III. EXPERIMENTAL DETAILS
3.7 Wetting test and analysis
3.7.3 Theoretical Analysis
3.7.3.3 High temperature viscosity of glass
From Vogel-Filcher-Tamman (VFT) equation, we can be estimate the glass viscosity at particular temperature range is shown in Eq. (32)
Log η = A + [B/(T-To)] (32)
Where A=5.87, B =234.6 and T =490 are constants.
4 RESULTS AND DISCUSSION
4.1 Variation of Al2O3 Coating Morphologies with Withdrawal Speed
Hydrolysis and polycondensation reactions of the alkoxy and hydroxyl group take place during the deposition and drying stages at the room temperature. The cross-sectional SEM image shows the dried Al2O3 film composed by aggregated nanoparticles with dense structure and smooth surface, as shown in Figure 4-1. A lower drawing speed favors the deposited Al2O3 film with a better uniformity and dense. The thickness of Al2O3 film was increased with the drawing speed (Figure 4-2) and the variation of film thickness is less than 10 % with respect to withdrawal speed was identified from the Graph between the withdrawal speed (mm/min) and film thickness (nm) as shown in Figure 4-3. The XRD results present nanocrystallined structure after annealed treatment. Annealing treatment allows the Al2O3 films to complete further densification by diffusion.
Figure: 4-1 Effect of drawing speed on the surface morphologies of Al2O3 films (a) 20 mm/min (b) 100 mm/min (c) 200 mm/min.
4.2 Thickness of Al2O3 Coating Vs Withdrawal Speed
Figure: 4-2 Effect of withdrawal speed on the thickness of Al2O3 films (a) 20 mm/min (b) 100 mm/min (c) 200 mm/min.
Figure: 4-3 Graph between the withdrawal speed (mm/min) and film thickness (nm).
Up to 2μm thickness of sol-gel derived films can be achieved through dip coating process.
Uniformity and cracks free surface coatings were observed from the OM analysis. These sol-gel derived SiO2 and Al2O3 coatings exhibit amorphous-crystalline phase transformation at 300°C and 350°C respectively. Heat treated at 650°C, both water based sol-gel SiO2 and Al2O3 coatings become fully crystalline structure and the grain size increases with increasing the heating duration as shown in Figure 4-4. The grain size increased to 500nm when the sintering temperature was up to 800°C; however, the surface becomes rough due to the crystal facet effect.
Figure: 4-4 SEM micrographs of (a) SiO2 and (b) Al2O3 coated substrates after heat treatment process carried out at 650°C for 30minutes.
A 3D optical imaging profiler was used to evaluate the topographies of the Al2O3 coated mold before and after wetting tested mold surface (shown in Figure 4-5). Based on the measurement, the average roughness of Al2O3 Coated mold (Ra) is 45.41nm and average roughness value (Ra) is 62.3nm at where the glass contacted with coated mold surface. Sharp peaks intimate raising the surface roughness of coated mold which is in contact with molten glass from left portion of Figure 4-5.
Figure: 4-5 Topography of coated mold surface after wetting test.
4.3 Differential Thermogravimetry (DTG) Analysis of Al2O3 Coatings
Hydrolysis and polycondensation reactions of the alkoxy and hydroxyl group take place during the deposition and drying stages at the room temperature. pH value of water based Al2O3
sol-gel coating is approximately 4.1. From the DTG curve at 100°C, weight loss peak appeared due to the evaporation of the solvent (water) from the Al2O3 coating surface as shown in Figure 4-6. From DTG spectra the temperature range 150 to 570°C, it has thermal effects due to shrinkage of aggregated nanoparticles in the coating. Moreover, during this range, polycrystallization processes associated inside the coating material. Above the 570°C temperature range, DTG curve manifested crystallization peaks from Figure 4-6.
Figure: 4-6 Differential thermogravimetry (DTG) spectra of Al2O3 coated glass preform.
4.4 Physico-Optical Properties of Al2O3 Coatings 4.4.1 Transmittance of Al2O3 Coated Glass Preform
Coating’s light transmittance was evaluated by UV-vis spectrophotometer. UV-spectra of transparency of glass preform before and after Al2O3 sol-gel coating. The traces are very similar, indicating that the transmittance remains unaffected by deposited Al2O3 sol-gel coating as shown in Figure 4-7.
Figure: 4-7 UV-spectra of transparency of glass gob before and after Al2O3 sol-gel coating. The traces are very similar, indicating that the transmittance remains unaffected by deposited Al2O3 sol-gel coating.
4.4.2 Al2O3 Coated Glass after Scratch Test
The surface morphology of Al2O3 film after scratch test was shown in Figure 4-8. The detached Al2O3 film presents obvious plastic deformation and ductile failure appearance. It is believed that the Al2O3 film composed by aggregated nanoparticles, which are able to flow to accommodate a large amount of ploughing and associated shear stress through densification and shear deformation. There is no stress concentration at flaws in nanoparticle aggregated ultra-thin film. The critical applied stress for fracture of ultra-thin Al O film may become infinite or
particles may dominate fracture behavior of Al2O3 film.
Figure: 4-8 Surface mophologies of Al2O3 film coated glass after scratch test.
4.5 Glass Wetting Test
4.5.1 Glass on Mold
4.5.1.1 Spreading Kinetics of Glass Preform on Mold Surface
Figure 4-9 shows the variation of contact angle and contact area radius with respect to holding time for glass ball on uncoated stainless steel substrate at 800°C. The contact angle rapidly decreases from initial and then gradually approaches a stable value after 2 minutes holding time. However, the contact radius keeps increasing with the holding time and finally spreads completely over the substrate. The final contact angle and contact radius are 29° and 5.5 mm respectively. The glass remains adhered with substrate after cooling down to room
temperature due to the formation of the oxide layer at interface; it constitutes strong chemical bonds at interface.
0 50 100 150 200 250 300
0 20 40 60 80 100 120 140 160
Contact Radius( mm)
Holding time ( Sec)
Contact angle ( degrees)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
(b) (a)
Figure: 4-9 Variation of (a) Contact angle and (b) contact area radius as a function of time for molten glass on the uncoated stainless steel substrate at 800°C for 5- minute holding time.
The interfacial interaction between the substrate and molten glass has been investigated through EDX analysis and elements mapping (Figure 4-11 and 4-12). The reaction products mainly consist of Ni, Cr, Ba, Fe, Zn and O mixed compounds. This indicates that high affinity reactive elements, Cr and Fe enhanced chemical reactions, resulting in the formation of Cr-O and Fe-O based compound layer at interface. It has also observed that some of the reactive elements Si and Ba diffused out from the molten glass into the substrate.
From elements mapping analysis of uncoated glass/substrate interface was good evidence of noticeable reactive elements diffusion at interface (representative view as shown in Figure 4-10). In generally, the nature of the interface is also influenced by external perturbations. Both side of reactive elements are actively involved into reaction at interface and formed mixed product through redox process. From EDX and mapping analysis, the redox processes enhanced between the stainless steel (uncoated) and the glass materials at interface as given below;
From the substrate
4/3 Cr(s) + O2(g) = 2/3 Cr2O3(s) (33)
2 Ni(s) + O2(g) = 2 NiO(s) (34)
2 Fe(s) + O2(g) = 2FeO(s) (35)
From the glass
Si(s) + O2(g) = SiO2(s) (36)
2 Ba(s) + O2(g) = 2 BaO(s) (37)
However, in reality some oxides may carry covalent bonds only, no actual electron transfer from the metal to the oxygen. In oxide systems, the change of standard Gibbs free energy can be represented by a general equation of the form [1, 2]
(38)
In our case, according to redox reactions for stainless steel and glass interaction the above eq. (38) was modified to [1, 2]
(39)
Where A, B and C are three experimentally determined constants for a particular system. The values of these constants for a number of oxide system of redox reactions as mentioned above is given in Table 4-1.
Table: 4-1 Representation of standard free energy of redox reactions in stainless steel/glass interface at 1098K.
Reaction -A C - (calories)
178500 41.1 133372.2 61.131
116900 47.1 65184.2 29.877
124100 29.9 91269.8 41.833
215600 41.5 170033 77.935
271600 6.4 220652.8 101.136
The free energy change is related to the equilibrium constant of any chemical reaction by the equation: [1, 2, 148]
ΔG°T = RT lnPO2 (40)
Thus by knowing of any oxidation reaction, the equilibrium oxygen pressure (lnPO2)
estimated that standard Gibbs free energy of formatted SiO2 elements in interface at 1073K is much smaller than the same at 298K (- =192100calories). In other hand, for barium oxide (BaO) pieces in interface at 1073K is higher than at 298K. It believed that barium oxide encourage the adhesion or strong hermetic bonding through diffusion at interface (example:
uncertainty of sodium (Na) and potassium (K) volatile elements in glass at high temperature). It believed that by reduce or avoid the BaO percentage in main glass composition, which discriminate the less tendency of glass sticking with substrate material from our study.
Figure: 4-10 Cross-sectional views at the interface of the molten glass/uncoated stainless steel substrate.
It was observed that the ratio of the work of adhesion and cohesion is 98% for the case of glass on the uncoated substrate at 800°C, indicating a strong adhesion, which is ensured by effective chemical bonding through transfer of the mass or forces. The value of surface energy for uncoated stainless steel substrate has been obtained from the sessile drop method is about 0.128 Nm-1 and the calculated work of adhesion is about 0.249 Nm-1. Molten glass completely
spread over the substrate and sticking is confirmed at room temperature. Spreading coefficient has been obtained as -0.00856 Nm-1. The surface tension (σ) for the test carried out in oxygen–
free environment is higher than that of oxygen-containing ones [148]. The occurrence of inter-diffusion usually results in good adhesion/wetting.
Figure: 4-11 Cross-sectional views at the interface of the molten glass/uncoated stainless steel substrate and EDX results.
Figure: 4-12 Cross-sectional views at the interface of the molten glass/uncoated stainless steel substrates with element mapping results.
4.5.1.2 Effect of Temperature on the Final Contact Angle
The variation of Contact angle with respective to temperature profile as a function of time for molten glass on the uncoated stainless steel substrate at 800°C for 5- minute holding time as shown in Figure 4-13. In this case, the temperature is inverse proportional to the contact angle.
During the holding stage, rapid decreased of contact angle curve has been observed. In cooling stage, fluctuation of contact angle curve exists due to contraction mismatch of the substrate and glass material during the cooling process and finally it reached to stable value at end of the cooling process.
Figure: 4-13 Variation of Contact angle with respective to temperature profile as a function of time for molten glass on the uncoated stainless steel substrate at 800°C.
4.5.1.3 Influence of Ridge Formation on the Spreading Kinetics
A strong reaction can lead to dissolution of the particles in terms of ridge formation.the ridges form by diffusion or solution/precipitation of the solid atoms in response to the groove formation at the intersection of a grain boundary and a free surface [85]. Through SEM investigations, potential ridge formation was observed on uncoated substrates. Ridge formation and ring of small glass islands formed at surrounding interface was observed by interface analysis shown in Figure 4-14. Formed ridge is very small compared with the radius of liquid curvature was observed at interface boundary and it controls the spreading kinetics.
Figure: 4-14 Analysis of interface conditions between the glass and uncoated stainless steel substrate; (a) microscopic image of glass adhesion at interface: chemical reaction takes place at edge of interface between the glass and uncoated substrate, (b) ridge formation indentified by optical microscopy, (c) SEM image: width of ridge formation at interface, (d) SEM image: ring
4.5.1.4 Formation of Oxide Layer
In this study, the oxide layer (FeO) formed on the uncoated stainless steel surface after high-temperature oxidation treatment serves as a glass-to-metal interface layer. It is investigated that the influence of oxide layer formation in between the uncoated mold substrate and glass preform. Form an oxide layer on substrate under heat treatment process at O2 environment (which is known as pre-oxidation process). The variation in rate of oxidation for uncoated mold substrate oxidized at 800°C for different isothermal holding periods. The rate of oxidation is defined by the difference in net weight per unit area of samples before and after heat treatment, which is also called as “net weight of oxidation” [150]. The net weight of oxidation is increases with increasing of isothermal holding as shown in Figure 4-15. Longer explores time helps higher oxidation rate of the substrate and also net weight of oxidation is linear proportional to holding time.
Figure: 4-15Graph represents relationship between the net weight of oxidation with respective to holding time.
The relationship between the thickness of oxide layer and isothermal holding time was shown in Figure 4-16. The change of the oxide layer thickness with increase in the isothermal hold time; it seems oxidations become saturated above 60min isothermal hold period, because of formatted oxide layer has very loose structure and becomes mechanically brittle nature and it will disappears or break easily during the handling process or long explore holding time at high temperature. (Figure 4-15). Formed oxide layer is more dense and brittle nature during the holding at 800°C (Figure 4-16). It can be concluded that oxide layer plays a crucial role in adhesion or sticking process between the uncoated mold and glass. Wetting at the interface indicates adhesion of glass to the substrate surface, which will facilitate good sealing and joining with help of interface oxide layer. The proposed optimum thickness of the oxide layer should be 2–6.5 µm for good quality of sealing at interface [150]. The thickness of the oxide layer formed on the pre-oxidized samples was observed using optical microscope. The obtained thickness of oxide layer at 800°C of uncoated mold substrate is 18µm as shown in Figure 4-17.
Figure: 4-16 Relationship between average thickness of oxide layer on uncoated mold and isothermal holding time at 800°C.
Figure: 4-17 Micrograph of oxide layer of uncoated mold substrate treated at 800°C.
4.5.2 Glass on Al2O3 Coated Mold
The wetting curve shows very interesting results for Al2O3 coated substrate. The variation of the contact angles varied from 152 to 136° with minor change in contact radius, presents anti-sticking or non wetting behavior (Figure 4-18).
0 50 100 150 200 250 300 134
136 138 140 142 144 146 148 150 152 154
Contact Radius( mm)
Holding time ( Sec)
Contact angle ( degrees)
0.0 0.5 1.0 1.5 2.0 2.5
(b) (a)
Figure: 4-18 The behavior of (a) Contact angle and (b) contact radius as a function of time for molten glass on the sol-gel Al2O3 coated substrate at 800°C.
The appearance of the area contacted by glass ball looks smooth and clean. Many elements such as Fe, Ni, Cr, C, Al, and O were observed from the Al2O3 coated substrate (Figure 4-19).
Figure: 4-19 SEM /EDX results of Al2O3 coated substrate after wetting test.
However, there are no active elements such as Zn and Ba can be found on the glass surface after wetting test. There is no Ni or Cr peak appears in the slumped glass ball surface from SEM/EDX analysis as shown in Figure 4-20. It means that there is no chemical interaction occurred at the interfaces between the Al2O3 coated substrate and glass. The sol-gel coated Al2O3 film acted like as an excellent diffusion barrier which effectively inhibit the element diffusion and reaction between the substrate and glass. This is the reason why the final contact angle of Al2O3 coated substrate has minor change from initials at 800°C. The glass ball still remains fully transparent after contacting with the Al2O3 coated substrate during the wetting test.
Figure: 4-20 SEM/EDX results of tested glass surface after wetting test when contacted with Al2O3 coated substrate.
4.5.3 Glass on SiO2 Coated Mold
In the case of molten glass on SiO2 coated steel substrate; it was observed that the contact angle decreased slowly from initial 100 seconds and again rapid change in contact angle occurred in the until 200 seconds because of the fluctuations at the low frequency limit due to van der wall forces between the contact surfaces. Heat treatment is qualitatively different in its effects because it is a time-dependent process. The contact radius increases with the time duration, as shown in Figure 4-21. The final contact angle is 77° and final contact radius is 3.9 mm. The appearance of the substrate contacted by glass ball looks clean from a low magnification SEM picture. The EDX results show Fe, Ni, Cr, C, Al, and O are the main elements observed from the surface of SiO2 coated substrate. It worth to note that there are a
few of Sb-rich particles can be observed on the glass ball contacted area from high magnification SEM/EDAX analysis (Figure 4-22). The glass contacted surface of SiO2 coated steel substrate becomes very rough after the test. The weak Ni peak was detected on the slumped glass surface which is eliminated from the substrate (Figure 4-23). This indicates the mass diffusion and redox reaction occurred at interfaces, which resulted in the obvious decrease of the contact angle.
0 50 100 150 200 250 300
70 80 90 100 110 120 130 140 150 160
Contact Radius( mm)
Holding time ( Sec)
Contact angle ( degrees)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
(b) (a)
Figure: 4-21 The variation of (a) contact angle and (b) contact area radius as a function of holding time for molten glass on SiO2 coated substrate at 800°C.
Figure: 4-22 SEM /EDX results of SiO2 coated substrate after wetting test.
Figure: 4-23 SEM/EDX results of tested glass surface after wetting test when contacted with SiO2 coated substrate.
The results show that the adhesion and contact angle can be attributed to two possible mechanisms. The interfacial chemical reaction dominates wetting behavior due to the occurrence of oxydo-reduction and complex basic-acid reactions. Second possible mechanism may be from the surface roughness of the substrates and other physical forces which need to be further studied.
4.5.4 Al2O3 Coated Glass on Mold
From our wetting experimental results, Al2O3 coated glass preform on stainless steel substrate (shown in Figure 4-24); the glass ball spreads from initial contact angle to reach a final value. It spreads from θ0 to θf value; the spreading rate is determined by viscous force or resistance. Means it is control by viscous resistance only, because there is limited reactive product formation at the interface.
From previous studies, coated mold materials performed better anti-sticking behavior than uncoated ones [101].However, exact reasons and mechanism behind the glass-to-mold sticking is not discussed yet. It’s important to understand the interaction of atoms and molecules at interface (for example: chemical reactions, adsorption/desorption or mass transformation etc,).This requires careful analysis to judge the wetting phenomenon. In general, wetting can be divided to partial wetting or non-wetting. If the probability of reaction is less; then there is very less probability of sticking at triple line. The variation of (a) Contact angle and (b) contact area
at 800°C was shown in Figure 4-25 and the final contact angle values is 136° as shown in Figure 4-26.
Figure: 4-24 The variation of contact angle as a function of holding time for sol-gel Al2O3 -coated glass preform on the stainless steel substrate with respect to temperature profile.
Figure: 4-25 Variation of (a) Contact angle and (b) contact area radius as a function of time for sol-gel Al2O3 coated glass preform on the stainless steel substrate at 800°C for 5 minutes holding period.
Figure: 4-26 Images of the final contact angle of sol-gel Al O coated glass preform on the
The variation of the contact angle with respect to the temperature profile from 550 to 800°C and processing time is shown in Figure 4-27. According to temperature profile, the contact angle curve can be classified into 3 stages, such as heating, holding and cooling stages.
In the first stage, the contact angle of molten glass slowly decreases while heating from temperature 550 to 800°C followed by a very slow spreading kinetics for uncoated glass balls.
In the second stage the temperature was kept at 800°C for five minutes. The contact angle was rapidly decreasing with respect to holding time. In cooling stage; the contact angle changes are negligible, small variation of contact angle due to expansion or contraction of glass material during the cooling process. The total cycle time is twenty five minutes
Figure 4-27 The variation of contact angle as a function of holding time and temperature for uncoated and sol-gel Al2O3 coated glass ball on stainless steel.
The Al2O3 coated glass ball still remains fully transparent after wetting test when contacted with stainless steel substrate (Figure 4-28(a)). The surface area of stainless steel when contacted with Al2O3 coated glass ball remains smooth and free of reactants (Figure 4-28(b)).
Although stainless steel was oxidized, there is no observable glass sticking products, such as Zn, Ba, Al and Si oxides to be detected on the glass contacted surface (Figure 4-28(c)). It means that there is no detectable chemical interaction occurred at the interfaces between the Al2O3 coated glass ball and stainless steel substrate. The sol-gel coated Al2O3 film acted as an excellent diffusion barrier which effectively inhibit the element diffusion and reaction between the substrate and glass. This is the reason why the final contact angle of Al2O3 coated substrate has little change at 800°C.
Figure: 4-28 (a) Appearance of Al2O3 coated glass ball (b) SEM image of stainless steel and (c)
4.5.5 Al2O3 Coated Glass on Al2O3 Coated Mold
The wetting behaviors between the substrate and glass preforms, which are coated by Al2O3, were determined by IR heating source at high temperature. The pair of this system is called as a “similar metals interaction”. The interaction of similar metals (Al-Al) at high temperature was investigated (Figure 4-29). It is observed that the final contact angle of Al-Al interaction is 134°, and this value is smaller than the glass on Al2O3 coated substrate (136°) and Al2O3 coated glass preform on substrate cases. The deviation of final contact angles in all these cases are all most negligible values. In case of Al-Al interaction, after 15 minutes holding time final contact angle is observed. The constant final contact angle is achieved dues to repulsive force of inter atoms or molecules of contacted aluminum and it depends on interatomic distance of interacted or contact materials. Either similar or dissimilar metal interaction at evaluated temperature plays an important role in wetting or non wetting systems. No interdiffused reactive elements from either substrate or glass preforms are observed. Measured values of wetting will differ from intrinsic work of adhesion (WA) values because of contributions of chemical interactions, inter-diffusion effects, internal film stresses, interfacial impurities, imperfect contact, etc at interface.
2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 1 1 5
1 2 0 1 2 5 1 3 0 1 3 5 1 4 0 1 4 5 1 5 0 1 5 5 1 6 0 1 6 5
Contact Angle ( ?)
H o ld in g T im e (m in )
Figure: 4-29 Variation of Contact angle as a function of time for the Al2O3 coated substrate and Al2O3 coated glass preform at 800°C for 5- minute holding time.
4.6 Molded Lens Analysis
The sets of samples have been prepared the first one having thin coating obtained by single dipping and the second corresponding to thicker layers produced by multidipping process.
4.6.1 Al2O3 Coated L-BAL 42 Molded Lens
Figure 4-30 shows the elements depth profile of Al2O3 coated lens after glass molding test at 580°C. It shows that the Al2O3 film has thermodynamic stable phase which can effectively hinder out diffusion of elements such as Si, Zn, B and Ba at 580℃. Carbon based
Figure : 4-30 Elements depth profile of Al2O3 coated glass lens produced by molding process at 580°C.
A desired optical lens is obtained by press molding a glass preform using stainless steel material as the mold (Figure 4-31(a)). When press molding of a glass materials having an Al2O3
film on the surface, deformation of the glass materials is accompanied by extension of Al2O3 film on the surface. When extension of the Al2O3 film cannot keep up with deformation of glass material, breach will occur. Thus, the glass material is exposed at the breached portions, resulting in the risk of fusion to the molding surface. Our results showing the surface of molded lens appear defectless within the designed aspherical aperture when the thickness of Al2O3 film is 34 nm (Figure 4-31 (b)). The breaches only appear near the edge of the molded lens due to a
greater glass deformation on this portion (Figure 4-31 (c)). The thickness of Al2O3 film less than 15 nm is suggested to completely avoid breaches on the whole molded lenses.