Ceramics

Lecture 4

January 29, 2009

Zirconia Data for HW1

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

N. Navruz, Phys of metals & metallography 105:6 (2008)

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

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.

Other Definitions

• Amorphous: 1. Lacking detectable crystallinity;

2. 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.

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.

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%

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

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)

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

Attachment

Four types of ceramic-tissue attachment are related to the tissue response to a material

1. 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

2. Biological fixation – porous, inert ceramics attach by bone ingrowth (into pores) resulting in mechanical attachment of bone to material

3. Bioactive fixation – dense, nonporous surface-reactive ceramics attach directly by chemical bonding with bone

4. Resorbable – dense, porous or nonporous resorbable ceramics are slowly replaced by bone

Types of Ceramics

1. Nonporous, nearly inert materials are very strong and stiff

2. Porous, inert materials have lower strengths, but are useful as coatings for metallic implants

3. Nonporous, bioactive materials establish bonds with bone tissue

4. Resorbable materials may be porous or nonporous and degrade with time

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

Processing of Ceramics

1. Compounding– Mix and homogenize ingredients into a water based suspension

= slurry or, into a solid plastic material containing water called a clay

2. Forming– The clay or slurry is made into parts by pressing into mold

(sintering). The fine particulates are often fine grained crystals.

3. Drying– The formed object is dried, usually at room temperature to the

so-called "green" or leathery state.

4. 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.

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 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

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

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)

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, < 4 m

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

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 (>99.5%)– Additives such as MgO added (<0.5%) to limit grain

growth

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

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

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

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

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

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

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

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

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.

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

• 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

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 >> 1016 P

melt

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

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

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

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

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

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 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

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

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

Glass-Ceramic implants for spinal repair

Glass-Ceramic cochlear implants

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

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 [Ca10(PO4)6(OH)2]

1. Ideal Ca:P ratio of 10:62. Crystallizes into hexagonal rhombic prisms3. This apatite form of calcium phosphate is closely related to the

mineral phase of bone and teeth4. Very low bulk solubility; can be used as a structural biomaterial

-Tricalcium Phosphate (TCP)

• Another widely used form is β-tricalcium phosphate [β-Ca3(PO4)2]– In aqueous environment surface reacts to form HA– 4Ca3(PO4)2 + 2H2O → Ca10(PO4)6(OH)2 + 2Ca2+ + 2HPO4

2-

• Often porous (partially sintered powders)

Tricalcium phosphate thin film (Osteologic) used in orthopedic applications

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 stablehigh rate of dissolution low rate of dissolutionTTCP > α-TCP > β-TCP > HA > 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)

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)

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