Papers by Author: Karel Slámečka

Abstract: Nowadays commonly used thermal barrier coatings (TBC) are based on yttria stabilized zirconia (YSZ). Addition of mullite phase into the YSZ coating can improve resulting high temperature properties. The contribution focuses on high temperature cyclic oxidation behaviour of two TBC systems with different top coats (TC) deposited by the means of atmospheric plasma spraying. The initial mullite-YSZ powder mixture consisted of 29 vol. % of mullite and 71 vol. % of YSZ. The conventional TBC system consisted of ~ 150 µm thick NiCoCrAlYHfSi bond coat (BC) and ~ 300 µm thick YSZ top coat. The experimental mullite-YSZ (MYSZ) TBC system consisted of ~ 150 µm thick NiCoCrAlYHfSi bond coat, ~ 100 µm thick YSZ interlayer and ~ 200 µm thick mullite-YSZ top coat. The experimental TBC proved higher lifetime, durability and phase stability and also lower grow rate of thermally grown oxide (TGO) compared to conventional TBC. Lifetime, phase stability and changes in the microstructure of TBCs after the furnace cyclic oxidation test were evaluated by the means of scanning electron microscopy equipped with EDX analyzer and X-ray diffraction techniques. Oxidation kinetics of TGO was calculated based on thickness determined utilizing digital image analysis.

Abstract: In order to increase the peripheral speed of grinding wheels of size of Ø 500 × 18 × Ø 200 mm for precise hard metallic surface finishing from conventional 80 m.s^-1 up to 138 m.s^-1 while still ensuring their safety and reliability, the critical locations in the grinding wheel were evaluated using the finite element analysis. The microstructure of grinding wheel was revealed using the materialographic techniques and the scanning electron microscope images were recorded in the back-scattered electrons mode. The image analysis was used on recorded micrographs for separation of individual material components, i.e. an abrasive, a binder, and pores, and to extract their geometries and spatial relationships. Subsequently, the influence of different filling agents (Young's modulus of 5, 10, 20, and 40 GPa) was studied, considering both surface dipping and bulk filling treatments.

Abstract: This paper deals with a testings of the Selective Laser Melting (SLM) process parameters on mechanical properties, material behavior, surface quality and on machinability of austenitic steel 316L. The SLM process conditions were changed to assess their effect on mechanical properties of the sintered material measured in tensile testing. The laser power, the scanning velocity of laser beam, the layer thickness and the laser focus plane were set to find the most appropriate set of process parameters to obtain the mechanical properties of parts.

Abstract: The main goal of this contribution is to investigate the influence of the substrate morphology on the resulting thermally sprayed coatings microstructure. Therefore, three different representative coating systems and/or thermal spray techniques were utilized to produce the coatings on grid-like structure substrates: (i) CoNiCrAlY bond coat (BC) sprayed by high velocity oxygen fuel (HVOF) technique and yttria stabilized zirconia (YSZ) top coat (TC) sprayed by means of atmospheric plasma spray (APS) technique, (ii) YSZ coating sprayed by means of APS and (iii) YSZ coating sprayed by means of nanoparticle colloid suspension plasma spraying (SPS). The shadowing effect of thermal spray coatings in relation with the grid-like substrate structure was investigated in detail. Resulting microstructure of sprayed samples was studied utilizing light microscopy, digital image analysis, scanning electron microscopy, energy-dispersive spectrometer and X-ray diffraction techniques.

Abstract: Asymmetric arrangements of stored crack-wake dislocations and low values of the size ratio SR, the plastic zone size / the characteristic microstructural distance, were found to be of basic importance for the shear misfit of crack flanks causing the roughness-induced crack closure in case of plain strain conditions. The crack wake dislocations produce also the plasticity induced crack closure as a result of a near-tip mismatch perpendicular to crack flanks. According to a recently published theoretical concept, an estimation of these extrinsic shielding effects in the threshold region of fatigue crack propagation was made for austenitic steel of Japan provenience. Related fatigue experiments were based on a standard load shedding technique associated with monitoring of the crack closure level. The surface roughness was analysed by means of the optical chromatography that enables a 3D reconstruction of fracture morphology. Calculated and measured effective threshold values of about 2.2 MPa.m1/2 are practically identical. Total levels of the extrinsic toughening induced by the austenitic microstructure are rather low when compared to those identified in ferritic- and ferritic-austenitic steels.

Abstract: High fatigue threshold values of duplex ferritic-martensitic steels are interpreted by using a unified model of roughness- and plasticity induced crack closure. Complex metallographical and fractographical analysis was performed in order to obtain characteristics of tortuous crack paths produced by crack deflection and branching mainly at austenite/ferrite interfaces. Calculated values of effective thresholds are in a good agreement with experimental data. The total level of extrinsic toughening (closure + shielding) induced by the duplex microstructure was determined to be as much as 70% of measured fatigue threshold values. This is the main reason for the high resistance to propagation of long fatigue cracks in the near-threshold region.

Abstract: The paper deals with a profile analysis of the fracture surface generated by pure cyclic torsion. Roughness and fractal characteristics studied in two mutually perpendicular directions were found to be sensitive to the crack front position. The investigation of surface topography revealed the presence of the opening Mode I in all investigated crack front locations.