July/August_AMP_Digital

iTSSe TSS 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 1 iTSSe TSS element, or RE, content) in the bond coating. Therefore, TGO thickness can easily reach a crucial value at shorter oxidation times, which effectively reduces TBC life [9] . Thick VPS (LPPS) coatings with the lowest oxygen con- tent have higher Y content due to the lack of tied-up fine ox- ides in the coating, which reduces TGO growth rate. This is due to greater incorporation of Y (in the form of Y 2 O 3 ) into the TGO layer during oxidation [9] . In general, a slow growing, continuous, and dense α- Al 2 O 3 TGO monolayer noticeably reduces the oxidation ki- netics of bond coatings and enhances the life expectancy of TBC systems during long thermal exposure in air (service) [1,6] . BOND COATING MODIFICATION Bond coatings are generally produced using air plasma spray, low pressure plasma spray, and high velocity oxygen fuel spray methods [10] . Numerous oxide stringers are present in the APS coating (Fig. 2a), resulting fromoxidation of powder particles during spraying. Extensive formation of spinels at the initiation of oxidation also occurs on the bond coating at high- er temperatures (Figs. 2b and c). The high process tempera- ture and relatively low particle impact velocity of APS leads to extensive melting of powder particles, resulting in maximum contraction upon cooling [11,12] . Over the past decade, LPPS has been used to produce bond coatings for APS TBC coatings. LPPS is an expensive process carried out in a large vacuum chamber. Production of bond coatings with minimum oxygen content (<500 ppm, or 0.05 wt%) and porosity, and acceptable surface rough- ness, is possible using optimal process parameters. However, TBC systems consisting of VPS (LPPS) bond coatings with RE over-doping have shorter service life [13] . RE (particularly Y) distribution in the VPS bond coating is significantly influenced by vacuum heat treatment before ex- tended thermal exposure in air. The heat treatment can lead to selective RE oxidation on the bond coating surface and exten- sive RE depletion from the bond coating (enhancing VPS coat- ing oxygen content). Gil et al. [14] reports that spinel formation and rapid TGOgrowth rate can be impeded by a VPS CoCrNiAlY bond coating that is ground after vacuum heat treatment. HVOF spray (without a vacuum chamber) was developed to produce bond coatings with low porosity, lower oxygen (ox- ide) content, higher Al content, excellent corrosion resistance, and good bond strength [10] (Fig. 3). HVOF spray bond coatings exhibit lower oxidation rates compared with bond coatings produced using VPS (LPPS) spray. This is due to lower flame temperature and higher in-flight particle velocities in theHVOF spray method, which could result in considerable retention of partially melted particles in the coating microstructure. More- over, oxygen content is higher in HVOF spray bond coatings compared with that in LPPS coatings. Therefore, a substantial portion of Y can be tied up in the bond coating in the form of oxide precipitates (during spraying), reducing its destructive role in TGO growth rate during oxidation. Better oxidation behavior of HVOF spray bond coatings is also attributed to the probable diffusion-blocking effect origi- nating from the presence of a small amount of Al 2 O 3 formed in the bond coating [15] . HVOF spray MCrAlY bond coatings primarily show para- bolic oxidation kinetics at elevated temperatures in air [7,16,17] . FEATURE 7 (a) (b) (c) Fig. 2 — Air plasma spray NiCoCrAlTaY coating with high oxygen content (>4 wt%): (a) considerable particle oxidation during spray process; (b and c) extensive spinel formation at the initiation of high-temperature oxidation of bond coating . Fig. 3 — As-sprayed high velocity oxygen fuel NiCoCrAlTaY coating with both lower oxygen content (<0.8 wt%) and porosity compared with air plasma spray NiCoCrAlTaY bond coating.