Application of High-Velocity Oxygen-Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings

Abstract

From the literature, it is known that due to their glass formation tendency, it is not possible to deposit fully-crystalline silicate coatings when the conventional atmospheric plasma spraying (APS) process is employed. In APS, rapid quenching of the sprayed material on the substrate facilitates the amorphous deposit formation, which shrinks when exposed to heat and forms pores and/or cracks. This paper explores the feasibility of using a high-velocity oxygen-fuel (HVOF) process for the cost-effective fabrication of dense, stoichiometric, and crystalline Yb 2 Si 2 O 7 environmental barrier coatings. We report our findings on the HVOF process optimization and its resultant influence on the microstructure development and crystallinity of the Yb 2 Si 2 O 7 coatings. The results reveal that partially crystalline, dense, and vertical crack-free EBCs can be produced by the HVOF technique. However, the furnace thermal cycling results revealed that the bonding of the Yb 2 Si 2 O 7 layer to the Silicon bond coat needs to be improved.

Environmental Barrier Coatings Made by Different Thermal Spray Technologies

“…With such high velocities dense coating might be possible even without complete melting of the feedstock. Such a particle with non-molten core is visible in Figure 2 An additional more detailed description of some of the results can be found in our previous papers [17][18][19][20].…”

“…With such high velocities dense coating might be possible even without complete melting of the feedstock. Such a particle with non-molten core is visible in Figure 2 An additional more detailed description of some of the results can be found in our previous papers [17][18][19][20]. Characteristics of the 4 investigated thermal spray processes (Axial III used for SPS, TriplexPro210 for APS, O3CP for VLPPS, Dj2700 for HVOF) showing plume temperatures and particle velocities, photos of the plumes plus conditions used (process gases, current in case of plasma processes, and stand-off distances), and photos of typical splats.…”

“…To reach such particle conditions, sensitive process optimizations are required in terms of fuel-oxygen stoichiometry, total gas flow, powder feed rate, stand-off distance, and particle size distribution. Details of these investigations can be found elsewhere [18] and HVOF microstructure will be further discussed in the following. Consequently, these coatings own higher crystallinity regardless of the lower deposition temperature in the process as un-melted part of the particles remain crystalline.…”

“…No vertical cracking was observed in the fairly dense HVOF layer (Figure 14c), presumably due to lower Yb 2 SiO 5 content and hence reduced thermal stresses in the process as a result of the lower particle and deposition temperatures, respectively. However, isothermal thermal cycling experiments revealed that the adhesion of the HVOF oxide layer on the Si bond coat is poor because the HVOF layer partially delaminated only after few cycles [18]. The short lifetime of the layer was attributed to the presence of un-melted particles at the interface wherein the cracks can easily propagate.…”

Ni‐Ti Shape Memory Alloy Coatings for Structural Applications: Optimization of HVOF Spraying Parameters

“…Among the several technologies for depositing metallic coatings on metallic substrates, thermal spray high-velocity oxygen-fuel (HVOF) is one of the most versatile [8] and efficient technologies, with many multiscale features [9,10], capacity to produce homogeneous [11] and very dense coatings [12][13][14][15][16][17] with porosity levels typically in the range 0.1-2% [13], low oxide content [11,12], high hardness [13,18], excellent bond strength frequently exceeding 69 MPa [11,13,19], and low decarburization [13,20]. Also, the low gas temperature of particles avoids superheating during flight and preserves the nanocrystalline structure of the starting powders of the coating [21].…”