Transluzente oxidfaserverstärkte Glasmatrix‐Verbundwerkstoffe

Translucent oxide fiber reinforced glasmatrix composites The aim of the work is the developement of transparent glass matrix composites. Therefore besides the mechanical properties also the optical properties of the components have to be adapted. In this study the influence of different fiber coatings (boron nitride, titanium oxide and a boron nitride/titanium oxide double coating) on the mechanical and optical properties of Nextel 440‐fiber reinforced glass was investigated. Micromechanical investigations (push‐in‐tests) and 3‐point‐bending tests have shown the best improvement of the fracture toughness for the binary boron nitride/titanium oxide coating. For single coatings of boron nitride or titanium oxide the transparency was characterized by the transmission spectra.     

Microstructure and mechanical properties of spark plasma sintered titanium‐added copper/reduced graphene oxide composites

Copper matrix composites were fabricated through mixing fixed amount of reduced graphene oxide and the different amounts of titanium. The dried copper/titanium/reduced graphene oxide mixture powders were firstly obtained by the wet‐mixing process, and then the spark plasma sintering process realized their faster densification. In the as‐sintered bulk composites, the layered reduced graphene oxide network, uniform titanium particles and copper‐titanium solid solution are observed in copper matrix. Investigations on mechanical properties show that the as‐prepared bulk composites exhibit the hardness and compressive yield strength compared with single reduced graphene oxide added composites. Increased titanium addition resulted into higher hardness and strength. The relevant formation and failure mechanisms of the composites and their influence on mechanical properties were discussed.     

Herstellung und komplexe Charakterisierung von Metall‐Keramik‐Verbundschichten

Formation and characterization of metal‐ceramic coatings The influence of the formation process and used materials of metal‐ceramic coatings on the structural properties of the deposited layers were investigated and optimized to increase the mechanical properties. There the deposition of the metal‐ceramic‐layers occurred by a combination of electrophoretic and galvanic deposition with siloxane as bonding compound. Layers with a high ceramic content were successfully created. As ceramic components commercial silicon carbide and silicon nitride were used. Nickel and Copper respectively were applied as metal component to fill the porous ceramic structure with the aim to increase the strength of the layers, where nevertheless a pre nickel‐plating or pre cupper plating of the steel substrate X6Cr17 before ceramic component deposition had to be done to increase the adhesion of the layers. The layer characterization was made by optical microscopy and scanning and transmission electron microscopy, where especially the bonding of the single particles by the siloxane was in evidence.     

Oxidation‐Resistant Coatings for Application on High‐Temperature Titanium Alloys in Aeroengines

High‐temperature application of titanium alloys in aeroengines is often limited by their insufficient resistance to the aggressive environment. Magnetron‐sputtered Ti–Al based coatings were developed in order to increase the maximum service temperature of conventional titanium alloys from the present 520–600 °C, the temperature limit set by the mechanical capabilities of most advanced alloys. The coatings not only demonstrated excellent oxidation resistance but also demonstrated beneficial effects on mechanical properties. Most importantly, the fatigue behavior of the substrate alloys was not degraded, a major hurdle for coating application on titanium alloys so far. Initial results on Cr‐containing Ti–Al based coatings indicated significant potential for application on titanium aluminides.     

Strangpressen von verschleißbeständigen Fe‐Basis MMC

Hot extrusion of wear resistant Fe‐base metal matrix composites (MMC) Increasing demands on technical surfaces, i.e. thermal load, corrosion or wear, often prompt the development of tailored materials or coatings. In highly abrasive environments the progress in powder metallurgy has lead to the production of highly wear‐resistant materials based on metal‐matrix composites (MMC). Such materials are produced from a metal matrix (MM) based on Fe, Ni or Co and additional hard phases (HP), such as carbides, nitrides, borides or oxides. Moreover, powder metallurgical techniques can be used to adapt the particle size, the distribution and the content of the hard phases to the wear system on a large scale. HIP cladding is an established method of producing such MMC, but due to its near net shape capsule technique it is quite expensive. Because of this reason hot direct extrusion of capsules filled with powder blends was researched in a DFG‐Project as a method of producing long cylindrical products. Aiming at a high abrasive wear resistance, powder blends of hardenable steels with additions of fused tungsten carbide (WSC) or titanium carbides (TiC) were used. The extruded MMC were investigated with respect to their densification and microstructure, their bending strength and their wear resistance.     

Microstructure and Compression Strength of Novel TRIP‐Steel/Mg‐PSZ Composites

Novel composites on basis of austenitic stainless TRIP‐steel as matrix with reinforcements of Mg‐PSZ are presented. Compact rods were produced by cold isostatic pressing and sintering, square honeycomb samples by the ceramic extrusion technique. The samples are characterized by optical and scanning electron microscopy before and after deformation, showing the microstructure and the deformation‐ induced martensite formation. The mechanical properties of samples with 5 vol% zirconia are superior compared to zirconia‐free samples and composites with higher zirconia contents in terms of bending and compression tests. The honeycomb samples exhibit extraordinary high specific energy absorption in compression.     

Carbon Dots as Fillers Inducing Healing/Self‐Healing and Anticorrosion Properties in Polymers

Self‐healing is the way by which nature repairs damage and prolongs the life of bio entities. A variety of practical applications require self‐healing materials in general and self‐healing polymers in particular. Different (complex) methods provide the rebonding of broken bonds, suppressing crack, or local damage propagation. Here, a simple, versatile, and cost‐effective methodology is reported for initiating healing in bulk polymers and self‐healing and anticorrosion properties in polymer coatings: introduction of carbon dots (CDs), 5 nm sized carbon nanocrystallites, into the polymer matrix forming a composite. The CDs are blended into polymethacrylate, polyurethane, and other common polymers. The healing/self‐healing process is initiated by interfacial bonding (covalent, hydrogen, and van der Waals bonding) between the CDs and the polymer matrix and can be optimized by modifying the functional groups which terminate the CDs. The healing properties of the bulk polymer–CD composites are evaluated by comparing the tensile strength of pristine (bulk and coatings) composites to those of fractured composites that are healed and by following the self‐healing of scratches intentionally introduced to polymer–CD composite coatings. The composite coatings not only possess self‐healing properties but also have superior anticorrosion properties compared to those of the pure polymer coatings.     

Untersuchung der Haftmechanismen kaltgasgespritzter Al‐Schichten auf Al2O3

The metallization of ceramics by means of cold gas spraying has been in the focus of numerous publications in the recent past. However, the bonding mechanisms of metallic coatings on non‐ductile substrates are still not fully understood. Former investigations of titanium coatings on corundum revealed that a combination of recrystallisation induced by adiabatic shear processes and heteroepitaxial growth might be responsible for the high adhesions strengths of coatings applied on smooth ceramic surfaces. In the present work, it is intended to examine the interface area of cold gas sprayed aluminum on alumina substrates. Besides a variation of powder fraction and substrate temperature, it is investigated if a downstream heat treatment has an influence on tensile strength and hardness of the coatings. The splat formation of single particles is investigated by means of scanning electron microscopy, while a high resolution transmission electron microscope is used to examine the Al/Al₂O₃ interface. First results suggest that mechanical clamping is the primary bonding mechanism on polycrystalline coatings with a sub‐micrometer‐scaled surface roughness, while heteroepitaxial growth is the main bonding mechanism for Al coatings on single‐crystalline, atomically smooth sapphire (α‐Al₂O₃) substrates. Heteroepitaxy is promoted by deformation‐induced recrystallisation of the cold gas‐sprayed aluminum.     

Tailor‐Made Composites Based on Titanium for Medical Devices

For special advanced applications in medicine biomaterials can be tailor‐made as composites if a single material is not able to fulfill the various requirements. In most cases a functional surface layer, which may have the required physical, mechanical, or biological properties, covers a structural material: titanium and its alloys for preference. Three classes of composites based on titanium materials offer special properties: Ti/porous Ti composites have special mechanical properties for isoelastic implants; Ti/ceramic composites show special biological properties used for improved osseointegration of the implants; and the special physical properties of Ti/ceramic composites makes them suitable for heart pacemaker leads.     

Oxidation protective coatings for γ‐TiAl – recent trends

Engine designers show continued interest in γ‐TiAl based titanium aluminides as light–weight structural materials to be used at moderately elevated temperatures. Although alloy development has made significant progress in terms of mechanical properties and environmental resistance, protective coatings have been developed that help to extend the lifetime of these alloys significantly. The major challenge of coating development is to prevent the formation of fast growing titania. Furthermore, changes of coating chemistries at high temperatures have to be considered in order to avoid rapid degradation of the coatings due to interdiffusion between substrate and coating. The paper describes recent work of the authors on different coatings produced by means of magnetron sputter technique. Thin ceramic Ti‐Al‐Cr‐Y‐N layers tested at 900 °C exhibited poor oxidation resistance. In contrast, intermetallic Ti‐Al‐Cr, Si‐based and aluminum rich Ti‐Al coatings were tested at exposure temperatures up to 950 °C for 1000h resulting in reasonable and partially excellent oxidation behaviour.     

Packaging‐related properties of protein‐ and chitosan‐coated paper

The mechanical and gas‐barrier properties of paper and paperboard coated with chitosan–acetic acid salt (chitosan), whey protein isolate, whey protein concentrate and wheat gluten protein were studied. Paper sheets were solution‐coated using a hand applicator. In addition, bi‐layer composites of wheat gluten and paper or paperboard were produced by compression moulding, and the chitosan solution was also applied on paperboard using curtain coating. Young's modulus, fracture stress, fracture strain, tearing strength, air permeance and oxygen permeability were assessed. The mechanical and air permeance measurements of solution‐coated paper showed that chitosan was the most effective coating on a coat weight basis. This was due to its high viscosity, which limited the degree of penetration into the paper. The proteins, however, also enhanced the strength and toughness of the paper. Compression‐moulded wheat gluten/paper or paperboard, as well as curtain‐coated chitosan paperboard laminates, showed oxygen barrier properties comparable to those of paper and paperboard coated with commercial barrier materials. None of the composites could be delaminated without fibre rupture, indicating good adhesion between the coatings and the substrates. Copyright © 2005 John Wiley & Sons, Ltd.     

Potentiality of artificially aged aluminium 6061 hybrid composites reinforced with granite and graphite micro–fillers for structural applications

The current study explores the feasibility of artificial aging treatment to aluminium alloy 6061 hybrid composites reinforced with graphite and fine granite particulates to suit structural applications. Aging at 100 °C retrieved a better response of the hybrid composites as compared to that at 150 °C in terms of peak hardness and strength. Among the five different stir‐cast compositions, the composition of aluminium 6061 with 2 wt. % graphite and 4 wt. % granite showed enhanced mechanical properties than that of the base alloy as well as mono reinforcement type composites which was attribued to the formation of rod‐shaped β‐Mg₂Si precipitates.     

Laser Deposition of In Situ Ti – TiB Composites

Due to their enhanced mechanical properties and potentially wide applicability, there is considerable interest in the development of metal‐matrix composites consisting of titanium borides in a titanium alloy matrix. Despite the development of a variety of different processing routes for these composites, there are relatively few ones capable of processing a fully dense, near‐net shape component with a relatively fine dispersion of boride precipitates. This paper will discuss the in situ laser deposition of Ti‐TiB composites using the laser engineered net‐shaping (LENS™) process from a blend of elemental titanium (or titanium alloy) and boron powders. The microstructure of the LENS™ deposited Ti‐TiB composite has been compared with that of a conventionally cast in situ composite of the same composition. The conventionally cast composite exhibits a significantly coarser scale microstructure. Thus, the ability to produce a fine dispersion of TiB precipitates in dense Ti‐TiB composites of near‐net shape using LENS™ processing can be attributed to the rapid solidification effects during such processing.     

石墨烯/钛基复合材料的界面反应控制、微观组织和压缩性能

将Ti6Al4V（TC4）粉末与少层石墨烯（GR）粉末进行三维机械旋转混合,实现了GR在TC4球形粉末表面的均匀包覆,经放电等离子烧结(SPS)得到增强相呈三维网络状分布的GR/TC4复合材料。对不同的SPS烧结温度、保温时间、升温速率和轴向压力对GR与钛基体原位界面反应程度的影响进行了研究,并对界面处不同GR/TC4比例的网状结构复合材料的物相结构、显微组织及室温压缩性能进行了系统性的研究。结果表明,烧结温度和升温速率是影响GR与基体反应程度的主要因素,压力主要影响材料致密度,低温高压快速烧结可以降低GR与基体的反应程度,但高比例的GR残留并没有带来力学性能的大幅提升。对于0.25wt%的GR添加量,GR的反应比例约为70%~80%能得到更加良好的异质界面的结合,获得综合力学性能优异的GR/TC4协同增强的钛合金基复合材料。GR在钛合金基体中的三维网络状分布能调控钛基复合材料的强度与塑性的矛盾。      

非连续增强金属基复合材料剧烈塑性变形行为研究进展

综述了非连续增强金属基复合材料剧烈塑性变形(SPD)行为的研究进展,系统阐述了等径弯曲通道变形(ECAP)、高压扭转(HPT)、多向锻造(MF)、累积叠轧(ARB)和循环挤压压缩(CEC)5种SPD的加工原理和方法。集中介绍了这些方法在铝基、镁基、铜基和钛基等金属基复合材料方面应用的研究进展。重点介绍了金属基复合材料SPD的微观组织演化和变形力学行为,详细阐明了金属基复合材料SPD机制以及超细晶形成机理,指出了金属基复合材料在SPD中存在的深层次问题及发展趋势,展望了利用SPD方法制备超细晶非连续增强金属基复合材料的应用前景。     

Mikrostrukturelle Pressschweißnahtcharakterisierung im strangpressten Zustand für Al‐Mg‐Si‐Legierungen

Weld seams form when profiles are extruded using porthole or bridge dies. These are inevitable when producing industrial relevant hollow profiles but imply the weak spot if the profiles are used for applications with high mechanical requirements. The characterization of formed weld seams together with their mechanical properties is problematic or sometimes even impossible due to the complex profile geometries. Expansion, bending or tensile tests of profiles or parts of the same were hitherto often used for their analysis lacking the possibility of basic assumptions about their properties. The investigations described herein circumvent the problem by using a flat profile exhibiting the weld seam in the middle. Flat profiles offer the possibility of standardised specimens for tensile testing. The extrusion experiments focused on the aluminium alloys EN‐AW 6060 and EN‐AW 6082. During the experiments billet temperature and extrusion speed have been varied. The microstructure was analysed subsequent to explain the obtained results and to offer a possibility of characterisation.     

A study on montmorillonite/polyethylene nanocomposite extrusion‐coated paperboard

Extrusion coating was used to obtain montmorillonite/polyethylene‐coated paperboard. The coating was prepared from a master batch containing maleated polyethylene, low‐density polyethylene and 32 wt.% polyvinylpyrrolidone‐surface‐modified montmorillonite clay, which was blended with different amounts of low‐density polyethylene to yield composites with 4 wt.% and 8.3 wt.% montmorillonite. X‐ray diffraction revealed that the clay stacks in the coating were more extensively intercalated than in the original surface‐modified clay. Transmission electron microscopy showed that the clay stacks were, to a large extent, separated by the high shear forces during extrusion into smaller evenly distributed entities. This was, unfortunately, achieved at the expense of the formation of a great many voids and pinholes, as revealed by transmission electron microscopy and dye staining. This had strong negative effects on the oxygen barrier properties but only a modest effect on the water vapour permeability. Tensile tests showed that the coating was always ductile and that the coating–paperboard adhesion decreased with increasing clay content. The creasability was good and unaffected by the presence of the filler in the coating. Thermogravimetry showed that the degradation temperature in air of the filled coatings were of the order of 10°C higher than that of unfilled polyethylene. Copyright © 2004 John Wiley & Sons, Ltd.     

Funktionale Oberflächen auf Duplex‐Stahl durch Laserfeinbeschichten

Functional surfaces on duplex stainless steel by lasercladding The product‐lubricated axial and radial bearings installed in multistage high‐pressure pumps inevitably encounter severe mixed friction conditions as the pumps start and stop. This leads to extremely high tribological loads on the bearing components, compounded by the effects of a highly corrosive pumped fluid. The present paper describes a laser cladding process which produces near‐net‐shape coatings of new, highly corrosion and wear resistant functional layers which can be deposited directly on high‐alloy stainless steels without requiring additional buffer layers and without affecting the mechanical properties and corrosion resistance of the substrate. The results cover the solidification behaviour of the coatings as well as the microstructure resulting from various heat treatment conditions. In addition, the technological properties of the coatings and the resulting composites are discussed. The coating systems are tested as to their corrosion resistance and tribological characterization in a pump‐specific tribological system.     

Compositional and Tribo‐Mechanical Characterization of Ti‐Ta Coatings Prepared by Confocal Dual Magnetron Co‐Sputtering

Titanium‐Tantalum coatings are deposited by magnetron co‐sputtering technique, using independently driven titanium and tantalum targets. The effect of the Ta content on the structure, mechanical, and wear properties of Ti films is investigated. It is found that the percentage of the added Ta varies linearly from 3.7 to 31.3 at% by increasing the power applied to the Ta target from 10 to 100 W. The XRD results show that the coatings are crystalline, and there is no evidence of the formation of intermetallic phases, instead formation of metastable phases of α″ and β depending on Ta content are observed, though the samples are deposited at low temperature (150 °C). It is shown that the elastic strain to failure (H/E_r; hardness to reduced elastic moduli ratio) can be increased by 40% through the formation of crystalline phases with a lower E, while the hardness remains constant. The tribological study shows that increasing the Ta content up to 14.9 at% causes a significant improvement in adhesion of the coating to a soft metallic substrate.

     

Laserstrahlschweißen von Mikrodrähten aus Nickel‐Titan‐Formgedächtnislegierungen und austenitischem Stahl

Laserwelding of microwires made of nickel‐titanium shape memory alloys and austenitic steel The special properties of nickel‐titanium shape memory alloys are currently used in micro engineering and medical technology. In order to integrate nickel‐titanium components into existing parts and modules, they often need to be joined to other materials. For this reason, the present contribution deals with the laser welding of thin pseudoelastic nickel‐titanium wires (100 μm) with a neodymium‐doped Yttrium Aluminium Garnet laser. Based on extensive parameter studies, joints without defects were produced. This study deals with the microstructure in the fusion and heat‐affected zones, the performance of the joints in static tensile tests and their functional fatigue. It can be shown that nickel‐titanium/nickel‐titanium joints reach about 75 % of the ultimate tensile strength of pure nickel‐titanium wires. In case of welding nickel‐titanium to steel no interlayer was used. The dissimilar nickel‐titanium/steel joints provide a bonding strength in the fusion and heat‐affected zones higher than the plateau stress level. Nickel‐titanium/steel joints of thin wires, as a new aspect, enable the possibility to benefit from the pseudoelastic properties of the nickel‐titanium component.