Advanced ceramics: electronic, structural and bioceramics

In short

Advanced (fine, technical or engineering) ceramics are made from synthetic, high-purity, closely sized powders and are processed under tight control so that the final microstructure — grain size, grain boundaries, porosity, phases — delivers a designed electrical, mechanical or biological function. They differ from traditional ceramics (clay-based whitewares, tiles, cement, container glass) in raw material, forming route, firing atmosphere and price per kilogram. The family splits broadly into electroceramics (capacitors, piezoelectrics, ferrites, sensors, solid electrolytes), structural ceramics (cutting tools, seals, armour, engine and bearing parts, refractory composites) and bioceramics (implant heads, dental crowns, bone grafts, coatings). In India the subject is examined in B.Tech Ceramic Technology at IIT (BHU), NIT Rourkela, Anna University, GCECT Kolkata and in GATE/UGC-NET; it is applied at CSIR-CGCRI Kolkata, ARCI Hyderabad, BARC, DRDO (DMRL), Cumi, Carborundum Universal, Murugappa, TDK-EPCOS India, Kemet, Elin, Bosch India and in the orthopaedic-implant industry. Numerically it is examined through phase/property relations, Weibull statistics, dielectric and piezoelectric constants, sintering shrinkage and fracture mechanics.

Explained

What makes a ceramic "advanced"

• Raw materials: synthetic, chemically prepared powders (calcined alumina, ZrO₂ co-precipitated with Y₂O₃, SiC by Acheson or carbothermal route, Si₃N₄ by nitridation/imide decomposition, BaTiO₃ by solid-state or hydrothermal routes) with purity typically 99–99.99% and particle size 0.1–5 µm.
• Powder preparation routes: solid-state calcination, co-precipitation, sol-gel, hydrothermal, spray pyrolysis, combustion synthesis, SHS and plasma routes — each chosen for purity, size distribution and sinterability.
• Shaping: dry pressing, isostatic pressing (CIP), slip casting, tape casting (multilayer capacitors and substrates), extrusion (honeycombs, tubes), injection moulding (complex small parts), gel casting, additive manufacturing.
• Densification: pressureless sintering with additives, hot pressing (HP), hot isostatic pressing (HIP), gas-pressure sintering, reaction bonding (RBSN, RBSC), spark plasma sintering (SPS), microwave sintering. Liquid-phase sintering (SiC with Al₂O₃–Y₂O₃; Si₃N₄ with MgO/Y₂O₃) is common because covalent ceramics have very low self-diffusion.
• Finishing: diamond grinding, lapping, laser machining; green machining is far cheaper than hard machining.
• Why control matters: strength and reliability are flaw-controlled, so the largest pore or inclusion, not the average, sets the property.

Electronic (electro-) ceramics

• Dielectrics / capacitors: BaTiO₃ (perovskite ABO₃) has εr of a few thousand near its Curie point (~120–130 °C); doping with Zr, Ca, Sn, rare earths flattens and shifts the peak to meet Class 2 codes (X7R, Y5V); Class 1 (NP0/C0G) uses low-loss CaZrO₃, TiO₂ or Nd/Ba titanates. MLCCs are made by tape casting hundreds of 1–3 µm layers with Ni (base-metal) or Pd–Ag electrodes, co-fired, terminated and tested.
• Ferroelectricity: below the Curie temperature the cubic perovskite distorts (tetragonal), the Ti⁴⁺ ion moves off-centre and a spontaneous polarisation appears; domains switch under field, giving the P–E hysteresis loop (remanent polarisation Pr, coercive field Ec).
• Piezoelectrics: PZT (Pb(Zr,Ti)O₃) near its morphotropic phase boundary (≈ 52/48 Zr/Ti) gives the highest d₃₃ and kp. Soft PZT (donor-doped, Nb⁵⁺ on B-site) = high d and permittivity, high loss — actuators, sensors; hard PZT (acceptor-doped, Fe³⁺) = high Q, low loss — ultrasonic power, sonar. Must be poled in DC field (1–4 kV/mm, near Tc) to be piezoelectric. Lead-free substitutes: (K,Na)NbO₃, (Bi,Na)TiO₃, BaTiO₃-based — driven by RoHS-type restrictions on lead.
• Pyroelectrics and electro-optics: LiTaO₃, PVDF composites for IR detectors; PLZT for optical shutters.
• Magnetic ceramics (ferrites): soft spinel ferrites MnZn (high-permeability, low-frequency transformer cores) and NiZn (high-resistivity, high-frequency, EMI suppression); hard hexagonal ferrites BaFe₁₂O₁₉ / SrFe₁₂O₁₉ for permanent magnets (motors, loudspeakers); garnets (YIG) for microwave devices. Ferrimagnetism: antiparallel but unequal sublattice moments.
• Semiconducting / sensing ceramics: ZnO varistors (non-ohmic I = kVⁿ, n ≈ 25–50) for surge arresters; PTC thermistors from donor-doped BaTiO₃ (resistance jumps above Tc — self-regulating heaters, motor starters); NTC thermistors from Mn–Ni–Co spinels (temperature measurement, inrush limiting); SnO₂, TiO₂, ZnO gas sensors; TiO₂/MgCr₂O₄ humidity sensors.
• Ionic conductors (solid electrolytes): YSZ (8 mol% Y₂O₃–ZrO₂) conducts O²⁻ by oxygen vacancies — lambda sensors in BS-VI vehicles, SOFC electrolytes; β-alumina conducts Na⁺ (sodium–sulfur batteries); LiPON/LLZO for solid-state Li batteries; gadolinia-doped ceria for intermediate-temperature SOFCs.
• Insulating and packaging ceramics: 94–99.8% alumina substrates and spark-plug insulators; AlN and BeO for high thermal conductivity (AlN ~150–200 W/m·K) with electrical insulation; LTCC and HTCC multilayer packages; cordierite honeycombs (low expansion) for catalytic converters and DPFs.
• Superconductors: YBa₂Cu₃O₇₋ₓ (Tc ≈ 92 K) and BSCCO — Type II, flux pinning, anisotropic; used in tapes, magnets, fault-current limiters.

Structural (engineering) ceramics

• Alumina (Al₂O₃): cheapest workhorse; 99.7% grade ~ 350–400 MPa flexural strength, hardness ~1,600–1,800 HV, KIC ~ 3–4 MPa·m^0.5; used for seals, wear tiles, textile guides, ballistic plates, insulators. Strength falls sharply with residual porosity and glassy grain-boundary phase.
• Zirconia (ZrO₂): polymorphs monoclinic → tetragonal (~1,170 °C) → cubic (~2,370 °C). The t→m transformation on cooling gives ~3–5% volume expansion, which cracks pure zirconia; stabilisers (Y₂O₃, MgO, CaO, CeO₂) retain cubic (FSZ) or metastable tetragonal (3Y-TZP, Mg-PSZ).
• Transformation toughening: a crack tip's tensile stress field triggers t→m in metastable grains; the resulting volume expansion closes the crack and adds compressive stress. 3Y-TZP reaches 900–1,200 MPa strength and KIC 5–10 MPa·m^0.5 — the toughest monolithic ceramic. Limitation: low-temperature degradation (LTD/ageing) in moist conditions around 150–300 °C, and loss of toughening above ~600–800 °C.
• Silicon carbide (SiC): sintered (SSiC), reaction-bonded (RBSiC/SiSiC), recrystallised; covalent, very hard (~2,500 HV), high thermal conductivity (~120 W/m·K), excellent oxidation and corrosion resistance; mechanical seals, pump parts, kiln furniture, burner nozzles, armour, bulletproof plates, heat exchangers, wafer-handling parts, abrasives. SiC power devices depend on single-crystal, not sintered, material.
• Silicon nitride (Si₃N₄) and SiAlON: elongated β-Si₃N₄ grains in a glassy boundary give in-situ whisker-like toughening (KIC 6–8 MPa·m^0.5) plus high thermal shock resistance (low α ≈ 3 × 10⁻⁶/K); bearings (hybrid bearings), turbocharger rotors, metal-forming tools, molten-aluminium handling (SiAlON riser tubes, thermocouple sheaths).
• Boron carbide (B₄C) and TiB₂: among the hardest and lightest — armour (B₄C density ~2.5 g/cm³), neutron absorber control rods (B-10), nozzles.
• Ceramic matrix composites (CMCs): C/C, C/SiC, SiC/SiC made by CVI, polymer impregnation and pyrolysis (PIP) or melt infiltration; fibre pull-out and debonding give "graceful" non-brittle failure — brake discs, rocket nozzles, nose cones, aero-engine shrouds; relevant to ISRO/DRDO work.
• Toughening mechanisms to list in exams: transformation toughening, crack deflection, crack bridging and fibre pull-out, microcrack toughening, residual compressive surface stress, whisker/platelet reinforcement, and (in glass-ceramics) controlled crystallisation.
• Design philosophy: ceramics have essentially no plasticity, so design is by probability of failure (Weibull statistics), proof testing and avoidance of tensile stress — not by a yield-stress factor of safety.

Bioceramics

• Classification by tissue response:
  1. Bioinert — alumina (hip femoral heads, dental), zirconia (3Y-TZP and ZTA heads, dental crowns), carbon; fibrous encapsulation only.
  2. Bioactive (surface-active) — hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂, Ca/P = 1.67), bioactive glass (45S5 Bioglass: 45 SiO₂, 24.5 Na₂O, 24.5 CaO, 6 P₂O₅ wt%), A/W glass-ceramic; form a carbonated apatite layer and bond chemically to bone (osteoconduction).
  3. Bioresorbable — tricalcium phosphate (β-TCP), biphasic HA/TCP, calcium sulfate — dissolve and are replaced by new bone.
  4. Porous for ingrowth — HA scaffolds, coralline HA; pore size > 100 µm (ideally 150–400 µm, interconnected) is needed for vascularised bone ingrowth.
• Why ceramics for implants: chemical inertness, hardness and wear resistance (ceramic-on-ceramic hip bearings have far lower wear than metal-on-polyethylene), no metal-ion release, radiopacity, and for HA a chemistry close to bone mineral.
• Limitations: brittleness and low tensile strength, risk of chipping/squeaking in ceramic hips, difficulty of machining, HA's low strength (cannot be load-bearing alone — hence plasma-sprayed HA coatings on Ti-6Al-4V stems).
• Processing specifics: wet precipitation of HA, sintering below ~1,250–1,300 °C to avoid decomposition to TCP/CaO, plasma spraying of coatings, freeze casting and foam replication for scaffolds, glass-ceramic routes for dental (lithium disilicate, leucite) and ZrO₂–alumina (ZTA) for strength.
• Standards and regulation (India): implants are notified medical devices under the Medical Devices Rules, 2017 (CDSCO, under the Drugs and Cosmetics Act) — manufacturing and import licences, risk classification A–D (orthopaedic implants are typically Class C/D); quality system to ISO 13485; biological evaluation to ISO 10993 series; HA and surgical implant ceramics are covered by ISO 13779 (hydroxyapatite) and ISO 6474 (alumina/zirconia implants), adopted as IS/ISO standards — quote the exact IS number only if verified, since several have been revised. General QMS reference is ISO 9001:2026 (current edition).

Properties, testing and standards

• Strength testing: 3-point and 4-point flexure (MOR) on ground, edge-chamfered bars; biaxial ring-on-ring for discs; compressive strength is 5–15 times flexural strength. Tensile testing is rare because of gripping/alignment problems.
• Fracture toughness: SENB, chevron-notch, SEPB, indentation fracture (Vickers crack length) and indentation strength — note that indentation methods give indicative values only.
• Hardness: Vickers/Knoop microhardness; the value depends on load (indentation size effect).
• Thermal shock: quench tests; resistance parameter R = σ(1−ν)/(Eα) and R′ = kσ(1−ν)/(Eα) — SiC and Si₃N₄ score high, zirconia and alumina low.
• Other: dielectric constant and loss (LCR bridge), resistivity, piezoelectric d₃₃ (Berlincourt meter), resonance method for kp, creep at temperature, wear (pin-on-disc), density/porosity by Archimedes.
• Indian standards and codes relevant in practice: BIS has IS standards for alumina ceramics, ceramic insulators (high-voltage porcelain/alumina), ferrite cores and electroceramic components; electrical safety now follows the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023. Occupational health in powder plants (silica, lead oxide in PZT, beryllia) is governed by the OSH&WC Code, 2020 (in force 21 November 2025, replacing the Factories Act regime); hazardous waste and lead handling under the environment rules of MoEFCC, with municipal waste under the Solid Waste Management Rules, 2026. Many old IS numbers in this field have been superseded, so cite them only when the current number and year are known.

Key points

• Advanced ceramic - high-purity synthetic powder, engineered microstructure, function-driven property, high value per kg.
• Perovskite ABO₃ - the structure behind BaTiO₃, PZT, PLZT; Curie temperature separates ferroelectric from paraelectric behaviour.
• MLCC - tape-cast, co-fired multilayer capacitor; capacitance scales with layer count and inversely with layer thickness.
• Poling - DC field alignment of domains; without it a PZT disc is piezoelectrically dead.
• MPB - morphotropic phase boundary in PZT (≈52/48) where d₃₃ and coupling peak.
• Soft vs hard PZT - donor-doped (high d, high loss) vs acceptor-doped (high Q, power ultrasonics).
• Ferrite split - MnZn soft (low frequency), NiZn soft (high frequency), Ba/Sr hexaferrite hard (magnets).
• Varistor - ZnO grain-boundary double Schottky barriers give I ∝ Vⁿ with n ≈ 25–50; clamps surges.
• PTC vs NTC - BaTiO₃ PTC resistance rises above Tc; Mn–Ni–Co spinel NTC resistance falls exponentially with T.
• YSZ - oxygen-vacancy conductor; lambda sensor and SOFC electrolyte.
• Transformation toughening - stress-induced tetragonal → monoclinic expansion closes cracks; basis of 3Y-TZP and PSZ.
• LTD/ageing - zirconia's hydrothermal t→m degradation around 150–300 °C in moisture.
• Covalent ceramics need help - SiC and Si₃N₄ require sintering aids, pressure or reaction bonding to densify.
• CMC - fibre pull-out converts catastrophic fracture into progressive, damage-tolerant failure.
• Weibull modulus m - scatter measure; m ≈ 5–15 for ceramics, 30+ for good engineering ceramics; higher m = more reliable.
• Griffith criterion - strength is set by the largest flaw, so surface finish and powder purity decide strength.
• Bioceramic classes - bioinert, bioactive, resorbable, porous-for-ingrowth; HA Ca/P = 1.67.
• Regulatory frame - implants under CDSCO Medical Devices Rules, 2017; ISO 13485 QMS and ISO 10993 biocompatibility testing.

Formulas and examples

Example 1 — Flaw size controls strength (zirconia vs alumina). A 3Y-TZP part has K_IC = 6 MPa·m^0.5 and a surface flaw of a = 30 µm = 30 × 10⁻⁶ m, with Y = 1.

• √(π a) = √(3.1416 × 30 × 10⁻⁶) = √(9.425 × 10⁻⁵) = 9.708 × 10⁻³ m^0.5.
• σ_f = 6 / 9.708 × 10⁻³ = 618 MPa.
• Same flaw in alumina with K_IC = 3.5 MPa·m^0.5 → σ_f = 3.5/9.708 × 10⁻³ = 361 MPa.
• Interpretation: toughness nearly doubles, strength nearly doubles for the same flaw — and quartering the flaw to 7.5 µm doubles strength again (σ ∝ 1/√a).

Example 2 — MOR from a 4-point bend test. A SiC bar 4.0 mm wide × 3.0 mm deep breaks at P = 480 N in 4-point bending, outer span L = 40 mm (quarter-point loading).

• MOR = 3PL/(4bd²) = (3 × 480 × 40)/(4 × 4.0 × 3.0²) = 57,600/144 = 400 N/mm² = 400 MPa.
• If the same bar were tested in 3-point bending at the same load, MOR = 3PL/(2bd²) = 800 MPa — so always state the test geometry; 3-point values are numerically higher for the same load, and measured 3-point MOR for a given material also runs above the 4-point value because less volume is under peak stress.

Example 3 — Weibull reliability and size scaling. A batch of alumina rings gives σ₀ = 400 MPa and m = 10.

• Probability of failure at 300 MPa: (300/400)¹⁰ = 0.75¹⁰ = 0.0563 → P_f = 1 − e^−0.0563 = 1 − 0.9453 = 5.5%.
• At 250 MPa: (0.625)¹⁰ = 0.00909 → P_f = 1 − e^−0.00909 = 0.90%.
• Design stress for P_f = 10⁻³: (σ/400)¹⁰ = −ln(0.999) = 1.0005 × 10⁻³ → σ/400 = (1.0005 × 10⁻³)^0.1 = 0.5012 → σ ≈ 200 MPa, i.e. a service stress of half the characteristic strength.
• A component of 10 times the stressed volume: σ₁/σ₂ = 10^1/10 = 1.259 → strength drops by about 21%.

Example 4 — MLCC design. An X7R chip has εr = 2,800, active overlap area A = 1.0 mm² = 1.0 × 10⁻⁶ m², dielectric thickness t = 2.0 µm = 2.0 × 10⁻⁶ m, and 201 electrode layers (N − 1 = 200 active dielectrics).

• C = (8.854 × 10⁻¹² × 2,800 × 1.0 × 10⁻⁶ × 200) / (2.0 × 10⁻⁶)
• Numerator = 8.854 × 10⁻¹² × 2,800 = 2.479 × 10⁻⁸; × 10⁻⁶ = 2.479 × 10⁻¹⁴; × 200 = 4.958 × 10⁻¹².
• C = 4.958 × 10⁻¹² / 2.0 × 10⁻⁶ = 2.479 × 10⁻⁶ F = 2.48 µF.
• Halving t to 1.0 µm and keeping the same stack height (so ~400 layers) raises C by about four times to ~9.9 µF — the whole history of MLCC miniaturisation in one line. Thinner layers mean higher field (V/t), so breakdown and reliability set the floor.

Example 5 — Density, porosity and shrinkage of a sintered alumina. A fired disc: dry mass 15.20 g, saturated mass 15.38 g, suspended mass 11.52 g, water density 1.00 g/cm³; theoretical density of Al₂O₃ = 3.98 g/cm³. Green diameter 50.0 mm, fired diameter 42.5 mm.

• Exterior volume = 15.38 − 11.52 = 3.86 cm³ → bulk density = 15.20/3.86 = 3.938 g/cm³.
• % theoretical = 3.938/3.98 × 100 = 98.9%.
• Apparent porosity = (15.38 − 15.20)/3.86 × 100 = 0.18/3.86 × 100 = 4.66% open porosity — inconsistent with 98.9% TD, which flags either a weighing/blotting error or surface water retention; in the lab the open porosity of a 98.9% dense body should be well under 1%, so the test is repeated. (Worked deliberately: examiners set such data to test whether the two results are cross-checked.)
• Linear shrinkage = (50.0 − 42.5)/50.0 × 100 = 15.0%; volume shrinkage = 1 − (42.5/50.0)³ = 1 − 0.614 = 38.6%.

Example 6 — Thermal shock comparison.

• Worked for Si₃N₄: R = 800 × 0.75/(310 × 10³ × 3.2 × 10⁻⁶) = 600/0.992 = 605 K; R′ = 30 × 605 ≈ 18,100 W/m.
• Conclusion: zirconia is strong but a poor conductor, so it survives slow cycling and fails in severe quench; Si₃N₄ and SiC dominate thermal-shock applications. Property values are typical grade ranges, not specification values.

Where it is used

• Electronics and components manufacturing: MLCC, ferrite core, varistor and thermistor plants (TDK-EPCOS India, Elin, Cosmo Ferrites, Mahendra Electricals); roles — process engineer (tape casting, co-firing), dielectric formulation, electrical test and reliability.
• Abrasives, wear and refractory engineering: Carborundum Universal (Cumi) and Grindwell Norton make SiC/alumina abrasives, wear-resistant linings, mechanical seals and kiln furniture for cement, steel, mining and power plants; roles — application engineer, failure analysis.
• Automotive and energy: cordierite/SiC substrates and DPFs, YSZ lambda sensors, spark-plug insulators (Bosch, NGK India), SOFC and solid-state battery R&D at ARCI Hyderabad and CSIR labs.
• Defence, space and nuclear: B₄C and SiC/alumina armour, C-SiC nose tips, nozzle throats and brake pads (DRDO-DMRL, ISRO-VSSC, BARC: B₄C control rods, nuclear-grade UO₂ and ThO₂ pellet technology).
• Medical devices: alumina/zirconia hip and dental components, HA-coated implants and bone graft granules (CSIR-CGCRI, SCTIMST Thiruvananthapuram, Meril, Sahajanand, Narang Medical); roles — implant process engineer, QA under ISO 13485, regulatory affairs with CDSCO.
• R&D and testing laboratories: CGCRI, ARCI, IIT ceramic engineering departments, NABL-accredited materials test labs — characterisation (XRD, SEM, dilatometry, impedance spectroscopy), new powder and CMC development.

Interview questions

• Q: What distinguishes an advanced ceramic from a traditional one? A: Synthetic high-purity sub-micron raw material, tightly controlled forming and sintering, near-zero porosity and a property designed for a specific function rather than for bulk and appearance.
• Q: Why must a PZT element be poled, and what destroys poling? A: Poling aligns the ferroelectric domains with a DC field so the net polarisation (and hence piezoelectric effect) survives after the field is removed; heating above the Curie temperature, a strong reverse field or high mechanical stress depoles it.
• Q: How does transformation toughening work in zirconia? A: Metastable tetragonal grains transform to monoclinic in the crack-tip stress field, expanding 3–5% in volume and putting the crack faces in compression, which absorbs energy and arrests the crack.
• Q: Why does pure zirconia crack on cooling? A: The tetragonal-to-monoclinic transformation near 1,170 °C is accompanied by a large volume expansion that the rigid body cannot accommodate; stabilisers such as Y₂O₃ or MgO are added to retain the cubic or metastable tetragonal phase.
• Q: Why is silicon carbide hard to sinter without additives? A: It is strongly covalent with extremely low self-diffusion and a high surface-energy-to-grain-boundary-energy ratio, so densification needs boron–carbon additives, oxide liquid phases, or applied pressure (HP, HIP, SPS), or a reaction-bonding route.
• Q: What does the Weibull modulus tell a design engineer? A: It measures the scatter of strength caused by the flaw-size distribution; a low m means wide scatter and a much lower allowable design stress for a given failure probability.
• Q: Why is a ceramic's compressive strength so much higher than its tensile strength? A: Fracture starts from flaws under tension; compression closes cracks instead of opening them, so compressive strength may be 5–15 times the flexural value.
• Q: Difference between bioinert, bioactive and resorbable ceramics with one example each? A: Bioinert forms only a fibrous capsule (alumina hip head); bioactive bonds chemically to bone through a surface apatite layer (45S5 bioactive glass, hydroxyapatite); resorbable dissolves and is replaced by bone (β-tricalcium phosphate).
• Q: Why is hydroxyapatite used as a coating rather than as a load-bearing implant? A: HA has excellent bioactivity but low strength and fracture toughness, so it is plasma-sprayed onto a strong Ti-6Al-4V or Co-Cr substrate that carries the load.
• Q: How does a ZnO varistor protect a circuit? A: Grain-boundary double Schottky barriers block current at normal voltage; above the breakdown voltage the barriers collapse and current rises as Vⁿ with n of 25–50, clamping the surge and dissipating its energy.
• Q: What is the function of YSZ in a lambda sensor? A: It is an oxygen-ion-conducting solid electrolyte that develops a Nernst potential between exhaust and reference air, telling the ECU whether the mixture is rich or lean.
• Q: Why do CMCs fail "gracefully" while monolithic ceramics fail catastrophically? A: A weak fibre–matrix interface lets cracks deflect, debond and bridge with fibre pull-out, so load is carried after matrix cracking and the stress–strain curve shows pseudo-ductility.
• Q: Which regulatory route applies to an orthopaedic ceramic implant made in India? A: It is a notified medical device under the Medical Devices Rules, 2017 administered by CDSCO, needing a manufacturing licence with an ISO 13485 quality system and biological evaluation to the ISO 10993 series.

Common mistakes

• Quoting a flexural strength without the test geometry (3-point vs 4-point) or specimen size — the numbers are not comparable, and 3-point values are optically "better".
• Treating a single strength value as a material constant and designing with a metal-style factor of safety instead of Weibull statistics and a stated failure probability.
• Confusing piezoelectric with ferroelectric: all ferroelectrics are piezoelectric after poling, but many piezoelectrics (quartz, AlN) are not ferroelectric and need no poling.
• Saying "zirconia is stabilised so it will not transform" — PSZ/TZP toughening *depends* on controlled transformation; full stabilisation (cubic FSZ) removes the toughening.
• Forgetting that hydrothermal ageing (LTD) and temperatures above about 600–800 °C both kill zirconia's transformation toughening.
• Mixing up soft and hard ferrites with soft and hard PZT — the terms mean different things (magnetic coercivity vs dopant valence and loss).
• Unit slips: εr is dimensionless but ε = ε₀εr in F/m; d₃₃ in pC/N must be converted to C/N; flaw size in µm must be converted to metres before computing √(πa).
• Sintering HA above about 1,300 °C and reporting it as HA — it decomposes to TCP and CaO, changing both strength and resorption behaviour.
• Assuming ceramic-on-ceramic hips are always better: they wear far less but can chip, squeak or fracture, and need precise surgical alignment.

Related topics

Glass technology: melting, forming and annealing · Whitewares, tiles, sanitaryware and glazes · Cement, glass and ceramics industries · Phase diagrams and ceramic phase equilibria · Powder processing and sintering science · Refractories and high-temperature materials