Properties of Ceramics PDF

Properties of Ceramics http://justfunfacts.com/interesting-facts-about-ceramics/ 1 Ceramic Properties Mechanical properties o brittle fracture of ceramics  crystalline or non-crystalline ceramics usually fracture before any plastic deformation occurs  brit...

Properties of Ceramics http://justfunfacts.com/interesting-facts-about-ceramics/ 1 Ceramic Properties Mechanical properties o brittle fracture of ceramics  crystalline or non-crystalline ceramics usually fracture before any plastic deformation occurs  brittle fracture process consists of the formation and propagation of cracks that is perpendicular to the applied load 2 Ceramic Properties Mechanical properties o brittle fracture of ceramics  crack growth in crystalline ceramics could be either transgranular (through grains) or intergranular (along grain boundaries)  for transgranular fracture, cracks propagates along planes of high atomic density 3 Ceramic Properties Mechanical properties o brittle fracture of ceramics 4 https://www.researchgate.net/publication/228689472_A_New_Model_of_Microcracks_Propagation_in_Granite_Rock/figures?lo=1 Ceramic Properties Mechanical properties o brittle fracture of ceramics  very small flaws in ceramics act as stress-raisers that amplify the magnitude of an applied tensile stress  no plastic deformation mechanism to divert the crack formation 5 Ceramic Properties Mechanical properties o brittle fracture of ceramics  for monolithic ceramics, the degree of stress amplification depends on crack length and tip radius of curvature 1/2 𝑎 𝜎0 : magnitude of the nominal applied tensile stress 𝜌𝑡 : radius of curvature of the crack tip 𝜎𝑚 = 2 ∙ 𝜎0 ∙ 𝑎: length of a surface crack, or half of the length of 𝜌𝑡 an internal crack 6 Ceramic Properties Mechanical properties o brittle fracture of ceramics  the measure of a ceramic material’s ability to resist fracture when a crack is present is specified in terms of fracture toughness 7 Ceramic Properties Mechanical properties o brittle fracture of ceramics  the plane strain fracture toughness 𝐾𝐼𝑐 = 𝑌𝜎 𝜋𝑎 𝑌: dimensionless parameter that depends on both specimen and crack geometries 𝜎: the applied stress 𝑎: length of a surface crack, or half of the length of an internal crack 8 Ceramic Properties Mechanical properties o brittle fracture of ceramics 9 Ceramic Properties Mechanical properties o brittle fracture of ceramics  when 𝐾𝐼𝑐 > 𝑌𝜎 𝜋𝑎 , crack propagation will NOT occur  when 𝐾𝐼𝑐 > 𝑌𝜎 𝜋𝑎 , crack propagation may occur slowly if stresses are static (static fatigue, or delayed fracture) 10 Ceramic Properties Mechanical properties o brittle fracture of ceramics  stress corrosion cracking (SCC) mechanism 11 Ceramic Properties Mechanical properties o brittle fracture of ceramics  fracture strength of a specific brittle ceramic material, depends on  the presence of a flaw that can initiate a crack  fabrication technique  treatment technique 12 Ceramic Properties Mechanical properties o brittle fracture of ceramics  fracture strength of a specific brittle ceramic material, depends on  specimen size, or volume  time of stress application 13 Ceramic Properties Mechanical properties o brittle fracture of ceramics  demonstrates higher strengths in compressions than in tension  fracture strength of a specific brittle ceramic can be further improved by thermal tempering 14 Ceramic Properties Mechanical properties o fractography of ceramics  acquires information regarding the cause of a ceramic fracture  a failure analysis focuses on determination of the location, type, and source of the cracking-initiating flaw 15 Ceramic Properties Mechanical properties o fractography of ceramics  examines the path of crack propagation and microscopic features of the fracture surface  after nucleation, a crack accelerates until a critical velocity is achieved. Then a crack branches 16 Ceramic Properties Mechanical properties o fractography of ceramics 17 Ceramic Properties Mechanical properties o fractography of ceramics  the site of nucleation can be traced back to the point where a set of cracks converges  the rate of crack acceleration increases with rising stress 18 Ceramic Properties Mechanical properties o fractography of ceramics  during propagation, a crack interacts with the microstructure of the material, generating distinctive features on the fracture surface and providing information of cracking origin, applied stress, and etc 19 Ceramic Properties Mechanical properties o fractography of ceramics 1 𝜎𝑓 ∝ 𝑟𝑚 0.5 𝜎𝑓 : stress level where fracture occurs 20 Ceramic Properties Stress-strain behavior o flexural strength  the stress-strain behaviors of ceramics is usually studied using transverse bending test, because universal ceramic specimen with required geometry cannot be achieved 21 Ceramic Properties Stress-strain behavior o flexural strength  three-point loading scheme 22 Ceramic Properties Stress-strain behavior o flexural strength  is also called modulus of rupture, fracture strength, or bend strength  for a rectangular cross section 3𝐹𝑓 𝐿 𝐹𝑓 : the load at fracture 𝜎𝑓𝑠 = 𝐿: distance between support points 2𝑏𝑑 2 23 Ceramic Properties Stress-strain behavior o flexural strength  is also called modulus of rupture, fracture strength, or bend strength  for a circular cross section 𝐹𝑓 𝐿 𝐹𝑓 : the load at fracture 𝜎𝑓𝑠 = 𝐿: distance between support points 𝜋𝑅3 𝑅: radius of specimen 24 Ceramic Properties Stress-strain behavior o flexural strength 25 Ceramic Properties Stress-strain behavior o elastic behavior  elastic stress-strain behavior for ceramics using flexure tests, is similar to the tensile test results for metals  moduli of elasticity of ceramics is higher than that of metals 26 Ceramic Properties Stress-strain behavior o elastic behavior 27 Ceramic Properties Mechanisms of plastic deformation o crystalline ceramics  plastic deformation occurs by the motion of dislocations  the difficulty to slip is the reason to make ceramics hardness and brittleness  bonding is mainly ionic  few slip systems that allow dislocation move 28 Ceramic Properties Mechanisms of plastic deformation o crystalline ceramics  the electrically charged ions create electrostatic repulsion, when ions of like charge are close to each other during slip 29 Ceramic Properties Mechanisms of plastic deformation o crystalline ceramics  if the bonding is highly covalent, slip is also difficult due to  the strong covalent bonds  limited numbers of slip systems  dislocation structures are complex 30 Ceramic Properties Mechanisms of plastic deformation o non-crystalline ceramics  plastic deformation occurs by viscous flow  the rate of deformation is proportional to the applied stress  atoms or ions slide past one another by the breaking and reforming of interatomic bonds 31 Ceramic Properties Mechanisms of plastic deformation o non-crystalline ceramics  viscosity – a measure of a non-crystalline ceramic’s resistance to deformation 𝐹 𝜏 𝐴 𝜂= = 𝑑𝑣 𝑑𝑣 𝑑𝑦 𝑑𝑦 32 Ceramic Properties Mechanisms of plastic deformation o non-crystalline ceramics  viscosity – a measure of a non-crystalline ceramic’s resistance to deformation  as the temperature increases, the magnitude of the bonding is diminished 33 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  influence of porosity – porosity inside ceramics is originated from the void spaces in between ceramic powder particles  porosity will have a deleterious influence on the elastic properties and strength 34 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  influence of porosity 35 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  influence of porosity – the magnitude of the modulus of elasticity E decreases with volume fraction porosity P 𝐸 = 𝐸0 1 − 1.9𝑃 + 0.9𝑃2 𝐸0 : modulus of elasticity of the nonporous material 36 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  influence of porosity 37 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  the deleterious influence is due to  pores reduce the cross-sectional area which a load is applied  act as stress concentrators – 10 vol% porosity decreases the flexural strength by 50% of the value of nonporous one 38 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations 𝜎𝑓𝑠 = 𝜎0 ∙ exp −𝑛𝑃 39 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  hardness – accurate hardness measurements on ceramics are hard to conduct  usually is measured using Vickers and Knoop techniques that employ indenters having pyramidal shapes 40 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  hardness 41 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations 42 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  creep – ceramics experience creep deformation as a result of exposure to stresses at elevated temperatures  time-deformation creep behavior of ceramics is similar to that of metals 43 Ceramic Properties Mechanisms of plastic deformation o miscellaneous mechanical considerations  creep 44