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Hot filament CVD diamond coating of TiC sliders
Diamond & Related Materials 16 (2007) 609 – 615 www.elsevier.com/locate/diamond
Hot filament CVD diamond coating of TiC sliders S. Konoplyuk a,⁎, T. Abe b , T. Takagi b , T. Uchimoto b a
Innovation Plaza Miyagi, Japan Science and Technology Agency, 6-6-5 Minami-Yoshinari, Aoba-ku, Sendai 989-3204, Japan b Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-Ku, Sendai 980-8577, Japan Received 3 May 2006; received in revised form 27 October 2006; accepted 13 November 2006 Available online 29 December 2006
Abstract Hot filament chemical vapour deposition (HF CVD) process with low filament temperature (∼ 1650 °C) was utilized for the diamond coating of TiC samples. Porous substrates were fabricated by pulse discharge sintering (PDS) to create more nucleation sites. Nucleation density and morphology of deposited diamond films were studied using scanning electron microscopy (SEM). It was found that the highest growth rate occurs at substrate temperature of 980 °C. Evaluation of the residual stress in deposited films was carried out by Raman spectroscopy. Ball on disk tests were performed with steel as a counterface material. After polishing diamond films demonstrated good sliding performance: friction coefficient of 0.08 and wear rate of 10− 17 m3/N m. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond film; Pulse discharge sintering; Porous TiC substrate; Wear and friction
1. Introduction One of the most developed industrial applications of diamond films is protective and wear resistant coatings which exploit outstanding mechanical properties of diamond. Diamond coatings are used for protection of cutting tools, medical implants, microelectronic elements, etc. considerably prolonging their operation life. To date, most of the research on protective diamond coating was dedicated to Co cemented carbide [1] which is the material of choice in cutting tool manufacture and to SiC [2] which is widely used in mechanical seals. Along with development of the diamond coating on the commercialized materials, the new advanced materials as well as their protection by diamond films are being elaborated [3,4]. Until recently, bulk TiC was mainly used as a material in navigation, space and medical bearings, and TiC films were used for chemical and wear protection [5]. High hardness and brittleness limit applications of TiC. Now, due to development of the consolidation techniques such as spark plasma sintering, high frequency hot pressing etc. it is possible to produce the ⁎ Corresponding author. Tel.: +38 0674507481; fax: +38 0444241020. E-mail address: [email protected] (S. Konoplyuk). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.044
materials with required mechanical characteristics which are varying in accordance with pressure, temperature and other parameters of the synthesis. We elaborated the synthesis of cost-effective TiC samples, which can be easily machined by conventional high-speed steel tools, and further protection of their surface by the diamond film for application as sliders in linear motors [6,7]. Usually in motors of this type sliders are installed in positioning stage driven by a servomotor or a stepping motor and move on steel rail. Crucial parameters controlling effectiveness of sliders are friction and wear. It should be remarked that our choice of material for sliders stems from the fact that deposition of diamond film on ceramic substrates is apparently easier to perform than deposition on metallic ones [8], which often requires intermediate layers between the substrate and the diamond film. In this paper our attention will be primarily focused on the substrate synthesis and selection, the characterization of films and their tribological behaviour. 2. Experimental details For fabrication of TiC substrates PDS (pulse discharge sintering) was used that is a relatively fast method of material
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synthesis [9]. Application of this consolidation technique is also reasonable because temperature and pressure applied to initial powders during the synthesis greatly affect density of the resultant specimens varying it in the range from being typical for porous to fully compact structures. Taking into account that porous materials contain many surface defects (pits), which improve adhesion of the diamond film to substrate we synthesized the TiC ceramic substrates having the porous structure. TiH2 and TiC of 97.9% and 99% purity were utilized as raw powders for this purpose. In order to find optimal composition they were mixed in different proportions and ground by ball milling for 10 h with 12 mm diameter steel balls. After milling average particle size was about 5 μm. The mixed powders were compacted into graphite mold with diameter of 50 mm for the following sintering. Pressure in chamber during the sintering was about 1 Pa. In order to produce porous TiC the mixtures were heated at a rate 50– 60 °C/min to the targeted temperatures of 1000, 1050, 1100, 1200 °C and conditioned at these temperatures for 5 min. The reaction zone in mold during the synthesis was under pressure 50 MPa. The sintered specimens were ground with diamond covered wheel to remove upper 0.3 mm carbon layer for further measurements and diamond deposition. Part of the fabricated
Fig. 2. Substrate surface after diamond nucleation for 30 min under gas pressure of 8 kPa: (a) porous TiC specimen, (b) compacted TiC specimen.
Fig. 1. XRD pattern of specimens obtained after PDS of TiC and TiH2 mixtures with (a) 10 wt.%, (b) 20 wt.% of TiH2.
samples was cut into parallelepiped specimens measuring 45 × 2.5 × 2 mm for evaluation of bending strength. After Xray diffraction study, measurements of bending strength, and SEM observations specimens were selected as shown in the next section for further deposition of diamond. Before deposition the TiC substrates were treated in nanocrystalline diamond slurry by ultrasonic agitation for 10 min followed by rinsing in ethanol for 10 min. A conventional hot filament system with CH4 and H2 as feeding gases was used for multifilament diamond deposition. Methane and hydrogen were fed at a flow rate of 2.42 and 99.8 sccm in all depositions that yielded the highest nucleation density for better adhesion of film to substrate. Molybdenum holder of 150 cm2 area for the substrates was mounted between two molybdenum electrodes. We used different number of tungsten filaments (diameter of 0.2 mm) from 8 to 12 depending on the desired temperature during deposition. Depositions were performed without substrate cooling. Substrate and filament temperatures were measured with chromel–alumel thermocouple and an optical pyrometer, respectively. Temperature of filaments was about 1650 °C. The distance from substrate to filaments array was 5– 6 mm in all experiments that provided deposition of homogeneous film as well as the highest atomic hydrogen
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concentration near substrate [10]. Afterwards the condition of the coated surface was studied with SEM and optical microscope. The films were also characterized by microRaman spectroscopy employing Jobin Yvon BX-40 apparatus with excitation line 632.8 nm of the Helium Neon laser. In order to gain better sliding performance diamond films were polished with TiAlX wheel, where X was Mn, Cr and Nb. While a rough surface of a CVD diamond contacts to the rotating TiAlX wheel (3000 rpm), contact points of the diamond tip, sub-micron size, warm up to high temperature, above 1000 °C, then react with a very reactive Ti element. As a result contact points of the CVD diamond are mirror polished both by chemical reaction and thermal decomposition of the diamond into TiC and graphite whereas, due to the fact that hardness of TiAlX is high and its maximum is observed above 600 °C, wear of wheel is very moderate. Tribological properties were evaluated in air under humidity of 30–55% and temperature of 20–25 °C. Specimens covered by diamond films were rotated against SUJ 2 (AISI-E-52100) and SUS 304 (AISI 304) steel balls with a diameter of 6 mm. Applied normal load was 1–5 N, radius, track length, and linear speed of sliding were 12–15 mm, 500–3000 m, and 20 cm/s, respectively.
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3. Results and discussion The specimens sintered from mixtures with various proportions of TiH2 to TiC possessed different phase content and mechanical properties. All of them which were synthesized from compositions with less than 8 wt.% of TiH2 were very brittle and often fractured whereupon they had been removed from graphite mold or during polishing. Toughness of specimens increased with percentage of TiH2 in initial mixtures. In spite of relatively high content of TiH2 powder (up to 18 wt.%), only TiC peaks were presented on XRD patterns after sintering (Fig. 1, a). In the case of specimens sintered from starting compositions with more than 18 wt.% of TiH2 additional peaks of Ti appeared in their diffraction spectrum (Fig. 1, b). Ti is undesirable phase because of the large mismatch in thermal expansion coefficient with diamond. By this reason, specimens with 10 wt.% of TiH2 were chosen for further characterization and diamond deposition. Continuing selection of specimens for application as substrates we preferred the ones with higher flexural strength, better machinability and higher pit density, which was proportional to porosity of specimen. Three point bending measurements of the fabricated porous specimens showed
Fig. 3. SEM images of diamond films after deposition at filaments temperature 1650 °C, under flow rate ratio of CH4:H2 = 2.42: 99.8, pressure of 11 kPa, and at (a) substrate temperature 780 °C, (b) substrate temperature 915 °C, (c) substrate temperature 970 °C, (d) substrate temperature 1060 °C.
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flexural strength of 100–150 MPa increasing with temperature of sintering. In order to check machinability drilling test with high-speed steel drill was carried out. The specimens sintered at 1200 °C turned out to be not machinable. The samples after the synthesis at 1000 °C were too soft with cracks on the surface and unsuitable for further utilization. The others could be used but since those sintered at 1050 °C demonstrated higher pit density at SEM examination, we selected them as substrates for diamond depositions. The typical length and width of the pits on them were in the range from a few to several tens micrometers and they were distributed with density about 3 × 106 cm− 2. For comparison, besides abovementioned porous TiC substrates with specific weight of 3.9 g/cm2, the fully compact TiC substrate with specific weight of about 4.9 g/cm2 was sintered. The compacted specimen was synthesized at 1300 °C for 15 min. By our estimate nucleation density of diamond on a porous substrate exceeded seven–ten times one on a compacted substrate (Fig. 2) after identical pretreatment. The possible reasons of the nucleation enhancement in the former case are higher number of residual diamond seed particles, which leave pits on a surface after pretreatment and pits serving as chemical active sites, which prefer to adsorb diamond precursors [11]. For diamond films were deposited on porous substrates with many pits, attention was given to the smoothness of films. It is well known that easiness and, therefore, time of polishing
depend on morphology of diamond so that cauliflower diamond film [12] is more compliant to mechanical treatment than the well-faceted one because of lower hardness [7]. In order to diminish time of routine polishing process cauliflower morphology is desirable in as-deposited films. Filament and substrate temperature, when other deposition parameters such as filament to substrate distance, concentration, pressure and flow rate of gases are fixed, determine quality of films. It is known [13] that low filament temperature prolongs operational lifetime of filaments and decreases filament impurities in diamond film. Although filament impurities are not detrimental for utilization of diamond as solid lubricant, long service lifetime of filaments is very important in mass production. Besides low filament temperature is beneficial for diamond films with cauliflower morphology. For reasons given temperature of filaments was set at 1650 °C. To find the optimal substrate temperature additional study was undertaken. Fig. 3, a and b demonstrate that cauliflower or ball-like morphology is intrinsic in the films deposited at lower substrate temperature. The reason of predominance of cauliflower diamond at lower substrate temperature can be explained in terms of gas-phase kinetic model suggested in [14] as a result of increased condensation rate of aromatic hydrocarbons. Small crystalline clusters form while depositing at the substrate temperatures between 780 and 940 °C (Fig. 3, a, b). As substrate
Fig. 4. Raman spectra from diamond films deposited under the next parameters: filaments temperature of 1650 °C, flow rate ratio of CH4:H2 = 2.42:99.8, pressure of 11 kPa and (a) substrate temperature of 780 °C, (b) substrate temperature of 915 °C, (c) substrate temperature of 970 °C, (d) substrate temperature of 1060 °C.
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temperature increases diamond crystals enlarge and finally acquire well-faceted shape (Fig. 3, d). This change in film morphology comes when depositions are carried out at a substrate temperature between 940 °C and 1000 °C on the condition that methane concentration and gas pressure are 2.4% and 11 kPa, respectively. Fig. 3, c shows that the diamond film grown within this substrate temperature range is comprised of crystalline aggregates with relatively large square (100) facets on the top of each of them. Another important characteristic of the diamond film for application as solid lubricant is the residual stress. It determines adhesion of film to substrate. In order to calculate the stress generated in diamond, Raman scattering analysis was carried out. Fig. 4 demonstrates Raman spectra of the films shown in Fig. 3. The film deposited at the lowest substrate temperature reveals broad band from 1300 to 1500 cm− 1 and no discernible diamond peak (Fig. 4, a). The former feature indicates high amorphous carbon content and the latter can be attributed to a very small size of diamond crystals [15] since XRD signature (not shown here) contains diamond peaks. Raman spectrograms taken from diamond deposited at higher substrate temperature (Fig. 4, b and c), besides amorphous carbon band, show broadened diamond peaks centered at 1335–1336 cm− 1. Shift in Raman line to high wavenumbers is indicative of the compressive stress in the diamond film. We performed three scans at different parts of the films to find average value of this shift. Diamond peak in each of Raman spectrum was fitted with Lorentzian line to find exact position. Bearing in mind Raman line of stress-free natural diamond at 1332.5 cm− 1 and using 0.348 GPa cm− 1 [16] as linear stress coefficient per cm− 1 of Raman shift from the stress-free diamond Raman line, we calculated the residual stress in these films. It amounted to − 1.2 and − 1 GPa for the films deposited at the substrate temperature of 915 and 970 °C, respectively. The film produced at the highest substrate temperature (Fig. 4, d) yields sharp Raman peak with no shift relative to stress-free diamond. Most likely residual stress relaxed due to high temperature of TiC substrate at deposition. As this takes place, the top of ceramic substrate becomes very prone to microcracking, and partial
Fig. 5. Relationship between substrate temperature and thickness of the films after 16 h of deposition at the filament temperature of 1650 °C and under gas pressure of 11 kPa.
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Table 1 Summary of the friction and wear rate of steel balls on diamond films tested at a normal load of 1 N Tested diamond films
Coefficient of friction
Ball wear rate, m3/N m
As-deposited faceted morphology As-deposited cauliflower morphology Polished films
0.65
3.4 × 10− 13
0.6–0.5
2.1 × 10− 13
0.07–0.15
10− 16 – 10 −17
delamination of diamond film following stress relaxation may occur. Along with morphology and residual stress thickness of films determine the ability of film to withstand polishing and durability at further application as slider coating. It is one of the reasons why diamond film as a solid lubricant is superior to diamond like carbon film that cannot be grown so thick. Thickness of diamond films deposited at different temperatures of substrate is depicted in Fig. 5. According to this plot thickness of film as a function of substrate temperature shows a peak value of 12 μm at a temperature of about 980 °C. Although substrate temperature does not so critically influence film thickness as filament temperature [17] does, its increase from 780 °C to 980 °C almost trebles film thickness. At the substrate temperature of 1060 °C low nucleation density of diamond yielded discontinuous film (Fig. 3, d). Apparently in this case higher mobility of ad-species and higher diffusivity of carbon atoms into substrate including dissolving nanocrystalline seeds as a result of high substrate temperature hinder the nucleation process. Tribological behaviour of the deposited films was also studied to ensure if they would show good performance for sliding. For this purpose three kinds of specimens were selected: as-deposited specimens having two different morphology of diamond film, and specimens after polishing with TiAlX wheel. The results of the tribological tests are presented in Table 1. As would be expected steel ball during sliding on the film of faceted morphology possesses the highest friction and wear rate because of abrasion of steel ball with sharper asperity tips of
Fig. 6. Variation of the friction coefficient during sliding SUS 304 and SUJ 2 steel balls against polished diamond film under load of 1 N.
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diamond crystals. When ball sliding, no reduction of friction with time was found at least after the distance of 3000 m. In the case of the film of cauliflower morphology the friction coefficient decreased from 0.6 to 0.5 during the test. After polishing the wear rate of steel balls SUS 304 and SUJ 2 and the friction against diamond film considerably diminished to about 10− 16 – 10− 17 m3/N m and 0.07–0.15 (Fig. 6) depending mainly on polishing time and, accordingly, on roughness. The wear rate of balls on some films was even lower than that reported for some nano-size diamond films [18]. Surface of the polished film (Fig. 7) consisted of alternating grooves and plateaus. Laser microscope measurements of films after polishing showed that average roughness Ra of surface was about 1 μm meanwhile roughness Ra within plateau was approximately 0.1 μm. Since difference in height of nearby plateaus does not exceed much amplitude of unevenness within a single plateau shown by arrows, we can claim that in our case low average roughness within plateau is a paramount reason of low friction and wear at ball sliding on polished diamond. It should be noted that the friction coefficient of SUS 304 exceeds that of SUJ 2 (Fig. 6) at sliding over the track with the same roughness because of higher hardness of the latter and, therefore, lower adhesion component of friction. The trace of SUJ 2 ball friction demonstrates spike after distance of 2000 m that can be attributed to removal of diamond coating, which occurred in one of the contact regions. As a result abrasive wear is enhanced in a film detached region by higher roughness of the porous TiC substrate and wear particles get trapped at the sliding interface until being dislodged from there [19]. This film damage presumably occurs due to excessive polishing resulted in film thinning in the film detached region. As normal load increases the friction slightly reduces but the wear rate enhances with load very sharply if load exceeds 3 N that is presumably caused by change of the wear mechanism (Fig. 8). According to our measurements (not shown here) the wear rate and friction of widely used steel ball/steel ring pair comprised 1.65·10− 14 m3/N m and 0.8 after the sliding distance of several hundreds meters. Therefore it can be concluded that
Fig. 8. (a) Variation of the friction coefficient during sliding SUS 304 steel ball against polished diamond films under different loads as a function of sliding distance, (b) the wear rate of SUS 304 steel ball after 500 m sliding as a function of load.
the substitution of TiC with diamond film on a sliding surface for steel significantly improves operational capability of a slider provided that polishing reduces its friction to the level not causing extensive abrasive wear of steel counterpart. 4. Conclusions
Fig. 7. Optical microscope image of polished diamond film surface before sliding test and its profile recorded by laser microscope.
TiC sintered by PDS was utilized as a substrate for hot filament diamond deposition. In order to increase number of surface defects the synthesis was performed at temperature 1050 °C that provided porous structure of the samples. The porous TiC substrate gave essential gain in nucleation density as compared to not porous one. Low filaments temperature (∼ 1650 °C) was utilized to increase service life of filaments. The morphology of diamond films changed from cauliflower diamond to faceted one at increasing substrate temperature. The optimal substrate temperature was found to be at the range from 900 to 1000 °C providing the highest deposition rate and low level of compressive residual stress about 1 GPa. Diamond film of cauliflower morphology demonstrated better workability than film of faceted morphology due to higher non-diamond carbon content and after polishing wear rate and friction of steel ball on it improved to 10− 17 m3/N m and 0.07, respectively.
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Acknowledgements We acknowledge Japan Science and Technology Agency for their financial support. The authors would also like to thank Prof. Endo of Miyagi National College of Technology for his assistance on SEM analysis. References [1] B.S. Park, Y.J. Baik, K.R. Lee, K.Y. Eun, D.H. Kim, Diamond Relat. Mater. 2 (1993) 910. [2] A. Erdemir, G.R. Fenske, A.R. Krauss, M. Gruen, T. McCauley, R.T. Csencsits, Surf. Coat. Technol. 121 (1999) 565. [3] T. Abe, T. Takagi, Z.M. Sun, T. Uchimoto, J. Makino, H. Hashimoto, Diamond Relat. Mater. 13 (2004) 819. [4] E. Cappelli, L. Esposito, F. Pinzari, G. Mattei, S. Orlando, Diamond Relat. Mater. 11 (2002) 1731. [5] Hugh O. Pierson, Handbook of Chemical Vapor Deposition (CVD). Principles, Technology, and Applications, Noyes Publications, New York, 1999, p. 249.
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[6] I. Nakamori, T. Takagi, T. Takeno, T. Abe, T. Uchimoto, Y. Kohama, Diamond Relat. Mater. 14 (2005) 2122. [7] T. Takeno, T. Komoriya, I. Nakamori, H. Miki, T. Abe, T. Uchimoto, T. Takagi, Diamond Relat. Mater. 14 (2005) 2118. [8] T.P. Ong, R.P. Chang, Appl. Phys. Lett. 58 (1991) 358. [9] S. Konoplyuk, T. Abe, T. Uchimoto, T. Takagi, Mater. Lett. 59 (2005) 2342. [10] M.A. Childs, K.L. Menningen, L.W. Anderson, J.E. Lawler, J. Chem. Phys. 104 (1996) 9111. [11] S.-Tong Lee, Z. Lin, X. Jiang, Mater. Sci. Eng. 25 (1999) 123. [12] J. Stiegler, T. Lang, M. Nyghd-Ferguson, Y. von Kaenel, E. Blank, Diamond Relat. Mater. 5 (1996) 226. [13] G.F. Zhang, V. Buck, Surf. Coat. Technol. 160 (2002) 14. [14] M. Frenklach, H. Wang, Phys. Rev., B 43 (1991) 1520. [15] M. Yoshikawa, Y. Mori, M. Maegawa, G. Katagiri, H. Ishida, A. Ishtani, Appl. Phys. Lett. 62 (1993) 3114. [16] H. Boppart, J. Vab Straaten, I.S. Silvera, Phys. Rev., B 32 (1985) 1423. [17] S. Zhou, Z. Zhihao, X. Ning, Z. Xiaofeng, Mater. Sci. Eng., B 25 (1994) 47. [18] A. Tanaka, K. Wazumi, Y. Koga, Proc. 6th Diamond Conf./2nd Frontier Carbon Technology Joint Conf. 1, NASA, 2001, p. 447. [19] D.H. Hwang, D.E. Kim, S.J. Lee, Wear 225–229 (1999) 427.
Hot filament CVD diamond coating of TiC sliders S. Konoplyuk a,⁎, T. Abe b , T. Takagi b , T. Uchimoto b a
Innovation Plaza Miyagi, Japan Science and Technology Agency, 6-6-5 Minami-Yoshinari, Aoba-ku, Sendai 989-3204, Japan b Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-Ku, Sendai 980-8577, Japan Received 3 May 2006; received in revised form 27 October 2006; accepted 13 November 2006 Available online 29 December 2006
Abstract Hot filament chemical vapour deposition (HF CVD) process with low filament temperature (∼ 1650 °C) was utilized for the diamond coating of TiC samples. Porous substrates were fabricated by pulse discharge sintering (PDS) to create more nucleation sites. Nucleation density and morphology of deposited diamond films were studied using scanning electron microscopy (SEM). It was found that the highest growth rate occurs at substrate temperature of 980 °C. Evaluation of the residual stress in deposited films was carried out by Raman spectroscopy. Ball on disk tests were performed with steel as a counterface material. After polishing diamond films demonstrated good sliding performance: friction coefficient of 0.08 and wear rate of 10− 17 m3/N m. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond film; Pulse discharge sintering; Porous TiC substrate; Wear and friction
1. Introduction One of the most developed industrial applications of diamond films is protective and wear resistant coatings which exploit outstanding mechanical properties of diamond. Diamond coatings are used for protection of cutting tools, medical implants, microelectronic elements, etc. considerably prolonging their operation life. To date, most of the research on protective diamond coating was dedicated to Co cemented carbide [1] which is the material of choice in cutting tool manufacture and to SiC [2] which is widely used in mechanical seals. Along with development of the diamond coating on the commercialized materials, the new advanced materials as well as their protection by diamond films are being elaborated [3,4]. Until recently, bulk TiC was mainly used as a material in navigation, space and medical bearings, and TiC films were used for chemical and wear protection [5]. High hardness and brittleness limit applications of TiC. Now, due to development of the consolidation techniques such as spark plasma sintering, high frequency hot pressing etc. it is possible to produce the ⁎ Corresponding author. Tel.: +38 0674507481; fax: +38 0444241020. E-mail address: [email protected] (S. Konoplyuk). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.044
materials with required mechanical characteristics which are varying in accordance with pressure, temperature and other parameters of the synthesis. We elaborated the synthesis of cost-effective TiC samples, which can be easily machined by conventional high-speed steel tools, and further protection of their surface by the diamond film for application as sliders in linear motors [6,7]. Usually in motors of this type sliders are installed in positioning stage driven by a servomotor or a stepping motor and move on steel rail. Crucial parameters controlling effectiveness of sliders are friction and wear. It should be remarked that our choice of material for sliders stems from the fact that deposition of diamond film on ceramic substrates is apparently easier to perform than deposition on metallic ones [8], which often requires intermediate layers between the substrate and the diamond film. In this paper our attention will be primarily focused on the substrate synthesis and selection, the characterization of films and their tribological behaviour. 2. Experimental details For fabrication of TiC substrates PDS (pulse discharge sintering) was used that is a relatively fast method of material
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synthesis [9]. Application of this consolidation technique is also reasonable because temperature and pressure applied to initial powders during the synthesis greatly affect density of the resultant specimens varying it in the range from being typical for porous to fully compact structures. Taking into account that porous materials contain many surface defects (pits), which improve adhesion of the diamond film to substrate we synthesized the TiC ceramic substrates having the porous structure. TiH2 and TiC of 97.9% and 99% purity were utilized as raw powders for this purpose. In order to find optimal composition they were mixed in different proportions and ground by ball milling for 10 h with 12 mm diameter steel balls. After milling average particle size was about 5 μm. The mixed powders were compacted into graphite mold with diameter of 50 mm for the following sintering. Pressure in chamber during the sintering was about 1 Pa. In order to produce porous TiC the mixtures were heated at a rate 50– 60 °C/min to the targeted temperatures of 1000, 1050, 1100, 1200 °C and conditioned at these temperatures for 5 min. The reaction zone in mold during the synthesis was under pressure 50 MPa. The sintered specimens were ground with diamond covered wheel to remove upper 0.3 mm carbon layer for further measurements and diamond deposition. Part of the fabricated
Fig. 2. Substrate surface after diamond nucleation for 30 min under gas pressure of 8 kPa: (a) porous TiC specimen, (b) compacted TiC specimen.
Fig. 1. XRD pattern of specimens obtained after PDS of TiC and TiH2 mixtures with (a) 10 wt.%, (b) 20 wt.% of TiH2.
samples was cut into parallelepiped specimens measuring 45 × 2.5 × 2 mm for evaluation of bending strength. After Xray diffraction study, measurements of bending strength, and SEM observations specimens were selected as shown in the next section for further deposition of diamond. Before deposition the TiC substrates were treated in nanocrystalline diamond slurry by ultrasonic agitation for 10 min followed by rinsing in ethanol for 10 min. A conventional hot filament system with CH4 and H2 as feeding gases was used for multifilament diamond deposition. Methane and hydrogen were fed at a flow rate of 2.42 and 99.8 sccm in all depositions that yielded the highest nucleation density for better adhesion of film to substrate. Molybdenum holder of 150 cm2 area for the substrates was mounted between two molybdenum electrodes. We used different number of tungsten filaments (diameter of 0.2 mm) from 8 to 12 depending on the desired temperature during deposition. Depositions were performed without substrate cooling. Substrate and filament temperatures were measured with chromel–alumel thermocouple and an optical pyrometer, respectively. Temperature of filaments was about 1650 °C. The distance from substrate to filaments array was 5– 6 mm in all experiments that provided deposition of homogeneous film as well as the highest atomic hydrogen
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concentration near substrate [10]. Afterwards the condition of the coated surface was studied with SEM and optical microscope. The films were also characterized by microRaman spectroscopy employing Jobin Yvon BX-40 apparatus with excitation line 632.8 nm of the Helium Neon laser. In order to gain better sliding performance diamond films were polished with TiAlX wheel, where X was Mn, Cr and Nb. While a rough surface of a CVD diamond contacts to the rotating TiAlX wheel (3000 rpm), contact points of the diamond tip, sub-micron size, warm up to high temperature, above 1000 °C, then react with a very reactive Ti element. As a result contact points of the CVD diamond are mirror polished both by chemical reaction and thermal decomposition of the diamond into TiC and graphite whereas, due to the fact that hardness of TiAlX is high and its maximum is observed above 600 °C, wear of wheel is very moderate. Tribological properties were evaluated in air under humidity of 30–55% and temperature of 20–25 °C. Specimens covered by diamond films were rotated against SUJ 2 (AISI-E-52100) and SUS 304 (AISI 304) steel balls with a diameter of 6 mm. Applied normal load was 1–5 N, radius, track length, and linear speed of sliding were 12–15 mm, 500–3000 m, and 20 cm/s, respectively.
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3. Results and discussion The specimens sintered from mixtures with various proportions of TiH2 to TiC possessed different phase content and mechanical properties. All of them which were synthesized from compositions with less than 8 wt.% of TiH2 were very brittle and often fractured whereupon they had been removed from graphite mold or during polishing. Toughness of specimens increased with percentage of TiH2 in initial mixtures. In spite of relatively high content of TiH2 powder (up to 18 wt.%), only TiC peaks were presented on XRD patterns after sintering (Fig. 1, a). In the case of specimens sintered from starting compositions with more than 18 wt.% of TiH2 additional peaks of Ti appeared in their diffraction spectrum (Fig. 1, b). Ti is undesirable phase because of the large mismatch in thermal expansion coefficient with diamond. By this reason, specimens with 10 wt.% of TiH2 were chosen for further characterization and diamond deposition. Continuing selection of specimens for application as substrates we preferred the ones with higher flexural strength, better machinability and higher pit density, which was proportional to porosity of specimen. Three point bending measurements of the fabricated porous specimens showed
Fig. 3. SEM images of diamond films after deposition at filaments temperature 1650 °C, under flow rate ratio of CH4:H2 = 2.42: 99.8, pressure of 11 kPa, and at (a) substrate temperature 780 °C, (b) substrate temperature 915 °C, (c) substrate temperature 970 °C, (d) substrate temperature 1060 °C.
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flexural strength of 100–150 MPa increasing with temperature of sintering. In order to check machinability drilling test with high-speed steel drill was carried out. The specimens sintered at 1200 °C turned out to be not machinable. The samples after the synthesis at 1000 °C were too soft with cracks on the surface and unsuitable for further utilization. The others could be used but since those sintered at 1050 °C demonstrated higher pit density at SEM examination, we selected them as substrates for diamond depositions. The typical length and width of the pits on them were in the range from a few to several tens micrometers and they were distributed with density about 3 × 106 cm− 2. For comparison, besides abovementioned porous TiC substrates with specific weight of 3.9 g/cm2, the fully compact TiC substrate with specific weight of about 4.9 g/cm2 was sintered. The compacted specimen was synthesized at 1300 °C for 15 min. By our estimate nucleation density of diamond on a porous substrate exceeded seven–ten times one on a compacted substrate (Fig. 2) after identical pretreatment. The possible reasons of the nucleation enhancement in the former case are higher number of residual diamond seed particles, which leave pits on a surface after pretreatment and pits serving as chemical active sites, which prefer to adsorb diamond precursors [11]. For diamond films were deposited on porous substrates with many pits, attention was given to the smoothness of films. It is well known that easiness and, therefore, time of polishing
depend on morphology of diamond so that cauliflower diamond film [12] is more compliant to mechanical treatment than the well-faceted one because of lower hardness [7]. In order to diminish time of routine polishing process cauliflower morphology is desirable in as-deposited films. Filament and substrate temperature, when other deposition parameters such as filament to substrate distance, concentration, pressure and flow rate of gases are fixed, determine quality of films. It is known [13] that low filament temperature prolongs operational lifetime of filaments and decreases filament impurities in diamond film. Although filament impurities are not detrimental for utilization of diamond as solid lubricant, long service lifetime of filaments is very important in mass production. Besides low filament temperature is beneficial for diamond films with cauliflower morphology. For reasons given temperature of filaments was set at 1650 °C. To find the optimal substrate temperature additional study was undertaken. Fig. 3, a and b demonstrate that cauliflower or ball-like morphology is intrinsic in the films deposited at lower substrate temperature. The reason of predominance of cauliflower diamond at lower substrate temperature can be explained in terms of gas-phase kinetic model suggested in [14] as a result of increased condensation rate of aromatic hydrocarbons. Small crystalline clusters form while depositing at the substrate temperatures between 780 and 940 °C (Fig. 3, a, b). As substrate
Fig. 4. Raman spectra from diamond films deposited under the next parameters: filaments temperature of 1650 °C, flow rate ratio of CH4:H2 = 2.42:99.8, pressure of 11 kPa and (a) substrate temperature of 780 °C, (b) substrate temperature of 915 °C, (c) substrate temperature of 970 °C, (d) substrate temperature of 1060 °C.
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temperature increases diamond crystals enlarge and finally acquire well-faceted shape (Fig. 3, d). This change in film morphology comes when depositions are carried out at a substrate temperature between 940 °C and 1000 °C on the condition that methane concentration and gas pressure are 2.4% and 11 kPa, respectively. Fig. 3, c shows that the diamond film grown within this substrate temperature range is comprised of crystalline aggregates with relatively large square (100) facets on the top of each of them. Another important characteristic of the diamond film for application as solid lubricant is the residual stress. It determines adhesion of film to substrate. In order to calculate the stress generated in diamond, Raman scattering analysis was carried out. Fig. 4 demonstrates Raman spectra of the films shown in Fig. 3. The film deposited at the lowest substrate temperature reveals broad band from 1300 to 1500 cm− 1 and no discernible diamond peak (Fig. 4, a). The former feature indicates high amorphous carbon content and the latter can be attributed to a very small size of diamond crystals [15] since XRD signature (not shown here) contains diamond peaks. Raman spectrograms taken from diamond deposited at higher substrate temperature (Fig. 4, b and c), besides amorphous carbon band, show broadened diamond peaks centered at 1335–1336 cm− 1. Shift in Raman line to high wavenumbers is indicative of the compressive stress in the diamond film. We performed three scans at different parts of the films to find average value of this shift. Diamond peak in each of Raman spectrum was fitted with Lorentzian line to find exact position. Bearing in mind Raman line of stress-free natural diamond at 1332.5 cm− 1 and using 0.348 GPa cm− 1 [16] as linear stress coefficient per cm− 1 of Raman shift from the stress-free diamond Raman line, we calculated the residual stress in these films. It amounted to − 1.2 and − 1 GPa for the films deposited at the substrate temperature of 915 and 970 °C, respectively. The film produced at the highest substrate temperature (Fig. 4, d) yields sharp Raman peak with no shift relative to stress-free diamond. Most likely residual stress relaxed due to high temperature of TiC substrate at deposition. As this takes place, the top of ceramic substrate becomes very prone to microcracking, and partial
Fig. 5. Relationship between substrate temperature and thickness of the films after 16 h of deposition at the filament temperature of 1650 °C and under gas pressure of 11 kPa.
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Table 1 Summary of the friction and wear rate of steel balls on diamond films tested at a normal load of 1 N Tested diamond films
Coefficient of friction
Ball wear rate, m3/N m
As-deposited faceted morphology As-deposited cauliflower morphology Polished films
0.65
3.4 × 10− 13
0.6–0.5
2.1 × 10− 13
0.07–0.15
10− 16 – 10 −17
delamination of diamond film following stress relaxation may occur. Along with morphology and residual stress thickness of films determine the ability of film to withstand polishing and durability at further application as slider coating. It is one of the reasons why diamond film as a solid lubricant is superior to diamond like carbon film that cannot be grown so thick. Thickness of diamond films deposited at different temperatures of substrate is depicted in Fig. 5. According to this plot thickness of film as a function of substrate temperature shows a peak value of 12 μm at a temperature of about 980 °C. Although substrate temperature does not so critically influence film thickness as filament temperature [17] does, its increase from 780 °C to 980 °C almost trebles film thickness. At the substrate temperature of 1060 °C low nucleation density of diamond yielded discontinuous film (Fig. 3, d). Apparently in this case higher mobility of ad-species and higher diffusivity of carbon atoms into substrate including dissolving nanocrystalline seeds as a result of high substrate temperature hinder the nucleation process. Tribological behaviour of the deposited films was also studied to ensure if they would show good performance for sliding. For this purpose three kinds of specimens were selected: as-deposited specimens having two different morphology of diamond film, and specimens after polishing with TiAlX wheel. The results of the tribological tests are presented in Table 1. As would be expected steel ball during sliding on the film of faceted morphology possesses the highest friction and wear rate because of abrasion of steel ball with sharper asperity tips of
Fig. 6. Variation of the friction coefficient during sliding SUS 304 and SUJ 2 steel balls against polished diamond film under load of 1 N.
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diamond crystals. When ball sliding, no reduction of friction with time was found at least after the distance of 3000 m. In the case of the film of cauliflower morphology the friction coefficient decreased from 0.6 to 0.5 during the test. After polishing the wear rate of steel balls SUS 304 and SUJ 2 and the friction against diamond film considerably diminished to about 10− 16 – 10− 17 m3/N m and 0.07–0.15 (Fig. 6) depending mainly on polishing time and, accordingly, on roughness. The wear rate of balls on some films was even lower than that reported for some nano-size diamond films [18]. Surface of the polished film (Fig. 7) consisted of alternating grooves and plateaus. Laser microscope measurements of films after polishing showed that average roughness Ra of surface was about 1 μm meanwhile roughness Ra within plateau was approximately 0.1 μm. Since difference in height of nearby plateaus does not exceed much amplitude of unevenness within a single plateau shown by arrows, we can claim that in our case low average roughness within plateau is a paramount reason of low friction and wear at ball sliding on polished diamond. It should be noted that the friction coefficient of SUS 304 exceeds that of SUJ 2 (Fig. 6) at sliding over the track with the same roughness because of higher hardness of the latter and, therefore, lower adhesion component of friction. The trace of SUJ 2 ball friction demonstrates spike after distance of 2000 m that can be attributed to removal of diamond coating, which occurred in one of the contact regions. As a result abrasive wear is enhanced in a film detached region by higher roughness of the porous TiC substrate and wear particles get trapped at the sliding interface until being dislodged from there [19]. This film damage presumably occurs due to excessive polishing resulted in film thinning in the film detached region. As normal load increases the friction slightly reduces but the wear rate enhances with load very sharply if load exceeds 3 N that is presumably caused by change of the wear mechanism (Fig. 8). According to our measurements (not shown here) the wear rate and friction of widely used steel ball/steel ring pair comprised 1.65·10− 14 m3/N m and 0.8 after the sliding distance of several hundreds meters. Therefore it can be concluded that
Fig. 8. (a) Variation of the friction coefficient during sliding SUS 304 steel ball against polished diamond films under different loads as a function of sliding distance, (b) the wear rate of SUS 304 steel ball after 500 m sliding as a function of load.
the substitution of TiC with diamond film on a sliding surface for steel significantly improves operational capability of a slider provided that polishing reduces its friction to the level not causing extensive abrasive wear of steel counterpart. 4. Conclusions
Fig. 7. Optical microscope image of polished diamond film surface before sliding test and its profile recorded by laser microscope.
TiC sintered by PDS was utilized as a substrate for hot filament diamond deposition. In order to increase number of surface defects the synthesis was performed at temperature 1050 °C that provided porous structure of the samples. The porous TiC substrate gave essential gain in nucleation density as compared to not porous one. Low filaments temperature (∼ 1650 °C) was utilized to increase service life of filaments. The morphology of diamond films changed from cauliflower diamond to faceted one at increasing substrate temperature. The optimal substrate temperature was found to be at the range from 900 to 1000 °C providing the highest deposition rate and low level of compressive residual stress about 1 GPa. Diamond film of cauliflower morphology demonstrated better workability than film of faceted morphology due to higher non-diamond carbon content and after polishing wear rate and friction of steel ball on it improved to 10− 17 m3/N m and 0.07, respectively.
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Acknowledgements We acknowledge Japan Science and Technology Agency for their financial support. The authors would also like to thank Prof. Endo of Miyagi National College of Technology for his assistance on SEM analysis. References [1] B.S. Park, Y.J. Baik, K.R. Lee, K.Y. Eun, D.H. Kim, Diamond Relat. Mater. 2 (1993) 910. [2] A. Erdemir, G.R. Fenske, A.R. Krauss, M. Gruen, T. McCauley, R.T. Csencsits, Surf. Coat. Technol. 121 (1999) 565. [3] T. Abe, T. Takagi, Z.M. Sun, T. Uchimoto, J. Makino, H. Hashimoto, Diamond Relat. Mater. 13 (2004) 819. [4] E. Cappelli, L. Esposito, F. Pinzari, G. Mattei, S. Orlando, Diamond Relat. Mater. 11 (2002) 1731. [5] Hugh O. Pierson, Handbook of Chemical Vapor Deposition (CVD). Principles, Technology, and Applications, Noyes Publications, New York, 1999, p. 249.
615
[6] I. Nakamori, T. Takagi, T. Takeno, T. Abe, T. Uchimoto, Y. Kohama, Diamond Relat. Mater. 14 (2005) 2122. [7] T. Takeno, T. Komoriya, I. Nakamori, H. Miki, T. Abe, T. Uchimoto, T. Takagi, Diamond Relat. Mater. 14 (2005) 2118. [8] T.P. Ong, R.P. Chang, Appl. Phys. Lett. 58 (1991) 358. [9] S. Konoplyuk, T. Abe, T. Uchimoto, T. Takagi, Mater. Lett. 59 (2005) 2342. [10] M.A. Childs, K.L. Menningen, L.W. Anderson, J.E. Lawler, J. Chem. Phys. 104 (1996) 9111. [11] S.-Tong Lee, Z. Lin, X. Jiang, Mater. Sci. Eng. 25 (1999) 123. [12] J. Stiegler, T. Lang, M. Nyghd-Ferguson, Y. von Kaenel, E. Blank, Diamond Relat. Mater. 5 (1996) 226. [13] G.F. Zhang, V. Buck, Surf. Coat. Technol. 160 (2002) 14. [14] M. Frenklach, H. Wang, Phys. Rev., B 43 (1991) 1520. [15] M. Yoshikawa, Y. Mori, M. Maegawa, G. Katagiri, H. Ishida, A. Ishtani, Appl. Phys. Lett. 62 (1993) 3114. [16] H. Boppart, J. Vab Straaten, I.S. Silvera, Phys. Rev., B 32 (1985) 1423. [17] S. Zhou, Z. Zhihao, X. Ning, Z. Xiaofeng, Mater. Sci. Eng., B 25 (1994) 47. [18] A. Tanaka, K. Wazumi, Y. Koga, Proc. 6th Diamond Conf./2nd Frontier Carbon Technology Joint Conf. 1, NASA, 2001, p. 447. [19] D.H. Hwang, D.E. Kim, S.J. Lee, Wear 225–229 (1999) 427.