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Simultaneous chemical vapor deposition and thermal strengthening
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of glass
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Peter Sundberg1, Lina Grund Bäck1, Robin Orman2, Jonathan Booth2, Stefan Karlsson1*
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1RISE Research Institutes of Sweden, Built Environment Division, Glass Section, SE-351 96 Växjö,
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Sweden
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2Johnson Matthey Technology Centre, Blounts court, Sonning Common,Reading, RG4 9NH, United
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Kingdom
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*Corresponding author: Stefan.Karlsson@ri.se
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Keywords: Chemical vapour deposition, thermal strengthening, crack resistance, contact angle,
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hardness
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Declaration of interest: none.
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Abstract
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In the current paper we present a concept combining metal organic chemical vapor deposition with
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thermal strengthening process of flat glass. As the flat glass is heated to be thermally strengthened,
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which takes a few minutes, there is an opportunity for performing a surface modification. We describe
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the application of transparent and amorphous Al2O3 thin films during the thermal strengthening process.
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Al2O3 was chosen due to the following desirable properties: increased surface mechanical properties
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and increased chemical durability, the latter has not been investigated in the current paper. The residual
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surface compressive stresses after performed strengthening of the coated glasses were quantified to
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be in the range of 80-110 MPa. The Al2O3 content in the surface was measured using the Surface
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Ablation Cell employed with Inductively Coupled Plasma Atomic Emission Spectroscopy and found to
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be at least doubled at the surface and having an increased Al2O3 content at least 0.5 µm underneath
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the glass surface. During the surface reaction, sodium is migrating to the surface giving a hazy salt layer
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on the glass which can easily be washed off with water. The applied coatings are transparent and
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provide increased surface hardness and crack resistance at low indentation loads. At higher indentation
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loads the interaction volume is larger and displays the same effect on the surface mechanical properties
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as for thermally strengthened glass. The contact angle with water compared to annealed float glass is
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significantly increased from 5° to 45° due to the different surface chemistry and surface structure.
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1. Introduction
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Glass is an essentially strong material. It is evident from its strength calculated from the bonds forming
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the vitreous network or its strength under compressive loads [1]. However, due to unavoidable surface
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defects, glass is brittle for tensile loads. The two most common methods of strengthening glass involve
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pre-stressing the glass surface with compressive stresses by essentially either physical or chemical
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means. Physical strengthening is more commonly called thermal strengthening (or tempering).
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Tempered glass is made by uniformly heating glass to a temperature of up to 700°C and immediately
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cooling down [2]. The hot glass undergoes a rapid cooling process by a uniform and simultaneous blast
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of air on both surfaces. This gives a solid surface that contracts lesser than the interior that still has a
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temperature above the glass transition which upon further cooling results in a permanent parabolic
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stress profile [3]. Chemical strengthening of glass most often means exchanging smaller ions in glass
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surface for larger ions by a molten salt bath [4] or by a spraying process [5]. It generally gives higher
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strength than thermally tempered glass but requires more time (hours instead of minutes), the upscaling
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is complicated, and therefore comprise a higher cost. Thermal strengthening of glass has in general two
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major drawbacks, the frangibility (the explosive behavior upon fracture) [1] and spontaneous fracture
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due to NiS inclusions [6]. Thinner thermally strengthened glass is possible to be made by using
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2(7)
increased rapidity in the cooling [7]; however, at some point thermally strengthened glass also needs
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increased surface mechanical properties. Therefore, it can be very favorable to combine thermal and
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chemical strengthening in the same process. The traditional chemical strengthening is though not
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possible to combine with thermal strengthening [8].
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There are also other methods to improve the strength of glass as well; these can also be categorized
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as chemical methods to strengthen glass and involves gases that react with the glass surface. Some of
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these methods are summarized in Tab. 1. To be able to thermally strengthen we require a heating cycle
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as shown in Fig. 1. At temperatures as high as 600-700 °C there is an opportunity for enhancing the
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surface mechanical properties. Rancoule describes benefits and problems of adding gaseous SO2
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during the tempering process of float glass [17]. The formation of sodium sulfate is said to “benefit for
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the contact surface longevity” i.e. being a lubricant [10] but can also lead to defects [17]. The yield of
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the sodium sulfate depends on moisture, the temperature and the concentration of the gas during the
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process. No details of the SO2 additions were given in the cited references. In the current paper we
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investigated combining chemical vapor deposition for providing amorphous Al2O3 thin films during the
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thermal strengthening process [18, 19]. Amorphous Al2O3 thin films provide various desirable properties
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including improved chemical durability and surface mechanical properties [18-20].
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2. Experimental
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Conventional soda-lime-silicate float glass in the size of 70×70 mm2 with thicknesses of 4 and 2 mm
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were used with a typical composition as given in ref [21]. The general experimental procedure is given
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in Fig. 2. The flat glass was heated in the furnace up to 640-680 °C. At higher temperatures the glasses
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became non-flat and at lower temperatures the strengthening level became too low due to insufficient
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difference between the temperature of the surface and the interior upon cooling. For general guidance
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about thermal strengthening in experimental details please have a look in Barr’s Handbook [22]. The
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Al2O3-precursor was Al-(acac)3, from Strem Chemicals, which is a commonly used precursor for
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Metalorganic Chemical Vapor Deposition (MOCVD) [19]. The Al2O3-precursor was partly dissolved in
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isopropyl alcohol (IPA) at elevated temperature (about 70 °C) using a magnetic stirrer with the
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proportions of approximately 3.5:96.5 weight ratio of Al2O3-precursor and IPA respectively. The solution
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was then sprayed manually and as homogeneous as possible on to the grating made of stainless steel
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using a Preval sprayer at the constant spray pressure 4.4 bar and driven by the propellant gases,
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dimethyl ether, propane and isobutane. The spray system is an aerosol-based spray system that allows
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custom-made solutions to be turned into sprayable material. The gratings were then dried in a heating
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cabinet at 105 °C (Thermoscientific’s Heratherm oven OMH100) making the IPA to evaporated and only
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leaving deposited Al2O3-precursor. The gratings’ difference in weight was recorded before and after the
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spray and drying process so that the amount of applied Al2O3-precursor on the grating could be
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determined. The amounts of applied Al2O3-precursor on the gratings, in dried state, were in the range
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of 100-200 mg. The size of the gratings was 10x11 cm2 for each side of the flat glass so approximately
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0.5-1 mg/cm2 Al2O3-precursor was deposited on the grating. The grating together with an untreated float
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glass sample were then put into a hot (640-680 °C) muffle furnace (Naber Industrieofen N2OH) with
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normal air atmosphere as illustrated in Fig. 2A, so that the flat glass on both sides was located <1 cm
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from the grating. In the furnace the glass and the grating heats up, the glass surface reaches the furnace
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temperature after approximately 5 minutes, the Al2O3-precursor vaporizes creating an Al-(acac)3/air
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atmosphere. The Al2O3-precursor starts to evaporate above its boiling temperature 189 °C [19]. As the
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Al2O3-precursor reaches the hot glass surface it immediately decomposes and forms an Al2O3 thin film.
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Please note that all Al2O3-precursor do not end up forming a thin film on the glass. The glass and the
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grating were let in the furnace for 20 minutes in total as was predetermined from thermal strengthening
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tests. The reaction was verified to be completed by weighing the grating afterwards. After 20 minutes
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the sample and grating were removed and placed into a stream of air having an ambient temperature,
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see Fig 2B. The air flow generators were leaf blowers from Ryobi (3 kW) providing an air flow of 16
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m3/min. The rapidly cooled glass reach room temperature within 10-20 seconds. The weight change of
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the samples was not recorded as the weight change of the glass could not only have been prescribed
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to be due to the thin film deposition but also sodium migration to the surface (see results section). The
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thermal strengthening level of the tempered samples was measured using SCALP-03, GlasStress Ltd
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[23]. All samples were cleaned with deionized water prior to all type of characterizations and in all
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characterizations was the air-side investigated.
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The UV-Vis transmittance spectra were measured using the spectrophotometer Lambda 25 from Perkin
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Elmer. A 1 nm slit width, a 2 nm data interval and a scan rate of 480 nm/min was employed for the
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spectral range 350 to 1100 nm.
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The Scanning Electron Microscopy (SEM) images were acquired using a Zeiss ultra 55 Field Emission
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Electron Microscope equipped with in-lens secondary electron and backscattered detectors. The
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compositional analysis and the low-resolution general imaging were acquired using an accelerating
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voltage of 20 kV, a working distance of 7.3 mm.
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The surface chemical composition of a treated glass was studied using Surface Ablation Cell (SAC) [24]
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employing Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) to quantify the glass
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constituents dissolved. The experimental procedure is principally the same as outlined in [25] but
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adjusted to give a better depth resolution using a less concentrated acid mixture.
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The surface hardness was measured using an Anton Paar Nanoindenter (model NHT2) and the
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indentation hardness was determined using the Oliver and Pharr method [26]. For each reported
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indentation hardness value at least 20 indents were made within an area of 1 mm2 (apart from 1 mN
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where 30 were made); outliers were removed giving an <6% error of the resulting values. The crack
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resistance (CR) was measured using CSM Instruments Micro-Combitester and was determined
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according to the description in [27] performing at least 15 indents. The uncertainty of the CR method is
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lower than the determination of indentation toughness, for a discussion see ref [28]. The contact angle
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with deionized water was recorded using a pocket goniometer (PGX+) from FIBRO System AB. The
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optical profilometry measurements were performed using a Veeco instrument operating in the phase
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shifting interferometry (PSI) mode with a magnification of 2.7x. The vertical resolution of the PSI
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measurements is about 0.1 nm and the error of the root mean square data about 10%.
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3. Results
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The applied Al2O3 films are transparent, see Fig. 3, and believed to have an amorphous structure in
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accordance of what is reported by others [12, 29]. During the thermal strengthening some of the
alkali-32
species leaves the glass due to its volatile nature at these temperatures (640-680 °C). We suppose that
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this process is enhanced by the deposition of the Al2O3 thin film similar to the results shown by Fonné
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et al. [30]. The outward diffusion of the alkali is shown by as a white haze on the glass, see left inset of
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Fig. 4A. The white haze is easily washed off with deionized water and it was chemically quantified to
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contain mostly Na2O. As shown in Fig. 4A the Al2O3 concentration is approximately increased in the
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surface by a factor of 2 compared to bulk glass concentration. The profile of Al2O3 show a diffusion-like
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profile and the diffusion coefficient was estimated using Green’s function [25] having an effective
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diffusion coefficient in the range of.2×10-13 to 3×10-14 cm2/s. The polariscope photo taken of two in-situ
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CVD thermally strengthened glasses with deposited Al2O3 thin film, hereinafter referred to Al-CVD, show
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a similar pattern as commercially tempered glasses. The Na2O concentration profile also shows
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indication of having a bit lower concentration at the outmost surface, see Fig. 4B.
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Repeated Scattered Light Polariscope (Scalp) measurements showed that the surface compressive
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stresses of the Al-CVD thermally strengthened glasses was in the level of 80 to 110 MPa. Conventional
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thermally strengthened glass is in similar range [31]. The surface hardness, as measured by
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nanoindentation, was determined by measuring the hardness and the maximum contact depth (hm) for
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different loads, see Fig. 5. The trend of the hardness curves (the three upper curves) with load reveals
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discrepancies at low loads (1-5 mN). The annealed float glass shows increasing hardness with
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4(7)
increasing load. The thermally strengthened and the Al-CVD thermally strengthened show a minimum
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in the hardness at the load of 5 mN. At the loads 1 and 3 mN the Al-CVD thermally strengthened glass
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show a significantly higher hardness compared to the thermally strengthened glass. This can be
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explained by the fact that the Al2O3 thin film gives a higher hardness than the glass. We believe that the
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reason for a small discrepancy between the annealed float glass and the thermally at low loads is due
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to the fact that some Na2O evaporates during the thermal strengthening process.
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Using microindentation it is possible to study the crack initiation behavior. We have used the
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methodology of CR which is based on statistical data and a sigmoidal curve-fitting, see Fig. 6. The
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annealed and the thermally strengthened glass gave quite similar statistics at lower loads but at higher
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loads these were significantly different. The combined Al-CVD thermally strengthened glass clearly
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show a significant difference at lower loads but show a similar value for 100% PCI as the thermally
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strengthened glass. The reported CR values are given from the fitted curve at 50% PCI. The significant
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differences at low loads give the resulting difference of CR: 0.7 N for annealed, 0.8 N for thermally
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strengthened and 1.3 N for combined Al-CVD thermally strengthened float glass. The significant
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differences at low loads are caused by the deposited Al2O3 thin films. However, the thin films are so thin
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that at higher loads the bulk glass interaction overshadows the effect of the thin film. This resembles the
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results presented in Fig. 4 as well.
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Al2O3 thin films are known to give a higher contact angle than soda-lime-silicate (SLS) glass e.g. see
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following ref for superhydrophobic Al2O3 thin films [32, 33]. The contact angle of float glass i.e. SLS
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glass is about 5° immediately after its been cleaned [34] but generally increases with time. The silicate
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network often forms Si-OH species at the surface which gives the hydrophilic behavior. We investigated
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the wetting behavior of annealed float glass and Al-CVD thermally strengthened glass, see Fig. 7.
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Analogous to its normal behavior the float glass shows a contact angle vs. water of about 5° while the
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Al-CVD thermally strengthened glass shows an average contact angle of 45°. The high difference for
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the latter is likely to be due to the surface topography and surface chemistry in addition to the general
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error of contact angle measurements. It can easily be explained by the different surface chemistry and
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possibly surface topography provided with the Al2O3 thin film. Two exemplary images of the surface
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topography are provided in Fig. 8 and Fig. 9. They show quite similar pattern, so the surface topography
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cannot be ruled out. The root mean square (Rq) data give a value of 0.8 nm for the Al-CVD toughened
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glass compared to 1.3 nm error of the root mean square data is about 10% over the surface taken as
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an average of two measurements of two samples. However, surface topography cannot be ruled out to
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have an impact on the contact angle data shown in Fig. 6, which is one of the reasons for the deviation
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of the contact angle data. The heat-treatment could possibly affect the contact angle as well but a
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previous study with heat-treatments in the range of 400-600 °C showed that SLS glass still gives a low
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contact angle [34], about 5°.
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4. Discussion
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There are many questions still to be answered and more samples to be investigated e.g. the
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homogeneity of the thin films has not been investigated so far. The method is not optimized but providing
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a concept approach to improve thermally strengthening process. However, the present results give a
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proof of the innovation of a process that potentially can increase the surface mechanical properties of
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thermally strengthened glass. On the other hand, it can possibly be used for giving the thermally
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strengthened glass other functional properties such as chemical barrier coatings for providing an inert
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surface [35], conductive coatings [36, 37], photoluminescence [38], photocatalytic [39] etc. There are
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plenty of metalorganic substances that can be suitable for being adopted into the presented concept
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process [11, 40]. Another interesting, but outstanding aspect, is that the applied Al2O3 thin films may
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also impose somewhat increased surface compressive stresses due to the slightly lower thermal
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expansion coefficient of Al2O3.
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5. Conclusions
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5(7)
In the current paper we present a concept of combined MOCVD with thermal strengthening process. As
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the glass is heated there is an opportunity for applying a coating immediately prior being rapidly cooled
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in the thermal strengthening process. In the current paper we have investigated the application of Al2O3
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thin films in the thermal strengthening process and its beneficial effect on the surface mechanical
4
properties. The residual surface compressive stresses of the coated glasses were quantified to be in
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the range same range as fully tempered glass. The Al2O3 content was quantified being at least doubled
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at the surface and having an increased Al2O3 content at least 0.5 µm into the glass surface. During the
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surface reaction, sodium migrates to the surface giving a hazy salt layer on the glass which can easily
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be washed off with water. The applied Al2O3 coatings are transparent and are assumed to be
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amorphous. They provide increased surface hardness and crack resistance, especially at low
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indentation loads which would statistically give a positive impact on the mechanical strength. At higher
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indentation loads the interaction volume is larger and therefor they display the same result as for
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conventionally thermally strengthened glass. Because of the different surface chemistry and surface
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structure the contact angle with water is significantly increased from 5° to 45° compared to annealed
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float glass.
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Acknowledgement
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The authors wish to acknowledge Solar-ERA.NET as well as Swedish Energy Agency (contract no.
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38349-1) and Technology Strategy Board (contract no. 620087) for providing funding for this research.
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We also wish to acknowledge fruitful discussions with Prof. Lothar Wondraczek, Otto Schott Institute of
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Materials Research, University of Jena and Dr. Paul A. Bingham, Sheffield Hallam University.
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Table 1: Overview of some reactive gases that have been applied to glass substrates with time and temperatures required to achieve some notable effect.
Gas Time min Temp (°C) Depth Application Reference
SO2 10-40 min ~ 680 Traces of
Na2SO4
Float glass – lubricant,
surface protection [9, 10] SO2 10-40 min 580-650 Dealkalisation - Bottles [11]
Al-R3 30-60 min 200 70 nm Training / Al2O3 Coating
[12] AlCl3 30-60 min 200 Training / Al2O3 Coating
AlCl3 10-30 min 700-1000 Surface protection –
bottles, stemware [13] NH3 30 hrs 600 200-400 nm soda-lime-silicate glass [14]
N2 1-20 hrs 375–425 100 nm lead (Pb) / soda-lime
1(1)
Figure Captions
Figure 1: Measured temperature in flat glass centre during the tempering process. The red box indicates the window for surface reactions to occur. The time domain of the reactive gas surface interaction starts after approximately a few minutes and continues until the quench begins.
Figure 2: Experimental setup for A) Heat-treatment and reactive gas atmospheric treatment and B) uniform rapid cooling of samples.
Figure 3: Transmittance spectra for thermally strengthened and Al-CVD thermally strengthened flat glass. The insets show SEM-images of the coated surface (the bottom material) and an EDS-spectra with three spot analysis at the coated surface showing an Al signal.
Figure 4: Surface concentration as measured using SAC-ICP-OES [24]: A) shows the Al2O3 and K2O concentration profile. The inset in A) shows the SiO2 profile. B) shows the concentration profile of CaO, MgO and Na2O. The inset in to the right shows a polariscope photo of two Al-CVD thermally strengthened samples and the inset to the left shows a sample immediately after the rapid cooling. Figure 5: Surface hardness as measured using nanoindentation. The three upper curves show the hardness as a function of load while the three lower shows the maximum contact depth (hm) as a function of load. The error of the hardness and maximum contact depth measurements are 0.2 GPa and 8 nm respectively, as estimated from the average of the standard deviations.
Figure 6: Crack resistance (CR) diagram showing values for annealed (CR=0.7 N), thermally strengthened (CR=0.8 N) and combined Al-CVD thermally strengthened float glass (CR1.3 N). The data were fitted with sigmoidal curves giving the crack resistance load at 50% probability of crack resistance (PCI).
Figure 7: A few contact angle data vs. water showing the significant difference of the surfaces of free energy behavior of annealed and combined Al-CVD thermally strengthened float glass.
Figure 8: Surface topography images of a thermally strengthened float glass (left) and of an Al-CVD thermally strengthened glass (right).
