Listed below are property definitions related to charts on our material properties pages.
The temperature at which internal stresses are substantially removed in a short period of time.
Glass Transition Temperature
The temperature range in which a material transitions from a true solid to a very viscous liquid. This definition applies to non-crystalline solids.
The temperature at which a material turns suddenly from solid to liquid.
The temperature at which a glass deforms at a specific rate under its own weight.
The amount of energy a body stores per unit mass for each degree increase in temperature (at constant pressure).
The temperature at which internal stresses in glass are substantially relieved in approximately 1 hour.
The rate at which heat flows through a unit area of homogeneous material for a given temperature difference.
The change in length per unit length of a material corresponding to a unit change in temperature.
A parameter that characterizes the material cracking resulting from the temperature gradients caused by rapid change in temperature. A reduction in temperature is usually used for brittle materials.
Ratio of the speed of monochromatic light in a vacuum to the speed of light through a material.
Wavelength range light will transmit through a unit length of optical material without significant optical attenuation.
A hardness test that forces a hardened steel or tungsten-carbide ball against the smooth material surface for a standard dwell time to create an indentation. The hardness is expressed as a Brinell Hardness Number (BHN) computed by dividing the force by the projected area of indentation.
Ratio of stress to change in volume of a material subjected to axial loading. It is related to the Modulus of Elasticity and Poisson's Ratio by the following equation: Bulk Modulus = (Modulus of Elasticity)/(3x(1-2xPoisson's Ratio)).
The maximum compressive stress a material can withstand before failure.
Mass per unit volume.
Greatest stress that can be applied to a material without causing permanent deformation.
Maximum stress developed in a specimen just before it cracks or breaks in a flexure test.
The resistance a material has to the propagation of a crack. The higher the fracture toughness, the more resistant a material will be to rapid crack growth.
The resistance a material surface offers to abrasion, scratching and indentation. Common measures of hardness are Mohs, Vickers, Brinell, and Knoop.
A hardness test that forces a pyramid-shaped diamond tip against the smooth surface of a material for a standard dwell time to create an indentation. This test differs from the Vickers test in that only a small indentation is made so that the test can be used with brittle materials or thin sheets. The hardness is expressed as a Knoop Hardness Number (HKN) computed by dividing the force by the projected area of indentation.
Scale of hardness that characterizes the scratch resistance of various materials through the ability of a harder material to scratch a softer material. Mohs hardness is based on a scale of ten minerals that are all readily available. As the hardest known naturally occurring substance, diamond is at the top of the scale. Talc is at the bottom of the scale.
The negative ratio of the thickness decrease divided by the length increase resulting from a tensile stress applied to a material.
The proportion of the non-solid volume to the total volume of material.
The proportionality constant between elastic shear stress and elastic shear strain of a solid material subjected to shear loading.
Shore Durometer is a measure of hardness commonly used with rubbers, elastomers and polymers. Like all hardness testers, the Shore Durometer test measures the depth of an indentation under a given test force. There are no less than 12 Shore Durometer scales, with the two most common being the Durometer Shore A and D scales. The Shore A scale used for softer materials and the Shore D scale is for harder ones. The primary differences between the two scales are the force range and indenter shape used during the tests.
All scales range in numbers from 0 to 100, with higher numbers indicating harder materials. For reference, provided below are the Durometer values for a few common materials:
The maximum tensile stress a material can withstand before rupture.
The maximum torsional stress that a material can withstand before rupture.
A hardness test that forces a pyramid-shaped diamond tip against the smooth surface of a material for a standard dwell time to create an indentation. The size of the indentation determines the hardness value. The Vickers Number (HV) is then determined by the force applied to the diamond and the projected surface area of the resulting indentation.
Maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of elastic limit. The amount of permanent deformation used depends on the material (for metals it is 0.2% strain).
The proportionality constant between elastic stress and elastic strain for a solid material subjected to uniaxial loading. This property describes the inherent stiffness of a material.
The property of the dielectric material that indicates the ability of a material to store electrical energy. It is expressed as a ratio relative to the dielectric constant of a vacuum.
The minimum electric field that produces a breakdown of the insulating properties of a dielectric material.
A measure of a material's resistance to electrical current per unit length for a uniform cross section.
Dielectric Loss Tangent
Product of the dielectric constant of a material and the tangent of its dielectric loss angle.
Dielectric Loss Angle
90 degrees minus dielectric phase angle.
Dielectric Phase Angle
Angular difference in phase between the sinusoidal alternating potential difference applied to a dielectric material and the component of the resulting alternating current having the same period as the potential difference.
Reference MIL-O-13830 Optical Components for Fire Control Instruments; General Specification Governing The Manufacturing, Assembly, and Inspection of Glass and BSR/OEOSC OP1.002 Optics and Electro-Optical Instruments - Optical Elements and Assemblies - Appearance Imperfections.
Scratch-Dig refers to the quality of optical surfaces. Owing to their relative complexity, many misconceptions exist about the meaning of scratch-dig specifications, and how they are applied. Conceptually, scratch-dig specifications attempt to set a limit on the amount of area surface defects occupy relative to the overall clear aperture of the optical element. Because assessments are made relative to the size of the part, a scratch that is unacceptable for a small part may be acceptable for a large part.
Much of the confusion about scratch/dig requirements finds its origins in the fact that there are actually two distinct standards - the "visibility method" and the "dimensional method". These two standards differ only in the way scratch widths are categorized. The dimensional method, which is less prevalent, characterizes scratches by width measurements; whereas, the visibility method uses comparisons to commercially available visual references.
Scratch: Any marking or tearing of the part surface.
Dig: A small rough spot on the part surface similar to a pit in appearance. A bubble is considered a dig. Surface stains are also considered digs.
Scratch/Dig: Surface quality is specified by a number such as 60/40. The 60 defines a scratch width according to a visual standard (it does not mean a scratch can be 60 um wide). For reference, scratch numbers of 60 typically refer to maximum allowable scratch widths of ~7um to 8um. With this in mind, the scratch part of the specifications includes the following four requirements:
The second number of the Scratch-Dig specification refers to digs, and establishes a limit to the actual size (diameter) of the digs in hundredths of a millimeter. The dig part of the specifications includes the following three requirements:
Scratch/Dig: Surface quality is to be specified by letters such as E/D. The first letter relates to the maximum width allowance of a scratch as measured in microns. The next digits indicate to maximum diameter allowance for a dig in hundredths of a millimeter. A surface quality callout of E/D would permit a scratch width of 60 microns (0.0024") and a dig diameter of 400mm (0.0158"). The table below provides the correspondence between letters and physical sizes. Note that the scratch and dig accumulation rules for the dimensional and visibility methods are the same, but are repeated below for completeness. With this in mind, the scratch part of the specifications includes the following four requirements:
The second letter of the Scratch-Dig specification refers to digs, and establishes a limit to the actual size (diameter) of the digs in hundredths of a millimeter. The dig part of the specifications includes the following three requirements:
(*) - Note that the length descriptor of the clear aperture is not always simple. MIL-O-13830 defines the length descriptor as the diameter of a circle with the same area as the clear aperture of the part being evaluated. Often times, however, optical industry prefers to use the smallest dimension of the clear aperture.
Maximum Dig or Bubble Diameter
Dig or Bubble Separation Distance
Surface roughness is a measure of the texture of a manufactured surface. Although there are many definitions of surface roughness, all of them are based on a statistical representation of the high frequency surface deviations (peaks and values) from the local mean surface height. Filtering is used to separate the high frequency texture data from lower frequency machining features.
Surface roughness can be measured using contact methods involving dragging a stylus across the part, or using non-contact optical methods. CiDRA® Precision Serivces, LLC uses both measurement methods.
Ra, the most commonly used surface roughness definition, is expressed mathematically by
where n is the total number of data points used in the calculation and Y is the vertical surface position measure from the average surface height. To be applied properly, this line measurement should be made perpendicular to machining lay marks. Visit Wikipedia.org/wiki/Surface_roughness for a more in-depth explanation of surface roughness.
We have developed capabilities to easily fabricate precise custom products from glass, ceramic, sapphire, hard metal, and many other materials. Our capabilities enable machining of a wide range of materials, of which the most common selections are listed below. If your material of choice is not listed, please don't hesitate to contact us. We realize that tomorrow's high-tech products will require the most advanced material technologies, and we at IDEX Health & Science are always up for a challenge!
For a list of definitions of terms used in the material charts on each page, please visit our Material Property Definitions section above.
All ceramics start as a mixture of powdered base material (Zirconia, etc.), binders and stabilizers. This mixture is "formed" into shapes and then fired (sintered) at high temperature to create hard, dense materials. Forming is done using standard processes such as pressing, extruding, injection molding, tape casting or slip casting. Ceramics can also be machined prior to being fired using standard machine tools in a process known as "green machining." Green machining is inexpensive because unfired material is soft. However, firing causes ceramics to lose 20% to 40% of their volume; therefore, green machining followed by firing is suitable only for those applications with loose tolerances (~1% of characteristic lengths). In contrast, tight tolerance parts must be machined using high speed, diamond tools after ceramics are fired.
Some of the better known ceramic manufacturing processes combine sintering with forming.
Ceramics are consolidated into dense material by exposing them to 1800°C - 2000°C for days or weeks at a time, depending on the ceramic and process details. The addition of the thermal energy promotes strong bonds between the raw ceramic particles, leading to densification. Green machined, near net shapes or raw stock material can be sintered. Knowledgeable ceramics manufacturers are very adept at accounting for volumetric shrinkage.
Hot pressing combines the forming and firing steps to produce relatively simple geometric shapes. The ceramic powder is simultaneously subjected to sintering temperatures and uniaxial pressure. Simple shapes are generated by placing the raw material in a high temperature die while under load.
Hot isostatic pressing is a uniform pressure assisted method of sintering ceramics into simple and complex shapes. The pressure, usually applied via an inert gas like Argon to prevent reactions, significantly reduces porosity and therefore improves physical properties. Often times, the pressurization process is preceded by evacuating all air to reduce moisture and impurities. In order for the hot isostatic press process to work, the green ceramic must be placed in a gas tight container. An alternative method is to pre-sinter the ceramic to remove porosity at the surfaces. In this way, the ceramic material itself acts as the pressure vessel. Hot isostatic pressing differs from isostatic pressing in that the former applies uniform pressure to the ceramic during sintering.
Chemical vapor deposition is the process of converting gases (called precursors) into solids by continuously depositing monolayers of material onto a heated substrate. This is a thermodynamically driven process, so control of substrate temperature and chamber pressure is critical. Certain ceramic materials, such as Silicon Carbide and Silicon Nitride, can be manufactured using chemical vapor deposition techniques. Shapes are formed using sacrificial targets premachined into the desired shape of the part. Although the resulting material is much more expensive than its conventionally made counterparts, the cost is warranted by applications requiring superior physical properties.
Reaction bonding uses a chemical reaction to bind ceramic powders into a solid form. After forming, the binder is burned off to create a porous preform, and then capillary pressure is used to infiltrate liquefied reactants (different reactants for different ceramics) into the preform at temperatures just above the ceramic melting point. The resulting reaction creates the solid ceramic form. For example, liquefied Si is used in reaction bonded Silicon Carbide. The main disadvantage of reaction bonded ceramics is that it leads to relatively high porosity.