Effect of Pulsed Laser Ablation and Continuous Laser ... - Springer Link

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JTTEE5 21:1322–1333 DOI: 10.1007/s11666-012-9812-8 1059-9630/$19.00 ASM International

Effect of Pulsed Laser Ablation and Continuous Laser Heating on the Adhesion and Cohesion of Cold Sprayed Ti-6Al-4V Coatings M. Perton, S. Costil, W. Wong, D. Poirier, E. Irissou, J.-G. Legoux, A. Blouin, and S. Yue (Submitted November 17, 2011; in revised form June 14, 2012) The individual and cumulative effects of in situ pulsed laser ablation and continuous laser pre-heating on adhesion and cohesion strength of cold sprayed Ti-6Al-4V coatings are investigated. Laser beams were coupled to a cold spray gun in order to ablate and pre-heat the substrate surface a few milliseconds prior to the impact of the spray particles. Cohesion and adhesion strength were evaluated by scratch test, standard ASTM C633 pull test and laser shock (LASAT) technique. The effects of laser ablation before and during cold spray operations were investigated. Results demonstrate that laser ablation of the substrate before cold spraying led to a smooth surface which improved adhesion strength. However, when laser ablation was maintained throughout the cold spray process, i.e., in between the coating layers, a reduction of cohesion and adhesion was observed. These negative effects were circumvented when laser ablation and laser pre-heating were combined.

Keywords

adhesion strength, cold spray, continuous laser pre-heating, gas dynamic spray, kinetic spray, LASAT, pulsed laser ablation, surface preparation, Ti-6Al-4V

1. Introduction Cold spray (CS) involves the acceleration of micronsized particles to extremely high velocities through the drag forces of a high pressure supersonic gas stream followed by the particle impacting onto a given substrate to build up a coating (Ref 1-7). The low temperature involved in this process together with the absence of a melting step offers several advantages regarding the manufacturing of coatings made from heat and oxidationsensitive materials. The spraying of ‘‘cold’’ solid particles at high velocities also induces compressive residual stresses, allowing the production of ultra-thick coatings and making cold spray extremely promising for spray forming and repair (Ref 4, 8, 9). Indeed, several studies have M. Perton, D. Poirier, E. Irissou, J.-G. Legoux, and A. Blouin, National Research Council Canada, Boucherville, QC Canada; S. Costil, Universite´ de Technologie de BelfortMontbe´liard, LERMPS, 90010 Belfort Cedex, France; W. Wong, National Research Council Canada, Boucherville, QC Canada; and Department of Mining and Materials Engineering, McGill University, Montreal, QC Canada; and S. Yue, Department of Mining and Materials Engineering, McGill University, Montreal, QC Canada. Contact e-mails: [email protected], and [email protected].

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remarked on the high interest of the aerospace industry in using cold spray to spray form alloys that are otherwise difficult and/or expensive to shape, such as Ti alloys (Ref 10, 11). In contrast to other thermal spraying processes where coatings are produced from droplets of semi-molten particles, cold spray coatings are formed due to the plastic deformation of solid state particles. Bonding between particles results from the localized heating and flash welding at the interface of these particles generated by the plastic deformation during impact (Ref 12-15). Several studies have suggested that adiabatic shear instability is indispensable to achieve successful bonding between the sprayed particles and the substrate or the previously deposited layer (Ref 2, 12-14, 16). As such, if the cold spray process has been easily implemented for ductile materials, the application of this process for harder materials, such as aerospace Ti alloys, has been more difficult. For instance, cold spraying of Ti-6Al-4V typically results in porous coatings with poor mechanical properties (Ref 11, 17-21). This would be especially unacceptable for spray forming applications, where the manufactured pieces are expected to exhibit bulk properties. Several operational parameters can be adjusted in order to optimize the coating structure in terms of physical-mechanical properties as well as interfacial bonding (Ref 22-26). Parameters typically considered for this are particle impact velocity [which depends on the propelling gas nature, temperature and pressure, and nozzle geometry (Ref 6, 7, 12, 14, 25, 27-29)], as well as the surface state prior to cold spraying (Ref 30-37). These parameters affect the way the solid state particles deform and bond to the substrate and to each other, involving mechanisms that

Journal of Thermal Spray Technology

2. Experimental Procedure The production of all samples had two main steps: first, the initial surface preparation of the substrate and second, the cold spraying of the coating. The cold spray process was further divided into: laser pre-heating, PLA process and the cold spray deposition itself. The two laser treatments occurred simultaneously on the same area just before the cold spray operation by attaching the lasers and the cold spray gun on the same flange of a fixed robot (Motoman HP20, Yaskawa, Kitakyushu, Japan) (Fig. 1). The substrates were placed in front of this robot and anchored to a sample holder mounted on another robot (Motoman HP6, Yaskawa, Kitakyushu, Japan). A schematic representation of the sample (disc with 25.4 mm diameter) in which the PLA spot (4 mm 9 13.5 mm rectangle), the heating laser spot (disc with 10 mm diameter) and the spray deposition spot (disc with 8 mm diameter) are illustrated, is shown in Fig. 2. The lasers and cold spray gun were fixed while the substrate was displaced following the pattern that is represented by the horizontal arrows. The spray deposition and the laser treatments occurred only during the left-to-right horizontal displacement with a constant velocity of 330 mm/s. After each traverse, the robot moved the substrate back to its horizontal original position and then moved it to the


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are not totally understood (Ref 12, 13, 15, 38-40). One way to modify the surface state is to use pulsed laser ablation (PLA). PLA is frequently used in thermal spray to remove the surface oxide and hydroxide layers on the substrate surface as well as other contaminants. Typically this is performed a few milliseconds prior to coating deposition and provides a fresh metallic surface for the impacting particles [a process known as PROTAL (Ref 41-44)]. Results have shown that enhanced bonding properties were achieved when using PLA compared to conventional surface preparation methods for arc spray, plasma spray, HVOF, and more recently, cold spray processes (Ref 34, 42, 44-48). To explain the enhanced adhesion between the coating and the substrate, bonding mechanisms involving physicochemical interactions were suggested. In some cases, a texturing effect was also found to contribute to the adhesion (Ref 43, 49). Another way to alter the surface state prior to cold spraying is through pre-heating. Even though the underlying phenomena for these effects are still unclear, several studies have shown that pre-heated substrates can either enhance or reduce coating adhesion (Ref 25, 49-54). The objective of this study was to evaluate the influence of two laser treatments, PLA and surface heating by laser, on the adhesion and cohesion strength of Ti-6Al-4V cold sprayed coatings. The influence of PLA combined with surface heating by laser has been used on substrates displaying various levels of roughness. Ti-6Al-4V was selected as the coating and substrate material because of the growing interest in the aerospace industry for potential spray forming applications with this high value alloy.

Fig. 1 Experimental setup of cold spray and laser equipment

start position of the next horizontal traverse which is at a 2-mm vertical step from the previous one until the entire substrate surface was coated. The deposition process following the described substrate displacement pattern is referred to hereafter as a pass. Three passes were performed on all samples to obtain the desired coating thickness of approximately 450 lm. Figure 2a shows the deposited particle profile along a diameter of the spray deposition spot. Figure 2b illustrates the cross-section of the final coating and how the coating built up one traverse after another (wavy lines). The intermediate layers obtained after each pass were also superimposed with bold lines. It is worth noting that, if the laser ablation was maintained throughout the build-up of the whole coating, the ablation was not only located on the original substrate surface but also within the coating, as illustrated by the wavy lines.

2.1 Feedstock Powder and Substrate Materials A commercially available plasma-atomized grade 5 Ti-6Al-4V feedstock powder provided by Raymor (Montreal, Quebec, Canada) was used in this investigation. The mean particle size was found to be 30 lm and the mean Vickers microhardness was found to be 386 HV10. More details about this powder may be found in Ref 18. Each sample consisted of a coating sprayed onto the flat surface of Ti-6Al-4V cylindrical substrates of 25 mm diameter and 10 mm thick.

2.2 Surface Preparation Before and During Cold Spray 2.2.1 Substrate Surface Finish. The surface finish of the substrates prior to cold spraying was obtained through four different methods: (i) grit blasted with grit 24 or 100 alumina, (ii) as-machined, (iii) ground with grit 400 or 800 papers, and (iv) mirror polished. 2.2.2 Surface Pre-heating. In conventional cold spray, the substrate is inevitably heated by the gas jet stream (Ref 55) and by the particle impacts. Since all spray conditions were kept constant, the surface temperature

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Peer Reviewed Fig. 2 Schematic of the sample, the laser spots, the particle spot, the substrate displacement pattern and the coating build-up pattern; (a) deposited particle profile as projected from the center of the cold spray gun nozzle and (b) the coating thickness build-up with the accumulation of the deposited particle profile displaced by 2-mm step for each pass and for a total of three passes (continuous black lines) and the laser ablated surface within the coating (wavy lines)

reached due to the process heating was assumed to be identical for all coatings. The effect of an elevation in surface temperature was investigated by subjecting samples to laser surface heating with a Nd:YAG continuous laser (model CW 020 from Rofin Sinar, Hamburg, Germany) set to the maximum power of 2 kW, throughout the entire cold spraying process . The laser beam was delivered through glass optical fibers. The laser spot size was adjusted to a 10-mm diameter (Fig. 2). The two laser power levels used during the study were: 750 and 1650 W. In order to give an estimate of the substrate surface temperature augmentation due to laser heating, the substrate displacement pattern used during cold spray deposition was repeated with only the heating laser on (both cold spray gun and ablation process off). An infrared camera (SC640, Flir, Boston, USA) was used to record the substrate surface temperature until steady state was achieved. It was found that a laser power of 750 W corresponded to a surface temperature augmentation of 95 C while 1650 W resulted in a surface temperature augmentation of 175 C.


2.2.3 Pulsed Laser Ablation Process. The PLA was carried out using four Q-switched Nd:YAG lasers. Each laser was calibrated to a mean power of 40 W, a 10-ns pulse duration, operating at a repetition rate of 150 Hz (Laserblast 1000, Quantel, Courtaboeuf, France). The four lasers were combined into two laser heads that provide a homogeneous spatial beam profile (Ref 41). The ablated area covered by one laser shot was approximately 4 9 13.5 mm2. Two pulse fluence conditions were used during the study: 1.3 and 2.2 J/cm2. According to the pulsed laser repetition rate and the horizontal traverse speed, about 90% of the surface would be ablated twice while 10% would be ablated once during one traverse. Also, regarding the vertical size of the ablated area (13.5 mm), the spray deposition spot size, and the 2 mm vertical step, the surface would receive between 2 and 5 additional laser ablation shots during one pass. As a consequence, during the overall PLA process, both the original substrate surface and the intermediate surfaces during coating build up (wavy lines in Fig. 2b) would have been ablated between 4 and 10 times.


All coatings were deposited using a KINETIKS 4000-M system with a MOC 24 WC nozzle (CGT-GmbH, Ampfing, Germany). This nozzle had a 5.3-mm diameter round outlet, an expansion ratio of 3.94, and a divergent section length of 120 mm. All spray experiments were performed using nitrogen as the propelling gas. The powder feed rate was approximately 20 g/min and the stand-off distance was 40 mm. The propelling nitrogen gas temperature and pressure conditions were 800 C and 4 MPa, respectively, which were the maximum stable operating conditions of the cold spray system. These selected conditions allowed greater plastic deformation to occur as they provided the highest particle impact velocity with this gas and nozzle design (Ref 56, 57). The average particle velocity measured by time-of-flight optical diagnostics (Tecnar Automation, St. Bruno, QC, Canada) (Ref 58) at the center of the jet at a distance from the nozzle exit corresponding to the standoff distance used for deposition was found to be approximately 840 m/s.

2.4 Sample Characterizations 2.4.1 Metallographic Preparation and Examination. Selected cold sprayed samples were sectioned with a coolant-assisted diamond wheel and were cold vacuum mounted in an epoxy resin. These specimens were ground with SiC papers (grit 320) and polished with 9 and 0.04 lm diamond suspensions. Polishing steps were performed at a low pressure (~25 N) and for prolonged duration (~5 min) to prevent smearing or pore obstruction. Microstructural observations were performed using a field emission gun scanning electron microscope, Hitachi S-4700 (FEGSEM) in both secondary electron imaging mode (SEI) and back-scattered electron imaging mode (BSEI). SEI (accelerating voltage ranging from 8 to 10 kV and working distance ranging from 8 to 12 mm) was used to characterize substrate top views, while BSEI (accelerating voltage of 15 kV and working distance of 10 mm) was used to characterize coating cross-sections and to measure coating porosity. The latter was performed on as-polished samples with the use of Visilog image analysis software (Noesis, Saint Aubin, France). A minimum of ten random images were taken and evaluated for porosity for each specimen. 2.4.2 Substrate Surface Roughness Measurements. The substrate surface roughness was measured using a mechanical stylus profilometer, Mitutoyo Model SJ-201P (Kanagawa, Japan). The reported results represent an average of three measurements on the same surface finish. 2.4.3 Coating Cohesion Strength Measurements (Scratch Test). The scratch test consisted of running a Vickers pyramid on a polished cross-section of a sample through the coating, from the substrate to the resin where the sample was embedded. The pyramid was applied with a constant force Fn, normal to the surface. When the pyramid passes from one material to another, a cone forms and an imprint remains. The cone is characterized by its area A as illustrated in Fig. 3. The scratch hardness


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2.3 Cold Spray Parameters

Fig. 3 Illustration of the cross-section scratch test used to evaluate the cohesion of samples

is defined by the ratio HV = Fn/A. Since the hardness of the resin is relatively weak compared to the hardness of the coating, hardness can be estimated as a measure of the coating cohesion strength (Ref 59). 2.4.4 Coating/Substrate Adhesion Strength Measurements. ASTM C633 pull test and LAser Shock Adhesion Test (LASAT) were performed for comparison purposes on selected samples. The high bond strength of the cold sprayed Ti-6Al-4V coatings typically resulted in a glue failure (Ref 60). Therefore, LASAT was used to evaluate the adhesion strength for coatings that exhibit bond strength values higher than approximately 80 MPa. Standard ASTM C633 adhesion strength tests consist of gluing the cold sprayed samples to grit 24 alumina blasted mild steel cylinders with FM1000 epoxy glue (Cytec Industries, Woodland Park, NJ) followed by a destructive tensile test which measures the coating-substrate adhesion strength. Prior to gluing the cold sprayed samples to the mild steel cylinders, the epoxy glue was first heat treated in an air furnace at 90 C for 3 h. In order to ensure proper adhesion between the glue and the metallic surfaces, the samples were heat treated in an air furnace at 200 C for 3 h and then air cooled at room temperature. All ASTM C633 adhesion strength tests were performed using an Instron 5582 universal testing machine (Burlington, Ontario, Canada) with a 100 kN dynamic load and a constant speed of 1.02 mm/min. The LASAT technique is based on the propagation of high amplitude ultrasonic waves that provide a local measurement of joint strength (Ref 61-66). First, a compression wave is generated by a short and powerful laser pulse and is converted after reflection on the assembly back surface into a tensile wave. The resulting tensile force, which is normal to the interfaces, can cause delaminations (adhesion rupture) or cracks (cohesion rupture) inside the sample. The strength is assessed by increasing the laser pulse energy until debonding occurs. Monitoring of the damage initiation and propagation is performed by measuring the sample back surface velocity by an optical interferometer. The velocimeter signals are


Peer Reviewed Fig. 4 Back surface velocity signals measured for laser shock pulse energies below and above damage threshold

also used to estimate the stress history inside the sample. For some applications, when the cohesive strengths of each material composing the sample are higher than the bond strength between the parts, the technique is noninvasive and can be used as a proof test to verify that the bond strengths are above a given threshold value. The generation laser used in this study was a powerful Q-Switched Nd:YAG laser which delivered optical pulses of 8 ns duration and up to 2 J of energy at a wavelength of 1064 nm. The laser beam was focused to a spot diameter of about 3 mm. To avoid surface damage and to increase the ultrasonic wave amplitude, the surface of the material was first covered with an optically absorbing tape (black electrical tape) and then with a constraining medium transparent to the laser wavelength (a water layer). To quantitatively evaluate the stresses inside the sample, an optical velocimeter based on a Fabry-Perot solid state etalon, was developed and used to monitor the back surface velocity, u(t). A detailed description of the interferometer can be found elsewhere (Ref 67). The mechanical properties of the sample were such that it was an ideal case for the LASAT method: no plastic transformation was observed, only a weak ultrasonic reflection occurred at the interface, and the ultrasonic attenuation was negligible. Indeed, the pressure level inside the sample was below the Hugoniot Elastic Limits (HEL) of titanium (830 MPa at a low strain rate). Thus, the wave propagation is in the weak or elastic shock regime, such that the waves would travel approximately at the elastic sound velocity c = 6310 m/s, and no plastic deformation would be induced. Also, since the two parts were composed of the same material but with different porosities, there is a weak acoustic impedance mismatch and thus, a weak acoustic reflection. Finally, while traveling back and forth through the coating, the wave propagated with almost no diffraction since the generation spot was much larger than the coating thickness and the acoustical attenuation in the titanium alloy was negligible in the operational acoustic frequency range. The substrate thickness was also reduced to approximately 2 mm after the CS process in order to avoid diffraction and, thus, energy lost in the substrate. When no cohesive rupture is observed, the LASAT method allows a quantitative and accurate measurement of the joint strength. Under these assumptions, a simple relationship is used to relate the


back surface velocity to the pressure at the rupture threshold Prupt (Ref 68): 1 Prupt ¼ qc u ti u ti 2zrupt =c 2

ðEq 1Þ

where q stands for the material density (q = 4.43 g/cm3), ti is the time on the velocity signal at which the damage was identified and zrupt is the distance between the damage and the back surface, i.e., the thickness of the coating. Since shock propagation is elastic, all velocity signals obtained with different laser energies superimpose when normalized, except for the signal in which a debonding signature appears. When gradually increasing the laser energy until debonding occurs, the time ti is obtained when a change is seen on the normalized velocity signals. Figure 4 shows the signals obtained for a very weak load and for loads just below and above the rupture threshold. The debonding signature was identified at about ti = 0.5 ls. The following small oscillations with a constant period and decreasing amplitude correspond to reverberations within the coating and the period of the oscillation confirms that the disbond occurred at the interface. Since the diameter of the spot is much smaller than the diameter of the sample, several measurements were taken. The rupture threshold had then been determined with precision and the adhesion strengths given hereafter were a result of an average of at least three measurements.

3. Results and Discussion This study attempts to clarify some aspects of the adhesion and cohesion mechanisms occurring during cold spray. First, the influence of the laser treatments on the coating cohesion and porosity will be evaluated. Then, the adhesion strength measured by the LASAT and ASTM C633-79 methods will be discussed. Finally, the influence of initial roughness and laser treatment parameters on adhesion strength will be investigated.

3.1 Influence of Laser Treatment on Coating Cohesion In order to evaluate the influence of laser treatment on the cohesion and on the porosity of cold sprayed coatings,


Table 1 Coating porosity and scratch hardness according to sample preparation

Coating porosity, % Scratch hardness, MPa

No PLA no heating

PLA (2.2 J/cm2) no heating

PLA (2.2 J/cm2) heating (1650 W)

7.5 ± 1.5

6.8 ± 1.8

6.5 ± 1.6

860 ± 90

520 ± 25

880 ± 65

The laser treatments were maintained throughout the spraying process, 3 passes

Fig. 5 Cross-section SEM micrographs obtained from coatings produced using PLA (2.2 J/cm2) and laser heating (1650 W)


increase (Ref 57). However, particle velocity is not the only parameter governing particle deformation. Mechanical and physical properties of the sprayed material (Ref 12) as well as particle (Ref 12) and substrate temperatures (Ref 25, 49-54) also influence how cold spray coatings develop. In this work, the influence of temperature of the substrate as well as of the previously deposited sprayed particle layers was investigated. Analysis of the coating porosity revealed that the laser treatments slightly lowered coating porosity. These results were not as pronounced as previously reported (Ref 25, 69) where it was shown that heating during the spray operation resulted in a denser coating. However, the heating power used was higher in those investigations. Scratch tests were replicated for several loads (10, 20, 40, 60 N) and the cone area over the load ratio was found to be linear. Thus, constant hardness values were obtained for each sample and the error was calculated as the standard deviation of the linear fits. The scratch tests clearly show that the PLA process reduced the cohesion strength of the coatings. This was unexpected as it was believed that the cleaning effect of PLA would allow an intimate and oxide-free bonding between the particles. Indeed, in previous studies, it has been demonstrated that the removal of oxide films on the particle and substrate surfaces, by the high velocity particle impact in these cases, allows a fresh contact at the atomic scale which favors intimate bonding in the cold spray process (Ref 20, 70, 71). In fact, the PLA process not only ablated the surface but it also produced high amplitude acoustic waves, such as in a LASAT test. Since the coating was porous, the compression wave generated during the PLA process could have damaged the particle/particle bonds due to stress concentration in the particles close to the pores, inducing cohesion loss in the coating. The PLA process is also very similar to laser peening which has been used for imposing compressive residual stress (Ref 72). It has also been shown that residual tensile stress can develop transversely on the edge of the area treated in order to compensate for the compressive residual stress. These tensile stresses could be another source of coating cohesion degradation. Nevertheless, the cohesion reduction caused by PLA is counterbalanced by the laser heating which improved coating cohesion as shown in Table 1. Based on the laser wavelength, the optical penetration, which heats up the surface, was calculated to be about 25 nm. The heat diffusion for the 40-ms time delay between the substrate surface heating and the laser ablation would penetrate the volume by roughly 10 lm. Thus, comparing to the 30 lm of particle diameter, the exposed particle would be heated and softened only partially. Consequently, this enhanced the cohesion between particles. However, this did not reduce the overall coating porosity because the average particle temperature remained relatively low and, thus, the plastic deformability improvement was limited. In conclusion, it has been shown that when the PLA process was maintained during the full deposition process, coating cohesion decreased, while surface heating promoted particle/particle bonding, which increased coating cohesive strength.


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three samples were prepared with the same surface finish, but under different laser conditions: without laser treatment, with the PLA process only (2.2 J/cm2 of fluence) and with two laser treatments (PLA at 2.2 J/cm2 of fluence and heating at 1650 W). In addition, the laser treatments were maintained throughout the entire spray operation, not only during the first deposited layer (first pass), but for the other two subsequent layers (three passes in total). It is worth noting that by considering the deposition pattern, the laser ablation was performed not only on the original substrate but also on each deposited traverse layer (cf. Fig. 2b). As a result, maintaining the laser treatments during all three passes affected the whole thickness of the coating (wavy lines in Fig. 2b). Porosity measurements and scratch tests were performed on these samples and the results are presented in Table 1. Coating porosities were evaluated from image analysis of the cross-section SEM micrographs such as the one presented in Fig. 5. One can observe that the coating surface roughness is about the size of a few particle diameters. The pores in the coatings resulted from a lack of particle deformation upon impact. Thus, the cold spray parameters used in this study were not able to produce fully dense coatings. It has been shown that coating density increases as particle velocities

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3.2 Influence of Adhesion Test Method on Adhesion Measurements The adhesion strength of the Ti coatings measured by the two methods, ASTM C633 pull test and LASAT, are presented in Table 2 as a function of initial substrate surface roughness. Five initial roughness conditions were considered, and no laser treatment was performed. The ‘‘>80’’ MPa value stands for the cohesion strength of the glue employed for attaching the sample during the ASTM C633 pull test. The coating/substrate adhesion is then expected to be higher than that value. The values are different for the two methods and the ASTM C633 pull test method cannot effectively provide measurement for highly adhesive coatings. These results illustrate how coating adhesion strength still faces several challenges. For example, the relationship between the adhesion parameters obtained with LASAT and with the standardized ASTM C633 methods is not obvious. On the one hand, the LASAT method is a dynamic measurement of bond strength at very high strain rate. On the other hand, the ASTM C633 pull test method is quasi-static. Since the constitutive laws of material behavior are very different with respect to strain rate, the adhesion strength values differ significantly (generally by a factor of ten). The best way to compare these methods would be to rely on the fracture energy, but this approach would be much more complex under high strain rates which can cause high measurement errors. Also, it is important to emphasize that the LASAT method, contrary to the pull test method, imposes a tensile stress in a small region far from the edges of the sample. Thus, the measurement region could be considered free of macroscopic defects (large voids and cracks) that are intrinsically present at the edge of the pull test coupons. Therefore, the LASAT bond strength measurement is related to the crack propagation initiated by microscopic defects at the coating/substrate interface. In contrast, the standard pull test method measurements are related to the crack propagation initiated by the stress concentration of the macroscopic defects on the edge of the coupon in combination with the stress concentration of the microscopic defects at the interface. However, the trends obtained by the two methods are similar: the adhesion is weak for a certain roughness range (2.56 lm < Ra < 3.21 lm). These trends are reported qualitatively on Fig. 6. The adhesion strength curve given

by the ASTM C633 pull test method had been extrapolated for values above 80 MPa and these values appear as gray. A deeper analysis of the role of the initial substrate roughness on adhesion mechanism, presented in the next section, is required to interpret these results. However, since for some conditions, we reached the limit of the pull test measurement method due to the limitation of the adhesive strength of the glue, only the LASAT method will be used henceforth.

3.3 Influence of Initial Roughness and Laser Treatments on Adhesion The SEM micrographs of the sample cross-sections constructed with various substrate surface preparations are presented in Fig. 7. The presence of porosities in the coatings (on top) differentiates them from the substrates (on bottom). The coating/substrate demarcation was varied, corresponding to the type of substrate surface preparation. For the coating produced on a grit-blasted substrate (Fig. 7a), the interface appears well defined, as a long continuous succession of microscopic defects in the form of voids and reveals that the coating particles did not completely fill in the roughness of the substrate surface. Contrary to other thermal spray processes where the molten particles can fully penetrate into the asperities, the particles in cold spray would need to be much more heavily deformed to achieve similar behavior. Interfacial microscopic voids were also observed for coatings produced on a ground grit 400 substrate (Fig. 7b). However, the size of these voids was smaller and they seem to be not interconnected. In contrast, the interface of the coating produced on PLA treated substrate (Fig. 7c) was much more difficult to distinguish. Under our experimental conditions, the adhesion strength principally results from chemical (metallic) and mechanical bonds. In addition, the measurement of adhesion strength considers in fact two effects: crack initiation and crack propagation. The chemical adhesion

Table 2 Influence of initial substrate roughness due to grit size used on the adhesion strength of cold sprayed Ti coatings measured by ASTM C633 and LASAT (without laser treatment) Initial substrate roughness Ra, lm 0.05 0.12 2.56 3.21 5.53

(miror) (grit 800) (as-machined) (grit 100) (grit 24)

Bond strength measured by ASTM C633, MPa

Bond strength measured by LASAT, MPa

>80 (glue failure) >80 (glue failure) 41 63 (adhesive failure) >80 (glue failure)

900 613 70 242 371


Fig. 6 (a) Schematic of the coating/substrate contact area as a function of the interfacial asperities sizes and (b) schematic of the adhesion strength and real surface contact area as a function of the surface roughness


Peer Reviewed Fig. 7 SEM micrographs of sample cross-sections around the coating/substrate interface for different surface finishes: (a) grit 24 alumina blasted, (b) grit 400, ground (c) with PLA at 2.2 J/cm2

Fig. 8 SEM micrographs of the top substrate (a) as-machined, (b) ground and (c) mirror polished surfaces

strength is proportional to the chemical links (metallurgical bonds) between the coating and the substrate and, as a consequence, to the area of the interface where the coating and the substrate are in contact. However, the real contact area is equal to the apparent contact area solely in the case when the surface is completely flat, without roughness. Otherwise, the contact surface can be smaller when all the asperities of substrate are not entirely filled by the particles, as is the case in Fig. 7(a) and (b), or higher if the asperities are completely filled, as is the case for higher roughness conditions (not shown here). The real contact surface variation as a function of surface roughness is schematically represented in Fig. 6(b) and the cross-sections of the three discussed cases are illustrated in Fig. 6(a). The trend of the curve is similar to the trend of the adhesion reported in the previous section. The absence of cracks and defects in the interface shown in Fig. 7c is then interpreted as an indication of better adhesion compared to the cases presented in Fig. 7(a) and (b). Furthermore, it is well known in fracture mechanics that mechanical bonds inhibit crack propagation, and as such, they will increase the measured adhesion. The mechanical anchors, which correspond to the sprayed particles that fill in the asperities, are mainly non-existent for smooth substrate surfaces (i.e., mirror polished), and numerous for rougher substrate surfaces. Therefore, the roughness value needs to be considered relative to the sprayed particles size. But if the crack propagation is reduced, the initial substrate roughness may also enhance the presence of microscopic defects, such as voids due to unfilled asperities, which facilitate the crack initiation and,


thus, reduce adhesion strength. A schematic view of these considerations is presented in Fig. 6b through the relationship between mechanical bond strength and surface roughness. There is, therefore, a range of roughness values where the bond is principally chemical since no mechanical bond can exist and where microscopic defects favor crack initiation. This range of roughness values correspond in Fig. 6b to the roughness values around the minimum adhesion strength observed for both adhesion measurement methods. Since the two methods are not equally sensitive to crack initiation and crack propagation, perhaps a finer measurement of adhesion strength as a function of the surface roughness would show that the minimum adhesion for the two methods would not be obtained for the same values. These observations will now be considered along with the quantitative adhesion strength measurements obtained with the LASAT method. Figure 8 shows the substrate surface with three different initial surface finishes, namely: (a) as-machined, (b) ground (grit 400), and (c) mirror polished. Their respective measured roughness (Ra) values were 2.56, 0.22, and 0.05 lm. The coating adhesion strengths of samples produced on these substrates with and without the PLA process (2.2 J/cm2) are presented in Fig. 9. No laser pre-heating was performed. These results show that a rougher substrate surface results in weaker adhesion. Furthermore, the PLA process (without laser heating) lowers coating adhesion strength except on mirror finish surface where the adhesion was found similar without PLA. This result confirms that the PLA generates defects, as was previously explained in section 3.1. Indeed,


Peer Reviewed Fig. 9 Adhesion strength measured by LASAT for three initial roughness conditions with and without PLA process (without laser heating treatment)

for the as-machined surface with PLA, the coating debonded during deposition. The results obtained on samples made with a mirror finished substrate surface, indicate that the removal of the oxide layer by the PLA has little or no effect on the adhesion process of cold sprayed Ti-6Al4V coatings. This is contrary to previous results obtained with other thermal spraying processes where higher bond strength was associated with the removal of the native oxide layer by the laser ablation treatment (Ref 43, 44, 73) and with cold sprayed aluminum coatings (Ref 34). It is possible that the removal of the fine surface oxide layer by the PLA does not provide a true fresh metallic surface to the impinging particles due to high oxidation kinetics. Further in-depth chemical and metallurgical interfacial studies would be required to confirm this hypothesis. Figure 10 displays the results obtained for the same surface finish (ground with grit 400) under three laser preheating conditions (no pre-heating, power of 750 W, and power of 1650 W) and three PLA conditions (no ablation, fluence of 1.3 J/cm2, and fluence of 2.2 J/cm2). Ablation laser treatment was performed during the first pass only. Similarly to the previous results, the ablation lowered coating adhesion strength when laser pre-heating was omitted. Without PLA, the adhesion strength did not vary significantly for the various pre-heating conditions. However, adhesion strength increased when the PLA process was combined with increasing laser pre-heating temperature. The reason suspected to explain this behavior is that the laser heating helped maintain the temperature and pressure of the plasma created by the PLA process and thus, lengthened its duration. The high temperature of the plasma would then melt the substrate to prepare a very smooth surface, equivalent to the one obtained with polishing. This hypothesis was confirmed by analyzing the surface finish of substrates subjected to the same laser treatment conditions. Figure 11 shows surface SEM micrographs of the substrates subjected to PLA treatments at 1.3 and 2.2 J/cm2 for 1 and 4 laser shots and for the three substrate pre-heating conditions. All the streaks


Fig. 10 Influence of laser treatment on adhesion strength measured by LASAT

produced by grinding with grit 400 became smoother with the laser heating and ablation and the surface finish appears similar to the one obtained by polishing. Thus, when PLA is performed on high temperature substrates, a smoother surface resulted. One should note that the ablation threshold was not significantly changed by the laser heating and consequently, the effect of laser heating is not equivalent to increasing the laser ablation fluence. In conclusion, the laser treatments (PLA and preheating) were beneficial for coating adhesion because they contributed to preparing a smooth surface with enhanced contact surface for particle adhesion. However, to prevent any degradation of the coating by the laser ablation in the case of porous Ti alloy coatings, it would be more suitable for the PLA process to be applied only before the cold spray operation on pre-heated substrates.

4. Conclusions The effects of initial surface roughness, PLA process and laser pre-heating on the adhesion and cohesion strength of cold sprayed Ti-6Al-4V coatings were investigated by metallographic examinations, the scratch cohesion strength test and LASAT and ASTM C633 adhesion strength tests. The LASAT method was successfully used to evaluate the effect of the different laser treatments on coating adhesion, overcoming the limitation of the standard ASTM C633 pull test on highly adhesive coatings. Contrary to what has been commonly observed for thermal spray coatings, the highest adhesion strength was obtained on the mirror finished substrates for the Ti-6Al4V cold spray coatings. Starting from the mirror finished state, increasing the substrate surface roughness yielded a greater surface contact area for adhesion but lowered coating adhesive strength because of the low deformability of the cold sprayed Ti-6Al-4V particles that fail to fill the asperities created by the roughening procedure. As the asperitiesÕ size increased (greater roughness) the particles


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Fig. 11 Influence of laser treatments (PLA and heating) on substrate roughness

filled in these asperities better and the true actual surface contact increased, yielding to an increase in coating adhesive strength. The PLA process showed a negative effect on the adhesion and cohesion of the deposited coating. This was attributed to the interaction of the laser beam with the deposited particles which in turn created particle/particle or particle/substrate interfacial defects. On the other hand, laser pre-heating was found to be beneficial for both coating adhesion and cohesion strength only when it was combined with the PLA process. The combination of the laser preheating with the PLA is suspected to significantly alter the laser ablation plasma-substrate interaction resulting in a smoother surface due to surface etching by the generated plasma. The initial substrate surface roughness, prior to laser treatments, was indeed found to be detrimental to the


coating adhesion with and without laser treatments. Contrary to results obtained for other materials, it was found that performing the laser ablation on pre-heated substrate surfaces prior to cold spray Ti-6Al-4V coating deposition, as opposed to in situ spraying, would be more efficient for improving coating adhesion and cohesion strength. The removal of the oxide layer by the laser ablation did not result in higher adhesion strength for Ti alloy. The high oxidation kinetics of this material might have contributed to this phenomenon. A more thorough analysis of the coating-substrate interfaces using MET or SIMS needs to be conducted in order to confirm this. In addition, contrary to other thermal spraying processes, it seems that the oxide layer does not affect the adhesion mechanisms in cold sprayed Ti-6Al-4V coatings, but this also needs to be confirmed.


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Acknowledgments The cold spray equipment used for this study was partially funded by CFI project number 8246 McGill University (Montreal, Canada) with the support of Cold Gas Technology GmbH, Tecnar Automation Ltd., and Polycontrols Technologies Inc. The authors would like to thank Mr. J.F. Alarie, F. Belval, B. Harvey, M. Lamontagne, J. Sykes, D. De LaGrave and M. Thibodeau, from the National Research Council Canada —Industrial Materials Institute (NRC-IMI, Boucherville, Canada) for performing the cold spray process, sample preparations and microscopic observations.

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