THE ROLE OF HAFNIUM IN MODERN THERMAL BARRIER COATINGS

Abstract

The world’s experience in using hafnium in two important parts of high-temperature thermal barrier coatings, such as the outer thermal barrier layer and bond coat layer, was analyzed. In the outer thermal barrier layer, hafnium is present as HfO₂ completely or partially stabilized by yttrium oxide (or other rare-earth oxides). Another approach is to use hafnium dioxide as an addition to conventional coatings based on ZrO₂ stabilized completely or partially. Electron-beam physical vapor deposition (EB-PVD) and air plasma spray process (APS) are most common techniques for applying thermal barrier coatings containing hafnium dioxide. Magnetron sputtering turned out to be successful as well. Compared to the 8YSZ coating, the 7.5YSH coating showed reduced Young’s modulus, 30% lower thermal conductivity (decreased to 0.5–1.1 W/(m · K)) at high temperatures for HfO₂ stabilized with 27 wt.% Y₂O₃, and higher sintering resistance and heat resistance. Doping of ZrO₂ and HfO₂ by several stabilizers proved to be promising: specifically a mixture of one trivalent ion larger than Y³⁺ and another trivalent ion smaller than Y³⁺, preserving the metastable structure of the t¢ phase. The importance of phase diagrams to choose correctly the composition of the thermal barrier layer and doping elements for the bond coat layer of thermal barrier coatings is shown. Doping the bond coat with a small amount (up to 1 wt.%) of hafnium improved its cyclic oxidation resistance and increased the adhesion of the thermally grown oxide layer to the bond coat layer and strength of the latter.