Youngstown State University 1 Lecture Summary Lecture Topic: Materials – Fundamentals of Material Properties, Part 3 Describing material characteristics of non-metals used in manufacturing Lecture Notes by: Darrell Wallace Allocated Time: 1 hr 15 minutes Teaching Objectives: (At the conclusion of this lecture, students should know ____) Required Materials / Preparation / Props LCD Projector and PC or overheads

Youngstown State University 2 Fundamentals of Material Properties - Part 3- Non-Metallic Materials for Manufacturing Darrell Wallace Youngstown State University Department 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

Youngstown State University 4 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.

Youngstown State University 5 Where do we find Ceramics? Naturally Occuring: Silica SiO 2 Silicates SiO 4 Oxides Man-Made Carbides Nitrides

Youngstown State University 6 Bonding and Structure Ceramic materials are predominantly bound by covalent and ionic bonds

Youngstown State University 7 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

Youngstown State University 8 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 -

Youngstown State University 9 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”

Youngstown State University 10 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.

Youngstown State University 11 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

Youngstown State University 12 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)

Youngstown State University 13 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.

Youngstown State University 14 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

Youngstown State University 15 Chain Polymerization - Polyethylene Polyethylene Monomer

Youngstown State University 16 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)

Youngstown State University 17 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 (6.02x10 23 molecules) Degree of Polymerization – average number of mers in a molecule Typical degrees of polymerization range from about 700 (LDPE) to 170,000 (UHMWPE)

Youngstown State University 18 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

Youngstown State University 19 Straight Chain Polymers

Youngstown State University 20 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.

Youngstown State University 21 Pendant-Forming Polymers

Youngstown State University 22 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

Youngstown State University 23 Naming Conventions for Pendant- Forming Polymers Syndiotactic – pendants alternate sides in a pattern

Youngstown State University 24 Naming Conventions for Pendant- Forming Polymers Atactic – pendants alternate sides randomly Tight packing is not achievable Amorphous Generally poor properties

Youngstown State University 25 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

Youngstown State University 26 Crosslinked Polymers (Thermosets) Occurs when bonds between molecules are covalent Polymer becomes “cured” and process cannot be reversed

Youngstown State University 27 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

Youngstown State University 28 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

Youngstown State University 29 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

Youngstown State University 30 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

Youngstown State University 31 Polymer Rheology Shear Stress, . Shear Strain rate,  Newtonian Bingham Dilatant Pseudoplastic

Youngstown State University 32 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.

Youngstown State University 33 Matrix Composites Matrix Material Polymer Metal Ceramic Embedded Material Particulate Composites Fiber Reinforcement

Youngstown State University 34 Composites that Utilize Deliberate Orientation Unidirectional composites

Youngstown State University 35 Composites that Utilize Deliberate Orientation Biaxial Composite Designed to resist stresses In two axes Not designed to be strong in the third direction

Youngstown State University 36 Composites that Utilize Deliberate Orientation Laminate Composites Stacks of planar material Planar subcomponents are usually varied in orientation to compensate for directionality.

Youngstown State University 37 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