XRD patterns of ZrO₂-SiO₂ films (~600 nm) with different SiO₂ contents are shown in Fig. 1. Pattern of pure ZrO₂ film (Zr100) is well indexed as a tetragonal structure. Patterns of other films with SiO₂ show no characteristic diffraction peak, which indicates that these films have an amorphous structure. The representative SEM images of the Sol-Gel derived coating films (Zr100, Zr50 and Si100) are shown in Fig. 2. Homogeneous, continuous films are observed.

Fig. 1

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	Fig. 1 XRD patterns for the films on glass substances after ion exchange process

Fig. 2

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	Fig. 2 SEM images of (a) ZrO2 film, (b) ZrO2-SiO2 composite film and (c) SiO2 film surface and cross-section

Hardness and Young’s modulus were obtained by nano-indentation test. This test is especially applicable for thin solid film samples^{[ 16]}. The average hardness and Young’s modulus of glass uncoated and coated with ZrO₂-SiO₂ films are shown in Fig. 3. Values were measured from penetration depth at ~20 nm, which is about 0.1 of the thickness of films.

Fig. 3

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	Fig. 3 Hardness and Young’s modulus of glass coated with ZrO2-SiO2 films

The nano-indentation hardness and Young’s modulus of ZrO₂-SiO₂ films (15-20 GPa, 145-185 GPa), ZrO₂ film (~22 GPa, ~190 GPa) are much higher than that of un-coated ion-exchanged glass (Blank, ~12 GPa, 120 GPa). As the composition of ZrO₂ in films increase, hardness and Young’s modulus increase significantly, reaching up to twice that of un-coated ion-exchanged glass. These results indicate that the improvement in surface hardness and accordingly scratch resistance of ion-exchanged glass could be achieved by coating with Sol-Gel films.

Average values of 3-point bending strength, ion-exchange depth of coated and un-coated ion-exchanged glass, elastic recovery ( W_e), hardness ( H), Young’s modulus ( E) and ( H/ E) of films are presented in Table 2. The bending test was performed with thinner (45 nm) film, but there was no discernible difference in the strength of thicker (200 nm) samples. Ion-exchange depth is commonly believed to have positive impact to strength of glass, i.e., strength increase with the increasing ion-exchange depth. However, in this study, glass coated with ZrO₂-SiO₂ films with smaller ion-exchange depth seems to shows similar 3-point bending strength with that of uncoated glass with greater ion-exchange depth (except for Zr100 sample). Complemental strength may be induced by coated films. Hard nanocomposite coatings with (i) a low value of the Young's modulus E satisfying high H/ E≥0.1 ratio and (ii) a high value of the elastic recovery ( W_e≥60%) were thought to enhance strength^{[ 17, 18]}.

Table 2

Table 2

Table 2 Three-point bending strength (MPa) and ion-exchange depth (μm) of ZrO2-SiO2 films coated glass Sample Zr100 Zr75 Zr50 Zr25 Si100 Blank Average strength/MPa 350±37 370±36 393±39 401±40 409±37 361±27 Ion-exchange depth/μm 15.9 16.2 16.7 17.0 17.2 17.6 Elastic recovery, We/% 72.2 70 71 69.3 67 - Film hardness, H/GPa 22 20 18.5 16.5 13.2 - Young’s modulus, E/GPa 193 185 166 145 122 - H/ E 0.114 0.108 0.111 0.113 0.108 -

Table 2 Three-point bending strength (MPa) and ion-exchange depth (μm) of ZrO2-SiO2 films coated glass

Weibull analysis was introduced to study the mechanism. Figure 4 shows the Weibull plots corresponding to strength values of Blank and Si100 samples. The failure probability was calculated as

P_i = ( i-0.5)/ N (1)

Weibull distribution equation:lnln[1/(1- P_i)] = m Ln σ_i + ln[1/( σ₀)^m] (2)

Fig. 4

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	Fig. 4 Weibull plot of the Blank and Si100 samples after ion-exchange process R: Correlation coefficient; m: Weibull modulus

Where N is the total number of samples, we assign failure probability P_i to each value of σ_i after ranking all the 3-point bending test measured values in ascending order, i takes value from 1 to N which corresponds to the number of measurements of the sample tested; σ_i and σ₀ are failure strength and characteristic strength, respectively.

Equation (2) can be plotted as a straight line. In present work, lnln[1/(1- P_i)] versus ln σ_i whose slope is the Weibull modulus m by linear fitting the measured strength values^{[ 19]}. In Fig. 4, Si100 demonstrates a higher rupture resistance than uncoated glass according the distribution, however, the Weibull modules m is less than that of uncoated glass. The result agrees with present coating strengthening theoretical model established by Carturan, et al^{[ 10]}. Strength- ening effect of coating films promoted by crack length reduction and flaws healing are more pronounced for partial cracks of a smaller dimension than the average length, and that caused the Weibull modules change^{[ 19]}.

It is worth noting that the values of strength and ion-exchange depth increase with increasing SiO₂ content. The SiO₂ content in the films seemed to enhance the finalglass strengthening effect. According to the previous investigation^{[ 20]}, a possible explanation is proposed: zirconia is a well-known barrier material against alkali corrosion because of low alkali diffusion, while silica possesses much larger alkali diffusion coefficient. When silica content increases in the films, composite alkali ion diffusion coefficient would be improved. This could weaken the obstacle effect of films during ion-exchange process between glass surfaces and molten salt^{[ 20, 21]}.

Figure 5 shows the transmittance spectra of un-coated and ZrO₂-SiO₂ films coated glass. Pure SiO₂ film coated glass shows a negligible optical loss, and Zr25, Zr50, Zr75 samples also show high transmittance reaching up to 85% at the wavelength of 350 nm and above. Pure ZrO₂ film coated glass exhibits lower transmittance compared to that of other samples. According to the XRD patterns shown in Fig. 1, Sol-Gel derived SiO₂ film and ZrO₂-SiO₂ films have amorphous structures, while pure ZrO₂ film shows a tetragonal structure. T-zirconia film performs greater optical scattering compared to that of amorphous film^{[ 13, 22]}, which could provide an explanation of the phenomenon described above. Figure 6 shows the refractive index versus wavelength of ZrO₂-SiO₂ films coated glass. Glass coated with more ZrO₂ content exhibit greater refractive index, which could cause more optical loss through reflec-tion^{[ 23, 24]}. This provides another aspect of the optical loss of pure ZrO₂ film coated glass. Moreover, notable decrease of transmittance could be found when increasing the coating thickness from 45 nm to 200 nm. More absorption loss would be happened in the thicker ZrO₂-SiO₂ film (typically: Zr50, shown in Fig. 7).

Fig. 5

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	Fig. 5 Transmittance spectra of the samples after ion-exchange process

Fig. 6

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	Fig. 6 Refractive index of the samples

Fig. 7

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	Fig. 7 Transmittance spectra of the samples (Zr50) with different film thickness