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May 2, 2013 - Monolayer TiN and a-BCN and nanolayer TiN/BCN coatings are deposited by dual magnetron sputtering. The effect of discharge power, flow ...
Powder Metallurgy and Metal Ceramics, Vol. 52, Nos. 1-2, May, 2013 (Russian Original Vol. 52, Nos. 1-2, Jan.-Feb., 2013)

STRUCTURAL AND MECHANICAL PROPERTIES OF TIN/BCN COATINGS P. L. Skrinskii,1 A. I. Kuzmichev,2 V. I. Ivashchenko,1,4 L. A. Ivashchenko,1 I. I. Timofeeva, 1 O. O. Butenko,1 O. Yu. Khizhun,1 T. V. Tomila,1 and S. N. Dub3 UDC 620.3:621.723:620.18.539:620.178 Monolayer TiN and a-BCN and nanolayer TiN/BCN coatings are deposited by dual magnetron sputtering. The effect of discharge power, flow rates of gases, and substrate bias voltage on the mechanical properties of the deposited nanolayer coatings is studied. The structure, chemical bonding, mechanical properties, adhesion, and friction coefficient of the coatings are analyzed. It is established that the deposited nanolayer coatings have nc-TiNx/a-BNO+a-C structure. The Knoop hardness of the nanolayer TiN/BCN coatings is found to reach 45 GPa on average. The nanohardness is lower than the Knoop hardness (by 20–30%). The deposited nanolayer coatings can be recommended as wear-resistant and protective coatings for mechanical engineering. Keywords: magnetron sputtering, wear-resistant coating, nanolayer coating, mechanical properties, friction coefficient, adhesion.

INTRODUCTION Nanocomposite TiN–BN coatings have been examined in sufficient detail. The nanocomposite nc-TiN/aBN, nc-TiN/a-BN/a-TiB2, and nc-TiN/a-TiBx/a-BN systems produced by plasma-enhanced chemical vapor deposition (PECVD) and arc vacuum evaporation are comprehensively studied in [1–4]. A specific structure of nanocomposite coatings forms depending on deposition parameters. It was established that structures with titanium nitride particles ranging from 3 to 6 nm [1, 2] showed the highest hardness (40–50 GPa), substantially exceeding that of TiN coatings (20–25 GPa). The hardness of coatings is also sensitive to the thickness of an amorphous layer between the TiN nanocrystallites. It is shown that 40–50 GPa hardness is reached by coatings with an amorphous layer whose thickness is close to one BN monolayer (1 ML) [2]. If the layer decreases to 0.5 ML or increases to 2 ML, hardness reduces to 25 GPa. The TiN–BN system was theoretically studied in [5], using the first-principle pseudopotential method. The solid TiN–BN solution is shown to decompose by spinodal mechanism. Nanolayer TiN/BN coatings are still to be examined. In particular, we have found only several papers focusing on their development. Nanolayer TiN/BN coatings were formed by PECVD in [6]. It is established that the cubic boron nitride phase formed with a layer ranging from 0.5 to 1 nm. The tribological properties of nanostructured

1Frantsevich

Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev, Ukraine. 2National Technical University of Ukraine ‘Kiev Polytechnic Institute’, Kiev, Ukraine. 3Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, Kiev. Ukraine. 4To

whom correspondence should be addressed; e-mail: [email protected].

Translated from Poroshkovaya Metallurgiya, Vol. 52, No. 1–2 (489), pp. 95–106, 2013. Original article submitted September 13, 2011. 1068-1302/13/0102-0073 2013 Springer Science+Business Media New York

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multilayer TiN/TiBN coatings were studied in [7] as a function of modulation period. The best tribological properties were demonstrated by coatings with a modulation period of 1.8 nm. They had a hardness of 29.5 GPa. Their low friction coefficient is ensured by hexagonal boron nitride, acting as a lubricant. Two-layer TiN/BN coatings were also deposited using pulsed arc plasma [8]. The emphasis was placed on the electrical properties of the coatings. X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) revealed the cubic and hexagonal boron nitride phases, the latter being predominant. The multilayer TiN/[BCN/BN]n/c-BN coatings were deposited by magnetron sputtering of Ti, B4C, and BN targets [9]. The best mechanical properties (hardness of about 30 GPa and elastic modulus of about 230 GPa) are exhibited by a multilayer coating with a modulation period of 80 nm [9]. Unfortunately, we have found no studies focusing on the multilayer TiN/BCN system. Therefore, its study is quite reasonable in terms of ascertaining the possibility to strengthen TiN/BN coatings by introduction of carbon.

EXPERIMENTAL Table 1 summarizes parameters for depositing mono- and nanolayer TiN and BCN coatings by dual magnetron sputtering (samples of SP series). The following deposition parameters were used: substrate temperature TS (350ºC for all samples); bias voltage UD on the substrate; argon and nitrogen flow rates FAr and FN2; working pressure Pc in the chamber; magnetron voltage U, magnetron current I, and time t for deposition of a specific layer.

Pressure in the chamber before deposition was 2.66  10–9 MPa. The nanolayer coatings contained 58 layers of TiN and BCN. The substrates were single-crystalline plates (KDB-10, ) and hard alloys VK8 (SP-37.1) and T15K6 (SP-37.2). Amorphous a-BCN coatings were deposited using a B4C target and TiN coatings using Ti and TiN targets. The Ti, TiN, and B4C targets were produced by hot pressing. Nanolayer coatings were deposited by alternate spraying of B4C and TiN targets. X-ray diffraction showed that the TiN and B4C targets corresponded to stoichiometric compositions. According to Table 1, the samples of SP-8, SP12, SP21, and SP23 series are silicon plates with TiN coatings, the samples of SP-32, SP-33, SP-35, SP-36, and SP-37 series are nanolayer coatings deposited on silicon (SP-32, SP-33, SP-35, SP-36) and cutting plates from hard alloys VK8 (SP-37.1) and T15K6 (SP-37.2), and the samples of SP-38 series are BCN coatings on silicon substrates. After deposition, 60 mm × 15 mm silicon plates were cut into four equal parts. Hence, each series includes four samples, the first sample being closer to the Ti(TiN) target and the fourth to the ceramic target. In case of SP-37 samples, both cutting plates were close to the titanium target. TABLE 1. Deposition Parameters and Thickness d of Coatings

Series

SP-8 SP-12 SP-21* SP-23 SP-32 SP-33 SP-35 SP-36 SP-37 SP-38 *

d, m

0.6 0.4 0.6 0.4 0.7–0.4 0.5–0.4 0.6–0.4 0.7–0.4 0.1 0.03

UD, V

0 0 –50 –50 –50 –50 0 –50 –50 –50

Titanium nitride target is used.

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FAr,

FN2,

cm3/min cm3/min 0.8 3.0 2.5 59.6 40.0 39.0 64.0 60.0 60.0 62.0

0.26 1.00 0 15.5 13.0 13.0 15.0 20.0 20.0 20.0

Pc, MPa 7.99 7.99 5.33 2.26 1.33 1.33 2.13 2.0 2.13 2.4

B4C target U, V – – – – 400 400 400 400 400 400

Ti target

I, mA

t, min





– – – 100 50 100 100 100 100

– – – 1 1 1 1 1 40

U, V

I, mA

t, min

500 500 370 450 400 400 400 400 380 –

200 200 250 250 200 200 200 200 200 –

60 60 40 30 2 2 2 2 2 –

The structure of all coatings was examined with X-ray diffraction (XRD). We provide the most informative XRD spectra. Some of the nanolayer coatings were also analyzed using low-angle X-ray reflection (LA-XRR) to verify their nanolayer structure. The experiment was performed with DRON-3 (XRD) and DRON-U (XRR) diffractometers with a Cu-K radiation source. The thickness of the coatings was determined with a Micron-Alpha Optical Shape Meter. X-ray photoelectron spectra (XPS) of layers in the nanolayer coatings were measured using a UHV-Analysis-System (SPECS, Germany). The device is equipped with a PHOIBOS-150 hemispherical analyzer. Pressure in the chamber was ~1.06  10–11 MPa. The XPS excitation source was Mg-K radiation (E = 1253.6 eV). The energy shift due to changes in charging of the sample was taken into account by XPS-C-1s (285.0 eV) of hydrocarbons. The nanohardness and elastic modulus of the films were determined using a G200 nanoindenter equipped with a Berkovich pyramid tip with a load of 9–13 mN. This loading range was chosen to ensure nanoindentation avoiding the influence of the substrate material. Four measurements were performed for each sample. Loading P and displacement L continuously increased; in particular, L increased to 200 nm at a constant strain rate of 0.05 sec–1. Nanohardness H and elastic modulus were determined with the Oliver–Pharr method [10]. The Knoop hardness (HK) was measured under a load of 100 mN with a Micromet 2103 Microhardness Tester (Buehler Ltd). Hardness was determined by five impressions. The nanohardness and Knoop hardness of a crystalline silicon plate (KDB-10) were H = 12.6 GPa and HK = 12.8 GPa (which is close to published data: 12–13 GPa). Coatings with the greatest thickness were also subjected to microindentation. The microhardness values matched the Knoop hardness with an accuracy to 5%. Scratch tests were performed with a Micron-Gamma Tester with a Vickers pyramid with a radius close to 1.0 µm. The tests were performed under a constant load of 0.01 N/sec at a distance of 158–230 µm. The results were used to evaluate the friction coefficient. The tests were conducted at room temperature and 50% humidity.

RESULTS AND DISCUSSION Monolayer TiN Coatings. Figure 1 shows typical Auger distribution of elements across the thickness of monolayer TiN coatings produced in different deposition conditions (Table 1). The coatings consist of about 60 at.% N, 30 at.% Ti, 7 at.% C, and 3 at.% O. Despite relatively high prevacuum in the chamber, there are carbon and oxygen impurities that are desorbed from chamber walls in the deposition process. Nitrogen content above 50 at.% may be due to penetration of molecular nitrogen between the grains. Figure 2 summarizes XRD results for the TiN coatings. All diffraction patterns show TiN(200) reflection at about 42.6. Intensity of the other peak, TiN (111) reflection, at about 36.8 depends on the gas flow rate. This peak tends to decrease with increasing FAr and FN2. The coatings are nanocrystalline, TiNx nanocrystallites reaching 15– 17 nm (SP-8, SP-12, and SP-21) and 4 nm (SP-23). Since no Ti–O or Ti–C system is revealed in XRD spectra, we assume that oxygen and carbon are mainly distributed along grain boundaries in the monolayer coatings produced with a titanium target. The nanohardness and elastic modulus of monolayer TiN coatings versus the nanoindenter penetration depth are shown in Fig. 3. It is evident that the nanohardness of the coatings deposited by sputtering a titanium target weakly depends on production conditions and is about 25 GPa. The situation is different for the elastic

Fig. 1. Auger distribution of elements versus sputtering time for TiN coatings 75

Fig. 2. X-ray diffraction spectra of TiN coatings deposited in different conditions

Fig. 3. Nanohardness and elastic modulus of TiN coatings depending on nanoindenter movement

modulus: it is the lowest in case of the smallest grains (SP-23) and reaches 250 GPa, compared with 280–300 GPa for other coatings. Refinement of nanocrystallites reduces the likelihood of cracking on the one hand but affects cohesion because the share of boundaries with weak bonds increases on the other hand. As a result, nanohardness would increase and elasticity decrease. The hardness and elastic modulus of the coating deposited with a titanium nitride target are the lowest. In our opinion, this is due to a high oxygen content of these coatings (to 6 at.% in SP-21 versus 3 at.% in other TiN coatings). Monolayer BCN Coatings. X-ray analyses indicate that the BCN coating is amorphous. Figure 4 shows the FTIR absorption spectrum of a monolayer coating of amorphous boron carbonitride (a-BCN). The absorption bands at 780 and 1376 cm–1 are due to B—N oscillations in hexagonal (h) BN between and inside the planes, respectively [11]. The band at 1106 cm–1 can be attributed to oscillations of B—N bonds in c-BN and B—C oscillations [9, 11] and the band at ~1500 cm–1 to B—C oscillations and C=N bonds [11]. The absorption band at 3400 cm–1 results from O—H oscillations [11]. Hence, the coatings formed on a silicon plate by sputtering a B4C target in the presence of nitrogen commonly represent a B–N network with some B—C and O—H complexes. The nanohardness and elastic modulus of the a-BCN coating are ~10 and ~100 GPa. Nanolayer TiN/BCN Coatings. Typical LA-XRR spectra of the nanolayer coatings (Cu-K radiation) are shown in Fig. 5. The peaks in spectra at