1 Prof C. H. XU School of Materials Science and Engineering Henan University of Science and Technology Chapter 4: Matrix: Ceramics Subject: Composite Materials Science and Engineering Subject code:

2 Matrix: Ceramics Contents Atomic bonding and crystal structures Mechanical Properties Application and processing

3 Matrix: Ceramics Atomic bonding and crystal structures Bond Most ceramics are composed of at least two elements Atomic bonding: ironic covalent, many ceramics exhibit a combination of these two bonding types For several ceramic materials, percent ionic character of the inter-atomic bonds

4 Matrix: Ceramics Atomic bonding and crystal structures Technical (Modern) ceramics – Crystal AX-type crystal structures: (A cation; X anion) NaCl crystal structure Coordination No is 6 NaCl; MgO, MnS, LiF, FeO CsCl structure Coordination No. is 8 ZnS structure Coordination No. is 4 ZnS, ZnTe, SiC

5 CaF 2 structure (AX 2 ) Coordination No. cation 8 Coordination No. anion 4 ThO 2 Perovskite crystal structure (ABX 3 ) Coordination No. 12 for Ba 6 for Ti and 6 for O SrZrO 3, SrSnO 3 Matrix: Ceramics Atomic bonding and crystal structures

6 Silica SiO 2 Polymorphism: a material has more than one crystal structures SiO 2 slowly cooling at T m melting point, Crystal structure: quartz, density of 2.65g/cm 3, melting point 1710  C, (a, b) SiO 2 fast cooling at T m melting point, e.g.Glass: amorphous (c) Si 4+ O 2- (a) A unit cell of Quartz (b) Crystal SiO 2 (c) Amorphous SiO 2 Traditional Ceramics: Amorphous

7 Carbon: In various polymorphic forms Diamond A metastable carbon polymorph at room T and atmospheric pressure C bonds to 4 other C (covalent) Hard, low electrical conductivity, high thermal conductivity Large single crystals are used as gem stone; diamonds are utilized to grind or cut other materials in industries. Produce synthetic diamonds have been developed since 1950’s, such as, singe crystal or diamond films SEM photo of a diamond thin film A unit cell for diamond Maatrix: Ceramics Atomic bonding and crystal structures

8 Graphite: a polymorph of carbon more stable than diamond at ambient temperature and pressure C atom bonds to three C within a layer by covalent bonds and the 4 th bonding electron participates in a weak van der Waals type of bond between the layers. The excellent lubricative properties; electrical conductivity in the directions parallel to the sheets; high resistance to thermal shock (thermal shock: damage due to change in temperature) Density: 2Mg/m 3 Application: Heating elements in electric furnace, motor brushes, casting molds. Structure of graphite Matrix: Ceramics Atomic bonding and crystal structures

9 Matrix: Ceramics mechanical properties Mechanical properties Brittle fracture of ceramics Very small flaws in materials Crack growth in crystalline ceramics is through the grains and along specific crystallographic planes with high atomic density Low strengths under tensile load, due to crack Higher strength under compressive load, due to no stress amplification with any existent flaws. σ m =2σ 0 (a/  t ) 1/2

10 Matrix: Ceramics-mechanical properties It is difficult to do tensile test for ceramics Difficult to prepare a tensile specimen (hard, brittle) Difficult to alignment in the tensile test. (only 0.1% strain) 3-point bend test: at the point of loading, the top surface in a state of compression and bottom surface in a state of tension stress A three-point loading for measuring the stress-strain behavior  ( stress) = Mc/I Where M=maximum bending moment c=distance form center of specimen to outer fibers I=moment of inertia of cross section F=applied load Rectangular circular

11 Matrix: Ceramics-mechanical properties Flexural strength ( 挠曲 强度 ): Stress at fracture in 3-point bend test. Elastic moduli for ceramics are slightly larger than those for metals. Typical stress-strain behavior fracture for aluminum oxide and glass

12 Matrix: Ceramics-mechanical properties E=E 0 (1-1.9P+0.9P 2 ) E: elastic modulus of materials E 0 : elastic modulus of nonporous materials P: volume fraction porosity  fs =  0 exp(-nP)  fs : the flexural strength  0 and n: are experimental constants P: volume fraction porosity Mechanical properties related to porous in materials

13 Matrix: Ceramics-mechanical properties Mechanisms of plastic deformation Crystalline ceramics Motion of dislocation Very few slip systems due to predominant ionic bonding. Complex dislocation structures Non-crystalline ceramics Plastic deformation be viscous flow, similar to liquid. Atom or ions slide past one another by the breaking and reforming of inter-atomic bonds.

14 Matrix: Ceramics -application and processing Ceramics: high melting point, Processing? Ceramic fabrication techniques Traditional ceramics Glass Cement Technical ceramics: Forming processes Initial shape, drying and firing Sol-gel Processes Vapor Processing

15 Matrix: Ceramics-application and processing (glass) Glass: SiO 2 + other oxide, non-crystalline No significant melting point, a glass becomes more and more viscous in a continuous manner with decreasing temperature T g : glass transform temperature Supercooled liquid at T>T g and glass at T< T g Contrast of specific volume versus temperature behavior of crystalline and non-crystalline materials

16 Matrix: Ceramics-application and processing (glass)

17 Matrix: Ceramics-application and processing (glass) Melting point: viscosity is 10 2 P. the glass is fluid enough. Working point: viscosity is 10 4 P. the glass is easily deformed. Softening point: viscosity is 4x10 7 P. Glass piece may be handled without causing significant dimensional change. Annealing point: Viscosity is P. Atomic diffusion is sufficiently rapid that residual stresses may be removed in 15 min. Strain point: viscosity is 3x10 14 P. Fracture occur before plastic deformation Logarithm of viscosity versus temperature for glasses

18 Matrix: Ceramics-application and processing (glass) Glass forming Melting: Quartz sand is heated at high temperature, adding other oxide materials. Forming: pressing (similar to metal), blowing, drawing Annealing: when material is cooled from high temperature to room temperature, internal stresses may be introduced as a result of the differences in cooling rate and contraction between the surface and interior regions. The internal stresses can weaken materials (especially for brittle materials). It’s important for slowly cooling. If cooling rate for a glass is too fast, annealing is needed. (T at annealing point)

19 Matrix: Ceramics-application and processing (glass) Glass ceramics glasses containing TiO 2 are changed the structure form noncrystalline to fine-grained polycrystalline materials (glass ceramics) after a proper high temperature heat treatment (devitrification). Glass ceramics have less thermal stress during cooling and high thermal conductivity. Application: ovenware, tableware

20 Matrix: Ceramics -application and processing Cements (to produce concrete): Produce cement: clay + lime; mixture is heated to 1400  C; grind materials to powders (composition: 2CaO-SiO 2 ) Portland cement concrete: = Gravel, sand + cement + water; 2CaO-SiO 2 +xH 2 O= 2CaO-SiO 2 - xH 2 O Cement function: bonding phase at room temperature Applications: as construction materials, such as building, bridge…

21 Matrix: Ceramics -application and processing Powder pressing for ceramic parts Materials: clay or non-clay powders Pressing: uni-axial, iso-static, or hot pressing Firing: atoms diffuse Micro-structural changes during firing (a)Powder particles after pressing (b)Pore forms during starting firing (c)Final structure after firing Steps in uniaxial powder pressing

22 Matrix: Ceramics -application and processing Processing: Tape casting A suspension of ceramic particles in an organic liquid (binders) Firing Schematic diagram showing the tape casting process using a doctor blade

23 Vapor Processing ： Matrix: Ceramics -application and processing

24 Further Reading: Text Book: Composite Materials: Engineering and Science (pages ). Reference Book: Introduction to Materials ( 材料概论 ) pages Other Reference: Lecture note 4