Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84; DOI: 10.3221/IGF-ESIS.22.08 79 condition[14]. However, the spray powders used are mostly polydisperse powders having particles of varying diameter from minimum diameter( dmin.) to maximum diameter (dmax.) [15] and as discuss before the particle velocity is inversely related to particle diameter, hence, particle with dmin. will achieve maximum velocity (Vmax.) and with dmax will achieve minimum velocity (Vmin.). Now if Vmin. is more than Vc then all the particles in the spray powder will be deposited with 100% DE and if Vmax. is less than Vc then no deposition with 0% DE. The maximum size of the particles (dc) that can be adhered to the substrate is: d c =(k/Vc) 1/ n , where Vc is the critical velocity for the particle to be deposited for a certain material[15]. The relationships between the deposition efficiencies and particle velocity is shown in Fig.12 [7]. These calculations of particle velocity are for normal angle between nozzle axis and substrate. As the spray angle decreases from the normal angle called off-normal angle (θ), then deposition of particle depend upon the normal component of the velocity and only the particles with the normal velocity components higher than the Vc will be deposited during impact as: dc = (k sin(θ)/Vc) 1/ n ,[15]. The variation of DE with angle is shown in Fig.13. Figure 12 : The effect of particle velocity on deposition efficiency (DE) in CS [7]. Figure 13 : Effect of spray angle on deposition efficiency (DE) [7]. Effect of material properties on DE The deposition of particles and DE also depends upon the plastic behavior of the particles and substrate and it is more for metals with high plasticity and the particle adhesion is assumed to be possible if the particle is substantially more plastic than the substrate [7]. Metals with the FCC lattice are highly plastic due to more number of slipping planes, hexagonal structure have much fewer slipping planes, which yield a lower plasticity; and metals with the BCC lattice have the lowest plasticity among the three types. During the impact the particle temperature raised to the glass transition temperature (Tg) of the amorphous alloys, leading to particle softening and making it possible to achieve very high densities in the coating. It is also reported that for the soft substrates and hard particles, the first impacts will primarily confine the deformation to the substrate material, and after the first layer of undeformed hard particles are created the subsequent impacts provide severe plastic deformation on both substrate and impacting particles [7]. Though it is a general postulate that as the substrate deformability decreased, the ease with which particles bond to the surface also decreased, as reported by Ghelichi et al. [7], but completely opposite results are also reported and bonding for Al particles are seen to be rapid on metallic surface with hardness higher than that of the particles, even when deformation of the substrate was not visible [7].