Manufacturing: Surface Finishing

Porcelain Enameling

Porcelain enamel is a specially formulated, highly durable glass permanently fused to metal under extremely high temperatures. The metal may be steel, cast iron, or aluminum. It may be a protective surface for cooking utensils, kitchen and bathroom fixtures, and appliances such as dishwashers, ranges, refrigerators, washers and dryers. It is acid resistant. Porcelain enamel may be a decorative surface of metal, glass or pottery.

Porcelain enamel while very durable is also expensive to apply and so is being replaced by synthetic enamels or plastics in many appliances today, in such places as cabinet sides, tops and doors. Range tops and ovens will remain porcelain enamel because there is no good substitute presently to withstand the high temperatures. Older appliances may have more porcelain enamel surfaces.

Process

Porcelain Enamel begins as a blend of minerals smelted in much the same manner as common glass. During this process (known as "fritting"), the red-hot, molten mixture is poured from a smelter and quenched between water-cooled rollers. This quick-cooled ribbon of glass is then shattered forming a particulate known as "frit".

Frit is applied to metal using either a wet or dry method and fired at very high temperatures - generally ranging from 1100 to 1600 degrees Fahrenheit. While in the furnace, the frit melts...bonding with the metal to create more than a coating. It forms an inseparable compound merging the chemical makeup of the frit and metal; resulting in a new, chemically unique, finish.

Mechanical and Physical Properties

INTRODUCTION
The porcelain enamel coating, the metal substrate and the design of the part to be coated all contribute to the mechanical and physical properties of the porcelain enamel. However, since porcelain enamel is glass, the glasslike properties are most influential. The development of thinner coatings has increased the role of the base metal's mechanical properties, providing more flexibility, less brittleness and greater chip resistance. Porcelain enamels, regardless of thickness, provide outstanding wear resistance and abrasion resistance, while contributing to the strength of the metal substrate.

HARDNESS
Depending on composition, hardness of porcelain enamels range from 3.5 to 6 on Mohs scale of mineral hardness. Most porcelain enamels for steel substrates fall in the range of 4 to 5.5. Organic finishes commonly fall in the 2 to 3 range. As a rough approximation, a typical porcelain enamel on steel has about the same hardness as plate glass. Rather uniquely, hardness of porcelain enamel does not vary greatly from one composition to another. Porcelain enamel surfaces are unaffected through the range of pencil hardness scratch tests commonly used to evaluate organic finishes. Comparable values on the Knopp scale range from 149 to 560. The Sward rocker rating is 100, the same as plate glass. The surface hardness property of porcelain enamel contributes a durability quality that is essential to a broad range of long-lasting products.

ABRASION AND WEAR RESISTANCE
Porcelain enamel coatings provide excellent abrasion and wear resistance, with every formulation being substantially more abrasion resistant than the hardest organic coating. Porcelain enamel's resistance to wear and abrasion is due to its resistance to gouging or crushing of the underlying enamel structure, its high surface hardness (surface abrasion resistance), its high surface gloss and its good lubricity. Porcelain enamels frequently provide better wear and abrasion resistance than metals. This is attested to by their use on bunker and silo discharge chutes, coal chutes, water lubricated bearings, screw conveyors and chalkboards. Sinks, lavatories, bathtubs and range tops are further examples where good abrasion and wear resistance is an important service requirement. Test reference: ASTM C448 Abrasion Resistance of Porcelain Enamels.

LUBRICITY
Lubricity of conventional glossy porcelain enamels is perhaps the highest of any known finish except the "no stick" fluorcarbons. Lubricity of porcelain enamels is particularly important in low friction applications. Examples include water lubricated bearings where the enamel mates with rubber, chutes and pipes conveying coal and other materials at low angles of inclination and pipe linings used in waste disposal systems. If desired, porcelain enamels can be specifically formulated to have a coefficient of friction finish which provides skid resistance. These are used on nonskid stairs, shower stall bases and bathtub bottoms. Similarly, a very low coefficient of friction and extremely wear-resistant porcelain enamel finish can be produced for such applications as package chutes, screw conveyors and food processing equipment. Test reference: Inclined plane tests are sometimes devised to quantify the lubricity property. With application of a "standard load" on a coated test panel, the angle of inclination is observed when sliding begins. From this data, a coefficient of friction can be calculated.

ADHESION
Adhesion may be viewed as (1) resistance to mechanical damage by impact, torsion, bending or heat shock; (2) "attraction" of enamel and metal; and (3) a relationship to substrate design. Good adhesion is produced by reaction and fusion of the porcelain enamel coating with the base metal at relatively high temperatures that may fall within a broad 932 degree F to 1652 degree F (500 degree C to 900 degree C) range. Glass is very strong in compression. When the porcelain enamel coating (glass) is applied to the metal substrate, the formulation of the coating is such that it has a lower coefficient of expansion than the substrate and thus is always in compression. The bond has many characteristics of a true chemical bond in combination with mechanical bond developed by fusion flow of the coating over the surface roughness of the substrate. Since moisture or rust cannot penetrate beneath the porcelain enamel coating, it will not flake away from exposed edges or damaged areas. The coating does not tend to "creep" under service conditions as can be shown by salt spray testing. Porcelain enameled metal will flex with the metal providing resistance to stresses that cannot be obtained in solid glass.

IMPACT RESISTANCE
Like glass, porcelain enamel will fracture when abused. It is difficult to predict the impact resistance of a specific porcelain enamel since it depends as much or more on the design of the part as on the properties of the porcelain enamel. However, a porcelain enamel can be very strong and flexible if applied to a properly designed part. As a general rule, porcelain enamel will not fracture due to impact unless the base metal is permanently deformed. Because of its high compressive strength, the enamel is rarely crushed at the point of impact. Porcelain enamel's compressive strength is in the range of 20,000 psi.

FLEXIBILITY
Thin porcelain enamel coatings have very good flexibility and adhesion when applied to thin metal substrates. For example, a 10-mil commercial steel sheet with two porcelain enamel coatings 5-mils thick is so flexible it can be shipped in 12-inch diameter coils without damage. Experimental porcelain enamels applied at a thickness of 1.5-mils to steel sheet 4-mils thick have been deformed to a radius of 1.5-inches without damage to the coating. Porcelain enameled coatings will flex with the base metal until the metal is permanently deformed. The modulus of elasticity is 10 x 10^6 for porcelain enameled steel. Tensile strength is approximately the yield point of the base metal. The impression of brittleness and lack of flexibility probably stems from the fact that heavy coatings applied to thick metal articles (bathtubs, for example) tend to fracture when lightly bent.

STIFFENING
Because of its low ductility and intimate bond, porcelain enamel increases base metal flexural strength. Thus, the stiffening effect of the coating can be used advantageously to reduce metal thickness in certain applications. The stiffening effect is more pronounced on lighter gages than on heavier gages of metal. Though thicker porcelain enamel coatings may be used to promote needed rigidity or offer added wear protection, thinner coatings are much less vulnerable to fracture and chipping. For instance, a 0.016-inch porcelain enamel under torsion test may be expected to show failure at 50-60 degree but 0.003-inch coatings have been torsion tested to 200 degree and beyond before any fracture occurred. For metal porcelain enameled on one side only, the effect is greater when the porcelain enamel coating is on the compression side. With equal coating thickness on opposite sides of a panel, the residual compressive stresses contribute a stiffening condition desirable for rigid designs.

THICKNESS
Porcelain enamel can be applied in a wide range of thickness, from 1-mil or less on steel or aluminum substrates to 125-mils (1/8 inch) or more on cast iron or heavy gage steel or plate. Optimum thickness depends on compositions of the porcelain enamel coating and the base metal and -particularly-on the expected service conditions. In general, thinner porcelain enamel coatings are more flexible and have greater resistance to fracture. Thicker coatings have better electrical properties; they also withstand and chemical attack for longer periods. The thickness of the porcelain enamel can be a factor in the mechanical strength of the product or component adding stiffness to it. For applications on steel, a base or ground coat of porcelain enamel 2 to 5-mils thick is commonly applied and followed with one or more finish coats. However, with modified pretreatment of steel substrates and decarburized steel quality, a one-coat porcelain enamel finish coat of 3 to 5-mils may be applied directly to the steel. If more than one cover coat is applied, each may be 2 to 10-mils thick. Multiple coats can be applied that interfuse to form a single heavy layer. Normally, a white cover coat (titania opacified) should be at least 3-mils thick to provide adequate opacity to hide light scratches in the metal. Other white cover coats, not titania opacified, require thicker coatings to produce satisfactory appearance and color. Certain colors and textural effects can be obtained with relatively thick coatings. However, purely functional coatings such as those for high temperature protection are usually applied quite thin (as thin as 1-mil) using special base metals. Thickness over 15-mils is not generally recommended for sheet metal parts because of warpage or chippage problems. Normally, heavier coatings are used on cast iron or steel plate where rigidity of the substrate resists deformation and reduces the danger of fracture. Such coatings are sometimes desirable to hide rough spots on the metal or to provide longer service life. Porcelain enamels for aluminum sheet are usually applied at a target thickness of 3-mils. However, thickness of 5 to 7-mils may be required for specified appearance or use requirements.

THERMAL EXPANSION
Coefficient of thermal expansion is largely determined by chemical composition. Coefficient of expansion is 8-14 X 10^-6 cm./cm./degree C formulated so glass is always in compression. Being glass-like, porcelain enamel is much stronger in compression than in tension. Hence, it is important to have the coefficient of expansion of the porcelain enamel coating lower than that of the metal substrate so that in cooling the coating will be in compression, not tension. The amount of compressive stress allowed to develop must be controlled carefully. If it becomes too high, fracture can occur at sharp radii. Excessive compressive stress can increase warpage tendencies, particularly with the metal substrate coated on one side only, or having unequal coating thickness on the two sides. Thus, residual compressive should be low in such applications as appliance parts and architectural panels. On the other hand, residual coating stresses should be kept high on parts subject to failures by thermal shock or bending. Test reference: ASTM C359 Linear Thermal Expansion of Porcelain Enamel and Glass Frits and Ceramic Whiteware Materials by the Interferometric Method.

THERMAL CONDUCTIVITY
Porcelain enamel is not a thermal insulator, but it is a relatively good heat conductor when applied in thin coats. Its emissivity characteristics are particularly good. Normally, porcelain enamels are applied so thin there is only a very small temperature gradient through them. Thermal conductivity is expressed as 0.001 - 0.003 cal./sq. cm./sec/degree C.