Final Exam - Ceramics II

What are *glass ceramics*?

"Ceramic that is formed to shape in the glassy state and subsequently heat treated to partially or completely crystallize the object" - Partially crystallized glasses produced by nucleation and growth of crystals in the glass matrix phase - Available in CAD-CAM

Review of ceramics:

"ceramics are strong, temperature-resistant, and resilient, these materials are brittle and may fracture without warning."

How many crystal structures does Zirconia have?

*3* different crystal structures: 1. Monoclinic 2. Tetragonal 3. Cubic

What is the *flexural strenght* of acid-resistant ceramics?

*450-1000 MPa*

What is the *flexural strength* of acid-sensitive ceramics?

*65-450 MPa*

Zirconia

- *ACID-RESISTANT CERAMIC* - biocompatible - Advances in the CAD/CAM technology - Framework for bridges or single units - Infrastructure for partial fixed prosthesis and dental implant abutments - Zirconia IF is generally veneered with an aesthetic ceramic such as feldspathic or lithium disilicate ceramic

Acid-etching

- *HF 5-9.5%* - The amorphous glass compounds are selectively removed - Creates a micro-retentive highly reactive surface for bonding - Increases the surface area - Clean, microretentive surface

Glass ceramics

- *Leucite, lithium disilicate, and hydroxyapatite* - can be mixed by *powder, hot pressed or solid blocks (CAD-CAM)*

Cast metal for metal-ceramic prothesis

- *To bond to alloys* porcelain *must have CTE and CT Contraction* that are closely matched to those of the alloys - *Metal oxide* = necessary to promote the chemical bonding (metal/ceramic) - *Alloy*s - HIGH MELTING RANGES prevent sag, creep or melting of the coping or framework - The *base metals form a surface oxide layers* during the oxidation treatment --> SURFACE OXIDE IS RESPONSIBLE FOR DEVELOPMENT OF A BOND WITH PORCELAIN

If zirconia has this weakening from the phase transformation of its crystals, then how can it be used in dentistry?

- *if an oxide is used it can stabilize zirconia in the TETRAGONAL phase at room temperature* this is what you load it with --> *3Y-TZP* (*this is the type of zirconia that is used MOST in dentistry*)

*3Y-TZP (3% Yttrium + Zirconia)*

- *the type of zirconia that is used MOST in dentistry* - *Regeneration heat treatment* - 900°C for 1 h - *Converts the monoclinic phase* within the surface *back to tetragonal phase*

Leucite-reinforced glass ceramics

- IPS Empress, Ivoclar Vivadent - Optec OPC 3G (Pentron Laboratory Technologies) - Approx. *65% to 70% by volume* of lithia disilicate as the principal crystal phase - Flexural strength = *350 Mpa* - Fracture toughness = *3.3 Mpa.m1/2*

*Disadvantages* of metal-ceramic systems:

- Metal allergy - Not best esthetic choice (dark line at the facial margin)** - Alloys: Au-based (contains elements In and Sn that can oxidize)

Silane coupling agent

- Silane is applied to the surface of the ceramic -- stronger chemical bond between the *glass ceramic* and the silane methacrylate - Silanization process generates a thin layer of covalently bonded methacrylate groups that co-polymerize with the luting composite when the restoration is seated

What is known as "*transformation toughening*" for zirconia?

- add Y2O3 - "Metastable tetragonal phase" - Tensile stress develop at crack tip - Volume expansion adjacent to the crack tip (around 3%) - *INCREASE in the localized FRACTURE TOUGHNESS* - Inhibition of the potential crack propagation (crack progression is arrested)

What are the three states that zirconia can be bought in?

1. products that are milled in the green 2. products that are milled in a partially sintered state 3. products that are milled in the fully sintered state

Preventing tensile stress from occurring:

Ideally - ceramic veneer should sustain slight compression in the final restoration (glaze/compressive stress on cooling) *PROTECTION OF COMPRESSIVE STRESS --> ↑ fracture resistance and survival probability*

Do we want zirconia to be monoclinic?

No, but we do need the system to stop the crack

Would you make a feldspathic porcelain crown for a posterior tooth?

No, it is not going to work well due to the loads and mechanical properties

Which ceramics CAN be etched?

lithium dislicate, leucite and feldsphatic porcelain why? because they have glass

What *phase transformation* can occur between the crystal structures of Zirconia?

phase transformation can occur *between monoclinic and tetragonal* - results in a *3 to 5% volume increase* - *crack* in bulk zirconia - reduction in strength and toughness --> *useless for dental application*

How does the *fracture toughness* of leucite-reinforced glass ceramics compare to that of feldspathic porcelain and zirconia?

they have a fracture toughness of 3.3 which is HIGHER than feldspathic porcelain and LOWER than zirconia

Airborne particle abrasion

- attempts to create a rough surface on acid-resistant ceramics for prepping (because they cannot be acid etched) - Alumina particles or alumina particles *coated with silica* - Increases the surface roughness - Modifies the surface energy and wettability of the ceramic - Improves the micromechanical retention

What are *glass infiltrated ceramics*?

"A crystalline core ceramic whose interconnected pore network is infiltrated during heating by the capillary inflow of a low-viscosity highly wetting glass." - Veneered with porcelain - Alumina, magnesia-alumina spinel

What is *porcelain*?

"Ceramic produced by sintering a mixture of feldspar, silica, alumina, other metal oxides, pigments and opacifying agents."

Dental cements

- attempts to create a rough surface on acid-sensitive ceramics for prepping (because they cannot be acid etched) - Have adequate resistance to dissolution in the oral environment - Develop an adequately strong bond through *mechanical interlocking and adhesion* - High strength in tension, shear, and compression - High fracture toughness to resist stresses at the restoration-tooth interface

Bonding promotors

- attempts to create a rough surface on acid-sensitive ceramics for prepping (because they cannot be acid etched) - The primers used for zirconia are typically either *based on methacryloxypropyltrimethoxysilane (MPS), MDP, or a combination of the two* - *Covalent bonds* between the crystalline ceramics and the resin cement

What are the *disadvantages* of an *all-zirconia crown (monolithic)*?

- difficult in adjusting occlusion - cutting difficult - heat generated in removing defective crowns/endodontic access

*Advantages* of metal-ceramic systems:

- high overall survival percentages - excellent esthetics - adequate strength (metal framework/core)

What have been some improvements to ceramic systems that have occurred throughout the years?

- improved fracture resistance - CAD/CAM technology - excellent esthetic capability

What results from zirconia being loaded with *3Y-TZP*?

- phase transformation is avoided - microcracks are prevented REMEMBER: *this is the type of zirconia that is used MOST in dentistry*

What are some methods for strengthening ceramics?

- prevent tensile stress from occurring - development of residual compressive stresses (veneering of all-ceramic crowns)

What is the *green-state* of zirconia?

- the semi-hard, prefired condition of a ceramic object - green ceramics are *always porous* - isostatic pressed to shape prior to firing

Fabrication of metal-ceramic prosthesis: *Porcelain condensation*

1. Fine powder + Water (liquid) 2. Slurry is condensed into the desired form 3. Packing or condensation - obtained through vibration, spatulation, and brush technique ***Properly condensed - densely packed porcelain = LOWER firing shrinkage and less porosity in the fired porcelain*** 4. Excess of water is blotted away with a clean tissue 5. *Glaze* - seal surface flaws and reduce stress concentration (helps with final appearance) 6. Provide a more lifelike appearance - *color match* to adjacent teeth or restorations 7. Adjusting the occlusion - diamond bur/gridding the porcelain surface 8. Internal staining

Steps for acid treatment of acid-sensitive ceramics:

1. Hydrofluoric acid → Micromechanical bonding 2. Chemical bond → *Silane* coupling agent 3. Resin cement

Development of residual compressive stresses (Veneering of all ceramic- crowns):

Veneering ceramics --> thermal expansion or contraction coefficient is slightly LESS than that of core ceramic

Dental porcelain: composition and manufacture

What is the *final product*? A: *feldspathic porcelain* Is there a *glassy phase*? A: Yes Is *leucite* added as a part of the *crytalline phase*? A: Yes What *controls coefficient of thermal expansion (CTE) in feldspathic porcelain*? A: *LEUCITE*

Are there different types of ceramics called *opaque*, *enamel* and *dentin*?

Yes, the translucences are different to make it closer to patient tooth

Do zirconia and feldspathic porcelain have different sintering temperatures?

Yes, zirconias is very high at 1500 degrees C

What is IPS e.max® cream?

a highly esthetic fluorapatite layering ceramic

What are the *concerns* with the use of airborne particle abrasion?

air-abrasion procedures *may cause zirconia to phase transform from tetragonal to monoclinic* --> which may be detrimental for the durability of this ceramic

Ceramics and adhesive bonding:

ceramics resistant to acid etching vs. etchable ceramics "The professional needs to know what type of ceramic will be used in order to define the treatment that will be applied on the ceramic surface."

Which ceramics CANNOT be etched?

zirconia and alumina why? because they do NOT have glass