Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84; DOI: 10.3221/IGF-ESIS.22.08 81  Al-Sn alloys are widely used as sliding bearing materials in automobile and shipbuilding industry. Tin is a necessary soft phase in the aluminum matrix which can provide suitable friction properties and shear surface during sliding. But due to deleterious effects of high temperature of other thermal processes there is coarsening of Sn in the coating. Ning et al. [27] successfully prepared Al-5Sn and Al-10Sn coatings by CS process, with low porosity and well bonded dense structures and fraction of Sn phase is consistent with the feedstock powder. Al–5Sn coating can be deposited by high pressure cold spray with nitrogen while Al–10Sn can only be deposited by low pressure cold spray with helium gas. [30]  CS can also be used for the fabrication of complex conductive patterns in solar cells, to enhance surface performance in components made of advanced polymer-matrix composites in wind power generation. Architects can create aesthetic metallic patterns on any metal or ceramic substrate by this process [1].  In the medical field, cold spray has already been demonstrated to effectively apply hydroxyapatite (Ca10(PO4)6(O H)2, also known as (HAP), to a number of substrates [1]. HAP is widely used in dental and orthopedic implants, due to its chemical and crystallographic similarity with bone minerals and due to lack of cytotoxic effects it can be bonded to the bone directly. But due to poor mechanical strength, the combination of bioactive HAP coating and mechanically strong metals is used for surgical implants. Though this bioceramic coating is usually done by plasma spray technique, but with deleterious effects of high temperature in the plasma, hence low temperature cold spray coating can remove these harmful effects. The composite coating of Ti-HAP is successfully deposited using ‘CGT Kinetiks 4000 cold spray system’ at ASB industries, Ohio, with bond strength comparable or better to that of plasma sprayed coating and it is reported that dense composite coatings, containing up to 30% HAP can be deposited by this technique. Hence the cold spray process has matured from an emerging technology to a viable alternative to thermal spray for selected applications [3,28].  Cold spray technology is successfully used to develop high temperature oxidation resistant CuCrAl and NiCrAlY coatings on GRCop-84 substrates, at ASB Industries, Ohio, using specially designed tungsten carbide nozzle. GRCop-84, a Cu-8CR-4Nb alloy, developed at NASA Glenn Research Center (GRC),for rocket engine liner applications, having very high temperature creep and fatigue capabilities. But for their maximum life additional oxidation protection is required to prevent blanching [21].  CS is used for application of aluminimum barrier coatings for protection from high rate of corrosion and also to restore dimensional tolerances of heat-sensitive materials such as Mg alloys ZE41A-T5 used in the fabrication of transmission gearboxes in helicopters and fixed-wing aircraft like UH-60 Seahawk and MH-60S Seahawk. But Mg alloys are highly susceptible to galvanic corrosion when coupled to another metal, because magnesium being the most electrochemically active structural metal. Also these alloys are very susceptible to damage from excess heat and if coated by conventional thermal spray methods like HVOF, then magnesium can reacts with molten material deposited,[22, 29].  Cold spray coatings are being used in the power generation plant. These are applied on the boiler tubes to provide resistance from the high temperature corrosion, to provide resistance from the cavitation wear of turbine blades, and the water pump housing, impeller fins, impeller seal section and wear rings. The cold spray of tungsten carbide, chromium oxide and Ni-Cr coatings can be applied on these parts. Also wear rings can be reconditioned by applying bronze by this low temperature spray process. The chromium carbide can be applied to the journal used in coal crusher[30].  One of the important application of cold spray is the coating of copper powder on the aluminium tips of the electric mains to prevent the electrochemical oxidation of contacting elements of copper wire of the transformer and aluminium tip of the cable. The presence of different materials results in circuit breakdown and can be prevented by this method. This problem also occurs in the automotive batteries with contact of copper wire and the aluminium battery terminal[30]. Nanostructured Coating and smart structures Cold spray can be also be used to embed micro-sensors, along with functional coatings, on surfaces for smart structures. These structures would have the ability to provide real-time information related to materials performance or environmental conditions[1]. Cold spray can effectively be used for nanostructured coating without compromising their beneficial microstructure. Nanostructured (or nanocrystalline) materials have achieved much attention in recent years due to their outstanding properties like having having higher hardness, strength and corrosion resistance as compared to those of micron-size counterparts. These materials are characterized by a microstructural length scale in the1–200 nm regime. It is found that these are sensitive to process temperature because particle size strongly influences the particle thermal history as small