For both protective and decorative purposes, ceramic coatings such as TiN, CrN, ZrN, TiCN, TiCNbN etc are usually used. For example, TiN and ZrN stoichiometric coatings have golden colour with difference between both coating materials being in brightness [1]. TiCNbN compound for example exhibits a white colour [2]. All these coatings play an important role in industrial sector because of the improvement of surface properties that they provide but also because of their good appearance. Figure 1 shows WC inserts (uncoated (1), coated with TiN (2) and coated with TiCN/TiCNbN (2)).
Magnetron sputtering, a physical vapor deposition (PVD) in which the coating vapor is created by energetic particle bombardment (sputtering) and is transported subsequently to the substrate, can be used to produce these coatings.
This can be done by sputtering a target (source of the coating) made of the compound material which is a good thing especially if the chemical composition of the desired colour is known, meaning that a target material of that composition can be made and will be reproduced exactly with that composition as a coating.
However, reactive magnetron sputtering can also be used in which a range of ceramic materials can be created by sputtering a low-cost metal target and admitting, in addition the inert working gas, a suitable reactive gas to the deposition chamber. This allows also to control the chemical composition and thus the colour.
However, as the reactive gas is introduced to the chamber, it reacts at all surfaces which includes the substrate, the chamber walls but also the target surface. This is because the chemical reaction that leads to compound formation is highly exothermic, and the excess energy can only be removed by a surface (as a third partner) since three body collisions in magnetron sputtering are rare because of the low reactive gas pressure and low density of the sputtered atoms [3].
The reaction of the reactive gas with the target has a negative effect on the deposition rate and this is illustrated in the figure 2 that shows schematically the deposition rate as a function of the reactive gas flow ϕ [4].
At low ϕ, the few surface atoms that have reacted with the reactive gas at the target surface are sputtered away by the ions that are accelerated towards the target. Sputtered metal atoms that react with the reactive gas at the substrate surface will form a coating that is sub stoichiometric as the system is still starved from the reactive gas. As ϕ is increased, more compound formation occurs at the target surface until the removal of compound material by sputtering cannot keep pace with the formation of that compound material. As a result, not only the yield of secondary electrons decreases which diminishes the sputtering rate as the ionization probability is decreased, but also the sputtering yield is reduced. This leads to an abrupt decrease in the deposition rate (figure 2 transition A-B) and the target becomes “poisoned”. Reducing the ϕ will bring the system to the high deposition rate mode where the target is not “poisoned” with the compound but only after the compound layer on the target has been removed (figure 2 transition C-D) resulting on a hysteresis effect (region A-B-C-D).
In the region A-B-C-D, stoichiometric compound can be formed with unstable deposition rate. For ϕ below point B and C, sub stoichiometric compound is formed and beyond point A and B over stoichiometric compounds are formed. The chemical composition and thus the stoichiometry is determined by the colour desired but also by the required mechanical properties.
Depositing a compound with optimized deposition rate implies the reduction of the hysteresis effect i.e., of the poisoning of the target should be reduced. Reactive High power impulse magnetron sputtering (R-HiPIMS) is a deposition technique that significantly lowers the poisoning of the target. This is since a low flow rate of reactive gas is needed in R-HiPIMS to produce the desired ceramic compound coating composition. This is explained by the high dissociation rate of the reactive gas that is achieved in R-HiPIMS because of the high electron density. Furthermore, the heavy sputtering of the target that occurs during the pulse on time keeps the target metallic free from compound layer.
The deposition rate can be further improved by proper design of pulsing configuration and magnetic field configuration [5] [6].
Furthermore, as said earlier, since the ceramic coatings produced for decorative applications have also protective purposes from external solicitations such as wear and corrosion. R-HiPIMS produces inherently denser coatings, as seen in the SEM image below (figure 3), which increases the performance of these protective coatings. In fact, the columnar structure in direct magnetron sputtering (figure 3 left) is inherently porous which is especially detrimental for corrosion protection as they are diffusion channels for the corrosive agent. This issue is eliminated in HiPIMS (figure 3 right).
Last but not least, an important aspect of HiPIMS in the decorative application is the possibility to tune the values of L*a*b. This is due to the additional process parameters the HiPIMS process has, that allow the change of the plasma density and energetics.
Therefore, HiPIMS and R-HiPIMS should be considered regarding production of coatings for decorative applications.
References:
[1] M. Nose, M. Zhou, E. Honbo, M. Yokota, S. Saji, Colorimetric properties of ZrN and TiN coatings prepared by DC reactive sputtering, (2001) 211–217.
[2] J.C. Caicedo, Re fl ectance and color purity obtained for novel Ti e C e N / Ti e Nb e C e N multilayers as function of niobium ( Nb ) modulation, J. Alloys Compd. 770 (2019) 875–885. https://doi.org/10.1016/j.jallcom.2018.08.222.
[3] K. Strijckmans, R. Schelfhout, D. Depla, Tutorial: Hysteresis during the reactive magnetron sputtering process, J. Appl. Phys. 124 (2018). https://doi.org/10.1063/1.5042084.
[4] D. Lundin, M. Tiberiu, G.J. Tomas, High Power Impulse Magnetron Sputtering. Fundamentals, Technologies, Challenges and Applications, Elsevier, 2020.
[5] A. Ghailane, Development of hard and corrosion resistant titanium nitride thin films using high power impulse magnetron sputtering, Univesity Koblenz Landau, 2022. https://kola.opus.hbz-nrw.de/opus45-kola/frontdoor/deliver/index/docId/2290/file/PhD+thesis+report_21032022_Anas_Ghailane.pdf.
[6] A. Ghailane, H. Larhlimi, Y. Tamraoui, M. Makha, H. Busch, C.B. Fischer, A. Jones, The effect of magnetic field configuration on structural and mechanical properties of TiN coatings deposited by HiPIMS and dcMS, Surf. Coatings Technol. 404 (2020) 126572. https://doi.org/10.1016/j.surfcoat.2020.126572.