Oxidation Behavior of Cu Doped CrAlN Coating Deposited by Magnetron Sputtering at 800°C

CrAlN coating with moderate Cu content was deposited on the surface of 1Cr18Ni9Ti stainless steel by DC reactive magnetron sputtering. The oxidation behavior under 800°C of the coatings was investigated emphatically in this research. The phase construction, microstructure and chemistry contents were analyzed by X-ray diffraction (XRD), field-emission electron scanning microscope (FESEM) and energy disperse spectroscopy (EDS). And the mechanical properties of the coatings were measured by microhardness tester and multi-function scratcher. As a comparison, the properties of CrAlN coating were also analyzed in the article. The results showed that the microstructure of CrAlCuN coating became smother and denser with a phase structure of CrN and Cu. With Cu doped in the CrAlN coating, the more excellent hardness which enhanced to 48.37 GPa was obtained. While the bonding strength of the coating was decreased because of the advanced microhardness usually released more internal stress. Both of the coatings were not complete failure and CrN diffraction peak was also tested. The oxidation rate of weight gain had significantly decreased with Cu doped, which improved the resistance of high temperature oxidation to a certain extent.


Introduction
The conventional binary nitride coatings have been paid extensive and close attention due to high hardness and excellent wear resistance over the past decades. TiN, as an early hard coating, has excellent mechanical properties but its moderate thermal stability limits its application areas [1]. In comparison, CrN coating exhibits enhanced performance under high temperature oxidation because of the formation of dense oxidation film. However, traditional binary coatings can't satisfy the demand of use due to the increasingly harsh working environment [2]. Alloying transition metal elements into these binary coatings can improve the mechanical properties and thermal stability which may expand their application scope. The element Al has been studied most. CrAlN coating is developed from binary Cr-based coating, which may obtain advanced hardness and high-temperature oxidation resistance by adding Al [3][4][5]. The main reason for improved performance is the formation of dense and adherent (Cr, Al) 2 O 3 mixed oxide film after high temperature annealing treatment. [6]. Previous studies indicated that the presence of Al in the coating formed a metastable phase in the cubic lattice of c-CrN coating. In addition, numerous works pointed that CrAlN coating had advanced resistance to high temperature oxidation, whereas its thermal stability is lower because of the presence of w-AlN during oxidation process [7]. And other researches pointed that the N-loss may be further aggravated with annealing process which has deteriorated the comprehensive practical properties and restricted its serviceable range. In certain studies that mixing the fourth element into CrAlN can constitute

Results and discussion
The phases of CrAlN and CrAlCuN coatings. Figure 2 shows the XRD patterns of the CrAlN and CrAlCuN coatings before oxidation. The diffraction patterns of CrAlN coating revealed that a crystal structure of CrN phase and a faint diffraction peak of Cu were detected in the CrAlCuN coating. Both of the coatings were presented a dramatically (200) preferred orientation with several major crystal orientation of (220), (222) and the preferential growth was not changed in the coatings [12]. However, the diffraction peaks of CrN phase were moved to the small angle that the main reason may be attributed to the adulteration of Cu. The mechanism of above reason was that Cu as a 3d transition metal element possibly changed the defect type so that cause lattice distortion [13]. What's more, the intensity of the diffraction peak has enhanced in the CrAlCuN coating. According to the Scherrer Formula that the full width at half maximum of the preferred orientation diffraction peak decreased indicated the grain size changed larger. And the surface morphology of the CrAlN and CrAlCuN    Figure 4 image (b were som topograp defection attribute emerged exposed to the st oxides an high tem    Figure 4 indicated that the grain had refined, and became more smother. Some research pointed that the Cu content in the coating had a limitation about 1%-5%. The Cu as soft phase played a leading role when it exceeded the limitation. In this work, the Cu content was 1.41at% and the result brought into correspondence with Pan's research [14][15][16][17]. And compared with the increasing trend of hardness, the bonding strength usually presented a reverse tendency. Table 1 enumerated the chemical composition of the coatings after oxidation annealing with surface scanning. From the Table 1, a variety of elements have been detects including several matrix elements such as Fe, Mn, Ni and O, whereas no N was discovered that the result was in agreement with the phase analysis in Figure 3. It can observe that the outer diffusion of the matrix elements appear, which may produce the crisp oxide and further cause the film to crack or fall off. And in the Table 2 the same situation was happened except that the N still preserved in the coating which account for that although the coating was oxidation, still kept excellent thermal stability and played a protective role to a certain degree.  Figure 6 shows the kinetic curve about the coatings. From the image that the general trend of the coatings after oxidation annealing is that as the oxidation time changes its oxidation rate increases first and then decreases and the main reason was that the initial phase of oxidation is dominated by the diffusion of oxygen so that it did not produce enough dense metal oxide protective film. As the oxidation time lengthened, the coatings surface were cladding with (Cr, Al) 2 O 3 oxide film which could postpone the process of oxidation [18]. However, the diffusion of oxygen aggravated that caused the bonding force of the coatings became weaken, what's more, crack of drop occurred followed. The matrix element such as Fe, Ni presented external diffusion in this process, which is consistent with the EDS analysis results. The oxidative weight curve of CrAlN coating is much lower than the CrAlCuN coating. The main reason is that the primary channel of the oxygen atoms diffuse depended on the grain boundary and defection [19], the Cu can refine the size of grains and reduce the boundary, what's more, it can change the type of the defection in the coatings to a certain extent, which reduced the diffusion rate of oxygen in the coatings and slow the oxidation process [20][21].

Conclusion
The phase structure of CrAlN and CrAlCuN coatings deposited by DC reactive magnetron sputtering was basically CrN and Cu with a preferred orientation of CrN (200) before oxidation. And after oxidation annealing, both of the coatings were appeared the diffraction peaks of metal oxide.
The size of grains in CrAlCuN coating was smaller than CrAlN coating. And the CrAlN coating oxidized seriously than CrAlCuN coating.
The hardness of the coatings was increasingly from 30.17 GPa to 4837 GPa with the adulteration of Cu. While the bonding force of the CrAlCuN coating was 8N that lower than the CrAlN coating with a value of 16N.
The oxidation process of the CrAlCuN coating was slower than CrAlN coating and trend of coatings after oxidation annealing is that as the oxidation time changes its oxidation rate increases first and then decreases.