lecture 13

what are metal ceramics?

they consist of a metal framework on which several layers of veneering ceramics are applied

what are requirements of metal ceramics

1) alloy must have high mpt

2) metal interval should be 100 deg higher

3) sintering temp of porcelain show be low to prevent deformation of infrastructure

4) porcelain should wet metal alloy well

5) contact angle should be below 60

6) bond between metal and porcelain must be strong

7) is obtained by a chemical reaction between opaque porcelain and metal oxides of alloy and by mechanical interlocking of the porcelain and micrograins

8) metal ceramic systems are designed so that the CTE of alloy is slightly higher than porcelain

9) very high stiffness and strength of metal infrastructure

10) resistance to thermal deformation of metallic infrastructure at low volumes is required

opaque porcelain

layers of opaque porcelain screen out the underlying metal oxide surface colour.

opaque porcelain contains approximately 15% tin oxide, zirconium oxide or titanium oxide.

can be aesthetically favourable but relatively weak

lies under porcelain restorations and above alloy

basic optical phenomena - fluorescence

fluorescence is the emission of light by an object at wavelengths diff from those of incident light

emission ceases on removal of incident light

natural teeth fluoresce in blue region when illuminated by UV light

dental porcelains are also fluorescent under UV.

quality of fluorescence depends on brand of porcelain

is considered for shade matching

is influenced by illuminant, optical parameters and interpretation of the observer

double layer effect on aesthetics

outer translucent layer acts as a light scattering filter over inner layer.

as the thickness of the outer layer increases, effect of inner layer is diminished.

opalescence

Scattering of Light:

When light encounters tiny structures or particles within the substance, the wavelengths of light scatter in various directions

Interference and Diffraction:

caused by the interactions of light waves with small particles or irregularities within the material. lead to the separation and recombination of different wavelengths of light, resulting in the display of various colours

alloy selection for metal ceramics

properties:

coefficient of thermal expansion (CTE). select that similar to ceramic used in restoration

modulus of elasticity:

picking one with similar stiffness and rigidity to metal

yield strength

hardness

corrosion resistance

castability:

easy manipulation

biocompatability:

ensuring is biocompatible with oral tissues. high noble and metal noble like gold or palladium based are used

ease of finishing and polishing

cost and availability

chemical bond between metal and porcelain

achieved by mechanical interlocking/ micromechanical retention rather than direct chemical bonds like covalent or ionic

1. during fabrication, metal substructure is created from noble or base metal alloy. surface is roughened to create an irregular surface texture.

2. porcelain powder applied to rough surface in multiple layers.

3. porcelain is fired in furnace

4. porcelain melts slightly and flows into irregularities of metal surface --> creates mechanical interlock

5. oxide layer forms and chemical interactions occur at interface

mechanical bond between metal and porcelain

critical factor influencing the longevity and stability of the restoration.

1. micromechanical interlocking - surface is roughened to create irregular surface --> improves bond strength by providing greater SA for adhesion

2. oxide layer formation: oxides act as a bonding agent when porcelain is fired onto metal substructure --? enhances bond strength

3. firing/cohesion: involved heating to high temp to fuse onto metal substructure. undergoes vitrification

4. compressive stresses: cooling phase means thermal expansion coefficients create compressive stresses. enhances mechanical bond

5. surface treatment: sandblasting, etching are used to enhance bond strength

coefficient of thermal expansion in metal ceramics

it affects the compatibility and durability of the restoration.

measures the rate at which a material expands or contracts with changes in temperature.

crystals have strong bonds so low CTE, polymers have weaker bonds so high CTE

Waxes have the highest coefficients of thermal expansion of any dental material.

values:

Porcelain or dental ceramics used in restorations typically have a CTE range of around 9 to 14 x 10^-6 per degree Celsius (μm/m°C).

Metal Alloys:

High Noble (Gold-Based) Alloys: CTE range approximately 14 to 15 x 10^-6/°C.

Noble (Palladium-Based) Alloys: CTE range approximately 14 to 16 x 10^-6/°C.

Base Metal (Non-Precious) Alloys (e.g., Nickel-Chromium or Cobalt-Chromium): CTE range approximately 13 to 15 x 10^-6/°C

linear coefficient of thermal expansion

Change in length per unit of original length for a 1°C temperature change

relationship between metal and porcelain - fractures

can be caused by mismatch in thermal expansion between metal + porcelain

can occur as porcelains are very brittle

can be caused by insufficient bonding or weak adhesion

can be caused by difference in mechanical properties like modulus of elasticity and hardness

TO MINIMISE:

- ensure proper matching of CTE

- employ appropriate surface treatments

- accurately cast metal

- consider occlusal forces

how the design of a metal and ceramic restoration can cause fractures

- Inadequate Thickness of Ceramic Layer

- Inadequate Metal Support or

Framework

- Sharp Angles or Thin Margins

- Poor Occlusal or Functional Design

- Improper Contour and Anatomy

- Lack of Proper Support from the Preparation

- Incorrect Cementation or Bonding Techniques

how preparations of the metal framework can cause fractures

- Inadequate Thickness of Metal Framework

- Sharp Angles and Undercuts

- Thin or Weak Connectors

- Inadequate Margin Design

- Surface Irregularities or Roughness

- Overcontoured or Undercontoured Framework

- uneven oxide layers

- contaminations on the surface

metal ceramic process - first 4 steps

1. Tooth preparation: dentist preparing the tooth by removing any decayed or damaged structure. The tooth is shaped to accommodate the restoration while preserving its natural structure.

An impression of the prepared tooth and the surrounding teeth is taken using dental putty or other impression materials. This impression serves as a mould for creating the restoration.

2. Model Fabrication:

Dental technicians pour dental stone or plaster into the impression to create a precise replica of the prepared tooth and adjacent teeth. This model accurately represents the patient's oral anatomy.

3. Wax Pattern Creation:

A wax pattern or a wax replica of the desired restoration is sculpted on the dental stone model. This wax pattern outlines the shape and contours of the final restoration.

4. Investing and Casting:

The wax pattern is encased in a material called investment, which forms a mould around the wax. Once the investment sets, the wax is melted or burned out (lost-wax technique) to leave a cavity in the investment mould.

The metal alloy (such as cobalt-chromium or nickel-chromium) is melted and cast into the cavity of the investment mould. This process creates the metal substructure or framework of the restoration.

metal ceramic process - last 4 steps

5. Metal Framework Finishing:

After casting, the metal framework undergoes various finishing processes. This includes removal of excess metal, polishing, and adjustment to ensure proper fit and contour.

6. Application of Ceramic Layers:

The metal substructure is returned to the dental technician for the application of ceramic layers. The technician applies specialized dental porcelain in multiple layers over the metal framework.

Each layer of porcelain is carefully applied, shaped, and fired in a high-temperature furnace (ceramic oven) to achieve the desired colour, translucency, and shape of the tooth.

7. Staining and Glazing:

Dental technicians may apply additional artistic techniques such as staining and glazing to further enhance the appearance of the restoration. Staining involves the application of pigments to mimic natural tooth characteristics, while glazing adds a protective and glossy finish.

8. Final Adjustment and Polishing:

Once the ceramic layering is complete, the restoration is returned to the dentist. The dentist checks the fit and occlusion (bite) of the restoration, making any necessary adjustments.

Excess material is trimmed, and the restoration is finely polished to achieve a smooth surface and a natural appearance.