Metallization

Abstract:

The process of metallization could be described as coating the surface of products with a metal layer to obtain coatings that are corrosion-resistant, abrasion-resistant and decorative.
This article describes the main processes and techniques of metallization.

The process of metallization could be described as coating the surface of products with a metal layer to obtain coatings that are corrosion-resistant, abrasion-resistant and decorative.

The main processes of metallization include:

  • galvanic metallization, so called galvanostegy,
  • metal spraying,
  • fire metallization, where the metal layer is coated by an immersion of a product in liquid metal,
  • diffusion metallization,
  • vacuum metallization, and
  • contact metallization.

The main techniques are:

  • aluminizing,
  • chromium plating,
  • zinc coating,
  • tin plating,
  • cadmium plating,
  • copperizing,
  • nickel plating,
  • leading,
  • silver plating, and
  • gold plating.

Vacuum metallizing

In vacuum metallizing, a metallic coating material placed in a vacuum chamber with the workpiece to be coated. The material that is being applied is then heated until it starts to evaporate, this vaporized metal condenses on the product or workpiece as a thin metallic film. As this is happening the part is being rotated for uniformity of the coat. The process characteristics are:

  • Done within a vacuum
  • Vaporizes the metal
  • Can be used on glass, plastic, metal, ceramic, and paper materials
  • Pieces being coated must be extremely clean
  • It produces a very thin coat of metal.

Thickness ranges for 0.01 to 0.2 micrometres. When coating a piece it changes its conductivity, it improves its corrosion resistance, and it enhances its appearance. Some target metals are aluminum, copper, platinum, titanium, chromium titanium, gold, lead, nickel, silver, tin and tantalum.

During the vacuum metallization one can observe three phases of the process of coating manufacturing:
a) metal evaporation, it means the process of changing the state of a metal (from solid, through liquid into gas one),
b) diffusion of a metal vapour from the source of evaporation to the coating product or the wall of a vacuum chamber,
c) condensing of a metal vapour on the product surface and creating the continuous metal layer.

The vacuum deposition process is a physical, rather than electrochemical, method of depositing metals onto a substrate. The deposition takes place within a vacuum chamber where metal is melted and then becomes gaseous when it reaches its vapor point. Gas molecules traveling by line condense on the desired substrate, creating a relatively uniform coating. The vacuum system itself consists of an airtight chamber where the deposition process occurs. Outside the chamber, multiple pumps evacuate air to the desired process pressure.

A power supply is used to deliver the required voltage to an electrode, which connects a series of standoffs holding tungsten filaments loaded with the desired metal for vaporizing. There are eight to ten major steps in the vacuum metallization process.

A typical application may take between two to three hours for completion. The substrates that work well for vacuum metallization include metal (tin, steel, aluminum, etc.), plastic and glass.

The structure of a layer and its features are depended on the conditions of coating processes, cleanness of a vacuum chamber and features of the atom and molecules flux. The main element that influences the structure of manufactured layer is the method of base preparation. It could largely change the properties of obtained the metal film.

To achieve a brilliant blemish-free coating, the raw substrate must be free of surface contamination, such as mold releases, fingerprints, dirt, dust, oil and grease. The first step in the process is to assemble the parts on production racks. Parts need to be held securely as they go through a variety of painting and metallization processes. Typically, parts for metallization can have holes, ribs, pins or a small section of a runner on them for holding in areas on the parts that are not critical Class A surfaces.

Metal spraying

The metal-spraying, also known as “flame coating”, “flame spray metallizing” or simply “metallizing”, is a process of spraying the molten metal onto a surface in order to repair wear or to provide a protective coating. During the process a material in form of wire or powder is melted in an oxy-acetylene flame and atomized using compressed air to form a fine spray. When the spray contacts the prepared surface of a substrate material, the fine molten droplets rapidly solidify forming a solid metal coating.

It is crucial to note that the work piece remains at very low temperatures during spraying (up to 150°C), which means that the metallizing is essentially a “cold” build-up process. Therefore, there is no damage, metallurgical changes or distortion of the work piece during or after the completion of the process. This gives metal spraying a tremendous advantage over other repair technologies such as brazing, welding, powder spraying etc. where working temperatures are near melting point of the base metal.

The other major advantage is its versatility. Any combination of ferrous and non-ferrous combinations of base and sprayed metals can be achieved. For example phosphorus bronze or copper can be sprayed on a mild steel base, stainless steel can be sprayed on aluminum base, ceramics can be sprayed on stainless steel base and so on.

Due to its versatility and a huge number of available materials the list of specific applications is virtually endless. Here are just a few situations where metal spraying process is frequently used:

  • Reclaimation of worn bearings and seals on all types of shafts
  • Reclaimation of bearing seat areas on various casings, motor covers, pumps etc
  • Repair of oil seal, mechanical seal drive, gland sections on shafts
  • Protection of seal areas on stainless steel shafts sleeves with ceramic coating
  • Repair of rotors, spindles, stirrer and other shafts
  • Reclamation of worn crankshaft journals on engines or compressors
  • Reclamation of worn-out surfaces on the wire drawing drums
  • Applying stainless steel (and others) coating on rollers and shafts for textile, printing, pharmaceutical, chemical and other industries
  • Reconditioning hydraulic rams
  • Providing tin based (90%) white metal coating on segmented bearing pads, liners for large ball mills, cement plants etc.
  • Applying copper on printing rolls for printing, electric, electronics, chemical, petro-chemical and other industries
  • Bimetal bearings
  • Rebuilding mis-machined or worn-out parts in general.

Some of the most commonly used materials are:

  • Ceramics for pumps and pump components
  • Nickel/aluminum composite wire for bond coats and self-bonding coatings
  • Molybdenum for bond coats
  • Molybdenum for hard bearing applications, excellent resistance to adhesive wear, used on piston rings, synchromesh cones and journals
  • High chromium steel for many applications requiring hard and wear resistant coating
  • Bronzes and Babbitt metal for bearing applications
  • Stainless steels, nickel and monel for anti-corrosion and wear
  • Aluminum, nickel/aluminum for heat and oxidation resistance
  • Zinc and aluminum for anti-corrosion cathodic coatings on steel.

Furthermore, metal spraying equipment is portable so the repairs can be carried out on-site if required, for instance for the repairing of capital machines.

Thermal Spraying

Thermal spray coatings are produced from either wire or powder materials, which are melted or softened into droplets and then propelled onto the selected substrate. On impact, the droplets bond to the surface, creating a dense protective coating with no alteration to the substrates structure. Typical thermal spray coating processes are the following.

1. Combustion Flame Spray, which is achieved by burning oxygen and acetylene in a torch with a flame accelerating nozzle. For materials in wire form the flame is concentric to the wire fed through the nozzle axis. The combustion gas melts, atomizes and propels molten particles onto the surface being coated.

2. Electric Arc Spray. In the arc spray process, the raw materials in the form of a pair of metallic or cord wires are melted by an electric arc. The molten material is atomized by a cone of compressed air and propelled towards the work piece; the molten spray solidifies on the component surface to form a dense, strongly adherent coating.

3. Plasma Spray. Plasma is the term used to describe gas that has been heated to such a high temperature that it ionizes and becomes electrically conductive. In the case of plasma spraying the plasma is created by an electric arc burning within the nozzle of a plasma gun and the arc gas is formed into a plasma jet as it emerges from the nozzle. Powder particles are injected into the jet where they melt and strike the surface at high velocity, to produce a strongly adherent coating. Almost any surface and be sprayed including metals and ceramics.

4. High Velocity Oxy-Fuel (HVOF), which is a relatively new flame spray process. Liquid fuel and oxygen are fed via a premixing system at high pressure into a combustion chamber where they burn to produce a hot, high pressure gas stream. The powders are injected Into this supersonic gas jet, which has spped of over 2000 m/sec, thus making contact with the substrate to form a very dense, good quality coating.

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