Overview
ISO 21821:2019 specifies a standardized test method to determine the densification behavior of ceramic powder compacts during natural (pressureless) sintering. Applicable to fine ceramics (advanced/technical ceramics) made from pure oxides, oxide mixtures, solid solutions and certain non‑oxides (e.g., carbides, nitrides) sinterable under vacuum or ≤1 bar gas to prevent oxidation. The method measures mass, dimensions, volume and apparent density before and after high‑temperature heat treatment and produces densification data such as shrinkage, mass variation and relative density (densification curves).
Key topics and requirements
- Scope and exclusions: Designed for pressureless sintering; not applicable to pressure‑assisted processes (HP, HIP, GPS, SPS). Inorganic sintering aids may be used if reported.
- Sampling and preparation: Powders are normally tested as‑received; optional drying at (110 ± 5) °C for ≥24 h if required (do not dry powders containing organic/volatile additives).
- Compaction: Compacts prepared per ISO 17172, with specified die geometries (floating or single‑acting) producing cylinders 10–32 mm diameter and controlled height‑to‑diameter ratios. Dies typically include a 5 mm ejection cone (~1% diameter increase).
- Apparatus: Hardened steel or tungsten carbide dies, press (±2% precision), furnace with hot‑zone temperature uniformity (±10 °C) and controlled heating rates (±2 °C/h), balance (±0.001 g), micrometer (±0.01 mm).
- Thermal cycle and temperatures: Measure over a temperature range where relative density spans ~0.9 up to the onset of de‑densification or substantial mass loss; range must be at least 100 °C. Typical oxide schedule: ramp to 600 °C at 60 °C/h, to test temperature at 180 °C/h, dwell ≥1 h, natural cooling.
- Measurements & output: Record mass, diameter and height (multiple positions) before and after sintering; calculate apparent density and express results as shrinkage (diameter/height/volume), mass variation, relative density and plot densification curves.
Applications and users
ISO 21821:2019 is valuable for:
- Ceramic R&D teams validating powder sinterability and comparing granulated vs ungranulated powders
- Process engineers optimizing sintering schedules for pressureless firing
- Quality control laboratories establishing production acceptance criteria for powder lots
- Materials scientists characterizing densification kinetics of oxides and non‑oxides
- Suppliers and manufacturers of advanced ceramics who need reproducible, comparable sintering performance data
Practical benefits include improved powder selection, reduced trial sintering cycles, and objective densification metrics to support scale‑up and production consistency.
Related standards
- ISO 17172 - Determination of compaction properties of ceramic powders (compaction procedures referenced in ISO 21821)
- ISO/IEC 17025 - Competence of testing laboratories (relevant for test lab accreditation)
- ISO 3611 - Micrometers for external measurements (dimensional measurement requirements)
Keywords: ISO 21821:2019, fine ceramics, densification properties, natural sintering, ceramic powders, sintering test method, densification curve, apparent density, shrinkage.