July/August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 8 4 2 iTSSe TSS iTSSe TSS Vacuum heat treatment (pre-oxidation) should be used to prevent nucleation and growth of non-alumina oxides on the MCrAlY bond coating at the onset of oxidation. Pretreat- ment maximizes the efficiency of HVOF spray bond coatings (as overlay or bond coatings) during thermal exposure in air (Fig. 4). The Al 2 O 3 layer (produced by vacuum heat treatment) at the interface of NiCoCrAlTaY-nanostructured YSZ coatings noticeably retards the descending trend of aluminum activity (a Al ) at the bond coating-TGO interface during extended ther- mal exposure in air. This helps to markedly reduce the ten- dency (activity) of other metallic cations (particularly Ta 5+ ) to diffuse outward through the Al 2 O 3 TGO layer. The high velocity air fuel (HVAF) spray method, a modi- fication of HVOF spray, produces MCrAlY coatings with a rela- tively lower oxidation rate compared with HVOF spray MCrAlY coatings. This is due to reducing in-flight oxidation of particles during the HVAF spray process, which can be operated at jet flame temperatures between those of HVOF spray and cold spray. In addition, a slow growing, continuous, and dense α -Al 2 O 3 layer forms on the HVAF spray MCrAlY coating during oxidation [18] . The high velocity air plasma spray (HV-APS) method pro- duces MCrAlY bond coatings with lower oxygen content simi- lar to that of HVOF spray bond coatings. The oxidation rate of HV-APS MCrAlY bond coatings is markedly lower than that of LPPS bond coatings. Better oxidation behavior is due to less Y incorporated in the TGO layer (in the form of very fine Y-rich oxide precipitates) and excellent adherence of a slow growing α-Al 2 O 3 TGO layer [19] . COLD SPRAY BENEFITS Cold spray is a developing process to produce MCrAlY bond coatings. Cold gas dynamic spray (CGDS) uses kinetic energy to produce the coating rather than a combination of thermal and kinetic energies as with HVOF spray. In the cold spray method, fine powder particles are propelled toward the substrate surface with supersonic velocity. Upon impact, the particles undergo adiabatic heating and plastically deform at very high shear rates, which leads to flattening and bonding them to the underlying surface. Cold spray operating tempera- tures are very low compared with those of thermal and plas- ma spray methods (APS, HV-APS, HVOF, HVAF, and warm spray). The low operating temperature of cold spray together with the use of inert gas prevents oxidation of the parti- cle surfaces, phase transformation, grain growth, and formation of undesirable phases in the coating during spray application [20] . These advantages make cold spray an- other suitable candidate to pro- duce MCrAlY bond coatings, as well as bond coatings for TBCs and overlay coatings (Fig. 5). CGDS and HVOF spray (mod- ified parameters) bond coatings (with 0.1 and 0.75 wt% O, respec- tively) have similar microstruc- tures. Further, the low porosity and lowoxide content coatings can sub- FEATURE 8 Fig. 4 — High velocity oxygen fuel spray NiCoCrAlTaY/Al 2 O 3 / nanostructured air plasma spray YSZ coating after oxidation at 1100°C for 120 h. Fig. 5 — Cold spray MCrAlY coating, which has the lowest oxygen content: (a) 200 × SEM magnification; (b) 500 × SEMmagnification. (b) (a)