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Effect of environmental dust particles on laser textured yttria-stabilized zirconia surface in humid air ambient
Optics and Laser Technology 101 (2018) 388–396
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Effect of environmental dust particles on laser textured yttria-stabilized zirconia surface in humid air ambient B.S. Yilbas a,b,⇑, H. Ali a, A. Al-Sharafi a, F. Al-Sulaiman a,b, C. Karatas c a
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 c Materials Science and Nanotechnology Engineering, Kastamonu University, Turkey b
a r t i c l e
i n f o
Article history: Received 8 October 2017 Received in revised form 31 October 2017 Accepted 29 November 2017
Keywords: Zirconia Laser texturing Hydrophobicity Environmental dust Adhesion
a b s t r a c t Zirconium nitride is used as a selective surface for concentrated solar heating applications and one of the methods to form a zirconium nitride is texturing of zirconia surface by a high intensity laser beam under high pressure nitrogen gas environment. Laser texturing also provides hydrophobic surface characteristics via forming micro/nano pillars at the surface; however, environmental dust settlement on textured surface influences the surface characteristics significantly. In the present study, laser texturing of zirconia surface and effects of the dust particles on the textured surface in a humid air ambient are investigated. Analytical tools are used to assess the morphological changes on the laser textured surface prior and after the dust settlement in the humid air ambient. It is found that laser textured surface has hydrophobic characteristics. The mud formed during condensate of water on the dust particles alters the characteristics of the laser textured surface. The tangential force required to remove the dry mud from the textured surface remains high; in which case, the dried liquid solution at the mud-textured surface interface is responsible for the strong adhesion of the dry mud on the textured surface. The textured surface becomes hydrophilic after the dry mud was removed from the surface by a desalinated water jet. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Solar energy harvesting has a crucial role for widening renewable energy applications around the Globe. Utilization of high temperature resistant ceramic components becomes unavoidable for solar thermal applications. Zirconium nitride can serve as a selective surface for solar energy harvesting in thermal systems, which is because of excellent optical properties in terms of absorption and emission [1]. On the other hand, climate change has imitated dust storms around the Middle East [2]. The dust settlement on solar energy harvesting surfaces lowers device performance in terms of output power and efficiency [3]. The environmental dust particles compose of various elements and compounds including alkaline and alkaline earth metals [4]. In humid air ambient, water condensates on the dust particles and some compounds of the dust particles dissolve into water condensate. This forms a chemically active liquid solution, which accumulates at the interface of the device surface and the dust particles under the gravity [4]. The chemically active liquid solution causes some asperities on 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). https://doi.org/10.1016/j.optlastec.2017.11.052 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
device surface, such as pin holes and pit sites, while causing permeant damages [5]. In addition, once the liquid solution dries at the device surface, adhesion between the dried liquid and the surface becomes significantly strong and the efforts required removing the dried solution and the mud from the surface increase significantly. One of the solutions to create self-cleaning characteristics at the surface is to improve surface hydrophobicity. In general, surface hydrophobicity is associated with the micro/nano scale surface texture and low surface free energy. However, plane yttria-stabilized zirconia surface has high surface free energy and demonstrates hydrophilic characteristics. Consequently, altering characteristics of zirconia surface through surface processing becomes essential to generate surface hydrophobicity. Several methods have been suggested and many techniques were reported for generating hydrophobic characteristics at the surfaces [6–11]. The techniques reported were involved with multi-steps processes and harsh conditions. Some of these processes include phase separation [6], electrochemical deposition [7], plasma treatment [8], sol-gel processing [9], electrospinning [10], and solution immersion [11]. Laser gas assisted texturing offers considerable advantages over the multi-step processes for hydrophobized surfaces, which is particularly true for ceramic surfaces [12]. Some of these advantages include high speed processing, the precision
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of operation, and low cost. In laser texturing process, a combination of melting and evaporation of ceramic surface takes place via ablation [12,13]. This in turn generates surface texture consisting of micro/nano poles and cavities. The use of high pressure assisting gas, such as nitrogen, generates nitride compounds on the laser textured surface [12]; in which case, nitride compounds have low surface energy than the oxides [12]. In addition, arrays of micro/nano size pole/pillar improve the water repelling capacity of the laser textured surface. However, dust settlement on the laser textured surface changes the surface characteristics such as surface hydrophobicity. Hence, investigation of environmental dust particle adhesion and influence of dried mud solution on the characteristics of laser textured zirconia surface becomes essential. Considerable research studies were carried out to examine laser processing of zirconia tiles. Laser micromachining of yttriatetragonal zirconia polycrystal ceramic was carried out by Li et al. [14]. They demonstrated that crack and asperity free machined surfaces were possible under the precise control of laser treatment parameters. Laser short-pulse processing of yttriastabilized tetragonal zirconia surfaces was carried out by Oyane et al. [15]. They indicated that submicro-/micro-structures on the zirconia surface were obtained after laser ablation and laser treated surface showed increased water wettability. A parametric study of laser surface patterning of zirconia was carried out by Roitero et al. [16]. They showed that increasing both fluence and number of pulses resulted in deep texture patterns at the surface. However, increasing number of pulses had a detrimental effect on the quality of the textured surfaces; in which case, surface damage, such as intergranular cracking, open porosity and nanodroplets formation, could be generated after the laser treatment process. Laser surface treatment of zirconia and the corrosion resistance of the resulting surface were studied by Ahmadi-Pidani et al. [17]. The findings revealed that the corrosion resistance of zirconia was enhanced more than twofold by laser surface glazing due to reducing the specific reactive area of the dense glazed surface layer and, consequently, decreasing the reaction between molten salt and zirconia stabilizers. The selective laser melting of yttriastabilized zirconia ceramic was investigated by Liu et al. [18]. They showed that high-temperature preheating in 10 mm diameter range was possible with the Nd-YAG laser, and that orderly cracks were transformed into disordered little cracks by the hightemperature preheating. On the other hand, zirconia is used in dentistry because of its durability and biocompatibility. Bacterial adhesion and growth are critically important in dental implants, which are associated with the surface characteristics such as texture and surface energy. Low surface free energy reduces the interfacial bonding between the surface and adhering particles. Considerable research studies were carried out to examine surface characteristics of zirconia for the improvement of surface resistance towards the bacterial adhesion. The correlation between the surface roughness and bacterial adhesion of zirconia-porcelain veneer were examined by Kang et al. [19]. They demonstrated that a positive correlation between surface roughness and bacterial adhesion was found in glazed porcelain surface while a negative correlation was observed in zirconia surface of Cerapol group. In addition, surface roughness and bacteria adhesion were significantly influenced by the polishing method and surface material. A study on the direct silanization of zirconia for increased bio-integration was carried out by Caravaca et al. [20]. They introduced 3-aminopropyldimethylethoxy silane (APDMES) directly on the surface of zirconia (3Y-TZP) and used a plasma of oxygen to clean the surface promoting hydroxylation of the surface and increasing silane density. Treated surface displayed a qualitatively higher spreading rate in opposition to the untreated zirconia surface. Surface treatment of zirconia towards achieving improved biocompatibility was studied by Hsu et al.
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[21]. They indicated that the hydrothermal treatment of the surface resulted in phase transition from tetragonal phase to monoclinic phase and proliferation and cellular activities on the treated surface were enhanced considerably. The wetting analysis and surface characterization of flax fibers modified zirconia by sol-gel method were carried out by Boulos et al. [22]. They engineered the surface chemically while forming a modified zirconia film, which improved the surface hydrophobicity. Although laser surface texturing of ceramic surface was investigated earlier [13], the study was focused on the surface characteristics of zirconia, and dust and dry mud effects on the surfaces of zirconia were left for the future study. In addition, when the ceramic surface, such as zirconia, exposed to the outdoor environment, surface characteristics are modified by dust settlement and mud formed in the humid air ambient. Consequently, in the present study, laser processing of zirconia surface and influence of environmental dust and mud on the properties of surface is investigated. In laser surface texturing, high pressure nitrogen gas is used. Morphological and metallurgical changes in the laser textured layer are analyzed incorporating the analytical tools, which include scanning electron microscope, energy dispersive spectroscopy, and Xray diffraction. The micro-tribometer is used to assess the surface scratch resistance and friction coefficient. Surface hydrophobic characteristics are evaluated through water contact angle measurements. Environmental dust settlement and mud formation on the laser treated surface is realized while mimicking the outdoor environment. Dry mud adhesion on the laser textured surface is measured and after effects of dry mud removal from the surface is analyzed.
2. Experimental A CO2 laser (LC-ALPHAIII) with a nominal output power of 2 kW was incorporated for laser texturing of workpiece surfaces. A focusing lens of 127 mm focal length was used during laser processing, which provided the irradiated spot diameter of about 0.2 mm at the workpiece surface. High pressure nitrogen gas is used as the assisting gas during laser texturing. The laser melting parameters are given in Table 1. Zirconia wafers of 25 mm 1 mm 3 mm were used as workpieces. JEOL JDX- 3530 scanning electron microscope (SEM) was used for the morphological examination of the laser textured zirconia surface. Bruker D8 Advance XRD, having Cu Ka radiation, was utilized to assess the compounds formed on the laser textured surface. 3-dimensional image and line scan of the surface was obtained through atomic-force microscope (AFM/SPM), by Agilent, in a contact mode. The atomic force microscope (AFM) was used to examine the surface texture of the zirconia samples. The AFM probe tip had a manufacturer specified force constant k = 0.12 N/ m and it was made of silicon nitride with the tip radius of r = 20–60 nm. Kyowa (model - DM 501) goniometer was used for the contact angle measurements of workpiece surfaces. The dispensing system was utilized to control the droplet volume with 0.1 lL steps. The water droplet contact angle measurements were repeated three times at different locations of the workpiece surfaces. It is estimated that the measurement errors are in the order of 5%. The free surface energy of the laser textured surface is determined from liquid droplet method [23–27] and water, Glycerol, and Diiodomethane are used in this regard. The surface free energy determined is in the order of 49.33 mJ/m2, which is slightly less than that presented in the early study (52.6 mJ/m2) for ZrN [28]. Since water condenses onto the dust particles in humid air ambient, experiments were conducted to investigate the formation of mud from the dust particles on the laser textured zirconia sur-
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Table 1 Laser processing parameters. Feed rate (m/s)
Power (W)
Frequency (Hz)
Nozzle gap (mm)
Nozzle diameter (mm)
Focus diameter (mm)
N2 pressure (kPa)
0.1
1800
1500
1.5
1.5
0.2
600
faces. In this case, the thickness of the dust layer was measured and it was found that the dust settled over one week period gave rise to the thickness of the dust layer in the order of 300 lm. The water condensate on the dust surfaces were also measured over one week period and it was observed that the one third of weight increase of the dust particles took place after this period due to water condensate. These tests were repeated five times and the error was estimated as 7%. Consequently, the actual condensate formation was resembled in the laboratory environment via mixing of desalinated water with the dust particles with same ratio that determined from the actual tests. Therefore, actual mud formation from the dust particles in open environment was resembled onto the surface of laser textured zirconia surfaces. The mud formed on the sample surfaces was left for during over the period of six hours. The tangential force measured from linear microscratch tester (MCTX-S/N: 01-04300) was used to measure the adhesion work required for the dry mud removal. The contact load of 0.03 N and the end load of 5 N were set during the scratch testing. After completion of the scratch tests, the pressurized desalinated water jet was used to remove the dry mud from the laser textured surfaces. The water jet emerged from a nozzle with a velocity of 2 m/s. The nozzle exit diameter was 2 mm. The process of water jet removal of the dry mud was continued for 20 min. The morphological and hydrophobic characteristics of the surfaces after the dry mud removal were assessed incorporating the analytical tools. 3. Results and discussion Laser texturing of zirconia surface is realized using the high pressure nitrogen assisting gas. Dry mud adhesion and after effects on the surface characteristics are analyzed using the analytical tools. 3.1. Characteristics of laser treated zirconia surface Fig. 1 shows SEM and AFM images of the laser treated zirconia surface. Because of high repetition rate of the laser pulses (1500 Hz of pulse repetition rate), the regular patterns are developed at the workpiece surface along the scanning direction of the laser irradiation (Fig. 1a). During the laser texturing, the molten flow across these patterns does not take place. Therefore, the excessive laser heating of the surface is avoided during the laser texturing of the workpieces. In general, the irradiated surface is free from surface asperities such as large size open cavities and cracks. The formation of the crack free texture is related to the self-annealing effect of the laser scanning tracks. In this case, the heat conducted from the lately formed irradiated spots towards the early formed spots modifies the cooling rates below the surface. This behavior creates a self-annealing effect while minimizing the high thermal strain levels and thermal stresses in this region. The close examination of the surface shows that surface texture consisting of micro/nano pillars and cavities. Although the surface melting and evaporation takes place at the irradiated spot, the evaporated region is limited to the small area at the surface. This is due to the laser power intensity distribution at the surface, which follows the Gaussian distribution. Therefore, laser power intensity remains high in the irradiated spot center region and reduces towards the edge of the irradiated spot. Consequently, evaporation occurs at the close
region of the irradiated pot center because of the high laser power intensity while melting takes place in the close region of the irradiated spot edges. Moreover, the melt flow from the irradiated spot edges towards the cavity modifies the cavity geometry at the irradiated spot center. This in turn gives rise to the formation of the micro/nano pillars and shallow depth cavities at the textured surface (Fig. 1b). The average roughness of the laser textured surface is about 0.85 lm. The small size cavity, which is formed at the surface, is also visible from Fig. 1c and d, in which AFM 3dimensional image (Fig. 1c) and line scan (Fig. 1d) are shown. The cavity depth is very shallow, which is in the order of 60 nm. The composition of the surfaces due to as received and laser treated zirconia are given in Table 2. The energy dispersive spectroscopy (EDS) data reveals that the elemental composition is almost uniform at the textured surface; in which case, laser processing does not significantly change the elemental composition of the substrate material. The quantification of light elements is involved with an error in the EDS data; however, nitrogen is observed in Table 2. Therefore, the presence of nitrogen is associated with the nitride compound formation at the surface under the high temperature surface treatment process. The nitride compound such as, ZrN, can be formed through two steps reactions. Firstly, the tetragonal structure of zirconia (t-ZrO2) transforms into cubic zirconia (c-ZrO2) at high temperature at the surface of the laser treated workpiece. Later, oxygen release through the dissociation process takes place. This initiates to the formation of zirconium nitride (ZrN) at the surface at high pressure nitrogen gas ambient. Consequently, the chemical processes taking place at the treated surface can be expressed as: t-ZrO2 ? c-ZrO2 and 2ZrO2 + N2 ? ZrN + O2 It should be noted that the reactions occurring in the surface vicinity results in formation of vacancies in the zirconia [29], which modifies the surface energy. The formation of ZrN is also evident from Fig. 2, in which X-ray diffractogram of the laser textured and as received zirconia surfaces are shown. Tetragonal ZrO2 (t-ZrO2) peaks are dominant for the surface of asreceived zirconia while ZrN peaks are present for the laser treated substrate surface. The water droplets images from the contact angle measurements are shown in Fig. 3. The surface texture and surface free energy influence the wetting properties of the surface [29]. Although hierarchical micro/nano structured texture is formed at the surface, texture profile and pillar distribution vary across the treated surface. This results in non-uniform like texture pattern at the surface. In addition, the nitride compounds formed in the surface vicinity is not uniformly distributed. Therefore, the surface free energy and the texture structures alter over the area of the laser treated surface. Consequently, the water droplet contact angle varies at the surface. However, this variation is within the range of 108 ± 5–122 ± 5°. The laser textured surface demonstrates hydrophobic characteristics. This behavior is due to (i) formation of ZrN, which has relatively lower surface free energy than as received zirconia and (ii) texture profile composing of arrays of micro/nano pillars. The free surface energy of ZrN is measured using the liquid droplet contact angle and it is found to be 49.3 mJ/m2, which close to the data reported in the literature (52.6 mJ/m2) for ZrN [28]. However, the water droplet contact angle reduces at the locally scattered few regions on the surface (Fig. 3), which is related to ZrN distribution and texture profile variation on the laser treated surface, i.e. the surface has a hetero-
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a)
391
b)
d)
c)
Fig. 1. SEM micrographs and AFM images of laser textured zirconia surface: (a) regular laser scanning tracks, (b) micro-size cavity (marked in circle), (c) AFM image of 3dimensional textured surface showing micro-size cavity, and (d) AFM line scan of the surface with cavity.
Table 2 EDS data for elemental composition of laser treated workpiece surface (wt%). Spectrum
Y
N
O
Zr
As received Laser textured
5.1 4.9
0 5
65.8 58
Balance Balance
geneous interface and confining water at the air/water interface occurs. However, the Laplace pressure remains critical in assessing the hydrophobic characteristics. The Laplace pressure can be expressed as [30]:
DP ¼
2c cosðh aÞ do þ h tan a
ð1Þ
where c is the surface tension of water, h is the contact angle, a is the inclination angle, h is the height of the groove, do is the groove width within the texture, and DP = P Po. Here, P represents the pressure in the liquid of the meniscus and Po is the ambient or atmospheric pressure [30]. The Laplace pressure enhances air volume trapped in the texture, which inhibits the meniscus of the water droplet touching the texture bottom surface and causes attainment of high droplet contact angle. The Laplace pressure estimated is in the range of 0.9 104–1 104 Pa and varies locally
because of the surface texture variation. However, the water droplet contact angle reduces at some locations at the surface and the Laplace pressure calculations lost its meaning. Consequently, the meniscus of the water droplet touches the laser textured surface; in which case, the Wenzel state becomes dominant in this region. On the other hand, the water droplet contact angle on the rough surface can be related to the smooth surface by an expression [31]:
cos h ¼ f 1 ðR1 cos h þ 1Þ 1
ð2Þ
here f1 is the fraction of the solid-liquid interface and f1 = 1 for homogeneous interface (without existence of air gap), R1 is the surface e roughness factor and it can be represented in terms of the ratio of the total surface area to its flat projection, and h is the contact angle. However, the solid-liquid interface factor (f1) always remains within 0 f1 1. The geometrically composite surface texture, due to non-uniform texture characteristics, gives rise to two hydrophobic states at the laser textured surface, namely, Cassie and Baxter, and Wenzel states. Therefore, uniform distribution of the surface texture with micro/nano pillars gives rise to Cassie and Baxter state while non-uniform distribution of the pillar heights and the pillar gaps causes Wenzel state at the laser textured surface. However, the coverage area at which the Wenzel state dominates is in the order of 5% of the total laser treated area, which is considerably small.
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a)
5 μm
b)
Fig. 2. X-ray diffractogram of laser textured and as received surface.
θ = 55o
As Received Surface
θ = 108o
θ = 122o
Laser Textured Surface
θ = 85o
5 μm Fig. 4. SEM micrographs of dust particles: (a) various sizes of dust particles, and (b) small dust particles attaches large size particles.
Laser textured Surface
Laser textured Surface and Dry Mud Removed Surface
Fig. 3. Water droplet contact angles on as received, two different locations on laser textured, and laser textured and dry mud removed zirconia surface.
3.2. Characteristics of environmental dust particles and dry mud formed from dust particles on laser treated zirconia surface The dust particles are gathered from the local area of Dhahran in Saudi Arabia and analytical tools are incorporated to characterize the dust particles. Initially, the size of the dust particles are measured and the dust particles size varies in between 0.01 lm and 30 lm with the average dust particle size of 1.2 lm. Fig. 4 shows SEM micrographs of the dust particles. The dust particles have various shapes with rounded and sharp edges (Fig. 4a and b). Some of the small size dust particles (submicrometer) are attached to large size dust particles and bright appearance of these small particles demonstrates the electron charging during the SEM imaging (Fig. 4b). Therefore, these particles have electrostatic charges, which cause attachment to the large size particles. It should be noted that small particles can reside in atmosphere longer periods and they interact with solar
Table 3 Elemental composition of the dust (wt%). Si
Ca
S
Mg
Na
K
Fe
Cl
O
12.5
7.4
1.8
2.6
3.6
1.1
1.2
1.1
Balance
radiation, particularly close to the seashore environment. The ionic compounds can attach to these particles while causing electrostatic charging. Table 3 gives the elemental composition of the dust particles. Oxygen, iron, sulfur, chlorine, calcium, silicon, sodium, magnesium, and potassium are evident from the EDS data. The elemental composition of the individual dust particles remains almost same; however, the quantified data (weight % of each element) varies for each individual dust particles. This may be associated with the geological characteristics of the regions where the dust particles are emanated. Nevertheless, elemental distribution remains almost same for all the dust particles. Fig. 5 shows the XRD diffractogram of the dust particles. The peaks of various elements are observed and these include sulfur, potassium, calcium, sodium, iron and chlorine. The aluminum and silicon peaks coincided with the iron peak. The sodium and potassium peaks can be related to the sea salt compounds, since the dust are collected in the close region of the Arabian Gulf. The presence of sulfur may be associated with a monomer layer during the aging process
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Fig. 5. X-ray diffractogram of dust particles.
in the atmosphere. Sulfur is related to calcium, such as the anhydrite or gypsum component (CaSO4), in the dust. The iron is associated with clay-aggregated hematite (Fe2O3). Since water condensates on the dust particles in humid air environments and the dust particles possess alkaline (Na, K) and alkaline earth (Ca) metallic compounds, the dissolution of these compounds in water alter the chemical state of condensate water.
In order to assess this situation, water is mixed with dust particles by 1:10 ratio (10 being water) and the liquid solution from this mixture is extracted. The solution extracted is dispensed on the laser textured zirconia surface. In addition, pH of the liquid solution is measured after 6 h of mixing and it is observed that the
a)
100 μm
b)
20 μm Fig. 6. SEM micrographs of dried liquid solution: (a) crystals formed, and (b) various size dry solution crystals.
Fig. 7. SEM micrographs of dry mud surface: (a) topology of dry mud surface, and (b) small porous formed on dry mud surface due to liquid solution sediments under gravity (marked in circles).
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pH of the liquid solution increases to +7.6 showing the basic state. The increase of pH of the liquid solution is because of the presence of OH ions in the solution, which are because of the dissolution of alkaline and alkaline earth metals in water condensate. In order ro assess the characteristics of the liquid solution, the dispensed solution on the laser textured zirconia surface is left for drying in a control chamber at room temperature. Fig. 6 shows SEM micrographs of the dried solution. Crystals are formed on the zirconia surface with varying sizes (Fig. 6a and b) after drying of the mud solution. Formation of crystals is associated with the presence of OH compounds in the liquid solution [32]. Hence, the dried liquid solution adheres to zirconia surface via crystal formation upon its drying. In order to investigate the mud formation on zirconia surface due to water condensate on the dust particles, actual humid air conditions resemble and mud is formed on zirconia surface accordingly. The mud formed on zirconia surface is dried, similar to that of the liquid solution, and the dry mud characteristics are analyzed. Fig. 7 shows SEM micrographs of the dry mud surface. The dust particles at the dry mud surface attach with the fine size bonding layers in between them (Fig. 7a). However, some small porous like structures are observed at the dry mud surface (Fig. 7b). The porous like structures are related to the liquid solution, which flows towards the laser textured zirconia surface under the gravity. Fig. 8 shows SEM micrographs of the cross-section of the dried mud on the laser textured zirconia surface. The dried mud solution forms an interfacial layer between the dry mud and the laser treated surface, which appears as bright color in the micrograph (Fig. 8a). In addition,
some of the liquid solutions is captured in a cavity in between the dust particles and forms a white region in the dry mud crosssection (Fig. 8b). In order to observe the effect of dry mud residues on the laser treated zirconia surface, the dry mud is removed from the surface with a desalinated water jet of 2 mm diameter and 2 m/ s velocity, which is similar to that reported in the previous study [32]. Fig. 9 shows SEM micrographs of the dry mud removed laser textured surface. Some dust residues are observed on the laser textured surface (Fig. 9a). This behavior shows the strong adhesion of the dry mud at the surface. However, no chemical attack, resulting in pit sites or pin holes, is observed at the laser textured surface after the dry mud removal. However, some closely attached structures at the surface are observed (Fig. 9b), which are associated with the mud solution formed after desalinated water jet cleaning of the surface. The tangential force is measured during the dry mud removal from the laser textured zirconia surface by using the microtribometer. The tangential forces measured for as received zirconia and the laser textured surfaces are also shown in Fig. 10 for comparison. Tangential force variation along the scanning length remains low, which is related to the low friction coefficient of as received surface. Tangential force for the laser textured surface is also relatively lower than that of the forced required removing the dry mud from the laser treated surface. However, the tangential force resulted due to the laser textured surface remains slightly lower than as received surface. The attainment of slightly low tangential force is related to the a: low surface energy of the laser trea-
a)
b)
Fig. 8. SEM micrographs of dry mud cross-section: (a) dry mud solution at the interface between dry mud and laser textured surface (marked in circle), and (b) dry mud solution in small cavities (marked in circle).
Fig. 9. SEM micrographs of laser textured surface after dry mud removal: (a) attachment of fine size mud residues on surface, and (b) structure of mud residue on surface.
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Fig. 10. Tangential force along the scratch length for dry mud removal, laser textured surface, and as received surface.
ted surface because of nitride compound formation at the surface, and surface hardness increase after the laser texturing process, i.e. it increases from 1600 ± 40 HV (as received surface) to 1960 ± 40 HV (laser treated surface), Moreover, some small variations of tangential force occur along the scanning direction. This variation is related to the micro/nano textures of the laser textured surface. The attainment of high values of the tangential force for the dry mud removal from the laser textured surface indicates the strong adhesion between the dry mud and the laser textured surface. The strong adhesion force is because of the interlayer of dried mud solution between the dry mud and the laser textured surface, which acts as an adhesion layer at the textured surface. The adhesion work is determined from the integration of the tangential force along the scanning distance. However, the area under the tangential force for the laser textured surface is considered to be the frictional work. Therefore, subtraction of the area under the tangential force due to the dry mud removal and the frictional work results in adhesion work of the dry mud on the laser textured surface. This is estimated as 0.145 mJ, which is similar to that presented in the early study (0.155 mJ) [32].
4. Conclusion Laser gas assisted surface texturing of yttria stabilized zirconia is reaized. The combination of surface evaporation and melting results in micro/nano size pillars at the textured surface. Since the repetitive laser irradiated spots have a high frequency (1500 Hz), they give rise to the irradiated spot overlapping ratio of 72% at the surface. This in turn gives rise to regular laser scanning tracks at the treated surface. The condensation of water onto the environmental dust particles in the humid air ambient is examined and its influence on the laser textured zirconia surface is analyzed. The dust particles are collected from the local area (Dhahran) of Saudi Arabia and humid air ambient is simulated incorporating the actual local environmental conditions. Adhesion force between the dry mud, which is formed from the dust particles mimicking water condensate onto the dust particles, and laser textured zirconia surface is measured using the micro-tribometer. The adhesion work required to remove the dry mud from the laser textured zir-
conia surface is determined from the tangential force data. In order to assess the influence of the dry mud on the laser textured surfaces, a desalinated water jet is used to clean the dry mud from the surface. The dust particles and the mud residues on the dry mud removed surface are characterized using the analytical tools. In addition, the morphological and elemental changes of laser treated surface prior and after removing the dry mud are investigated. In general, the laser treated surface is free from large size asperities such as large size cracks and open voids. The nitride compounds are formed at the laser textured surfaces after laser texturing under the ambient of high pressure nitrogen assisting gas. The laser textured surfaces demonstrate hydrophobic characteristics because of the arrays of micro/nano pillars and nitride compounds formed at the surface. The dust particles possess various elements including silicon, alkaline (Na, K) and earth alkaline (Ca) metals, sulfate, oxide, and chloride compounds. The dust particles have various shapes and sizes and the average dust particles are in the order of 1.2 lm. Small dust particles have electrostatic changes and they attach to the large size particles. The alkaline and earth alkaline metallic compounds in the dust particles dissolve in water condensate while forming a chemically active liquid solution, which sediments on the laser texture surface under the gravity. The crystals with various sizes are formed on the laser textured surface upon drying of the liquid solution. The tangential force required for removal of the dry mud from the textured surface is significantly higher than the frictional force. This behavior is related to the strong adhesion between the dry mud and the textured surface. In this case, dried liquid solution in between the dry mud and the laser textured surface plays an important role towards increasing adhesion. This study provides useful information on the zirconium nitride formation through laser surface processing, which can be used for solar absorption as a selective surface. In addition, it also gives insight into the characteristics of laser textured yttriastabilized zirconia surface when subjected to the dusty environments in a humid air condition. Acknowledgements The authors acknowledge the financial support of King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia, through Project# MIT11111-11112 to accomplish this work.
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Full length article
Effect of environmental dust particles on laser textured yttria-stabilized zirconia surface in humid air ambient B.S. Yilbas a,b,⇑, H. Ali a, A. Al-Sharafi a, F. Al-Sulaiman a,b, C. Karatas c a
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 c Materials Science and Nanotechnology Engineering, Kastamonu University, Turkey b
a r t i c l e
i n f o
Article history: Received 8 October 2017 Received in revised form 31 October 2017 Accepted 29 November 2017
Keywords: Zirconia Laser texturing Hydrophobicity Environmental dust Adhesion
a b s t r a c t Zirconium nitride is used as a selective surface for concentrated solar heating applications and one of the methods to form a zirconium nitride is texturing of zirconia surface by a high intensity laser beam under high pressure nitrogen gas environment. Laser texturing also provides hydrophobic surface characteristics via forming micro/nano pillars at the surface; however, environmental dust settlement on textured surface influences the surface characteristics significantly. In the present study, laser texturing of zirconia surface and effects of the dust particles on the textured surface in a humid air ambient are investigated. Analytical tools are used to assess the morphological changes on the laser textured surface prior and after the dust settlement in the humid air ambient. It is found that laser textured surface has hydrophobic characteristics. The mud formed during condensate of water on the dust particles alters the characteristics of the laser textured surface. The tangential force required to remove the dry mud from the textured surface remains high; in which case, the dried liquid solution at the mud-textured surface interface is responsible for the strong adhesion of the dry mud on the textured surface. The textured surface becomes hydrophilic after the dry mud was removed from the surface by a desalinated water jet. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Solar energy harvesting has a crucial role for widening renewable energy applications around the Globe. Utilization of high temperature resistant ceramic components becomes unavoidable for solar thermal applications. Zirconium nitride can serve as a selective surface for solar energy harvesting in thermal systems, which is because of excellent optical properties in terms of absorption and emission [1]. On the other hand, climate change has imitated dust storms around the Middle East [2]. The dust settlement on solar energy harvesting surfaces lowers device performance in terms of output power and efficiency [3]. The environmental dust particles compose of various elements and compounds including alkaline and alkaline earth metals [4]. In humid air ambient, water condensates on the dust particles and some compounds of the dust particles dissolve into water condensate. This forms a chemically active liquid solution, which accumulates at the interface of the device surface and the dust particles under the gravity [4]. The chemically active liquid solution causes some asperities on 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). https://doi.org/10.1016/j.optlastec.2017.11.052 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
device surface, such as pin holes and pit sites, while causing permeant damages [5]. In addition, once the liquid solution dries at the device surface, adhesion between the dried liquid and the surface becomes significantly strong and the efforts required removing the dried solution and the mud from the surface increase significantly. One of the solutions to create self-cleaning characteristics at the surface is to improve surface hydrophobicity. In general, surface hydrophobicity is associated with the micro/nano scale surface texture and low surface free energy. However, plane yttria-stabilized zirconia surface has high surface free energy and demonstrates hydrophilic characteristics. Consequently, altering characteristics of zirconia surface through surface processing becomes essential to generate surface hydrophobicity. Several methods have been suggested and many techniques were reported for generating hydrophobic characteristics at the surfaces [6–11]. The techniques reported were involved with multi-steps processes and harsh conditions. Some of these processes include phase separation [6], electrochemical deposition [7], plasma treatment [8], sol-gel processing [9], electrospinning [10], and solution immersion [11]. Laser gas assisted texturing offers considerable advantages over the multi-step processes for hydrophobized surfaces, which is particularly true for ceramic surfaces [12]. Some of these advantages include high speed processing, the precision
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of operation, and low cost. In laser texturing process, a combination of melting and evaporation of ceramic surface takes place via ablation [12,13]. This in turn generates surface texture consisting of micro/nano poles and cavities. The use of high pressure assisting gas, such as nitrogen, generates nitride compounds on the laser textured surface [12]; in which case, nitride compounds have low surface energy than the oxides [12]. In addition, arrays of micro/nano size pole/pillar improve the water repelling capacity of the laser textured surface. However, dust settlement on the laser textured surface changes the surface characteristics such as surface hydrophobicity. Hence, investigation of environmental dust particle adhesion and influence of dried mud solution on the characteristics of laser textured zirconia surface becomes essential. Considerable research studies were carried out to examine laser processing of zirconia tiles. Laser micromachining of yttriatetragonal zirconia polycrystal ceramic was carried out by Li et al. [14]. They demonstrated that crack and asperity free machined surfaces were possible under the precise control of laser treatment parameters. Laser short-pulse processing of yttriastabilized tetragonal zirconia surfaces was carried out by Oyane et al. [15]. They indicated that submicro-/micro-structures on the zirconia surface were obtained after laser ablation and laser treated surface showed increased water wettability. A parametric study of laser surface patterning of zirconia was carried out by Roitero et al. [16]. They showed that increasing both fluence and number of pulses resulted in deep texture patterns at the surface. However, increasing number of pulses had a detrimental effect on the quality of the textured surfaces; in which case, surface damage, such as intergranular cracking, open porosity and nanodroplets formation, could be generated after the laser treatment process. Laser surface treatment of zirconia and the corrosion resistance of the resulting surface were studied by Ahmadi-Pidani et al. [17]. The findings revealed that the corrosion resistance of zirconia was enhanced more than twofold by laser surface glazing due to reducing the specific reactive area of the dense glazed surface layer and, consequently, decreasing the reaction between molten salt and zirconia stabilizers. The selective laser melting of yttriastabilized zirconia ceramic was investigated by Liu et al. [18]. They showed that high-temperature preheating in 10 mm diameter range was possible with the Nd-YAG laser, and that orderly cracks were transformed into disordered little cracks by the hightemperature preheating. On the other hand, zirconia is used in dentistry because of its durability and biocompatibility. Bacterial adhesion and growth are critically important in dental implants, which are associated with the surface characteristics such as texture and surface energy. Low surface free energy reduces the interfacial bonding between the surface and adhering particles. Considerable research studies were carried out to examine surface characteristics of zirconia for the improvement of surface resistance towards the bacterial adhesion. The correlation between the surface roughness and bacterial adhesion of zirconia-porcelain veneer were examined by Kang et al. [19]. They demonstrated that a positive correlation between surface roughness and bacterial adhesion was found in glazed porcelain surface while a negative correlation was observed in zirconia surface of Cerapol group. In addition, surface roughness and bacteria adhesion were significantly influenced by the polishing method and surface material. A study on the direct silanization of zirconia for increased bio-integration was carried out by Caravaca et al. [20]. They introduced 3-aminopropyldimethylethoxy silane (APDMES) directly on the surface of zirconia (3Y-TZP) and used a plasma of oxygen to clean the surface promoting hydroxylation of the surface and increasing silane density. Treated surface displayed a qualitatively higher spreading rate in opposition to the untreated zirconia surface. Surface treatment of zirconia towards achieving improved biocompatibility was studied by Hsu et al.
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[21]. They indicated that the hydrothermal treatment of the surface resulted in phase transition from tetragonal phase to monoclinic phase and proliferation and cellular activities on the treated surface were enhanced considerably. The wetting analysis and surface characterization of flax fibers modified zirconia by sol-gel method were carried out by Boulos et al. [22]. They engineered the surface chemically while forming a modified zirconia film, which improved the surface hydrophobicity. Although laser surface texturing of ceramic surface was investigated earlier [13], the study was focused on the surface characteristics of zirconia, and dust and dry mud effects on the surfaces of zirconia were left for the future study. In addition, when the ceramic surface, such as zirconia, exposed to the outdoor environment, surface characteristics are modified by dust settlement and mud formed in the humid air ambient. Consequently, in the present study, laser processing of zirconia surface and influence of environmental dust and mud on the properties of surface is investigated. In laser surface texturing, high pressure nitrogen gas is used. Morphological and metallurgical changes in the laser textured layer are analyzed incorporating the analytical tools, which include scanning electron microscope, energy dispersive spectroscopy, and Xray diffraction. The micro-tribometer is used to assess the surface scratch resistance and friction coefficient. Surface hydrophobic characteristics are evaluated through water contact angle measurements. Environmental dust settlement and mud formation on the laser treated surface is realized while mimicking the outdoor environment. Dry mud adhesion on the laser textured surface is measured and after effects of dry mud removal from the surface is analyzed.
2. Experimental A CO2 laser (LC-ALPHAIII) with a nominal output power of 2 kW was incorporated for laser texturing of workpiece surfaces. A focusing lens of 127 mm focal length was used during laser processing, which provided the irradiated spot diameter of about 0.2 mm at the workpiece surface. High pressure nitrogen gas is used as the assisting gas during laser texturing. The laser melting parameters are given in Table 1. Zirconia wafers of 25 mm 1 mm 3 mm were used as workpieces. JEOL JDX- 3530 scanning electron microscope (SEM) was used for the morphological examination of the laser textured zirconia surface. Bruker D8 Advance XRD, having Cu Ka radiation, was utilized to assess the compounds formed on the laser textured surface. 3-dimensional image and line scan of the surface was obtained through atomic-force microscope (AFM/SPM), by Agilent, in a contact mode. The atomic force microscope (AFM) was used to examine the surface texture of the zirconia samples. The AFM probe tip had a manufacturer specified force constant k = 0.12 N/ m and it was made of silicon nitride with the tip radius of r = 20–60 nm. Kyowa (model - DM 501) goniometer was used for the contact angle measurements of workpiece surfaces. The dispensing system was utilized to control the droplet volume with 0.1 lL steps. The water droplet contact angle measurements were repeated three times at different locations of the workpiece surfaces. It is estimated that the measurement errors are in the order of 5%. The free surface energy of the laser textured surface is determined from liquid droplet method [23–27] and water, Glycerol, and Diiodomethane are used in this regard. The surface free energy determined is in the order of 49.33 mJ/m2, which is slightly less than that presented in the early study (52.6 mJ/m2) for ZrN [28]. Since water condenses onto the dust particles in humid air ambient, experiments were conducted to investigate the formation of mud from the dust particles on the laser textured zirconia sur-
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Table 1 Laser processing parameters. Feed rate (m/s)
Power (W)
Frequency (Hz)
Nozzle gap (mm)
Nozzle diameter (mm)
Focus diameter (mm)
N2 pressure (kPa)
0.1
1800
1500
1.5
1.5
0.2
600
faces. In this case, the thickness of the dust layer was measured and it was found that the dust settled over one week period gave rise to the thickness of the dust layer in the order of 300 lm. The water condensate on the dust surfaces were also measured over one week period and it was observed that the one third of weight increase of the dust particles took place after this period due to water condensate. These tests were repeated five times and the error was estimated as 7%. Consequently, the actual condensate formation was resembled in the laboratory environment via mixing of desalinated water with the dust particles with same ratio that determined from the actual tests. Therefore, actual mud formation from the dust particles in open environment was resembled onto the surface of laser textured zirconia surfaces. The mud formed on the sample surfaces was left for during over the period of six hours. The tangential force measured from linear microscratch tester (MCTX-S/N: 01-04300) was used to measure the adhesion work required for the dry mud removal. The contact load of 0.03 N and the end load of 5 N were set during the scratch testing. After completion of the scratch tests, the pressurized desalinated water jet was used to remove the dry mud from the laser textured surfaces. The water jet emerged from a nozzle with a velocity of 2 m/s. The nozzle exit diameter was 2 mm. The process of water jet removal of the dry mud was continued for 20 min. The morphological and hydrophobic characteristics of the surfaces after the dry mud removal were assessed incorporating the analytical tools. 3. Results and discussion Laser texturing of zirconia surface is realized using the high pressure nitrogen assisting gas. Dry mud adhesion and after effects on the surface characteristics are analyzed using the analytical tools. 3.1. Characteristics of laser treated zirconia surface Fig. 1 shows SEM and AFM images of the laser treated zirconia surface. Because of high repetition rate of the laser pulses (1500 Hz of pulse repetition rate), the regular patterns are developed at the workpiece surface along the scanning direction of the laser irradiation (Fig. 1a). During the laser texturing, the molten flow across these patterns does not take place. Therefore, the excessive laser heating of the surface is avoided during the laser texturing of the workpieces. In general, the irradiated surface is free from surface asperities such as large size open cavities and cracks. The formation of the crack free texture is related to the self-annealing effect of the laser scanning tracks. In this case, the heat conducted from the lately formed irradiated spots towards the early formed spots modifies the cooling rates below the surface. This behavior creates a self-annealing effect while minimizing the high thermal strain levels and thermal stresses in this region. The close examination of the surface shows that surface texture consisting of micro/nano pillars and cavities. Although the surface melting and evaporation takes place at the irradiated spot, the evaporated region is limited to the small area at the surface. This is due to the laser power intensity distribution at the surface, which follows the Gaussian distribution. Therefore, laser power intensity remains high in the irradiated spot center region and reduces towards the edge of the irradiated spot. Consequently, evaporation occurs at the close
region of the irradiated pot center because of the high laser power intensity while melting takes place in the close region of the irradiated spot edges. Moreover, the melt flow from the irradiated spot edges towards the cavity modifies the cavity geometry at the irradiated spot center. This in turn gives rise to the formation of the micro/nano pillars and shallow depth cavities at the textured surface (Fig. 1b). The average roughness of the laser textured surface is about 0.85 lm. The small size cavity, which is formed at the surface, is also visible from Fig. 1c and d, in which AFM 3dimensional image (Fig. 1c) and line scan (Fig. 1d) are shown. The cavity depth is very shallow, which is in the order of 60 nm. The composition of the surfaces due to as received and laser treated zirconia are given in Table 2. The energy dispersive spectroscopy (EDS) data reveals that the elemental composition is almost uniform at the textured surface; in which case, laser processing does not significantly change the elemental composition of the substrate material. The quantification of light elements is involved with an error in the EDS data; however, nitrogen is observed in Table 2. Therefore, the presence of nitrogen is associated with the nitride compound formation at the surface under the high temperature surface treatment process. The nitride compound such as, ZrN, can be formed through two steps reactions. Firstly, the tetragonal structure of zirconia (t-ZrO2) transforms into cubic zirconia (c-ZrO2) at high temperature at the surface of the laser treated workpiece. Later, oxygen release through the dissociation process takes place. This initiates to the formation of zirconium nitride (ZrN) at the surface at high pressure nitrogen gas ambient. Consequently, the chemical processes taking place at the treated surface can be expressed as: t-ZrO2 ? c-ZrO2 and 2ZrO2 + N2 ? ZrN + O2 It should be noted that the reactions occurring in the surface vicinity results in formation of vacancies in the zirconia [29], which modifies the surface energy. The formation of ZrN is also evident from Fig. 2, in which X-ray diffractogram of the laser textured and as received zirconia surfaces are shown. Tetragonal ZrO2 (t-ZrO2) peaks are dominant for the surface of asreceived zirconia while ZrN peaks are present for the laser treated substrate surface. The water droplets images from the contact angle measurements are shown in Fig. 3. The surface texture and surface free energy influence the wetting properties of the surface [29]. Although hierarchical micro/nano structured texture is formed at the surface, texture profile and pillar distribution vary across the treated surface. This results in non-uniform like texture pattern at the surface. In addition, the nitride compounds formed in the surface vicinity is not uniformly distributed. Therefore, the surface free energy and the texture structures alter over the area of the laser treated surface. Consequently, the water droplet contact angle varies at the surface. However, this variation is within the range of 108 ± 5–122 ± 5°. The laser textured surface demonstrates hydrophobic characteristics. This behavior is due to (i) formation of ZrN, which has relatively lower surface free energy than as received zirconia and (ii) texture profile composing of arrays of micro/nano pillars. The free surface energy of ZrN is measured using the liquid droplet contact angle and it is found to be 49.3 mJ/m2, which close to the data reported in the literature (52.6 mJ/m2) for ZrN [28]. However, the water droplet contact angle reduces at the locally scattered few regions on the surface (Fig. 3), which is related to ZrN distribution and texture profile variation on the laser treated surface, i.e. the surface has a hetero-
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a)
391
b)
d)
c)
Fig. 1. SEM micrographs and AFM images of laser textured zirconia surface: (a) regular laser scanning tracks, (b) micro-size cavity (marked in circle), (c) AFM image of 3dimensional textured surface showing micro-size cavity, and (d) AFM line scan of the surface with cavity.
Table 2 EDS data for elemental composition of laser treated workpiece surface (wt%). Spectrum
Y
N
O
Zr
As received Laser textured
5.1 4.9
0 5
65.8 58
Balance Balance
geneous interface and confining water at the air/water interface occurs. However, the Laplace pressure remains critical in assessing the hydrophobic characteristics. The Laplace pressure can be expressed as [30]:
DP ¼
2c cosðh aÞ do þ h tan a
ð1Þ
where c is the surface tension of water, h is the contact angle, a is the inclination angle, h is the height of the groove, do is the groove width within the texture, and DP = P Po. Here, P represents the pressure in the liquid of the meniscus and Po is the ambient or atmospheric pressure [30]. The Laplace pressure enhances air volume trapped in the texture, which inhibits the meniscus of the water droplet touching the texture bottom surface and causes attainment of high droplet contact angle. The Laplace pressure estimated is in the range of 0.9 104–1 104 Pa and varies locally
because of the surface texture variation. However, the water droplet contact angle reduces at some locations at the surface and the Laplace pressure calculations lost its meaning. Consequently, the meniscus of the water droplet touches the laser textured surface; in which case, the Wenzel state becomes dominant in this region. On the other hand, the water droplet contact angle on the rough surface can be related to the smooth surface by an expression [31]:
cos h ¼ f 1 ðR1 cos h þ 1Þ 1
ð2Þ
here f1 is the fraction of the solid-liquid interface and f1 = 1 for homogeneous interface (without existence of air gap), R1 is the surface e roughness factor and it can be represented in terms of the ratio of the total surface area to its flat projection, and h is the contact angle. However, the solid-liquid interface factor (f1) always remains within 0 f1 1. The geometrically composite surface texture, due to non-uniform texture characteristics, gives rise to two hydrophobic states at the laser textured surface, namely, Cassie and Baxter, and Wenzel states. Therefore, uniform distribution of the surface texture with micro/nano pillars gives rise to Cassie and Baxter state while non-uniform distribution of the pillar heights and the pillar gaps causes Wenzel state at the laser textured surface. However, the coverage area at which the Wenzel state dominates is in the order of 5% of the total laser treated area, which is considerably small.
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a)
5 μm
b)
Fig. 2. X-ray diffractogram of laser textured and as received surface.
θ = 55o
As Received Surface
θ = 108o
θ = 122o
Laser Textured Surface
θ = 85o
5 μm Fig. 4. SEM micrographs of dust particles: (a) various sizes of dust particles, and (b) small dust particles attaches large size particles.
Laser textured Surface
Laser textured Surface and Dry Mud Removed Surface
Fig. 3. Water droplet contact angles on as received, two different locations on laser textured, and laser textured and dry mud removed zirconia surface.
3.2. Characteristics of environmental dust particles and dry mud formed from dust particles on laser treated zirconia surface The dust particles are gathered from the local area of Dhahran in Saudi Arabia and analytical tools are incorporated to characterize the dust particles. Initially, the size of the dust particles are measured and the dust particles size varies in between 0.01 lm and 30 lm with the average dust particle size of 1.2 lm. Fig. 4 shows SEM micrographs of the dust particles. The dust particles have various shapes with rounded and sharp edges (Fig. 4a and b). Some of the small size dust particles (submicrometer) are attached to large size dust particles and bright appearance of these small particles demonstrates the electron charging during the SEM imaging (Fig. 4b). Therefore, these particles have electrostatic charges, which cause attachment to the large size particles. It should be noted that small particles can reside in atmosphere longer periods and they interact with solar
Table 3 Elemental composition of the dust (wt%). Si
Ca
S
Mg
Na
K
Fe
Cl
O
12.5
7.4
1.8
2.6
3.6
1.1
1.2
1.1
Balance
radiation, particularly close to the seashore environment. The ionic compounds can attach to these particles while causing electrostatic charging. Table 3 gives the elemental composition of the dust particles. Oxygen, iron, sulfur, chlorine, calcium, silicon, sodium, magnesium, and potassium are evident from the EDS data. The elemental composition of the individual dust particles remains almost same; however, the quantified data (weight % of each element) varies for each individual dust particles. This may be associated with the geological characteristics of the regions where the dust particles are emanated. Nevertheless, elemental distribution remains almost same for all the dust particles. Fig. 5 shows the XRD diffractogram of the dust particles. The peaks of various elements are observed and these include sulfur, potassium, calcium, sodium, iron and chlorine. The aluminum and silicon peaks coincided with the iron peak. The sodium and potassium peaks can be related to the sea salt compounds, since the dust are collected in the close region of the Arabian Gulf. The presence of sulfur may be associated with a monomer layer during the aging process
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Fig. 5. X-ray diffractogram of dust particles.
in the atmosphere. Sulfur is related to calcium, such as the anhydrite or gypsum component (CaSO4), in the dust. The iron is associated with clay-aggregated hematite (Fe2O3). Since water condensates on the dust particles in humid air environments and the dust particles possess alkaline (Na, K) and alkaline earth (Ca) metallic compounds, the dissolution of these compounds in water alter the chemical state of condensate water.
In order to assess this situation, water is mixed with dust particles by 1:10 ratio (10 being water) and the liquid solution from this mixture is extracted. The solution extracted is dispensed on the laser textured zirconia surface. In addition, pH of the liquid solution is measured after 6 h of mixing and it is observed that the
a)
100 μm
b)
20 μm Fig. 6. SEM micrographs of dried liquid solution: (a) crystals formed, and (b) various size dry solution crystals.
Fig. 7. SEM micrographs of dry mud surface: (a) topology of dry mud surface, and (b) small porous formed on dry mud surface due to liquid solution sediments under gravity (marked in circles).
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pH of the liquid solution increases to +7.6 showing the basic state. The increase of pH of the liquid solution is because of the presence of OH ions in the solution, which are because of the dissolution of alkaline and alkaline earth metals in water condensate. In order ro assess the characteristics of the liquid solution, the dispensed solution on the laser textured zirconia surface is left for drying in a control chamber at room temperature. Fig. 6 shows SEM micrographs of the dried solution. Crystals are formed on the zirconia surface with varying sizes (Fig. 6a and b) after drying of the mud solution. Formation of crystals is associated with the presence of OH compounds in the liquid solution [32]. Hence, the dried liquid solution adheres to zirconia surface via crystal formation upon its drying. In order to investigate the mud formation on zirconia surface due to water condensate on the dust particles, actual humid air conditions resemble and mud is formed on zirconia surface accordingly. The mud formed on zirconia surface is dried, similar to that of the liquid solution, and the dry mud characteristics are analyzed. Fig. 7 shows SEM micrographs of the dry mud surface. The dust particles at the dry mud surface attach with the fine size bonding layers in between them (Fig. 7a). However, some small porous like structures are observed at the dry mud surface (Fig. 7b). The porous like structures are related to the liquid solution, which flows towards the laser textured zirconia surface under the gravity. Fig. 8 shows SEM micrographs of the cross-section of the dried mud on the laser textured zirconia surface. The dried mud solution forms an interfacial layer between the dry mud and the laser treated surface, which appears as bright color in the micrograph (Fig. 8a). In addition,
some of the liquid solutions is captured in a cavity in between the dust particles and forms a white region in the dry mud crosssection (Fig. 8b). In order to observe the effect of dry mud residues on the laser treated zirconia surface, the dry mud is removed from the surface with a desalinated water jet of 2 mm diameter and 2 m/ s velocity, which is similar to that reported in the previous study [32]. Fig. 9 shows SEM micrographs of the dry mud removed laser textured surface. Some dust residues are observed on the laser textured surface (Fig. 9a). This behavior shows the strong adhesion of the dry mud at the surface. However, no chemical attack, resulting in pit sites or pin holes, is observed at the laser textured surface after the dry mud removal. However, some closely attached structures at the surface are observed (Fig. 9b), which are associated with the mud solution formed after desalinated water jet cleaning of the surface. The tangential force is measured during the dry mud removal from the laser textured zirconia surface by using the microtribometer. The tangential forces measured for as received zirconia and the laser textured surfaces are also shown in Fig. 10 for comparison. Tangential force variation along the scanning length remains low, which is related to the low friction coefficient of as received surface. Tangential force for the laser textured surface is also relatively lower than that of the forced required removing the dry mud from the laser treated surface. However, the tangential force resulted due to the laser textured surface remains slightly lower than as received surface. The attainment of slightly low tangential force is related to the a: low surface energy of the laser trea-
a)
b)
Fig. 8. SEM micrographs of dry mud cross-section: (a) dry mud solution at the interface between dry mud and laser textured surface (marked in circle), and (b) dry mud solution in small cavities (marked in circle).
Fig. 9. SEM micrographs of laser textured surface after dry mud removal: (a) attachment of fine size mud residues on surface, and (b) structure of mud residue on surface.
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Fig. 10. Tangential force along the scratch length for dry mud removal, laser textured surface, and as received surface.
ted surface because of nitride compound formation at the surface, and surface hardness increase after the laser texturing process, i.e. it increases from 1600 ± 40 HV (as received surface) to 1960 ± 40 HV (laser treated surface), Moreover, some small variations of tangential force occur along the scanning direction. This variation is related to the micro/nano textures of the laser textured surface. The attainment of high values of the tangential force for the dry mud removal from the laser textured surface indicates the strong adhesion between the dry mud and the laser textured surface. The strong adhesion force is because of the interlayer of dried mud solution between the dry mud and the laser textured surface, which acts as an adhesion layer at the textured surface. The adhesion work is determined from the integration of the tangential force along the scanning distance. However, the area under the tangential force for the laser textured surface is considered to be the frictional work. Therefore, subtraction of the area under the tangential force due to the dry mud removal and the frictional work results in adhesion work of the dry mud on the laser textured surface. This is estimated as 0.145 mJ, which is similar to that presented in the early study (0.155 mJ) [32].
4. Conclusion Laser gas assisted surface texturing of yttria stabilized zirconia is reaized. The combination of surface evaporation and melting results in micro/nano size pillars at the textured surface. Since the repetitive laser irradiated spots have a high frequency (1500 Hz), they give rise to the irradiated spot overlapping ratio of 72% at the surface. This in turn gives rise to regular laser scanning tracks at the treated surface. The condensation of water onto the environmental dust particles in the humid air ambient is examined and its influence on the laser textured zirconia surface is analyzed. The dust particles are collected from the local area (Dhahran) of Saudi Arabia and humid air ambient is simulated incorporating the actual local environmental conditions. Adhesion force between the dry mud, which is formed from the dust particles mimicking water condensate onto the dust particles, and laser textured zirconia surface is measured using the micro-tribometer. The adhesion work required to remove the dry mud from the laser textured zir-
conia surface is determined from the tangential force data. In order to assess the influence of the dry mud on the laser textured surfaces, a desalinated water jet is used to clean the dry mud from the surface. The dust particles and the mud residues on the dry mud removed surface are characterized using the analytical tools. In addition, the morphological and elemental changes of laser treated surface prior and after removing the dry mud are investigated. In general, the laser treated surface is free from large size asperities such as large size cracks and open voids. The nitride compounds are formed at the laser textured surfaces after laser texturing under the ambient of high pressure nitrogen assisting gas. The laser textured surfaces demonstrate hydrophobic characteristics because of the arrays of micro/nano pillars and nitride compounds formed at the surface. The dust particles possess various elements including silicon, alkaline (Na, K) and earth alkaline (Ca) metals, sulfate, oxide, and chloride compounds. The dust particles have various shapes and sizes and the average dust particles are in the order of 1.2 lm. Small dust particles have electrostatic changes and they attach to the large size particles. The alkaline and earth alkaline metallic compounds in the dust particles dissolve in water condensate while forming a chemically active liquid solution, which sediments on the laser texture surface under the gravity. The crystals with various sizes are formed on the laser textured surface upon drying of the liquid solution. The tangential force required for removal of the dry mud from the textured surface is significantly higher than the frictional force. This behavior is related to the strong adhesion between the dry mud and the textured surface. In this case, dried liquid solution in between the dry mud and the laser textured surface plays an important role towards increasing adhesion. This study provides useful information on the zirconium nitride formation through laser surface processing, which can be used for solar absorption as a selective surface. In addition, it also gives insight into the characteristics of laser textured yttriastabilized zirconia surface when subjected to the dusty environments in a humid air condition. Acknowledgements The authors acknowledge the financial support of King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia, through Project# MIT11111-11112 to accomplish this work.
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