Composites, polymers and ceramics

In short

This topic covers the three non-metallic families of engineering materials: polymers (long-chain organic molecules), ceramics (ionically/covalently bonded inorganic compounds) and composites (two or more materials combined so the mix beats either one alone). They matter because almost every weight, corrosion, temperature or cost problem that metals cannot solve is now solved by one of these — wind blades, uPVC pipes, cutting-tool inserts, brake pads, refractory linings, aircraft structures. In exams they carry steady marks in GATE ME (rule of mixtures, polymer classification, ceramic brittleness) and in university Material Science / Engineering Materials papers. In placement rounds they are asked by Reliance and Supreme Industries (polymers), Kajaria and CUMI (ceramics and abrasives), HAL, ISRO, Tata Advanced Systems, Kineco and Suzlon (composites), and by every automotive interviewer asking "why plastic here and not steel?"

Explained

Polymers: structure and classification

• Mer / monomer: the repeating chemical unit; polymer = many mers joined. Degree of polymerisation (DP) = number of mers per chain = Mn / (mer molecular weight).
• Molecular weight: chains differ in length, so a polymer has a distribution. Mn (number-average) and Mw (weight-average); polydispersity index PDI = Mw/Mn ≥ 1. Higher molecular weight → higher strength, higher melt viscosity, harder to mould.
• Polymerisation routes: (i) addition / chain-growth — initiation, propagation, termination of C=C double bonds, no by-product (PE, PP, PVC, PS); (ii) condensation / step-growth — functional groups react, releasing a small molecule such as water (nylon, PET, phenolic, epoxy).
• Chain architecture: linear, branched (branching lowers packing and density — LDPE vs HDPE), cross-linked, network.
• Tacticity: isotactic (all side groups one side — regular, crystallisable, strong), syndiotactic (alternating), atactic (random — amorphous, soft). Commercial PP is isotactic; atactic PP is a sticky wax.
• Copolymers: random, alternating, block, graft — e.g. ABS (acrylonitrile-butadiene-styrene), SBR rubber.
• Crystallinity: polymers are semi-crystalline at best (lamellae + amorphous regions). Simple, linear, regular chains crystallise; bulky or cross-linked ones do not. Crystallinity raises density, stiffness, strength, chemical resistance and opacity.

Three polymer classes, thermal behaviour and processing

• Thermoplastics: linear/branched chains held by secondary bonds; soften on heating, remeltable, weldable, recyclable. PE, PP, PVC, PS, PET, PMMA, PC, nylon, ABS, PTFE; engineering grades PEEK, PPS, POM.
• Thermosets: three-dimensional covalent network formed by curing; do not melt, char on overheating, cannot be reshaped or easily recycled; stiffer, more creep- and heat-resistant. Epoxy, phenolic (Bakelite), unsaturated polyester, melamine, polyurethane (can be either).
• Elastomers: lightly cross-linked, coiled chains above Tg; huge recoverable strain. Vulcanisation = cross-linking rubber with sulphur; more sulphur → harder, less extensible. Natural rubber (NR), SBR, nitrile, neoprene, silicone, EPDM. Hardness measured in Shore A.
• Transitions: Tg (glass transition) — amorphous regions change from glassy/brittle to leathery/rubbery; Tm (melting) applies only to the crystalline fraction. Service temperature must sit on the right side of Tg: natural rubber Tg ≈ −70 °C, PVC ≈ 80 °C, polycarbonate ≈ 150 °C.
• Viscoelasticity: response depends on time and temperature — creep (strain grows at constant stress), stress relaxation (stress decays at constant strain), hysteresis. Modelled by Maxwell (spring + dashpot in series) and Kelvin-Voigt (parallel) elements.
• Deformation: glassy polymers craze and fracture; above Tg they draw (necking with chain alignment) and strain-harden; elastomers show non-linear rubber elasticity.
• Processing: extrusion (pipes, profiles, film), injection moulding (mass-produced parts), blow moulding (bottles), rotational moulding (water tanks), thermoforming (trays), calendering (sheet), compression and transfer moulding (thermosets), FDM 3D printing (ABS, PLA, PETG).
• Additives: plasticisers (flexible PVC), fillers (CaCO₃, talc — cost and stiffness), UV and heat stabilisers, antioxidants, flame retardants, pigments, lubricants, blowing agents.
• Degradation: UV photo-oxidation, thermal and oxidative ageing, hydrolysis, swelling in solvents, environmental stress cracking, and permanent creep. Resin identification codes 1–7 (1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 other).
• Indian practice: BIS product standards — IS 4984 (PE pipes for water), IS 4985 (uPVC pipes), IS 15778 (CPVC), IS 14333 (HDPE sewerage pipes); IS 13360 is the multi-part plastics test-method series (many parts are adoptions of ISO methods). Regulation: the Plastic Waste Management Rules, 2016 under the Environment (Protection) Act, 1986, as amended — EPR obligations on producers, importers and brand owners, the ban on identified single-use plastic items from 1 July 2022, and a 120-micron minimum for carry bags since 31 December 2022. Several polymers have also been brought under BIS Quality Control Orders (mandatory ISI marking); the covered list changes, so check the current notification.

Ceramics: structure, properties and processing

• Definition: inorganic, non-metallic compounds of metals with O, N, C, B, Si — ionic and/or covalent bonding, no free electrons.
• Consequences of that bonding: high melting point and hardness, high stiffness, chemical and wear resistance, electrical/thermal insulation (except SiC, AlN, BeO which conduct heat well) — but brittleness, near-zero plasticity at room temperature (dislocation slip is blocked by charge and directional bonds) and low fracture toughness.
• Traditional ceramics: clay-based — bricks (IS 1077), tiles (IS 15622, tested to IS 13630), whiteware, sanitaryware, cement (IS 269 covers OPC 33/43/53 grades) and concrete (design to IS 456).
• Refractories: acidic (silica, fireclay), basic (magnesia, dolomite), neutral (chromite, alumina, carbon); specified by PCE, RUL, porosity and slag resistance (BIS IS 1528 series carries the test methods). Used in blast-furnace, ladle, cement-kiln and glass-tank linings — Orient Refractories, IFGL, Vesuvius India.
• Advanced / engineering ceramics: Al₂O₃ (insulators, wear parts, armour), ZrO₂ (toughened, oxygen sensors, thermal barriers), SiC (seals, heat exchangers, armour, abrasive), Si₃N₄ and SiAlON (bearings, turbine parts), B₄C (armour, nozzles), WC-Co cermet (cutting tools), PZT (piezoelectric), ferrites (magnetic), hydroxyapatite and alumina (bioceramics, implants).
• Glass: amorphous (no Tm, only Tg plus annealing, softening and working points). Soda-lime float glass, borosilicate (low α), silica. Tempering puts the surface in compression so the glass must overcome that before a flaw opens. Glass-ceramics are glasses deliberately crystallised (controlled nucleation + growth) for low α and high strength.
• Powder processing route (standard exam sequence): powder preparation and milling → mixing with binder/plasticiser → forming (uniaxial die pressing, cold/hot isostatic pressing, slip casting, tape casting, extrusion, injection moulding) → drying → binder burnout → sintering (solid-state diffusion or liquid-phase, typically 0.7–0.9 Tm) with 10–20% linear shrinkage → finishing by diamond grinding/lapping → glazing where needed.
• Densification variants: hot pressing and HIP (pressure + heat, near-zero porosity), reaction bonding (RBSN, RBSC), sintering aids.
• Why strength scatters: failure starts at the worst pre-existing flaw (Griffith criterion), so strength is a statistical quantity described by the Weibull modulus m — low m means wide scatter. Ceramics are 5–10 times stronger in compression than in tension, so designs load them in compression.
• Toughening: transformation toughening (tetragonal → monoclinic ZrO₂ at the crack tip, as in PSZ and ZTA), fibre/whisker reinforcement, microcrack and crack-deflection toughening, compressive surface layers.
• Testing: strength measured in 3- or 4-point flexure (modulus of rupture), not tension; hardness by Vickers/Knoop; toughness by indentation or notched-beam methods.

Composites: definition, classification and micromechanics

• Definition: a matrix (continuous, transfers load, protects) plus a reinforcement (discontinuous, carries load) that remain chemically distinct, giving properties neither has alone. The interface decides whether the design works.
• Classification by reinforcement: (i) particle-reinforced — large-particle (concrete, WC-Co, filled polymers) and dispersion-strengthened (fine oxide particles blocking dislocations, e.g. SAP, TD-nickel); (ii) fibre-reinforced — continuous aligned, discontinuous aligned, discontinuous random; (iii) structural — laminates and sandwich panels.
• Classification by matrix: PMC (polymer matrix — 90% of production; GFRP, CFRP, aramid), MMC (Al-SiC, Al-Al₂O₃ by stir casting, squeeze casting or powder metallurgy — pistons, brake rotors, electronic packaging), CMC (SiC/SiC, C/SiC — hot sections, brake discs), CCC (carbon-carbon — rocket nozzles, aircraft brakes).
• Fibres: E-glass (cheap, 72 GPa), S-glass (stronger), carbon (standard modulus ~230 GPa, high modulus up to ~400 GPa), aramid/Kevlar (impact and ballistic), boron, basalt, and natural fibres (jute, coir, sisal, banana, bamboo) which India has in volume and which suit low-cost, low-load panels.
• Matrix resins: unsaturated polyester (cheap, general FRP), vinyl ester (chemical plant), epoxy (aerospace, best adhesion and properties), phenolic (fire/smoke/toxicity performance), plus thermoplastic matrices (PP, PA, PEEK) for recyclability and toughness.
• Micromechanics: stiffness along the fibres follows the isostrain (Voigt) rule of mixtures; across the fibres it follows the isostress (Reuss) rule and is dominated by the weak matrix — hence strong anisotropy. Fibres carry almost all the load when Ef ≫ Em.
• Short fibres: load builds up along the fibre by interfacial shear, so a fibre shorter than the critical length lc pulls out instead of breaking. Design rule of thumb: l > 15 lc behaves as continuous.
• Coupling: silane coupling agents on glass fibre, surface oxidation/sizing on carbon fibre, matrix wetting — poor interface gives low transverse and shear strength.
• Laminates: plies stacked at chosen angles; 0/90 (cross-ply), ±45 (shear/torsion), quasi-isotropic 0/±45/90; stacks are made symmetric and balanced about the mid-plane to avoid warping on cure. Classical laminate theory assembles the A, B, D stiffness matrices (derivation usually only outlined at UG level).
• Sandwich construction: stiff thin faces (FRP, aluminium) carry bending stress; a light core (honeycomb, PU/PVC foam, balsa) carries shear and keeps the faces apart — very high bending stiffness per unit weight.
• Manufacturing: hand lay-up and spray-up (cheap, labour-intensive, Vf ~0.3), filament winding (pressure vessels, CNG cylinders), pultrusion (constant-section profiles, gratings), RTM / VARTM (closed-mould, good finish), prepreg + autoclave (aerospace, Vf 0.6, best quality, costly), SMC/BMC compression moulding (automotive panels), short-fibre injection moulding.
• Failure modes: fibre fracture, matrix cracking, fibre-matrix debonding, fibre pull-out, delamination (the critical one; caused by impact and free edges, often invisible on the surface) and buckling in compression.
• Advantages: very high specific strength and specific stiffness, excellent fatigue and corrosion resistance, properties tailorable by direction, part-count reduction, low thermal expansion, no magnetic signature.
• Limitations: high raw-material and tooling cost, anisotropic and low transverse/interlaminar strength, poor impact damage tolerance, difficult joining (bolt holes cut fibres; bonding preferred), moisture and UV ageing, limited recyclability of thermosets, and inspection needing ultrasonic/thermographic methods.

Key points

• Thermoplastic vs thermoset - secondary bonds, remeltable and recyclable vs covalent network, infusible and non-recyclable.
• DP - mers per chain; Mn/(mer weight). Strength and melt viscosity both rise with it.
• Tg - the amorphous softening transition; Tm belongs only to crystalline regions. Glass and thermosets show Tg only.
• Tacticity - isotactic chains pack and crystallise; atactic chains stay amorphous and weak.
• Vulcanisation - sulphur cross-links that turn sticky rubber into an elastomer; more sulphur → harder.
• Viscoelasticity - polymers creep and relax; design on long-term, not short-term, modulus.
• Ceramic brittleness - ionic/covalent bonds block slip, so failure is flaw-controlled and scattered (Weibull modulus).
• Compression rule - ceramics are far stronger in compression than tension; never load them in tension or bending if avoidable.
• Sintering - densification by diffusion below the melting point, with 10–20% shrinkage that must be allowed for in the green part.
• Transformation toughening - metastable tetragonal zirconia transforming at the crack tip absorbs energy and closes the crack.
• Thermal shock resistance ∝ strength × thermal conductivity / (modulus × expansion) — why SiC survives quenching and soda-lime glass does not.
• Rule of mixtures - isostrain gives the upper bound (along fibres), isostress the lower bound (across fibres).
• Critical fibre length - lc = σf·d/(2τc); shorter fibres pull out and waste their strength.
• Load sharing - Pf/Pm = EfVf/(EmVm); in a 60% carbon/epoxy the fibres take about 99% of the load.
• Specific properties - composites win on strength/density and stiffness/density, not on absolute values; glass/epoxy is no stiffer per kg than steel, carbon/epoxy is 3–4 times stiffer per kg.
• Laminate design - symmetric and balanced stacks; quasi-isotropic 0/±45/90 where direction is unknown.
• Sandwich panel - faces take bending, core takes shear; maximum stiffness per unit mass.
• Indian regulation - Plastic Waste Management Rules, 2016 (as amended) with EPR and the single-use plastic ban; BIS product standards for pipes, tiles, cement and refractories.

Formulas and examples

Example 1 — unidirectional glass/epoxy vs carbon/epoxy (Vf = 0.6). Take Ef = 72 GPa, ρf = 2.54 g/cm³ (E-glass); carbon Ef = 230 GPa, ρf = 1.80; epoxy Em = 3.5 GPa, ρm = 1.20.

• Glass/epoxy: El = 72(0.6) + 3.5(0.4) = 43.2 + 1.4 = 44.6 GPa.
• Transverse: 1/Et = 0.6/72 + 0.4/3.5 = 0.008333 + 0.114286 = 0.122619 → Et = 8.16 GPa — only 18% of El, the anisotropy penalty.
• Density: ρc = 2.54(0.6) + 1.20(0.4) = 1.524 + 0.48 = 2.004 g/cm³; specific modulus = 44.6/2.004 = 22.3 against steel's 210/7.85 = 26.8 — glass/epoxy is *not* stiffer per kg than steel.
• Carbon/epoxy: El = 230(0.6) + 3.5(0.4) = 139.4 GPa; ρc = 1.80(0.6) + 1.20(0.4) = 1.56 g/cm³; specific modulus = 89.4, i.e. 3.3 times steel.
• Load share in the carbon laminate: fibres take EfVf/El = 138/139.4 = 99.0% of the load.

Example 2 — weight to volume fraction, and strength. A prepreg is 70% carbon fibre by weight in epoxy.

• Vf = (0.7/1.8) / [(0.7/1.8) + (0.3/1.2)] = 0.3889/(0.3889 + 0.25) = 0.3889/0.6389 = 0.609 (60.9% by volume).
• With σf* = 3,500 MPa and σm' = 30 MPa: σcl = 3500(0.609) + 30(0.391) = 2,131.5 + 11.7 ≈ 2,143 MPa.
• Specific strength = 2143/1.566 ≈ 1,368 MPa/(g/cm³) versus mild steel 400/7.85 ≈ 51 — a 27-fold advantage, which is why pressure vessels and blades are wound, not forged.

Example 3 — critical fibre length and a short-fibre moulding. Carbon fibre σf* = 3,500 MPa, d = 7 µm, interfacial τc = 35 MPa.

• lc = 3500 × 7 / (2 × 35) = 24,500/70 = 350 µm = 0.35 mm; fibres should exceed 15 lc ≈ 5.25 mm to act as continuous.
• A short-glass-fibre polypropylene with Vf = 0.30, random in-plane (K = 0.375), Ef = 72 GPa, Em = 1.5 GPa: Ecd = 0.375(72)(0.30) + 1.5(0.70) = 8.10 + 1.05 = 9.15 GPa — six times neat PP, but only about 40% of the aligned value 72(0.3)+1.5(0.7) = 22.65 GPa.

Example 4 — ceramic strength, flaws, porosity and a bend test. Alumina: KIC = 3.5 MPa·m^½, E0 = 400 GPa, σ0 = 400 MPa.

• Largest surface flaw a = 25 µm = 25 × 10⁻⁶ m, Y = 1: σ = KIC/√(πa) = 3.5/√(7.854 × 10⁻⁵) = 3.5/0.008862 = 395 MPa. Halving the flaw to 12.5 µm raises it by √2 to 559 MPa — polishing is a strengthening operation.
• Porosity P = 0.05: E = 400(1 − 0.095 + 0.00225) = 400(0.90725) = 362.9 GPa; σ = 400 e^{−4 × 0.05} = 400(0.8187) = 327.5 MPa.
• Three-point bend bar, F = 200 N at fracture, span L = 40 mm, b = 5 mm, d = 4 mm: σfs = 3(200)(40)/[2(5)(16)] = 24,000/160 = 150 MPa.
• Weibull check, m = 10, σ0 = 400 MPa, design stress 300 MPa: Ps = exp[−(0.75)¹⁰] = exp(−0.0563) = 94.5% survival — i.e. 1 part in 18 fails, so ceramics are designed to a survival probability, not a single allowable stress.

Example 5 — polymer molecular weight. PVC mer C₂H₃Cl = 2(12.01) + 3(1.008) + 35.45 = 62.49 g/mol.

• If Mn = 100,000 g/mol, DP = 100,000/62.49 = 1,600.
• For polyethylene (mer C₂H₄ = 28.05) with Mn = 280,000, DP = 9,982; if Mw = 420,000 then PDI = 420,000/280,000 = 1.50.

Where it is used

• Pipes, tanks, fittings and electricals: uPVC/CPVC/HDPE plumbing and irrigation (Supreme, Astral, Finolex, Jain Irrigation) to IS 4984/4985/15778; GRP pipes for effluent and seawater lines; moulded switchgear in phenolic and PC.
• Automotive and two-wheeler: PP bumpers and interior trim, nylon intake manifolds, SMC body panels, ceramic-matrix and resin-bonded brake friction material, Al-SiC MMC brake rotors, ceramic glow plugs and oxygen sensors; design and CAE roles at Maruti, Tata Motors, Mahindra, Bajaj.
• Aerospace, space and defence: carbon/epoxy wing and fuselage structures (HAL's Tejas airframe is reported at roughly 45% composites by weight), ISRO/VSSC composite payload fairings, motor casings and carbon-carbon nozzle throats; B₄C/SiC ceramic plates and aramid in bullet-resistant jackets (DRDO/BARC designs).
• Energy and process industry: GFRP wind-turbine blades (Suzlon and blade suppliers in Tamil Nadu and Gujarat), filament-wound CNG cylinders, FRP cooling-tower parts, refractory linings for steel, cement and glass plants (Tata Steel, JSW, UltraTech).
• Manufacturing and tooling roles: WC-Co and alumina/SiAlON cutting inserts, abrasive wheels and seals (Carborundum Universal, Grindwell Norton), process engineering in sintering, injection moulding and mould design; materials/QC engineers running flexural, hardness and Vf burn-off tests.
• Infrastructure and consumer: concrete and vitrified tiles (Kajaria, Somany), sanitaryware, FRP doors and railway coach interiors and water tanks, CFRP/GFRP wrapping for strengthening RCC members.

Interview questions

• Q: What is the difference between a thermoplastic and a thermoset? A: A thermoplastic has separate chains held by secondary bonds, so it softens and can be remelted and recycled; a thermoset is one covalently cross-linked network that cannot be remelted and degrades instead.
• Q: Why is the transverse modulus of a unidirectional laminate so much lower than the longitudinal one? A: Along the fibres both phases strain equally and the stiff fibres dominate (isostrain); across them both carry the same stress and the compliant matrix controls the deformation (isostress).
• Q: What is critical fibre length and why does it matter? A: lc = σf·d/(2τc) is the minimum length over which interfacial shear can build the fibre up to its failure stress; shorter fibres pull out, so their strength is wasted.
• Q: Composites are "stronger than steel" — qualify that. A: On specific strength and specific stiffness, yes; in absolute modulus, E-glass/epoxy at about 45 GPa is far below steel's 210 GPa, and its transverse and interlaminar strengths are low.
• Q: Why are ceramics brittle? A: Ionic and covalent bonds are strong and directional and slip would bring like charges together, so dislocations cannot move; a pre-existing flaw concentrates stress until it propagates.
• Q: Why is ceramic strength quoted from a bend test with a Weibull modulus? A: Gripping a brittle bar in tension is impractical, and strength depends on the worst flaw in the stressed volume, so results scatter and must be reported statistically.
• Q: How does partially stabilised zirconia get its toughness? A: Metastable tetragonal grains transform to monoclinic at the crack tip with a volume expansion that absorbs energy and puts the crack in compression.
• Q: Why does silicon carbide resist thermal shock far better than glass? A: It has high strength and high thermal conductivity with low expansion, so R = σk/(Eα) is around two orders of magnitude higher — the thermal gradient and the resulting stress stay small.
• Q: What happens to a ceramic part's dimensions during sintering? A: It shrinks typically 10–20% linearly as pores close, so the green part is made oversize and critical surfaces are ground afterwards.
• Q: Why is Tg so important in selecting a polymer? A: Above Tg an amorphous polymer is rubbery and creeps badly; below it, it is glassy and can be brittle — the service temperature must lie on the intended side of Tg.
• Q: Why are composite joints bonded rather than bolted wherever possible? A: Drilling cuts load-bearing fibres and creates stress concentration and delamination sites; a bonded joint spreads load over an area and keeps the fibres continuous.
• Q: What are the main regulatory obligations on a plastic product manufacturer in India? A: Compliance with the applicable BIS product standard (and any Quality Control Order making ISI marking mandatory) plus Extended Producer Responsibility registration and targets under the Plastic Waste Management Rules, 2016 as amended, including the single-use-plastic ban list.

Common mistakes

• Using weight fractions in the rule of mixtures — it needs volume fractions; convert first.
• Applying El = EfVf + EmVm to a random short-fibre or woven part; the efficiency factor (0.375 or 0.2) and fibre length must be included.
• Treating a composite as isotropic and using a single E in stress calculations, or forgetting that an unsymmetric lay-up warps on cure.
• Confusing Tg with Tm, or claiming a fully amorphous polymer or a glass has a melting point.
• Quoting ceramic tensile strength from compressive data, or ignoring that strength falls sharply with flaw size and porosity.
• Mixing units in flexure and fracture work — mm with m, MPa with Pa, µm flaw sizes left unconverted in √(πa).
• Saying thermosets can be recycled by remelting, or that adding more fibre always helps (above about Vf 0.7 there is too little matrix to wet the fibres and properties fall).

Related topics

Engineering materials and heat treatment · Material testing and non-destructive testing · Surface engineering and coatings · Strength of materials: stress, strain and elasticity · Manufacturing: moulding and polymer processing · Design against fatigue and fracture