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Metals I

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  • 1. Ceramics Lecture 4 January 29, 2009
  • 2. Zirconia Data for HW1 N. Navruz, Phys of metals & metallography 105: 6 (2008) Monoclinic values a = 0.5181 nm b = 0.5200 nm c = 0.5365 nm  = 98.6 ° Tetragonal a = 0.5126 nm c = 0.5206 nm
  • 3. Ceramics in Medicine
    • Historically common in medical industry – glass beakers, slides, thermometers, eyeglasses, etc.
    • Ceramic materials exist in the body
      • Bone and teeth
    • Thus, they are useful in devices and implants
  • 4. Ceramic vs Glass
    • Ceramic : an inorganic, nonmetallic, typically crystalline solid, prepared by application of heat and pressure to a powder
      • Most ceramics are made up of two or more elements.
      • Contain metallic and non-metallic elements, ionic and covalent bonds
    • Glass : (i) An inorganic product of fusion that has cooled to a rigid condition without crystallization; (ii) An amorphous solid
    • Glass-ceramic : Product formed by the controlled crystallization (devitrification) of a glass-forming melt. Consists of two-phases: crystals in a glass matrix.
  • 5. Other Definitions
    • Amorphous :
      • Lacking detectable crystallinity;
      • Possessing only short-range atomic order; also glassy or vitreous
    • Bioactive material : A material that elicits a specific biological response at the interface of the material, (usually) resulting in the formation of a bond between the tissues and the material.
  • 6. Crystalline vs Glassy (Amorphous) Ceramics
    • Crystalline ceramics have long-range order, with components composed of many individually oriented grains.
    • Glassy materials possess only short-range order, and generally do not form individual grains.
    • The distinction is based on x-ray diffraction characteristics.
    • Most of the structural ceramics are crystalline and oxides.
  • 7. Atomic Bonds
    • Ionic
      • Large differences in electronegativity
      • Non directional strong bonds
    • Covalent
      • Small differences in electronegativity
      • Strong, directional bonds
    • All ionic, all covalent or covalent-ionic bonds possible
    Ceramic Name Melt Point °C % Covalent Char. % Ionic Char. Magnesium Oxide 2798° 27% 73% Aluminum Oxide 2050° 37% 63% Silicon Dioxide 1715° 49% 51% Silicon Nitride 1900° 70% 30% Silicon Carbide 2500° 89% 11%
  • 8. Properties
    • High melting temperature – bond type (ionic-covalent)
    • Low thermal conductivities and thermal expansion coefficients
      • Strong ionic - covalent bonding
      • Imperfections (grain boundaries, pores)
    • High heat capacity and low heat conductance – good thermal insulators
    • Low density
    • High strength, compressive strength usually ten times > tensile
    • Very high elastic modulus (stiffness greater than metals)
    • Very high hardness
    • Brittle – due to ionic bonds
    • Wear resistant – because of high compressive strength and hardness
    • Corrosion resistant and/or unreactive– oxides do not oxidize further
    • High melting point, chemical inertness, high hardness and low fracture strength can make it difficult to make ceramic components
  • 9. Ceramics as Biomaterials
    • Advantages
      • Inert in body (or bioactive in body); chemically inert in many environments
      • High wear resistance (orthopedic & dental applications)
      • High modulus (stiffness) & compressive strength
      • Esthetic for dental applications
    • Disadvantages
      • Brittle (low fracture resistance, flaw tolerance)
      • Low tensile strength (fibers are exception)
      • Poor fatigue resistance (relates to flaw tolerance)
  • 10. Applications
    • Orthopedics:
      • bone plates and screws
      • total & partial hip components (femoral head)
      • coatings (of metal prostheses) for controlled implant/tissue interfacial response
      • space filling of diseased bone
      • vertebral prostheses, vertebra spacers, iliac crest prostheses
    • Dentistry
      • dental restorations (crown and bridge)
      • implant applications (implants, implant coatings, ridge maintenance)
      • orthodontics (brackets)
      • glass ionomercements and adhesives
    • Other
      • inner ear implants (cochlear implants)
      • drug delivery devices
      • ocular implants
      • heart valves
  • 11. Attachment
    • Four types of ceramic-tissue attachment are related to the tissue response to a material
    • Morphological fixation – dense, inert, nonporous ceramics attach by bone (or tissue) growth into surface irregularities, by cementing the device into the tissues, or by press fitting into a defect
    • Biological fixation – porous, inert ceramics attach by bone ingrowth (into pores) resulting in mechanical attachment of bone to material
    • Bioactive fixation – dense, nonporous surface-reactive ceramics attach directly by chemical bonding with bone
    • Resorbable – dense, porous or nonporous resorbable ceramics are slowly replaced by bone
  • 12. Types of Ceramics
    • Nonporous, nearly inert materials are very strong and stiff
    • Porous, inert materials have lower strengths, but are useful as coatings for metallic implants
    • Nonporous, bioactive materials establish bonds with bone tissue
    • Resorbable materials may be porous or nonporous and degrade with time
  • 13. 1. Nonporous, Nearly Inert Ceramics
    • Alumina (Al 2 O 3 ) and Zirconia (ZrO 2 )
      • The two most commonly used structural bioceramics.
      • Primarily used as modular heads on femoral stem hip components.
      • Wear less than metal components, and the wear particles are generally better tolerated.
    • Pyrolytic Carbon
      • Coatings for heart valves, blood contacting applications
  • 14. Processing of Ceramics
    • Compounding
      • Mix and homogenize ingredients into a water based suspension = slurry or, into a solid plastic material containing water called a clay
    • Forming
      • The clay or slurry is made into parts by pressing into mold (sintering). The fine particulates are often fine grained crystals.
    • Drying
      • The formed object is dried, usually at room temperature to the so-called "green" or leathery state.
    • Firing
      • Heat in furnace to drive off remaining water. Typically produces shrinkage, so producing parts that must have tight mechanical tolerance requires care.
      • Porous parts are formed by adding a second phase that decomposes at high temperatures forming the porous structure.
  • 15. Solid State Sintering
    • Sintering is a diffusional process that combines distinct powdered grains below the melting point into one cohesive material
    • Powder particles are pressed together forming a compacted mass of powder particles
      • Powders are milled or ground to produce a fine powder (d ≈ 0.5 – 5.0  m)
        • Smaller grain size = greater strength
    • Powder compact is then heated to allow diffusion to occur and the separated powder particles become fused together
      • Usually T > ½ T m in Kelvin
        • Higher temperature = smaller pore size
    • Final product consists of grains with boundaries containing a mixture
    • of atoms from two separate particles
    • Material also becomes denser as it is sintered
  • 16. Energy Minimization
    • Sintering is driven by a reduction in surface energy
      • Two surfaces are replaced by one grain boundary (s/g to s/s)
      • Atoms diffuse from the grain boundary to the void surface
        • Fast diffusion occurs at grain boundary
      • Voids are filled and the part is more dense with less surface energy
  • 17. Liquid Sintering
    • Heat the compacted powder up just above the eutectic melting point
      • Eutectic melting point is the minimum melting point of a combination of two or more materials
    • On heating a small proportion of the ceramic material melts to form a highly viscous liquid
      • Occurs at the periphery of the particles
    • The liquid draws the ceramic particles together
    • On cooling the viscous phase transforms to either:
      • Glass state (poor high-temp properties)
      • Crystalline state (better high-temp properties)
  • 18. Alumina
    • Al 2 O 3
    • Single crystal alumina referred to as “Sapphire”
    • Most used in polycrystalline from
    • Unique, complex crystal structure
    • Strength increases with decreasing grain size
    • Elastic modulus (E) = 360-380 GPa
    • Low friction and wear properties
      • Good for joint bearings
      • Grain size must be very small, < 4  m
  • 19. Zirconia
    • ZrO 2 (same compound as CZ, but a different crystal)
    • Good mechanical properties
      • Stronger than alumina (2-3 times stronger)
      • Less stiff than alumina
      • Surface of the zirconia can be made smoother than that of an alumina
      • Zirconia-PE wear rates are ½ of alumia-PE wear rates
    • Properties only good for tetragonal crystals
      • Tetragonal form is unstable, may transform to other crystal structure with poor properties
    • Must be stabilized to be useful, much to learn still
  • 20. Fabrication with Al 2 O 3 and ZrO 2
    • Devices are produced by pressing and sintering fine powders at temperatures between 1600 to 1700ºC.
    • High purity alumina used in biomedical applications (>99.5%)
      • Additives such as MgO added (<0.5%) to limit grain growth
    a – Alumina sintered 180 minutes at 1580 °C b – Zirconia sintered 120 minutes at 1400 °C
  • 21. Alumina & Zirconia Applications
    • Orthopedics – femoral head, bone screws and plates
      • Alumina at a bone interface: bone will grow right up to it, but will not grow in
      • Ceramic-UHMWPE contact used in hip and knee replacements
      • Ceramic-ceramics contact also used
      • Problem with stiffness of alumina…
    • Dental restorations – crowns, bridges, brackets
      • Good mechanical and aesthetic properties
  • 22. Elemental Carbon
    • Elemental, non-metal, many forms possible
    • Properties depend on atomic structure
      • Diamond, graphite, fullerenes, etc.
    • Carbons generally have good biocompatibility
    • Forms used in bio-applications
      • Graphite – lubricating properties
      • Diamond-like carbon – hard, wear-resistant
      • Glassy carbon – temp and chem resistant, low strength and poor wear resistance
      • Pyrolytic carbon – wear-resistant, fairly strong, brittle
  • 23. Pyrolytic Carbon
    • Most successful and commonly used form
    • “ pyrolysis” – thermal decomposition
      • Occurs at high temp, with an inert gas (N or He)
      • Instead of “burning,” the carbon “polymerizes” due to the absence of oxygen
    • Often used as a coating material
      • Preforms are coated, then machined and polished before assembly
      • Diamond plated grinders and tools are needed because PyC is very hard
      • Finish can be made very smooth
  • 24. Applications
    • Very good blood-contacting properties
    • Used to coat
      • Heart valve components
      • Stents
    • Compatibility not perfect
      • Anticoagulants needed
      • Blood compatibility not completely understood
    • Other applications
      • Joint components
      • solid PyC parts are possible
  • 25. 2. Porous Ceramics
    • Porous ceramics have very limited properties due to the porosity (reduced solid volume)
      • Generally restricted to non-load bearing applications
        • Coatings for metal or other materials
        • Structural bridge for bone formation
      • Increasing porosity results in
        • Bone ingrowth to fix the component to tissue
        • Decreased mechanical properties
        • Increased surface area (more environmental effects)
      • Pore size is critical to tissue growth & angiogenesis
    • Calcium hydroxyapatite is the most common
      • Converted from coral or animal bones
  • 26. Calcium Hydroxyapatite (HA)
    • Ca 10 (PO 4 ) 6 (OH) 2
    • HA is the primary structural component of bone.
      • consists of Ca 2+ ions surrounded by PO 4 2– and OH – ions.
    HA microstructure
  • 27. HA
    • Gained acceptance as bone substitute
    • Repair of bony defects, repair of periodontal defects, maintenance or augmentation of alveolar ridge, ear implant, eye implant, spine fusion, adjuvant to uncoated implants.
    • Properties
      • Dense HA (properties are similar to enamel – stiffer and stronger than bone)
        • Elastic modulus = 40 – 115 GPa
        • Compressive Strength = 290 MPa
        • Flexure Strength = 140 MPa
      • Porous HA not suitable for high load bearing applications
  • 28. Bioceramic Coating
    • Coatings of hydroxyapatite are often applied to metallic implants (most commonly titanium/titanium alloys and stainless steels) to alter the surface properties.
    • In this manner the body sees hydroxyapatite-type material which it appears more willing to accept.
    • Without the coating the body would see a foreign body and work in such a way as to isolate it from surrounding tissues.
    • To date, the only commercially accepted method of applying hydroxyapatite coatings to metallic implants is plasma spraying.
  • 29. Bone Fillers
    • Hydroxyapatite may be employed in forms such as powders, porous blocks or beads to fill bone defects or voids.
    • These may arise when large sections of bone have had to be removed (e.g. bone cancers) or when bone augmentations are required (e.g maxillofacial reconstructions or dental applications).
    • The bone filler will provide a scaffold and encourage the rapid filling of the void by naturally forming bone and provides an alternative to bone grafts.
    • It will also become part of the bone structure and will reduce healing times
  • 30.
    • Certain types of ceramics have been shown to bond to bone
      • Bioactive glass
      • Bioactive glass-ceramics
      • Bioactive crystalline ceramics and bioactive composites exist also
    • Have relatively high melt temperatures are (1300 – 1450ºC)
    • Can be cast into intricate shapes (in glass form)
    • Can be ground into powders, sized, and used for packing material, etc.
    3. Bioactive Ceramics
  • 31. Glass
    • Structure is isotropic, so the properties are uniform in all directions
    • Brittle
      • No planes of atoms to slip past each other
      • No way to relieve stress
      • Often more brittle than (crystalline) ceramics
    • Typically good electrical and thermal insulators
    • Transparent (amorphous)
    • A supercooled liquid or a solid?
      • Viscosity of water at room temp is ~ 10 -3 Poise
      • Viscosity of a typical glass at room temp >> 10 16 P
  • 32. Glass Processing
    • Completely melting ingredients to a homogeneous liquid and cooling to a homogeneous material.
    • Glasses are most commonly made by rapidly quenching a melt
      • Elements making up the glass material are unable to move into positions that allow them to become crystalline
      • Result is a glass structure – amorphous
    melt
  • 33. Glass Structure
    • Glass-forming oxides
      • e.g., SiO 2 ; B 2 O 3 ; P 2 O 5 ; GeO 2
      • glass-forming network: often the major component
    • Glass-modifying oxides
      • e.g., Na 2 O; CaO; Al 2 O 3 ; TiO 2
      • modify glass network: add positive ions to the structure and break up network
      • minor to major component: alter glass properties (e.g. softening pt)
    • Even when molten, chains not free to move, very viscous
  • 34. Types of Glass
    • Silicate glass (fused silica)
      • SiO 2
      • Each silicon is covalently bonded to 4 oxygen atoms
    • Soda-lime glass
      • 70 wt% SiO 2 ; 15 wt% Na 2 O; 10 wt% CaO
      • Window glass, bottles, etc.
    • Borosilicate glass:
      • Some SiO 2 replaced by B 2 O 3
      • 80 wt% SiO 2 ; 15 wt% B 2 O 3 ; 5 wt% Na 2 O
      • Pyrex glass; cooking and chemical glass ware
  • 35. Glass-Ceramics
    • Composite structure consisting of a matrix of glass in which fine crystals have formed
      • Crystals can commonly be very fine (avg. size < 500 nm)
    • Glass-ceramics are 50 to 99% crystalline
    • The result is a mixture of glass-like and crystalline regions that:
      • Prevents thermal shock
      • Lowers porosity
      • Increases strength
  • 36. Glass-Ceramic Processing
    • Glass with nucleating agent like TiO 2 is formed into the desired shape
    • Nucleating agents aide in the formation of the crystals
      • Barely soluble in the glass
      • Remain in solution at high temperatures
      • Precipitate out at low temperature
      • Act as nuclei for crystal growth at elevated temperatures
    • Conversion takes part in two phases
      • First glass is seeded with nuclei
        • The formed material may be lowered to the nucleating temperature after forming OR
        • It may be lowered to room temperature, then reheated to the nucleating temperature.
      • Second crystals grow around the nuclei
        • Following nucleation the temperature is then raised to the crystal growth temperature
  • 37. Bioactivity
    • Bioactivity is very sensitive to composition
      • Both in glasses and glass-ceramics
      • Less than 60 mol% SiO 2
      • High Na 2 O and CaO content
      • High CaO/P 2 O 5 ratio, minimum 5:1
    • Composition makes the surface highly reactive when it is exposed to an aqueous environment
    Bioactive glass implants (45S5) and matching drill bits used to replace the roots of extracted teeth
  • 38. Bioactive Ceramic Interfacial Reactions
    • When the bioactive glass is immersed in body fluids sodium ions leach from the surface and are replaced by H + through an ion exchange reaction.
      • This produces a silica rich layer
    • An amorphous calcium-phosphate layer is formed on the silica rich layer due to migration of the calcium and phosphate ions from the bulk of glass.
      • Biological moieties such as blood proteins, growth factors and collagen are incorporated into the layer.
    • The amorphous layer crystallizes into carbonate hydroxyapatite (equivalent to natural bone mineral).
    • Body’s tissues are able to attach directly to the crystallized layer
    • Layer grows to be approximately 100-150  m in depth.
    • Occurs within 12-24 hr
    • Cells arrive within 24 to 72 hr and encounter a bonelike surface, complete with organic components
  • 39. Bioactive Applications Glass and Glass-Ceramic
    • A/W Solid Glass-Ceramics (Cerabone®)
      • Vertebral prostheses (for spinal fractures)
      • Vertebral spacers (for lumbar instability)
      • Iliac crest prostheses (restoration after bone graft removal)
    • Solid Bioglass®
      • Douek cochlear implants (100% effective after 10 years vs. 72% failure for metallic and polymeric implants of same type)
    • Particulate Bioglass®
      • PerioGlas® – for treatment of periodontal disease
      • NovaBone – bone grafting material for orthopedics & maxillofacial repair
    Glass-Ceramic implants for spinal repair Glass-Ceramic cochlear implants
  • 40. 4. Resorbable Ceramics
    • Degrade upon implantation in the host
    • Rate of degradation varies from material to material
      • rate needs to be equal to rate of tissue generation at specific site of application
    • Almost all bioresorbable ceramics (except Biocoral and Plaster of Paris – calcium sulfate dihydrate) are variations of calcium phosphate
    • Uses of biodegradable bioceramics:
      • Drug-delivery devices
      • Repair material for bone damaged by trauma or disease
      • Space filling material for areas of bone loss
      • Material for repair and fusion of spinal and lumbosacral vertebrae
      • Repair material for herniated disks
      • Repair material for maxillofacial and dental defects
      • Ocular implants
  • 41. Calcium Phosphate
    • Calcium phosphate compounds are abundant in nature and in living systems.
    • Biologic apatites which constitute the principal inorganic phase in normal calcified tissues (e.g., enamel, dentin, bone) are carbonate hydroxyapatite, CHA.
    • In some pathological calcifications (e.g., urinary stones, dental tartar, calcified soft tissues – heart, lung, joint cartilage)
    • Form of calcium phosphate depends on Ca:P ratio
      • Most stable form is crystalline hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ]
        • Ideal Ca:P ratio of 10:6
        • Crystallizes into hexagonal rhombic prisms
        • This apatite form of calcium phosphate is closely related to the mineral phase of bone and teeth
        • Very low bulk solubility; can be used as a structural biomaterial
  • 42.  -Tricalcium Phosphate (TCP)
    • Another widely used form is β -tricalcium phosphate [ β -Ca 3 (PO 4 ) 2 ]
      • In aqueous environment surface reacts to form HA
      • 4Ca 3 (PO 4 ) 2 + 2H 2 O -> Ca 10 (PO 4 ) 6 (OH) 2 + 2Ca 2+ + 2HPO 4 2-
    • Often porous (partially sintered powders)
    Tricalcium phosphate thin film (Osteologic) used in orthopedic applications
  • 43. Stability
    • Resorption caused by 3 factors
      • Physiologic dissolution (depends on environment pH, type of CaP)
      • Physical disintegration into small particles as a result of preferential chemical attack of grain boundaries (enhanced by porosity)
      • Biological factors, such as phagocytosis, which causes a decrease in local pH concentration
    • Apatite forms are the most stable
    • high rate of dissolution  low rate of dissolution
    • TTCP > α -TCP > β -TCP > HA > Fluorapatite
    • Substitution of F- for OH- in HA greatly increases the chemical stability
      • Get fluorapatite [Ca 10 (PO 4 ) 6 (F) 2 ]
      • Found in dental enamel
      • Principle is used in dental fluoride treatments (~ 1 in 100 OH- replaced)
  • 44. Mechanical Properties
    • Dense HA (properties are similar to enamel – stiffer and stronger than bone)
      • Elastic modulus = 40 – 115 GPa
      • Compressive Strength = 290 MPa
      • Flexure Strength = 140 MPa
    • Porous HA
      • not suitable for high load bearing applications
    • TCP
      • Generally poor (more of a packing material)
  • 45. Summary
    • 4 groups of ceramic for biomedical applications
      • Nonporous, nearly inert – structural components
      • Porous, inert – non-load bearing, coatings, fillers
      • Nonporous, bioactive – coatings, dental applications, strong attachment to bone
      • Resorbable – fillers, spinal/defect repair, drug delivery
    • Function greatly affected by
      • Composition – bioactivity
      • Structure (crystal and grains) – mechanical properties
      • Processing – mechanical properties
      • Porosity – reactivity, degradation
      • In vivo environment – reactions with tissue/fluids