Many tend to assume that as long as the final coating is AlCrN, TiAlN or CrN with similar composition, the internal stress of the film should be comparable. In fact, this is far from the case.
The coating composition defines what the material is, whereas the deposition method determines how atoms grow into the film. Stress, in fact, is highly sensitive to the growth mode.
First, the ion energy varies, leading to different degrees of atomic implantation. Cathodic arc, conventional magnetron sputtering and HiPIMS can all deposit the same AlCrN, yet the state of particles arriving at the workpiece surface differs drastically.
In conventional magnetron sputtering, most metallic species remain neutral atoms, which arrive at the substrate and accumulate in a relatively slow, adductive manner. For cathodic arc and HiPIMS, the metal ionization rate is much higher. When a bias voltage is applied to the workpiece, a greater number of metal ions bombard the surface with kinetic energy.
At this stage, the process is no longer simple deposition. Instead, it involves atomic displacement, densification, defect generation, resputtering and lattice compression.
If such bombardment is intense, the coating generally develops higher intrinsic compressive stress. A simple analogy: it is like bricklaying. In one case, bricks are gently placed into position; in the other, every brick is forcibly pressed inward as it is laid. The final wall consists of the same material, yet how tightly its interior is packed is entirely different.
Second, deposition rate also alters stress. Cathodic arc deposition typically proceeds at a high rate. If metal species arrive rapidly while ion bombardment and surface diffusion cannot keep pace, the coating tends to develop pronounced columnar growth and defects. Conversely, excessively intense ion bombardment yields a dense coating, yet atoms may be over-packed into lattice interstices, causing a sharp rise in compressive stress.
Therefore, it is not the deposition rate per se that governs stress, but the balance between deposition rate and ion bombardment. This explains why, for the same AlCrN coating, one system can reliably reach 3 μm, while another starts to show edge chipping at only 2 μm.
Third, stress varies with grain size and microstructure. Even with identical composition, deposition techniques can produce coatings with entirely different grain sizes, textures and columnar structures. Conventional magnetron sputtering tends to form well-defined columnar grains. Thanks to stronger ion bombardment, HiPIMS more readily yields refined, dense microstructures. Cathodic arc can also achieve highly dense coatings, yet droplets, localized high-energy bombardment and high deposition rates introduce another type of defect state. Accordingly, an EDS reading of 65 at.% Al, 35 at.% Cr and comparable nitrogen content does not guarantee identical internal structures for two coatings.
Different microstructures naturally lead to different stress states.
Fourth, bias voltage directly modulates stress, and this effect is particularly pronounced. Take the same AlCrN coating as an example: at −50 V bias, the coating may be relatively porous with low stress. When the bias is increased to −100 V or −150 V, ion bombardment intensifies, the coating becomes denser and harder, while compressive stress generally rises.
Further raising the bias may trigger intensified resputtering, increased defects, excessive stress and edge chipping. It is therefore not uncommon to observe that hardness rises from 2800 HV to 3300 HV, yet service life declines. This happens because the coating may become over-compressed as hardness increases.
Fifth, temperature variation also results in different stress states. Deposition stress is not of a single type. One component is the intrinsic stress generated during film growth. The other is thermal stress, which develops after cooling due to the mismatch in thermal expansion coefficients between the coating and the substrate.
Take AlCrN as an example. If one batch is deposited at 350 °C and another at 500 °C, the final residual thermal stress can differ substantially after cooling down to room temperature owing to the different temperature excursions.
Hence, it is insufficient to consider only the stress during deposition. Once the finished component is taken out, the final stress state is determined by the combined effect of intrinsic stress and thermal stress.
Sixth, working pressure also modifies stress. At low pressure, particles have a longer mean free path and retain more energy upon arriving at the workpiece. As pressure rises, particle collisions increase, altering both their direction and energy distribution.
Therefore, even with identical bias voltage and composition, coatings deposited at 0.3 Pa and 2 Pa can exhibit entirely different stress states. If the pressure is too low, bombardment may become excessive, leading to high compressive stress. If the pressure is too high, particles lose excessive energy, and the coating tends to be porous with columnar microstructure.
Stress generally has an optimal process window.
Why can the service life vary greatly for the "same coating" produced on different equipment? Customers only see a name: AlCrN. What actually takes place inside the machine, however, is that metal ionization rate, ion energy, ion flux, deposition rate, pressure, temperature, bias voltage and grain growth collectively define the final coating. Similar composition only means close chemical makeup; it does not mean identical crystal structure, density, defect population or stress state. This is also why PVD evaluation cannot rely solely on composition reports. A truly mature process aims neither to reduce compressive stress to zero nor to maximize hardness.
Instead, it keeps stress within a suitable range: moderate compressive stress is leveraged to improve densification and crack resistance, while the stress must not become so high that it causes spontaneous coating delamination.
In summary: for the same material, different deposition techniques pack atoms into the coating in distinct ways. The packing density, impact intensity and thermal contraction upon cooling all differ, which naturally results in vastly different residual stress.
