Basic physical properties of materials. Test methods for materials in dental material science. 2nd lecture Dr. Kinga Turzó 11th of September 2012
Subject of the lecture: Basic properties (bulk and surface) of materials: Overview of the: Solid state Interatomic bonds Atomic structure Physical properties of materials: Mechanical Thermal Bulk properties Electrical and electrochemical Surface properties of materials: Surface energy Wetting Methods of testing materials in dental material science.
Bulk properties of materials Solid state: Their constituent atoms are held together by strong/primary interatomic forces/bonds (Pauling, 1960). Interatomic primary bonds: Ionic Covalent Metallic Interatomic secondary bonds: Hydrogen bonding van der Waals forces The electronic and atomic structures, and almost all the physical properties of solids depend on the nature and strength of the interatomic bonds.
Overview of interatomic primary(strong) bonds Ionic bonding (A) • Attraction of + and – charges, characterized by electron transfer from one element (+, cation, metal) to another (-, anion, nonmetal) • Examples: Na+Cl-, NaF, MgCl2, certain crystalline phases of some dental materials (gypsum and phosphate cements) • Poor electrical conductors (tightly bound e-), low chemical reactivity (low overall energy state) Covalent bonding (B) • Characterized by electron sharing and very precise bond orientations, particular atomic arrangement or crystal structure • Examples: H2, diamond, silicon, many organic compounds (dental resins) • Poor electrical conductors (localization of the valence electrons in the covalent bond) Metallic bonding (C) • Characterized by electron sharing and formation of „gas” of electrons that bonds the atoms (which become positively charged because of the electron gas formation) together in the lattice. • Examples: metal crystals, gold, cobalt • The nonlocalized bonds permit plastic deformation and the electron gas accounts for the chemical reactivity and thermal conductivity of metallic systems
Overview of interatomic secondary(weak) bonds Hydrogen bonding (H...O) • Shared electrons (covalent bond), forming a permanent dipole, an asymmetric molecule (e.g.: water molecule) • Associated with the positive charge of hydrogen caused by polarization • Due to this polarity: intermolecular reactions in many organic compounds (sorption of water by synthetic dental resins) Van der Waals forces • These forces form the basis of a dipole attraction • E.g.: in a symmetric molecule (inert gas) the electron field is constantly fluctuating, its charge becomes momentarily positive and negative • The fluctuating dipole will attract other similar dipoles • Quite week Strength: 3-10% of the primary C_C covalent bond Significantly influence the properties of some solids, especially polymers
Atomic structure of dental materialsCrystalline structure: The atoms are bonded by either primary or secondary forces. In the solid state, they combine in a manner that ensures minimal internal energy (e.g.: Na+Cl-). They do not simply form only pairs, all the positively charged ions attract all of the negatively charged ions. As a result they form a regularly spaced configuration: space lattice or crystal (14 possible lattice types).Noncrystalline structure: Some of dental materials can occur in this form, for example: waxes may solidify as amorphous materials such that the molecules are distributed randomly. Glass is also considered to be a noncrystalline solid, since its atoms tend to develop a short range order instead of a long-range order characteristic of crystalline solids. The internal energy is not so low as for crystalline arrangements. They do not have a definite melting temperature, but rather they gradually soften as the temperature is raised. Synthetic dental resins are examples of materials that often have glassy structures.
Space lattice typesSingle cells of cubic space lattices: Other simple lattice types of dental interest: Simple cubic Body centered cubic Face- centered cubic
Atomic structure of metals Metallic bonding in the solid state B Alloys: mixtures or solutions of different metals A 85% of all metals have one of the crystal structures shown in FigureA: Face-centered cubic (FCC) C DB: Full size atoms in FCCC: Hexagonal close packed (HCP)D: Body-centered cubic (BCC)
Atomic structure of ceramicsCombination of ionic orcovalent bondingTightly packedstructures, but withspecial requirements forbonding, such as fourfoldcoordination for covalentsolids and chargeneutrality for ionic solidsMore open and complexcrystal structures Different physical Carbon, two different crystal structures: properties: hardness, A – diamond (cubic, tetrahedral coordination) color, electrical B – graphite (hexagonal, t.arrang. is distorted into conductivity, density a nearly flat sheet, stacking of these sheets)
Atomic structure of polymers Constituent atoms are usually carbon, joined in a linear chainlike structure by covalent bonds. The bonds within the chain require two of the valence electrons of each atom, leaving the other two bonds available for adding a great variety of atoms (H, etc.), molecules, functional groups, etc. Based on this organization: 2 classes of polymers: Thermoplastic polymers: the basic chains have little or no branching, can be melted and remelted without a basic change in structure (amorphous th.p.p: van der Waals and hydrogen bonding holds the chain together). Thermosetting polymers: if side chains are present and form links (covalent) between chains, a three-dimensional network structure is formed; strong structures and once formed by heating will not melt uniformly on reheating.
Mechanical properties Strength is one of the most important properties of a material. Elastic behavior, Hooke’s law (1678), tensile test: Load – A solid material subjected (Newtons) to a tensile (distraction) force would extend in the direction of traction by an amount that was Extension (mm) proportional to the load. – Most solids behave like a spring if the loads are not to great.
Stress and strain.Tension and compression. The extension for a given load varies withgeometry of the specimen as well as with itscomposition. To resolve this confusion, the load anddeformation can be normalized: STRESS = Normalized load σ = F/A (unit: N/m2 or Pa) STRAIN = Normalized deformation ε = ∆l/lo (measured applying reference marks) In tension and compression the area (A)supporting the load is perpendicular to theloading direction and the change in length isparallel to the original length.
Example:Stresses induced in a three-unit bridge (A) and in atwo-unit cantilever bridge (B) by a flexural force (P).• Tensile and compression stress is produced.• In case of the three-unit bridge the tensile stress develops on the gingival side.• In case of the cantilever bridge the tensile stress develops on the occlusal side.• These areas of tension represent potential fracture initiation sites!
Shear stress, elastic constants For cases of shear the applied load is parallel to the area supporting it - shear stress, τ − and the dimensional change is perpendicular to the reference dimension - shear strain, γ Quantitative expression of Hooke’s law, using the definitions of stress and strain:σ=E.ε for tension or compressionτ=G.γ for shearE – tensile constant (Young’s modulus)G – shear modulus Represent inherent properties of the material (since all geometric influences have been removed)
Explanation σ = F/A STRESS = Normalized load, force/area SI unit: [N/m2] or [Pa] ε = ∆l/lo STRAIN = Normalized deformation no SI unit (m/m), usually given in % If, F is proportional with ∆l (based on Hooke’s law), and A (area) is constant and also lo (initial length of the solid) is constant, than σ will be also proportional with ε! The factor of proportionality is called tensile constant (E). The same explanation is valid for the shear modulus (G).
Atomic model illustrating elastic sheardeformation (A) and plastic deformation (B)• Bonded two material system: material A – white atoms, material B – shaded atoms• Figure A: the shear force is applied far from the interface, as it increases produces anelastic or reversible shear strength, that would return to zero if the shear force isremoved.• Figure B: if the shear force is closer to the interface and increased sufficiently, apermanent or plastic deformation will be produced.
Stress-strain plot Measurement of stress and strain on an object being stretched Elastic deformation region: stress is proportional to strain (proportional limit) Plastic deformation region: stress and strain are no longer proportional Ultimate tensile strength: the maximum stress just before the object breaks Percent elongation: the total amount that the object stretches (the total strain), the sum of the elastic and the plastic deformation.
Ductility, brittle fracture, toughness,resilience, fatigue Ductile: materials that experience a large amount of plastic behavior or permanent deformation. Brittle: materials that undergo little or no plastic behavior; in real materials, elastic behavior does not persist indefinitely; microscopic defects will begin to grow under the applied stress, and the specimen will fail suddenly. Toughness: the entire area under the stress- strain curve; a measure of the energy required to fracture the material. Resilience: the area under only the elastic region of the stress-strain curve; a measure of the ability of the material to store elastic energy. Fatigue: when materials are subjected to cycles of loading and unloading (e.g.: mastication) they may fail due to fatigue stresses below the ultimate tensile strength.
Stress-strain plot for a stainless steel orthodonticwire that has been subjected to tensionPoints: measured valuesPL: proportional limitYS: yield strength at a 0.2%strain offset from the origin (O)UTS: ultimate tensile strengthE: Young or elastic modulus iscalculated from the slope ofthe elastic region
Mechanical properties of Ti and its alloys (ASTMF136, 1992 and Davison et al., 1994)Properties Grade 1 Grade 2 Grade 3 Grade 4 Ti6Al4V Ti13Nb13ZrTensile strength (MPa) 240 345 450 550 860 1030Yield strength (0.2% 170 275 380 485 795 900offset) (MPa)Elongation (%) 24 20 18 15 10 15Reduction of area (%) 30 30 30 25 25 45
Comparison of dentin and enamel Dentin is capable of sustaining significant plastic deformation (ultimate compressive strength CS = 234 MPa) under compressive loading before it fractures. Dentin is more flexible and tougher than enamel. Elastic modulus E for enamel is ~three times greater than that of dentin. Enamel is stiffer and more brittle material than dentin.
Elastic modulus of different biomaterialscompared to bone
Hardness Hardness: the resistance of a material to indentation or penetration (mineralogy: ability to resist scratching).Properties that arerelated to the hardnessof a material: – strength – proportional limit – ductilityMethods for measuringhardness: – Brinell, Vickers, Knoop, Rockwell, Shore A
Thermal propertiesThe rate at which heat flows through amaterial is expressed as: Thermal conductivity: kIs a measure of the speed at which heattravels through a given thickness ofmaterial, when one side of the material ismaintained at a constant temperature thatis 1oC higher than the other side.Unit: [Cal / cm . sec . oC] Thermal diffusivity: hTells us how rapidly the interior of thematerial will reach the temperature of theexterior.h = k/(Cp . ρ) , [mm2/ sec]Cp – heat capacity, ρ -density
Thermal propertiesThere are several situations indentistry in which the thermalexpansion of materials is important: Linear coefficient of thermalexpansion = change in length perunit original length for 1oCtemperature changeSome restorative materials havecoefficient of thermal expansion thatare markedly different from toothstructure.In such cases, temperaturefluctuations that occur in the mouthcan cause percolation at the tooth-restoration interface as the Relative thermal expansions of severalrestoration contracts and expands. restorative materials and tooth structure.
Electrical and electrochemicalproperties - If there is a difference between the electrode Electrode potentials: potentials of two metals in contact with the - An electrochemical series is a listing of same solution (Au, Al), an electrolytic cell may elements according to their tendency to develop, discomfort in the mouth! gain or lose electrons in solution. - The series is referenced versus the potential of a standard Hydrogen electrode, which is arbitrarily assigned a value of 0 Volts. - If the elements are listed according to their tendency to lose electrons: oxidation potentials, if they are listed according to the tendency of their ions to gain electrons, reduction potentials.Standard electrode potential: reduction potentials at 25oC and 1 atmosphere of pressure.Examples: - metals with a large positive electrode potential (platinum, gold) are more resistant to oxidation and corrosion in the oral cavity.
Electrical and electrochemicalproperties Electrical resistivity: ρ : measures the resistance of a material to the flow of an electrical current. Relationship: R = ρ . (l/A) R -resistance, l -length, A -area Examples: Metallic restorative materials: low resistivity (Cu: 1.7x10-6 Ω m), discomfort for the pulp. Cements: good insulating properties (zinc phosphate cement: 2x105 Ω m).
Surface properties of materials Important question in biocompatibilty: - how the device or material „transduces” its structural makeup to direct or influence the response of proteins, cells and organisms? This transduction occurs through the surface structure–the body „reads” the surface structure and responds. The surface region is known to be uniquely reactive and is different from the bulk. Surface energy or tension: the increase in Adhesion: attraction (secondary energy per unit area of surface. bonding) across the interface between The energy at the surface of a solid is unlike molecules (silver, gold adsorbs greater than that of its interior oxygen). (interatomic distances are equal, minimal Chemisorption: primary bonding is energy, while at surface the outermost involved (chemical bond is formed atoms are not equally attracted in all between the adhesive and the directions). adherent).
Surface properties of materials Important parameters to Contact angle (θ) of wetting: biological reaction: - roughness, wettability, surface - measuring method of the extent to mobility, chemical composition, which an adhesive wets the surface. crystallinity and heterogeneity. Wetting: to produce adhesion - when θ = 0ο , the liquid will spread between two solid surfaces, a completely over the surface liquid most flow easily over the - large θ formed by poor wetting entire surface and adhere to the solid. The ability of an adhesive to wet the surface is influenced by a number of factors: – cleanliness of the surface, etc. Teflon (polytetrafluoroethylene, commercial synthetic resin) is often used to prevent the adhesion of films to a surface.
Mechanical requirements The mechanical requirements of dental materials can be different, depending from their function.Example: the masticatory forces applying on dental implants is quite high (500-800 N), therefore their design and mechanical strength has to be adequate for such loading.
Technological requirements The most important aspects of technological suitability are: – easy processing, machining – precise and good elaboration – cheep and economic raw materialOther practical aspects:- easy sterilization- good radiographic image- simple surgery/implantation and prosthodontic procedure- easy extraction if the implant fails
Testing methods in dental materialsscienceBiomechanical tests Methods measuring hardness Finite element analysis Birefraction analysis Load compression test Testing osseointegration between bone and dental implant: – Pull-out – Push-out tests – TorqueMethods characterizing dental material surfaces
Methods for measuring hardnessBrinell hardness number:The indenter is a small hardened steel ball(Ø1.6 mm), which is forced into the surfaceunder a specified load (123 N, 30 s). Leavesa round dent in the material.Vickers hardness number:The indenter is a square pyramid-shapeddiamond (136o); hardness is determined bymeasuring the diagonals of the square and Rockwell hardness:taking the average of the two dimensions. Used mostly for determining the hardnesses ofKnoop hardness: steels; different hardened steel balls or diamond cones and different loads.the indenter is also made of a diamond, butone diagonal is longer than the other; only Shore A hardness :the long diagonal is measured. Used to measure the hardness of rubbers and soft plastics. Scale between 0 and 100 units: 0 complete penetration, 100 no penetration.
Finite-element analysis for determination of stresses indental implants The implant is divided in small, grid-like structures/elements. Elastic (Young) modulus (E): Al2O3 conical Implants: 380 000 MPa and step-form Cortical bone: 20 000 MPa implant Trabecular bone: 2 000 MPa
Finite element testing of the design of dental implants with different connectionsExternal hexagonal connection: Ø 3,75 mm Tube in tube connection: Ø 3,5 mmStress: 32 N·cm Stress: 15 N·cmLoading: 300 N/30° Loading: 300 N/30° Merkle H. et al. 1998
Mechanical testingDiametral compression test:for materials that exhibit elastic deformationand little or no plastic deformation. Controlled load-deflection (stress-strain) test: The load frame is much stiffer and stronger than the specimen; the cross head is moved up or down by a screw or a hydraulic piston; the loadCompressive force (P) applied along the disk, a cell monitors the force being applied.tensile fracture is produced. Compression testing: the direction of theTensile strength depends from the fracture load (P), cross–head is reversed.disk diameter (D) and the thickness (t). Torque test, push-out test: for implant tests
Push-out and torque tests Methods for the investigation of the osseointegration of implants (LLoyd Instrument)
Common Methods of CharacterizingBiomaterial Surfaces Depth Spatial Analytical Method Principle Costa analyzed resolution sensitivity Low or high Liquid wetting of surfaces is used toContact angles 3-20 Å 1 mm depending on the $ estimate the surface energy chemistry X-rays wetting cause the emissionESCA 10-250 Å 10-150 µm 0.1 Atom % $$$ of electrons of characteristic energyAuger electron A focused electron beam causes the 50-100 Å 100 Å 0.1 Atom % $$$spectroscopyb emission of Auger electrons Ion bombardment leads to theSIMS 10 Å-1 µmc 100 Å Very high $$$ emission of surface secondary ions IR radiation is adsorbed in excitingFTIR-ATR 1-5 µm 10 µm 1 Mole % $$ molecular vibrations Measurement of the quantumSTM tunneling current between a metal 5Å 1 Å Single atoms $$ tip and a conductive surface Secondary electron emission High, but notSEM caused by a focused electron beam 5Å 40 Å typically $$ quantitative is measured and spatially imaged