Papers by Author: Michael M. Gasik

Abstract: Bioimplants composed of metal and ceramic parts are recently widely used in medicine. It was shown that properties of these implants might be substantially improved with functionally graded materials (FGM). In this work ceramics FGM are considered with emphasis on the optimal processing technique. For ceramic balls and liners of total hip replacement prosthesis optimal sintering procedure is very important irrespectively on the initially selected graded composition profile to get beneficial stresses distribution after sintering and assembling. Different examples are shown and influence of the sintering and gradient profile selection on final properties is discussed.

Abstract: For FGM thermal barrier coatings thermal-mechanical fatigue due to temperature cycling is one of the critical parameters which determine the lifetime of the TBC. In Tohoku University, a method known as “hot burner test” has been developed and was implemented into the standard JIS H 7851. For further optimization of the testing conditions and selection of the parameters for comparison of different test results knowledge of temperature and heat flux inside the test jig is necessary. FEM analysis of the heat flux and temperature inside the 14 mm diameter test stainless steel jig has been performed. Homogenous YSZ top coat with NiCrAlY bondcoat as well as FGM TBC with different gradation profiles were compared. Distributions of the temperature and heat flux are critical at the edges of the test piece and thermocouples locations, which must be taken into account when comparing test data for coatings with rather different composition and structure. The largest stress gradients develop at the ceramic-metal interface (duplex) and near the edges of the TBC.

Abstract: FGM thermodynamics has been mostly based on adaptation of classical Gibbs-Helmholtz approach for infinite systems to locally “homogeneous” zones. A statistical sum calculation in this theory cannot predict inhomogeneous distributions. A new approach to the statistical description of solid solutions is suggested, which takes into account possible formation of spatially inhomogeneous simultaneous particle and field distributions in finite space domains. The formation of new periodical or gradated structure in binary system is described. The effective free energy of system was determined and the condition of formation of such spatially inhomogeneous distribution of interacting particles was obtained. New method may be applied to FGM to calculate ab initio free energy of these systems without usual limitations of classical theory.

Abstract: Computer-integrated safe design of FGM component for hip replacement prosthesis was presented based on principle of optimal stresses distribution in the FGM component. The goals were to create an FGM structure with reasonable compressive stresses on the surfaces (subjected to wear) and to keep them during the entire manufacturing cycle (sintering, machining, assembling and application). The residual stresses developed in the different parts of the composite were implemented into the subsequent process of the surface grinding and assembly to simulate properly the whole processing route to ensure optimal combination of the processing parameters.

Abstract: Homogeneous and FGM environmental barrier coatings (EBC) made of alumina – NiCr on the steel substrates by high-velocity oxygen fuel (HVOF) spraying technique were studied. At room temperature, these coatings were characterized by DC polarization measurements and by electrochemical impedance spectroscopy (EIS) after various exposure times in highly aggressive basic solutions. At high temperatures (400-750°C) corrosion resistance was tested in respect to sulfide – sulfate – chloride – carbonate melts in air, which simulates environments in incinerators, kraft recovery boilers and turbines. After testing, the samples were examined by SEM and EDS and their corrosion resistance was compared.

Abstract: New functionally graded Al2O3/Y-ZrO2 ceramics are developed as a new material for production of hip prosthesis, especially all-ceramic hip bearings. The used concept of graded ceramics is expected to provide better long-term performance based on improved mechanical properties and lower clinical risks. Mechanical properties are strongly dependent on the distribution of residual stresses resulting mainly from phase specific stresses after cooling from the sintering temperature. However, these stresses could be optimized by an appropriate selection of a concentration profile of constituent phases. The non-destructive neutron diffraction mapping of residual stresses has been used to investigate this problem experimentally. In addition, the smallangle neutron scattering technique was applied to study the porosity in the ceramics as function of the production parameters.

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.