Hard chromium coatings have been around for a long time and can be used to increase wear and corrosion resistance of tools and machinery components, e.g. piston rings, hydraulic cylinders, and moulds. Very thin chromium films are often used for decorative purposes in car or furnishing industry. Another type of application of chromium is the chrome-on-glass masks for photolithography in the microelectronics industry. The traditional deposition method for Cr is chromium plating, a wet electrolytic method. However this method uses hexavalent chromium that is carcinogenic and it is therefore necessary to replace it by health and environmental friendly deposition methods, for instance a PVD method. Sputtered or cathodic arc evaporated Cr, CrN, and CrC, but also chromium free coatings like diamond-like carbon (DLC), are considered as possible substitutes for electroplated hard chromium coatings in the large scale industrial applications.
The sputtering of chromium is quite slow. In magnetron sputtered Cr/CrN and Cr/Cr2N multilayer coatings the chromium layers was sputtered by a φ150 mm magnetron at a rate of 10 µm/h (≈170 nm/min) onto -20V biased steel substrates at a target current of 4 A (≈ 23 mA/cm 2 ).
The development of texture in RF sputtered Cr films is discussed in a work by Feng et al. where a model based on the minimization of the surface and interfacial energies is proposed. The model was tested in Cr depositions on glass substrates at different conditions. The films always had Cr (110) texture when deposited on glass substrates at room temperature but when preheated to 250 °C the (110) or (002) texture was determined by the amount of deposited energy from Ar ions or Cr atoms. The Cr (110) preferred orientation was favored by bombardment of the glass substrate. Control of the preferred orientation is important e.g. when the Cr films are used as an under-layer for cobalt-based magnetic films, where the Cr (200) texture is desirable.
Chromium nitride films
Chromium nitride films exhibit excellent corrosion and wear properties and a high thermal stability. It is possible to deposit thick (several 10 µm) CrN films thanks to the fine grained and a low stress structure. This fact together with that CrN is less brittle than TiN, but still quite hard, makes CrN more suitable for surface protection at relatively soft substrates such as aluminum alloys and stainless steels. The adhesion to steel is often good but it can be enhanced by an intermediate Cr-layer. Stoichiometric or near-stoichiometric CrN coatings have cubic NaCl-structures. With low nitrogen content the harder hexagonal Cr2N phases can appear. Chromium is a less reactive metal than titanium and this has a consequence for reactive PVD. The required nitrogen partial pressure to form stoichiometric CrN films is higher than for stoichiometric TiN. Typical properties of a commercial coating are a hardness of 1750 HV and a thermal stability up to 700 °C.
The high thermal stability makes CrN-coatings very suitable for wear and corrosion protection in working processes at elevated temperatures, e.g. in die casting under pressure. Examples of the CrN coated components are plastic moulds, extrusion dies, and tools for machining and cold forming of metals as Cu and Ti.
The common deposition methods for CrN films are the reactive magnetron sputtering and the arc evaporation. The DC magnetron sputtering was used in to investigate an effect of preferred orientation on mechanical properties of the CrN coatings. Two coatings were produced at a total pressure of 0.27 Pa (2 mTorr), a target current of 2.5 A, OEM controlled N2 flow, and at different DC bias voltages a) 70 V and b) 120 V. The deposition rate was ~18 and ~28 nm/min respectively. The resulting films were a) CrN with a preferred orientation of (200), columnar structure and a hardness of 2300 HV and b) Cr2N with a preferred orientation of (111), dense structure and a somewhat higher hardness (2400 HV) but with a weaker adhesion to the steel (SKD11) substrates.
A high rate deposition of CrNx by DC magnetron sputtering with a pulsed DC bias was studied by Nam et al. The films were sputtered with a target power density of 13 W/cm2 at a constant argon pressure of 0.24 Pa (1.8 mTorr) and a nitrogen flow varied from 0 to 45 sccm and a varied bias voltage. This made it possible to control the microstructure and phase composition of the CrNx films. The maximum deposition rate was 210 nm/min for Cr2N (89% of the rate for pure Cr deposition) and the maximum hardness was 2250 kg/mm2 (Knoop) for a mixed phase CrN+Cr. The same group has also made a study of properties of the CrNx films deposited at different deposition rates. In this study they used a constant bias voltage of -100V and a constant argon pressure of 0.2 Pa (1.5 mTorr) and used the target power densities 5, 10, and 13.2 W/cm2 and the nitrogen flow was varied from 0 to 160 sccm. They concluded that the deposition rate of CrN increased linearly with the target power density (max 430 nm/min at 13.2 W/cm2 ) and that the film stress was changed from tensile to compressive with increasing deposition rate. Further the highest hardness and best adhesion was found for the film deposited at highest target power density owing to a high compressive stress and high adatom mobility.
Carbide tools coated with CrxNy films by RF magnetron sputtering have been tested in wood machining. For structural and chemical analysis the films were deposited on Si substrates. Depositions were made at RF powers of 450 W and 650 W and a varied total pressure from 0.1 to 1 Pa. Deposition times were selected between 15 and 80 minutes with a maximum deposition rate of 4.4 µm/h (73 nm/min) for Cr2N. The Cr2N films had a columnar structure while the CrN films seemed to be featureless with a maximum hardness of 2100 HV. Cr2N films were found to be harder but less adherent than the CrN films.
An RF magnetron sputtering was also used for a study of CrNx films deposited within a wide nitrogen partial pressure range 0.005 – 30 Pa where the chemical and mechanical properties were analyzed. The target power was kept constant at 300 W (the target power density was 6.8 W/cm2 ) and the Ar partial pressure constant at 0.3 Pa. Stoichiometric Cr2N was obtained for nitrogen partial pressures between 0.02 and 0.04 Pa and a stoichiometric CrN was obtained for 0.3 Pa, while for other pressures the CrN and Cr2N phases were mixed. The conclusion was that the nitrogen content in CrNx films can be controlled by changing the nitrogen partial pressure, but not independently of the deposition rate and the microstructure. The Cr2N films were very hard (27.1 GPa) and stiff (E = 348 GPa), a single phase CrN was almost as hard as Cr2N but more elastic (E = 300 GPa) and the deposition rate was lower.
The microstructure and mechanical properties of chromium nitride films deposited on high speed steel substrates by reactive arc evaporation were studied by Odén et al. The 10 µm thick films were deposited for 220 min at a nitrogen partial pressure of 8 Pa and different negative substrate biases from 20 to 400 V. The microstructure of the films was dense and columnar, the preferred orientation was CrN (220) and the CrN (220) texture coefficient increased with an increasing negative bias up to 200V. A maximum nanohardness of 29 GPa was reached for a substrate bias of -100 V.
The CrN coatings for a dedicated application, cutting tools for machining of copper, were produced by a cathodic arc ion plating. These films were deposited at nitrogen partial pressure of 4 Pa and different negative substrate biases, 0 – 200 V. The preferred orientation was CrN (111) and the micro structure was dense and columnar. The grain size decreased with an increasing bias and a maximum Vickers micro hardness was reached for a bias of 100 V as well as the maximum compressive residual stress. The cutting performance tests indicated that the film hardness and the residual stress could not be taken as a measure of the performance in the milling of copper.