This situation is common on production sites: a customer has an urgent order, requiring only a few dozen tools, but the furnace capacity is several thousand. Should it be turned on or off?
To save time, often the tools are put directly into the furnace. The result? Color difference, film cracking, or even softening of the substrate after annealing.
At this point, the technician often adjusts parameters, checks gases, and suspects the target material, but overlooks the most inconspicuous factor-the furnace is underloaded.
Today, we'll discuss a "divine assist" in tool plating (especially sensitive coatings like DLC) that's easily mistaken for scrap metal-the dummy load.
The so-called "process parameters" are actually "full-load parameters."
Many coating engineers have a misconception that if they set the voltage, current, gas pressure, and temperature correctly, the coating quality will be consistent.
This is completely wrong.
PVD vacuum coating is essentially a dynamic equilibrium within a plasma environment. All the parameters you set (such as bias current and target current) are adjusted based on a preset "load."
When the furnace is full, the plasma evenly "envelops" thousands of workpieces; but what happens when the furnace contains only one-tenth of the workpieces, given the same power density?
The plasma will relentlessly attack the few remaining workpieces.
It's like using a fire that was originally meant to cook a pot of meat, but now only allowing you to cook two slices; the inevitable result is-the meat burns, and the pot might even burn through.
The Three Core Functions of Dummy Loads
In high-end tool plating, especially when applying hydrogen-free DLC or high-hardness coatings, the use of dummy loads (discarded tool holders, specialized fixtures, or substitutes) is not for aesthetic purposes, but for survival.
1. Stabilize "thermal inertia" to prevent substrate annealing
Heat conduction is very slow in a vacuum environment; it relies mainly on radiation.
If there are too few workpieces in the vacuum chamber, the total heat capacity is greatly reduced. Under the same heating power or bombardment energy, the workpiece's heating rate will be like a rocket.
Consequence: Your instrument may show a temperature of only 450°C, but the local temperature of the sharp corners or cutting edges of the workpiece may have already exceeded the tempering temperature of high-speed steel.
Outcome: The coating was fine, but the tool's hardness decreased, causing it to break immediately upon customer use. The purpose of the dummy load is to act as a large "heat absorber," controlling the temperature and making the heating curve smooth and manageable.
2. "Deceiving" the Power Supply for Stable Bias Voltage
Bias power supplies need to detect load impedance during operation.
If there are too few workpieces, the load impedance will vary greatly. The power supply's matching network may not operate optimally, and may even frequently detect false "arcing," leading to frequent power-offs or waveform distortion.
Consequences: Poor coating adhesion, and even pitting caused by instantaneous high voltage breaking down the workpiece edges.
Outcome: By adding a dummy load, the power supply sees a "familiar" impedance environment, resulting in a smooth and stable output waveform.
3. Improving Gas Flow Distribution and Plasma Density (Especially DLC)
DLC (Diamond-like Carbon) coatings are extremely sensitive to internal stress.
During carbon film deposition, if the loading is too low, excess carbon ions have nowhere to go, leading to excessively rapid local deposition rates and a sharp increase in internal stress within the film.
Consequences: The film may crack in the vacuum before leaving the chamber, or automatically peel off after being left for several days after leaving the chamber.
Solution: A dummy load, by occupying space, simulates the gas flow disturbances and consumption under full load conditions, ensuring that the consumption rate of the reactive gases is maintained within the process window.
A dummy load is not a "garbage can."
Since a dummy load is so important, can't you just throw in any old scrap metal?
This is the second common mistake many factories make.
1. The dummy load must be "clean." Many operators simply throw in rusty iron blocks or oil-stained jigs to fill the space.
These dirty things, when heated in a vacuum, will release large amounts of water vapor and oil vapor (outgassing).
Recommendation: Dummy loads should also undergo a standard cleaning process, at least to ensure they are free of oil and rust.
2. The geometry should be "similar".
If you are coating a slender end mill, the dummy load should ideally also be a similar rod-shaped object.
If you place a large flat plate as the dummy load, its electric field distribution (edge effect) will be completely different from that of the slender rod, severely interfering with the coating uniformity of the nearby workpiece.
