COATINGS -surface finishing technologies
Post 238 ⇒ by Gautam Shah →
Coatings are thin surfacing. A coating is mainly created through a change in the physical state of a material (change of a phase); first from higher or solid to liquid (or even gas) for the purpose of application, and than reconversion to a solid (or even a very heavy liquid phase such as ‘creamy or pasty’).
At an application stage, a lower phase helps in many ways:
- Easy, uniform and thin spread over the surface.
- Efficient dispersion of costly or rare constituents.
- Easy and thorough mixing of constituents such as film forming materials and other additives, during production and application.
- Less energy is required for application.
- A controlled rate of deposition.
We tend to identify Coatings as Paints and Varnishes. Coatings as technology, however, include diverse methods of surface formation. The most primitive coatings were daubing of clay over walls and floors to create a smooth surface. The next major use was finishing the ceramics in pre and post firing processes. Coatings of oils and waxes were applied on materials like leathers, furs, and fabrics, to make them, water and fungus resistant, soft and supple. Lime and other minerals were used as colourant through coatings. Starch, gums, and other protein materials were used as stiffening or bodying coatings. Application of melted tin over metal surfaces is very ancient coating system. Ceramic slips were applied to create glaze over pottery items. Metal oxides were fired over metal surfaces to form ceramic enamels. Coal tar and Bitumen have been used in waterproofing applications. Metal plating, metallizing and surface alloying are fairly ancient technologies of coatings.
Metal coating, enamelling, Minakari, etc. Ceramics or metal objects are coated with heat fusing coats of metallic and non metallic compounds. Vitreous enamel coatings are glassy, but non-crystalline coatings are matt. In a wet application a slip is prepared through a water suspension of crushed glass, flux, suspending agent, refractory compound and colouring agents or opacifiers. The slip is applied on a raw product or half-baked product by dipping or flow coating. It is then fired at temperatures for the slip to fuse and flow into a glassy surface. The Dry process involves spraying the enamel powder onto a heated product, to allow the slip to fuse. Enamelling has been used as a coating system for iron surfaces in sign boards, utility items like dishes, tumblers, cooking ranges, hospital wares, ceiling panels, partition panels etc.
Conversion coatings are chemical surface coating treatments which provide some degree of corrosion protection to the surface, mostly very temporary and an appropriate substrate for subsequent treatments or coatings. Two principal processes are used, namely Phosphating and Chromating.
Phosphating: Phosphate coatings are applied to ferrous metals and zinc surfaces. The phosphate coating is an excellent pretreatment before painting, and as a base for impregnation with enamels, paints, lacquers, oils and waxes. The electrical inertness of the coating helps in localizing corrosion breaks. A light or rapid phosphate treatment is used when a paint film is to be applied but a heavy phosphate coating is used when the coating is intended to function as a barrier film and carrier for oil impregnation. Phosphate coatings may be produced by immersion, brush application or spraying.
Zinc phosphate coatings: Zinc phosphate coatings are smooth and fine grained, and are excellent surface preparative treatments prior to painting, for improved adhesion.
Chromating: Chromating process is used to increase the atmospheric corrosion resistance of metals such as aluminium, magnesium, tin, zinc and cadmium primarily, although it is sometimes also used to increase the tarnish resistance of copper and silver.
Diffusion coatings are also known as cementation coatings, differ from other forms of metal finishing in that, part of the coating interacts and alloys with the substrate. Cementation coating process is very similar to carburising of iron to produce surface-hardened steel, where iron is heated with carbon particles for the diffusion to occur. Common processes that fall into this category are: aluminizing (calorising), chromising and Sherardising (zinc cementation coating). Hot dip galvanizing, tinning and aluminizing as well as terneplating also involve alloying between coating and substrate. Diffusion coating can be made in two ways, namely in the gas phase at high temperatures by exposing the substrate to a volatile (usually halide) compound of the coating metal and through a solid phase by packing the substrate metal in a container with the powdered coating metal and heating them together.
Aluminizing: Aluminizing can be done by both solid state and pack diffusion. This surface alloy however, is porous and brittle so the parts are heated in air at 815-980° C for 12 to 48 h, to allow Interdiffusion of aluminium and iron to occur.
Chromising: Chromising is formation of a diffusion coating on iron or steel by chromium to create a surface with enhanced oxidation, corrosion and wear resistance.
Sherardising: Sherardising is zinc diffusion coating of steel, most widely applied diffusion treatment. Commercially sherardising is less important than hot dipping or galvanizing treatments, and electro deposition of zinc. Sherardising is though very useful for treating nuts, bolts, screws and chains. Treated articles are characterized by high abrasion resistance and uniformity of coating thickness irrespective of the article shape. The sherardised coating is a hard matt grey finish.
Galvanising: Galvanising is a term commonly given to molten zinc dipped coatings on steel. Steel can also be coated by zinc through electro plating. The steel is cleaned and then dipped in heated hydrochloric acid which produces iron chloride on the surface of the metal that can act as a flux. However if the coating is cut or scratched, then the zinc flows to decay preferentially to the steel and provide continuous protection.
Vapour deposition: A thin specular coating is formed on metals, plastics, paper glass and even fabrics through a deposition by means of vaporization. Coatings formed by condensation of metal vapour originating from molten metal, high voltage discharges between electrodes (cathode sputtering), or from chemical means such as hydrogen reduction or thermal decomposition (gas plating) of metal halides.
Siliconising of steel by done by heating in contact with silicon carbide powder in an atmosphere of silicon tetrachloride vapour. The treatment primarily provides high surface hardness and wear resistance.
SPRAYED METAL COATINGS
Sprayed coatings are formed by the impingement and subsequent flattening of large numbers of molten metal particles on the surface. The deposit has high intrinsic tenacity due to mechanical interlocking. The deposits are laminated or stratified since they are built up in layers. Sprayed metal coatings have a matt finish and are porous and as a result function as good substrates for paint films. Metal spraying is possible at very low and short duration of temperature so that metals, plastics, rubbers and even paper can be sprayed without damage.
There are basic Four types of spray metal processes: 1. Metals in powder form are heated to liquefy during spraying, 2. Hot liquidized metals are atomized during spraying, 3. Metal powders are sprayed on heated objects, and 4. Objects are coated or stuffed with metal powders, and then sintered.