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Lecture 04 Lecture 04 Presentation Transcript

  • Today’s objectives-Mechanical Properties
    • How do mechanical characteristics (stress v strain) of metals and ceramics compare at room Temperature?
    • What are typical ceramic failure mechanisms?
    • Why are stress-strain characteristics of ceramic materials determined using transverse bending tests rather than tensile tests?
    • Be able to compute the flexural fracture strength of a ceramic from a flex test.
    • Be able to use fracture toughness to determine the max stress for a given ceramic with flaws of a known size and radius of curvature.
    • Why is there normally significant scatter in the fracture strength for specimens of the same ceramic material?
    • Why are crystalline ceramic materials so brittle?
  • Abrasives
    • Ceramics are generally extremely hard
    • Applied as abrasives
      • alumina
      • SiC
      • WC
      • sand
      • cubic BN, Diamond
    http://www.abrasiveengineering.com/eurostus.pdf Almost 3000 tons a day! 700 Sand 550 Glass 600 Steel 80 Nickel 2500 Silicon carbide (SiC) 2100 Alumina (AL2O3) 2100 Tungsten carbide (WC) 800 Quartz (SiO2) 1 Graphite 7000 Diamond (C) Knoop Hardness (100g load) Material 60 1150 550 Super (no gems) 850k 310 580 Loose 70k 850 1600 Coatings 65k 860 450 Bonded tons Europe $ US $ Abrasive use, 2002 http://www.chm.bris.ac.uk/pt/diamond
  • Stress vs. Strain
    • For many metals:
    • Elastic and significant plastic deformation
    • For most ceramics:
    • No appreciable plastic deformation
    Brass Brass strains to 35% = Fracture Strength (stress at failure) Ceramics strain to 0.1%
  • Fracture strength Callister, Appendix B Note that the fracture strength for ceramics is about 10* better in compression than in tension. Body armor 69 Soda lime glass 13.8-69 Graphite 230-825 SiC (sintered) 1050 Diamond (natural) 800-1400 Diamond (synthetic) 37.3-41.3 Concrete 130 Si [100] cleaved 282-551 Al 2 O 3 (99.9% pure) Strength (MPa) Ceramic
  • Failure Mechanisms-single xtals
    • For single crystals, cleavage occurs
      • Very rapid crack propagation along specific crystallographic planes.
      • Creates exceedingly flat surfaces (even atomically flat).
        • Examples?
    http://www.theimage.com/crystalinfo/crystal_page2.htm
  • Failure Mechanisms-polycrystals
    • Two possibilities:
      • Transgranular (through grains)
        • Rough surface everywhere
      • Intergranular (along grain boundaries)
        • Rough surface of many flat faces
    • In rare cases (usually nanoscale polycrystalline ceramics), there can be limited ductility at room temperature.
    • At higher temperatures, plastic deformation may occur.
  • Typical mechanical property measurements
    • Standard tensile tests are problematic:
      • Failure usually occurs at low strains (<0.1%), where bending stresses can be significant unless the sample is perfectly aligned in the tensile stage.
      • Gripping brittle materials like ceramics often leads to fracture at the grips.
      • Test geometry is difficult to prepare.
  • Measuring ceramic mechanical properties
    • We can’t use the standard tensile test, but we still need elastic modulus and fracture strength.
    Solution: bend test. Most appropriate for bars, rods, plates, and wafers. Where will cracks form? Which part is under tension and which is under compression?
  • MEASURING FRACTURE STRENGTH • 3-point bend test to measure room T strength. Adapted from Fig. 12.29, Callister 6e. • Flexural strength: Flexural fracture strength is higher than the tensile fracture strength. Why? Test specimens undergo compressive and tensile loads instead of pure tension. Si carbide Al oxide glass (soda) 550-860 275-550 69 Data from Table 12.5, Callister 6e. circ.
  • MEASURING ELASTIC MODULUS • Room T behavior is usually elastic, with brittle failure. • Determine elastic (Young’s) modulus according to slope: Adapted from Fig. 12.29, Callister 6e. Si carbide Al oxide glass (soda) 430 390 69 Data from Table 12.5, Callister 6e.
  • Measured Fracture Strengths
    • Practically, measured fracture strengths of ceramic materials are usually much lower than predicted.
      • Is the strength equation wrong?
    • NO. Omnipresent flaws concentrate stresses locally.
      • Pores
      • Grain boundary grooves
      • Internal grain corners
      • Surface cracks / scratches
        • Particularly enhanced by humidity and contaminants
    • So how can we ever apply ceramics structurally?
  • Fracture Toughness
    • The mode I plane strain fracture toughness (K Ic ) guides engineers when trying to know whether a brittle material is applicable for a given tensile load.
      • If the product of the applied stress ( σ ), crack length ( a), and geometric factor (Y ≈1 ) is greater than the fracture toughness, the part will fail .
    Mode I a 2.7-5 Aluminum Oxide usually greater metals 0.7-1.1 Soda-lime glass 0.2-1.4 Concrete K Ic (MPa*m ½ ) Ceramic
    • An applied stress is amplified at crack tips and/or pores in ceramics (from σ o to σ m ) , depending on:
    • Crack tip radius (  t ).
    • Crack length (a) or length of a pore/2 (a/2).
    • Since fracture toughness related to max stress:
    • The resulting maximum tensile stress (ie. strength) applicable to the ceramic part before failure is:
    Fracture Toughness-stress concentration Note: no such stress concentration occurs for compression.
  • Journal of Irreproducible Results?
    • Measurements of fracture strength for multiple specimens usually leads to a significant variation and scatter in the results.
    • Related to huge number of flaws, primarily pores (cracks).
      • Fracture occurs when K Ic is surpassed.
      • K Ic depends on the maximum stress within the specimen, a function of flaw size and radius.
      • The flaw size and radius of curvature is governed by probability laws .
      • Thus, so must be the fracture strength for multiple specimens.
  • Weibull statistics
    • By controlling pore size, the flexural strength can be controlled (statistically).
    • With fewer flaws, strength is improved.
    130 MPa Si [100] cleaved 81.8 MPa Si [100] laser scribed
  • Minimizing Failures
    • Take advantage of statistics and the behaviour of ceramics to minimize failures of parts you sell.
      • There will be a few failures (statistically), but these can be replaced through customer service as long as there aren’t too many.
      • Limit the ‘rated load’ to somewhere low on the Weibull response curve.
    rated load
  • Guaranteeing No Failures
    • Take advantage of statistics and the behaviour of ceramics to guarantee no failures :
      • If the sample contains flaws that are too large, it will fail. No problem since this is in the factory, not the flying airplane.
      • All parts that survive are good to the rated load—but don’t surpass that load since then failures will begin to occur.
      • Load a particular critical component (e.g. airplane engine turbine blade) to a ‘rated load.’
    rated load
  • Where failures matter
    • Ceramic Hips: “Modern medical grade ceramic is individually tested before use with weights 60 times greater than the patient body weight…
    • The Reported fracture rate = 0.004% or 4 in 100 000.
    • Not bad, except > 200,000 implants per year.
    • www.totaljoints.info/ceramic_total_hips.htm
  • Caveat for brittle materials: delayed fracture
    • If a static load is applied, even if below K Ic , fracture may still eventually occur.
    • “ Delayed Fracture” is caused by slow crack propagation below fracture toughness (<K Ic )
      • Caused by “stress-corrosion cracking”
        • Combination of crack tip, stress, and corrosion sharpens and elongates a crack.
        • Eventually, K IC is surpassed as crack size and radius changes.
      • Very sensitive to chemical environment (esp. humidity).
      • A greater problem with increased porosity (more surface area for chemical reactions and thus crack growth).
  • Ceramics at higher temperatures
    • Dislocation motion (slip) is extremely difficult in ceramics due to their ionic nature.
      • hardness and brittleness are extremely high.
    • For covalent ceramics, the covalent bonds are also very strong and difficult to overcome.
    • Still, plastic deformation does occur in ceramics, but:
      • less than for metals.
      • usually only near the melting point.
    Ionic Bonding Slip is impossible: Too much electrostatic repulsion + - + - + - + - + - + -
  • Improvements for mechanical applications
    • Use in compression
    • Decrease influence of internal flaws
      • Decrease size by enhanced processing and optimal raw materials
      • Increase radius
      • Decrease number
    • Decrease number of surface flaws
      • Surface polishing
    • Decrease influence of surface flaws
      • Add a compressive layer at the surface
    • Minimizing stress-corrosion cracking
      • Protect the component from the environment
    • Use below the Weibull ‘rated’ load
    • Decrease component size (fewer flaws)
    • Keep temperature as low as possible
  • SUMMARY Reading for next class Phase diagrams Chapter sections 12.6, 12.7
    • Room temperature mechanical response of ceramics is elastic, but fracture is brittle with negligible ductility.
    • Elastic modulus and fracture strength are determined differently than for metals.
    • Ceramic materials are stronger in compression than in tension.
    • Elevated temperature properties are generally superior to those of metals.
    • Viscosity is the mechanism for deformation for amorphous ceramics.
    • Porous ceramics exhibit a strong variation in properties—why, and how can this be overcome?
    • Many ceramics are extremely hard. Why?