Materials Science Forum Vols. 492-493

Abstract: This paper gives a short overview of tests applied for the investigation of long term behaviour of thermal barrier coating systems. A variety of tests has been conducted on an exemplary material system with the coatings applied by electron beam physical vapour deposition. Damages and damage evolution in different tests are compared. Since the observed damage mechanisms are different, it is proposed to design laboratory tests as realistic as possible, especially if the test data are used for lifetime assessment. In order to get reasonable testing times, the damage accumulation has to be described as a function of loading history, long time before failure. For the case of final failure by spallation of the ceramic top coat, it is proposed to use the apparent interfacial fracture toughness as damage parameter. Several methods for measuring the apparent fracture toughness of brittle coatings are discussed with respect to their application to thermal barrier coating systems.

Abstract: Application of FGM concept for thermal barrier coatings (TBC) provides a superior thermal stress relaxation over homogeneous or duplex coatings. It was demonstrated that FGM TBC have better oxidation resistance and longer lifetime during test conditions. In this work, new FGM TBC system was designed using high-velocity oxygen flame (HVOF) coating process. After coatings optimisation, coating layer was subjected to a hot burner test for thermal fatigue cycling with increasing heat load. It was found that FGM TBC has successfully withstood thermal cycling and prevented visible delamination or transverse cracks. New coating design may give an opportunity to develop a cost-effective FGM TBC system for gas turbine applications.

Abstract: Electrically conductive and wear resistant Si3N4-based composites were developed in order to facilitate electrical discharge machining (EDM). The microstructural and mechanical properties of Si3N4-based composites with different amounts of TiC0.5N0.5, fabricated by hot pressing at 1650°C for 1 hour, are investigated and evaluated. The hardness of the micron-sized TiC0.5N0.5 powder based composites increased with increasing TiC0.5N0.5 content from 20 up to 40 vol. %, whereas the bending strength decreased. The fracture toughness reached a maximum at 30 vol. % TiC0.5N0.5 and exhibits a strong anisotropy with respect to the hot-pressing direction. The EDM behaviour of the composites is strongly influenced by the TiC0.5N0.5 content. The composites with a higher TiC0.5N0.5 content have a lower material removal rate but a better surface quality.

Abstract: Functionally graded aluminium matrix composites reinforced with SiC particles are attractive materials for a broad range of engineering applications whenever a superior combination of surface and bulk mechanical properties is required. In general, these materials are developed for the production of high wear resistant components. Also, often this kind of mechanical part operates in the presence of aggressive environments, such as marine atmospheres. In this work, aluminium composites with functionally graded properties, obtained by centrifugal cast, are characterised by reciprocating pin-on-plate sliding wear tests against nodular cast iron. Three different volume fractions of SiC reinforcing particles in each functionally graded material were considered. Sliding experiments were performed with and without the presence of a lubricant (3% NaCl aqueous solution). In the case of the lubricated tests, electrochemical parameters (corrosion potential) were monitored during sliding. Friction values were in the order of 0.42 for unlubricated conditions, but varied between 0.22 and 0.37 when the aqueous solution was present. For all test conditions, relatively high wear rates (over 1×10-6 gm-1) were obtained, particularly for the cast iron pin. The volume fraction of SiC particles exerted a net effect on the tribological response of the composites, although conditioned by the presence or absence of the aqueous solution. The worn surface morphology of the composites indicated that the presence of the aqueous solution modifies the protective action promoted by the combined effect of the presence of reinforcing particles as load bearing elements and the formation of adherent iron-rich tribolayers. The evolution of the corrosion potential during the sliding action is in accordance to the degradation mechanisms proposed for these systems.

Abstract: Highly porous open- cell materials on the base of various metals and alloys are of increasing interest as they combine structural and functional properties. There is a wide range of possible applications for such materials, e.g. as heat exchangers, filters or catalysts. A new and promising method to produce open- cell metallic foams on base of iron powder, low and high alloyed steel powders as well as nickel alloy powder is the SlipReactionFoamSintering (SRFS)- process. In comparison to other production processes of metallic foams, the SRFS- process provides several advantages: foaming at room temperature, allowing a very good process control by various parameters, foams of a great variety of metals are possible and a broad spectrum of properties is achievable.

Abstract: The influence of grinding surface stresses in Y-TZP doped with 2.5% molar yttria on the contact loading response has been studied. It is shown that by grinding with different pressures compression residual stresses are induced in a surface layer of about 10 µm in depth. Their effect is manifested by shorter indentation cracks in ground specimens, but they recover the original lengths of annealed specimens after a surface thin layer is removed by polishing. The response to cyclic contact loading with a ball of 1.96 mm in diameter has been studied by looking to the damage that takes place below the indenter, as well as by measuring the flexure biaxial strength after 1, 103 and 105 cycles and for different load amplitudes. It is found that there is a threshold load, that is, a maximum contact load below which there is no degradation in the strength. This threshold is lower when transformability is higher and it depends on the number of contact cycles. But after a given large number of cycles, no further important additional degradation takes place. Under repeated contact loading in annealed specimens at loads for which a marked degradation in strength is detected, nucleation and growth of like-radial cracks is the main cause for the loss of strength, while for of ground specimens the extension of these defects is impeded by the compression surface residual stresses.

Abstract: The aim of the research project described here is to increase the durability of components under cavitation erosion by electrodeposition of gradient layers. Parallel to the material development, a simulation tool is elaborated to determine the optimal gradient layers for improved load resistance. This helps in minimising the number of prototype layer systems to be manufactured within the project. The initial experiments were executed with the nickel-phosphorus system, which is well-known for its favourable properties regarding hardness, corrosion resistance and tribology. Single and gradient layers are characterised and evaluated in view of the following properties: hardness, internal stresses, ductility, coefficient of friction, wear resistance, elastic modulus and tensile strength.