Tribology of SiCp reinforced Al-12Si matrix composite coatings in water

Author’s Accepted Manuscript Tribology of SiCp reinforced Al-12Si matrix composite coatings in water Ahmet Hilmi Paksoy, Ozde Deprem, Onur Tazegul, Huseyin Cimenoglu www.elsevier.com/locate/jtri

PII: DOI: Reference:

S0301-679X(16)30402-9 http://dx.doi.org/10.1016/j.triboint.2016.10.037 JTRI4427

To appear in: Tribiology International Received date: 6 August 2016 Revised date: 22 October 2016 Accepted date: 25 October 2016 Cite this article as: Ahmet Hilmi Paksoy, Ozde Deprem, Onur Tazegul and Huseyin Cimenoglu, Tribology of SiC p reinforced Al-12Si matrix composite coatings in water, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2016.10.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Tribology of SiCp reinforced Al-12Si matrix composite coatings in water

Ahmet Hilmi Paksoy, Ozde Deprem, Onur Tazegul, Huseyin Cimenoglu*

Department of Metallurgical and Materials Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

*

Corresponding author. Huseyin Cimenoglu, Postal address: Istanbul Technical University,

Ayazaga Campus, Chemistry-Metallurgy Faculty, Metallurgical and Materials Engineering Department, Sariyer, Istanbul, Turkey.. Tel.:+90 212 285 68 34; fax:+90 212 285 34 27; [email protected],

Abstract This study compares the tribological behaviour, of cold sprayed (CS’ed), SiC particle (SiCp) reinforced Al-12Si matrix composite coatings deposited on 1050 grade aluminium, in two different testing environments (dry and water). The increase in hardness with incorporation of SiCp caused the coatings to exhibit lower wear rate and friction coefficient in both testing environments. As an indication of water lubricity, sliding in water imposed superior tribological performance (i.e. lower wear rate and lower friction coefficient) as compared to the dry sliding condition especially at lower wear testing load. In summary, finding of this study revealed that, water could be a promising lubricant for metallic engineering components after coating with SiCp reinforced Al-12Si matrix composites by CS process.

Keywords: Coating, Friction, Sliding, Wear

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1. Introduction The deposition of thick coatings by CS process has many advantages over thermal spray processes especially for the high temperature sensitive substrates and spraying powders [1-3]. Since deposition of powders by CS process relies on their plastic deformation as they impact on the substrate; hard and brittle ceramic powders can remain in the coating when they are sprayed together with soft and ductile metallic powders. Hence, there are many studies in the open literature focusing on the characteristics of metal (aluminium or copper) matrix ceramic (oxide or carbide) particle reinforced composite coatings deposited by CS process [4-10]. In terms of mechanical properties, hardness and wear resistance appear to be of prime interest for CS’ed composite coatings. While ceramic reinforcement particles contribute to hardness and wear resistance; composite coatings could not exhibit sufficient wear performance especially at relatively high wear loads [8]. J. Singh, and A. Chauhan [11], who reviewed the effect of wear parameters (such as wear load, sliding speed, sliding distance) on the dry sliding wear behaviour of ceramic particle reinforced aluminium matrix bulk composites, concluded that protective tribolayer formation at the contact surface play crucial role on behaviour of the composites under sliding contact. Deterioration of the tribolayer at higher contact pressures causes the composites to exhibit similar wear performance with that of matrix metal as reinforcements are removed from the matrix, which can be eliminated or minimized upon lubrication of the matting surfaces. Thus, lubrication remarkably enhances the tribological performance of composites as compared to the dry sliding condition similar to other engineering materials [12]. It should be noted that, the most of the lubricants used in engineering applications are oil based and harmful for the environment [13-15]. Owing to the possibility of water, soil and air pollution by oil-based lubricants, environmental acceptability of lubricants appeared as one of the major industrial concerns. In this respect, it has been shown that pure water has a potential to be utilized as a lubricant particularly for silicon-based

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Si3N4/SiC ceramics for wear related applications such as bearings of drainable pumps and hydraulic systems [16-21]. Although, water could have major drawback for metals due to corrosion and inconsistent tribological properties. However, beneficial effect of water lubrication can come into consideration for corrosion resistant metallic components when they are reinforced with silicon-based Si3N4/SiC ceramics (i.e. metal matrix ceramic reinforced composites). In this respects, there are some studies in the open literature focusing on the tribological performance of SiC reinforced aluminium matrix bulk composites [12,22,23]. Enhanced strength to weight ratio and good corrosion resistance of Al–Si alloys [24] and water lubricity characteristics of SiC ceramics motivated the authors of this study to deposit SiCp reinforced Al-12Si matrix composite coatings on 1050 grade aluminium by CS process. More specifically, tribological behaviours of the fabricated coatings have been investigated in water under different wear testing loads to examine the possibility water lubricity for SiCp reinforced composite coatings as an environmental friendly technical solution for metallic engineering components. 2.

Materials and Methods

In the scope of this study, coatings were deposited on 1050 grade aluminium with feedstock containing different volume fractions (0, 10, 20, 30 vol.%) of SiC powder (Alfa-Aesar, irregular, <325 mesh, purity of %99) in Al-12Si powder (Alfa-Aesar, spherical, <325 mesh, purity of %99) by CS process [25]. For the fabrication of the coatings, RUSONIC Model K201 CS equipment with converging-diverging type tubular nozzle having an expansion ratio of 2.3 was utilized. 6 bar (600 kPa) inlet pressure of air was used as a process gas and traverse speed were fixed at 1 mm/s for all the cases. Stand–off distance, beam distance and powder feeding rate were set as 10mm, 1.5mm and 5 (equipment setting scale of 8), respectively.

3

Phase analyses and microstructural appearances of the coatings were done by X-Ray Diffraction (XRD) and optical microscope examinations, respectively. XRD analyses were executed with Cu Kα radiation between the angles of 10◦-90◦ on GBC MMA XRD equipment. Optical microscope (Leica DM750M) equipped with image analyser (Clemex Vision PE 6.0.027) was utilized to visualise the microstructure and interface characteristics of the coatings. Superficial hardness measurements were conducted on the coatings by using of Zwick ZHR Rockwell Hardness tester with the method of HR15T, where 15 denotes the maximum indentation load in the unit of kg and T denotes 1/16” steel ball used as the indenter. The results were determined as the average of ten successful measurements, which were performed on the randomly selected regions of the coatings. Wear tests were conducted in dry and water condition by TribotechTM reciprocating wear tester with different test loads (between 1 and 5N) against Al2O3 ball, which has a diameter of 6mm. Surfaces of the coatings were ground with 1200 grit SiC emery paper before the wear tests. For all the cases, sliding stroke, total sliding length and sliding velocity were set as 5mm, 30m and 10mm/s, respectively. Coefficients of friction, were recorded throughout the wear testing. After the wear tests, the wear loss of the coatings were determined from the 2-D profiles of the wear tracks which were obtained by Veeco Dektak 6M surface profilometer. Worn surfaces of the coatings and contact surfaces of the Al2O3 ball were examined by scanning electron microscope (SEM, Hitachi TM-1000) and optical microscope, respectively. Additionally, cross-sections of wear tracks were also investigated by SEM. To clarify the possible chemical reactions, which could take place between coating and water, SiCp free Al12-Si matrix and SiCp reinforced composite coatings were kept in water for three months and deposits form on the surfaces were examined by the Horiba Jobin Yvon Micro Raman Spectrometer and SEM.

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3.

Results

3.1 Structural Features of the Coatings XRD patterns and the cross section optical microscopic images of the coatings deposited by feedstock containing Al-12Si and Al-12Si+SiCp (30 vol.%) are presented as an example in Figure 1 and 2, respectively. No other peaks different from the feedstock appeared on the XRD patterns (Figure 1). Al and Si peaks exist as the matrix constituents. While reinforcing SiCp’s are spread homogenously, any evidence of porosity was hardly identified in the structure of the examined coatings (Figure 2). Coating/substrate interfaces are free from discontinuities indicating good bonding between the coatings and substrate. Table 1 presents the average hardness values and the volume fraction of SiCp’s entrapped in the coatings. SiCp in the coatings were about 55-60% of the feedstock as there is no 1:1 relationship for SiCp volume fractions in the feedstock and coatings. Since the main mechanism for deposition of CS coatings is the plastic deformation of sprayed particles as they impact on the substrate, limited deposition efficiency of SiCp can be associated with their poor deformation capability leading to bounce back [1-5]. The hardness of the coatings increased with increasing volume fraction of SiCp’s entrapped in the coating. Furthermore, hammering effect of the SiCp’s during CS process also contributes to the hardness of the composite coatings owing to the strain hardening of the Al-12Si matrix [7-10]. 3.2 Tribological Characterisation In Figure 3, 2-D wear track profiles and friction curves of the SiCp free and 18.3 vol.% SiCp containing Al-12Si matrix coatings are given as an example for dry and water sliding conditions worn under load of 1N. As compared to the dry sliding condition, testing in water generated smaller wear tracks with smoother contact surfaces, while reducing the fluctuations on the friction curves. Unlike SiCp free coating, remarkable reduction in friction coefficient is detected in water for the 18.3vol.% SiCp reinforced coating after the running-in period.

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Wear rate and friction coefficient values of the examined coatings obtained under dry and water sliding conditions are presented in Figures 3 and 4, respectively. For each testing load, wear rates of the coatings were calculated from 2-D profiles of the wear tracks and converted into wear rate in the unit of mm3/m by considering the total sliding distance. Friction coefficient values of the coatings were quantified as the steady state friction coefficient value when the recorded friction curves became almost stable. In accordance with the increment in hardness (Table 1); wear rate (Figure 4) and friction coefficient (Figure 5) values tended to decrease with the presence of the SiCp’s in the coatings. SiCp’s in the coating as low as 5.3 vol.% is sufficient for considerable enhancement in tribological performance as compared to SiCp free Al-12Si matrix coating. Further increase in the volume fraction SiCp’s caused better tribological properties. Altering of the testing environment from dry sliding condition to that of sliding in water resulted in reduction of both wear rate and friction coefficient, indicating efficiency of water as a lubricant particularly at lower wear testing loads. In order to establish the correlation between water lubricity and wear testing load; the wear rates and friction coefficicient values of each coating obtained in water were normalized by dividing with the corresponding values obtained under dry sliding condition. In Figure 6a and b normalized wear rate and normalized friction coefficient values are presented with respect to wear testing load, respectively. For the SiCp free Al-12Si matrix coating, testing in water provided 50% reduction in wear rate at all testing loads as compared to the dry sliding condition, while normalized friction coefficient tended to decrease about 10% (from 1.0 to 0.9) with increasing wear testing load from 1N to 5N. Unlike SiCp free coating, normalized wear rate and friction coefficient of the SiCp reinforced composite coatings increased with increasing wear testing load. At wear testing loads higher than 3N normalized friction coefficient values of composite coatings and SiCp free coating remained almost in the same range. Wear testing loads higher than 3N also

6

favoured higher normalized wear rates than SiCp free coating for the 18.3 vol.% SiCp containing composite coating. In the case of 5.3 vol.% SiCp containing composite coating, wear testing loads higher than 2N yielded higher normalized wear rate than that of SiCp free coating. The worn surfaces of the coatings and contact surface of the Al2O3 balls used as a counterface are depicted in Figures 7 and 8 after the wear tests conducted under the wear testing loads of 1N and 5N. In general dry sliding conditions were dominated by adhesive wear mechanism (Figure 7). The rubbing action of the counterface caused local delamination and material transfer to the contact surface of the Al2O3 ball. Presence of SiCp reduced the substantial delamination of the coating and heavy material transfer to the counterface. In the case of wear testing in water, deterioration of the coating and material transfer to the contact surface of the Al2O3 ball has been significantly eliminated. SiCp’s were clearly identified on the worn surface of the composite coatings worn in water. The increase of testing load from 1N to 5N caused severe fragmentation of the SiCp’s, while generating radial type cracks on the worn surface of the SiCp free coating. It is important to note that contact surfaces of the Al2O3 balls contained parallel scratch marks suggesting heavy abrasive effect of SiCp’s on the counterface under water sliding conditions at higher testing loads (i.e. 5N). Cross-sectional SEM micrographs of the wear tracks formed on 18.3 vol.% SiCp reinforced coatings under dry and water sliding conditions at wear testing load of 5N are depicted in Figure 9. Beneath the wear track formed under dry sliding condition, subsurface cracks that accelerated material removal by delamination were detected, while some of the SiCp have also undergone cracking (Figure 9a). In the case of water sliding condition, cracking of SiCp’s was observed without excessive delamination beneath the worn surface (Figure 9b). Although SiCp were cracked, some of them still remained on the worn surface indicating contact of the counterface predominantly with SiCp’s rather than the matrix under sliding contact in water.

7

Figure 10 presents the contact pressure values (derived from Appendix) acted on the examined coatings during wear tests. Depending on the wear testing load and volume fraction of SiCp in the coating, contact pressure varied in wide range (500 and 1100 MPa). Higher wear testing loads and higher volume fraction of SiCp in the coatings imposed higher contact pressures. In accordance with the SEM examinations conducted on the worn surfaces of composite coatings (Figures 7-9), fragmentation of the SiCp’s is more likely at higher wear testing loads as the contact pressure gets closer to the compressive strength (max.1400 MPa) of bulk SiC [26]. 3.3 Raman analyses Raman analyses conducted on the worn surfaces were not conclusive about the possible chemical reactions that might have taken place during wear testing in water. For this reason SiCp free and 18.3 vol.% SiCp containing coatings were held in water to make the products of chemical reactions that might form on the surfaces of the coatings in water more prominent. SEM micrographs and Raman patterns taken from the surfaces of these coatings are presented in Figures 11 and 12, respectively. The surface of the SiCp free Al-12 Si coating was covered by the rod shaped deposits. On the surface of the 18.3 vol.% SiCp containing coating small needle like deposits were also observed in addition to the rod shaped ones (Figure 11). In this study Raman patterns presented in Figure 12 were analysed according to the data obtained from the literature [27-30]. On the Raman pattern of SiCp free coating, Raman peaks have been assigned to SiO2 and Al(OH)3. In the case of 18.3 vol.% SiCp containing coating additional peaks of SiO2 and Si-OH have appeared. Si-OH stretching vibrations indicates bonding of hydroxyl group to a silicon atom as the result of dissolution of SiO2 in water [28]. The simplest soluble form of SiO2 stable in water at room temperature is Si(OH)4, which has silicon tetrahedrally coordinated to four hydroxyl groups [31]. Therefore, detection Si-OH on the Raman pattern can be associated with Si(OH)4. In this respect, we suggested formation of

8

SiO2 and Si(OH)4 (over SiCp’s) in addition to SiO2 and Al(OH)3 (over Al-12Si matrix) on the surfaces of the 18.3 vol.% SiCp containing coating held in water. It should be stated that, the deposits covering the surfaces of the examined coatings (Figures 11 and 12) are similar to those reported to form on the worn surfaces of aluminium [32] and SiC ceramics [19,21] during sliding wear in water.

4.

Discussion

CS’ed Al-12Si matrix coatings exhibited an enhancement in both hardness and tribological performance upon incorporation of SiCp’s. As compared to the dry sliding condition, testing in water provided further enhancement in tribological performance below a critical wear testing load, which seems to depend upon the volume fraction of SiCp’s. As per our findings, the critical wear testing load is quantified as about 2N (corresponds to contact pressure of about 650 MPa) for 5.3 vol.% SiCp containing coating. Increase of SiCp volume fraction to 18.3 vol.% shifted this critical testing load to above 3N (corresponds to contact pressure of about 900 MPa) . According to the literature, contact of water with aluminium and silicon initiates following reactions [33,34]; 2 Al + 2H O = 2 Al(OH) + H    3 3

∆G

!

∆G

Si + 2H O = SiO + 2H

= −285.4 kJ/mol !

= −382 kJ/mol

Eq.1 Eq.2

respectively. Gibbs free energy (∆G298) calculations for Eqs. 1 and 2 also confirms these reactions to take place on the surface of the Al-12Si matrix coatings separately at room temperature, as Al(OH)3 on the aluminium constituent of the matrix and SiO2 on the silicon constituent of the matrix. It has been documented that formation of Al(OH)3 at the contact surfaces during sliding in water is protective against wear without causing a reduction in friction coefficient [32]. In accordance with this statement, as the wear testing media water 9

imposed about 50% reduction in wear rate of Al-12Si matrix coating as compared to the dry sliding condition, while keeping the friction coefficient almost in the same range. It has been reported that water triggers spontaneous reactions during sliding contact of SiC ceramic tribo-pairs [19,21,32,35]; ∆G

SiC + 2H O → SiO + CH$ SiO + 2H O → Si(OH)$

∆G

!

!

= −358.9 kJ/mol

= −273 kJ/mol

Eq.3 Eq.4

∆G298 calculations also confirms formation of SiO2 and Si(OH)4 on the surfaces of SiC ceramics contacting with water at room temperature. During sliding wear tests in water formation of SiO2 at the contact surface according to Eq. 3 is assumed as the key phenomenon for detection of water lubricity for silicon based Si3N4/SiC ceramics. Although further reactions between SiO2 and water are not well defined it is widely accepted that formation of viscous silica gel at the contact surface promotes hydrodynamic lubrication and therefore provides an enhancement in tribological performance [19,32,35]. On the bases of above explanations, the model presented in Figure 13a has been proposed to explain the enhanced tribological behaviour of the composite coatings in water at low wear testing loads (i.e.<2N for 5.3 vol.% SiCp containing coating and <3N for 18.3 vol.% SiCp containing coating). As the wear testing environment, water imposed a tribo-layer on the contact surface of the composite coatings composed of Al(OH)3 + SiO2 (on Al-12Si matrix) and SiO2 + Si(OH)4 (on SiCp’s). Since formation of Al(OH)3 and SiO2 as the tribo-layer on the contact surface of the SiCp free coating did not cause a considerable reduction in the friction coefficient, the enhancement in tribological performance of the composite coatings can be attributed to the formation of SiO2 + Si(OH)4 (over SiCp’s), which are reported to be play crucial role in reducing the wear rate and friction coefficient of bulk SiC ceramics worn in water [17,19,32,35]. Different from the bulk SiC ceramics, SiCp’s, of the composite coatings are exposed to higher stresses as the counterface interacts with limited number of

10

SiCp’s. In this respect, at certain wear testing load, reduction in the SiCp volume fraction in the composite coating causes exposure of SiCp’s to stress levels approaching to their fracture strength. Therefore, critical wear testing load above which water lubricity effect disappears tends to decrease with decreasing volume fraction of SiCp’s in the composite coating. As an extreme case, maximum wear testing load applied in this study (5N) imposed very high contact pressure levels (in between 900 - 1100 MPa depending on the volume fraction of SiCp’s in the coating) causing fragmentation of the SiCp’s during sliding contact. It should be emphasized that fragmentation of SiCp’s hinders the lubricity effect of water due to deterioration of the tribo-layer (as schematically shown in Figure 13b) and favours abrasion at contact surface of the counterface as the result of direct contact with SiCp’s. According to the findings of this study it can be suggested that; the technological applicability of water as a lubricant could be more promising for CS’ed SiCp reinforced Al-12Si matrix composite coatings if; i.

The formation of a thicker tribo-layer on the contact surfaces is encouraged, by covering the both of the mating surfaces with composite coatings and/or increase SiCp volume fractions in the coating and

ii.

The integrity of the tribo-layer is maintained during sliding contact by reducing the contact pressure (i.e. <650 MPa for 5.3 vol.% SiCp containing coating and <900 MPa for 18.3 vol.% SiCp containing coating).

iii. 5. Conclusion Tribological performance of Al-12Si matrix CS’ed coatings under sliding contact conditions, increased upon incorporation of SiCp’s. Contribution of SiCp’s on tribological performance became more dominant in water as compared to dry sliding condition due to protective and lubricous nature of the tribo-layer formed over SiCp’s. Hence, water could be a promising

11

lubricant for SiCp incorporated composite coated metallic engineering components operating below the critical contact stress levels causing deterioration of the tribo-layer and/or fragmentation of the reinforcing SiCp’s.

Acknowledgement Authors would like to thanks to Prof. Dr. Mustafa URGEN and M.Sc. Dilek DEMIROGLU for the Raman analyses conducted in the scope of this study. Technical cooperations of Assoc. Prof. Dr. Erdem ATAR and Mr. Shaikh Asad Ali DILAWARY are highly appreciated.

Appendix Contact pressure (Pmax) values presented in Figure 10 for each wear testing load (F) were calculated by [14]; *

P%&' = ,&-

Eq .A1

where a is the contact area dimension expressed as; a=(

016 ;/ ) :6

Eq. A2

R¢ and E¢, which represent the reduced radius of curvature and reduced Young Modulus, respectively, were obtained from the following equations ;

16

;

=1

<>

;

+1 ;

:6

(for ball on plate systems)


; ;Bʋ-<

= @

:<

+

;Bʋ-E :E

F

RAx : Reduced radii of curvature in ‘x’ direction of the counterface (Al2O3 ball) RAy : Reduced radii of curvature in ‘y’ direction of the counterface (Al2O3 ball) EA: Young Modulus of the counterface (Al2O3 ball) EB: Young Modulus of the coating. uA: Poisson ratio of the counterface (Al2O3 ball) 12

Eq. A3 Eq. A4

uB: Poisson ratios of the coating. In this study, the material data listed in the Table A1 were used in calculation of the contact stresses. For the composite coatings poisson ratio and young modulus were determined according to the rule of mixture [26, 36] by considering volume fractions of SiCp and Al-12Si matrix in the coatings.

Table A1. Material data utilized in calculation of contact pressure. Material Al-12Si SiC Al2O3

Young Modulus [GPa] 75 [36] 401 [26] 344 [26]

Poisson Ratio 0.33 [36] 0.18 [26] 0.21 [26]

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Fig 1. XRD patterns of the coatings deposited by (a) SiCp free and (b) 30 vol.% SiCp containing feedstock. Fig 2. Optical microscope images of the coatings deposited by (a) SiCp free and (b) 30 vol.% SiCp containing feedstock. Fig 3. 2-D wear track profiles and friction curves of the SiCp free and 18.3 vol.% SiCp containing Al-12Si matrix coatings worn under dry and water sliding conditions at wear testing load of 1 N. Fig 4. Wear rates of the examined coatings tested in (a) dry and (b) water sliding conditions. Fig 5. Friction coefficient values of the examined coatings tested in (a) dry and (b) water sliding conditions. Fig 6. (a) Normalized wear rate and (b) normalized friction coefficient of the examined coatings. Fig 7. Contact surface appearances of the tribo-couples tested under dry sliding conditions. Fig 8. Contact surface appearances of the tribo-couples tested under water sliding conditions. Fig 9.

Cross-sectional SEM micrographs of the wear tracks for the 18.3 vol.% SiCp

reinforced coating worn under (a) dry and (b) water sliding conditions at wear testing load of 5 N. Fig 10. Contact pressure values corresponding to wear testing loads.

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Fig 11. SEM images of the surfaces of (a) SiCp free and (b) 18.3 vol.% SiCp containing coatings after immersion in water for three months. Fig 12. Results of the raman analyses conducted on the SiCp free and 18.3 vol.% SiCp containing coatings after immersion in water for three months. Fig 13. Schematic representation of wear on composite coatings under water sliding conditions at wear testing loads (a) below and (b) above the critical value.

Table 1: Superficial hardness and volume fraction of SiCp’s entrapped in the examined coatings. Feedstock Al-12 Si 10 vol.% SiCp 20 vol.% SiCp 30 vol.% SiCp

Hardness of the coating (HR15T) 80±0.5 85±1.0 87±0.5 91±0.5

SiCp in the coating (vol. %) 5.3 11.6 18.3

Highlights ·

SiCp reinforced Al-12Si matrix composite coatings deposited by CS process.

·

Water provided better tribological performance then dry sliding condition.

·

Water is promising lubricant for composite coating at low contact pressure.

18

Figure 13

(a)

(b) Figure 13

Figure 12

Figure 12

Figure 11

(a)

(b) Figure 11

Figure 10

Figure 10

Figure 9

(a)

(b) Figure 9

Figure 8

Figure 8

Figure 7

Figure 7

Figure 6

(a)

(b) Figure 6

Figure 5

(a)

(b) Figure 5

Figure 4

(a)

(b) Figure 4

Figure 3

Friction Curve

Sliding in Water

Dry Sliding

2-D Wear Track Profile

Figure 3

Figure 2

(a)

(b) Figure 2

Figure 1

Figure 1