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Scanning Electron Microscopy (SEM)

Definition: Scanning electron microscopy (SEM) is an imaging analysis method in which a surface is scanned in a raster pattern with a focused electron beam. Interactions between the electrons and the sample generate signals such as secondary electrons (SE) or backscattered electrons (BSE). The method enables high-resolution investigations down to the nanometre range.

Practical relevance: SEM is used for fractography, microstructure analysis, particle characterisation and coating assessment. In combination with EDX, spatially resolved elemental analysis is possible, and with EBSD, grain orientation analysis. Sample preparation, conductivity and coating significantly influence image quality and the analysis result.

Decision-making perspectives:

Technical decision-makers: Identification of crack causes, inclusions or interface problems.
Purchasing/project management: Commissioning of well-founded failure or material analyses.
Science: Investigation of micro- and nanoscale structures and phase distributions.
Insurance/law: Court-proof documentation of fracture surfaces and damage mechanisms.

Typical testing or verification methods: SE/BSE imaging, SEM-EDX analysis, EBSD mapping, fractography.

FAQ:

What is the advantage of an SEM compared with a light microscope?
The SEM offers significantly higher resolution and greater depth of field for detailed microstructure and fracture surface analyses.

Special tests

Definition: Special tests are individually developed or adapted testing methods for clarifying specific technical questions that are not sufficiently covered by standardised normative tests. They frequently combine several analysis and testing methods. The aim is a robust, problem-oriented assessment.

Practical relevance: Fields of application include complex failure cases, novel materials, atypical loading scenarios or special regulatory requirements. Special tests can comprise modified mechanical tests, combined environmental stresses or special specimen geometries. A traceable documentation of the test set-up, boundary conditions and measurement uncertainty is essential.

Decision-making perspectives:

Technical decision-makers: Clarification of specific failure mechanisms or suitability verifications beyond standardised tests.
Purchasing/project management: Definition of individual acceptance criteria for innovative products.
Science: Development of validatable test designs and comparison with reference methods.
Insurance/law: Court-proof documentation in exceptional failure or dispute cases.

Typical testing or verification methods: Combined climatic and load tests, modified fatigue tests, special corrosion or media tests.

FAQ:

When are special tests required?
When normative standard tests do not sufficiently represent the specific technical question.

Spectral analysis

Definition: Spectral analysis is an analytical method for determining the chemical composition of a material on the basis of characteristic emission or absorption spectra. It uses the element-specific spectral lines for qualitative and quantitative analysis. In materials engineering, it is used to determine alloying and trace elements.

Practical relevance: Typical methods are optical emission spectroscopy (OES), X-ray fluorescence analysis (XRF) or ICP-OES. Detection limits and accuracy depend on the method and the sample preparation. Applications include incoming goods inspections, PMI testing and failure analyses. Results are compared with normative material specifications.

Decision-making perspectives:

Technical decision-makers: Ensuring the correct material composition in safety-relevant components.
Purchasing/project management: Verification of supplier assurances in accordance with material standards.
Science: Validation of analytical accuracy and comparison of different spectrometry methods.
Insurance/law: Documented proof of material deviations in liability cases.

Typical testing or verification methods: OES, XRF, ICP-OES, comparison with reference materials.

FAQ:

What is the difference between OES and XRF?
OES uses emission lines after spark excitation, whereas XRF measures fluorescent X-rays without removing material.

Standard Tests (Streicher, Strauss)

Definition: The standard tests according to Streicher and Strauss are standardised corrosion testing methods for assessing the resistance of stainless steels to intergranular corrosion. They serve to verify whether sensitisation due to chromium carbide precipitation is present. The methods are governed by DIN EN ISO 3651.

Practical relevance: The Strauss test (copper sulphate-sulphuric acid test) and the Streicher test (ferric sulphate-sulphuric acid test) examine the susceptibility to corrosion along grain boundaries. Mass loss, crack formation or microstructural attack are assessed after a defined exposure time. The tests are relevant after welding or heat treatment processes of austenitic steels.

Decision-making perspectives:

Technical decision-makers: Ensuring the intergranular corrosion resistance of welded components.
Purchasing/project management: Requiring standard-compliant test certificates for corrosion-critical applications.
Science: Analysis of sensitisation mechanisms and carbide precipitation.
Insurance/law: Verification of specification-compliant corrosion resistance in cases of damage.

Typical testing or verification methods: Strauss test, Streicher test, metallographic microstructure examination after exposure.

FAQ:

When are Streicher or Strauss tests carried out?
After welding or heat treatment processes to test for susceptibility to intergranular corrosion.

Stylus Profilometry Method

Definition: The stylus profilometry method is a tactile measurement technique for determining surface roughness, in which a diamond stylus tip traces the surface along a defined measurement length. The height profile is recorded and evaluated mathematically. The normative bases are DIN EN ISO 4287 and ISO 16610.

Practical relevance: Parameters such as Ra, Rz or Rt are calculated from the profile. The choice of cut-off wavelength (λc), filter type and measurement length significantly influences the result. The method is well established for the quality control of sealing faces, bearing locations and tribologically loaded components. The stylus tip radius and force must be matched to the surface.

Decision-making perspectives:

Technical decision-makers: Ensuring function-appropriate roughness parameters for friction and sealing systems.
Purchasing/project management: Clear specification of roughness parameters and measurement conditions in drawings.
Science: Analysis of filtering methods and comparison with optical 3D topography.
Insurance/law: Traceable verification of surface requirements in the event of functional deviations.

Typical testing or verification methods: Profile acquisition with a stylus instrument, filtering in accordance with ISO 16610, parameter evaluation (Ra, Rz).

FAQ:

Why is the choice of filter important in the stylus profilometry method?
It separates roughness from waviness and significantly influences the calculated parameters.

Surface measurement

Definition: Surface measurement is the quantitative recording of roughness, waviness and three-dimensional topography of a technical surface. It serves to evaluate functional properties such as friction, tightness or coating adhesion. Normative bases include DIN EN ISO 4287 and ISO 25178.

Practical relevance: Characteristic values such as Ra, Rz or Sa describe height deviations in the micro and macro range. Measurement methods are tactile stylus instruments or optical 3D profilometers. Filter settings (e.g. in accordance with ISO 16610) influence the evaluation. Surface quality is decisive for tribological systems, sealing surfaces and joined connections.

Decision-making perspectives:

Technical decision-makers: Ensuring functionally appropriate surfaces for wear and tightness requirements.
Purchasing/project management: Definition of unambiguous roughness characteristic values in drawings and specifications.
Science: Analysis of topography structures and their influence on friction and wetting.
Insurance/law: Verification of surface deviations in functional or liability matters.

Typical testing or verification methods: Stylus profiling method, optical white-light interferometry, 3D laser scanning, profile analysis.

FAQ:

What is the difference between roughness and waviness?
Roughness describes fine microstructures, waviness larger periodic deviations of the surface.