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/mm2, which is omitted herein accordance with common usage.
Notes 2:	The HK value obtained by the above equation using SI units is equal to that obtained by the calculation equation using kgf units.
Notes 3:	1N = 1.01972×10-1kgf
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 9cm² 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 = 1.01972×10² kgf / mm²

It is measured by the method of applying to JIS R1602:1995.

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 / mm²