Technik und Potenziale des Verschleißschutzes mittels thermisch gespritzter Beschichtungen

Technology and Potential of Wear Resistant Thermal Spray Coatings Thermal spray technologies are used for coating of component parts with a large variety of materials, e. g. for protection against wear and corrosion. During the last 20 years, the comparatively novel processes in thermal spraying have been established in surface technologies and will increasingly develop their market. Continuous equipment developments as well as new technologies for thermal spraying guarantee increasing importance of this technology, and will have a sustainable effect on industries with typical wear and corrosion‐caused problems. Therefore, thermal spray coated components possess an outstanding suitability and a high application potential for the construction of stone machining machines / plants and the mining industry (as well as associated vehicle and equipment technology), which still has hardly been developed in the area of mining industry and stone machining. A short introduction of thermal spray technology and processes and an overview is given for typical applications and already established coating solutions besides some recent examples from the area of stone machining and mining. This article shows the potential of thermally sprayed coatings as solutions for the stone machining and mining industry.     

Zum Korrosions‐ und Verschleißverhalten von Nickeldispersionsschichten mit Nanopartikeln

Corrosion and Wear Behaviour of Nickel Dispersion Coatings with Nano Particles The present study describes the results of the electrochemical deposition of Nickel dispersion coatings from a Watts electrolyte with nano‐scaled particles. Subsequently, the incorporated particles lead to different microstructures and have impact on the mechanical properties. The corrosion and wear behaviour of these layers are of high interest regarding their applications. By means of potentiodynamic measurements and optical characterisation, the corrosion behaviour can by evaluated in correlation to the applied particles and additives. Unfortunately, an improvement of corrosion protection could not be measured. From investigations by means of oscillation tribometer, it can be shown that the incorporated particles improve the wear behaviour.     

Mikrostrukturieren von thermisch gespritzten Mo‐Schichten für Anwendungen im Bereich hoher Reib‐ und Verschleißbeanspruchungen

The properties of thermally sprayed coatings are dependent on many parameters such as the spraying material, substrate properties, and the injection parameters. In this study, the influence of two variable spray parameters (spraying distance and current) were investigated on molybdenum‐containing thermal spray coatings. Particularly, materials and surface characterizing properties were analyzed, and the dependence on each other was examined. The important surface parameters studied in this case are the porosity and the pore depth of the coatings. Following the correlation between spray parameters and coating properties, the influence of surface properties on the tribological behavior will be discussed, in comparison to an uncoated steel surface.     

Untersuchung der Einflüsse von Substratrauheit und Spritzpartikelgröße auf die Haftung thermisch gespritzter Schichten

About the influence of substrate roughness and spray particle size on the adhesion of thermal spray coatings The influence of substrate roughness and spray particle size on the adhesion of thermal spray coatings was researched systematically. In addition to established spray materials (Cr₂O₃, WCCo, NiCr) and spraying processes (atmospheric plasma spraying (APS), high velocity flame spraying (HVOF)) different substrate materials (steel, stainless steel, aluminum) were included in the research work as well.     

Verschleißschutz für Titanbauteile

Wear Protection for Titanium Components The use of titanium and its alloys in the last decades keeps on increasing due to its material‐specific characteristics like high firmness, good corrosion characteristics and very high thermal maximum stress. However nowadays, the use of titanium components in systems where wear resistance is important is limited by titanium’s relatively low wear resistance. Surface wear is in principle a characteristic, conditioned by chemical and physical effects of the elements involved as well as collective stress. The necessity for new systems where good wear resistance and excellent mechanical properties are combined keeps on showing up. Due to titanium’s tendency to react with surrounding media, titanium alloys are difficult to be welded. Embrittlement by admission of hydrogen and oxygen can occur at high temperature processes or even changes on titanium’s microstructure may appear. Brazing techniques, which are actually applied to steel, have been modified and adapted for using them to titanium materials. Here, commercially available braze pastes and hard materials where combined and applied on titanium.     

Untersuchungen zur Beständigkeit thermisch gespritzter Schichten im Vergleich zu Hartchromschichten bei durch Abrasion dominierter tribologischer Beanspruchung

Investigations on thermal spray coatings resistance against abrasion dominated tribological load in comparison to hard chromium coatings HVOF iron and nickel based hard alloy as well as WC/Co(Cr) and Cr₃C₂/Ni20Cr coatings are compared to APS Al₂O₃/TiO₂ and Cr₂O₃, powder flame sprayed and fused composite coatings consisting of NiCrBSi and WC/Co and electrolytically deposited hard chromium coatings concerning their wear behavior for tribological load by lose abrasive particles (ASTM G65 and ASTM G75). Thereby the influence of newly developed HVOF torch combustion chambers with reduced critical diameter and divergent expansion nozzles that both permit increased combustion gas and therefore also particle velocities on microstructure and wear resistance of the produced coatings is studied. While there is no improvement of wear resistance for hard alloy coatings compared to mild steel substrates for the specific tribological boundary conditions of these tests, especially the carbide reinforced coatings permit improvement by more than one order of magnitude in ASTM G65 tests and even more than two orders of magnitude in ASTM G75 tests. Also, for both types of tribological load HVOF coatings with WC as reinforcing phase are clearly superior to electrolytically deposited hard chromium coatings. Both use of the combustion chamber with reduced critical diameter and the expansion nozzles with divergent contour result in improved wear resistance of the thereby produced coatings. The specific wear mechanisms are deduced based on SEM examination of worn specimen surfaces.     

Herstellung und Untersuchung des Verschleißverhaltens von induktiv eingeschmolzenen Hardpaint‐Schutzschichten

Friction and wear processes generate significant economic damage annually in industrial production due to maintenance and repair costs as well as loss associated with production downtime. Wear‐resistant coatings are a measure for reducing wear. In this context, the novel hardpaint technology for coating of parts with a fusible metal powder composition is described. Components with complex geometries as well holes or undercuts can be coated easily and inexpensively. Two protective layers are discussed and characterized in terms of their microstructure. Density measurements, hardness tests and scanning electron microscopic investigations were carried out. Both powder layers were inductively melted after application and are based on a hard alloy (iron‐based) commonly used for plasma transferred arc powder surfacing. Abrasion wear resistance is evaluated based on the results of the wear tests and microstructural investigations. Furthermore the results are discussed in comparison to a martensitic fine‐grain steel (Hardox 450), which is commonly used in abrasively stressed areas. Compared to Hardox 450 both hardpaint coatings have a much more wear‐resistant behavior due to their hard phases. In addition, it was confirmed that the hardpaint technology is able to embed thermally sensitive fused tungsten carbides which are significantly increasing the wear resistance against abrasion.     

Herstellung HVOF gespritzter,feinststrukturierter Cermetschichten unter Verwendung von feinen WC‐12Co Pulvern

Manufacturing of HVOF sprayed, finest structured cermet coatings using fine WC‐12Co powders The continuous increase in productivity and performance of modern sheet metal forming processes combined with the employment of novel, high strength materials cause high wear on tool systems. Coating technologies like thermal spraying provide a high potential to functionalize and to protect the surface of forming tools. However, it has to be ensured that the high shape and dimensional accuracy of the tool contour is preserved after the application of a wear protective coating. This aim cannot be achieved using currently applied, thermally sprayed coating systems with conventional, coarse grained microstructure. To solve this problem, novel finest structured coatings have been developed in this study by thermal spraying of fine WC‐12Co powders using the HVOF technique. For this purpose the influence of varying HVOF combustion gas compositions on the spray process as well as on the corresponding coating properties has been investigated. Next to a high surface quality the focus was placed on achieving coatings with high hardness and corresponding high wear resistance, low porosity as well as a good adhesive strength on the substrate material.     

Einsatz des Ritzpendels zur Quantifizierung des Verschleißverhaltens bei der Bearbeitung von Naturstein

Use of a Scratch Pendulum for Quantification of the Abrasive Behaviour in Machining Stones The machining of natural stones is one of the most demanding applications of today´s cutting technique. Modern stone processing is based on grinding and it is usually done with diamond based tools. The achievable material removal rate is process dependant and mainly determined by tool speed and infeed. The tool durability depends on the adjustment of the parameters to the wear resistance of the processed material. This resistance is a material characteristic value. Using a scratch pendulum, it is possible to determine the energy needed to create a defined scratch and relate it to the induced material loss and the chipped material volume. The analysis provides a value which, after verification by industrial production data, can be seen as characteristic for the material. It therefore enables the quantification of a material dependant scratch resistance.     

Verifizierung numerischer Verfahren zur Modellierung abrasiver Verschleißprozesse durch Berechnung von Scratchtests

Verification of numerical methods for modeling abrasive wear processes by calculating scratchtests Abrasion is a fundamental wear mechanism. It occurs whenever surfaces are loaded tribologically by hard particles or roughness peaks. The scratch test shows the behavior of a surface during abrasive stress. For that purpose a diamond tip (indenter) moves onto the material with either constant or progressive normal force. At first the material will be deformed elastically; with increasing force plastic deformation occurs till crack‐ and chip‐forming, depending on the ductility of the material. Using elastic and plastic indentation depth, friction, acoustic emission through cracks and the three‐dimensional analysis of the scratch, wear resistance to scratch‐stress can be characterized. The simulation of scratch tests is useful to verify material models comparing the relative easily and quickly accomplishable tests with the simulation results. Based on this verification new or existing material approaches can be adapted even better to applications. The article describes problems and solutions during implementation of the simulation of the scratch test for the steel C45 (normalized). The comparison with experimental results shows that this approach is successful. In the future a closed material model for this type of stress will be developed.     

Virtuelles Werkstoff‐ und Prozessdesign funktionaler Schichten

Virtual Material‐ and Processdesign of Functional Coatings Thermally sprayed and plasma transferred arc clad coatings are often used to improve the surface properties of mechanical parts with regard to an improved wear and corrosion behavior. New coating processes and applications can be developed, if it is possible to control the coating microstructure by a defined management of the process parameters. Simulation can be used to get a detailed understanding of the process‐material interaction for a defined controlling of the process parameters with less experimental effort. This allows a systematic variation of the coating structure and to calculate the parameter set which represents the best compromise between a high deposition rate and low residual stresses in the coating. In order to model thermal spraying, the following sub‐processes are considered: gas flow, material supply, heating and accelerating of particles, particle impact on the substrate, coating formation, solidification and formation of residual stresses. The results presented in this paper will demonstrate the influence of the process parameters on particle properties and subsequently on the splat formation, the coating formation and the coating microstructure. Controlling different process parameters like material injection conditions and substrate properties, the heating, cooling and solidification behavior of the particles and the coating structure can be influenced significantly.     

Erhöhung der Verschleißfestigkeit von Elastomerverarbeitungsmaschinen durch innovative Beschichtungsverfahren

Increasing the resistance to wear of elastomer processing machines by innovative coating processes A goal of the subsequent work is the evaluated of selected surface coating processes with respect to the attainable abrasion characteristics of the generated layers regarding to their suitability for the abrasion protection of elastomer processing equipment (e.g. interior kneading machines). Additionally the specifications of various commercial coating companies were layers compared to data concerning to the wear resistance of the created sections. On this base own experimental tests are performed dedicated to designated application cases and were evaluated by an abrasion test on its wearing properties. Apart from classical deposition‐welding, the Plasma‐Transferred‐Arc process, above all the innovative thermochemical techniques were applied. Due to of their steadily increasing market shares, particularly the arc and HVOF‐Spraying got in the center of attention thereby.     

Neuentwicklungen für den Verschleiß‐ und Korrosionsschutz beim Plasma‐Pulver‐Auftragschweißen

New developments for wear an corrosion protection by weld surfacing with plasma transmitted arc process Highly wear‐resistant claddings which contain carbides can be applied by weld surfacing with the PTA process. The use of vanadium carbide prevents undesirable reactions with the matrix material. Thus, highly corrosion‐resistant Fe‐based claddings can be produced for applications in the food and marine industries, and Ni‐based claddings can be applied to components exposed to inorganic acid attack. A combined test is applied for determining the relative effect of corrosion under combined exposure to abrasive wear and corrosion and indicates the primacy of abrasive wear for behaviour in operation.     

Prozessintegriertes Pulverbeschichten durch Radial‐Axial Ringwalzen

Process‐integrated powder coating by radial axial rolling of rings Powder metallurgically produced parts, that are exposed to high stresses and/or intense abrasive loads do not tolerate any residual porosity. The process of hot isostatic pressing (HIP) used to ensure a reliable compaction is expensive and limitates the producible component size because of the dimensions of existing HIP‐furnaces. The VolkswagenStiftung is funding the development of an alternative production process to apply powder metallurgical wear protective layers consisting of metal‐matrix composites (MMC) to ring shaped parts using radial‐axial ring rolling. The rolling blanks used for this process feature a circumferential recess on the outer surface, closed by a piece of sheet metal. The thereby generated chamber contains the powder blend that will later form the coating layer. The rolling process leads to compaction of the powder and a firm bonding to the substrate rings surface. Furthermore, the deformation forces are used for expanding the ring’s diameter.     

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.     

Untersuchungen zum Verschleißverhalten von Metall/Metall – Gleitpaarungen aus CoCrMo‐Legierungen

Investigation of the Wear Behaviour of Metal/Metal Bearings of Co Cr Mo – Alloys CoCrMo‐alloys are successfully used for long‐term implants, because of their corrosion and wear resistance as well as their mechanical properties. In order to improve CoCrMo‐alloys for metal‐on‐metal bearings the influence of carbon content on wear behaviour is investigated. Casted or forged CoCrMo‐alloys with a carbon content from 0.008 to 0.48 wt % were studied in ring on disc oscillating tests. Friction torque, weight loss and surface roughness, as well as light and scanning electron microscopic investigations of the sample surface were used to characterize the wear behaviour. – All alloys show similar friction torque and weight loss. But the surface roughness and the wear mechanisms are depended on the carbon content.     

Eigenschaften und Anwendungen von Chemisch Nickel‐Dispersionsschichten

Properties and applications of electroless nickel composite coatings This paper discusses the variety of composite electroless coatings used in different industrial applications. The inclusion of particulate matter within electroless nickel deposits can add entirely new properties to the plated layer. Composites with hard particles like diamond, silicon carbide and boron carbide provide greater wear resistance and the possibility for adjustable friction properties. Composite electroless nickel with diamond or ceramics has found wide applications in the textile, automotive and mechanical engineering industry. Friction joints in automotive engines constitute an important field of application for diamond coatings. Modern internal combustion engine designs require that the crankshaft and camshaft be fitted at a specific relative angle. In order to establish the correct angle during assembly and maintain it over the life of the engine, axial press‐fit joints in combination with centrally located retention bolts are employed. Failure of either the joints or the bolts can result in serious damage to the engine. The torque transfer ability of these engine components can be significantly increased by incorporating a friction foil that is diamond‐coated on both sides. Composite coatings with coarser diamond particles can be used for the coating of precision tools in the semiconductor industry. Enhanced lubricity can be achieved by incorporating solid lubricants in electroless nickel deposits. Composite coatings with PTFE or PFA offers non‐stick surfaces with antiadhesive properties and good resistance against adhesive wear. Because of the temperature and softness limitations these coatings are best suited for lower temperature and light loading applications. Electroless nickel boron nitride coatings can withstand temperatures up to 600 °C. This coating reduces coefficient of friction and wear in dynamic applications. A further application is the coating of molds for rubber and plastic components.     

Potenziale von ausgewählten Randschichtbehandlungsverfahren sowie deren Kombinationen zur Verbesserung des Verschleiß‐ und Korrosionsverhaltens von Gusseisenwerkstoffen

Die typischen hohen C‐ und Si‐Gehalte von Gusseisenwerkstoffen und der weiche Graphit limitieren die Behandel‐ und Beanspruchbarkeit nach dem Nitrieren und der Hartstoffbeschichtung. Wenn die Gusseisenoberfläche vor den genannten Randschichtbehandlungen mittels Elektronstrahls umgeschmolzen wird (Kombinationsbehandlung) und eine harte, graphitfreie ledeburitische Randschicht gebildet wird, dient diese als Stützschicht für die harte und dünne Verbindungs‐ bzw. Hartstoffschicht. Vergleichende Verschleißtests (Stift‐Scheibe) zeigten, dass bei geringen Lasten die Verschleißrate aller Einzel‐ und Kombinationsbehandlungen auf einem vergleichbar niedrigen Niveau wie der unbehandelte und beschichtete Grundwerkstoff liegen. Bei höheren Lasten kommt das überragende Verschleißverhalten der Kombinationsbehandlungen gegenüber den Einzelbehandlungen voll zum Tragen. Die Bildung defektfreier Randschichten nach der Kombinationsbehandlung resultiert außerdem in einer deutlichen Verbesserung der Korrosionsbeständigkeit in chloridhaltiger Lösung. Im Vergleich zum Grundwerkstoff und den Einzelbehandlungen wurden die relevanten Potenziale zu deutlich positiveren Werten verschoben.     

Verschleißreduzierung an Matrizen für das Präzisionsschmieden von Zahnrädern durch Mehrlagenhartstoffbeschichtung (TiN‐TiCN‐TiC)

Wear reduction on dies for precision forging of gear wheels by means of multi‐layer coating (TiN‐TiCN‐TiC) Due to high thermal, mechanical and tribological loads the tool life quantity of hot forging dies compared to other manufacturing processes is relatively low. Depending on the number of forged parts the mentioned loads lead to different failure causes of the dies. In this connection wear is the main failure cause of hot forging dies. Especially in the precision forging process of gear wheels with its exacting tolerances the tool life quantity is low, which leads to often interruptions of the production process. Because wear concentrates on the near‐surface die areas, these problems can be reduced by increasing the wear resistance of these areas by the Duplex PACVD‐Method, which means nitriding and multi‐layer coating (TiN‐TiCN‐TiC).