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Biomaterials, Artificial Organs and Tissue Engineering Chapter 3 Ceramics Aldo R. Boccaccini Imperial College London
Aim To describe the atomic structure, microstructure and properties of ceramics and develop an understanding of the correlation between the processing, microstructure and properties of ceramics.
Ceramics are solid inorganic compounds with various combinations of ionic or covalent bonding. They have more complex crystal structures than metals. Ceramics are: ü Hard ü Brittle ü Incombustible ü Corrosion resistant ü Poor conductors of heat and electricity ü Light (in general lighter than metals) Ceramics have: ü High melting points ü Good resistance to heat (e. g. they do not deform) ü Good chemical stability at high temperatures (do not oxidise)
The Solid State Solids are distinguished from other states of matter (liquids and gases) : The constituent atoms are held together by strong interatomic forces Almost all of the physical properties of solids depend on the nature and strength of the interatomic bonds. There are three types of strong or primary interatomic bonds: ü Ionic ü Covalent ü Metallic CERAMICS METALS
Interatomic Bonds IONIC BONDING: Electron donor (metallic) atoms transfer one or more electrons to an electron acceptor (non-metallic) atom. The atoms then become a cation (e. g. metal) and an anion (e. g. non-metal), which are strongly attracted by electrostatic effect (typical example K+Cl-). Bound electrons are not available to serve as charge carriers Ionic solids are poor electrical conductors Fig. 3. 1
Interatomic Bonds COVALENT BONDING: ü Occurs in such elements that fall along the boundary between metals and nonmetals (e. g. C, Si). ü Equal tendency to donate and to accept electrons. ü Share valence electrons. The localisation of the valence electrons renders these materials poor electrical conductors Fig. 3. 2
Interatomic Bonds METALLIC BONDING: ü Atoms are arranged in an orderly, repeating, three-dimensional pattern. ü Valence electrons migrate between the atoms like a gas. The non-localised bonds permit plastic deformation. The electron “gas” accounts for the chemical reactivity and high electrical and thermal conductivity of metallic systems. Fig. 3. 3
Interatomic Bonds Weak secondary bonds ü Van der Waals’ bonding ü Hydrogen bonding
Atomic structure and microstructure Material Crystal/Grain boundaries Atom Fig. 3. 4 Microstructure: array of crystals or grains (0. 5 - ~500 mm) Atomic structure: 3 -D array of atoms (this arrangement usually constitutes a crystal)
Crystalline structure The atoms or ions are arranged in an orderly repeating pattern in three dimensions Often represented by repeating elements or subdivisions of the crystal called unit cells
Face Centered Cubic (FCC) Fig. 3. 5
Body Centered Cubic (BCC) Fig. 3. 6
Hexagonal Close Packed (HCP) Fig. 3. 7
Time-dependence on Processing Fig. 3. 8 (a) Phase diagram of silica-MO glass/ ceramic. (b) Time-temperature profiles of processing steps for (1) glass, (2) cast polycrystalline (large grained) ceramic, (4) solid-state sintered ceramic; (5) polycrystalline glass-ceramic, (6) polycrystalline coating from liquid. From Introduction to Bioceramics, L. L. Hench, J. Wilson, Singapore, World Scientific, 1993.
Melt-derived v Sol-gel process for glass synthesis Fig. 3. 9 Processing steps for sol-gel derived glass production compared to melt-processing. From Introduction to Bioceramics, L. L. Hench, J. Wilson, Singapore, World Scientific, 1993.
Microstructure Scanning electron microscope (SEM) SEM of a CERAMIC microstructure Fig. 3. 10 Fig. 3. 11
Microstructural Features ü Grain size ü Grain shape ü Grain orientation ü Grain boundaries ü Porosity ü Microcracks especially in ceramics compared to metals The properties of ceramics strongly depend on their microstructure
Microstructural Features Aggregate of randomly oriented crystallites (small single crystals of less than 100 mm diameter) intimately bonded together to form a solid Porosity Fig. 3. 12 Grain boundaries
Types of Ceramic Materials OXIDE CERAMICS Silicates (also inorganic glasses and glass-ceramics) Al 2 O 3 Zr. O 2 Ti. O 2 NON-OXIDE CERAMICS Carbides Nitrides (Carbon)
Ceramic Processing Ø Not produced from the molten state Ø Usually produced via a powder (sintering route) Ø Can be also produced by various deposition techniques (e. g. CVD, EPD, etc. ) and by colloidal and sol-gel processing
Powder Technology and Sintering (Fine) Ceramic Powder Shaping Slip or tape casting Cold-pressing Sintering (High-T densification) Hot-pressing Densification by simultaneous application of temperature and pressure Machining to final shape and dimensions Processing defects: internal and external microcracks, abnormal grain growth, porosity THEY ALL AFFECT THE FINAL PROPERTIES Fig. 3. 13
Inorganic Glasses are distinguished from polycrystalline ceramics by their amorphous nature or lack of long-range internal structure. A glass is a product of fusion, which has been cooled to a rigid condition without crystallising. Fig. 3. 14
Atomic Structure of Glasses The atomic structure of glass is similar to that of a liquid, as shown by X-ray diffraction, i. e. an orderly repeating arrangement of atoms is not maintained over long distances. The following oxides form glasses readily: Si. O 2, Ge. O 2, B 2 O 3, P 2 O 5 Fig. 3. 15 Silicon-oxygen tetrahedron – Silicon atom O - Oxygen atom Glasses are favoured for several applications because of their wide availability, low cost, and ease of fabrication. Fig. 3. 16 Schematic representation of a random-network glassy structure.
Glass-Ceramics Glass-ceramics are crystalline materials obtained by the controlled crystallisation of an amorphous parent glass. Controlled crystallisation requires: - specific compositions - usually a two-stage heat-treatment Fig. 3. 17 Fig. 3. 18 Glass-ceramics have better thermomechanical properties than glasses
Medical Applications of Ceramics and Glasses ü EYEGLASSES ü DIAGNOSTIC INSTRUMENTS ü CHEMICAL WARE ü THERMOMETERS, TISSUE CULTURE FLASKS ü CARRIERS FOR ENZYMES, etc. ü RESTORATIVE MATERIALS IN DENTISTRY ü IMPLANTS BIOCERAMICS: REPAIR OR REPLACE SKELETAL HARD CONNECTIVE TISSUES BIOCERAMICS ARE CLASSIFIED ACCORDING TO THE TYPE OF ATTACHMENT TO TISSUE Nearly inert crystalline ceramics (e. g. Al 2 O 3, Zr. O 2). (Load-bearing hip prosthesis, dental implants). Bioactive ceramics, glasses and glass-ceramics (e. g. hydroxyapatite, Bioglass®, A-W glass-ceramic, etc. ). (Middle ear replacement, repair of periodontal defects, vertebral surgery, etc. ). Resorbable calcium phosphates.
Mechanical Properties Stress BRITTLE MECHANICAL BEHAVIOUR: very low fracture toughness METAL Fracture strength CERAMIC Elastic Region Fig. 3. 19 Strain
Effect of Microstructure on mechanical properties POROSITY - Open or closed - Decrease Young’s modulus - Decrease strength - Decrease hardness - Decrease thermal conductivity Fig. 3. 20 GRAIN BOUNDARIES - Collectors of impurities - Glassy phase - Stress concentration - Microcracking source
Fracture strength is dependent on Porosity Two effects: Reduction of the load-bearing cross-section Stress concentration Fig. 3. 21 s= s 0 (1 -P)K K = f (shape and orientation of pores) [stress concentration]
Fracture strength of porous ceramics with spherical pores Fig. 3. 22
Porous Ceramics and Glasses HOWEVER, FOR SEVERAL APPLICATIONS, INCLUDING SEVERAL BIOMEDICAL APPLICATIONS, WE WANT TO HAVE POROUS CERAMICS ! • Non-loading implant applications • Scaffolds for tissue engineering • etc. Bone will grow within interconnecting pore channels near the surface maintaining its vascularity and long-term viability Fig. 3. 23 Bioactive Glass Foams for Tissue Engineering (Jones, Sepulveda and Hench, 2002)
Porosity Effects on Mechanical Properties Porosity can be used to tailor the Young’s modulus of a glass or ceramic to that of bone üThe Young’s modulus of ceramics and glasses is much higher than that of bone üFor implant applications, the Young’s modulus of the implant must be matched to that of bone (avoid stress shielding) üAdding porosity to decrease the Young’s modulus of the implant Range of variation of the Young’s modulus of cortical bone Fig. 3. 24
Summary • Atomic bonding determine the physical properties of materials. • Ceramics contain ionic and covalent bonding • Ceramics, glasses and glass-ceramics are strong, hard and brittle • Microstructure: array of grains and phases in the range (0. 5 – 500 mm) • Microstructural features: grain size, shape, orientation, grain boundaries, porosity, microcracks, impurities • Mechanical properties of ceramics are strongly dependent on the microstructural features • Processing of ceramics affect the microstructure • Strong interaction: Processing Microstructure Properties