Powder Metallurgy Surface Finish: What to Expect From Sintered Parts
Understand what surface finish is achievable from powder metallurgy parts as-sintered, after sizing, and with secondary operations. Includes Ra values and practical guidance for drawing callouts.
Yao Qingpu
Powder Metallurgy Manufacturing Expert at SinterWorks Technology
Table of Contents
# Powder Metallurgy Surface Finish: What to Expect From Sintered Parts
Surface finish is one of the areas where PM parts differ most noticeably from machined or cast parts. The porous, powder-derived surface of a sintered part looks and feels different from machined steel, and the achievable Ra values depend heavily on which operation last contacted the surface. Understanding what PM can and cannot achieve - and when secondary operations are needed - prevents misspecification and unexpected cost.
Why PM Surface Finish Is Different
A sintered PM part surface has two characteristics that distinguish it from machined metal:
Porosity. The surface has open pores - the tops of the interconnected void network that runs through the part. Even at high density (90-92% TD), the surface has visible pores under magnification. These pores set a practical floor on achievable surface roughness: you cannot polish a porous surface to the same smoothness as a fully dense machined surface using the same operations.
Particle-derived texture. The surface at the microscopic level reflects the original powder particle shape. Even with sizing, the surface has a distinctive dimpled or granular character different from the machined lay marks of a turned or ground surface.
These differences matter for some applications (sealing faces, precision bearing bores, cosmetic surfaces) and are irrelevant for most (structural interfaces, enclosed surfaces, oil-impregnated bearings).
Surface Finish by Process Stage
As-sintered
After sintering, the surface contacts only the die wall during compaction - a smooth carbide or hardened steel surface. The resulting as-sintered surface finish is driven by:
- Die wall surface quality (polished carbide gives better surface than worn tool steel)
- Powder particle size (finer powder → smoother surface)
- Sintering atmosphere and temperature (some slight surface roughening from oxide reduction reactions)
Typical as-sintered Ra values for iron-based PM:
| Condition | Typical Ra Range |
|---|---|
| As-sintered, standard iron-copper alloy | 0.8-3.2 µm |
| As-sintered, fine powder / polished die | 0.4-1.6 µm |
| As-sintered, coarse powder or worn die | 1.6-6.3 µm |
The as-sintered surface is adequate for many structural applications where surface finish is not a functional requirement.
After sizing
Sizing cold-works the surfaces that contact the sizing die. Cold working partially closes surface pores and improves surface finish on contact faces:
| Feature | Typical Ra After Sizing |
|---|---|
| Bore diameter (mandrel sized) | 0.4-1.6 µm |
| OD surface (die sized) | 0.4-1.6 µm |
| Flat face (punch coined) | 0.2-0.8 µm |
Sizing does not dramatically improve surface finish - the improvement is typically one roughness grade. The benefit of sizing is primarily dimensional, with surface finish improvement as a secondary effect.
After secondary machining
Turning, boring, and grinding PM parts produces surface finishes similar to equivalent operations on wrought steel, but with the complication of pores interrupting the cutting surface:
| Operation | Typical Ra on PM |
|---|---|
| Turned bore (carbide insert) | 0.4-1.6 µm |
| Ground bore (cylindrical grinding) | 0.1-0.4 µm |
| Face turned or milled | 0.4-1.6 µm |
| Surface ground | 0.1-0.4 µm |
| Honed bore | 0.1-0.4 µm |
Honing and cylindrical grinding on high-density PM achieve very fine surface finish, but pores interrupt the ground surface - producing a finish that measures well on a profilometer but looks porous under a microscope. For sealing faces and precision bearing applications, this must be accounted for.
Surface Finish After Common Treatments
Steam treatment
Steam treatment creates a magnetite (Fe₃O₄) layer that slightly smooths the surface by partially filling pores near the surface. As-sintered Ra of 1.6-3.2 µm typically improves to 0.8-1.6 µm after steam treatment. The improvement is modest; steam treatment is not a surface finishing operation in the Ra sense.
Electroless nickel plating
A 10-25 µm electroless nickel deposit partially fills surface pores and creates a smooth, dense nickel-phosphorus surface over the porous substrate. After plating:
- The nickel surface itself has Ra 0.1-0.4 µm
- Underlying pores are partially bridged; cosmetic appearance is smooth
- Note: thin deposits over very open porous PM may show pinholes - resin impregnation before plating is standard
Electropolishing (stainless PM only)
Electropolishing removes surface material selectively, smoothing high points on the surface. For stainless PM (316L, 304), electropolishing achieves Ra 0.1-0.4 µm and removes surface contamination. It is the standard surface treatment for food-grade and hygienic stainless PM parts.
Grinding and lapping after sintering
Precision grinding or lapping of PM faces achieves Ra values of 0.025-0.1 µm (comparable to precision machined surfaces), but the pores remain. For applications requiring both smooth surface and dense material (precision sealing faces), high-density PM (>95% TD) or resin-impregnated PM before grinding is recommended.
When Surface Finish Matters
Bearing surfaces: Bore surfaces for rotating or sliding shafts typically require Ra ≤ 0.8 µm. This is achievable with sizing (mandrel) on standard PM density. For high-speed or precision bearings, sizing to 0.4 µm or internal grinding may be required.
Sealing surfaces: O-ring grooves, face seals, and press-fit sealing surfaces require dense, smooth surfaces. For metal-to-metal seals, the porous PM surface is generally inadequate without resin impregnation and grinding. For O-ring sealing, the groove dimensions and surface finish must be specified to match the O-ring standard, and porosity in the groove wall must be managed.
Gear tooth flanks: PM gear flanks after sizing typically have Ra 0.8-1.6 µm - adequate for moderate-speed industrial gears. For high-speed precision gearing, flank grinding may be required.
Cosmetic surfaces: PM surfaces are not cosmetically attractive in the as-sintered or as-sized condition. They are matte, slightly porous in appearance, and not suitable for high-gloss applications without plating, powder coating, or painting over a sealed (impregnated) base.
Press-fit and interference surfaces: The slightly porous surface of a PM OD or bore in a press-fit assembly can be an advantage - the interface has slightly higher effective friction than fully dense metal, which can improve press-fit retention. However, the surface must be sized to the correct tolerance for the fit class.
Drawing Callout Guidance
When specifying surface finish on a PM drawing:
Match the requirement to the achievable process:
- As-sintered surface: 1.6-3.2 µm (Ra or Rz) is reasonable
- Sized surface: 0.4-1.6 µm is reasonable
- Machined/ground surface: 0.1-0.8 µm depending on operation
Use surface finish symbols, not just notes: ASME Y14.36 surface texture symbols communicate finish requirements clearly and prevent ambiguity about which surfaces are controlled.
Distinguish finished vs. as-sintered surfaces: Not all PM surfaces are controlled to the same finish. Mark surfaces that require sizing or machining explicitly; leave non-functional surfaces at "as-sintered."
Consider the functional requirement, not a default number: Calling out Ra 0.8 µm on a surface that will never contact another part or seal anything adds inspection and process cost with no functional benefit. Apply finish requirements only where they matter.
Summary
| Stage | Typical Ra |
|---|---|
| As-sintered | 0.8-3.2 µm |
| After sizing | 0.4-1.6 µm |
| Ground bore | 0.1-0.4 µm |
| Electroless nickel | 0.1-0.4 µm |
| Electropolished (stainless) | 0.1-0.4 µm |
PM surface finish is adequate for most structural, gear, and bearing applications with sizing. Sealing, cosmetic, and precision contact surfaces typically require secondary operations. Specify surface finish only where the function requires it.
For questions about achievable surface finish on a specific PM geometry or application, contact our engineering team.
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Expert Review
Yao Qingpu
Powder Metallurgy Manufacturing Expert at SinterWorks Technology
Yao Qingpu works with global buyers on powder metallurgy design review, material selection, tolerance planning, cost-down opportunities, and production feasibility. His experience covers PM gears, automotive components, structural parts, and practical DFM support for long-run manufacturing programs.