MIM Surface Finishing and Heat Treatment Guide

Start with the service requirement, not the finish name

Finish specifications often arrive as a single word on a drawing — "polished", "nickel plated", "hardened". Each of those words can mean several different operations with different costs, and choosing the wrong one produces a part that passes inspection and fails in service.

The reliable approach is to state the service condition first, then let the finish follow. Examples: "surface will contact stainless steel under sliding motion at low load", "part will be washed with alkaline cleaner weekly", "part is visible to the end customer and must read as uniformly matte", "insulating coating must survive 400 °C". Those sentences resolve into specific operations far more reliably than an adjective does.

Mechanical finishing

Tumbling and vibratory finishing

The standard first operation after sintering. Parts are tumbled in abrasive media to remove flash, round edges and produce a uniform matte surface. It is inexpensive, runs in bulk and improves fatigue behaviour by removing sharp edges. It does not correct dimensions, and it works best on parts without fine features that might be rounded off.

Machining after sintering

Where a feature needs a tolerance tighter than as-sintered capability — bores, bearing journals, sealing faces, threads — material is removed after sintering. Typical operations are boring, reaming, face milling, thread cutting and drilling cross-holes that could not be moulded economically. This is the most controllable way to hold tight tolerances and it should appear explicitly in the quotation.

Polishing and buffing

Used where appearance matters. Because MIM parts sinter to 95–99% of theoretical density, remaining surface porosity can be revealed as tiny pits during aggressive polishing. If a cosmetic finish is required, density and process control should be discussed at RFQ stage rather than discovered at the buffing wheel.

Blasting

Grit or bead blasting produces a uniform, slightly textured matte surface and is often used to mask minor surface variation. Bead blasting gives a softer, more satin appearance than grit. Confirm the finish, material condition and allowable stock removal; even a finishing step can affect small features and mating dimensions.

Chemical and coating treatments

Passivation

An acid treatment that removes free iron and surface contamination from stainless steel, restoring the passive oxide layer. It is standard on 316L and 17-4PH parts for medical, food-contact and corrosive-service applications. Passivation does not change dimensions, does not build a layer, and does not improve the corrosion resistance of the alloy itself — it only removes what would otherwise cause localised attack.

Electroless nickel plating

Deposits a nickel-phosphorus alloy uniformly, including inside bores and on complex geometry, without the current-density variation of electroplating. It adds typically 5–25 µm per surface and improves corrosion and wear resistance. Because it is uniform, it is often the chosen coating for complex MIM geometry where electroplating would build unevenly.

Electroplating

Chrome, zinc and nickel plating are all used on MIM parts where appearance or corrosion resistance requires it. The limitation is throwing power — plating builds faster on exposed surfaces than inside recesses — which matters on parts with deep pockets or blind holes. Where uniformity is critical, electroless nickel is the better answer.

PVD coating

Physical vapour deposition applies thin, hard coatings such as TiN, CrN or DLC. Coating thicknesses are typically 1–5 µm, much thinner than plating, and hardness is substantially higher. Used on wear surfaces, cutting edges and sliding components. PVD is a line-of-sight process, so it does not coat internal surfaces facing away from the target.

Black oxide

A conversion coating that darkens the surface and provides mild corrosion resistance with minimal dimensional change. Common on low-alloy steel parts where appearance and light corrosion protection are wanted without a plating line. It offers little protection on its own in wet environments without an oil or wax topcoat.

Heat treatment by material family

Heat treatment availability is determined by the grade, not by preference. Specifying a hardening treatment on a grade that does not respond to it is one of the most common errors in MIM drawings.

A note on austenitic stainless

The most frequent misunderstanding in MIM finishing is asking for 316L to be hardened. It cannot be, because the austenitic structure does not transform on cooling. The practical alternatives are to change grade — 17-4PH or a martensitic stainless if hardness is essential — or to apply a hard coating such as PVD over 316L where corrosion resistance must be preserved.

Stress relief

Where a part has been heavily machined after sintering, a stress-relief treatment is sometimes specified before final finishing to reduce the risk of distortion during subsequent operations. It is relatively inexpensive and worth considering on thin or asymmetric machined parts.

How finishing changes tolerances

Every finishing operation that removes, adds or moves material interacts with the tolerances on the drawing. The interactions worth planning for:

• Tumbling rounds edges and slightly reduces sharp feature dimensions; fine details and sharp corners suffer most.
• Machining is the only reliable way to bring a feature to tight tolerance — and it must be sequenced before coating, not after.
• Plating and coating add thickness on all surfaces unless masked. Threads, press fits and bores typically need allowance or masking.
• PVD adds only 1–5 µm, which is negligible for most dimensions but can matter on precision fits.
• Heat treatment causes dimensional movement through phase change and stress relief; parts needing both hardness and tight tolerance should be hardened first, then finish-machined.
• Passivation and black oxide are effectively dimensionally neutral, which makes them safe on parts with tight tolerances.

Common specification mistakes

• Calling for hardening on 316L. Not achievable; the specification will either be ignored or produce a rejected lot.
• Specifying "17-4PH hardened" without a range. Hardness outcome depends on the aging temperature, and the supplier needs a target to work to.
• Ignoring coating thickness on threaded parts. A 20 µm coating on an M6 thread changes the fit and can prevent assembly.
• Requesting a mirror polish on an as-sintered surface. Residual porosity limits the achievable finish; density should be discussed first.
• Specifying PVD for internal surfaces. PVD is line-of-sight and cannot coat recesses facing away from the target.
• Sequencing coating before final machining. This removes the coating from the machined features and wastes the operation.

What to include in the RFQ

A finish specification that produces an accurate quotation states: the service environment, the required function of each finished surface, the target hardness with a range if hardness matters, the acceptable surface roughness where it matters, and any standard the finish must meet. Adding the sequence you expect — for example "harden, then grind the bore" — prevents the two most expensive finishing errors: operations in the wrong order, and coatings applied to features that then need machining.

Conclusion

Finishing and heat treatment are where a physically correct MIM part becomes a functional one. The operations themselves are well established; the failures come from specification rather than execution — hardening a grade that will not harden, plating a thread that then will not assemble, or polishing a surface whose porosity was never controlled.

The discipline that avoids all three is to specify the service requirement and the sequence, not just the operation name. State what the surface must do, in what environment, and in what order the operations should occur. A supplier can work with that; a single adjective is a guess.

Related component families

These pages show where specific finishing decisions typically arise.