The Role of Hafnium in Modern Thermal Barrier Coatings

The world’s experience in using hafnium in two important parts of high-temperature thermal barrier coatings, such as the top thermal barrier layer and bond coat layer, was analyzed. In the top thermal barrier layer, hafnium is present as HfO2 completely or partially stabilized by yttria (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, doping by 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 for a correct choice of the top coat composition and doping elements for the bond coat 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 and strength of the latter.