Dental ceramics are specific ceramic materials for dental application, i.e. essentially visible and therefore aesthetic structures. These structures can be formed in all-ceramic or in an aesthetic coating of a metal substructure. To describe these materials, it is necessary to take stock of the characteristics of ceramic materials.

Ceramic materials are brittle and, unlike metallic materials, break without plastic deformation so after the initiation of a crack (beginning of cracking) the material will break.

Hanno un carico di snervamento alto che equivale al carico di rottura quindi non si ha una deformazione prima della rottura, questo perché i legami sono ionici o covalenti.

These vitreous systems are used for the realization of the capsule in traditional porcelain or by glass infiltration of ceramic systems, then in the presence of a ceramic matrix with glass infiltration we will have a matrix tending to alumina whose porosity are replaced with a vitreous system.

In the case of a covalent bond, which is an extremely oriented and strong bond, or in the case of an ionic bond, which involves the bonding of ions of opposite charge, if you try to slide the planes, two equal charges will approach and repel each other, thus causing the material to break without deformation.

Ceramic materials are characterized by brittle fracture and displacement of dislocations and plastic deformation is essentially impossible due to the characteristics of atomic bonds.

Brittle fracture is caused by defects that lead to breakage: porosity, surface defects or interstitial atoms.

The defects disturb the lattice of the material which therefore cannot deform because of these defects and lead it to breakage.

A small defect, but above all a "sharp" defect, causes a point of concentration of stresses, so that even if the load applied externally is less than the breaking load of the material, failure may occur because locally the stresses exceed the limit of the material.

This type of breakage is particularly sensitive to tensile stresses.

In figure a we see that if we apply traction efforts on the material we are going to "open" the defect and therefore facilitate the progression of the crack; on the contrary with compression efforts we are going to "close" this defect.

This is why ceramic materials are resistant in compression but not very resistant in tension or shear or bending. When making a ceramic material one must make it to work in compression for this reason. With reference to the metal sub-coating of the ceramic capsule it is necessary to have thermal expansion coefficients that are not too different in order not to have too many differences between the materials (since they have to be compatible anyway) but the thermal expansion of the metal has to be a bit higher in order to keep the ceramic material in a compression situation.

Ceramic materials are of interest for the aesthetic part of restorations such as crowns, bridges, etc.

Dental ceramics must have some basic requirements :

• To cover the structure of the crown of the tooth they must resist the masticatory load because they are the surfaces directly exposed to mastication, although the masticatory load is a compressive load so the ceramic responds well to this type of stress.
• They must withstand the corrosive and aggressive oral environment but being an oxide based material it is inert and corrosion resistant.
• They must have an aesthetic requirement because it must replicate in the most natural way possible the color of a tooth as a visible component of the prosthesis; ceramic is an excellent material to be used for this aesthetic performance just think that even the enamel is a ceramic material.

Dental ceramics are inert, therefore they remain unaffected by contact with biological fluids and do not release substances during their lifetime, therefore they are biocompatible. They are resistant to chewing and are durable throughout their time of use.

While with a metallic material, even with a thin section, you can have a good mechanical strength, with ceramic materials the section varies a lot the mechanical strengths. This is why with many ceramics used are used only as a coating on a metal structure to have adequate mechanical strengths.

There are two options to choose from: Metal-ceramic or all-ceramic.

The metal-ceramic has a metallic substructure (titanium, gold alloys or chrome-nickel) that guarantees mechanical resistance even with reduced thicknesses. It is possible to have good thermal compatibility with ceramics by appropriately selecting the metal-ceramic coupling. The complex has a good resistance to fracture but an exposure of the metal edge may occur following gingival recession.

This could cause aesthetic problems by exposing the metal (gray tends to be) and allergenic problems in case the release of metal ions causes allergies.

In all-ceramic you have an aesthetic similar to the natural tooth and, not having any metal substructure, you do not risk the exposure of a metal edge. If an appropriate composition and section of the ceramic is chosen, an adequate compressive strength can be obtained.

The ceramic is biocompatible so there is no risk of ionic release in the oral cavity and has a good resistance to wear.

Ceramics can be classified according to their microstructure:

• Glass systems, primarily silica-based, amorphous materials. (ceramic particles within a glassy matrix)
• Glassy systems with crystalline fillers (crystalline particles within an amorphous glassy matrix)
• Crystalline systems with glassy infiltration, you have a system opposite to the previous point, that is, the preponderant part is formed by ceramics whose porosity is filled by a glassy structure
• Polycrystalline Ceramic Systems

Ceramics can also be classified according to processing technology.

Glass systems are made using traditional techniques or there are technologies in which ceramic blocks are mechanically processed.

There are also fully automated technologies based on CAD/CAM, i.e. on a digital acquisition of the impression and a transformation into a model that is made with an appropriate transformation in ceramic material.

Glass systems typically contain silica, feldspars, opacifying pigments to modulate the color of the final product, and modifying oxides to modulate melting temperatures and color.

The typical composition of a dental porcelain sees some amount of silica, alumina, boron oxide, potassium oxide, sodium oxide and other oxides.

These vitreous systems are used to make the traditional porcelain capsule or by glass infiltration of ceramic systems, so in the presence of a ceramic matrix with glass infiltration you get a matrix that tends to be alumina whose porosity is replaced with such a vitreous system.