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Ti-B-N coatings deposited by magnetron arc evaporation
SUI/ace and Coatings Technology. 54/55 (1992) 255-256
255
Ti-B-N coatings deposited by magnetron arc evaporation M. Tamura and H. Kubo Advanced Materials and Tech. Res. Labs .. Nippon Steel Corporation, 1618 Ida. Nakahara-ku, Kawasaki-shi, Kanagawa 211 (Japan)
Abstract Ti-B-N films were deposited by reactive ion plating using magnetron arc evaporation and their fundamental coating properties were characterized. Titanium and boron were evaporated and ionized by the ionization electrode above the electron beam evaporator. Nitrogen and argon were mainly activated in a hot-cathode plasma-discharge within a parallel magnetic field near the substrates. The structure and crystallinity of the Ti-·B-N coatings were strongly influenced by the activation process and the chemical composition of the coatings. Deposition using both ionization of titanium in the arc plasma and activation of nitrogen in the hot-cathode plasma was very elTective for obtaining crystalline cubic Ti-B-N. With increasing addition of boron to TiN, the X-ray amorphous phase became dominant. The Ti-B-N films showed a lower friction coefficient and were more resistant against corrosion than TiN.
1. Introduction
The increasingly widespread use of hard coatings such as TiN is accompanied by ever stricter demands on their mechanical, physical and chemical properties. In order to meet these requirements, it is necessary to develop new, higher performance coating systems more closely matched to particular applications. The development of deposition techniques for BN-based ultrahard coatings is presently a subject of intensive research [IJ. However, these films are affected by high intrinsic compressive stress and turn out to be very brittle for practical application. In order to increase the hardness of Ti-N films and yet maintain good toughness, better results were obtained with coatings of the type Ti-B-N of composition based on TiB 2 . Mitterer et ai. [2J deposited various film types within the system Ti-B-C-N by sputtering from a TiB 2 target and obtained very hard films of up to 4800 Hv for the Ti-B-N type. These coatings had a hexagonal TiB 2 -xNx phase if prepared with a low nitrogen flow. Selbach et al. [3J reported that d. sputtered cubic Ti-B-N films could be obtained at a high total pressure and a low bias voltage. Matthes et al. [4J showed that the wear behavior of coated tools was remarkably improved by coating with the Ti-B-N films. Although Ti-B-N coatings of composition based on TiB 2 have been prepared by various workers, there are only a few results published concerning its behavior with respect to chemical composition and crystallographic phases [5]. The aim of this investigation was to develop a process for arc evaporation of ultrahard coatings within the system Ti-B-N. In addition, the fundamental coating
0257-8972/92/$5.00
properties of the Ti-B-N coatings were characterized. Addition of the element boron is expected to cause a further improvement in properties such as morphology, hardness, friction and corrosion resistance of the TiN films.
2. Experimental details 2.1. Film deposition Reactive ion plating with a magnetron arc evaporation unit was used to deposit the Ti-B-N coatings. The experimental system used in this study is shown in Fig. 1. The evaporation rate of boron to titanium was controlled in the range 0.0-1.0 by changing the power of the electron beams. The Ti-B-N film was deposited onto the d. biased (300 W) substrate in an argon-N, gas mixture. Titanium and boron were evaporated and ionized by the ionization electrode above the electron beam evaporators. Nitrogen and argon were mainly activated in a hot-cathode plasma-discharge within a parallel magnetic field near the substrates. Both plasmas could be operated separately and most experiments were done using both. Nitrogen partial pressure was varied in the range 0.02-0.08 Pa and the films were deposited mostly under 0.04 Pa nitrogen partial pressure. The total pressure was 0.08 Pa, the substrate temperature was held at 773 K, the deposition time was 60 min in every experiment. The deposition rate was 0.05 urn min - 1, giving a total film thickness of 3 urn. Several substrates were covered simultaneously: (1) silicon single crystals for glow discharge spectroscopy (GDS), X-ray diffraction (XRD) and scanning electron
©
1992 - Elsevier Sequoia. All rights reserved
256
M. Tamura, H. Kubo / Ti-B-N coatings
I~ 1
3. Results and discussion
2
2
IG 3
4
!;
mel reference electrode. Coated samples were prepared with the same exposed area, 1 em". The test solution was 0.1 N H zS0 4 .
5
l
l
Fig.T, Experimental set-up used to deposit Ti-B-N film: I, substrate;
2, water-cooled magnet; 3, tungsten filament: 4, inlet of argon and N 2 ; 5, ionization electrode; 6, r.f. bias; 7, evaporant source of titanium; 8, evaporant source of boron.
microscopy (SEM); (2) alloy tool steels (Cr12Mo) for friction coefficient measurement and XRD; (3) stainless steels (Crl8) for corrosion behavior; (4) NaCl single crystals for transmission electron microscopy (TEM); (5) cemented carbides for microhardness measurement. Prior to coating, the substrates were mechanically polished to an average roughness R. less than 0.1 urn. All the substrates were ultrasonically cleaned with methanol and acetone. The substrates were sputter etched before deposition for a period of 15 min. 2.2. Film characterization X-ray diffraction analysis and transmission electron diffraction (TED) were performed to analyze phases of the coatings. GDS was used to study the chemical composition of the coatings. The structure of the films was studied using SEM; The microhardness was measured using a Knoop microhardness tester and a load of 0.245 N. At least five indentations were made on each film. A CSEM scratch tester was used to measure the adhesion using a Rockwell C diamond and the friction coefficient using a steel pin (tip radius 2 mm). The latter was measured at a speed of 10 mm min- 1 and load of 10 N. Polarization measurements were performed to study the corrosion behavior in a conventional electrical cell with a platinum counter-electrode and a saturated calo-
3.1. Structure and crystallinity of the coatings The structure and crystallinity ofthe Ti-B-N coatings were strongly influenced by the activation process and chemical composition of the coatings. Deposition using both ionization of titanium in the arc plasma and activation of nitrogen in the hot-cathode plasma was very effective in obtaining sharp diffraction peaks of TiB-N as shown in Fig. 2. These corresponded to the peaks of the well known crystalline f.c.c. TiN. The broad peaks obtained by either ionization of titanium or activation of nitrogen, indicating an amorphous structure as shown later, agree with the peak positions of cubic TiN. No peaks of hexagonal TiB z or hexagonal BN were found. It is difficult to ionize boron itself since the ionization potential of boron is much higher than that of titanium [6]. Electrons could be released by ionization of titanium vapor and these could activate the boron vapor in the arc plasma by electron impact collisions or ion reactions as seen in the plasma-etching process [7]. In addition to this, the activation of nitrogen also contributes to form a highly crystalline cubic phase. This was studied by Murakawa et al. [8J, who showed that cubic stoichiometric BN could be synthesized using a hot-cathode plasma discharged within a parallel magnetic field. Both ionization of titanium in the arc plasma and activation of nitrogen are necessary to form a highly crystalline cubic Ti-B-N. Broadening and lowering of the diffraction peaks were found with decreasing partial pressure of nitrogen or increasing addition of boron to TiN. This was indicated by the loss of intensity of the main peaks of the films as shown in Fig. 3. Crystalline XRD peaks were not observed from Tio.5Bo.5No.5 films (Fig. 6). TED results are also indicative of a change in structure from crystalline to amorphous (Fig. 4). With increasing boron content in the film, the diffraction pattern changed from sharp rings of spots for TiN to diffuse rings for the Ti-B-N film. The diffraction pattern of sharp rings of spots matches the f.c.c. ring pattern of a cubic TiN-like structure. Indexing of (hkl) is possible from the inner spots as (Ill), (200), (220), (311) and (222). TEM images of the TiN and Tio.8Bo.zNo.z films are shown in Fig. 5. It can be seen that the Ti-B-N fihn is dense with a very fine grain size ranging from 5 to 50 nm (b), whereas that of the TiN film ranges from 20 to 200 nm (a). Smaller grains were observed with increasing boron content in the film. No grain boundaries were
M. Tamura. H. Kllbo / Ti-B-N coatings (a)
Ti-B-N ( 111)
•
......,.
~
257
4000,...---------------, pN2 ~ O.08Pa ~ O.04Pa --+- O.03Pa 3000 - t r - O.02Pa
t"I.l
.....= ~
=
2000
~
1000
0.1
0.2
x in (TiN)
0.3
I-x
0.4
0.5
Bx
Fig. 3. Effect of partial pressure of nitrogen during deposition and chemical composition of the coatings upon the XRD intensity of the (111) peak of the Ti-B-N films.
30
40
28
(deg.)
(b)
• (c)
Fig. 2. XRD diagrams of Ti-B-N coatings: (a) both ionization of titanium in the arc plasma and activation of nitrogen in the hotcathode plasma in the parallel magnetic field; (b) activation of nitrogen only; (c) ionization of titanium only.
observed in the Tio.sBo.sNo.s film using the same magnification as in Fig. 5. It is concluded that the Tio.sBo.sNo.s film has an amorphous structure from the lack of crystalline XRD peaks (Fig. 6), from the homogeneous structure and lack of grain boundaries observed using TEM, and from the diffraction pattern of diffuse rings (Fig. 4). SEM examination of a fracture cross-section revealed a fine, dense fibrous crystal structure within the film and good bonding of the film to the substrate. This finecolumnar structure changed to a fracture-amorphous structure with increasing boron content in the film.
3.2. Chemical composition
Stable phase compositions were estimated at 773 K and the chemical compositions of Ti-B-N films studied are plotted in Fig. 7. The GDS depth profile of a film is shown in Fig. 8. Almost homogeneous films are obtained. In Fig. 7 the chemical compositions are plotted along the line between TiN and boron, and the ratio of Ti: B of films deposited is close to unity. Therefore, the chemical composition of the films could be written as (Ti'N}, -xBx, while this does not attest a solid solution between TiN and boron. According to the ternary phase diagram [9J, at 1227 K the compositions of films deposited in this study are in the stability region of a cubic TiN, a hexagonal BN and a hexagonal TiB2 phase. However, no diffraction peaks derived from hexagonal BN or hexagonal TiB 2 were observed, as shown before. One may conclude that the cubic Ti-B-N films are deposited with substitution of titanium and nitrogen atoms by boron atoms in the cubic TiN framework in a highly activated state of non-equilibrium enhanced by the plasmas. The amorphous phase identified by XRD and TED with increasing boron content in the films may imply that the cubic TiN framework could not be sustained in films containing a large amount of boron atoms. 3.3. Microhardness
The Knoop microhardness values of selected coatings deposited onto cemented carbide substrates were measured. The hardness value of the substrates was 1500. The results are plotted against the chemical composition of the films in Fig. 9. The maximum value of hardness was 3800, which is higher than that of TiN, and was
258
:\J. Tamllra. H . Kubo I Ti-B -N coutings
'I
(al
5 01r.l
Fig. 5. TEM images of deposited films: (a) T iN , (b) Tio.sDo.2No.B'
obtained for the film containing small amounts of boron in TiN . With increasing boron content in TiN, the hardness value decreases to 170. No cracks appeared in the hard Ti-B-N films with a relatively high load of 0.98 N, thu s indicating that the film has good toughness, whereas some cracks were observed around the indentation on the soft and glassy film whose hardness value was below 1000, It is suggested that the micro hardness is greatly affected by the crystallinity of the film. A high hardness value is obtained on the film composed of crystalline cubic Ti -B-N. Fig. 4. Selected area diffraction patterns: (n)- TiN. (b) Tio.sBo. 2No.B' (c) Tio.sBo .sNo .s·
3.4. Fri ction coefficient The relationship between the sliding friction coefficient and boron content in the films is shown in Fig. 10. The average roughness values of the Ti-B-N films are similar
259
M. Tamura. H. Kuoo / Ti-B-N coatings
1.0 r--
(a)
r
o
::::
- - - - - - - - - - - - ---,
0.8
~
~
u
s
I
I
o -..
~LA.d-.JI:...-L::i=b.,..kd,=L-.b=IC.::L.L-l
--0--0--II-
TI
--fi-
Si
n N
0.6 0.4
~
o. 0 l-Q--6-1Cr--lJ--tr-b-ler--llr-tr-tr-'ir-f.~_tx'__fr_{j;:a..::l--...J 2.0
1.0
0 .0
3 .0
Dept h (m icr o-m eter)
(b)
Fig. R. G DS depth profile of a Ti-B-N film.
-..
4000
I
..::.=
;,
3000
I--...
~
(
40
50
28 (deg.)
2000
-,
~
Fig. 6. XRD diagrams of (a) steel substrate and (b) Tio.sBo.sNo.5 coatings deposited onto steel substrates. The vertical axis is the X-ray intensity in arbitrary units.
~\
1000
o
0.0
B
\) 0.1
0.2
0.3
x in (TiN)
I-x
0.4
B
0.5
x
Fig. 9. Microhardness of coatings as a function of the chemical composition.
(R a = 0.02 urn] and independent of boron content. The
addition of boron to TiN is very effective in decreasing the friction coefficient. However, the wear resistance of the films containing a large amount of boron is insufficient because of the low hardness of the surface as shown in Fig. 9. The friction coefficient and microhardness of the films can be chosen to match particular applications by controlling the chemical composition.
Ti
TaN
TiN
N
Fig. 7. Chemical compositions studied plotted on a ternary phase diagram for the Ti-B-N system at 773 K.
3.5. Corrosion resistance
Figure 11 shows a potentiostatic polarization curve of a Ti- B- N coating. It is demonstrated by a comparison
M. Tamura, H. Kubo
260
.....
= ......t::
0.5
~
0.4
1\
e
~
e ........= ... ~
0.3 0.2
~
\ r--..)
I.
~
0.1
0.0
0.1
0.2
0.3
x in (TiN)
1-x
0.4
B
0.5
x
Fig. 10. Relationship between the sliding friction coefficient and chemical compositions of Ti-B-N films.
1.5
--. ~
1.0
CZl
00
;>
0.5
~
.~ 0
1 Non coating 2 TiN 3 TiO.5BO.sN 0.5
A.. 0
-0.5
of passive currents that the Ti-B-N coating is more corrosion resistant than the TiN coating, and both of them are superior to stainless steel. No corrosion pits were observed on the tested surface of either coating. Some pin-holes [10J or small particles were reported in solid crystalline films and they induce and enhance the corrosion of the hard surface of the films. It is considered that the amorphous structure obtained with increased boron content in the films decreases "defects" such as pin-holes and as a result the Ti-B-N coating becomes more corrosion resistant than the TiN coating.
4. Conclusions
The following conclusions may be drawn from the observations presented above. (1) A cubic Ti-B-N film is effectively obtained by using both the ionization of titanium in the arc plasma and the activation of nitrogen in the parallel magnetic field. (2) The X-ray amorphous structure becomes dominant with increasing boron content in the Ti-B-N films. (3) The Ti-B-N films showed a lower friction coefficient and were more resistant against corrosion than TiN.
References
~
:> "-"
I Ti-B-N coatings
i--
10-3
_
10-2
10-1
toe
101
Current density (Ajm2) Fig. 11. Anodic polarization curves of Ti-B-N coated samples and stainless steel in corrosion medium 0.1 N H 2S04 ,
I S. Mineta, M. Kohata, N. Yasunaga and Y. Kikuta, Thin Solid Films, 189 (1990) 125. 2 C. Mitterer, M. Rauter and P. Rodhammer, Surf. Coat. Techno!', 41 (1990) 351. 3 E. Selbach, K. Schmidt and M. Wang, Thin Solid Films. 188 (1990) 267 . 4 B. Matthes, E. Broszeit and K. H. Kloos, Surf. Coat. Technol., 43/44 (1990) 721. 5 L. S. Wen, X. Z. Chen, Q. Q. Yang, Y. Q. Zheng and Y. Z. Chuang, Surf. Eng.• 6 (1990) 41. 6 C. E. Moore, Analyses of Optical Spectra, NSRDS-NBS 34, (Office of Standard Reference Data, National Bureau of Standards, Washington, DC). 7 M.1. Kushner, J. App!. Phys., 53 (4) (1982) 2923. 8 M. Murakawa, S. Watabe and Y. Sugimoto, J. Jpn. Soc. Powder Powder Metall., 37 (1990) 283. 9 Nowotny, Benesovsky, Brukl and Schob, Monatsh. Chem., 92 (1961) 409. 10 K. Takizawa, M. Fukushima and H. Imai, J. Surf Finish. Soc. Jpn., 42 (1991) 102.
255
Ti-B-N coatings deposited by magnetron arc evaporation M. Tamura and H. Kubo Advanced Materials and Tech. Res. Labs .. Nippon Steel Corporation, 1618 Ida. Nakahara-ku, Kawasaki-shi, Kanagawa 211 (Japan)
Abstract Ti-B-N films were deposited by reactive ion plating using magnetron arc evaporation and their fundamental coating properties were characterized. Titanium and boron were evaporated and ionized by the ionization electrode above the electron beam evaporator. Nitrogen and argon were mainly activated in a hot-cathode plasma-discharge within a parallel magnetic field near the substrates. The structure and crystallinity of the Ti-·B-N coatings were strongly influenced by the activation process and the chemical composition of the coatings. Deposition using both ionization of titanium in the arc plasma and activation of nitrogen in the hot-cathode plasma was very elTective for obtaining crystalline cubic Ti-B-N. With increasing addition of boron to TiN, the X-ray amorphous phase became dominant. The Ti-B-N films showed a lower friction coefficient and were more resistant against corrosion than TiN.
1. Introduction
The increasingly widespread use of hard coatings such as TiN is accompanied by ever stricter demands on their mechanical, physical and chemical properties. In order to meet these requirements, it is necessary to develop new, higher performance coating systems more closely matched to particular applications. The development of deposition techniques for BN-based ultrahard coatings is presently a subject of intensive research [IJ. However, these films are affected by high intrinsic compressive stress and turn out to be very brittle for practical application. In order to increase the hardness of Ti-N films and yet maintain good toughness, better results were obtained with coatings of the type Ti-B-N of composition based on TiB 2 . Mitterer et ai. [2J deposited various film types within the system Ti-B-C-N by sputtering from a TiB 2 target and obtained very hard films of up to 4800 Hv for the Ti-B-N type. These coatings had a hexagonal TiB 2 -xNx phase if prepared with a low nitrogen flow. Selbach et al. [3J reported that d. sputtered cubic Ti-B-N films could be obtained at a high total pressure and a low bias voltage. Matthes et al. [4J showed that the wear behavior of coated tools was remarkably improved by coating with the Ti-B-N films. Although Ti-B-N coatings of composition based on TiB 2 have been prepared by various workers, there are only a few results published concerning its behavior with respect to chemical composition and crystallographic phases [5]. The aim of this investigation was to develop a process for arc evaporation of ultrahard coatings within the system Ti-B-N. In addition, the fundamental coating
0257-8972/92/$5.00
properties of the Ti-B-N coatings were characterized. Addition of the element boron is expected to cause a further improvement in properties such as morphology, hardness, friction and corrosion resistance of the TiN films.
2. Experimental details 2.1. Film deposition Reactive ion plating with a magnetron arc evaporation unit was used to deposit the Ti-B-N coatings. The experimental system used in this study is shown in Fig. 1. The evaporation rate of boron to titanium was controlled in the range 0.0-1.0 by changing the power of the electron beams. The Ti-B-N film was deposited onto the d. biased (300 W) substrate in an argon-N, gas mixture. Titanium and boron were evaporated and ionized by the ionization electrode above the electron beam evaporators. Nitrogen and argon were mainly activated in a hot-cathode plasma-discharge within a parallel magnetic field near the substrates. Both plasmas could be operated separately and most experiments were done using both. Nitrogen partial pressure was varied in the range 0.02-0.08 Pa and the films were deposited mostly under 0.04 Pa nitrogen partial pressure. The total pressure was 0.08 Pa, the substrate temperature was held at 773 K, the deposition time was 60 min in every experiment. The deposition rate was 0.05 urn min - 1, giving a total film thickness of 3 urn. Several substrates were covered simultaneously: (1) silicon single crystals for glow discharge spectroscopy (GDS), X-ray diffraction (XRD) and scanning electron
©
1992 - Elsevier Sequoia. All rights reserved
256
M. Tamura, H. Kubo / Ti-B-N coatings
I~ 1
3. Results and discussion
2
2
IG 3
4
!;
mel reference electrode. Coated samples were prepared with the same exposed area, 1 em". The test solution was 0.1 N H zS0 4 .
5
l
l
Fig.T, Experimental set-up used to deposit Ti-B-N film: I, substrate;
2, water-cooled magnet; 3, tungsten filament: 4, inlet of argon and N 2 ; 5, ionization electrode; 6, r.f. bias; 7, evaporant source of titanium; 8, evaporant source of boron.
microscopy (SEM); (2) alloy tool steels (Cr12Mo) for friction coefficient measurement and XRD; (3) stainless steels (Crl8) for corrosion behavior; (4) NaCl single crystals for transmission electron microscopy (TEM); (5) cemented carbides for microhardness measurement. Prior to coating, the substrates were mechanically polished to an average roughness R. less than 0.1 urn. All the substrates were ultrasonically cleaned with methanol and acetone. The substrates were sputter etched before deposition for a period of 15 min. 2.2. Film characterization X-ray diffraction analysis and transmission electron diffraction (TED) were performed to analyze phases of the coatings. GDS was used to study the chemical composition of the coatings. The structure of the films was studied using SEM; The microhardness was measured using a Knoop microhardness tester and a load of 0.245 N. At least five indentations were made on each film. A CSEM scratch tester was used to measure the adhesion using a Rockwell C diamond and the friction coefficient using a steel pin (tip radius 2 mm). The latter was measured at a speed of 10 mm min- 1 and load of 10 N. Polarization measurements were performed to study the corrosion behavior in a conventional electrical cell with a platinum counter-electrode and a saturated calo-
3.1. Structure and crystallinity of the coatings The structure and crystallinity ofthe Ti-B-N coatings were strongly influenced by the activation process and chemical composition of the coatings. Deposition using both ionization of titanium in the arc plasma and activation of nitrogen in the hot-cathode plasma was very effective in obtaining sharp diffraction peaks of TiB-N as shown in Fig. 2. These corresponded to the peaks of the well known crystalline f.c.c. TiN. The broad peaks obtained by either ionization of titanium or activation of nitrogen, indicating an amorphous structure as shown later, agree with the peak positions of cubic TiN. No peaks of hexagonal TiB z or hexagonal BN were found. It is difficult to ionize boron itself since the ionization potential of boron is much higher than that of titanium [6]. Electrons could be released by ionization of titanium vapor and these could activate the boron vapor in the arc plasma by electron impact collisions or ion reactions as seen in the plasma-etching process [7]. In addition to this, the activation of nitrogen also contributes to form a highly crystalline cubic phase. This was studied by Murakawa et al. [8J, who showed that cubic stoichiometric BN could be synthesized using a hot-cathode plasma discharged within a parallel magnetic field. Both ionization of titanium in the arc plasma and activation of nitrogen are necessary to form a highly crystalline cubic Ti-B-N. Broadening and lowering of the diffraction peaks were found with decreasing partial pressure of nitrogen or increasing addition of boron to TiN. This was indicated by the loss of intensity of the main peaks of the films as shown in Fig. 3. Crystalline XRD peaks were not observed from Tio.5Bo.5No.5 films (Fig. 6). TED results are also indicative of a change in structure from crystalline to amorphous (Fig. 4). With increasing boron content in the film, the diffraction pattern changed from sharp rings of spots for TiN to diffuse rings for the Ti-B-N film. The diffraction pattern of sharp rings of spots matches the f.c.c. ring pattern of a cubic TiN-like structure. Indexing of (hkl) is possible from the inner spots as (Ill), (200), (220), (311) and (222). TEM images of the TiN and Tio.8Bo.zNo.z films are shown in Fig. 5. It can be seen that the Ti-B-N fihn is dense with a very fine grain size ranging from 5 to 50 nm (b), whereas that of the TiN film ranges from 20 to 200 nm (a). Smaller grains were observed with increasing boron content in the film. No grain boundaries were
M. Tamura. H. Kllbo / Ti-B-N coatings (a)
Ti-B-N ( 111)
•
......,.
~
257
4000,...---------------, pN2 ~ O.08Pa ~ O.04Pa --+- O.03Pa 3000 - t r - O.02Pa
t"I.l
.....= ~
=
2000
~
1000
0.1
0.2
x in (TiN)
0.3
I-x
0.4
0.5
Bx
Fig. 3. Effect of partial pressure of nitrogen during deposition and chemical composition of the coatings upon the XRD intensity of the (111) peak of the Ti-B-N films.
30
40
28
(deg.)
(b)
• (c)
Fig. 2. XRD diagrams of Ti-B-N coatings: (a) both ionization of titanium in the arc plasma and activation of nitrogen in the hotcathode plasma in the parallel magnetic field; (b) activation of nitrogen only; (c) ionization of titanium only.
observed in the Tio.sBo.sNo.s film using the same magnification as in Fig. 5. It is concluded that the Tio.sBo.sNo.s film has an amorphous structure from the lack of crystalline XRD peaks (Fig. 6), from the homogeneous structure and lack of grain boundaries observed using TEM, and from the diffraction pattern of diffuse rings (Fig. 4). SEM examination of a fracture cross-section revealed a fine, dense fibrous crystal structure within the film and good bonding of the film to the substrate. This finecolumnar structure changed to a fracture-amorphous structure with increasing boron content in the film.
3.2. Chemical composition
Stable phase compositions were estimated at 773 K and the chemical compositions of Ti-B-N films studied are plotted in Fig. 7. The GDS depth profile of a film is shown in Fig. 8. Almost homogeneous films are obtained. In Fig. 7 the chemical compositions are plotted along the line between TiN and boron, and the ratio of Ti: B of films deposited is close to unity. Therefore, the chemical composition of the films could be written as (Ti'N}, -xBx, while this does not attest a solid solution between TiN and boron. According to the ternary phase diagram [9J, at 1227 K the compositions of films deposited in this study are in the stability region of a cubic TiN, a hexagonal BN and a hexagonal TiB2 phase. However, no diffraction peaks derived from hexagonal BN or hexagonal TiB 2 were observed, as shown before. One may conclude that the cubic Ti-B-N films are deposited with substitution of titanium and nitrogen atoms by boron atoms in the cubic TiN framework in a highly activated state of non-equilibrium enhanced by the plasmas. The amorphous phase identified by XRD and TED with increasing boron content in the films may imply that the cubic TiN framework could not be sustained in films containing a large amount of boron atoms. 3.3. Microhardness
The Knoop microhardness values of selected coatings deposited onto cemented carbide substrates were measured. The hardness value of the substrates was 1500. The results are plotted against the chemical composition of the films in Fig. 9. The maximum value of hardness was 3800, which is higher than that of TiN, and was
258
:\J. Tamllra. H . Kubo I Ti-B -N coutings
'I
(al
5 01r.l
Fig. 5. TEM images of deposited films: (a) T iN , (b) Tio.sDo.2No.B'
obtained for the film containing small amounts of boron in TiN . With increasing boron content in TiN, the hardness value decreases to 170. No cracks appeared in the hard Ti-B-N films with a relatively high load of 0.98 N, thu s indicating that the film has good toughness, whereas some cracks were observed around the indentation on the soft and glassy film whose hardness value was below 1000, It is suggested that the micro hardness is greatly affected by the crystallinity of the film. A high hardness value is obtained on the film composed of crystalline cubic Ti -B-N. Fig. 4. Selected area diffraction patterns: (n)- TiN. (b) Tio.sBo. 2No.B' (c) Tio.sBo .sNo .s·
3.4. Fri ction coefficient The relationship between the sliding friction coefficient and boron content in the films is shown in Fig. 10. The average roughness values of the Ti-B-N films are similar
259
M. Tamura. H. Kuoo / Ti-B-N coatings
1.0 r--
(a)
r
o
::::
- - - - - - - - - - - - ---,
0.8
~
~
u
s
I
I
o -..
~LA.d-.JI:...-L::i=b.,..kd,=L-.b=IC.::L.L-l
--0--0--II-
TI
--fi-
Si
n N
0.6 0.4
~
o. 0 l-Q--6-1Cr--lJ--tr-b-ler--llr-tr-tr-'ir-f.~_tx'__fr_{j;:a..::l--...J 2.0
1.0
0 .0
3 .0
Dept h (m icr o-m eter)
(b)
Fig. R. G DS depth profile of a Ti-B-N film.
-..
4000
I
..::.=
;,
3000
I--...
~
(
40
50
28 (deg.)
2000
-,
~
Fig. 6. XRD diagrams of (a) steel substrate and (b) Tio.sBo.sNo.5 coatings deposited onto steel substrates. The vertical axis is the X-ray intensity in arbitrary units.
~\
1000
o
0.0
B
\) 0.1
0.2
0.3
x in (TiN)
I-x
0.4
B
0.5
x
Fig. 9. Microhardness of coatings as a function of the chemical composition.
(R a = 0.02 urn] and independent of boron content. The
addition of boron to TiN is very effective in decreasing the friction coefficient. However, the wear resistance of the films containing a large amount of boron is insufficient because of the low hardness of the surface as shown in Fig. 9. The friction coefficient and microhardness of the films can be chosen to match particular applications by controlling the chemical composition.
Ti
TaN
TiN
N
Fig. 7. Chemical compositions studied plotted on a ternary phase diagram for the Ti-B-N system at 773 K.
3.5. Corrosion resistance
Figure 11 shows a potentiostatic polarization curve of a Ti- B- N coating. It is demonstrated by a comparison
M. Tamura, H. Kubo
260
.....
= ......t::
0.5
~
0.4
1\
e
~
e ........= ... ~
0.3 0.2
~
\ r--..)
I.
~
0.1
0.0
0.1
0.2
0.3
x in (TiN)
1-x
0.4
B
0.5
x
Fig. 10. Relationship between the sliding friction coefficient and chemical compositions of Ti-B-N films.
1.5
--. ~
1.0
CZl
00
;>
0.5
~
.~ 0
1 Non coating 2 TiN 3 TiO.5BO.sN 0.5
A.. 0
-0.5
of passive currents that the Ti-B-N coating is more corrosion resistant than the TiN coating, and both of them are superior to stainless steel. No corrosion pits were observed on the tested surface of either coating. Some pin-holes [10J or small particles were reported in solid crystalline films and they induce and enhance the corrosion of the hard surface of the films. It is considered that the amorphous structure obtained with increased boron content in the films decreases "defects" such as pin-holes and as a result the Ti-B-N coating becomes more corrosion resistant than the TiN coating.
4. Conclusions
The following conclusions may be drawn from the observations presented above. (1) A cubic Ti-B-N film is effectively obtained by using both the ionization of titanium in the arc plasma and the activation of nitrogen in the parallel magnetic field. (2) The X-ray amorphous structure becomes dominant with increasing boron content in the Ti-B-N films. (3) The Ti-B-N films showed a lower friction coefficient and were more resistant against corrosion than TiN.
References
~
:> "-"
I Ti-B-N coatings
i--
10-3
_
10-2
10-1
toe
101
Current density (Ajm2) Fig. 11. Anodic polarization curves of Ti-B-N coated samples and stainless steel in corrosion medium 0.1 N H 2S04 ,
I S. Mineta, M. Kohata, N. Yasunaga and Y. Kikuta, Thin Solid Films, 189 (1990) 125. 2 C. Mitterer, M. Rauter and P. Rodhammer, Surf. Coat. Techno!', 41 (1990) 351. 3 E. Selbach, K. Schmidt and M. Wang, Thin Solid Films. 188 (1990) 267 . 4 B. Matthes, E. Broszeit and K. H. Kloos, Surf. Coat. Technol., 43/44 (1990) 721. 5 L. S. Wen, X. Z. Chen, Q. Q. Yang, Y. Q. Zheng and Y. Z. Chuang, Surf. Eng.• 6 (1990) 41. 6 C. E. Moore, Analyses of Optical Spectra, NSRDS-NBS 34, (Office of Standard Reference Data, National Bureau of Standards, Washington, DC). 7 M.1. Kushner, J. App!. Phys., 53 (4) (1982) 2923. 8 M. Murakawa, S. Watabe and Y. Sugimoto, J. Jpn. Soc. Powder Powder Metall., 37 (1990) 283. 9 Nowotny, Benesovsky, Brukl and Schob, Monatsh. Chem., 92 (1961) 409. 10 K. Takizawa, M. Fukushima and H. Imai, J. Surf Finish. Soc. Jpn., 42 (1991) 102.