ABSTRACT Cold spray process is a high velocity-low temperature variant of thermal spray technologies wherein the coating deposition takes place due to supersonic impact of metallic/alloy or composite powders onto a suitable substrate

ABSTRACT
Cold spray process is a high velocity-low temperature variant of thermal spray technologies wherein the coating deposition takes place due to supersonic impact of metallic/alloy or composite powders onto a suitable substrate. Supersonic gas and particle velocities are realized by employing a converging-diverging nozzle and feeding it with high pressure gas. However, design of the nozzle is very critical in cold spray as far as coating properties and overall cost of the process is concerned. In the present study, an attempt is made to identify the most energy efficient nozzle out of three designs that differ only in the absolute values of throat and exit dimensions (1.7×1.7, 2×2 and 3×3). Other design features such as mach number (M), shape of exit and throat and length of diverging portion and converging portion have been maintained constant. Process parameters were also maintained constant. Coatings deposited were characterized and the anomaly in their properties and microstructure has been addressed by looking at the particle velocity data and boundary layer domination in three nozzles.
1. INTRODUCTION TO CENTRE FOR ENGINEERED COATINGS
Overview of Surface Engineering
Surface engineering refers to the modification of the surface of a component to confer surface properties, which are different from the bulk properties. This need for surface modification is derived from the fact that most of the engineering failures e.g., corrosion, wear, friction, surface adhesion, fatigue etc. initiate from the surface of failed products or components. It is also realized that in some cases the properties of bulk materials alone will not provide the level of performance demanded. For instance, the most detrimental failures encompass aircraft accidents, automobile accidents, rupture of pipelines, erosion and corrosion of ship propellers and hulls and, ample number of other engineering components and structures. The remedy for the majority of these kinds of failures is implicitly the alteration of the surface discipline by finishing or coating. At present there are diverse techniques and several coating species which not only serve to overcome friction losses but also provide thermal insulation, electrical insulation, act as diffusion barrier or simply improve the aesthetic appearance. For instance, the performance of aircraft and moter vehicles are dependent on surface engineering components and about 85% of these industries are affected by coatings.

Generally today’s era of advanced tool industrial world, a substantial need for better processes with higher performance and productivity has arisen. Many of industrial components are subjected to harsh environments and hence are prone to rapid degradation and failure ultimately. In some cases the properties of bulk materials alone do not provide the level of performance desired as evident from detrimental failures of aircrafts, automobiles, pipelines erosion and corrosion of ship propellers and numerous other engineering components. Surface engineering is used to incorporate an engineered surface to combat degradation over the surface while retaining the toughness and ductility of the bulk component, leading to reduction in the replacement frequency and enhancing the decorative and aesthetic appeal. It involves all scientific details and problems related to the surface layer, its manufacture, and enhancement prior to or during service. Benefits such as material savings, enhanced efficiencies and environmental friendliness make this branch of science, an attractive prospective for metallurgists.
The manufacturing industries are constantly decrease it is cutting costs and increase the quality of the machined parts as the demand for high manufactured is rapidly increasing. The increasing need to be boost productivity, to machine more hard materials and to improve quality in high volume by the manufacturing industries has been the driving force behind the develop of cutting tool materials.
For all this the answer could be Surface Engineering. The ‘surface engineering’ all coating techniques and processes which are utilized, modify and enhance the performance such as wear test, fatigue and corrosion resistance of the surfaces, and these three are major categories of interrelated activities:
(1) Optimization of surface coating properties: the optimization of the performance of (surfaces and substrates, i.e. in coating system) in terms of corrosion, wear and other properties are (physical and mechanical properties).

(2) Coatings technology: This coating technologies more traditional of painting, electroplating, weld surfacing, plasma and higher velocity spraying, various thermal and thermo chemical treatments such as nitriding and carburizing,
(3) Characterization of coatings: Evaluation of composition, microstructure, and mechanical, electrical properties.

These coating processes are successful exploitation. The coating processes are vary simpler, cheaper substrate or base materials, with substantial reduction in costs, minimizing of demands for strategic materials and improve productivity and performance. Surface engineering practices are major roles in long blasting problems and enhancing prospects for advancement in three major places of technology corrosion, wear and manufacture.
The overall surface engineering coatings approached by the fact those modifications can be metallurgical, mechanical, chemical or physical properties. Generally, the surface treatments are broadly classified into three types, namely the physical treatment, the chemical treatment and coatings, of which the latter two have been developed vastly since the few last decades. The processes of nitriding, and carburizing are some of the thermo-chemical treatments are improving the surface properties of alloys, performance at temperatures up to 1200°C in difficult to create wear-resistant surfaces.

Therefore with all this developments in the field of materials and its treatment, it has been observed that the impact of materials is very much when it comes to the aerospace industry. It is in constant search of a lighter, stronger and thermally stable material for an aircraft`s framework and its engine. The ever increasing fuel prices and the impact made on the ecology are the most important problems on the agenda for the aerospace industry. But, as the search for the toughest and hardest material is just one side of the coin the other problem that is encountered is the ability to machine (cut) these materials to a precise shape. The conventional tools that are used will fail easily against these hard materials and the cost of special tools is not an economic idea, therefore a conventional tool treated with a surface coating is the need of the hour.

Surface Engineering at ARCI
The different coating techniques commonly employed in ARCI are
Chemical Deposition
Micro Arc Oxidation.

Pulsed Electro Deposition.

Thermal Spraying
Solution Precursor Plasma Spraying
Cold Gas Dynamic spray
Detonation Spray
Physical Vapor Deposition
Electron Beam Physical Vapor Deposition.

Cathodic Arc Physical Vapor Deposition.

2. Literature Survey
2.1 Cold Gas Dynamic Spray Technique:
Cold spray process has been emerged as a novel deposition process in which micron sized metallic particles are accelerated to supersonic velocities to obtain deposition upon impact onto a substrate. This process has been drawing attention among industries since the technique is a low temperature deposition process and is unique among the conventional thermal spray techniques. Materials prone to oxidation and phase transformation can be easily deposited without any change in properties since cold spray is a solid-state deposition process. Variety of metals has been successfully deposited with high density using this technique. Bonding mechanism in cold spraying was well established which can be referred elsewhere. Plastic deformation induced adiabatic shear instability is one of the well-established factors for successful inter-splat bonding. It was reported to cold spray technology, so many worn out / damaged part should not be reclaimed due to the lack of restoration technology. Several articles were published emphasizing the importance of cold spray technology for structural repair applications. For instance, Champagne et al. compiled a critical assessment of different repaired parts for different industries such as nuclear, aerospace, automobiles etc.

2.1.1 CGDS Process Description
CGDS is a process of applying coatings by exposing the metallic or non-metallic substrate to a high velocity (300 to 1200 m/sec) jet of small metallic powder particles (10 to 50 microns) accelerated by a supersonic jet of compressed gas (Air/Nitrogen/Helium). The high velocity impact of the particles disrupts the oxide films on the metal particle and provides intimate conformal contact under high local pressure, thus permitting bonding to occur by allowing a kind of “explosive welding” of the materials. This hypothesis was consistent with the fact that wide ranges of ductile materials, such as metals and polymers, have been cold spray deposited. On the other hand, experiments with non-ductile materials are ceramics, have not been successful unless they are co-deposited along with a ductile matrix materials. A suitable combination of particle velocity, temperature and size leads to the deposition of reliable coatings. The principle behind the formation of thermally sprayed coatings is to melt material feedstock (wire or powder) with a heat source (electric/gas) and then accelerate the molten droplets to impact onto the substrate, where rapid solidification at the rate of 104 to 108 K/sec occurs. In contrast to the conventional thermal spraying processes, the particles are accelerated by a supersonic gas jet at a temperature much lower the melting point of the materials in the CGDS technique, enabling only solid-state deposition of coating materials. The relative comparison of particle velocity and gas temperatures typical of the CGDS process with that of other different thermal spray processes is shown in Fig. 2.1.1.1
More problems associated with conventional thermal spray coating methods because of the high temperatures required to heat (or) partially melt the coating materials. The high deposition temperatures often preclude the spraying of temperature-sensitive materials like Titanium. Moreover materials prone to phase transformations, excessive oxidation, evaporation, Additional problems arise due to possible residual stress effects and deformations induced by the thermal expansion mismatch between the coating and substrate. The coating partials are bonded to the substrate, the inherent residual stresses may be unacceptable distortions to week them bond strength and accelerate coating failure. Many of the above mentioned problems can be suppressed by using the low temperature CGDS process
2.1.1.1 Comparison Cold spray and other process

Figure 2.1.1.2, 2.1.1.3 and 2.1.1.4 show as a schematic of the CGDS systems. The operating gas (Compressed Air/Nitrogen/Helium) at certain pressure is introduced to amanifold system containing typically a ceramic cartridge gas heater, where the heating of the gas is accomplished electrically. A metered quantity of powder is fed by a high-pressure powder feeder to an intermediate mixing chamber, from where the heated operating gas carries the powder particles through a de Laval type converging-diverging supersonic nozzle. The compression of the heated gas and the corresponding powder particles happens at the throat region. It may be pointed out that the particles are significantly accelerated as they pass through the nozzle.
The warm/hot particles that are accelerated to supersonic velocities by the time they exit the nozzle proceed to impact the part to be coated, where they flatten. Manipulation of the gun and the job and the deposition of successive layers are eventually responsible for the formation of a coating of desired thickness on large areas and on complex geometrical shapes.
Fig. 2.1.1.2: Schematic diagram of cold spray process.

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Fig 2.1.1.3 Cold spray gun

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Fig. 2.1.1.4 Cold Spray Robot (6 Axis) setupGun assembly housing nozzle and heaterCompressive residual stresses
Low oxide and porosity

2.1.2 Mechanism of Coating Formation
The overall coating development process can be divided into four distinct stages as per a recently proposed mechanism. These four stages are depicted in Fig. 2.1.2.1. with the transition between individual stages being governed by the incident particle velocity. The four stages can be briefly listed as follows,
Stage 1: Substrate creating and first layer develop up the particles.
Stage 2: Particle deformation and multilayer coating as described.

Stage 3: Metallic particles bond formation between particle–particle and void reduction.

Stage 4: After densification and work hardening of coating.

The basic coating formation is transformation of the kinetic energy to plastic deformation and thermal energy. This transformation occurs rapidly, on the order of 108 S-1.
Fig.2.1.2.1. Coating formation

2.1.3 Influence of Process Parameters
The typical operating regimes for obtaining cold-sprayed coatings are
Gas Pressure: 14-20 kg/cm2 Gas Temperature: 100- 6000C
Gas Flow Rate: 10-70 cfm Powder Feed Rate: 1-15 kg/hr
Spray Distance: 8-30 mm Powder size: 1-50 µm
The influence of each individual process variable on the quality of cold-sprayed coatings has been the subject of considerable research interest. Nozzle design, too, has attracted significant attention. Nozzles made of high strength die steels are commonly used. While different nozzle shapes have been studied, few results reported in the literature suggest that the rectangular nozzles produce higher deposition efficiency than other shaped nozzles. The influence of nozzle size on coating formation has also been a subject of investigation.

2.1.4 Advantages, Limitations and Applications of CGDS
Advantages of CGDS:
Low Temperature Process
Below Melting Point of Metals
No Combustion Fuels, Gases
Solid State Bonding
Mechanical Mixing of Particles and Substrate
Similar to Explosive Bonding
Plastic Deformation of Particles Disrupt Oxide Films
High Density Deposits
Form Thick Coatings at High Deposition Rates
Low Oxide content
Low Porosity Content (‹1%)
Compressive Residual Stresses
High hardness and cold worked microstructures can be obtained.

Limitations ofCGDS:
Coatings at present are limited to ductile materials like Aluminium, Silverand Copper.
Metals like Titanium, Tantalum and Niobium need gas velocities more than 1100 m/s.

Hard and brittle materials like ceramics cannot be sprayed in the pure form, but may be applied as composites with a ductile matrix phase.
It is a line of sight process, special job manipulators may be needed to coat on complex shaped substrate.

Substrate materials are also limited to those that can withstand the aggressive action of the spray particles. Soft or friable substrates will erode rather than be coated.
The critical velocity requirements of certain feedstock cannot be achieved with air or nitrogen, forcing the use of expensive helium.
The characteristic cold working of particle does not allow the much-needed ductility to the coatings. Consequently, post-treatment of coatings is required in many cases.
The use of inert atmospheres for highly reactive materials like titanium adds marginally to the operational coat.
2.1.5 Applications of CGDS:
Biomedical – prostheses with improved wear
Aerospace – fatigue-resistant coatings
Chemical – improved corrosion resistance
Mineral processing – improved corrosion and erosion resistance
Die casting – extending die life
Electronics – creating a heat sink or superconductive, magnestostrictive surfaces
Printing – copper coating on rollers
Oil and gas – improved corrosion resistance
Glass – platinum coating.

3. EXPERIMENTAL DETAILS
3.1. Material Selection:
Substrate:
The substrate materials chosen for the experiments are mild steel which was fabricated into samples of 30mm by 30 mm dimensions, 5 mm thickness respectively.

Powder Materials:
The powder material chosen for coating is Copper (Cu) powder. The size of the powder was 10-45?m. And the shape of the copper powder particles is dendritic as shown in fig.3.1.1
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Fig.3.1.1. Dendritic morphology of copper powder
3.2. Sand Blasting
The samples that are to be coated were sandblasted in a sand blasting machine (M/s Metallurgical Equipment Company, Jodhpur, India) to make the surface of the substrate rougher which helps in proper interlocking of the coating material to the substrate during thermal spraying process. This would result in very good bonding between the coating and the substrate. The abrasive media for shot blasting was silica sand with a particle size of 60 Mesh. The air pressure was kept around 3 kg/cm2.Samples were cleaned in acetone in an ultrasonicator after blasting.

3.3. Coating with Cold Spray
Cold spray coating of electrolytic copper were deposited as per table-1. The standoff distance was set at 15 mm and powder feed rate was set at 18 g/min for all experiments. Three nozzles with same design Mach number (M=2.75) but with different throat areas was employed in the present study as shown in Table-1.

Table-1.
S.No Nozzle Parameter Mass flow rate (kg/s) Heater power (kW)
Pressure, (bar) Temperature,
oC 1 1.7×1.7 15 250 2 1.7×1.7 15 350 3 1.7×1.7 15 450 4 1.7×1.7 20 250 5 1.7×1.7 20 350 6 1.7×1.7 20 450 7 2×2 15 250 8 2×2 15 350 9 2×2 15 450 10 2×2 20 250 11 2×2 20 350 12 2×2 20 450 13 3×3 15 250 14 3×3 15 350 15 3×3 15 450 16 3×3 20 250 17 3×3 20 350 18 3×3 20 450 Coatings were also deposited as single and multiple beads by rastering the spray gun to study the boundary layer effects.
3.4. Particle Diagnostic study
A powder particle velocity at all the above parameters was measured experimentally using Oseir spray watch equipment as shown in fig.3.4.1. Using high speed camera and laser particles are captured in illuminated condition in flight at three different instances. Distance travelled by the particle divided by the pulse interval determines the in flight particle velocity.

Spraywatch setup3810340360
Fig.3.4.1. Spray watch setup to measure particle velocity
3.5. Preparation of Metallographic Specimen
A metallographic specimen preparation for macroscopic and microscopic investigation compulsory includes a representative plane surface area of the material. To clearly distinguish the structural details, this area must be free from changes by surface deformation, the material (smears), plucking (pullouts), and scratches.
3.5.1 Specimen Sectioning:
Following proper steps, most of metallographic samples need to be a certain area of interest and for easyb to handling. Depending upon the material, the certain section will be cut in to the abrasive cutting machine (metals and metal matrix composites). The mild steel sample coated with Cu were cut into 2cm square specimens by using the BUEHLER make ISOMET-1000 model linear precision saw machines at 300 rpm and 150gm load with diamond cut-off wheel show in fig 3.5.1.1. The coolant is used to prevent more heat that is affect the 1704975790575microstructure of the specimen.
Fig 3.5.1.1 Sectioning machine

3.5.2. Mounting
The mounting machine can be used for easy to handling the specimen.

Hot mounting procedure is adopted (Metamount 3 Automatic Pneumatic Mounting Press M/s Scientific Tech). The majorly of metallographic specimens (Cu coated) we are mounted by the specimen into a compression mounting machine. The Heating time was around 10 minutes and cooling time was around 12 minutes. Bakelite and dually phthalate resins are thermosetting while acrylic resins are thermoplastic, both requiring heat and pressure during molding. After completion of heating and cooling cycle, the mount was taken out. Parameters of mounting experiment are:
Temperature -1800C
Heating – 10min
Cooling -12min
Mounting powder -Bakelite
Fig. 3.5.2.1 Mounted Specimen
Fig. 3.5.2.2 Hot mounting machine

3.5.3 General polishing:
There are three main steps in mechanical polishing.

(A) Grinding:
In this process we have to chamfer the all sides of the specimen i.e. blend the sharp edges of the mounted specimen holding purpose. This process is generally carried out on belt grinder or rough abrasive paper (60-100 grit). If we do not remove the edges there is a chance of cutting the polishing cloth and hands also.
(B) Rough polishing:
Fig. 3.5.3.2 Sample rotation
-3048001213485 In this process the specimen was polishing using the emery papers size of 60, 220, 340, 400, 600, 800, 1000, 2000 and 4000 grit papers for 5-10 minutes/grit. Polishing is done on grit papers to remove the deformed layer and produce a flat surface with unidirectional scratches. Rotate the mount through 90 0 on each successive grinding paper steps 3.5.3.1 and 3.5.3.2 shows the rough polishing process.

Fig.3.5.3.1 Rough polishing

(C) Diamond Polishing or Fine polishing:
3.5.3.3 Directions for polishing
Wheel and sample rotation.

After completed rough polishing, starts the disc polishing using diamond compound of 3?m, 1?m, and 0.5?m to remove scratches left over after polishing on emery papers. In this we have to gradually decrease the load from 10 um to 1um. . After every step we have to examine the specimen under the ordinary Microscope or in optical Microscope. Fine polishing of the mounted samples was carried out on automated disc polishers with a velvecloth (BUEHLER make polisher ; model: ECOMET – 4)

3.5.4 Ultrasonic Cleaning:
 Ultrasonic cleaner is a cleaning device that used for ultrasound (usually from 20–300 kHz) and an appropriate cleaning solvent (water). The ultrasound can be use only water but use of a solvent appropriate for the samples to be cleaned and the soiling enhances the effect. Cleaning normally two and three minutes.
Operating principle:
The ultrasonic cleaning for a solvent (water) fills in to the certain level of chamber. The water fills after the sample keep into chamber will be start machine. the machine can be start that time produces ultrasonic waves in the fluid by changing size in concert with an electrical signal oscillating at ultrasonic frequency.
4. EVALUATION OF PROPERTIES
4.1 Electrical Conductivity measurement:
The conductivity of the coating is measured before and after IR treatment. The conductivity of the coating is measured by using FORESTER SIGMATEST 2.069 machine which has an eddy current conducting gauge. Before measuring conductivity the coating has to be lightly polished.

The principle behind this measurement is the generation of skin effect as per the formula given below
= (?2 f µ)-1
Where
?= Standard Depth of Penetration (mm)
= 3.14
f = Test Frequency (Hz)
µ = Magnetic Permeability (H/mm)
= Electrical Conductivity (% IACS)

Fig.4.1. Conductivity Measuring Machine
4.2 Vickers Micro Hardness:
Micro hardness testing of metals, ceramics, and composites is useful for a variety of applications. Micro-hardness testing per ASTM E-384 gives an allowable range of loads for testing with a diamond indenter; the resulting indentation is measured and converted to a hardness value. The actual indenters used are Vickers (more common; a square base diamond pyramid with an apical angle of 136°) or Knoop (a narrow rhombus shaped indenter). The result for either Vickers or Knoop micro hardness is reported in kg/cm2 and is proportional to the load divided by the square of the diagonal of the indentation measured from the test. A schematic diagram showing a Vickers hardness sample indentation and Vickers micro hardness tester Fig.5.2
Fig.5.2.1 Schematic of Vickers indentation for calculation of hardness

Fi Fig.5.2.2 Photograph of Vicker
s micro hardness tester at ARCI
The hardness test is done on UHL VMHT Vickers hardness tester shown in fig.5.3. The load on the Vickers micro-hardness indenter usually ranges from a few grams to a kilogram. Vickers hardness is also sometimes called Diamond Pyramid Hardness (DPH) owing to the shape of the indenter. The test samples should have a smooth surface and be held perpendicular to the indenter.
The indentation of the different coated samples were done by a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 100gm and 300gm load. The full load is normally applied for 15 seconds.
The Vickers hardness is the quotient obtained by dividing the load by the area of indentation.

The Vickers hardness is calculated using the Equations

Where HV is Vickers Hardness, F is Load in kgf and d is average of two diagonals, d1 and d2 of indentation in mm. When the mean diagonal of the indentation has been determined the Vickers hardness may be calculated from the formula, but is more convenient to use conversion tables. The Vickers hardness should be reported like 800 HV/10, which means a Vickers hardness of 800, was obtained using a 10 kgf force.

5.RESULTS AND DISCUSSIONS
RESULTS:
Particle velocity data calculated for different parameters is shown in fig.5.1. The particle velocity increases as a function of process parameter (increasing pressure and temperature). However, the average particle velocity is not the same for all the nozzles despite the fact that the experiments were carried out at the same pressure-temperature combination. The velocity was the highest in 3×3 nozzle followed by 2×2 and 1.7×1.7.

Fig.5.1 raster horizonalvelocity f process parameter fParticle velocity as a function of process parameter for 3 nozzles

Electrical conductivity of the coatings shows an expected trend of increase with an increase in pressure and temperature as shown in fig.5.2 below. Although, the process parameters employed to deposit coatings were the same for the three nozzles (also same design mach number M), there is a huge discrepancy in the coating electrical conductivity especially for coatings deposited using 1.7×1.7 nozzles. Coatings deposited using 2×2 and 3×3 nozzles showed electrical conductivities close to each other. Hardness of the coatings also exhibited a similar trend, wherein the 2×2 and 3×3 showed hardness values close to each other and 1.7×1.7 nozzle had inferior hardness especially so at lower pressure and temperature as shown clearly in fig.5.3.These discrepancies in coating properties can be understood by observing the microstructures of the coating cross section. Polished coating cross sections for three parameters are shown in fig.5.4 (15 250, 15 450 and 20 450).

Fig.5.2. Electrical conductivity as a function of process parameters for three nozzles
Fig.5.3.Hardness as a function of process parameters for three nozzles
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15,250
25400153670 15,250
31750153670 15,250
13970153670
15,450
25400132080 15,450
31750100330 15,450
6477049530
20,450
19050118745 20,450
-1905118745 20,450
64135118745
1.7×1.7
2×2
3×3

Fig.5.4 Cross section microstructure for three process parameters for three nozzles

Coating cross section micrographs also exhibit a contrast between three nozzles similar to that of the measured properties as shown in fig.5.2 and fig.5.3. The coatings developed using 1.7x.1.7 nozzle shows higher porosity, insufficient bonding, lower build up etc. especially at lower parameter (15, 250) and intermediate parameter (15,450). In contrast, microstructure of coatings deposited using 2×2 and 3×3 nozzles show minor differences as is the case with their properties.

DISCUSSIONS:
The anomaly in the coating properties and microstructure as described in the previous section can be explained by looking at the average particle velocities at different process parameters using the three kinds of nozzles. In cold spray, coating property or even for that matter, coating deposition is intimately related to the particle velocity before impact. It is well known that deposition in cold spray is possible only when the particle reaches or crosses “critical velocity (Vcr)” which is a function of the thermo-mechanical properties of the powder being sprayed.
Fig.5.5.Electrical conductivity variation with particle velocity for three nozzles
In this particular case, fig.5.5 shows the variation of coating electrical conductivity with average particle velocity measured experimentally as detailed in experimental part. It can be clearly seen

that, although there is an accepted trend of increase in conductivity with particle velocity, there is a disparity in the particle velocity measured at the same parameter for the three nozzles (as shown in fig.5.1 as well). This disparity can only be rationalised by invoking the boundary layer theory in compressible fluid flow through converging-diverging nozzles or venturis. Fig.5.6 shows hardness data of coatings as a function of experimental particle velocity and it can be observed clearly that even in this case the 1.7×1.7 nozzle has inferior hardness values at lower particle velocities akin to electrical conductivity in fig.5.5. However, as the particle velocity is further increased, the hardness data of coatings obtained using all three nozzles fall on a single line indicating that hardness being a structure sensitive property, once there is a considerable amount of strain hardening caused by the cold spray process (say beyond a particle velocity of 480 m/s for 1.7×1.7) the further increase in particle velocity or tamping effect will only induce low amount of hardening as the saturation of hardening that can be caused by depositing coating at that parameter has already taken place. In fact, further increase may induce partial dynamic recrystallisation which will reduce overall hardness to some extent.Fig.5.6. Hardness variation with particle velocity for three nozzles

In order to understand the discrepancies in properties, coating microstructure and particle velocities and also to ascertain and estimate the boundary layer effects, a separate set of experiments were carried out. Apart from velocity contours obtained from spray watch, it is interesting to look at the horizontal single raster coating beads. Horizontal coating beads by moving parallel to the longer dimension of the respective nozzles were carried out and fig.5.7 shows that the coating thickness variation across coating width is drastically different for 1.7×1.7 nozzle in comparison to 2×2 and 3×3 (see fig.5.7) nozzle clearly establishing the fact that the boundary layer formation near nozzle walls is reducing particle velocities and thereby reducing the coating build up across the width and in turn reflecting in final coating properties.

Fig.5.7.Cross section microstructure of single and double raster horizontal beads for 15 bar, 350oC (a) and (b) 1.7×1.7 nozzle, (c) and (d) 2×2 nozzle and (e) and (f) for 3×3 nozzle and (g) coating thickness profile for three nozzles
1000 µm
a
1000 µm
b
1000 µm
c
1000 µm
d
e
1000 µm
f
1000 µm
g
Particle velocity contours as shown below in fig.x10 also support the observations made in the above fig 5.7.104775788670
Hence, it can be concluded that the reduction in particle velocities brought in by the dominance of boundary layers in 1.7×1.7 nozzle results in coatings that are significantly inferior to that of 2×2 and 3×3 nozzles. Therefore, in the pursuit of making the cold spray process energy efficient, it is mandatory that the boundary layer effects are also taken into consideration.

CONCLUSIONS
Successfully deposited dendritic copper coatings using three different nozzle with same design mach number (M=2.75) but different throat areas
Employing 2×2 and 1.7×1.7 nozzle results in reduction of gas and power consumption by 70% and 60% respectively.

However, closer look at coating deposition efficiency, coating properties such as hardness, electrical conductivity, porosity, coating microstructure reveals that the coating quality is not maintained despite depositing coatings at same parameters.

Particle velocity data, single raster horizontal beads revealed that the boundary layer effects starts to dominate in 1.7×1.7 nozzle and hence the coating characteristics in coatings deposited using 1.7×1,7 nozzle are markedly inferior to that of 2×2 and 3×3 nozzles.

From the above analysis and taking into consideration the overall savings on the process, 2×2 nozzle should be used for making the process energy efficient.