PVD thermal barrier coating systems for Mo–Si–B alloys

Abstract

Mo-Si-B alloys possessing melting points exceeding 2000°C are promising candidates for high temperature structural applications outperforming the service temperatures of the currently employed Ni-based superalloys. However, their oxidation behaviour is poor, in particular at temperatures below 1000°C, suffering from evaporation of MoO 3 . Application of protective coatings forming continuous silica/borosilicate scales is a viable approach to enhance the oxidation resistance. In the combustion environments of gas turbines, however, the oxide scales require environmental protection by ceramic topcoats which can moreover operate as thermal barrier coatings (TBCs). In the present feasibility study, Mo-9Si-8B (at.%) specimens were coated with different 5 -10 µm thick oxidation protective layers produced by magnetron sputtering. Their chemical compositions in at.% were Mo-70Al, Mo-46Si-24B, Mo-37Si-15B and Mo-47Si-24Al. On the pre-oxidised coated samples, ceramic topcoats of 7 wt.% yttria partially stabilized zirconia (YSZ) and gadolinium zirconate (GZO) were applied using electron-beam physical vapour deposition. The thickness of the TBCs was in the range between 145 and 160 µm. The specimens with the different TBC systems were exposed to air at 1000°C for periods between 20 and 100 h. Post-oxidation cross-sectional analyses were carried out to study microstructural characteristics of the coated specimens.Both as-deposited YSZ and GZO topcoats exhibited good adhesion to the pre-oxidised bond coats. After 20 h of exposure to air at 1000°C, the Mo-70Al bond coat was entirely degraded in the region near the suspension hole related to oxidation of uncoated substrate material. In contrast, the YSZ topcoat was tightly-adherent to the borosilicate scale grown on the Mo-46Si-24B bond coat. Similar results were obtained for GZO topcoats deposited on Mo-46Si-24B and Mo-37Si-15B bond coats. The TBC system consisting of GZO topcoat and Mo-47Si-24Al bond coat, which formed a mixed scale of silica and mullite-like oxides, survived 100 h at 1000°C. However, after this exposure time, the bond coats were approaching their lifetime due to the low layer thickness. Oxidation of the Mo-Si-B substrate occurring at unprotected areas around the suspension hole of the sample caused substantial degradation of the outer region of the GZO topcoat due to chemical reaction between MoO 3 and Gd 2 Zr 2 O 7 . This corrosion attack led to complete destruction of the TBC systems after 10 h of exposure at 1300°C.

Oxidation Protection of High-Temperature Coatings on the Surface of Mo-Based Alloys—A Review

“…In addition, Anton et al Prepared Mo-Si thin film coating on the surface of Mo-Si-B alloy by magnetron sputtering. The oxidation test shows that the coating can be used for 300 h at 1200 • C, while the inhibition time of medium temperature pulverization can reach 100 h at 800 • C. Moss-Mo 3 Si-Mo 5 SiB 2 phase is formed on the coating surface, which significantly improved its antioxidation activity [79][80][81]. Liquid-phase deposition technology inserts the refractory metal alloy into the alloy melt and prepares the intermetallic compound coating by a thermal diffusion reaction in a vacuum or inert gas atmosphere [82,83].…”

Influence of the Sputtering Technique and Thermal Annealing on YSZ Thin Films for Oxygen Sensing Applications

“…Yttria-stabilized zirconia (YSZ) is widely employed in technical applications such as thermal barrier coatings [1][2][3], solid oxide fuel cells (SOFC) [4][5][6][7], and oxygen gas sensors [8,9]. Besides its high thermal and mechanical stability, the ionic conductivity of YSZ plays a major role in most of its applications.…”

Magnetron Sputtered Silicon Coatings as Oxidation Protection for Mo‐Based Alloys

“…as well including the application of thermal barrier coatings on top. [15] In this research, a different approach has been chosen. To establish a larger coating thickness by magnetron sputtering, boron was avoided in the procedure because it possesses a poor sputter rate.…”

“…Mo 5 SiB 2 (T2) was introduced as diffusion barrier due to its high atomic packing density and to avoid early coating degradation by interdiffusion with the alloy . In addition, aluminum containing coatings based on Mo–70Al and Mo–47Si–24Al have been investigated as well including the application of thermal barrier coatings on top …”