Science of Dental Materials PDF

2 General Properties of Matter CHAPTER SURVEY States of matter Fatigue, failures, endurance limits Interatomic bondings...

2 General Properties of Matter CHAPTER SURVEY States of matter Fatigue, failures, endurance limits Interatomic bondings Surface tension, contact angles Crystal structure Wetting, adhesion, and failures Deformations—stress, strain Surface hardness, testing methods Stress–strain relations Rheological—viscosity Elastic and non-elastic deformations Viscoelasticity (creep) Strengths, moduli of elasticities Thermal properties Testing of mechanical properties Conduction, expansion Complex stresses Colour—hue, value, chroma Dynamic forces Impact strengths Colour matching STATES OF MATTER Applications 1. At higher pressures, the melting point decreases and Matter is made up of a large number of electrically boiling point increases. The boiling point of MMA neutral atoms of the particular structure with definite monomer while curing the dentures in a flask under number of neutrons and protons forming the nuclei high pressure will be more than 100.8°C, which is its and electrons in different shells (orbits) around them. normal BP (refer to boiling monomer porosity in The long range and short range electrostatic cohesive dentures). forces make them approach each other, forming 2. The latent heats of fusion and vaporisation of water crystalline (metals), semi-crystalline (polymers) and are 80 cal per gm at 0°C and 540 cal/gm at 100°C amorphous (vitreous—noncrystalline, ceramics, respectively. Large latent heat of fusion is absorbed waxes), etc. solid structures at low temperatures, i.e. from heat sources when the alloy pellets are melted, below their solidification temperatures. When the and an equal amount of heat is released during temperature of the solid is raised, the kinetic energy solidification in the casting, brazing, and welding of vibrations of atoms increases, and at one stage, the procedures. atoms get debonded and solid is converted into a Depending on the internal energies or characteristic liquid. The thermal energy required to break temperatures, matter exists mainly in three states, solid, completely or debond this solid structure, is the latent liquid and gas (or vapour) heat of fusion. This is also, the heat energy required Heat Heat Heat (Melt) (Vaporise) (above Tc) to convert one gram of a solid to liquid state or liquid Vapour Gas Solid Liquid to solid state, by liberating heat at its normal melting Cool Cool Cool temperature and at normal pressure. Similarly, heat (Fuse) (Condense) (below Tc) required to debond atoms of liquid or convert one When the vapour is heated above a critical tempera- gram of a liquid into its vapour state at normal ture (triple point), Tc, it becomes a gas above which it pressure and boiling point, is the latent heat of cannot be liquefied only by pressure, e.g. CO2 has its vaporisation (refer to Appendix, Table 9). Tc = 31°C and below this temperature, it can be liquefied 14 t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 15 only by pressure. At very high temperatures, the electrons Composites: These have combined properties of escape from orbits and the plasma state is reached. ceramics and polymers. These varied properties of different groups of MATERIALS USED IN DENTISTRY materials are mainly due to their interatomic bonding These can be broadly classified into energies, crystal lattice structures, compositions, etc. Metals and alloys (solid solutions—ordered and disordered) used for fabrication of permanent 1. INTERATOMIC BONDING appliances such as DFG, silver amalgam, noble and All the physical, chemical, mechanical and thermal base metal alloys. properties of materials depend on their atomic Ceramics: Inorganic salts, like gypsum, cements, structures, the interatomic cohesive binding energies, dental porcelains, etc. and the solid-state conditions. When two atoms are at Polymer resins: PMMA and other resins, soft waxes a certain distance, like charges (+ve and +ve, or –ve and impression materials, hard denture base material and –ve) repel and unlike charges attract each other. resins, clinical instruments, etc. The resultant force is attractive above, and repulsive Composites: Ceramic–resin combinations like below, a certain minimum distance. This represents composite restorative resins. the interatomic distance, at equilibrium. The energy These dental materials can also be broadly required to bring these atoms from infinity or separate classified according to their applications as them completely is the interatomic bonding energy. If this bonding energy is more, the material has a higher Materials used for the prevention and intervention of oral diseases. melting temperature, lower thermal expansion, etc. The Restorative materials used for direct filling and various properties depend on this, as well as the types fabrication of indirect restorations. of atomic bondings, namely primary and secondary. Auxiliary materials used in laboratory procedures. 1. Primary bondings: These are due to long-range Clinical and laboratory equipment and facilities. electrostatic forces between the opposite charges. Ionic bonding takes place between the +ve and GENERAL PROPERTIES –ve ions by strong electrostatic forces of attraction. Metals: These are characterised by metallic bonding, In NaCl, the sodium atom donates its single formation of positive ions in solutions, good electron of outermost shell, to chlorine, to conductors of heat and electricity, ductility, completely fill its outermost shell. Sodium and malleability, higher density, COTE, definite melting chlorine atoms become +ve and –ve ions points, opacity, etc. respectively and are held together firmly by Alloys (disordered solid solution—ordinary alloys): electrostatic forces of attraction. Such ionic These have a certain range of melting temperatures bonded substances are chemically stable, lower ductility and malleability, higher strength, insoluble in organic solvents and fusible, e.g. opacity, heat hardenability, etc., e.g. high noble, noble, Na+ + Cl– → NaCl predominantly base metal alloys. Covalent bonding is due to the sharing of one or Intermetallic compound (ordered solid solution): more electrons of the outermost orbit of the atoms. These are more brittle than ordinary alloys, Hydrogen bonding takes place in this manner to form containing different phases in the microstructure, e.g. a stable hydrogen molecule. The organic molecules, dental amalgam, ordered Au, Cu, etc. such as CH4, C2H4, etc. are formed in this manner. In Ceramics: There are non-crystalline amorphous polymers, the chain structure of polymers is formed (vitreous) solid structures having high compressive by this strong covalent bonding with long-range strengths, brittleness, hardness, and fusion attractive forces. These materials are generally, temperatures. These soften above their glass transition temperatures. These have low thermal conduction and thermal expansions. Polymers: Polymers are soft, flexible or rigid, fibrous properties, crystalline, semi-crystalline or amorphous structures, low density, less brittleness, lower glass transition temperature, higher COTE, thermal and electrical insulators, transparent inorganic glasses (PMMA). t.me/Dr_Mouayyad_AlbtousH  16 Science of Dental Materials with Clinical Applications soluble in organic solvents and infusible (i.e. valency electrons can move to and fro between the ends decompose on heating, without melting). of polymer chains inducing an opposite charges in the Metallic bonding: The valence electrons of the adjacent chains. This causes fluctuating dipoles and outermost shell are rather loosely bound to the nuclei binds the polymer chains with weak short range and hence get knocked out by thermal energies, even secondary bonding (Fig. 2.2). When a polymer is heated at low temperatures. These free electrons can move above its characteristic glass transition temperature, Tg, at random, in the space between the +ve ion lattices, the weak secondary bonds are broken, the polymer as electron gas cloud and form a strong electrostatic chains can easily slip one over the other, or it becomes field-bonding. The free electron gas density is soft (thermoplastic). Tgs become higher if the chains responsible for the characteristic properties of metals, are more cross-linked by the strong covalent bonds. such as conductivity, ductility, malleability, opacity, polishability, etc. Gold, silver, platinum, palladium, etc. have high, free electron gas densities and therefore have excellent conductivity, malleability, ductility, etc. Organic compounds with hydrogen bondings, ceramics with oxygen bondings, do not have the free electrons, and hence, these have low thermal and electrical conductivities, ductilities, and high transparencies (Fig. 2.1). A. Ionic bond formation e– transfer from one Fig. 2.2: “Secondary bonding fluctuating dipole bonds”. atom to another Fluctuating positions of electrons relative to an atom’s nucleus (the dipole is not fixed in direction) Chemisorption of gases by alloy liquids is followed by B. Covalent bonding by attraction due to van der Waals forces and get absorbed. sharing of electrons Adhesion between two materials is by secondary between two atoms. bonding or by primary–chemical valency bondings. e– transfer from one element to another 2. CRYSTAL STRUCTURE The attraction between the atoms, make them approach C. Metallic bond to the minimum equilibrium distance during solidifica- formation tion. They arrange themselves in crystalline ordered e– sharing manner, so that each atom is positioned, symmetrically Formation of gas of e– with the neighbouring atoms, to have minimum that bonds the atoms energies. Six different geometric arrangements have together in a lattice been recognized. These are cubic, tetragonal, orthor- hombic, monoclinic, triclinic and hexagonal, depending Fig. 2.1: Primary atomic bondings on the spacing between the atoms, a, b, and c on the x, Secondary bondings: These are weak, short range, y and z axes and the angles α, β, γ formed between the van der Waals attractive forces due to dipole faces. moments between the molecules having temporarily Bravias has shown that there can be 14 varieties of asymmetric charge distributions. networks of lattice points so that each point has identical Examples surroundings as shown in Fig. 2.3. The electric charge distribution in a water molecule Most common varieties belong to cubic types. is asymmetric. The proton (H+) side is less –ve or more Simple cubic crystal (SCC) has 8 atoms positioned +ve (δ+) than the oxygen side (δ–), forming an electric at the corners of a cube (e.g. NaCl). dipole. The molecules rotate and become loosely The face-centered cubic (FCC) structure has one bonded liquid structure by these short-range forces. atom at the center of each face in addition to those at In polymers, the mers join by long-range strong the corners, e.g. Au, Ag, Cu, Pt, Pd, Al, Pb, Fe (γ), Co covalent bonding forces to form long chains. The (β), Ni (β), etc. t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 17 In the precipitation (order–disorder) heat treatment of gold alloy castings, the FCT Au-Cu phase precipitates causing lattice inhomogeneity and increase in strength. Changes in the allotropic forms, at certain temperatures cause different mechanical properties. Sudden cooling of Austenite (FCC) steel precipitates very hard martensitic steel, which has distorted FCT structure. Similarly the properties of α -Ti (HCP) change when heated above 885°C forming β -Ti (BCC). This β-Ti has more suitable properties for active orthodontic appliances, and β (BCC) structure can be retained by alloying with aluminum and vanadium. Superelasticity and elastic memories of Ni-Ti alloys are due to changes in the lattice structures, by changes in temperatures or stresses and then recovering back, to earlier structure (refer to Ni-Ti alloys), by twinning recovery. AMORPHOUS (NON-CRYSTALLINE) STRUCTURES Many dental materials like waxes, resins, composite resins, glass-ceramics, etc. are non-crystalline. The atoms have only short-range attractive forces, by which atoms may tend to have crystallinity just in small sites. During solidification, there is no conduction of heat due to low thermal conductivity. They solidify, without any arrangements, like molecules in a liquid state. Hence, these are known as supercooled liquids, or vitreous solids. Properties of amorphous materials Fig. 2.3: Bravias unit cells of 14 space lattices No definite single melting point When heated, first softens above a certain The body-centered cubic (BCC) lattice has one temperature known as glass-transition temperature, extra-atom at the center of the cube in addition to those the flow increases with temperature and finally at the corners, e.g.: Na, K , Ba, Li, Mo, Ta, Fe (α), Cr (α), becomes liquid. W(α), β-Ti, etc. No free-electron gas cloud: Hence, these are poor conductors of heat and electricity. Note The coefficient of thermal expansion suddenly However, as the solidification process progresses, the increases above Tg and varies with temperatures. diffusion of atoms is obstructed. Perfect crystals are not formed, during casting procedures. Lattice defects Examples (point, line, dislocation and voids) make the as-cast Dental waxes, impression compounds structure soft, ductile and malleable. When work Denture resins, composite resins hardened, it becomes brittle and loses ductility. Ceramics (glasses) Applications MECHANICAL PROPERTIES (DEFORMATIONS) One of the conditions to form solid solutions, is that the metals should have same type of lattice External forces or energies acting on the materials, cause structures. Au, Ag, Cu, Pt, Pd, etc. are the elastic and non-elastic deformations, and fractures. constituents of noble metal casting alloys, as all these Large dynamic masticating forces, frequently acting, have FCC structures. many times, damage and fracture oral appliances and t.me/Dr_Mouayyad_AlbtousH  18 Science of Dental Materials with Clinical Applications restorations in service. With the knowledge of stresses, Applications strains, stress–strain relationships, the reasons for Large masticating forces (0–100 kg) acting on small areas failures can be understood. Suitable remedial measures of contact between tooth and restorations or appliances, or selection of suitable materials and techniques can be cause high stress (force/area) which can fracture weak adopted for the patients’ benefit. restorations or damage the appliances. The restorative Units and measurements of force, work (energy), power materials should have a high compressive strength of and stress more than 200 MPa and other mechanical properties. 1. Force: It is measured by the product, mass (m) × acceleration (a), F = ma STRESS AND STRAIN (Fig. 2.5) Units: Dyne = gm·cm·sec –2 The atoms in their equilibrium positions have minimum Poundal = pound·feet·sec–2 energies. External forces displace these atoms and cause Newton (kg·m·sec–2) = 105 dynes micro distortions, which manifest as macroscopic kg·m force = kg 9.8 m·sec–2 = 9.8 N structural deformations. These deformations are 1 Newton = 0.102 kgf internally resisted, by the atomic bonding forces When a force, F acts in a certain direction it has its (Figs 2.6a to c). rectangular components F cos θ and F sin θ (Fig. 2.4). Stress is defined as the internal resistance developed per unit area opposing the external deforming force per unit area, to retain equilibrium. Greater deforming forces produce greater stresses and also greater dimensional changes (strain) Stress = Force/area. Fig. 2.4: Rectangular components of force 2. Work = Energy = Force × distance moved by the point of application of the force in its direction. W = F. S cos θ Units: Erg = dyne × cms Foot pound = poundal × foot Newton-metre = Joule = 107 ergs 3. Power: Rate of doing work = Work Time Units: Ergs/sec, foot poundal/sec. Watt = Joules/sec = Newton-metre/sec = 107 ergs/sec. 1 watt-hour = Energy consumed in one hour at the rate of 1 Joule/sec. 1 kWh = 1000 watt hours 1 Horse-power = 1 HP = 550 foot – poundal/sec = 746 watt 4. Stress: Force per unit area Units: Dynes/cm2 and psi (pounds per sq. inch) Pascal = Newton/metre2 = 10 dynes/cm2 Mega Pascal = 1 MPa = 106 Pascals = 106 N/m2 = 107 dynes/cm2 1 MPa = 10.2 kgf·cm–2 = 145 psi or 1 kgf·cm–2 = 0.098 MPa Fig. 2.5: Stresses and strains t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 19 Figs 2.6a to c: Complex stresses: (a) Flexure, (b) compression, (c) tension Types of Stresses Types of Strains 1. Compressive stress is developed internally to 1. Compressive strain = contraction = x oppose the two external equal and opposite original length L compressive forces acting along the same axis elongation 2. Tensile strain = =x mg original length L = Force = Area Area 3. Shear strain = shear angle = θ = x L 2. Tensile stress is developed internally to oppose two external equal and opposite tensile forces, acting The strain causes the stress at the same instant, and along the same axis both disappear simultaneously. = Force Complex Stresses and Strains Area Pure tensile, compressive and shear stresses and their 3. Shearing stress is developed internally to oppose corresponding strains do not individually occur in practice. the two external equal and opposite shearing forces These combine and form complex stresses and strains. acting along different axes 1. Consider a rectangular bar, supported by two knife edges at distance ‘l’ apart and a deforming force F act at the = Force middle (refer, three-point bending test). The upper Area surface contracts producing compressive stress, lower surface elongates producing tensile stress and the layers Dynes at right angles, get sheared (Fig. 2.6a). Units of stress correspond to force/unit area, i.e. cm 2 The combined strains cause bending or flexure. In Newton = (Pascal), Kgf , psi, etc. irregular crown and bridge structures or cast RPDs, m2 cm2 the inhomogeneous stress distributions are studied, 1 MPa = 145 psi with models by optical methods. These complex stresses or strains are created due to the resistance Strain is the deformation taking place per unit between the lattices and cause slipping one over the dimension. This has no units and is expressed as the other. ratio or percentage. Strain is dimensionless quantity 2. Distribution of stresses when a cylindrical sample is usually expressed in percentage. compressed and elongated shows the tensile and t.me/Dr_Mouayyad_AlbtousH  20 Science of Dental Materials with Clinical Applications shear strains formed during deformations (Figs 2.6b Applications and c). Impression materials should have large flexibility If the metal is malleable, it can be deformed into a or elastic deformations to withdraw through severe thin sheet. Semi-ductile or semi-malleable material undercuts without permanent deformation. bulges at right angles due to the lateral tensile stress Flexibility is compared by applying a constant load, and gets drum-shape. The tensile axial forces cause say, 100 to 1000 gm on identical cylindrical samples compressive stress at right angles, as shown by the and finding the strains in different samples. narrow neck region before fracture. Orthodontic wires used for active appliances should have high flexibility or elastic range (low stiffness or STRESS–STRAIN RELATIONS modulus of elasticity). Nickel-titanium and β-Ti are the materials of choice. By clamping a wire at one end, and loading at the other Maxillofacial reconstructive materials and soft end in increments, it is possible to measure the denture reliners should have high flexibility. elongations for different loads, until it fractures. This is done with tensile, compressive or shear loading of samples using universal testing machines. Graphs Modulus of Elasticity representing stress (or load) and strain (elongation) It is defined as the ratio of stress and strain (within can be obtained as shown. Mainly there are two elastic limit) or stress required to produce unit strain. portions, elastic range, and non-elastic range, as It represents rigidity, stiffness or resistance to shown in Fig. 2.7. deformations. 1. ELASTIC DEFORMATION Young's Modulus of Elasticity (Y, E or Q) It is the ratio of compressive or tensile stresses and Proportional Limit (PL) corresponding strains. It is obtained from the slope of The first part of the graph OP is a straight line, the straight line portion of the stress-strain graph, i.e. showing that the stress is directly proportional to LM strain up to a certain limit, known as proportional or the ratio limit. KM It is the maximum stress up to which, the stress is Stress Proportional limit P Y=Q=E= = = directly proportional to strain (up to this strain, the Strain Flexibility F1 internal structure is not deformed permanently). Units: dynes/cm 2 , MPa, GPa (=1000 MPa), psi For tooth enamel, dentin, acrylics, stainless steel, PL = 225, 147, 27.5, 1630 MPa respectively. Applications Elastic Limit (EL) Oral appliances and restorative materials should have high proportional limit and high modulus of According to Hooke's law, within the elastic limit, elasticity, E (as very large dynamic compressive stress is directly proportional to strain masticating forces are acting on them frequently) to Stress LM resist permanent deformations. E for acrylics (2,500), = constant = modulus of elasticity = slope = =E Strain KM enamel (40,000), dentin (16,000), bones (18,000), ZnPO4 cement (14,000), gold alloys (100,000) Cr-Co The elastic limit is determined, in a similar manner. alloys (220,000), stainless steel (200,000) Ni-Ti The incremental load is applied, or load is increased (41,000), β-Ti (71,400), etc. in MPa. step by step after removing each time, to check the Ideally, restorative materials should have the same complete elastic recovery. E as that of dentin (i.e. about 14,000 MPa). ZnPO4 The elastic limit can be defined as the maximum cement has nearly same value. stress up to which the elastic recovery is complete or Orthodontic wires for active appliances should have there is no permanent strain. high flexibility or low E (i.e. Ni-Ti and β-Ti) and for The elastic and proportional limits have nearly same reactive or passive appliances, high stiffness or E (like values, as they represent the same phenomena. 18-8 stainless steel, elgilloy, Cr-Co or Cr-Ni alloys). Flexibility or elastic range (F1) Alloys of metal-ceramic appliances should have It is the maximum recoverable strain or the strain at high modulus of elasticity, to avoid sagging, at high the proportional limit, F1 = P/E. temperatures. t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 21 Modulus of rigidity or shear (η η) absorbed by a unit volume of material when stressed It is the ratio = η = Shear stress/shear angle or strain. up to the proportional limit. It can be shown, R = ½ This is the resistance to shearing (torsion) forces. In stress × strain or the area of the triangular part of the liquids, the viscosity represents the ratio of shear stress stress–strain graph up to the proportional limit (PL), to the shear strain rate or resistance to flow. Bond i.e. ( OAP. strengths of adhesive cements are measured by tensile 1 1 P P P2 R = stress × strain = P × F = × = J/m 3 or shear strengths. 2 2 2 E 2E Modulus of rupture or flexure strength for brittle Applications materials like ceramics is of importance. It is measured Tooth enamel and dentin have R = 0.5 and 0.93 by the resistance to flexure and is related to complex J/m3 respectively. Higher R and lower E of dentin, stresses and strains. show that dentin is tough and acts like a cushion underneath, the hard, brittle enamel. Dynamic Young's modulus The elastomers have large R and permanent resilient Dynamic Young's modulus of elasticity is more properties. Highly plasticized PMMA have high realistic than the static values. Dynamic Young's flexibility and resilience. They are chosen for resilient- modulus ED is obtained by measuring the velocity of denture liners, tissue conditioners and maxillofacial ultrasonic waves (v) in the material of density d. reconstructive materials. Dynamic modulus = v2d. Large modulus of resilience is a requirement for orthodontic wires, to store a large amount of Modulus of Resilience (R) potential energy of deformation and supply small It is the ability of a material to absorb deforming constant forces for a long time. energies without undergoing permanent deformation. Poison's ratio In other words, it is the ‘cushion-like’ or ‘springiness’ When a tensile force is applied along one axis to produce property. It is measured by the work done on a material elongation, compressive strain is produced at right or energy stored, when stressed up to the proportional angles, proportionately. If D and d are initial and final limit. Modulus of resilience, R is the amount of energy diameters, l1 and l2 are initial and final lengths (Fig. 2.6c). G = True breaking stress US = Ultimate strength BS = Breaking stress YS = Yield strength P = Proportional (elastic) limit E = Modulus of elasticity = slope R = Modulus of resilience = area OS = Offset value 0.2% OA = Flexibility (F) OC = Elongation (%) OB = Strain at fracture OC = OB–OA = AB Fig. 2.7: Stress–strain relationship t.me/Dr_Mouayyad_AlbtousH  22 Science of Dental Materials with Clinical Applications Table 2.1 ADA specification 1997 or ISO Draft 2002 for yield strengths of casting alloys ADA specification 1997 ISO Draft 2002 (minimum) (minimum) Casting alloys Yield strength Elongation Yield strength Elongation (MPa) (%) (MPa) % Type I—soft 80 18 80 18 Type II—medium 180 12 180 10 Type III—hard 240 12 270 5 Type IV—extra hard 300 10 360 3 Lateral contractional strain Yield strength is slightly more than the proportional Poison's ratio ) = Longitudinal, elongational strain limit. It indicates the possibility of permanent deformation when higher stress is produced. D–d l 2 – l1 Yield strength value can be measured accurately = / D l1 and hence is often referred for selection. The dental For most of the dental materials ) = 0.3 casting alloys are classified according to their For ideal isotropic medium ) = 0.5 and can be found minimum yield strengths at 0.2% offset value and by ultrasonic testing methods. percentage elongations (ability to undergo plastic deformation) in the quenched conditions, as per The shear modulus η and Young's modulus E are ADA specification No. 5, (1997) or ISO Draft (2002) related by: (Table 2.1). E η= = 0.38 E The orthodontic wires of the following alloys have 2 (1 + )) their yield strengths at 0.2% offset values, ultimate tensile strengths, modulus of elasticities (Table 2.2). 2. NON-ELASTIC DEFORMATION These values, vary according to the compositions, The stress at which the change from elastic to non- diameters, work hardened, annealed and tempered elastic deformations takes place cannot exactly be conditions of the wires. measured. If the strain measuring instruments are more sensitive, lower PL value can be identified. This yield stress is measured by finding the stress required to Ultimate Strength initiate permanent strains, of certain percentages. This is the maximum stress up to which the material Yield stress, yield strength or proof stress is the resists fractures and is represented by the peak value stress required to initiate a definite amount of in the graph. Ultimate strengths are different under permanent strain says 0.2%. Sometimes, it is known as tension, compression, bending (flexure or transverse) proof stress for 0.2% offset value. In bending testing and shear. These values are quite important for selection methods, the offset value is given in degrees, say 2.9 of restorative materials to withstand fracture, by large degrees (or 0.5 radians) offset values. biting forces. Table 2.2 YS, UTS, and modulus of elasticity for materials used in orthodontia Wrought alloys Yield strength Ultimate tensile Modulus of (MPa) strength (MPa) elasticity Q (MPa) 18-8 Stainless steel 1580 2100 180,000 Elgilloy (Cr-Co-Ni-Fe,Mo,Be) 1410 1680 184,000 Nickel-titanium 430 1490 41,400 β-Titanium 930 1275 71,700 PGP 1000 1200 110,000 PSC 750 1000 120,000 t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 23 The ultimate tensile strengths of alloys are given material of unit volume or the energy absorbed before in Table 2.2. For some restoratives, the UTS values in fracture, which is a measure of toughness. MPa are; tooth enamel = 10, dentin = 51, human bone = A material is said to be tough if the plastic 140, GIC-type I = 5.5, GIC type II = 6–15, composite deformation or this area is more, i.e. the ductility and resins = 40–60, amalgam = 30–50, porcelain = 25–35 MPa, malleabilities are higher, e.g. metals and alloys. etc. A material is said to be brittle if the area is small that is negligible plastic deformations, e.g. gypsum Breaking Stress products, ceramics, glass, etc. These have low tensile, When the stress slightly exceeds the ultimate strength shear and flexure strengths, but higher compressive value, the atoms begin to tear out or separate at the V- strengths (Fig. 2.8). shaped micro notches, surface defects, and lattice defects and finally fracture. Fracture Toughness The actual stress beyond the proportional limit is It is defined as the amount of energy or the stress more than that measured. Due to plastic deformations, intensity required to fracture, or the ability of the the area of cross sections decreases and the true stress material to get plastically deformed without fracture. is not calculated. That is why the breaking stress Brittle materials get easily fractured by deforming appears less than the ultimate strength value. forces or trauma. The surface flaws such as V-shaped cracks, notches, and internal cracks produced during True Breaking Stress solidification of insulators like porcelain, become the By substituting actual area of cross-section at different centers of stress concentrations, specially, at the tip of loads, true stress can be calculated. Graph OPG the cracks, imperfections, and voids. The large stress represents the true stress–strain relation and G gives concentrations tear the lattices or separate the atoms true breaking stress (TBS). causing crack propagations. Fracture toughness is also defined as the energy required to produce a fine crack Toughness and Brittleness of unit length (MPa m or Newton/meter 3/2. It is The total area under the entire stress-strain graph with measured by Charpy and Izod impact testing, punch the strain-axis refers to the work done to fracture the out shear testing; three-point flexure-bending method, Stress–Strain Relationship–Mechanical Properties of Materials: Comparison Fig. 2.8: Significance of stress–strain graphs t.me/Dr_Mouayyad_AlbtousH  24 Science of Dental Materials with Clinical Applications Vicker's hardness indentation methods. Types: Ductile, condition, should be 18%, 12%, 12% 10% or minimum brittle, shear, tensile, flexure, impact, etc. 18%, 10%, 5%, 3% respectively (ISO Draft 2002). Type The fracture toughness depends on the size of the iv alloys, after hardening, percentage elongation cracks inside, or notches and flaws on the surface. The should have a minimum value of 3%. Base metal values are expressed in MPa·m½, where m is in meters. alloys. Cr-Co-Ni, etc. have very low values of % elongation 1.5–8% (Table 2.1). Examples The percentage elongation depends upon the alloy The fracture toughness of composite resins, porcelain, ceramics, tooth enamel and dentin are components, work hardened and annealed 0.75–2.2, 2.2, 1.5–2.1, 0.61–1.8, 3.08 units respectively. conditions, as well as the method of heat treatments. High copper silver amalgam restorations fracture Large values are useful for burnishing, bending or more easily than low copper variety, as the latter has deforming, clasp adjustments of appliances, etc. higher fracture toughness. Dispersion toughening of porcelain: Addition of Ductility and Malleability hard (tough) materials like zirconia (ZrO2), alumina Metals and alloys have a large number of free-electrons (Al 2O 3), leucite, lithia disilicate crystals, etc. to in the lattice space due to metallic bonding. These are porcelain, can resist crack propagation and increase responsible for their ability to undergo non-elastic fracture toughness, up to about 3.3. MPa·m½ (refer permanent deformations when stressed above the to ceramics). elastic or proportional limits. Ductility is defined as the ability to undergo Percentage Elongation (Plastic Strain) permanent deformations, by tensile loading (or stress), When a wire is stretched to fracture, initial elastic or it is the ability to be drawn into a wire. The ductilities deformation is completely recoverable up to elastic limit can be compared by their percentage strain (elongation), OA in Fig. 2.9. After the fracture of the wire, the parts percentage decrease of areas of cross-sections, or the are joined and the increase in the distance between any number of cold bends, at fracture. two points marked on the wire earlier can be measured. Malleability is the ability to undergo plastic The percentage of this increase in length (per original deformations by compressive loading (stresses), or it length) gives the percentage of non-elastic, permanent is the ability to be beaten into a thin sheet. These can elongation (OC = OB–OA = AB, if, elongation is plotted), otherwise, strain (AB) × 100%. be compared by measuring the percentage increase in the areas at fracture. Applications Metals used in dentistry, having their ductilities in According to the ADA No. 5, specification (1997), decreasing order are Au, Ag, Pt, Pd, Fe, Ni, Cu, Al, Zn, classification of dental casting alloys, the types 1, 2, Sn, Pb. and malleabilities in decreasing order are, Au, 3, and 4, a minimum % elongation, in annealed Ag, Al, Cu, Sn Pt, Pb, Zn, Fe, Ni. Wires Y.S. MPa E. MPa 1. Cr-Co 1300 200,000 2. Elgiloy 1400 184,000 3. 18-8 1600 180,000 4. α-Ti 450 112,000 5. Au alloy PGP, PSC 1,000 110,000 6. β-Ti 930 71,700 7. Ni-Ti 430 41,400 Fig. 2.9: Stress–strain relationships representing properties of different orthodontic wires t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 25 Ductilities and malleabilities decrease, by STRENGTHS OF MATERIALS increasing slip resistance, i.e. by The ultimate strength can be defined as the maximum Alloying, solution hardening stress the material can withstand by compressive, Work hardening tensile, shear, flexure loading, before it fractures. Age hardening heat treatment The ultimate, compressive, tensile, torsion and shear Precipitation heat treatments. strengths are measured by using universal testing machines, tensiometers, etc. In all these cases, proper Applications sized samples are to be prepared (Figs 2.10a and b). High ductility and malleabilities are useful in The instruments also show the stress–strain adapting metallic restorations, to the margins by relations, proportional limits, modulus of elasticity, etc. burnishing direct filling gold foil, type I and II, casting and the values can be easily computed, at different rates gold alloys, silver amalgam restorations, etc. of loadings. Gold and silver have highest ductility and For viscoelastic materials, the properties depend on malleabilities. Very thin, pure direct filling gold foils the rate of loading. For example, modulus of elasticity of submicron thickness (no. 3, 4, 5, 6, etc.) are available of silver amalgam can vary from about 40,000 to 80,000 for restorations. MPa, at a higher rate of loading. Very thin platinum foils are used in the procedure of fabrication of porcelain jacket crowns. 1. Tensile Strength Very thin tin foils were used earlier as separating For ductile materials, the ultimate tensile strength can medium in the denture-fabrication procedures. be measured directly using a tensiometer. A dumbbell- Orthodontic wires are drawn from cast ingots into shaped cylindrical specimen is clamped rigidly at the very thin wires. ends and pulled apart to fracture. UTS is the maximum force per unit area required to fracture. Brittle materials DETERMINATION OF MECHANICAL PROPERTIES may fracture at clamping points due to stress Many varieties of sophisticated instruments involving concentrations. Diametral compression test for tension, different principles are used for testing the different Brazilian or indirect tensile testing method is used for properties of dental materials. brittle materials. In this, a circular disc of diameter D, ADA has specified the methods of preparing the test thickness t, is loaded (P) diametrically until it fractures. samples and the methods of measurements for various This method does not give reliable values for strain- dental materials. Even then, it is very difficult to control rate sensitive (viscoelastic) materials. Many dental many variable parameters such as voids, lattice restorative materials—cements, ceramics, composite imperfections in cast alloys, work hardened or heat- resins, silver amalgam, etc. can be tested by this method. treated conditions, etc. That is why, very rarely This UTS value is important, as brittle materials, have low consistent values, but often—different values are UTS and fracture easily. For 18-8 stainless steel wire, UTS obtained for different trials. The values of the = 2100 MPa, tooth enamel = 10 MPa, dentin = 51 MPa. properties of various dental materials, obtained by different authors, are not concurrent or exact, but only 2. Compressive Strength representative figures, helping the dentist to compare Many dental materials are brittle, have high compressive and select the suitable materials and techniques. (The strengths and low tensile strengths. When a compressive values given in this book are also, therefore, some force is applied, complex stresses, created inside average representative values.) (cylindrical sample diagram, Figs 2.6a to d) give Figs 2.10a and b: Tensile strength testing. (a) Uniaxial tension, (b) diametral compressive test for tension t.me/Dr_Mouayyad_AlbtousH 26 Science of Dental Materials with Clinical Applications (c) (a) (b) (d) Figs 2.11a to d: UTM fractured testing samples t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 27 unreliable values. If the too long cylinder is used, the Bond strengths are measured by shear or sometimes buckling in the middle takes place. Usually a cylindrical by tensile testing. The samples sometimes are subjected sample of a length twice its diameter is used for testing. to thermal cycling between 5°C and 50°C, several times, The universal testing machines give stress–strain graphs to study the bond strengths. Zinc phosphate cement (Fig. 2.11e). From this modulus of elasticity, resilience, easily gets debonded compared to zinc polycarboxylate PL, EL, and yield strengths can be found. The Young’s cement which is chemically bonding with enamel. modulus (E) is nearly same for compressive and tensile methods. The restorative materials should have higher compressive and tensile strengths comparable to tooth enamel and dentin (380 and 10 MPa, 350 and 51 MPa, respectively). Examples: Compressive strengths in MPa: Heat cure acrylics = 75, ZnPO4 cement = 80–100, ZnPO4 base = 110–130, zinc polycarboxylate cement = 60–70, GIC = 60–150, porcelain = 150, amalgam = 300–500, composite resins = 300–450, etc. (pages 355–357). 3. Shear Strength It is the maximum shearing stress at shear failure. Push- 1 out or punch test method (i.e. applying an axial load to push out a sample, through the other side), is used. For cylindrical punch of diameter (d), Load P Shear strength = = MPa π × Diameter × Thickness πdt For rectangular punch of cross-section l and b, shear P strength = MPa 2(l + b)t For the punch of square cross-section of side(s) shear P strength = MPa. 4st Applications 2 The interfacial force of retention, under shear, is quite important for bonding metal-ceramics, composite-resin restorations, etc. The shear strengths for amalgam, dentin, acrylics, porcelain, enamel, ZnPO4 cement are 188, 138, 122, 111, 90 and 13 MPa, respectively (Fig. 2.11f). 3 Fig. 2.11f: (1) Punch-pushout test for shear strength, (2) shows measurement of pushout bond strength of MTA cement with root dentin. The dentin of thickness 3 mm and with a hole (apical foramen) 1 mm diameter embedded in the acrylic and filled with MTA. The specimen is placed in a special jig and load is applied with a plunger of 1 mm diameter. The load required to shear is Fig. 2.11e: Universal testing machine recorded, (3) shows the sheared specimen during testing t.me/Dr_Mouayyad_AlbtousH  28 Science of Dental Materials with Clinical Applications 4. Complex Stresses: Testing 2. Bending: Many instruments used in clinical practices Flexure (Transverse) Strength, Modulus of Rupture are bent frequently. (For example, files and reamers, during root canal preparations.) If the applied stress This is a bending test. The load, P is applied at the middle is more than a critical value (yield strength), it is of a rectangular bar of breadth (b), depth (thickness) (d), permanently bent and damaged. The bending supported by two knife edges at a distance—l apart, moment increases with the angular deflections as and the depressions (δ) are measured (3 point-bending shown. Thicker reamers can have higher angular test). If P is the fracture load (Fig. 2.12). deflections. The initial straight line portion indicates 3Pl the elastic deflections for safe use without causing Flexure strength = Modulus of rupture = 2bd2 permanent bending, or damage. Elastic memory— Pl 3 Ni-Ti alloys are used nowadays (Figs 2.13a and b). The deflection = δ = 4Ebd3 3. Permanent bending is to be done in many cases, such Pl 3 as clasp adjustments for cast partial dentures, Flexure modulus E = fabrication of orthodontic appliances, etc. In these 4bd3 δ Photoelastic method of stress analysis is used for the cases also complex stresses, compressive on the inner 3-point bending test of irregular samples. A model of part and tensile on the outer part, are introduced. the sample (say crown and bridges, cast partial dentures, 4. Torsion or twisting is another type of loading, etc.) is prepared in a transparent optically isotropic producing complex stresses. The endodontic files are material. Plane polarized light is passed while twisted through a handle. The angle of twist θ is (stressing) loading and viewed through a Nicol Prism directly proportional to the torsional moment. analyzer. Isochromatic fringes representing constant Graphs of torsional moments against the angle of stresses, obtained in different concentrations are analysed rotation, for files of different numbers, are represented to get the stress distributions and properties. The finite as shown. Twisting, above the corresponding elastic element analysis method is used for this study. limit angle, should not be done to prevent permanent The flexure strength, rather than diametral tensile damage (Fig. 2.14). stress testing, gives more realistic values, as it involves 5. Tear strength: It is the minimum force required to complex stresses and real clinical situations. initiate tearing of a crescent or trouser shaped specimen of unit thickness, with a right-angled V- Applications notch. 1. For stability of a (cantilever) bridge, the deflection δ High tear strength is required for impression should be small for large dynamic loads. Hence, the materials to use in thin sections (thick impressions get bridge is to be designed, such that: distorted while setting or cooling, by relaxation of l, the distance between the abutments should be internal stresses). Agar-agar and alginates have low tear small (i.e. short span bridges) strengths about 900 gm/cm and 700 gm/cm, respectively. b, the breadth, must be large Elastomers have higher tear strengths about 2000– d, thickness, must be large (d3 is more effective). 7000 gm/cm and hence can be used in thinner sections. Figs 2.12a to c: (a) Three-point bending test, (b) stress distribution by photoelastic method, (c) example: Crown and bridge t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 29 Figs 2.13a and b: (a) Bending moment and angular deflection for endodontic reamers, (b) Ni-Ti flexible file DYNAMIC FORCES Dental restorations and many oral appliances, like dentures, crowns, bridges, cast partial dentures, cements, etc. are subjected to large dynamic impact forces, which are more damaging, than the applied static forces. Dynamic mechanical properties, even though depending on the static properties, sometimes are measured by different methods. Properties measured are the dynamic modulus of elasticity, resilience, Poison's ratio, etc., by different techniques for brittle and resilient materials. Dynamic modulus of elasticity is defined as Stress ED = for small cyclic deformations by a given Strain stress at a certain frequency. Fig. 2.14: Torsion moment and angular rotation graph for This is measured by forced oscillations technique, endodontic files, sizes 15 to 60 using a vibrating yoke of definite mass striking on a sample at a certain frequency. The ultrasonic method involves measuring its velocity (v) in a sample of density d, using piezoelectric transducers, (E D=v 2d). This method is very suitable for viscoelastic materials like acrylics, silver amalgam, dentin, athletic mouth protectors, etc. The testing also can be done, after Fig. 2.15: Tear strength testing samples of thickness t cm. Tear thermal cycling between (5°C and 50°C) to find the strength is expressed as T = F/t gm/cm variations. Elastomeric materials also can be tested by Polysulphides have higher tear strengths but greater this non-destructive method. softness or flexibility (strain in compression). Polyethers The cyclic stretching method can be used to find the have lower tear strengths 1800–4800 gm/cm and lower dynamic modulus of resilience (RD) of elastomers which flexibility (or high stiffness) (Table 3.17). is the ratio of, RD = energy lost/energy applied. t.me/Dr_Mouayyad_AlbtousH  30 Science of Dental Materials with Clinical Applications IMPACT STRENGTHS FATIGUE Impact strength is defined as the energy required to When a material is loaded repeatedly (cyclically) several fracture a material sample by dynamic impact forces, times it can undergo fracture by stresses even at much i.e. by a sudden blow. lower values than the proportional limits. Separation In the Charpy testing method, a rectangular sample of atoms (fracture) takes palce progressively in cyclic with a V-shaped notch is rigidly clamped at its ends, loading. This progressive fracture under repeated and hit by a wedge shaped heavy bob of a swinging loading is known as fatigue. To fracture a material by pendulum. lower stresses, it should be applied repeatedly number In the Izod testing method, U-notched rectangular of times. The fracture strength when plotted against shaped specimen, rigidly clamped at one end, is the number of cyclic operations shows a minimum suddenly hit, at the other end by the bob of a swinging stress required to be applied infinite number of times. pendulum. Impact strength is calculated by the energy This endurance limit is defined as the minimum stress lost (by the pendulum) or energy absorbed per unit area required to fracture or maximum stress which cannot of the specimen to fracture. Impact strength is measured fracture the material even if applied infinite number in kg cm or joules (Fig. 2.16). of times. Fatigue strength can be defined as the maximum stress which is resisted when it is cyclically Applications applied for a definite number of times (Fig. 2.17). 1. Denture base materials should have high impact Factors which decrease the fatigue strengths or strengths to protect it from fractures by dynamic endurance limits and life of restorations are: masticating forces, accidental falls or trauma. The 1. Surface flaws (rough surface) which initiate fracture, PMMA denture base resin has low impact strengths due to surface cracks of about 0.25 Joules, which is its main drawback. 2. Internal flaws (microcracks) When it is modified by butadiene rubber, the 3. Solvents entering the systems (resins, ceramics) impact strength becomes more than double, about 4. Brittleness of materials 0.6 Joules. 5. Work hardened conditions 2. Silver amalgam cavity design: The energy required 6. Dynamic forces. to fracture a material by impact forces is directly proportional to the modulus of resilience (R) and Applications volume of the restoration V. It is estimated that the biting forces, on the average Hence, resistance to fracture is directly proportional are applied about 3,00,000 times per year. Acrylic to V × R, i.e. = KVR where K is a constant of geometric denture resin has fatigue strength about 17 MPa configuration or cavity design factor. which can be applied about 1.5 million times. That is This shows that the cavity designed, should have the lifetime is about 5 years for this load. a large volume and no thin sections (ledges) on the Gold alloy restorations have higher fatigue strength occlusal surface. However, at present, cavity prepara- and can withstand about 1 million to 25 million tion is minimized as silver amalgam posterior flexures, and can have life (service) time around 5–50 composite resins gradually replace. years or more. Fig. 2.16: Impact strength testing, samples t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 31 2. Indentation methods Brinnel’s, Rockwell's and Brale methods Micro Vicker’s and Knoop’s hardness—diamond point hardness. 3. Penetration methods: Shore-A and Barcol's duro- meters. 1. Moh’s Scratch Tests (Refer to page 349) Scratching one ore by another ore: Moh could compare the hardnesses of various mineral ores available and arranged them according to numbers 1 for softest talc and 10 for hardest diamond. Fig. 2.17: Fatigue and failure stress—number of cycles Hardness of tooth enamel lies in between 5 and 6. As the method was inaccurate and could not be used Dental instruments sometimes undergo fracture, for many materials, Beirbaum modified the method suddenly, due to fatigue. by scratching a sample by a sharp hard indentor with definite loads and measuring the depths and width’s of scratches. This method is now obsolete. SURFACE HARDNESS It can be defined as the resistance offered by the 2. Indentation Methods surface of a material to scratching, abrasion, Brinnel’s Hardness Test (BHN) indentation or penetrations. Theoretically, any method of measurement should involve only the surface layer A hardened steel or tungsten carbide ball of diameter of atomic thickness, which of course is not practicable. D mm is pressed on the flat surface of a sample with a In micro-hardness testing methods, attempts have been definite load, P, for 30 sec. The diameter of the indenta- made to reduce the depth or area of indentations to tion, ‘d‘ mm is measured with the help of calibrated minimum values and also to get the hardness of eye piece of a traveling microscope. Brinnel hardness localised small sites of particular phases of alloys. number is computed by formula (force/unit area) For higher abrasive resistance, materials should have 2P BHN = higher surface hardnesses and surface integrities. As πD [D–(D 2 – d2 )½ ] diamond has the highest surface hardness (>7000 KHN) diamond instruments are used for cutting, abrading The applied load can be varied according to the or finishing of all other materials without any attrition. hardness of the materials (Fig. 2.18). For measuring the hardness of materials used in The surface hardness of a material depends upon dentistry, small Baby-Brinnel testing instrument 1. Atomic binding forces having D = 1.6 mm and P = 123 kg is used. 2. The composition of phases of cast alloys 3. Work hardened and annealed conditions Advantage 4. Heat treatments This method is quite simple and certain relationships 5. Proportional limits and ultimate strengths between BHN and proportional limit or ultimate tensile 6. Modulus of elasticity and resilience strengths of gold alloys have been found. 7. Ductility and malleabilities. But there are no exact relationships (or proportionali- ties) between the mechanical properties and surface hardnesses. Certain relationships established between surface hardness and ultimate tensile strengths for semi ductile materials, are also not reliable. METHODS OF MEASUREMENTS OF SURFACE HARDNESS These involve the principles of scratching (abrasion), indentation, penetration and elastic rebounds 1. Moh's scratch test and Bierbaum’s modifications Fig. 2.18: Brinnel's testing method t.me/Dr_Mouayyad_AlbtousH  32 Science of Dental Materials with Clinical Applications Disadvantages This method cannot be used for: Brittle materials like ceramics, gypsum products, dental cements, etc. Elastically recovering materials, as ‘d’ decreases on removal of indentor Small sites of different phases of alloys Getting accurate real values of the hardness of surfaces, as the depth or area of indentation is quite large Rubber-like materials like elastomers, hydrocolloids. Some BHN values (approximate) Fig. 2.19: Rockwell's testing method Direct filling gold foil = 24 BHN and after condensation = 68 BHN, Gold alloys type I (45), Type II (95), Type Small sites of different phases of alloys III-H (120), Type IV-H (220), Cr-Co alloys. 265, etc. in Getting accurate real hardness of surface, as depth BHN (kg/mm2) (refer to page 350). of indentation is quite large. Rockwell’s Hardness Test Some RHN values (approximate) A hardened steel ball of 12.7 mm diameter or a conical Gypsum products: Type III stone = 60 RHN diamond point indentor is held on the surface under a Type IV die stone = 80 RHN minor load of 3 kg. Then the major load of 30 kg is Type V HE, HS die stone = 90 RHN applied for 10 min and the depth 'a' of indentation can be directly measured with a micrometer dial gauge. Microsurface Hardness: RHN is calculated by a certain formula or read on the Vicker’s Diamond Pyramid Test calibrated scales. (For different applied loads P = 60, 90, 120, 150 kg, different scales are referred.) The earlier explained methods do not give the real If b is the depth of indentation left after 10 minutes surface hardness. As the depths of indentations are of removal of the major load, the percentage elastic more, the resistance offered has the influence of internal lattice structures and properties. To get more realistic a–b (Fig. 2.19) recovery = ×100. values, the depths of indentation must be reduced to a a minimum. This is done by using, diamond pyramids Advantages of large angles and measuring the diagonals of Rockwell’s method is also quite simple and RHN is indentation. directly obtained from different scales (for different Vicker used a square-based diamond pyramid of a loads), i.e. RA, RB, RC, RD, RE, RF, RG, etc. large angle 136°. The metal work surface has to be well Can be used for hard and brittle materials polished. Indentor is held for 10 seconds on this surface Can also be used for ductile materials with definite loads, P (5 to 120 kg) and the average Can be used for comparing hardness of elastomers, diagonal d = d1 + d2 is measured in mm and k is a rubbers, etc. constant second. Percentage elastic recovery can be found If the angle of indentor is α (Fig. 2.20a). α 2P Sin Disadvantages: Cannot be used for: DPH = VHN = 2 = P kg/mm 2 Very hard base metal alloys d2 Kd2 Figs 2.20a and b: (a) Vicker's diamond point indentor, (b) Knoop's diamond point indentor t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 33 Advantages Some KHN values (approximate) VHN can be directly found from tables for different Tooth enamel = 340 KHN, dentin = 68 KHN, cementum diagonals ‘d’ and for different loads. = 40 KHN and hard calculus = 80 KHN, heat cure As the depth of indentation is negligible, more acrylics = 18–20 KHN, diamond = 7,000–10,000 KHN, realistic values are obtained. silica/quartz = 800 to 820 KHN. Can be used for very hard (base-metal alloys) and Note (i): This is the most common method, for testing very brittle (ceramic) materials. almost all materials used in dentistry, approximate Can be used for small sites of metallurgical phases values are given in separate tables (refer to Appendix, of alloys. page 349). Disadvantages Note (ii): Comparison of surface hardnesses of different Elaborate methods to obtain polished alloy surfaces materials should be done only with reference to During polishing, work hardening taking place is to particular testing method and the load-scales. be taken care off by annealing Cannot be used for elastically recovering materials 3. Penetration Method: and elastomers (refer to pages 350 and 351). Barcol Shore-A Durometer Some VHN values (approximate) The instrument consists of a blunt pointed stainless- steel indenter of 0.8 mm tip diameter, tapering to a Tooth enamel = 300 VHN cylinder of 1.6 mm. This is connected through a lever Tooth dentin = 60 VHN to an adjustable loading spring and a pointer, moving Titanium alloys = 125–350 VHN on a scale. The scale is graduated from 100 to 0. The Noble metal alloys (N) = 175–400 VHN spring load can be adjusted so that when the depth Nickel-Chromium alloys = 210–380 VHN of penetration is maximum (softest material) pointer Cobalt-Chromium alloys = 300–465 VHN reads 0 and when there is no penetration (hardest Porcelain = 400–700 VHN material) it reads 100 (Fig. 2.21). Alumina (Al2O3-recrystallised) = 1800 VHN A sample of definite thickness say, 5 cm, is prepared Microsurface hardness; Knoop’s method—KHN: and allowed to harden completely. The durometer A rhombic based diamond point indentor with indenter is placed on it, for some time until the opposite sides at 130° and 172° 30' is used under viscoelastic material shows maximum recovery and different loads (P = 10 kg,......... 100 kg). Indentor is hardness number is noted in Table 2.3 where (l = light, held under the load for 10 min. time, on the highly m = medium, h = high, p = putty consistencies). polished surface of the material. These form a rhombic indentation of negligible depth. The major diagonal ‘l’ is measured, under scattered light, after the indentor is removed (Fig. 2.20b). P kg KHN = , where K is a constant Kl 2 mm 2 Advantages KHN values can readily be found from reference tables Can be used for very hard and brittle materials Due to negligible depth more realistic values are obtained Alloys having elastic recoveries can be tested. This is because, when the indentor is removed, the elastic recovery, contraction takes place, along the minor diagonal only, leaving the major diagonal unaffected. Can be used for small sites of alloy phases. Disadvantages Very high polishing needed, may cause work hardening which requires annealing. Rubber-like elastomers cannot be tested. Fig. 2.21: Barcol Shore-A Durometer t.me/Dr_Mouayyad_AlbtousH  34 Science of Dental Materials with Clinical Applications Table 2.3 Consistencies and hardness of elastomers Material Consistencies Shore-A hardness no. 1. Elastomer Polysulphides (l, m, h) 20, 30, 35 respectively 2. Elastomer Add. polysilicones (l, m, h, p) 35, 50, 60, 70 respectively 3. Elastomer Polyethers, (l, m, m + thinner, h) 38, 48, 43, 55 respectively 4. Resilient liners...................................... 45–85 5. Maxillofacial polysilicones....................................... 25 6. Polyethylene, polyvinyl acetate........................................ 65 l = light, m = medium, h = heavy, p = putty consistencies Applications in dentistry materials (e.g. water molecules and glass surface Abrasive resistance of the surfaces, mostly depend molecules, adhesives). Due to cohesion, the molecules upon the surface integrity and hardness. Cutting, well inside a liquid, are attracted equally by nearby abrasion, polishing, finishing, etc. are the various molecules from all directions. The resultant force procedures, involved in the finishing of fabricated oral becomes zero (energy becomes minimum) and the appliances, preparation of tooth cavities, restorative molecules are ‘free’ to move, within the liquid. procedures, maintenance of oral hygiene, etc. Even However, the molecules on the free surface, are acted though tooth enamel is the hardest part of the body, by unbalanced cohesive forces towards the interior, with a surface hardness around 340 KHN, it undergoes tending to retain a minimum number of molecules on attrition by harder foods, opposing metallic, and the surface or minimum surface area. This surface ceramic appliances, and chemical attacks. tension causes the liquid surface to act like a stretched 1. Suitable restorative materials of same SH as enamel, elastic membrane, e.g. horizontal liquid surfaces, should be chosen, to avoid abrasion of tooth or spherical—small liquid drops, raindrops, mercury material. drops, etc. have a minimum area of surface. 2. Diamond points and carbide burs are used for cutting Surface tension is defined as the unbalanced force tooth enamel and hardened steel burs for dentin acting on the free surface of a liquid, tending to reduce the surface area to a minimum. It is measured by force preparations (SH = 68 KHN). acting on unit length of a straight-line imagined on 3. Dentifrices should have mild abrasives and not hard the free surface of a liquid, stretching the molecules levigated alumina, silica, etc. Prophy pastes should apart. It is measured in dynes/cm or Newton/meter. contain harder abrasives, to remove the hard calculus deposits. Factors affecting surface tension 4. Suitable coarse and then finer abrasives are used in Nature of the liquid molecules, i.e. the cohesion succession for obtaining a smooth surface of oral between the molecules. appliances like crown and bridges, cast partial For example: Water at 20°C = 72.8 dynes/cm, Benzene dentures, acrylic dentures, etc. = 29 dynes/cm, Mercury = 465 dynes/cm, at 20°C, 5. Base metal alloys are finished with the hard diamond Saliva = 40–45 dynes/cm at 37°C, Blood = 50–54 points, discs or carbide burs, and finally by electro- dynes/cm at 37°C polishing. Temperature: ST decreases as temperature rises. 6. Abrasive resistance of cast and die (gypsum Water has surface tensions 76, 72, 68, 59, dynes/cm, products) materials are increased by using surface at 0°C, 20°C, 50°C and 100°C, respectively. hardeners, or electroforming methods. Chemical impurities: Which decrease surface tension are known as surfactants, detergents, wetting agents or debubblizers, e.g. soap solutions or detergents SURFACE PHENOMENA OF LIQUIDS like sodium lauryl sulphate, sodium stearate, sodium laureates, etc. having –COONa hydrophilic groups. COHESION (SURFACE TENSION) AND These deplete the number of water molecules on ADHESION (WETTING) the free surface and reduce surface tension and Cohesion is the attraction between the molecules of angle of contacts. One in 5,000 parts, i.e. 0.02% of same substance (e.g. molecules of water) and adhesion sodium laureate in water decreases surface tension is the attraction between the molecules of different from 72 to 35 dynes/cm (1 N/m = 1000 dynes/cm). t.me/Dr_Mouayyad_AlbtousH  General Properties of Matter 35 SURFACE ENERGY The wax patterns are coated with surfactants or The surface molecules have higher energies due to washed with detergents, leaving a thin film of unbalanced forces. Surface energy is defined as the wetting agent on the surface. work required to bring the molecules from inside, to Some elastomeric impression materials are form a new surface of unit area or the energy of hydrophobic, and air bubbles get collected, when the molecules in unit area of the free surface. It is expressed stone cast is poured. Hence, hydrophilic elastomers in erg/cm2 which has the same magnitude (since ergs/ are preferred (page 342). cm2 = dynes/cm) of surface tension. SE for water at 20oC Fluxes are applied on base-metals soldering surface = 72.8 ergs/cm2. to remove the oxide layers (fluoride fluxes for stainless steel) by reducing or dissolving the oxides Surface energy of tooth enamel = 84 ergs/cm2 (0.084 J/m2) and help to wet the surface with the solder liquid. ANGLE OF CONTACT AND WETTING Fluoride applications on tooth enamel reduce the surface energy, wetting, and collection of debris. When a liquid is placed on a solid surface, it spreads out Surfactants are included in type V die stones for better and wets the surface. The surface energies of solid, induce wetting and decrease W/P to improve strength. secondary bonding between the solid and liquid molecules. The surface tension of liquid, oppose the ADHESION spreading, an area of the liquid surface should be The attraction between molecules of different materials minimum (i.e. it tries to retain spherical shape). These causes adhesion. The secondary bonding causes weaker reduce the wetting and form contact angle of more than 0°. adhesion. The primary bonding causes stronger The angle of contact (θ θ ) is the angle made by the adhesion. When the molecules of a gas or liquid tangent, drawn to the liquid surface at the point of approach the surfaces, they are attracted by secondary contact with the solid surface, the angle being bonding and get adsorbed on the surface. If these atoms measured within the liquid (Fig. 2.22). then chemically combine with the surface molecules, it Smaller θ, shows greater wetting, due to the higher is known as chemisorption. Oxygen, hydrogen, etc. surface energy of solid and lower ST of liquid, and a gases get adsorbed in molten gold alloy liquid during clean surface. casting procedures. Passivation of base metals by Cr, Wetting of solid surface by the liquid is more, when Al, Ti are by chemisorption. The contact angle is small The weak secondary bonding, cannot hold two solid The surface energy of solid is more surfaces together since very few molecules come close The surface tension of the liquid is less to each other (less than 0.0007 microns). If a drop of Surface is clean without oxide layer or contaminations. water/liquid is placed in between the two glass slides, the wetting causes strong adhesion. Applications Waxes are hydrophobic and do not wet when wax Conditions for strong adhesions are patterns of dentures, inlays, crowns, etc. are invested, Greater wetting, i.e. smaller contact angle causing collection of air-bubbles on their surfaces. The higher surface energy of adherend Fig. 2.22: Angle of contacts of water and mercury with different solid surfaces t.me/Dr_Mouayyad_AlbtousH  36 Science of Dental Materials with Clinical Applications The lower surface tension of adhesive mucosa (θ2) cause adhesion and retention of the Clean surfaces, without oxide layers denture to the oral tissues with force, The larger area of surfaces (by acid etching) AT Chemisorption of adhesive (dentin bonding poly- F= (cos θ1 + cos θ 2 ) dynes t carboxylate cements). Where A = area of the surface of the denture, t = the Applications thickness of saliva film. For better retention, area A 1. Capillarity: Penetration of liquids through must be large, better close fitting (i.e. small thickness capillaries is due to the creation of negative pres