Review of Nitride-Based Multifunctional PVD-Deposited Coatings

Deposition Methods

Multifunctional nitride coatings can be successfully produced by employing physical vapor deposition techniques such as sputtering and cathodic arc techniques. Coatings based on the magnetron sputtering technique are carried out in vacuum chambers and with the use of DC or RF sources. Inside the chamber, the substrate is connected to a negative potential, or left floating, and the target is used as a cathode (Figure 1). The process starts with the evacuation of the air in the chamber at pressures ranging from 0.75 to 7.5e-4 mTorr. The chamber is usually filled with argon gas (with p.d = 0.3 torr·cm) due to the argon’s low reactivity and appreciable mass. Then, a potential difference of 300 to 5000 V is applied between the anode and the cathode, which produces an electrical discharge due to the emission of electrons from the cathode and argon gas ionization (Baptista et al., 2018; Borra, 2006).

Figure 1: Schematic diagram of a DC sputtering system.

The positive ions are accelerated against the target (negatively charged), thus producing sufficient momentum transfer to sputter their atoms, which then condense on the substrate surface. The advantage of this process lies in the wide variety of materials that can be used as target. The disadvantages are low deposition rates and the contamination of the deposited films, mainly due to particles generated by the heat of the plasma on the chamber walls and the components of the vacuum system.

The basic sputtering process has been used for several years, and many materials have been deposited using this system. To improve the efficiency of the sputtering process, a magnetron is included. Magnetrons are configured in such a way that one pole is positioned at the central axis of the target, and the second pole is formed by a ring of magnets around the outer edge of the target (Mattox, 2010). In this way, the electrons are confined, and the probability of collision with atoms is increased. The increased ionization efficiency of magnetron results in a dense plasma in the target region, which is associated with the ion bombardment of the target, this yielding higher sputtering and deposition rates, as well as and a decrease in the working pressures. In fact, in normal magnetron conditions, the discharge could be maintained at lower operating pressures (typically, 10~3 mbar) and lower operating voltages, typically -500V (Kelly and Arnell, 2000).

In recent years, the density and reactivity of this process have significantly increased through the inclusion of high-power pulsed magnetron sputtering (HIPIMS). This process can employ a DC generator source to charge the capacitor bank of a pulsed unit, which is connected to the magnetron.

The charging voltage of the capacitor bank typically ranges from several hundred V to several kV. The stored energy is released in pulses of defined frequency and width, using transistors with a switching capacity in the μs range, which are located between the capacitors and the cathode. The pulse width (also referred to as pulse-in-time) varies, typically from 5 to 5000 μs, while the pulse repetition frequency ranges from 10 to 10 kHz. Under these conditions, the target peak current density can reach values of 3-5 A/cm², which are up to three orders of magnitude higher than conventional sputtering densities. Furthermore, the ionization levels in the pulsed process are very high, reaching values close to 90% of the eroded particles on the target. This is due to the appearance of power density zones on the target, whose behavior obeys a stochastic process that cannot be predicted.

The second process (Figure 2) is the cathodic arc, which has considerable potential in the present and future of the tool coating industry, mainly because of its simplicity, easy implementation, low operating cost under industrial production conditions, and especially because of its high ionization rate, which accelerates the deposition process and also generates excellent adhesion to substrates (Curtins, 1995; Randhawa and Johnson, 1987).

Figure 2: Schematic diagram of the cathodic arc PVD process.

Arc deposition is a PVD (physical vapor deposition) technique that involves the vaporization of an electrode by an electric arc as a plasma source (Lindfors et al., 1986; PalDey et al., 2004; Vetter and Perry, 1993). The arc conditions require high currents (˷̴ 100 A) and voltages close to the ionization potential of the gas or vapor (> 25 volts) (Mattox, 1998).

The ion bombards the cathode, and the electron bombards the anode, thus heating both electrodes. Most of the evaporated material becomes part of the plasma. However, a small portion of the material is ejected in the form of molten droplets. These small droplets (not vaporized) on the surface cause a substantial increase in roughness. This roughness is key in the case of cutting tools, as it increases the coefficient of friction with the chip and, therefore, the cutting forces. For a stable arc to form, a minimum current must be present. The current value varies from 50 to 100 A for low melting point materials such as copper and titanium. Meanwhile, for refractory materials such as tungsten, it reaches 300-400 A. The arc voltage can be between 15 and 100 volts, depending on the ease of electron movement from the cathode to the anode (Mattox, 1998).

Nanostructured and functional MeBN coatings deposited by PVD seek to protect the material from degradation, and their effects positively impact a nation's economy. From an environmental perspective, the degradation of a material by corrosion and wear phenomena becomes a source of pollution, as it reduces its useful life. For example, deterioration due to oxidation or pitting prevents the recycling of a product and causes the emission of substances that affect the quality of the air or water, thus affecting the health conditions of species. A second factor is related to the consumption of energy and resources needed to replace or recover a degraded material. In addition, the manufacture of a new product produces gases and liquid waste that can pollute the environment.

On the other hand, in the metalworking industry, where the machining of a product for an aeronautical, automotive, or biomedical application is based on the detachment of material due to wear with a cutting tool, it is important to note that machining processes constitute approximately 15% of the economic cost of the product in industrialized countries (Markopoulos, 2013). In the same way, tool wear has additional environmental effects, such as increased energy consumption, wear in other parts of the machine, increased emission of toxic gases due to evaporation of coolants or lubricants, and increased noise production. This means that, if functional coatings such as PVD techniques are developed, more durable materials can be produced, and better use will be made of the available resources, resulting in reduced energy consumption and less damage to the environment.

Binary Coatings

Transition metal nitrides have a considerable content of metal bonds, although nitrogen is covalently bonded to metals. In a densely packed in FCC cubic structure, there are two interstitial sites: the tetrahedral and octahedral sites. Nitrogen occupies only the octahedral sites because the tetrahedral sites are too small to accommodate nitrogen. Nitrides form, as groups 4 and 5 have refractory characteristics, with melting points above 1800 °C; while the refractory properties of group 6 nitrides, such as CrN, MoN, and WN, are reduced because they have low melting points and dissociate rapidly to N₂ and MT at temperatures near 1000 °C (Capote-Rodríguez et al., 2015).

Metal nitrides have been applied in the industry due to their excellent response when deposited on metallic surfaces. Metal nitrides from groups 4 to 6 are the most widely applied because they exhibit the following properties:

Low friction coefficients (kinetic)

Good surface finish

High surface hardness, good adhesion, and abrasion resistance

Increased corrosion resistance

They are chemically stable and inert

Titanium Nitride (TiN) Coatings

Titanium nitride (TiN) is classified as a gold-colored ceramic material with 1800-2100 HV (Table 1). It is one of the first thin films obtained by using the PVD technique (Xi et al. 2017) to improve the surface properties of substrates such as steels, titanium alloys, hard metals, aluminum, and carbides. It is generally applied in cutting tools, elements for medical applications, parts for the food industry, and mold components, among others (Domínguez-Crespo et al., 2018).

Table 1: Comparison of nitride-based nanostructured coatings properties (MatWeb - Material Property Data, 2020).

Titanium nitride (TiN) has a friction coefficient between 0.4 and 0. 9 without lubrication, depending on the surface finish and substrate material. Its melting point is 2930 °C, and it oxidizes at a temperature of approximately 600 °C in a normal atmosphere. Its typical formation has a crystalline NaCl-type structure with a determined stoichiometric ratio of 1:1. TiNx compounds are thermodynamically stable in the x-range of 0.6 to 1.2 (C. F. Wang et al., 2014).

It has been reported that the bonding of these two materials is a combination of three types of bonds (Barshilia et al., 2010): (i) the ionic bond resulting from the transfer of electrons from titanium to nitrogen, since the latter has a higher electronegativity (the electronegativity of N is 3.04 and that of Ti is 1.54); (ii) metallic bonding, with a finite density of states at the Fermi energy level, (iii) covalent bonding, which occurs in a higher proportion and is generated by the superposition between the d states of titanium and the p states of nitrogen, with some metal-metal interaction.

Chromium Nitride (CrN)

This coating, obtained via the PVD process, exhibits hardness in the range of 2000 to 2500 Vickers (Table 1), has a higher temperature resistance than TiN, and is mainly applied in precision metal parts (Kabir et al., 2016). CrN thin films are ideal in industrial applications where corrosion protection and high adhesive wear resistance are required (Olaya and Cifuentes, 2018; T. Wang et al., 2016).

It is generally used in dies for metal forming, plastic injection molds, and cutting tools for machining titanium alloys, as well as in non-ferrous alloys such as aluminum (Sui et al., 2018). The coating forms a metal bond with the substrate which does not form chips, flakes, or delamination. In several nanostructured coatings, CrN or pure chromium is deposited on the substrate as a thin adhesion film to prevent the delamination caused by residual stresses typical of PVD techniques or the mismatch of the thermal expansion coefficient between substrate and coating (Ou et al., 2016).

Ternary Coatings

Titanium-Silicon Nitride (TiSiN) Coating

Although TiN is widely used in the cutting tool sector, the need to improve the cutting speed and increase mechanized parts production led to research aimed at enhancing properties. In this sense, the addition of Si arises as an alternative to increase hardness and/or reduce its friction coefficient. Therefore, in the last 20 years, there has been an increase in the studies on this TiSiN material (An et al., 2008; Chawla et al., 2010; Ding et al., 2004; Diserens et al., 1998, 1999; Jiang et al., 2004; Liu et al., 2018a; Mei et al., 2005). With X-ray diffraction, the incorporation of silicon atoms was observed to cause a decrease in the peak high of TiN, which suggests crystallite size reduction or a tendency towards amorphous phase formation (Mei et al., 2005). This result proves the existence of Si₃N₄ in contrast to the multiple studies carried out by other authors who, based on X-ray analysis, did not find the presence of this nitride. The chemical composition of TiSiN, identified via the X-ray photoelectron spectroscopy technique (XPS), exhibits Ti 2p, Si 2p, and N 1s. In these results, it was observed that, for the Si 2p peak, the binding energy 101.7 eV demonstrates the existence of Si₃N₄ (Jiang et al., 2004). The N 1s peak exhibited a binding energy of 397.1 eV and showed the presence of TiN and, at 398 eV, the existence of Si₃N₄ (Jiang et al., 2004).

Regarding mechanical properties, Jiang et al. (2004) found a hardness of 3400-3500 Vickers at a 9% Si concentration. Mei et al. (2005) found a maximum hardness value of 3420 HV at a 7% Si concentration. Diserens et al. (1999), like the previous authors, evaluated the hardness, finding a maximum value of 3800 HV at a Si concentration of 5%. These values vary with a small silicon modification, which shows the remarkable effect of silicon addition on mechanical properties.

TiSiN is applied in cutting tools and is designed to achieve high-speed machining operations on hard materials such as nickel- and titanium-based alloys with no or minimal lubrication conditions. TiSiN-coated cutting tools are extremely wear-resistant. The coating could operate at temperatures of up to 1100 °C (Arab Pour Yazdi et al., 2014). The TiSiN layers are deposited on the cutting tool with the help of the PVD arc evaporation process or by reactive magnetron sputtering.

Titanium-Aluminium Nitride (TiAlN) Coatings

Titanium aluminum nitride has a complex structure consisting of TiN and AlN bonds in a crystalline matrix (Figure 3). Cutting tools with this violet-black coating are high-performance and ideal for machining abrasive and difficult-to-machine materials such as tool steels, aluminum alloys, cast iron, and nickel alloys (Keunecke et al., 2010).

Figure 3: Location of atoms in a crystalline structure (Ti, Al)N (Dobrzanski et al., 2010).

AlTiN thin films exhibit higher temperature resistance than TiN (Barshilia et al., 2008) and slightly higher hardness, especially in hot work (Table 1). AlTiN coatings were developed from the tribological needs and characteristics of titanium nitride TiN (Liu et al., 2018b). The presence of high percentages of aluminum makes AlTiN harder and more thermally stable, even at high temperatures, where the formation of alumina favors heat dissipation, thus allowing for the reduced use of synthetic refrigerants (W. Zhou et al., 2014).

Nanostructured TiAlN coatings with a cubic NaCl-type structure, where aluminum replaces titanium in the TiN base structure (Ti1-XAlXN) by up to 65%, have been employed in metalworking due to their better oxidation resistance at elevated temperatures, higher hardness, and thermal stability in comparison with TiN. However, for aluminum contents exceeding the maximum solubility limit of x = 0.65, a mixture between a B1-NaCl-type cubic structure and a Wurtzite-type structure is created, which improves the oxidation resistance but degrades the mechanical and thermal properties of the TiAlN coating. Figure 4 shows a schematic of the structure of these coatings with increasing aluminum.

Figure 4: Transformation of the cubic B1 structure into the hexagonal B4 structure as an aluminum content (Mayrhofer et al., 2014).

It is also known that the substitution of Ti atoms by smaller atoms such as Al in TiN not only affects the size of the structure, but also its density, leading to a decrease in the crystal lattice. From this, the number of defects increases, thus leading to an increase of up to 65% in the hardness of the coatings, i.e., to a value close to 3300 HV. In addition, several authors have reported an improvement in oxidation resistance at elevated temperatures due to the formation of protective products such as aluminum oxide (Al₂O₃) (Oliveira et al., 2008; Stueber et al., 2005). Studies on industrial TiAlN coatings in machining processes at high cutting speeds show the formation of an aluminum oxide layer that helps to prevent high-temperature degradation of the nitride and acts as a diffusion barrier that reduces the chip, making it an excellent coating for machining abrasive materials. Thus, the use of TiAlN has been limited to applications for machining materials at medium to low speeds, as its performance depends on the formation of aluminum oxide.

TiAlN coatings are used to avoid extemporaneous consumption of wear-exposed cutting tools due to their excellent tribological properties, high-temperature oxidation resistance at a temperature range of 600-800 °C, and high chemical stability (Sarin, 2014), but the requirements of high-speed and dry cutting applications, for which a high hardness and a high-temperature oxidation resistance above 1000 °C are of great importance, place high demands on coatings that must possess high wear resistance. The improvement of these properties can be obtained by incorporating Si into TiAlN coatings during processing (Park et al., 2004; Veprek et al., 2004).

Miletic et al. (2014a) alternate TiAlN and TiSiN coating layers, expecting them to exhibit nanocoatings with a high hardness (approximately 4200 HV), an oxidation resistance of up to 900 °C, and a thermal stability around 600 °C under working conditions. This type of coating would be useful in the high-speed dry milling of materials that have been hardened.

Chromium Aluminium Nitride Coatings (CrAlN)

CrAlN, like TiAlN, can be synthesized in a cubic B1-NaCl-type structure, in which Al replaces Cr up to a maximum solubility of 75% (x = 0.75). After reaching these values, its structure changes to a B4-hexagonal configuration of the Wurtzite type, as detailed in Figure 5.

Figure 5: Different crystal structures of (CrAl)N and (AlCr)N coatings as a function of the Al content (Vetter et al., 2021).

This behavior is similar to that of AlTiN coatings, which implies that this type of cubic structure will exhibit similar behaviors despite the number of substitutional atoms in the crystal structure. However, is important to highlight that introducing atoms produces changes in lattice parameters, micro-deformations, residual stresses, and so on, modifying, among others, the mechanical properties, tribological performance, and corrosion resistance.

In this way, aluminum-chromium-nitride (AlCrN) coatings allow more aluminum particles to be added to its network without entailing structural deformations, which helps the formation of alumina (Al₂O₃), whose main function is heat dissipation. This results in a coating with excellent performance under extreme temperature conditions, which represents machining with higher cutting speeds, even with little or no lubrication (Wu et al., 2015).

Among its main applications are high-speed milling, the machining of special steel alloys, the coating of end mills and hobs for the manufacture of pinions, punches for cutting and punching sheets, and coating of injection molds, which increases oxidation resistance, all as a result of optimized process parameters and an optimum coating microstructure (Gong et al., 2017).