- Email: [email protected]
Effect of normal scratch load and HF etching on the mechanical behavior of annealed and chemically strengthened aluminosilicate glass
Journal Pre-proof Effect of normal scratch load and HF etching on the mechanical behavior of annealed and chemically strengthened aluminosilicate glass Zhen Wang, Tianhao Guan, Tengfei Ren, Haokang Wang, Tao Suo, Yulong Li, Takeshi Iwamoto, Xiang Wang, Yinmao Wang, Guozhong Gao PII:
S0272-8842(19)33088-3
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.214
Reference:
CERI 23282
To appear in:
Ceramics International
Received Date: 26 August 2019 Revised Date:
13 October 2019
Accepted Date: 22 October 2019
Please cite this article as: Z. Wang, T. Guan, T. Ren, H. Wang, T. Suo, Y. Li, T. Iwamoto, X. Wang, Y. Wang, G. Gao, Effect of normal scratch load and HF etching on the mechanical behavior of annealed and chemically strengthened aluminosilicate glass, Ceramics International (2019), doi: https:// doi.org/10.1016/j.ceramint.2019.10.214. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of normal scratch load and HF etching on the mechanical behavior of annealed and chemically strengthened aluminosilicate glass Zhen Wanga, Tianhao Guana, Tengfei Rena, Haokang Wanga, Tao Suoa, b, c, , Yulong Lia, b, c , Takeshi Iwamotod, Xiang Wange, Yinmao Wange, Guozhong Gaoe a
School of Aeronautics, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, PR China. b
Joint International Research Center of Impact Dynamics and Its Engineering Application. c
Shaanxi Key Laboratory of Impact Dynamics and Engineering Application (IDEA), Northwestern Polytechnical University, Xi’an 710072, PR China.
d
Academy of Science and Technology, Hiroshima University, Higashi-Hiroshima 739-8527, Japan e
Jiangsu Tie Mao Glass Company Limited, Nantong 226600, China
Corresponding author: Tao Suo Email address: [email protected] Address: 127 West Youyi Road, Beilin District, Xi’an Shannxi, 710072, P. R. China
Abstract Micro-cracks generated by hard body scratch are a major cause of strength decrease for silicate glass. The influence of normal scratch load on the cracking patterns and flexural strength of annealed glass (AG) and chemically strengthened glass (CSG) were studied. With the increase of the normal load, the load capacity of scratched AG specimens decreased to about 40MPa at 20gf immediately. However, the residual strength of CSG decreased to a steady value of 145MPa as the scratch load increased to 500gf. Then the effect of hydrofluoric acid (HF) etching on the surface morphology and mechanical properties of the 500gf scratched glass were investigated. After 8min (for CSG) and 16 min (for AG) acid treatment, the flexural strength of CSG and AG increased to a considerable value of 900MPa, which is 3.6 and 5.5 times higher than the flexural strength of undamaged specimens. Microscopic observations show that the blunting and eliminating of median cracks as well as the formation of new
surfaces are the main causes of strength enhancement. Keywords: aluminosilicate glass; scratch; HF etching; flexural strength; morphology 1. Introduction Aluminosilicate glasses are commonly used both in military and civil fields, such as transparent armor systems, aircraft windshields and touch covers of electronic devices[1]. However, brittleness is a main issue to these brittle materials and will cause sudden fracture of glass structures. The actual strength of glass differs by several orders of magnitude from the theoretical strength, which is determined by the macro- and microstructural defects[2]. Also, the tensile strength of glass is only 8-10% of its compressive strength[3, 4], which is mainly caused by the surface flaws and defects of silicate glass[5]. In most applications, they face the risk of surface damage, such as indentation, scratch and wear. Surface damage is very dangerous for glass sheets acted as load bearing components. Chemical strengthening, or ion-exchange, is a promising method to improve the mechanical strength and deformation bearing capacity of silicate glass[6, 7]. It is an ion-exchange process that the glass substrates are dipped into the bath of molten salt to replace smaller ions in the glass with bigger ions in the salt bath. A uniform compressive layer will be built in the surface of chemically strengthened glass. The residual compressive stress can reduce the severity of surface flaws and make the glass stronger. The studies about scratch behavior of silicate glass and its residual strength have attracted several researchers. Sharp indenters like Vickers diamond pyramids produce two basic types of crack pattern: the radial-median system and the lateral one[8]. The scratch hardness was defined recently, showing this parameter as a quantitative parameter for the first time.[9] It’s suggested that the scratch load, scratch speed, indenter geometry, environmental humidity as well as glass composition will influence the scratch behavior of silicate glass[10-12]. The flexural strength of silicate glass is very sensitive to surface scratches. Nasreddine[13] found that the residual strength of Vickers indenter scratched soda-lime-silica glass decreased to about 40MPa when the scratch load was above 0.5N, while the flexural strength of intact specimens was about 90MPa. J.J. Swab[14] utilized a scribe embedded with diamond
chips for scratching tests. It was found that tempered glass was more resistant to surface scratch. A stable strength was also gained when the scratch load exceeded 1N. Blunt (quasi-spherical) indenter scratching is another specific case of scratching process. A transition from ductile fracture to plastic yielding in scratched region was recently reported[15]. It is found that structural cohesion controls yielding, scratch-induced fracture and micro-abrasion are dominated by volume density of bond energy. Our previous work[16] showed that the flexural strength of spherical indenter scratched glass decreased 20%-40% for different loading speeds. Also, chemically strengthened glass showed better resistance to blunt surface scratch than annealed glass. From the experiment observations summarized above, it can be seen that different results were presented due to different testing techniques, material composition and scratch methods. Meanwhile, the Vickers indenter scratch behavior of chemically strengthened glass and the influence of surface scratch on residual strength are still not very clear. Hydrofluoric acid (HF) etching is also a well-known method to strengthen glass [17-19]. HF etching of silica leads to the formation of hexafluorosilicic acid following the reaction[20]: SiO2+6HF=SiF6H2+2H2O
(1)
The chemical reaction between HF and SiO2 facilitates blunting of crack tips and the removal of glass surface layers with flaws. Li[21] conducted nanoindentation tests on HF etched glass and found the hardness and Young’s modulus increased after 1min corrosion. M. Kolli[22] had conducted an interesting test. Several glass specimens were sandblasted and then put to the HF etching process. It was found that the flexural strength of etched specimens reached 181MPa, which was much higher than the strength of eroded specimens and the as received specimens. Microscopic observations showed that surface cracks blunting is the main cause of the strengthening effect. In the present work, annealed and chemically strengthened aluminosilicate glasses were scratched by standard Vickers indenter. Three-point bending tests were conducted to get the residual flexural strength. Also, the effect of HF etching on the
strength of AG and CSG previously scratched by Vickers indenter was examined.
2. Materials and methods 2.1 Glass Characteristics and specimens preparation The annealed aluminosilicate glass (AG) and corresponding chemically strengthened glass (CSG) after the ion-exchange process are both provided by the Tiemao Glass Co. Ltd in China. The chemical composition of annealed glass is shown in Table 1. The strengthened layer of CSG was detected by the FEI Verios G4 Scanning Electron Microscope (SEM) with Thermo-NS7 Energy Dispersive Spectrometer (EDS). The energy resolution of the employed EDS is 129eV and the thickness of strengthened layer is about 10µm[23].
Table 1 Chemical composition of annealed glass Oxides
SiO2
Al2O3
Na2O
MgO
K2O
CaO
Others
Wt%
64.3
17.7
10.4
3.8
2.5
0.5
0.8
The electronic scratch machine shown in Fig. 1 was utilized in this work. The glass sheet with a dimension of 50mm×50mm×2mm was fixed by the fixture on the movable platform. A Vickers indenter with a tip radius of about 2µm was placed under the loading weight. Before the loading weight was placed on the machine, the balance weight was adjusted to a proper place to make sure that there was no contact force between the indenter and glass sheet. In this way, the normal scratch load depends on the loading weight. Different combinations of loading weights (20g, 50g, 100g, 200g, 500g, and 700g) were used to generate different normal scratch loads. The moving speed of the platform was adjustable and set as 2mm/s in this work. During the scratching test, the temperature and humidity in the laboratory were 25±2℃ and 40±5% RH respectively. After the glass sheets were scratched, a diamond wire cutting machine was utilized to cut the sheet to specimens with a dimension of 18mm×4mm×2mm. The scratches were kept in the middle of the specimens and
perpendicular to the 18mm long edges, as shown in Fig. 1.
Fig. 1. Scratching and Machining of glass specimens
2.2 Etching procedures The scratched AG and CSG specimens with the normal scratch load of 500gf were etched by hydrofluoric acid. The etching procedure can be summarized as follows: i) The scratched glass specimens were first clean ultrasonically in the bath of ethanol for 3 minutes to remove the chips after the scratch process. ii) Put the specimens to the electronic vacuum drying oven for 10 minutes at the temperature of 60℃. iii) The samples were immersed in a solution containing HF acid (10 wt%) and sulphuric acid (18 wt%) for different periods (0.25, 0.5, 1, 2, 4, 6, 8, 12, and 16min) at room temperature (25±2℃). The solution was stirred by the magnetic stirring apparatus to ensure uniform corrosion. iv) Wash the etched samples by distilled water for 1 minute. v) Clean the specimens ultrasonically in the bath of ethanol for 3 minutes. vi) Dry the specimens in the electronic vacuum drying oven for 10 minutes at the temperature of 60℃. After these steps the samples were placed in sealed plastic bags to avoid external damage to the new corroded surfaces. All the subsequent flexural tests were
conducted within 10 hours after the etching process. The corrosion thickness was obtained by measuring the thickness of specimens etched for different times with micrometer.
2.3 Three-point bending tests The flexural strength of glass specimens was obtained by three-point-bending tests using an Instron-5848 universal testing machine. The maximum load capacity is 2000N with the precision of 0.1N. The specimens were loaded in the thickness direction and the scratch was in the unloading (lower) surface bearing the most severe tensile stress, as shown in Fig. 2. The quasi-static loading speed of 0.1mm/min was used for all the tests. During experiments, a high-speed camera was utilized to capture the fracture mode of the specimens. The flexural strength is calculated as follows:
σf =
3Fmax L 2bh2
where
(2)
is the flexural strength,
is the peak load, b, L, and h are the width,
span length, and thickness of the specimen separately.
Fig. 2. Schematic of three-point-bending tests
3. Influence of the normal scratch load on cracking and flexural strength of aluminosilicate glass 3.1 Scratch behavior of annealed and chemically strengthened glass Different normal loads were used to scratch the annealed and chemically strengthened glass. Fig. 3 illustrates the evolution of micro-cracks in silicate glasses
during Vickers indenter scratching. There are mainly two types of crack systems under different scratch loads: radial-median cracks and lateral cracks[24]. At a critical load, one or more nascent flaws within the deformation zone are activated and median cracks initiate along the scratching direction. After scratched under evaluated loads, a lateral crack may appear while unloading the indenter, corresponding to the residual stress field behind the indenter in the scratching region[25]. The lateral cracks form around the interface of the elastic-plastic deformation and propagate to the glass surface. When they emerge at the free surface, chips are produced.
Fig. 3 Schematic of crack systems by Vickers indenter scratching With a monotonous increasing scratch load, three damage regimes can be defined: micro ductile regime, micro-cracking regime and micro abrasive regime[10]. For annealed aluminosilicate glass, plastic flow appeared beneath the sharp indenter and a plastic groove was observed when the scratch load was 20gf, as shown in Fig. 4 (a). When the scratch load increased to 50gf, the deformation mode transmitted into the micro-cracking regime. The radial-median and lateral cracks could be seen clearly in the SEM images. For 100gf scratch load, the lateral cracks became denser with a smaller spacing. Then the deformation mode turned to the micro abrasive regime from
micro-cracking regime when the scratch load increased to 200gf, as shown in Fig. 4 (d). In micro abrasive regime, the lateral cracks were not obvious and the damage zone narrowed. Lots of chips formed in the scratch region and an obvious groove formed corresponding to the material removal process. The scratch region turned to micro abrasive regime totally when the normal scratch load was 500gf. It can be seen from Fig. 4 (e) that the damage degree and cracks density was much higher. The scratched groove was full of dense radial cracks. It is worth noting that median cracks can also be observed in the abrasive region.
Fig. 4. Micrographs of scratched annealed glass samples under different normal loads: (a) 20gf, (b) 50gh, (c) 100gf, (d) 200gf, (e) 500gf. The scratch direction is from left to right.
Contrast scratch tests were also performed on CSG. The surface micrographs captured by SEM are shown in Fig. 5. Micro ductile regime is also observed at 20gf. However, when the scratch load increased to 50gf, there were lateral cracks but no radial-median cracks in Fig. 5 (b). It is illustrated that the threshold load for the formation of radial-median cracks is increased for CSG. The reason for the delay of radial-median cracks can be contributed to the compression layer of CSG. The radial-median cracks form in the direction perpendicular to the pre-compression stress along in-plane direction, so these cracks are depressed while the lateral cracks along the compression layer direction are not influenced. Noting that the suppressed radial-median cracks are usually strength-limit cracks for silicate glass. At 100gf, only sporadic radial-median cracks initiated in Fig. 5 (c). When the scratch load reached 200gf, the scratch behavior of CSG also turned to the micro abrasive regime from micro-cracking regime. The median cracks could also be seen clearly from the SEM images in Fig. 5 (d) and (e) at 200gf and 500gf.
Fig. 5. Micrographs of scratched chemically strengthened glass samples under different normal loads: (a) 20gf, (b) 50gh, (c) 100gf, (d) 200gf, (e) 500gf. The scratch direction is from left to right.
Fig. 6 shows the three-dimensional optical morphology and its corresponding two-dimensional profile of scratched glass. The depth information of scratched region is marked with different colors. We can notice that radial-median cracks and lateral cracks initiated for AG scratched at 50gf, while only lateral cracks appeared for CSG. The scratch depth of CSG is much lower than AG at 50gf. However, the depth of scratched grooves at 500gf was nearly the same shown in Fig. 6 (c) and (d). It can be also seen that more chips along the scratch line formed for CSG scratched at 500gf due to pre-stress relief.
Fig. 6. 3D optical morphology and its corresponding 2D profile of scratched glass: (a) AG at 50gf, (b) CSG at 50gf, (c) AG at 500gf, (d) CSG at 500gf
3.2 Flexural strength of scratched specimens The flexural strength of AG and CSG specimens and the residual strength of
scratched specimens were obtained by three-point-bending tests. During tests, a high-speed camera with a frame rate of 100000fps was used to capture the failure mode. As shown in Fig. 7, the flexural strength of unscratched CSG specimens was much higher than AG for the existence of surface compression layer. The flexural strength of AG reduced sharply (from 162.2MPa to 43.4MPa) when scratched at 20gf and then remained a constant value around 40MPa at higher scratch loads up to 500gf. However, with the increase of scratch load, the flexural strength of CSG decreased gradually. It can be seen from the high-speed images that the post-failure crack damage area decreases as the scratch load increases, corresponding to the decrease of flexural strength. A 700gf scratch was introduced to CSG additionally and the average residual strength reduced from 149.5MPa at 500gf to 146.0MPa at 700MPa. That means a nearly constant flexural strength was gained when the scratch load was over 500gf for CSG. Micro abrasive scratch behavior was shown for CSG scratched at 500gf and 700gf with similar scratch depth and crack patterns.
Fig. 7. Flexural strength of scratched glass
3.3 Discussion Fig. 8 shows the fracture surface of unscratched and scratched annealed glass
specimens. The lower edge of the fracture surface corresponds to the tension surface during three-point bending tests. It can be concluded from the mirror and mist region that the fracture origin is near the right side of lower edge for the unscratched specimen, as shown in Fig. 8 (a). However, for scratched specimens, there was stress concentration in scratched region during the loading process. In Fig. 8 (b), the main crack initiated from the scratch line and propagated into a decreasing stress field. The fracture surface is very smooth without mist region and hackles, which is a telltale sign of low-stress fracture.
Fig. 8. The fracture surface of annealed glass specimens (a) without scratch; (b) scratched at 200gf It can be seen clearly that the scratch resistance of CSG is much higher than AG. The chemically strengthened layer can suppress the formation of median crack, which
is a main source for strength declining under flexural loading. The flexural strength of CSG decreased slightly from 246.6MPa to 240.4MPa at 20gf and 233.1MPa at 50gf, because the chemically strengthened layer was not broken totally by indenter at this stage. Then a much sharper decrease to 202MPa at 100gf was obtained. From the SEM images in Fig. 5 (c), the median and radial cracks just appeared when the scratch load was 100gf. The post-failure image in Fig. 7 also shows a positive correlation between flexural strength and cracks region area. High strength specimens can store more strain energy and promote the cracks propagating in glass. It is worth noting that the flexural strength of CSG was still much higher than AG when the scratch load was higher enough to generate median cracks in both glasses. For chemically strengthened glass, compression layers were formed in both surfaces due to the extrusion effect of ion-exchange. Then tensile stress in the inner part of glass sheet to achieve a mechanically-equilibrium state will be formed based on the deformation coordination theory of solids. However, this equilibrium state will be broken when defects are introduced to the surface of CSG, i.e. scratch damage. The residual compressive stress near the scratched area is released for scratched CSG, leading to less compressive stress to balance the expanding of the other intact surface. Finally, there is compression stress around the scratched grooves for scratched CSG specimens[23]. Under flexural loading, the compressive stress around the median cracks should be conquered before the tip is under tensile stress. In this case, a higher flexural strength for CSG specimens under the same scratch load than AG specimens is expected. A further quantitative explanation of this effect can be found in our previous paper[23]. For AG scratched at 20gf, although there were no apparent median cracks initiated, the flexural strength decreased sharply and a major crack initiated from the scratched line of the specimen, as can be seen from Fig. 7. It is believed[26, 27] that the inelastic part of indentation/scratch region is induced by a combination of densification and plastic flow. Yoshida et al.[28] show results that blunt contacts promote more densification whereas sharp contacts promote more plastic flow. Plastic flow is a volume conserving process along with volume dilatation, which is more
likely to produce high density of strength limiting shear crack systems in the subsurface of scratched area at low loads[29, 30]. Also, stress concentration happened in the plastic groove with a small tip radius when the samples were loaded. These two reasons results in the low residual strength of AG scratched at 20gf. Then at 50gf and higher scratch loads, continuous and severe median cracks were generated and the flexural strength maintained a nearly constant value. It indicates that the stress concentration of median cracks limits the strength of AG. However, for CSG scratched at 20gf, the compression stress can suppress the formation of crack systems in the subsurface. There was no material removal and the strengthened layer was not removed during the scratch process, resulting in a small decrease of flexural strength.
4. HF etching effect of scratched glass 4.1 HF reaction kinetics Silicate glass is mainly composed by SiO2, which consists of tetragonal SiO4 units connected at all four corners with the same SiO4 units by covalent siloxane bonds. The chemical reaction of SiO2 and HF is based on the ionization of the acid. Fluoride irons react with the undissociated HF to form bifluoride anion for the attack of silica matrix[31]. These processes are expressed as follows: HF=H++F-, Ka=6.7×10-4 (mol/L)
(3)
HF+F-=HF2-, Ka=3.96 (mol/L)
(4)
where Ka represents the equilibrium constant at 25℃. The relationship between corrosion thickness and corrosion time for AG and CSG is shown in Fig. 9. A similar corrosion thickness rate for both glasses was obtained and the corrosion rate decreased for longer corrosion time. These observations are in consistent with published results[21, 32]. The surface of glass contains multiple flaws, which can increase the effective surface area and result in a higher corrosion rate initially. The reaction rate decreases when the flaws are removed. Another reason for the decreased reaction rate is the continuous reduction of hydrofluoric acid and fluoride irons concentration according to reactions (3)-(4), especially for longer reaction periods[33].
Fig. 9. Corrosion thickness of AG and CSG for different corrosion times
4.2 Surface morphological analysis The AG and CSG specimens scratched at 500gf were put into the chemical treatment process described in section 2.2. The morphology evolution of scratched area for different HF erosion periods is shown in Fig. 10 and Fig. 11. It can be seen from Fig. 4 (e) and Fig. 5 (e) that the untreated glass is characterized by dense radial cracks. Though not very clear, the main median cracks can also be distinguished. For scratched annealed glass after 0.25min HF treatment (Fig. 10a), the dense cracks opened up to many small grooves. The ends of the cracks presented circular fronts. After 0.5min treatment (Fig. 10b), some shallow cracks were eliminated and raised ridges formed between these cracks. Also, the median cracks could be seen clearly after acid treatment. When the treatment time increased to 1min, 2min, 4min and 6min (Fig. 10c-f), radial cracks were removed. Median cracks became more shallow and wider during erosion. After 8min treatment (Fig. 10g), the median crack was not very clear and the front was blunted, corresponding to a corrosion thickness of 81µm according to Fig. 9. The median cracks were removed after 10min and 12 min chemical reaction shown in Fig. 10 (h) and (i).
Fig. 10. Micrographs of scratched annealed glass treated in HF acid during different durations: (a) t=0.25min, (b) t=0.5min, (c) t=1min, (d) t=2min, (e) t=4min, (f) t=6min, (g) t=8min, (h) t=12min, (i) t=16min
Similar morphology evolution process can be observed for scratched CSG specimens during chemical treatment shown in Fig. 11. It’s noting that the median cracks begin to be indistinguishable in Fig. 11 (e), which is a similar state as Fig. 10 (g) shows. However, the treatment during is only 4min for Fig. 11 (e), with a corrosion thickness of 49µm. That means the depth of median crack of CSG is much smaller than AG scratched at the same normal load.
Fig. 11. Micrographs of scratched chemically strengthened glass treated in HF acid during different durations: (a) t=0.25min, (b) t=0.5min, (c) t=1min, (d) t=2min, (e) t=4min, (f) t=6min, (g) t=8min
The cracks blunting effect can be seen more clearly from three-dimensional optical morphology of etched specimens shown in Fig. 12. After 0.5min (Fig. 12a) or 1min (Fig. 12c) treatment, the crack tips were blunted, but the median cracks were still very obvious. As the etching process median cracks disappeared and quasi spherical shape caps appeared at the scratched region (Fig. 12b and d), which was also reported in Ref. [22, 32]. The observed craters depth decreased a lot with the increase of treatment duration. The surface flatness also improved with much lower stress concentration around the scratch region during loading process. This has a positive effect on the mechanical strength as we can see in the next section.
Fig. 12. Three-dimensional optical morphology of etched scratched glass: (a) AG for 0.5min, (b) AG for 16min, (c) CSG for 1min, (d) CSG for 8min
4.3 Flexural strength of etched specimens The flexural strength of etched specimens for different acid treatment durations is shown in Fig. 13. The untreated AG and CSG specimens have an average strength of 37.3MPa and 149.5MPa respectively. There are two reasons for the much higher value of scratched CSG specimens. Besides the stress balance effect of CSG illustrated in section 3.3, the other reason is much shallower median cracks resulting from the compression layers of CSG, which can be proved from the HF etching tests above. It can be seen from Fig. 13 (a) that the flexural strength of AG rises slowly from the beginning to 6min treatment. The specimens broke into two pieces during loading according the high-speed images. The strength increased to 292.4MPa after 8min of HF reaction and a crack zone near the scratch area was observed after breakage, corresponding to the specimens with unclear median cracks shown in Fig. 10 (g). After 12min and 16min treatment, the strength increases to 765.0MPa and 904.5MPa respectively. For CSG, a similar “slow-fast-slow” strength increasing trend was obtained from tests. The treatment time for CSG is much shorter because of the shallow median cracks and faster median cracks elimination process shown in Fig. 11. Finally the strength of AG and CSG is nearly the same around 900MPa after 16min
and 8min treatment. The initial surface is removed at this period and the strength improvement is mainly governed by the bulk defects instead of surface flaws[18, 22]. In this way, the flexural strength of CSG and AG increased to a considerable value of 900MPa, which is 5.5 and 3.6 times higher than the flexural strength of undamaged AG (162.2MPa) and CSG (246.6MPa) specimen. Fig. 13 (b) is a local enlarged drawing of Fig. 13 (a) for the HF treatment period less than 1min. For annealed glass, the flexural strength increased slightly by 10.7%, 13.4% and 17.2% after 0.25min, 0.5min and 1min treatment. In this period, although the median crack front also opened and blunted through acid reaction, the crack tip radius is still at micro scale. The main strength limiting factor in this period is stress concentration of median cracks. However, the strength of CSG increased from 149.5MPa to 165.3MPa after 0.25min treatment but decreased to 153.1MPa at 0.5min. The HF reaction will remove the strengthened layers of CSG first and release the compression stress, which is the main reason for the strengthening effect of CSG. After 0.25min acid reaction, the corrosion thickness is 4.1µm (Fig. 9), while the strengthened layer thickness is about 10µm. The strengthened layer is partly removed in this condition and the strengthening effect still exists. However, after 0.5min treatment, the corrosion thickness increases to 10.6mm and the compression layer is totally removed. The strength decrease is observed at this time (Fig. 13b). Then the strength increases monotonously because of the cracks opening and blunting effect, as well as the surface flatness restitution process[22].
(a) Flexural strength versus corrosion time
(b) Local enlarged drawing of (a) Fig. 13. Flexural strength of scratched glass etched by HF acid for different periods
5. Conclusions In this study, the influence of normal scratch load on the cracking pattern and flexural strength of annealed and chemically strengthened glass were studied. Then scratched specimens were put into the HF acid treatment procedure to investigate the effect of HF etching on the mechanical behavior of scratched glass. The relation between macroscopic mechanical behavior and micro-cracks of scratched glasses were established. The following conclusions were drawn: (1) Micro ductile regime, micro-cracking regime and micro abrasive regime were observed with increasing scratch load for both AG and CSG. The strengthened layer of CSG can suppress and delay the formation of median-radial cracks during Vickers indenter scratching. (2) The flexural strength of scratched AG declined sharply while that for CSG decreased gradually. The stress balance and redistribution of scratched CSG and the suppression of median cracks contributes to the much higher strength of CSG than AG at the same normal scratch load. (3) During HF reaction process, a similar corrosion thickness rate for AG and CSG was obtained and the corrosion rate decreased for longer reaction time. Morphological changes including cracks opening and blunting were observed after HF treatment. (4) A “slow-fast-slow” strength increase process was obtained for etched AG and CSG with the increase in reaction period. The “fast growth” stage of flexural strength corresponds to the disappearance of median cracks. The final strength of the glasses was nearly the same while shorter reaction time was taken for CSG.
Acknowledgements This work is financially supported by National Natural Science Foundation of China (grant number 11772268, 11522220, 11527803 and 11627901). We also would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for SEM characterization of the hydrofluoric acid etched glass specimens.
References 1.
Jannotti, P., et al., Influence of ultra-high residual compressive stress on the static and dynamic indentation response of a chemically strengthened glass. Journal of the European Ceramic Society, 2012. 32(8): p. 1551-1559.
2.
Wondraczek, L., et al., Towards ultrastrong glasses. Advanced Materials, 2011. 23(39): p. 4578-4586.
3.
Sheikh, M.Z., et al., Dynamic failure of annealed and chemically strengthened glass under
4.
Sheikh, M.Z., et al., Static and dynamic Brazilian disk tests for mechanical characterization of
compression loading. Journal of Non-Crystalline Solids, 2018. 499: p. 189-200. annealed and chemically strengthened glass. Ceramics International, 2019. 45(6): p. 7931-7944. 5.
Tsien, L.C., On Surface Cracks in Glasses. Proceedings of the Royal Society of London, 1937. 159(898): p. 346-354.
6.
Varshneya, A.K., Chemical Strengthening of Glass: Lessons Learned and Yet To Be Learned. International Journal of Applied Glass Science, 2010. 1(2): p. 131-142.
7.
Varshneya, A.K., Stronger glass products: Lessons learned and yet to be learned. International Journal of Applied Glass Science, 2018. 9(2): p. 140-155.
8.
Lawn, B.R., Fracture of brittle solids. Physics Bulletin, 1976. 27(5): p. 220-221.
9.
Sawamura, S. and L. Wondraczek, Scratch hardness of glass. Physical Review Materials, 2018. 2(9): p. 092601.
10.
Houérou, V.L., et al., Surface damage of soda–lime–silica glasses: indentation scratch behavior. Journal of Non-Crystalline Solids, 2003. 316(1): p. 54-63.
11.
Schneider, J., S. Schula, and W.P. Weinhold, Characterisation of the scratch resistance of annealed and tempered architectural glass. Thin Solid Films, 2012. 520(12): p. 4190-4198.
12.
Li, K., Y. Shapiro, and J.C.M. Li, Scratch test of soda-lime glass. Acta Materialia, 1998. 46(15): p. 5569-5578.
13.
Bensaid, N., et al., Influence of the normal load of scratching on cracking and mechanical
14.
Swab, J.J., et al., Influence of Surface Scratches on the Flexure Strength of Soda-Lime Silicate
strength of soda-lime-silica glass. Journal of Non-Crystalline Solids, 2018. and Borosilicate Glass. Experimental Mechanics, 2013. 53(1): p. 91-96. 15.
Sawamura, S., et al., Scratch‐induced yielding and ductile fracture in silicate glasses probed by nanoindentation. Journal of the American Ceramic Society, 2019.
16.
Zhen, W., et al., A comparative study on the effect of loading speed and surface scratches on the flexural strength of aluminosilicate glass: Annealed vs. chemically strengthened. Ceramics International, 2018. 44(10): p. 11239-11256.
17.
Saha, C.K. and A.R. Cooper Jr, Effect of etched depth on glass strength. Journal of the American Ceramic Society, 1984. 67(8): p. C‐158-C‐160.
18.
Sglavo, V., R. Dal Maschio, and G. Soraru, Effect of etch depth on strength of soda-lime glass rods by a statistical approach. Journal of the European Ceramic Society, 1993. 11(4): p. 341-346.
19.
Dabbs, T.P. and B.R. Lawn, Acid‐Enhanced Crack Initiation in Glass. Journal of the American Ceramic Society, 1982: p. C37-C38.
20.
Mikeska, K.R., S.J. Bennison, and S.L. Grise, Corrosion of Ceramics in Aqueous Hydrofluoric Acid. Journal of the American Ceramic Society, 2000. 83(5): p. 1160-1164.
21.
Li, X., et al., Effects of HF etching on nanoindentation response of ion-exchanged aluminosilicate float glass on air and tin sides. Journal of materials science, 2017. 52(8): p. 4367-4377.
22.
Kolli, M., et al., HF etching effect on sandblasted soda-lime glass properties. Journal of the European Ceramic Society, 2009. 29(13): p. 2697-2704.
23.
Wang, Z., et al., Quasi-static and dynamic flexural behavior of annealed and chemically strengthened aluminosilicate glass with notch defects. Journal of Non-Crystalline Solids, 2019. 521: p. 119479.
24.
Li, X., et al., Nucleation location and propagation direction of radial and median cracks for brittle material in scratching. Ceramics International, 2019. 45(6): p. 7524-7536.
25.
Wang, W., et al., Elastic stress field model and micro-crack evolution for isotropic brittle materials during single grit scratching. Ceramics International, 2017. 43(14): p. 10726-10736.
26.
Yoshida, S., J.C. Sanglebœuf, and T. Rouxel, Quantitative Evaluation of Indentation-Induced Densification in Glass. Journal of Materials Research, 2005. 20(12): p. 3404-3412.
27.
Peter, K.W., Densification and flow phenomena of glass in indentation experiments. Journal of Non-Crystalline Solids, 1970. 5(2): p. 103-115.
28.
Yoshida, S., et al., Effects of indenter geometry on indentation-induced densification of
29.
Hagan, J.T., Shear deformation under pyramidal indentations in soda-lime glass. Journal of
soda-lime glass. Journal of Materials Research, 2010. 25(11): p. 2203-2211. Materials Science, 1980. 15(6): p. 1417-1424. 30.
M, C.T., Scratch Damage in Ion-Exchanged Alkali Aluminosilicate Glass: Crack Evolution and the Dependence of Lateral Cracking Threshold on Contact Geometry. 2012: John Wiley & Sons, Inc. 113-122.
31.
Ezz-Eldin, F., T. Abd-Elaziz, and N. Elalaily, Effect of dilute HF solutions on chemical, optical, and mechanical properties of soda–lime–silica glass. Journal of materials science, 2010. 45(21): p. 5937-5949.
32.
Tso, S.T. and J.A. PASK, Reaction of glasses with hydrofluoric acid solution. Journal of the
33.
Knotter, D.M., Etching mechanism of vitreous silicon dioxide in HF-based solutions. Journal of
American Ceramic Society, 1982. 65(7): p. 360-362. the American Chemical Society, 2000. 122(18): p. 4345-4351.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)33088-3
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.214
Reference:
CERI 23282
To appear in:
Ceramics International
Received Date: 26 August 2019 Revised Date:
13 October 2019
Accepted Date: 22 October 2019
Please cite this article as: Z. Wang, T. Guan, T. Ren, H. Wang, T. Suo, Y. Li, T. Iwamoto, X. Wang, Y. Wang, G. Gao, Effect of normal scratch load and HF etching on the mechanical behavior of annealed and chemically strengthened aluminosilicate glass, Ceramics International (2019), doi: https:// doi.org/10.1016/j.ceramint.2019.10.214. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Effect of normal scratch load and HF etching on the mechanical behavior of annealed and chemically strengthened aluminosilicate glass Zhen Wanga, Tianhao Guana, Tengfei Rena, Haokang Wanga, Tao Suoa, b, c, , Yulong Lia, b, c , Takeshi Iwamotod, Xiang Wange, Yinmao Wange, Guozhong Gaoe a
School of Aeronautics, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, PR China. b
Joint International Research Center of Impact Dynamics and Its Engineering Application. c
Shaanxi Key Laboratory of Impact Dynamics and Engineering Application (IDEA), Northwestern Polytechnical University, Xi’an 710072, PR China.
d
Academy of Science and Technology, Hiroshima University, Higashi-Hiroshima 739-8527, Japan e
Jiangsu Tie Mao Glass Company Limited, Nantong 226600, China
Corresponding author: Tao Suo Email address: [email protected] Address: 127 West Youyi Road, Beilin District, Xi’an Shannxi, 710072, P. R. China
Abstract Micro-cracks generated by hard body scratch are a major cause of strength decrease for silicate glass. The influence of normal scratch load on the cracking patterns and flexural strength of annealed glass (AG) and chemically strengthened glass (CSG) were studied. With the increase of the normal load, the load capacity of scratched AG specimens decreased to about 40MPa at 20gf immediately. However, the residual strength of CSG decreased to a steady value of 145MPa as the scratch load increased to 500gf. Then the effect of hydrofluoric acid (HF) etching on the surface morphology and mechanical properties of the 500gf scratched glass were investigated. After 8min (for CSG) and 16 min (for AG) acid treatment, the flexural strength of CSG and AG increased to a considerable value of 900MPa, which is 3.6 and 5.5 times higher than the flexural strength of undamaged specimens. Microscopic observations show that the blunting and eliminating of median cracks as well as the formation of new
surfaces are the main causes of strength enhancement. Keywords: aluminosilicate glass; scratch; HF etching; flexural strength; morphology 1. Introduction Aluminosilicate glasses are commonly used both in military and civil fields, such as transparent armor systems, aircraft windshields and touch covers of electronic devices[1]. However, brittleness is a main issue to these brittle materials and will cause sudden fracture of glass structures. The actual strength of glass differs by several orders of magnitude from the theoretical strength, which is determined by the macro- and microstructural defects[2]. Also, the tensile strength of glass is only 8-10% of its compressive strength[3, 4], which is mainly caused by the surface flaws and defects of silicate glass[5]. In most applications, they face the risk of surface damage, such as indentation, scratch and wear. Surface damage is very dangerous for glass sheets acted as load bearing components. Chemical strengthening, or ion-exchange, is a promising method to improve the mechanical strength and deformation bearing capacity of silicate glass[6, 7]. It is an ion-exchange process that the glass substrates are dipped into the bath of molten salt to replace smaller ions in the glass with bigger ions in the salt bath. A uniform compressive layer will be built in the surface of chemically strengthened glass. The residual compressive stress can reduce the severity of surface flaws and make the glass stronger. The studies about scratch behavior of silicate glass and its residual strength have attracted several researchers. Sharp indenters like Vickers diamond pyramids produce two basic types of crack pattern: the radial-median system and the lateral one[8]. The scratch hardness was defined recently, showing this parameter as a quantitative parameter for the first time.[9] It’s suggested that the scratch load, scratch speed, indenter geometry, environmental humidity as well as glass composition will influence the scratch behavior of silicate glass[10-12]. The flexural strength of silicate glass is very sensitive to surface scratches. Nasreddine[13] found that the residual strength of Vickers indenter scratched soda-lime-silica glass decreased to about 40MPa when the scratch load was above 0.5N, while the flexural strength of intact specimens was about 90MPa. J.J. Swab[14] utilized a scribe embedded with diamond
chips for scratching tests. It was found that tempered glass was more resistant to surface scratch. A stable strength was also gained when the scratch load exceeded 1N. Blunt (quasi-spherical) indenter scratching is another specific case of scratching process. A transition from ductile fracture to plastic yielding in scratched region was recently reported[15]. It is found that structural cohesion controls yielding, scratch-induced fracture and micro-abrasion are dominated by volume density of bond energy. Our previous work[16] showed that the flexural strength of spherical indenter scratched glass decreased 20%-40% for different loading speeds. Also, chemically strengthened glass showed better resistance to blunt surface scratch than annealed glass. From the experiment observations summarized above, it can be seen that different results were presented due to different testing techniques, material composition and scratch methods. Meanwhile, the Vickers indenter scratch behavior of chemically strengthened glass and the influence of surface scratch on residual strength are still not very clear. Hydrofluoric acid (HF) etching is also a well-known method to strengthen glass [17-19]. HF etching of silica leads to the formation of hexafluorosilicic acid following the reaction[20]: SiO2+6HF=SiF6H2+2H2O
(1)
The chemical reaction between HF and SiO2 facilitates blunting of crack tips and the removal of glass surface layers with flaws. Li[21] conducted nanoindentation tests on HF etched glass and found the hardness and Young’s modulus increased after 1min corrosion. M. Kolli[22] had conducted an interesting test. Several glass specimens were sandblasted and then put to the HF etching process. It was found that the flexural strength of etched specimens reached 181MPa, which was much higher than the strength of eroded specimens and the as received specimens. Microscopic observations showed that surface cracks blunting is the main cause of the strengthening effect. In the present work, annealed and chemically strengthened aluminosilicate glasses were scratched by standard Vickers indenter. Three-point bending tests were conducted to get the residual flexural strength. Also, the effect of HF etching on the
strength of AG and CSG previously scratched by Vickers indenter was examined.
2. Materials and methods 2.1 Glass Characteristics and specimens preparation The annealed aluminosilicate glass (AG) and corresponding chemically strengthened glass (CSG) after the ion-exchange process are both provided by the Tiemao Glass Co. Ltd in China. The chemical composition of annealed glass is shown in Table 1. The strengthened layer of CSG was detected by the FEI Verios G4 Scanning Electron Microscope (SEM) with Thermo-NS7 Energy Dispersive Spectrometer (EDS). The energy resolution of the employed EDS is 129eV and the thickness of strengthened layer is about 10µm[23].
Table 1 Chemical composition of annealed glass Oxides
SiO2
Al2O3
Na2O
MgO
K2O
CaO
Others
Wt%
64.3
17.7
10.4
3.8
2.5
0.5
0.8
The electronic scratch machine shown in Fig. 1 was utilized in this work. The glass sheet with a dimension of 50mm×50mm×2mm was fixed by the fixture on the movable platform. A Vickers indenter with a tip radius of about 2µm was placed under the loading weight. Before the loading weight was placed on the machine, the balance weight was adjusted to a proper place to make sure that there was no contact force between the indenter and glass sheet. In this way, the normal scratch load depends on the loading weight. Different combinations of loading weights (20g, 50g, 100g, 200g, 500g, and 700g) were used to generate different normal scratch loads. The moving speed of the platform was adjustable and set as 2mm/s in this work. During the scratching test, the temperature and humidity in the laboratory were 25±2℃ and 40±5% RH respectively. After the glass sheets were scratched, a diamond wire cutting machine was utilized to cut the sheet to specimens with a dimension of 18mm×4mm×2mm. The scratches were kept in the middle of the specimens and
perpendicular to the 18mm long edges, as shown in Fig. 1.
Fig. 1. Scratching and Machining of glass specimens
2.2 Etching procedures The scratched AG and CSG specimens with the normal scratch load of 500gf were etched by hydrofluoric acid. The etching procedure can be summarized as follows: i) The scratched glass specimens were first clean ultrasonically in the bath of ethanol for 3 minutes to remove the chips after the scratch process. ii) Put the specimens to the electronic vacuum drying oven for 10 minutes at the temperature of 60℃. iii) The samples were immersed in a solution containing HF acid (10 wt%) and sulphuric acid (18 wt%) for different periods (0.25, 0.5, 1, 2, 4, 6, 8, 12, and 16min) at room temperature (25±2℃). The solution was stirred by the magnetic stirring apparatus to ensure uniform corrosion. iv) Wash the etched samples by distilled water for 1 minute. v) Clean the specimens ultrasonically in the bath of ethanol for 3 minutes. vi) Dry the specimens in the electronic vacuum drying oven for 10 minutes at the temperature of 60℃. After these steps the samples were placed in sealed plastic bags to avoid external damage to the new corroded surfaces. All the subsequent flexural tests were
conducted within 10 hours after the etching process. The corrosion thickness was obtained by measuring the thickness of specimens etched for different times with micrometer.
2.3 Three-point bending tests The flexural strength of glass specimens was obtained by three-point-bending tests using an Instron-5848 universal testing machine. The maximum load capacity is 2000N with the precision of 0.1N. The specimens were loaded in the thickness direction and the scratch was in the unloading (lower) surface bearing the most severe tensile stress, as shown in Fig. 2. The quasi-static loading speed of 0.1mm/min was used for all the tests. During experiments, a high-speed camera was utilized to capture the fracture mode of the specimens. The flexural strength is calculated as follows:
σf =
3Fmax L 2bh2
where
(2)
is the flexural strength,
is the peak load, b, L, and h are the width,
span length, and thickness of the specimen separately.
Fig. 2. Schematic of three-point-bending tests
3. Influence of the normal scratch load on cracking and flexural strength of aluminosilicate glass 3.1 Scratch behavior of annealed and chemically strengthened glass Different normal loads were used to scratch the annealed and chemically strengthened glass. Fig. 3 illustrates the evolution of micro-cracks in silicate glasses
during Vickers indenter scratching. There are mainly two types of crack systems under different scratch loads: radial-median cracks and lateral cracks[24]. At a critical load, one or more nascent flaws within the deformation zone are activated and median cracks initiate along the scratching direction. After scratched under evaluated loads, a lateral crack may appear while unloading the indenter, corresponding to the residual stress field behind the indenter in the scratching region[25]. The lateral cracks form around the interface of the elastic-plastic deformation and propagate to the glass surface. When they emerge at the free surface, chips are produced.
Fig. 3 Schematic of crack systems by Vickers indenter scratching With a monotonous increasing scratch load, three damage regimes can be defined: micro ductile regime, micro-cracking regime and micro abrasive regime[10]. For annealed aluminosilicate glass, plastic flow appeared beneath the sharp indenter and a plastic groove was observed when the scratch load was 20gf, as shown in Fig. 4 (a). When the scratch load increased to 50gf, the deformation mode transmitted into the micro-cracking regime. The radial-median and lateral cracks could be seen clearly in the SEM images. For 100gf scratch load, the lateral cracks became denser with a smaller spacing. Then the deformation mode turned to the micro abrasive regime from
micro-cracking regime when the scratch load increased to 200gf, as shown in Fig. 4 (d). In micro abrasive regime, the lateral cracks were not obvious and the damage zone narrowed. Lots of chips formed in the scratch region and an obvious groove formed corresponding to the material removal process. The scratch region turned to micro abrasive regime totally when the normal scratch load was 500gf. It can be seen from Fig. 4 (e) that the damage degree and cracks density was much higher. The scratched groove was full of dense radial cracks. It is worth noting that median cracks can also be observed in the abrasive region.
Fig. 4. Micrographs of scratched annealed glass samples under different normal loads: (a) 20gf, (b) 50gh, (c) 100gf, (d) 200gf, (e) 500gf. The scratch direction is from left to right.
Contrast scratch tests were also performed on CSG. The surface micrographs captured by SEM are shown in Fig. 5. Micro ductile regime is also observed at 20gf. However, when the scratch load increased to 50gf, there were lateral cracks but no radial-median cracks in Fig. 5 (b). It is illustrated that the threshold load for the formation of radial-median cracks is increased for CSG. The reason for the delay of radial-median cracks can be contributed to the compression layer of CSG. The radial-median cracks form in the direction perpendicular to the pre-compression stress along in-plane direction, so these cracks are depressed while the lateral cracks along the compression layer direction are not influenced. Noting that the suppressed radial-median cracks are usually strength-limit cracks for silicate glass. At 100gf, only sporadic radial-median cracks initiated in Fig. 5 (c). When the scratch load reached 200gf, the scratch behavior of CSG also turned to the micro abrasive regime from micro-cracking regime. The median cracks could also be seen clearly from the SEM images in Fig. 5 (d) and (e) at 200gf and 500gf.
Fig. 5. Micrographs of scratched chemically strengthened glass samples under different normal loads: (a) 20gf, (b) 50gh, (c) 100gf, (d) 200gf, (e) 500gf. The scratch direction is from left to right.
Fig. 6 shows the three-dimensional optical morphology and its corresponding two-dimensional profile of scratched glass. The depth information of scratched region is marked with different colors. We can notice that radial-median cracks and lateral cracks initiated for AG scratched at 50gf, while only lateral cracks appeared for CSG. The scratch depth of CSG is much lower than AG at 50gf. However, the depth of scratched grooves at 500gf was nearly the same shown in Fig. 6 (c) and (d). It can be also seen that more chips along the scratch line formed for CSG scratched at 500gf due to pre-stress relief.
Fig. 6. 3D optical morphology and its corresponding 2D profile of scratched glass: (a) AG at 50gf, (b) CSG at 50gf, (c) AG at 500gf, (d) CSG at 500gf
3.2 Flexural strength of scratched specimens The flexural strength of AG and CSG specimens and the residual strength of
scratched specimens were obtained by three-point-bending tests. During tests, a high-speed camera with a frame rate of 100000fps was used to capture the failure mode. As shown in Fig. 7, the flexural strength of unscratched CSG specimens was much higher than AG for the existence of surface compression layer. The flexural strength of AG reduced sharply (from 162.2MPa to 43.4MPa) when scratched at 20gf and then remained a constant value around 40MPa at higher scratch loads up to 500gf. However, with the increase of scratch load, the flexural strength of CSG decreased gradually. It can be seen from the high-speed images that the post-failure crack damage area decreases as the scratch load increases, corresponding to the decrease of flexural strength. A 700gf scratch was introduced to CSG additionally and the average residual strength reduced from 149.5MPa at 500gf to 146.0MPa at 700MPa. That means a nearly constant flexural strength was gained when the scratch load was over 500gf for CSG. Micro abrasive scratch behavior was shown for CSG scratched at 500gf and 700gf with similar scratch depth and crack patterns.
Fig. 7. Flexural strength of scratched glass
3.3 Discussion Fig. 8 shows the fracture surface of unscratched and scratched annealed glass
specimens. The lower edge of the fracture surface corresponds to the tension surface during three-point bending tests. It can be concluded from the mirror and mist region that the fracture origin is near the right side of lower edge for the unscratched specimen, as shown in Fig. 8 (a). However, for scratched specimens, there was stress concentration in scratched region during the loading process. In Fig. 8 (b), the main crack initiated from the scratch line and propagated into a decreasing stress field. The fracture surface is very smooth without mist region and hackles, which is a telltale sign of low-stress fracture.
Fig. 8. The fracture surface of annealed glass specimens (a) without scratch; (b) scratched at 200gf It can be seen clearly that the scratch resistance of CSG is much higher than AG. The chemically strengthened layer can suppress the formation of median crack, which
is a main source for strength declining under flexural loading. The flexural strength of CSG decreased slightly from 246.6MPa to 240.4MPa at 20gf and 233.1MPa at 50gf, because the chemically strengthened layer was not broken totally by indenter at this stage. Then a much sharper decrease to 202MPa at 100gf was obtained. From the SEM images in Fig. 5 (c), the median and radial cracks just appeared when the scratch load was 100gf. The post-failure image in Fig. 7 also shows a positive correlation between flexural strength and cracks region area. High strength specimens can store more strain energy and promote the cracks propagating in glass. It is worth noting that the flexural strength of CSG was still much higher than AG when the scratch load was higher enough to generate median cracks in both glasses. For chemically strengthened glass, compression layers were formed in both surfaces due to the extrusion effect of ion-exchange. Then tensile stress in the inner part of glass sheet to achieve a mechanically-equilibrium state will be formed based on the deformation coordination theory of solids. However, this equilibrium state will be broken when defects are introduced to the surface of CSG, i.e. scratch damage. The residual compressive stress near the scratched area is released for scratched CSG, leading to less compressive stress to balance the expanding of the other intact surface. Finally, there is compression stress around the scratched grooves for scratched CSG specimens[23]. Under flexural loading, the compressive stress around the median cracks should be conquered before the tip is under tensile stress. In this case, a higher flexural strength for CSG specimens under the same scratch load than AG specimens is expected. A further quantitative explanation of this effect can be found in our previous paper[23]. For AG scratched at 20gf, although there were no apparent median cracks initiated, the flexural strength decreased sharply and a major crack initiated from the scratched line of the specimen, as can be seen from Fig. 7. It is believed[26, 27] that the inelastic part of indentation/scratch region is induced by a combination of densification and plastic flow. Yoshida et al.[28] show results that blunt contacts promote more densification whereas sharp contacts promote more plastic flow. Plastic flow is a volume conserving process along with volume dilatation, which is more
likely to produce high density of strength limiting shear crack systems in the subsurface of scratched area at low loads[29, 30]. Also, stress concentration happened in the plastic groove with a small tip radius when the samples were loaded. These two reasons results in the low residual strength of AG scratched at 20gf. Then at 50gf and higher scratch loads, continuous and severe median cracks were generated and the flexural strength maintained a nearly constant value. It indicates that the stress concentration of median cracks limits the strength of AG. However, for CSG scratched at 20gf, the compression stress can suppress the formation of crack systems in the subsurface. There was no material removal and the strengthened layer was not removed during the scratch process, resulting in a small decrease of flexural strength.
4. HF etching effect of scratched glass 4.1 HF reaction kinetics Silicate glass is mainly composed by SiO2, which consists of tetragonal SiO4 units connected at all four corners with the same SiO4 units by covalent siloxane bonds. The chemical reaction of SiO2 and HF is based on the ionization of the acid. Fluoride irons react with the undissociated HF to form bifluoride anion for the attack of silica matrix[31]. These processes are expressed as follows: HF=H++F-, Ka=6.7×10-4 (mol/L)
(3)
HF+F-=HF2-, Ka=3.96 (mol/L)
(4)
where Ka represents the equilibrium constant at 25℃. The relationship between corrosion thickness and corrosion time for AG and CSG is shown in Fig. 9. A similar corrosion thickness rate for both glasses was obtained and the corrosion rate decreased for longer corrosion time. These observations are in consistent with published results[21, 32]. The surface of glass contains multiple flaws, which can increase the effective surface area and result in a higher corrosion rate initially. The reaction rate decreases when the flaws are removed. Another reason for the decreased reaction rate is the continuous reduction of hydrofluoric acid and fluoride irons concentration according to reactions (3)-(4), especially for longer reaction periods[33].
Fig. 9. Corrosion thickness of AG and CSG for different corrosion times
4.2 Surface morphological analysis The AG and CSG specimens scratched at 500gf were put into the chemical treatment process described in section 2.2. The morphology evolution of scratched area for different HF erosion periods is shown in Fig. 10 and Fig. 11. It can be seen from Fig. 4 (e) and Fig. 5 (e) that the untreated glass is characterized by dense radial cracks. Though not very clear, the main median cracks can also be distinguished. For scratched annealed glass after 0.25min HF treatment (Fig. 10a), the dense cracks opened up to many small grooves. The ends of the cracks presented circular fronts. After 0.5min treatment (Fig. 10b), some shallow cracks were eliminated and raised ridges formed between these cracks. Also, the median cracks could be seen clearly after acid treatment. When the treatment time increased to 1min, 2min, 4min and 6min (Fig. 10c-f), radial cracks were removed. Median cracks became more shallow and wider during erosion. After 8min treatment (Fig. 10g), the median crack was not very clear and the front was blunted, corresponding to a corrosion thickness of 81µm according to Fig. 9. The median cracks were removed after 10min and 12 min chemical reaction shown in Fig. 10 (h) and (i).
Fig. 10. Micrographs of scratched annealed glass treated in HF acid during different durations: (a) t=0.25min, (b) t=0.5min, (c) t=1min, (d) t=2min, (e) t=4min, (f) t=6min, (g) t=8min, (h) t=12min, (i) t=16min
Similar morphology evolution process can be observed for scratched CSG specimens during chemical treatment shown in Fig. 11. It’s noting that the median cracks begin to be indistinguishable in Fig. 11 (e), which is a similar state as Fig. 10 (g) shows. However, the treatment during is only 4min for Fig. 11 (e), with a corrosion thickness of 49µm. That means the depth of median crack of CSG is much smaller than AG scratched at the same normal load.
Fig. 11. Micrographs of scratched chemically strengthened glass treated in HF acid during different durations: (a) t=0.25min, (b) t=0.5min, (c) t=1min, (d) t=2min, (e) t=4min, (f) t=6min, (g) t=8min
The cracks blunting effect can be seen more clearly from three-dimensional optical morphology of etched specimens shown in Fig. 12. After 0.5min (Fig. 12a) or 1min (Fig. 12c) treatment, the crack tips were blunted, but the median cracks were still very obvious. As the etching process median cracks disappeared and quasi spherical shape caps appeared at the scratched region (Fig. 12b and d), which was also reported in Ref. [22, 32]. The observed craters depth decreased a lot with the increase of treatment duration. The surface flatness also improved with much lower stress concentration around the scratch region during loading process. This has a positive effect on the mechanical strength as we can see in the next section.
Fig. 12. Three-dimensional optical morphology of etched scratched glass: (a) AG for 0.5min, (b) AG for 16min, (c) CSG for 1min, (d) CSG for 8min
4.3 Flexural strength of etched specimens The flexural strength of etched specimens for different acid treatment durations is shown in Fig. 13. The untreated AG and CSG specimens have an average strength of 37.3MPa and 149.5MPa respectively. There are two reasons for the much higher value of scratched CSG specimens. Besides the stress balance effect of CSG illustrated in section 3.3, the other reason is much shallower median cracks resulting from the compression layers of CSG, which can be proved from the HF etching tests above. It can be seen from Fig. 13 (a) that the flexural strength of AG rises slowly from the beginning to 6min treatment. The specimens broke into two pieces during loading according the high-speed images. The strength increased to 292.4MPa after 8min of HF reaction and a crack zone near the scratch area was observed after breakage, corresponding to the specimens with unclear median cracks shown in Fig. 10 (g). After 12min and 16min treatment, the strength increases to 765.0MPa and 904.5MPa respectively. For CSG, a similar “slow-fast-slow” strength increasing trend was obtained from tests. The treatment time for CSG is much shorter because of the shallow median cracks and faster median cracks elimination process shown in Fig. 11. Finally the strength of AG and CSG is nearly the same around 900MPa after 16min
and 8min treatment. The initial surface is removed at this period and the strength improvement is mainly governed by the bulk defects instead of surface flaws[18, 22]. In this way, the flexural strength of CSG and AG increased to a considerable value of 900MPa, which is 5.5 and 3.6 times higher than the flexural strength of undamaged AG (162.2MPa) and CSG (246.6MPa) specimen. Fig. 13 (b) is a local enlarged drawing of Fig. 13 (a) for the HF treatment period less than 1min. For annealed glass, the flexural strength increased slightly by 10.7%, 13.4% and 17.2% after 0.25min, 0.5min and 1min treatment. In this period, although the median crack front also opened and blunted through acid reaction, the crack tip radius is still at micro scale. The main strength limiting factor in this period is stress concentration of median cracks. However, the strength of CSG increased from 149.5MPa to 165.3MPa after 0.25min treatment but decreased to 153.1MPa at 0.5min. The HF reaction will remove the strengthened layers of CSG first and release the compression stress, which is the main reason for the strengthening effect of CSG. After 0.25min acid reaction, the corrosion thickness is 4.1µm (Fig. 9), while the strengthened layer thickness is about 10µm. The strengthened layer is partly removed in this condition and the strengthening effect still exists. However, after 0.5min treatment, the corrosion thickness increases to 10.6mm and the compression layer is totally removed. The strength decrease is observed at this time (Fig. 13b). Then the strength increases monotonously because of the cracks opening and blunting effect, as well as the surface flatness restitution process[22].
(a) Flexural strength versus corrosion time
(b) Local enlarged drawing of (a) Fig. 13. Flexural strength of scratched glass etched by HF acid for different periods
5. Conclusions In this study, the influence of normal scratch load on the cracking pattern and flexural strength of annealed and chemically strengthened glass were studied. Then scratched specimens were put into the HF acid treatment procedure to investigate the effect of HF etching on the mechanical behavior of scratched glass. The relation between macroscopic mechanical behavior and micro-cracks of scratched glasses were established. The following conclusions were drawn: (1) Micro ductile regime, micro-cracking regime and micro abrasive regime were observed with increasing scratch load for both AG and CSG. The strengthened layer of CSG can suppress and delay the formation of median-radial cracks during Vickers indenter scratching. (2) The flexural strength of scratched AG declined sharply while that for CSG decreased gradually. The stress balance and redistribution of scratched CSG and the suppression of median cracks contributes to the much higher strength of CSG than AG at the same normal scratch load. (3) During HF reaction process, a similar corrosion thickness rate for AG and CSG was obtained and the corrosion rate decreased for longer reaction time. Morphological changes including cracks opening and blunting were observed after HF treatment. (4) A “slow-fast-slow” strength increase process was obtained for etched AG and CSG with the increase in reaction period. The “fast growth” stage of flexural strength corresponds to the disappearance of median cracks. The final strength of the glasses was nearly the same while shorter reaction time was taken for CSG.
Acknowledgements This work is financially supported by National Natural Science Foundation of China (grant number 11772268, 11522220, 11527803 and 11627901). We also would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for SEM characterization of the hydrofluoric acid etched glass specimens.
References 1.
Jannotti, P., et al., Influence of ultra-high residual compressive stress on the static and dynamic indentation response of a chemically strengthened glass. Journal of the European Ceramic Society, 2012. 32(8): p. 1551-1559.
2.
Wondraczek, L., et al., Towards ultrastrong glasses. Advanced Materials, 2011. 23(39): p. 4578-4586.
3.
Sheikh, M.Z., et al., Dynamic failure of annealed and chemically strengthened glass under
4.
Sheikh, M.Z., et al., Static and dynamic Brazilian disk tests for mechanical characterization of
compression loading. Journal of Non-Crystalline Solids, 2018. 499: p. 189-200. annealed and chemically strengthened glass. Ceramics International, 2019. 45(6): p. 7931-7944. 5.
Tsien, L.C., On Surface Cracks in Glasses. Proceedings of the Royal Society of London, 1937. 159(898): p. 346-354.
6.
Varshneya, A.K., Chemical Strengthening of Glass: Lessons Learned and Yet To Be Learned. International Journal of Applied Glass Science, 2010. 1(2): p. 131-142.
7.
Varshneya, A.K., Stronger glass products: Lessons learned and yet to be learned. International Journal of Applied Glass Science, 2018. 9(2): p. 140-155.
8.
Lawn, B.R., Fracture of brittle solids. Physics Bulletin, 1976. 27(5): p. 220-221.
9.
Sawamura, S. and L. Wondraczek, Scratch hardness of glass. Physical Review Materials, 2018. 2(9): p. 092601.
10.
Houérou, V.L., et al., Surface damage of soda–lime–silica glasses: indentation scratch behavior. Journal of Non-Crystalline Solids, 2003. 316(1): p. 54-63.
11.
Schneider, J., S. Schula, and W.P. Weinhold, Characterisation of the scratch resistance of annealed and tempered architectural glass. Thin Solid Films, 2012. 520(12): p. 4190-4198.
12.
Li, K., Y. Shapiro, and J.C.M. Li, Scratch test of soda-lime glass. Acta Materialia, 1998. 46(15): p. 5569-5578.
13.
Bensaid, N., et al., Influence of the normal load of scratching on cracking and mechanical
14.
Swab, J.J., et al., Influence of Surface Scratches on the Flexure Strength of Soda-Lime Silicate
strength of soda-lime-silica glass. Journal of Non-Crystalline Solids, 2018. and Borosilicate Glass. Experimental Mechanics, 2013. 53(1): p. 91-96. 15.
Sawamura, S., et al., Scratch‐induced yielding and ductile fracture in silicate glasses probed by nanoindentation. Journal of the American Ceramic Society, 2019.
16.
Zhen, W., et al., A comparative study on the effect of loading speed and surface scratches on the flexural strength of aluminosilicate glass: Annealed vs. chemically strengthened. Ceramics International, 2018. 44(10): p. 11239-11256.
17.
Saha, C.K. and A.R. Cooper Jr, Effect of etched depth on glass strength. Journal of the American Ceramic Society, 1984. 67(8): p. C‐158-C‐160.
18.
Sglavo, V., R. Dal Maschio, and G. Soraru, Effect of etch depth on strength of soda-lime glass rods by a statistical approach. Journal of the European Ceramic Society, 1993. 11(4): p. 341-346.
19.
Dabbs, T.P. and B.R. Lawn, Acid‐Enhanced Crack Initiation in Glass. Journal of the American Ceramic Society, 1982: p. C37-C38.
20.
Mikeska, K.R., S.J. Bennison, and S.L. Grise, Corrosion of Ceramics in Aqueous Hydrofluoric Acid. Journal of the American Ceramic Society, 2000. 83(5): p. 1160-1164.
21.
Li, X., et al., Effects of HF etching on nanoindentation response of ion-exchanged aluminosilicate float glass on air and tin sides. Journal of materials science, 2017. 52(8): p. 4367-4377.
22.
Kolli, M., et al., HF etching effect on sandblasted soda-lime glass properties. Journal of the European Ceramic Society, 2009. 29(13): p. 2697-2704.
23.
Wang, Z., et al., Quasi-static and dynamic flexural behavior of annealed and chemically strengthened aluminosilicate glass with notch defects. Journal of Non-Crystalline Solids, 2019. 521: p. 119479.
24.
Li, X., et al., Nucleation location and propagation direction of radial and median cracks for brittle material in scratching. Ceramics International, 2019. 45(6): p. 7524-7536.
25.
Wang, W., et al., Elastic stress field model and micro-crack evolution for isotropic brittle materials during single grit scratching. Ceramics International, 2017. 43(14): p. 10726-10736.
26.
Yoshida, S., J.C. Sanglebœuf, and T. Rouxel, Quantitative Evaluation of Indentation-Induced Densification in Glass. Journal of Materials Research, 2005. 20(12): p. 3404-3412.
27.
Peter, K.W., Densification and flow phenomena of glass in indentation experiments. Journal of Non-Crystalline Solids, 1970. 5(2): p. 103-115.
28.
Yoshida, S., et al., Effects of indenter geometry on indentation-induced densification of
29.
Hagan, J.T., Shear deformation under pyramidal indentations in soda-lime glass. Journal of
soda-lime glass. Journal of Materials Research, 2010. 25(11): p. 2203-2211. Materials Science, 1980. 15(6): p. 1417-1424. 30.
M, C.T., Scratch Damage in Ion-Exchanged Alkali Aluminosilicate Glass: Crack Evolution and the Dependence of Lateral Cracking Threshold on Contact Geometry. 2012: John Wiley & Sons, Inc. 113-122.
31.
Ezz-Eldin, F., T. Abd-Elaziz, and N. Elalaily, Effect of dilute HF solutions on chemical, optical, and mechanical properties of soda–lime–silica glass. Journal of materials science, 2010. 45(21): p. 5937-5949.
32.
Tso, S.T. and J.A. PASK, Reaction of glasses with hydrofluoric acid solution. Journal of the
33.
Knotter, D.M., Etching mechanism of vitreous silicon dioxide in HF-based solutions. Journal of
American Ceramic Society, 1982. 65(7): p. 360-362. the American Chemical Society, 2000. 122(18): p. 4345-4351.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: