Sol-Gel/Ion Exchange Composite Process Strengthening Glass
|Keywords||composited strengthening process ion exchange Sol-Gel Zirconia film high-speed train window Nano hardness|
High-performance equipments and materials are required with the rapid development of high-speed railway technology. As an important component of high-speed train system, the window glass material demands high strength, security and stability. Chemical strengthening glass demonstrates high strength, cuttale, barely optical defect, hardly any deformation and plays an important role in high-speed train window glass. Therefore, researches in this territory gained much attention and achieved many breakthroughs both at home and abroad. However, because of the relatively low surface hardness, chemical strengthening glass is susceptible to weakness from micro defects. Surface damages penetrated thin compressive stress layer would lead to lowering the overall bending strength remarkably, and hence lowered security and reliability of high-speed train window-shields or cockpit.Sol-Gel coating/ion exchange composite strengthening process were adopted to improve wear-resistance of chemical strengthing glass. Recent studies shown that glass surface could be enhanced by coating with transparent hard Sol-Gel films. However, few reports were concerned on jointly enhancing strength and wear-resistance by composite strengthening process. In our research, based on optimization experiments of process parameters, molten salt formula of the ion exchange process and exploring new type of propylene epoxide-assisted Sol-Gel route, wear-resistant chemical strengthening glass were obtained by both ion exchange-Sol-Gel coating and Sol-Gel coating-ion exchange composite strengthening methods. At the same time, Main influencing factors and mechanism of all processes were investigated.The main research contents and conclusions are as follows:(1) Parameters and fused mixed salts formula were optimized according to ion exchange strengthening theory.The results showed that:After pretreatment in chromic acid lotion for15min, raw glass ion exchanged in molten potassium nitrate salt at450℃for10-11hours can achieve maximum bending strength. Optimized molten mixed salt formula obtained by the orthogonal experiment is (in mass):KNO3:K2CO3:KOH:alumina:diatomite=100:2:0.5:3.5:1.5. Ion exchange time would be reduced to9hours by using this molten salt formula instead of pure potassium nitrate, and obtained a bending strength as high as-362MPa. Visible light transmittance was unchanged, while nano hardness was greater than the glass surface in the depth less than～650nm and almost the same in deper thickness after ion exchange. (2) Mechanism and main influencing factors were studied in manufacturing coating sols with low cost propylene epoxide-assisted Sol-Gel route.We found that:stable and uniform unitary or binary composite sols (solid content:2wt%, viscosity:6.5-7mPa·s) could be obtained when the reaction solution which consisted of Zirconium Chloride or Tin Chloride or Ethyl Orthosilicate as precursor, mixed water and ethanol as solvent (water/alcohol:VW/E=1/3, in volume), assisted with propylene epoxide (PO/Cl or-CH2CH3=1.5/1, in mole ratio) and dilute nitric acid (5wt%,5ml/100ml), stirred well at room temperature after30h. The brief reaction mechanism is that:propylene epoxide is a gel agent witch promotes precursors’s hydrolyzation and polymerization reaction by ring opening reaction; adding suitable amount of dilute nitric acid could stabilize the pH value of the system at the same time; moderate control VW/E of solvent improves dielectric constant of dispersion medium and so does the electrostatic repulsion between colloidal particles. This method not only ensures the formation and stability of the sol, but also avoids fast aggregation of the colloidal particles to become gel.(3) Transparent and wear-resistant films coated chemical tempering glass prepared by ion exchange-Sol-Gel coating composite strengthening method.Chemical tempered glass coated with ZrO2-SiO2, ZrO2-SnO2series thin films in the surface obtained by Sol-Gel method. Influences of heat treatment temperature and film composition on the mechanical and optical properties of the coated glass were investigated. Results showed that: Homogeneous, continuous and dense films are obtained. With increasing heat treatment temperature from300℃to500℃,～45nm pure ZrO2film gradually transformed from amorphous into tetragonal phase, whose nano hardness were increased. Meanwhile, refractive index of film also increased which resulted in reflection and scattering loss and reduced glass transmittance. During the high temperature heat treatment, obvious compressive layer relaxation caused rapidly bending strength reduction. As a result, films coated chemical tempering glass should not be heat treated exceed400℃. Besides, as coating films thickness increased from45nm to200nm, transmittance will fall sharply too. However, there was no significant influence on bending strength of the coated glass.ZrO2-SiO2series thin films containing Si are amorphous structure after heat treating at400℃for1h. When increase of Si content in the films, transmittance would be improved, so did the bending strength of coated glass, but nano hardness and young’s modulus decreased. Named1Zr-1Si thin film (thickness～45nm) coated sample exhibited comprehensive better performance:surface hardness=14GPa, bending strength=342MPa, visible light transmittance is greater than88%. ZrO2-SnO2series thin films with more SnO2content could found more significant tetragonal phase SnO2and increased visible light transmittance, while nano hardness and bending strength of coated glass almost remained the same. Pure SnO2film (-45nm thickness) coated sample performs optimal properties:surface hardness=16.4GPa, bending strength=322MPa, visible transmittance>90%.(4) Preparation of transparent and wear resistant films coated chemical strengthening glass through the Sol-Gel coating-ion exchange composite strengthening method.To avoid high heat treatment temperature limitations in ion exchange-thin film coating process, an alternative path was developed. Raw glass was coated by Sol-Gel method and heat treated at550℃for2hours, then, followed by an ion-exchange strengthening process. Investigation results revealed that:Homogeneous, continuous are obtained. Pure ZrO2film belongs to tetragonal structure, other films with SiO2have an amorphous and with SnO2have tetragonal SnO2, ZrO2and orthorhombic ZrSnO4polycrystalline structure. All obtained films with high H/E≥0.1ratio and elastic recovery (We≥60%) were thought to benefit glass3-point bending strength. Light transmittance increased, while refractive index, hardness and Young’s modulus decreased with the increasing SiO2content in films. Nano hardness and bending strength change trend are not obvious when SnO2content increased in ZrO2-SnO2series composite coated films and lowered the refactive index to improve transmittance. Values of bending strength of ZrO2-SnO2coated glass are in the range of360MP-380MPa. Pure SnO2film (-45nm thickness) coated sample performs optimal nano hardness=24GPa and visible transmittance>91%. Surface film was considered as obstacles during ion exchange. Ion exchange depth was reduced and resulted in bending strength reduction while the film thickness increased. However, reinforcement brought by Sol-Gel films could make up the strength loss caused by exchange of depth in less than the film thickness of200nm.In conclusion, high film-glass bonding wear-resistant chemical tempering glass could be easily and expediently obtained by Sol-Gel coating/ion exchange composited strengthening process with low cost. Those coated films protect the vulnerable surface compressive stress layer from scratch damage. Meanwhile, the excellent properties of original chemical tempering glass were able to keep. We think composited strengthening process is quite suitable when applied in preparation of high strength wear-resistant high-speed train window materials.