SiCp coatings on AZ91 magnesium alloy by HVOF

Surface & Coatings Technology 261 (2015) 130–140

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Al/SiCp and Al11Si/SiCp coatings on AZ91 magnesium alloy by HVOF B. Torres ⁎, C. Taltavull, A.J. López, M. Campo, J. Rams Departamento de Matemática Aplicada, Ciencia e Ingeniería de Materiales y Tecnología Electrónica, ESCET, Universidad Rey Juan Carlos, C/Tulipán s/n, Móstoles, 28933 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 1 September 2014 Accepted in revised form 18 November 2014 Available online 26 November 2014 Keywords: Thermal spray Aluminum matrix composite Silicon carbide Coating Magnesium alloy HVOF

a b s t r a c t High velocity oxygen-fuel (HVOF) thermal spray has been used to fabricate Al, Al11Si and metal matrix composite (Al/SiCp and Al11Si/SiCp) coatings on the AZ91 Mg alloy. Taguchi design of experiment (DOE) methodology was used to analyze the influence of the HVOF spraying conditions (% SiCp in feedstock, spraying distance, number of layers and gun speed) in the main characteristics of the coatings (actual amount of reinforcement in the coating, porosity and thickness) and in some properties of the coatings such as hardness and adhesion. In general, for the same HVOF spraying conditions, the coatings fabricated using Al11Si as matrix presented higher thickness and lower incorporation of reinforcement, as well as higher hardness and adhesion values in comparison with those of pure Al matrix coatings. Independently of the matrix used, the % SiCp in feedstock and spraying distance seem to be the most important spraying parameters in controlling the properties of the sprayed coatings. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Magnesium alloys are the lightest of all metals used as the basis for constructional alloys. This lightness, high strength to weight ratio and its demonstrated versatility makes this material a great choice in aerospace, transportation and in civil and military applications [1]. However, several drawbacks restrict the application of unprotected magnesium alloys, especially their low wear and corrosion resistance. The use of coatings is one of the most effective strategies to protect these light alloys against corrosion and wear. Among others, aluminum and aluminum composite coatings present a density which is only 1.5 that of magnesium and have high resistance to corrosion in many environments although they have limited wear resistance. As a consequence, the use of aluminum alloys and the addition of ceramic particles such as SiCp into the coatings are required to increase their mechanical behavior [2–7]. The aluminum matrix composite reinforced with ceramic particles allows obtaining simultaneously good wear and corrosion resistances, while low density of the substrate–coating system is maintained [8–12] and its coefficient of thermal expansion is reduced without deterioration of its thermal conductivity [13]. Different thermal spraying techniques have been used to deposit Al/SiCp coatings on magnesium alloys such as oxy-acetylene flame spray, cold spray and high velocity oxy-fuel (HVOF). Among the different spraying techniques, oxy-acetylene thermal spray is the simplest and cheapest one, therefore it is available even for small industries [14,15]. Its main limitation is the low kinetic energy of the sprayed material, which usually results in highly porous coatings. A reduction on ⁎ Corresponding author. E-mail address: [email protected] (B. Torres).

http://dx.doi.org/10.1016/j.surfcoat.2014.11.045 0257-8972/© 2014 Elsevier B.V. All rights reserved.

porosity may be achieved by post-processing routes. Arrabal et al. [16] and Carboneras et al. [17] used oxy-acetylene flame spray to deposit Al/SiCp coating on different magnesium alloys and cold-pressing posttreatment was used to reduce the porosity of the coating up to values b0.5%. Cold spray is another alternative to deposit Al/SiCp coating on magnesium alloy, but some limitations are inherent to the technique. Due to the low temperature and the high velocity of the sprayed particles, there is a limited contact time between Al and SiC particles during the spraying process. These characteristics may cause the breaking of the ceramic reinforcement, as Leshchynsky et al. reported [18], which results in high porosity coatings and lower adhesion strength than that achieved with thermally sprayed coatings. This phenomenon increases as the reinforcement percentage does. These factors hinder in most cases the good mechanical properties of the coating in wear applications. HVOF combines the low heat input of the low pressure spraying technique with high kinetic energy of the sprayed powder, giving rise to low porosity coatings. This high energetic spraying technique has not been usually used on magnesium alloys because of their low melting point, although some successful examples can be found in the literature [19,20]. This processing technology is an interesting route to develop efficient corrosion and wear protective coatings over magnesium alloys, promoting its applications in the transport industry to reduce vehicle weight and fuel consumption. In the present work, Al, Al11Si, Al/SiCp and Al11Si/SiCp HVOF coatings have been sprayed with the aim to determine the influence of the spraying parameters (spraying distance, number of deposited layers, volume fraction of reinforcement in the feedstock and gun speed) in the characteristics (thickness, actual volume fraction of the reinforcement and porosity) and mechanical properties (adhesion and hardness) of the sprayed coatings. In addition, the effect of the nature of the

B. Torres et al. / Surface & Coatings Technology 261 (2015) 130–140

131

Fig. 1. SEM micrograph of (a) pure Al powders and (b) Al11Si powders.

matrix, i.e. Al and Al11Si, has been also evaluated. The relationship between the HVOF parameters and the characteristics and properties of the fabricated coatings has been analyzed by a Taguchi design of experiment (DOE) method. 2. Experimental procedure 2.1. Materials AZ91 was supplied by Magnesium Elektron with the following nominal composition (in wt.%): 9 Al, 0.67 Zn, 0.23 Mn and balance Mg. The material was supplied in trapezoidal ingot of 7.5 kg and was cut into specimens of 30 mm × 25 mm × 3 mm for the HVOF process. All samples were sand blasted with corundum particle size of 1–3 mm, degreased with propanol and dried in warm air. Pure aluminum powder was supplied by Flame Spray Technologies with an average size of 20 μm (Fig. 1a). The Al11Si powder was supplied by Sulzer with the following chemical composition (in wt.%): 0.01 Cu, 0.14 Fe, 0.01 Mn, 0.01 Mg, 11.4 Si, 0.01 Zn, 0.03 others and balance aluminum with an average size of 50 μm (Fig. 1b). Al/SiCp and Al11Si/SiCp composite coatings were obtained by using feedstock powder blends of Al and Al11Si powders with different proportions (0, 30 and 50 wt.%) of SiC particles (Navarro S.A.) with an average size of 26 μm. These powder blends were obtained in a rotational ball mixing with alumina balls after 1 h of mixing. 2.2. Spraying conditions Al, Al11Si, Al/SiCp and Al11Si/SiCp coatings were fabricated using HVOF thermal spray equipment from Sulzer Metco (Unicoat, DS2600). The HVOF gun was placed on an anthropomorphic robot ABB IRB2400/16 to control the spraying distance and the gun speed over the surface of the substrate. Oxygen and hydrogen were used as oxidizing and fuel gas, respectively. Proportions of both gases (635 NLPM – normalized liter per minute – for hydrogen and 214 NLPM for oxygen)

and of air used as shielding gas (344 NLPM) were previously optimized to protect Al powder from oxidation and to reach supersonic flame. Nitrogen was used as transport gas to feed the powder into the gun. To evaluate the influence of the HVOF parameters into the coating features and properties, a Taguchi DOE method has been used. Four factors (distance, wt.% SiCp in feedstock, number of layers in the coating and gun speed) with three levels (350, 450, 550 mm; 0, 30, 50% SiCp; 3, 6, 9 layers; 150, 200, 250 mm s− 1 respectively) were selected. The factors and levels were used to design an orthogonal array L9 (34) for experimentation. The nine Taguchi experiments for each of the two aluminum coating matrices are presented in Table 1. To investigate the coating formation process single layer of the Al and Al11Si matrices was deposited on a F4000 grounded magnesium substrate by the so-called wipe test [21] using a spray gun transverse speed of 1500 mm s−1 at spraying distances of 350, 450 and 550 mm. 2.3. Specimen characterization The cross-sections of the coated substrates were prepared by using a diamond disc cutter on hot conductive resin mounted samples, followed by grinding with SiC emery paper up to 1200 grit and polished with alumina suspension up to 0.5 μm. Thickness, porosity and actual reinforcement of the coatings were measured by means of an image analysis software (Image Pro Plus) on the captured images obtained by a light microscope (Leica DMR). The adhesion strength of the coating to the substrate was evaluated by means of a PosiTest AT-Pull-Off Adhesion Tester following the ASTM D4541-02 procedure E standard [22]. Coating roughness measurements were performed by stylus profilemeter (SJ-210 Mitutoyo) with a resolution of 0.01 μm and according to DIN4776 specification [23]. Hardness tests were carried out on the cross-sections of the samples using a Vickers Buehler Micromet 2103 micro-hardness tester with a load of 500 g (HV0.5). Averages of 10 tests for each coating were used to obtain representative values.

Table 1 Spraying condition for the different fabricated coatings according to Taguchi DOE method. Also given are the coating designations used throughout this paper. Coating denomination

Spraying conditions

Condition

Al

Al11Si

Distance (mm)

SiC in feedstock (vol.%)

Layers

Gun speed (mm s−1)

C1 C2 C3 C4 C5 C6 C7 C8 tC9

Al-C1 Al-C2 Al-C3 Al/30SiC-C4 Al/30SiC-C5 Al/30SiC-C6 Al/50SiC-C7 Al/50SiC-C8 Al/50SiC-C9

Al11Si-C1 Al11Si-C2 Al11Si-C3 Al11Si/30SiC-C4 Al11Si/30SiC-C5 Al11Si/30SiC-C6 Al11Si/50SiC-C7 Al11Si/50SiC-C8 Al11Si/50SiC-C9

350 450 550 350 450 550 350 450 550

0 0 0 30 30 30 50 50 50

3 6 9 6 9 3 9 3 6

150 200 250 250 150 200 200 250 150

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B. Torres et al. / Surface & Coatings Technology 261 (2015) 130–140

C2

C4

C7

a)

b)

c)

d)

e)

f)

Al

Al11Si

Fig. 2. Macroscopic top view of the sprayed coatings (scale in cm).

Scanning Electron Microscope (SEM; Hitachi S-3400N) was used to characterize the cross-section of the coated specimens. The accelerating voltage was 10–15 kV and the working distance was about 10 mm. 3. Results and discussion 3.1. Coating characterization Table 1 presents the HVOF conditions used following Taguchi DOE array. The coating denomination shown in Table 1 will be used to simplify the results presentation. Fig. 2 presents a general view of the

HVOF Al, Al11Si, Al/SiCp and Al11Si/SiCp coatings fabricated on AZ91 magnesium alloy, Al-C2 and Al11Si-C2 (pure Al; Fig. 2a and Al11Si; Fig. 2b), C4 (Al/30SiC; Fig. 2c and Al11Si/30SiC; Fig. 2d) and C7 (Al/50SiC; Fig. 2e and Al11Si/50SiC; Fig. 2f). Based on the macroscopic images, it can be determined that the HVOF coatings were homogeneously deposited over the AZ91 Mg substrate, independently of the matrix used, i.e. Al or Al11Si. However, it can be observed that the color of the sprayed coatings differs in function of the matrix used. Al and Al metal matrix composite (MMC) coatings appeared as a white coating, while Al11Si and Al11Si based MMC coatings showed pink tone. A detailed analysis of the sprayed coatings with both matrices was

a)

b)

c)

d)

Fig. 3. Cross-section SEM micrographs of Al and Al11Si coatings (a) Al-C2, (b) detail of a, (c) Al11Si-C2 and (d) detail of c.

B. Torres et al. / Surface & Coatings Technology 261 (2015) 130–140

a)

b)

c)

d)

133

Fig. 4. Cross-section SEM micrographs of Al/SiC and Al11Si/SiC coatings (a) Al/30SiC-C5, (b) detail of a, (c) Al11Si/30SiC-C5 and (d) detail of c.

developed by observing the corresponding cross-section micrographs presented in Figs. 3 and 4. Fig. 3 presents the representative images of pure HVOF sprayed coatings, made of Al (a and b) and Al11Si (c and d), under equivalent HVOF conditions (conditions C2 indicated in Table 1). It can be observed that in both cases, homogeneous and continuous coatings were fabricated, accordingly with the top view of the coatings previously shown (Fig. 2). To determine the differences in the coating morphology obtained by using both matrices, magnifications of the surrounded zone crosssection micrographs are presented in Fig. 3b for pure Al and in Fig. 3d for Al11Si coatings. Comparing the cross-sections of Al and Al11Si coatings, it can be detected that the thickness of the Al one is significantly lower than that corresponding to the Al11Si coating (thickness data are collected in Tables 2 and 3). Besides thickness, it can be observed that the surface of both coatings differs from one another. Pure Al coating presents a smooth top surface while Al11Si coatings show a rough one. The average roughness values measured were 5 μm for pure Al coatings (Table 2) while an average value of 11 μm has been determined for the Al11Si matrix (Table 3). In terms of porosity, although the total values are similar it can be observed that the distribution of porosity is different in both coatings. In the pure Al coating the porosity is

concentrated in the top of the sprayed coating (Fig. 3b) while in the Al11Si pure coating the residual porosity is homogeneously distributed along the whole section (Fig. 3d). Using both matrices, i.e. Al and Al11Si, HVOF MMC coatings have been fabricated with SiCp ceramic particles as reinforcement. Continuous coatings have been produced for both matrices (Fig. 2). To determine the main coating characteristics in each case, cross-section SEM micrographs of HVOF MMC coatings sprayed under equivalent conditions are shown in Fig. 4 for the two studied matrices, i.e. pure Al matrix in Fig. 4a and b and Al11Si matrix in Fig. 4c and d. The porosity of the composite coatings is minimum for both matrices used, Tables 2 and 3. Establishing a comparison between those figures, it can be stated that SiCp incorporation in pure Al matrix was higher than in Al11Si matrix. This observation is in agreement with the actual SiCp incorporation calculated in both systems, i.e. pure Al (Table 2) and Al11Si (Table 3). A good bonding was observed between the matrix and the SiCp reinforcement in both matrices used where no dissolution of SiCp into the aluminum matrices has been detected (Fig. 4). In terms of thicknesses, as it was previously observed in the pure coatings, sprayed using pure Al matrix presented lower thicknesses than those fabricated using Al11Si matrix, as is reflected in Tables 2 and 3.

Table 2 Characteristics of the Al and Al/SiC sprayed coatings. Coating denomination

Thickness (μm)

Coating characterization % SiCp in coating (vol.%)

Porosity (%)

Roughness Ra (μm)

Adhesion (MPa)

Vickers hardness (HV0.1)

Al-C1 Al-C2 Al-C3 Al/30SiC-C4 Al/30SiC-C5 Al/30SiC-C6 Al/50SiC-C7 Al/50SiC-C8 Al/50SiC-C9

61.47 76.48 106.58 47.52 100.64 44.47 62.61 19.46 63.94

– – – 15.69 17.50 14.23 15.30 10.58 20.50

4.53 ± 0.13 5.09 ± 0.18 5.83 ± 0.20 1 ± 0.5 1 ± 0.5 1 ± 0.5 b0.5 b0.5 b0.5

6.56 5.03 7.40 7.65 7.47 9.21 7.10 8.34 6.54

7.60 5.73 5.04 12.84 12.64 12.89 12.31 12.26 9.07

58.26 ± 11.11 50.04 ± 12.12 33.24 ± 5.68

a

± ± ± ± ± ± ± ± ±

4.53 2.64 5.04 3.04 3.25 3.40 7.68 2.33 4.53

± ± ± ± ± ±

1.02 1.05 1.04 0.40 0.90 1.15

For thickness b50 μm, standard deviation is higher than the average value.

± ± ± ± ± ± ± ± ±

0.28 0.27 0.38 1.30 1.11 1.50 0.74 0.71 0.38

± ± ± ± ± ± ± ± ±

1.58 2.38 0.35 2.96 1.34 0.41 2.47 1.31 1.02

a

47.77 ± 13.03 a

139.50 ± 52.92 a

76.22 ± 41.06

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B. Torres et al. / Surface & Coatings Technology 261 (2015) 130–140

Table 3 Characteristics of the Al11Si and Al11Si/SiC sprayed coatings. Coating denomination

Al11Si-C1 Al11Si-C2 Al11Si-C3 Al11Si/30SiC-C4 Al11Si/30SiC-C5 Al11Si/30SiC-C6 Al11Si/50SiC-C7 Al11Si/50SiC-C8 Al11Si/50SiC-C9 a

Thickness (μm)

87.49 128.74 165.25 64.36 174.15 53.05 74.79 30.93 80.57

± ± ± ± ± ± ± ± ±

Coating characterization

5.04 6.28 9.72 4.93 6.60 6.62 3.26 2.12 6.93

% SiCp in coating (vol.%)

Porosity (%)

Roughness Ra (μm)

Adhesion (MPa)

Vickers hardness (HV0.5)

– – – 4.73 ± 0.19 5.78 ± 0.19 2.84 ± 0.35 21.90 ± 4.45 – 9.94 ± 3.65

2.87 ± 0.18 3.20 ± 0.24 6.09 ± 0.47 1 ± 0.5 1 ± 0.5 1 ± 0.5 b0.5 b0.5 b0.5

13.22 11.20 12.65 8.99 11.34 9.07 10.11 8.93 9.38

10.20 10.50 8.71 12.71 12.60 11.88 12.43 12.87 11.08

91.10 ± 16.10 103.90 ± 7.30 96.06 ± 18.55 113.37 ± 26.10 88.47 ± 11.45 104.8 ± 24.45 129.20 ± 33.62

± ± ± ± ± ± ± ± ±

2.16 2.02 1.33 0.71 0.54 0.43 0.59 0.27 2.14

± ± ± ± ± ± ± ± ±

0.84 1.45 0.40 2.30 1.50 0.72 1.64 1.07 0.83

a

95.53 ± 18.05

For thickness b50 μm, standard deviation is higher than the average value.

3.2. Influence of the HVOF parameters in the coating morphology by Taguchi DOE method Fig. 5 presents the relationship between the coating features (thickness, % SiCp in coating and porosity) with the HVOF parameters (% SiCp in feedstock, spraying distance, number of layers and gun speed) by

a)

b)

Al

Coating thickness %SiCp in feedstock (vol%)

means of Taguchi DOE method. The horizontal line represents the mean value of the obtained data. As a general trend, it can be stated that the relationship between the HVOF parameters and the final coating features is equivalent for the two studied matrices, i.e. Al or Al11Si. Based on the analysis developed by Taguchi DOE method, the thickness value of the Al and Al11Si coatings: (i) increases as the vol.% SiCp

Coating Thickness

Spraying distance (mm)

%SiCp in feedstock (vol%)

150 80

Al11Si Spraying distance (mm)

125 100

60 75 40

50 0

30 Layers

50

350

450 Gun speed (mm/s)

550

0

80

30

50

350

Layers

150

450

550

Gun speed (mm/s)

125 100

60

75 40

50 3

6

9

150

c)

200

Al

250

3

6

d)

9

150

% SiCp in Coating %SiCp in feedstock (vol%)

16

200

250

Al11Si Spraying distance (mm)

12 8 4 0 0

30

50

350

Layers

16

450

550

Gun speed (mm/s)

12 8 4 0 3

e)

Al

f)

6

9

150

200

250

Al11Si

Fig. 5. Taguchi analysis of the effect of: vol.% SiCp in feedstock; spraying distance; number of layers and gun speed on the (a) coating thickness of Al and Al/SiC, (b) coating thickness of Al11Si and Al11Si/SiC, (c) vol.% SiC in coating of Al and Al/SiC, (d) vol.% SiC in coating of Al11Si and Al11Si/SiC, (e) porosity of Al and Al/SiC, and (f) porosity of Al11Si and Al11Si/SiC.

B. Torres et al. / Surface & Coatings Technology 261 (2015) 130–140

decreases; (ii) increases as the spraying distance increases; (iii) increases as increasing the number of layers applied and (iv) increases as decreasing the gun speed (Fig. 5a and b). As it can be seen, the most relevant parameters in the coating thickness are vol.% SiCp and number of layers. HVOF is characterized by high velocity and thermal energy of the sprayed particles deposited over the substrate. Due to thermal softening, aluminum powder is able to deform and be well adhered to the substrate. However, reinforcement particles of SiCp do not have the ability to deform. Therefore, when they reach the substrate most of them rebound. This might be associated with the decrease of the thickness of the coating related with the increase of the amount of SiCp in the initial powder blend [24]. On the other hand, the increase of thickness due to the increase of spraying distance may be correlated with the fact that as the spraying distance increases, the amount of particles that rebound decreases. This is because of the fact that lower kinetic energy is introduced in the system in comparison with that of the ones sprayed at shorter spraying distances. As the gun speed increases, the spraying time decreases and therefore the amount of mass deposited, so the thickness of the coating decreases. As expected, an increase of the number of applied layers is translated into an increase of the coating thickness. Fig. 5c and d presents the relationship between the actual % SiCp into Al and Al11Si MMC coatings in the vertical axis with the HVOF parameters (% SiCp in feedstock, spraying distance, layers and gun speed) determined by the Taguchi DOE method. It can be observed that the mean value of the real % SiCp into the coating of the Al matrix is higher than in the Al11Si (13% and 7%, respectively). However, the relationship between the HVOF parameters and the incorporation of SiCp does not present a dependence on the matrix used. As expected, the amount of reinforcement into the final coatings increases as the amount of reinforcement into the initial powder blend increases. The other spraying parameters: layers, spraying distance and gun speed, do not present a relevant influence on the incorporation of ceramic reinforcement into the coatings (Fig. 5c and d).

a)

Adhesion

The deposition efficiency of the process in terms of vol.% SiCp was 50%, when using Al matrices, decreasing to 25% for Al11Si matrices. This low deposition efficiency obtained when Al11Si/SiC blends are used as powder feeder, has been previously described by some authors using plasma or HVOF thermal spray process [8,25–27]. The reason given by these authors is based on: (i) the large difference in melting temperatures between the metallic particle and the SiCp and (ii) the poor wettability of the SiCp with aluminum. After reaching the substrate in molten or semi-molten state, the metallic particle solidifies, but SiC particles reach the substrate in solid state, and rebound off the surface or are entrapped in the sprayed material. In addition, the low contact time between molten Al and SiC particles has to be taken into consideration as a key factor for the low deposition efficiency when using high velocity deposition techniques, i.e. HVOF [2–16]. Plasma spray technique generates higher deposition efficiency when using Al/SiCp blends. In fact, Gui et al. [28] obtained 87% of deposition efficiency when using blends with 55 vol.% of SiCp reinforcement, but also great porosity was obtained, up to 3.5 vol.%. In order to increase the proportion of the reinforcement particles in the coating, higher amount of SiCp should be added in the blending powder. Nevertheless, some authors have previously found a limit to this increment [28,29] because of the lack of molten metallic phase to retain the SiCp in the coating. Gui et al. [28] found that when increasing the amount of ceramic reinforcement beyond 55 vol.%, the actual amount of SiCp in the final coating decreased. In our investigation we have lost 50% with 30 and 50 wt.% of SiCp in the blend when using Al matrix and 75% when using Al11Si matrix. Fig. 5e and f presents the relationship between the porosity with the HVOF parameters (% SiCp in feedstock, spraying distance, number of layers and gun speed) for the two matrices. The horizontal line represents the mean value of the obtained data. Based on the Taguchi DOE method, it can be stated that the porosity value of the Al and Al11Si coatings: (i) decreases as the vol.% SiCp increases; (ii) slightly increases as the spraying distance increases; (iii) slightly increases as increasing the number of layers applied and (iv) slightly increases as the gun speed increases.

b)

Al

%SiCp in feedstock (vol%)

135

Adhesion

Spraying distance (mm)

Al11Si

%SiCp in feedstock (vol%)

12,0

Spraying distance (mm)

12

10,5 11

9,0 7,5

10

6,0 0

30 Layers

50

350

450 Gun speed (mm/s)

550

12,0

0

30 Layers

50

350

450 Gun speed (mm/s)

550

3

6

9

150

200

250

12

10,5 11

9,0 7,5

10

6,0 3

c)

6

9

150

Al

200

250

d)

Al11Si

Fig. 6. Taguchi analysis of the effect of: vol.% SiCp in feedstock; spraying distance; number of layers and gun speed on the mechanical properties (a) adhesion strength of Al and Al/SiC, (b) adhesion of Al11Si and Al11Si/SiC, (c) hardness of Al and Al/SiC and (d) hardness of Al11Si and Al11Si/SiC.

136

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The critical parameter in the coating porosity was the vol.% SiCp. The porosity reduction observed as the amount of reinforcement increases may be related with the fact that the kinetic energy of the SiCp promotes the compactness of the coating. The mean value of porosity in the Al11Si coatings is lower than for the Al coating as shown in Fig. 5e and f, 1.9% to 2.4% respectively, because the higher fluidity of the Al11Si in comparison with Al favors the wettability of the splats into the substrate and between the subsequent splats. The Al11Si matrix is more sensitive to spraying conditions. 3.3. Influence of the HVOF parameters in the coating properties by Taguchi DOE method Fig. 6a and b presents the relationship between the adhesion strength of the sprayed coatings with the HVOF parameters (% SiCp in feedstock, spraying distance, number of layers and gun speed) for the two studied matrices. The horizontal line represents the mean value of the obtained data (10 MPa for the Al matrix and 11.5 MPa for the Al11Si matrix). Based on these figures, it can be determined that adhesion of the Al and Al11Si coatings: (i) increases with the incorporation of SiCp and (ii) decreases as the spraying distance increases; and does not seem to depend on the number of layers applied and the gun speed. The most relevant parameter in the adhesion strength is the presence of SiCp in the coating. An increase of 100% has been observed between pure and 30% of SiCp in feedstock (8 vol.% of SiCp in coating). In the literature, it has been established that several factors influence the adhesion of a coating such as particle speed (higher particle velocity is related with higher adhesion values), particle temperature (higher particle temperature is associated with higher adhesion values), residual stress [29,30] in the interface coating–substrate and size of semi-solid particle (lower size particles tend to decrease adhesion values) [31]. Higher adhesion values of pure Al11Si in comparison with pure Al coatings might be attributed to the fact that Al11Si particles are bigger than pure Al ones. In addition, due to Si incorporation, the fluidity of the powders increases so it is easier to fulfill all the voids, as indicated by its lower porosity value. The higher adhesion of MMC coatings is

because of the impact of SiCp onto the magnesium substrate. When these particles reach the substrate or the Al first-layer coatings, they hit them and increase the surface roughness of the pre-coating. After that, the SiCp rebound although the hole formed can be refilled by next sprayed Al or Al11Si powders. Therefore, the impact of the ceramic particles promotes a higher roughness of the substrate, improving the mechanical coating–substrate bonding [32]. Fig. 6c and d presents the relationship between the hardness with the HVOF parameters (% SiCp in feedstock, spraying distance, layers and gun speed) for the two studied matrices. The horizontal line represents the mean value of the obtained data (70 HV0.5 for the Al matrix and 103 HV0.5 for the Al11Si matrix). As a general trend, the hardness value of the Al and Al11Si coatings: (i) increases as the vol.% SiCp increases; (ii) decreases as the spraying distance increases; (iii) slightly increases with the number of layers applied and (iv) presents a maximum value at 200 mm s−1. As expected, the matrix with Si incorporation presents higher hardness values than the pure Al ones. This is in agreement with other studies developed using low velocity thermal spray [20]. Based on previous studies [33], there is a strong inverse relationship between porosity and hardness; as porosity decreases the hardness of the coating increases. All the spraying parameters that promote a decrease of the coating porosity would reflect a harder coating. In the present work, it has been observed that the critical spraying parameter in the porosity was vol.% SiCp, the porosity decreases as the vol.% SiCp increases, similarly the hardness of the coating increases as the amount of reinforcement of the MMC coatings increases (Tables 2 and 3). Spraying distance is a relevant parameter in the hardness of the coating. When the spraying distance decreases, the hardness of the coating increases, doubling the value between 450 and 350 mm. At the smallest spraying distance, the powder is hotter and deforms more upon getting to the substrate, so the coating is more compact and shows lower porosity, which is reflected into an increase in hardness. On the other hand, it is important to indicate that the hardness values obtained in the sprayed HVOF coatings (pure Al and Al11Si) are

Fig. 7. SEM micrograph of the splat morphology based on wipe test at a gun speed of 1500 m s−1 (a and b) pure Al and (c and d) Al-11Si.

B. Torres et al. / Surface & Coatings Technology 261 (2015) 130–140

most important differences have been detected in pure coatings, where Al11Si coatings present higher thicknesses in comparison with that of Al coatings. An increase of 30% in thickness was detected when using the Al11Si matrix (95 μm) in comparison with pure Al one (65 μm). This may be related with the fact that, incorporations of Si in the aluminum matrix introduce an increase of fluidity (pure aluminum melts at 660.4 °C and silicon forms a eutectic with aluminum at 11.7% Si, 577 °C). This is why aluminum–silicon alloys are usually used in casting processing routes because they fill the mould more easily. This phenomenon is translated into a higher deformation ability of the Al11Si powders in comparison with pure Al, Fig. 2, and a reduction of the rebounding of these particles when reaching the Mg substrates [24]. This is also the reason for the higher adhesion strength of the Al11Si to that of the Al (Tables 2 and 3). In the case of MMC coatings the trend remains the same, i.e. the thickness is higher for the MMC coatings using Al11Si as matrix, although the difference is smaller than that observed for the pure coatings. This phenomenon is also related with the incorporation of SiCp ceramic particles which will be analyzed below. Fig. 8b presents a comparison between the actual SiCp incorporated into the Al/SiC and Al11Si/SiC coatings. In all studied conditions the actual vol.% of SiCp for pure Al matrix was higher than that of Al11Si matrix. The higher reinforcement of pure aluminum in comparison with Al11Si might be related with their respective hardness values. Pure Al presents a hardness of around 70 HV0.5 while the corresponding value for Al11Si is of 100 HV0.5. It has been indicated in the literature that incorporation of ceramic reinforcement into a coating using thermal spray technologies requires a pre-layer of aluminum which acts as somewhat bonding layer [24]. As a consequence, when reinforcement of SiCp is sprayed in combination with pure Al powders, these particles are able

twice as the ones obtained by low velocity thermal spray [2,24]. This phenomenon cannot be only justified by the reduction of coating porosity of HVOF coatings in comparison with other thermal spray techniques such as flame spray (FS). The compression shock waves created in the HVOF technique are also increasing the hardness of the coating. In fact, similar porosity values, i.e. around 1.5–1.7%, were obtained after FS and cold compactness post-processing route. As a consequence, the energy of the spraying process also increases the hardness of the final coating obtaining similar values than those observed in casting techniques [34,35]. 3.4. Influence of the matrices in the coating morphology To understand the deposition mechanisms of both matrices, wipe tests were carried out as shown in Fig. 7. In both cases the geometry of a splash splat suggests that they have reached the substrate in molten state. The results revealed different splat morphologies for each studied matrix; the geometry of an Al matrix-splat, Fig. 7a and b was round while the geometry of the Al11Si splash, Fig. 7c and d was like fingers. To evaluate the influence of the matrix used (pure Al and Al11Si) in the different coating features, Fig. 8 shows a comparison of the thicknesses, actual incorporation of SiCp into the MMCS coatings, porosity and roughness of the different sprayed coatings. The dotted line represented in all plots indicates the conditions for equality. As can be seen, for all spraying conditions used, the Al11Si coatings present higher thicknesses than Al coatings. Fig. 8a plots a comparison between the calculated thicknesses in pure and MMC coatings using both matrices, i.e. Al and Al11Si. The

Pure 30 % SiCp (MMC) 50 % SiCp (MMC)

150

30

a)

% SiCp in Al11Si Coatings (vol %)

Al11Si Coating Thickness (µm)

175

125 100 75 50 25

30 % SiCp (MMC) 50 % SiCp (MMC)

b)

25

20

15

10

5

0

0

7

25

50

75

100

125

Al Coating Thickness (µm)

150

5

10

15

20

25

30

% SiCp in Al Coatings (vol %)

Pure 30 % SiCp (MMC) 50 % SiCp (MMC)

6

0

175

15

c)

5 4 3 2 1

Al11Si Coatings Roughness, Ra (µ m)

0

Al11Si Coatings Porosity (%)

137

Pure 30 % SiCp (MMC) 50 % SiCp (MMC)

d)

10

5

0

0 0

1

2

3

4

Al Coatings Porosity (%)

5

6

7

0

5

10

Al Coatings Roughness, Ra (µm)

Fig. 8. Correlation between Al and Al11Si and Al/SiC and Al11Si/SiC in terms of (a) thickness, (b) SiC vol.% in coating, (c) porosity, and (d) roughness.

15

138

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to easily adhere to a previously soft Al sprayed layer. However, as the hardness of the matrix is higher in the Al11Si, the bond of the ceramic reinforcement is more difficult, so higher rebounding is probably produced. The porosity of the coatings for the matrices used is shown in Fig. 8c. In the case of pure coatings, it can be observed that porosity of the Al coatings is higher than that of Al11Si coatings. In the case of MMC coatings, the porosity values were in most cases lower than our measurement sensitivity (Tables 2 and 3). As a consequence, equivalent values have been observed independently of the matrices used. The roughness of the coatings was similar for each system (Fig. 8d). However, as a general trend, it can be determined that pure Al coatings presented lower roughness values (between 5 and 7 μm) than pure Al11Si coatings (11 μm). This difference decreases with the incorporation of SiCp. Based on the above, it can be determined that the morphology of the coatings is highly dependent of the matrix used. 3.5. Influence of the matrices in the coating properties The influence of the matrices in the coating properties such as adhesion and hardness has been evaluated. Based on Tables 2 and 3 an increase of adhesion strength has been achieved with the incorporation of SiCp into the coating from 5–7.6 MPa of Al coatings to 12–13 MPa for Al-MMC coatings and from 8.7–10.5 MPa of Al11Si coatings to 11–13 MPa for Al11Si MMC coatings. Fig. 9 presents a comparison between the two matrices (pure Al and Al11Si); the dotted line represented in all plots indicates the conditions for equality. As can be seen in Fig. 9a, the adhesion of the Al11Si matrix is higher than that of the pure Al, although similar values have been observed for the MMC coatings, independently of the matrix used. This indicates that the addition of Si in the pure Al to form the Al11Si matrix increases the adhesion of the sprayed powders to the substrate in comparison with the pure Al powders. A comparison of the microhardness values of the different sprayed coatings, pure and MMCs, in relation with the matrix used is presented in Fig. 9b. The hardness of the Al11Si coatings is higher than that of pure Al. Only for the C7 coating the hardness of the Al MMC coating is higher than that of the Al11Si MMC, probably related with a significant increase of the actual amount of reinforcement in this coating (Tables 2 and 3). The higher hardness of the matrix Al11Si was expected. According to the literature the hardness of as-cast ingot AlSi alloy is 68 HV, after spark plasma sintering 98 HV and after electrospark deposition 120 HV [36]. The hardness of as-cast ingot pure Al is 25 HV [37,38]. In the case of a

Table 4 HVOF spraying condition of the Al/30SiC sprayed coatings for verification of the Taguchi DOE method. Coating denomination Condition Al C10 C11

Al/30SiC-C10 350 Al/30SiC-C11 350

Layers Gun speed (mm s−1)

30 30

9 20

200 200

3.6. Validation of the Taguchi DOE method in HVOF processing technologies From all the analysis made by using the Taguchi DOE method, the relationship between the coating properties and the HVOF parameters used can be determined without the need of having all the tests. Because of using a DOE method, the number of sprayed coatings has been reduced from 81 to 9 for each aluminum matrix using an orthogonal array L9 (34). However, it is important to validate the analysis provided by this methodology in order to apply it into the development of optimum HVOF coatings, based on the requirement of each application. Based on the literature, porosity is a key issue to obtain efficient coatings to improve the corrosion and wear behavior of magnesium alloys [16]. Using the relationship between the HVOF parameters and the characteristics and properties of the fabricated coatings after being analyzed by a Taguchi DOE method showed that the optimum conditions to obtain minimum porosity for Al coating on AZ91 Mg alloy substrate are the following: (i) 50 vol.% SiCp powder, (ii) 350 mm of spraying distance, (iii) 9 layers and (iv) 150 mm s−1 of gun speed (Fig. 5e). Equivalent trend has been observed for the Al11Si powder. To verify these conditions, C10 coating was fabricated (Table 4 indicates the spraying conditions used). Small changes have been introduced as 150 mm s− 1 of gun speed introduces high specific heat for a low temperature magnesium alloy so 200 mm s− 1 has been used as no high influence has been detected in the Taguchi DOE method (Fig. 5e and f). In addition, C11 coating (Table 5) was produced applying 20 layers to determine if the coating properties could be extrapolated from the Taguchi DOE method. From the characterization of these coatings (shown in Fig. 10), as expected, it was observed that there was an increase in coating

b) Pure 30 % SiCp (MMC) 50 % SiCp (MMC)

Pure 30 % SiCp (MMC) 50 % SiCp (MMC)

140

Al11Si Coatings Hardness (HV)

Al11Si Coatings Adhesion (MPa)

SiC in feedstock (vol.%)

composite material, the hardness is determined by the nature of the reinforcement, the amount of the reinforcement and the reinforcementmatrix bonding. In the present investigation we found a significant increase in hardness with increasing the degree of reinforcement.

a) 15

Spraying conditions Distance (mm)

10

5

120 100 80 60 40 20 0

0 0

5

10

Al Coatings Adhesion (MPa)

15

0

20

40

60

80

100

Al Coatings Hardness (HV)

Fig. 9. Correlation between Al and Al11Si and Al/SiC and Al11Si/SiC in terms of (a) adhesion and (b) hardness.

120

140

B. Torres et al. / Surface & Coatings Technology 261 (2015) 130–140

139

Table 5 Characteristics of the Al/30SiC sprayed coatings. Coating denomination

Al/30SiC-C10 Al/30SiC-C11

Thickness (μm)

89.1 ± 4.02 148.4 ± 5.12

Coating characterization % SiCp in coating (vol.%)

Porosity (%)

Adhesion (MPa)

Vickers hardness (HV0.5)

13.12 ± 1.14 18.32 ± 1.36

1 ± 0.5 1 ± 0.5

12.50 ± 1.81 14.62 ± 1.27

109.12 ± 16.23 116.19 ± 19.63

Fig. 10. Cross-section SEM micrographs of Al/30SiC coatings (a) C10 condition and (b) C11 condition.

thickness related with the increase of number of layer deposited; from 89 μm for 9 layers to almost 148 μm for 20 layers (Table 5). The incorporation of SiCp into the Al/30SiC-C10 coatings was 12.7 vol.% SiCp in the coating (Table 5) which is similar to those previously observed using a 30 vol.% SiCp in the feedstock with other spraying conditions (Table 2). While, for the Al/30SiC-C11 coating, where 20 layers were applied, a slight increase in the SiCp incorporation into the coating has been detected. The same trend has been observed in terms of adhesion, 14.6 MPa, hardness, 116.2 HV, and porosity, ~1%. A summary on the coating thickness, vol.% SiCp into the coating, porosity and adhesion values obtained for C10 and C11 is presented in Table 5. The good agreement between C10 coating with the previously fabricated coatings (Table 2) in terms of SiCp incorporation, porosity and adhesion reveals reproducibility of the spraying technique. Based on the above, it can be stated that the Taguchi DOE method is a valid analysis tool to determine the optimum HVOF spraying conditions to obtain high quality coating even if the optimum condition is extrapolated from the set of experiments selected. 4. Conclusions Pure Al, Al11Si and MMC (Al/SiCp and Al11Si/SiCp) HVOF coatings were sprayed on the AZ91 Mg alloy. The set of spraying parameters was selected using a Taguchi DOE array. The most relevant conclusions are: 1. Similar relationships have been observed between the spraying parameters analyzed (% SiCp in feedstock, spraying distance, number of deposited layers and gun speed) and the final coating properties for both sprayed matrix compositions, i.e. Al and Al11Si. 2. The deposition efficiency (in terms of vol.% SiCp in coatings) of the process was 50% higher for pure Al rather than for Al11Si. 3. The coatings exhibited low porosity levels and the use of postspraying treatments is not required. 4. The most influential spraying parameters into the coating properties (from higher to lower) are the following: % SiCp in the feedstock, number of deposited layers, spraying distance and gun speed. 5. The adhesion strength of the coating can be maximized using: (i) Al11Si 30 vol.% SiCp in feedstock, (ii) 350 mm of spraying distance, (iii) 3 layers and (iv) 200 mm s−1 of gun speed.

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