SiC particulate reinforced Al–12Si alloy composite coatings produced by the pulsed gas dynamic spray process: Microstructure and properties

Surface & Coatings Technology 203 (2009) 3260–3270

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

SiC particulate reinforced Al–12Si alloy composite coatings produced by the pulsed gas dynamic spray process: Microstructure and properties M. Yandouzi ⁎, P. Richer, B. Jodoin Department of Mechanical Engineering, University of Ottawa, Ottawa, Ontario, Canada

a r t i c l e

i n f o

Article history: Received 30 January 2009 Accepted in revised form 3 April 2009 Available online 9 April 2009 Keywords: Al–12Si SiC particle reinforcement Al alloy matrix composite coatings PGDS process

a b s t r a c t The work presented in this paper examine the possibility of using the pulsed gas dynamic spraying (PGDS) process as a new production technique of Al–12Si alloy coatings and (Al–12Si + SiCp) composite coatings using feedstock powder containing 20, 40, and 60 (vol.%) reinforcement particles. In this study the effect of the processing conditions, in particular the gas preheating temperature and pressure and the feedstock particle preheating temperature on the coatings microstructure is investigated in order to optimize the spray parameters. Different analysis techniques (OM, SEM, XRD, and microhardness) as well as adhesion strength testing are used to characterize the coatings produced. Detailed comparisons of powders and coatings microstructure, phase composition, microhardness and adhesion strength are presented and discussed. Moreover similarities and differences with coatings prepared by other techniques, namely thermal spray (TS) and cold gas dynamic spray (CGDS), are presented and discussed. It was found that PGDS allows producing dense Al–12Si alloy coatings and (Al–12Si + SiCp) composite coatings with good adhesion properties while retaining a large fraction of the SiCp present in the feedstock powder. Using the shear test standard (EN 15340), analysis showed that the adhesion strength of the PGDS coatings was slightly superior to the CGDS coatings, but still lower than TS coatings. It was also demonstrated that the adhesion strength decreases as the SiC content in the coating increases. © 2009 Elsevier B.V. All rights reserved.

1. Introduction For many applications, metal matrix composite (MMC) materials are considered more suitable than their conventional matrix alloy counterparts as they generally have enhanced mechanical, thermal and frictional properties. MMCs combine properties typically found in metals such as ductility and toughness with those of ceramics namely strength and Young's modulus. As a result, MMCs exhibit greater strength in shear and compression as well as higher service temperature capabilities [1]. Interest in MMCs for aerospace, automotive and other structural applications has increased over the last twenty years due to the availability of relatively inexpensive reinforcements combined with the development of various processing routes. As such, reproducible microstructures and properties are now possible [2–4]. MMCs can be tailored to yield superior properties by incorporating a controlled amount of reinforcement material within a metal matrix. Among these matrix materials, aluminum (Al) based alloys are interesting on account of their processing flexibility, low density, high wear resistance and high thermal conductivity [5]. Compared to unreinforced Al alloys, Al matrix composites (AMCs) reinforced with ceramic particles such as SiC and Al203 exhibit ⁎ Corresponding author. 161 Louis Pasteur, Room B210B, Ottawa, ON, Canada K1N6N5. Tel.: +1 613 562 5800x6536; fax: +1 613 562 5177. E-mail address: [email protected] (M. Yandouzi). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.04.001

significant improvements in yield strength, elastic modulus, wear and fatigue resistance [6–8], high-temperature mechanical properties and low thermal expansion coefficient [9,10]. Al–SiC composites have emerged as an important class of high-performance structural elements in the electronic and aerospace industries. Al–SiC materials have been used as base plates and heat sinks for powder modules. The incorporation of SiC within the aluminum matrix strengthens the material and lowers the coefficient of thermal expansion. A reduction of weight, vibration fatigue and thermal cycling were obtained without reducing the thermal dissipation [11]. Processing techniques have evolved over the last two decades in an effort to optimize the microstructure and thus properties of particulate reinforced AMCs [12]. Major fabrication methods of these Al-based composites, typically reinforced with silicon carbide particles (SiCp), include casting, extrusion, spray deposition, and powder metallurgy [13]. The selected process and its parameters as well as the type and amount of feedstock materials play an important role on the resulting composites quality and properties. Defects such as porosity, oxide inclusions, shrinkage and degradation of the reinforcement significantly influence the composite properties [13]. Although the process has been developed for several years, manufacturing AMCs with a high reinforcement particle volume fraction remains a challenge. To date, the existing methods capable of producing composites with high levels of particle reinforcement include the infiltration process aided by externally applied forces [14] and powder

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metallurgy [15]. In the case of thermally sprayed (TS) coatings, the reinforcement volume fraction may be varied widely by modifying the ratio of reinforcement to matrix material in the feedstock powder [16,18]. It has been reported that plasma spraying (PS) is capable of producing AMCs with large reinforcement volume fraction. Gui et al. [17] used Al powders mixed with 55 and 75 vol.% SiCp as feedstock and have obtained plasma sprayed composite coatings with 48 and 40 vol. % SiCp and porosity level of 2.6% and 3.5% respectively. High velocity oxy-fuel (HVOF) has also been used to manufacture MMCs, In that case the coatings exhibited high porosity levels and required post spray treatments [18]. It has been reported that during TS processing of composites, several problems may arise. For low processing temperature and short interaction time between Al and SiC particles, high porosity coatings are obtained. When the processing temperature and interaction time are increased, a reaction takes place between molten Al and SiC producing aluminum carbide (Al4C3). The later is a brittle compound that degrades in humid environments [19,20]. Furthermore, decomposition of SiC and Si phase formation occur due to the high temperature of the plasma plume [16]. Porosity and aluminum carbide formation reduce the coating corrosion resistance and its mechanical properties and should thus be avoided. Recent studies have reported the possibility of producing composite coatings using the Cold Gas Dynamic Spraying (CGDS) process [21–26]. This process uses a supersonic gas jet to accelerate solid fine powders (micron size) of various materials above a critical velocity at which particles impact, deform plastically and bond to the substrate to form the coating [27–29]. As opposed to TS processes, CGDS does not involve any significant heating of the powder particles. Consequently,

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the process temperature never reaches the sprayed material melting temperature. This suggests that CGDS coatings are produced solely as a result of intense local plastic deformation [30,31], and the technique is thus often referred to as a solid-state process. It has been shown that coatings of hard materials such as SiC, AlN and Al2O3 incorporated in a ductile Al matrix could be successfully produced by the CGDS process without a major phase degradation of the feedstock materials [22,25]. However, the percentage of the hard particles confined within the deformed ductile matrix particles in the coating is low compared to that of the original feedstock powder. Eesley et al. [24] found that inclusion of SiCp within the cold sprayed (Al + SiCp) coatings saturated in the 30–40% volume fraction range. More recent work concluded that depending on the initial SiCp volume fraction of the blend, between 33% and 50% of the SiCp in the feedstock powder is retained in the (Al–12Si + SiCp) cold sprayed coatings [20]. It has also been shown that changing the content of SiCp within an Al matrix produces predictable changes in thermal properties such as thermal conductivity and coefficient of thermal expansion [22]. The present study is undertaken with the objective of investigatingthe possibility of producing Al–12Si alloy coatings as well as (Al–12Si+SiCp) composite coatings using the Pulsed Gas Dynamic Spraying (PGDS) process [32]. One of the advantages of the latter technique is its capability to spray different types of materials at various and controlled impact temperatures, in contrast with the CGDS process which is limited due to its gun geometry. Using the PGDS technique, it has been demonstrated that conventional, nanocrystalline, and amorphous soft and hard coatings can be produced [31,33,34]. In this work, Al–12Si alloy coatings and (Al– 12Si+SiCp) composite coatings using feedstock powder containing 20,

Fig. 1. (a) Low magnification OM image showing the spherical morphology of the Al–12Si matrix powder. (b) SEM image of the reinforcement SiC particles revealing their irregular shaped. (c) SEM image of the starting (Al–12Si + 20% SiCp) mechanically mixed composite powder. (d) Measured size distribution of the matrix and the reinforced feedstock powders.

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40, and 60 (vol.%) reinforcement particles content are produced. Microstructure, phases, porosity, composition and SiCp distribution within the coatings as well as their mechanical properties (in particular hardness and bond strength) are studied. Moreover similarities and differences with coatings prepared by other techniques, namely TS and CGDS, are presented and discussed. 2. Experimental procedures 2.1. Feedstock material The feedstock powder used in this study and illustrated in Fig. 1 is a mechanical mixture of SiC ceramic and Al–12Si matrix alloy particles. This alloy is the commercially available Al–12%Si (in wt.%) powder (Al-111, Praxair Surface Technologies, Indianapolis, IN, USA). It is produced by atomization, and is composed of spherical particles ranging between 5 to 65 µm in diameter and has an average of 21.5 ± 9.3 µm. Since the composition of the Al–12Si alloy is very close to that of the eutectic reaction in Al–Si system at 12.2%Si, the particles exhibit a microstructure typical from a eutectic reaction. The light areas in Fig. 1a are aluminum-rich while the darker areas are silicon-rich. The reinforcement phase used in the current work is SiC powder (Sika, Arendal Smelteverk, Norway). The morphology of the SiC, as seen in Fig. 1b, is angular with sharp edges as a result of the crushing and grinding of SiC lumps that occurred during their production. Note that cracks within some SiC feedstock particles are observed. The particles size distribution, outlined in Fig. 1d, indicates that the selected SiC powder is in the range of 9–44 µm and has an average of 22.1 ± 8.7 µm. Even though the SiC particles have a slightly higher density than the Al–12Si particles (3.2 g/cm3 and 2.7 g/cm3 for SiC and Al– 12Si respectively), both are expected to reach similar velocities prior to impact on the substrate as irregular shaped particles experience larger drag coefficients than spherical particles [35]. The reinforcement particles were sieved below 32 μm and were mixed to the matrix alloy powder to create feedstock powder blends containing 0, 20, 40, and 60% (in vol.) of SiCp. Examination of the composite feedstock powder prior to spraying revealed that the mechanical mixing resulted in a homogeneous distribution. An example of the starting (Al–12Si + 20% SiCp) composite powder is shown in Fig. 1c. 2.2. Coating process The aluminum alloy and SiCp-reinforced aluminum alloy coatings were produced using the Pulsed Gas Dynamic Spraying process. This coating process has recently been developed at the University of Ottawa Cold Spray Laboratory [31]. In the PGDS process the feedstock particles are accelerated and heated in a high velocity unsteady flow induced by a series of shock wave generated at a fixed frequency before impacting the substrate to be coated. Similarly to the CGDS process, the particles impact the substrate and deform plastically to produce a coating. However, as opposed to CGDS, it is possible to achieve high particle impact temperatures when using PGDS due to the gas compression generating the driving flow. As such, it is envisioned that this process would allow the particles to be accelerated to high impact velocities and intermediate impact temperatures, which should lead to a lower critical velocity compared to CGDS and thus enhance plastic deformation upon impact on the substrate for similar impact velocity. For a more detailed description of the process and its physics, the readers are referred to previous publications [31–33]. For the present work, the deposition parameters and conditions are summarized in Table 1 and discussed in more details in the results section. Grit-blasting was used to prepare the substrates prior to the application of PGDS coatings, thus removing contaminants and roughening the surface to provide a favorable surface finish for coating adhesion to the substrate.

Table 1 Summary of pulse gas dynamic spray (PGDS) process parameters. Feedstock powder

Al–12%Si alloy

Al–12%Si + SiCp (20, 40, and 60 vol.%) composite

Substrate Propellant gas Gas pressure (Pgas) Preheating gas temperature .(Tgas) Preheating powder temperature (Tpowder) Substrate temperature Frequency

Al6061–T6 He 1.5 and 3.0 (MPa) 423–773 (K) RT–423 (K)

Al6061–T6 He 3.0 (MPa) 773 (K) 423 (K)

RT 10 (Hz)

RT 10 (Hz)

2.3. Characterization techniques Microstructural characterization of the powders and coatings was performed using both metallurgical optical microscopy (Olympus Metallurgical Microscope) and scanning electron microscopy (Philips XL 30, SEM). The latter is equipped with electron dispersion spectroscopy (EDS, Oxford, UK) for chemical analysis. The powder samples were examined directly. Prior to microstructural observations, the coatings were sectioned and prepared for cross-section investigation, following standard metallographic preparation techniques. The microstructural features such as the coating thickness, porosity, and the SiCp volume fraction measurements were done on the polished coatings cross-section images, using image analysis technique (Clemex Vision-Lite software). The images were taken at various positions within the coatings. A quantitative separation of the coating's structural elements was performed based on the grey level distribution of the SEM images. The porosity (black contrast) and the SiCp (deep grey contrast) could be distinguished by setting grey scale threshold cutoff points. The percent areas of the marked regions for porosity and for the SiCp could then be measured independently. Five measurements were performed per sample using 500× magnification images, while the intensity range and thresholds were standardized on reference materials. Phase identification of the feedstock powder and the coatings were investigated by X-ray diffraction (XRD). The XRD analyses were carried out using a Philips X-Pert model 1830 X-ray diffractometer equipped with a graphite monochromator using Cu kα (λ = 0.15406 nm) radiation. Detailed scans were performed over a 20–90° 2θ range, 0.02° step width and 2 s per step acquisition time. Vickers microhardness tests were performed on the cross-section of the deposits using a 300 gf (HV300) load and a dwell time of 10 s using a Duramin-1 microhardness tester (Struers Inc., Cleveland, OH, USA). To avoid the effect of stress field, the distance between two indentations was kept greater than three times the length of the indentation diagonal. The reported values are the average of 10 indentations for each sample. Coating bond strength refers to the degree of adhesion of a coating to a substrate and can have strong effects on the coating performance. In this study, characterization of the adhesive and cohesive strength of PGDS sprayed coatings is performed using the shear test standard (EN 15340) [36]. This technique was recently developed for rapid evaluation of the adhesion/cohesion strength of a coating on its substrate without the need of gluing and curing, and better describes the behavior of the coating when subjected to shearing loads which are frequently encountered in industrial applications [37–39]. In this technique, a coating sample is submitted to shear loading in a direction parallel to the substrate/coating interface using a commercial hard metal plate as a punch, thus pressing against the coating while a sample holder maintains the substrate fixed during testing. In this study, the load is increased by using a commercially available tensile testing machine (Instron model 4482 equipped with a static load cell of 100 kN) until coating delamination or failure occurs. The

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necessary force to produce failure is defined as the shear force. Adhesion strength (MPa) is obtained by dividing the measured shear force by the sample sheared area. The reported values are the average of 5 measurements for each sample. 3. Results and discussion 3.1. Al–12Si alloy coatings microstructure Al–12Si alloy coatings were first sprayed onto Al6061 substrates using the PGDS process at two different gas pressures (1.5 MPa and 3 MPa), gas preheating temperatures (423 K and 773 K) and powder preheating temperatures (293 K and 423 K). The objective of this part of the study was twofold: First, confirm the possibility of producing Al–12Si alloy coatings using the PGDS process and second optimize the spaying parameters for this type of coating. These optimized parameters are then used for the subsequent deposition of Al–12Si based composite coatings. Cross-sections of the coatings produced using the various spray parameter combinations are shown in Fig. 2. OM observation reveals that, in all coatings, the sprayed particles have preserved their microstructure/phase features resulting from an eutectic reaction; Al-rich (light areas) and Si-rich (dark areas) areas are observed as in the original feedstock powder (Fig. 1a). The absence of phase degradation or transformation of the sprayed material during the deposition process was confirmed by XRD analysis. The effect of the spray process parameters, in particular the increase of gas preheating temperature, gas pressure and particle preheating temperature on the coating quality, namely the porosity level, was first observed. Fig. 2a shows an OM image of a typical

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coating sprayed at the lower gas pressure and preheating temperature (Pgas = 1.5 MPa, Tgas = 423 K) and without preheating the feedstock powder. The resulting coating was found to be very porous (with a porosity level around 12%) with the majority of the particles seemingly undergoing insufficient plastic deformation and consequently retaining their original spherical morphology in the coating. This coating was also characterized by low adherence to the substrate; peeling of the coating at the substrate/coating interface and presence of micro cracks within the coating was observed. Increasing the gas preheating temperature to 773 K and preheating the feedstock powder to 423 K while maintaining the gas pressure at 1.5 MPa resulted in a significant decrease of the porosity level (below 3%), as can be observed in Fig. 2b. In this case, nearly all particles within the coating appear well deformed. It was determined that in PGDS, the particles are heated by the gas flow and that increasing the gas preheating temperature as well as particle preheating temperature leads to higher particle impact temperature [33]. Thus the enhanced particle deformation is attributed to the increased ductility at time of impact resulting form the higher particle impact temperature. Increasing the gas pressure from 1.5 MPa to 3.0 MPa while keeping the gas preheating temperature at 423 K and not preheating the particles seems to have resulted in enhancing significantly the particle deformation and consequently reducing the porosity level. Comparing Fig. 2a and c, it can be seen that the particle deformation level increased with the increasing gas pressure, and the coating porosity level was reduced from 12% to less than 5%. As it has been shown that in PGDS, the particle impact velocity increases with the gas pressure [32,33], it is concluded that the enhanced particle deformation can be attributed to higher particle impact velocity.

Fig. 2. OM images of Al–12Si alloy sprayed by the PGDS process revealing the effect of the gas pressure and preheating temperature on the coatings density. (a, b) Coatings sprayed at (Pgas = 1.5 MPa, Tpowder = 293 K and Tgas = 423 K) and (Pgas = 1.5 MPa, Tpowder = 423 K and Tgas = 773 K) respectively. (c, d) Coatings sprayed at (Pgas = 3.0 MPa, Tpowder = 293 K and Tgas = 423 K) and (Pgas = 3.0 MPa, Tpowder = 423 K and Tgas = 773 K) respectively.

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Fig. 3. Cross-section OM images showing the microstructure of the PGDS coatings onto Al6061 substrate sprayed at (Pgas = 3.0 MPa, Tpowder = 423 K and Tgas = 773 K) and using (a) Al–12Si feedstock powder (b, c, and d) Al–12Si feedstock powder reinforced with 20, 40, and 60 (vol.%) of SiCp respectively.

The best coating obtained is shown in Fig. 2d and was achieved using a gas pressure and preheating temperature of 3.0 MPa and 773 K respectively as well as preheating the feedstock powder to 423 K. This coating exhibits a low porosity level (less than 1%). The interface

between the deposited particles could not be easily distinguished within the OM images, suggesting very good particle cohesion and coating adhesion. Thus it can be concluded that operating at a gas pressure of 3.0 MPa and preheating temperature of 773 K as well as

Fig. 4. Plan-view SEM image shows the surface of Al–12Si and SiC particles onto the Al6061 substrate obtained with one pulse PGDS process. The schematic shows zone two, where the SEM image is taken.

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preheating the feedstock powder to 423 K provides a sufficient particle impact velocity and temperature required for the coating production. 3.2. (Al–12Si + SiCp) composite coating microstructure Cross-section OM images of the PGDS composite coatings sprayed onto Al6061 substrates using mechanically mixed feedstock powders containing respectively 0, 20, 40, or 60 (in vol.%) of SiCp with the Al– 12Si matrix are presented in Fig. 3. The coatings were sprayed using the process parameters that resulted in the best the aluminum alloy coating (Pgas = 3.0 MPa, Tgas = 773 K, and Tpowder = 423 K), as presented previously. Fig. 3a shows a dense coating consisting of deformed Al–12%Si particles. As mentioned previously, the latter is a result of significant plastic deformation of the feedstock particles upon impact on the substrate. Low porosity level and absence of delamination at the coating/substrate interface are the major characteristics of this coating. Fig. 3b shows the coating produced using the 20 vol.% SiCp mixture as feedstock powder. It is observed that the coating consists of ceramic particles (darker grey spots) imbedded in matrix particles. It can be observed from Fig. 3, that the SiC particles are randomly distributed at the matrix particle boundaries and homogeneously dispersed within the aluminum alloy matrix, thus suggesting the absence of particle segregation in the flow propelling the particles. While the matrix material exhibits a large deformation level, the SiCp present in the coating has maintained the angular morphology and features as those observed in the initial feedstock powder (see Fig. 1). This suggests that upon impact on the substrate/ coating, the SiC particles did not deform but rather distorted the matrix material and became confined within the matrix material. This is confirmed by Fig. 4. The later is a plan-view SEM image showing the surface of the substrate after one pulse deposition. The image is taken at the edge of deposited spot, where no coating built-up takes place due to the reduced velocity of the particles and also due to the high impacting angle with respect to the substrate surface. The later area was selected in order to see individual sprayed particles within the substrate. As can be observed, the Al alloy particle deformed upon impact and imbedded on the surface while the SiC particles retained their shape while distorting the surface and getting imbedded. Fig. 3(c & d) illustrate the coatings sprayed using the feedstock powders with 40 and 60 vol.% SiCp content respectively. It is observed that, by using the PGDS process, high amounts of reinforcement particles are retained within the coating. Conversely, although it is difficult to distinguish between porosity (black area) and SiC particles (dark grey area) in the OM images, the presence of voids in the coatings can be observed, especially on those with high SiC content. SEM observations of these two composite coatings reveal that the particulate reinforcements are homogeneously dispersed at the aluminum alloy matrix particle boundary, as illustrated in Fig. 5. SEM observations confirm that the coatings consist of deformed Al– 12Si particles into which are imbedded SiCp. The SEM images also confirm the previous OM observation (see Fig. 3): increasing the volume percentage of SiCp in the feedstock powder leads to an increase of the volume percentage of SiCp in the composite coatings as well as an increase in the coating porosity level. It is hypothesized that the latter phenomenon could be explained as follow: During the deposition process, the SiC particles do not plastically deform but either create voids/craters in the coating and bounce off or plow their way into the existing coating. These voids or craters, due to their different sizes and shapes could not be filled up by either the subsequent incoming SiC particles as they do not plastically deform upon impact or by the softer matrix particles due to insufficient impact velocity to achieve the required deformation level to fill up those voids/craters and also due to the absence of melting. Furthermore, the presence of SiCp in the coating also constitutes an extra obstacle for the softer matrix particles which cannot deform

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enough to completely fill the space around the SiCp, similar to the particle shading effect often seen in thermal spray processes. As a result increased porosity levels were obtained when using higher SiC content in the feedstock powder. The coating percentage porosity as a function of SiCp content in the feedstock powder has been analyzed using the SEM technique and is presented in Fig. 6. The results demonstrate that the coatings become more porous when the SiCp content in the feedstock powder is increased. Increasing the percentage of SiCp from 0 to 20% in the feedstock powder resulted in a minimal difference in the coating porosity level (1.30 ± 0.74%) compared to the pure Al–12Si alloy coating (0.78 ± 0.52%). Furthermore, even at a level of 40% of SiCp in the starting powder, the observed porosity level in the sprayed coatings only increases to (2.10 ± 0.85%). However, increasing the percentage of the reinforced particles content in the feedstock powder from 40% to 60% has resulted in a threefold increase of the coating porosity to an average value of (6.30 ± 2.15%). These porosity level values must be taken with caution as the polishing process may have contributed to increase the number of voids within the coating due to SiCp pullout. Pullouts have been observed and reported in other studies on (Al + SiCp) composite coatings [40]. Due to the presence of a large number of voids at grain boundaries, particularly between SiCp, microcracks within the coating and also at the interface with the substrate were observed. Investigation of the coatings composition (vol.% of SiC) as function of the feedstock powder composition was performed and results are reported in Fig. 7. It is observed that approximately 14.5, 27.7 and 41.0% of SiCp content in the coating were obtained when using 20, 40 or 60% of reinforced particles in the feedstock powder respectively, also reported in Table 2. Upon impact on the sprayed surface, the SiCp

Fig. 5. (a and b) SEM images of the cross-sections of the reinforced Al–12Si composite coatings sprayed with the feedstock powders containing 20 and 40% of SiCp respectively. The dark grey and white spots correspond to SiC particles and spray gun material fragments respectively.

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M. Yandouzi et al. / Surface & Coatings Technology 203 (2009) 3260–3270 Table 2 Summary of PGDS coatings results.

Fig. 6. Porosity percentage within the coating as function of the SiCp content in the feedstock powder measured using image analysis technique.

did not deform and as such a large fraction of the particles simply created voids/craters on the surface and bounced off and were lost. Some particles implanted themselves in the coating by deforming the matrix material upon impact. This is similar to surface grit contamination that occurs during the grit blasting process. Grit residual content has been found to reach a maximum when the blasting angle is perpendicular to the surface to be cleaned [41] and increases with the blasting pressure [42]. Fig. 7 also reports the results of a previous study which studied coatings manufactured by the CGDS process using the same feedstock composite materials [21]. It is observed that although the same feedstock composition was used, the coatings sprayed using the PGDS process exhibit the presence of a higher proportion of SiCp compared to those deposited by the CGDS process. It has been reported that when using the CGDS process, approximately 10, 17 and 22% of SiCp within the coating were obtained by using 20, 40 and 60% of the reinforced particles in the starting powder respectively [21]. Thus, composite coatings manufactured by the PGDS process seem to lose fewer hard particles during spraying than in the CGDS process. This could be explained based on the contribution of two factors. First, the particle impact temperature. The particle impact temperature used in PGDS is closer to the melting point of the Al–12Si alloy and therefore contributes significantly in softening the matrix particles, thus enhancing the percentage of SiCp that are trapped within the coating during deposition. The second factor is attributed to the nature of the PGDS process, where the (Al–12Si + SiCp) particles are projected towards the substrate as a series of clusters of powder particles at a set frequency during deposition as opposed to a continuous flow of

Starting powder

Porosity (%)

SiC (vol.%) in the coating

Hardness (HV300)

Shear force/bond strength

Al–12%Si alloy

0.78 ± 0.52

N/A

112 ± 5

Al–12%S + 20% SiCp

1.30 ± 0.74

14.5 ± 2.7

158 ± 12

Al–12%S + 40% SiCp

2.10 ± 0.85

27.7 ± 3.5

212 ± 16

Al–12%S + 60% SiCp

6.30 ± 2.15

41.0 ± 4.3

198 ± 27

(1522 ± 266) N/ (21.7 ± 3.8) MPa (1465 ± 303) N/ (20.9 ± 4.3) MPa (1167 ± 233) N/ (16.7 ± 3.6) MPa –

particles in the case of CGDS. It is hypothesized that as the feedstock particles travel as clusters, or lumps, more hard particles are trapped in the matrix even if they were initially bouncing off the coating surface since it is possible that there rebound motion is very rapidly interrupted by the impact with incoming soft matrix particles that are part of the same cluster. In CGDS, due to its continuous spray nature, some of the hard SiCp will be entrapped in the coating while others will bounce off the surface without being restrained or impacted by other incoming particles. This is illustrated in Fig. 8. Compared to (Al + SiCp) coatings thermally sprayed as shown in Fig. 7, the PGDS process seems to preserve a similar amount of hard particles than those prepared by the LVOF process [16]. Gui et al. [17] have obtained AMCs with a high volume fraction of reinforcement particles (48% SiCp) by plasma spraying using mechanically mixed (Al + 55% SiCp) feedstock. These coatings reveal a porosity level of 2.6% combined with the decomposition of SiCp and formation of Si and Al4C3 phases due to the high temperature of the plasma flame. Using 75 vol.% SiCp mechanically mixed to the Al matrix as feedstock, Gui et al. [17] have obtained TS coatings with lower retained SiC particles (40 vol.%) and with higher porosity level (3.5%). Other than deformed Al–12Si and SiC particles, the SEM analysis also revealed the presence of small particles (white regions) within PGDS coatings as shown in Fig. 5. These small white particles were observed at the particles boundaries in all coatings except those of pure Al–12Si alloy. The EDS analysis revealed that the white regions observed in the coating images were fragments of the spray gun material (see Fig. 9). This can be attributed to the use of silicon carbide particles, one of the hardest blasting media available, flowing through the spray gun. Fig. 9 shows a cross-section SEM image under secondary electron mode of (Al–12Si + 20% SiC) coating as well as three EDS spectrum confirming the chemical composition of SiCp (dark Grey), Al–12Si particles (grey), and nozzle fragments made from stainless steel 304 (white area) taken respectively. Due to their high hardness, the SiC particles are likely to erode the walls of the spray gun during the spraying process. The wear debris are entrained and deposited with the mixed feedstock powders. Furthermore, increasing the SiC content of the feedstock powder accentuated the erosion of the wall of the spray gun. The coatings contained from 1.5 to 5.0 vol.% of wear debris as the SiCp content in the feedstock powder was varied from 20 to 60% in vol. respectively. Similar results were also obtained in the case of the CGDS process [20]. In order to overcome this inconvenience, the gun materials used in both CGDS and PGDS processes should be made from harder material than the hardest sprayed material. 3.3. (Al–12Si + SiCp) composite coating phase analysis

Fig. 7. Volume fraction of SiC in the PGDS coatings as function of the SiCp content in the feedstock powder, shown together with those obtained by other spray techniques (LVOF, PS, and CGDS) according to [16,17,21].

A detailed phase analysis of the composite feedstock powder and coatings was undertaken using X-ray diffraction to observe the influence of the PGDS process on the phase change. Fig. 10 shows typical X-ray diffraction scans from the (Al–12Si + 20% SiCp) composite feedstock powder and PGDS coatings sprayed at Pgas = 3.0 MPa, Tgas = 773 K and Tpowder = 423 K. It is apparent that almost no

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Fig. 8. Schematics illustrating the continuous and the pulsed deposition processes during CGDS and PGDS deposition respectively.

degradation of the feedstock powder took place during the PGDS process. Similarly to the feedstock powder, XRD scans from the coating revealed the presence of a distinct SiC peak in addition to the α-Al and α-Si peaks. The XRD analyses did not indicate any possible reaction between Al and SiC particles, in particular the formation of Al4C3. It is known that carbides, especially SiCp, tend to react and dissolve in molten Al, leading to the formation of Al4C3 and ternary Al– Si–C carbides during solidification [18]. Al4C3 has been widely reported in Al/SiCp composites produced by melt-stir casting, in which the temperatures involved are relatively low (under 1200 K) [43]. The later phase, which was the result of the interfacial reaction between SiC particle and the Al matrix, is very brittle and hence severely detrimental to the mechanical properties. In the case of thermally sprayed Al/SiCp composite coatings, contradictory results have been reported. Ghosh et al. [44] reported that Al-matrix composite coatings with 20–75 vol.% SiCp by plasma spraying show the presence of Al4C3 phase within the coating. Meanwhile, Gui et al.

[45] reported that the period from melting to solidifying of the powders is very short during plasma spraying, and therefore the reactions between Al and SiC can be avoided completely. In this study, the absence of the Al4C3 within the coating suggests that the matrix particles did not melt during the PGDS spray although the used gas temperature was close to the melting point of the Al–12Si alloy (850 K). 3.4. Hardness measurements Microhardness measurements, reported in Table 2, were first performed on Al–12Si coatings sprayed at different process conditions such as gas pressure and temperature. It was observed that at constant powder temperature (Tpowder = 423 K), increasing gas pressure from 1.5 MPa to 3.0 MPa and gas temperature from 423 K to 773 K increased the hardness of the coating from 77 ± 14 to 112 ± 5 HV300 in the case of optimized parameters. The low hardness observed in the coating

Fig. 9. SEM image of the (Al–12Si + SiCp) composite coating, and the EDS spectrograms taken at different locations revealing the chemical composition of the light gray region, dark grey particles and white spots which correspond Al–12Si, SiCp and Fe-based materials (fragments of gun material) respectively.

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Fig. 10. XRD scans of the (Al–12Si / 20% SiCp) powder as well as the corresponding coating prepared by PGDS process at (Pgas = 3.0 MPa, Tpowder = 423 K and Tgas = 773 K), and reveal the phase preservation during deposition.

sprayed at low pressure and temperature is attributed to the low cohesion between the sprayed particles and to the high porosity level within the coating (≥12%). Meanwhile, the high deformation of sprayed particles, low porosity level (≤1%) within the coatings, and the strong particle cohesion and adhesion to the substrate observed in the coating prepared at optimal conditions (Pgas = 3.0 MPa, Tgas = 773 K and Tpowder = 423 K), are all factors contributing to its hardness. The measured hardness of the coating obtained by the optimized PGDS system seems to be similar to the one obtained previously by CGDS process, as a hardness of 110 ± 6 HV300 was measured for cold sprayed Al–12Si coating [20]. In the case of composite coatings, the SiC particles present in the coatings initiated significant changes in the coating hardness, as illustrated in Fig. 11. This figure shows the average values of hardness measurements of PGDS coatings performed during this study and those previously reported with the CGDS process. The average hardness of Al–12Si coatings reinforced with SiCp increases from 112 ± 5 to 158 ± 12 and up to 212 ± 16 HV300 when the content of SiCP in the feedstock powder is increased from 0% to 20% and 40% in volume. As such, coatings with 14.5% in volume of SiCp were found to be 41% harder than pure Al–12Si alloy coatings. The highest coating hardness was of 212 ± 16 HV300 and was obtained in the coating with 27.7% of SiCp. Increasing the contents of the SiCp within the coating up

Fig. 11. Vickers' hardness measurement of SiCp-reinforced Al–12Si coatings as a function of the SiCp content in the feedstock powder.

to 40.3% by using (Al–12Si + 60% SiCp) feedstock powder did not contribute to additional hardening of the PGDS coatings; in fact the average coating hardness appeared to slightly decrease (198 ± 27 HV300) compared to the one obtained with the 40% SiCp feedstock powder. This can be attributed to the increasing level of voids (high porosity) in the coating and to the decreasing level of cohesion between deposited particles. It suggests that the spray parameters used in this study are not optimized to spray composite coatings with large percentage of reinforcement particles. Compared to the coatings hardness obtained by the CGDS process (Fig. 11), analogous result was obtained. It was found that the deposition of coatings with an average

Fig. 12. (a) The schematic drawing of the test arrangement (according to EN15340 standards) as well as the typical-load-displacement curves showing the coating failure. (b) Shear strength (MPa) of SiCp-reinforced Al–12Si coatings sprayed by CGDS and PGDS process and the corresponding maximum forces (N) as function of the SiCp content in the feedstock powder.

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of 20 to 25% of SiCp content contributes in doubling the hardness of the Al–12Si coatings. 3.5. Shear strength measurements Adhesion strength analysis of the PGDS coatings was performed using the newly developed shear test method [36–38]. Shear strength tests were performed on (Al–12%Si + SiCp) coatings deposited onto Al6061 substrates. The test was performed on as sprayed samples that have rectangular geometry as illustrated in Fig. 12. Coating samples with 10 mm length, 7 mm width, and an average coating thickness of 500 μm were tested. The reported values are the average of five measurements. The test was performed on pure Al–12Si coatings and composite coatings produced using feedstock powder containing 20% and 40% of SiC particles. The shear strength method used in this work is a relatively new technique for rapid evaluation of the adhesion/cohesion strength. It was selected as it describes the coating behavior when subjected to shearing loads, which are often encountered in industrial applications. However, most of the bond strength testing for coatings has been historically done with bond strength ASTM C 633-01 standard. In order to compare results obtained in the case of CGDS (Al–12Si + SiCp) coatings [21], which was conducted using the ASTM C 633-01 standard [46] with the ones obtained here, (Al–12Si + 20%SiCp) coatings were also prepared using the CGDS process and parameters used in [21] and tested using the shear strength method. To ensure that CGDS coatings with similar properties as those in [21] were produced, coatings were also sprayed using CGDS onto gritblasted standard test cylindrical samples having a 25.4 mm diameter and an overall length of 38.1 mm. Several passes at a 50% overlap were carried out to cover the entire surface of the samples. The top portion of the coatings was then machined flat and glued to an uncoated test samples. An adhesive (Master Bond EP-15, Hackensack, NJ, USA) was used for bonding the test specimens. For those specimens, a bond strength of (43.4 ± 1.9) MPa was obtained, similar to the one reported [21] (44.0 ± 7.0 MPa), thus confirming that the CGDS process and spray parameters used reproduced properly those in [21]. It should be noted that during bond strength testing, all coatings remained attached to the specimens on which the bonding agent was applied. As such, the failure occurred at the coating–substrate interface, and therefore the measurement does reflect well the adhesion strength of the coating on the substrate. Using the shear testing standard, shear strength values of 16.4 ± 1.3 MPa and 15.5 ± 2.9 MPa were measured in the CGDS coatings prepared by feedstock powder containing 20 and 40% of SiCp respectively (Fig. 12). Consequently, it can be concluded that a bond strength of 43.4 ± 1.9 MPa obtained using the ASTM C 633 standards (tensile test) corresponds to a shear strength of 16.4 ± 1.3 MPa using the EN15340 European Standard (shear strength test). Note that the observed small decreasing of the adhesion strength of the CGDS coatings by increasing the volume percent of the SiCp within the coating is similar to what is reported previously [21]. In the case of PGDS composite coatings, the shear strength measurement as a function of the SiCp content in the feedstock powder is summarized in Table 2. For all the samples tested, the coatings detached completely (the failure occurred at the coating–substrate interface). Shear strength values of 19.3 ± 3.6 MPa and 16.7 ± 3.6 MPa were obtained as the volume fraction of SiCp in the coating was increased from 15.4% and 27.7% respectively. Note that the shear strength test was also performed on pure Al–12Si alloy coating and was determined to be 20.3 ± 5.3 MPa. According to the shear strength analysis results, a few observations can be made. First, the adhesion of (Al–12Si + 20% SiC) coating prepared by the PGDS process are slightly stronger compared to those sprayed using the CGDS process. The high temperature (close to the melting point of the matrix) used during PGDS deposition could

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contribute significantly to the acceleration and the deformation of the matrix particles therefore contribute to a better adhesion to the substrate. Secondly, an increase of the SiCp content in the PGDS coating reduced the level of adhesion of the coatings on the substrates. The later is similar to what is reported in the case of coatings prepared by the CGDS technique. It is foreseen that the presence of SiCp at the interface has affected the adhesion strength. These SiCp reduced the surface area available for the matrix particles to deform. As more SiCp impinged on the substrate, fewer Al–12Si particles come in contact with the substrate, thus reducing the bond strength of the coating. Results of a bond strength analysis of thermally sprayed (Al–12Si + SiCp) coatings have not been reported so far. The adhesion strength of (Al6061 + SiC) coatings produced by PS was found to vary from 68 MPa to 76 MPa by using the ASTM C 633 tensile test [44]. Although the reported bond/shear strength values in this study are somewhat higher to those prepared by CGDS, with consideration of the hard particle content within the coating, they are still below the adhesion strength values in coatings produced by PS due to the nature of the bond. In the PGDS and CGDS processes, the bonding mechanism is based on mechanical anchoring since no bulk particle melting occurs and no metallurgical reactions take place between the sprayed material and the substrate [47]. On the other hand, it was reported that a decrease in the metal content at the interface also reduced the bond strength in the TS coatings, similar to what was observed in this study. 4. Conclusions Al–12Si alloy and SiCp-reinforced Al–12Si matrix composite coatings were successfully produced using the Pulsed Gas Dynamic Spray process. It was found that increasing the gas pressure, as well as powder and gas preheating temperature contributes significantly to the quality of the obtained coatings. The optimized parameters allowed obtaining dense Al–12Si coatings with a porosity level below 1%. These coating reveal a good cohesion between deformed particles and good adhesion to the substrate. (Al–12Si + SiCp) composite coatings were also successfully produced. It was observed that approximately 14.5, 27.7 and 41.0% of SiCp content in the coating were obtained when using 20, 40 or 60% of reinforced particles, with porosity levels of 1.30% 2.10% and 6.30% respectively. The SiCp exhibited a reasonably uniform distribution within the matrix. It is envisioned that due to its pulse nature and the higher particle impact temperature during spraying when compared to CGDS, the PGDS process shows to better preserve the original composition of the feedstock powder. Adhesion strength analysis showed that the average adhesion strength of the PGDS coatings was slightly superior to that of coatings prepared by the CGDS process, but still lowers than thermally sprayed coatings. Shear strength tests demonstrated that the adhesion strength decreases as the SiC content in the coating increases. No phase degradation of the feedstock powder was observed when deposited either by CGDS or PGDS, whereas TS deposition of this feedstock material caused decomposition of SiCp and Si phase formation. Acknowledgement Authors acknowledge the financial support from the NSERC. They also like to thank Mr. C. Bourgogne for helping with the coating preparation. References [1] J.W. Kaczmar, K. Pietrzak, W. Wlosinski, J. Mater. Process. Technol. 106 (1–30) (2000) 58. [2] T.F. Klimowicz, J. Met. 46 (11) (1994) 49. [3] T.W. Chou, A. Kelly, A. Okura, Composites 16 (3) (1985) 187. [4] I.A. Ibrahim, F.A. Mohamed, E.J. Lavernia, J. Mater. Sci. 26 (1991) 1137.

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