1 Introduction

Transition metal nitrides such as TiN, CrN and HfN, with the NaCl (B1) type lattice structure possess excellent tribological and chemical properties, which enable their use as wear resistant, corrosion resistant, and diffusion barrier coatings. The effect of additions of Al, Si, and, Va on the mechanical and tribological properties of CrN coatings using cathodic arc ion plating [1, 2] cathodic arc evaporation [3] and DC/RF magnetron sputtering [4–9] has been reported in the literature. A significant increase in hardness value up to 27 GPa [4] to 35 GPa [10] or even higher values to 40 GPa [11], has been achieved for the CrAlN coatings when compared to CrN coatings. The coatings also exhibited higher thermal stability than pure CrN coatings.

Shah et al. [12] deposited CrAlN coatings with different Al contents on SA304 substrates prepared by a magnetron sputtering technique and found that the coatings are highly textured with a dominating peak of CrN(111), with its position shifted to higher 2θ angle with increasing Al content deposited on SA 304. Sun et al. [13] investigated CrAlN coatings deposited on different substrate materials such as Si (100), commercial aluminum alloy, AA6061, and M42 high speed steel (HSS). They found that the substrate material influences phase evolution in a reactively co-sputtered CrAlN films with Al contents favorable for B1 phase formation. The films deposited on Si (100) substrate showed the formation of single B1 phase (CrAl)N, which correlated well with theoretical predictions. A clustering of Al atoms in the B1 CrN {200} planes seems to occur, which leads to the large contraction of inter-planar spacing for the films deposited on the HSS substrate. The coatings became more compact and denser, and the microhardness and fracture toughness of the coatings increased correspondingly with increasing substrate bias voltage [14]. Cr1_xAlxN coatings deposited on Si and Stainless Steel substrates using RF magnetron sputtering with different atomic concentrations of aluminum (0.51 < x < 0.69) showed the evolution of (111), (200), and (102) crystallographic orientations associated to the cubic Cr1_xAlxN and w-AlN phases, respectively [15]. The insertion of Al or AlN in the B1 type nitride coatings beyond solubility limits leads to phase transition from B1 to B4. Also, the solubility limit of AlN in B1 CrN has been calculated theoretically and found to be 77 % as reported in the literature [16]. The B4 Wurtzite phase showed low hardness and poor ductility, which are not desirable for many industrial applications [13].

The CrAlN coatings were characterized by using the techniques such as XRD, FE-SEM/EDS, and AFM. The mechanical properties were measured using microhardness tester while the wear rate and coefficient of friction of the coatings were measured by using Pin on disk tribo tester in the present work.

1.1 Experimental Procedure

CrAlN coatings were deposited on SA 304 steel (92 HRb) of 15 × 15  and 0.9 mm thick with a surface finish of 0.01 μm Ra by using DC/RF magnetron sputtering (Model: DCSS—12, Manufactured by Excel Industries, Mumbai). Chromium target (cathode power fixed at 60 W) and aluminum targets (cathode power varied from 60 to 110 W) in a mixed Ar/N2 atmosphere were used for the deposition of coatings. The Ar/N2 flow ratio was maintained constant at 1:1, while the argon and nitrogen flows set to value 5 sccm. The thickness of as the deposited coatings varied from 4.0 to 4.5 μm. The SA304 samples were ground and mechanically mirror polished and then cleaned with acetone in an ultrasonic container for 15 min. The samples were mounted on the rotational substrate holder cum heater and rotated in forward and reverse manner. The rotation angle were set (30°) in such a way that it covers maximum plasma region. Prior to deposition, the targets were sputter cleaned in Ar gas (1.33 Pa) for 10 min. The deposition parameters are summarized in Table 1.

Table 1 Deposition condition for CrAlN coatings
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The coating thickness was measured by using its cross sectional scanning electron micrographs and stylus profile meter (XP-200 Ambios Technology Inc., USA). The microstructural and topographical analyses were made by field emission scanning electron microscopy (FESEM, Model: 200F, FEI Quanta) and atomic force microscopy (AFM, NTMDT). The films composition was determined by the energy-dispersive X-ray (EDS) technique. The coatings were also analyzed by X-ray diffraction (XRD, Model: D8 Advance, Brucker), to determine the phase composition and orientation using a Ni-filtered CuKα X-ray source.

1.2 Result and Discussion

1.2.1 Chemical Analysis

For the elemental compositions EDS is performed and the Fig. 1a shows the Al/Cr atomic ratio of as deposited CrAlN film as a function of the ratio of sputtering currents applied onto the Al targets and Cr targets (IAl/ICr). The result shows that the Al/Cr atomic ratio in the coatings increased monotonously with the increase with the sputtering current applied to the Al targets. The sputtering current ratio IAl/ICr is much higher than the Al/Cr atomic ratio for each sample, indicating the much lower sputtering yield of aluminium target in compare to that of chromium target. This fact could be ascribed to the formation of an insulating AlN layer on the Al target surface during the reactive sputtering process, which is responsible for reduction in sputtering yield of Al. The nitrogen content in all of the as-deposited CrAlN coatings, as shown in Fig. 1b, was vary between 31 and 35 at %. It was found that the N content increased slightly with the increase of sputtering current applied on the Al targets, indicating that the aluminum incorporation is beneficial for the improvement of stoichiometric composition of the coatings [1].

Fig. 1
figure 1

a Al/Cr atomic ratio and b N content in the as-deposited CrAlN coatings as a function of the sputtering current ratio IAl/ICr

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1.2.2 Microstructural Analysis of CrAlN Coating Deposited on SA304 Substrate

With the addition of aluminum, the CrN coatings gradually crystallize, rocksalt-type cubic structure showing a CrAlN(111) as a preferential orientation along with lower intense CrAlN(200) as shown in Fig. 2. The lower intensity of CrAlN(200) mainly due to higher nitrogen content in the chamber during deposition which favors (111) preferential growth due to comparatively lower strain energy. The Wurtzite-type (B4) structure phase was not observed as the AlN content in the coatings is below the critical composition for the phase transition from B1 to B4. It is reported that the solubility of AlN in cubic CrN has been predicted to go as high as 70 mol% [9, 10], the supersaturated CrAlN phase is thermodynamically metastable and hence the actual solubility limit finally depends on deposition condition [11].

Fig. 2
figure 2

The X-ray diffraction profiles of the as deposited CrAlN with different Al/Cr atomic ratio. The diffraction peaks marked with S comes from the stainless steel SA304 substrate

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The XRD result shows mainly two diffraction peaks (111) and (200). The intensity of both the peak increased and reach maximum then it is reduced to lower intensity value, which shows that the initially with the incorporation of Al content, the crystallinity increased or the crystallites in the coatings become increase. However after reaching maximum the peak intensity is reduced with further increase in Al content which leads to reduction in overall crystallinity of the film. The peak position of CrAlN(111) is shifted to higher angle side, indication a decrease in lattice constant gradually with increase in the aluminium content as shown in Table 2.

Table 2 The effect of Al content in CrAlN coating
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This shifting indicates the formation of the alloy nitride CrAlN and the fact could be attributed to lattice distortion caused by smaller aluminium atom incorporation which substitutes the chromium atom in the film. The incorporation of Al into the CrN coating, the full width at half maxima (FWHM) of the (111) diffraction peak increased to a maximum and then decreases with the increasing AlN content. But the abnormal result found for FWHM of CrAlN at 0.83 Al/Cr atomic ratio which has a lowest FWHM value among all. The abnormal result found may be due to formation of c-AlN. Generally the peak broadening of XRD profiles means the finer grain size and large residual micro stress. Also on the other side higher deposition rate will result in a finer grain size and a higher microstress [5, 6–7]. The deposition rate increased and touch at maximum at (44 nm/min) for the coating CrAl(30.06 at %)N having Al/Cr atomic ratio 0.86. Also the microstress is increased with deposition rate as mention in past literature [7], the higher kinetic energy of the incident ions distort the crystal lattice which leads to highest FWHM but in our case the CrAl(30.06 at %)N film having lowest FWHM. For clarification of above contra result, the grain size (Fig. 3) was calculated using Sherrrer formula.

Fig. 3
figure 3

FWHM and deposition rate of CrAlN film deposited on SA304 substrate as a finction of Al/Cr atomic ratio

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The lattice constant ‘a’ is calculated from CrAlN(111) peaks for coating deposited on SA304 substrates and lattice microstrain ‘ε’ were calculated by using Eqs. 1 and 2 respectively [17–19].

$$ \frac{1}{{d^{2} }} = \frac{{h^{2} + k^{2} + l^{2} }}{{a^{2} }} $$
(1)

where ‘d’ is the interplanar distance obtained from the position of the (111) peak using the Bragg condition, and

$$ \varepsilon = \frac{{a - a_{0} }}{{a_{0} }} \times 100 $$
(2)

where ‘a’ is the lattice constant in the strained condition and ‘a0’ is the unstrained lattice constant (4.140 Å). The lattice constants were found to be decreasing with increasing Al content as shown in Table 2.

It has been reported in the literature [20] that the microstress increases with the deposition rate and hence coating thickness, because of the distortion of crystal lattice of the coating due to the comparatively high kinetic energy of the incident ions. The CrAlN coatings were deposited at the same temperature (573 K), and therefore thermal stresses of all the thin films were presumed to be the same in the present work. The grain size and deposition rate of CrAlN coating as a function of Al content is shown in Fig. 4. The grain size calculation of all the CrAlN film found that initially it is reducing and reach minimum for CrAl(30.06 at %)N film. The minimum grain size at 0.86 Al/Cr atomic ratio is due to higher deposition rate as shown in Fig. 4 at that atomic ratio. Also, the microstrain is found highest among all films as its deposition rate is highest.

Fig. 4
figure 4

Grain size and deposition rate of CrAlN film as a function of Al/Cr atomic ratio

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Two dimensional surface topography of CrAlN coatings deposited at different Al content, were characterized by using atomic force microscopy (AFM) and the morphology of the films were obtained by using FE-SEM (Fig. 5). The coating becomes more compact and denser as seen in these micrographs. The overall roughness of the coating deposited on SA304 substrate was summarized in Table 2. The surface roughness is initially reducing up to 30.04 at % Al and then increasing with increasing Al content. Initially the reduction in surface roughness and high density of the coating may be due to increasing, applied voltage to Al target, which increase mobility of the atoms and results in higher nucleation density [21] during the formation of denser coating. The higher mobility of the adatoms can move into inter granular voids and diffuse in the films. Higher nucleation density helps to produce denser coating and atoms with high mobility diffuse into the intern granular voids [22, 23].

Fig. 5
figure 5

2D and 3D AFM surface morphologies of CrAlN films deposited with different Al/Cr atomic ratio

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2 Conclusion

CrAlN coatings with different Al content were deposited on SA304 substrates prepared by a magnetron sputtering technique. The deposited coatings are highly textured with dominating peak of CrN(111), with position being shifted to higher 2θ angle with increasing Al content deposited on SA304. The grain size is reduced initially up to 30.0 at % Al content and then grain size increases with further increase in Al content up to 7.15 %. The surface roughness is comparatively low, between 6 and 10 nm in all the coatings with varying Al content. The low surface roughness and high density of the coating may be due to increasing applied voltage to Al target, which increases mobility of the atoms and results in higher nucleation density leading to denser coating.