PVD‐Nanocomposite‐Beschichtungen für die Zerspanungs‐ und Umformtechnik

PVD Nanocomposite coatings for the machine cutting and deformation technique Physically and chemically separated wear‐protection layers exhibit increasingly more complex structures. Similarly to the a coil of compact materials increase also here solutions develop as composite materials, which are offered in the form of Nanocomposites. An industrially available solution represents the system MeN/Si₃N₄. This is to be introduced concerning its characteristics as well as its application type within the range of the machine cutting and deformation technique.     

Untersuchung des Sputter‐Ätzens auf die Eigenschaften von PVD‐CrN Hartstoffschichten auf Magnesium AZ91hp

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.     

Neue Kapseltechnik für Diamantverbundwerkstoffe mittels PVD‐Verfahren

Novel encapsulation technique for diamond composites using PVD‐process For machining of mineral materials diamond tools consisting of a steel body combined with diamond impregnated segments are used. Frequently, these segments are hot pressed. Other process routes are pressureless sintering of green compacts partly combined with hot isostatic pressing and hot isostatic pressing of encapsulated powder mixtures. The compaction effect of hot isostatic pressing require a low porosity of sintered components realized by using ultra‐fine metal powder or an impermeable capsule made of metal or glass. The Institute of Materials Engineering pursues a novel process route by physical vapor deposition of a coating on pressureless sintered composites. The thin coating acts as a capsule and guarantees the pressure transfer in the following hot isostatic pressing process. Although bronze powders with particle sizes up to 90 μm are used, the manufacturing of diamond composites with low porosities is possible. In comparison to conventional encapsulation‐techniques the main advantages of this novel process route are the use of comparatively coarse metal powders and a larger geometric flexibility.     

Elektronenstrahlbehandlung von PVD‐Hartstoffschichten

Electron beam treatment of PVD – hard coatings Coatings of the type CrN_x, (Ti, Cr)N, (Ti, Al)N, Ti(C, N) and Ti(B,N) were deposited on the quenched and tempered steel C45 to investigate the effect of electron beam treatment on the structure and the properties of hard coated steels. A controlled energy input by electron beams was used to investigate the thermal behaviour of hard coatings with fixing the transformation levels by self‐quenching. Simultaneously a different case hardening of the substrate was caused providing a different effect of supporting the hard layer. There are big differences in the thermal stability of the investigated coatings. The surface hardness, adhesion and wear resistance of the composit hard coating/steel was improved in dependence on the energy input. The use of electron beam technologies enables the generation of support layers which locally increase the working behaviour of hard coated 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.     

X‐ray diffraction (XRD)‐studies on the temperature dependent interface reactions on hafnium,zirconium, and nickel coated monocrystalline diamonds used in grinding segments for stone and concrete machining

Diamond impregnated metal matrix composites are the state of the art solution for the machining of mineral materials. The type of interface reactions between the metal matrix and diamond surface has an essential influence on the tool performance and durability. To improve the diamond retention, the diamonds can be coated by physical vapour deposition with metallic materials, which enforce interface reactions. Hence, this paper focuses on the investigation of the interfacial area on metal‐coated monocrystalline diamonds. Hafnium and zirconium, both known as carbide forming elements, are used as coating materials. The third coating, which is used to determine its catalytic influences when applied as a physical vapour deposition (PVD)‐layer, is nickel. Additionally, the coated diamond samples were heat‐treated to investigate the starting point of the formation of new phases. X‐ray diffraction‐analyses revealed the assumed carbide formation on hafnium and zirconium coated samples. The formation temperature was identified between 800 °C and 1000 °C for hafnium and zirconium coatings.     

Tailoring protective coatings for all‐oxide ceramic matrix composites in high temperature‐/high heat flux environments and corrosive media

The application range of porous all‐oxide ceramic matrix composites (CMCs) can be significantly extended through deposition of protective coating systems. Typical applications include protection against erosion, wear and foreign object damage as well as a reduced permeability. Environmental barrier coatings (EBC) are mandatory in order to guarantee sufficient lifetime of the CMC components under high temperature‐, high heat flux conditions and corrosive attack (combustor liners, thermal protection systems for atmospheric reentry). Limited thermal stability of today’s oxide fibers requires additional thermal barrier functionality for EBCs in order to keep the effective CMC bulk temperatures below 1200 °C. Depending on the specific application DLR’s coating concept for all‐oxide CMCs is based on either a single reaction‐bonded aluminium oxide (RBAO) coating or a hybrid coating system consisting of a RBAO bond coat followed by an EB‐PVD YSZ/FSZ top coat and is highlighted for three case studies. Deposition techniques (magnetron sputtering, MOCVD) alternative to EB‐PVD as well as the suitability of fibrous and cellular materials for thick EBC/TBC layers are explored.     

Durability‐based design for endurance strength of hydroforming tools

Hydroforming tools must be designed fatigue resistant because in case of tool rupture high costs for repair and production downtime occur. Available guidelines for lifetime estimation of cyclically loaded parts usually are of general nature. In order to examine their applicability to the special concerns of hydroforming tools, stress states occuring during operation were investigated by means of numerical simulation. Basing on the results, specimen geometries were developed, representing a typical stress state. The estimated durability was calculated using the “FKM‐guideline”. The real durability was determined experimentally in form of Wöhler‐graphs. Eventually calculation and experiments were compared. Hence, the qualtity of the lifetime prediction was evaluated and correction factors were proposed.     

Lifetime‐determining spalling mechanisms of NiCoCrAlRE / EB‐PVD zirconia TBC systems

The mechanisms that control the lifetime of thermal barrier coating (TBC) systems have been traced by two particular overlay bondcoats serving as model systems: superalloy pins (IN100, CMSX‐4) with two alternative NiCoCrAlRE (RE: Hf, Y) bond coat compositions (i) NiCoCrAlY without and (ii) with co‐dopants of silicon and hafnium. On top an electron‐beam physical‐vapor deposited (EB‐PVD) yttria partially stabilized zirconia (YPSZ) TBC commonly mixed with 2 wt.% hafnia, or, rarely with 10 wt.%, was applied. The test pins were thermo‐cycled at 1100 and 1150 °C until failure. Identical lifetimes in cyclic tests on YPSZ TBCs with 2 (relatively high sintering rate) and 10 wt.% hafnia (relatively low sintering rate) preclude an effect of diffusion mechanisms of the YPSZ TBC on lifetime. The fit of lifetimes and test temperatures to Arrhenius‐type relationships gives activation energies for failure. These energies agree with the activation energies for anion and cation diffusion in alumina for the respective bondcoat variant: (i) for the NiCoCrAlY/TBC system for O^2‐ diffusion in alumina, (ii) for the NiCoCrAlYSiHf/TBC system for Al³⁺ diffusion in alumina. SEM and EDS investigations of the thermally grown oxides (TGOs) confirm the mechanisms responsible for TBC failure as indicated by activation energies. Two categories of failure can be distinguished: (i) NiCoCrAlY coatings fail by an “adhesive mode of failure” along smooth bond coat/TGO interfaces driven by a critical TGO thickness. (ii) NiCoCrAlYSiHf coatings fail later and more reluctantly by a “cohesive” crack mode via de‐cohesion at the TGO/TBC interface. In the latter case a quasi‐integrity of the crack‐affected TGO is lengthily maintained up to failure by a crack‐pinning mechanism which runs via Al³⁺ supply from the bondcoat.     

Mikrostruktur‐Untersuchungen zum Schwingungsverschleißverhalten von Nickelkomposit‐ und Ni‐P‐Schichten

Microstructural and oscillating sliding wear studies of nickel composites and electroless Ni‐P layers In many industrial applications, oscillating sliding wear leads to serious damage of construction components. To avoid this, different layers of electroplated nickel and nickel composites as well as chemically deposited nickel phosphorus layers are used and/or tested. The performance of these layers under oscillating sliding wear was characterized. Additionally microstructure characterisations took place regarding grain size, particle content and distribution as well as concerning crystallization and development of tetragonal phase nickel phosphide. These results correlate well with the Martens hardness of the layers and contribute to understand the oscillating sliding wear characteristics of the examined layers. Heat treated Ni‐P layers achieve twice the hardness of nickel dispersion layers; however, fail under oscillating sliding stress by embrittlement, cracking and debonding. On the other hand dispersion‐hardening nano composites with TiO₂ clearly exhibit a more favourable tribological behaviour. A solid content of approx. 3 vol‐% leads to dispersion and fine grain hardening effects, which cause good protection against oscillating sliding wear; thereby the Ni‐matrix remains ductile. The incorporation of very hard particles (SiC) intensifies the oscillating sliding wear process due to the abrasive effect of the particles.     

The influence of the electroplating parameters on the conditions of deposited nickel‐iron coatings

Several specific electroplating parameters are varied to develop a manufacturing process for nickel‐iron coatings with low residual stresses and favourable structures and hardness values. Here, the produced coating conditions on the surface are characterised according to alloy compositions, microstructures, residual stresses, texture and microhardness as well as the elastic indentation modulus. The manufacture of the coatings was carried out in a laboratory electroplating plant. The process parameters are: electroplating in electrolytic baths with and without saccharin as an additive, with different iron content as well as with various current parameters. The results show significant influences of saccharin, as a bath additive, and of the pulse frequency during the reverse current pulse procedure on the coating conditions. Supplementary investigations are scheduled for determining the distributions of the conditions through the coating thickness. Similarly, micromechanical tests are planned. Process parameters with optimal conditions can then be selected from the determined process‐condition property relation. This is to subsequently serve as the basis for improving the manufacture of micro gear‐wheels. The micro gear‐wheels are produced by electroforming using the direct LIGA process.