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JTTEE5 22:1348–1357 DOI: 10.1007/s11666-013-0034-5 1059-9630/$19.00 Ó ASM International

The Elastic Modulus of Cold Spray Coatings: Influence of Inter-splat Boundary Cracking G. Sundararajan, Naveen M. Chavan, and S. Kumar (Submitted March 30, 2013; in revised form September 28, 2013) It is well established that cold spray coatings exhibit substantially lower elastic modulus as compared to bulk material of the same composition. It has also been observed that the heat treatment of the cold spray coatings results in a significant increase in the elastic modulus of the coating. To check whether the presence of inter-splat cracks is responsible for the above behavior, a wide variety of metallic materials (Cu, Ag, Zn, Nb, Ta, Ti, and 316L stainless steels) in the powder form have been deposited on a mild steel substrate using the cold spray technique. These coatings in both as-coated and heat-treated conditions have been characterized for their porosity, extent of inter-splat boundary cracking, hardness, and elastic modulus. Results indicate that the elastic modulus of the coatings are substantially lower than the bulk value and also that the heat treatment of the coatings consistently increase their elastic modulus values. It has been shown that the reduction in elastic modulus of cold spray coatings can be related to the extent of inter-splat boundary cracking. Further, it has been shown that the standard models relating elastic modulus to the crack density are capable of explaining the observed modulus in the case of cold spray coatings in the as-coated and heat-treated conditions.

Keywords

cold spray, elastic modulus, inter-splat boundaries, nano indentation, porosity

1. Introduction Cold spray coatings, belonging to the family of thermal spray coatings, are formed through repeated impact of powder particles on to the substrate at high velocities (500-1200 m/s) and as a result exhibit very low porosities. Since the powder particles are heated only to maximum temperature of few hundred degrees (please note that the powder particle temperature is considerably lower than the carrier gas temperature) in the cold spray technique, the composition and phases present in the powder feedstock are essentially retained in the final coating formed. In particular, the oxidation of powder particles during their flight (prior to impact onto the substrate) is minimal leading to near zero oxide content in the coating with the attendant advantages like high electrical and thermal conductivity.

This article is an invited paper selected from presentations at the 5th Asian Thermal Spray Conference (ATSC 2012) and has been expanded from the original presentation. ATSC 2012 was held at the Tsukuba International Congress Center, Ibaraki, Japan, November 26-28, 2012, and was organized by the Japan Thermal Spray Society and the Asian Thermal Spray Society. G. Sundararajan, Naveen M. Chavan, and S. Kumar, International Advanced Research Centre for Powder Metallurgy & New Materials (ARCI), Hyderabad 500005, India. Contact e-mail: [email protected].

1348—Volume 22(8) December 2013

Cold spray coatings with attractive properties have been obtained in the case of pure metals like Cu, Al, Ti, Ni, Fe, and Ta (Ref 1-9) and also in the case of metallic alloys like stainless steels, Inconel, Ni-Cr alloys, Ti-6Al4V, and steels (Ref 10-17). In all these coatings, the coating property and performance is substantially influenced by the extent of bonding between the splats (powder particles which flatten on impacting the substrate are defined as splats) that make up the coating. It is believed that the bonding between the splats is essentially the result of high pressure generated in the contact surface, which helps in breaking and pinching out the oxide layer on splat surfaces in contact and also due to adiabatic shearing effects which concentrate the deformation on impact to the contact region causing a substantial temperature rise in the contact area which again aids bonding (Ref 18-24). In spite of the fact that the above two factors aid intersplat bonding, in a typical cold spray coating, only a fraction of the splats are well bonded with the neighboring splats. Further, the fraction of well bonded splats depends on the material being coated and on the coating process parameters. The fraction of debonded or cracked splats, on the other hand, has considerable influence on the elastic modulus and electrical conductivity of the cold spray coatings. In fact, in the as-coated condition, all the cold spray coatings exhibit elastic modulus values which are lower than that of bulk material of the same composition by 40-80% (Ref 25-29). As a result, cold spray coatings invariably require subsequent heat treatment to recover properties closer to that of bulk material (Ref 27-29). It also follows from the above discussion that the elastic modulus of cold-sprayed coatings is an excellent indicator of the extent of inter-splat cracking in the coating, and thus the quality of the coating. Elastic modulus can be

Journal of Thermal Spray Technology

2. Experimental 2.1 Coating Procedure and Heat Treatment The powders for cold spraying were chosen not only on the basis of their sprayability but also because they represented a wide range with regard to their melting point and elastic modulus as illustrated in Table 1. Thus, it was felt that use of such divergent powders as feedstock for cold spraying will provide a more general validity for the use of elastic modulus to correlate the extent of inter-splat cracking. All the coatings were deposited using the in-house facility for cold spraying. A De Laval nozzle with a rectangular exit was used and compressed air was used as both the process and powder carrier gas. Commercially available powders of silver, zinc, copper (Innomet, India), SS 316L (Prax Air, USA), tantalum (Inframat, USA), niobium (H.C. Starck Gmbh), and titanium (Medicoat, Switzerland) were used as the feedstock. The silver, copper, and zinc powders were manufactured by water atomization, while SS 316L was air atomized. Niobium and titanium powders were mechanically crushed, whereas tantalum was derived chemically. Figure 1 illustrates the morphology of all the powders used in the present study. The measurement of powder particle size distribution of all the powders indicated their average sizes (i.e., diameter at 50%) to be in the range of 25-35 lm.

Journal of Thermal Spray Technology

The substrate samples were grit blasted and subjected to ultrasonic cleaning prior to the coating deposition. To optimize the cold spray coating process parameters for each powder, the stagnation pressure was varied over the range 10-20 bar and gas preheat temperature from 300 to 475 °C. The cold spray process was then optimized in terms of stagnation pressure and gas preheat temperature on the basis of deposition efficiency. The duration of the coating was adjusted so as to get a coating thickness of 550 ± 50 lm in all cases. Table 1 provides the optimized process parameters for each coating. All the seven coatings were heat treated at various temperatures to observe its effect on coating properties and in particular elastic modulus and its correlation to the evolving micro structure (i.e., inter-splat cracks). The heat treatment conditions are provided in Table 2. All the coatings were furnace-cooled after the soaking period, except SS 316L coatings which were air-cooled.

2.2 Coating Characterization The cold-sprayed coatings were sectioned in a direction perpendicular to the coating plane and then polished. These sectioned surfaces were examined using Scanning Electron Microscope (SEM, Hitachi-S-4300N, Japan) and in conjunction with the image analysis software (analysis, Olympus, USA), the porosity was also estimated for both as-coated and heat-treated cold spray coatings and was found to have an associated scatter of up to ±20% around the mean value. The sectioned and polished surfaces were also etched to reveal the inter-splat structure of the coating. The flattening of the feed-stock powder during the cold spray process was also estimated using the image analysis software. The area of the individual splats was determined and diameter dc of a circle of equivalent area was calculated. The longest dimension of each splat due to flattening (df) was also obtained from the micrographs. The ratio df to dc was then estimated as the flattening ratio (defined as a). Since the original feedstock particles were not spherical, the a values computed in the above manner are only approximate values. The values of a were obtained only on the coatings in the as-coated condition. The hardness of the coatings was measured on sectioned and polished surfaces of the coating using a Vickers microhardness tester (Walter UHL, Germany). In each sample, at least 10 hardness measurements were carried out at a load of 200 g and the average value of the hardness (H) has been reported. The average Hvalue had an associated scatter of up to ±11.8%. The hardness of the coatings were obtained on the as-coated and heat-treated cold spray coatings. The elastic modulus of the coatings (Ec), in the as-coated and heat-treated conditions, was obtained using a nanoindentor (MTS System Corporation, USA) at a 200 g load utilizing the technique described by Oliver and Pharr (Ref 34). The modulus measurements were taken at ten different locations on the sectioned and polished coating sample surfaces and the average value has been reported. The scatter in the measured value was up to ±8% around the mean value.

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measured on a wide range of metals and alloys unlike electrical conductivity which works only on materials exhibiting reasonably high values of electrical conductivity. However, it should be admitted that the elastic modulus is an indirect and qualitative indicator of the extent of inter-splat cracking in the cold spray coating unlike techniques like SAXS and SANS, which are able to quantify the volume fraction of inter-splat cracks, pores etc. (Ref 30-33). The objective of the present work is to assess and demonstrate the usefulness of elastic modulus as a parameter capable of quantifying the extent of inter-splat cracking in the case of cold spray coatings. Towards the above purpose, a wide variety of materials (Cu, Ag, Zn, Ta, Nb, Ti, and 316L stainless steel) with varying physical and mechanical properties were deposited using cold spray technique. In addition, these coatings have been heat treated at various temperatures to improve the intersplat bonding, and thus study the resultant effect on elastic modulus. It is shown that elastic modulus appears to be strong indicator of the extent of inter-splat bonding as observed independently through SEM micrographs of sectioned and polished surfaces of the cold spray coatings. It is also shown that the existing models relating the elastic modulus to crack density in solid materials can explain the observed reduction in the modulus of cold spray coatings as compared to bulk material, and also the observed increase in the elastic modulus with heat treatment of the cold-sprayed coatings.

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Table 1 Properties of coating materials in bulk form Optimzed spray parameters Material

Melting point, K

Elastic modulus, (Eb) (a), GPa

Stagnation pressure, bar

Gas preheat temperature, K

Ag Cu Zn SS316L Ta Nb Ti

1233 1356 693 1623 3287 2740 1943

82.7 129.8 104.5 193.0 185.7 104.9 120.2

20 20 20 20 20 20 10

723 673 623 748 723 723 673

Stand-off distance: 15 mm The coating duration was adjusted so as to get a final coating thickness of 550 ± 50 lm in all materials (a) Ref 45

3. Results The SEM micrographs of the cold-sprayed Cu, Ag, SS 316L, Zn, Ta, and Nb coatings are presented in Fig. 2 to 4, respectively. In these figures, the micrographs representing the coatings in the as-coated condition and the coatings heat-treated at the highest temperature have been presented for each of the coatings under study except Ti. In these micrographs, the dark line contrast represents the inter-splat boundary some of which are also cracked. During the course of etching of these coated samples, the inter-splat crack opens up first and the inter-splat boundary without cracks is subsequently etched. Therefore, by careful control of etching time it is possible to get an idea of the extent of inter-splat cracking. However, quantification of inter-splat cracking is difficult since the etching procedure is somewhat subjective. In the as-coated condition, all the coatings exhibit a substantial fraction of cracked (i.e., opened up by etching) inter-splat boundaries. Among the coatings, Ag coatings (Fig. 2c) exhibits the least proportion of cracked inter-splat boundaries in the as-coated condition, while SS 316L (Fig. 3a) and Ti coatings (not shown) exhibit the highest proportion of cracked inter-splat cracks. The remaining coatings exhibit intermediate inter-splat crack densities. The influence of heat treatment in reducing the density of cracked inter-splat areas is obvious in all the coatings. However, the effect is more dramatic in Cu, SS 316L, and Ta coatings (Fig. 2b, 3b, and 4b, respectively). In copper, silver, Ti (not shown), and SS 316L heat treatment heals the inter-splat cracks by solid-state diffusion as in diffusion bonding (Ref 1, 35) and leaves behind remnant pores/ short cracks along the original cracked inter-splat cracks. These pores/short cracks represent regions wherein the splats were too widely separated to be brought together by mass diffusion (Ref 1). In case of Ag (Fig. 2d), since the coating in the as-coated condition itself exhibits low density of inter-splat cracks, the improvement by heat treatment is only marginal. The flattening ratio (a) was estimated using the procedure described earlier. Measurements were carried out on at least 25 splats for each coating and the average values of a and the associated scatter were obtained and provided in Table 3. The Ag coatings and the 316L SS

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coatings exhibit the highest and lowest values of a, respectively. The other coatings exhibit intermediate a values. The porosity of the as-coated and heat treated coatings, obtained on sectioned, polished, and unetched surfaces, using the procedure described earlier, are presented in Table 4. The porosity lies in the range 0.8% (SS316L coating in the as-coated condition) to 0.03% (Ag coating in the heat-treated condition). It is also obvious that porosity decreases continuously with increasing heat treatment temperature in all the coatings as illustrated in Fig. 5. The hardness (H) and the elastic modulus (Ec) of all the coatings in the as-coated and heat-treated conditions are presented in Table 4. It is clear that in the as-coated condition, all the cold spray coatings exhibit elastic modulus values substantially lower than that of the corresponding bulk material (compare Tables 1 and 4). In addition, the hardness generally decreases with increasing heat treatment temperature.

4. Discussion In this section, the property data obtained on all the cold spray coatings will be compared to arrive at possible correlations. In Fig. 5, the variation of porosity of all the coatings (as-coated and heat-treated) is presented as a function of heat treatment temperature normalized by melting point of the coating material (Tht/Tmp; Fig. 5). From Fig. 5, it is clear that porosity generally decreases with increasing normalized heat treatment temperature. However, the rate of decrease of porosity is more dramatic in coatings with high initial porosity (SS 316L and Zinc) as compared to other coatings having low initial porosity (Cu, Ag, Ta, Nb, or Ti). It has been observed (Fig. 2-4) that the extent of intersplat cracking and porosity level in the coating decreases with increasing heat treatment temperature. To see the effect of such a reduction in crack density and porosity on the elastic modulus of the coating (Ec), the variation of normalized elastic modulus of the coating (Ec/Eb; Eb = elastic modulus of bulk material) with normalized heat treatment temperature is illustrated in Fig. 6. This

Journal of Thermal Spray Technology

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(b)

(a) Silver powder

Zinc powder

(d)

(c)

Copper powder

SS316L powder

(e)

(f) Tantalum powder

Niobium powder

(g) Titanium powder

Fig. 1 SEM micrographs of the feedstock powders (a) silver, (b) zinc, (c) SS 316L, (d) copper, (e) tantalum, (f) niobium, and (g) titanium

figure indicates that there is a general trend of increasing Ec/Eb with increasing heat treatment temperature in all the coatings. However, the effect of heat treatment is less dramatic in the case of Ag coatings since in the as-coated condition, the elastic modulus of Ag coating is quite close to the bulk value and hence no scope for further improvement of modulus by heat treatment. In contrast, in the case of Zn coatings, as indicated elsewhere (Ref 28),

Journal of Thermal Spray Technology

the presence of oxides on the Zn particle surface inhibited the consolidation during heat treatment. The above increase in Ec/Eb with Tht/Tmp, is most likely related to the fact that inter-splat crack density decreases with heat treatment and such a postulate is clearly supported by the coating micrographs presented earlier (Fig. 2-4). To further quantify the relationship between crack density/porosity and the elastic modulus, it is

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appropriate to take advantage of the large number of analytical and numerical models already available in the literature for the determination of elastic modulus of cracked and porous solids (Ref 36-43). These models differentiate between solids with randomly oriented cracks vis-a`-vis horizontally or vertically aligned cracks. In the case of cold spray coatings, the inter-splat cracks are generally oriented parallel to the coating plane and thus can be treated as horizontally aligned cracks (Ref 1, 25, 27). Unlike in the case of plasma spray coatings, cold spray coatings do not exhibit vertical cracks associated with solidification shrinkage since the splats never become molten. Thus, for the cold spray coating comprising horizontally aligned inter-splat cracks and porosity, the

Table 2 Heat treatment parameters of the coatings

Material Ag Cu Zn SS316L Ta Nb Ti

Heat treatment temperature, K

Soaking time, h

673 673 873 1073 623 673 1073 1373 773 1273 773 1273 1393

Atmosphere

1 1 1 1 10 1

Argon Vacuum

2

Vacuum

2

Vacuum

2

Vacuum

Argon Air

analytical model due to Zimmerman (Ref 36) is the most appropriate. As per this model, the elastic modulus of the cracked and porous cold spray coating (Ec) should be related to the elastic modulus of the uncracked, non-porous material (Eb) by the following equation: Ec 1 ¼ P C Eb 1 þ A 1P þ B 1P

ðEq 1Þ

In Eq 1, P represents the volume fraction of porosity in the coating and C the crack density in the coating. The constants A and B have been derived as (Ref 36), A ¼ 3ð1  tÞð9 þ 5tÞ=½2ð7  5tÞ

ðEq 2Þ

B ¼ 8ð1  t2 Þ ð1  3t=8Þ=½3ð1  t=2Þ

ðEq 3Þ

In Eq 2 and 3, t represents the PoissonÕs ratio. For a typical value of 0.3 for t, the constants A and B take the value of 2 and 5, respectively. In Eq 1, the second term in the denominator represents the influence of porosity on the modulus. In the case of cold spray coatings, the highest porosity value is around 0.8% while the average value is P around 0.3%. For these values of P, the parameter A 1P has a negligible value (1). Thus, to a good approximation, Eq 1 can be simplified as, Ec 1 1 ¼ ¼ Eb 1 þ B  C 1 þ 5C

ðEq 4Þ

Thus, the elastic modulus value of the cold spray coating depends primarily on the parameter C defined as the crack density, i.e., crack volume per unit volume of the material.

Fig. 2 SEM microstructure of cold-sprayed coatings (a) Cu in as-coated condition, (b) Cu heat-treated at 673 K, (c) Ag in as-coated condition, (d) Ag heat-treated at 673 K

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Journal of Thermal Spray Technology

Peer Reviewed Fig. 3 SEM microstructure of cold-sprayed coatings (a) SS316L as-coated condition, (b) SS316L after heat treatment at 1073 K, (c) Zn coatings as-coated condition, and (d) Zn after heat treatment at 623 K

Fig. 4 SEM microstructure of cold-sprayed coatings (a) Tantalum in the as-coated condition, (b) Tantalum after heat treatment at 1273 K, (c) Nb in the as-coated condition, and (d) Nb after heat treatment at 1273 K

The value of C for a cold spray coating composed of splats is estimated as follows: assume the splat to be a circular disk of radius aR (a > 1) and thickness bR (b < 1), wherein R is the radius of the spherical particle

Journal of Thermal Spray Technology

which impacts the surface to form a splat and a is the flattening ratio. The volume of the splat (Vs) is obtained as,
ðEq 5Þ Vs ¼ p a2 R2 ðbRÞ ¼ p R3 a2 b

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Table 3 Average flattening ratio of splats in various cold spray coatings Material

Flattening ratio

Ag Cu Zn SS316L Ta Nb Ti

2.055 1.597 1.63 1.44 1.89 1.66 1.55

± ± ± ± ± ± ±

0.38 0.42 0.28 0.34 0.34 0.19 0.21

a2 b ¼ 4=3

ðEq 6Þ

Let us assume A and t as the area and thickness of the coating. Then, the number of splats in the coating (Ns) is obtained as, Ns ¼ At=Vs ¼ At=pR3 a2 b

ðEq 7Þ

Number of horizontal splat boundaries (Nb) is one more than the number of splats and hence equals Ns to a very good approximation. Therefore, the number of splat boundaries per unit volume of the coating is obtained as 1/pR3a2b. A crack of radius R is associated with a volume of a3R3 since the effect of a crack is not limited to its areas (/ a2R2) but to the adjoined volumes (/ a3R3) as indicated by many investigators (Ref 36, 37, 44). If we further assume that only a fraction f of the inter-splat boundaries are cracked,

Table 4 The porosity, hardness, and elastic modulus of the coatings as a function of heat treatment temperature Heat treatment, Tht, K

Copper Copper Copper Copper Silver Silver 316L SS 316L SS 316L SS 316L SS Zinc Zinc Tantalum Tantalum Tantalum Niobium Niobium Niobium Titanium Titanium

As-coated 673 873 1073 As-coated 673 As-coated 673 873 1073 As-coated 673 As-coated 773 1273 As-coated 773 1273 As-coated 1393

Tht/Tmp

Porosity, %‘

Hardness, GPa

Elastic modulus of the coating, GPa

0.221 0.496 0.644 0.791 0.243 0.546 0.185 0.415 0.538 0.661 0.433 0.899 0.091 0.235 0.387 0.109 0.282 0.465 0.154 0.717

0.20 0.10 0.08 0.07 0.09 0.03 0.80 0.76 0.36 0.20 0.47 0.15 0.28 0.14 0.04 0.23 0.12 0.06 0.12 0.05

1.66 1.07 0.92 0.71 1.31 0.85 2.92 2.60 2.20 2.11 0.57 0.53 3.76 3.28 3.16 1.47 2.18 1.60 1.32 2.25

82.7 94.0 99.4 115.0 73.5 74.6 98.0 123.0 135.0 164.0 62.0 69.4 101.5 113.0 140.3 74.6 85.1 95.9 51.5 97.2

0.9 Copper Silver 316L SS Zinc

0.8 0.7

Tantalum Niobium Titanium

0.6 0.5 0.4 0.3 0.2 0.1

Normalised Elastic Modulus of the Coating (Ec /Eb )

Material

Porosity, %

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However, conservation of volume during flattening also implies that Vs = 4/3 pR3. Therefore, a and b are interrelated as,

1.0

0.9

0.8

0.7

Copper Silver 316L SS Zinc Tantalum Niobium Titanium

0.6

0.5

0.4 0.0

0 0.0

0.2

0.4

0.6

0.8

1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Normalised heat treatment temperature (Tht / Tmp )

Normalised heat treatment temperature

Fig. 5 The variation of porosity with heat treatment for all the coatings

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Fig. 6 The variation of the normalized elastic modulus of the coatings as a function of the normalized heat treatment temperature (Tht/Tmp)

Journal of Thermal Spray Technology

Normalised Elastic Modulus of the Coating (Ec /Eb )

Copper Silver 316L SS Zinc Tantalum Niobium Titanium

0.9

0.8

Material

0.7

0.6

0.5

0.4 0

2

4

6

8

10 12 14 16 18 20 22 24 26 28 30 32

Fraction of cracked inter-splat boundaries, f (%)

Fraction of cracked inter-splat boundaries, f (%)

Fig. 7 The variation of the normalized elastic modulus of the coatings (as-coated and heat-treated) with the fraction of cracked inter-splat boundaries (f)

32 Copper Silver

30 28 24

316L SS Tantalum Niobium

22

Titanium

26

20

Copper As-coated HT 673 HT 873 HT 1073 Silver As-coated HT 673 SS 316L As-coated HT 673 HT 873 HT 1073 Zinc As-coated HT 623 Tantalum As-coated HT 773 HT 1273 Niobium As-coated HT 773 HT 1273 Titanium As-coated HT 1393

Eb, GPa

Ec, GPa

Ec/Eb

a

f, %

129.8 129.8 129.8 129.8

82.7 94 99.4 115

0.637 0.724 0.766 0.886

1.6 1.6 1.6 1.6

11.65 7.79 6.25 2.63

82.7 82.7

73.5 74.6

0.889 0.902

2.06 2.06

1.20 1.04

0.508 0.637 0.699 0.850

1.44 1.44 1.44 1.44

27.19 15.96 12.05 4.96

193 193 193 193

98 123 135 164

104.5 104.5

62 69.4

0.593 0.664

1.63 1.63

13.26 9.78

185.7 185.7 185.7

101.5 113 140.3

0.547 0.609 0.756

1.89 1.89 1.89

10.29 7.98 4.01

104.9 104.9 104.9

74.6 85.1 95.9

0.711 0.811 0.914

1.66 1.66 1.66

7.44 4.26 1.72

120.2 120.2

51.5 97.2

0.428 0.809

1.55 1.55

30.00 5.32

18 16 14 12 10 8 6 4 2 0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Normalised heat treatment temperature (Tht/ Tmp )

Fig. 8 The influence of normalized heat treatment temperate on the fraction of inter-splat boundaries (f)

the crack density C (i.e., volume of the crack per unit volume of the coating) is obtained as, C ¼ f ðNb per unit volume) a3 R3 ¼

3 3 fa 4p

ðEq 8Þ

Substitution of Eq 8 in Eq 4 gives, Ec =Eb ¼ 1=ð1 þ 1:194 f a3 Þ

ðEq 9Þ

Eqn. 9 indicates that the ratio Ec/Eb depends only on the flattening ratio (a) and fraction of cracked inter-splat boundaries (f). Using Eq 9, the best-fit value of f can be obtained for each coating since the values of Ec/Eb and a are available. It is important to note that even in heat treated coatings, wherein both porosity and inter-splat crack density have reduced substantially, the influence of porosity on Ec is negligible compared to the short cracks still present. The variation of normalized elastic modulus of the coating (Ec/Eb), as a function of the calculated f value, is

Journal of Thermal Spray Technology

presented in Fig. 7. The general trend of decreasing Ec/Eb with f in general follows Eq 9 with the scatter being due to the a values being different for different coatings (Table 4). Finally, the variation of calculated values of f with the normalized heat treatment temperature is presented in Fig. 8. As expected, increasing heat treatment temperature reduces the best-fit values of f signifying a decrease in inter-splat crack density, consistent with the microstructural observations of the coating (Fig. 2-4). On the basis of the analysis of the results carried out, it can be stated that the measured elastic modulus of the coatings can be rationalized on the basis that it depends on the extent of cracking of the inter-splat boundaries. Comparison with the existing models predicting the reduction in elastic modulus due to the presence of horizontally aligned cracks indicate that the extent of reduction of modulus experimentally observed can be explained using reasonable values of f, the fraction of cracked intersplat boundaries. However, there is an urgent need to independently measure the value of f to fully validate the model proposed here. The process adopted by us, i.e., etching, to reveal the cracked inter-splat boundaries is adequate only as a qualitative technique. To quantify the value of f more rigorously, techniques like SAXS and SANS needs to be used. Already, Herman and co-workers (Ref 30-33) have demonstrated that SAXS/SANS techniques can be utilized to quantify the volume fraction of pores and cracks in thermal spray coatings. While estimating the best-fit f value (Table 5), it has been assumed that the flattening ratio (a) does not change with heat treatment of the coating. This is a reasonable

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Table 5 The normalized elastic modulus and the best-fit value of the parameter ‘‘f’’ for cold spray coatings

1.0

Peer Reviewed

assumption since heat treatment only heals/closes the inter-splat cracks and the original splat boundaries are still visible. The only exception seems to be the Ti coating heat-treated at a very high temperature of 1393 K where the original splats have been replaced by equiaxed grains.

5. Conclusions 1.

2.

3.

4.

The cold spray coatings of a wide range of metallic materials (Cu, Ag, Zn, Nb, Ta, Ti, and 316L stainless steels) exhibit elastic modulus values considerably lower than that of the bulk material of the same composition. The heat treatment of the cold spray coatings causes a decrease in the extent of inter-splat cracking and porosity in the case of all the coatings with a concomitant increase in the elastic modulus. The lower modulus values obtained in the as-coated condition and the increase in the modulus value with heat treatment can be explained on the basis of existing models relating elastic modulus to crack density by assuming a reasonable value for f, the fraction of cracked inter-splat boundaries. The elastic modulus is a good indicator of the quality and integrity of the cold spray coatings.

Acknowledgements Authors acknowledge the help of Dr. G. Ravichandra and P. Anil for elastic modulus measurement, Mr. S. Chandra Sekhar for heat treatment and Mr. G.V.R. Reddy for SEM.

References 1. W.-Y. Li, C.-J. Li, and H. Liao, Effect of Annealing Treatment on the Microstructure and Properties of Cold Sprayed Copper Coating, J. Therm. Spray Technol., 2006, 15, p 206-211 2. P. Sudharshan Phani, D. Srinivasa Rao, S.V. Joshi, and G. Sundararajan, Effect of Process Parameters and Heat Treatments on Properties of Cold Sprayed Copper Coatings, J. Therm. Spray Technol, 2007, 16, p 425-434 3. P. Sudharshan Phani, V. Vishnukanthan, and G. Sundararajan, Effect of Heat Treatment on Properties of Cold Sprayed Nanocrystalline Copper Alumina Coatings, Acta Mater., 2007, 55, p 4741-4751 4. R.C. McCune, W.T. Donion, O.O. Popoola, and E.L. Cartwright, Characterization of Copper layers produced by Cold Gas Dynamic Spraying, J. Therm. Spray Technol., 2000, 9(1), p 73-82 5. T. Novoselova, P. Fox, R. Morgan, and W. OÕNeill, Experimental Study of Titanium/Aluminium Deposits Produced by Cold Gas Dynamic Spray, Surf. Coat. Technol., 2006, 200, p 2775-2783 6. H.-R. Wang, W.-Y. Li, L. Ma, J. Wang, and Q. Wang, Corrosion Behavior of Cold Sprayed Titanium Protective Coating on 1Cr13 Substrate in Seawater, Surf. Coat. Technol., 2007, 201, p 52035206 7. C.-J. Li and W.-Y. Li, Deposition Characteristics of Titanium Coating in Cold Spraying, Surf. Coat. Technol., 2003, 167, p 278283

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8. L. Ajdelsztajn, B. Jodoin, and J.M. Schoenung, Synthesis and Mechanical Properties of Nanocrystalline Ni Coatings Produced by Cold Gas Dynamic Spraying, Surf. Coat. Technol., 2006, 201, p 1166-1172 9. T. Van Steenkiste and D.W. Gorkiewicz, Analysis of Tantalum Coatings produced by the Kinetic Spray Process, J. Therm. Spray Technol., 2003, 13(2), p 265-273 10. N. Bala, H. Singh, and S. Prakash, High-Temperature Oxidation Studies of Cold-Sprayed Ni-20Cr and Ni-50Cr Coatings on SAE 213-T22 Boiler Steel, Appl. Surf. Sci., 2009, 255, p 6862-6869 11. H.Y. Lee, S.H. Jung, S.Y. Lee, and K.H. Ko, Alloying of ColdSprayed Al-Ni Composite Coatings by Post-Annealing, Appl. Surf. Sci., 2007, 253, p 3496-3502 12. W.-Y. Li, C. Zhang, H. Liao, J. Li, and C. Coddet, Characterization of Cold-Sprayed Nickel-Alumina Composite Coating with Relatively Large Nickel-Coated Alumina Powder, Surf. Coat. Technol., 2008, 202, p 4855-4860 13. X. Guo, G. Zhang, W.Y. Li, L. Dembinski, Y. Gao, H. Liao, and C. Coddet, Microstructure, Microhardness and Dry Friction Behavior of Cold-Sprayed Tin Bronze Coatings, Appl. Surf. Sci., 2007, 254, p 1482-1488 14. G. Sundararajan, P. Sudharshan Phani, A. Jyothirmayi, and R.C. Gundakaram, The Influence of Heat Treatment on the Microstructural, Mechanical and Corrosion Behaviour of Cold Sprayed SS 316L Coatings, J. Mater. Sci., 2009, 44, p 2320-2326 15. W.-Y. Li, H. Liao, G. Douchy, and C. Coddet, Optimal Design of a Cold Spray Nozzle by Numerical Analysis of Particle Velocity and Experimental Validation with 316L Stainless Steel Powder, Mater. Des., 2007, 28, p 2129-2137 16. P. Richer, A. Zu´n˜iga, M. Yandouzi, and B. Jodoin, CoNiCrAlY Microstructural Changes Induced During Cold Gas Dynamic Spraying, Surf. Coat. Technol., 2008, 203, p 364-371 17. H.-T. Wang, C.-J. Li, G.-J. Yang, and C.-X. Li, Effect of Heat Treatment on the Microstructure and Property of Cold-Sprayed Nanostructured FeAl/Al2O3 Intermetallic Composite Coating, Vacuum, 2009, 83, p 146-152 18. F. Gartner, C. Borchers, T. Stoltenhoff and H. Kreye, Numerical and Microstructural Investigations of bonding Mechanisms in Cold Spraying, Thermal Spray 2003: Advancing the Science and Applying the Technology, C. Moreau and B. Marple, Ed. (Materials Park, OH, USA), ASM International, 2003, p 1-7 19. H. Assadi, F. Gartner, T. Stoltenhoff, and H. Kreye, Bonding Mechanism in Cold Gas Spraying, Acta Mater., 2003, 51, p 43794394 20. M. Grujicic, J.R. Saylora, D.E. Beasleya, W.S. DeRossetb, and D. Helfritch, Computational Analysis of the Interfacial Bonding Between Feed-Powder Particles and the Substrate in the ColdGas Dynamic-Spray Process, Appl. Surf. Sci., 2003, 219, p 211-227 21. M. Grujicic, C.L. Zhao, W.S. DeRosset, and D. Helfritch, Adiabatic Shear Instability Based Mechanism for Particles/Substrate Bonding in the Cold-Gas Dynamic-Spray Process, Mater. Des., 2004, 25, p 681-688 22. W.-Y. Li, C. Zhang, X. Guo, C.-J. Li, H. Liao, and C. Coddet, Study on Impact Fusion at Particle Interfaces and Its Effect on Coating Microstructure in Cold Spraying, Appl. Surf. Sci., 2007, 254, p 517-526 23. D. Zhang, P.H. Shipway, and D.G. McCartney, Particle Substrate Interactions in Cold Gas Dynamic Spraying, Thermal Spray 2003: Advancing the Science and Applying the Technology, C. Moreau and B. Marple, Ed. (Materials Park, OH), ASM International, 2003, p 45-52 24. A.N. Papyrin, S.V. Klinkov, and V.F. Kosarev, Modelling of Particle Substrate Adhesive Interactions Under Cold Spray Process, Thermal Spray-2003: Advancing the Science and Applying the Technology, C. Moreau and B. Marple, Ed. (Materials Park, OH), ASM International, 2003, p 27-35 25. V. Luzin, K. Spencer, and M.-X. Zhang, Residual Stress and Thermo-Mechanical Properties of Cold Spray Metal Coatings, Acta Mater., 2011, 59, p 1259-1270 26. S. Sampath, X.Y. Jiang, J. Matejicek, L. Prchlik, A. Kulkarni, and A. Vaidya, Role of Thermal Spray Processing Method on the Microstructure, Residual Stress and Properties of Coatings: An

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Integrated Study for Ni-5 wt.% Al Bond Coats, Mater. Sci. Eng., 2004, A364, p 216-231 N.M. Chavan, M. Ramakrishna, P. Sudharshan Phani, D.S. Rao, and G. Sundararajan, The Influence of Process Parameters and Heat Treatment on the Properties of Cold Sprayed Silver Coatings, Surf. Coat. Technol., 2011, 205, p 4798-4807 G. Sundararajan, N.M. Chavan, G. Sivakumar, and P. Sudharshan Phani, Evaluation of Parameters for Assessment of InterSplat Bond Strength in Cold Sprayed Coatings, J. Therm. Spray Technol., 2010, 19, p 1255-1266 N.M. Chavan, B. Kiran, A. Jyothirmayi, P. SudharshanPhani, and G. Sundararajan, The Corrosion Behaviour of Cold Sprayed Zn Coatings on Mild Steel Substrate, J. Therm. Spray Technol., 2013, 22(4), p 463 Z. Wang, A. Kulkarni, S. Deshpande, T. Nakamura, and H. Herman, Effects of Pores and Interfaces on Effective Properties of Plasma Sprayed Zirconia coatings, Acta Mater., 2003, 51, p 5334-5391 A. Kulkarni, Z. Wang, T. Nakamura, S. Sampath, A. Goland, H. Herman, J. Allen, J. Ilavsky, G. Long, J. Frahm, and R.W. Steinbrech, Comprehensive Microstructural Characterization and Predictive Property Modeling of Plasma-Sprayed Zirconia Coatings, Acta Mater., 2003, 51, p 2457-2475 A.J. Allen, J. Ilavsky, G.G. Long, J.S. Wallace, C.C. Berndt, and H. Herman, Microstructural Characterization of Yttria-Stabilised Zirconia Plasma-Sprayed Deposits Using Multiple Small-Angle Neutron Scattering, Acta Mater., 2001, 49, p 1661-1675 S. Deshpande, A. Kulkarni, S. Sampath, and H. Herman, Application of Image Analysis for Characterization of Porosity in Thermal Spray Coatings and Correlation with Small Angle Neutron Scattering, Surf. Coat. Technol., 2004, 187, p 6-16