Dental Ceramics Lecture 9 PDF

DENTAL CERAMICS OR DENTAL PORCELAIN Lecture 9  Keramos (Greek) = pottery (“burnt earth”; so a product obtained through the action of fire upon earthy materials. ) Ceramics = formed from nonmetallic materials fired at high temperature Porcelain (“porcelino/porc...

DENTAL CERAMICS OR DENTAL PORCELAIN Lecture 9  Keramos (Greek) = pottery (“burnt earth”; so a product obtained through the action of fire upon earthy materials. ) Ceramics = formed from nonmetallic materials fired at high temperature Porcelain (“porcelino/porcellana”= cowrie shell - also called the Venus shell) = refers to ceramics composed mainly of feldspar, quartz and kaolin fired at high temperatures =Powders which fuse through heating = a solid mass high melting/annealing T Brittle (a high resistance to compression than tensile strength) Hard Thermal and electrical isolators Resistance to chemical factors Large range of colours  Molecular structure of ceramics Metallic elements: Al, Ca, Mg, K + Non-metallic elements: Si, O, B, F  oxides, nitrites, silicates with the following structure: - amorphous = glasses - crystalline = ceramic  Molecular structure of ceramics Most of ceramics contain metalloid oxides – SiO2 = silica “Unconventional” ceramics used in dentistry: NaCl, ZnO Investment materials Inorganic filler of composites Glass ionomer cements  Molecular structure of ceramics Intermolecular bonds from ceramics can be : Ionic bonds = crystalline structure =crystalline ceramic with a more complex structure than that of metals Covalent bonds = amorphous structure = glass with a more complex three-dimensional structure than that of the polymers Ceramics need to be heated at high T for the rearrangement of its atoms Ceramics= brittle materials at room T  Molecular structure of ceramics Most of ceramics generally used = ceramics based on clay (kaolin = clay) Clay paste - modeling – burning – dehydration and clay particles elimination - vitreous material (glass) The object maintain its shape due to vitreous phase The higher the T is – the more vitreous phase will be formed The whole transformation of the clay into vitreous phase = glasses  Molecular structure of the glass Glass = a material in which the disorderly structure of the atoms from the liquid state remains at a sufficient low T for the material to have the properties of a solid. Glass = an overcooled liquid Glasses flow also at the room temperature (creep)  Molecular structure of the glass The material in the molten phase is cooled very quickly = the atoms don’t have time to arrange themselves from a disorderly structure of a liquid, into an orderly structure of a solid = glass (vitrification) Any material if is cooled sufficiently fast can form glasses The more complex the material’s structure is , the more easier the glasses are formed Can be formed also several grains with a very slow rate of growth Crystallization of a glass = devitrification (maintaining the glass at a high T enough time)  Molecular structure of the glass Dental ceramics – mineral silicates with a very complex structure Cooling rate of several 0C/hour –> vitrification Silica (SiO2) = versatile – three-dimensional pyramidal structure = tetrahedron Every O atom (valence 2) is bond by to 2 Si atoms  Molecular structure of the glass Bonding of tetrahedrons into orderly structure = crystal lattice = quartz, tridymite (SiO2), cristobalite Bonding of tetraedrons into disorderly structures = amorphous structure = silica glass polymorphism  Substances which can form the glasses’ composition Network formers = complex molecular structure Silica silica + Al oxide (alumino-silica glasses), silica + feldspar (feldspar glasses) Germanium oxide Boric oxide  Substances which can form the glasses’ composition Intermediates= cannot form glasses, but they can be inserted into network Alumina (Al2O3) Network modifiers/stabilizers = very important additives, big metallic ions which substitute covalent bonds from glass with ionic bonds Ionic bonds are weaker – annealing T (sintering) more reduced, mechanical and chemical strength more reduced Na, K, Ca oxides or carbonates  COMPOSITION OF DENTAL PORCELAIN  =is formed of 2 different phases :  A glassy amorphous matrix - A pyramidal network of silica +/- modifiers in which is dispersed  Crystalline phase - Formed by the leucite, alumina, zirconia, fluoride, mica, spinell - Determines the mechanical, physical, chemical and optical properties -A mixture of feldspar, quartz and kaolin -The composition differs from that of industrial porcelain through increasing the feldspar ratio and decreasing the quartz and kaolin ratio FELDSPAR ceramics Industrial porcelain KAOLIN QUARTZ - Conventional feldspar dental ceramic: feldspar (75%); quartz(22-25%); kaolin (0-3%), formed themselves of metallic oxides : - SiO2 – 50-70% - Al2O3 – 10-20% - Na2O – 4-10% -K oxide – 8-10% -Ca oxide– 1-2% Feldspar – an anhydrous aluminosilicate, of mineral origin (SiO2, Al2O3,CaO, Na2O) -orthose – potash feldspar -albite - soda feldspar (Na2O·Al2O3·6SiO2) - ratio between orthose and albite influence the properties of feldspar: - Na2O (sodium oxide) reduces sintering T - K2O (potassium oxide) – increases viscosity of the molten mixture (benefit – ceramics won’t flow during sintering) - when the amount of K2O is insufficient into natural feldspar, there will add network modifiers ( Na and K carbonates) -During sintering feldspar will melt and through cooling: - forms o glassy mass -a part of it will be crystallized = leucite Role: - assures ceramic’s translucency -influences viscosity of molten ceramics Quartz = Si (silicone) dioxide - into ceramics under a fine crystalline dispersion within the glassy phase produced by the feldspar Role: -assures mechanical strength, keeping its shape during sintering -assures translucency Kaolin = hydrous alumino-silicate Role: -bonding agent within the formed paste, mouldable (+water) - opacity - into dental ceramics its contain is very reduced or absent, its role being taken by the other components. Inorganic pigments = metallic oxides Fe -brown, Cu - green, Co - blue, Ti – yellow-brown Organic binders = starch, glucose - bond the particles of powder and facilitate the modeling of the material - burn without residues Traditional ceramic porcelain (feldspathic porcelain) (disadvantages): -Sintering shrinkage 30% -Stiff -Brittle -Very hard Firing and sintering of the dental ceramics  Dental ceramics’ firing Industrial 1. increase the T at which the mixture becomes liquid and all the components are fusing together 2. rapid cooling into water (=fritting) transformation into solid glass (frit) accumulation of internal stresses build up in the glass – resulting in extensive cracking 3. milling – a very fine powder (packaged into recipients) Powders – 3 types - base layer = ground = opaquer; -dentine layer; -enamel layer - accessory masses: correction masses; for coloring; transparents Liquid – distilled water mixed with glycerine and dextrin (organic binders) enamel layers clear fluorescence layer dentin layer opaque dentin primary dentin opaque layers Sintering 1. the mixture powder with water/special liquid (+organic binders) 2. appliance of the material onto the die and modeling the tooth’s morphology 3. heating till the annealing T of the glass = sintering - fusing of powder’s particles  Dental “porcelain”= sintered glass Technical stages to obtain a full ceramic crown 1. compaction 2. firing 3. glazing  1. Compaction of the ceramic mass -the material is applied onto the die through brushing = high risk of air or excess water inclusion - increasing the firing shrinkage through decreasing the density of particles - ceramic masses with a mixture of different size of particles - decreased shrinkage -Compaction made through: - vibration - spatulation - whipping + dabbing (press lightly with absorbent material) excess water  2. Firing (Sintering) - gradual heating with the open door of the furnace- drive off the excess water and combustion by products – otherwise explosion - the furnace door is closed and it continues the T increasing - burning of the organic binders =light shrinkage - bisque stage =begins the fusing process, but only at certain points (porous, fragile) - glass melting will help the particles’ fusion – shrinkage on firing due to fusion of particles (till -20%)  2. Firing - prolonging the firing period – losing of the morphology, highly glazed (pyroplastic flow = flow of the molten glass) Firing furnace +/ - vacuum Vacuum-firing: -reduces porosity from 5.6% to 0.5% -increases flexure strength from 20-30MPa to 50-60MPa -increases translucency 20x (times) (without pores, a more translucent material) !! Cooling - slowly because ceramic is a poor thermal conductor - rapid cooling => fissures => decreasing of the mechanical properties -but cracks always appear inside the ceramic mass - in time will be propagated to exterior - cooling under pressure = reduces the dimension of existing porosities within the ceramic mass 3. Glazing -after firing any ceramic body has a certain degree of porosity - glazing => smooth, shiny and impervious outer layer Glazing made through: 1) appliance of a glazing layer+ sintering it at low T, for short time, OR 2) a final firing stage at high T, carefully controlled =>fusion of the particles from the superficial layer Properties of dental porcelain Physical properties Density -medium (1- 3.8g/cm3) – medium weight (between noble and non-noble alloys, heavier than polymers) - easy casting Melting T -very high - firing T can be reduced by adding of the network stabilizers/modifiers -firing at low T = reduced hardness -important especially at metal-ceramic (MC) Coefficient of thermal expansion (CTE) -1- 15 x 10-6/0C -reduced values = similar enamel = advantage = disadvantage for MC (12-15 x 10-6/0C) = disadvantage for mechanical working –high risk of increasing the local T and cracks Ceramics can be worked only with diamond burs, under water cooling Aesthetics - excellent -it is an almost perfect material similar to natural dental structures -ceramics has an amorphous structure - isotropic; - enamel has a crystalline structure - anisotropic - fluorescent, translucent (vacuum firing) -unlimited possibilities of coloring -stable chromatics due to inorganic pigments (oxides) - casting ceramics (glassy) superior aesthetics due to its modulus of reflection inside the material by the mica crystals Visible differences from tangential incidence Translucency (decreasing): Dicor (glassy ceramic) Empress (pressed ceramic) In-Ceram Spinell (infiltrated glassy ceramics) In-Ceram Alumina In-Ceram Zirconia Chemical properties -they have reduced chemical reactivity ~ chemically inert, they can be attacked only by the fluorhydric acid (used to etch the inner layer of all-ceramic restoration) - it isn't affected by the large variations of the oral pH - glazing reduces phenomena of solubility of the constituents or water absorption There are contraindicated mechanical workings over ceramics after glazing !!!!! Biological properties - the glazed ceramic masses don’t retain plaque -are very well tolerated by the marginal parodontium and dental tissues - thermal isolator for the dental pulp - excellent biocompatibility for the patient - for the dental technician: - uranium powders = can be toxic - fluorhydric acid for porcelain etching = toxic Mechanical properties -very stiff (E= 380GPa) Crack -compressive strength (150-900MPa) -tensile strength (20-60MPa) = brittle - inner layers – tensile = micro-cracks = fragile -without fracture strength(or it has very low values) -maximum plastic strain 0,1% -on cooling within inner layer may appear micro- cracks due to accumulation of internal stresses through a very low coefficient of thermal contraction -the glaze coat fills partially the cracks -the inner side can’t be glazed because the adaptation to the dental abutment will be lost - it has a very reduced impact strength (because it is brittle) Mechanical properties can be influenced by the irregularities of the internal and pores surface or ceramic powders vacuum firing polishing the with different sizes reduces the pores substrate with of particles, but rounded angles reduced produce a without more compact irregularities material All these reduce the risk of appearance and propagation of micro-cracks Hardness -Classical ceramics (460-600VHN) have higher hardness than that of the enamel (343VHN) -hardness can affect the antagonist natural teeth - ceramics with low sintering T has a more reduced hardness (380VHN), similar to that of dental enamel Vickers’ hardness In-Ceram 1200 Aluminous ceramics 410 For veneering 380 Enamel 343 Dentine 70 Ni-Cr alloys 330 Fatigue occurs in time due to: -alkaline hydrolysis of the Si oxides - corrosion appeared at the interface with the metallic substrate - it is a limited phenomenon, but occurs especially in those zones without glaze layer: - interface with the metallic substrate - inner surface of the full ceramic crowns, due to marginal percolation - this phenomenon leads to micro-cracks