01-3-1 - materials, part 3 (wallace).ppt

Youngstown State University2

Fundamentals of Material Properties

- Part 3-Non-Metallic Materials for Manufacturing

Darrell Wallace

Youngstown State UniversityDepartment of Mechanical and Industrial

Engineering

January 14, 2006

Youngstown State University 3

Non-Metals in Manufacturing Long History

Organics Wooden Tools Textiles Rope

Ceramics Pottery

Very Different Properties from Metals Some Overlap of Processes Key to many “cutting edge” manufacturing

processes


Ceramics

What is a ceramic? Narrow Definition:

A compound composed of both metallic and non-metallic components

Broader Definition: Everything that is not a metal or organic and that

is subjected to very high temperature during manufacture or use.


Where do we find Ceramics?

Naturally Occuring:

Silica SiO2

Silicates SiO4

Oxides

Man-Made Carbides

Nitrides


Bonding and Structure

Ceramic materials are predominantly bound by covalent and ionic bonds


Covalent Bonds in Ceramics Covalent Bonds - Electrons are shared by adjacent

atoms Very Strong Has associated directionality Significant factor in atomic

spacing and crystalline structure

Associated Characteristics High melting point, strength, brittleness and hardness Low thermal expansion, thermal and electrical conductivity


Ionic Bonds in Ceramics Ionic Bonds: Electron transfer leads to ionization of

atoms. Attraction based on opposing electrical charges.

•Creates a smaller (denser) molecule than covalent bonding

•Brittle and nonconductive at lower temperatures, but exhibits some movement of dislocations and charge carriers at elevated temperatures.

•Deformation is particularly possible under elevated temperature and hydrostatic pressure

•Example: Na+Cl-


Crystalline Structure Most ceramics exhibit a crystalline structure in

their solid state Some ceramics exhibit different crystalline

structures (polymorphs) under different pressure or temperature conditions. Changes in crystalline structure lead to changes in

properties, especially density Volumetric changes tend to be more pronounced in

ceramics than in allotropic metals Ceramics that don’t have a crystalline structure

(amorphous) are called “glasses”


Glasses Glasses are formed when a ceramic is heated

above its melting point and cooled at a rate faster than the crystallization can occur.

Ceramic glasses can be held at elevated temperature for extended periods to allow stable crystalline structures to form. This is called “devitrification”

Amorphous glasses tend to be isotropic whereas crystalline ceramics can be very anisotropic.


Mechanical Properties of Ceramics Ceramics are VERY sensitive to stress risers

(notch sensitivity) Material tests must take great care not to damage the

surface Cracks are naturally occurring, so tests must be statistical

in nature.

Ceramics are less sensitive to crack formation in compression than in tension (including bending)

Excellent hot-hardness and dimensional stability


Improving Mechanical Properties of Ceramics

Reduce Particle Size Retard the Propagation of Large Cracks

Incorporate particles that suffer phase transformation

Introduce microfractures Guide the crack propagation with fibers

Induce Compressive Residual Stresses Reduce Creep (improve hot hardness)


Polymers and Plastics

From the Greek: Polymer:

Poly = many Meros = parts

Plastic: Plastikos = able to be molded or formed

Most polymers are based on Carbon chains and are, therefore, organic compounds.


Chain Polymerization Monomer (“one part”) Initiator is used to open up double bonds and allow it to

bond to adjacent atoms Polymerization occurs in the entire batch almost

simultaneously Most commonly forms hydrocarbon chains (aliphatic

hydrocarbons) or benzene rings (aromatic hydrocarbons) Additional elements may bond covalently

in place of a carbon atom (N, O, S, P, Si) In place of a hydrogen atom (Cl, F, Br)

Some of these polymers can be recycled through a process called high-temperature cracking


Chain Polymerization - Polyethylene

Polyethylene Monomer


Step-Reaction Polymerization Joining of two dissimilar monomers into short groups Pattern increases, usually releasing a low molecular

weight byproduct (for example, water in the case of nylon-6,6)

Such polymers can sometimes be recycled by depolymerization (unless cross-linked)


Degree of Polymerization The polymers form lengthy chains. The length of these chains has a significant

influence on mechanical properties. Measures of this characteristic include:

Molecular weight – average weight in grams of 1 mole (6.02x1023 molecules)

Degree of Polymerization – average number of mers in a molecule

Typical degrees of polymerization range from about 700 (LDPE) to 170,000 (UHMWPE)


Linear Polymers (Thermoplastics) “Straight” chains

Not truly straight, since bond angle of C-C bonds is 109.5˚

Chains twist and tangle together like sticky spaghetti

Shorter chains will not develop sufficient order to create crystalline patterns, thus amorphous (simple PE has lengths of only about 18nm)

Long straight chains (HDPE) may allow for more entanglement


Straight Chain Polymers


Linear Polymers (Thermoplastics)

Some polymers form pendant groups Polypropylene (PP), for example These pendant groups grow off of the sides of

the backbone of the polymer and increase “tangling”

Such polymers are characterized by the pattern of these pendant groups.


Pendant-Forming Polymers


Naming Conventions for Pendant-Forming Polymers

Isotactic – all pendants form on one side of the molecule Can develop highly ordered, compact, crystalline structure Wide use in engineering applications



Syndiotactic – pendants alternate sides in a pattern



Atactic – pendants alternate sides randomly Tight packing is not achievable Amorphous Generally poor properties


Bonding Between Polymer Molecules

Entanglement (mechanical bonding) Adds limited strength

Secondary Bonds Van der Waals (weak) Dipole bonds (polar molecules) Hydrogen bonds (strong)

H with O, N, or F


Crosslinked Polymers (Thermosets)

Occurs when bonds between molecules are covalent

Polymer becomes “cured” and process cannot be reversed


Characteristics of Thermosets

Strong High elastic modulus High temperature resistance Relatively brittle Bonds can only be broken by overheating,

and result is burning with carbon residue Scrap cannot be recycled except as filler


Elastomers

Capable of elastic deformation of 200% or more Thermoset Elastomers – crosslinked amorphous

linear polymers (e.g. natural rubber crosslinked with sulfer – ‘vulcanized’)

Thermoplastic Elastomers – semi-crystalline with glassy regions


Fillers and Additives

Polymer properties are often enhanced by the addition of other compounds Additives: agents designed to change properties

UV stabilization, flame retardant, plasticizers, dyes, lubricants

Fillers: reinforcing agents Add structural stability in a two-phase structure Effectively a composite material


Typical Mechanical Characteristics of Polymers Strength

Stress-strain characteristics are widely varied and typically are very sensitive to temperature

Range from pure elastic to nearly perfect-plastic Creep

Polymers are generally susceptible to creep, especially at elevated temperatures

Deflection temperature Residual Stresses

Anisotropy, particularly related to thermal expansion, often leads to residual stress considerations in polymer processing

Rheology Polymers can exhibit a wide range of viscosity behaviors

depending on formulation and applied process


Polymer RheologyS

hea

r S

tres

s,

.Shear Strain rate,

Newtonian

Bingham

Dilata

nt

Pseudoplastic


Composites Two or more distinct materials combined

such that the identities and properties of the constituent materials are retained.

Composites are usually “engineered” materials

Utilize materials with materials with complementary properties to compensate for weaknesses individually.


Matrix Composites Matrix Material

Polymer Metal Ceramic

Embedded Material Particulate Composites Fiber Reinforcement


Composites that Utilize Deliberate Orientation

Unidirectional composites



Biaxial Composite Designed to resist

stresses In two axes

Not designed to be strong in the third direction



Laminate Composites Stacks of planar

material Planar

subcomponents are usually varied in orientation to compensate for directionality.


Familiar Composites

Fiberboard, OSB, and Plywood Fiberglass Concrete / Steel-reinforced concrete Steel-belted radial tires Carbon-fiber

Bike frames, fishing poles, skis

Rice Krispy Treats

01-3-1 - materials, part 3 (wallace).ppt

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