Surface & Coatings Technology 204 (2009) 503–510

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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

Microstructure and wear properties of aluminum/aluminum–silicon composite coatings prepared by cold spraying Srinivasa R. Bakshi a, Di Wang a, Timothy Price b, Deen Zhang b, Anup K. Keshri a, Yao Chen a, D. Graham McCartney b, Philip H. Shipway b, Arvind Agarwal a,⁎ a b

Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, USA School of Mechanical, Materials and Manufacturing Engineering and Management, University of Nottingham, University Park, Nottingham, NG7 2RD, UK

a r t i c l e

i n f o

Article history: Received 14 May 2009 Accepted in revised form 17 August 2009 Available online 21 August 2009 Keywords: Aluminum coating Metal matrix composite Wear Delamination Cold spraying

a b s t r a c t Composite coatings containing aluminum and aluminum–11.6 wt.% silicon eutectic alloy phases of varying compositions were fabricated using cold spraying. Coating contained a uniform distribution of the two phases. The hardness of the coatings increased as the volume fraction of Al–Si in the coating increased. The length to width ratio of the splats was found to be larger for Al particles compared to Al–Si particles. Dry sliding ball-on-plate wear tests indicated that the wear volume loss was similar for the Al and Al/Al–Si composite coatings in spite of the increase in microhardness. This discrepancy is explained by the inter-splat delamination mechanism. The coefficient of friction of aluminum coating reduced on Al–Si addition. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Cold gas dynamic spraying or, more simply, cold spray is a coating technology which involves the acceleration and impact of solid particles on a substrate to form a coating. Typical cold sprayed coating thicknesses range between 100–1500 μm. The particles are accelerated in a supersonic gas jet which can be produced by the use of a converging–diverging de Laval nozzle [1]. Particles impinging on a substrate will either rebound from the substrate (with or without causing erosion), or bond with the substrate depending on the material type and particle velocity on impact with the substrate. In this way, coating formation is provided via plastic impact at high velocity (300–1200 m/s) and a temperature much below the melting point of the starting powder (ambient temperature to 700 °C). Bonding occurs due to rapid and significant deformation under high pressures and strain rates. It has been observed that for a range of metals, particle deposition only occurs when the particle velocity exceeds a critical value. It has been widely reported that many metallic materials such as Cu, Al, Ni, Ti, Fe- and Ni-based alloys can be readily deposited by cold spray [2–6] and recently there has been a growing interest in the use of cold spray to deposit composite coatings. A disadvantage of the process is that it is believed that only materials that are plastically deformable can form a coating.

⁎ Corresponding author. Tel.: +1 305 348 1701; fax: +1 305 348 1932. E-mail address: agarwala@fiu.edu (A. Agarwal). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.08.018

Dispersion of hard and brittle phases in a ductile matrix has been achieved by co-deposition of powder mixtures. Examples include TiO2–Zn [7], WC–Co [8,9], Al2O3–Cu [10], TiB2–Cu [11], Al–SiC [12] and Al2O3–Al [12,13], Cr3C2–Ni [14] and SiC–Al–Si [15]. The majority of these studies have been aimed at investigation of the coating microstructure and deposition characteristics. Although a suitable distribution of reinforcement phase was obtained in most of the cases, only a few attempts have been made to study the effect on the mechanical properties of the composites. Kim et al. [9] found that the cold sprayed nanosized WC–Co coatings had extremely high hardness of around 2050 VHN. Phani et al. [10] found that addition of nanoalumina inhibited grain growth of nanocrystalline copper coatings and an increase in the hardness was observed. Hall–Petch hardening was found to occur and be the dominant mechanism of hardening. Wolfe et al. [14] have carried out preliminary wear experiments on cold sprayed Cr3C2–Ni coatings and found that the wear resistance was slightly better than 4140 steel substrate. Guo et al. [16] have studied the dry sliding wear of cold sprayed tin–bronze coatings and found that the wear properties deteriorated on annealing the coatings although it was observed that the porosity decreased. However, despite its importance in terms of engineering applications of cold spray coatings; there have been limited studies on the wear behavior of cold sprayed ceramic–metal composite deposits. Two of the main applications of cold spraying are deposition of wear resistant coatings and repair of defective structural components. Aluminum–silicon alloys and its composites are important materials for applications in the automobile industry. One of the main reasons for their use is their excellent wear resistance. Clarke and Sarkar have

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shown that the wear rate of as-cast Al–Si binary alloys decreases with an increase in Si content and is lowest for the eutectic composition for various normal loads [17]. Addition of Si beyond the eutectic composition was shown to increase the wear rate. Improvement in wear resistance of Al by addition of Si in the range of 4–24% was observed by Pramila Bai and Biswas [18]. The dry sliding wear properties of particulate reinforced aluminum matrix composites have been studied in detail by a number of researchers. For AA 5043 alloy-B4C composites fabricated by hot isostatic pressing, Tang et al. [19] found that the wear rate of 10 wt.% B4C composite was approximately 40% lower than that of the composite with 5 wt.% B4C under the same test conditions. Sudarshan and Surappa [20] have fabricated fly ash reinforced A356 alloy composites by stir casting. It was found that the wear properties were similar to SiC and Al2O3 reinforced composites. So there exists a potential for improving the wear properties of aluminum coatings by addition of hard particles. The purpose of the present study was to establish whether an Al/ Al–Si composite coating could be cold sprayed to give a low porosity deposit, to elucidate the microstructures formed following spraying and to determine the dry sliding wear properties of the deposit. In the work reported here, soft Al powder was blended with hard Al– 11.6 wt.% Si eutectic pre-alloyed powder (referred as Al–Si hereafter) in varying proportions and cold sprayed in order to form composite coatings with compositions ranging from 100% pure Al to 100% Al–Si eutectic. 2. Experimental techniques 2.1. Materials Aluminum powder of 99.7% purity having a particle size of 26±13 μm was obtained from Alpoco Ltd (Minworth, UK). Aluminum–silicon gasatomized, pre-alloyed powder of composition Al–11.6 wt.% Si–0.14 wt.% Fe and mean particle size 14±9 μm was obtained from Valimet Inc. (Stockton, CA, USA). The powders were mixed together in the required weight proportions and blended in a turbula mixer for 1 h to obtain the cold spray feedstock. The compositions that were employed as feedstock materials for cold spraying were as follows: pure Al; Al+ 25 wt.% Al–Si; Al + 50 wt.% Al–Si; Al+ 75 wt.% Al–Si; and Al–Si. The above powders were deposited onto samples cut from AA6061 Al–Mg–Si alloy plate, sample dimensions were 100 × 25 × 3 mm. The aluminum substrates were prepared by grit blasting with Al2O3 having a particle size ~500 μm and then degreased with alcohol prior to spray deposition. The coatings obtained by deposition of pure Al, blended Al–25 wt.% Al–Si, Al–50 wt.% Al–Si, Al–75 wt.% Al–Si and Al–Si powders are designated as A, A-25AS, A-50AS, A-75AS and AS respectively.

2.2. Cold spraying Cold gas spraying was performed with an in-house built cold gas spraying system at the University of Nottingham comprising a high pressure gas supply, high pressure powder feeder, a converging– diverging nozzle and an X–Y traverse unit [21]. The martensitic steel de Laval nozzle had a throat diameter of 1.35 mm, with an area expansion ratio of ~8.8. The system utilized room temperature helium at 2.9 MPa for the primary accelerating gas and nitrogen as the powder carrier gas. The pressure of the carrier gas was set approximately 1 bar higher (~3.0 MPa) than the primary gas pressure to ensure powder transport into the main flow. A high pressure powder feeder (Praxair 1264HP, Indianapolis, IN, USA) was used during the cold spraying process. The nozzle-substrate standoff distance for all the spray runs was fixed at 20 mm. The nozzle was fixed to a frame and the substrates were fixed onto an X–Y traverse table, the movement of which was computer controlled. Eight passes at a traverse speed of 100 mm s− 1 were used to build up the coating thickness for each composition. 2.3. Microstructural characterization Coating cross sections were prepared by cutting samples with a diamond slitting wheel. The cross section was hot mounted using bakelite and sections were sequentially ground using SiC paper and then polished to a 1 μm diamond surface finish. Final polishing was undertaken using 100 nm colloidal silica suspension. For microscopy, polished samples were lightly etched using Keller's reagent (5 ml HNO3, 3 ml HCl, 2 ml HF and 190 ml H2O). A JEOL JSM 630OF FEG-SEM operating at 20 kV operating in the secondary electron (SE) image mode was used to study the feedstock powder and wear debris morphology. The powder morphology was investigated by examining a small quantity of material which had been sprinkled onto an adhesive mount. SEM was also used to examine the cross section of the wear track through the mid-plane, containing the sliding direction. An FEI Phenom SEM was used to observe the top surface of the wear track. The microhardness of coatings was measured using a LECO M-400 microhardness tester with a 2 N load and 15 s dwell time. Values quoted are the average of ten indents taken along the mid-plane of a coating cross-section. Optical microscopy was employed to measure coating porosity and to quantify selected microstructural features. To measure porosity, optical micrographs were analyzed using the image analysis software Image J. At least five optical micrographs at magnifications of 200× and 400× were used in this measurement. Porosity was taken equal to the area fraction of pores in the micrographs. Projected inter-particle distances of Al–Si powder particles in the coating were measured parallel and perpendicular to the coating surface. All distances were measured from center of one particle to another. The center of an oval particle was taken to be the point of intersection of its major and minor axis. Very small particles were not considered since it is believed that most of the particles would be below the polished

Table 1 Microstructural parameters and microhardness of the cold sprayed coatings.

Fig. 1. SEM micrograph of 1:1 mixture of Al and Al–Si powder.

Sample name

Vol. % of Al–Si in powder mixture

Measured vol. % of Al–Si in sprayed coating

Vol. % of porosity

Coating thickness, µm

Vickers hardness kg/mm2

A A-25AS A-50AS A-75AS AS

0 25.3 50.4 75.3 1

0 18 ± 2 29 ± 3 47 ± 2 100

1.6 ± 0.1 1.1 ± 0.3 1.1 ± 0.3 0.8 ± 0.2 0.5 ± 0.1

1100 950 600 450 350

56 ± 3 59 ± 2 67 ± 5 80 ± 3 127 ± 6

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Fig. 2. Optical micrographs of the cross section of the cold sprayed coatings: (a) 100% Al, (b)Al-25wt.% Al–Si, (c) Al-50wt.% Al–Si, (d) Al-75wt.% Al–Si and (e)100% Al–Si.

surface. Lines between centers having a deviation of ±2.5° from horizontal/vertical were also admitted. More than 100 values were measured from at least 4 micrographs and the results were reported as mean ± standard deviation of the values. 2.4. Wear testing Dry sliding wear experiments were conducted using Nanovea Tribometer (Micro Photonics Inc., Irvine, CA, USA). A ball-on-plate geometry was employed with a 6 mm diameter martensitic stainless steel ball (440C) used as the counter body to slide against the cold sprayed deposits. The samples were clamped to the sample holder which rotated with a speed of 200 rpm. A normal load of 5 N was applied on the ball and the ball was stationary. The wear track was circular with a diameter of

12 mm. The lateral force was measured via the deflection of a LVDT, which was previously calibrated using known loads. Coefficient of friction was calculated by the ratio of lateral force to applied normal load. All materials were prepared for wear testing by grinding and polishing down to 1 μm diamond finish. The weight loss of the sample was measured after 5000 revolutions which corresponds to a sliding distance of 188.52 m. For a proper comparison of the wear behavior, the mass loss values were converted to volume losses using an estimated density for each coating. The density of the coatings was calculated using rule of mixtures where the values of volume fraction of Al and Al–Si were taken as measured by image analysis and density of Al and Al–Si were taken to be 2700 kg m− 3 and 2650 kg m− 3 respectively. Average volume loss for two wear experiments has been reported. After the wear test, the wear debris was collected on a wax

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Fig. 3. Volume fraction of Al–Si in powder feedstock and in the coating.

paper by slightly tapping the sample on the paper so that almost all of the loosely held particles generated due to wear fell on the paper. A small part of the wear debris was taken with a spatula and spread over a double side adhesive carbon tape attached on a stub. This was gold coated and examined under SEM. 3. Results and discussion 3.1. Microstructure formation Fig. 1 shows the SEM micrograph of a 1:1 powder mixture of Al and Al–Si by weight. The larger and elongated powders are mostly Al whilst the smaller and spherical ones are mostly Al–Si powders. Different coating thicknesses were obtained for the five different powder blends sprayed as listed in Table 1. It is seen that the pure Al powder resulted in the thickest coating, 1100 μm, and the thickness deposited decreased as the proportion of Al–Si powder in the blend increased. Fig. 2 shows optical micrographs of the cross section of the five coating types. It is seen that pure Al coating had the highest porosity and the composite coatings generally exhibited lower porosity values. All the composite coatings shown in Fig. 2(b)–(d) are composed of a relatively uniformly distributed mixture of deformed Al and Al–Si powder particles. Table 1 tabulates various processing and microstructural measurements for the coatings. It can be seen that the coatings have low porosity, less than 2% in all cases. It can also be seen from the data in Table 1 that the volume fraction of the Al–Si powder particles in the coatings is found to be lower than the volume fraction of Al–Si powder in the blended mixture. In Fig. 3 the volume fraction of Al–Si in the coating versus the volume fraction of Al–Si added to the powder mixture is plotted. The relationship is approximately linear with a slope of 0.61, which means that only 61 vol.% of the Al–Si is retained in the microstructure. This is attributed to the lower deposition efficiency of Al–Si in the blends as compared to that of pure Al. This is supported by the fact that pure Al powder forms a 1100 μm thick coating whereas Al–Si powder forms a

Fig. 4. Flattening ratio and porosity fraction of the coatings.

350 μm thick coating for the same spraying conditions and almost identical powder feed rates. Table 1 also shows that the Vickers hardness of the coatings increases with addition of Al–Si particles. The ratio of the length to width of powder particles is a measure of the degree of flattening due to plastic deformation undergone by the particles on impact. The values of the flattening ratio for Al and Al–Si particles are given in Table 2. In Fig. 4 the flattening ratio for Al and Al– Si particles and the porosity are plotted as a function of the Al–Si content in the coatings. As is typically observed in cold sprayed coatings, the powder particles have been deformed and flattened during deposition; this is the case for both the pure Al and the Al–Si alloy powder particles. It is seen that the amount of plastic deformation of the Al particles is higher than the Al–Si particles. Also, it is observed that the flattening ratio of the Al particles increased as the Al–Si content of the blend was increased. This could be due to the fact that the lower deformability of the Al–Si particles leads to larger energy transfer to Al particles during impact. Fig. 4 also shows

Table 3 Experimentally determined hardness and wear volumes of the coatings. Sample name

Wear volume loss, mm3

Average coefficient of friction

A A-25AS A-75AS AS

6.08 ± 0.02 6.52 ± 0.92 6.17 ± 1.02 3.96 ± 0.74

0.80 ± 0.02 0.67 ± 0.05 0.55 ± 0.03 0.23 ± 0.01

Table 2 Flattening ratio (L/W) of Al and Al–Si particles in the coatings. Sample

L/W for Al

L/W for Al–Si

A A-25AS A-50AS A-75AS AS

2.4 ± 0.7 2.4 ± 0.6 3 ± 0.9 3 ± 0.8 –

– 1.9 ± 0.4 1.9 ± 0.5 2 ± 0.4 2 ± 0.3

Fig. 5. Calculated wear volume loss of the coatings after 5000 revolutions.

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The deformation or strain undergone by individual particles depends on the hardness and the velocity of the particles according to the following empirical relationship [22]

ɛp = exp −1:4

Fig. 6. Variation of coefficient of friction of the coatings with sliding distance.

Hp

!

ρp ν2p

ð1Þ

Here εp is the strain in the particle, Hp is the hardness of the particle (MPa) and ρp and νp are the density and velocity of the particles on impact respectively. Using the measured values of the hardness of pure Al (A) and Al–Si alloy (AS) tabulated in Table 3 and estimating the velocity to be around 500 m/s, the strain undergone by the Al and Al–Si particles is calculated to be 0.66 and 0.11 respectively. So it is expected that the flattening ratio for Al be higher than Al–Si particles. 3.2. Wear behavior

that the porosity decreased, somewhat, as the flattening ratio of Al increased. Therefore it seems that the higher the deformation of the particles the lower is the porosity of the coating.

Fig. 5 shows the volume loss of the coatings after the dry sliding wear tests. It is observed that the volume loss for A, A-25AS and A-

Fig. 7. SEM images of the wear surface for a) Al, b) Al-25%Al–Si, c) Al-75%Al–Si, and d) Al–Si coating.

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75AS coatings are similar whereas AS coating showed the least volume loss. A simple model for adhesive wear mechanism in case of metals has been given by Archard [23]. The wear rate according to this model is given by the following equation, W=K

P 3H

ð2Þ

where W is the wear volume per unit sliding distance, P is the applied load, H is the hardness of the material and K is a wear coefficient which corresponds to the severity of wear. According to Eq. (2) the wear volume loss must reduce from A to AS since the hardness goes on increasing. But it is observed that the wear volume in fact for A25AS and A-75AS is similar to Al coating. This is due to the delamination mechanism of wear which will be discussed in later section. The Al–Si coating shows the least wear volume which is expected since it has the highest hardness. This shows that the addition of Al–Si to Al leads to coatings with increased hardness but with comparable wear performance with Al. Similar wear behavior has been found for Al–Al2O3 composites prepared by cold spraying [13]. It was found that the even though the hardness increased by a factor of 2, the abrasion wear volume was almost same for composites with up to 26 vol.% Al2O3 particles. Haque et al. [24] have measured the wear resistance of cast Al–Si alloys by pin-on-disc method under a normal load of 5 N. They found the wear loss to be equal to 3 mg (~1.1 mm3) at a sliding distance of 1800 m. This is quite low compared to the present Al–Si coatings. One of the reasons for the higher wear rates of the cold sprayed Al–Si coatings is its cold worked nature which makes it brittle. The other reason is the poor bonding between Al–Si particles which leads to debonding and larger wear volumes. Fig. 6 shows the variation of coefficient of friction with the sliding distance. It is observed that the coefficient of friction reduces with addition of Al–Si. This is attributed to the increased hardness of Al–Si phase. As the wear progresses, the Al phase gets plastically deformed and results in an increase of the contact area. Presence of hard Al–Si phase reduces the effective contact area between the ball and the wear track and hence reduces the frictional force. The wear volume loss and the average coefficient of friction have been tabulated in Table 3. It has been reported that the friction coefficient of Al–Si alloys is insensitive to Si content [18]. However, Blau has found that the rule of mixtures predicts the friction coefficient of Al–Si alloys well [25]. Using similar approach, the friction coefficient of the composite coatings can be given by the following formula, μ C = μ Al VAl + μ Al–Si VAl–Si

three indicating that the wear is milder. This is due to the higher hardness of the coating. Delamination craters are observed in Al–Si which is due to adhesive pullout of debonded particles. Adhesive wear and delamination kind of phenomena are also seen in the Fig. 7a and b. It is observed that there is considerable plastic deformation on the wear surface in Fig. 7a and b and to some extent in Fig. 7c as compared to Al–Si coating (Fig. 7d). Fig. 8 shows SEM images of the cross section of the wear track for the A and A-75AS samples. In the pure Al coating (A) there is very little evidence for cracking occurring preferentially along inter-particle boundaries in the cold sprayed coating. However, in Fig. 8(b), taken from the A-75AS coating, it is clearly evident that there is significant cracking especially along the boundaries of unlike particles in the coating i.e. the Al/Al–Si interfaces. This leads to the delamination of splats and resulting in considerable wear loss in spite of higher hardness of A-75As coating. Fig. 9 shows the SEM images of the wear debris obtained for the coatings. It is observed that the wear debris for Al (Fig. 9a) is flake like and has gone considerable deformation. The particle has delaminated from the coating after being plastically deformed underneath the ball. The fracture surface as shown in the inset is quite smooth indicating that it was delaminated. The wear debris of Al–Si as shown in Fig. 9d on the other hand shows a different morphology. Some of the particles are flake like indicating that they have formed due to delamination and debonding. Several wear particles show corrugated and layered structure. This is similar to the ‘roof top laminate’ morphology observed on the pin surface of Al–Si alloys in pin-on-disc experiments [26]. The inset shows the high magnification image of the laminate. The steps observed could be due to slip at the Al and Si interfaces due to constant rubbing between the wear surfaces. It is possible that the

ð3Þ

where µ stands for friction coefficient and V stands for volume fraction of the phase. Using Eq. (3) and the values of volume fraction from Table 1 and friction coefficients of Al and Al–Si from Table 3, the coefficient of friction of A-25AS and A-75AS comes out to be equal to 0.70 and 0.53 respectively which is very close to the experimentally measured values. Thus it is seen that addition of Al–Si leads to reduction in friction and increase in hardness and hence is desirable in applications like repair of structural components. 3.3. Wear morphology and mechanism Fig. 7(a)–(d) show SEM images of the wear surfaces of the coatings. It is observed that a combination of adhesive and abrasive wear is observed for the coatings. Adhesive delamination is observed in Fig. 7a. Fig. 7b shows the presence of gouging by Al–Si particles in A-25AS which might lead to the slightly larger volume loss as observed. Pile up of softer Al phase is observed ahead of the Al–Si particle. Presence of grooves and scratches due to micro-cutting are also observed in all the coatings which is indicative of abrasive wear. The wear surface of Al–Si coating is smoother than the rest of the

Fig. 8. SEM micrographs of the cross section of wear track showing a) good bonding between particles in coating A, and b) interface debonding in A-75AS coating.

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Fig. 9. SEM images of the wear debris obtained from a) Al, b) Al-25%Al–Si, c) Al-75%Al–Si, and d) Al–Si coating.

flake like debris might undergo rubbing between the ball and the Al– Si surface and eventually transform into a laminate. Each of the steps could then be caused by one rotation of the ball. The wear debris of the composite coatings contains a mixture of Al and Al–Si wear debris. Deuis et al. [27] provide a comprehensive review of the dry sliding wear of aluminum composites. It has been found that particle delamination is one of the important mechanisms controlling wear of particle reinforced aluminum composites. The important factors affecting wear performance of the composites are the (i) second phase particle size, (ii) inter-particle spacing and (iii) particle/matrix interfacial bonding. Jahanmir and Suh have found that during delamination wear, crack nucleation was favored by the second phase particles and the inter-particle spacing played an important role [28]. Saka et al. have found that delamination wear is the dominant mechanism during dry sliding wear of two-phase metals [29]. For a uniform distribution of second phase particles in a coating the interparticle spacing is given by the following equation [30] λ=

4ð1 − f Þr 3f

thickness of the coating. This is due to the fact that the Al particles form splats due to plastic deformation and tend to separate the Al–Si particles along the direction of the coating surface. It is also seen that the observed inter-particles spacing is only slightly different than that calculated by Eq. (1) indicating a uniform distribution of the Al–Si particles. This inter-particle spacing could therefore be considered as the mean free path that a wear crack can travel before it meets an Al/ A–Si interface. When a wear crack reaches an Al/Al–Si interface, it could develop into an inter-particle crack and lead to delamination. It is evident from Fig. 3 that the coating labeled A-75AS had in fact only 45% of Al–Si powder particles present in it due to the different

ð4Þ

where λ is the inter-particles spacing, ‘f’ is volume fraction and ‘r’ is the radius of second phase particles respectively. Fig. 10 plots the measured and calculated values inter-particle distances for the coatings. The inter-particle distances were measured centre to centre of a particle by drawing straight lines using image analysis software Image J. At least 120 distances were taken to compute the average value. It is found that the inter-particle spacing of Al–Si reduces as the volume fraction of Al–Si in the coating increases. Also the spacing of the Al–Si particles parallel to the coatings is larger than that along the

Fig. 10. Inter-particle spacing parallel and perpendicular to coating surface.

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deposition efficiencies of the Al and Al–Si powders during the cold spray process. Thus, of the two composite coatings produced, the A75AS deposit had the largest interfacial area per unit volume associated with Al/Al–Si boundaries. It also had the smallest Al–Si inter-particle spacing. It seems very likely therefore that both of these factors increase the ease with which delamination cracking (Fig. 8b) can occur during dry sliding wear tests. In this coating there is the shortest separation between Al–Si particles and the largest boundary area between dissimilar particles. This failure mechanism seems, therefore, to offer an explanation for the larger wear loss for the A-25AS and A-75AS coatings as compared to Al coating. The increased hardness of A-75AS might be the reason for the slight decrease in wear volume loss. The decrease in wear volume associated with the coating which was 100% Al–Si could then be attributed to both the significant hardness increase and the elimination of unlike particle boundaries, i.e. the removal of easy crack propagation paths. Increase in the interfacial strength or bonding between the Al and Al–Si particles could lead to improvement in wear resistance. Such increase can be brought about by annealing treatments which is in consideration for future work. 4. Conclusions Cold spraying was used successfully to prepare 350–1200 μm thick composite coatings containing aluminum and aluminum–silicon alloy with a density of 98% or higher. The Al–Si particles were distributed uniformly in the aluminum matrix. Image analysis revealed that only 61% of the Al–Si particles present in the powder was retained in the coating due to poor deposition efficiency of the coating. Porosity reduced with increase in flattening ratio of Al. Wear volume was found to be similar for Al and Al–Si composite coatings. This is due to the delamination between of dissimilar splats in the composite coatings, which caused increased wear volume even though the hardness was higher. Coefficient of friction of the coatings was found to reduce with addition of Al–Si. Acknowledgments A. Agarwal and S. R. Bakshi would like to acknowledge funding from the National Science Foundation CAREER Award (NSF-DMI0547178) and International Research and Education in Engineering

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