Ceramics

1
Ceramics

• Lecture 4
• January 29, 2009

2
Zirconia Data for HW1
N. Navruz, Phys of metals metallography 1056
(2008)
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.

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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.

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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.

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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

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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 gt 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

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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)

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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

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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

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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

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1. Nonporous, Nearly Inert Ceramics

• Alumina (Al2O3) and Zirconia (ZrO2)
• 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

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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.

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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 mm)
• 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 gt ½ Tm 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

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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

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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)

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Alumina

• Al2O3
• 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, lt 4 mm

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Zirconia

• ZrO2 (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

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Fabrication with Al2O3 and ZrO2

• Devices are produced by pressing and sintering
  fine powders at temperatures between 1600 to
  1700ºC.
• High purity alumina used in biomedical
  applications (gt99.5)
• Additives such as MgO added (lt0.5) to limit
  grain growth

a Alumina sintered 180 minutes at 1580 Cb
Zirconia sintered 120 minutes at 1400 C
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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

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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

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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

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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

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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

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Calcium Hydroxyapatite (HA)

• Ca10(PO4)6(OH)2
• HA is the primary structural component of bone.
• consists of Ca2 ions surrounded by PO42 and OH
  ions.

HA microstructure
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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

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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.

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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

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3. Bioactive Ceramics

• 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.

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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 gtgt 1016
  P

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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

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Glass Structure

• Glass-forming oxides
• e.g., SiO2 B2O3 P2O5 GeO2
• glass-forming network often the major component
• Glass-modifying oxides
• e.g., Na2O CaO Al2O3 TiO2
• 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

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Types of Glass

• Silicate glass (fused silica)
• SiO2
• Each silicon is covalently bonded to 4 oxygen
  atoms
• Soda-lime glass
• 70 wt SiO2 15 wt Na2O 10 wt CaO
• Window glass, bottles, etc.
• Borosilicate glass
• Some SiO2 replaced by B2O3
• 80 wt SiO2 15 wt B2O3 5 wt Na2O
• Pyrex glass cooking and chemical glass ware

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Glass-Ceramics

• Composite structure consisting of a matrix of
  glass in which fine crystals have formed
• Crystals can commonly be very fine (avg. size lt
  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

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Glass-Ceramic Processing

• Glass with nucleating agent like TiO2 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

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Bioactivity

• Bioactivity is very sensitive to composition
• Both in glasses and glass-ceramics
• Less than 60 mol SiO2
• High Na2O and CaO content
• High CaO/P2O5 ratio, minimum 51
• 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
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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).
• Bodys tissues are able to attach directly to the
  crystallized layer
• Layer grows to be approximately 100-150 mm in
  depth.
• Occurs within 12-24 hr
• Cells arrive within 24 to 72 hr and encounter a
  bonelike surface, complete with organic components

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Bioactive ApplicationsGlass 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

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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

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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 CaP ratio
• Most stable form is crystalline hydroxyapatite
  Ca10(PO4)6(OH)2
• Ideal CaP ratio of 106
• 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

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b-Tricalcium Phosphate (TCP)

• Another widely used form is ß-tricalcium
  phosphate ß-Ca3(PO4)2
• In aqueous environment surface reacts to form HA
• 4Ca3(PO4)2 2H2O ? Ca10(PO4)6(OH)2 2Ca2
  2HPO42-
• Often porous (partially sintered powders)

Tricalcium phosphate thin film (Osteologic) used
in orthopedic applications
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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 gt a-TCP gt ß-TCP gt
  HA gt Fluorapatite
• Substitution of F- for OH- in HA greatly
  increases the chemical stability
• Get fluorapatite Ca10(PO4)6(F)2
• Found in dental enamel
• Principle is used in dental fluoride treatments
  ( 1 in 100 OH- replaced)

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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)

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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