PRMs, Particle Reinforced Composites
Titanium alloys exhibit high specific strength and stiffness that fit structural lightweight applications. However ceramic reinforcements like SiC, B4C, TiC, TiB, TiB2 can improve specific stiffness and also wear resistance.
But the preparation of Ti-SiC by conventional powder metallurgical methods like mixing and sintering, SPS or hot pressing gives high porosity and pronounced silicides formation. Hot extrusion of powder mixtures performed out to be the best suitable process ending in negligible reactions between Ti and SiC and full densification.
ODS-Ag Contact Materials
Contact materials have to withstand extreme conditions due high loads of thermal and electrical power and must therfore guarantee a bundle of important properties like high resistance to arc erosion, resistance against welding, high elevated temperature-hardness and low creep rates. Silver based ODS-materials can fullfil those demands, as the matrix contributes the high thermal and electrical conductivity and the oxides donate to high temperature stability of the microstructure. Materials are produced by internal oxidation of Ag-matrix RE-metal or light metal alloys and subsequent oxidizing heat treatment of thin sheet materials for several hours.
Alternative processing routes are also under evaluation like co-precipitation of aqeous Ag-solutions together with aqeous RE-metal solutions to form composite powders after the addition of suitable precipitation or reducing agents. The resulting powders can be easily handled and must be furthermore heat treated under oxidizing conditions to establish the RE-oxides formation. The advantage lies in the enormeous reduction of the heat treatment times compared to the internal oxidation processing.
WC/Ag Contact Materials
Composite materials based on tungsten carbide and silver (WC/Ag) are well known and widely used electrical contact materials. Most commonly infiltration methods are used for production as they are superior to mixing-pressing-sintering. Nevertheless, infiltrated WC/Ag shows certain inhomogeneities and more or less porosity. Moreover, the silver content and particle size of tungsten carbide is limited. Therefore new methods were investigated applying electroless deposition techniques. Aqueous solutions containing different complexing agents for silver, such as ammonium hydroxide, benzoic acid and ammonium acetate have been tested. It was found that the use of aqueous suspensions of tungsten carbide in solutions of ammonium hydroxide and ammonium bicarbonate shows best coverage. The silver coated tungsten carbide powders were consolidated by uniaxial cold pressing and liquid phase sintering. The sintered WC/Ag had distinct higher homogeneity than infiltrated products.
Research in this field is performed in cooperation with AC2T, the Austrian Competence Center for Tribology, Wr. Neustadt, Austria and focusses on the preparation of fused tungstencarbide composites in Ni and Fe-based matrix by sintering and by plasma arc transfer welding (PTA). The work concentrated on the study of reaction between the constituents and to prevent it by using diffusion barrier layers.
ODS-Pt and ODS-Pt-alloys (e.g. ODS-PtRh10) can be produced either by powdermetallurgical techniques like high energy ball mixing/milling of sub-µm and µm-range Pt-, or Pt-alloy powders together with nm-scale oxides like Y2O3 or ZrO2. The metal powders are produced by wet chemical means (see focus of research of recovery of valuable metals) and are subsequently consolidated by degassing and hot isostatic pressing.
Alternatively a homogeneous mixture of precious metal powders and dispersoids can be produced by a simultaneous precipitation of Y-, or Zr-compounds together with Pt directly from aqueous Pt-solutions followed by the subsequent thermal conversion to the oxides and the decomposition of the precious metal salt to the metallic state.
A different method produces the oxides by internal oxidation of Pt-Y, Pt-Zr, resp. complex Pt-alloys (e.g. Pt-La-Y, Pt-Zr-La-Y or Pt-Hf-La... alloys). Investigations concern the influences of composition, temperature, time, atmosphere on the formation and the size of the reinforcements for the internal oxidation method. Materials following the PM-method are characterized by their mechanical and thermal properties (creep behaviour).