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ISO 14798-12:2016 Road Vehicles — Brake Lining Friction Materials — Part 12: Determination of Thermal Conductivity by Guarded Hot Plate Method
A comprehensive guide to the international standard for measuring thermal conductivity of brake lining materials
1. Introduction and Scope
ISO 14798-12:2016 is part of the ISO 14798 series of international standards that specify test methods for brake lining friction materials used in road vehicles. This part (Part 12) focuses on the determination of thermal conductivity of friction materials using the guarded hot plate (GHP) method. The standard provides a reproducible and accurate procedure for measuring the steady-state heat transfer through a specimen, which is critical for understanding the thermal behavior of brake pads during service.
The standard applies to all types of brake lining friction materials, including disc brake pads and drum brake linings, that are intended for use in passenger cars, light commercial vehicles, and heavy trucks. The test method is suitable for both laboratory research and quality control purposes, providing a consistent basis for comparing materials from different manufacturers or batches.
Note: ISO 14798-12:2016 does not cover the measurement of thermal diffusivity or specific heat capacity, nor does it apply to materials that are non-homogeneous or contain large inclusions (e.g., metallic fibers) that may bias the measurement.
2. Technical Requirements and Test Setup
2.1 Test Apparatus
The standard specifies the use of a guarded hot plate apparatus conforming to the general principles of ISO 8302 (now replaced by ISO 22007-2 for some aspects). The apparatus consists of:
- A heated central metering section (hot plate) surrounded by a guard heater to eliminate lateral heat losses.
- Two cold plates maintained at a constant, lower temperature, positioned symmetrically on both sides of the hot plate.
- Temperature sensors (thermocouples or resistance temperature detectors) embedded in the plates and on the specimen surfaces.
- Thermal insulation around the assembly to minimize environmental heat exchange.
2.2 Specimen Preparation
Specimens must be representative of the friction material and have flat, parallel surfaces. Typical dimensions are 300 mm × 300 mm with a thickness of 10 mm to 30 mm, depending on the material’s thermal conductivity. Multiple specimens may be required to achieve the necessary thickness ratio. The specimen must be conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for at least 24 hours prior to testing.
2.3 Measurement Procedure
The test involves placing the specimen between the hot plate and the cold plates, applying a controlled clamping pressure (typically 0.1 kPa to 1.0 kPa), and establishing a steady-state temperature gradient. The thermal conductivity λ is calculated using Fourier’s law of heat conduction:
λ = (Q × d) / (A × ΔT)
where Q is the heat flow rate through the metering area, d is the specimen thickness, A is the metering area, and ΔT is the temperature difference across the specimen.
Critical: The standard requires that the temperature difference across the specimen be maintained between 10 K and 30 K to ensure accurate measurement. The mean test temperature is typically 100 °C, but alternative temperatures (e.g., 150 °C or 200 °C) may be used to simulate different service conditions.
2.4 Test Parameters
| Parameter | Requirement (ISO 14798-12:2016) |
|---|---|
| Metering area (A) | ≥ 0.01 m² (typically 300 mm × 300 mm) |
| Specimen thickness (d) | 10 mm to 30 mm |
| Temperature difference (ΔT) | 10 K to 30 K |
| Mean test temperature | 100 °C (other values optional) |
| Clamping pressure | 0.1 kPa to 1.0 kPa |
| Conditioning | 23 °C ± 2 °C, 50 % ± 5 % RH for 24 h |
| Steady-state criterion | Δt < 0.1 K over 30 min |
3. Implementation Highlights
3.1 Apparatus Validation
Before routine testing, the guarded hot plate apparatus must be validated using reference materials with known thermal conductivity (e.g., glass fiber board, PMMA, or NIST SRM 1450d). The measured values must agree within ± 2% of the certified values. Inter-laboratory comparisons (ILC) are recommended to ensure consistency across testing facilities.
3.2 Edge Loss Correction
The standard includes guidelines for correcting edge heat losses when the specimen’s lateral dimensions are smaller than the metering area. Numerical modeling or empirical correction factors may be applied, provided they are documented and validated.
3.3 Moisture Sensitivity
Friction materials are hygroscopic and their thermal conductivity can change significantly with moisture content. The standard mandates conditioning at standard laboratory atmosphere, but additional testing under controlled moisture levels may be specified by agreement between parties.
Best Practice: For quality control, perform triplicate measurements on each material batch. The coefficient of variation for repeated measurements should not exceed 5% for well-prepared specimens.
3.4 Reporting
The test report must include the following information per ISO 14798-12:2016, Clause 7:
- Specimen identification and preparation details
- Mean test temperature and temperature difference
- Applied clamping pressure
- Measured thermal conductivity (in W/(m·K))
- Any anomalies or deviations from the standard procedure
- Date of test and personnel responsible
4. Compliance Notes
4.1 Mandatory vs. Informative Elements
ISO 14798-12:2016 contains both normative (mandatory) and informative (optional) provisions. Clauses 4 to 6 are normative and must be followed to claim compliance with the standard. Annex A (informative) provides additional guidance on apparatus design and correction methods.
4.2 Deviations
Any deviation from the specified test parameters, such as using a different specimen thickness or mean temperature, must be clearly stated in the report. The standard does not require a specific temperature for all applications; however, a change in temperature may affect the measured thermal conductivity, especially for materials with strong temperature dependence.
4.3 Reference to Other Standards
ISO 14798-12:2016 is linked with other parts of the ISO 14798 series, such as Part 1 (general requirements), Part 3 (shear strength), and Part 5 (compressibility). When combined, these standards provide a comprehensive characterization of brake lining materials. Additionally, ISO 22007-2 (guarded hot plate method for plastics) may be consulted for general principles.
Non-Compliance Risk: Failure to use a validated guarded hot plate apparatus can lead to errors of 10% or more in thermal conductivity values, potentially compromising brake system thermal modeling. Always verify that your equipment meets the geometric and thermal stability requirements of ISO 14798-12:2016.
4.4 Certification
Third-party certification bodies may offer accreditation for laboratories performing ISO 14798-12 tests. While the standard itself does not require external certification, many automotive OEMs mandate compliance as part of their material approval process. The standard is recognized within the International Automotive Task Force (IATF) framework.
5. Frequently Asked Questions
Q: What is the main difference between ISO 14798-12:2016 and older methods like ASTM E1530 or the traditional hot plate method?
A: ISO 14798-12:2016 is specifically tailored for brake lining friction materials, which have unique characteristics such as heterogeneity, moderate thermal conductivity (0.2–2.0 W/(m·K)), and sensitivity to compression. The specimen size and clamping conditions are optimized for automotive friction materials, while ASTM E1530 is more general for plastics and rubbers. The standard also specifies a narrower conditioning and test regime to match typical brake operating conditions.
A: ISO 14798-12:2016 is specifically tailored for brake lining friction materials, which have unique characteristics such as heterogeneity, moderate thermal conductivity (0.2–2.0 W/(m·K)), and sensitivity to compression. The specimen size and clamping conditions are optimized for automotive friction materials, while ASTM E1530 is more general for plastics and rubbers. The standard also specifies a narrower conditioning and test regime to match typical brake operating conditions.
Q: Can ISO 14798-12:2016 be used for materials with thermal conductivity above 2.0 W/(m·K), such as certain carbon-ceramic composites?
A: The standard does not restrict thermal conductivity range, but the guarded hot plate method becomes less accurate for high-conductivity materials (λ > 5 W/(m·K)) because lateral heat losses become significant relative to the in-plane heat flow. For such materials, alternative methods like laser flash analysis (ISO 22007-4) are recommended. However, for many typical organic and semi-metallic brake linings, the GHP method works well.
A: The standard does not restrict thermal conductivity range, but the guarded hot plate method becomes less accurate for high-conductivity materials (λ > 5 W/(m·K)) because lateral heat losses become significant relative to the in-plane heat flow. For such materials, alternative methods like laser flash analysis (ISO 22007-4) are recommended. However, for many typical organic and semi-metallic brake linings, the GHP method works well.
Q: What should I do if my specimen thickness is less than 10 mm? Can I stack multiple layers?
A: Stacking multiple layers is not recommended because the thermal contact resistance between layers will bias the measurement. If a specimen thickness less than 10 mm is unavoidable, the standard allows testing at a reduced thickness provided that the aspect ratio (lateral dimension / thickness) is at least 10:1. However, the results must be reported with a clear note indicating the deviation from the standard thickness range.
A: Stacking multiple layers is not recommended because the thermal contact resistance between layers will bias the measurement. If a specimen thickness less than 10 mm is unavoidable, the standard allows testing at a reduced thickness provided that the aspect ratio (lateral dimension / thickness) is at least 10:1. However, the results must be reported with a clear note indicating the deviation from the standard thickness range.
Q: How often should the guarded hot plate apparatus be recalibrated?
A: ISO 14798-12:2016 recommends that the apparatus be validated at least once per year using certified reference materials. More frequent checks (e.g., every 6 months) are advisable if the apparatus is used heavily or if a change in measurement environment occurs. Records of all calibration and validation activities must be maintained as part of the quality system.
A: ISO 14798-12:2016 recommends that the apparatus be validated at least once per year using certified reference materials. More frequent checks (e.g., every 6 months) are advisable if the apparatus is used heavily or if a change in measurement environment occurs. Records of all calibration and validation activities must be maintained as part of the quality system.
© 2026 — This article is provided for informational purposes and does not substitute the official ISO standard. Always refer to the current edition of ISO 14798-12:2016 for authoritative requirements.