Study of strengthening by chemical tempering of lanthanum oxide-containing aluminoborosilicate glasses
Aluminoborosilicate glasses with rare-earth oxides have been the object of study for applications in dental porcelains and in solid-state lasers due to high mechanical properties and high refractive index. The objective of this work was to verify the viability of ion-exchange toughening (chemical tempering) as an alternative to add value and further improve the mechanical properties of this type of glass. Compositions of the SiO2–B2O3–Al2O3–La2O3–TiO2 and SiO2–B2O3–Al2O3–La2O3–TiO2–Na2O systems were investigated in order to evaluate the need to include alkali metal oxide for the ion-exchange process. The sodium oxide content was varied by 0, 5 and 10 mol%. The glasses obtained by melting in an electric oven were chemically tempered by immersion and deposition of KNO3 paste at 650 oC and 700 oC at intervals of 30 minutes, 3 hours and 24 hours. The X-ray diffraction analysis (XRD) did not identify the presence of crystalline phases in any of the compositions, while the energy-dispersive spectroscopy (EDS) analysis detected phase segregation in the composition with 10% Na2O, confirmed by Raman spectroscopy. The evaluation by differential scanning calorimetry (DSC) resulted in glass transition temperature (Tg) ranges of 724 and 694 oC for compositions with 0 and 5% Na2O, respectively. Tg1 at 653 oC and Tg2 at 718 oC were found for 10% Na2O. After tempering, there was no positive variation in Vickers hardness, remaining the same or less for all conditions analyzed. There was a growth of up to 47% in fracture toughness (KIc) in compositions with 5% and 10% Na2O, jumping from 1.35 to 1.98 MPa.m1/2 in the composition with the highest Na2O content, according to the Indentation Fracture method (IF). The analysis of the surfaces by EDS detected K+ with intensity proportional to the time of exposure in the compositions with the addition of Na2O, indicating the effectiveness of the ion exchange between sodium and potassium and its time dependence.