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 2 0 5 8 iTSSe TSS iTSSe TSS combinations, increasing particle impact temperature is gen- erally desirable in CS. With a similar objective of enhancing deformation to promote creation of fresh oxide-free surfaces on the substrate, studies demonstrate that awarmer substrate is beneficial [17-19] . However, the studies underscore the challenge of maintaining substrate temperature at a fixed value as it is exposed to large convection heat transfer rates from the impinging jet. This tends to quickly bring the local substrate temperature to the impinging gas jet stagnation temperature. As such, one of the best approaches to locally maintaining substrate temperature at a desired value is to use a laser [20-23] . RECENT STUDIES In recent work [4] , the CW bonding mechanismwas imple- mented in a finite element method (FEM) model. This model has been used as a tool to study the effect of substrate tem- peratureonmetallicbonding in theCSprocess. In this research, 55-µm copper particles impacting onto a 304 SS substrate at 673 m/s were modeled at two initial temperatures (300 and 873 K). Figure 5 shows particle bonding status after impact. With the 300 K substrate, the particle created some temporary bonding but due to the large rebound energy, which causes higher induced stress compared to bond strength, the particle debonded completely from the substrate. By increasing the substrate temperature to 873 K, the particle remained bond- ed to the substrate. Larger deformations were achieved for the latter case and therefore a larger area was bonded. These re- sults confirm that increasing substrate temperature decreases critical velocity and potentially increases adhesion strength by providing a larger bonded area. While powder and substrate heating challenges are ad- dressed by many studies, one remaining hurdle is the need to properly characterize particle in-flight temperature. If one wants to monitor and control the CS process to ensure a set particle temperature, it is mandatory to measure it. Although particle in-flight velocities have been characterized using particle imaging velocimetry, laser two-focus velocimetry, and doppler picture velocimetry, measuring CS particle in- flight temperature proves to be technically challenging due to (a) (b) Fig. 4 — NiCoCrAlTaY coatings deposited using CS on nickel-based single-crystal superalloy substrate. (a) Lowmagnification secondary electron image and (b) high magnification backscattered electron image. Fig. 5 — Status of local bonding for 55 µm copper particle impacted at 673 m/s to 304 SS substrate at (a) 300 K and (b) 873 K. Bottom view of particle in contact with substrate upon impact. (a) (b) FEATURE 8