Issue 8

R. Ghelichi et alii, Frattura ed Integrità Strutturale, 8 (2009) 30-44; DOI: 10.3221/IGF-ESIS.08.03 32 - Printing – copper coating on rollers - Oil and gas – improved corrosion resistance - Glass – platinum coating. Figure 2: an example of the CGDS applications [6]. C OLD SPRAY IMPORTANT PARAMETERS Deposition efficiency ne of the most important characteristics of cold gas dynamic spray (CGDS), as well as of any other methods of powder spray, is deposition efficiency; and also for many reasons, there are many reasons that make it practically impossible to obtain a deposition efficiency that is equal to unity. First, polydisperse powders are usually used. As the jet during its impingement is spreading along the substrate surface, the finest particles either do not reach the surface at all or impact at acute angles, which deteriorate particle attachment. Although the largest particles are incident at a close-to-normal angle (i.e., the angle between the particle impact velocity and the substrate surface is close to 90°), their velocities may be insufficient for particle attachment. Second, the particle velocity at the jet periphery can be lower than it must be for particle attachment. In addition, if the velocity is not sufficiently high, the surface should be self- activated by the impacted particles. Due to the sufficiently complicated nature of CGDS, it is rather difficult to measure the deposition efficiency; first, three main stages (see Fig. 3) of the spray process may be identified. At the initial stage, some time is required for surface preparation (i.e., the induction time), when only erosion occurs without any deposition. At the second stage, a thin layer of the particle material (hereinafter referred to as the first layer) is formed on the substrate surface. This stage is characterized by the interaction of particles with the substrate surface, and it depends on the preparation level and properties of the surface material. The third stage, which can be conventionally called the build-up stage, is characterized by the growing thickness of the coating layer. In this case, the particles interact with the surface formed by previously incident particles. Thus, it is clear why there is some uncertainty in measuring the powder deposition efficiency. [7] Second, adhesion of particles depends on many factors: area of the contact surface, crater depth, plastic strain, yield stress, pressure and temperature at the contact boundary, etc. In turn, these factors are affected by the impact velocity of the particle. Therefore, it seems logical to assume that these parameters reach their critical values at velocities close to the critical one or their dependence on velocity becomes different. Postulating of some adhesion criterion requires these critical values and conditions to be determined [8]. But in the simplest way the deposition efficiency calculated experimentally as follow: kd = Δms/Mp (1) where Δms is change of weight of a substrate and Mp is weight of all particles interacting with a substrate. O