Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ISO 26424:2008 Abrasion Resistance of Coatings Micro-Scale Abrasion Test Method
Determination of the abrasion resistance of ceramic coatings by a micro-scale abrasion wear test
1. Introduction to ISO 26424 and Micro-Scale Abrasion Testing
ISO 26424:2008 specifies a method for measuring the abrasive wear resistance of ceramic coatings using a micro-scale abrasion wear test based on the crater-grinding technique. This standard is a natural complement to ISO 26423 (coating thickness measurement), extending the ball-cratering methodology to quantify wear behaviour rather than just dimensional properties.
Abrasion resistance is a critical performance parameter for protective coatings used in applications ranging from cutting tools and moulds to biomedical implants and aerospace components. The micro-scale abrasion test offers the advantage of requiring only small test specimens (a few square millimetres) while providing quantitative wear rate data for both the coating and the substrate.
Unlike macro-scale abrasion tests (ASTM G65, ASTM B611) that require large flat specimens, the micro-scale abrasion method can be applied to real components with curved surfaces, making it invaluable for quality control and failure analysis of industrial coated parts.
2. Test Principle and Methodology
2.1 Principle of the Ball-Cratering Wear Test
A rotating ball is pressed against the coated test piece with a controlled normal load while an abrasive slurry is fed into the contact zone. This produces a spherical wear crater. By measuring the crater dimensions and knowing the test conditions (sliding distance, normal load), the abrasive wear coefficient K (volume removed per unit sliding distance per unit load) is calculated using the Archard wear equation.
| Parameter | Type A Test (No Perforation) | Type B Test (With Perforation) |
|---|---|---|
| Test objective | Coating wear rate only | Coating + substrate wear rates |
| Crater configuration | Single crater, coating not penetrated | Series of craters, coating penetrated |
| Measured dimensions | b (total crater diameter) | a (substrate crater), b (total crater) |
| Wear coefficient | Kc = b4 / (64RSN) | Kc, Ks from linear regression |
| Coating thickness | Not required for calculation | Required (measured independently) |
2.2 Type A Test: Non-Perforation Method
In Type A tests, the test duration is controlled so that the crater does not penetrate through the coating. The wear volume V approx = b4/(64R) is calculated from the crater diameter b and ball radius R. The abrasive wear coefficient Kc for the coating is then obtained from V = KcSN (Archard equation), where S is sliding distance and N is normal load.
2.3 Type B Test: Perforation Method
In Type B tests, the coating is deliberately perforated, exposing the substrate. A series of craters is produced at various test durations, and both the total crater diameter b and the substrate crater diameter a are measured for each crater. By plotting SN/Vc against Vs/Vc and applying linear regression, both Kc (from the intercept) and Ks (from the slope) can be determined from a single test series.
For Type B tests, the substrate crater diameter measurement should only be used when the depth of penetration into the substrate exceeds the abrasive particle size. The condition is: a > sqrt(8Rd), where R is ball radius and d is mean abrasive particle size.
3. Engineering Design Insights for Reliable Testing
3.1 Test Apparatus and Ball Conditioning
The test system can be either a free-ball or fixed-ball configuration. Critical design considerations include:
- Ball conditioning: New balls must be conditioned by running for at least 300 revolutions on a non-critical surface under normal test conditions, repeated at five different orientations. Conditioned balls are usable for approximately 50 craters.
- Free-ball system geometry: A test piece tilt angle of 60 to 75 degrees and shaft groove width of 10 mm produce the smallest result variability.
- Drive shaft run-out: Must be less than 20 um at ball contact points.
- Load control: For free-ball systems, use a load cell to measure true normal force as friction alters the effective load. Recommended load: 0.2 N, maximum 0.4 N.
3.2 Abrasive Slurry Selection
The type and concentration of abrasive slurry critically influence the wear mode and results:
- Dilute slurry (2% vol SiC): Promotes grooving abrasion
- Concentrated slurry (20% vol SiC): Promotes rolling abrasion
- Abrasive type: F1200 SiC (nominal 4 um) or F1200 alumina, average size <=5 um
- Corrosion prevention: Add 1 g NaNO2 per 100 cm3 water for steel substrates
Interlaboratory validation (13 labs) for TiN coatings on HSS substrates showed Type A Kc = 5.35×10-13 m3N-1m-1 (repeatability sr = 7.7%, reproducibility sR = 17.6%), and Type B Kc = 8.00×10-13 m3N-1m-1 (sr = 24.3%, sR = 26.1%). These statistics provide engineers with realistic expectations for measurement variability.
4. Data Analysis and Limitations
The method applies the Archard wear equation, which assumes linear proportionality between wear volume, sliding distance, and normal load. For Type B tests, the data analysis relies on plotting SN/Vc against Vs/Vc and performing linear regression to extract coating and substrate wear coefficients. Results should be reported from at least two complete series of craters on each test piece.
Key limitations: The method applies to homogeneous single-layer coatings on flat surfaces. Non-planar specimens require more complex analysis (see References [4] and [5] of the standard), and inhomogeneous coatings may produce erroneous results.
5. Frequently Asked Questions
Q1: Can this test be used for quality control on real components?
Yes. Although designed for quantitative measurement, the standard notes that the test can be adapted as a quality control test for use on real components, including those with curved surfaces.
Yes. Although designed for quantitative measurement, the standard notes that the test can be adapted as a quality control test for use on real components, including those with curved surfaces.
Q2: How does the slurry concentration affect the wear mechanism?
Dilute slurries (2% by volume) promote grooving wear where abrasive particles are embedded in the ball and scratch the surface. Concentrated slurries (20% by volume) promote rolling wear where particles roll between the ball and surface, producing a different wear topography.
Dilute slurries (2% by volume) promote grooving wear where abrasive particles are embedded in the ball and scratch the surface. Concentrated slurries (20% by volume) promote rolling wear where particles roll between the ball and surface, producing a different wear topography.
Q3: What should be done if the crater is not circular?
If the crater diameter measured parallel and perpendicular to the ball rotation direction differs by more than 10%, the crater shall not be used for wear rate calculation. Non-circular craters indicate problems with the test setup or ball condition.
If the crater diameter measured parallel and perpendicular to the ball rotation direction differs by more than 10%, the crater shall not be used for wear rate calculation. Non-circular craters indicate problems with the test setup or ball condition.
Q4: Can results from different laboratories be compared?
Only when identical test conditions are used same abrasive material, particle size, concentration, load, speed, and ball material. The standard explicitly warns that results from different tests should only be compared when test conditions are the same.
Only when identical test conditions are used same abrasive material, particle size, concentration, load, speed, and ball material. The standard explicitly warns that results from different tests should only be compared when test conditions are the same.
