Metals and Ceramics in Prosthodontics
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Table of Contents
MATERIAL SCIENCE REVIEW - METALS
It is important to understand the fundamental properties of metals and how it affects clinical decision making. Strength is measured in megapascals (MPa).
Compressive strength is the capacity of a material to withstand loads tending to reduce size.
Tensile strength is the maximum amount of tensile stress that a material can withstand before failure (breaking). It can also be referred to as ultimate tensile strength.
Shear strength is a measure of a material’s resistance against a shear load before the component fails.
Yield strength (identified on a stress-strain curve as the yield point) indicates the limit of elastic behavior and the beginning of plastic behavior. A material stressed beyond its yield point will undergo permanent deformation. Optimally a metal-ceramic restoration or framework will have a high yield strength to minimize permanent deformation under occlusal forces.
The elastic modulus is a measure of a material’s resistance to elastic deformation when a stress is applied. It can be visualized as the slope of its stress–strain curve (prior to the yield point). It is measured in gigapascals (GPa). A stiffer/more rigid material will have a higher elastic modulus.
Hardness is the property of the material which enables it to resist plastic deformation by penetration/indentation. Hardness is measured in kg/mm2. A hard material will be more difficult to polish, finish and cut (e.g. zirconia). It will also cause more wear on an opposing softer material, for example the wear seen on teeth opposing a porcelain crown.
The Coefficient of thermal expansion (CTE) describes how the size of an object changes with a change in temperature. A material with a higher coefficient of thermal expansion will expand more when heated. In a perfect world the CTE of different restorative materials will match each other, and would match the CTE of dentine, enamel and cementum. The CTE of a material is important because a CTE mismatch between restorative materials and tooth structure can introduce stresses at the tooth-restoration interface when the surrounding temperature changes. And the CTE of porcelain must be compatible with the CTE of the underlying metal in a metal-ceramic restoration because the two bonded materials will experience stress at the bonded interface in response to temperature changes.
When porcelain is bonded to metal, the bond is micromechanical and chemical, created by adhering porcelain to the oxide layer formed on the outside of the metal alloy. A weak bond or defects will cause delamination (adhesive failure), more likely if the oxide layer did not form properly or was contaminated. Voids or inclusions can cause internal failure (cohesive failure). When metal-ceramic restorations are fabricated the CTE of the metal is intentionally left a little higher than that of the porcelain. This keeps the porcelain in compression which adds strength to the metal ceramic restoration.
Specific Gravity is a dimensionless unit defined as the ratio of density of the material to the density of water at a specified temperature. Gold alloys have a higher specific gravity compared to base metal alloys.
HEAT TREATMENTS AND SURFACE MODIFICATION
Work hardening (strain hardening or cold working) is the increase in hardness of a metal induced by plastic deformation (hammering, rolling, drawing etc.) at room temperature. This is different to exerting the same forces on a heated alloy, termed forging. Work hardening can be used intentionally to alter the hardness, but is often an unintended consequence of cyclic loading. Strain hardening increases surface hardness, strength and proportional limit, but decreases ductility and the resistance to corrosion. Work hardening does not tend to change the elastic modulus significantly. Constant loading and unloading of a clasp arm may cause an increase in hardness, making the material more brittle, and can ultimately lead to failure. If you were to see elongated grains in the microstructure of a wrought wire it would indicate strain hardening has occurred.
Heat treatment is used to alter the microstructure of metals/alloys by controlling the heating and cooling. Examples include quenching, annealing, and tempering.
Quenching is when metal is brought to an elevated temperature and is cooled rapidly. This is done to preserve at room temperature a phase and it’s associated mechanical properties ordinarily only stable at higher temperatures. Quenching can be used to stop a process triggered by elevated temperatures. Quenching type III gold alloys or noble metal alloys will leave them more malleable and ductile, able to be manipulated (burnished, polished etc.).
Annealing is the process of slow controlled cooling of a heated metal or glass. Annealing increases ductility and strength. Annealing relieves residual internal stresses introduced during the manufacturing process.
Burnishing occurs when the surface of the metal is drawn or moved. Burnishing is easier when using metals that are thin or soft (like Type I gold). A round steel point may be rubbed over the margins of a restoration to close any small discrepancies.
Soldering is the procedure of heating a piece of metal (solder) to join metal components. A solder joint should be circular in form and occupy the region of the contact area. The recommended distance between the parts to be joined is 0.25 mm. The solder has a lower melting temperature than the metals to be joined together. Gold solders are generally used for fixed bridgework, and silver solders for orthodontic appliances. Soldering flux dissolves surface oxides and allows the melted solder to wet and flow onto the alloy surfaces. Flux is composed of borax, potassium fluoride, silica, and sodium pyroborate. Fluoride functions to dissolve the passivating chromium oxide film, and is used when soldering stainless steel or cobalt-chromium alloys. Antiflux restricts the flow of solder, applied on areas such as occlusal grooves and margins. Graphite (soft graphite pencil) and iron oxide are antifluxes. A clean surface is very important for surface wetting and the success of a soldered joint.
Pickling is a process that involves carefully heating the casting in an acidic solution, used to remove impurities such as stains, rust, contaminants or scale. 50% hydrochloric acid is commonly used, but the fumes are very corrosive and hazardous to health. Pickling removes the dark surface oxide film that forms on a gold casting. The metal can either be heated first then dropped into an acidic solution, or placed in the solution then heated. The former risks distorting the alloy.
BASE AND NOBLE METALS
Noble metals consist of gold (Au), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os). They are naturally corrosion/oxidation resistant but are more expensive than base metals. Silver (Ag) is used to improve castability, but is not considered noble and can cause porcelain “greening”. Noble metals are classified as “precious”, but not all previous metals are noble (silver).
Base metals (non-precious) include copper (Cu), lead (Pb), nickel (Ni), tin (Sn), aluminum (Al), and zinc (Zn). Base metals are more common, more readily extracted, and less expensive than noble metals. Compared to gold alloys, base metal alloys have:
- Low material cost (cheaper for patients).
- Higher resistance to deflection in thin sections.
- Higher modulus of elasticity (stiff) and high yield strength. About half as flexible as noble alloys.
- Much higher melting point (2300-2600°F or 1260-1426°C)
- Lower specific gravity and density. A lower density makes them lighter for patients, but also makes them more difficult to cast.
- Lower casting accuracy.
Chromium is responsible for a complex surface oxide layer forming when firing a cobalt-chromium alloy. This may be beneficial when used in a bonding process (bonding porcelain to metal copings) and makes the alloy naturally corrosion and tarnish resistant. Cobalt increases the alloy’s rigidity and contributes to the framework’s strength and hardness. Nickel increases ductility, but is the most likely metal to induce an allergic response, seen in about 17% of females and 3% of males. Metals are blended to form alloys, which can be classified according to their hardness.
There are four types of high-gold alloys:
- ADA Type I – soft – highest percentage gold content (~83% noble metals) mostly used for small inlays (intracoronal). Easily burnished.
- ADA Type II – medium – greater than 78% noble metals, used for larger inlays and some onlays (mostly intra-coronal). Can also be burnished.
- ADA Type III – hard – greater than 75% noble metals, used for onlays and crowns. Capable of being heat treated.
- ADA Type IV – extra hard – greater than 75% noble metals, used for RPDs (frame) or bridges. Also capable of being heat treated.
Any metal melted and cast into a mold is termed “cast metal”. When the casting is cold worked into a desired shape or property it’s called “wrought metal”. Generally any physical property of wrought metal is superior to cast metal (i.e tensile strength, hardness etc.). Compared to cast clasp arms, wrought-wire clasps exhibit greater flexibility, adjustability, ductility, toughness, and tensile strength. But the yield strength can be negatively impacted by excessive heat (by recrystallization, aka grain growth).
Flexibility of a retentive clasp arm depends on:
- Clasp length – flexibility increases with increased length (cube ratio).
- Clasp thickness – flexibility decreases with increased thickness (cube ratio).
- Clasp width – flexibility decreased with increased width (1:1 ratio).
- Clasp cross-section – a round form will be equally flexible in all directions. A half round shape will be more flexible if bent perpendicular to the flat surface.
MATERIAL SCIENCE REVIEW - CERAMICS
There are many different dental ceramics, but they usually contain the following:
- Feldspar – undergoes fusion to form a glassy material which gives porcelain its translucency. It is used as a fluxing agent to form a glassy phase at low temperatures and as a source of additional alumina (Al2O3) and silica (SiO2). Feldspar improves the strength, toughness, and durability of the ceramic, and cements the crystalline phase of other ingredients (acts like a matrix).
- Quartz/silica – composed generally of silicon and oxygen (SiO). It helps to modify thermal expansion, regulates drying and shrinkage, and improves the structural integrity (strengthener) and appearance.
- Kaolin – a mixture consisting principally of the mineral kaolinite, but contains varying amounts of other minerals such as muscovite, quartz, feldspar, and anatase. This clay acts as a binder.
- Fluxes or glass modifiers, and metallic colorants.
Dental porcelain usually contains silicone dioxide (65-69%), aluminum oxide (8-19%), potassium oxide (8%), and sodium oxide (2-5%). Ceramics are hard, brittle materials that exhibit very high compressive strength (~300-500MPa) and relatively weak tensile (20-60MPa) or shear strength. Generally ceramics are known for their low flexural strength and tendency to fracture under tension due to crack propagation (brittle, low fracture toughness). Preparations for all ceramic restorations must be well rounded, with no sharp angles, to avoid stress concentration. They are not able to undergo much plastic deformation and have a low coefficient of thermal expansion. Ceramics exhibit low chemical reactivity, low absorption and solubility, and are relatively inert, but can be severely damaged by acidulated fluoride.
Ceramics are classified into different families according to their contents, microstructure, manufacturing process and resultant properties. Though confusing, there are some key differences you need to be able to identify. If ceramics are organized according to the composition at the microstructural level:
- Category 1 – glass-based systems (mainly silica).
- Category 2 – glass-based systems (mainly silica) with fillers, usually crystalline (typically leucite or, more recently, lithium disilicate).
- Category 3 – crystalline-based systems with glass fillers (mainly alumina).
- Category 4 – polycrystalline solids (alumina and zirconia).
CLASSIFICATIONS OF CERAMICS
Feldspathic porcelain is a low-to-moderate leucite-containing feldspathic glass (a subcategory of category 2). Even though other categories have a feldspathic-like glass, this category is what most people mean when they say “feldspathic porcelain.” They are very aesthetic but are prone to mechanical wear and damage. Feldspathic porcelain is reserved for esthetically demanding, non-load bearing areas (veneers).
Aluminous porcelains contain alumina instead of quartz. When alumina is used as a strengthener instead of quartz the porcelain is considerably stronger. Strength is determined by the amount of alumina reinforcement.
All ceramic restorations do not contain an underlying metal layer. The ceramic can be formed in many ways:
- Pressed ceramic restorations are fabricated by a lost wax technique. A pattern is waxed onto a die, which is then invested and burned out. Small ceramic discs (ingots) are melted and pressed into the pattern. Alternatively, a block can be generated and milled into the final restoration. Restorations made from pressed ceramics have better margins and are much stronger than feldspathic porcelain restorations. They are also more aesthetic than metal-ceramic restorations. Examples of pressed ceramic restorations include IPS Empress (Ivoclar Vivadent).
- Cast glass ceramics are made by investing a wax pattern and casting. Heating the reinvested crown for six hours at 1070°C carries out controlled crystallization, termed ‘ceraming’. Examples include Dicor systems.
- Sintered porcelains are built up from a slurry of porcelain particles condensed onto a refractory die. Sintering changes the porcelain from a powder to a solid and occurs at a temperature above the softening point of porcelain, causing the glassy matrix to melt and the powder particles to coalesce. There is volume shrinkage of 30–40%. Porosity can be reduced from 5.6 to 0.56% by vacuum firing. Sintering is the most commonly used technique for making metal-ceramic and veneering restorations. There are a number of different materials available, including Aluminous porcelain (Vitadur-N, Hi-Ceram), feldspathic porcelain reinforced with Zirconia (Mirage II), and feldspathic porcelain reinforced with leucite (Optec HSP).
- Machined glass ceramics are milled from blanks. Leucite-type, lithium disilicate-type, and zirconia (ZrO2) ceramics can be used, utilizing CAD/CAM technology. Leucite-type glass-ceramics exhibit high translucency and preferable optical properties. Lithium disilicate glass improves on mechanical parameters, especially strength and toughness. Zirconia shows the highest toughness and strength, roughly 10 times stronger than human enamel. There are many examples of machined glass ceramics including IPS Empress CAD, IPS e.max CAD, and IPS e.max ZirCAD (Ivoclar Vivadent).
Ceramics can be classified according to the fusion (vitrification) temperature:
- High-fusing porcelains – 1280-1370°C – used to fabricate denture teeth.
- Medium-fusing porcelains – 1090-1280°C – used to fabricate all-ceramic and porcelain jacket crowns. In addition to silicon dioxide, aluminum oxide, potassium oxide, and sodium oxide, medium fusing porcelains also contain lithium, magnesium, and phosphate.
- Low-fusing porcelains – 870-1090°C – used to fabricate metal-ceramic (PFM) crowns. Calcium, potassium, sodium, and chromium oxides are added as modifiers to lower the fusing temperature. However, this also decreases the viscosity and may make the porcelain “slump down” during firing. Aluminum oxide is added to minimize slumping.
PORCELAIN FIRING
If porcelain is fired too many times it may devitrify. The ceramic appears “milky” and becomes very hard to glaze. Metal-ceramic restorations are fabricated in two separate phases. First, the production of a metal coping/casting, and second, the layering and firing of porcelain. Porcelain adheres to the metal via a strong covalent chemical bond, formed by sharing oxygen elements found in the porcelain (silicon dioxide) and the metal alloy (oxidizing elements of silicon, indium and iridium). Porcelain attaches to metal via mechanical (roughness from sandblasting) and chemical bonding. The elements tin (Sn), Indium (In), Iron (Fe) and Chromium (Cr) all contribute to metal oxidation for chemical bonding. In gold-based alloys, iron (Fe) is the most important for bonding. In base-metal alloys, chromium (Cr) is the most important.
Degassing (degasification) is the removal of dissolved gases from liquids. While the alloy is molten it must be degassed in order to:
- reduce/eliminate dissolved gases (especially hydrogen and nitrogen).
- reduce dissolved carbon which improves ductility.
- form an oxide layer for an adequate metal-ceramic bonding.
Porcelain is heated to 980°C to burn off any impurities. As time and temperature increases, the number of bubbles formed at the interface decreases. After degassing the casting is ready for porcelain addition. Common causes of porcelain fracture (delamination) in a metal-ceramic restoration include:
- Poor metal framework design.
- Inadequate oxide layer formation due to degassing temperature being too low.
- Contamination of metal prior to porcelain application.
- Using too low temperature or too short time to fuse the opaquing porcelain to the metal.
After degassing, porcelain can be added to the metal metal casting. An opaquer (very opaque porcelain) is added first to mask the metal color. Subsequent layers of dentine/body and enamel shades are layered and fired to attain the proper form and function. The melting temperature of the metal should be at least 300-500 degrees (F) higher than the fusing temperature of the porcelain, otherwise the metal coping may distort.
The final step is glaze firing. Glazing removes the porosity and makes the porcelain more abrasion resistant, as well as improving the optical properties and improving soft tissue tolerance. The following is a list of dental materials, ordered according to how they impact soft tissues, from least irritating to most irritating:
- Glazed porcelain.
- Polished gold.
- Unglazed porcelain.
- Polished acrylic resin.
Compared to any other polished restoration, glazed porcelain is the least irritating dental material to gingival tissues. A “natural” glaze is formed by a separate glaze firing, the rapid heating and maintenance of the porcelain at its fusion temperature causing the glass grains to flow over the surface to form a vitreous layer. This glaze is much more permanent than overglazes.
Overglazes are ceramic powders that are added to porcelain after it has been fired. Overglazes are cured by heating the restoration to a maturing temperature, which is lower than the body porcelain’s fusion temperature, resulting in a glossy/semi-glossy non-porous surface. Overglazing is more susceptible to erosion of the glaze, leaving a rough and sometimes porous surface.
Fritting is the process of producing frit, a ceramic material that has been fused, quenched, and granulated (ground into very fine powder). Frits are important constituents of most ceramic glazes which mature at temperatures below 1150 °C (low of medium fusing).
