ISO 23458:2020 download free.Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for determining thermal expansion coefficient and residual stress of CVD ceramic coatings.
ISO 23458 specifies a test method for determining thermal expansion coefficient up to 2 300 K and the residual stress of chemical vapour deposition (CVD) ceramic coatings (thickness > 0,03 mm) at room temperature. Procedures for test piece preparation, test modes, heat rate, data collection, property calculations and reporting procedures are given.
ISO 23458 applies to CVD ceramic coatings on metal or ceramic substrates. This test method can be used for material research, quality control, characterization and design data-generation purposes.
2 Normative references
The following documents are referred to in the text in such a way that some or all o their content constitutes requirements of ISO 23458. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (Including any amendments) applies.
Iso 3611, Geomet rica! product specifications (GPS) — Dimensional measuring equipment: Micrometers for external measurements — Design and metrological characteristics
ISO 17139, Fine ceramics (advanced ceramics, advanced technical ceramics) — Thermophysical properties of ceramic composites — Determination of thermal expansion
150 19603, Fine ceramics (advanced ceramics, advanced technical ceramics) — Test m et hod for determining elastic modulus and bending strength of thick ceramic coatings
IEC 60584-1, Thermocouples — Part 1: EMFspec,fications and tolerances
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
4 I’rrnciple
The deformation of a coated bar during heating or cooling involves two effect factors, i.e. thermal expansion coefficient and residual stress in the coating and the substrate. The thermal expansion coefficient is a material characteristic. However, the residual stresses in the coating are not only due to a material property but depend also on the size of the component and the cross-sectional area ratio of the coating and substrate.
The thermal expansion coefficient of CVD ceramic coatings is deduced by the relative method, i.e. it is related with the expansion coefficient of the substrate and the coefficient of the coated bar, Two types of test piece, a bar with CVD ceramic coating (composite) and a bar without coating (substrate), shall be used to measure the thermal expansion coefficient according to Iso 17139. After determining the elastic modulus of the substrate and coating, the thermal expansion coefficient of both the composite and the substrate are obtained. The thermal expansion coefficient of CVD ceramic coating shall then be calculated.
The sample length change and environment temperature are measured continuously at regular. frequent intervals during the imposed cycle. The residual stress of CVD ceramic coatings is usually generated due to the mismatch of thermal expansion coefficient between the coating and the substrate. Based on the uniform strain model, geometric compatibility analysis and internal stress equilibrium, the residual stress calculation formula of CVD ceraniic coatings is deduced by the relative method, which is a function of many factors including elastic modulus, expansion coefficient, configurations and temperature. After getting the thermal expansion coefficient of the coating, the residual stress of CVD ceramic coatings shall be calculated from elastic modulus, expansion coefficient, cross-sectional area ratio of the coating to substrate and the temperature during coating preparation.
5 Apparatus
5.1 General
Recommended testing equipment is shown in Figure 1. This can be used to measure the thermal expansion coefficient of two testing pieces in the same environment, If not, the two testing pieces shall be tested respectively in the same environment using the normal thermal expansion equipment in accordance with ISO 17139.
5.5 Data recording system
A system capable of measuring displacements to an accuracy better than 0.000 1 mm. The system shall allow recording of the test piece temperature and the displacement simultaneously.
An analogue chart recorder or digital data collection system should be used. The error of the recording system shall be 1 % or lower. The minimum data collection frequency shall be 15 Hz and a response frequency of 50 Hz is deemed adequate.
5.6 Dimensional measuring devices
The dimensions of the test piece shall be measured using a Vernier calliper conforming with ISO 3611 and with precision of 0.02 mm or better, or other calibrated measuring device providing the same or better measurement accuracy.
Coating thickness shall be measured by using a calibrated optical microscope with magnification of 1 000 times or better. Sample displacement shall be measured using a calibrated electronic micrometer with a precision of at least 0,001 mm and resolution of 0,000 5 mm or better, or other measuring device providing the same or better measurement accuracy. All calibrations shall be traceable to national standards.
6 Specimens
6.1 Test piece
In order to simplify the preparation of test pieces, three different symmetrical structures of coating configurations are considered:
a) coating on upper and lower surfaces of the test pieces only (two-face coating, Figure2 a);
b) coating on four surfaces of the test pieces (four-face coating. Figure 2 b);
c) coating on cylinder surface of the test pieces (around coating, Figure 2 c). Any of the three coating configurations may be used for evaluating the properties of the coating layer.
6.3 Test piece preparation
6.3.1 General
The test piece preparation is only for the measurement of the expansion coefficient of the CVD coating. Two samples are required for comparison, one bar with uniform CVD coatings and another without coating, conforming with ISO 17139. The non-uniformity of the coating thickness for the test piece shall be lower than 10 %.
Since residual stress is not a material constant, no test piece is required for it.
6.3.2 Test piece handling and storage
The test pieces shall be handled with care to avoid the introduction of damage after test piece preparation.
Test pieces shall be stored separately and not allowed to impact or scratch each other.
6.3.3 Number of test pieces
A minimum of three pairs of test pieces are required for the test.
7 Test procedure
7.1 Measurement of the test piece dimensions
Measure the dimensions of the test pieces using a Vernier caliper conforming with ISO 3611 and with a precision of 0,02 mm or better, or another calibrated measuring device providing the same or better measurement accuracy. Coating thickness shall be measured by using a calibrated optical microscope with magnification of 1 000 times or better.
The variation in coating thickness between the thickest and thinnest measured values, ‘max hmin )/h man’ shall be less than or equal to 10 %. If the variation is more than this, prepare new test pieces meeting this requirement.
9 Test report
The test report shall contain at least the following information:
a) the name and address of the testing establishment;
b) the date of the test and customer name, address and signature;
c) a reference to ISO 23458, i.e. Iso 23458:2020;
d) the test piece shape, size and coating thickness;
e) a description of the test material (material type, manufacturing code);
f) the maximum testing temperature;
g) the testing atmosphere;
h) the minimum, mean and maximum values of residual stress;
i) the minimum, mean and maximum values of linear thermal expansion coefficient;
j) the temperature and humidity of the laboratory.