Structure and properties of CrAlSiN Nanocomposite coatings deposited by lateral rotating cathod arc
Introduction
Nowadays, thin wear-resistant hard coatings are extensively applied on various kinds of cutting and forming tools to improve their lifetime and performance, enhance productivity, and enable specific engineering applications as well. Hard coating has now become a routing processing step in tools industry. Currently, a wide range of hard coatings is available for a variety of applications. TiN is the first generation of a physical vapor deposited (PVD) hard coating and is now still being widely used as protective coatings for bearings, gears, and cutting and forming tools. However, the fracture toughness and oxidation resistance of TiN coatings are not satisfactory for many advanced engineering applications. A second generation of PVD coatings was developed by the addition of further elements to produce ternary systems. It has been widely reported that ternary nitride films generally have a higher hardness than binary nitrides. For example, the mechanical properties of PVD TiN coatings can be effectively improved through partial substitution of N with C (TiCN) or B (TiBN) [1], [2], [3], or partial substitution of Ti with Al (TiAlN), Cr (TiCrN), Zr (TiZrN) or Nb (TiNbN) [4], [5], [6], [7], [8], [9], [10], [11], [12]. These substitutions provide a solid solution strengthening effect that results in a higher hardness (~ 30 GPa) and an associated increase in wear resistance. The substituting atoms can also impart higher chemical stability and improved oxidation resistance.
The most commonly employed element to alloy transition metal nitride (MeN) films for mechanical applications is aluminum, such as TiAlN and CrAlN coatings for typical examples [4], [5], [6], [7], [8], [9], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. In the last decade, TiAlN coatings represent state-of-the-art for commercial tool coatings. They have become the standard choice for many industrial applications. In the past few years, (Cr,Al)N coatings have gained much attention as a promising substitute for (Ti,Al)N. CrAlN coatings have been demonstrated with superior toughness, higher corrosion and oxidation resistance, better tribological properties with both lower friction coefficient and wear rate, and as a consequence, a better cutting performance for high speed dry machining, than the TiAlN coatings [17], [18], [19], [20], [21], [22].
Recently, much attention has been paid to Si-containing coatings. Like aluminum, silicon is a light element that forms highly hard and stable oxides. Adding silicon to transition metal nitrides, such as (Ti,Si)N [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], (Cr,Si)N [32], [33], [34], and (Ti,Al,Si)N [30], [35], [36], [37], [38], [39], [40], [41], [42], has been reported to significantly improve coatings' properties, including hardness, toughness, erosion and oxidation resistance, which are desirable in most wear-resistant coatings. Most microstructural studies on these Si-containing nitrides [27], [28], [29], [30], [31], [41] report that incorporation with Si resulted in refinement of the grain size of the crystalline MeN phase and the formation of an amorphous SiNx phase. This consequently led to the formation of two-phase nanocomposite structures composed of nanocrystalline MeN grains embedded in an amorphous silicon nitride (a-SiNx) matrix, which conformed well to the generic conceptual microstructure model of nanocomposite coatings proposed by S. Veprek et al. [24], [25], [26], [27]. The formation of such a nanocomposite structure is generally attributed to thermodynamic spinodal phase separation during coating deposition, due to the very low solubility of silicon in the lattice of B1-structured nitrides.
Since the pioneering work of S. Veprek et al., nanocomposite coatings have attracted great attention all over the world. Most of the initial research works on nanocomposite coatings were focused on the (Ti,Si)N system [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. Later on, based on the consideration of the superior properties of TiAlN over the binary TiN, the nanocomposite coating system was extended to quaternary Ti–Al–Si–N coatings. Tanaka et al. [36] reported a greatly improved oxidation resistance of Ti–Al–Si–N coatings, with a few atomic percentages of Si, up to 1100 °C in air. Better cutting performance, particularly under high speed machining conditions, for TiAlSiN coated cermet cutting tools has been demonstrated relative to both TiAlN and commercial CVD multilayer TiCN/Al2O3/TiN coated tools [37]. Regarding the analogy between the Ti–Al–Si–N and Cr–Al–Si–N systems, more recently, quaternary Cr–Al–Si–N coatings started to be explored [43], [44], [45], [46]. It has been reported that (Cr,Al,Si)N coatings for cutting tools are more wear resistant than (Cr,Al)N [43]. As CrAlN coatings have been demonstrated with superior properties over TiAlN, it can be expected that CrAlSiN nanocomposite coatings have excellent properties and promising applicability in modern industry and manufacturing technology. Recently, Chang and Hsiao [47] deposited both CrAlSiN and CrTiAlSiN nanocomposite coatings by the lateral rotating cathode arc (LARC). They found that the CrAlSiN coating without Ti incorporation exhibited a better oxidation resistance than the CrTiAlSiN coating.
Initially, Si-containing nanocomposite coatings were synthesized by plasma enhanced chemical vapor deposition processes [23], [24], [25]. In our previous work [32], superhard Ti–Si–N nanocomposite coatings were prepared the by a combined DC/RF reactive unbalanced magnetron sputtering process. The as-deposited nanocomposite coatings exhibited an excellent erosion wear resistance in comparison with other hard/superhard coatings. We also prepared CrAlN coatings showing better tribological properties and cutting performance in high speed machining than TiAlN coatings [20]. In this work, a series of CrAlSiN coatings were deposited by the lateral rotating cathode arc technique. Structure and properties of the as-deposited coatings were investigated.
Section snippets
Experimental
A series of CrAlSiN coatings, with a thickness of 3.1–3.6 μm, were deposited using a Platit π80 LARC system, which has been described in detail elsewhere [48], [49]. For depositing the CrAlSiN coatings, one elemental Cr cathode and one AlSi alloy cathode (with ~ 11 at.%Si) were used in this work. These were laterally rotating during the coating deposition process. The coating deposition was carried out in a flowing pure nitrogen atmosphere under a working pressure controlled at 1.5 Pa. In order to
Results and discussion
EDX analysis results for as-deposited coatings are shown in Fig. 1, Fig. 2, Fig. 3. The nitrogen content (Fig. 1) was between 53.0 and 54.5 at.% in all samples under study, indicating a satisfactory stoichiometric composition of all the as-deposited coatings. Fig. 2 shows the Al+Si to Cr atomic ratio in as-deposited CrAlSiN coatings as a function of the current ratio applied onto the AlSi and Cr cathodes. Results were as expected; the (Al+Si)/Cr atomic ratio in coatings increased monotonously
Summary
A series of CrAlSiN coatings with different (Al+Si)/Cr atomic ratios, from 0 up to 3.1, were deposited by a vacuum arc reactive deposition process from two lateral rotating chromium and aluminum–silicon cathodes in a flowing pure nitrogen atmosphere. All of the as-deposited CrAlSiN coatings exhibited a higher hardness than pure CrN, showing a maximum hardness of about 40 GPa at around (Al+Si)/Cr = 1.62. The abrasive wear resistance of the CrAlSiN coatings is better than for TiAlN coatings
Acknowledgements
The authors would like to thank Mr Anthony Yeo and Mr K.C. Shaw for their technical assistance in this work.
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