STRUCTURAL CERAMICS

Mechanical propertiesStructural ceramics IStructural ceramics encompass all ceramic materials that fulfil mechanical functions.

Advantages of ceramic materials over metals and polymers:

excellent temperature resistance high hardness high corrosion resistance low density

Disadvantages

lower fracture toughness higher price mechanical properties can only be indicated statistically

Mechanical propertiesStructural oxide ceramics

Aluminum oxide, aluminaAl2O3Zircon oxide, zirconiaZrO2Partially stabilized zirconia (with CeO, CaO, MgO or Y2O3)Zr0.9Mg0.1O1.9Aluminum titanateAl2TiO5 (ATi, AlTi)Cordierite Mg2Al4Si5O18MulliteAl6Si2O11SpinelMgAl2O4Lithium-aluminum-silicateLi2O-Al2O3-SiO2 - Basis (LAS)SIALONSi3N4-Al2O3-Al-SiO2 - Basis

Mechanical propertiesStructural carbide and nitride ceramics

Elastic deformationMechanical propertiesuniaxial compression in 2-D of a isotropic bodystrain:E: Youngs modulus : Poissons ratiosimple shear deformationstress strain relationshipG: Shear modulusMicroscopically the elastic deformation is due to the reversible stretching of atomic bonds.

Ceramics have much larger elastic moduli than metals, e.g. they are much less elastically deformed than metals

The theoretical strength of alumina shouldtherefore be between 40 and 50GPa, the measured values however are only 0.27GPa! Why?ceramicceramic compositemetalStress-strain relationships for different materials at ambient temperatureelastic deformationplastic deformationMaterial strengthMechanical propertiesYoungs modulus(GPa)Materialdiamond 1210, 970Al2 O3 (s,p) 460, 390MgO(s) 250SiC (p) 560Glasses 70 - 80Aluminum 60 -75 Steel 190

: surface energya0 : av. atomic distanceThe theoretical strength of a material(for a flawless single crystal) is related to the elastic modulus:

Fracture strength IMechanical propertiesThe maximum strength is based on the assumption that a body fails by simultaneous separation of all bonds, actual fracture in brittle material however occur by enlargement of preexisting flaws (cracks).

l2csEnergy promoting crack growth Energy resisting crack growth= elastic energy release stored at = surface energy the crack tip For a stable crack of length c e.g. one which does not open more the elastic energy release must be equal to the surface energy or less e.g.

Above a certain size the crack will start to selfpropagate for constant or even decreasing stress. The critical stress for a certain crack size c already present in the material is given by: KICis called the fracture toughness for opening mode loading, e.g.tensile stresses perpendicular to the crack axis. Fracture propagation prevention = toughening through microstructural adjustments

- Transformation toughening- Multiphase ceramics- Fiber reinforcement Fracture toughness(MPa /m2)MaterialAl2 O3 (s,p) 4.5, 3.5 - 4MgO(s) 1SiC(p) 4 - 6Glasses 0.7 - 2Aluminum 35 -45Steel 40 - 60

Fracture strength IIMechanical propertiesThe much lower than theoretically predicted strength of ceramic materials is due to the fact, that it is impossible to manufacture perfect ceramic parts which contain no cracks. A second problem is, that the number and the size of cracks present in a ceramic part are usually not known.

Although metals have a lower Young modulus than most ceramic materials, their actual strenght is much larger. Moreover, at a certain strenghth the deformation of metals becomes partly irreversible. The higher strength and the plastic behaviour is due to the dissipation of stress at the crack tips by the creation and movement of defects called dislocation.A linear disruption of the periodicity of a crystal structure is a linear defect, also called a dislocation. Three types of dislocations are known: pure edge, pure screw and mixed dislocations.

Edge dislocationsAn edge dislocation is the boundary of an extra half plane of atoms (unit cells) inserted into a perfect crystal:extra half plane of atomslower boundary of half plane = edge dislocation (dislocation line) running perpendicular to the paper foil perfect structuredisturbedstructureperfect structureMechanical propertiesPlasticity of metals

Burger vectorCharacterization of dislocations: Burgers vector loopIf such a loop does not close, one or more line defects are present in the interior of the loop. The line defect is characterized by the closure failure. For edge dislocations, the Burgers vector is perpendicular to the dislocation line. If the Burgers vector has the direction and the size of a lattice translation, the dislocation is perfect.Closing gap = Burgers vectorstart point6 latticetranslationsto the right6 latticetranslationsdown6 latticetranslationsup6 latticetranslationsto the leftMechanical properties

Screw dislocationScrew dislocationsAn screw dislocation has a Burgers vector parallel to the dislocation line.dislocation linestart of Burgersvector loopBurgers vectorMixed dislocationsDislocations with Burgers vector orientations oblique to the dislocation line are called mixed.Mechanical properties

Movement of dislocations Dislocation glideShear stress may initiate dislocations. Under continuous stress the dislocation will move through the crystal.

Edge dislocations: glide plane is always parallel to dislocation line and burgers vector.

Screw dislocations: glide plane can have different orientations, because Burgers vector and dislocationline are parallel. t1t2t3t4t5t6t7t8Mechanical properties

Energy of dislocationsThe elastic energy of dislocations are proportional to the square of the Burgers vector:

Eel = Gb2

: const.G: material elastic property, shear modulusb: Bugers vector

The most frequent Burgers vectors in a deformed material are, therefore, usually equal to the smallest lattice vectors of the phase.

Shortest lattice vectors of

MetalsCeramic Materials

Fe0.248 nmAl2O3 0.479nmAg0.288 nmZrO2 0.363nmNi0.248 nmBaTiO3 0.399nmMechanical propertiesThe stress necessary to activate dislocations in ceramic materials is thus much higher in ceramics than in metals. Glide activation in ceramics is only possible at high temperatures.

Dislocation examples High resolution electron transmissionmicroscopy (HRTEM) image of a edge dislocation in Si (arrow). The vertical lines correspond to lattice planes.Conventional TEM images of dislocation lines in MgO deformed under different stress. Straight dislocation lines have either pure screw or edge character. Curved lines and loops have mixed character. Mechanical properties

Mechanical propertiesHardness IHardness is the property of a material to withstand indentation and surface abrasion by another hard object. It is an indication of the wear resistance of a material. Alumina is very hard, metals however have a lower hardness, despite having a higher fracture toughness. When a sharp tip is imprinted on a metal, the surface will be deformed by the creation and glide of dislocations, not so ceramic surfaces. The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load F of 1 to 100 kg. The two diagonals d of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.cracksc

MohsMaterialVickersHardnessHardness1 Talc 12 Gypsum 33 Calcite 94 Fluorite 215 Apatite 486 Orthoclase 727 Quartz 1008 Topaze 2009 Corundum 400SiC 600TiC 60010 Diamond1500finger nail (2.5)coin (3.5)

steel (5.5)glass (6)Mechanical propertiesHardness IIhttp://www.gordonengland.co.uk/hardness/vickers.htm

Mechanical propertiesHardness III

c0a1a2Corundum structure, hexagonal unit cell setting, only the cation sublattice is shown. The oxygen form an hexagonal dense packed array.(210) projection of the corundum structure. Aluminum ions in adjacent face-sharing octahedra mutually repell each other.Al siteempty sitec0Structure of alpha-Al2O3Mechanical properties

Mechanical propertiesAlumina as structural ceramic Properties of reactive grade alumina:impuritiesNa2O0.08wt%melting temperature2050Csurface area6.8m2g-1sintering temperature 1550 - 1600Csintered density 3.92 (2h 1650C)fracture toughness 4 -4.5MPam1/2bend strength500 - 600 MPa Applications: hip protheses, cutting tools (zirconia-toughened)

http://www.cncmagazine.com/Cutting elements made of aluminaTriangular alumina-based cutting element used to machine metallic parts

5mm Dense hot pressed alumina without (top) and with addition of MgO (bottom) Grain growth is detrimental to the fracture strength of ceramics:

d: grain diam.Doping alumina with MgO leads to the formation of precipitates of spinell along the grain boundaries, which lowers the grain boundary mobility. (Bennison et al., 1983)Alumina: microstructure and strength IMechanical propertiesControlling the microstructure of alumina ceramics to enhance mechanical properties 5mm

Mechanical propertiesAlumina: microstructure and strength IIPorosity is detrimental to the mechanical strength:

Doping alumina with periclase reduces also the internal residual porosity. The picture (Geskovich et al.500x) shows an alumina body sintered without dopant. There is a large number of entrapped pores.When sintered with a dopant, the reduced grain boundary mobility allows the filling of the pores when they are at the grain boundaries, whereas fast grain growth encloses the pores quickly into the interior of the grain, where it is difficult to eliminate them. 0: strength at zero porosityb: constant.Pure alumina has a low fracture toughness. Mixing ca. 10% of zirconia (BSE image, zirconia: white) into the alumina doubles the fracture toughness.

Mechanical propertiesExample: Hip implantshttp://www.wmt.com/ceramichttp://www.ceramic-hip.com/healthcare/index.phpThe articulation of hip implants require:

Mechanical strength. Typical maximal loads within the human body are 10 to 15 kN.Wear resistance e.g. high hardnessBiocompatibility

Alumina is the material of choice. It is biocompatible e.g. no rejection reaction nor degradation in physiological liquids. The mechanical strength, though not very high, is 10 to 20 times higher than required for the maximum loads expected. The high hardness of alumina results in average wear rates for alumina-alumina coupling that are up to 50 times lower than for alumina - polyethylene or alumina - chrome cobalt alloys.

Pepper / Salt GrinderProcessingNet-shape Injection Molding PropertiesHigh HardnessResistance against NaCl AdvantagesNo CorrosionLong lifetimeCheaperMechanical properties

Zirconia as structural ceramicMechanical propertiesProperties of partially stabilized zirconia:dopantY2O3, CaO 3 - 10wt%melting temperature2500Csintered density 6.05 gcm -3 sintering temperature 1800CYoungs modulus170 -210 GPafracture toughness 6 - 20 MPam1/2Bend strength400 - 700 MPa

Applications: die material in the metall industry, thermal barrier coatings, piston caps, cutting tools

Piston parts (valves, sealings etc. made of stabilized zirconia.valvesealingSchematic drawing of a piston.

Polymorphs of ZrO2 cacaSchematic structures of the three zirconia polymorphscubic c-phase 2370C - 2680Ctetragonal t-phasec/a = 1.02! 1240C - 2370Cmonoclinic c-phase < 1240C- The cubic phase can be stabilized by doping with MgO, CaO or Y2O3- The tetragonal - monoclinic phase transformation involves a 4.7% volume increase.- This volume increase is the basis for transformation toughening.

Mechanical properties

Partially stabilized zirconia (PSZ)Manufacturing of partially stabilized zirconia

Add about 10% MgO Sinter in the cubic phaseLower temperature and heat treat (age) to nucleate small precipitates of t-phaseThese are growing below the critical size for t-m transformationCool to room temperatureRemaining c-phase has no time to transformZrO2-MgO phase diagram

Mechanical properties

Mg-PSZ MicrostructuresAfter sintering at 1800C an annealing stage at 1400C is introduced:

-After 4-5 hours tetragonal precipitates, grow by conventional diffusion processes as coherent spheroids along {001} cube planes

Below a well defined critical size of about 200 nm the t-particles remain tetragonal down to room temperature

- Optimum microstructures contains about 25% - 30% by volume of tetragonal phaseMechanical properties

tetragonal ZrO2 inclusion transformed to monoclinicstructurestress orientation around the crack tip cracktransformation zone 1. The stresses concentrated at the crack tip transform the surrounding tetragonal ZrO2 inclusions to the monoclinic polymorph. The transformation absorbs fracture energy and slows down crack propagation. Lense-shaped tetragonal inclusions ina matrix (black) of cubic zirconia (A. Heuer). Transformation toughening IMechanical properties

2. Microcracking around the transformed inclusions: The volume stresses resulting from the tetragona- monoclinic transformation delocalize also the stresses from the crack tip volume of the tetragonal zirconia inclusion volume after transformation to monoclinic stresses due to the volume increase microfracture due to the volume stresses3. Crack deflection due to volume stresses: The deflection of cracks increases the crack surface.The stress releave per unit penetration is, therefore, larger then for an inclusion free zirconia.

Transformation toughening IIMechanical properties

Initially tetragonal zirconia inclusion in a cubic zirconia matrix, which are completely transformed to the monoclinic structure. The bands within theinclusions are twin lamellae. 100nmTransformation toughening IIIMechanical properties

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