Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84; DOI: 10.3221/IGF-ESIS.22.08 82 particles rapidly heat up and also rapidly cool down. It also affects the interlamellar adhesion of the splats and hence influencing mechanical properties of the coating. Nano structured WC-Co cermets coatings shows better wear resistance due to the increase of hardness of coatings by the decrease in the particle size of the feedstock powder [31]. Jianhong et al. [32] found that the nanostructured coating of Cr 3 C 2 -25( Ni20Cr) exhibits a 20.5% increase in microhardness of 1020 HV300 as compared with the corresponding conventional coating of 846 HV 300 . Also these coatings consists of a ductile and a brittle phase that has high potential to improve the corrosion resistance even in the almost hopeless case of high temperature corrosive environment. However, the main hurdle in the nano powder coating is the spray technique, due to the requirement of heating by other thermal spray process like HVOF and plasma spray, the properties of nano-sized particles changed. During nano-powder coating by thermal spray process, the feed stock material should exhibit a nanosized structure to get the properties of a nano-structured coating after the spray process .The processing of a nanopowder by means of the standard thermal spray procedure, first of all requires the agglomeration of the nanopowders, in order to form spray particles with appropriate grain size that are suitable for a standard powder feeding device. The agglomerated particles lose their nanostructure, so it is not yet clear if there is any benefit of using nanostructured feed stock materials as compared to using conventional powders. Also due to decreasing particle size, the fluidization of the spray powder gets more and more challenging, so it is difficult to decrease the coating thickness below a value in the range of 30 μm by thermal spray process and a reduction of particle sizes in the nano range of below 5 μm needs improvement in powder feeding technique [31]. Therefore, cold spray coating technique, being a low temperature and also a solid state process, is foreseen as a viable solution to development of nano-structured coatings, using cold spray process [9, 31, 33, 34]. During the last two decades, research and development on cold spray process and technology, leads to significant progresses on both coating process and technologies [14, 33]. With such progresses, most metals and their alloys can be deposited by cold spraying including coatings using nanostructured powders like cermets WC-Co which is a most important wear-resistant coating materials. Nanostructured WC-12Co coatings are deposited successfully by cold spraying using a nanostructured feedstock with 1800 Hv0.3 microhardness of sprayed coating, with critical velocities of about 915 m/s is reported, by nozzle of downstream length 100mm, throat diameter of 2mm and exit diameter of 4mm, standoff distance 20mm and using helium gas at 2MPa pressure and 600 o C temperature in the prechamber [33]. Lima et al. [9] produce pure and well bonded nanostructured WC-12%Co coatings on low carbon steel substrate via cold-spray processing. The coating has a high density and around1225 kgf/mm 2 Knoop microhardness compared to around 42 kgf/mm 2 of the nanostructured feedstock i.e around 2800% increase. The impact of the particles against the substrate at supersonic velocities, promotes a densification in each nanostructured particle, without the presence of porosity. However there is no significant difference between the average grain size of the nanostructured feedstock 109 nm and coating 103nm is reported in the study [9]. Hence, the ability of cold spray process to deposit advanced materials onto a diversity of substrates will define the direction and opportunities lying ahead for this technology [1]. C ONCLUSION old spray technology is an emerging technology and it should be clear that it is not here for replacement of any of the well-established thermal spray methods. Instead, cold spray technology is expected to supplement and expand the range of applications for thermal spray processes as a greener alternative according to stringent environmental and health safety regulations. A number of materials have already proven to be suitable for deposition by cold spray from decorative articles to biomedical, automotive, power plants and space industries. Extensive research is required to design the optimum parameters like nature of gas, temperature control, nozzle design and its material and also prediction of critical velocity for different particle/substrate combinations. The research area of mathematical modeling to optimize various design parameters is still open to expand this process to more non-traditional applications. Research is required on deposition of hard and brittle ceramic materials by cold spray. The cold spray coating process has huge potential of growth with more applications in new areas like in boiler industry, to increase the life of boiler tubes by prevention from high temperature corrosion in aggressive chlorine and sulphate based environments. Coatings by cold spray technique can be beneficial for protection from high temperature oxidation failure of boiler tubes especially of waste-to-energy plants, which are still running at very less efficiency as compared to fossil fuel based plants. This area is explored by the author with a project sponsored by UGC, Govt. of India and in collaboration of ASB Industries Ohio and results will be published in future. It is expected that this decade will saw exponential growth of cold spray technology around the globe, through the development of CS coatings for specific applications. C