GB/T 41062-2021Test method for thermal transport properties to friction materials and brakes (English PDF)

3.1 Thermal diffusivity

The rate of heat diffusion inside the friction material when the friction material is heated or cooled.

Note: in square centimeters per second (cm2/s). The larger the thermal diffusivity, the faster the temperature diffusion and heat flow through the material.

3.2 Specific heat

The heat absorbed or released by a unit mass of a substance when the temperature rises or falls under the condition of constant pressure.

Note: in Joules per gram Celsius [J/(g·°C)].

3.3 Thermal conductivity

The time rate of heat flow per unit temperature gradient in the direction perpendicular to the area, through the unit area, under stable conditions.

Note: in watts per meter kelvins [W/(m·K)].

4.1 Determination method of thermal diffusivity

4.1.1 Method overview

The laser flash method is a method used to determine the thermal diffusivity. Expose a test piece of known thickness to an intense laser or xenon flash lamp, to obtain a short […]

4.1.3 Test piece

4.1.3.1 Flash method (method A)

4.1.3.1.1 The commonly used sample is a thin disk-shaped sample whose surface area for receiving pulsed energy radiation is smaller than the energy beam spot. The typical diameter of a test piece is 6 mm ~ 18 mm; the optimal thickness of a test piece depends on the estimated thermal diffusivity; this thickness of test piece shall be selected to ensure that the time required to reach the maximum temperature varies within 40 ms ~ 200 ms. A thinner test piece shall be used for high temperature measurements, so as to minimize the heat loss correction. The test piece shall be of sufficient thickness, so that the to-be-tested material is more representative. The typical thickness of a test piece is 1 mm ~ 6 mm. Since the thermal diffusivity is proportional to the square of the thickness of test piece, different thicknesses of test piece should be used in different temperature ranges. The optimal thickness of test piece required for low temperature testing is inconsistent with that required for high temperature testing.

4.1.3.1.2 Improperly selected thickness of test piece will not only cause unnecessary test failures, but also be the main cause of test errors. Generally, a test piece with a thickness of 2 mm ~ 3 mm can be selected at the beginning; then, the thickness of test piece can be changed based on the obtained temperature recording curve (the signal cannot be observed if the test piece is too thick).

4.1.3.1.3 The surface of the prepared test piece shall be flat and the parallel error shall be within 0.5% of the thickness. There shall be no surface defects (sandish holes, scratches, streaks) as they would seriously affect the test results.

4.1.3.2 Plane heat source method (method B)

4.1.3.2.1 The effective diameter of the test surface of the test piece shall not be less than twice the diameter of the probe; the surface shall be flat, and the surface material and density shall not be modified.

4.1.3.2.2 Block samples shall meet the requirements of 4.1.3.2.1, and can be prepared into cylinders, cubes, etc. The thickness of the upper and lower samples shall be the same, and the thickness should be greater than the diameter of the selected probe, but not less than the radius of the probe.

4.1.3.2.3 Sheet samples shall meet the requirements of 4.1.3.2.1. During preparation, the thickness of the upper and lower samples shall be the same, and the deviation shall not be greater than 0.01 mm.

4.1.3.2.4 Film samples shall meet the requirements of 4.1.3.2.1. During preparation, the thickness of the upper and lower samples shall be the same, and the deviation shall not be greater than 0.001 mm.

4.1.3.2.5 The uniaxial anisotropy sample shall meet the requirements of 4.1.3.2.1. During preparation, the test surface of the sample shall be parallel to the plane determined by the x-axis and the y-axis.

4.1.3.2.6 Dry the sample to constant mass and adjust it to the test temperature; when there are special requirements, the sample shall be adjusted according to the product requirements.

4.1.4 Test procedure

4.1.4.1 The flash method (method A) shall be determined according to GB/T 22588.

4.1.4.2 The plane heat source method (method B) shall be determined according to GB/T 32064.

4.1.5 Test report

The test report shall at least include the following information:

a) sample identification;

b) sample thickness;

c) test temperature;

d) thermal diffusivity, under the test temperature, that is calculated when X = 50% (percentage of temperature rise), m2/s;

e) a statement related to the thermal diffusivity calculated for X = 25%, X = 75% and X = 50%, or a comparison of the data at each temperature in the descending part of the test curve with the theoretical model;

f) report of repeated test results at each temperature;

g) Whether the thermal expansion is corrected, if yes, the thermal expansion data used shall be reported;

h) correction process for heat loss and finite pulse time effects and analysis of errors;

i) environmental conditions around the sample; […]

4.2.2 Instruments and apparatuses

The instruments and apparatuses required for the determination of specific heat are as follows:

a) DSC instrument: capable of heating or cooling at a constant rate of 0.5 °C/min ~ 20 °C/min; capable of keeping the test temperature constant within ±0.5 °C for at least 60 minutes; capable of sub-programmed heating or temperature rise of other modes;

b) crucible: of good thermal conductivity, capable of being covered and sealed, and capable of withstanding the pressure generated during the measurement process;

c) analytical balance: weighing accuracy of ±0.01 mg.

4.2.3 Test piece

Test pieces can be classified as powders, particles, granules, or pieces cut from the sample. Test pieces shall be representative of the test sample and shall be prepared and handled with care. If the test piece is cut from the sample, care shall be taken to prevent the polymer from reorienting or other phenomena that may change its properties from happening when heated. Grinding and similar operations shall be avoided to prevent exposure to heat, or reorientation, or heat history of changing the test piece. For granular or powder samples, two or more test pieces shall be taken. The method of sampling and the preparation of the test pieces shall be stated in the test report.

4.2.4 Test procedure

The specific heat shall be determined according to GB/T 19466.4.

4.2.5 Test report

The test report shall include the following information:

a) test date;

b) all necessary details for complete description of the test sample, including heat history;

c) manufacturer's model and type (power compensation type or heat flow type) of the DSC instrument used;

d) shape, size and material of the crucible and lid of the sample for test;

e) test atmosphere and flow rate;

f) calibration substances, including information on the printed matter, nature of the material, mass of usage and other characteristics relevant to calibration;

g) shape, size and mass of the test piece;

h) details of sampling and conditioning of test pieces;

i) temperature program parameters, namely: initial temperature, heating rate, final temperature, time interval of isothermal section, and temperature increment in the step-by-step method; if cooling is adopted, the cooling rate shall also be specified;

j) test results, including specific heat and corresponding temperature;

k) other required information.

4.3 Determination method of thermal conductivity

4.3.1 Method overview

The steady-state measurement of heat, or the speed of heat passing through a material, is an important indicator for evaluating the thermal transport of a material. The thermal conductivity is the unit heat transferred through a material of unit thickness under stable heat transfer conditions, and is calculated according to Formula (3):

Where: lambda - thermal conductivity, in watts per meter kelvin [W/(m·K)];

H - heat flow through the area A, in watts (W); A - heat transfer area, in square meters (m2);

- temperature gradient, in Kelvins per meter (K/m). […]

4.4 Determination method of coefficient of thermal expansion

4.4.1 Method overview

The differential pushrod dilatometers is used to determine the coefficient of linear expansion. The thermal expansion of the test sample is determined from the thermal expansion of a standard reference sample (e.g., quartz). Place two samples side by side in a furnace to heat slowly while pressing two pushrods extending from the furnace to a Linear Variable Displacement Transducer (LVDT) with thermal isolation in order to measure their length change. The difference in expansion between the two samples results in different movements of the pushrod, allowing the determination of the unknown sample's coefficient of linear thermal expansion, which is the ratio of the change in length to the original length at the initial temperature. Use a thermocouple to measure the temperature of the sample. Calculate according to Formula (5):

Where: alpham - coefficient of thermal expansion, per degree Celsius (°C-1); deltaL - length change, in millimeters (mm);

L0 - original length of the test piece, in millimeters (mm); deltaT - temperature rise, in degrees Celsius (°C).

4.4.2 Instruments and apparatuses

The instruments and apparatuses required for the determination of coefficient of thermal expansion is as follows: […]