Mechanical properties and surface properties of th

  • Detail

Mechanical properties and surface properties of glass

1 Physical and technological significance of glass surface tension

glass surface tension refers to the work done by adding a unit surface at constant temperature and constant volume on the phase interface between glass and another phase (generally refers to air), and the units are n/m and j/m2 The surface tension of silicate glass is (200-380) *n/m. the surface tension of glass plays an important role in the process of glass clarification, homogenization, molding, and the interaction between liquid glass and refractory materials

2. Relationship between glass surface tension and composition and temperature

various oxides have different effects on the surface tension of glass, such as Al2O3, La2O3, Cao, MgO, which can improve the surface tension If the amount of K2O, PbO, B2O3, Sb2O3, etc. is large, the surface tension can be greatly reduced At the same time, the surface tension can also be reduced when the amount of Cr2O3, V2O3, Mo2O3 and WO3 is small

The influence of composition oxides on the surface tension at the interface between glass melt and air can be divided into three categories Section "The influence of class II and class III composition oxides on the surface tension of the melt conforms to the law of additivity.

the relationship between class II and class III composition oxides on the surface tension of the melt is a composite function of composition, which does not conform to the law of additivity. Due to the adsorption of these compositions, the composition of the surface layer is different from that in Jiang body.

cyanide such as Na2SiF6 and Na3AlF6, sulfate such as mirabilite, chloride such as NaCl, can significantly reduce the surface tension of glass Surface tension, therefore, the addition of these compounds is conducive to the clarification and homogenization of glass

the surface tension decreases with the increase of temperature, and the relationship between them is almost linear. In fact, it can be considered that when the temperature increases by 100 ℃, the surface tension decreases by 1%. However, in the presence of surface active components and some free oxides, the surface tension energy increases slightly with the increase of temperature

3. Mechanical properties of glass

3.1 theoretical strength and actual strength of glass

generally, the mechanical strength of glass is expressed by compressive strength, flexural strength, tensile strength and impact strength Glass is widely used because of its high compressive strength and hardness. Its application is limited because of its low tensile strength, flexural strength and brittleness

the theoretical strength of glass is calculated as 11.76gpa according to Orowan hypothesis. The average strength of glass fiber without serious defects on the surface can reach 686mpa The tensile strength of glass is generally between 34.3-83.3mpa, while the compressive strength is generally between 4.9-1.96gpa However, the flexural strength of the actual glass is only 6.86mpa, which is 2-3 orders of magnitude smaller than the theoretical strength This is due to the existence of microcracks (especially surface microcracks) and uneven areas (equal division) in actual glass

at present, the commonly used methods to improve the mechanical strength of glass mainly include annealing, tempering, surface treatment and coating, microcrystallization, and making composites with other materials These methods can make the strength of glass increase and clear time extremely short, increase several times or even ten times

3.1.1 relationship between glass strength and chemical composition

the bond strength between glass structures with different chemical compositions is also different, which affects the mechanical strength of glass Quartz glass has the highest strength The order of the effect of various oxides on improving the tensile strength of glass is: CSO B2O3 Bao Al2O3 PbO K2O Na2O (MgO, fc2o3)

the order of the effect of various oxides on improving the compressive strength of glass is: Al2O3 (MgO, SiO2, ZnO) B2O3 Fe2O3 (B2O3, Cao, PBO)

3.1.2 defects in glass

macroscopic defects, such as solid inclusions, gaseous inclusions, chemical inhomogeneity, etc., cause internal stress because their chemical composition is inconsistent with that of the main glass At the same time, some micro defects (such as point defects, local crystallization, grain boundaries, etc.) are often concentrated in the place of macro defects, resulting in microcracks in the glass, which seriously affect the strength of the glass

3.1.3 temperature

the strength of glass is different at different temperatures. According to the measurement results in the range of -20 ℃ - 500 ℃, the lowest strength is around 200 ℃

it is generally believed that with the increase of temperature, the thermal fluctuation phenomenon increases, which makes more strain energy accumulate at the defect and increases the probability of fracture When the temperature is higher than 200 ℃, the viscosity flow of the glass increases, which passivates the crack of the microcrack, alleviates the stress effect, and increases the strength of the glass

3.1.4 stress in glass

the residual stress in the glass, especially the uneven distribution of residual stress, greatly reduces the strength However, after the glass is tempered, there is compressive stress on the surface and tensile stress inside, and it is regularly and evenly distributed. At this time, the light should be symmetrical, so the strength of the glass can be improved

3.2 hardness of glass

hardness is the ability of an object to resist the invasion of other objects There are many ways to express hardness, including Mohs hardness, microhardness, grinding hardness and scoring hardness. The Mohs hardness of glass is 5-7 The hardness of glass depends on the radius, charge and bulk density of the constituent atoms. The complex ions make the glass hard, while the complex ions reduce the hardness of the glass The effect of various compositions on the increase of glass hardness is roughly: SiO2> SiO2 (MgO, ZnO, Bao)> Al2O3 Fe2O3 K2O Na2O PbO. The hardness of glass decreases with the increase of temperature

3.3 brittleness of glass

brittleness of glass refers to the characteristic of breaking immediately when the load exceeds the ultimate strength of glass The brittleness of glass is usually expressed by the impact strength when it is destroyed The measured value of impact strength is related to the thickness and thermal history of the sample. The impact strength of quenched glass is 5-7 times greater than that of annealed glass

3.4 elasticity of glass

in modern technology, glass is more and more widely used as structural materials, so the research on the elasticity of glass is also increasing High altitude and high-speed flight requires high elastic modulus materials with certain stiffness. The structural compression and relaxation caused by high-power laser passing through the glass medium will lead to changes in density and refractive index Elasticity has become an important physical property of glass

4. Density and density calculation of glass

4.1 density of glass

the density of glass indicates the mass per unit volume of glass, which mainly depends on the chemical composition, temperature and thermal history of glass, and is also related to the atomic packing compactness and coordination number of glass. It indicates that the density of glass is closely related to the chemical composition The density of glass varies greatly with its composition Among all kinds of glass products, the density of quartz glass is the smallest, 2000kg/m3, and that of ordinary soda lime silica glass is 2500-2600kg/m3 The density of glass containing PbO, Bi2O3, Ta2O5 and WO3 can reach 6000kg/m3 Even the density of some anti radiation glasses can be as high as 8000kg/m3

the density of glass decreases with the increase of temperature For general industrial glass, when the temperature rises from 20 ℃ to 1300 ℃ under the action of many favorable factors mentioned above, the density decreases by about 6% - 12%. Within the range of elastic deformation, the decrease of density is related to the thermal expansion coefficient of glass

the thermal history of glass refers to the experience of glass cooling from high temperature and passing through the TF TN region, including the specific conditions such as residence time and cooling rate in the region Thermal history affects the structure of solid glass and many properties related to structure Its effect on glass density is:

4.1.1 when glass is cooled from high temperature, the density of quenched glass is smaller than that of annealed glass

4.1.2 after holding at a certain annealing temperature for a certain time, the glass density tends to balance

4.1.3 the faster the cooling rate is, the more the deviation from equilibrium density is, and the higher the TG value is Therefore, the quality of annealing in production can be clearly reflected in density

accidents often occur in glass production, such as formula calculation errors, batch weighing errors, raw material chemical composition fluctuations, etc., which can cause changes in glass density Therefore, glass factories often take measuring density as a means of controlling glass production

Copyright © 2011 JIN SHI