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Top1. Introduction
There are many coating deposition techniques available, and choosing the best process depends on the functional requirements, adaptability of the coating material to the technique intended, level of adhesion required, size, shape, and metallurgy of the substrate, and availability and cost of the equipment. The commonly employed coating deposition techniques have been enlisted in Figure 1 (Goyal, 2010).
Figure 1. Various coating deposition processes in commercial use
In the early nineteen hundreds, a young Swiss inventor named Dr. Max Schoop invented thermal spraying, after watching his son playing with his toy cannon. Dr. Schoop observed that the hot lead shots that were projected out of the cannon, stuck to almost any surface, the result of which gave him the idea that if metal could be melted and projected in a spray like manner, then a surface could be built up with that material.
The technology continued, but expanded in the 70s due to development of the thermal plasmas and the increasing demand of high-temperature and wear resistant materials and coating systems (Knotek, 2001). Thermal spraying is one of the most versatile hard facing techniques available for the application of coating materials used to protect components from abrasive wear, adhesive wear, erosive wear or surface fatigue and corrosion (such as that caused by oxidation or seawater) (Marceau, 1995; Groshart, 1995; Ishikawa, 1993).
Cold spray as a coating technology was initially developed in the mid-1980s at the Institute for Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Science in Novosibirsk (Bishop, 1993; Alkhimov, 1994; Tokarev, 1996). The cold-gas dynamic-spray process, often referred to as simply “cold spray,” is a high-rate material deposition process in which fine, solid powder particles (generally 1–50 µm in diameter) are accelerated in a supersonic jet of compressed (carrier) gas to velocities in a range between 500 and 1000 m/s. As the solid particles impact the target surface, they undergo plastic deformation and bond to the surface, rapidly building up a layer of deposited material. A simple schematic of a typical low- pressure cold-spray set up is shown in Figure 2 (Maev, 2008).
Figure 2. A typical scheme of LPCS device
In this paper, surface coatings have been produced by low-pressure cold spray process (Figure 2) by varying input parameters using Taguchi L18 array. The design parameters selected for producing the coatings were selected to be type of substrate material, stagnation pressure, stagnation temperature of the carrier gas, type of powder feeding arrangement, and stand-off distance. The paper talks about the development of coatings produced by the process for electro-technical applications and measuring raw data and calculating S/N ratio for the micro hardness of the developed coatings. Similar technique had been used for optimization of coating thickness, coating density and surface roughness of the developed coatings (Goyal, 2012; Goyal, 2012b; Goyal 2011). Many researchers have used Taguchi optimization for optimization of their coating processes(Jones, 2008; Sahu, 2010; Mishra, 2009; Kucuk, 2012).