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

Metallic and ceramic materials were sprayed using atmospheric arc (Ni and Mo on mild steel and Fe) and plasma (Al2O3 on NiCr20 substrate) spraying techniques. The aim of the investigations was to analyse the interfacial phenomena and particle microstructure and morphology. The phase composition of sprayed particles and substrates in the interface area was investigated by optical and electron microscopical (SEM, TEM) methods. The bond quality was also determined by bond strength measurements. It is possible to correlate the results of microscopical investigations of interface phenomena with results of quantitatively measured bond strength of plasma sprayed Ni coatings.

H.-D. Steffens

Papers

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

Interfacial phenomena in thermally sprayed composites