Karel Slámečka

Plasma-sprayed thermal barrier coatings: numerical study on damage localization and evolution

Thermal barrier coatings (TBCs) are advanced material systems used to enhance performance and in-service life of components operated at high temperatures in gas turbines and other power-generation devices. Because of complexity, numerical methods became important tools both for design of these coatings and for in-service life estimations and optimization. In this contribution, two main features that affect the TBCs’ performance, namely the roughness of the bond coat and the microstructure of the ceramic top coat, are discussed based on Finite Element Method (FEM) and Finite Element Microstructure MEshfree (FEMME) simulations that were used to calculate stresses and assess damage within the coating. Roughness data obtained from plasma-sprayed CoNiCrAlY + YSZ coated samples are supplemented to discuss assumptions and results of employed numerical models.

A Fractographic Study of Bending/Torsion Fatigue Failure in Metallic Materials with Protective Surface Layers

Results are given of a fractographic study of biaxial in-phase bending/torsion fatigue fractures in specimens made of nitrided steel and nickel-based superalloy with protective coatings (diffusion coatings and plasma-sprayed thermal barrier coatings). Fracture surfaces were examined by optical and scanning electron microscopes while stereophotogrammetry and optical profilometry were employed to obtain 3D surface data of selected fracture surface regions. The studied materials exhibited a wide range of fracture mechanisms depending on the microstructure and applied mechanical loading.

Critical Weak Points of Atmospheric Plasma Sprayed Thermal Barrier Coatings

The 8 wt. % yttria stabilized zirconia top coat (TC) and the CoNiCrAlY bond coat (BC) were sprayed onto the surface of newly developed fine-grained cast polycrystalline nickel-based superalloy Inconel 713LC by means of atmospheric plasma spraying (APS). As-prepared samples were isothermally exposed at the temperature of 1050 °C for 200 hours in an ambient atmosphere. Structural changes in the thermal barrier coatings (TBC) system after thermal exposure were studied by means of scanning electron microscope equipped with an energy dispersive microanalyzer. Critical weak points were identified on both the substrate-bond coat and bond coat-top coat interfaces.

Application of sacrificial coatings and effect of composition on Al-Al3Ni ultrafine eutectic formation

This paper introduces an unconventional method designed for forming hypereutectic alloys via coating deposition onto the substrate surface and subsequent heat treatment of such systems. The coating was produced from 99.7 wt% nickel powder by means of high velocity oxyfuel (HVOF) spraying onto the surface of 99.999 wt% aluminium sheet. The specimens were manufactured immediately after the spraying. Specimens were heat-treated using a differential thermal analysis (DTA) apparatus up to a temperature of 900°C and then cooled down to the room temperature in an argon atmosphere with constant heating and cooling rates, under which the NiAl3 intermetallic phase formed within the initial substrate. Two different alloy microstructures consisting of a coarse eutectic and an ultrafine well-dispersed eutectic were produced. The formation processes and resultant microstructures were studied by means of differential thermal analysis, metallography, scanning electron microscopy, energy dispersive microanalysis, and image analysis techniques.

Bending Fatigue Behaviour of Diffusion and Thermal Barrier Coating Systems

Room temperature fatigue behaviour of two variants of diffusion aluminide coatings and conventional air plasma sprayed CoNiCrAlY + ZrO2-Y2O3 thermal barrier system, which were deposited on a cast nickel-based supperalloy substrate, was studied under symmetrical bending loading regime. The study elucidates the differences in a fatigue crack initiation micromechanism with respect to applied stress level and its impact on the fatigue performance of studied coating systems.

Analysis of Fracture Morphology and Local Loading Modes in Torsional Fatigue

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.

Investigations of Wettability of Wear Resistant Coatings Produced by Atmospheric Plasma Spraying

Changes in fluids contact angle in the interaction with materials surface can play a critical role in enhancement of hydro-machine components and pipelines efficiency and/or service lifetime. However most nowadays used materials and/or coatings are made from polymers or ceramic polymer composites produced by highly sophisticated and/or very expensive techniques. Unfortunately there are a lack of mechanical properties. With the aim to study the role of the surface topography on the water contact angle changes, the representatives of wear resistant coatings (WC10Co4Cr, Cr2O3+5SiO2+3TiO2 and Al2O3) were produced by means of atmospheric plasma spraying. Wettability of the coatings surface was studied by adding the liquid droplet on as sprayed, grinded and polished coatings surface by measuring the changes of its contact angle. To estimate the coatings phase composition and topography XRD technique and optical profilometer were used. The contact angle of water was measured by sessile droplet method. To obtain the complex information of the cross-sectional coatings microstructure the conventional metallographic analysis approaches and optical microscopy were also used.

Powder Processing and Thermal Spraying of Barium-Magnesium-Aluminium-Silicate Environmental Barrier Coatings

As a potential candidate for the top coating in novel Environmental Barrier Coating systems, one representative of a Barium-Magnesium-Aluminium-Silicate family was produced in the form of the powder. Initial compounds were heat-treated to synthesize and the product was crushed in the ball mill device down to the fraction of 20 micrometers. In the next step, the atmospheric plasma spray (APS) technique was used to form a coating on a steel sheet substrate. The aim of this study was to obtain the most favorable technological parameters for the thermal spraying process and therefore two plasma spraying parameters for pure alumina or yttria stabilized zirconia, and other three experimentally designed ones were tested. Despite the same stand-off distance used for coatings manufacturing, thickness and porosity differed in order of tens of micrometers and several percent, respectively. Resulting coatings consisted of a mixture of amorphous and crystalline Al2O3, SiO2 and MgO phases.

Simple criterion for predicting fatigue life under combined bending and torsion loading

Multiaxial fatigue is a challenging problem and, consequently, a number of methods has been developed to aid in design of components and assemblies. Following the complexity of the problem, these approaches are often elaborate and it is difficult to use them for simple loading cases. In this paper, an empirical approach for constant amplitude, proportional axial and torsion loading is introduced to serve as a basic engineering tool for estimating fatigue life of rotational structural parts. The criterion relies on a quadratic equivalent-stress formula and requires one constant parameter to be determined from experiments. The comparison with similar classical stressbased approaches using data on diverse materials (several steels, aluminium alloy, and nickel base superalloy) reveals very good agreement with experimental data.

Analysis of Selective Laser Melting Process Parameters Effect on Mechanical and Material Properties for Stainless Steel 316L

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.