Exhaustive Guide to Metal, Ceramic, and Polymer Matrix Materials

Metal Matrix Materials

Aluminium (Al) Alloys * General Properties * Density is approximately $2700\,kg/m^3$ , which is roughly one-third the density of steel ( $7900\,kg/m^3$ ). * Materials are characterized as being quite reactive. * Oxidation Resistance: Aluminium resists progressive oxidation (unlike steel which rusts away). It forms an inert aluminum oxide film on the exposed surface upon contact with oxygen. This film is approximately "a few ten-millionths of an inch" thick and serves to block further oxidation. * Casting Alloys: Often alloyed with Silicon (Si); these are noted for being easy to process. * Wrought Alloys: Often alloyed with Magnesium (Mg); these possess moderate mechanical properties. * Structural Alloys: Often alloyed with Copper (Cu) or Zinc (Zn) / Magnesium (Mg) for higher performance.
Magnesium (Mg) Alloys * General Properties: These possess the lowest density among the metals ( $1700\,kg/m^3$ ) but are very reactive and harder to process than Al alloys. * Cast Alloys: Alloyed with Al and Manganese (Mn). Characterized by being less strong, less stiff, and less tough. * Aircraft Forging Alloys: Alloyed with Zinc (Zn) and Zirconium (Zr).
Titanium (Ti) Alloys * General Properties: Higher density than Al and Mg alloys ( $4500\,kg/m^3$ ), but significantly stiffer and stronger. * Pure Titanium: Used for its higher temperature resistance. * Alpha Titanium: Alloyed with Al and Tin (Sn). These are quite reactive at high temperatures. * Beta Titanium: Alloyed with Vanadium (V), Iron (Fe), and Al.
Comparative Summary of Metals * Al alloys: Cheapest, tough, and easiest to process. * Mg alloys: Lightest but most brittle. * Ti alloys: Strongest, tough, highest melting point, but the most expensive.

Ceramic Matrix Materials

Aluminium Oxide (Alumina) * Density: $3900\,kg/m^3$ * Melting Point: $2045^\circ C$ * Strength: Approximately $400\,MPa$ * Young’s Modulus: $340\,GPa$ * Toughness ( $K_{1c}$ ): $4\,MPam^{1/2}$ * Applications: High temperatures in air and high stiffness requirements.
Zirconium Oxide (Zirconia) * Alloying: Often alloyed with magnesium oxide ( $MgO$ ) or yttrium oxide ( $Y_2O_3$ ) for extra toughness. * Density: $5700\,kg/m^3$ * Melting Point: $2660^\circ C$ * Strength: Approximately $400\,MPa$ * Young’s Modulus: $205\,GPa$ * Toughness ( $K_{1c}$ ): $4\,MPam^{1/2}$ * Applications: Higher temperature applications than Alumina.
Glass Ceramics * Composition: Most commonly lithium aluminosilicates (LAS). * Density: $3500\,kg/m^3$ * Melting Point: Approximately $1300^\circ C$ * Strength: Approximately $150\,MPa$ * Young’s Modulus: $80\,GPa$ * Toughness ( $K_{1c}$ ): $1.5\,MPam^{1/2}$ * Characterization: Easier processing due to lower melting temperatures; however, they are less stiff and less tough than other ceramics.
Carbon * Structure/Properties: Large variations in structure and performance. * Primary Benefit: Very high temperature strength, provided no oxygen is present. * Oxidation: Oxidizes in the presence of oxygen starting from $400^\circ C$ .

Thermosetting Polymer Matrix Materials

Step Reaction Polymerisation * Involves reactions between functional groups located on both ends of a monomer. * Crosslinking: If a monomer has more than $2$ functional groups, branching and crosslinking occur, producing a thermoset. * Initiation: The reaction is started by mixing two components or by applying high temperatures ( $T$ ). Extended pre-polymers can be crosslinked using double bonds, such as with styrene, often requiring peroxide-based catalysts.
Polyester Resins * Mechanism: Ester links are formed between acid and alcohol groups. Formation requires a di-alcohol (glycol) and a di-acid. If these are both simple, they produce linear polyesters (thermoplastics). * Useful Resin Chemistry: For cross-linkable resins, an acid anhydride is used to introduce double bonds, creating an unsaturated polyester. This is then cross-linked with styrene. * Reaction Components: Propylene glycol + Maleic anhydride $\rightarrow$ Unsaturated polyester. * Practical Systems: Consist of unsaturated polyester supplied mixed with styrene (the styrene acts as a solvent to reduce viscosity and as a crosslinking agent). A peroxide-based catalyst starts the reaction, typically at room temperature. * Advantages: Low cost, easy processing, and reasonable mechanical properties. * Disadvantages: Poor toughness, low temperature resistance, and poor resistance to alkalis. * Applications: General purpose resin for automotive, marine, and construction sectors.
Polyvinyl Esters * Structure: Similar to polyesters but double bonds are located only at the ends of the chains. * Properties: Fewer cross-links compared to standard polyesters; more flexible and tougher. * Chemical Performance: Better chemical resistance (due to fewer double bonds) but higher water absorption (due to more $-OH$ groups). * Cost: Slightly more expensive than polyesters; used where higher toughness is required.
Epoxy Resins * Pre-polymers: Require molecules with $2$ epoxy groups. Examples include: * Diglycidyl ether of bisphenol-A (DGEBA) * Tetraglycidyl ether of diamino-diphenyl methane (TGDDM) * Epoxy novalac * Crosslinking Agents: Di-amine, tri-amine, or boron tri-fluoride ( $BF_3$ ). * Delivery: Suppled as $2$ -part liquids or $1$ -component heat-activated systems. * Advantages: Superior mechanical properties, low shrinkage, and better temperature resistance (up to $180^\circ C$ ). * Disadvantages: More expensive, harder to process, and higher water absorption. * Applications: Aerospace resins and wherever high strength, stiffness, toughness, and thermal stability are required.
Phenolic Resins * Types: Novalacs or Resoles. * Structure: Dominated by benzene ring structures. * Curing: Resoles crosslink by heating; Novolacs require curing agents. * Advantages: Excellent temperature resistance (up to $220^\circ C$ ), low flammability, and low cost. * Disadvantages: Low toughness. * Applications: High-temperature and flame-resistant environments.
Polyimides * Description: Similar to epoxies but offer higher temperature resistance. * Trade-offs: Lower toughness and more expensive than epoxies.

Thermoplastic Polymer Matrix Materials

Semi-Crystalline Polymers * Structure: Composed of regions where chains are arranged in a regular crystalline manner (held by secondary bonds) and amorphous regions where chains are random. * Melting Point ( $T_m$ ): These materials have a definite melting point. * Crystallinity Effects: Higher crystallinity leads to increased stiffness, increased strength, better chemical and thermal resistance, reduced light transmission, and better barrier properties.
Polyethylene (PE) * General: Highly versatile, cheap, easily molded/recycled, low strength but high ductility and impact strength. Melting points range from $120\,^\circ C$ to $180\,^\circ C$ . * High Density Polyethylene (HDPE): Density $\ge 0.941\,g/cm^3$. Low degree of branching allows linear molecules to pack tightly, leading to stronger intermolecular forces and high tensile strength. * Medium Density Polyethylene (MDPE): Density range $0.926\text{--}0.940\,g/cm^3$ . Known for good shock and drop resistance. * Low Density Polyethylene (LDPE): Density range $0.910\text{--}0.940\,g/cm^3$ . High degree of short- and long-chain branching results in low tensile strength and high ductility.
Polypropylene (PP) * Properties: Commodity thermoplastic, cheap, easily molded/recycled. Not particularly strong or stiff. Melting point $\sim 170\,^\circ C$.
Nylons * Properties: Stiffer and stronger than polypropylene. More expensive and harder to process. Notably affected by water. Melting points are between $200\,^\circ C$ and $260\,^\circ C$ .
Poly-aryl-ethers (e.g., PEEK) * Properties: Strong, stiff, tough, and chemically resistant. Melting point up to $340\,^\circ C$ . It is an expensive material.
Polysulphones & Polyimides * Properties: Complex structures with high melting points and good mechanical properties. They are expensive materials.

Technical Specification Reference Table

Material	Density ( $kg/m^3$ )	Stiffness ( $GPa$ )	Tensile Strength ( $MPa$ )	Failure Strain ( $\%$ )	$K_{1c}$ ( $MPa.m^{0.5}$ )	Melting Point/Max Temp ( $^\circ C$ )
Al casting alloys	$2700$	$75$	$230$	$3.5$	-	$660$
Al wrought alloys	$2700$	$75$	$230$	$15$	-	$660$
Al structural alloys	$2700$	$75$	$500$	$11$	-	$660$
Mg casting alloys	$1700$	$45$	$200$	$11$	-	$650$
Mg forging alloys	$1700$	$45$	$350$	$5$	-	$650$
Ti	$4500$	$107$	$450$	$25$	-	$1668$
Alpha Ti	$4500$	$107$	$800$	$16$	-	$1668$
Beta Ti	$4500$	$107$	$1200$	$10$	-	$1668$
Alumina	$3900$	$340$	$400$	<2	$4$	$2045$
Zirconia	$3900$	$205$	$400$	<2	$4$	$2660$
Glass ceramics	$3500$	$80$	$150$	<2	$1.5$	$1300$
Polyesters	$1100\text{--}1500$	$1.3\text{--}4.5$	$45\text{--}85$	$\sim 5$	$0.5$	$80^\dagger$
Polyvinyl esters	$1100\text{--}1500$	$1.0\text{--}2.5$	$30\text{--}60$	$\sim 10$	$1.0$	$80^\dagger$
Epoxies	$1100\text{--}1400$	$2.1\text{--}6.0$	$35\text{--}90$	$5\text{--}15$	$0.6\text{--}1.0$	$180^\dagger$
Phenolics	$1300$	$4.4$	$50\text{--}60$	$\sim 2$	$0.3$	$220^\dagger$
Polyimides	$1200\text{--}1900$	$3.0$	$80\text{--}190$	$\sim 3$	$0.5$	$250^\dagger$
Polypropylene	$950$	$1.2$	$30$	>50	-	$170$
Nylons	$1140$	$3.0$	$80$	>50	-	$220\text{--}260$
PEEK	$1300$	$3.8$	$90$	$50$	-	$320$
(Steel Reference)	$7900$	$210$	To $600$	$10$	-	$\sim 1500$

* $\dagger$ Note: For thermosets marked with *, no melting point exists; instead, the maximum use temperature is quoted.*