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ISO 28279: Cleanliness Testing for Powder Metallurgy Parts
Determination of surface contamination levels in sintered metal materials
Introduction to ISO 28279: Cleanliness Testing for PM Parts
ISO 28279:2010 specifies a standardized method for determining the amount and nature of surface contamination on powder-metallurgy (PM) parts. Developed by ISO/TC 119 (Powder metallurgy), this standard provides manufacturers and users with a reliable, repeatable procedure for assessing the level of cleanliness — a critical quality parameter for PM components used in automotive, aerospace, and industrial applications.
Surface contamination in PM parts can originate from residual lubricants, machining fluids, handling oils, or environmental particles. Left unchecked, contamination can cause adhesion failure in subsequent coating operations, interfere with sealing surfaces, or introduce reliability issues in precision assemblies.
The test principle is straightforward: PM parts are pressure-rinsed with a filtered solvent, the rinseate is re-filtered to capture contaminants, and the captured residue is weighed and examined microscopically. The result is expressed as contaminant mass per part (mg/part) with identification of particle types and sizes.
| Apparatus | Specification | Purpose |
|---|---|---|
| Filter funnel and flask | Minimum 2 L capacity | Solvent filtration |
| Vacuum pump | Standard laboratory | Filtration assistance |
| 5 um filter | Polyester or polyamide | Primary contaminant collection |
| 1 um filter | Polyester, polyamide, or cellulose | Solvent pre-cleaning |
| Analytical balance | 0.0001 g accuracy, 20 g capacity | Weighing contaminants |
| Drying oven | 100 C +/- 10 C | Filter drying |
| Pressure gun | 200 +/- 50 kPa | Part rinsing |
| Stereomicroscope | >=10x magnification | Particle examination |
Engineering Design Insights for Contamination Testing
The procedural rigor in ISO 28279 reflects the challenges of measuring low-level contamination. The solvent (white spirit, or alternative as agreed) must be pre-filtered through a 1 um filter to establish a clean baseline. The 5 um collection filter is dried at 100 C and weighed before use, then re-weighed after contaminant collection. The difference, divided by the number of parts tested, yields the contaminant mass per part.
A minimum total surface area of 1,000 cm2 is required for each test. For very large or very small parts, the surface area should be agreed between user and supplier. This ensures statistically representative results across different part geometries.
Critical Process Parameters
Several parameters must be carefully controlled: solvent pressure (200 +/- 50 kPa), drying temperature (100 C +/- 10 C), drying time (minimum 30 minutes), and analytical balance accuracy (0.0001 g). The standard notes that an ultrasonic extraction technique may be suitable as an alternative to pressure rinsing, provided both parties agree. However, ultrasonic extraction should not be used for materials that can be chemically attacked by ultrasound, such as aluminium.
For PM parts intended for vacuum service or applications exceeding 10 hPa pressure, ultrasonic extraction is the recommended technique. This recognizes that pressure rinsing may not adequately dislodge contaminants from deep pores in parts destined for demanding environments.
Expression of Results and Quality Control
Results are reported as contaminant amount C (mg/part), calculated as C = (m2 – m1) * 1000 / N, where m1 is the initial filter mass, m2 is the filter mass with contaminants, and N is the number of parts. Particle nature is documented by size (um) and type. The test report must reference ISO 28279:2010 and include the part identification, contaminant amount, maximum particle size, and contaminant nature. Any deviation from the standard parameters (filter mesh, solvent type, extraction method) must be agreed between parties and documented.
Frequently Asked Questions
Q1: Why is the 5 um filter specified as the primary collection medium?
A: The 5 um pore size captures the majority of process-related contaminants while allowing fine oil and solvent residues to pass through. This provides a practical balance between sensitivity and test practicality for typical PM part contamination levels.
A: The 5 um pore size captures the majority of process-related contaminants while allowing fine oil and solvent residues to pass through. This provides a practical balance between sensitivity and test practicality for typical PM part contamination levels.
Q2: Can solvents other than white spirit be used?
A: Yes, but any alternative solvent must be agreed between the user and supplier. The solvent must effectively dissolve residual oils without attacking the PM part substrate or introducing artefacts.
A: Yes, but any alternative solvent must be agreed between the user and supplier. The solvent must effectively dissolve residual oils without attacking the PM part substrate or introducing artefacts.
Q3: What is the minimum surface area required for testing?
A: At least 1,000 cm2 total surface area of the parts being tested. For very large or small parts, an alternative surface area may be agreed between the parties.
A: At least 1,000 cm2 total surface area of the parts being tested. For very large or small parts, an alternative surface area may be agreed between the parties.
Q4: How is the contaminant nature assessed?
A: The contaminant particles are visualized between glass microscope slides using a stereomicroscope with at least 10x magnification. The maximum length/diameter and nature of each contaminant particle are recorded.
A: The contaminant particles are visualized between glass microscope slides using a stereomicroscope with at least 10x magnification. The maximum length/diameter and nature of each contaminant particle are recorded.
