Strengthening by chemical tempering of lanthanum oxide-containing aluminoborosilicate glasses
Aluminoborosilicate glasses containing rare earth oxides have high values of mechanical properties and refractive index. The aim of this study was to verify the feasibility of strengthening by ion-exchange (chemical tempering) this type of glass as an alternative to counterbalance the high cost of rare earth oxides and to improve the mechanical properties in an aluminoborosilicate glass with La2O3. SiO2-B2O3-Al2O3-La2O3-TiO2 and SiO2-B2O3-Al2O3-La2O3-TiO2-Na2O glass compositions were investigated in order to verify the necessity of an addition of an alkaline earth oxide to promote the ion exchange. The sodium oxide content was varied as 5, 10 and 15 mol%. The glasses were chemically tempered with KNO3 salt at 600, 650 and 700 oC. Before and after the tempering, the X-ray diffraction analysis (XRD) did not identify the presence of crystalline phases and the energy dispersive spectroscopy analysis (EDS) detected K in the second composition, indicating the effectiveness of ion exchange between sodium and potassium. After the tempering, the Vickers's hardness raised to approximately 7.5 GPa. In sodium-containing composition, compressive residual stress was observed and the fracture toughness (KIc) determined by indentation fracture analysis increased.