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Deposition of B, C, N coatings on WC–Co substrates—Analytical problems indicating c-BN formation
International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379 www.elsevier.com/locate/ijrmhm
Deposition of B, C, N coatings on WC–Co substrates—Analytical problems indicating c-BN formation Ronald Weissenbacher, Roland Haubner
¤
University of Technology Vienna, Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, A-1060 Vienna, Austria Received 30 October 2005; accepted 23 November 2005
Abstract BCN-layers were deposited in a cold-wall reactor by decomposition of Tris(dimethylamino)borane on inductively heated hardmetal (WC–Co) substrates. The substrate temperature was varied between 800 and 1200 °C. XRD measurements of the deposited layers showed weak peaks, that might be explained by crystalline BN-phases, beside strong peaks of the substrate phases WC and Co. Indications for the deposition of the BN-phases could also be found in IR spectra. For an explicit BN phase identiWcation thicker layers were deposited by changing certain deposition parameters, which should provide higher intensities of the XRD-peaks of the BN phases. However, with increasing layer thickness, the BN related peaks decreased, which indicates that the phase causing the XRD peak is located near the substrate/coating interface. Through investigations of substrate reactions the eta-phase (Co6W6C) was observed which showed XRD peaks similar to c-BN and h-BN. It could be proved, that the eta-phase is responsible for the XRD peaks but an explanation for the detected IR peaks of the BN phases could not yet be found. © 2005 Elsevier Ltd. All rights reserved. Keywords: Hardmetal; B, C, N coatings; Analytical characterization
1. Introduction Since the deposition of diamond with low-pressure methods has been reported, strong eVorts were made to Wnd a similar process for the deposition of c-BN layers. PVD methods like ion-beam assisted deposition [1], ion plating [2], laser induced deposition [3], RF- or magnetron sputtering [4] succeeded in depositing nano-c-BN Wlms. In consideration of the deposition conditions which are necessary for the c-BN formation, the c-BN areas are very small [5]. The deposition of nano-c-BN was also reported by using diVerent plasma CVD methods to activate the gas phase. Typically used plasma CVD methods are for example ICP
* Corresponding author. Tel.: +43 1 58801 16128; fax: +43 1 58801 16199. E-mail address: [email protected] (R. Haubner).
0263-4368/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.11.010
CVD [6], bias enhanced plasma CVD [7] or ECR plasma CVD [8]. Up to now, several BN phases could also be deposited by using conventional thermal CVD for gas activation, but the deposition of c-BN or nano-c-BN could never be reported. In the Wrst phase diagram, which was published by Bundy and Wentorf [9], c-BN was considered to be the stable phase at standard conditions. In 1975 Corrigan and Bundy [10] postulated that the c-BN was metastable according to the carbon system (in which diamond—the cubic phase—is metastable at standard conditions). That fact was accepted until the late 1980s. Since 1987 a lot of papers have been published [11–14] showing c-BN to be the stable phase at standard conditions, but with lots of diVerences concerning the correct position of the equilibrium line between the h-BN and c-BN. First decomposition experiments of Tris(dimethylamino)borane on inductively heated hardmetal substrates
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
were carried out by Haubner and Tang [15]. Deposition of BCN layers in an argon atmosphere at standard pressure were carried out at substrate temperatures above 800 °C. It was possible to increase the growth rate by increasing the precursor Xow rate or by increasing the substrate temperature. The analysis of layers deposited at 800 °C showed some hints of the deposition of c-BN in XRD and IR-spectroscopy but TEM-measurements showed no c-BN crystallites. The intention for the present paper was to reproduce and to upgrade the results published earlier [15]. 2. Experimental set-up and sample characterization 2.1. Decomposition process For the deposition of BCN-layers by the decomposition of the single source precursor Tris(dimethylamino)borane, a quartz tube cold wall reactor with a diameter of 50 mm was used (Fig. 1) [16]. To adjust the precursor concentration for the reaction an argon stream was saturated with the precursor in an evaporation unit [17,16] and afterwards diluted with an additional pure argon stream. Using this procedure the partial pressure of the precursor in the gas stream was varied but the mass transport was constant. The two argon streams were regulated by mass Xow controllers. The gas mixture was introduced into the reactor from the top. Hardmetal substrates (K20 SPGN WC 6% Co) were placed on a graphite block inside the reactor, which was heated up to 1200 °C by medium frequency induction (10 kHz) in an argon atmosphere at atmospheric pressure. The precursor decomposed on the substrate surface and formed BCN layers. The substrate temperature was measured by a Wlament pyrometer (IRCON IR-UC). Scanning electron microscopy (JEOL 6400) was used to investigate the morphology of the layers. The deposited phases were characterized by X-ray diVraction (CuK radiation), infra-
375
red spectroscopy (Biorad FTS 175 equipped with a microscope) and Raman spectroscopy. All examinations were normally carried out in the center of the samples. The layer growth rate was determined by gravimetric analyses. As the density of the layers vary with the chemical and phase composition, the layer thickness was not calculated in m. 2.2. Characterization of BN phases The characterization of diamond layers is rather simple by using Raman spectroscopy [18] but a lot of diYculties occur by the analyses of phases of BN or BCN systems. The task becomes especially challenging without the knowledge of the elemental composition of the samples [15,19]. X-ray diVraction can be used to characterise c-BN and h-BN, but many cubic phases (of the B, C, N system or of substrate materials) show peaks at the same position as c-BN. The peak positions and their intensities are mainly inXuenced by the grain size, stress within the layers or other phases present. The characterisation of layers deposited on hardmetal substrates is additionally disturbed by very strong WC peaks. Infrared spectroscopy or Fourier transformed infrared spectroscopy peaks for c-BN are reported in literature as follows: for c-BN the transversal optical (TO) mode at a wavenumber of 1065 cm¡1 and the longitudinal optical (LO) mode at 1340 cm¡1 are described by Gielisse et al. [20]. If commercially available c-BN is investigated, usually one peak between 1050 and 1100 cm¡1 is observed. Peaks of w-BN, B4C and B2O3 can be found in the same area. Raman spectroscopy can also be used to identify c-BN or to distinguish between the c-BN and h-BN. For c-BN two peaks are described in literature; a TO-mode at 1055– 1057 cm¡1 and a LO-mode at 1305–1306 cm¡1 [21,22]. For h-BN a peak is observed at 1367 cm¡1 [22].
TC…temperature control C…gas cleaning R…rotation pump (needed for evacuation/ flushing)
P
P Sample TC
Precursor
R
MFC
C C
Fig. 1. Apparatus for the deposition of BCN-layers by the decomposition of a B, C, N containing single source precursor on inductively heated samples.
376
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
Fig. 2. Characterization of BCN layers deposited at a precursor Xow rate of 0.53 mg/min, a total argon Xow rate of 84 sccm and a substrate temperature of 800 °C. (a, b) SEM pictures, (c) XRD diagram, (d) IR spectrum.
The aim of this work was to identify crystalline BN-phases in layers, which were deposited on hardmetal substrates by thermal decomposition as described elsewhere [15]. At Wrst layers of higher thickness were grown and in a second step substrate reactions were screened. 3.1. Reproducing of literature data At Wrst the experiments described in literature [15] were reproduced and achieved similar results. The substrate was heated to a surface temperature of 800 °C, the precursor Xow rate was 32 mg/h, the total argon Xow rate was 84 sccm and the pressure in the reactor chamber 1013 mbar. After 6 h a layer with a growth rate of 0.23 mg h¡1 cm¡2 was deposited. Fig. 2(a) and (b) shows typical SEM pictures of the layers, which were deposited under the described conditions. The surface is covered with spherical, ball like structures with a diameter up to 5 m. X-ray diVractograms (Fig. 2(c)) of the deposited layers were similar to the ones observed earlier [15]. Beside the strong peaks of the WC phase and a rather small Co peak, two additional peaks could be found at 2 40.7° and 43.2°, which are corresponding to the strongest peaks of h-BN and c-BN in XRD. Other peaks of h-BN or c-BN could not be identiWed due to the strong WC peaks. In the IR spectra, peaks at 814 and 1088 cm¡1 were found (Fig. 2) which cor-
respond to the h-BN peaks observed at 780 and 828 cm¡1 and the c-BN peak at 1065 cm¡1 [20]. 3.2. Growth of BCN layers with increasing layer thickness The following experiments were carried out to grow thicker layers which should result in more explicit peaks of the BN phases in XRD. Several parameters can be changed to achieve thicker layers. The substrate temperature and the partial pressure of the precursor in the gas stream were varied (Fig. 3). The partial pressure was changed by mixing the precursor Xow 20
layer thickness [mg/cm2]
3. Results and discussion
16
1000°C
12 8
800°C
4 0 0.5
1
1.5
2
2.5
3
3.5
partial pressure of the precursor [mbar]
Fig. 3. Changes in the deposition rate as a function of the partial pressure of the precursor in Ar at two diVerent temperatures (precursor Xow rate: 0.53 mg/min, total Ar Xow rate 28–84 sccm).
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
377
Fig. 4. Layer thickness and SEM morphology as a function of the deposition times 1, 4 and 6 h (800 °C, precursor Xow rate: 0.53 mg/min; total Ar Xow rate: 84 sccm).
with additional argon in the ratio of 1:0, 1:1 or 1:2 (constant precursor Xow at increased velocity). It could be shown that the inXuence of the substrate temperature on the growth rate is rather strong. A temperature increase from 800 to 1000 °C results in a three times higher deposition rate. The highest deposition rates in these series of experiments could be reached at 1000 °C with the highest partial pressure of the precursor in the Ar stream (lowest Ar Xow rate). Also experiments with increasing deposition time from 1 to 6 h were carried out. These experiments were run at a precursor Xow rate of 32 mg/h, a total argon Xow of 84 sccm and a substrate temperature of 800 °C (Fig. 4). An almost linear relationship between the deposition time and layer thickness was observed. SEM investigations of the layers, which were deposited within one hour still show the surface of the hardmetal substrate, where scratches are visible. The nucleation of the BCN layer starts along the scratches on the surface. After 6 h of deposition a close layer on the substrate surface is observed (Fig. 4). However, the increased layer thickness never resulted in an increase of the BN peaks. The ratio c-BN/WC got even worse, and at a certain layer thickness no peaks of the BN phases could be detected any more. X-ray diVractograms of the deposited layers showed higher peaks of the BN phases for thinner layers. In Fig. 5 the ratio between the peak at 43.2° and the highest peak of the tungsten-carbide phase at 35.6° is shown as a function of the layer thickness. Although X-ray measurements of layers, which were grown at other conditions (diVerent substrate temperature, deposition time, partial precursor pres-
sure) indicated, that the ratio between the two peaks increases with decreasing layer thickness. This indicates that the phase, which causes the peaks at 43.2° and 40.7°, is located near the substrate/layer interface and not in the layer. The IR-spectra of the deposited layers are similar for all diVerent layer thicknesses, with only small changes in the wavenumber of the peaks (Fig. 5). Two of the observed peaks correspond to the cubic and the hexagonal BN phases. No satisfying explanation for these peaks could be found. 3.3. Reactions of the hardmetal substrate during BCN deposition From the experiments with samples of increasing layer thickness it could be shown that the phase described b the two unknown peaks is located near the substrate/layer interface. Possible compounds, which can be formed during the deposition, were checked by comparing diVraction peaks (e.g. ASTM 03-0959 CoB, 25-0241 Co2B, 12-0443 Co3B) [23]. Only in case of Co6W6C the peaks were similar to the XRD diagrams observed during BCN deposition (ASTM 23-0939 Co6W6C). Nevertheless the formation of Co6W6C during BCN deposition is surprising, because eta-phases are the result of a decarburisation of the hardmetal substrate, and the precursor used for deposition contains C:N:B in a ratio of 6:3:1. Therefore a hardmetal substrate was heated to 800 °C in an argon stream without a precursor Xow. XRD of the
378
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
ratio c-BN peak/WC peak [%]
30 25 20 15 10 5 0 0
5
10
15
20
25
30
deposited layer [mg] thin layer (9.3 mg deposited)
thin layer (2.9 mg deposited)
intensity
intensity
WC
X R D
c-BN
35
40
45
50
30
35
c-BN
(780) (828) (1065)
700
C (sp2)
1122,80 823,80
1566,50 h-BN
40
45
50
2*theta
absorption
h-BN
1066,80
absorption
IR
812,22
2*theta
1585,80
30
(1380) (1530-1590)
900 1100 1300 1500 1700 1900 2100
wavenumber [cm-1]
700
900 1100 1300 1500 1700 1900 2100
wavenumber [cm-1]
Fig. 5. The ratio of the c-BN peak and the highest WC peak in XRD as a function of the layer thickness and XRD diVractograms and IR spectrums of layers with a thickness of 2.9 mg, respectively, 9.3 mg.
substrate which was treated as described showed peaks which correspond to c-BN and h-BN, but with higher intensity. The peaks at 43.2° and 40.7° as well as additional peaks at 32.9° and 47.2° belong to the phase Co6W6C, a cubic so called eta-phase in the tungsten/carbon/cobalt system. To prove the existence of the eta-phase on a hardmetal substrate, the surface was etched with Murakami solution
(10% NaOH solution mixed with 10% K3[Fe(CN)6] solution) for 10 s. Thereby the eta-phase Co6W6C is oxidized selectively, resulting in a tarnished substrate surface of a dark gray color. Rough structures growing out of the substrate surface are observed by scanning electron microscopy (Fig. 6). The eta-phase was observed on the substrates treated without a precursor as well as on a substrate where the deposited layer was removed by polishing.
Fig. 6. SEM pictures of substrate surfaces showing the eta-phase after Murakami etching: (a) substrate heated in pure argon to 800 °C, (b) substrate after removing the BCN layer.
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
At least layers were deposited on Mo substrates. No XRD peaks corresponding to c-BN were found but with IR the peaks were observed again. 4. Conclusions By the use of PVD methods or plasma assisted CVD methods high accelerated ions are used to deposit nano-c-BN layers with a rather high stress within the layers. A simple chemical way for the low-pressure deposition of c-BN layers would be of high interest for industrial applications. Compared with the low-pressure diamond deposition some diYculties appear in the BN or BCN system. Firstly there are still very inconsistent thermodynamic data available, secondly the analyses of BN phases, especially of c-BN, is rather diYcult. In our case a rather simple way was used to produce BCN layers from the single source precursor Tris(dimethylamino)borane by thermal decomposition. XRD which is a good method to identify crystalline phases, was used for Wrst investigations of the deposited layers. The received peaks, gave Wrst hints for the deposition of h-BN and c-BN. But one analysis method is not enough to prove c-BN, especially if the elemental composition of the sample is unclear. IR and Raman spectroscopy were additionally used to characterize the layers. While Raman only showed weak peaks for carbon containing phases and no faceted crystals were found in SEM, IR spectroscopy gave another hint for the deposition of c-BN, showing a peak at 1088 cm¡1. Thicker layers were deposited to get XRD peaks with higher intensity, but the opposite eVect was reached. Thicker layers showed no XRD peaks for BN phases, but the IR peaks were still observed. Additional peaks in the XRD of the hardmetal substrate, treated without precursor and Murakami etching, led to the identiWcation of Co6W6C. Moreover it was not possible to detect the peaks for the BN phases in XRD, when the layers were deposited on Mo. The conclusion of these experiments show, that once more the characterization and identiWcation of c-BN by two diVerent methods give no explicit result. Especially in
379
the BCN system a lot of phases exist showing a similar behavior in some of the commonly used analyzing methods. Due to the Wnal results it seems to be clear, that the XRD plots are caused by Co6W6C. But the results of IR measurements, which show peaks in the area of c-BN and h-BN independently from the layer thickness and substrate are still unclear and have to be the aim of further investigations. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
Kulisch W, Reinke S. Diamond Films Technol 1997;7:105–38. Saitoh H, Yarbrough WA. Diamond Relat Mater 1992;1:137–46. Reisse G, Weissmantel S. Thin Solid Films 1999;355–366:105–11. Seidel KH, Reichelt K, Schaal W, Dimigen H. Thin Solid Films 1987;151:243–9. Shtansky DV, Tsuda O, Ikuhara Y, Yoshida T. Acta Mater 2000;48:3745–59. Kulisch W, Freudenstein R, Klett A, Plass MF. Thin Solid Films 2000;377–378:170–6. Chayahara A, Yokoyama H, Imura T, Osaka Y. Jpn J Appl Phys 1987;26:L1435–6. Okamoto M, Yokoyama H, Osaka Y. Jpn J Appl Phys 1990;29:930–3. Bundy FP, Wentorf RH. J Chem Phys 1963;38(5):1144–9. Corrigan FR, Bundy FP. J Chem Phys 1975;63(9):3812–20. Leonidov VYa, Timofeev IV, Solozhenko VL, Rodionov IV. Russ J Phys Chem 1987;61:1503–4. Solozhenko VL, Leonidov VYa. Russ J Phys Chem 1988;62:1646–7. Maki J, Ikawa H, Fukunaga O. New Diamond Sci Technol 1991:1051–5. Will G, Nover G, von der Gönna J. J Solid State Chem 2000;154(1):280–5. Haubner R, Tang X. Int J Refract Met Hard Mater 2002;20:129–34. Aigner K, Tang X, Haubner R, Lux B. New Diamond Front Carbon Technol 2000;10(4):181–9. Tang X, Haubner R, Lux B, Zechmann A, Hengge E. J Phys IV-C5 1995:777–84. Bühlmann S, Blank E, Haubner R, Lux B. Diamond Relat Mater 1999;8(2–5):194–201. Kolber T, Piplits K, Dreer S, Mersdorf E, Haubner R, Hutter H. Appl Surf Sci 2000;167(1–2):79–88. Gielisse PJ, Mitra SS, Plendl JN, GriYs RD, Mansur LC, Marshall R, et al. Phys Rev 1967;155(3):1039–46. Brafman O, Lengyel G, Mitra SS, Gielisse PJ, Plendl JN, Mansur LC. Solid State Commun 1968;6:523–6. Huong PV. Diamond Relat Mater 1991;1:33–41. ASTM PCPDFWIN, Version 1.20, 1996.
Deposition of B, C, N coatings on WC–Co substrates—Analytical problems indicating c-BN formation Ronald Weissenbacher, Roland Haubner
¤
University of Technology Vienna, Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164, A-1060 Vienna, Austria Received 30 October 2005; accepted 23 November 2005
Abstract BCN-layers were deposited in a cold-wall reactor by decomposition of Tris(dimethylamino)borane on inductively heated hardmetal (WC–Co) substrates. The substrate temperature was varied between 800 and 1200 °C. XRD measurements of the deposited layers showed weak peaks, that might be explained by crystalline BN-phases, beside strong peaks of the substrate phases WC and Co. Indications for the deposition of the BN-phases could also be found in IR spectra. For an explicit BN phase identiWcation thicker layers were deposited by changing certain deposition parameters, which should provide higher intensities of the XRD-peaks of the BN phases. However, with increasing layer thickness, the BN related peaks decreased, which indicates that the phase causing the XRD peak is located near the substrate/coating interface. Through investigations of substrate reactions the eta-phase (Co6W6C) was observed which showed XRD peaks similar to c-BN and h-BN. It could be proved, that the eta-phase is responsible for the XRD peaks but an explanation for the detected IR peaks of the BN phases could not yet be found. © 2005 Elsevier Ltd. All rights reserved. Keywords: Hardmetal; B, C, N coatings; Analytical characterization
1. Introduction Since the deposition of diamond with low-pressure methods has been reported, strong eVorts were made to Wnd a similar process for the deposition of c-BN layers. PVD methods like ion-beam assisted deposition [1], ion plating [2], laser induced deposition [3], RF- or magnetron sputtering [4] succeeded in depositing nano-c-BN Wlms. In consideration of the deposition conditions which are necessary for the c-BN formation, the c-BN areas are very small [5]. The deposition of nano-c-BN was also reported by using diVerent plasma CVD methods to activate the gas phase. Typically used plasma CVD methods are for example ICP
* Corresponding author. Tel.: +43 1 58801 16128; fax: +43 1 58801 16199. E-mail address: [email protected] (R. Haubner).
0263-4368/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.11.010
CVD [6], bias enhanced plasma CVD [7] or ECR plasma CVD [8]. Up to now, several BN phases could also be deposited by using conventional thermal CVD for gas activation, but the deposition of c-BN or nano-c-BN could never be reported. In the Wrst phase diagram, which was published by Bundy and Wentorf [9], c-BN was considered to be the stable phase at standard conditions. In 1975 Corrigan and Bundy [10] postulated that the c-BN was metastable according to the carbon system (in which diamond—the cubic phase—is metastable at standard conditions). That fact was accepted until the late 1980s. Since 1987 a lot of papers have been published [11–14] showing c-BN to be the stable phase at standard conditions, but with lots of diVerences concerning the correct position of the equilibrium line between the h-BN and c-BN. First decomposition experiments of Tris(dimethylamino)borane on inductively heated hardmetal substrates
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
were carried out by Haubner and Tang [15]. Deposition of BCN layers in an argon atmosphere at standard pressure were carried out at substrate temperatures above 800 °C. It was possible to increase the growth rate by increasing the precursor Xow rate or by increasing the substrate temperature. The analysis of layers deposited at 800 °C showed some hints of the deposition of c-BN in XRD and IR-spectroscopy but TEM-measurements showed no c-BN crystallites. The intention for the present paper was to reproduce and to upgrade the results published earlier [15]. 2. Experimental set-up and sample characterization 2.1. Decomposition process For the deposition of BCN-layers by the decomposition of the single source precursor Tris(dimethylamino)borane, a quartz tube cold wall reactor with a diameter of 50 mm was used (Fig. 1) [16]. To adjust the precursor concentration for the reaction an argon stream was saturated with the precursor in an evaporation unit [17,16] and afterwards diluted with an additional pure argon stream. Using this procedure the partial pressure of the precursor in the gas stream was varied but the mass transport was constant. The two argon streams were regulated by mass Xow controllers. The gas mixture was introduced into the reactor from the top. Hardmetal substrates (K20 SPGN WC 6% Co) were placed on a graphite block inside the reactor, which was heated up to 1200 °C by medium frequency induction (10 kHz) in an argon atmosphere at atmospheric pressure. The precursor decomposed on the substrate surface and formed BCN layers. The substrate temperature was measured by a Wlament pyrometer (IRCON IR-UC). Scanning electron microscopy (JEOL 6400) was used to investigate the morphology of the layers. The deposited phases were characterized by X-ray diVraction (CuK radiation), infra-
375
red spectroscopy (Biorad FTS 175 equipped with a microscope) and Raman spectroscopy. All examinations were normally carried out in the center of the samples. The layer growth rate was determined by gravimetric analyses. As the density of the layers vary with the chemical and phase composition, the layer thickness was not calculated in m. 2.2. Characterization of BN phases The characterization of diamond layers is rather simple by using Raman spectroscopy [18] but a lot of diYculties occur by the analyses of phases of BN or BCN systems. The task becomes especially challenging without the knowledge of the elemental composition of the samples [15,19]. X-ray diVraction can be used to characterise c-BN and h-BN, but many cubic phases (of the B, C, N system or of substrate materials) show peaks at the same position as c-BN. The peak positions and their intensities are mainly inXuenced by the grain size, stress within the layers or other phases present. The characterisation of layers deposited on hardmetal substrates is additionally disturbed by very strong WC peaks. Infrared spectroscopy or Fourier transformed infrared spectroscopy peaks for c-BN are reported in literature as follows: for c-BN the transversal optical (TO) mode at a wavenumber of 1065 cm¡1 and the longitudinal optical (LO) mode at 1340 cm¡1 are described by Gielisse et al. [20]. If commercially available c-BN is investigated, usually one peak between 1050 and 1100 cm¡1 is observed. Peaks of w-BN, B4C and B2O3 can be found in the same area. Raman spectroscopy can also be used to identify c-BN or to distinguish between the c-BN and h-BN. For c-BN two peaks are described in literature; a TO-mode at 1055– 1057 cm¡1 and a LO-mode at 1305–1306 cm¡1 [21,22]. For h-BN a peak is observed at 1367 cm¡1 [22].
TC…temperature control C…gas cleaning R…rotation pump (needed for evacuation/ flushing)
P
P Sample TC
Precursor
R
MFC
C C
Fig. 1. Apparatus for the deposition of BCN-layers by the decomposition of a B, C, N containing single source precursor on inductively heated samples.
376
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
Fig. 2. Characterization of BCN layers deposited at a precursor Xow rate of 0.53 mg/min, a total argon Xow rate of 84 sccm and a substrate temperature of 800 °C. (a, b) SEM pictures, (c) XRD diagram, (d) IR spectrum.
The aim of this work was to identify crystalline BN-phases in layers, which were deposited on hardmetal substrates by thermal decomposition as described elsewhere [15]. At Wrst layers of higher thickness were grown and in a second step substrate reactions were screened. 3.1. Reproducing of literature data At Wrst the experiments described in literature [15] were reproduced and achieved similar results. The substrate was heated to a surface temperature of 800 °C, the precursor Xow rate was 32 mg/h, the total argon Xow rate was 84 sccm and the pressure in the reactor chamber 1013 mbar. After 6 h a layer with a growth rate of 0.23 mg h¡1 cm¡2 was deposited. Fig. 2(a) and (b) shows typical SEM pictures of the layers, which were deposited under the described conditions. The surface is covered with spherical, ball like structures with a diameter up to 5 m. X-ray diVractograms (Fig. 2(c)) of the deposited layers were similar to the ones observed earlier [15]. Beside the strong peaks of the WC phase and a rather small Co peak, two additional peaks could be found at 2 40.7° and 43.2°, which are corresponding to the strongest peaks of h-BN and c-BN in XRD. Other peaks of h-BN or c-BN could not be identiWed due to the strong WC peaks. In the IR spectra, peaks at 814 and 1088 cm¡1 were found (Fig. 2) which cor-
respond to the h-BN peaks observed at 780 and 828 cm¡1 and the c-BN peak at 1065 cm¡1 [20]. 3.2. Growth of BCN layers with increasing layer thickness The following experiments were carried out to grow thicker layers which should result in more explicit peaks of the BN phases in XRD. Several parameters can be changed to achieve thicker layers. The substrate temperature and the partial pressure of the precursor in the gas stream were varied (Fig. 3). The partial pressure was changed by mixing the precursor Xow 20
layer thickness [mg/cm2]
3. Results and discussion
16
1000°C
12 8
800°C
4 0 0.5
1
1.5
2
2.5
3
3.5
partial pressure of the precursor [mbar]
Fig. 3. Changes in the deposition rate as a function of the partial pressure of the precursor in Ar at two diVerent temperatures (precursor Xow rate: 0.53 mg/min, total Ar Xow rate 28–84 sccm).
R. Weissenbacher, R. Haubner / International Journal of Refractory Metals & Hard Materials 24 (2006) 374–379
377
Fig. 4. Layer thickness and SEM morphology as a function of the deposition times 1, 4 and 6 h (800 °C, precursor Xow rate: 0.53 mg/min; total Ar Xow rate: 84 sccm).
with additional argon in the ratio of 1:0, 1:1 or 1:2 (constant precursor Xow at increased velocity). It could be shown that the inXuence of the substrate temperature on the growth rate is rather strong. A temperature increase from 800 to 1000 °C results in a three times higher deposition rate. The highest deposition rates in these series of experiments could be reached at 1000 °C with the highest partial pressure of the precursor in the Ar stream (lowest Ar Xow rate). Also experiments with increasing deposition time from 1 to 6 h were carried out. These experiments were run at a precursor Xow rate of 32 mg/h, a total argon Xow of 84 sccm and a substrate temperature of 800 °C (Fig. 4). An almost linear relationship between the deposition time and layer thickness was observed. SEM investigations of the layers, which were deposited within one hour still show the surface of the hardmetal substrate, where scratches are visible. The nucleation of the BCN layer starts along the scratches on the surface. After 6 h of deposition a close layer on the substrate surface is observed (Fig. 4). However, the increased layer thickness never resulted in an increase of the BN peaks. The ratio c-BN/WC got even worse, and at a certain layer thickness no peaks of the BN phases could be detected any more. X-ray diVractograms of the deposited layers showed higher peaks of the BN phases for thinner layers. In Fig. 5 the ratio between the peak at 43.2° and the highest peak of the tungsten-carbide phase at 35.6° is shown as a function of the layer thickness. Although X-ray measurements of layers, which were grown at other conditions (diVerent substrate temperature, deposition time, partial precursor pres-
sure) indicated, that the ratio between the two peaks increases with decreasing layer thickness. This indicates that the phase, which causes the peaks at 43.2° and 40.7°, is located near the substrate/layer interface and not in the layer. The IR-spectra of the deposited layers are similar for all diVerent layer thicknesses, with only small changes in the wavenumber of the peaks (Fig. 5). Two of the observed peaks correspond to the cubic and the hexagonal BN phases. No satisfying explanation for these peaks could be found. 3.3. Reactions of the hardmetal substrate during BCN deposition From the experiments with samples of increasing layer thickness it could be shown that the phase described b the two unknown peaks is located near the substrate/layer interface. Possible compounds, which can be formed during the deposition, were checked by comparing diVraction peaks (e.g. ASTM 03-0959 CoB, 25-0241 Co2B, 12-0443 Co3B) [23]. Only in case of Co6W6C the peaks were similar to the XRD diagrams observed during BCN deposition (ASTM 23-0939 Co6W6C). Nevertheless the formation of Co6W6C during BCN deposition is surprising, because eta-phases are the result of a decarburisation of the hardmetal substrate, and the precursor used for deposition contains C:N:B in a ratio of 6:3:1. Therefore a hardmetal substrate was heated to 800 °C in an argon stream without a precursor Xow. XRD of the
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ratio c-BN peak/WC peak [%]
30 25 20 15 10 5 0 0
5
10
15
20
25
30
deposited layer [mg] thin layer (9.3 mg deposited)
thin layer (2.9 mg deposited)
intensity
intensity
WC
X R D
c-BN
35
40
45
50
30
35
c-BN
(780) (828) (1065)
700
C (sp2)
1122,80 823,80
1566,50 h-BN
40
45
50
2*theta
absorption
h-BN
1066,80
absorption
IR
812,22
2*theta
1585,80
30
(1380) (1530-1590)
900 1100 1300 1500 1700 1900 2100
wavenumber [cm-1]
700
900 1100 1300 1500 1700 1900 2100
wavenumber [cm-1]
Fig. 5. The ratio of the c-BN peak and the highest WC peak in XRD as a function of the layer thickness and XRD diVractograms and IR spectrums of layers with a thickness of 2.9 mg, respectively, 9.3 mg.
substrate which was treated as described showed peaks which correspond to c-BN and h-BN, but with higher intensity. The peaks at 43.2° and 40.7° as well as additional peaks at 32.9° and 47.2° belong to the phase Co6W6C, a cubic so called eta-phase in the tungsten/carbon/cobalt system. To prove the existence of the eta-phase on a hardmetal substrate, the surface was etched with Murakami solution
(10% NaOH solution mixed with 10% K3[Fe(CN)6] solution) for 10 s. Thereby the eta-phase Co6W6C is oxidized selectively, resulting in a tarnished substrate surface of a dark gray color. Rough structures growing out of the substrate surface are observed by scanning electron microscopy (Fig. 6). The eta-phase was observed on the substrates treated without a precursor as well as on a substrate where the deposited layer was removed by polishing.
Fig. 6. SEM pictures of substrate surfaces showing the eta-phase after Murakami etching: (a) substrate heated in pure argon to 800 °C, (b) substrate after removing the BCN layer.
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At least layers were deposited on Mo substrates. No XRD peaks corresponding to c-BN were found but with IR the peaks were observed again. 4. Conclusions By the use of PVD methods or plasma assisted CVD methods high accelerated ions are used to deposit nano-c-BN layers with a rather high stress within the layers. A simple chemical way for the low-pressure deposition of c-BN layers would be of high interest for industrial applications. Compared with the low-pressure diamond deposition some diYculties appear in the BN or BCN system. Firstly there are still very inconsistent thermodynamic data available, secondly the analyses of BN phases, especially of c-BN, is rather diYcult. In our case a rather simple way was used to produce BCN layers from the single source precursor Tris(dimethylamino)borane by thermal decomposition. XRD which is a good method to identify crystalline phases, was used for Wrst investigations of the deposited layers. The received peaks, gave Wrst hints for the deposition of h-BN and c-BN. But one analysis method is not enough to prove c-BN, especially if the elemental composition of the sample is unclear. IR and Raman spectroscopy were additionally used to characterize the layers. While Raman only showed weak peaks for carbon containing phases and no faceted crystals were found in SEM, IR spectroscopy gave another hint for the deposition of c-BN, showing a peak at 1088 cm¡1. Thicker layers were deposited to get XRD peaks with higher intensity, but the opposite eVect was reached. Thicker layers showed no XRD peaks for BN phases, but the IR peaks were still observed. Additional peaks in the XRD of the hardmetal substrate, treated without precursor and Murakami etching, led to the identiWcation of Co6W6C. Moreover it was not possible to detect the peaks for the BN phases in XRD, when the layers were deposited on Mo. The conclusion of these experiments show, that once more the characterization and identiWcation of c-BN by two diVerent methods give no explicit result. Especially in
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the BCN system a lot of phases exist showing a similar behavior in some of the commonly used analyzing methods. Due to the Wnal results it seems to be clear, that the XRD plots are caused by Co6W6C. But the results of IR measurements, which show peaks in the area of c-BN and h-BN independently from the layer thickness and substrate are still unclear and have to be the aim of further investigations. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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