Competitive effects of free volume, rigidity, and self‐adaptivity on indentation response of silicoaluminoborate glasses

Abstract

Lithium aluminoborate glasses have recently been found to feature high resistance to crack initiation during indentation, but suffer from relatively low hardness and chemical durability. To further understand the mechanical properties of this glass family and their correlation with the network structure, we here study the effect of adding SiO2 to a 25Li2O–20Al2O3–55B2O3 glass on the structure and mechanical properties. Addition of silica increases the average network rigidity, but meanwhile its open tetrahedral structure decreases the atomic packing density. Consequently, we only observe a minor increase in hardness and glass transition temperature, and a decrease in Poisson's ratio. The addition of SiO2, and thus removal of Al2O3 and/or B2O3, also makes the network less structurally adaptive to applied stress, since Al and B easily increase their coordination number under pressure, while this is not the case for Si under modest pressures. As such, although the silica‐containing networks have more free volume, they cannot densify more during indentation, which in turn leads to an overall decrease in crack resistance upon SiO2 addition. Our work shows that, although pure silica glass has very high glass transition temperature and relatively high hardness, its addition in oxide glasses does not necessarily lead to significant increase in these properties due to the complex structural interactions in mixed network former glasses and the competitive effects of free volume and network rigidity.

Structure and Properties of Mixed Former Aluminum Borate Glasses Modified With Silver Oxide

“…It can be seen that the hardness number increases with increasing Al 2 O 3 content and there is a large increase in hardness value of the investigated system from about 2.3 to 4 GP as aluminum oxide content increases from 0 to 30 mo%. It is known that the increase in hardness is related to the increase in the rigidity of glass [21,22].…”

Indentation deformation and cracking behavior of hydrated aluminoborate glasses

“…We used the traditional melt-quenching technique to prepare the three different aluminoborate glasses with the composition 25M We moved the obtained glasses into a preheated annealing furnace at an estimated glass transition temperature (T g ) based on the previous studies 21,25,26 . Subsequently, the T g of the glasses was determined from differential scanning calorimetry measurements (STA 449 F3 Jupiter, Netzsch) at 10 K/min on specimens with a preceding cooling rate of 10 K/min.…”

“…To clarify the relation between the extent of water penetration (as tuned by the glass composition) and CR, we here study the indentation cracking behavior of different alkali aluminoborate glasses subjected to varying hydration conditions. We focus on aluminoborate glasses 20,21,25 as they are well known for their soft and highly adaptable network structure and relatively low chemical durability compared to silicate glasses. These characteristics allow easier water penetration in aluminoborate glasses that will have a significant impact on their mechanical properties.…”

Structural densification of lithium phosphoaluminoborate glasses

“…In a recent work, we studied the effect of adding SiO 2 (up to 20 mol%) to a 25Li 2 O-20Al 2 O 3 -55B 2 O 3 glass on the structure and mechanical properties. 29 Addition of silica increases the average network rigidity, but meanwhile its open tetrahedral structure decreases the atomic packing density and also makes the network less structurally adaptive to applied stress. This is because Al and B easily increase their CN under pressure, while this is not the case for Si under modest pressures.…”

“…These glasses generally exhibit high crack initiation resistance upon indentation, including for a lithium aluminoborate glass 23 that shows a dramatic increase in the CNs of both boron and aluminum upon high‐temperature densification. In a recent work, we studied the effect of adding SiO 2 (up to 20 mol%) to a 25Li 2 O‐20Al 2 O 3 ‐55B 2 O 3 glass on the structure and mechanical properties 29 . Addition of silica increases the average network rigidity, but meanwhile its open tetrahedral structure decreases the atomic packing density and also makes the network less structurally adaptive to applied stress.…”