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Chapter 2

LITERATURE REVIEW

Literature review relevant to the present study has been divided into three parts.

First part contains a comprehensive review of the existing literature on electro

conductive materials and their characteristics. In the second part, literature review

concerning the protective coatings and Cold spray (CS) process has been described.

The studies related to the behavior of the copper coatings have been reviewed in the

third part of this chapter.

PART-I

2.1 ELECTRO-CONDUCTIVE MATERIALS

Electrical resistivity (also known as resistivity, specific electrical resistance, or

volume resistivity) is a measure of how strongly a material opposes the flow of

electric current. A low resistivity indicates a material that readily allows the

movement of electric charge. It is commonly represented by the Greek letter ρ (rho)

and its SI unit is the ohm metre (Ω. m).

Electrical conductivity or specific conductance is the reciprocal quantity, and

measures a material's ability to conduct an electric current. It is commonly

represented by the Greek letter σ, and its SI unit is Siemens per metre (S·m−1

):

σ = 1/ ρ

2.1.1 Resistivity of various materials

The conductivity of a solution of water is highly dependent on its concentration of

dissolved salts, and other chemical species that ionize in the solution. Electrical

conductivity of water samples is used as an indicator of how salt-free, ion-free, or

impurity-free the sample is; the purer the water, the lower the conductivity (the higher

the resistivity). Conductivity measurements in water are often reported as specific

conductance, relative to the conductivity of pure water at 25 °C.

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The effective temperature coefficient varies with temperature and purity level of the

material. The 20 °C value is only an approximation when used at other temperatures.

For example, the coefficient becomes lower at higher temperatures for copper, and the

value 0.00427 is commonly specified at 0 °C.

The extremely low resistivity (high conductivity) of silver is characteristic of metals.

George Gamow (1947) tidily summed up the nature of the metals' dealings with

electrons in his science-popularizing book, One, Two, Three...Infinity: "The metallic

substances differ from all other materials by the fact that the outer shells of their

atoms are bound rather loosely, and often let one of their electrons go free. Thus the

interior of a metal is filled up with a large number of unattached electrons that travel

aimlessly around like a crowd of displaced persons. When a metal wire is subjected to

electric force applied on its opposite ends, these free electrons rush in the direction of

the force, thus forming what we call an electric current". Table 2.1 shows the

resistivity, conductivity and temperature coefficient of various materials at 20 °C (68

°F).

The cold spray process offers advantages to produce, with minimum thermal

exposure, coatings from functional materials such as thermoelectric, magneto-caloric,

photo-voltaic, piezo-electric, super-magnetic, and high-temperature superconductive

formulations.

Electrical resistivity of CS Al was higher than the bulk Al sample measured, and

anisotropic. These characteristics can be attributed to intrinsic defects within Al splats

(e.g. high dislocation density) and oxidized interfaces between splats. Anisotropy, on

initial approximation, could be attributed to the quasi-lamellar microstructure

containing more interfaces per unit length through the thickness of the coating versus

in-plane. Nevertheless, examining the data from Choi (2007) on the thickness

dependence of resistivity (as specimen thickness approaches splat size this effectively

confines electrons to a single interrupted conductive path). This concept was fully

explored in Sharma (2006); the imperfect interfaces between splats (interface normal

perpendicular to substrate normal) have a non-trivial effect on anisotropy. It could

further be argued that these interfaces are less well-bonded than those above and

below particles, but this assertion must be certified with direct measurements.

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Table 2.1: Resistivity, conductivity and temperature coefficient of various materials

Material ρ [Ω·m] at

20 °C

σ [S/m] at 20

°C

Temperat

ure

coefficient1

[K−1

]

Reference

Silver 1.59×10−8 6.30×107 0.0038 Serway, 1998; Griffiths, 1999

Copper 1.68×10−8 5.96 × 107 0.0039 Griffiths, 1999

Annealed Copper2 5.80 × 107

Gold3 2.44×10−8 4.52 × 107 0.0034 Serway, 1998

Aluminium4 2.82×10−8 3.5 × 107 0.0039 Serway, 1998

Calcium 3.36x10−8 0.0041

Tungsten 5.60×10−8 0.0045 Serway, 1998

Zinc 5.90×10−8 0.0037 http://physics.mipt.ru/

Nickel 6.99×10−8 0.006

Lithium 9.28×10−8 0.006

Iron 1.0×10−7 0.005 Serway, 1998

Platinum 1.06×10−7 0.00392 Serway, 1998

Tin 1.09×10−7 0.0045

Lead 2.2×10−7 0.0039 Serway, 1998

Titanium 4.20x10−7 X

Manganin 4.82×10−7 0.000002 Giancoli, 1995

Constantan 4.9×10−7 0.000008 O’Malley, 1992

Mercury 9.8×10−7 0.0009 Giancoli, 1995

Nichrome5 1.10×10−6 0.0004 Serway, 1998

Carbon (amorphous) 5-8×10−4 −0.0005 Serway, 1998; Pauleau, 1997

Carbon (graphite)6

2.5-5.0×10−6

⊥ basal

plane

3.0×10−3 //

basal plane

Pierson, 1993

Germanium7 4.6×10−1 −0.048 Serway, 1998; Griffiths, 1999

Sea water7 2×10−1 4.8 Physical properties of sea

water

Drinking water8 0.0005 to 0.05

Deionized water9 5.5 × 10−6 Pashley et al., 2005

Silicon7 6.40×102 −0.075 Serway, 1998

Glass 1010 to 1014 ? Serway, 1998; Griffiths, 1999

Hard rubber approx. 1013 ? Serway, 1998

Sulphur 1015 ? Serway, 1998

Air 3 to 8 × 10−15 Pawar et al., 2009

Paraffin 1017 ?

Quartz (fused) 7.5×1017 ? Serway, 1998

PET 1020 ?

Teflon 1022 to 1024 ?

1 The numbers in this column increase or decrease the significant portion of the resistivity. For example, at 30 °C (303 K), the

resistivity of silver is 1.65×10−8. This is calculated as Δρ = α ΔT ρo where ρo is the resistivity at 20 °C (in this case) and α is the

temperature coefficient. 2 Referred to as 100% IACS or International Annealed Copper Standard. The unit for expressing the conductivity of nonmagnetic

materials by testing using the eddy-current method. Generally used for temper and alloy verification of aluminium. 3 Gold is commonly used in electrical contacts because it does not easily corrode. 4 Commonly used for high voltage power lines 5 Nickel-Iron-Chromium alloy commonly used in heating elements. 6 Graphite is strongly anisotropic. 7 Corresponds to an average salinity of 35 g/kg at 20 °C. 8 This value range is typical of high quality drinking water and not an indicator of water quality. 9 Conductivity is lowest with mono-atomic gases present; changes to 1.2 × 10-4 upon complete de-gassing or to 7.5 × 10-5 upon

equilibration to the atmosphere due to dissolved CO2.

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2.1.2 Resistivity density products

In some applications where the weight of an item is very important resistivity density

products are more important than absolute low resistivity- it is often possible to make

the conductor thicker to make up for a higher resistivity; and then a low resistivity

density product material (or equivalently a high conductance to density ratio) is

desirable. For example, for long distance overhead power lines— aluminum is

frequently used rather than copper because it is lighter for the same conductance.

Silver, although it is the least resistive metal known, has a high density and does

poorly by this measure. The calcium and the alkali metals have the best products, but

are rarely used for conductors due to their high reactivity with water and oxygen.

Aluminum is far more stable. Table 2.2 shows the resistivity-density products for

various materials.

Table 2.2: Resistivity density product for various materials

Material Resistivity [nΩ·m] Density [g/cm³] Resistivity-density product [nΩ·m·g/cm³]

Sodium 47.7 0.97 46

Lithium 92.8 0.53 49

Calcium 33.6 1.55 52

Potassium 72.0 0.89 64

Aluminum 26.50 2.70 72

Copper 16.78 8.96 150

Silver 15.87 10.49 166

PART-II

2.2 PROTECTIVE COATINGS

In a wide variety of applications, materials have to operate under severe conditions

such as erosion, corrosion and oxidation in hostile chemical environments. Therefore,

surface modification of these components is necessary to protect them against various

types of degradation (Pawlowski, 1995).

Only composite materials are able to meet such a demanding spectrum of

requirements, the base material provides the necessary mechanical strength and

coatings provide a way of extending the limits of use of materials at the higher

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temperatures (Sidky and Hocking, 1999; Li et al., 2003). Even if the material

withstands environmental degradation without a coating, the coating enhances the

lifetime of the material.

A coating is a layer of material, formed naturally or synthetically or deposited

artificially on the surface of an object made of another material, with the aim of

obtaining required technical or decorative properties. Heath et al. (1997) have

summarized the main advantages of coatings as follows:

Very high flexibility concerning alloy selection and optimization for specific

resistance to corrosion environments and particle abrasion/erosion. Surface

properties can be separated from required mechanical properties of the structural

component.

Coating systems (multi-layered or functionally graded) can be used, combining,

for example, good adhesion with optimized corrosion and erosion behavior.

Unique alloys and microstructures can be obtained with thermal spraying which

are not possible with a wrought material. These include continuously graded

composites and corrosion resistant amorphous phases.

Costs of a coating solution are normally significantly lower than those of a highly

alloyed bulk material; thermal spray coatings are especially interesting for their

cost/performance ratio.

Thermal spray coatings additionally offer the possibility of on-site application and

repair of components, given a sufficient accessibility for the sprayer and his

equipment. However, thermal spraying is preferred, whenever possible, to achieve

optimum results.

2.2.1 Use of Coatings for various applications

The growth of power-generation facilities throughout the world has been

unprecedented. With this expansion, tremendous of effort is expended behind the

scenes to keep these power generating facilities running smoothly. These facilities

face numerous corrosion and/or wear issues, and maintenance must regularly be

performed on various machines and systems. In an effort to reduce downtime, thermal

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spray technology had emerged to extend the life of power-generating components and

systems. Coatings manufactured by this technology are being used throughout the

power-generating industry in applications such as water pumps, conveyor screws,

boiler tubes, coal crushers and contacting elements of electro-technical applications.

2.2.1.1 Pump Repair-Housings, Impeller Fins, Seal Sections, and Wear Rings

Pumps are used in almost every facet of a power-generating facility and must often

endure abrasion as well as cavitations wear. The double suction pump is commonly

used to move river water through a power plant. As river water commonly contains

fine sand and even small stones, several sections of a pump can be attacked including

the impeller fins, the pump housing, the impeller’s seal section, and the wear ring.

The fins of the impeller are abrasively worn by the fine sand and the small stones and

broken down by cavitations. Over time, pump efficiency will be reduced. Similarly,

the housing of the pump also faces wear from the sand and/or rocks as water is

pumped through it. If left unchecked, the housing will eventually wear away to the

point where the pump may rupture. In both cases, the cold spray solution is to apply a

very hard and wear-resistant tungsten carbide coating onto the fins.

The seal section of the impeller shaft undergoes abrasive wear when fine sand slips

into the packing material and scores the journal. If the journal becomes too worn,

water will eventually penetrate beyond the seal section and start corroding the

bearings. An effective solution is to undercut the journal, cold spray a chromium

oxide coating, and finish the journal to size.

Lastly, the wear rings begin to erode as fine sand flows through the gap between the

impeller and the housing. As it wears and widens, the efficiency of the pump is

reduced. These wear rings can be efficiently reconditioned by applying bronze onto

the rings and machining them to size.

2.2.1.2 Coal Crusher Roll Repair-Journals and Seal Sections

In this case, a coal crusher required repair on the seal section and the bearing section.

A quick analysis of the seal section reveals that the coal dust generated during

grinding penetrated the gap between the packing material and the journal, imbedding

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itself into the packing material and creating abrasion on the journal. The solution was

to undercut the section, cold spray chromium carbide, and then finish the journal to

the size.

2.2.1.3 Reconditioning an abrasively worn conveyor screw

Conveyor screws are used in power plants to transport limestone into the boilers.

Conveyor screw manufactured from carbon steel, needed to be replaced/repaired once

a year due to the abrasion of the limestone. The thermal spray solution was to apply a

thin layer of wear-resistant tungsten carbide on the shaft and both sides of the flights

using the Cold spray system.

Table 2.3 lists the general coating material that are cold sprayed for different

applications.

Table 2.3: General coating material criterion for different applications (Sanderson, 2007)

Application Coating Material Industry Sector

Cd- plating alternative.

Corrosion mitigation

Controlled potential coatings

Al alloys Aerospace

Oil and gas

Petrochemical

Pb-free bearings e.g. connecting

rods, turbochargers

Al, Cu alloys Automotive

Motorsport

Aerospace

Thermal management e.g. power

hybrid devices, switchgear

Conductive tracks

Cu, Al, Cu-W Electronic

Automotive

Corrosion mitigation Ti, Ta, Nb, Ni, Cr,

Fe, Mo Oil and gas

Petrochemical

Power generation

High temperature corrosion and

oxidation mitigation e.g. gas turbines

Ni alloys,

MCrAlYs Aerospace

Power generation

Biocompatible coatings for medical

devices

Ti Medical

2.2.1.4 Contacting elements for electro-technical applications

Important structural elements of power engineering systems are tips of connecting

cables and connecting plates. The contact between the copper wire of the transformer

and the aluminum tip of the cable of the electric mains is a typical situation as well.

Under the action of atmospheric moisture and electric current, intense electrochemical

oxidation processes occur in such a contact pair, which increases the resistance of the

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contact and leads to contact and circuit breakdown. Aqueous solutions of acids, alkali,

and salts are electrolytes, i.e., liquids capable of conducting the electric current

(Papyrin et al., 2008). The problem is also common with the contact of copper wires

and brass terminals in automotive batteries. To prevent oxidation of contacting

elements, it is necessary to avoid the presence of different materials in contacts. The

same may be achieved by coating copper powder on aluminum tips/terminals by cold

spray process.

A variety of engineering problems have been solved using thermal spraying

applications. The use of thermal spraying ranges across many manufacturing

processes, from the automotive (Nakagawa et al., 1994; Nicoll, 1994) through to the

space exploration industry (Nguyentat et al., 1992). Metals, carbides and cermets are

the most widely used coating materials; however the spraying of polymers has also

been researched (Varacalle et al., 1996; Kawase and Nakano, 1996). The HVOF

coatings have been widely used in various engineering components for combating

wear and corrosion including propellers, pump impellers and casings, super-heaters

and pre-heaters of boilers, valve bodies/trim and pipe systems (Tan et al., 2005).

Eliminating the deleterious effects of high temperature on coatings and substrates like

oxidation, evaporation, melting, crystallization, residual stresses, debonding, gas

release and other common problems for traditional thermal spray methods, offers

significant advantages and new possibilities for cold spray for thermal and electro-

conductive applications.

2.3 COLD GAS DYNAMIC SPRAY (CGDS)/ COLD SPRAY (CS)

PROCESS

The literature on Cold Spray that deals with strategies and technologies for effectively

optimizing a cold spray process is quite vast. Results of research in the following

basic areas are presented: modeling and analysis of cold spray, effect of process

parameters, interaction of high-speed particle with the substrate, bonding mechanism,

heat treatment effects and technologies and applications.

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2.3.1 Process modeling of CS process

A detailed analysis shows that particle velocity is known to be one of the most

important parameter affecting the coating properties. Modeling of cold spray process

so as to improve the process capability and reliability for variety of coating

applications has been made. A brief analysis of some research carried out is presented

in this section:

A detailed analysis of the effects that the type of the carrier gas, the inlet gas

temperature and the shape of the cold-spray nozzle have on the impact velocity of the

feed-powder particles is carried out by Dykhuizen and Smith (1998) using an

isentropic, one-dimensional gas-flow model.

In the work by Grujicic et al. (2003), the analysis of Dykhuizen and Smith (1998) was

extended in order to include the effects of finite values of the particle velocity and the

effect of variability of the gas/particle drag coefficient. While the one-dimensional

model of Dykhuizen and Smith (1998) has been found to be quite successful, its

numerical nature does not enable an easy establishment of the relationships between

the gas, process and feed-powder parameters on one side and the gas and the particle

velocities at the nozzle exit and the velocity at which particles impact the substrate

surface, on the other. The research also provides analytical equations that can be used

to estimate the gas dynamics of the cold-spray process. Equations are also presented

that allow calculation of the particle velocity. It is shown how the spray particle

velocity depends on particle size and density, gas stagnation pressure, total gas

temperature, gas molecular weight, and nozzle shape. Use of the equations derived in

this research allows determination of an optimal nozzle shape given the gas

conditions, particle properties, and nozzle length. However, it is shown that the spray

particle velocity is relatively insensitive to the nozzle shape. Thus, a single nozzle can

be used for a variety of operational conditions.

An empirical equation for particle velocity based on the experimental data was

derived by Alkhimov et al. (2001). Numerical and experimental research of wedge-

shaped nozzles shows different types of nozzle dimensions for a given type of

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particles that produces the maximum possible particle velocity at the moment of

impact on a target surface.

Grujicic et al. (2003) were able to employ a one-dimensional model to predict the

particle velocity at nozzle exit and particle behavior upon impact with a substrate

during CGDS. One-dimensional models generally used to analyze the dynamics of

dilute two-phase (feed-powder particles suspended in a carrier gas) flow during the

cold-gas dynamic-spray process require the use of numerical procedures to obtain

solutions for the governing equations. The researchers also found that a relatively

simple function is defined which relates the gas velocity at the nozzle exit with the

nozzle expansion ratio and the carrier gas stagnation properties.

Numerical analysis for the accelerating behavior of spray particles in cold spraying

using a computational fluid dynamics program, FLUENT was conducted by Li and Li

(2005). The optimal design of the spray gun nozzle is achieved based on simulation

results to solve the problem of coating for the limited inner wall of a small cylinder or

pipe. They found that the nozzle expansion ratio, particle size, accelerating gas type,

operating pressure, and temperature are main factors influencing the accelerating

behavior of spray particles in a limited space. Owing to the axi-symmetrical

characteristic of the flow in this study, a two-dimensional model was used in this

paper.

Jen et al. (2005) reported the effect of friction on the gas dynamic flow through the

Convergent-Divergent (CD) nozzle. The effect of particle size on the acceleration of

the particles was also reported. The acceleration process of micro scale and sub-micro

scale copper (Cu) and platinum (Pt) particles inside and outside De-Laval-type nozzle

is investigated. A numerical simulation is performed for the gas-particle two phase

flows with particle diameter ranging from 100 nm to 50 μm, which are accelerated by

carrier gas nitrogen and helium in a supersonic De-Laval-type nozzle. The carrier gas

velocity and pressure distributions in the nozzle and outside the nozzle are illustrated.

Raletz et al. (2006) began with an overview of critical particle velocity under cold

spray conditions. The system described in study makes it possible to simultaneously

measure particle velocity and number of rebounds. They concluded that the particle

velocities calculated with the one-dimensional isentropic model under the same

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operating conditions are close to the values found in the literature. The difference

between predicted and experimental values arises from the use of real powders in the

experiments and not ideal ones used in the model.

The comparison between the Convergent-Barrel (CB) and Convergent-Divergent

(CD) nozzle was made by Li (2006). Nitrogen (N2) was employed as driving gas at a

pressure of 1.4 M Pa and a temperature of 460 0C, and the spherical Cu particles of 20

μm in diameter were used. Author found that the main factors influencing

significantly the particle velocity and temperature include the length and diameter of

the barrel section, the nature of the accelerating gas its operating pressure and

temperature, and the particle size when using a CB nozzle. Particles achieve a

relatively lower velocity but a higher temperature using the CB nozzle than a CD

nozzle under the same gas inlet pressure. The gas flow rate, and subsequently the

operating cost for cold spraying, could be reduced.

Jodoin et al. (2006) described a framework for cold spray modeling and validation

using an optical diagnostic method. In this study, an axi-symmetric two-dimensional

mathematical model is presented which was used to predict the flow inside a cold

spray nozzle as well as the particle velocity in the vicinity of the nozzle exit. The

model results are compared with those obtained using the one-dimensional isentropic

theory and with particle velocity measurements (using laser diagnostic techniques)

made on a commercial cold spray system. The study has allowed coming to the

following conclusions: The measured nozzle mass flow rate follows closely the

overall behavior predicted by the one-dimensional isentropic theory. The predictions

of the axi-symmetric two-dimensional mathematical model are more accurate than the

one-dimensional approach due to the inclusion of viscous effects in the model. The

proposed model is more accurate than the one dimensional theory in predicting the

particle velocity inside the nozzle and in the free exiting jet. It is concluded that this is

due to the proper treatment of the shock waves in the nozzle by the proposed model.

For the two propellant gases used (helium and nitrogen), the proposed model predicts

oblique shock waves in the diverging section of the nozzle. The shocks reduce

drastically the gas velocity and Mach number while the gas temperature and pressure

are increased sharply. The proposed model results reveal that the maximum gas

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velocity is always reached just in front of the first oblique shock wave. With the use

of nitrogen as the propellant gas, the jet has a lower gas velocity and takes longer to

reach the aerodynamic equilibrium state (jet pressure equal to external pressure). The

experimental study reveals that increasing the stagnation temperature or pressure

leads to an increase in particle velocity. The values predicted by the proposed model

are close to the measured ones, confirming that the mathematical model is accurate

even in the presence of shock waves.

Li (2007) carried out simulations using FLUENT to study the effect of throat & exit

area and diverging section length on the particle flow and to optimize the nozzle

design for different inlet conditions. Numerical modeling was performed by using

commercial software FLUENT (Ver. 6.1) to determine the flow field of driving gas

inside and outside the nozzle, and subsequently the acceleration of particles in cold

spraying. It was concluded that the optimization of nozzle exit diameter is influenced

by the gas conditions, particle size, and nozzle divergent section length and throat

diameter.

A one-dimensional isentropic gas flow model in conjunction with a particle

acceleration model to calculate particle velocities was employed by Pardhasaradhi et

al. (2008). A laser illumination-based optical diagnostic system is used for validation

studies to determine the particle velocity at the nozzle exit for a wide range of process

and feedstock parameters such as stagnation temperature, stagnation pressure, powder

feed rate, particle size and density. The relative influence of process and feedstock

parameters on particle velocity is presented in this work. It was found that the

stagnation temperature have greater influence on particle velocity in comparison to

stagnation pressure and powder feed rate (particle loading) have negligible effect on

particle velocity for the range employed in the present study due to the lower particle

concentrations in the present study.

Lupoi and O’Neill (2010) analyzed the powder stream characteristics in CGS

supersonic nozzles. The powder injection location was varied within the carrier gas

flow, along with the geometry of the powder injector, in order to identify their relation

with particles trajectories. Computational Fluid Dynamic (CFD) results are presented

along with experimental observations. Three nozzles configurations were examined,

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characterized by a different acceleration channel lengths and powder injector’s

geometry and locations. When powder is released axially and upstream the nozzle

throat, particles trajectories do not stay close to the centre line, but tend to spread over

the entire volume of the channel. The particle stream diameter at impact with

substrate is comparable with the nozzle exit cross-section diameter. A theoretical

(CFD) analysis has shown that one of the reasons for this effect is a relatively high

gas turbulence level generated at the vicinity of the nozzle throat. The beam geometry

by the CFD results compares well against experimental observations. On the other

hand, when powder is released axially and straight into the supersonic region of the

nozzle, a more focused stream can be achieved. In this case, CFD results were in

accordance with the experiments, but failed to provide an accurate prediction of the

powder beam geometry.

2.3.2 Effect of CS process parameters

2.3.2.1 Gas temperature effects

Lee et al. (2007) found that increasing the gas pressure caused an increase in particle

velocity, while increasing the gas temperature not only affected the particle velocity

but also the particle temperature. Increasing the particle temperature could enhance

thermal softening, which is important for bonding. The deposition efficiency at the

same particle velocity became higher at higher process gas temperatures.

2.3.2.2 Powder properties

The irregular shape particle presents higher in-flight velocity than the spherical shape

particle under the same condition as showed by Ning et al. (2007). It was also found

that the larger size powder presents a lower critical velocity in the study.

2.3.2.3 Spray parameters

VanSteenkiste et al. (1999) found that kinetic sprayed coatings have relatively low

porosity values, hardness comparable with the corresponding bulk materials, adhesive

strengths as high as 68-82 M Pa, and oxide contents essentially the same as in the

powders. They observed that powders essentially do not stick to the substrates below

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a certain inlet air temperature range. Substrate effects appeared to be relatively weak

in this experiment.

2.3.2.4 Incidence angle

The contact area between the deformed particle and substrate decreases and the crater

depth in the substrate reduces with increasing the tilting angle at the same impact

velocity was found by Gang et al. (2007).

2.3.2.5 Stand-off distance

Li et al. (2008) found that the deposition efficiency was decreased with the increase of

standoff distance from 10 mm to 110 mm for both Al and Ti powders used in study.

However, for Cu powders, the maximum deposition efficiency was obtained at the

standoff distance of 30 mm, and then the deposition efficiency decreased with further

increasing the standoff distance to 110 mm.

Pattison et al. (2008) found that the bow shock formed at the impingement zone not

only reduces the velocity of the gas, but also that of the entrained particles. Therefore

at small standoff distances, when the strength of the bow shock is high, deposition

performance is reduced. While at large standoff distances, when the bow shock has

disappeared, deposition can continue unhindered. Three distinct standoff regions were

identified that affect deposition performance: the small standoff region, where the

presence of the bow shock adversely affects deposition performance, and is limited by

the length of the nozzle's supersonic potential core; the medium standoff region,

where the bow shock has disappeared and, if the gas velocity remains above the

particle velocity (positive drag force), the deposition efficiency continues to increase;

and the large standoff region, where the gas velocity has fallen below the particle

velocity (negative drag force), and the particles begin to decelerate. For optimal

deposition performance, the standoff distance should be set within Region 2.

2.3.2.6 Impact velocity analysis

A simple function, which relates the gas velocity at the nozzle exit with the nozzle

expansion ratio and the carrier gas stagnation properties, was defined by Grujicic et

al. (2004).

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Wu et al. (2005) showed that particle velocity increases with increasing the process

gas pressure and temperature. At higher temperature (or pressure), gas pressure (or

temperature) do more effect particle velocities. It is believed that gas pressure and

temperature have little effect on the flux distribution of flying particles. Further, it

was concluded that with increasing particle velocity, two critical velocities are

observed: one is for particle deposition onto the substrate (Vcr1) and the other for

particle–particle (Vcr2) bonding.

It is true that small particles exit the nozzle at high velocity; their velocity at impact

can be significantly lower because of the bow shock wave. It was shown by Helfritch

and Champagne (2008) that impact velocity increases as the particle diameter

decreases until a diameter of 4 to 8 μm is reached. Impact velocity then decreases as

the particle diameter is further reduced.

Critical velocities decrease with increasing particle size, which can be attributed to

effects by heat conduction or strain rate hardening, respectively. By defining the

influences of particle temperature and velocity on bonding, a parameter window for

cold spray deposition can be developed, which for successful spray experiments must

be met by the respective particle impact conditions was demonstrated by Schmidt et

al. (2006).

2.3.2.7 Impact phenomenon

Klinkov et al. (2005) showed that results of particle impacts depend not only on

impact velocity but also on particle size. Impacts result in two contrasting

phenomena: destruction (also called erosion, cratering etc.) and adhesion (also called

attachment, sticking). At hyper-velocity impacts, cratering and destruction are typical

features which exhibit minor scale effects. At low impact velocities, there is a

transition from erosion to adhesion (sticking) when the particle size decreases. Here,

the nature of adhesion is due to van der Waals and electrostatic forces. At high

velocities, the results of impact depend not only on size and velocity but also on other

parameters (e.g. plasticity, particle flux concentration, etc.).

Wu et al. (2006) concluded that a rebound phenomenon was observed in which a high

particle velocity caused a high fraction of rebound particles. A maximum impact

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velocity was found for the particle deposition onto the substrate. The deposition of

individual particles was controlled by the adhesion energy and the rebound (elastic

recovering) energy. The impacting particles could only be attached to the substrate

when the adhesion energy was higher than the rebound energy.

2.3.3 Bonding mechanism of CS process

The two main factors contributing to the observed higher deposition efficiency in the

case of copper deposition on aluminum are larger particle/substrate interfacial area

and higher contact pressures as indicated in literature (Grujicic et al., 2003). Both of

these are the result of a larger kinetic energy associated with a heavier copper feed-

powder particle. They concluded that the critical velocity, above which cold-spray

deposition takes place, is associated with the attainment of a condition for the

formation of a particle/substrate interfacial jet composed of both the particle material

and the substrate material. An interfacial instability due to differing viscosities and the

resulting interfacial roll-ups and vortices may promote interfacial bonding by

increasing the interfacial area, giving rise to material mixing at the interface and by

providing mechanical interlocking between the two materials.

Assadi et al. (2003) found that the bonding of a particle can be attributed to adiabatic

shear instabilities which occur at the particle/substrate or particle/particle interfaces at

high velocities. The modeling also shows a very non-uniform development of strain

and temperature at the interface, suggesting that this bonding is confined to a fraction

of the interacting surfaces. The analysis also suggests that density and particle

temperature have significant effects on the critical velocity and are thus two of the

most influential parameters in cold gas spraying.

Li et al. (2006) indicated that both the flattening ratio and compression ratio of

particles increase with the increase in particle velocity. The compression ratio is more

convenient for characterizing the extent of particle deformation owing to the easy

estimation and its independency on meshing size.

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2.3.4 Adiabatic shear instability in CGDS process

The minimal impact particles velocity needed to produce shear localization at the

particles/substrate interface correlates quite well with the critical velocity for particles

deposition by the cold-gas dynamic-spray process in a number of metallic materials

was shown by Grujicic et al. (2004). This finding suggests that the onset of adiabatic

shear instability in the particles/substrate interfacial region plays an important role in

promoting particle/substrate adhesion and, thus, particles/substrate bonding during the

cold-gas dynamic-spray process.

2.3.5 Properties of Cold Sprayed coatings

Stoltenhoff et al. (2006) demonstrate that at negligible porosities, microstructures of

cold-sprayed and thermally sprayed coatings are mainly determined by high degrees

of deformation or oxidation. Here, cold-sprayed coatings show a more uniform

appearance and respectively higher electrical conductivity. Overall, a solid-state

deformation results in higher hardness of cold-sprayed and HVOF-sprayed coatings,

similar to that of cold rolled bulk material after 90% reduction in thickness.

Subsequent annealing experiments and respective analyses demonstrated that in cold-

sprayed coatings mainly recovery and recrystallization determine further micro-

structural developments.

2.3.6 Effect of heat treatment

Gartner et al. (2006) demonstrate that cold-sprayed coatings, which are processed

with helium, show a similar performance as highly deformed bulk material. Also after

subsequent annealing, strength and elongation to failure develop in a similar manner

as for cold rolled sheets. In the as sprayed state, cold-sprayed coatings, processed with

nitrogen, and thermal spray coatings show brittle failure already under comparatively

low tensile stress. Whereas the performance of cold-sprayed coatings, processed with

nitrogen, can be substantially improved by annealing, mechanical properties of

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thermal spray coatings are influenced only to a minor extend by following heat

treatments.

Copper alumina coatings are found to be resistant to grain growth and softening even

upon heat treatment at 950 0C which is close to the melting point of copper (1083

0C)

owing to the presence of fine alumina particles. Grain size is found to be the most

dominant factor affecting the electrical conductivity of the coatings. The present study

by Phani et al. (2007) clearly indicates the potential of cold sprayed copper alumina

coatings in high-strength, high conductivity applications.

PART III

2.4 ROLE OF THE COATINGS

The thermal spray coatings used for corrosion resistance must be dense enough so that

the protective oxides can form within and fill voids, and thick enough to postpone the

diffusion of corrosive species to the substrate material before the protective oxide can

form within the coating (Bluni and Marder, 1996). Guilemany et al. (2002) have also

reported that a thicker coating provides better resistance against corrosion.

Gurrappa (2003) suggested that the coating used for corrosion resistance should have

a composition that will react with the environment to produce the most protective

scale possible, provide corrosion resistance with long term stability and have

resistance to cracking or spallation under mechanical and thermal stresses induced

during operation of the components. Further, the protective oxide scale should not

react with the corrosive environment and at the same time, it should not allow the

corrosive species to diffuse through the coating.

Alloys and coatings designed to resist oxidizing environments for corrosion resistance

should be able to develop a surface oxide layer, which is thermodynamically stable,

slowly growing and adherent (Brandl et al., 2004).

Table 2.4 lists the benefits of using thermal spray process.

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Table 2.4: Benefits of using thermal spray process (Sidhu, 2006)

Coating benefit Main reason for the benefit

Higher density (lower porosity) Higher impact energy

Improved corrosion barrier Less porosity

Higher hardness ratings Better bonding, less degradation

Improved wear resistance Harder, tougher coating

Higher bond and cohesive strengths Improved particle bonding

Lower oxide content Less in-flight exposure time to air

Fewer unmelted particle content Better particle heating

Greater chemistry and phase retention Reduced time at higher temperatures

Thicker coatings Less residual stress

Smoother as-sprayed surfaces Higher impact energy

2.4.1 Behavior of copper coatings

Pratt & Whitney used deposition of a copper coating by the cold spray to improve the

structure and functioning of a new aircraft engine (Haynes and Karthikeyan, 2003;

Cooley et al., 2002). A thick layer of the copper coating was necessary to improve

heat removal from the combustion-chamber zone. If the coating thicker than 1 mm is

applied by the traditional electrolytic technology, the adhesion is very low; in

addition, the process takes a long time (two weeks). Application of a copper coating

applied by the cold spray allows one to significantly reduce the time of the process,

improve adhesion, and avoid the use of hazardous chemicals necessary in the

conventional electrolytic method.

The requirements to the coating are rather severe: it has to withstand cyclic thermal

loads, including connection (welding) of tubes, etc. The highest thermal load is the

heating up to 1200 K in vacuum, and the coating has to be still operational under

these conditions (have no bubbles and exfoliation). High temperature gradients arise

when the engine is started and shut down; therefore, the coating has to withstand high

thermal impacts without exfoliation. To check the coating operation in this regime,

the coated samples were cooled in liquid nitrogen and water.

An analysis of various methods for coating application showed that it is possible to

obtain a copper coating with prescribed parameters by the electrolytic method,

vacuum plasma deposition, and cold spray. The researchers from Pratt & Whitney

first decided to use the electrolytic technology to obtain the required coating. It turned

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out, however, that this method requires several processes of treatment to obtain a

high-quality coating and that this method should be significantly upgraded.

The plasma coating satisfied the tests (Hickman and Mckechnie, 2001), but the

process was too long and too expensive. Moreover, masking and mechanical post-

processing (improvement of accuracy to a necessary level) are rather labor-

consuming, because the article to be coated has a very intricate shape, which makes it

difficult to reach the necessary dimensions within admissible margins.

A preliminary analysis of the cold spray showed that it is possible to obtain dense

coatings without inclusions and with a low content of oxides (in some cases, even

lower than in the initial material) (Smith et al., 1999). Moreover, obtaining very thick

coatings with good adhesion during a short time (Karthikeyan, 2002) and with

acceptable expenses is the advantage of the cold spray, which evoked the interest in

this method.

A series of experiments (Haynes and Karthikeyan, 2003) were performed with copper

spraying onto stainless steel under different conditions: the varied parameters were

the surface roughness, spraying parameters (pressure and temperature), etc. Initially,

the coatings exfoliated from most samples. A micro structural study showed that the

coating behavior during the tests is affected, in addition to spraying parameters, by the

powder purity and the state of the surface. Therefore, an attempt was made to obtain

coatings from powders of higher purity and with precise control of the spraying

parameter (stagnation pressure and temperature of the gas). Testing of the samples

obtained showed that they can withstand thermal cycles without exfoliation of

coatings.

A metallographic analysis of coatings applied with optimized parameters revealed

good adhesion and low porosity. After this encouraging result was obtained, an engine

mock-up was fabricated, and a copper coating more than 2.5 mm thick was applied.

Then the coating was treated by grinding and drilling holes to demonstrate that the

coating can withstand stresses caused by machining. Thus, it was demonstrated that

the cold spray can be used to obtain dense, phase-pure, and thick coatings with good

adhesion, which can withstand significant thermal loads. In addition, the cold spray

was shown to be promising for production of Hi-Tech articles/elements. In the latter

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activity, the cold spray advantages are the high production efficiency, acceptable cost,

environmental safety, etc. (Haynes and Karthikeyan, 2003) over other methods.

Obtaining an aluminum-bronze coating on a stainless-steel substrate from aluminum-

bronze powder with a particle size smaller than 40µm was considered in Xiong et al.

(2004). Detailed inspection of the element composition in the vicinity of the

coating/substrate interface showed that copper and aluminum in the coating migrate to

the substrate material. At the same time, iron and chromium from the substrate

migrate to the coating material. Moreover, the farther from the interface, the lower the

diffusion of copper and aluminum to the substrate and the lower the diffusion of iron

and chromium to the coating. The width of the diffusion zone is several microns.

The effect of the spraying parameters (stagnation pressure and temperature, velocity

of substrate motion) on the properties of copper coatings (from a copper powder with

a particle size of −22+5 µm) on an aluminum substrate was examined in Galla et al.

(2004). Prior to spraying, the substrate was cleansed by ethyl alcohol denatured by

methyl alcohol. Spraying was performed with the use of helium. An increase in

deposition efficiency with increasing stagnation pressure was noted, as well as a

certain (from 90 to 85%) decrease in deposition efficiency with increasing flow rate

of the powder from 45 g/min (21 wt% of the gas flow rate) to 76 g/min (36 wt% from

the gas flow rate). It was noted that the coating starts exfoliating from the surface after

it reaches a certain thickness owing to compressive stresses. The tendency to

exfoliation can be prevented or significantly reduced in many cases by means of gas

heating. The coating hardness increases with increasing pressure from 1.1 to 2.2 M Pa

and then stabilizes with a further increase in pressure to 2.9 M Pa. An increase in

temperature (from 300 to 700 K) leads to a decrease in hardness.

Obtaining coatings from a copper powder (5–25 µm) on aluminum, copper, steel, and

stainless steel pre-processed by sandblasting was considered in Hukanuma and Ohno,

(2004). Coatings approximately 600 µm thick were applied onto samples 20 mm in

diameter; before the tensile tests by the glue method, however, the coating was

polished off to a thickness of 300 µm. The experiments showed that the adhesion of a

copper coating applied with the use of nitrogen was higher on the aluminum substrate

than on the copper substrate. In spraying the particles onto the carbon steel and

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stainless steel substrates, the coating exfoliated from the substrate in the course of

spraying, as soon as the coating thickness reached approximately 500 µm, whereas no

exfoliation occurred if helium was used. In addition, the adhesion was much higher

than that in the case of spraying by nitrogen. The authors also noted that the adhesion

force depends on the working gas pressure (in the range of 1–3 M Pa): the higher the

pressure, the higher the adhesion. If the threshold pressure is exceeded, the coating

starts separating along the epoxy resin/coating interface rather than along the

coating/substrate interface. Depending on the substrate material, the adhesion force is

distributed as follows (in increasing order): stainless steel, steel, aluminum, and

copper.

The influence of the spraying angle on application of coatings of copper particles was

considered in Li et al. (2003). The deposition efficiency increases with increasing

spraying angle from zero at a critical angle to 100% at 900. The size of this region

depends on the particle-velocity distribution. The most important parameter, however,

is the velocity of the impact onto the substrate. The critical velocities for copper are

given in Alkhimov et al. (1991): approximately 560–580 m/s. The critical velocity is

affected by the particle size, the particle-size distribution (Van Steenkiste et al. 2002),

and the substrate material. On the other hand, the particle velocity for a certain

material is determined by the type of the accelerating gas, its pressure and

temperature, and the nozzle structure. The particle properties (density, size, and

shape) also affect the acceleration of particles and, correspondingly, the spraying

process (Van Steenkiste et al. 1999).

If the particles hit the substrate surface at an angle other than 900, the normal

component of the particle velocity is smaller than that in the normal impact. As the

particle deformation depends on the normal velocity, the angle can be assumed to

affect the spraying process and the coating microstructure. Though the influence of

the angle on the microstructure and properties of coatings obtained by thermal

spraying are known, such effects have different features in the cold spray, because the

sprayed particles are in a non-melted state. There are only a few papers where the

influence of the spraying angle in cold gas-dynamic spraying was considered

(Gilmore et al., 1999; Li et al., 2003).

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Commercial copper powders (with a particle size of 15–37µm) were used in

experiments. These copper powders were obtained by atomization and contained

spherical particles. Stainless steel was used as a substrate material. The substrate was

subjected to sandblasting by a corundum powder to study the process of copper

deposition and was polished to study the copper-coating microstructure. The nozzle

had a throat diameter of 2 mm, an exit diameter of 6 mm, and a length of the

supersonic section of 100 mm; the powder was injected along the nozzle centerline.

Nitrogen with a pressure of 2.0 M Pa and a temperature of 220 0C was used to

accelerate copper particles. The distance between the nozzle exit and the substrate

was 15 mm, and the velocity of substrate motion was 80 mm/s. The microstructure

was analyzed with the use of a scanning electron microscope (JEOL, JSM5800). It

was demonstrated that the dimensionless deposition efficiency has a maximum at

angles of 80-900. This means that the spraying angle has an insignificant effect within

these limits. With a further decrease in spraying angle, however, the dimensionless

deposition efficiency rapidly decreases and vanishes at an angle of 400. Thus, there is

an angle below which no deposition occurs. As the particles in the normal impact are

attached at a velocity higher than the critical value, the normal component of velocity

can be assumed to be the main factor. As the spraying angle decreases, the normal

component of the impact velocity also decreases; when the normal velocity

component becomes lower than the critical velocity, the particle is not attached.

The cold spray method simultaneously involves deposition of particles and erosion of

the substrate. Adhesion of particles occurs when they reach a velocity higher than the

critical value, while erosion occurs because there are particles whose velocity is lower

than the critical value. As there is some distribution of particles in terms of their

velocity, the critical velocity can turn out to be inside this distribution and only some

portion of particles attach to the substrate. On the other hand, particles with lower

velocities rebound and destroy the coating. If the particle hits the substrate at a certain

angle, the impact velocity of the particle can be divided into the normal component

and the tangential component with respect to the substrate surface. If the effect of the

tangential component is assumed to be small, the deposition is mainly determined by

the normal component of velocity. Correspondingly, the dimensionless deposition

efficiency changes insignificantly as the angle decreases from 900 to an angle at

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which the normal component reaches the critical velocity. With a further decrease in

spraying angle, the dimensionless deposition efficiency decreases from 100 to 0%.

The experimental results obtained support the validity of this model. Following this

model, we can assume that the range of transitional angles depends on the particle-

velocity distribution. The transitional region in terms of the spraying angle is wide for

particles with a wide velocity distribution and narrow for particles with a narrow

velocity distribution. The transitional region for copper is approximately 400. A wide

distribution of the particle size leads to a wide distribution of the particle velocity.

The effect of various parameters of the substrate, such as the substrate thickness,

surface roughness, substrate temperature, number of passes, and velocity of substrate

motion, was considered in Sakaki et al. (2004) by an example of copper coatings

(mean particle size of 8 µm) on soft steel. Nitrogen with a pressure of 3 M Pa and a

temperature of 623 K was used as a working gas. The deposition efficiency for copper

increases (from 60 to 70%) with increasing substrate thickness (from 6 to 32 mm).

The deposition efficiency also slightly increases with increasing substrate roughness

(from 0.2 to 7 µm Ra). Pre-heating of the substrate from room temperature to 450 K

and an increase in the number of passes (from 1 to 3) exert the same effect as an

increase in the substrate thickness. The deposition efficiency significantly decreases

(from 70 to 55%) with increasing velocity of substrate motion (from 20 to 100 mm/s).

Unfortunately, the authors did not give any comments on these effects.

The copper coating on an aluminum substrate (polished or etched) obtained in Voyer

et al. (2003) displays a low level of porosity. Moreover, the content of oxygen in the

coating is approximately 0.1%, which coincides with the content in the initial material

of the powder. It follows from here that cold spray does not oxidize the initial

material. The electrical conductivity of coatings is approximately 90% of the

electrical conductivity of copper proper, which is much higher than the electrical

conductivity of coatings obtained by gas-thermal methods, such as the gas-plasma and

the electric-arc methods (only 30–40% of the electrical conductivity of copper

proper). The Vickers 0.3 hardness of the cold spray coating is 150, and the

coating/substrate interface does not contain defects. Mechanical tests were performed

for two types of coatings: immediately after spraying and after annealing at 400 0C

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during 1 hour to examine the influence of annealing on mechanical properties. After

annealing, the coating becomes more plastic. The coating sample without annealing

after spraying broke under a load of 66 M Pa and a strain of 0.06%, whereas the

annealed sample could withstand a load of 195 M Pa with a total strain of 1.04%. A

comparison with characteristics of copper shows that Young’s moduli differ

insignificantly, but the coating strength is lower. Composite coatings from copper and

aluminum were also dense and, which is especially important, had a uniform

distribution of materials. Thus, the cold spray allows obtaining composite coatings

consisting of mixtures of materials (Voyer et al., 2003). The properties of the copper

coating on an aluminum substrate and the degree of the influence of spraying

parameters on the coating density were considered in Xiong et al. (2005). In the

examined range, the greatest effect on the coating quality is exerted by the standoff

distance, and then there follow the gas temperature and pressure. The influence of

voltage on the powder feeder (of the drum type), which occupies the last position, is

also noted. Yet, the authors commented that the degree of the influence of

temperature and pressure could be prevailing if the range of parameters is expanded.

Thus, the tests performed are primarily useful for optimization of a particular cold

spray setup rather than for optimization of the cold spray process proper. A decrease

in micro hardness with increasing coating annealing temperature from 160 to 90

(Hv0.2) is noted. Coating adhesion turned out to be 18 MPa. The authors assumed that

the basic mechanisms of adhesion are sub melting and mechanical adhesion. The

experiments aimed at determining the thermal and electrical conductivity of coatings

showed that these quantities reach approximately 70% of the corresponding values for

the cast material. This ratio is almost unaffected by annealing.

The effect of thermal treatment of copper coatings on their structure and properties

was considered in Calla et al. (2005). The grain size increases from 60 to 120 nm with

increasing annealing temperature from 25 to 200 0C. The micro hardness of copper

coatings remains roughly unchanged. As the annealing temperature increases to 300

0C and higher, recrystallization with a further increase in the grain size occurs

(dislocations related to micro strains form new strain-free grains), which impairs the

hardness of coatings. It was assumed in Meyers et al. (1992) that grains of an

approximately identical size are formed under conditions of dynamic recrystallization

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(i.e., under high strains and strain rates), but the recrystallization itself is initiated at

temperatures above 0.4Tm. Dynamic recrystallization can occur in the course of

deposition because of the formation of a high density of dislocations and a local

increase in temperature owing to adiabatic shear and heating of the substrate. The

grain size in the deposited coating is too small for optical microscopy; annealing at

200 0C reveals several very small grains, and annealing at 500

0C leads to noticeable

recrystallization with a grain size from 1 to 5 µm. The feature most frequently

encountered in tensile tests of coatings is the plastic fracture in the case of annealing

at 600 0C. This indicates that there exists a clear boundary with good adhesion

between the particles already during the spraying process, and the microstructure is

improved during annealing. Directly after deposition, the coatings display fracture

close to brittle fracture. The tensile force, however, is very high, which again

confirms the hypothesis about high-quality adhesion of particles.

The influence of thermal treatment on electrical resistance and hardness of copper

coatings (spherical particles 24 µm in diameter on the average) on tough copper

substrates, which were obtained with the use of nitrogen at a temperature of 150 0C

and a pressure of 2 M Pa, was considered in Li and Li (2005). It turned out that the

coating resistance immediately after deposition in the direction parallel to the

substrate surface is approximately half its value in the perpendicular direction (47%

against 81% of the value for the cast material). After annealing, however, they

become very close and reach 95–96% of the value for the cast material. The coating

hardness decreases with increasing annealing temperature.

The effect of thermal treatment of coatings obtained from copper powders (10–36

µm) on electrical resistance and coating hardness was examined in Lagerbom et al.

(2005). It was noted that annealing of the copper coating at a temperature of 300 0C or

higher decreases the coating hardness (from 140 to 80 Hv0.1). The electrical resistance

of the entire composition (steel substrate and copper coating) also decreases with

increasing annealing temperature. At an annealing temperature of approximately 200

0C and higher, the resistance reaches the value of 1.8×10

−8Ωm (the resistance of the

cast material is 1.6–1.7×10−8

Ωm) against 2.3×10−8

Ωm in the coating without thermal

treatment. The resistance of the coating proper without the substrate is 3.7×10−8

Ωm.

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The influence of the flow rate of the powder was examined in Taylor et al. (2005).

Significant concentrations of the powder in the gas flow reduce the impact velocity of

particles. If the powder concentrations are insufficient, the coating formed has some

holes. In both extreme cases, the quality of coatings is not as good as that desired. The

effect of the copper-powder concentration in a helium flow with deposition onto an

aluminum substrate on the coating thickness and microstructure and on deposition

efficiency was considered. A copper powder with angular particles (similar to crushed

stone) with a mean size of 25–30µm was used in the experiments. An axi-symmetric

nozzle with an exit diameter of 6.3 mm was used. The stagnation temperature of

helium was equal to room temperature, its stagnation pressure was 1.7 M Pa, and the

difference in pressure between the feeder and the pre-chamber was 35 k Pa. During

the deposition, the flow rate of the powder was changed (from 0.9 to 5.0 g/min),

whereas the substrate velocity was constant; one pass was made. The modeling

showed that the particle velocity should be higher than the critical velocity (equal to

450 m/s for copper); therefore, the deposition efficiency was expected to be fairly

high. In addition, the influence of particles on the gas can be neglected even for the

highest concentrations of the powder. Interesting results were obtained. The coating

thickness and mass (per unit length of the deposition band) increase if the flow rate of

the powder is low, but their values decrease if the flow rate of particles exceeds 4.1

g/min. Moreover, the coating starts exfoliating. The deposition efficiency thereby

remains approximately unchanged (about 85–90%). If the substrate velocity is

increased, however, no exfoliation occurs, and the coating is as dense as that obtained

with lower flow rates of the powder. The authors attribute exfoliation to more intense

cold working owing to enhanced bombardment of the surface, i.e., residual stresses in

the coating become higher, which is the reason for exfoliation.

The influence of the temperature and impact angle of copper particles with a mean

particle size of 56µm on the critical velocity of adhesion on a copper substrate was

studied experimentally and theoretically studied in Li and Li (2005). Nitrogen and

helium was used for acceleration in an axi-symmetric nozzle with the supersonic

section 100 mm long. The motion of the gas and particles is modeled by the FLUENT

+ DPM software (LS-DYNA, 1998), which takes into account the effects of

hardening and thermal softening but ignores heat transfer. The theoretical value of the

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critical deposition velocity is related to the emergence of adiabatic shear instability.

Experimentally, the critical deposition velocity is determined on the basis of the

measured deposition efficiency. The values obtained, however, were slightly lower

(from 295 to 355 m/s) than those mentioned in other publications (about 410 m/s).

This difference was attributed to the presence of an oxide film, but this phenomenon

was not modeled.

The properties of cold spray produced copper coatings were examined in Borchers et

al. (2003) and compared with copper coatings obtained by various gas-thermal

methods. It was found that the electrical resistance of the coatings is commensurable

with that of the cold-drawn cast material (1.7µΩcm), which turned out to be lower

than the resistance of gas-thermal coatings. The hardness of cold spray produced

copper coatings was estimated at 140–160 Hv0.3, which coincides with the hardness of

cold-drawn copper; the adhesion of cold spray produced copper coatings was

estimated at 30–40 M Pa, which coincides with the adhesion of coatings obtained by

gas-thermal methods (HVOF, HVCW (combustion wire)).

Copper coatings obtained by the cold spray and plasma methods were compared in

Barradas et al. (2005): porosity 0.5% against 9%; surface roughness Ra 8µm against

13µm; roughness of the coating/substrate boundary Ra 1.3µm against 13µm (the first

and second values in each pair refer to cold spray and plasma-produced coatings,

respectively). The coating was produced by the cold spray with the use of nitrogen

with stagnation parameters of 2.8 M Pa and 823 K and spheroidized copper powder

with a particle size of −22+5µm on a 2017 Al substrate.

Restrictions of thermal spraying in the automobile industry are often related to a

comparatively low quality of electro conducting elements with coatings applied by

conventional methods. The cold spray method can find its application for contact

joints, etc. The study of the microstructure of copper coatings (McCune et al. 2000 a;

McCune et al. 2000 b) showed that their properties depend on the powder

characteristics and on the spraying regime and that the coating becomes more plastic

after annealing.