Metals are normally crystalline solids at room temperature,although there are some exceptions such as hydrogen(a gas) , gallium and mercury(liquids).
Metals are noted for the ease with which they give up their valance elecrtons; this accounts for certain characteristics such as good electrical and thermal conductivity and the tendency for metals to form cations(positive ions) in solution;metals are also strong, lustrous and ductile.All metals have crystal lattice, it can be:cubic, body-centered cubic and face- centered cubic.
Alloys:two or more metals that are mutually soluble in each other in the molten state.
Liquidus temperature :the temperature below wich a metal is completely solid(i.e..the temperature at which it begins to melt.)
Solidus temperature : the temperature above which a metal is completely melted .
Grain: a single crystal of metal as seen in the metal‘s microstructure.Grain growth-enlargement of grains by heating.
Pure metals such as gold, silver are not used in prosthodontic(their relatively poor physical properties make them a poor choise for intraoral use).
.Generally alloys are used in dentistry because alloying strengthens a metal.
Grain refiner:a metal such as iridium that is added to a metal or alloy because of its high melting temperature.Since it remains solid while the rest of the metal is molten, its small particles act as seeds around which grains of the solidifyring metal form.This enhances the physical properties of the solid metal.
Sag deformation: of metal or alloy; for our discussion, this occurs at high temperatures such as during porcelain firing.
Yield strength- the stress at which dislocation motion, permanent deformation begin.
During solidification(i.e. when a metal or alloy goes from liquid to solid),the following process occurs:
Random atoms to embroys(temporary nuclei) to nuclei to dendrites to grains.As the grains grow, they contact each other.The border or junction between them is called a grain boundary.
Generally, a metal to bee used under intraoral conditions should have a fine-grained structure because it tends to be more resistant to permanent deformation.
Smaller grain size can be achieved by rapidly cooling the molten metal and by inclusion of a grain-refiner such as iridium(as noted previously).When iridium is added in a small concentration(0.005%=50ppm), the tensile strength and ductility increas.The is little, if any, effect on hardness and yield strength.
Effects of some metals:
Gold: melting temp.-1064˚C, increases resistance to tarnish and corrosion,increases ductility and malleability;
Copper: principal hardener, is necessary for heat treatment(when added in a concentratio>12wt%)
Silver: melting temp.-960.5˚C; main purpose is to modify the red color produced by gold and copper,reduces melting temperature,increases ductility and malleability.
Platinum: melting temp- 1773˚C; raises melting temperature, increases tensile strength, decreases the coefficient of thermal expantion, reduces tarnish and corrosion.
Palladium:melting temp.-1555˚C; raises melting temperature, increases hardness, acts to absorb hydrogen gas which can cause porosity in the castting, prevents tarnish and corrosion, has a very strong whitening effect on gold alloys even when used at a low concentration(i.e.5wt%)
Zinc: acts as a scavenger and prevents oxidation of the other metals during the manufacturing process, increases fluidity and decreases surface tension, which improves castability.
Iridium: acts as a grain refiner, this increases tensile strength and percentage elongatio(i.e. ductility); it also increases, to a much lesser degree(if at all), hardness and yield strength.
Titanium: derives corrosion resistance by forming a surface layer of titanium oxide.
Chromium: it is included as a component of many alloys in order to improve corrosion resistance. Passivating effect-Chromium is electrochemically active it reacts readily forming a layer of chromic oxide which protects the metal or alloy from further decomposition.Pasive elements are: Cr, Al and Ti.
CORROSION
Surface deterioration of a metal because of its interaction with the surrounding environmentis called corrosion.Resistance to corrosion is criticaly important for a dental alloys because corrosion can lead to roughening of the surface, weakening of the restoration, and liberation of elements from the metal or alloy.Liberation of elements can produce discoloratio of adjacent soft tissues and allergic reactions in susceptible patiens.Commonly, noble metals are added to an alloy to reduce its tendency to corrode.These include gold, platinum, palladium,ruthenium, rhodium, osmium and iridium.
There are many types of corosion: galvanic, pitting, crevice, errosion, intergranular, stress corrosion, biological.
· Galvanic corossion: The potential difference that exist when two dissimilar metals are immersed in a corosive or conductive solution is responsible for galvanic corrosion.The less-resistant(anodic) metal is corroded relative to the cathodic metal(it is called galvanic or electrolytic cell).The difference of electrical potential created by two or more dissimilar metals in the mouth or even by a single metal that conflicts with the body‘s own bioelectrical current is called oral galvanism.Other names for oral galvanysm are galvanic mouth currents, “mouth battery“, and metal tension fields.All regulating events in the human body are communicated by electrical charges.Therefore, any conflicting electrical charges that emanate from dissimilar metals in the oral cavity create an imbalance that can lead to pathogenicity.Variuos fluids in the mouth- saliva, bone fluid, and dentinal plasma- act as conductors for electrolytes(minerals that dissolve in a fluid medium into electrically charged ions).Whenever a non-precious metal post is placed in a root canal for reinforcement,or an amalgam buildup is installed ,or a gold crown constructed as a final restoration, measurable electrical currents emanate from that tooth. This leads to a disruption of the body‘s own internal electrical currents, which in turn has a negative impact on the functioning of the immune system, rendering us more vulnereble to inflammation and infection elsewhere in the body.The infection and toxity that can indirectly result from this „battery effect “ in the mouth, silently permeating the body, and causing untold damage and ill health.Can be metalic pain or/and taste in the mouth too.These problems are exacerbated by using different metals in the mouth, as they cross-react with one another.For example, even more galvanic currents will be created if a gold crown is instaled, followed by a non- precious metal partial(a removable bridge) that contacts with gold crown.Should be used only one kind of alloy in the mouth for avoiding „battery effect“.The gold being the most desirable choise as it does not readely oxidise.
· Pitting corrosion: pitting is a form of extremely localized attack that produces damage characterized by surface cavities.(i.e. produces holes- pits in the metal).This corrosion is caused by atacts of chlorides on a protective(passive) layer, which is enhanced by presence of oxidising chemicals.It is an especially insidious form of corrosion because in cause restorations to fail after only a small percentage of the designed for weight loss.Use materials with alloying elements designed to minimize pitting susceptibility.Other methods for preventing pitting corrosion include reducing exposure to aggressive ions be shielding the part, reducing the concentration of these ions in the environment surrounding the part,and minimizing the effects of and exposure to corrosive factors of those design elements that must not be weakened by pitting.To reduce corrrosion by this mechanism, metals and alloys used in the mouth should be polished to remove surface irregularities.
· Crevice corrosion: an intense localized corrosion frequently occurs within crevise and other shieled areas on metal surface exposed to corrosive attack.This type of attack usually is associated with small volume of stagnant liquid at design details such as holes, gaskes surfaces, lap joints, and crevices under bolt and rivet heads.Crevice corrosion like pitting could be very damaging because the destructive action is extremely localized.Crevice corrosion or concentration cell occurs in poorly gasketed flanges and under bolt heads and attachments components immersed in liquids.
· Errosion- corrosion: deterioration at an acceleated rate that is caused by relative movement between a corrosive fluid and metal surface.Then the metal tries to grow a new layer via corrosion, which is removed again.
· Intergranular corrosion: It is the result of heating an alloy(e.g. stainless steel) can suffer local(normally the grain boundaries) depletion of an essential element for corrosion protectio(e.g. chromium).This is called sensitation.When such an alloy is exposed to oxiding media, the corrosion proceeds along the grain boundaries and the alloy disintegrates into grain.
· Stress corrosion cracking(SCC) : Specific combinations of alloys and environment can lead SCC.When the metal is mechanically stressed while being exposed to this environment(the stress can be also result from the fabrication process).The metal then fails at a load far below its nominal mechanical strength.
· Biological corrosion: When the metalic surface is immersed in water a biofilm will being to form if there is any bacteria in the water.A biofilm is a microbial mass composed of aquatic bacteria, algae, or other micro-orgamizms.The biofilm begins when organic material is absorbed onto the surface of the metal, bacteria grow, oxygen is exluded, which creates a place where the passive film may break down.
PROBLEMS OF METALS BIOTOLERATION
Ideally, a dental material that is to be used in the oral cavity should be harmless to all oral tissues-gingiva, mucosa, pulp and bone.It should contain no toxic, leachable, or diffusible substance that san be absorbed into the circulatory system, causing systemic toxic responses, including teratogenic or carcinogenic effects.The material also should be free of agents that could elicite sensitication or an allergic response in a sensitized patient.Allergic reactions can be local and /or systemic.
It is important to use material that will not trigger an immune response(i.e. biocompatible material).They are called biomaterials.
Titanium
is used in implantology as it is inert and has a very high level of compatibility with human tissues.Importan that the bone cells grow well around implant.Others biomaterials are used in imlpantology too:bioglass,titanium plasma spray, titanium nitrid,aluminium oxide, zirconium oxide,silicium carbide,dens hydroxyapatite, tricalcium fosfate,calcium fosfate ceramic.
Dental amalgam
can cause local soft tissues reaction.Large amalgam particles that are embedded accidentally in the gingiva during placement of a restoration may elicit chronic inflammation.Pigmentation of oral mucosa is called „amalgam tattoo“An increased content of mercury has been observed in gingival biopsies from areas in close contact with amalgam, and in oral lichen planus lessions.
Gold
foil is a stable and relatively insoluble restorative material.There are patiens sensitized to gold may react to gold restorations.These reactions iclude burning sensation of oral mucous membrane in contact with the gold alloy, lichenoid lessions, and general systemic reactions.The insertion of gold may result in pulpal reactions, but these are generally thought to be caused by the conductivity, cavity preparation, dehydration of the cavity and micro leakage.
Allergic reactions have been described for many of metals, including palladium,nickel,chromium cobalt and berylium.
Approximately 10% of women and 1% of man are sensitive to nickel. Allergy to gold-based restorations is reported more commonly than allergic reactions to nickel-containing dental alloys.
Palladium
based alloys have been reported as causative agents in cases of stomatitis, oral lichenoid reactions, and disseminated urticaria. Palladium allergy seems to occur in patients who are sensitive also to nickel.
Chromium
cobalt alloys have an excellent history of biocompatibility, although there are some reports of tissue sensitivity in a very limited population.
Beryllium and cadmium
(toxic metals) can be harmful too. There is no known skin hazard connected with touching or handling solid forms of beryllium oxide, metallic beryllium or beryllium containing alloys. The hazard associated with chronic beryllium disease, so called “berylliosis” is due exclusively with inhalation. It is very important for dental technicians.
DENTAL ALLOYS CLASSIFICATION:
Before discussing alloys characteristics, it is worth reviewing the terms high-noble, noble, base-metals ;
Classification according to the cost -precious, semiprecious and non-precious metals. In dental advertisements precious usually refers to silver, a metal that is precious but not noble; silver is usually a major ingredient in most alloys advertised as precious. Nonpresious alloys are made from ignoble(base) metals(Co,Cr,Be,Fe)
Classification according to alloys composition:
High -noble metals must have a noble metal content of at least 60%, of which at least 40% is gold. They usually contain a small amount of tin, indium, and/or iron which provides for oxide layer formation. This increases the alloy’s bond strength to porcelain. The disadvantage of these alloys include their lower rigidity and poor sag resistance during porcelain firing. There are three subclasses of high noble alloys(Au-Pt; Au-Pd; Au-Cu-Ag-Pd)
Noble metals are defined on the basis of their chemical properties; that is they resist oxidation and are not attacked by acids .There are seven noble metals but widely are used gold, palladium, platinum(other-Os, Ru, Ir, Si, Rh). The alloys must contain at least 25% noble metal. They may be yellow or white in color. There are three subclasses of noble alloys(Au-Cu-Ag-Pd; Pd-Cu-Ga; Pa-Ag)
Base-alloys contain less than 25% noble metal. They are used for full-cast and porcelain fused to metals restorations, for removable partial denture frameworks. As a group they are much harder, stronger, and have twice as high an elastic modulus as do the high-noble and noble-metal alloys. This latter property is advantageous because casting can be made thinner and still retain rigidity. For metal- ceramic purposes, this means that porcelain need not be over-bulked for esthetics, and long- span fixed dentures will be more resistant to flexure, which can lead to porcelain fracture. For removable partial dentures, greater rigidity is advantageous because major connectors can be made thinner(and therefore less objectionable to the patient) and still be rigid. The base-metals alloys also have excellent sag resistance. But they have disadvantages too. They are more difficult to cast than non base-metal alloys because of their very high liquidus temperatures They also exhibit a greater casting shrinkage (about 2.3%)that must be compensated for because of their lower ductility and greater hardness, they are less burnish able and more difficult to finish and polish than the high-noble and noble-metal alloys. They are more prone to corrosion under acid conditions, difficult to solder, and can cause an allergic reaction in susceptible patients. Finally, beryllium and nickel can cause allergic reactions and may have carcinogenic effect .There are three subclasses of base-metal alloys(Ni-Cr-Be; Ni-Cr; Co-Cr)
Dental alloys classification according to the main element:
· Gold
· Palladium
· Silver
· Nickel
· Cobalt
· Titanium
Dental alloys classification according to their use:
1. Gold- based alloys for full- cast restorations
2. alloys for porcelain- fused –to- metal restorations
3. Alloys for removable partial dentures
Gold- based alloys classification according to physical properties:
· type I –soft(for casting subjects to non bearing stress or very slight stress bearig- two surface inlays)
· type II- medium( for inlays, MOD, crowns)
· type III- hard( for inlays, crowns, short-span fixed partial dentures)
· type IV extra-hard( for crowns, long-span fixed partial dentures, removable partial dentures)
The most commonly used type III which composition is:gold-75%,silver-10%, copper-10%, palladium-3%, zinc-2%.
As the gold-based full casting alloys go from type I to type IV, their composition changes: gold content, temperature, and ductility decrease, white copper content, proportional limit, hardness, tensile strength, and yield strength increase.
Alloys for porcelain-fused- to-metal(PFM) restoration:
· High gold alloys(74-88%-Au and total noble metals content is about 97%), small amount of indium, tin, iron, gallium are added for strength and to promote a good porcelain bond to metal oxide. Corrosion resistance is excellent because of high nobility.
· Gold- palladium alloys(no silver, 50%- gold, 40%- palladium)Disadvantage- the silver free alloys tend to have lower expansion values than the silver-containing group. So, gold palladium group thermal expansion is incompatible with some of the higher- expansion porcelains. Corrosion resistance is excellent.
· Gold-palladium- silver alloys(10-15%Ag,20-30% palladium)Disadvantage-have tendency to change the porcelain color(yellow-green) because of their silver content
· Palladium- copper alloys(70%-80%-palladium, contain little or no gold,15%-copper,9%- gallium ).They are extremely rigid and are used for full-cast and metal ceramic restorations.
· Palladium-silver alloys (50-50%Pd)Good corrosion resistance, solder well and have the lowest sag tendency of the precious metal alloys. Porcelain bond strength is also excellent.
· Nickel-chromium alloys (70%Ni,16%Cr,2%Al,0.5%Be). Al and Ni form an intermetallic compound, that contributes to strength and hardness, and beryllium lowers the melting range, enhances fluidity, and improves grain structure.
· Nickel- chromium-beryllium alloys
· Cobalt- chromium alloys (60%Co,25-30%Cr,which imparts corrosion resistance.)They may also contain minor amount of molybdenum, aluminum, iron, gallium, copper, silicon, carbon, platinum. Manganese and silicon enhance fluidity of the molten alloys.
· Titanium and titanium alloys
These alloys differed from the gold-based alloys used for full-cast restorations in the following ways:
1. they had higher melting temperature(due to greater palladium content) to accommodate the temperatures required for porcelain application;
2. they contained little, if any, silver which could cause the porcelain to turn green;
3. they contain little, if any, copper because it could cause the alloy to sag during porcelain firing;
Alloys for removable partial dentures
· Type IV gold- based alloys
· Chromium-based alloys
· Cobalt-based alloys
The type IV gold-based alloys were strong enough to function adequately when used to make frameworks. The Cr- and Co- based alloys differed from metal-ceramic counterparts in that they contained a small amount of carbon(0.1-0.5%) for hardening.
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