Structure and Mechanical Properties of PVD and CVD TiAlSiN Coatings Deposited on Cemented Carbide

: This work reports the results of our investigation of the structure and mechanical properties of physical vapor deposition (PVD) and chemical vapor deposition (CVD) TiAlSiN coatings deposited on cemented carbide substrates. For the ﬁrst time, a novel nanocomposite of Ti 0.13 Al 0.85 Si 0.02 N coating deposited from TiCl 4 -AlCl 3 -SiCl 4 -NH 3 -H 2 gas precursors was prepared by low pressure chemical vapor deposition (LPCVD) at 780 ◦ C and a pressure of 60 mbar, while PVD Ti 0.31 Al 0.60 Si 0.09 N coating was prepared using the arc ion plating method. The investigation results including morphology, microstructure, chemical composition, phase component, and hardness were carried out by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nano-indentator. TEM results revealed that both PVD and CVD TiAlSiN coatings consisted of nanocrystalline embedded in SiN x amorphous. The nanohardness of CVD Ti 0.13 Al 0.85 Si 0.02 N coating obtained in this work was 31.7 ± 1.4 GPa, which was 35% higher than that of the PVD Ti 0.31 Al 0.60 Si 0.09 N coating.


Introduction
With the increasing demands for materials used in dry and high speed cutting and machining in the metalworking industry, it is imperative to develop a coated cemented carbide cutting tool with improved performance. As a traditional hard coating, TiN single layer coating has been widely used due to the similar thermal expansion coefficient with high speed steel tools. However, it has been reported that TiN coatings become oxidized at the temperature of 500-600 • C which limits its application in high temperature working environment [1]. Subsequently, TiAlN coating, which was evolved from the Ti-N single layer coating, has developed dramatically in recent decades due to their superior stability, even at elevated temperatures of approximately 800 • C and advanced mechanical properties [2][3][4][5]. In general, the increase of Al content in fcc-Ti 1-x Al x N coating can improve the oxidation resistance of the coating. A large number of studies have revealed that the maximum Al content of fcc-Ti 1-x Al x N coating prepared by PVD technology is 0.6~0.7, while Endler et al. [6] prepared fcc-Ti 1-x Al x N coating with Al content of 0.9 by means of CVD method. Meanwhile, much attention was also recently paid to the development of TiSiN coating, which consists of nano-crystalline TiN and amorphous SiN x with high physical and mechanical properties, such as superhardness (>40 GPa), high oxidation resistance (around 850 • C), and high thermal stability (up to 1100 • C) [7][8][9][10]. Qiu et al. [11] reported that the TiSiN coating prepared at 850 • C and 3.0 kPa was composed of amorphous Si 3 N 4 coated nanocrystalline TiN, with small grain size (17.7 nm) and high hardness of 30.4 ± 0.5 GPa. More recently, the quaternary Ti-Al-Si-N system started to be explored, since it can have the merits of both TiAlN and TiSiN coatings. Due to its high hardness, thermal stability, and oxidation resistance, TiAlSiN superhard coatings are eagerly expected to be applied widely in high speed processing conditions to improve industrial efficiency.
In this work, a quaternary CVD TiAlSiN coating was deposited on cemented carbide cutting tools in a new LPCVD process for the first time. Meanwhile, PVD TiAlSiN coating was deposited on the same substrates using arc ion plating. The microstructure and mechanical evaluations such as hardness, Young's modulus, and other mechanical characteristics of Ti-Al-Si-N coating were systematically investigated. Another focus of this work was to compare the microstructure and properties of PVD and CVD TiAlSiN wear resistant coatings.

Deposition
The nanocomposite CVD TiAlSiN coating which contained a 0.57 µm thick TiN bonding layer was deposited on the cemented carbide substrate in a hot wall CVD reactor filled with a gaseous mixture of TiCl 4 (purity 99.90%), AlCl 3 (purity 99.90%), SiCl 4 (purity 99.90%), NH 3 (purity 99.995%), and H 2 (purity 99.95%). The deposition temperature was 780 • C, the pressure was 60 mbar, and the deposition time was 60 min. The TiN bonding layer was prepared using the process gases TiCl 4 -H 2 -N 2 as a carrier gas applying a deposition temperature of 900 • C and a pressure of 200 mbar. The deposition time was 100 minv for TiN layer. The cemented carbide substrates employed were composed of 86.5 wt.% WC, 10 wt.% Co, and 3.5 wt.% cubic (Ta,Nb)C, which were provided by Ganzhou Achteck Tool Co., Ltd of China.
The PVD Ti-Al-Si-N coating was arc ion plated from Ti 0.3 Al 0.6 Si 0.1 compound targets (powder-metallurgically manufactured by Plansee Composite Materials GmbH, Lechbruck, Austria) on a cemented carbide substrate. During the 150 min deposition (Metaplas Domino L industrial scale deposition facility) the arc current was set to 130 A. The nitrogen pressure was about 8.5 Pa along with a substrate temperature of 450 • C. The DC bias potential applied to the 3-fold rotating substrate carousel was set to −60 V.

Characterization
The thickness and surface morphology of the coatings were investigated using a field-emission scanning electron microscope (FESEM, Supra 55, Zeiss, Germany), and the elements content variation was characterized by energy dispersive spectrometer (EDS). Phase identification and crystal structure were performed by X-ray diffraction (Rigaku D-max 2500 system) with Cu-Kα radiation, wavelength λ = 0.1540598 nm, U = 40 kV, I = 40 mA, and diffraction angle 2θ = 20 • -90 • . Structure studies were carried out by transmission electron microscopy (TEM, FEI Tecnai G2 F20) under an operating voltage of 300 kV. TEM specimens were prepared using a dual-beam focused ion beam (FIB) system (FEI Helios Nanolab 600i) working under a vacuum of 5 × 10 −4 kPa. Final surface cleaning was conducted at 5.0 kV and 41 pA to remove the amorphous layer induced by FIB. Hardness and elastic modulus were measured in a nano-indentation equipment (Anton Paar, TTX-NHT2) at a load of 20 mN with a Berkovich diamond head. The indentation test was carried out in 20 locations, and the average value was taken as the coating hardness.

Chemical Composition and Structure of the As-Deposited CVD TiAlSiN and PVD TiAlSiN Coatings
The chemical compositions of CVD and PVD TiAlSiN coatings measured by EDS are listed in Table 1. For the CVD TiAlSiN coating, it was reasonable to detect a small amount of Cl in the coating due to the low deposition pressure and three chlorides TiCl 4 -AlCl 3 -SiCl 4 contained in the gaseous mixture. Moreover, Al, Co, and Ti elements could be detected in the TiN adhesive layer since Al atoms of the top layer and Co and C atoms of the substrate diffused into the TiN layer within 160 min of deposition process. Only four elements Ti, Al, Si, and N were detected in the PVD TiAlSiN coating. Thus, the EDS results show that the nominal compositions of CVD and PVD coatings were Ti 0.13 Al 0.85 Si 0.02 N and Ti 0.31 Al 0.60 Si 0.09 N under the condition that the content of Cl and W was ignored and the content of (Al + Ti + Si) was approximately equal to the content of N. It should be noted that CVD-TiN in Table 1 is not an additional third coating, but the underlying base layer of CVD TiAlSiN coating.   Figure 1, while the standard peak value of TiN (PDF#38-1420) is 36.665 • . It is evident that the diffraction peaks of these coatings are shifted to the right side in a small range, which implies that the c-TiN has a solid solution microstructure. Zou et al. [34] and Zhao et al. [35] also reported that when Al or Si atoms replaced part of Ti in c-TiN lattice, a small shift would take place in XRD peaks. As mentioned in [34], the substitution of Al and Si atoms in c-TiN causes a change in the lattice constant, which finally leads to a slight shift of the peak. Considering that Si atoms might also contribute to this shift, the shifted TiN peak in the XRD pattern can also be marked as the peak of (Ti,Al,Si)N solid solution. Furthermore, the strong peaks of WC and Co 3 W phases were identified in the XRD pattern since the coatings were deposited on WC-Co substrate.
As for PVD Ti 0.31 Al 0.60 Si 0.09 N coating, it is noteworthy that the w-AlN (100) peak at 33.874 • shows a remarkable broadening with an approximate integral breadth of 4.5 • in Figure 1. The coherently diffracting domain size of w-AlN was calculated to be approximately 9.8 nm from FWHM (full width at half maximum) using the Scherrer equation, which demonstrates that the apparent size of w-AlN is relatively small. It should be noted that the determination of coherently diffracting domain size using the Scherrer equation is a rough estimation, since the contribution of micro-strain to the peak broadening is neglected [36]. Compared with CVD TiAlSiN coating, no c-AlN and c-TiN phase peaks were observed in the XRD pattern of PVD Ti 0.31 Al 0.60 Si 0.09 N coating, and only the peak of w-AlN can be clearly observed apart from the peaks of WC-Co substrate.  It is obvious that Ti 0.13 Al 0.85 Si 0.02 N coating prepared using the LPCVD method exhibits a featureless morphology, while PVD Ti 0.31 Al 0.60 Si 0.09 N coating includes many shallow pits and irregular particles on the surface, which is a typical surface morphology feature of coating prepared using multi arc ion plating technology [37]. The formation of particles is mainly due to the solidification of molten metal droplets on the coating surface. Under the negative bias, the particles with poor adhesion on the coating surface are peeled off by high energy ion bombardment, resulting in the formation of shallow pits.   Figure 2b). In case of the PVD TiAlN coating, several groups of authors [38][39][40][41] have reported that PVD TiAlN coating exhibits a pronounced columnar structure. With the addition of Si into the TiAlN coating, the amorphous structure SiN x is formed, which will wrap around the TiAlN grains and make the grains unable to grow and force TiAlN to nucleate continuously. As a result, the grains of TiAlSiN coating are refined and the columnar grains disappear, showing a cross-section structure similar to amorphous state [26,42]. The cross-section morphology of PVD Ti 0.31 Al 0.60 Si 0.09 N was in agreement with that from the literature [26]. In addition, compared with Figure 2b, some micro particles which originate in the arc process at the target side appear in the PVD TiAlSiN coating. Compared with the PVD TiAlSiN coating, the CVD coating has an additional TiN layer, which resulted in a more obvious TiN peak identified in the CVD coating (see Figure 1).
TEM investigations were employed in order to compare the microstructure of the CVD Ti 0.13 Al 0.85 Si 0.02 N and PVD Ti 0.31 Al 0.60 Si 0.09 N coatings obtained from the present work. Figures 3 and 4 present the HAADF-STEM images and EDS mappings of two different kinds of TiAlSiN coatings, respectively. It can be found from mapping images that Ti and N are uniformly distributed mainly in the first layer with light gray contrast, and Ti, Al, Si, and N are uniformly distributed in the second layer with dark gray contrast. The results indicate that the CVD Ti 0.13 Al 0.85 Si 0.02 N coating is composed of a TiN bonding layer and a TiAlSiN layer, which is consistent with the FESEM result that can be seen in Figure 2. Otherwise, we also found that the element Co was distributed into the first layer, which is caused by the high deposition temperature of CVD method and the rapid diffusion of Co. For PVD Ti 0.31 Al 0.60 Si 0.09 N coating, Figure 4 indicates that there are some pores existing in the coatings and Ti and Si are enriched in the micro particles near the pores. Chen et al. [37] suggest that the growth defects of metallic macro-particles on the coating surface are caused by the difference of melting points for Al, Ti, and Si, where alloying with Si causes an apparent increase in growth defects of metallic macro-particles. As a result, it can be roughly inferred that the enrichment of Ti and Si around the pores is caused by TiSi alloying brought about by arc ion plating technology. In other areas, the four elements are evenly distributed.   Figure 1. There is no information corresponding to the SiN x crystalline from the SAED patterns. Nevertheless, the amorphous ring observed in fast Fourier transform in Figure 5b can still indicate the existence of amorphous SiN x . From the fast Fourier transformation (FFT) images in Figure 5b, we can see that the interplanar spacing of A is 2.63 Å which is close to that of w-AlN (100) plane (2.695 Å according to JCPDS 25-1133). Therefore, region A in Figure 5b is believed to be (100) planes of w-AlN. The interplanar spacing for region B and C is 2.30 Å and 2.50 Å, respectively, while the interplanar spacing of c-AlN and TiN (111) planes is 2.2736 Å and 2.4492 Å (c-AlN, JCPDS 46-1200 and TiN, JCPDS 38-1420). In addition, the area within the black circle does not have the characteristics of a crystalline structure, so it should be amorphous SiN x . Thus, the CVD Ti 0.13 Al 0.85 Si 0.02 N coating was composed of some nano-crystallines and amorphous structures. Meanwhile, the nanocrystals were embedded in the amorphous matrix, which was similar to the results for PVD TiAlSiN coatings [28,43].   (Figure 6b), it can be seen that the size of amorphous SiN x (circled by black circles) is between 1 and 3 nm. Meanwhile, the possibility that Si atoms are accommodated in the (Ti, Al, Si)N solid solution was not ruled out, since Si atoms exists in the solid solution in TiSiN, TiSiCN, and other coatings [12,44,45]. This proved that the structure of TiAlSiN is a kind of nano-composite structure, in which the amorphous SiN X matrix contains nano-crystals.

Mechanical Properties of the As-Deposited CVD TiAlSiN and PVD TiAlSiN Coatings
The hardness (H) and effective elastic modulus (E*) were measured by nanoindentation. Moreover, the elastic modulus (E) was calculated using the formula of E* = E/(1 − ν 2 ), where ν is the Poisson's ratio [46,47]. Figure 7 manifests the nano-hardness of CVD Ti 0.13 Al 0.85 -Si 0.02 N and PVD Ti 0.31 Al 0.60 Si 0.09 N coating. The hardnesses of CVD Ti 0.13 Al 0.85 Si 0.02 N and PVD Ti 0.31 Al 0.60 Si 0.09 N coating were 31.7 ± 1.4 GPa and 23.5 ± 0.8 GPa, respectively. The result shows that the hardness of the CVD TiAlSiN coating was significantly higher than that of PVD TiAlSiN coating, which is related to the higher Si content in PVD coating. Higher Si element content coating leads PVD coating to form a higher content of amorphous phase in the coating, as a result, the excessive amount of amorphous phase will decrease the hardness of the coating. In addition, the higher Al content in the CVD coating contributes to the formation of face centered c-AlN phase in the coating, and the increase of the face-centered cubic structure in the coating is conducive to the increase of hardness of the coating. With the aim of comparing the toughness of two kinds of different coatings, H/E (a description in terms of 'elastic strain to failure') and H 3 /E* 2 (a indicator of a surface's resistance to plastic deformation) [48][49][50] were employed to characterize the resistance against the elastic strain and plastic deformation, respectively. Figure 8 displays the H/E* and H 3 /E* 2 ratios of the TiAlSiN coating deposited with different methods. The results point out that the ratios of H/E* and H 3 /E* 2 of CVD Ti 0.13 Al 0.85 Si 0.02 N coating reached the maximum value (0.085 and 0.227). Since a higher H/E* and H 3 /E* 2 ratio means good toughness, the CVD TiAlSiN coating obtained in this work can have both higher hardness and toughness, which also means that the CVD coating could be widely applied in industry.

Conclusions
Ti 0.13 Al 0.85 Si 0.02 N and Ti 0.31 Al 0.60 Si 0.09 N coatings were deposited on cemented carbide substrate using the chemical vapor deposition and arc ion plating methods, respectively. XRD, FIB-SEM, TEM, and Nano-indenter were employed to study the effect of deposition method on microstructure and mechanical properties of TiAlSiN coating. The main conclusions of this work are summarized below.
The CVD Ti 0.13 Al 0.85 Si 0.02 N coating was composed of c-AlN, c-TiN phases, and a dominant amount of w-AlN according to the XRD patterns. In the PVD Ti 0.31 Al 0.60 Si 0.09 N coating, only the peak of w-AlN could be clearly observed apart from the peaks of matrix phase.
The CVD Ti 0.13 Al 0.85 Si 0.02 N coating exhibited a featureless morphology, while the PVD Ti 0.31 Al 0.60 Si 0.09 N coating had shallow pits and irregular particles on the surface, based on the SEM images. In addition, the cross-sections of these two coatings show a typical amorphous structure.
TEM analysis illustrated that these two coatings consist of a nanocrystalline embedded in amorphous SiN x .
The CVD Ti 0.13 Al 0.85 Si 0.02 N coating not only had higher hardness and Young's modulus than the PVD Ti 0.31 Al 0.60 Si 0.09 N coating, but also has higher H/E* and H 3 /E* 2 values than the PVD Ti 0.31 Al 0.60 Si 0.09 N coating. Therefore, we conclude that the CVD TiAlSiN coating has the better wear resistance, fracture toughness, and plastic deformation resistance compared to the PVD TiAlSiN coating.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.