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 2 0 5 7 iTSSe TSS ture and pressure) increases η by increasing v pi . However, use of such high process parameters may not be always possible; one of the many potential effects is development of tensile stresses. Alternatively, η can be increased by lowering v pi . Particle and substrate material properties, particle size and geometry, substrate, and particle impact temperature have been shown to affect v cr . An increase in particle impact temperature can directly decrease critical velocity [13] . As pro- posed by Assadi et al., the following equation provides an esti- mate of critical velocity, based on experimental results [11] , (Eq. 2) where T m is the particle melting temperature, T p is the particle impact temperature, σ is the tensile strength of the particle material at 293 K, k 1 is a particle-size-dependent fitting param- eter, ρ p is the density of the particle material, and c p is the spe- cific heat of the particle material. Equation 2 shows that an increase in T p can significantly decrease v cr and is attributed to an increasedmaterial ductility that enhances particle deformation, thus increasing the sur- face that becomes free from the native oxide layer. This effect of particle impact temperature has been observed for many powder-substrate combinations. Figure 3 shows the deposi- tion efficiency of pure aluminum under a set particle impact velocity while varying the particle temperature injected in the downstream portion of the CS nozzle. The exact particle temperature prior to injection is assumed to be at the feedline gas temperature. While it was not possible to measure particle impact temperature, it is reasonable to assume it follows a similar trend to particle temperature at injection, although colder due to convection cooling experienced in the nozzle. This increase in particle injection temperature (thus particle impact temperature) is shown to have a dras- tic effect on deposition efficiency as the 400°C test resulted in a nearly tenfold increase com- pared to spray with room temperature parti- cle injection. The beneficial effect of powder preheating has also been observed on materials that are more challenging to CS, such as NiCoCrAlTaY sprayed on nickel-base single-crystal super- alloy. Without particle preheating, depositing the powder using nitrogen at a gas pressure of 3.5 MPa and gas temperature of 400°C was unsuccessful, resulting in substrate erosion. However, powder preheating (400°C) facilitated particle deformation and promoted dense and well-bonded coatings (Fig. 4). While the notion that particle preheating is beneficial to CS is generally accepted, some studies show that increasing particle temperature can actually lower ad- hesion strength. This can be attributed to either increased particle oxidation [14] , decreased crater depth [15] , or decreased tamping effect of non-adhering particles [16] . While the effect of impact temperature on the deposition process and coating quality varies depending on spray conditions and material Fig. 2 — Schematic of metallurgical bonding in the CW process proposed by Bay [3,4] . (a) (b) (c) (d) Fig. 3 — Deposition efficiency vs. preheating gas temperature of the feedline (thus particle temperature at injection location) for aluminum powder. Particles are kept at same impact velocity. FEATURE 7