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

Thermal barrier coatings (TBCs) must successfully resist damage from a variety of environmental and mechanical mechanisms to be viable on turbine airfoils. This paper reviews engine-operative TBC damage mechanisms and the requirements for life methods that enable a designer and engine operator to achieve acceptable TBCed component lives. Understanding TBC damage mechanisms facilitates development of advanced TBCs. The following damage mechanisms are discussed: • growth and cracking of the bond coating's thermally grown oxide and its interfaces • molten deposit (sulfate salt and calcia–magnesia–alumina–silicate) wicking into TBC • sintering shrinkage, particle impact, bond coating creep

Damage mechanisms, life prediction, and development of EB-PVD thermal barrier coatings for turbine airfoils

The use of thermally sprayed thermal barrier coatings (TBC) has permitted the increase of combustion temperatures of gas turbines and diesel engines in the last decades. In terms of system performance, the possibility of increasing in working temperature is always welcome. It is well known that the residual stresses due to the mismatch between metallic bond coat and ceramic top coat as well as the stresses due to the oxidation of the bond coat generate debonding of the TBC, leading to a collapse of all thermal barrier system. Several attempts have been done to minimize or even to solve this kind of problems. One of the possibilities that have been tried is to apply graded or multilayered coatings, or to induce compressive stresses that can compensate the tensile residual stresses due to spraying process. In this work, two TBC systems were obtained by applying two distinct bond coats and two distinct top coats. The commercial bond coats were applied by High Velocity Oxygen Fuel (HVOF) thermal spraying. Residual stresses were measured by the Modified Layer Removal Test. Results show that the residual stresses are mainly influenced by the thermal history regarding the quenching of individual splats and the plastic deformation of ceramic deposits.

Evaluation of residual stresses of thermal barrier coatings with HVOF thermally sprayed bond coats using the Modified Layer Removal Method (MLRM)

Preparation and oxidation behaviour of nanocrystalline Ni + CrAlYSiN composite coating with AlN diffusion barrier on Ni-based superalloy K417

The thermal expansion behaviors of thirteen plasma-sprayed MCrAlY and related coatings, plus Tribomet TM-309 and its NiCo matrix were determined. Replicate runs to 1100 °C in argon for all coatings allowed the experimental variability and reliability of differences between coatings to be described. Wet chemical analysis of eight samples allowed multiple correlations between coating expansion and composition to be found. The model equations give calculated expansions at 525 and 825 °C within a few 0.1 mm/m of the original measurement for most coatings. The model and the original expansion data both show a NiCrAlY alloy, such as LN-33 has significantly lower expansion than current CoNiCrAlY coatings. Using low expansion NiCrAlY as a bondcoat is predicted to generate longer thermal cycle life of TBC coatings due to reduced expansion-induced interface stress with the YSZ ceramic.

T.A. Taylor

Papers

Thermal expansion of MCrAlY alloys