Characteristics of a solar selective absorber surface subjected to environmental dust in humid air ambient

Solar Energy Materials and Solar Cells 172 (2017) 186–194

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Characteristics of a solar selective absorber surface subjected to environmental dust in humid air ambient

MARK

⁎

B.S. Yilbasa,b, , H. Alib, A. Al-Sharafib, N. Al-Aqeelib, N. Abu-Dheirb, F. Al-Sulaimana,b, M. Khaledc a b c

Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Center of Research Excellence in Renewable Energy, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia

A R T I C L E I N F O

A B S T R A C T

Keywords: Environmental dust TiN coating Adhesion Dry mud removal

Solar refractory selective absorber surfaces, such as TiN coatings, suffer from dust settlements, which become critically important in terms of the efficient operation of a solar receiver. In the present study, environmental dust characteristics and dust settlement on TiN coating surface are investigated. Water condensation on the dust particles in humid air ambient is simulated while mimicking the actual environmental water condensation. The size distribution of the dust particles is analyzed and the geometric features of the particles are assessed via introducing the shape factor and the aspect ratio. The tangential force required removing the dust particles and dry mud, which is formed from the dust particles and water condensate, from TiN coating surface is measured incorporating the micro-tribometer. It is found that the dust particles have various shapes and the sizes with the average size of the dust particles in the order of 1.2 µm. Dust particles compose of various elements including alkaline and alkaline earth metals. The dissolution of alkaline and alkaline earth metal compounds in water condensate results in a liquid solution, which flows towards the coating surface under the gravitational force. The liquid solution does not form a continuous film at the interface of the dust particles and the coating surface because of the spreading coefficient. The liquid solution increases the tangential force required to remove the dry mud from the coating surface once it dries out.

1. Introduction Titanium nitride (TiN) coating finds wide applications in industry because of its superior resistance towards corrosion and wear. Titanium nitride (TiN) coated surfaces are also proposed as a solar selective absorber for high temperature air-stable solar receivers [1]. High temperature air-stable solar selective absorbers, in general, operate in air ambient with temperatures over 400 °C to provide cost effective energy harvesting and efficiency improvement in solar thermal power plant and solar cooling applications [2]. However, research towards utilization of air-stable solar thermal selective surfaces is in progress, particularly for high-temperature applications [3]. On the other hand, recent environmental dust storms, particularly in the Middle East, influence significantly on the performance of solar energy harvesting devices because of dust settlement on the device active surfaces [4]. The dust particles have different sizes and shapes and compose of various elements including alkaline and earth alkaline metals such as Na, K, and Ca [5]. In humid air ambient, where water condensates onto the dust particles, an extra effort is required removing the dust particles from the surfaces. Alkaline and earth alkaline compounds in the dust ⁎

particles dissolve in water condensate and form a chemically active solution. The liquid solution flows towards the surface under the gravity where the dust particles are settled and it forms a liquid interlayer between the surface and the dust particles with progressing time. Once the liquid solution dries out, the adhesion between the surface and the dust particles increases significantly because of the dried interlayer solution [6]. In general, adhesion between the dried solution and the surface is governed by the surface free energy of the substrate material and interfacial energy between the dried solution and the solid surface. The efforts required to remove the dust particles from the surface are, therefore, associated with the adhesion of the particles on the solid surface. Consequently, investigation of the adhesion of the dust particles on TiN coated surface is fruitful in terms of maintaining solar energy harvesting device efficiency in harsh environments. Considerable research studies were carried out to examine surface characteristics of TiN coating in relation to solar energy applications. The optical properties of TiN-based spectrally selective solar absorbers were studied by Gao et al. [7]. They showed that the SS/TiN/Al2O3 coating exhibited good thermal stability in a vacuum at 500 °C for 5 h with enhanced absorptance and reduced emittance. The

Corresponding author at: Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia. E-mail address: [email protected] (B.S. Yilbas).

http://dx.doi.org/10.1016/j.solmat.2017.07.038 Received 17 April 2017; Received in revised form 19 July 2017; Accepted 26 July 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

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testing sites could be used for reliable way to verify the corrosion resistance of new materials and products and the evolution of optical properties degradation of absorber surfaces in the presence of high concentration of contaminants. A degradation study for selective solar absorber surfaces in solar thermal collectors was carried out by Fernandes et al. [20]. The findings revealed that electrochemical impedance spectroscopy (EIS) allowed for the assessment of mechanistic information on the degradation processes, especially if equivalent circuits were used, providing quantitative data that could easily relate to the kinetic parameters of the system. On the other hand, environmental dust and its effects on surfaces become an interest to maintain the performance of solar energy harvesting systems in harsh environments. Effect of mud drying temperature on surface characteristics of a polycarbonate PV protective cover was studied by Yilbas et al. [21]. They showed that compounds of alkaline (Na, K) and alkaline earth metals (Ca) in dust particles dissolved in condensed water while forming chemically active mud solution, which settled at the interface between mud and polycarbonate surface under the gravitational force. This had a detrimental effect on cleaning of dusted polycarbonate surface because of the mud solution; in which case, upon drying, it increased adhesion between dry mud and surface as well as modified microhardness and surface texture of polycarbonate surface. A study on the wear, optical and electrical characteristics of dry cleaned PV solar panels was carried out by Al-Shehri et al. [22]. They indicated that no permanent or significant negative impact occurred affecting the solar panels performance when the brush-based dry cleaning was introduced removing dust from the panel surfaces. The mechanics of dust removal from rotating disk in relation to self-cleaning applications of PV protective cover were examined by Rifai et al. [23]. The findings revealed that centrifugal force remained higher than the adhesion, friction, drag, lift, and gravitational forces in the region away from the rotational center. The dust particle size and rotational speed significantly influenced the rate of dust removal from the disk surface. The surface characteristics of the laser textured silicon wafer and effect of mud adhesion on hydrophobicity were investigated by Yilbas et al. [24]. They demonstrated that laser textured surface composed of micro/nano poles and fibers, which in turn improved the surface hydrophobicity significantly. In addition, the formation of nitride species contributed to microhardness increase and enhancement of surface hydrophobicity due to their low surface energy. The mud residues did not influence the fracture toughness and microhardness of the laser textured surface; however, they reduced the surface hydrophobicity significantly. Characterization of dust particles collected from PV modules in the area of Dhahran, Kingdom of Saudi Arabia, and its impact on protective transparent covers for photovoltaic applications was studied by Mehmood et al. [25]. They showed that dried mud films required large tangential force removing the dry mud from glass PV cover, which was higher than that of the polycarbonate PV cover. The effect of environmental dust particles on laser gas assisted nitriding and sol–gel coating of alumina surfaces was examined by Yilbas et al. [26]. They demonstrated that the laser treated and sol–gel coated alumina surfaces provided superior surface characteristics in the harsh environments because of weak adhesion between the mud formed from the dust particles and the coating surface. This was associated with the small texture height of the sol–gel coating, which lowered the area of the interfacial contact between the mud and the coated surface, and relatively lower surface energy of the sol–gel coating as compared to that of the laser treated surface. The sol–gel coating did not alter the optical characteristics of the laser treated surface. Laser gas assisted texturing of alumina surfaces and effects of environmental dry mud solution on surface characteristics were studied by Yilbas et al. [27]. The findings revealed that laser texturing resulted in superhydrophobic surface. The mud solution modified the surface texture characteristics of the laser treated workpieces; in which case, surface hydrophobicity reduced significantly. The residual stress was compressive in the surface region of the laser textured workpiece and the mud solution increased

characterization and performance evaluation of Ti/AlTiN/AlTiON/ AlTiO high temperature spectrally selective coatings for solar thermal power applications was investigated by Barshilia [8]. He demonstrated that the absorber coating displayed improved adhesion, UV stability, corrosion resistance, and thermal stability in air and vacuum with high absorptance and low emittance. A review on physical vapor deposited (PVD) spectrally selective coatings for mid and high temperature solar thermal applications was presented by Selvakumar and Barshilia [9]. They indicated that solar selective coatings based on transition metal nitrides, oxides and oxynitrides hold great potential for high-temperature applications because of their excellent mechanical and optical properties, which were yet to be commercialized. Investigation of optical properties of solar absorber based on cermet of titanium nitride in SiO2 deposited on lanthanum aluminate was studied by Cao et al. [10]. The findings revealed that the optimized cermet contains TiN with a volume fraction of 60% and 65% demonstrated an absorptance higher than those of ~95% before annealing and ~94% after annealing at 700 °C, which appeared to be useful for especially concentrated solar applications. A refractory selective solar absorber incorporation TiN thin coating was examined by Jiang et al. [11] for the high performance thermochemical steam reforming process. They showed that the selective surface of TiN coating resulted in superior performance for steam reforming and hydrogen production rate at high surface temperatures. A study of porous titanium nitride microspheres in relation to dyesensitized solar cells was carried out by Wang and Liu [12]. They indicated that the dye-sensitized solar cell with this unique structure resulted in a high conversion efficiency of 6.8%, which was favorably comparable with the cell based on a conventional platinum counter electrode. Solar selective absorbing coatings TiN/TiSiN/SiN prepared on stainless steel substrates were investigated by Feng et al. [13]. They demonstrated that the solar selective absorbing performance of the Cu/ TiN/TiSiN/SiN did not show significant changes after they were heattreated up to 700 °C in vacuum. An optimization study of TiAlN/ TiAlON/Si3N4 coatings for solar absorber applications was carried out by An et al. [14]. They analyzed the reflectance spectrum of the coatings and indicated that the optimized coating resulted in a solar absorptance and thermal emittance of 94.6% and 5.2% at 400 K, respectively. A new TiN coating combining broadband visible transparency was introduced by Smith et al. [15]. They showed that by depositing thin films resulted in the surface reflection in the near infrared region. This, in turn, made it possible to produce films which transmit daylight neutrally at reasonably high levels, while still maintaining low emittance and solar control in the region of the near infrared radiation. A new structure of durable metallic thin film contacts for solar cells was investigated by Matenoglou et al. [16]. They used stoichiometric titanium nitride (TiN) with a thin non-stoichiometric titanium nitride (TiNx) buffer layer giving the desirable mechanical properties and metallic behavior. They showed that the metallic contacts remained cohered during the scratch test and coating strongly adhered to the substrate surface. Solar selective absorber coatings, consisting in a TiAlNx / TiAlNy tandem absorber with low (metallic-like) and high (semiconductor-like) nitrogen content, and an Al2O3 antireflective coating, were studied by Soum-Glaude et al. [17]. They presented the total hemispherical emittance of the coatings in terms of hemispherical directional reflectance spectra measured at different angles and indicated that using the near normal hemispherical emittance could be a good approximation for the estimation of the total hemispherical emittance. Nanostructured thin films for solar selective absorbers and infrared selective emitters were examined by Ollier et al. [18]. They showed that novel pathway for producing low cost nanostructured materials created new opportunities for solar capture in concentrated solar power, solar thermophotovoltaics, solar thermo-electrical generators and for infrared emission control in thermophotovoltaic technologies. Durability of different selective solar absorber coatings in environments with different corrosivity were investigated by Diamantino et al. [19]. They demonstrated that the outdoor exposure 187

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considered for the contact angle measurement. Droplet volume was controlled with an automatic dispensing system having a volume step resolution of 0.1 μl. Still images were captured, and contact angle measurements were performed after one second of deposition of a water droplet on the surface. In order to simulate the effect of mud formed from the dust particles in humid air ambient on the characteristics of TiN coating, an experiment was carried out to resemble the actual mud formation, due to condensation of water vapor on the accumulated dust particles, at TiN coating surfaces. Firstly, the dust particles were collected from outdoor PV panel surfaces in Dhahran area of Saudi Arabia. Soft brushes were used to collect the dust particles from the PV panel surfaces and the collected particles were stored in air-tide sealed container. Since dust layer thickness was in the order of 300 µm on the PV panel surface after two weeks, a layer of the environmental dust particles with 300 µm thickness was formed on TiN coating surface while mimicking the outdoor conditions. The desalinated water with the same volume of the dust was particles were dispensed gradually on to the dust layer. It should be noted that the initially condensation tests were carried out in a local outdoor humid air ambient to estimate the amount of condensate accumulated on dust particles over six hours period. It was found that the amount of water condensate had the same volume of dust over six hours period. In order to resemble the water condensation in the outdoor humid air ambient, dispensed water was left on the dust layer without mechanical mixing. This results in the natural formation of mud on the TiN coating surface. The workpieces were, then, kept in a local normal ambient air for three days to dry. The coating surfaces with the presence of dry mud were tested for adhesion work measurements. Later, the dry mud was removed from the coating surface with a desalinated water jet having 2 mm diameter and 1.5 m/s velocity. The cleaning process was continued for 15 min for each coating surface. Finally, the dry mud residues after water jet cleaning of the coating surface were analyzed using the analytical tools.

slightly surface hardness and residual stress in the surface region. Although the dust particles and mud adhesion on hydrophobic surfaces were carried out previously [21,24,27], the main focus was surface texturing towards achieving self-cleaning characteristics of the surfaces. However, the environmental dust and mud effect on selective surfaces for solar energy harvesting was left for the future study. In solar energy harvesting system, surfaces are exposed to outdoor conditions and they suffer from dust accumulation and mud formation in harsh environments within dusty and humid ambient. Since the environmental dust particles possess various elements, their effects on the selective surface properties become critically important for the sustainability of solar harvesting system performance. Consequently, investigation of the dust and the mud effects on the selective surfaces become essential. In the present study, environmental dust settlement and the mud formation on TiN coating are investigated. The dust particles and the dry mud formed on TiN coating are characterized. The influence of mud formed on TiN surface, due to water condensate on the dust particles, is examined using the analytical tools including electron scanning and atomic force microscopes, Fourier transforms infrared spectroscopy, X-ray diffraction, and energy dispersive spectroscopy. The tangential force required to remove the dry mud from the surface of TiN coating is measured incorporating the micro-tribometer. Surface energy and hydrophobic characteristics prior and after the dry mud removal were determined using the goniometer and water droplet technique. 2. Experimental TiN coating was deposited onto 3 mm thick Ti6Al4V alloy surface using the PVD coating unit as described in the previous study [28]. The PVD coating results in a uniform thickness of TiN coating, which can be seen from Figs. (1a) and (1b), in which scanning electron microscope (SEM) micrograph of a cross-section of the coating is shown. The coating surface was examined thoroughly using the optical microscope and SEM. The findings revealed the coating surface was free from coating defects such as pinholes, voids, and micro-cracks. mm thickness is used in the experiments. Material characterization of the laser treated surfaces was carried out using Jeol 6460 electron microscope and atomic-force/scanning-force microscopes (AFM/SPM), by Agilent, in contact mode. The atomic force microscope (AFM) tip was made of silicon nitride probes (r = 20 − 60 nm) with a manufacturer specified force constant, k, of 0.12 N/m. XRD. Jeol 6460 electron microscopy was used for SEM examinations and Bruker D8 Advanced having CuKα radiation was used for XRD analysis. A typical setting of XRD was 40 kV and 30 mA and scanning angle (2θ) was ranged 20°−80°. A linear micro-tribometer (MCTX-S/N: 01–04300) was used to determine the tangential force friction coefficient of the dry mud formed TiN coating surface. The equipment was set at the contact load of 0.03 N and end load of 5 N. The scanning speed was 5 mm/min and loading rate was 1 N/s. The wetting experiment was performed using Kyowa (model - DM 501) contact angle goniometer. A static sessile drop method was

3. Results and discussion Environmental dust effects and mud formation in a local humid air ambient on the surface of TiN coating is carried out and adhesion of the dust particles and the dry mud on the coating surface is investigated. Elemental composition and geometric features of the dust particles are analyzed. Fig. 2 shows SEM micrographs of the dust particles collected in Dhahran area of Saudi Arabia. The dust particles have various shapes (Fig. (2a)) and sizes (Fig. (2b)) within the range of 20–0.1 µm having the average particle size of 1.2 µm. In general, the small particles attach to the large size particle surfaces (Fig. (2c)). The bright appearance of some small dust particles demonstrates that they are charged particles; in which case, the static charge causes the small particles attachment to the large particles. The small dust particles stay longer time periods in atmosphere and interaction of these particles with solar radiation during the prolonged exposure allows the attachment of ionic compounds in regions near the Arabian Sea. Hence, these particles once Fig. 1. a and 1b. SEM micrographs of the cross-section of TiN coated Ti6Al4V alloy.

a)

Coating

b)

Coating

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Fig. 2. SEM micrographs of dust particles: a) various shapes of dust particles, b) various sizes of dust particles, c) small dust particles attach to the large size dust particle, and d) clustered small size dust particles.

settle on the surfaces, they adhere to the large particles while forming dust clusters (Fig. (2d)). Moreover, the geometric feature of the dust particles can be classified by the shape factor and the aspect ratio. The

Table 1 Elemental composition of the dust particles (wt%).

P2

shape factor can be defined by the ratio RShape = 4πA , where P is the perimeter of the dust particle and A is the cross-sectional area [5]. The shape factor is inversely related to the particle circularity and it is attributed to the complexity of the particle; in which case, the shape factor of unity corresponds to a perfect circle. The aspect ratio represents the ratio of the major-to-minor axes of an ellipsoid that is best

Dust Particle Size > 2 µm Dust Particle Size < 2 µm Mud Residues

Si

Ca

Na

S

Mg

K

Fe

Cl

O

11.4 11.5 10.5

7.6 7.1 6.3

2.8 5.1 3.4

1.8 1.1 –

2.2 2.9 2.1

1.2 2.1 2.1

1.1 0.9 –

0.9 2.1 1.8

Balance Balance Balance

π (Lproj)2

fit to the particle and it.is defined through AAspect = 4A , where, and Lproj is the longest projection length of the dust particle [5]. The aspect ratio is associated with the approximate particle roundness. Developing the mathematical correlation between the aspect ratio and the shape factor is difficult because of the complex shapes of the dust particles. Nevertheless, an inverse relationship occurs between the particle size and the aspect ratio and the direct relationship is observed between the particle size and the shape factor. In this case, with increasing particle size, the aspect ratio reduces as the shape factor increases. The shape factor approaches unity for the small size particles (≤ 2 µm) and the median shape factor approaches 2.8 for the large size particles (≥ 10 µm). Moreover, elemental composition of the dust particles are given in Table 1. The dust particles possess various elements including, Ca, Si, O, Na, K, Mg, Fe, Cl, and S, which can be associated with the local geological structure of the desert environment. The presence of chlorine is associated with the prolonged exposure of small particles in

Fig. 3. X-ray diffractogram of dust particles.

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Table 2 ICP data for the mud solution after 6 h dissolution time of dust particles in desalinated water. Ca

Na

Mg

K

Fe

Cl

280500

45600

70500

30800

1500

40100

atmosphere close to sea shore. Fig. 3 shows X-ray diffractogram of the dust particles. The various peaks including Na, K, Cl, Ca, Fe, and S are evident from the diffractogram. Iron peaks coincide with Si and Al peaks. However, the presence of S is associated with anhydrite or gypsum (CaSO4) components and iron is related to clay-aggregated hematite (Fe2O3). In order to assess the amount of dissolved alkaline (Na, K) and alkaline earth (Ca) metals in water condensate, dust particles are mixed with the desalinated water mimicking the water condensation on the dust particles in humid air conditions. The liquid solution from the mixture of desalinated water and the dust particles is extracted. Inductively Coupled Plasma (ICP) is carried out in the liquid solution and findings are given in Table 2. ICP data reveals that Ca, Mg, Na, K are present in the extracted liquid solution. The alkalinity of the liquid solution is also measured via pH meter. The results show that the liquid solution is basic with pH = 8.6. Consequently, alkaline and alkaline earth metal compounds dissolve in water condensate and form the liquid with a basic characteristic, which, then, flows towards the TiN coating surface under the gravitational force. To examine the effect of liquid solution on the coating surface, the liquid solution is deposited onto the coating surface and left for drying in six hours. Fig. 4 shows SEM micrographs of the dried liquid solution on TiN coating surface. The crystals with varying sizes are observed from the micrographs (4a) and (4b). The variation of crystal sizes is associated with the concentration of dissolved compounds and temperature variation at the surface. The dissolved alkaline and alkaline earth metal compounds are responsible for the formation of the crystal structures on the surface. Consequently, these structures increase the adhesion of the dust particles to the coating surface when the liquid solution dries out in between the dust particles and the coating surface. Fig. 5 shows FTIR data for dried mud solution on the surface of the TiN coating. The strong peak at 670 cm−1 corresponds to Ti-N bonding [29]. Because of the residues of mud in the mud solution, silica particles appear on the dried liquid solution (Figs. (4a) and (4b)). In this case, the peaks at 777 and 1077 cm−1 corresponds to the Si–O bonds, which is associated with stretching of SiO2 [30]. The peaks at 867 and 1431 cm−1 are attributed to the stretching vibrations of CO3−2 [31]. These peaks are attributed to

Fig. 5. FTIR data for dried mud solution on the coating surface.

calcite (CaCO3) residues in the dried liquid solution on the TiN coating surface. To resemble the mud formation on the TiN coating surface, a film of dust particles of 300 µm is formed and desalinated water is dispensed onto the dust particles with an amount obtained via mimicking the water condensate onto the dust particles in the humid air ambient. The mixture of dust particles and the desalinated water is left for drying over six hours period. Figs. (6a) and (6b) show SEM micrographs of the dry mud surface. In general, the dry mud surface composes of dust particles and condensed cement like structures formed in between the particles. In addition, some small voids like textures are also observed on the surface. The voids like textures are formed by the dissolved dust compounds. These compounds form a liquid solution, which flows through the dust particles towards the interface between the dust particles and the coating surface while leaving the void like the texture on the dry mud surface. Moreover, some of the liquid solution is captured in the cavity like structures in the mud cross-section and upon drying it appears like a bright region across the dry mud layer (Fig. (7a)). The liquid solution spreads on the coating surface while forming a film at the interface between the coating surface and the mud. However, the continuity and the uniformity of the liquid film depend on the spreading coefficient over the coating surface. In general, the spreading coefficient of liquid depends on the interfacial tension between TiN coating liquid, and air according to Ssl (a) = γla − γsa − γsl , where γla is the surface tension of liquid solution in air, and γsa is the surface free energy of TiN coating in air, and γsl is the interfacial tension between liquid solution and TiN coating. The surface energy of TiN coating is Fig. 4. SEM micrographs of crystals formed during drying of liquids solution: a) crystal formed on TiN coating. Since spreading coefficient allows island of liquid solution on TiN coating surface, locally scattered crystals are formed at the surface, and b) two crystals with different sizes on TiN coating surface.

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Fig. 6. SEM micrographs of dry mud surface: a) dust particles and cement like the texture at the surface, and b) porous like structures, marked by dotted circles, on the dry mud surface because of mud liquid flowing towards TiN coating surface under gravitational force.

arrangements yield:

determined by using the liquid contact angle measurements method [32]. In this case, water, glycerol, and ethylene glycol are used to measure the droplet contact angle of each fluid. The brief analysis for the surface energy formulation is presented in association with the previous study [32]. The surface energy of solids and liquids was presented by van Oss et al. [32,33] and the surface energy can be written as:

γ = γL + γP

γL (cos θ + 1) = 2 γSL. γLL + 2 γS+. γL− + 2 γS−. γL+

Eq. (5) could be used to determine the incorporating the contact angle data and However, the data for γLL , γL+, and γL− can be found in the open literature for water, glycerol, and ethylene glycol, which are given in Table 3 [32–35]. Eqs. (1), (2), and (5) are applicable only for smooth surfaces, which is the case for TiN coating surface. Incorporating Eqs. (1), (2), and (5) with the data is given in Table 3 for water, glycerol, and ethylene glycol, the surface energy can be obtained, which are given in Table 4. In addition, the surface tension of the liquid solution is measured using the capillary tube method [36]. The measurement reveals that the surface tension of the liquid solution is in the order of 0.085 N/ m. The interfacial tension between TiN coating and a liquid solution is taken as same of interfacial tension between TiN coating and water, which is in the order of 80.41 mJ/m2 (Table 4). It should be noted that the liquid solution has the similar surface tension of that of water; therefore, the consideration for interfacial tension between the liquid solution and TiN coating surface can be justified. Since the surface energy of TiN coating is determined as in the order of 120 mJ/m2 (Table 4) and surface tension of liquid solution is 0.085 mJ/m2, the spreading coefficient of liquid solution on TiN surface (Ssl (a) ) becomes in the order of – 200 mJ/m2, which is Ssl (a) < 0 . Therefore, the liquid solution partially wets the surface rather forming a liquid film between the dust particles and the TiN coating surface. The partial wetting of the liquid solution can be observed from SEM micro-graph of dry mud cross-section (Fig. (7b)). In this case, bright region at the interface of the dry mud and TiN coating surface represents the dried liquid solution. However, the dried liquid solution does not completely cover the

(1)

Where γL is the apolar component due to Lifshitz-van der Waals intermolecular interactions and γP is due to electron acceptor and electron donor intermolecular interactions. The apolar component γP due to electron acceptor and electron donor intermolecular interactions yield [32,33]:

γ P = 2 γ +. γ −

(2)

where γ and γ are the electron acceptor and electron donor parameters of an acid-base component of the solid and liquid surface free energy, respectively. However, the interfacial free energy for a solidliquid system can be expressed as [28,29]: +

-

γSL = γS + γL − 2 γSL. γLL − 2 γS+. γL− − 2 γS−. γL+

(3)

where subscripts S and L represent solid and liquid phases, respectively. The Young's equation for the surface free energy of a solid is [34]:

γL cos θ = γS − γSL − PeL

(4)

where γS is the solid surface free energy, γSL is the interfacial solidliquid free energy, γL is the liquid surface tension, θ is the contact angle, and PeL is the pressure of the liquid film, which is negligibly small and considered to be zero [32]. Combining Eqs. (3) and (4) and re-

a)

Fig. 7. SEM micrographs of dry mud cross-section: a) island of dried liquid solution at the interface of the dry mud and TiN coating surface, marked by dotted circles, and b) cavities across the dry mud crosssection, which are marked in dotted circle. The dried liquid solution, appearing as bright color, is captured in some cavities.

b) TiN Coated Workpiece

Dry Mud

(5)

values of γSL , γS+, and γS− while the values of γLL , γL+, and γL−.

Cavities

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were formed from the small size particles, which are strongly attached together like bonded structures. This can be attributed to the bonding of small size dust particles with the liquid solution prior to drying on the coating surface. These structures strongly attach the coating surface after drying and become difficult to remove from the surface via water jet. Elemental composition of the dry mud residues on the coating surface is given in Table 1. The presence of Ca, Si, Na, Mg, Cl, and O indicates that the dry mud residues most likely compose of CaCO3, NaCl, and MgO. Moreover, Fig. 9 shows AFM images of the coating surface after removal of the dry mud by a water jet. The dry mud residues form a texture on the coating surface (Fig. (9a)) and the residues of the small size dust particle attach to the coating surface. The texture height is observed to be varies on the surface (Fig. (9b)), which results in the average surface roughness, due to presence of the dry mud residues, in the order of 1.15 µm. In order to assess the dry mud adhesion on the TiN coating surface, the tangential force required to remove the dry mud from the surface is measured using the micro-tribometer. Fig. 10 shows the tangential force along the scanning (scratch) distance on the TiN coating. The friction force on the plain TiN coating is also shown in Fig. 10. The tangential force required to remove the dry mud from the coating surface is considerably larger than that of the frictional force on the as received coating surface. This behavior is related to the adhesion of the dry mud on the coating surface. In addition, the friction force on the coating surface also contributes to the tangential force. Some small peaks are observed along the tangential force, which is associated with

Table 3 Lifshitz-van der Walls components and electron-donor parameters used in the simulation [28,31].

Water Glycerol Ethylene glycol

γL (mJ/m2)

γLL (mJ/m2)

γL+ (mJ/m2)

γL− (mJ/m2)

72.8 64 48

21.8 34 19

25.5 3.92 0.41

25.5 57.4 1.28

Table 4 Surface energy and Lifshitz-van der Walls components and electron-donor parameters determined for TiN coating surface. γS (mJ/m2)

γS+ (mJ/m2)

γS− (mJ/m2)

γ P (mJ/m2)

γSL (mJ/m2)

120

35.93

10.93

39.63

80.41

entire interface, but it is present locally. The local appearance of the dried mud solution demonstrates the non-wetting characteristics of the liquid solution at the interface prior to drying. In order to assess the after effects of the dry mud on the surface of TiN coating, the dry mud is removed with a desalinated water jet of 2 mm diameter and 2 m/s jet velocity. Figs. (8a) and (8b) show SEM micrographs of the dry mud removed surface. A cluster of dry mud residues is evident on the coating surface. The close view of the dry mud residues demonstrates that they

Fig. 8. SEM micrographs of dry mud removed surface: a) mud residues on TiN coating surface, b) clustered small size mud residues on TiN coating surface, and c) various sizes of mud residues on water jet cleaned TiN coating surface.

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Fig. 9. AFM images of the dry mud removed surface: a) 3-D micrograph of dry mud removed surface, and b) line scan of mud removed surface.

Table 5 Adhesion work obtained from the tangential force. TiN Coating

Adhesion Work (mJ)

As Received Dust Particles Dry Mud

0.119 0.387 1.244

dry dust particles from the surface, the tangential force measured is repeated for the removal of the dust particles from the coating surface. The tangential force findings for the dust particles are also shown in Fig. 10 for comparison. It is evident that the average tangential force required to remove the dry dust particles is almost 0.3 of the tangential force required to remove the dry mud from the coating surface. On the other hand, the tangential work for removing the dry mud from the surface can be determined through the integration of tangential force over the scanning length in Fig. 10. The friction work can also be calculated in a similar manner. Therefore, subtraction of friction work from the tangential work gives the adhesion work. The adhesion work determined from the integration and subtraction of the tangential and the friction forces. The adhesion work for removing the dry mud when liquid solution totally wets the coating surface while forming a thin film at the interface is almost twice of the adhesion work required to remove dry mud from the surface. In addition, the adhesion work required to remove the dry dust particles is almost 0.3 of the adhesion work of the dry mud removal from the coating surface. Table 5 outlines the adhesion work required for the removal of the dry dust particles, the dry mud, and the dry mud with the dried liquid solution at the interface. Consequently, the dust particles removal from TiN coating surface in

Fig. 10. Tangential force measured to remove the dry mud and dust particles from TiN coating surface. Frictional force on the TiN coating surface is also included for comparison.

the strong adhesion of the dust particles on the coating surface. Since the liquid solution partially wets the surface, upon drying small islands of dried solution film occurs at the interface between the dust particles and the coating surface. These islands of dried liquid solution at the interface are responsible increasing the tangential force. Moreover, the ratio of the maximum tangential force to the average value of the tangential force is in the order of 1.5, which demonstrates that the adhesion increases almost 30% more as the liquid solution forms locally dried regions at an interface between the dust particles and the coating surface. In order to assess the tangential force required to remove the 193

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the dry air ambient is favorable in terms of the efforts required for surface cleaning. 4. Conclusion TiN coating on Ti6Al4V alloy is carried out and influence of the environmental dust particles on the coating surface is examined mimicking the humid air ambient. The environmental dust particles are collected and characterized using the analytical tools including scanning electron microscope, energy dispersive spectroscopy, and X-ray diffraction. The dust particle size is analyzed and geometric features of the dust particles are classified via introducing the shape factor and the aspect ratio. Water condensation on the dust particles in outdoor humid air ambient resembles in the laboratory environments and the resulting liquid solution is analyzed. Since the liquid solution, which is formed by the dissolution of alkaline and alkaline earth metals in water condensate, accumulates on the TiN coating surface under the gravitational force, the wetting characteristics of the coating surface are examined and spreading coefficient of the liquid solution is determined. The influence of the dry mud residues, after removing the dry mud from the coating surface by a water jet, is also examined. The adhesion between the dry mud and the TiN coating surface is estimated by incorporating the micro-tribometer and adhesion work is determined from the microtribometer data. It is found that the dust particles have various shapes and sizes with the average size of 1.2 µm. The small size dust particles attach to the large size particles forming the dust clusters on the coating surface. The dust particles compose of various elements including alkaline (Na, K) and alkaline earth metals (Ca). Compounds of alkaline and alkaline earth metals dissolve in water condensate while increasing alkalinity of water condensate (pH = 8.6). The liquid solution formed due to the mixture of water condensate and dissolved alkaline and alkaline earth metal compounds flows towards the TiN coating surface under the gravitational force while forming a liquid layer in between the dust particles and the coating surface. The spreading coefficient of the liquid solution on the coating surface remains less than zero while demonstrating the partial wetting of the coating surface. The liquid solution forms various sizes of crystals on the coating surface after drying. The dried liquid solution does not form a complete film at the surface rather locally scattered liquid islands and upon drying they form regional interfacial layers between the dust particles and the coating surface. The tangential force required to remove the dry mud from the coating surface increases significantly when interlayer of dried liquid solution is present between the coating surface and the dust particles. Therefore, the dry mud adhesion on the coating surface increases significantly when the liquid fluid is present at the interface. Consequently, the dust removal from the coating surface is favorable in a dry air ambient in terms of the minimum efforts, otherwise the efforts required to remove the dry mud formed on the coating surface become substantial. The present study provides useful information on the environmental dust particles and gives insight into the adhesion of the environmental dust particles on TiN coating. Acknowledgements The authors acknowledge the financial support of King Fahd University of Petroleum and Minerals (KFUPM) and King Abdulaziz City for Science and Technology (KACST) through project # 11ADV2134-04 to accomplish this work. References [1] G.B. Smith, A. Ben-David, P.D. Swift, A new type of tin coating combining broad band visible transparency and solar control, Renew. Energy 22 (2001) 79–84. [2] S. Ishii, R.P. Sugavaneshwar, T. Nagao, Titanium nitride nanoparticles as plasmonic solar heat transducers, J. Phys. Chem. C 120 (2016) 2343–2348. [3] Y. Yin, L. Hang, S. Zhang, X.L. Bui, Thermal oxidation properties of titanium nitride and titanium–aluminum nitride materials — A perspective for high temperature air-stable

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