Influence of high temperature exposure on the adhesion of a micro-composite refractory enamel to a Ni–18Cr–12W superalloy

Abstract

A new micro-composite refractory enamel (MCRE) was designed to play a multifunctional protective role (thermal barrier coating, corrosive and erosive protection) for the hot working parts made of EI868 superalloy sheets i.e. burning chambers, fire tubes, volets, eclises, etc. The effect of the isothermal heating at elevates temperature (800 °C, 900 °C) were, for the first time, investigate for MCRE/EI868 couple. The adhesion strength has shown an enhancement of about 100% when the couple was heated at 800 °C or at 900 °C for 10 h compared with the unexposed one. SEM-EDS, OM, XRD, EDP-XRFS and SDAR-OES investigations support a chemical bonding mechanism between MCRE and EI868 substrate via a transient interface layer. Elemental EDS distributions show that the Cr selective oxidation is responsible for the formation of the transient layer. The XRD measurements can be correlated with the contribution of the spinellic transformation of the SiO2, Cr2O3, NiO to the adhesion enhancement. The most important result reported herein is the adhesion strength enhancement of the MCRE coatings to the EI868 supperalloy by isothermal treatment. Accordingly, the MCRE behaves as a self-healing coating. Noteworthy, the working lifetime of the MCRE-EI868 couple can be significantly enhanced if an optimum pre-treatment is applied before it comes into service.