Failure mechanisms of magnesia alumina spinel abradable coatings under thermal cyclic loading

Abradable coatings have been used in low- and high-pressure sections of jet engine compressors for more than 40 years. Today, they are also used in the high-pressure turbine of jet engines and are gaining more interest for applications in industrial gas turbines. They minimise the clearance between the rotating blade tips and the stationary liners. Aside from being abradable, the coatings have to be mechanically stable and withstand high thermo-mechanical loadings. A typical material used in engines today is yttria-stabilised zirconia (YSZ). This material advantageously combines a suitable thermal conductivity with a high thermal expansion coefficient, but shows a temperature capability limited to 1200 °C in long-term applications. Typical abradable coating thicknesses are above 1 mm. With increasing coating thickness and limited cooling efficiency leading to high surface temperatures, there is a risk of premature failure. As a result, new ceramic materials have been developed with better high-temperature capability. The present work investigates an atmospheric plasma sprayed ceramic double-layer coating system composed of 7YSZ as an intermediate layer and magnesia alumina spinel as a top layer.This double-layer system was sprayed onto disc-shaped Inconel 738 superalloy substrates, which were coated with a vacuum plasma sprayed MCrAlY bondcoat. The lifetime of the coating system was assessed via thermal gradient cycling testing with surface temperatures above 1400 °C. During cycling, the samples showed a typical failure mechanism with exfoliation of thin coating lamellae starting from the coating surface. This failure mechanism was not observed in thermal barrier or abradable coatings in the past. The failure mechanism was analysed and mismatch stress calculations were carried out.     

Effects of direct electric current and electrode reactions on vinyl chloride degrading microorganisms

The use of electro-bioremediation has gained increasing interest during recent years. In these hybrid technologies bioremediation is stimulated by electrochemical or electrokinetic techniques to increase pollutant biodegradation efficiency. It is a pre-requisite for successful application of the bio-electro-processes that the microorganisms are not negatively affected by the electric fields or electrode reactions.In this study, for the first time microbial activity of aerobic vinyl chloride (VC) degrading microorganisms was assessed after exposure to constant currents ranging from 0.04 to 14 mA cm⁻². Viability and degradation kinetics were monitored during electrolysis for 4 h using two different types of electrodes: stainless steel and dimensionally stable electrodes (DSA). When the mixed microbial culture was exposed in the electrode compartments, inhibiting effects were observed with stainless steel and DSA electrodes beyond doses of 100 and 50 kJ/L, respectively. Incubation of the VC degrading microorganisms in the mineral medium that was pretreated in the electrode compartments with similar doses, resulted in slower VC degradation kinetics thus demonstrating that the inhibition was due to electrochemical reaction products. When electrodes were separated from the microorganisms by bipolar membranes, no inhibition by the electric field was observed.     

Time-dependent performance evaluation for loss-waiting queues with arbitrary distributions

This paper presents an analytical approach to evaluate queues with time-dependent, generally distributed inter-arrival times, generally distributed service times, and finite buffer capacities. A stationary backlog carryover (SBC) approach is developed to analyse the probability of blocking and other time-dependent performance measures. We further improve the general SBC approach by the analysis of load-dependent period lengths used in the approximation. The numerical study shows that this approach is very accurate for both transient and time-dependent loss-blocking systems.     

Modified Hyaluronic Acid-Laminin-Hydrogel as Luminal Filler for Clinically Approved Hollow Nerve Guides in a Rat Critical Defect Size Model

Hollow nerve guidance conduits are approved for clinical use for defect lengths of up to 3 cm. This is because also in pre-clinical evaluation they are less effective in the support of nerve regeneration over critical defect lengths. Hydrogel luminal fillers are thought to improve the regeneration outcome by providing an optimized matrix inside bioartificial nerve grafts. We evaluated here a modified hyaluronic acid-laminin-hydrogel (M-HAL) as luminal filler for two clinically approved hollow nerve guides. Collagen-based and chitosan-based nerve guides were filled with M-HAL in two different concentrations and the regeneration outcome comprehensively studied in the acute repair rat sciatic nerve 15 mm critical defect size model. Autologous nerve graft (ANG) repair served as gold-standard control. At 120 days post-surgery, all ANG rats demonstrated electrodiagnostically detectable motor recovery. Both concentrations of the hydrogel luminal filler induced improved regeneration outcome over empty nerve guides. However, neither combination with collagen- nor chitosan-based nerve guides resulted in functional recovery comparable to the ANG repair. In contrast to our previous studies, we demonstrate here that M-HAL slightly improved the overall performance of either empty nerve guide type in the critical defect size model.     

Sequential reductive and oxidative biodegradation of chloroethenes stimulated in a coupled bioelectro-process

This article for the first time demonstrates successful application of electrochemical processes to stimulate sequential reductive/oxidative microbial degradation of perchloroethene (PCE) in mineral medium and in contaminated groundwater. In a flow-through column system, hydrogen generation at the cathode supported reductive dechlorination of PCE to cis-dichloroethene (cDCE), vinyl chloride (VC), and ethene (ETH). Electrolytically generated oxygen at the anode allowed subsequent oxidative degradation of the lower chlorinated metabolites. Aerobic cometabolic degradation of cDCE proved to be the bottleneck for complete metabolite elimination. Total removal of chloroethenes was demonstrated for a PCE load of approximately 1.5 μmol/d. In mineral medium, long-term operation with stainless steel electrodes was demonstrated for more than 300 days. In contaminated groundwater, corrosion of the stainless steel anode occurred, whereas DSA (dimensionally stable anodes) proved to be stable. Precipitation of calcareous deposits was observed at the cathode, resulting in a higher voltage demand and reduced dechlorination activity. With DSA and groundwater from a contaminated site, complete degradation of chloroethenes in groundwater was obtained for two months thus demonstrating the feasibility of the sequential bioelectro-approach for field application.