Progress update on failure mechanisms of advanced thermal barrier coatings: A review

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Processing and design methodologies for advanced and novel thermal barrier coatings for engineering applications

Effect of substrate roughness induced by grit blasting upon adhesion of WC-17% Co thermal sprayed coatings

Substrate melting during thermal spray splat quenching

Metal and ceramic materials were sprayed on polished steel and iron substrates using atmospheric arc (nickel, molybdenum) and plasma (Al 2 O 3 , ZrO 2 -7Y 2 O 3 ) spraying. The bonding quality at the substrate-coating interface depends on the size and depth of the contact zones. The chemical-metallurgical interactions (diffusion, reactions) between the sprayed particles and the metal substrate as a function of the properties of the sprayed materials, contact temperature and solidification time were analysed. Higher contact temperature and longer interaction time for spraying of metals result in better bonding. Moreover, oxidation of the metal substrate to a limited degree by spraying of oxides allows one to improve the bonding quality. It is possible to correlate the results of microscopical investigations of the interface phenomena with the results of quantitative measurement of the bonding tensile strength.

Interface phenomena and bonding mechanism of thermally-sprayed metal and ceramic composites

Diese Arbeit liefert einen Beitrag zur Beschreibung der Haftmechanismen von metallischen und keramischen Spritzschichten. Dazu wurden die Spritzwerkstoffe auf Armco-Fe und St37 atmospharisch lichtbogen- (Ni, Mo und Cu) und plasmagespritzt (NiCr-Al, Al2O3, ZrO2-7Y2O3). Morphologie, Mikrostruktur und Fliesverhalten der Spritzteilchen auf polierten Substratoberflachen werden als Funktion der Eigenschaften der Partikel (z.B. Temperatur, Warmeinhalt, Grose, Geschwindigkeit, Oberflachenspannung und Viskositat) und des Substrates (Zusammensetzung, Temperatur, Oberflachenbeschaffenheit u.a.) herangezogen. Die Erkenntnisse zum Haftmechanismus der Spritzteilchen lassen sich am Beispiel von Nickel mit Ergebnissen zur Haftzugfestigkeit der Spritzschichten korrelieren.

Grenzflächenphänomene und Haftung bei thermisch gespritzten Verbundwerkstoffen

This paper reports on the use of Auger electron spectroscopy (AES)/ depth profile analysis for the investigation of plasma-sprayed coatings. Prior to spraying the St 37 substrates are heated to 300 °C or 500 °C for ceramic or metallic layers, respectively. Studies of the starting materials and of the interfaces are important if the adhesion mechanism is to be understood. Therefore the initial components—the unheated and heated substrates and the powder particles NiCrAl, Al2O3 and ZrO2-7.25Y2O3—are analyzed. Depth profiles obtained from two coatings St 37/NiCrAl and St 37/Al2O3 show the influence of plasmaspraying on substrate surfaces and sprayed particles. Plasma-spraying mainly causes a decrease of superficial carbon contamination for both coating layers. In the case of St 37/NiCrAl incorporation of carbon in the sprayed layer is observed. The whole layer is almost completely oxidized except for some areas where substrate and particle material are present. It is assumed that these areas are identical with so-called adherence zones.

Contribution to the analytical characterization of coatings produced by thermal spraying

Composites are materials with an increasing importance for the industry. A newly developed method for fabrication of fibre-reinforced metal matrix composites permits the production of composites with metal or ceramic fibres by thermal spraying. An improvement of coatings properties and the removal of flaws by post treatment is possible. This paper presents the thermal treatment by laser beam, electron beam and hot isostatic pressing (HIP) process and the mechanical treatment by shot peening.

Thermal spraying of composites—Manufacturing and post treatment

Siliziumnitrid (Si3N4 · Y2O3) wurde in verschiedenen Starken mit einem 1500 W CO2− Laser getrennt. Die beim Schneiden mit unterschiedlichen Bearbeitungsgasen (Sauerstoff, Luft, Stickstoff und Argon) auf der Schnittflache entstandenen festen Reaktionsprodukte wurden rasterelektronen-, lichtmikroskopisch und rontgenographisch untersucht. Weiterhin wurde das Festigkeitsverhalten getrennter Keramik im 4-Punkt-Biegeversuch gepruft, um den Materialschadigungsgrad zu bestimmen. Die durchgefuhrten Parameterstudien belegen, das die besten Trennergebnisse bezuglich der Biegefestigkeit und der Schnittflachenbeschaffenheit nur mit auserst kurzen Laserpulslangen sowie mit Sauerstoff als Bearbeitungsgas zu erzielen sind.



CO2-Laser Cutting of Silicon Nitride - Possibilites and Limits



Silicon nitride (Si3N4 · Y2O3) in different thicknesses was cutted with a CO2− laser beam (1500 W). The reaction products on the cut surface by cutting with oxygen, air, nitrogen and argon as processing gases have been analysed by scanning electron microscopy, optical microscopy and x-ray. Moreover, the 4-point bending strength of cutted ceramics was examined to determine the grade of material damage. The parameter studies show that the highest bending strength and the optimal surface structure appear by cutting of silicon nitride with extremly short laser pulses and only with oxygen as processing gas.

J. Drozak

Papers

Interface phenomena and bonding mechanism of thermally-sprayed metal and ceramic composites

Grenzflächenphänomene und Haftung bei thermisch gespritzten Verbundwerkstoffen

Contribution to the analytical characterization of coatings produced by thermal spraying

Thermal spraying of composites—Manufacturing and post treatment

CO2‐ Laserschneiden von Siliziumnitrid – Möglichkeiten und Grenzen