Metallizing is the application of a metallic coating to a ceramic to permit subsequent brazing to a mating part. Various techniques are employed but the basic steps follow the common practices of ceramic decorating except that materials must be carefully chosen.
Metallizing Systems
Metals. Reactivity being desirable, most metal powders are purchased as -325 mesh to ensure a high surface-to-volume ratio.
Copper. May be mixed, as flake, with coarse glass particles that melt and seal to the ceramic preserving the integrity of the flake. This continuous electrical path through the glass seal is useful in the manufacture of spark plugs. Copper, as the oxide, will bond to a ceramic under precise firing conditions and, when mixed with silver, may be used as a metallizing preparation.
Gold. Frequently combined with reactive glass for uses in the air-fired paste and solder category.
Iron. Reactive material originally combined with tungsten. Very common additive for sintered powder process.
Manganese. Reactive material combined with molybdenum and is a very common additive.
Molybdenum. Most used as basic metal for sintered powder metallizing. Oxidation potential allows control of oxidation state in controlled atmosphere furnace. Coefficient of expansion of the metal and its reaction products are favorable.
Palladium. Similar to platinum and sometimes added to platinum-frit mixtures for air firing.
Platinum. Used with reactive glasses in air-fired metallizing.
Silver. Basic metal for many air-fired pastes. Mixed as granular or flake material with a reactive glass; e.g., borosilicate.
Tin. Basis for direct chemical bonding to numerous ceramics and high-temperature metals.
Titanium. Basis of the active metals process. May be used as powder or foil prior to braze alloying. Titanium-bearing brazes will wet and flow over ceramic; in vacuum, almost as well as solder over copper. Frequently applied as the hydride, which dissociates at <800°C, providing nascent hydrogen, which tends to scour the surface to be wetted. May be added to sintered powder compositions to promote reaction.
Tungsten. Similar in metallizing properties to molybdenum.
Zirconium. Performs similarly to titanium but with less activity. Has lower coefficient of expansion.
Miscellaneous Reactive Powders. Many companies have evolved proprietary sintered powder metallizing systems that may be very complicated. Some of the likely additives to these compositions are aluminum, barium, boron, cadmium, magnesium, rare earths, and silicon. Most of these would be added as an oxide.
Glass Compositions. Most metallizing suppliers purchase glass-bearing metallizing compositions from commercial vendors. The glass compositions are regarded as proprietary and are not generally known.
Binders. Binders are fugitive materials that are required to increase viscosity and density of the vehicles developed for suspending metal powders during application of the ceramic. They must leave no deleterious residue following the firing operation and must hold the metal in situ until the particles become somewhat adherent. Typical binders are represented by acrylic polymers, commercial binders, nitrocellulose, and pyroxyline resins.
Solvents. Solvents are required to dissolve the binder material and usually have a high vapor pressure so they are effectively lost before firing of the metallizing begins. These materials, with the binders, constitute the vehicle. The best vehicles allow a smooth application with controlled thickness, and dry to a dense abrasion-resistant layer prior to firing. Solvents are selected typically from the following families: acetates, alcohols, commercial solvents, ethers, and ketones.
Application Methods
Pastes, when used, are prepared by selecting the metal powders and blending with additives in a ball mill. Acetone is frequently used as the carrier during the milling operation. Attention is given to the desired degree of particle size reduction. The milled material is removed, dried, pulverized, and combined with the chosen binder and solvent. This preparation may be milled further to ensure complete blending.
The paste is then ready to be applied to the ceramic.
Many methods of application are in use. They vary depending on size of the order, configuration of the part, the precision required, and operating economy.
Brushing. A paintable slurry is prepared and applied by brush to the desired area of the ceramic. Useful for small lots or unusual configurations.
Decalcomania. Commercial sources offer patterns prepared for transfer to ceramic.
Dipping. Applicable where complete coverage is permissible or where subsequent methods of recovering a pattern are feasible.
Evaporation. Vacuum evaporation of metals offers a readily controllable metal thickness for thin-film and other applications not requiring high strength.
Printing. Banding or printing equipment may be successfully used with certain pastes. Limitations result from ink thickness requirements.
Screening. A slurry is forced through a fine-mesh screen by a squeegee. This technique yields excellent patterns on cylindrical or flat surfaces.
Solution Metallizing. A liquid solution of the desired metal is applied to the ceramic, subsequently dissociated, and then reduced to provide a metallic coating.
Spraying. Paint-spray techniques are adapted to metallizing. Masking is usually required on the part. High-volume production may be maintained.
Tapes. Metallizing compositions manufactured as a tape of controlled thickness are being offered commercially. The tape may be applied to the ceramic, adhered by solvent action, and fired.
Vapor Deposition. Reduction of chemical vapors, e.g., nickel carbonyl, produces a uniform metallic layer on the ceramic. Deposition can be conventional or electrostatic firing.
Air Furnaces. Useful for glass-fritted pastes containing noble metal powders. Firing temperatures vary up to 1000°C.
Controlled Atmosphere Furnaces. Hydrogen or cracked ammonia atmospheres, with dew point and temperature controls, are typically used to react sin-tered-powder metallized coatings with ceramic. These furnaces operate at 1300 to 1700°C.
Vacuum Furnaces. Vacuum of 10-4 torr, or less, is required for metallizing and sealing by the active metals method. Vacuum evaporators operating in the 10-3 to 10-10 torr range are at present in use, experimentally, for studies of ultrapure metals deposited for two-dimensional circuitry requirements.
Others. While not yet of full commercial significance, work in plasma-jet technology and electron beam machining have significant potential.
Electroplating. Following firing, the active metal assembly hardware may be electroplated for ease of installation by user. Air-fired silver may be electroplated to improve solderability. Sin-tered-powder metallized ceramics must ordinarily be electroplated to allow wetting by solders or brazes. Many methods of electroplating are in use. Barrel plating is more economical than rack plating and is the preferred method. The thickness, purity, and type of electroplated metal has a significant effect on the effective strength of sin-tered-powder metallizing.
Etching. A chemical process used in conjunction with a masking system to create fine lines.
