Porcelain enamels Flashcards
What are the qualities of porcelain enamels?
> low temp fusing
glass forming
coatings on metal surfaces
durable
washable
abrasion resistant
What are porcelain enamels composed of?
typically BOROSILICATE, and sometimes contains lead
How are porcelain enamels prepared?
> 100% frit
milled with clay and opacifier
small amounts of transition metal oxides (Co3O4) are added to increase adherence
What does clay do in porcelain enamel slips?
> it is a suspending agent
What do transition metal oxides do in porcelain enamels? (Co3O4)
> Co promotes Fe dendrites to grow into the enamel, functioning as anchors
Co forms an adherent oxide on metal, onto which the enamel adheres to
Why are two coats applied in porcelain enamel slips?
- a base/ground coat contains the adhesion facilitation transition metal oxide
- an opaque cover coat with opacifier and stain particles
What do opacifiers do in porcelain enamel slips?
maximize Rayleigh scattering
What is a typical opacifier used in porcelain slips?
Titania: high difference in index of refraction as compared to glass, strongly scattering white light: “titania white”
How are opacifying particles added to porcelain enamels?
Either not allowed time to be soluble in the glass or are induced to devitrify during cooling
How are porcelain enamels applied?
> Spraying slip for smaller parts
Sifting on as a dry powder for larger parts
–> surface is de-greased, pickled, and heated so the enamel fuses on contact
What is pickling?
etching to form a roughened surface with anchoring points
What is the contemporary method of applying porcelain enamels?
> electrostatic deposition: dry frit encapsulated in an organic silane, allowing the aerosoiled frit to hold an electrical charge so it’s attracted to the grounded metal substrate
What are ground coats?
70% > 74um baked on for 2.5 minutes at 850 C
What are cover coats?
3-6% > 74um, baked for 3 min at 800-850C
What are the qualities of thinner coatings?
> more flexible
25um-3mm
