EN 15042-2:2006

Thickness measurement of coatings and characterization of surfaces with surface waves - Part 2: Guide to the thickness measurement of coatings by photothermic method

This document describes methods for the measurement of the thickness of coatings by means of thermal waves generated by a radiation source.
The method can be used for coatings whose thermal properties (e.g. thermal conductivity) are different from those of the substrates in a range from a few microns to some hundred microns.

Schichtdickenmessung und Charakterisierung von Oberflächen mittels Oberflächenwellen - Teil 2: Leitfaden zur photothermischen Schichtdickenmessung

Dieses Dokument legt ein Messverfahren fest, das eine Werkstoffprüfung mittels Wärmewellen erlaubt, die durch eine Strahlungsquelle erzeugt werden.
Das Messverfahren kann bei Beschichtungen angewendet werden, deren thermische Eigenschaften (z. B. Wärmeleitfähigkeit) sich von denen des Substrates unterscheiden, in einem Messbereich von einigen µm bis einige hundert µm.

Mesure de l'épaisseur des revêtements et caractérisation des surfaces à l'aide d'ondes de surface - Partie 2 : Guide pour le mesurage photothermique de l'épaisseur des revêtements

Le présent document décrit des méthodes de mesurage de l’épaisseur des revêtements à l’aide d’ondes
thermiques produites par une source de rayonnement.
La méthode peut être utilisée pour les revêtements dont les propriétés thermiques (par exemple, la
conductivité thermique) diffèrent de celles des substrats pour une plage d’épaisseur comprise entre quelques
microns et plusieurs centaines de microns.

Merjenje debeline nanosa prevlek in karakterizacija valovitih površin – 2. del: Vodilo za merjenje debeline prevlek s fototermično metodo

General Information

• Status: Published
• Publication Date: 11-Apr-2006
• Withdrawal Date: 30-Oct-2006
• ICS: 17.040.20 - Properties of surfaces
• Technical Committee: CEN/TC 262 - Metallic and other inorganic coatings
• Drafting Committee: CEN/TC 262 - Metallic and other inorganic coatings
• Current Stage: 9093 - Decision to confirm - Review Enquiry
• Start Date: 03-Jan-2023
• Completion Date: 14-Apr-2025
• Ref Project: SIST EN 15042-2:2006 - Thickness measurement of coatings and characterization of surfaces with surface waves - Part 2: Guide to the thickness measurement of coatings by photothermic method

Overview

EN 15042-2:2006 - "Thickness measurement of coatings and characterization of surfaces with surface waves - Part 2: Guide to the thickness measurement of coatings by photothermic method" (CEN) is a technical guide for non‑destructive, contactless measurement of coating thickness using photothermic / photothermal methods. The standard describes how thermal waves generated by a radiation source can be used to determine the thickness of coatings whose thermal properties differ from their substrates, over a range from a few microns up to some hundred microns.

Keywords: EN 15042-2:2006, photothermic method, coating thickness measurement, thermal waves, non‑destructive testing.

Key topics and technical requirements

• Measurement principle: generation of thermal waves by modulated or pulsed radiation and detection of surface thermal response (amplitude and phase) to infer layer properties.
• Depth control: use of modulation frequency to adjust the thermal diffusion length (penetration depth) for depth‑resolved measurements.
• Thermal parameters: interpretation relies on thermal effusivity, thermal diffusivity, photothermal efficiency (absorption), and the thermal reflection coefficient at layer interfaces.
• Layer applicability: suitable for coatings with a thermal contrast relative to the substrate; layer thicknesses from microns to several hundred microns.
• System components: excitation source, detector (point detectors or IR cameras), and signal processing/measurement evaluation; comparison with calibration data is required for quantitative results.
• Measurement considerations: optical accessibility and conversion of incident radiation to heat are prerequisites; accuracy requires knowledge or estimation of layer thermal diffusivity and proper calibration.
• Uncertainty and traceability: normative references include uncertainty guidance (ISO/GUM supplement referenced in the standard), so measurement uncertainty evaluation and documentation are expected.

Keywords: thermal effusivity, thermal diffusivity, modulation frequency, photothermal efficiency, measurement uncertainty.

Applications

• Quality control of coatings (metallic, polymeric, ceramic) in production lines for layer-thickness verification.
• On‑line process monitoring and feedback control to ensure uniform coating deposition.
• Materials characterization in R&D: depth profiling, porosity or adhesion inference when correlated with thermal properties.
• Failure analysis and surface treatment assessment (e.g., heat treatment, implantation) where thermal contrast exists.

Keywords: non‑destructive testing (NDT), on‑line monitoring, coating quality assurance, depth profiling.

Who should use this standard

• Coating manufacturers and process engineers
• Quality control and inspection teams
• NDT and metrology laboratories
• Materials scientists and surface engineers
• Instrument manufacturers developing photothermal measurement systems

Related standards / references

• EN 15042-2:2006 references measurement uncertainty guidance (ISO/GUM supplement). Follow CEN implementation procedures for national adoption.

Using EN 15042-2:2006 helps organizations implement reliable, non‑destructive photothermic thickness measurement for coatings where thermal contrast permits accurate, depth‑resolved evaluation.

Standards Content (Sample)

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Thickness measurement of coatings and characterization of surfaces with surface waves - Part 2: Guide to the thickness measurement of coatings by photothermic methodMesure de l'épaisseur des revetements et caractérisation des surfaces a l'aide d'ondes de surface - Partie 2 : Guide pour le mesurage photothermique de l'épaisseur des revetementsSchichtdickenmessung und Charakterisierung von Oberflächen mittels Oberflächenwellen - Teil 2: Leitfaden zur photothermischen SchichtdickenmessungTa slovenski standard je istoveten z:EN 15042-2:2006SIST EN 15042-2:2006en17.040.20ICS:SLOVENSKI
STANDARDSIST EN 15042-2:200601-september-2006

EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 15042-2April 2006ICS 17.040.20 English VersionThickness measurement of coatings and characterization ofsurfaces with surface waves - Part 2: Guide to the thicknessmeasurement of coatings by photothermic methodMesure de l'épaisseur des revêtements et caractérisationdes surfaces à l'aide d'ondes de surface - Partie 2 : Guidepour le mesurage photothermique de l'épaisseur desrevêtementsSchichtdickenmessung und Charakterisierung vonOberflächen mittels Oberflächenwellen - Teil 2: Leitfadenzur photothermischen SchichtdickenmessungThis European Standard was approved by CEN on 2 March 2006.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Central Secretariat or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania,Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2006 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 15042-2:2006: E

According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

∆∆∆∆T0
maximum local temperature variation of the oscillating part for periodic-harmonic heating processes NOTE See Equation 2. 3.2 penetration depth of thermal waves depth at which the temperature variation below a modulated heated surface is still measurable. NOTE In general, the penetration depth is of the order of magnitude of the thermal diffusion length 3.3 modulation frequency
f frequency at which the intensity of the heating radiation varies periodically 3.4 phase (phase shift) of the thermal wave
∆ - measure of the temporal delay of the temperature oscillation relative to the excitation for periodic-harmonic heating processes NOTE See Equation 3. 3.5 photothermal efficiency
η proportion of the incident radiation intensity that is converted into heat NOTE
In most technical applications it is approximately identical to the absorption.

µ characteristic length of the thermal diffusion with pulsed heating or periodically modulated heating, where the temperature amplitude has decreased to about 1/e or 37 %
NOTE 1 1/e, with natural number e = 2,71828. NOTE 2 See Equation 4. 3.7 thermal diffusion time
τ characteristic time that a thermal wave or a temperature pulse requires for penetrating a layer of finite thickness NOTE See Equation 7. 3.8 thermal diffusivity
α thermal parameter characterizing heat propagation in a body with time-dependent heating NOTE See Equation 6. 3.9 thermal effusivity
e thermal parameter determining the surface temperature of a body with time-dependent heating NOTE See Equation 5. 3.10 thermal wave spatiotemporally variable temperature field that is set up in a body (or medium) with time-dependent heating and is described by the heat conduction equation NOTE 1 see Equation 1. NOTE 2 The thermal wave is generated in one limiting case by a periodic-harmonic excitation, in the other limiting case by a pulsed excitation. 3.11 thermal reflection coefficient
Rls
thermal parameter that is a degree of the reflection of the thermal wave at the boundary interface between two layers of different effusivity and thus describes the heat transfer across this boundary interface NOTE See Equation 8.

Symbol Unit Description See Equation ∆T(x,t) K amplitude of the temperature oscillation of the thermal wave 1 ∆T0(x) K amplitude of the temperature oscillation of the thermal wave at the surface (x = 0) 2 ∆ -
rad phase of the temperature oscillation of the thermal wave
3 µ m thermal diffusion length 4 e Ws1/2/(m2K) thermal effusivity 5 α m2/s thermal diffusivity 6 τ s thermal diffusion time 7 F0 W/m2 heat flow/excitation power density 7 η
photothermal efficiency
k W/(m⋅K) thermal conductivity
12 ρ Kg/m3 mass density 13 c J/(kg⋅K) specific heat capacity
13 f s-1 modulation frequency
i0 W/m2 incident radiant power density
2 x m location below the boundary interface
t s time
5 Foundations of photothermal materials testing 5.1 Physical foundations 5.1.1 Thermal waves The concept "thermal wave(s)" describes a spatially and temporally variable temperature field that is generated in a body by time-dependent heating. Besides the concept thermal wave the term "temperature wave" is also used in technical literature. The excitation of the spatiotemporally variable temperature field mathematically described by a diffusion equation - the heat conduction equation - can occur in the one limiting case periodic-harmonically and in the other limiting case pulsed.
The physical foundations [1], [5], [6], [7] can be derived both for the harmonic excitation and for the pulsed excitation, and are related by a Fourier transformation. This clause considers primarily the harmonic excitation; the derivation for the pulsed excitation can be found in [8].

The thermal effusivity (thermal penetration coefficient), e, is given by the equation:
ckeρ= (5) and the thermal diffusivity, ., is given by the equation: ()ckρα= (6) Accordingly, frequency-dependent measurements of the amplitude and phase of the thermal wave provide depth-resolved information on these combined thermal parameters. In Equations (5) and (6), k is the thermal conductivity, ρ the mass density and c the specific heat capacity. The amplitude of the thermal wave measurable at the surface is proportional to the photothermal efficiency η, which specifies the proportion of the incident radiant power converted into heat. With layered systems the amplitude and the phase shift of the temperature oscillation are determined on the one hand by the ratio of the thermal effusivity of layer and substrate elayer/esubstrate, and on the other hand by the thermal diffusion time for the layer: layerlayerlayerlατ2= (7)

llayer
is the geometrical thickness of the layer; .layer
is the thermal diffusivity of the layer. Given a known value of the thermal diffusivity of the layer and a sufficiently large thermal contrast, describable according to [11] by the thermal reflection coefficient:
substratelayersubstratelayerlseeeeR+−= (8) contactless and non-destructive layer thickness determination is possible by means of thermal waves (see Clause 6). 5.1.2 Thermal Properties The significance of the thermal effusivity and the thermal diffusivity can be made especially clear by means of special time-dependent heating (step function). According to [12], the thermal effusivity e (Equation 5) is a measure of the time-dependent heating of a surface: ()teFtxTπ02,0==∆ (9) where F0 is the constant heat flow absorbed at the surface and ∆T(x = 0,t) represents the heating of the surface at time t after the start of heating. The thermal effusivity determines the contact temperature between bodies and layers having different thermal properties. An example is the contact temperature: ()()212211eeTeTeTcontact+⋅+⋅= (10) occurring at the boundary interface between two semi-infinite bodies having different thermal effusivity e1 and e2 and different initial temperatures T1 and T2, after these bodies have been brought into contact with one another. The thermal diffusivity α (Equation 6) is a measure of the propagation of the temperature through a homogenous body:
()()=∆=−=∆∫txtxTtttxeFtxTαπαπ4,0''/'4exp,020ierfcd (11) Given measurements of the thermal effusivity and thermal diffusivity by means of thermal waves, the heat conductivity and the heat capacity per unit volume can be determined using Equations (5) and (6): ek⋅=α (12) ()αρec= (13)

For the surface study of test objects, thermal imaging methods are used; here, thermal waves are used pointwise to form a surface raster, or in the case of large-area modulated heating, to make time-dependent recordings with an IR camera. 5.2 Structure of a photothermal measuring system A photothermal measuring instrument comprises components to carry out the following functions: excitation, detection and measured value processing. As a rule, the measurement data are compared with calibration data, so that quantitative statements on deviations in the production process or on deviations from the desired functional properties can be made. The only requirement on the test object (besides optical accessibility) is the supply of energy (through the absorption of radiation, for example) and its conversion into a detectable form (by increasing the thermal radiation to the level of IR detection, for example). The detectable signals (amplitude and phase) then provide information on material properties and/or their modifications in layers near the surface. Figure 1 shows one possible way of integrating a photothermal measuring system in a production line or in the production process. Feedback coupling serves to control the production process and ensure the functional properties of a component. As a non-destructive and contactless procedure, thermal waves are suitable for testing materials and components. They can be used as an on-line measuring procedure for production monitoring and quality testing.

5.3 General information on measuring methods with thermal waves Common to all photothermal studies is the precise periodic or pulsed introduction of a quantity of heat into a test body and the detection of local heating. Because of their special properties, light sources are especially suitable for efficient excitation (i.e. heating) of the test object. In the following, therefore, the optical excitation is considered to exemplify the possible ways of introducing heat into the test object.
The various photothermal methods of measurement differ in the temporal structure of the excitation and in the manner in which the thermal response is measured (e.g. surface temperature, heat flow or changes of other physical effects). In the case of periodic-harmonic excitation, the amplitude and phase are registered as accessible variables. In the case of pulsed excitation, the temporal development of the temperature is determined. As shown in Figure 2, the introduction of thermal energy (excitation) in areas near the surface of a test object leads to a series of effects that directly or indirectly can be used for testing material properties. These effects include the thermal and acoustic waves emitted at the surface of the test object and in the surrounding medium, increased thermal radiation, deformation of the surface, modification of the reflectivity as well as changes in the magnetic, electric and dielectric properties.

The introduced heat is diffused through the test object, causing changes in temperature, the propagation of which through the interior of the material can be described by Equation (1). The diffusion process itself depends on the thermal properties of the material and is described in terms of the thermal diffusivity α and the thermal effusivity e, which can be computed from the thermal conductivity k, the heat capacity c and the density ρ of the material according to Equations (5) and (6).
Owing to the severe damping, the amplitude of the temperature oscillation in the material depth strongly decreases. This feature is described by the thermal diffusion length µ (Equation 4). For measuring techniques with thermal waves, the thermal diffusion length µ approximately and intuitively describes the range in which information from the interior of the material is contained in the surface temperature. The characteristic feature of this form of materials testing thus lies in changing the thermal diffusion length through the selection of the modulation frequency (compare Figure 3). This provides an overview of the photothermally accessible measuring range. Key X Thermal diffusion length, µ [m] Y Frequency, f, [Hz] 1. 1 Ag, Cu, Al, Si,
SiC, Zn,
Cr, air, steel Si3N4 2. Silica, ceramics 3. PE, water, paint
10-310-410-510-11101021031014123XYFigure 3 — Diffusion length as a function of the frequency (data from [17]; further data can be found in [18] and [13])
In the case of pulsed excitation, the measurable depth range basically depends on the time delay of detection following excitation.
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