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Mechanical Properties

Mechanical properties include Knoop Hardness, Elasticity Modulus, Torsion Modulus, Poisson’s Ratio, Density and Grindability. They provide valuable information on the behavior of the grinding and polishing process.

Knoop Hardness (HK) represents the resistance of an optical glass to indentation and is measured with a microhardness tester, which has a rhomboidal diamond point with exactly defined angles. The penetration leaving an indentation on a polished surface is measured under a microscope. The SCHOTT’s Knoop Hardness is calculated with the following formula:

 

where

 

The values are listed in the SCHOTT’s date sheets for a test force of 0.9807 N and an effective test period of 20 s at room temperature. The N-BK7 has a Knoop Hardness equal to 610.

Grindability is very important for the grinding process. Here is the method used by SCHOTT, twenty samples of the glass to be classified are ground for 30 seconds in a standardized diamond pellet tool under predetermined conditions. Then the samples are compared by weighing the samples and considering the density of the removed volume of the glass with that of a reference glass, N-SK 16. The following formula is used to calculate the grindability HG as a comparison value:

 

where

 

  Grindability Class
HG1
HG2
HG3
HG4
HG5
HG6
  Grindability Limit Value
≤30
 >30
≤60
>60
≤90
>90
≤120
>120
≤150
>150
  Table 1: Grindability classification.

Table 1 shows the grindablity classification. Other Properties include density, elasticity modulus, torsion modulus and poisson’s ratio.

  Mechanical Properties
ρ (g/cm3)
E (103N/mm2)
μ
HK0.1/20
HG
  N-BK7
2.51
82 0.206 610 3
  N-SF11
3.22
 92 0.257 615 4
  Table 2: Mechanical Properties of N-BK7 and N-SF11.

Properties:  

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