Flashcards in Biomaterials 1 Deck (65):
There is a challenge for end users to critically evaluate the claims made on the reported _________ and to relate these claims carefully to well- established physical properties for the specific classes of materials.
performance of dental materials
The dentist needs to understand the fundamental principles of dental materials to be able to properly and critically evaluate ________.
Advances in dental materials
Dentists need to be able to critically evaluate ________, and a large body of dental literature is related to dental materials.
Materials used in the construction of a dental prosthesis but that do not become a part of the structure. These include acid-etching solutions, impression materials, casting investments, gypsum cast and model materials, dental waxes, acrylic resins for impression and bleaching trays, acrylic resins for mouth guards and occlusion aids, and finishing and polishing abrasives.
Auxiliary Dental Materials
Metallic, ceramic, metal-ceramic, or resin-based materials used to replace, repair, or rebuild teeth and/or to enhance esthetics. The ideal restorative material would be biocompatible, bond permanently to tooth structure or bone, match the natural appearance of tooth structure and other visible tissues, exhibit properties similar to those of tooth enamel, dentin, and other tissues, and/or be capable of initiating tissue repair or the generation of missing or damaged tissues.
Cements, metals or resin-based composites that are placed and formed intraorally to restore teeth and/or to enhance esthetics. A composite is when a monomer resin contains inorganic or polymeric filler particles that are bonded to the matrix resin by means of an organosilane coupling agent.
Direct Restorative Materials
Ceramics, metals, metal-ceramics, or resin-based composites used extraorally to produce prostheses that replace missing teeth, restore damaged teeth, and/or enhance esthetics.
Indirect Restorative Materials
Cement- or resin-based composites used for a period of a few days to several months to restore or replace missing teeth or tooth structure until a more long-lasting prosthesis or restoration can be placed.
Temporary Restorative Materials
Cements, coatings, or restorative materials that either seal pits and fissures or release therapeutic agents such as fluoride and/or other mineralizing ions to prevent or arrest the demineralization of tooth structure.
Preventive Dental Materials
Name the property: Esthetics is critical in dental restorative materials, especially with the increased use of bleaching and whitening technologies. Thus, the interaction of light with the restorative materials must mimic the interaction of light with the natural tooth, and the knowledge of the underlying scientific principles of color and other optical effects is essential.
Name the property: Materials differ markedly in their ability to conduct electric current. The current may be carried either by free electrons (in metals) or by ions in solution. This effect is pronounced in metals that are widely used in dentistry. Thus, electrical properties, especially the electrical conductivity or resistivity, are important to understand because the pulp is sensitive to microcurrents. Electrical properties are also important because the electrochemical properties of a material govern how corrosive a material may be in the oral environment.
Electrical and Electrochemical
Name which property includes
Reflection Refraction Absorbance Transmittance Translucence Opacity Color Fluorescence
Name which property includes Electrical Resistivity Electrode Potential Electromotive Series Corrosion
Electrical and Electrochemical
______ plays a role in economic considerations. E.g., gold alloys are twice as dense as non-noble (base metal) alloys used for the same restorative purposes. Since the same volume of alloy is used to cast a restoration, the actual cost difference is not just the difference of the cost of alloys per unit weight. _____ also may play a role in function, e.g., the retention of upper dentures.
Name the property: If a restorative placed in a deep cavity transmits too much heat to the dental pulp, it can cause thermal shock and trauma. If it thermally expands and contracts much more than the tooth it is placed in, marginal failure and leakage can result. Yet denture bases need to be able to transfer some heat to convey the sensations of heat and cold from food and beverages
This is the ability of a material to conduct heat. Another way of saying it is how MUCH heat will flow through the material, and is defined as the rate of heat (cal/sec) passing through a material 1 cm thick with a cross section of 1 cm2 having a temperature difference of 1 K (or ̊C). In general, _______ increase in the following order: polymers < ceramics < metals.
With restorative materials, we are most concerned with responses to transient temperature changes (e.g., when a hot or cold food or beverage is ingested), and the material experiences a change in the environmental temperature. In these instances, the temperature is not constant but changes, depending on how well the heat is conducted and how much heat is absorbed by the material in raising the temperature of the material. This is where _______ plays a part. ________ is the measure of how quickly the material or tooth reaches equilibrium temperature. It takes into account how much energy the material itself needs to heat up. Here, Cp is the specific heat (the number of calories required to raise the temperature one degree) and ρ is the density in grams per cm3. Thus, a material with low specific heat and high thermal conductivity has high diffusivity (Δ) and temperature easily passes through the material. Gold has about one tenth the specific heat of dentin and about 500 times the thermal conductivity. A pure gold filling would thus protect the tooth pulp very poorly from a thermal transient compared to natural dentin.
When materials undergo a temperature increase, the vibrational motion of atoms and the mean interatomic (bond) distances increase resulting in an increase in volume. The increase is described by the coefficient of ________. Here L is the original length, ΔL is the change in length and ΔT is the temperature change. This parameter is extremely important in applications as broad-ranging as producing cast restorations that fit, and maintaining the seal at a restoration margin.
Thermal energy required to covert a liquid to a vapor
Heat of Vaporization
Thermal energy required to convert a solid to a liquid
Heat of Fusion
Name the property which includes Melting Point
Softening Point Eutectic Temperature Glass Transition Temperature Liquidus Temperature Specific Heat
Unlike atoms in the interior of a material that are surrounded by other atoms that are equally attracted to each other, atoms on the surface are not surrounded by other atoms that are equally attracted to each other, leading to an increase in energy that is needed to form a surface. This energy is called the _______. These _______ per unit area are known as surface tension for liquids or surface free energy for solids. All systems attempt to minimize this energy. Liquids do so by assuming spherical shapes in the absence of other forces and minimizing the exposed surface area. Among solids, metals have high ________ while hydrocarbon polymers tend to have low ________.
One way for a system to decrease surface energy is for a low surface tension liquid to wet and spread on a high surface energy solid. The net energy of the system is reduced by the elimination of the solid-air high-energy interface. ______ occurs most readily when the solid has a high surface free energy or when the liquid has a low surface tension.
A third free energy, the interfacial energy between the solid and liquid, is also important in quantifying the degree of wetting, which is measured by the angle formed between a drop of liquid and a solid surface (Figure 1). This _______ decreases with improved wetting.
Much of the _______ in dentistry depends directly on wetting. In fact, a general definition of an adhesive is a liquid that wets the substrate (adherend) of a different material well and then transforms to a solid. ______ is when this happens in two materials of the same species.
That action that produces or tends to produce motion in a body, or which changes motion in a moving body. Force is a vector quantity having both magnitude and direction.
The response of the body to the force depends not only on the magnitude of the force, but also on the magnitude of the area upon which the force is acting. For the measurement of mechanical properties, this dependence is accounted for by normalizing the force to the cross-sectional area over which the force is applied. This normalized force is called _____. The units are, e.g., pounds per square inch (lb/in2 or psi), meganewtons per square meter (MN/m2 or MPa). All ____ fields, regardless of their complexity, may be resolved into three basic _____ components: tensile, compressive or shear.
results when a body is subjected to two forces acting along the same line but directed away from each other. In reality, most mechanical failure happens through tensile stress, especially in brittle materials.
results when a body is subjected to two forces acting along the same line but directed toward one another.
results when a body is acted upon by two forces not acting along the same line.
The application of a stress to a body tends to produce a deformation (e.g., when a tensile force is applied, the body tends to stretch or elongate along the line of the force application). Again, the deformation is normalized to eliminate the dependence on the size of the body. This normalized deformation is called the ____ and is found by dividing the change in length by the original length. The units are thus, e.g., mm/mm. Note that while the dimensions cancel out and make strain a dimensionless quantity, it is customary to retain the units in expressions of _____.
If, upon removal of the force, the body returns to its original dimensions, the deformation is said to be elastic and no permanent deformation has occurred.
the amount of strain when a material is elastically deformed.
If, upon removal of the force, the body does not return to its original dimensions,
permanent deformation has occurred and the body has undergone plastic deformation.
the irreversible deformation that remains when the external force or stress has been re-
Stress-Strain Diagram: the slope of the linear portion of the curve. It is a measure of the stiffness of a material.
Modulus of Elasticity (Young’s Modulus)
Stress-Strain Diagram: Within the linear region, the stress is directly proportional to strain. That stress beyond which devia- tion from linearity (and hence proportionately) occurs is called the ________.
Proportional Limit (P)
Stress-Strain Diagram: Within the linear region, the deformation is elastic and recoverable. That stress beyond which permanent (plastic) deformation is induced is called the _______. (Note that proportional limit and ______ are two defini- tions of the same point on the curve.)
Elastic Limit (E)
Stress-Strain Diagram: To make measurement easier, it is frequently convenient to define a point where the material exhibits a
specified deviation from proportionality. The specified deviation is defined in terms of a fixed permanent deformation (e.g.,
0.1 or 0.2% offset). The ______ is defined by the intersection of the stress-strain curve and a line drawn parallel to the linear portion and passing through the offset.
Yield Strength (Y)
Stress-Strain Diagram: the relative inability of a material to plastically deform before fracture. As such, the yield strength is not measurable in a perfectly brittle material since there can be no permanent deformation.
Stress-Strain Diagram: the greatest stress reached anywhere on the stress-strain diagram
Ultimate Strength (U)
Stress-Strain Diagram: the stress at the point of fracture
Breaking Strength (B)
Stress-Strain Diagram: the deformation resulting from the application of a tensile stress. The permanent ______ (plastic strain) may be determined by subtracting the strain at the elastic limit from the strain at the stress in question.
Stress-Strain Diagram: the relative ability of a material to plastically deform under a tensile stress. The higher the strain between the elastic limit to the breaking strength (ultimate plastic strain), the more _____ the material.
Stress-Strain Diagram: the ability to be hammered or compressed into thin sheets without fracture. It is the ability to sustain considerable permanent deformation without rupture under compression. Gold is the most ductile and ______ pure metal and silver is second. Of the metals of interest to the dentist, platinum ranks third in ductility and copper ranks third in _____.
Stress-Strain Diagram: the energy per unit volume absorbed by a material undergoing elastic deformation. The modulus of ______ is the area under the stress-strain curve up to the elastic limit.
Stress-Strain Diagram: the ability of a material to absorb elastic energy and to deform plastically before fracturing. Thus, it is the area under the entire stress-strain curve including both elastic and plastic regions.
Toughness (Fracture Toughness)
Name the test: Because of the difficulty of preparing samples for conventional tensile testing of brittle materials, tensile strengths are frequently calculated from data generated by testing a cylindrical sample in compression along a diameter. While the load is compressive, the internal stresses are tensile and the tensile stress may be calculated from
s= 2P/(pi)DT where P is load, D is sample diameter and T is thickness.
Diametral Tensile Test
Name the test: To more closely approximate the complex state of flexural stress that leads to failure in many practical materials applications, samples are frequently fabricated as rectangular beams and tested by applying the load to the center while supporting the beam at both ends (three point or four point bending tests, Figure 5). The resulting breaking strength (the transverse strength or flexure strength or modulus of rupture) is calculated from
s = 3Pl/2bd where P is load, l is length between supports, b is width and d is thickness.
Transverse (Flexural) Strength
Name the test: Frequently, materials that exhibit significant plastic deformation at low rates of load application behave increasingly more brittle as the rate is increased. Because impact loads are frequently encountered in practice (e.g., when a denture is dropped), specialized tests were developed to determine impact resistance. A weighted pendulum is allowed to swing through a specimen of defined geometry, and the difference be- tween the initial pendulum height and that reached after fracturing the specimen is used as a measure of the energy absorbed by the specimen in impact fracture.
At the other end of the load rate scale, some materials that exhibit elastic behavior in conventional tests behave plastically when loaded slowly or when the load is static (unchanging) (Figure 7). For example, roofing tar will fracture in a brittle manner if bent with the hands at room temperature, but will flow as a viscous liquid under its own weight over long periods of time. Such ______ is an important source of deformation in dental materials ranging from amalgam to impression materials.
creep or flow
_______ is usually evaluated by statically loading a sample of prescribed geometry and measuring the dimensional change (disregarding the initial elastic deformation) over a prescribed period of time. The load rate dependence of materials that exhibit ______ is described as viscoelasticity, which describes viscous (plastic) behavior at low strain rates and elastic behavior at high strain rates.
Creep or flow
Any material stressed once below the elastic limit will not permanently deform and, of course, will not fail. However, repeated cyclic stresses below the elastic limit may lead to fracture. This phenomenon is known as _____. It is an important mode of failure in dentistry (e.g., clasp arms and denture bases) where forces involved in repeated insertion or mastication are encountered. _____ tests are usually described by a plot of stress (peak stress during the cycle) versus the log of the number of cycles of load application at which failure occurred. Some materials exhibit a _____ limit where below a limiting stress no further failures occur regardless of the number of cycles.
a measure of the resistance of a material to permanent indentation or penetration. Numerous tests have been developed that differ in the magnitude of the load applied and shape or size of the in- denter.
Which test involves forcing a steel ball into the surface of the specimen under a fixed load
-Nearly all metals can be tested
-As long as the relationship between the load and the size of the indenter ball remains constant, the results of the test will accurately measure the hardness of the material tested.
-less influenced by surface scratches and roughness because of large indentations.
-can be used with hetergeneous materials to get bulk hardness since indentation is large.
Advantages of Birnell Hardness
1) Most widely used.
2) Different scales and loads allow for it to be used with different classes of materials. 3) Very quick.
4) Can be used for both hard and soft material.
Neither Brinell nor the Rockwell hardness tests are suitable for brittle materials.
Advantages of Rockwell Harndess Test
In which test is the depth of penetration measured rather than the area of the penetration. Different penetrators (balls and cones) are used and a pre- load may or may not be used. Each combination of indenter size and shape, and preload and final loads defines a unique ______ scale.
In which test is the indenter a square based pyramid with 136o angles between opposite sides
1) Leaves only tiny indentations.
2) Very accurate.
3) Only one type of indenter is used. It does not have a number of different scales and indenters, as does the Rockwell and Brinnell scales.
Which test is also measured with a pyramidal indenter. However, the pyramid is asymmetrical having a ratio of long to short diagonal of about 7:1
Both ____ and _____ tests can be used to determine the hardness of brittle materials.
Vickers and Knoop
Only a very small sample of material is required, and it is valid for a wide range of test forces.
Advantage of Knoop
1) Difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 μm).
2) Takes a longer time to prepare and test sample.
Disadvantages of Knoop