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Competition of nitrogen doping and graphitization effect for field electron emission from nanocrystalline diamond films J.

Syntactic metal foams -a special foam type consisting of a metal matrix with integrated hollow elements like micro glass bubbles -offer an interesting spectrum of properties, like low densities, high capacity for crash energy absorption, vibration dampening capacity, and pressure-tightness. [1,2] For low melting matrix alloys infiltration techniques can be used for production, whereas for higher melting alloys other techniques are needed. Iron-based (e.g., pure Fe, FeNi2, FeCu3) micro-porous syntactic foam components were produced by means of metal injection moulding (MIM) showing very good deformation energy absorption capabilities in compression but quite low fracture elongations in quasi-static tensile tests (<1%)). [3,4] Several reasons for this brittle behavior can be assumed: micro-cracks formed during cooling from sintering temperature due to the different thermal shrinkage behavior of matrix and glass spheres, insufficient bonding at the interface between iron matrix and hollow glass spheres or stress and strain concentrations in soft matrices near the hard glass spheres in the early stages of deformation which can cause premature failure of the test component.Therefore, experiments with Invar (FeNi36) alloy and S60HS micro hollow glass spheres were recently done in order to address two of the above-mentioned three points: Invar shows a considerably reduced thermal expansion reducing the risk of crack formation during cooling down from sintering temperature and has a higher strength in comparison to pure iron which might lead to a better adapted deformation of matrix and hollow spheres. Indeed, an enormous increase of tensile elongation could be observed, corroborating the initial assumptions. [5] Density values, deformation behavior, and metallographic sections of FeNi36-foams indicated a complex interplay of matrix powder particles and hollow glass spheres during sintering. Glass hollow spheres like 3M-S60HS are very soft at the sintering temperatures employed, they shrink and preserve their shape only because of the mechanical support of the adjacent metal powder particles. [5] Considering this interplay the present work's objective was to investigate the influence of the metal powder particle size on the resulting foam structure and mechanical behavior. The use of coarse powder is economically and ecologically advantageous especially for Ni. On the other hand fine powders improve sintering behavior and sphere shape stability.

Syntactic metal foams, i.e., foams which are composed of a metal matrix with integrated hollow elements, offer a unique spectrum of properties, like pressure-tightness, high capacity for crash energy absorption and comparably low densities. The production techniques are mostly based on the pressureassisted infiltration of solid structures or loose bulks of hollow elements like, e.g., ceramic hollow spheres and microglass bubbles (mGBs) by metal melts. [1][2][3] For higher-melting alloys like iron or steel this approach is, however, very difficult to realize because only few pressure-assisted techniques like gas-pressure infiltration are available. Moreover, the combined impact of high pressures and temperatures during the filling stage might lead to the destruction of the hollow elements.Therefore, tests have recently been performed in order to examine the feasibility of the production of syntactic foams based on iron matrices and mGBs by metal powder injection moulding (MIM), a process, which is closely related to polymer injection moulding. [4] After proving the feasibility of this approach a more systematic investigation was done with Fe99.7% as most simple matrix in order to better understand the process limits, the material structure, and its mechanical behavior. Furthermore, the corrosion behavior of the foam material was investigated not only as it is important for the material application but also as it can give indications of interactions between the mGBs and the matrix during the MIM-process. Experimental ProceduresFor the experiments blends of 50 vol% Fe powder (d 50 ¼ 1.4 mm, Fritsch) were mixed with polymers and waxes (binder). Then up to 13 wt% 3 M-S60HS (d 50 ¼ 35 mm) and 3 M-IM30 K (d 50 ¼ 17 mm) borosilicate glass bubbles were added and the mixture was injection moulded. For the injection a mould with tensile test specimen geometry was used. After moulding the binder was removed from the samples by a solvent (hexane, 12 h at 35 8C) and a thermal debinding step (0.