ISO 26423:2009 Coating Thickness Determination Crater-Grinding Method for Fine Ceramics

1. Introduction to ISO 26423 and the Crater-Grinding Method

ISO 26423:2009 specifies a method for determining the thickness of ceramic coatings on fine ceramics (advanced ceramics, advanced technical ceramics) using the crater-grinding technique. This method involves grinding a spherical cavity into the coated surface and subsequently examining the crater under a microscope to measure coating thickness.

Coating thickness is a critical parameter that directly influences the performance, durability, and reliability of coated components in demanding engineering applications. From cutting tools and wear-resistant surfaces to thermal barrier coatings in turbine blades, precise thickness measurement is essential for quality assurance and process control.

2. Technical Principles and Measurement Methodology

2.1 The Crater-Grinding Principle

A rotating ball wetted with an abrasive slurry is pressed against the coated test piece, creating a spherical wear crater that penetrates through the coating into the substrate. The coating thickness (h) is derived from the geometry of the crater specifically the outer crater diameter (D, at the coating surface) and the inner crater diameter (d, at the coating-substrate interface) combined with the known ball radius (rb).

2.2 Thickness Calculation

The simplified equation for thin coatings (where penetration depth is small compared to ball radius) is:

h = (D2 – d2) / (8 x rb)

Or equivalently, using X and Y measurements: h = (X x Y) / (2 x rb)

Equation using D and d is preferred because it is less sensitive to measurement errors than the X-Y method. For non-flat specimens (where the specimen curvature radius rs < 100 x rb), a corrected formula incorporating both ball and specimen curvature is required.

3. Engineering Design Insights for Reliable Measurement

3.1 Test Parameters and Optimization

The quality of crater-grinding results depends critically on the selection of appropriate test parameters. Typical parameters for thin (3-5 um) hard coatings on metallic substrates include:

• Ball diameter: 25 mm hardened steel (ISO 3290-1 Grade G200)
• Contact load: 0.25 N
• Rotational speed: 100 r/min
• Abrasive slurry: 1 um diamond paste in ethanol (1:4 concentration)
• Feed rate: 20 drops/min
• Test duration: 5 min (adjust based on coating wear resistance)

It is often necessary to make trial craters under a range of conditions to determine the optimal parameters for producing circular craters of sufficient depth with clearly delineated interfaces.

3.2 Sources of Measurement Uncertainty

Three primary sources of uncertainty must be managed:

• Crater dimension measurement: Uncertainty in D and d measurements propagates to h. The uncertainty decreases as the crater penetrates deeper into the substrate (larger d).
• Curvature mismatch: If the crater radius of curvature differs from the ball radius, the percentage error in h equals the fractional difference. For example, a 10% larger crater curvature produces a 10% error in h.
• Non-circular craters: An elliptical crater (parallel vs. perpendicular dimensions differing by >10%) introduces errors exceeding 10% such craters should be discarded.

4. Applicability and Limitations

The method applies to both flat and cylindrical specimens. For flat specimens, diameter measurements are taken both parallel and perpendicular to the direction of ball rotation. For cylindrical specimens, only the largest crater dimension parallel to the cylinder axis is measured. The method can also determine individual layer thicknesses in multilayer coating systems by applying the same principles to successive crater circles.

ISO 26423 was developed by ISO/TC 206 for fine ceramics. The crater-grinding method offers distinct advantages over alternative techniques such as cross-sectioning and microscopy because it requires minimal sample preparation and can be performed on actual components without destructive cutting. The spherical crater geometry provides a natural magnification effect where small coating thicknesses produce measurable diameter differences, enabling accurate measurement of sub-micrometre coatings using standard optical microscopes. This makes the method particularly attractive for quality control laboratories that need rapid, cost-effective thickness verification for coated production parts.

ISO 26423 was developed by ISO/TC 206 for fine ceramics. The crater-grinding method offers distinct advantages over alternative techniques such as cross-sectioning and microscopy because it requires minimal sample preparation and can be performed on actual components without destructive cutting. The spherical crater geometry provides a natural magnification effect where small coating thicknesses produce measurable diameter differences, enabling accurate measurement of sub-micrometre coatings using standard optical microscopes. This makes the method particularly attractive for quality control laboratories that need rapid, cost-effective thickness verification for coated production parts.

5. Frequently Asked Questions