MSE 351 Engineering Ceramics I

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1 Kwame Nkrumah University of Science & Technology, Kumasi, Ghana MSE 351 Engineering Ceramics I Ing. Anthony Andrews (PhD) Department of Materials Engineering Faculty of Mechanical and Chemical Engineering College of Engineering Website: Defect structure is the term used to describe types and concentrations of atomic defects in ceramics. Vacancies and interstitials are likely for either ion (cation or anion) High concentration of anion interstitials is unlikely! Conditions of electroneutrality needs to be maintained in the defect structure Defect in ceramics usually occur in pairs: Frenkel defect: cation vacancy/cation interstitial, no change in charge. Cation maintains its charge as interstitial. Schottky defect: found in AX ceramics. Cation vacancy/anion vacancy. The ratio of cations to anions is not altered by presence of either defect Material maintains its Stoichiometry exact cation to anion ratio as predicted by the chemical formula 1

2 Iron oxide (FeO): Fe 2+ & Fe 3+ Formation of an Fe 3+ ion disrupts the electroneutrality of the crystal The crystal is no longer stoichiometric because there is one more O ion than Fe ion; however, the crystal remains electrically neutral. Computation of number of defects N fr = Nexp Q fr 2kT N s = Nexp Q s 2kT Q fr = energy required to form each Frenkel defect Q s = energy required to form each Schottky defect N = total number of lattice sites k = Boltzmann s constant T = absolute temperature Example Calculate the number of Schottky defects per cubic meter in potassium chloride at 500 o C. The energy required to form each Schottky defect is 2.6 ev, whereas the density for KCl (at 500 o C) is g/cm 3. Impurities in Ceramics [k = 8.62 x 10-5 ev/k; N A = x atoms/mol; A K = 39.10g/mol; A Cl = 35.45g/mol] N s = Nexp Q s 2kT Impurities in Ceramics Defects in Ceramics - Example Assume the following point defects: 1. Mg 2+ ions substitute for yttrium atoms in Y 2 O 3 2. Fe 3+ ions substitute for magnesium in MgO Indicate for each case, what needs to be removed or added and indicate the type of point defect. 2

3 Ceramic Phase Diagrams Phase diagrams for ceramic materials obey the same rules as for metal systems. An important difference is that the terminal end phases are usually themselves binary compounds, rather than pure elements. A number of binary systems contain oxygen as a common element. Cr 2 O 3 - Al 2 O 3 SiO 2 - Al 2 O 3 MgO - Al 2 O 3 Binary Materials Systems 3 possible outcomes: complete solid solubility partial solid solubility without an partial solid solubility with an Solid solution types: Substitutional Interstitial Complete Solid Solubility Complete Solid Solubility For substitutional solid solution: Similar crystal structure Same valence Similar size ions Similar electronegativity Due to these restrictions, complete solid solutions are the exception, rather than the rule 3

4 Partial solid solubility, no Components only partially soluble in each other The limited solubility can be a consequence of the difference in ion sizes Partial solid solubility, no Formation of a eutectic point on the phase diagram Cooling at the eutectic point results in two phases Example CaO-MgO Partial solid solubility with Intermediate compounds may form if the end members have a strong affinity for each other Intermediate phases can be line compounds or compounds with a wider stoichiometric range Partial solid solubility with Binary system with a congruent melting intermediate phase Congruent melting s show no change in composition, and can be line phases or otherwise Incongruent melting s give intermediate compounds which dissociate before melting into a liquid and a different solid, across a peritectic reaction Partial solid solubility with Binary system with incongruent melting CaO-SiO 2 How many s? Congruent or incongruent?? 4

5 SiO 2 -Al 2 O 3 How many eutectics?? Mechanical Properties of Ceramics Very brittle in tension brittle fracture limited energy absorption Stronger in compression than in tension Limited load carrying capacity Strength strongly dependent on the processing Flaws limit strength 29 Brittle Fracture of Ceramics At room T, both crystalline and amorphous ceramics fracture before plastic deformation occurs Ductile fracture Brittle Facture less than 10% plastic deformation at failure Brittle Fracture of Ceramics Brittle fracture consists of the formation and propagation of cracks through the cross section of material in a direction perpendicular to the applied load Fracture is usually transgranular rather than intergranular For transgranular fracture, cracks often grow along high density crystallographic (or cleavage) planes

6 Strength Limitations in Ceramics Brittle ceramic strength is lower than expected Theoretical Strength: (e.g., TS = 6000 MPa) Observed Strength: (e.g., TS = 0.10 to of theoretical) Why the big difference?? Brittle Fracture Ceramic deformation entails very localized or short range deformation mechanism GRIFFITH-OROWAN Theory Flaws are stress concentrators (e.g., pores) and form cracks Fracture occurs in sequential steps Crack tips move at low stresses Crack tip radius of curvature is atomic size Strength of Ceramics Griffith 1920 s proposed that fine elliptical flaws exist and that concentrate stress Crack Initiation Cracks are always perpendicular to the applied stress Pores are most detrimental under tensile stress (not compression) Water tends to propagate existing cracks in most materials Flaws behave as stress magnifiers Applied stress may be fairly low, but effective local stress is very HIGH Crack Propagation Tensile stresses tend to propagate crack tip Water or other observed liquids tend to open crack tip Crack Management 1. Crack Prevention: Eliminate internal defects (e.g., eliminate pores). Eliminate surface defects. Eliminate design feature that concentrate stress (e.g., sharp corner). Prevent tensile stresses. 2. Crack Slowing or Stopping Mechanisms: Design material to be only under compressive stress. Pre-stress material in compression to cancel early tensile stresses Add crystalline phases that are stronger to slow or stop cracks. Add crystalline phases that make path more tortuous. Add phases that interact with cracks and compress the tip 36 6