BENG 186A Lecture 3: Metals and Ceramics

What are important design properties?

• Material selection

• Shape

• Bulk properties

• Chemical composition

• Physical properties

• Mechanical properties

• Surface properties

• Chemistry

• Topography

• Degradative properties

What are the properties of metals?

• Inorganic

• Non-directional metallic bonds

• Conduct electricity

• Strong

• Relatively easily formed into complex shapes

What is an FCC cubic?

• 4 atoms per unit cell, very ductile

• Atom on each corner and then in middle of each face

• Nickel, gold, platinum

• a = edge

• R = atomic radius

• a = 2Rsqrt2

What is a BCC cubic?

• 2 atoms per cubic face, less ductile

• Atom on each corner and then one in the middle

• Chromium, iron, tungsten

What is a HCP cubic?

• 6 atoms per cubic face

• Cobalt, zinc, titatnium

What is atomic packing factor and what does it depend on? What cubics is it highest in?

• Atomic packing factor = volume of atoms in a unit cell/ total unit cell volume

• Depends on crystal structure

• Highest in FCC and HCP

What is gray tin crystal structure?

• Gray tin top - diamond cubic

• Brittle material with poor electrical conductivity

What is white tin crystal structure?

• White tin bottom - body centered tetragonal

• Good electrical conductivity and thermal conductivity

What are defects?

• It is impossible to grow perfect crystals

• Defects change material properties

• Defects can lead to local lattice strain

What are intrinsic defects?

• Vacancy

• Self-Interstitial

What are extrinsic defects?

• Substitutional

• Interstitial

What are alloys? What properties do they improve?

• Intentional addition of impurities to engineer materials properties

• Alloys can improve mechanical strength and corrosion resistance, and can change electrical properties

• Almost all metals used as biomaterials are alloys

What are ceramics?

• Inorganic

• Non directional ionic bonds

• Typically poor conductors and good insulators

What are the advantages of the properties of ceramics?

• High modulus and compressive strength

• Aesthetic properties, dental applications

What are the disadvantages of the properties of ceramics?

• Brittle, sudden failure

• Low tensile strength

• Poor fatigue resistance

What is ceramic crystal structure?

• Most common ceramic materials are compounds with equal number of cations and anions

• Properties are affected by magnitude of ionic change and relative size of ions

How do ceramic defects occur?

• Charge unpairing is highly energetically unfavorable

• Defects in ceramics occur in charge-balanced pairs

What are ceramic impurities?

• Substitutional and interstitial impurities are possible

• Typically impurities maintain electronuertrality and replace matched charges

What are material linear defects?

• Linear defects can have impacts on final strength

• They are created during materials growth, and internal stresses from other defects

• Dislocations an deformations occur at linear defects

• Defects occur in more complex shapes

What is edge dislocation?

Burgers vector is perpendicular to dislocation

What is screw dislocation?

Planes form helical ramp, Burgers vector is parallel to dislocation

What is Burgers vector?

Denotes direction and magnitude of dislocation

What is the slip system and the slip plane?

• Dislocations do not move with the same propensity in all directions

• Slip system is the preferred plane of dislocation and specific directions

• The slip plane is determined by the plane that has the most dense atomic packing

What is slip plane for FCC?

• Slip planes are {1 1 1} family

• The slip direction is in the direction with the highest linear density of atoms

• Slip systems is number of planes x number slip directions

• 12 for FCC

How does slip compare in metals and ceramics?

• Metals have a higher number of slip systems and are more deformable

• Metals with few slip systems are brittle

• Ceramics require electroneutrality and therefore have less slip and are brittle

What are external planar defects in crystals?

• Every material has some planar defect

• Atoms at planar defects do not have the same bonding configuration as in the bulk of the material

• Results in higher energy at the surface, for example: surface tension and increased chemical interaction

What are internal planar defects?

• Most metals and ceramics are polycrystalline with a large number of randomly oriented crystals called grains

• Interface between grains are called grain boundaries

• Increased energy at the boundary, and therefore more reactivity

What is plastic deformation of polycrystalline materials?

• Most materials are polycrystalline

• Grains are randomly oriented

• Each grain has its own slip system

• Deformation leads to elongated grains

• Grain deformation is constrained by neighboring grains, and therefore an increased level of stress is required for deformation

• A dislocation must change direction when it encounters a grain boundary

• Polycrystalline materials are stronger than their single crystal equivalents

How does grain size reduction in polycrystalline materials impact strength and toughness?

Decreasing grain size leads to increased strength and toughness

What are grain boundaries?

• Small angle is less than 10-15 degrees

• Tilt boundaries are aligned edge dislocations

• Twist boundaries are aligned screw dislocations

• High angle is greater than 10-15 degrees

What are volume defects in crystals?

• Volume defects are 3D regions where long range order of crystal is lost

• Precipitate is a cluster of substitutional or interstitial impurities

• Voids are 3D aggregate of vacancies

• Volume defects lead to decreases in strength and increases in degradation

Why are the struts made of a titanium alloy material?

Stabilize bones of spine while they heal or fuse together. Strong but lightweight, biocompatible, corrosion-resistant, compatible with imaging,

elasticity closer to bone (less stiff than stainless steel)

What material properties would you want to engineer for these struts?

Combination of mechanical performance (high strength, high fatigue resistance,

appropriate stiffness, good toughness), biological compatibility (corrosion resistance,

low wear and particle generation, osseointegration ability), long-term durability,

manufacturability

What point defects are possible in NaCl if a Ca2+ ion is substituted for the Na+ ion?

• Substitutional defect – Ca2+ ion replaces Na+ ion at sodium lattice site

• Cation vacancy defect – Na+ vacancy created to compensate for extra positive charge

of Ca2+

• Charge-compensated substitutional defect pair

Why does plastic deformation occur by dislocation glide?

• Moving dislocations requires much less energy than sliding an entire plane of atoms

past another

• A dislocation allows atoms to shift gradually in small steps as the defects moves

through the crystal, reducing the applied stress needed for permanent deformation

Name one slip system for a BCC crystal.

There are 48 total slip systems possible

{110}<111> 6 unique {110} planes with 2 <111> directions/plane = 12

{112}<111}> 12 unique {112} planes with 1 <111> direction per plane = 12

{123}<111> 24 unique {123} planes with 1 <111> direction per plane = 24

*Some planes have one direction and others have 2, depending on how many <111>

directions fit inside that particular plane

*A slip system is one specific plane + one specific direction in that plane and crystal

symmetry determines how many unique choices exist