Thomas Schubert

Effects of high energy milling on densification behaviour of Mo–Si powder mixtures during pressureless sintering

Abstract The present paper reports on basic investigations of the controlled reaction sintering of MoSi 2 -based materials, including particle reinforced composites. It is shown, that certain amounts of pre-formed silicide phase in the powder mixtures are useful to achieve high densities by pressureless sintering of green compacts. This partial phase formation of αMoSi 2 can already take place during the mechanical treatment of the elemental powder mixtures by milling. In this context the role of volume changes linked with the phase formations is discussed.

Fabrication and Properties of Copper/Carbon Composites for Thermal Management Applications

The ideal thermal management material working as heat sink and heat spreader should have a high thermal conductivity combined with a reduced and tailorable thermal expansion. To meet these market demands copper composites reinforced with diamond particles were fabricated by a powder metallurgical method (powder mixing with subsequent pressure assisted consolidation). In order to design the interfacial behaviour between copper and the reinforcement different alloying elements, chromium or boron, were added to the copper matrix. The produced composites exhibit a thermal conductivity up to 700 W/mK combined with a coefficient of thermal expansion (CTE) of 7-8 x 10-6/K. The copper composites with good interfacial bonding show only small decrease in thermal conductivity and a relatively stable CTE after the thermal cycling test.

Rapidly solidified Fe-base alloys as electrode materials for water electrolysis

Abstract Amorphous ribbons of the systems Fe60Co20Si10B10 and Fe50Co20V10Si10B10 have been prepared via melt spinning to be used as electrode materials in alkaline water electrolysis. It turned out that the gas atmosphere during melt-spinning has a significant impact on the surface morphology and microstructure of the ribbons. Furthermore, a comprehensive characterization of the microstructure depending on the peripheral speeds of the cooling wheel was conducted. Moreover, the transition from the amorphous to the crystalline state was investigated via differential scanning calorimetry measurements. Preliminary electrochemical investigations suggest a noticeable improvement of the electrocatalytic activity of the melt-spun amorphous ribbons after an anodic–cathodic pretreatment.

Silver/Diamond Composite Material - Powder Metallurgical Route and Thermo-Physical Properties

To meet the need of high-performance thermal management materials in the field of electronic applications, heat sink materials reinforced with synthetic diamonds have been prepared via powder metallurgy. A matrix of a silver alloy with a silicon content of 0.45 wt.% was chosen out of the prediction of the thickness of a final carbide layer of about 180 nm. The volume content of the diamonds and the diamond size were kept constant. The mixed powders were consolidated by Spark Plasma Sintering (SPS) using different sintering temperatures between 800 and 870 °C with a holding time of 30 min. The maximum thermal conductivity of 680 W/(mK) measured at room temperature and 620 W/(mK) at 275 °C was obtained at 810 °C sintering temperature. The degradation of the most promising sample after one thermal cycle up to 275 °C was determined below 1 percent of the value after sintering.

Efficient Processing of Metal-Matrix-Composites by Combining Direct Pressure Sintering and Subsequent Thixoforging

The paper describes a cost effective and innovative combination of direct pressure sintering and subsequent thixoforming to produce MMC-components in (near-) net shape quality and, thus, to make these interesting materials attractive to mass production.First results of some combinations of aluminium matrix alloys with different ceramic reinforcements, consolidated by fast pressure sintering show the efficiency of this technology.The further processing of the consolidated billets has been performed by thixoforging. It can be shown, that the homogeneous microstructure from the direct pressure sintering stage with uniformly distributed ceramic reinforcements can be maintained over the semi-solid state and a full densification can be achieved. Form filling was complete and surface quality was comparable to forgings from conventional alloys.This new process flow shows advantages regarding the material yield in each of the processing steps. The (near-) net shape quality of thixoforged components allows a reduced effort for machining, which is of special importance for composite materials with a high content of wear resistant hard phases like SiC-particles.

Metal/FRP Connection Module – A Powder Metallurgical Approach

The Development of Fiber Reinforced Plastics (FRP) offers a great opportunity for applications in automobile industry, aeronautics and consumer goods to achieve light weight structures. However, the connection technology between FRP and mainly metallic based structures is the key to use the full potential of the FRP. Out of this motivation recent developments address this aspect.Using the powder metallurgical approach to generate a metal/ FRP connection module by spark plasma sintering a great variety is possible by integration of different metal and/ or fiber components. In this work aluminum and stainless steel was chosen for the upper and lower metallic side. The fibers integrated into the metal were glass, basalt and carbon fiber in one layer, two layer and mixed layer configuration. To connect the sintered module to greater CF weaves an infiltration process with a room temperature curing resin was used in a modified vacuum infusion (MVI) setup. In not optimized configuration the shear test after infiltration indicated an initial value for module shear strength above 20 MPa which can be enhanced in future developments by optimized armor between the upper and lower metal side and the number of integrated fiber layers of the connection module. A model is predicted to calculate the module shear strength in sintered state by multiplication of the armor area with the shear strength of the armor material. First experiments additionally show the possibility to weld the connection module directly to metallic structures.