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 HfO 2 completely or partially stabilized by yttria (or other r...

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Bibliographic Details
Published in:	Powder metallurgy and metal ceramics Vol. 60; no. 1-2; pp. 78 - 89
Main Authors:	Lakiza, S.M., Hrechanyuk, M.I., Red’ko, V.P., Ruban, O.K., Tyshchenko, Ja.S., Makudera, A.O., Dudnik, O.V.
Format:	Journal Article
Language:	English
Published:	New York Springer US 01-05-2021 Springer Springer Nature B.V
Subjects:	Air plasma Analysis Barrier layers Bonds Ceramics Characterization and Evaluation of Materials Chemistry and Materials Science Clothing and dress Coatings Composites Doping Electric properties Electron beams Electron-beam physical vapor deposition Glass Hafnium Hafnium oxide Heat resistance High temperature Magnetron sputtering Materials Science Metallic Materials Modulus of elasticity Natural Materials Oxidation resistance Oxides Partial stabilization Phase diagrams Physical vapor deposition Plasma physics Protective and Functional Powder Coatings Sintering Thermal barrier coatings Thermal conductivity Thermal resistance Trivalent ions Yttrium oxide Zirconium dioxide hafnium dioxide corrosion bond coat thermally grown oxide thermal conductivity thermal barrier coatings mechanical properties
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Summary:	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 HfO 2 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 ZrO 2 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 HfO 2 stabilized with 27 wt.% Y 2 O 3 , and higher sintering resistance and heat resistance. Doping of ZrO 2 and HfO 2 by several stabilizers proved to be promising: specifically, doping by a mixture of one trivalent ion larger than Y 3+ and another trivalent ion smaller than Y 3+ , 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.
ISSN:	1068-1302 1573-9066
DOI:	10.1007/s11106-021-00217-1