Korrosionsuntersuchungen an mittels PVD mit Zn und Zn/Mn beschichteten Stählen

Corrosion Studies of Steels Coated by means of PVD with Zn and Zn/Mn Alternative methods for hot dip‐ or electrogalvanic deposition of zinc coatings on steel are gas phase depositions (PVD). They posess high flexibility with respect to alloy composition, and are environmentally harmless. However, a PVD‐coated steel must have at least the same corrosion resistance than steels with “classical” surface finishing. Therefore, the corrosion behaviour of Zn‐coatings and Zn/Mn/system‐coatings deposited by electron beam evaporation without and with ion beam assistance (IBAD) on low alloy steel, was determined by means of salt spray test and electrochemical potential/time measurements. At first the influence of chemical and irradiation pre‐treatment and ion bombardment during deposition on the corrosion resistance of the coatings was investigated. Than the effect of the Zn‐layer thickness was determined in comparison with an 8μm thick electrogalvanized reference coating. Finally Zn/Mn‐alloys, Zn/Mn‐multilayers and Zn‐coatings with Mn‐ or Zn/Mn‐surface layers (top layers) were investigated. By means of optimised pre‐treatment and ion bombardment conditions one obtains, considering the layer thickness, PVD‐Zn coatings with corrosion resistance comparable with the reference layer. The best Mn‐containing coatings are Zn‐coatings with Mn‐toplayer. They surpass the corrosion resistance of the reference layer considerably. Additionally it could be shown that in tendency the potential/time measurements agree very well with the results of the salt spray test.     

Mechanische und tribologische Eigenschaften von hochstickstoffhaltigen Austeniten

Mechanical and tribological Properties of High‐Nitrogen Austenitic Steels Austenitic stainless steels provide a fair combination of strength, toughness and corrosion resistance. Undergoing tribological stresses – in particular under self‐mating contact situations ‐ their performance is not sufficient. Thus the more wear resistant austenitic Co‐base alloys with different carbon contents are applied under these circumstances, which may prevail in medical applications. Austenitic high‐Nitrogen Steels might be an alternative under these circumstances. Strength, corrosion resistance and tribologcial properties are similar to those of CoCrMo‐alloys, while their toughness is higher. This contribution presents the metallurgical mechanisms, which bring about this combination of properties.     

Innovationen durch PACVD Duplex Schichtsysteme im Leichtmetall‐Druckguss

Innovative PACVD Duplex Layer Systems applied for the Light Metal Die Casting Process Duplex‐PACVD hard coatings are well‐known for increasing the tool performance in terms of adhesion, wear, fatigue, and corrosion resistance of the steel. The developments made in synthesizing duplex nanostructure and nanocomposite, mono and gradient layers based on borides are described. The aim of the investigation is to optimize the surface capability by plasma process combinations: duplex process, gradient‐layer. Within this work different types of duplex hard coatings produced by PACVD were investigated in terms of their tribological behavior and were tested in aluminum and magnesium die casting applications. Practical tests have been carried out by automobile producers and part suppliers. All coatings tested on die casting tools showed a significant increase of lifetime and a reduced metal adhesion tendency. The economic efficiency of coated die casting tools could be proved.     

Einfluss der Mikrostruktur auf das Verschleiß‐ verhalten der hartanodisierten Aluminium‐legierungen EN AW‐6082 und EN AW‐7075

Microstructural effect on the wear behaviour of the hard‐anodised aluminium alloys EN AW‐6082 and EN AW‐7075 The suitability of hard‐anodising of high‐strength Al alloys (EN AW‐7075‐T651) for the fabrication of protective coatings which are also applicable on screws was investigated. A medium‐strength AlSi1MgMn alloy (AA60682‐T6), generally rated as applicable for anodising, was used as reference material. After possible setting phenomena of a screw joint, the load‐bearing surface of the screw can be subjected to an oscillating relative movement. The damaging tribological load was simulated in an oscillation wear test. The resulting wear appearances have revealed that the untreated oxide coatings on the EN AW‐6082 substrate are not capable of providing protection against tribological load. Since hot‐water sealing increases the hardness of the coating but also contains the technology‐induced risk of softening the substrate material, other tribological protection methods have been looked for. The analysis of the tribological tests (characterisation of the structure and the resulting properties of the material, measurement of the wear amount and analysis of the wear appearance) have shown that the films sealed with wax emulsion on both substrate materials are the most promising candidates for the application of devices under oscillation wear. The obtained roughness, friction coefficients and hardness values confirm the positive behaviour of the anodically oxidised EN AW‐7075‐T651 alloy under the chosen tribological load.     

Influence of nitriding on corrosion resistance of martensitic X17CrNi16‐2 stainless steel

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440 °C and 525 °C and salt bath nitrocarburizing were carried out on X17CrNi16‐2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X‐ray diffraction (XRD) and glow‐discharge optical emission spectrometry (GD‐OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16‐2 steel.     

Erosions‐Korrosions‐Untersuchungen an gradierten Multilayer‐Chromkarbidschichten

Erosion corrosion of graded chromium carbide coatings in multi layer structure So far PVD‐ and PECVD‐Layers have proved their value as wear protection mainly on cutting tools for machining. Depending on the composition of the layers not only a reduction in wear but also a reduction in friction is possible, e.g. by integration of hydrogen containing carbon. Furthermore such carbon containing layers use to be electrochemically inert. Thus they don’t corrode in aqueous media. Because they do also have a very dense structure, an application as corrosion protection seems to be promising. For the intended investigations under service‐like erosiv‐corrosiv loading a new testing rig was developed and constructed. The erosiv‐corrosiv loading was achieved by exposure of coated specimen to a flowing medium, that contains abrasive corund‐particles. Thus the erosion‐corrosion‐behaviour of new graded Multilayer‐Chromiumcarbide‐Coatings should be investigated. The aim was to identify the mechanisms of deterioration to promote a further developement of these layers. In addition the potential of PVD/PECVD‐coated low‐alloy steel to be in‐service under such conditions should be evaluated. For comparison an up‐to‐date industrial DLC‐coating and a high‐alloy duplex‐steel were also investigated. As a result of the conducted investigations an application of PVD‐/PECVD‐coated low‐alloy steel under erosive‐corrosive conditions with impingement wear could not yet be recommended. However the graded Multilayer‐Chromiumcarbide‐Coatings have the potential for a good erosion‐corrosion‐protection, if erosion promoting flaws are avoided. Because hard PVD‐ and PECVD‐coatings are relative brittle, a loading with hard particles, which hit the surface under a high angle, is very tough. Hence the question is, if the investigated layers possibly have a better wear behaviour under more abrasive loading in a more tangential flowing medium, which is also typical for in‐service‐conditions. This is intended to be investigated in future tests.     

Investigation of wear resistance and microstructure of a newly developed chromium and vanadium containing iron‐based hardfacing alloy

Plasma transferred arc (PTA) welded Ni and Co‐based alloys have gained high acceptance in many industrial applications for the wear protection of components. Recently, the cost of nickel and cobalt is rising drastically. This paper presents the development of a cost‐effective high chromium and vanadium containing iron‐based hardfacing alloy with high hardness and wear resistance. The welding processing of the alloy is carried out by PTA welding of atomized powders. Investigations on powder production as well as on weldability are presented. The coatings are metallographically studied by optical microscopy, SEM, EDX and micro‐hardness measurements. The wear resistance properties of the coatings are examined using pin on disk, dry sand rubber wheel and Miller testing, the corrosion properties are determined by immersion corrosion tests. The newly developed iron‐based alloy has nearly the same wear resistance as Ni‐based alloys with fused tungsten carbides at a higher level of corrosion resistance and much lower cost.     

Design of wear and corrosion resistant PVD‐coatings for magnesium alloys

Although magnesium alloys became popular in the first half of the 20^th 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.     

Tribologische Untersuchung bionischer und mikrostrukturierter Funktionsflächen

An established concept adjusting tribological properties and for increasing the wear resistance is presented by coatings. In addition to the material adaption of surfaces, there are efforts of applying structures on tool active parts in order to allow a further adjustment on the property profile. For this reason, the presented article investigates the influence of bionic and technologically textured surfaces on the friction and wear behavior with and without near‐net shaped wear‐resistant PVD coatings. Based on the example of nature, a honeycombed surface structure discovered on the head of scarab beetles as well as a dimple structure optimized for the manufacturing time were transferred on HSS steel by means of micro‐milling. The analyses focus on the influence of the surface structures, the effects of PVD coatings and their interactions on the friction and wear behavior. The investigations show that the tribological properties depend on each surface structure and the material pairing. Both the technological and the bionic structures show a reduction of the friction coefficient in combination with the material pairing 100Cr6 and WCCo compared to polished samples. Furthermore, it is shown that the CrAlN coating has no influence on the friction behavior, but rather leads to the desired increase in the wear resistance.     

Einfluss der HiPIMS‐Parameter beim PVD‐Verfahren

Effect of PVD process parameters on structural properties of CrN layers Commonly, imperfections on substrate surfaces influence layer nucleation unfavorably. They cause growth defects in the coating structures prepared by physical vapor deposition. In consequence this leads to local loss of adhesion, higher friction, voids and thus favoring pitting corrosion. CrN‐coatings are known for their high hardness and good wear resistance. Further they have a better resistance to corrosion than Ti‐based nitrides. Among other parameters, the structure and the mechanical properties of those coatings can be influenced by varying bias voltage and gas flow during film growth. Due to variation of those parameters during reactive magnetron sputtering CrN‐coatings were deposited with preferred crystallized lattice orientation (111) and (200). The main objective of investigation is the potential to cover imperfections.     

Earthworm‐Inspired Rough Polymer Coatings with Self‐Replenishing Lubrication for Adaptive Friction‐Reduction and Antifouling Surfaces

Earthworms are able to pass through sticky soil without inducing stains through a self‐forming thick lubricating layer on their rough skins. To mimic this earthworm‐like lubricating capability, an attempt to create a textured structure on the surface of liquid‐releasable polymer coatings by a “breath figure” process is described herein. The resulting coatings exhibit fast and site‐specific release behavior under external triggers such as solid‐based friction. The released oil is then stabilized by the surface texture to form thick lubricating layers, reducing friction and enhancing wear resistance. Moreover, the coatings also exhibit excellent antifouling property in a sticky soil environment. Because the lubricating layer can be regenerated after consumption, the potential of this self‐replenished lubricating mechanism in preparing friction‐reduction, antiwear, and antifouling coatings used in solid‐based environments is therefore envisioned.     

Influence of carbon diffusion on microstructure and wear behaviour of duplex stainless steel surface layers on lamellar grey cast iron

Surface welding with duplex stainless steel was performed to enhance the wear and corrosion properties of grey cast iron, which is used as material for applications as pump components in maritime and chemical environments. The method used for surface welding and the corresponding process parameters determine the chemical composition and microstructure, which both determine the corrosion and wear properties of the surface layer. High heat input leads to high chemical dilution and thus, reduced corrosion resistance. Slow cooling rates, which are recommended for welding of grey cast iron components, facilitate the formation of carbides in the fusion zone of the chromium‐rich duplex stainless steel surface layer. On the one hand, carbides lead to increased hardness and thus, improved wear resistance of the surface layers. On the other hand, carbides and high chemical dilution rates reduce the corrosion resistance and therefore should be avoided. Under high cooling rates, the risk of cracking in the heat affected zone of the grey cast iron increases due to martensitic phase transformations. The paper describes the correlation of process parameters, microstructure and chemical composition with a focus on carbon diffusion and carbide formation, ever considering the effect on the wear behaviour in an oscillation tribometer and under erosion‐corrosion conditions.     

Elektronenstrahl‐Randschichtbehandlung für die Herstellung verschleißbeständiger Auftragschichten auf nichtrostenden Stählen

Stainless steel components exposed to mechanical stresses are subjected not only to corrosion, but to abrasive wear. There are several possibilities for enhancing the wear resistance of stainless steels; however, such processes are very often associated with a reduction in corrosion resistance. This paper presents an electron beam surface treatment technology to significantly improve the wear resistance of austenitic steels (e.g. X6CrNiMoTi17‐12‐2) and duplex steels (e.g. X2CrNiMoN22‐5‐3), without a negative influence on the corrosion behavior. Fe‐ and Co‐additive wires were deposited thermally by electron beam cladding. The cladding layers produced were free of defects such as cracks and pores, and were well metallurgical bonded to the base materials. Microstructural analysis, hardness measurements, wear tests and corrosion tests were carried out. The wear rate k was reduced by a factor of 100 compared to the base materials for electron beam cladding with Fe‐based wire and by a factor of 10 with Co‐based wire. Corrosion resistance was preserved for the Fe‐based cladding layers and slightly increased (by a factor of 3) for the Co‐based cladding layers.     

Dry sliding wear behavior and corrosion resistance of NiCrBSi coating deposited by activated combustion-high velocity air fuel spray process

NiCrBSi is a Ni-based superalloy widely used to obtain high wear and corrosion resistant coatings. This Ni-based alloy coating has been deposited onto 0Cr13Ni5Mo stainless steel using the AC-HVAF technique. The structure and morphologies of the Ni-based coatings were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectrometer (EDS). The wear resistance and corrosion resistance were studied. The tribological behaviors were evaluated using a HT-600 wear test rig. The wear resistance of the Ni-based coating was shown to be higher than that of the 0Cr13Ni5Mo stainless steel because Fe₃B, with high hardness, was distributed in the coating so the dispersion strengthening in the Ni-based coating was obvious and this increased the wear resistance of the Ni-based coating in a dry sliding wear test. Under the same conditions, the worn volume of 0Cr13Ni5Mo stainless steel was 4.1 times greater than that of the Ni-based coating. The wear mechanism is mainly fatigue wear. A series of the electrochemical tests was carried out in a 3.5 wt.% NaCl solution in order to examine the corrosion behavior. The mechanisms for corrosion resistance are discussed.     

Tribological property and drilling application of Ti-C:H and Cr-C:H coatings on high-speed steel substrates

This study investigates the tribological properties and cutting performance of Ti-DLC and Cr-DLC doped metal coatings. The tribological properties of the coatings are evaluated by testing coated disks against an AISI 1045 steel counterbody under dry conditions using an oscillating friction wear tester, and then measuring the subsequent wear depth on the coated disk, the wear width on the steel counterbody, and the friction coefficient. The cutting performance of the coatings is evaluated by using coated high-speed drills to machine stainless steel workpieces, and then measuring the resulting flank wear and hole surface roughness. The results of the wear tests show that the Ti-C:H and Ti-C:H/TiC/TiCN/TiN coatings possess excellent tribological properties, including low coefficients of friction, low wear depths, and low wear widths. Regarding the machining tests, the Ti-C:H/TiC/TiCN/TiN coating has the lowest flank wear and yields the highest hole surface quality under both dry and cutting fluid drilling conditions. The single Ti-C:H coating has excellent tribological properties, but demonstrates a relatively poorer performance in the drilling of stainless steel. Finally, the Cr-DLC coatings all exhibit a poor cutting performance under dry cutting conditions.     

Verformungsverhalten nanostrukturierter HPPMS‐Hartstoffschichten

Elastic‐Plastic Deformation Behavior of Nanostructured HPPMS Hard Coatings Nitride hard coatings deposited via HPPMS (High Power Pulsed Magnetron Sputtering) or HiPIMS (High Power Impulse Magnetron Sputtering) are widely used in tribological applications due to their promising wear and corrosion resistance. During the application, the coated tools or components may be exposed to significant mechanical loads. Therefore, investigations on deformation behavior of the coatings under mechanical loading are of great importance. The objective of the present study was a comprehensive investigation on deformation behavior of nitride hard coatings from the coating system M‐Al‐O‐N (M = Cr, V) using nanoindentation und nanoscratch tests. In this regard, both nanoscale multilayer (nanolaminate) and monolayer coatings were investigated. All the coatings were deposited using HPPMS technology. Contrary to the expectations regarding a brittle behavior of ceramic‐like coatings, the results depict a considerable plastic deformation of the investigated hard coatings. Furthermore, in addition to a high strength, the applied coatings show a high crack resistance under mechanical loading.     

Entwicklung einer niedrigschmelzenden Legierung und deren Applikation zum Korrosionsschutz hochfester Stähle

Development of low‐temperature galvanizing and its application for corrosion protection of high‐strength steels Apart from reliability and quality, vehicle safety and cost efficiency are the decisive criteria for automobile manufacturers. Corrosion protection plays a decisive role because it increases the service life. The ultra‐high‐strength steels are materials which exhibit high lightweight potential as well as a very good energy absorption capacity because of their mechanical properties. In connection with the possibility of hot forming, they are predestined for the fabrication of complicated, load‐compatible shapes in the crash‐relevant frame and body construction. The application of these steel qualities has been carried out in structural parts which are protected from corrosion by a hot‐dip coat of FeAl7 – the so‐called Usibor. However, at the moment there is no ready‐for‐production solution for later corrosion protection of already hot‐formed parts. Therefore, a corrosion protection system on the basis of conventional low‐temperature galvanizing processes has been developed and utilized. First, the softening behavior of the highly‐resistant 22MnB5 substrate was analyzed. Afterwards, a galvanizing system was developed and applied. The corrosion protection coatings were characterized with regard to their structure and corrosion protection potential. As a result, a significant improvement of the corrosion behaviour has occurred.     

Cytotoxicity study of plasma-sprayed hydroxyapatite coating on high nitrogen austenitic stainless steels

Stainless steel has been frequently used for temporary implants but its use as permanent implants is restricted due to its low pitting corrosion resistance. Nitrogen additions to these steels improve both mechanical properties and corrosion resistance, particularly the pitting and crevice corrosion resistance. Many reports concerning allergic reactions caused by nickel led to the development of nickel free stainless steel; it has excellent mechanical properties and very high corrosion resistance. On the other hand, stainless steels are biologically tolerated and no chemical bonds are formed between the steel and the bone tissue. Hydroxyapatite coatings deposited on stainless steels improve osseointegration, due their capacity to form chemical bonds (bioactive fixation) with the bone tissue. In this work hydroxyapatite coatings were plasma-sprayed on three austenitic stainless steels: ASTM-F138, ASTM-F1586 and the nickel-free Böhler-P558. The coatings were analyzed by SEM and XDR. The cytotoxicity of the coatings/steels was studied using the neutral red uptake method by quantitative evaluation of cell viability. The three uncoated stainless steels and the hydroxyapatite coated Böhler-P558 did not have any toxic effect on the cell culture. The hydroxyapatite coated ASTM-F138 and ASTM-F1586 stainless steels presented cytotoxicity indexes (IC_50%) lower than 50% and high nickel contents in the extracts.     

Entwicklung von Diffusionsschichten auf Nickelbasiswerkstoffen für den Einsatz in chlorhaltigen Hochtemperaturprozessen

Development of diffusion coatings on nickel base alloys for the use in chlorine‐containing high temperature processes To open up the possibility of using sewage sludge ashes as fertilizers the removal of their heavy metal contents is obligatory. A process newly developed at the BAM Berlin executes this separation in highly chlorine‐containing atmospheres at temperatures of up to 1000 °C [1]. Unfortunately there are no materials available which can withstand such conditions over longer periods of time. This project deals with the development of materials that allow the operation in highly corrosive environments. The corrosion resistance of nickel base alloys against chlorine‐induced high‐temperature corrosion will be optimized by application of aluminum‐ and/or silicon‐containing diffusion coatings. As coating method the pack cementation process was selected. In this process, the metal to be coated is embedded in a powder, consisting of the coating metal, a halogen‐distributor (e.g. ammonium chloride) and aluminum oxide as filler material. During an annealing process of several hours at temperatures of 800 to 1000 °C, gaseous metal halides form. They diffuse through the powder pack and decompose at the substrate surface, thereby depositing the coating metal. Subsequent solid phase diffusion results in the formation of a protective diffusion layer. From the thermodynamic point of view, materials with a high content of aluminum and silicon show best prerequisites to build up slow‐growing, stable oxide layers with a high potential to protect the material against corrosive attacks. The actual performance of the materials will be examined in long‐time tests under simulated field conditions (high temperatures and chlorine‐containing atmospheres).     

The effect of PVD coatings on the tensile strength and low‐cycle fatigue resistance of stainless steel and titanium alloys

PVD coatings applied to components form hard, stronger layers and generate high residual compressive stresses that limit the plastic deformation in surface layers of the base metal thus increasing its tensile strength and resistance to fatigue loading. The purpose of this paper is to experimentally determine the influence of the deposition of 2 to 16.5‐μm‐thick PVD coatings of TiN, Cr, (Cr+TiN), (TiC)N, (TiAl)N onto specimens of stainless steel 321 and titanium alloys of types MILT‐81556A and (10‐2‐3; 4966) on their tensile strength and low‐cycle fatigue resistance when the development of large elastic–plastic strains takes place. The tensile and low‐cycle fatigue tests were conducted under conditions of axial zero‐to‐tension cycle of the stress‐controlled loading on flat 1‐ to 1.5‐mm‐thick specimens in the initial state (uncoated specimens) and after application of a PVD coating, including those after pretensioning or after cyclic prestraining in the low‐cycle fatigue range. The deposition of PVD coatings is found to enhance the characteristics of tensile strength and low‐cycle fatigue resistance in the quasi‐static fracture range. The deposition of PVD coatings on specimens cyclically prestrained to the values of 53–86% of the number of cycles to fracture, changes the cyclic properties of the material and predetermines the fatigue fracture mode only.