Influence of deposition conditions on structure parameters and mechanical properties of multi-element coatings based on niobium nitride

. The paper presents the results of studies of the influence of the deposition conditions, coatings NbTiZrN, NbTiAlN on structure parameters and mechanical properties. It is shown that the changes of these parameters depend on the composition of coatings. It was found that the condensation temperature affects the phase composition of the coating. Data obtained can be used when choosing functional layers of multi-element coatings.


Introduction
Wear-resistant coatings boost the efficiency of cutting tools [1][2][3][4][5]. Multi-element coatings are increasingly applied [6][7][8][9]. To boost the efficiency of functional layers of multi-element coatings it is necessary to have data on the influence of various factors accompanying their deposition process on their mechanical properties.
Objective of this research is to study the influence of the composition and deposition conditions of coatings on their structural parameters and mechanical properties

Methodology of Experiment
Multi-element coatings based on niobium nitride NbTiZrN and NbTiAlN were studied. They were applied on carbide blades МК8 and plates of 20Х13 and Р6М5 steel using «Bulat-6» unit. Chemical composition of coatings was determined by the quantitative X-ray spectroscopic analysis method using the apparatus MAP-4 taking into consideration ZAFmodifications. Structural parameters of coatings (the lattice spacing а, the half-width of the X-ray diffraction line β004) and residual compressive stresses σ0 were studied using the Xray diffraction meter «ДРОН-3M», sizes of coherent scattering regions (CSR) D were calculated using the method described in the paper [10]. Micro-hardness Hϻ, elasticity modulus Е and stress intensity coefficient KIC of coatings were determined using methods described in the paper [11]. Adhesion strength of coatings with the instrumental base was estimated by the stripping coefficient K0 [12].

Research Results and Discussion
NbTiZrN coatings were applied using two cathodes of niobium and a composite cathode of titan with a zirconium insertion. Coating composition was changed by varying the area of zirconium insertions. When applying NbTiAlN coating, we used cathodes of niobium, alloy of niobium, titan and aluminum and alloy of titan and aluminum. Coating composition was changed by varying the layout diagram of the unit -the combination of cathodes used. Chemical composition of coatings is presented in Table 1.  [13,14]. As Table 2 shows, coatings NbTiZrN and NbTiAlN are characterized with higher parameter values of the lattice, smaller value of the X-ray line half-width and substantially smaller compression residual stresses (by 1,6…2,7 times smaller compared to NbN), slight increase of CSR block sizes.  (100) Study of mechanical properties (Table 3) showed that micro-hardness and elastic module of three-element coatings do slightly differ from niobium nitride. NbTiZrN coating is characterized with higher values of micro-hardness and elastic module. Stress-intensity factor of multi-element coatings is higher compared to NbN coating by 8.4…23 %. This indicates higher crack resistance. Resistance to abrasive wear Hμ /Е and plastic straining Hμ 3 /Е 2 of multi-element coatings and NbN differ slightly. Adhesion strength of multielement coatings is lower compared to NbN. It is evidenced by higher values of the delamination coefficient. Change of the coating composition slightly influences its phase composition. For NbTiAlN coatings of various composition, the diffraction peak of NbN (004) of maximum intensity is observed almost at one and the same angle 2θ=41.4 grad. For NbTiZrN coatings, reducing Nb and increasing Ti and Zr respectively lead to displacement of the diffraction peak of NbN (004) from the angle 2θ=41.2 grad. to the angle 2θ=35.4 grad.
Comparing of data, presented in Tables 2 and 3, shows that change of composition differently influences structural parameters of coatings. For NbTiZrN coatings, parameter values of the lattice, X-ray line half-width, CSR block sizes almost do not depend on the composition. At the same time, the value of compression residual stresses in coatings with the lower content of niobium is by 2.0…2.8 times higher. Influence of the condensation temperature on the chemical composition of coatings is presented in Table 5. It has been established that change of the condensation temperature slightly influences the ration of deposited elements in the coating. At the same time, if the condensation temperature in NbTiAIN coating is higher, the aluminum composition will be slightly increased provided that the titan composition is decreased. .57 -Note: *, ** -at the temperature 300 0 С and 600 0 С, respectively Tables 6 and 7 show data on the influence of the condensation temperature on the phase composition of coatings. Figures 1 and 2 show diffractograms of NbTiZrN coating at different condensation temperatures.
It has been established that the condensation temperature has a great influence on the phase composition of the coating. When comparing data showed in Tables 6 and 7, it turns out that for NbTiZrN coating, obtained at the temperature 300 °С, in the diffractogram (Fig.  1, a) the diffraction peak of the highest intensity which is identified as NbN (112), common to the tetragonal structure, appears at the angle 2θ=35.4 grad. In the diffractogram of the coating, obtained at the temperature 600 °С (Fig. 1, b), the additional diffraction peak appears at the angle 2θ=41 grad. This fact is also common to the tetragonal structure and identified as NbN (200). Similar regularities of influence of the condensation temperature on the phase composition were also obtained for NbTiAlN coatings.   Influence of the condensation temperature on structural parameters and mechanical properties of coatings is shown in Tables 8 and 9 and in Fig. 2-4. It has been established that decreasing of the condensation temperature from 600 °С to 300 °С changes structural parameters and the value of compression residual stresses. The X-ray line half-width is increased (by 11%...64%). CSR block sizes are decreased (by 1.2…1.6 times). Compression residual stresses are increased (by 1.1…1.4 times). Maximum change of abovementioned parameters is common to NbTiAlN coatings. -5273±290 -3834±548 Note: data in the numerator -for 300 °С, in the denominator -for 600 °С  Decrease of the condensation temperature leads to increase of mechanical properties of coatings. Coatings obtained at the low condensation temperature are characterized with higher values of the micro-hardness, elastic module, stress-intensity factor provided that the adhesion strength is decreased. Condensation temperature has the maximum influence on the adhesion strength. Delamination coefficient, characterizing the adhesion strength, of coatings obtained at the high temperature, is lower by 1.3…1.4 times. This fact suggests that their adhesion strength is higher. Results obtained are well within data of works [15,16].

Conclusion
It has been established that deposition conditions of coatings based on niobium nitride influence the phase composition, structural parameters, residual stresses and mechanical properties. It is testified that changes of parameters mentioned above depend on the coating composition. Data obtained can be used when choosing functional layers of multi-element coatings.
This paper is drawn up with the aid of the grant from the Russian foundation for basic research (RFBR)the project № 18-48-730011