Will surface treatment affect part tolerances?

1. The link between different types of surface treatment and their effects on tolerance
There are four main types of surface treatment processes: surface modification, surface alloying, surface conversion coating, and surface coating. The extent of impact of various processes on tolerances varies considerably.
Technology for changing the surface
For example, sandblasting makes the surface rougher by hitting it with high-speed sand particles, but it can make the part smaller by 0.01–0.03mm. Rolling hardens the surface by changing its shape, which can make the diameter of shaft parts bigger by 0.005–0.015mm. Laser phase transformation strengthening has almost no effect on size because its heat-affected zone is so small.
Technology for surface alloying
Carburizing and nitriding make alloy layers by diffusion. Liquid nitriding will make the shaft 0.01mm wider and the aperture 0.01mm narrower, so you need leave a space of 0.01mm on one side while processing. Ionic nitridation, on the other hand, can keep the size change to within ± 0.002mm without using a liquid phase.
Technology for Surface Conversion Coating
Phosphating treatment creates a phosphate film on the surface of steel that is usually 2 to 10 μm thick and has a small effect on tolerances. On the other hand, anodizing (like hard anodizing of aluminum alloys) creates an oxide film that is 30 to 50 μm thick, which makes the parts bigger in one direction. To make up for this, the "lower difference smaller" strategy must be used.
Technology for covering surfaces
The thickness of the electroplating layer has a direct effect on the tolerance. For instance, if the screw length is less than or equal to five times the diameter, the maximum coating thickness should be kept to 8 μm. If it is not, a non-standard stop gauge inspection is needed. The thickness of the hot-dip galvanized layer is 30–80 μm, which will change the pitch diameter of fasteners a lot. To make sure they fit, the pre-plating dimensions need to be changed.
2. The little mechanism of tolerance change and industry data
There are three main physical and chemical processes that affect tolerances when you treat the surface:
Change in volume and phase of the material
When steel is blackened, it generates a Fe∝₄ oxide layer that makes the volume expand by 1.3 times, which causes surface protrusions. When aluminum alloy is anodized to form Al ₂ O ∝, the volume shrinks by around 15%, which can cause microcracks.
Anisotropy in the deposition of coatings
During the electroplating process, uneven current density can cause the coating to be different thicknesses. For instance, the electroplating coating on internal threads is normally 30% to 50% thinner than the outer surface. To make sure it fits, the "internal thread tolerance zone maintenance 6H" standard is needed.
Releasing residual stress during mechanical processing
Sandblasting treatment puts compressive force on the surface, which makes the parts creep when they are used again. According to experiments, sandblasted 45# steel shaft components can expand in diameter by 0.008mm after being held at 100 °C for 24 hours.
Data from the industry:
A specific aircraft firm says that the size variation rate of uncompensated 316L stainless steel parts after electric polishing is 12%. By leaving a 0.02mm leeway, the qualification rate has gone up to 98%.
The automotive sector has strict rules about how much tolerance galvanized bolts can have. For M12 bolts, the coating thickness must be kept between 8 and 2 μm, or else the torque coefficient will change by more than 15%.
3. Common Problems and Their Solutions in the Industry
In the field of aerospace
When making fuel nozzles for LEAP engines, GE Aviation uses the SLM (Selective Laser Melting) method and the HIP (Hot Isostatic Pressure) treatment. By optimizing the scanning strategy (spiral scanning) and layer thickness (30 μm), the surface roughness is kept within Ra12 μm. HIP treatment removes pores (from 0.8% to 0.02%), which increases the fatigue life by three times and meets the strict tolerance requirements of aviation standards.
Field of medical devices
Johnson&Johnson Medical has created a composite process called "vacuum annealing+chemical polishing" for 3D-printed hip joint implants. This process gets rid of residual stress by using vacuum annealing, and then it uses a citric acid-based polishing solution to smooth the surface from Ra50 μm to Ra0.8 μm while keeping it biocompatible. This method gives the implant a fatigue life of more than 20 years, which is more than what is needed in the clinic.
The field of making cars
Volkswagen makes engine cylinder blocks using a method called "phosphating+electrophoresis." The roughness of the cylinder's inner wall went from Ra3.2 μ m to Ra0.4 μ m by changing the thickness of the phosphating film (2–3 μ m) and the electrophoretic coating film (20–25 μ m). This also lowered the friction coefficient by 30% and increased fuel economy by 2%.
4. New technology and strategies for controlling tolerance
Design of reverse compensation
By making a database of changes in the size of surface treatment processes, allowances are set aside during the CAD modeling phase. For instance, one company has produced a "tolerance pre-compensation module" for electroplating technology. This module can automatically change the model size based on the thickness of the coating, which raises the first pass rate to 95%.
Detection and closed-loop control over the internet
Using 3D scanning technology, you can see changes in size in real time following surface treatment. Siemens' "digital twin" technology, for instance, can do virtual assembly verification on galvanized parts, which lowers the chance of tolerance deviation by 70%.
New method for treating surfaces
Plasma electrolytic oxidation (PEO) makes a ceramic film on the surface of aluminum alloy. The thickness of the film can be controlled between 5 and 200 μm, and the accuracy of the dimensions is ± 1 μm. It has been used in the structural parts of spacecraft.
Cold spraying technology: This method uses high-speed impacts of solid particles to deposit coatings. The heat affected zone is less than 50 μm, making it good for fixing and reinforcing precise parts.