Strengthening of soda lime glasses using molten salt bath and salt paste applications via ion exchange

Usage areas of glass materials are widening day by day in daily life. Studies with glass materials are increased as a natural consequence of becoming an important part of human life and they are grew into more important to gain different surface properties to the glass. For glass, it is possible to modify mechanical, optical, electrical and chemical properties. By mechanical properties it is especially hardness and strength which are considered. Hardness and strength are connected in the sense that strength of glass is most often determined by the present surface defects. If the hardness is increased, the strength is normally increased. The use of glass is frequently restricted by its mechanical fracture. The inherent strength of soda lime silicate glass is around 7000 MPa based on the strength of the covalent bonds forming the vitreous network. The actual strength and fracture behaviour is mostly determined by surrounding environmental factors causing surface flaws. These reduce the strength to approximately 1 % of the theoretical value. The strength of glass is dependent on the distribution of surface flaws. The problem of researchers is to find means for strengthening glass that eliminate the large difference between the theoretical and actual strength values. Researchers are endeavoring to develop new technological processes for increasing the strength of real glass in the direction of improving glassmaking processes, obtaining defect-free glass structure, depositing protective coatings during the production process, creating compressive stresses on the glass surface during heat treatment and modification of the glass surface. Methods that increase glass strength by means of compressive stresses of the glass surface have been studied the most. Methods of strengthening glass directed at the strength characteristics owing to compressive stresses created in the surface layers have been worked out and are now being developed. Compressive stresses on the glass surface are creating by thermal strengthening and chemical strengthening. Strengthening processes in glass mainly prevent surface flaws or cracks from propagating when external forces are applied to the surface. Since glass is stronger in compression than in tension, the introduction of surface compressive stress profiles in glass is a well known approach for strengthening. Chemical strengthening is a way to induce compressive stresses in the glass surface, which counteracts the stress concentrations of the crack tips of the distributed surface flaws. Two basic principles of chemical strengthening are differences in thermal expansion coefficient between the surface and subsurface glasses and ion stuffing. The greater part of the literature relating to strengthening of glass has been devoted to the ion stuffing method. A smaller ion is exchanged and substituted by a larger ion in a glass, the larger ion is squeezed into the surface, obtaining residual compressive stresses at the surface with a balancing tensile stress in the interior. Various ion exchange systems have been investigated for strengthening. Examples are exchange of Na+ for K+, Rb+, Cs+. Some basic factors which influence the efficiency of the ion exchange process and the glass strength are temperature effect on the interdiffusion coefficient, time of exchange, interface between glass and salt, glass composition, exchanging pair of ions, temperature influence on relaxation. Ion exchange process is carried out molten salt bath, salt paste and vapor phase applications in studies of literature and industrial areas. Chemically strengthened glass products have been successfully marketed, such as aircraft cockpit windshields, transparent armor, glass substrates for harvesting solar energy, high strength to weight ratio glass containers, auto injector cartridges, photocopier glass, computer disks, and thin display windows in electronic communication devices (monitors, cell phones, tablets and MP3 players). Chemical strengthening is one of the most common practical methods in the industrial process. Chemical strengthened glass has demonstrated a higher impact resistance, flexibility and thermal shock resistance than thermal strengthened glass particularly in a thin and irregular shape glass, and moreover the chemical strengthened glass has no optical distortion and can be cut. The scope of this study; soda lime glass has been chosen, because it is cheap, easy to manufacture and has wide usage area. The chemical strengthening of soda lime glass has been provided by the Na+ - K+ ion exchange with the molten salt bath and salt paste applications. Finally, the obtained glasses are characterized and the results are compared. According to results, the using of produced glass in washing machine door glass will be determined whether it is appropriate. In the experiments, commercial soda lime glasses which has molar composition of 72,50 % SiO2 12,97 % Na2O 9,21 % CaO 1,32 % MgO 2,26 % Al2O3 1,50 % K2O; 50*50 mm in size and 3 or 6 mm thickness have been used. In the molten salt bath applications, the 3 and 6 mm thick glass samples have been immersed in a molten KNO3 bath at 450°C by using high purity potassium nitrate (KNO3) salt. The ion exchange process has been carried out for 6, 12 and 24 hours. Additionally, the composition of KNO3:KCl (2:1) and KNO3:KCl (1:2) by weight has been prepared by weighing the high purity potassium nitrate (KNO3) and potassium chloride (KCl) salt with precision scales. The salt pastes have been obtained by the addition of distilled water to two different salt compositons and then, they are heated to 80°C. The heated salt pastes are coated to the surface of 3 and 6 mm thick glass samples by spatula. 5-10-20 % of kaolin by weight in the the composition of KNO3:KCl (2:1) and 10 % of kaolin by weight in the the composition of KNO3:KCl (1:2) have been added and obtained 4 salt pastes are heated to the 80°C. The heated salt pastes with kaolin has been coated to the surface of 6 mm thick glass samples by spatula. The ion exchange process has been carried out all salt paste coated samples for 6, 12 and 24 hours at 450°C in furnace in order to chemical strengthening. Thermal analysis of reference sample were performed by using differential thermal analysis (DTA) to determine process temperature of chemical strengthening. The strength has been determined by equibiaxial flexure strength, the hardness has been established by micro Vickers hardness tester and the diffusion depth has been analyzed by SEM/EDS line scan analysis of all of the samples obtained by chemical strengthening using molten salt bath and salt paste applications. Glass transition temperature of the reference glass is obtained as 549°C and the ion exchange temperature is determined as 450°C according to the thermal analysis results. Strength of the 3 and 6 mm thick glass samples which is treated with molten salt bath and salt paste applications has increased according to reference sample. It has been established that the obtained strength increases with the increasing process time. Hardness of the 3 and 6 mm thick glass samples which is treated with two different application has increased according to reference sample. It has been determined that the obtained hardness increases with the increasing process time. The diffusion depth has expanded with the increasing Na+ - K+ ion exchange time for all of the 3 and 6 mm thick glass samples obtained by two different methods. The highest strength and diffusion depth values has been obtained for the molten salt bath.