A study of adhesive strength of cold spray coatings H. Fukanuma, N. Ohno, Toda/JPN This paper presents the adhesive strength results of copper and titanium deposits produced by cold spray processes on steel, stainless steel, aluminum and copper substrates. We investigated how the combinations of the particles and the substrate, and the pressure in the cold spray nozzle chamber affect the adhesive strength between the deposits and substrates. We used nitrogen and helium as cold spray process and powder carrier gasses in the processes. We found that helium gas produced much higher adhesive strength than nitrogen and that the strength of the deposits produced by using both helium and nitrogen gases was almost proportional to the chamber pressure. The adhesion produced by cold spray processes appeared to be dependent on the combination of hardness of powder and substrate metals. We also present cross section micrographs of the deposits and substrate observed after tensile strength tests. They show the substrate which is soft or easily deformed by particle collisions generates strong adhesion. 1 Introduction Mechanisms of adhesion are still in mystery in cold spray coatings as well as thermally sprayed deposits. It is generally accepted that the adhesive strength of thermal spray coatings is controlled by three main forces caused by mechanical, physical and metallic interactions [1-3]. In cold spray impact processes, R.C. Dykhulze, et al. referred to the similarity to the explosive welding process in that the generation of jet formation during these deformation processes [4]. Several researchers have reported particle substrate interaction studies that showed the formation of jet on some part of their interface when the particle impacts on the substrate at a velocity that is higher than the so called critical velocity [4-7]. On the other hand, to analyse mechanisms of the adhesion produced by inter-particle-substrate interaction in cold spray processes, we can adopt another way to approach understanding the mechanisms of adhesion, taking into consideration the conventional concepts of the particle substrate interaction in thermal spray. Let’s think about following ideal cold spray processes;

(a) A spherical plastic metal particle impinges on a rigid metal substrate at velocity v0 that is higher enough than the critical velocity, and both surfaces of the particle and the substrate are smooth.

(b) A spherical plastic metal particle impinges on a roughened rigid metal substrate at velocity v0 and the particle surface is smooth.

(c) A spherical rigid metal particle impinges on a plastic metal substrate at velocity v0 and both surfaces of the particle and the substrate are also smooth.

(d) A spherical plastic metal particle impinges on a plastic metal substrate at velocity v0 and both surfaces of the particle and the substrate are smooth.

In the case (a) where only the particle deforms because the particle is plastic and the substrate is rigid as is shown in (a) of Fig.1. In the case (b) where the plastic particle deforms on the rigid substrate, deformation processes are more complicated than that in the case (a) because of the substrate roughness. On the other hand, in the case (c) where

the particle penetrates without its deformation into the substrate as is shown in (c) of Fig 1, only the substrate deforms. The case (d) is an actual cold spray process and both the particle and the substrate are deforming as is shown in (d) of Fig. 1. The shape of the interface between the particle and the substrate depends on mechanical and physical properties of the metals. We always ask a question as to whether or not metallic or physical bond is produced on the particle substrate interface during the interaction, or whether major force is produced by anchorage. In a particle substrate interaction system, the particle kinetic energy changes partly into heat produced by the particle and substrate internal friction, and inter particle substrate friction, and partly into strain energy stored inside both particle and substrate during the deformation. The heat dissipates by conducting into the substrate and ambient air. Can the heat generate chemical bonding on the interface? During the interaction, the pressure is generated on the interface. Can the pressure working on the interface produce metallic or physical bonding? Can it tangle the interface to make strong mechanical bonding. We would like to approach the questions through our studies. The case (a) is practically the combination of a soft particle and a hard metal. In this case, only the particle deforms and produces friction and viscous heat. Is bonding force generated along the interface between the deformed particle and the rigid substrate in the case (a)? If adhesive force of some kind exists between them, what does it come from? Since the substrate is smooth, there is no anchor effect along the interface, therefore, mechanical bonding is not generated. Is metallic or physical bond of some kind produced between them? Is the heat generated enough to cause diffusion or to melt both metals to produce metallic bond? If adhesive strength strong enough is obtained in the case (a), the major bonding force would be metallic or physical bond, or both of them. On the other hand if there is no bonding between the splat and the substrate, the chemical bonding force would not be generated If the adhesion in the case (b) is much stronger than that in the case (a), mechanical anchorage is more dominant than the other interactions because the

substrate roughness affects the strength. In the case (c) where the first layer of particles is formed, but the second layer is not deposited because the second particle bounces off because particles are rigid. The idea is not practical. So we can change the idea to another one that the particle is extremely harder than the substrate. In this case, the particle deforms quite small and penetrates into the substrate. In this case, the friction heat is mainly produced in the substrate because the particle is too hard to deform and then quickly spreads into the substrate. The temperature could be very low on the interface. The case (d) is a common cold spray process in which both particle and substrate deform. It is difficult to eliminate anchor effect from cold spray processes, because in the initial stage in cold spray, particles roughen the substrate at impact as not every particle adheres to it. It is also not easy to find evidence of metallic and physical interaction. We carried out experiments to understand how much anchor effect works in cold spray processes. Cold spray experiments of combination of different particle and substrate metals in their hardness would bring better understanding on mechanisms of adhesion. The four combinations of particles and substrates are soft and soft, soft and hard, hard and soft, and hard and hard. The impinging velocity would significantly affect particle and substrate deformation processes. Higher impact velocity causes rapid deformation that also produces higher temperature, pressure and shear stress on the interface between the particle and the substrate. So we changed chamber pressure to vary velocity. We carried out experiments under the above concepts. 2 Experimental Procedure We investigated adhesive strength of copper and titanium metal deposits on different metal substrates, changing gas pressure in the gas nozzle chamber. Copper was selected as a soft particle and titanium as a hard one. The term of soft means easily plastically deforming, hard means difficult to deform. The term is a relative not absolute concept. Aluminum, copper, steel and stainless steel were chosen for substrate metals to measure adhesive strength of deposits formed by cold spray. Aluminium and copper metals were selected as representatives of soft metals and steel and stainless steel as hard metals. The dimensions of the specimens are 20 mm in diameter and 20 mm in length. They were blasted with alumina grit before cold spraying under conditions shown in Table 1. The oxygen content in the copper powder is 0.069 in mass percentage. The purity of the titanium powder is more than 99.4 %. Particle size distributions of copper and titanium were 5-25 and 10-70 µm, respectively. The morphology of copper and titanium particles is shown in Fig. 2 and 3, respectively. Both copper and titanium particle shapes are nearly spherical, and the particle size distribution of titanium is broad, on the

other hand, copper powder has a good narrow size distribution. Titanium powder includes a lot of fine particles under 10 µm as is shown in the figure. The cold spray nozzle geometry is that the throat and exit diameters are 2 mm and 6 mm, respectively, and that the length is 100 mm. The process gases were nitrogen and helium. Spray conditions are shown in Table 2. All of the deposit thickness was over 600 µm and every deposit was grinded to adjust to the thickness of approximately 300 µm before epoxy resin bonding for tensile tests. Tensile tests were carried out

plastic particle plastic particle

0v 0v0v 0v

(a) (b)

rigid particle plastic particle

0v 0v0v0v 0v0v

(c) (d) Fig. 1 Schematic of deformation processes indifferent combination of particles and substrate incold spray coatings

rigid substrate rigid substrate

soft substrate soft substrate

in accordance with Japanese Industrial Standard H8664 except that the specimen diameter of 20 mm was smaller due to the shortage of loading strength of the equipment. 3. Experimental Results Figure 4 shows the results of adhesion tests of copper deposits on aluminium and copper substrates sprayed with nitrogen as process and powder gases. In the graph, the adhesive strength on both aluminium and copper substrates appears slightly dependent on the chamber pressure at 1 to 3 MPa in the case of nitrogen as process gas, although the adhesion on the aluminium substrate at 2 MPa is lower than at the other pressure conditions of I and 2 MPa. It is also shown that the adhesive strength on the aluminium is a little higher than that on the copper substrates. Copper was also sprayed on steel and stainless steel substrates, but all the deposits on these substrate sprayed under the conditions shown in Table 2 were separated from the substrates at the thickness of around 500 µm in the middle of coating. The reason for the separation appears that the internal stress accumulated to a large amount, since the coating traverse speed of 20 mm/sec was too slow, the deposits could be added excessive stress for short time to maintain adhering to the substrates. On the other hand, separation did not occur during the coating using helium as process gas. Table 1 Grit material Blasting air pressure Stand off distance Traverse speed Traverse pitch The number of blasting passes Grit feed rate

Alumina #36 0.48 MPa 120 mm 200 mm/sec 5 mm 4 passes 700 g/min

Table 2

Spray metal Cu Ti Process gas N2 He N2 He

MPa MPa Chamber pressure 1-3 1-2.5 1-3

C C Gas temperature 250 200 300

Stand off distance 20 mm Traverse speed 20 mm/sec No. of passes 1 pass

Figure 5 shows the adhesive strength of copper on steel, stainless steel, aluminium and copper substrates. The graph shows that the adhesion of coating on copper and aluminium with helium gas is much stronger than that with nitrogen shown in Fig. 4. The copper deposits on the aluminium substrate coated at higher pressure than 1.5 MPa of chamber pressure were separated at epoxy resin layer not at the interface between the deposits and the substrate. The copper deposits on the copper substrate sprayed at higher pressure than 2.0 MPa were also removed at

epoxy resin layer. The true adhesive strength of these coatings must be higher than the values shown in Fig. 5. The adhesive strength of the deposits on the steel and the stainless appears to be proportional. That means that higher particle impact velocity produces stronger adhesion. The graph shows that the adhesion of the deposits on aluminium and copper is stronger than those on steel and stainless steel. As for copper deposit, the adhesion sprayed on soft metals such as aluminium and copper is stronger than that on hard metals of steel or stainless steel. In the case of nitrogen process gas, the cupper removal on steel and stainless steel in the middle of coating means that the adhesion weaker that on aluminium and copper on which the deposits were completed. Figure 6 shows the cross sections of the copper deposits removed from the aluminium substrates. The deposit on aluminium was sprayed at I MPa with helium gas. The figure shows that, when the copper deposit was removed in the tensile tests, the deposit tore off aluminium fragments out of the substrate and copper fragments remain on the surface of the substrate. Figure 7 shows the relationship of the adhesion of titanium deposits on the aluminium and stainless steel substrates with the chamber pressure. It is shown that, when the chamber pressure is at 3 MPa, the deposits on stainless steel sprayed in helium gas process have

Fig. 2. Copper powder

Fig. 3. Titanium powder

much stronger adhesion than those in nitrogen, although the adhesion at 2 MPa is almost the same in both helium and nitrogen processes. The adhesion of titanium on the aluminium substrate is similar to that on the stainless steel substrate, when being coated in helium gas processes. Titanium deposit was not formed on aluminium substrates in nitrogen gas processes. Fig. 8 shows the aluminium substrates hit by high velocity titanium particles accelerated in the nozzle at 2 and 3 MPa of nitrogen gas in chamber pressure. A small amount of titanium remains on the substrate. White areas on the substrate surfaces show titanium remaining. Titanium

on the substrate sprayed at 3 MPa remains much more than that at 2 MPa. Fig. 9 shows a micrograph of the aluminium substrate on which a small number of titanium particles remain and many craters produced by titanium particle impingements are seen. The titanium particles remaining on the surface appear to

be little deformed. That means that titanium do no deform at a low impact velocity. It is interesting that titanium deposits were not formed on aluminium substrates in nitrogen gas process, although copper deposits were easily formed on aluminium in nitrogen gas process. It is also interesting that titanium was

Fig. 6. The micrograph of the cross sections of theseparated Cu deposit and the Al substrate.

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10

20

30

40

50

60

70

0.5 1 1.5 2 2.5 3 3.5

Ti on stainless steel with N2Ti on stainless steel with HeTi on aluminium with He

Chamber Pressure [ MPa ]

Fig. 7. Adhesion of the titanium deposits onstainless steel and aluminium substrates

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5

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15

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0.5 1 1.5 2 2.5 3 3.5

Cu deposit on Al substrateCu deposit on Cu substrate

chamber pressure [ MPa ]

Fig. 4. Adhesion of copper deposits on aluminiumand copper substrates in nitrogen gas processes.

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Cu on steelCu on stainless steelCu on aluminiumCu on copper

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Fig. 5. Adhesion of copper deposits on steel,stainless steel, aluminium and copper substratesin helium gas processes.

copper deposit

aluminium fragments

aluminium substrate

copper fragments

deposited on the stainless substrate even in nitrogen process.

Figure 9 shows the cross section micrographs of the separated titanium deposit from the aluminium substrate at 2 MPa in helium gas process and the substrate. Aluminium fragments torn off out of the aluminium substrate are found on the deposit bottom line. Titanium fragments also remain on the substrate surface. 3. Discussions Copper metal as a representative of soft metals obtained good adhesion on soft substrates of aluminium and copper metals. Even at a lower impact velocity in nitrogen process, the cupper deposits on aluminium and copper obtained the adhesive strength between 15 to 25 MPa. In helium process, the adhesion on every substrate metal rises as the chamber pressure is increases as is shown in Fig. 5. And the bond strength on aluminium and copper is higher than that on steel and stainless steel in a wider extent of chamber pressure that relates to particle impact velocity. The bonding of soft metals on soft metals appears to be stronger than that of soft metals on hard metals. And soft metal deposits can be easily formed on any metals whether soft or hard and whether the adhesion is strong or week. A soft particle impacting on a soft substrate such as a

copper particle impingement on a copper or aluminium substrate corresponds to the model shown in (d) of Fig.1. A copper particle impingement on a steel or stainless steel corresponds to the model (a) shown in Fig. 1. The term of hard changes to soft as a particle impact velocity becomes faster. The concept of hard or

soft depends on the collision velocity of a particle. On the other hand, the adhesion of titanium on the aluminium of a soft metal was lower than that on the stainless steel of a hard metal. No titanium deposit was formed on the aluminium at 1 to 3 MPa in nitrogen gas process. This case corresponds to the model (c) in Fig. 1. When the impact velocity is not high enough, the particle bounces off or do not deeply penetrate into the soft substrate, and then the next particle strikes the first particle to flick out. The particle needs a velocity high enough for the particle itself to be able to deform, when a hard particle impacts on a soft metal. Even in nitrogen process, the titanium deposits were formed on the stainless steel substrate even at the nozzle chamber pressure, 1 MPa, namely at a low impact velocity, probably because titanium particles might have deformed enough, impinging on the hard metal of stainless steel. Even if the particle is hard, the high impact velocity makes the particle deform enough to produce the deposit and the strong adhesion. The

Fig. 9. The cross section of the titanium depositremoved from the aluminium metal and substrate.

Fig. 8. The aluminium substrate surfaces hit by Tiparticles in nitrogen gas processes at 2 and 3 MPaand the micrograph at 3 MPa

titanium fragments

alminium substrate

aluminium fragments

titanium substrate

high speed impacting process results in the model (d) of Fig. 1. The adhesion is shown in Table 3 as to the combinations of the particles and the substrate, summarizing the experimental results Table 3

combination In N2 process In He process particle substrate adhesion adhesion

soft soft 10-20 MPa 40- over 60 MPa soft hard separation 10-40 MPa hard soft not formed 10-50 MPa hard soft 10-20 MPa 10-50 MPa

The combination of soft-soft seems to produce strong adhesion. It could be said that, since the particle impact velocity becomes higher, the particle and the substrate becomes softer. Both cupper and titanium deposits tore off aluminium metal fragments out of the substrate. This could suggest the generation of metallic bonding. Father research is necessary to clarify bonding mechanisms. We can also not exclude anchor effect as a bonding mechanism, because the adhesion of the combination of sort-soft is higher than that of soft-hard. The titanium deposits were formed on the aluminium in nitrogen processes, but the aluminium deposits were not. This suggests that, if a particle is too hard compared with a substrate, the particle does not receive reaction from the substrate to make deformation, because it is too soft, like the case of the titanium impacts on the aluminium substrate in nitrogen process.. 4. Conclusions The experimental results showed that the concept of soft and hard for particles and substrates seems useful, when we analyse the adhesion of cold spray coatings. It was also shown that impact velocity significantly affects the adhesion. Stronger adhesion was obtained in helium gas process than in nitrogen gas.

The copper deposits and the titanium deposits took fragments of aluminium metals of the substrate, which appears to suggest the possibility of metallic bond. This at least means that the place where aluminium fragment is held by the deposits has strong bonding force. In particular, when substrate metals are soft, metallic bond could be produced. 5. Literature [1] Matejka, D. and Benko, B: Plasma Spraying of Metallic and Ceramic. John Wiley and Sons Ltd. 1989, pp. 91/95. [2] Heimann, R.B.: Plasma-Spray Coating. Published by VCH, 1996, PP. 167/168. [3] Pawlowski, L.: The Science and Engineering of Thermal Spray Coatings. Published by John Wiley and Sons, 1995, pp. 127/130. [4] Dykhuizen, R.C., Smith, M.F., Glimore, D.L., Neiser, R.A., Jiang, X. and Sampath, S.: Impact of High Velocity Cold Spray Particles. Journal of Thermal Spray Technology, Vol. 8(4) (1999), pp. 559/564. [5] Gartner, F., Borchers, C., Stoltenhoff, T., Kreye, H. and Assadi, H.: Numerical and Microstructural Investigations of the Bonding Mechanisms in Cold Spraying. Thermal Spray 2003, Advancing the Science & Applying the Technology edited by Moreau, C. and Parple, B., 2003, pp. 1/8. [6] Zhang, D., Shipway, P.H. and McCartney, D.G.: Particle-Substrate Interactions in Cold Gas Dynamic Spraying. Thermal Spray 2003, Advancing the Science & Applying the Technology edited by Moreau, C. and Marple, B., 2003, pp. 45/52. [7] Papyrin, A.N., Kosarev, V.F., Klinkov, S.V. and Alkhimov, A.P.: On the interaction of high speed particles with a substrate under the cold spraying. International Thermal Spray Conference 2002 Proceedings. Edited by Lugscheider. 2002, pp.380/384.