Pavel Gejdoš

Degradation of YSZ/EUCOR TBC Coating System during High Temperature Low Cycle Fatigue Tests

Thermal barrier coatings are widely used to protect the substrate from high temperature and extremely aggressive environments in gas engines. In the present article, authors have been studied degradation of complex thermal barrier coating system deposited on polycrystalline nickel superalloy IN 713LC. The substrate material was grit blasted with alumina (Al2O3) particles prior to air plasma deposition of CoNiCrAlY bond coat. Top coat consists of conventional zirconia (ZrO2) stabilized by yttria (Y2O3) -YSZ ceramic in combination with a eutectic nanocrystalline ceramic Eucor made of zirconia (ZrO2), alumina (Al2O3) and silicia (SiO2) –in the ratio of 50/50 in wt. %. The top coat was deposited using water stabilized plasma. Test specimens with the TBC coating system were fatigued under strain control condition in fully reversed symmetrical push-pull cycles at 900°C in air. The microstructure of TBC was characterized with scanning electron microscopy and energy dispersion X-ray analysis. The coating hardness and thickness were measured. Fracture surface and polished sections parallel to the specimen axis were examined to study damage mechanisms in coatings under cyclic loading at high temperature. TBC delamination was observed at the top coat/bond coat interface after cyclic loading at high temperature. Fatigue crack initiation sites are documented. Majority of fatigue cracks start from the surface and top coat/bond coat interface.

Tricalcium Phosphate - Magnesium Interface: Microstructure and Properties

The fabrication of a composite material based on magnesium (Mg) and tricalcium phosphate is reported in this work. Rods of β-tricalcium phosphate (β-TCP) were processed and consolidated together with pure Mg powder through spark plasma sintering (SPS). The microstructure at the interface, the chemical composition and transformation of the components and the microhardness were analysed. The microstructure of the composite shows two zones with well-defined and continuous interface between them: a ceramic zone composed by β-TCP filled with Mg and the metallic zone constituted by Mg and Mg rich eutectic. Vickers hardness shows the excellent mechanical interaction between the two zones.

High-Temperature Low Cycle Fatigue Resistance of Inconel 713LC Coated with Novel Thermal Barrier Coating

Inconel 713LC was developed in the 1950s and is still widely used in power generation especially because of favourable price in conjunction with satisfying properties. However, the need for higher efficiency of high-temperature facilities leads to increase operating temperature that causes severe degradation of the material. In order to enhance the life-time of material, the protective coatings are applied. For the purpose of this study, nineteen cylindrical specimens were cut from rods manufactured using investment castings technique and subsequently, 10 specimens were coated with novel complex thermal barrier coating (TBC) system. The TBC system comprises a metallic CoNiCrAlY bond coat (BC) and a complex ceramic top coat (TC). The TC is a mixture of conventional YSZ ceramic and a eutectic nanocrystalline ceramic Eucor in the ratio of 50/50 in wt%. Eucor is made of zirconia (ZrO2), alumina (Al2O3) and silica (SiO2). Low cycle fatigue tests were performed in symmetrical push-pull cycle under strain control at 900 °C. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves of coated and uncoated material were obtained. Fracture surfaces and polished sections parallel to the loading axis of specimens in as-coated conditions and after cyclic loading were observed by means of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to study degradation mechanisms during high-temperature low cycle fatigue. TBC delamination was observed at the TC/BC interface and rafting of precipitates occurred after high-temperature exposure. The microstructural investigations help discuss the differences in the stress-strain response and fatigue life of coated and uncoated superalloy.

Analysis of Causes of Fractures of Terminal Board Bolts

The paper sums up the results of a complex analysis, using metallographic, microfractograpic and other methods, with the aim of establishing the cause of fractures appearing in bolts of a terminal board. The analysis has shown unambiguously that the bolt failure was a synergic effect of inadequately performed heat treatment and hydrogenation that occurred in course of surface treatment via pickling and subsequent zinc plating.

Analysis of Damage of Solder Joint of Heat Exchanger

The paper focus on the metallographic analysis of damaged heat exchanger made of high‑alloy austenitic steel by soldering. The object in question is a soldered joint of main mounting plate of the heat exchanger and first heat exchanging plate of it. In this part of the heat exchanger after the vibration and pressure test crack appeared. The subject of the analysis is the evaluation of the microstructure of the solder joint (high-alloy austenitic steel and copper), and evaluation of the appeared crack. The problematic is solved with aid of metallographic analysis of the microstructure of the material, using light microscopy and scanning electron microscopy and the EDS microanalysis of chemical composition.

Failure Analysis of Corrugated Hoses

Stainless steel corrugated hoses are widely used in many areas of industry. They are used in application such as distribution of drinking and supply water, gas and also various working fluids, for example in cooling systems. During the assembly of AISI 316L corrugated hoses for cooling systems defects form due to deformation of the hoses during assembly. The absolute tightness of such systems is required and any defects of the hoses are therefore undesirable. This study deals with evaluation of most common defects of corrugated hoses via light and scanning electron microscopy and proposes measures to minimize such defects. It was revealed, that defects form due to deformation of the hoses during assembly or, in case of unfavorable microstructure, during the service life. In later case, failure of the hose was caused by higher content of deformation-induced martensite in microstructure as a result of intense cold forming. Component with such microstructure exhibited less favorable mechanical properties and hoses were more prone to the failure.

Influence of Microstructure on Machinability of Material and its Final Surface Quality

Mechanical properties, such as hardness, yield strength and tensile strength, are very often considered as the only key factor for selecting technological parameters of materials machining. Analysis of the microstructure and specific microstructural parameters is not performed until serious problems occur during otherwise common machining processes. This study reports a few case studies, dealing with such problematics, which resulted from underestimation of the knowledge of macrostructure and microstructure of material. It deals with problems such as uncommon surface defects after machining or impossibility to use current technological machining processes when machining new batch of material.

Causal Analysis of Damage of a Cover

In this article, the causes of damage of a cover were investigated by metallographic and fractographic analysis. The component part was made from non-alloy quality steel for cold forming DC04. The failure occurred during high temperature pulsation (90 °C, 7b, 1 Hz) after 180,000 cycles. This component part was fabricated by deep drawing. After this process’s step, outlets were soldered at 1100 °C/5 min. to this part and then whole component part was coated using method without any specification. The coating layer was formed of Zn-Ni. The last step in this process was hemming where all component parts were assembled together. This case study was solved using light and scanning electron microscopy. The chemical composition was detected by energy dispersive X-ray analysis.

Evaluation of Surface Degradation of Deoxidized AW-AlMg0.7Si Alloy

Corrosion of aluminium alloys due to chlorides is not rare in industrial practice. When detected, it is necessary to decide whether and how to remove the corroded areas and also to find the appropriate method for protection against further degradation. Appearance and quality of final surface is key factor in such cases. This study deals with final surface quality of aluminium alloy components and the extent of degradation after corrosion attack and subsequent chemical surface cleaning. Two AW-AlMg0.7Si alloy pipes, fabricated by two individual producers, were analyzed in order to reveal extent of atmospheric corrosion after the storage in the industrial area. Further analysis evaluated the influence of such degradation on final surface quality after surface cleaning by chemical way, using two different methods, namely deoxidizing and subsequent passivation. Special attention was paid to the influence of initial surface roughness on quality of final surface.

Influence of Microstructural Parameters on the Corrosion Resistance of 7075 Aluminum Alloy

The aim of this analysis was to identify the cause of poor corrosion resistance of final product, after the slight change of heat treatment of initial material. Although the hardness of material prior and after the change was the same, significant difference in microstructure was revealed by microstructural analysis. Change in heat treatment process led to heterogeneous distribution of precipitates and increase in the precipitate size which caused deterioration in corrosion resistance. Based on the findings, further optimization of heat treatment was suggested. More importantly, initial material inspection based on microstructural analysis, not only mechanical testing, was recommended.