Robert Vassen

Spray-drying of ceramics for plasma-spray coating

The spray-drying process of ceramics which are candidate materials for thermal barrier coatings (TBCs), i.e. 3YSZ+0, 2, 4, 6 wt.% Al2O3, is discussed in this paper. The two most important properties of spray-dried powders to determine the coating quality are density and particle size. Polyethyleneimine (PEI) acts as both an organic binder and a dispersant giving low viscosity in the suspension. The optimised suspension composition is: ⩾ 33.6 vol.% powder+1.8 wt.% PEI+ethanol, and operational parameters of the spray-dryer: drying temperature 175°C, feeding rate 55 cm3/min, feeding pressure 1.013×104 Pa.

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Hot hardness and creep of Si3N4SiC micro/nano- and nano/nano-composites

Abstract In the present study, mixtures of conventional α-Si3N4 with β-SiC or nanosized β-SiC and ternary Si-C-N (15–20 nm) powders without and with Y 2 O 3 Al 2 O 3 as additive were used as starting materials. By Hot Isostatic Pressing of these powders, micro/nano-composites and nano/nanocomposites were obtained. Dynamic hot hardness at 800, 1000 and 1100 °C and creep behavior at 1600 °C for pressures of 50, 100 and 150 MPa were used to characterize the materials. The influence of grain size and SiC content on these properties is discussed.

Metal-glass based composites for application in TBC-systems

A new type of thermal barrier coating (TBC) based on metal-glass composite (MGC) consisting of an ordinary container glass and a NiCoCrAlY-alloy has been recently presented. This TBC material provides the possibility to easily adjust its thermal expansion coefficient to match the substrate by changing the metal to glass ratio of the composite. Vacuum plasma spraying (VPS) has been applied as a possible technologies for deposition of MGC coatings. Isothermal oxidation tests were carried out in air at temperatures of 950, 1000, and 1050 °C, respectively. Thermal cycling tests were carried out by applying a temperature gradient across the sample thickness by heating with an open flame of natural gas followed by removal of the burner and air cooling. Changes in the microstructure were examined by means of microscopy, microanalysis, and x-ray powder diffraction. For long-time annealing at high temperatures, a progressive degradation of the glass matrix as well as oxidation of the metal phases cannot be fully suppressed up to now. By lowering the effective temperature at the MGC layer when used as an intermediate layer, the degradation of the MGC can be reduced without losing its advanced features with respect to creeping and gas-tightness. Additional concepts for improved oxidation resistance of the MGC based on suitable heat treatments and on alternative glass compositions have been developed, and primary results are shown. Evaluation of results from isothermal oxidation experiments and from thermal cycling in burner-rig facilities validates a clear improvement of the lifetime of the coatings compared with earlier results.

Heat treatment of ultrafine SiC powders to reduce oxidation sensitivity and grain growth

Abstract Ultrafine SiC powders of 20 nm were synthesized by a CO 2 laser induced reaction between silane and acetylene followed by heat treatment at different temperatures and in different atmospheres. These powders were examined by TGA, IR-spectroscopy, XRD and also densified by HIP. Results of different oxidation behavior of the powders are presented. Densification of the different heat treated powders lead to different grain sizes in the dense body. Heat treatment of ultrafine SiC in nitrogen improves oxidation resistance of the powders and reduces grain growth during densification significantly.

Manufacturing of Composite Coatings by Atmospheric Plasma Spraying Using Different Feed-Stock Materials as YSZ and MoSi2

AbstractYttria-stabilized zirconia (YSZ) is the state-of-the-art material for the top coat of thermal barrier coatings. To increase the efficiency and lifetime of gas turbines, the integration of MoSi2 as a healing material was proposed. A new method of manufacture was explored in order to enable the spraying of a homogeneous mixed layer of YSZ and MoSi2. As the chemical and physical properties of these powders are very different, they require contrasting process conditions. Due to the evaporation of Si from MoSi2 at spraying conditions suitable for YSZ, more moderate conditions and a shorter time of flight are required for depositing MoSi2. At the same time, the spraying conditions still need to be sufficient for melting the YSZ particles in order to produce a coating. To obtain a homogeneous mixture, both conditions can be matched using an injection system that allows powder injection at two different locations of the plasma jet. Two-color pyrometry during flight (DPV-2000, Tecnar) was used to monitor the actual particle temperature. By optimizing the injection point for the MoSi2, a mixed coating was obtained without decomposition of the MoSi2, which has been analyzed by means of XRD and SEM.

Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders

Abstract The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O2 and Ar-4%H2-2%H2O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H2–H2O than in Ar–O2. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H2/H2O containing gas.

Special Focus on Next Generation Coatings for Gas Turbines-1 (Editorial)

This special issue of the Journal of Thermal Spray Technology (JTST) includes selected papers on Next Generation Coatings for Gas Turbines. This current issue includes 9 papers, and additional papers on this topic will appear in other regular issues. Thermal spray coatings are vital to aircraft engines as well as large-scale industrial gas turbines (IGTs) for energy generation. With the introduction of silicon-based ceramic matrix composite (CMC) hot-section components to reduce fuel consumption in aircraft engines, thermal spray environmental barrier coatings are critically needed to protect CMC components from silica volatilization and surface recession in water–vapor containing combustion gas. Next generation IGTs will have approximately 65% combined efficiency with turbine inlet temperatures on the order of 1700 C, which will require durable and phase stable thermal-sprayed coatings. This special issue contains the most recent advances in thermal spray technology for turbine applications, including thermal barrier coating materials and architecture, oxidation and corrosion resistant coatings, abradable, robust coating manufacturing, and advanced thermal spray techniques, including, for example, suspension plasma spraying. The Guest Editors would like to express their sincerest gratitude to the reviewers who provided invaluable feedback and to the authors who submitted manuscripts.

JTST Special Issue on “Coatings for Energy Applications”

The supply of energy at affordable prices is a key-requisite for all modern societies. Especially electricity plays a dominant role and has to be produced in an efficient and environmentally friendly way. With the proven, detrimental impact of increasing human CO2 emissions on the climate world-wide activities have been initiated to reduce these emissions by energy conversion processes with improved efficiency or by avoiding them using renewable energy sources as wind, water, sun light, or biomass.

Analysis of the Microhardness and the Porosity in Graded Thermal Barrier Coatings

Thermal barrier coatings (TBC’s) are used in a number of energy-related applications, such as protective layers in aero and land based gas turbine components at very high temperature.