Fig. 1. Refrigeration site glasses


There are areas of heat treatment that have been around for a long time, but the knowledge about them is now known by relatively few. Processes for glass-to-metal sealing and ceramic-to-metal sealing are good examples. Let’s learn more.

Glass-to-metal seals (Fig. 1) are very important to the construction of vacuum tubes, bomb detectors, electric discharge tubes, incandescent light bulbs, glass-encapsulated semiconductor diodes, reed switches, pressure-tight glass windows in metal cases and metal or ceramic packages for electronics. Glass-to-metal sealing encompasses two types of hermetic (airtight) seals: compression and matching. In a compression seal, the outer member has the highest expansion factor and the inner components have a slightly lower expansion. In a matching seal, all components have a somewhat similar expansion. Heavy-gauge metals are used in a compression-type glass-to-metal seal, while thin-gauge metals are used for matching seals.

A metal that can be attached to glass to produce an airtight (hermetic) seal must have the following general characteristics:
  • The melting point must be higher than the working temperature of the glass
  • The material should be free of as many non-metallic inclusions as possible
  • The thermal-expansion characteristics of both the metal and the glass should be similar (in the case of matched seals)
  • No allotropic transformations creating significant changes in the thermal-expansion rate should occur over as wide a range as possible, typically 50-2000°F (10-1090°C)
  • Any oxide layer formed during the process of making the glass-to-metal seal should adhere firmly to both the metal and the glass
  • High electrical and thermal conductivity are required if the metal has to carry substantial electrical current (otherwise the strain rate may increase)
  • Ease of joining to other metals by welding or soldering is highly desirable (and often essential)
Glass is used because it has several outstanding properties, including:
  • Glass is stable. It is homogeneous, non-porous, smooth, hard and resistant to a wide variety of chemical attack.
  • Glass has good electrical properties. Glass has very high dielectric strength (greater resistance to high voltage surges).
  • Glass is affordable. It comes in a wide range of dielectric-constant and power-loss-per-unit-volume values at little cost differential.
  • Glass has very high volume and surface resistivity values.
One of the most important characteristics of glass-to-metal or ceramic-to-metal seals is that each step of the manufacturing process is critically important. Failure to meet optimum specification requirements will destroy any chance of success. For glass-to-metal sealing (and joining), the following manufacturing steps are generally employed:
  • Preparation (cleaning)
  • Decarburization (if necessary)
  • Oxidation (glass-to-metal seals only)
  • Glass application
  • Fixturing
  • Sealing
In some cases, a stress relief (annealing) operation (for the metal alloy) is also used. For ceramic-to-metal sealing, the following additional steps are also included:
  • Metallizing (of the ceramic to provide a wettable surface suitable for joining)
  • Cleaning (of the metal component)
  • Plating (typically Ni or Cu)
  • Joining (soldering or brazing of the metalized ceramic to the metal alloy)


Preparation

Pre-cleaning
A number of techniques are in use including several multiple-step processes. For example, parts are washed in a 50/50 solution of water and hydrochloric (alternatively hydrofluoric) acid at 150°F then rinsed in SK-250 followed by an additional wash for 30 seconds in a 50/50 solution of hydrochloric (hydrofluoric) acid at room temperature. They are then rinsed in 104 detergent followed by a deionized water rinse and perchloroethylene rinse and dry. Surfaces are sometimes grit or vapor blasted (using 180-260 grit).

Decarburization/Degassing[1]
The removal of carbon from glass-to-metal sealing alloys is a prerequisite for the production of good glass-to-metal seals. On sealing glass to metal, the presence of carbon in the metal causes bubbles of CO2 and CO to form at the glass-to-metal interface. These bubbles cause blistering and, therefore, a poor seal. To remove the carbon from the glassing operation, metal parts are usually decarburized in wet hydrogen, dissociated ammonia or nitrogen/hydrogen mixtures provided the hydrogen content is at least 10% and the dew point is carefully controlled. The chemical reaction responsible for decarburization/degassing is:

(1) C (in solution) + H2O = CO + H2

Processing temperatures in the 900-1100°C (1650-2010°F) range are typical up to a maximum of about 1205°C (2200°F). A dew point range of 20° ±2°C (68° ± 4°F) is common with about 26°C (79°F) being a reported maximum. Exposure time is usually less than one hour.

Fig. 2. Equilibrium data for oxides of iron[8]

Oxidation[2,3]

Factors that influence glass-to-metal or ceramic-to-metal seals involve adherence issues, including:

1. Oxide thickness – adherence is not developed when the oxide is too thick or too thin. There is a range of oxide thickness where good adherence is developed in a reasonable time. Too thick/thin an oxide requires an unreasonable sealing time.
2. Oxidation temperature – of minor importance as long as oxidation time is sufficient to give an oxide thickness that falls in the optimum range.
3. Optimum sealing time (time to effect the seal, or glass-oxide contact time) – a function of the sealing temperature, and both sealing time and temperature are a function of oxide thickness.

The optimum range of oxide thickness depends on the type of metal and has been reported to be approximately 0.6-1.1 mg/cm2. The oxide is composed of two parts an oxide scale and intergranular oxide. The residual intergranular oxide layer should be in the 2.0-6.5 µm (80-255 µinch) range for best results. Oxidation below optimum thickness will cause the glass to dissolve the oxide and contact the base metal directly. A weak bond and poor seal will result. Over-oxidation is also undesirable because the oxide itself has little shear strength and the glass may become saturated with oxide. These factors also contribute to a poor seal.

In summary, to develop good adherence in a short time, the oxide must not contain alpha Fe2O3 and must be thicker than the minimum stated above (Fig. 2). If the oxide contains appreciable amounts of alpha Fe2O3, the sealing process is not nearly so straightforward and the conversion of alpha Fe2O3 into Fe3O4 must be allowed for. Thus, formation of a stable, glass-to-metal seal oxide depends on the creation of a layer of magnetite (Fe3O4). A well-oxidized part will have a dark-gray appearance.

One method of oxide formation is air oxidation. Oxidation in air involves reactions that are irreversible and not controllable due in part to the fact that the relative humidity change is constantly affecting the oxygen potential.

(2) 2Fe + O2 ® 2FeO
(3) 4Fe + 3O2 ® 2Fe2O3
(4) 3Fe + 2O2 ® Fe3O4

The result of thermal oxidation in air is almost always unsatisfactory with respect to thickness and composition uniformity. IH