC). The films produced using the HMDS precursor were rich in carbon, while the films synthesized from the BTSC precursor were high in nitrogen content. All films were amorphous and exhibited a similar hardness to SiC or Si 3 N 4 . Fundamental properties of the films were determined in tests such as ultra micro-hardness, elastic modulus, thickness and adhesion. To investigate the composition of the films, composition depth profiles were determined by GDOES. The wear behaviour was studied in sliding tests under oscillating motion. To investigate the wear mechanisms, the wear tracks were examined with topographical analysis, REM as well as EDX.

Different types of PVD coatings and plasma treatments were applied for the surface treatment of magnesium die cast alloy AZ91 specimens. The different types of surface treatment were all developed by the authors and the fundamental properties are described elsewhere. The coating systems were: • 9 μm CrN hard coating • 3 μm TiN coating • 0.5 μm plasma anodisation layer and 3 μm Al2O3 coating The wear and corrosion properties of the differently surface-treated AZ91 samples show crucial influence on the coating characteristics. While the well known metalloid CrN coatings show good wear properties due to high hardness and high coating thickness (9 μm), their corrosion properties are very poor due to crucial micro galvanic corrosion at coating defects. The plasma anodised and additionally Al2O3-coated specimens show good corrosion properties and are also prospective for tribological applications although their wear characteristics yet cannot reach the values for CrN-coated specimens. The publication shows the results of a novel immersion technique to determine the time dependent corrosion behaviour of coated magnesium alloys [B.L. Tiwari, J.J. Bommarito, Magnesium Technology 2 (2002) 269.]. Furthermore, the galvanic corrosion properties of differently coated AZ91 samples in contact with untreated AZ91, different aluminium alloys, roller bearing steel 100Cr6 and a titanium alloy TiAl6V4 were determined. The galvanic corrosion properties were tested according to DIN 50019.

The PVD plasma anodisation method developed by the authors allows the anodisation and subsequent PVD deposition of magnesium alloys in a process step using a conventional PVD sputtering unit. In the standard salt spray test, these surfaces show corrosion behaviour superior to conventional hard coatings, and they also show good wear characteristics [H. Hoche, H. Scheerer, E. Broszeit, C. Berger, Mater. Wiss. Werkst. Tech. 30 (2002) 395 [1] ; H. Hoche, H. Scheerer, D. Probst, E. Broszeit, C. Berger, Surf. Coat. Technol. 173 (2003) 1018 [2] ]. To determine the corrosion behaviour, different test methods were applied to plasma-anodised and Al2O3-coated specimens of the magnesium die cast alloy AZ91D and CrN hard coatings with optimised deposition parameters for magnesium alloys. The investigation of the macroscopic electrochemical corrosion behaviour with current density–potential curves (10 mm testing diameters) does not allow any satisfactory description of the quality of the different surface treatments. Measurements with a microprobe (0.1 mm testing diameter) supply a suitable description of the electrochemical corrosion behaviour of PVD-coated magnesium alloys by comparing intact coating areas with specific coating defects produced by Vickers indentations. The results of the three testing methods are compared and brought into agreement with each other. It is shown that conventional macroscopic current density–potential measurements are not suitable to describe the corrosion behaviour of PVD-coated magnesium alloys.

Tools for cutting operation are subject to very high abrasive, adhesive, and thermal loads. Process security and preparation quality as well as a sufficiently high tool life demand for the development of hard coatings with reduced seizure tendency, high hot hardness, and increased wear resistance especially under dry machining conditions. TiAlN coatings are the state-of-the-art coatings for machining operations. Hard materials on chromium basis like CrN or CrAlN are preferably used for bending and stamping tools. Thus, these coatings could have a high potential in machining operations. The aim of using chromium-based coatings is to redirect the heat from the workpiece and tool into the chip, thus acting as a thermal barrier. Due to their high oxidation resistance, CrN and Cr x Al y N coatings were deposited on WC/Co inserts. In order to find improved coatings for dry machining operations, mechanical characterizations and turning tests were performed. The characterization of the coatings includes fundamental properties such as thickness, hardness, and critical load. With Glow Discharge Optical Emission Spectroscopy (GDOES), depth profile analyses were executed. SEM analyses supplemented the picture of the scratch tests. Turning operations were performed in model tests to characterize the dry machining characteristics. The wear properties of CrN and Cr x Al y N coatings were compared with industrially coated (Ti,Al)N carbide inserts. Calculations of the friction coefficient supplemented the tests.

Developing PVD coating systems with better corrosion resistance leads to a duplex process, consisting of a plasma anodisation and an Al 2 O 3 top coating which can both be performed in a modified commercial PVD unit. The tested specimens were investigated by means of optical microscopy, SEM and EDX. Furthermore a novel immersion technique to determine the time dependent corrosion behavior of coated magnesium alloys is introduced.

Sufficient corrosion resistance of technical magnesium alloys is generally ensured by anodising the components in aqueous electrolytes. In the majority of cases, these processes bear environmental risks due to the highly toxically electrolytes and are subjected to substantial legal regulations. Considering these facts, the plasma anodisation seems to be a prospective method to obtain comparable corrosion resistance without using toxic substances. The main goal of the method of the plasma anodisation is the substitution of the aqueous electrolyte by an oxygen plasma. The plasma anodised surfaces were well characterized and the corrosion resistance was proved in the salt spray test according to DIN 50021 SS. For the tests the magnesium die cast alloy AZ91hp was used. The tests included plasma anodised, plasma anodised and PVD coated and only PVD coated specimens.

Extensive investigations of the authors showed the possibility of producing reliable plasma vapour deposition (PVD) hard coatings on magnesium alloys with good wear properties comparable to PVD coated steel or titanium based alloys. In presence of corrosive mediums crucial contact and pitting corrosion, induced by process related coating pinholes are the main failure mechanisms of coated magnesium alloys, even without any tribological load applied. The authors developed the new method of plasma anodisation to ensure acceptable corrosion resistance. In contrast to conventional anodisation processes the aqueous electrolyte is substituted by an oxygen plasma. The main advantages are that the process uses no toxic electrolytes, there is no waste and anodising and PVD-coating can be done in one process. The article gives a brief view of our extensive work starting from the first approach in PVD deposition of magnesium alloys up to the present work in establishing the plasma anodisation.

Sufficient corrosion'resistance of technical magnesium alloys is generally ensured by anodising the components in aqueous electrolytes. In the majority of cases these processes bear environmental risks due to the highly toxically electrolytes and are subjected to substantial legal regulations. Considering these facts the plasma anodisation seems to be a prospective method to obtain good corrosion resistance without using toxic substances. The main goal of the plasma anodisation is the substitution of the aqueous electrolyte by an oxygen plasma. The plasma anodised surfaces were well characterized and the corrosion resistance was proved in the salt spray test according to DIN 50021 SS. For the tests the magnesium die cast alloy AZ91 hp was used. The tests included plasma anodised, plasma anodised and PVD coated and only PVD coated specimens. After the salt spray test the surface of plasma anodised specimens showed sporadic filliform corrosion. The plasma anodised specimens showed superior corrosion behaviour in comparison to the only PVD- coated ones. The polished magnesium as well as the as casted samples showed more corrosion attack than the anodised specimens.

Eine gangige Methode, zu beschichtende Oberflachen von Verunreinigungen zu befreien und die Adhasionseigenschaften von PVD-Schichten zu verbessern, ist das sogenannte Sputter-Atzen vor dem eigentlichen PVD-Beschichtungsprozess. Beim Sputter-Atzen wird der Beschichtungsprozess umgekehrt und die Oberflache des Substrates selbst wird gesputtert. Im Rahmen dieser Veroffentlichung wurden vor der Abscheidung von PVD-CrN Schichten die Substrate der Magnesium Druckgusslegierung AZ91 unterschiedlichen Atzzeiten unterzogen und deren jeweiliger Einfluss auf die Grenzflache Schicht-Grundwerkstoff, auf die Schichtoberflache und auf die mechanischen Eigenschaften des beschichteten Systems hin untersucht. Die Grenzflachenuntersuchungen wurden mit tiefenaufgeloster XPS und mit SIMS durchgefuhrt. Die Oberflachen wurden mit hochauflosender Rasterelektronenmikroskopie und AFM untersucht. Die Charakterisierung der mechanischen Eigenschaften beinhaltet Schichtdicke, Schichtharte, Hartetiefenprofile, Schichthaftung, Schichteigenspannungen und die Schichtstruktur. Es wurden zum Teil sehr starke Eigenschaftsanderungen mit zunehmender Atzzeit festgestellt. Dies betrifft vor allem einige mechanische Eigenschaften sowie die Oberflachenrauheit der beschichteten Systeme. Beim Schichtsystem AZ91-Cr/CrN fuhren langere Atzzeiten zu einer Verbesserung der Schichtqualitaten. Investigating the Influence of the Sputter Etching Process on the Properties of PVD-CrN Coatings on Magnesium Die Cast Alloy AZ91hp A common method prior to the PVD deposition is the sputter etching process of the substrate itself to clean the surface from adhesion products and to improve the coating adhesion. This report deals with the sputter etching of magnesium die cast alloy AZ91hp to investigate the influences on the coating-substrate interface, the surface properties and the mechanical properties of PVD-CrN hard coatings. The coating-substrate interface of the Cr-AZ91 coating systems was investigated with XPS and SIMS. Surface studies were carried out by high resolution electron microscopy and AFM. The characterization of the mechanical properties of the CrN-AZ91 compound systems includes thickness, coating hardness and hardness depth profiles, coating adhesion, structure and residual stresses. Some properties show a strong dependency of the etching time, especially the mechanical properties and the coating roughness. Increasing etching times lead to an improvement of the coating quality.

The tribological behaviour of PVD Chromium-Nitride coated magnesium alloy AZ91hp was investigated in wear tests by variation of temperature and load. A combination of different investigation methods revealed information about the failure modes and wear mechanisms. The authors show that failure of the CrN-AZ91 systems is due to the unsatisfactory load-carrying capacity of the magnesium.

In the future dry running machine elements may play a more important role, due to environmental legislation. The economical demand for cost and timesaving causes higher mechanical and thermal stresses for tools in high-speed production processes. Is it possible to improve the wear resistance by the use of thin hard coatings and will they find an area of application especially at elevated temperatures where lubricants fail? Therefore, oscillating sliding motion dry wear experiments at specific temperatures ranging from 30 to 700°C (900°C is possible) were performed on PVD-coated steels and light metal alloys. The characterisation of the coatings includes fundamental properties such as thickness, hardness, structure and residual stresses. The tribological tests were done with an SRV3 apparatus from Optimol, the coatings were tested against 100Cr6 balls using a ball-on-disk arrangement. The friction and wear behaviour was documented. The wear traces were characterised using a Hommel Tester T 8000. Topographical microanalysis was done with SEM and EDX.

An PVD-Chromnitrid beschichteten Proben der Magnesiumlegierung AZ91hp und dem WA¤lzlagerstahl 100Cr6 wurden vergleichende Untersuchungen hinsichtlich Struktur, mechanischen Eigenschaften, Haftfestigkeit und Eigenspannungen durchgefA¼hrt. FA¼r die Beschichtungen wurden die Parameter Schichtdicke und Substrat-BIAS variiert. Beide Substratwerkstoffe wurden in einer Charge zusammen beschichtet. Bei der rA¶ntgenographischen Analyse der Eigenspannungen in den Schichten zeigten sich signifikante Unterschiede zwischen dem beschichteten Magnesium und dem beschichteten Stahl. Beim Stahl war eine AbhA¤ngigkeit der Eigenspannungen vom Substrat-BIAS erkennbar, beim Magnesium war dies nicht der Fall. Die Schichtstruktur wurde im Rasterelektronenmikroskop untersucht. An den beschichteten Substraten wurden mit SIMS Element-Tiefenprofile aufgenommen.

Thin films (50‐200 nm) of SnO2TiO2 were deposited on SiO2:(001)Si substrates by RF-sputtering and by molecular beam before they were annealed in vacuum at 200‐900°C. In-situ TEM, XRD, SEM, Raman and IR-spectroscopy were used to analyze the structure transformations in the SnO2TiO2 films. In the as-deposited state, the films are amorphous. They crystallize at higher temperatures (starting at about 500°C) forming nanosized grains. The problem of the spinodal decomposition in the SnO2TiO2 system observed earlier at high temperatures is discussed also for low-temperature processing. The stoichiometry of the films of both groups (reactive ion sputtered and high-vacuum e-gun sputtered) is being compared. © 2000 Elsevier Science S.A. All rights reserved.