Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84; DOI: 10.3221/IGF-ESIS.22.08 71 with enough kinetic energy to induce mechanical and/or metallurgical bonding. The spray efficiency in this HPSC system is very high, reaching up to 90% as compared to 50 % in LPSC system. Moreover, the temperature of particles remain substantially below the initial gas preheat temperature due to short contact time of spray particles with the hot gas called dwell time and hence the name cold spray coating [4]. Figure 1 : Operating principle of low-pressure coldspray [1]. Figure 2 : Operating principle of high-pressure coldspray [1]. H ISTORY OF THE I NVENTION (1900-2011) he basic idea behind cold spray is fairly simple and the concept has already been patented in the early 20 th century. The invention of the cold spray process can be dated back to the time of the invention of the thermal spray process by Schoop. Though, the idea of using the impact energy of the particles to produce a coating using a carrier gas jet to accelerate solid particles had been in use for a century or more. However, the technology needed to put the idea into practical use was complex at the time and required further development. Consequently, it took 80 years for its first demonstration that happened accidentally during a two-phase flow gas dynamic experiment in Russia. The cold spray technique can also be viewed as an offshoot of the cold war between the superpowers. While studying models subjected to a supersonic two-phase flow (gas + solid particles) in a wind tunnel at Russian Academy of Science, scientists observed that during the impact of particles of various sizes, impact angles, and velocities on various materials, the solid particles deposited onto the surface when the velocity of impact was higher than certain critical value. Since then, efforts have been devoted for the development of this process into a practical industrial technology and the number of related patents and research publications has grown exponentially. A brief view of the development of this process from 1900 to present is as follows [5]: 1902: On August 12, 1902, Thurston patented, "A method for carrying out the process of coating one metal with another”. 1915: March 30, 1915 Schoop patented thermal spray process. Schoop developed the process wherein molten metal droplets are sprayed onto a surface to produce the coating, viz., the thermal spray process, when he observed that if mud balls are shot onto a wall, they get deposited on impact onto the wall forming a mud deposit. However, during this invention he overlooked the impact energy in the mud balls that led to deposit formation. If Schoop had taken note of the basic phenomenon of his invention, solid state deposition by impact energy, Cold Spray would have been the first thermal spray process and not the most recent one. T