1. Knoop Hardness* (H_{K})
Knoop hardness is used to characterize the hardness of the surface of optical glass against penetration.For this measurement a pyramid-shaped diamond indenter with vertex-angles 172°30' and 130°and with a rhombic base is applied to the polished specimen surface.Indentation loads of up to 0.9807N are applied for 15 seconds. The size of the resulting indentation is then measured. Knoop hardness, H_{K}, is calculated using the following equation.
Where F(N) denotes the applied load and l (mm) is the length of the longer diagonal of the resulting indentation.
Notes 1: | The Knoop hardness is expressed in terms of MPa or N/mm^{2}, which is omitted herein accordance with common usage. |
Notes 2: | The H_{K} value obtained by the above equation using SI units is equal to that obtained by the calculation equation using kgf units. |
Notes 3: | 1N = 1.01972×10^{-1}kgf |
Notes 4: | The Knoop hardness measurements shown in this catalog are based on the classifications in table 12 . |
Table12 Classes of Knoop Hardness (H_{K})
Class | Knoop Hardness |
---|---|
1 | < 150 |
2 | ≥ 150 - < 250 |
3 | ≥ 250 - < 350 |
4 | ≥ 350 - < 450 |
5 | ≥ 450 - < 550 |
6 | ≥ 550 - < 650 |
7 | ≥ 650 |
2. Abrasion Factor* (F_{A})
The abrasion factor*, F_{A}, is a relative measure for lapping. A glass specimen with a surface area of 9cm^{2} is placed 80mm from the center of a cast iron circular plate. The plate is then rotated horizontally at 60 r.p.m., and a 9.807N lapping weight is vertically loaded on the specimen. Lapping is continued for five minutes, with a continuous supply of lapping compound composed of 10g aluminum oxide (grain size: 20μm) in 20ml of water. The mass loss of the specimen, m, is then measured and compared to that of the standard reference material (BSC7), m_{o}, specified by JOGIS. The abrasion factor is then determined by the following equation:
where d is the specific gravity of the test specimen and d_{o} is the specific gravity of the standard reference material (BSC7). See the attached H_{K}-F_{A} diagram for details.
3. Elastic Properties (E, G, μ)
Young's modulus E and the modulus of rigidity, G, are measured by an acoustic method on a well-annealed 20×20×100(mm) specimen placed in an isothermal chamber. The velocity of both the
longitudinal and transverse waves of 5 MHz ultrasonic waves are measured. Young's modulus E and the modulus of rigidity G are then calculated by the following equations:
where V_{l} = velocity of the longitudinal wave
ρ = density of the glass
ratio μ is obtained by the following equation:
Note1:
Young's modulus is termed the modulus of longitudinal elasticity. The modulus of rigidity is also termed the modulus of transverse elasticity or shear modulus.
Note2:
1GPa = 1.01972×10^{2} kgf / mm^{2}
4. Flexural Strength (Modulus of Rupture)(σ_{b})
A well-annealed specimen of 4mm in width, 3mm in thickness, and 40mm in total length with polished upper and lower surfaces and a chamfered edge of C0.2 is used to measure its breaking load P(N) by the "3-point bending test" according to JIS R 1601-1981 and the flexural strength, σ_{b}, is calculated by the following equation:
where L is the support span(mm), W is the width(mm) of the specimen and t is the thickness(mm) of the specimen. The measured value is expressed in MPa.
Note.1: Mpa = 1.01972 × 10^{-1} kgf / mm^{2}