PEEK (polyether ether ketone) is a semi-crystalline thermoplastic in the PAEK (polyaryletherketone) family, valued in defense programs for retaining mechanical strength at elevated temperature, surviving aggressive fuels and hydraulic fluids, and meeting low smoke / low toxicity targets on board vehicles and aircraft [S1][S2].
Defense selection is not a single resin decision. It is a chain of linked calls: unfilled, glass-filled, or carbon-filled grade; stock-shape or additive route; lot-level traceability; and documentary evidence (resin certificates, test reports, qualification to a known program) that the part will actually fly or sail [S1][S2].
Why defense engineers reach for PEEK over commodity polymers
PEEK sits at the top of the engineering-polymer pyramid because it keeps useful mechanical properties well past the ceiling of Nylon (PA12 / PA11) and most high-performance nylons, which is the point at which aerospace, medical, and oil and gas programs start to consider it [S1]. For defense, three engineering facts drive the call. First, PEEK delivers a high strength-to-weight ratio that lets it substitute for machined aluminum or titanium in selected brackets, housings, and electrical insulators, cutting mass on platforms where every kilogram is budgeted [S1]. Second, it is inherently flame resistant with low smoke emission, which matters for interior and under-armor applications where the platform fire-safety standard is binding [S1]. Third, it resists aggressive solvents, fuels, and hydraulic fluids that would swell or stress-crack lower-grade thermoplastics, so it survives the long dwell times seen in stored and deployed equipment [S1][S2].
Grade map: unfilled, glass-filled, carbon-filled, and bearing grades
Selection starts with the grade, and the grade drives both the data sheet and the cost. Unfilled (virgin) PEEK is the baseline: highest elongation, best toughness, and the grade used for seals, gaskets, and thin-wall electrical insulation where ductility and dielectric strength dominate. Glass-filled PEEK (typically ~30% glass fiber by weight) trades elongation for higher stiffness, lower thermal expansion, and better creep resistance at temperature, which is why it shows up in structural brackets and pump bodies. Carbon-filled PEEK (often ~30% carbon fiber) goes further on stiffness and density, and is the grade frequently chosen where mass plus conductivity are both wanted. Bearing-grade PEEK variants add solid lubricants (PTFE, graphite, carbon fiber) to push the PV (pressure-velocity) limit and remove the need for external lubrication in dry-running bushings and thrust washers on land platforms [S1][S2]. For adjacent decision context on how a different industry grades the same resin family, see the PEEK grade selection for medical devices guide.
Processing route: stock shapes vs. PEEK 3D printing
Defense programs historically qualified PEEK via machined stock shapes (rod, plate, tube) extruded or compression-molded from qualified resin lots, because that route has the longest pedigree of mechanical and traceability data. PEEK 3D printing is now used to produce functional, end-use parts rather than visual prototypes, with two main paths: FDM with PEEK filament, and SLS with PEEK powder [S1]. The trade is not just shape complexity; it is the thermal envelope the machine can hold. PEEK is a demanding semi-crystalline polymer whose crystallization must be strictly controlled during cooling, so PEEK 3D printers require heated build chambers, high-temperature extruders, or high-powered CO2 lasers, plus controlled platform temperatures, to keep part density and crystallinity within spec [S1]. A useful side-by-side reading is the PEEK selection for oil and gas grades, ratings, and sour-service rules, where the same resin is judged against harsher corrosion and NACE MR0175-style sour-service demands than defense typically imposes. For design engineers who also weigh weight, the steel strand selection for renovation guide is a useful reference point for how mass-sensitive projects document load math on the bill of materials.
Key characteristics engineers must verify on the data sheet
Before any defense PEEK is released to a drawing, four numbers on the resin data sheet should be checked against the application, not against the marketing brochure. Continuous service temperature, typically rated near 250 deg C for unfilled PEEK and slightly lower for filled grades, sets the thermal ceiling for under-hood, near-engine, and avionics-bay parts. Tensile and flexural modulus, which climb sharply when glass or carbon fiber is added, decide whether the part can replace a metal insert at the same wall thickness or needs geometry rework. Limiting PV (for bearing grades) and coefficient of friction against the mating surface, which together tell you whether the part can run dry at the expected load and sliding speed. Inherent flame, smoke, and toxicity performance, which is a release requirement for crew compartments, not a nice-to-have [S1][S2]. If any of those four cannot be backed by a lot-traceable certificate, the resin should not enter the part number.
Who should specify PEEK, and who should look elsewhere
PEEK is the right call when the part sits in a hot, chemically aggressive, or weight-sensitive zone and must be documented: avionics brackets, fuel and hydraulic system insulators, dry-running bearings, connectors, and select under-armor components. It is the wrong call when the duty is ordinary structural plastic at room temperature, when the program cannot absorb the resin and machining cost versus a glass-filled nylon, or when a qualified supply chain does not exist at the required volume. PEEK also is not a drop-in for a designed-in metal where the design relies on metal-only behavior such as high thermal conductivity for heat sinking, magnetic permeability for sensors, or true electrical grounding; the substitution needs a re-design, not just a material swap [S1][S2].
Selection checklist before the part is released
A practical, defense-flavored PEEK selection flow: (1) freeze the duty envelope (temperature, chemical exposure, load, fire/smoke class); (2) pick the grade family (unfilled, glass-filled, carbon-filled, bearing) and reject any grade that cannot show lot-traceable certification; (3) pick the forming route (machined stock shape, FDM, or SLS) and confirm the machine envelope covers PEEK-class temperatures; (4) confirm the supply chain can deliver the same grade and lot format at the program's forecast volume; (5) require a sample coupon program that tests tensile, flexural, and fire/smoke on the actual lot before the first article. Materials and standards bodies to track include ASTM and ISO for polymer test methods, and the OEM or platform-specific qualification lists that the program office maintains. Two signals worth watching over the next quarter are the publication of new defense-flavored PEEK 3D printing case studies from the major SLS and FDM vendors, and any program-office update that lists additively manufactured PEEK as a qualified process for a specific part class [S1][S2].
For the relevant spec sheets and selection criteria, see peek, pressure transmitter, and flow meter.