Chemical Strengthening Research Articles

Mechanistic understanding of the fracture toughening in chemically strengthened glass—experiments and phase-field fracture modeling

The use of silicate glass in engineering applications has become widespread in recent years due to its superior mechanical and optical properties. However, glass, being a brittle material, poses extremely poor resistance against fracture. Chemical strengthening through ion-exchange has emerged as a promising technique to improve its fracture resistance. In this study, the relationships between chemical strengthening parameters and the resulting fracture toughness of chemically strengthened (CS) glass are investigated using experiments, analytical and numerical simulations. Our results show that optimal fracture resistance for a CS glass requires a proper combination of chemical strengthening parameters, namely the surface compressive stress (σCS), and depth of the compression layer (DOL). The present study establishes the fact that unlike glass, the fracture toughness (KICCS) of a CS glass is not a constant material parameter, instead depends on the initial crack/notch length. The experimental data have also been supported with analytical calculations based on linear elastic fracture mechanics and numerical simulations based on the phase-field fracture theory. These findings provide a novel understanding towards the fracture toughness improvement of CS glass, which should facilitate their widespread engineering applications.

Chemically strengthened glass for architectural applications

Chemically strengthened glass is commonly used for applications that require high strength, such as boat and train windshields, control towers and the like. The strengthening process provides the glass with a surface residual stress that outperforms other strengthened glass types, such as thermally toughened glass. However, its use has been limited, probably for two main reasons: its high cost compared to other strengthening method and the perceived loss of the strengthening if glass surface is damaged. With the recent developments in the field of thin glass, where chemical strengthening is the only possible pre‐stressing method, there is the opportunity to better understand the process and the procedure to make it a safe material for architectural applications. In this paper, an extensive investigation on chemically strengthened glass is presented. Different parameters are considered, such as different thicknesses and chemical strengthening process conditions. The work consists of photoelastic measurement and destructive test. The data arising from the tests are elaborated with the purpose of suggesting a method to calculate the design strength of chemically strengthened glass according to the existing design standards. It is highlighted the importance to qualify chemically strengthened glass in terms of its residual stress profile.

COMPARATIVE ANALYSIS OF NATM CONSTRUCTION TECHNOLOGIES OF DNIPRO METRO ESCALATOR TUNNEL

Purpose. On the basis of the comparative analysis to carry out a substantiation of the most expedient and rational way of strengthening of a weak massif during a construction of Dnipro metro escalator tunnels by NATM. Methodology. To achieve this goal, an analysis of construction technologies in weak soils was conducted. The most used technologies are Forepoling Umbrella System (FUS), artificial ground freezing and chemical cementation. The peculiarities of carrying out each of the technologies for the conditions of inclined production were analyzed. It is determined how each of the technologies is applied to escalator tunnels and implements the strengthening of weak soil. Findings. The advantages and disadvantages of three technologies for fixing weak soil around the escalator tunnel are identified. Based on comparative analysis, it was found that the only technology that provides increased strength parameters of loamy soils, characteristic for the upper part of the escalator tunnel of the Dnipro metro, is the technology of chemical strengthening (cementation). In some cases, if necessary, short sections of sloping course, characterized by particularly weak soil, can be supported by several pipes, without creating a continuous leading mount. The results of the analysis are the basis for further substantiation of cementation, which creates a multilayer system "reinforced soil massif – temporary fastening – permanent lining". Originality. Based on the results of comparative analysis of three technologies for escalator tunnel construction by NATM, it is proved that the use of cementation not only increases the strength of the soil during drilling, but also further in operation serves as an additional element of the multilayer system "reinforced soil massif – temporary fastening – permanent lining". Practical value. In the course of research, the substantiation of cementation as the most rational and effective technology of strengthening of the surrounding weak massif at construction of the Dnipro metro was carried out.

Lithium aluminosilicate (LAS) glass-ceramics: a review of recent progress

ABSTRACT The lithium aluminosilicate system (LAS) is being explored for almost seven decades due to its anomalous and attractive properties (especially low/negative thermal expansion and fast ion conduction) and its commercial significance in consumer as well as strategic sectors. This review introduces current commercial applications of LAS systems, necessary background science, structural features of different LAS crystal systems, including their polymorphs and solid-solutions, and the origin of unusual properties. Significant emphasis is provided on processing transparent, nanocrystalline, low thermal expansion glass-ceramic (LEGC), the role of chemical constituents and additives, the effect of heat-treatment, and microstructural evolution while processing LEGC. Detailed discussions are provided on the following areas: LAS matrix composites, chemical strengthening of glass and glass-ceramic, low temperature co-fired ceramics (LTCC) and associated joining technologies (hydrolysis catalysis bonding, anodic bonding, brazing, etc.) that would further extend the application portfolio of LAS system.

Effect of chemical strengthening residual stress on the flexural performance and fracture behavior of aluminosilicate glass

In this hybrid experimental, theoretical and numerical study the effect of chemical strengthening residual stress on the flexural strength and fracture behavior of aluminosilicate glass was investigated. Three-point bending tests were conducted on annealed and chemically strengthened glass specimens to obtain the flexural strength of both specimens and with the aid of high-speed cameras, the fracture and failure modes of these two kinds of specimens were analyzed and compared. A theoretical analysis based on the elastic fracture mechanics theory was employed to explain the strengthening effect of chemically strengthened glass. It is shown that the mechanical strength of chemically strengthened glass is highly related to the relation of the chemically strengthened layer depth and the surface flaw depth. Furthermore, this study provides an effective numerical approach to replicate the mechanical strength and failure modes for both annealed and chemically strengthened glass. The proposed finite element numerical method includes modeling techniques which consider the presence of pre-stress, random distributed surface flaws and the fracture onset in glass specimens. The residual stress pattern was defined exploiting the dynamic relaxation process for temperature-induced expansion of the elements before the loading process. Discrete distributed surface flaws were inserted in the model, the brittle fracture of glass was represented by the smeared crack method and the numerical results were compared to experimental data. It’s illustrated that the proposed method can be used to represent both the discrete fracture behavior and the chemically-strengthening effect of silicate glass.

Chemical short-range order strengthening mechanism in CoCrNi medium-entropy alloy under nanoindentation

The strengthening effect of chemical short-range order (SRO) structure in CoCrNi medium-entropy alloy (MEA) was investigated using molecular dynamics (MD) simulations of nanoindentation. The quantitative correlation between SRO parameters and mechanical properties was established. Results show that the strength and hardness of CoCrNi MEA increase with increasing chemical SRO parameters and reach a stable value with steady SRO structure. Compared with random solid solution (RSS) state model, the average hardness increases 8.1% in an intermediate SRO model and 13.7% in a stable SRO model. The dislocation nucleation force of SRO model is 55% larger than RSS model. Moreover, dislocation pinning induced by local Ni SRO structure, as well as the promoted unique dislocation interaction were observed during the nanoindentation process. Finally, results also show that as the temperature rises, the enhancement of hardness becomes more significant (11.4% at 70K, 17.24% at 300K, and 23.8% at 800K).

Investigation of optimum nucleation temperature and heating rate of cordierite glass-ceramics

Abstract In this survey, cordierite glass-ceramic was prepared by melting in a platinum crucible at 1 580 °C. The effects of temperature and heating rate were studied on the nucleation and crystallization. Nucleation and crystallization temperatures were obtained by differential thermal analysis, scanning electron microscopy and X-ray diffraction. The results showed that the optimum nucleation and crystallization occurred at 810 and 1 150 °C, respectively. According to the X-ray diffraction patterns, cordierite, rutile, quartz and MgTi2O5 phases were obtained in the crystallized glassceramics. The maximum bending strength of 178 MPa was obtained after the chemical strengthening of the crystallized glass ceramic.

Effects of alkali oxides and ion-exchange on the structure of zinc-alumino-silicate glasses and glass-ceramics

Types and contents of alkali metal ions play important role on crystallization and ion-exchange properties in glasses. In this work, effects of Na2O/K2O ratio on crystallization and ion-exchange properties of zinc-alumino-silicate glasses were investigated. The crystalline phases precipitated in glasses changes from ZnO to β-Zn2SiO4 with the progressive replacement of K2O by Na2O in parent glasses. Ion-exchange depth of layer (DOL) decreases gradually with the increase in Na2O content in parent glasses. Precipitation of ZnO and β-Zn2SiO4 nanocrystals facilitated the ion-exchange and enlarged the DOL. Na+ and K+ ions were doped into ZnO and β-Zn2SiO4 nanocrystals during heat-treatment, and the extent of doping was facilitated by ion-exchange. Vickers hardness were improved significantly with the crystallization and ion-exchange. Results reported here are valuable for the controlled preparation and chemical strengthening of ZnO and β-Zn2SiO4 glass-ceramics.

Coupling of diffusion and chemical stress: The case of ion exchange in glass

AbstractThe chemical strengthening of glass results from an ion exchange process in which smaller alkali ions in a glass are replaced with larger alkali ions from a molten salt bath. This interdiffusion process leads to a buildup of chemical stress in the glass. However, traditional modeling of the ion exchange process has not fully accounted for interaction effects between mass diffusion and the chemical stress developed during the process. In this study, we develop the general theory of coupling between diffusion and stress, resulting in a single flux equation with a concentration‐ and stress‐dependent interdiffusion coefficient. We apply the theory to the specific cases of chemically strengthened soda lime silicate and aluminosilicate glasses, demonstrating the impact of interaction terms on concentration profiles and interdiffusion coefficient. Following a phenomenological approach, this study demonstrates the effect of the interdiffusion on stress generation and vice versa to account for deviations from the simple expressions published hitherto in the literature.

Chemical strengthening of Li+-containing phosphosilicate glass via a two-step ion-exchange process

Chemical strengthening of Li+-containing phosphosilicate glass via a two-step ion-exchange process

Chemical short range order strengthening in BCC complex concentrated alloys

Atomistic methods are used to anneal two body-centered cubic (BCC) chemically complex alloys (CCAs) in order to assess the effect of chemical short-range order on alloy strength. The two alloys, a model quaternary Co16.67Fe36.67Ni16.67Ti30 and ternary Nb33.33Ti33.33Zr33.33, are represented using simple Zhou interatomic potentials. Chemically random cells are annealed at temperatures 65% of the average melting temperature of the individual elements using a Monte Carlo approach, and the critical stress required to move a/2[111] screw dislocations in the initial and annealed cells are estimated using molecular dynamics simulations. It is shown that annealing leads to a softening of the BCC quaternary alloy relative to the random state, especially for deformation at low temperatures. On the other hand, short-range order has minimal effect on solid solution strengthening in the ternary alloy. These results are modeled using an extension of the Suzuki model of substitutional solid solution strengthening developed for BCC chemically complex alloys. Here the model is modified to account for the effects of chemical short-range order. Good agreement is shown between the model results and direct atomistic simulation data. The model presented should be useful in predicting BCC CCAs with adequate high temperature strength, which will accelerate development of high temperature, high strength BCC alloys for aerospace applications.

Lightweight Zr1.2V0.8NbTi Al high-entropy alloys with high tensile strength and ductility

High-entropy alloys (HEAs) provide a new perspective to design novel lightweight alloys, whereas the complex precipitates in current lightweight HEAs (LHEAs) generally limit the mechanical properties. By controlling Ti and Al, the B2 and C14 Laves precipitates in Zr 1.2 V 0.8 NbTi x Al y (Ti x Al y ) LHEA system were successfully suppressed, and a series of LHEAs with outstanding specific yield strength-ductility matching were designed. Especially, with the same deformability (tensile ductility over 25%), the specific yield strength of Ti3.6Al0.6 LHEA is over 40% higher than current developed HEAs. The excellent mechanical properties are attributed to be benefited by a combination of solid solution strengthening and chemical short-range order strengthening. Our results bring new insights into the phase stability and precipitation suppression of HEAs, which is constructive for the novel lightweight alloy development. • Ti–Al control achieved both precipitate suppression and lightweight in HEA design. • A specific strength improvement of 40% was achieved in HEAs with ductility over 25%. • Strengthening of solid solution and chemical short-range order benefit the property.

Chemical strengthening of zirconia/swelling mica composites by ion-exchange in molten salts

ABSTRACT In this study, strengthening of machinable zirconia/mica composites was attempted by exchanging interlayer ions of mica in composites for larger ions, like chemically strengthened glasses. So the sintering behavior and ion-exchange of the composites fabricated using swelling mica and the mechanical properties of the ion-exchanged composites were investigated. The dense composite was obtained by sintering at 1200°C for 1 min and was ion-exchanged in molten potassium salts at 800°C. Na+ ions in the interlayer of mica near surface of the composites were exchanged for K+ ions. However, K+ ions were not inserted in the interlayer of mica. Mica reacted with molten salts to form an amorphous phase near surface of the composites. Simultaneously with the reaction, Na+ ions in mica were discharged into molten salts and K+ ions in molten salts were penetrated into the amorphous phase. Then other crystalline phases were formed in the amorphous phase. By the ion-exchange, the cracks developed from Vickers indentation corners were obviously shortened, which is one of the evidence that the compressive stress was generated near surface. By the compressive stress, the fracture toughness (3.5 MPa・m0.5) and the bending strength (322 MPa) of the composites were increased to 5.6 MPa・m0.5 and 447 MPa, respectively.

The effect of substitution of Al2O3 and B2O3 for SiO2 on the properties of cover glass for liquid crystal display: Structure, thermal, visco‐elastic, and physical properties

AbstractBy the substitution of Al2O3 and B2O3 for SiO2 of the glass network former, the structural and thermal, visco‐elastic and physical properties of aluminosilicate glass were investigated. 27Al NMR results showed that in the glass structure of the system, Al3+ basically existed in the form of 4‐coordinated [AlO4] tetrahedron as the network former. Judged by Raman spectra, the bridging vibration Si‐Ob increased with the Al2O3/SiO2 substitute. And the total non‐bridging vibration Si‐Onb decreased, indicating the enhancement of the structure compactness. While for B2O3/SiO2 substitute, the vibration at around 1010 cm−1 and the bridging vibration Si‐Ob in Q4 unit both decreased and the structure got loosened. Based on the study of the structure of glass, the thermal, visco‐elastic, and physical properties such as viscosity, coefficient of the linear thermal expansion, high‐temperature resistance, strain point, annealing point, and softening point, as well as density and elastic modulus were investigated. The evaluation of the melting and fining process was also investigated by high‐temperature observation test. Furthermore, the chemical strengthening properties of glass samples were also investigated. The introduction of B2O3 benefited for the ion‐exchange. In this study, the glass composition of sample 4 was suitable for the floating process under comprehensive consideration.

Surface damage resistance and yielding of chemically strengthened silicate glasses: From normal indentation to scratch loading

AbstractWe report on surface elasticity, plastic deformation and crack initiation of chemically strengthened soda‐lime silicate and sodium aluminosilicate glasses during lateral indentation and scratch testing. Instrumented indentation using a normal indenter set‐up corroborated previous findings on the effects of chemical strengthening on surface Young's modulus, hardness, and indentation cracking. Using lateral indentation in the elastic‐plastic regime, we find a pronounced increase in the scratch hardness as a result of chemical strengthening, manifest in higher work of deformation required for creating the scratch groove. Thereby, the glass composition is found to play a stronger role than the absolute magnitude of surface compressive stress. Using a blunt conical stylus for instrumented scratch testing reveals three distinct modes of scratch‐induced surface fracture, which occur during scratching or after unloading. Occasional micro‐cracking caused by pre‐existing surface flaws at low scratching load can be completely suppressed through chemical strengthening. The intrinsic defect resistance to microcracking is reduced as a result of ion stuffing, depending on the initial glass composition, whereas the resistance to abrasive yielding is enhanced by several hundred MPa.

Chemical Strengthening of Sodium Aluminosilicate Glasses Containing P2O5 and B2O3

Sodium alumino-silicate (SAS) glasses were synthesized with varying P2O5 and B2O3 contents, and their effect on chemical strengthening properties such as surface compressive stress (CS) and depth of layer (DOL) was examined. When P2O5 replaced SiO2, CS decreased, while DOL increased with P2O5 content. In contrast, B2O3 improved CS at the expense of DOL. The simultaneous addition of P2O5 and B2O3 with varying ratios also changed CS and DOL depending on the P2O5 and B2O3 content. The interdiffusion coefficients of K+ ions in SAS glasses were estimated using the Boltzmann–Matano equation and supported the observed effects of P2O5 and B2O3. Structural modifications related to changes in chemical strengthening properties were monitored by Raman spectroscopy and discussed.

Research on chemical strengthening technology for K9 optical glass

Research on chemical strengthening technology for K9 optical glass

Enhanced technology of phytoremediation

The enhanced technology of phytoremediation has the advantages of low treatment cost, good purification effect, low environmental risk and environmental aesthetics. However, some hyperaccumulators grow slowly and their biomass is generally low; the activity of heavy metals in the soil is very low; the roots of plants are distributed in the surface of the soil, and the remediation effect of deep soil is poor; the nutrients of the soil to be repaired are seriously insufficient. It is necessary to take a series of strengthening measures to improve the efficiency of phytoremediation. The strengthening technologies of phytoremediation include chemical strengthening, microbial strengthening, animal strengthening, carbon dioxide strengthening, agronomic and management measures strengthening, etc.

Measurement of Dynamic Elastic Modulus and Poisson's Ratio of Chemically Strengthened Glass.

Glass with strong durability and transparency has been in the spotlight in various fields, including displays. Elastic and shear moduli and Poisson’s ratio are important properties of glasses. The purpose of this study is to evaluate the change in mechanical properties, such as the dynamic elastic modulus and Poisson’s ratio, with respect to the chemical strengthening time of glass for display applications, as measured by static and dynamic methods. The basic measurement principle of the dynamic method is to measure acoustic speed or resonant frequency using an ultrasonic generator. The mechanical properties of both non-strengthened and chemically strengthened glasses were investigated. It was found that the strength of the chemically strengthened glass decreased when chemical strengthening time increased. Chemical strengthening increased the bending strength and decreased the elastic modulus due to the introduction of compressive residual stress at the surface.

Physical properties of chemically strengthened thin glass prepared by the spray method using an original KNO3–Al2O3 slurry

In this study, we developed a novel KNO3–Al2O3 slurry for the chemical strengthening of thin glass sheets using a spray process, and investigated the physical properties of the strengthened glass. A 1 μm particle size α-Al2O3 powder is mixed with a KNO3 solution to obtain the novel slurry. Controlling the temperature and duration of the chemical strengthening process effectively tunes the compressive stress and depth of layer. The strengthened thin glass sheet produced in this study exhibits similar strength properties as commercial products. The highest compressive stress (CS) value (868 MPa) is obtained for the sample heated at 400 °C for 1.5 h, and the highest depth of layer value (60 μm) is observed for the sample at 480 °C for 4 h. The strengthened glass shows good hardness and bending strength. The glass sample heated at 400 °C for 1.5 h exhibits the highest hardness (680 HV) and bending strength (630 MPa).