Mechanical Properties: Stress Strain Relations (Stress Strain Curves p.40) Stress Strain Stress strain Curves Stress is the internal resistance Change in length per unit length of It is preferential to report the of the body in of force per the body when it is subjected to a stress-strain relations of a stress, Deformation resulting from a material rather than the forceunit area, Types: (Tension {A tensile or pulling force is an deformation characteristics, body is subjected to two sets of elongation of a body in the direction Studied by measuring the load forces that are directed away of applied force, whereas a and deformation, and then from each other in the same compressive or pushing force causes calculating the corresponding straight line}, Compression {A compression or shortening of the stress and strain, Relationship body is subjected to two sets of body, between stress and strain is forces in the same straight line often used to characterize the and directed towards each other}, Strain(E)=Deformation/original length mechanical properties of Shear {Result of two sets of =(L-Lo)/ Lo =∆L/L materials, (Such data are forces directed parallel to each generally obtained using a other}). mechanical testing machine which enables strain to be measured as a function of stress and recorded automatically). Proportional Limit Elastic Limit The greatest stress that a material will sustain without a Maximum stress that a material will withstand deviation from the proportionality of stress to strain, without permanent deformation, Region of the (Below the proportional limit, no permanent deformation stress-strain curve below the proportional limit is occurs in a structure, When stress is removed, the called the elastic region, or reversible strain will structure will return to its original dimensions), occur, Region of the stress-strain curve beyond Application of a stress greater than the proportional the proportional limit is called the plastic region, limit results in a permanent or irreversible strain in the Both the proportional and elastic limits are quite sample. different in values for different materials. Yield Strength ( Stress) γS Ultimate Strength (Stress) UTS Fracture Strength (Stress) Stress at which a material exhibits a Maximum stress that a material can Stress at which a specified limiting deviation from withstand before failure in tension, whereas material fractures, proportionality of stress to strain, A the ultimate compressive strength or A material does property used to describe the stress at stress (UCS) is the maximum stress a not necessarily which the material begins to function in material can withstand in compression, An fracture at the a plastic manner, Amount of permanent alloy that has been stressed to near the point at which the strain may be referred to as the percent ultimate strength will be permanently maximum stress offset, (Many specifications use 0.2% as deformed. occurs, For the a convention, Offset Yield Strength), specific cases of Determined by selecting the desired many dental alloys offset and drawing a line parallel to the subjected to linear region of the stress-strain curve, tension, the (The point at which the parallel line ultimate and intersects the curve is the yield stress). fracture strengths are the same. Elongation Elastic Modulus (Young's modulus) Flexibility The deformation that results from the Represents the stiffness of a material Describes the application of a tensile force, Total within the elastic range, It can be amount of strain percent elongation includes both the determined from a stress-strain curve by up to the elastic elastic elongation and the plastic calculating the ratio of stress to strain, or limit, Thus elongation, (Such a material, as many the slope of the lineal- region of the curve flexibility is the dental gold alloys, has a high value for & is reported in Mpa or Gpa, Elastic total amount of plastic or permanent elongation and, in Modulus=Stress/Strain or E=δ/Є, The elastic strain general, is a ductile type of alloy, stronger the basic attraction forces, the (deformation) in a whereas a material with only 1 % greater the values of the elastic modulus material. elongation would possess little and the more rigid the material will be.

Physical properties: Mass properties Density Specific Gravity Mass per unit volume (gm/cm3), Construct an upper Ratio of the mass of a particular body to the mass of an equal denture from a low density material, and a lower volume of water at the same temperature (Units: none), Since denture from a higher density material (to enhance the density of water is 1 q/cm3, density and specific gravity the stability of complete dentures). may be numerically equal, although they are not the same. Thermal Properties Adhesion Thermal Expansion Specific Heat Definition Adhesion Types Linear coefficient of thermal expansion: Quantity of heat The force that binds two Mechanical adhesion (change in length per unit length of the needed to raise dissimilar materials is through the material for 1°C change in temperature), the temperature together when they are roughness and Rate at which a material expands upon of one gram of brought into intimate irregularities of the heating or contracts upon cooling is the substance by the attraction adherend's surface, dependent upon the coefficient of thermal 1°C, Units are of atoms or molecules at Chemical adhesion expansion, Absolute coefficients of cal.gm-1. °C-1 , surfaces (cohesion is the theory, which includes thermal expansion of materials are of Symbol used is attraction between atoms all kinds of chemical small importance for clinical practice; their , How rapidly or molecules within one bonds between the relative values, however, are extremely the temperature substance), An adhesive and the important, For restorative materials the increases intermediate layer, adherend, including coefficient of thermal expansion should be depends upon called an adhesive, has to primary (ionic and the same or close to the tooth structure, the specific heat be placed between 2 covalent bonding) and (For example, when hot or cold beverages of the material. materials, Surfaces or secondary valence are taken, there is expansion of both the substrates that are forces (hydrogen, tooth structure and restorative materials, If adhered to, are termed dipole interaction, and they do not contract or expand at the same the Adherend. Vander Waals forces), time, the difference results in cracks Combination of both between the restoration and tooth types of adhesion. structure, This results in failure of the restoration). Thermal Conductivity Thermal Diffusivity Adhesion Principals Rate of transfer of heat Transfer of heat from a hot to a Surface Energy Surface Tension (calories or joules) by cold source is dependent on both The free energy at the Since the liquid conduction, through a unit the thermal conductivity surface of a liquid or solid is surface atoms are thickness (1 cm), across a (regulating the rate at which the greater than in its interior, not attracted unit area (1 cm/). Per unit heat enters and es through the {In the interior of the liquid equally in all time (1 sec), for a unit material) and the specific heat (or solid), atom A is directions, a difference of temperature (determining the rate at which the surrounded by its normal stronger attraction (1°C), Thermal temperature will rise as heat complement of 'near results between the insulation base is used enters the material), This is neighbours', In contrast, surface atoms under a large metallic presented by the thermal atom B at the surface is only themselves, restoration (e.g. amalgam) diffusivity, D, such that D=K/ bonded to a few other atoms, causing tightness to protect the pulp from (ρ) where (K=thermal Thus, there is a great of atoms at the temperature changes, conductivity, =heat capacity, tendency for atom B to surface, much like Law: {Q=KA((θ1-θ2)/d)} ρ=material density), The thermal satisfy its bonding a tough skin, The where (K=Constant, diffusivity gives a clear requirements (high surface force tending to termed material thermal indication of the rate of rise of energy) by bonding with produce this skin conductivity, d=material temperature at one point due to other substances more to effect is called bar length, A=cross heat source of another point, and itself}. surface tension. sectional area, may be considered the most Wetting θ1,θ2=difference between relevant in dental application, In Describes the liquid's tendency to spread out over 2 ends temp, Q=heat many circumstances a low value the solid surface, (↑ solid surface energy + ↓ liquid quantity ing per of diffusivity is preferred, A surface tension = ↑ wetting), In order to produce second), Units: watts per denture base material, ideally, adhesions, the liquid film must flow easily over the meter per degree Kelvin. should have a high value of entire surface and adhere to the solid. In other thermal diffusivity in order that words, complete (or total) wetting must occur, the patient retains

permanent elongation and would be considered brittle),, An alloy that has a high value for total elongation can be bent permanently without danger of fracture, (Clasps can be adjusted, orthodontic appliances can be prepared, and crowns or inlays can be burnished if they are prepared from alloys With high values for elongation). Ductility & Malleability Gold is the most ductile and malleable metal, and silver is second, Dentist manually closes the open margins by a process called burnishing (3 distinct steps, denting, bending, and adapting by flattening). Ductility Malleability Ability of a material to Ability of a material to withstand permanent withstand permanent deformation under a tensile deformation without rupture load without rupture, (A under compression, (as in metal which may be drawn hammering or rolling into a readily into a wire is said sheet). to be ductile), A property related to the workability of a material in the mouth, & to burnishability of the margins of a casting.

Resilience & Toughness Resilience Toughness Resistance of a material to Resistance of a permanent deformation, material to Indicates the amount of fracture, energy necessary to deform Indication of the material to the the amount of proportional limit, energy Measured by the area necessary to under the elastic portion of cause fracture, the stress-strain curve, The area under Units MN/m3, which the elastic and represents energy per unit plastic portions volume of material, of a stress-strain Resilience has a particular curve represents importance in the the toughness of evaluation of orthodontic a material Units wires in moving a tooth. of toughness are the same as resilience. Tensile Properties of Brittle Materials: (A variety of brittle restorative materials including dental amalgam, cements, ceramic materials, plaster and stone, and some impression materials, is important to dental practice, In many instances the material is much weaker in tension than in compression, which may contribute to failure of the material in service, However, brittle materials must be gripped with caution, and any stress concentrations at the grips can lead to premature fracture, An alternative method of testing brittle materials, in which the ultimate tensile strength of a brittle material is determined through compressive testing, The method is described as the diametral compression test for tension, the Brazilian test, or the indirect tensile test, In this test method, a disk of the brittle material is compressed diametrically in a testing machine until fracture occurs, (tensile stress) σx = 2P {Load}/ (πDT) {Diameter × Thickness}). Transverse Strength Impact Strength Tear Strength Transverse strength of a material is Energy required to fracture a A measure of the resistance obtained when a load is applied in of a material to tearing forces, material under an impact force, A the middle of a simple beam, which Important property of dental Charpy-type impact tester is is ed at each end, Such a test polymers used in thin usually used, (A pendulum is is called a three-point bending test, released which swings down to sections, as flexible and transverse strength is often impression materials in interfracture the specimen), Energy lost described as the modulus of proximal areas maxillofacial by the pendulum during the fracture rupture or flexure strength, Useful of the specimen can be determined materials, and soft liners for in comparing denture base materials dentures, Depends on the rate by a comparison of the length of its in which a stress of this type is of loading, as the viscoelastic swing after the impact with its free applied to the denture during nature of the materials tested, swing when no impact occurs, mastication, also in long bridge More rapid loading rates Energy units are joules, spans in which the biting stress may result in ↑ values of tear inch/pounds, In Izod impact tester, be severe, When applying load, strength, Clinically, the rapid the specimen is clamped vertically specimen bends, so principal (or snap) removal of an at one end instead of at the center of stresses applied on upper surface are alginate impression is imp to the specimen ed at both ends compressive & those on lower ↓ the tear strength & as for the Charpy. surface are tensile, (Law p.42). permanent deformation. Particularly to water absorption and Important, (It is possible for a To penetration, (Thus a material may

A satisfactory response to hot and cold stimuli in the mouth.

The degree of spreading of a liquid on a solid, or the wetting tendency, is expressed by the quantity called the angle or the angle of between the liquid and solid surfaces, (the lower the angle, the greater the wetting tendency will be), Surface tension of liquids generally decreases with increased temperature, (This is also true for molten metals, This is the reason why an increased pouring or casting temperature for a cast crown will aid in producing sharp mold details, Requirements for Formation of Strong Adhesive t: (Cleanliness, Penetration and wetting rough surface, Formation of strong chemical or mechanical bond, Minimizing thermal stresses, High surface energy of solid and low angle of adhesive), It is best to use low surface energy materials for the restoration, and to adequately polish margins of restorations to lower-surface energies. Optical Properties Light is a form of electromagnetic radiant energy that can be detected by the human eye, (The eye is sensitive to wavelengths from approximately 400 nm (violet) to 700 nm (dark red), The combinations of wavelengths present in a beam of light determine the property usually called color). Perception of color 3 Dimensions of color Color Measurement Metamerism In order for an object Quantitatively, color is described as a One frequently used Objects that to be visible, it must three-dimensional quantity specified by method of specifying appear to be either emit light or it values for three variables, hue, chroma, and colors is the Munsell color color must reflect or transmit value. coordinate system, (On the matched light incident upon it vertical axis is plotted the under one Hue Chroma Value from an external value, The hue is type of light Refers to the Refers to A source, (The latter is represented by a circle, may appear property the photometric the case for objects that commonly The chroma is the distance very strength parameter are of dental interest, from the centre), different associated or degree associated Light from an object Clinically in the dental under with the of with total which is incident on an lab, color matching is another light color of an saturation reflectance eye is focused onto the usually done by the use of source, object, for a or retina and converted shade guide, (An (color whether it is particular luminance, into nerve Impulses, alternative system is to matching red, green, hue, The (the which are transmitted express color in the should be blue, etc, higher the brightness or to the brain, ConeL*a*b* system designated done under This refers chroma, darkness of shaped cells in the by the Cornrnission two or more to the the more an object). retina are responsible Internationale de different dominant vivid is for color vision, l'Eclairage (ClE), where light sources, wavelengths the color. Reflected light is L* is the lightness factor one of which present. absorbed by retina and and a* and b* are should be perceived in brain as chromaticity coordinates; sunlight). "red", Individuals chromaticity includes both vary greatly in their hue and chroma), ability to distinguish Appearance of an object colors). depends on the nature of the light by which the object is viewed, (Sunlight & florescent lamps are common sources of light in the dental lab). Water Sorption Solubility & Disintegration Setting Time Represents the amount of water A measurement of the extent to Setting time is related to the time adsorbed on the surface and which it will dissolve in a given taken for the material to reach its final absorbed into the body of the fluid, for example, water or saliva, set state or develop properties which material during fabrication or while A high solubility will severely limit are considered adequate for that the restoration is in service, A the effective lifetime of the application, Associated with the serious warpage and dimensional restoration, Degradation may reaction rates and affect the practical change in the material are associated follow a sequence of absorption, applications of many materials in with a high percentage of water disintegration, and solution, restorative dentistry, Does not sorption, Polymers such as those Variables such as cement indicate the completion of the used in resin composites, dentures composition, thickness, molarity, reaction, One convenient and and soft liners are susceptible & ph of the medium are commonly used method is resistance

Hardness Resistance to permanent surface indentation or penetration. Hardness Tests Brinell Hardness Test Rockwell Knoop Hardness Test Hardness Test Depends on the resistance to the penetration of a small steel or tungsten carbide ball (typically 1.6 mm in diameter), Penetrator remains in with the sample tested for a fixed time of 30 seconds, Then it is removed and the indentation diameter is carefully measured, Formula (P.45, The smaller the area of indentation, the harder the material and the larger will be the BHN value, This test yields a relatively large indentation area. and as a result, this test is good for determining average hardness values and poor for determining localized values.

Basically similar in principle to the Brinell test, Utilizes a cone or metal ball, the depth of the penetration is read directly from a sensitive dial micrometer on the instrument, Initially, the indenter is placed under a small static load (minor load), and the dial is set at zero, Load is then increased (major load) and maintained for a certain period of time, Depth of indentation is measured after the load has been decreased to the minor value, Although it is not suitable for brittle materials, a large tip with a light load may be used for hardness testing of some plastics.

Developed to fulfil the needs of a microindentation test method, A load is applied to a carefully prepared diamond indenting tool with a pyramid shape, and the lengths of the diagonals of the resulting indentation in the material are measured, Knoop hardness number (KHN) is the ratio of the load applied to the area of the indentation calculated from the following formula: KHN=L/(l2), (L) load applied, (I) length of the long diagonal of the indentation, () constant relating (I) to the projected area of the indentation, Units for KHN are also kq/MM2, Used to examine materials that vary In area hardness, (For example, in testing the hardness of both enamel and dentin in extracted teeth, and in determining the hardness of metals and alloys that have isolated hard and soft phase throughout the material), Disadvantages: need for a highly polished and flat test sample, and the time required to complete the test operation.

Vickers Hardness Test The 136degree diamond pyramid, or Vickers hardness test, is also suitable for testing the surface hardness of materials, Indenter produces a square indentation, Loads are varied from 1 to 120 kg, depending on the hardness of the tested material, Useful in measuring the hardness of small areas and for very hard materials.

Shore A Hardness Test Hardness measurements used to determine the hardness of rubbers called a Shore A Durometer, Instrument consists of a blunt-pointed indenter 0.8 mm in diameter, The indenter is attached by a lever to a scale that is graduated from 0 to 100 units, If the indenter completely penetrates the sample, a reading of 0 is obtained, and if no penetration occurs, a reading of 100 units results, Used to evaluate soft denture liners, mouth protectors, and maxillofacial elastomers.

Tendency of a metal to give away electrons is called its electrode potential, Voltages are tabulated, and the list is called the electrode potential series, Table indicates the propensity of an element to give up electrons and go into solution as positive ions, i.e. the element's susceptibility to corrosion, {Large positive numbers indicate nobility of metals (e.g. gold and platinum), and large negative numbers indicate the tendencies toward corrosion}, When two sheets of metal in close are immersed in a solution (electrolyte), the metal with lower electrode potential becomes the anode that gives up the electrons and corrodes, while the metal with higher electrode potential becomes the cathode that receives the electrons and remains intact).

the loss of soluble components, It is desirable for the water sorption to be as small as possible. (This ensures that the polymer retains its characteristic properties, and that no components are leached out which might adversely influence the biocompatibility of the material), The simplest method of assessing the water sorption of a polymer is to monitor the weight change of a sample when immersed in water.

material to be stable at near neutral be considered set when it is able to pH values but to disintegrate rapidly resist penetration by a probe of known at extremes of either acidity or weight and tip diameter). alkalinity, This partially explains why certain materials perform adequately with some patients but not with others), Standard tests of solubility often involve the storage of disc specimens of materials in water for a period of time, the result being quoted as the percentage weight loss of the disc. Biological Properties Any material used in the mouth should be: (non-toxic, non-irritant, no allergic reactions, not carcinogenic). Oral Mucosa Enamel & Dentine Dental Pulp Systemic Effects Short-term with Presence of a Harmful effects on the It is possible for a impression materials: (These restoration in a tooth or pulp are very significant, dental material, or a materials rarely produce any of an appliance attached (Pulp is very easily component of a dental significant harmful effect), to the tooth can lead to damaged by irritants which material, to produce Medium-term with an increased may gain access to it from systemic toxic effects, periodontal dressings: (These susceptibility of the restorative materials, The e.g. mercury used in the dressings should promote adjacent tooth substance histological changes that preparation of healing after periodontal to caries, Secondary can occur range from mild amalgam, The surgery However, dressings decay around a hyperaemia to pulp problem of systemic made of zinc oxide/eugenol, do restoration is common, necrosis), The process of effects generally only cause mucosal irritation which Restorative materials cutting cavities in teeth is becomes manifest in can be severe), Long-term can have a therapeutic itself potentially harmful to dental personnel who with dentures: Denture effect by increasing the the pulp, Since all are exposed to mercury sore-mouth, or stomatitis, is resistance to caries if restorative materials are vapour over a long usually caused by the traumatic they are able to release potentially damaging, period of time, (Strict effects associated with illfluoride into the lining materials are mercury hygiene is fitting dentures; but there may adjacent enamel where a normally used to minimize essential). be a hypersensitivity reaction decrease in acid the irritation to the pulp. involved in response to a solubility of the tooth denture component). substance results. Failure of Materials in Oral Environment Tarnish & Corrosion Tarnish: (A surface discoloration on a metal, or even loss of surface finish and luster, Associated with the formation of hard or soft deposits {mostly sulfides of metals} on the surface, It is the first step leading to corrosion in most cases), Corrosion: (Destruction of a material by chemical or electro-chemical interaction with its environment, Such a reaction starts at the surface and gradually reaches the interior of the material causing irreversible and possible natural tissue damage, {Chemistry of saliva, Wide range of temperatures & pH values of ingested food make the oral environment very hostile to materials}, Corrosion can cause breakdown of the restoration itself, and its products may be harmful to the oral tissues both physiologically and aesthetically). Types of Corrosion (Dry corrosion: {Occurs in the absence of water or another fluid electrolyte, An ionized solution that conducts electricity}, Electrochemical corrosion: {Wet corrosion, Requires the presence of water or some other fluid electrolyte and also requires a pathway for the transport of electrons, i.e. an electrical current}). Electrochemical Corrosion Most corrosion is electrochemical, A combination of a chemical reaction & the flow of an electric current, Common situation is when a metal oxidizes, (Electrons are liberated from the metal to form positive metal ions, Liberated electrons are carried to another place, usually a metal surface in with a liquid electrolyte, where a second chemical reaction can occur consumes the electrons, This second reaction is called reduction), Basis of electrochemical corrosion of dental alloys is the electromotive series of the metals, which classifies the metals by their equilibrium values of electrode potential, (Arranging them in the order of their dissolution tendencies in water, Metals with more positive potential have a lower tendency to dissolve in aqueous environments, Potential of hydrogen to give up electrons is considered zero in water solution, and is used as reference,

slowly), Consider a liquid occupies the space between two metal plates; the lower plate is fixed and the upper plate is being moved to the right then, Shear stress =F/A, Shear strain rate, or rate of change of deformation = v/d. where (d) is the distance between the two plates and (V) is the velocity of the liquid, An ideal fluid demonstrates a shear stress that is proportional to the strain rate, and thus the plot is a straight line, (Such behaviour is called newtonian), Viscosity is measured in units of MPa per second (centipoise), the higher the value, the more viscous is the material, Many dental materials exhibit pseudo plastic behaviour, (Their viscosity decreases with increasing shear rate until it reaches a nearly constant value), Liquids that show the opposite tendency are called dilatants, Some materials behave like a rigid body until some minimum value of shear stress is reached (Referred to as plastic), viscosity of most liquids decreases rapidly with increasing temperature, A liquid that becomes less viscous and more fluid under pressure is referred to as thixotropic, (Thixotropic nature of these materials is beneficial because the impression material does not flow out of a tray until placed over dental tissues), Viscosity is measured also by use of a small container in the bottom of which is an orifice of specified dimension (A specified volume of liquid is poured into the container, and the time required for the liquid to run out of the orifice is measured, Viscosity is then computed from a standard table), During the pouring or casting of a gold alloy crown, the viscosity of the liquid metal or glass plays an important role (The metal must be fluid enough to fill all the parts of the mold), Increasing the temperature of a liquid decreases its viscosity so Increases its fluidity, Likewise, in the use of dental impression materials, the chosen material must have a sufficient degree of fluidity to adequately flow around the fairly complex areas of tooth structure and cavity preparations in a reasonable amount of time, prior to setting.

instantaneous change in shape, However, with some materials, the recovery is often time-dependent, When an Impression of this material is removed from the mouth, several seconds are required before portions distorted by removal from undercut areas return to their original shape, This time-dependent lagging of strain behind stress is termed Viscoelasticity (or anelasticity), Examples of viscoelastic behaviour in dentistry are the "elastic" impression materials.

Corrosion Of Dental Restoration Oral Environment Variations in diet, Bacterial activity, Drugs, Smoking, Oral hygiene habits. Wear Removal of constituents from a material's surface due to mechanical action, All materials are involved in a wear process, depending upon the nature of the material, In

periods of time at stresses below their elastic limits, Occurs only at temperatures near the softening point of a material, so that most metallic and ceramic restorations do not creep in the oral environment, An exception is dental amalgam (Under traumatic occlusal stresses, an amalgam filling may creep and thereby lose its marginal adaptation).

application of loads below the elastic limit, The higher the applied stress (still, below the elastic limit), the lower the number of cycles is necessary to cause failure, The stress below which fatigue failure does not occur is called the Endurance limit, Most metals, however, do not exhibit an endurance limit, and fatigue failure will occur even at low stresses when given enough stress cycling, Ceramics, in general, do not exhibit fatigue behaviour.

Dental Structures Composition of dental alloys Craze Fracture Polymer that has been under A detrimental tensile load for a long period of process of time, its surface is covered with creating new

Types of Electrochemical Corrosion Heterogeneous Surface Composition Concentration Cell Corrosion (Stress Corrosion) Energy level of the material is not A combination of corrosion and stress uniform throughout its entirety, conditions can cause failures of a metal by (Surface at a high energy level will stress corrosion, (At an anodic portion of a tend to give up electrons (becomes surface, electrolytic action can form a anodic) and corrodes, while the low minute crack, which can increase in size as energy surface receives the electrons a result of stress concentration, More (becomes cathodic) and remains corrosion can occur in the enlarged crack, intact), A metallic restoration with and so on, until eventual failure occurs), one polished region and one Burnishing of dental restorations induces unpolished region can become a localized stress on the surface, and corrosion cell in itself, (The excessive carving may form deep and unpolished area corrodes and the sharp pits or grooves on the surface; all polished area remains intact, these defects will serve as anodes, causing However, dental polishing is loss of margins or deep pits due to thought to be minimal enough that it corrosion, (Uneven polishing, severe does not effectively change the working of a material, unnecessary energy level on the restoration burnishing, and excessive carving should surface), Commercial dental alloys be avoided), In the case of a removable generally contain more than three partial denture, the retentive arm of the elements and they can have complex clasp is subjected to repeated insertion and microstructures that result in even removal (fatigue), which creates a severe more heterogeneous surface stress concentration build up at the grain compositions, (Corrosion resistance boundaries, (Such stress build up occurs in of multiphase alloys is generally less an electrolyte (e.g. saliva) and often leads than that of a single-phase solid to a corrosion classified as stress corrosion, solution), Solder ts between and with eventual fracture of the clasp), dental alloys also corrode because of Most common cause for concentration cell differences in compositions of the corrosion is a small pit or scratch on the alloy and solder, Impurities in surface of the metal, (The pit on the alloys enhance corrosion, (Mercury surface of this amalgam restoration will impurities that can contaminate gold entrap debris, Because the pH and oxygen alloys have electrode potentials concentration of the debris at the bottom of different from those of the bulk the pit is different from that of the saliva, a grains of the gold alloys), Pure concentration cell is created, The resulting metals which do not contain corrosion produces further disintegration significant quantities of impurities, of the amalgam, enlarging the pit and corrode at much slower rates than propagating the process, If such an area is alloys. placed under masticatory stress, the combined fatigue and concentration cell corrosion will accelerate the destruction of the underlying metal), Certain metals develop a protective coating which protects them from corrosion, (The superior resistance of aluminium and chromium to corrosion is due to the protective film of oxide which forms on the surface, Such metal is said to be ive). Time-Dependent Mechanical Properties Viscosity Viscoelasticity Creep Fatigue Study of the flow of materials, (For liquids, flow Application or Slow flow Failure of a is measured by the viscosity, whereas for solids removal of loads causing material by one considers creep and viscoelasticity), below the elastic permanent breaking or Caused by the internal friction of a fluid, A limit was deformation fracturing measure of the consistency of a fluid and its accompanied by a of materials after inability to flow, (A highly viscous fluid flows corresponding and held for long repeated Galvanic Corrosion (Dissimilar metals) Most common mechanism by which metals corrode, Involves electron flow from one place to another, and requires a solution medium that will allow electron flow: such a medium is called electrolyte, (As ing bridge or orthodontic appliance by a solder which differs in composition from that of the castings or wires, Corrosion may occur at the interface between the dissimilar metals), Due to variation in composition of the metals on the surface of a single restoration, Electrolytic corrosion cause galvanic pain as postoperative pain usually occurs immediately after insertion of a new restoration, although generally it gradually subsides and disappears in a few days after the pulp heals, Varnish is used to coat the outside of the restoration to isolate it from saliva, If any evidence of corrosion exists on the surface of the amalgam restoration, it should be polished, (Production of a more uniform surface will reduce the magnitude of the current and the sensitivity).

dentistry divided into three divisions: (Wear in natural dentition {Due to three causes: Wear resulting from normal masticatory functions, considered as a physiologic cause, is called attrition, [degree of attrition depends upon diet, Use of teeth for functions other than mastication, such as tearing, cutting and holding certain materials, Wear may occur due to pathological causes, such as xerostomia [dryness of the oral cavity, which causes brittleness of the enamel, as well as absence of lubricating saliva, Followed by high wear rates] and bruxism [Non-functional mandibular movement that is manifested by habitual occlusal grinding, which results in tooth wear and accelerated alveolar bone loss]}, Wear due to cutting the preparation for the restoration, Wear in restorative materials), Wear of the dentition by tooth-brush and dentifrice, is inevitable, This type of wear is termed abrasion, Cutting and polishing are also two wear processes, Except for dental porcelain, restorative materials are usually subjected to greater wear than natural dentition, due to their surface hardness, Factors Influencing Wear: (Surface roughness [increases the actual ], Temperature [Increases the flowability of materials especially low-heat resistant polymers, and results in higher wear], Environment (e.g. moisture) [Provides natural lubrication that reduces wear, Soft foods reduce wear because they serve as cushions during mastication, Resulting wear debris may be washed away by saliva, or may be trapped between the two opposing surfaces, creating the so-called three-body abrasive wear, This phenomenon occurs when the diet is impregnated with fine, solid particles, and during brushing of the teeth with an abrasive substance]), Improvement of Wear Resistance: (Hardness can be altered by either using harder materials or by using softer materials that have been treated to gain surface hardness, For polymers there are 2 ways to improve wear resistance, [To increase the molecular weight during polymerization, & to incorporate fine particles of high strength and hardness], Wear resistance of composite restorative materials is Improved by the addition of fine quartz particles or glass beads, [Polymer matrix of composite may be worn away due to its softer nature, Such resin wear may continue to such a degree that the hard structure (filler particles) no longer has any matrix to hold it in place, Filler particles will then break off and the restoration will show an even greater wear], Therefore, the selection of the opposing material is of paramount importance, [As porcelain teeth should not be allowed to rub against natural dentition or composite restorations], Polishing to attain a smooth surface is highly advisable to reduce wear.

cracks, Fine cracks are called craze, Microscopic examination reveals that there are fibrous connections between the two surfaces of a craze, [Craze does not show clear separation, while a crack is achieved when a clear separation occurs], As in a crack, the direction of craze usually lies perpendicular to the direction of the pulling force, Craze occurs strictly in polymers, Cause of Craze: [Non-homogeneity of materials, {Occurs most commonly in thin regions of polymeric dental restorations or devices, After processing of the denture at 6570°C, it is allowed to cool to room temperature, As the denture cools, the denture base material contracts faster than the teeth, As such, tension is built into the thin resin layer surrounding the teeth and craze lines occur}, Craze occurs also due to surface chemical attack], Prevention of Craze: [Highly crazed regions still possess relatively high strength, but are somewhat weaker than non-crazed polymers, In the case of denture base, it is obvious that because the residual stresses developed during cooling are not great, craze can be eliminated by increasing the thickness at the margin, or by cooling the denture at a slower rate, to allow the residual stresses to dissipate through the entire denture].

surfaces within a body, Occurs at stresses well below the elastic limit of materials, and is usually associated with cracks or structural flaws, Cause of Fracture: [Modes of fracture are: tensile mode, controlled crack propagation, fatigue, creep fracture, and stress (corrosion) cracking], Prevention of Fracture: [Main reason for fracture is the occurrence of stress concentration, Minimizing stress concentrations will definitely reduce the chance for premature fracture, Craze formation induces more stress concentration and eventually results in fracture].