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Tensile and fatigue strength of free-standing CVD diamond
Diamond & Related Materials 14 (2005) 6 – 10 www.elsevier.com/locate/diamond
Tensile and fatigue strength of free-standing CVD diamond A.R. Daviesa, J.E. Fielda,*, K. Takahashib, K. Hadab a
Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, UK b Japan Atomic Energy Research Institute (JAERI), 801-1, Naka, Ibaraki 311-0193, Japan Received 12 June 2003; received in revised form 1 June 2004; accepted 3 June 2004 Available online 30 July 2004
Abstract Chemical vapour deposited (CVD) diamond is finding increased application and it is important to study its strength and fatigue properties. The present paper describes research on a batch of di-electric (optical) grade CVD material. The procedure in the experiments was to use some samples from a batch to obtain an average failure load. Other samples were then stress cycled to a chosen fraction of this average load. If they survived 107 cycles, they were considered to be immune to fatigue at that load, and their strengths were measured using a three-point bend test. Some samples survived at least 95% of their critical fracture stress for 107 cycles without fatiguing. In other experiments, a 4-point (double torsion) rig was used to stress samples with macro-flaws in different environments. The observation that fatigue does not reduce the strength of CVD diamond in normal environments adds to its attractiveness for various applications. D 2004 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Strength; Fatigue and stress corrosion
1. Introduction Chemically vapour deposited (CVD) diamond is finding increased application where the exceptional properties attained by diamond are required. For example, in applications where high wear or erosion resistances are important, or as optical, infra-red and microwave bwindowsQ in aggressive environments [1–4]. The stimulus for the present research was the use of CVD diamond as a window in a millimetre wave heating system for fusion research. Attractions are the low loss target (tand=2105 at the 100 GHz band), high thermal conductivity (1800–2000 W m1 K1) and high mechanical strength [5–7]. Additionally, the window can also act as a barrier to tritium and radioactive dusts from the heating system [7,8]. Clearly it is important for the window design to have data on the mechanical and fatigue properties. This paper reports
* Corresponding author. Tel.: +44 1223 337 336; fax: +44 1223 350 266. E-mail address: [email protected] (J.E. Field). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.06.015
experiments on the cyclic loading of CVD diamond and its response to fatigue. There are several techniques for measuring the strength of a material. In the case of single crystal diamond, the choice is almost exclusively restricted to indentation methods due to the small specimen size and cost of samples. However, indentation stresses a small volume of material and only part of the flaw distribution is sampled [9]. Thus the strength value obtained depends on the indenter size with the measure strength value increasing as the indenter size decreases; see the reviews of Field [10,11], Field and Pickles [12] and the data of Howes [13,14], Ruoff and Wanagel [15] and Ikawa et al. [16,17]. A further disadvantage with diamond indenting diamond is that the tip of the indenter is susceptible to damage. Chemical vapour deposited (CVD) diamond is potentially easier to study since it can be obtained in disc or plate geometries of up to millimetre thickness. The bursting disk technique is an excellent method for determining the strength of most ceramic materials since the maximum tensile stresses are at the center of the disk and are much smaller near the sample edges [18]. Jilbert [19]
A.R. Davies et al. / Diamond & Related Materials 14 (2005) 6–10
was able to obtain a sufficient number of CVD diamond samples to utilise the bursting disk test. She obtained strength measurements for the nucleation and the growth side, and showed that the nucleation side (smaller grain size) was consistently stronger. These strength measurements were reliable since they were measured from samples that failed in the center, negating any edge effects. Her data are reviewed in Field and Pickles [12]. However, the bursting disk test is often impractical for routinely measuring the strength of CVD diamond due to the fact that only small specimen sizes are readily available. Instead, a three-point bend test is often employed since it may be performed on smaller sections [4,20]. The volume stressed in the three-point bend test is larger than in an indentation test but usually smaller than using the bursting disc methods. A further potential disadvantage with the three-point test is that specimen edges are stressed and unless the specimen sides are polished, as well as its faces, misleading results may be obtained. This is certainly the case with most brittle materials. However, recent research [3,4,21,22] has shown that the strength of CVD diamond is not affected by surface polishing. This is because the bintrinsicQ flaws are so large (the equivalent flaw size, assuming a half-penny geometry, may be 100 Am or more in radius depending on grain size; see [3,19,22]). That a material can be so strong with such large intrinsic flaws shows the potential of CVD if the material could be produced with smaller flaws!
2. Experimental 2.1. Strength The strength of di-electric (optical) grade CVD diamond was determined using a three-point bend test [4]. The geometry and sample size are shown in Fig. 1. The three-point bend test apparatus itself, is illustrated in Fig. 2. The two halves of the three-point-bend test apparatus were held together by four spring-repelled screws, ensuring that the specimens were constrained vertically but not
Fig. 1. Schematic of three-point bend test arrangement.
7
Fig. 2. Three-point-bend test apparatus; photograph and schematic.
horizontally. The side components were fixed to a base stand and the central loading beam was attached to an Instron loading machine. Since the apparatus was immobile, the loading points could be accurately positioned on the specimen with good reproducibility. Note that load bpointsQ are in reality line contacts extending across the top and bottom surfaces of the specimen. The loading beams were made from toughened steel drill blanks of 1 mm diameter. The two lower beams were positioned 10 mm apart, allowing a proportion of the specimen to protrude at the sides. The Instron loading device was first calibrated using a 2kg and a 5-kg weight, and the friction force was measured to be less than 0.01 N when the loading rate was V0.1 mm per minute. The load rate used during the experiments was 0.02 mm per minute. Typical loading curves are shown in Fig. 3. The load at the point of failure for each sample was used to calculate the fracture stress, r. The fracture stress for each CVD diamond sample was determined by inserting the failure load into Eq. (1) [23,24]. r¼
3PL ; 2H 2 W
ð1Þ
where P is the mid-point load failure load, L is the span interval between the two outer support beams, and H and W
Fig. 3. Load to failure for di-electric grade CVD diamonds, in a three-point bend test. The curves are shifted along the time axis to avoid overlap.
8
A.R. Davies et al. / Diamond & Related Materials 14 (2005) 6–10
Table 1 Strengths of CVD diamond samples
Average strength
Strength of nucleation side (MPa)
Strength of growth side (MPa)
727F20 526F20 712F20 813F20 660F20 690F95
305F20 266F20 312F20 235F20 262F20 280F30
are the specimen thickness and width, respectively. The strengths of 1.89 mm thick, di-electric grade CVD diamonds are shown in Table 1. The flaws within the bulk determine the strength of CVD diamond [19,20]. These intrinsic flaws are limited to the size of the grains, and are therefore smaller on the nucleation side. Correspondingly, the CVD diamond samples are stronger if the nucleation side is stressed. 2.2. Fatigue The apparatus for the three-point-bend test was also used in the fatigue studies. However, the middle loading point was connected to an Instron Fatigue loader, model 8872. A 1.89 mm thick sample of CVD diamond was cycle loaded, stressing the growth side, from 10 to 100 N at a rate of 20 Hz. This was 70% of the average growth side strength. The amplitude of the loading cycle was 45 N, centred around a mean load of 55 N. Not only did the sample survive the specified 107 cycles, but it was still intact after 1.5107 cycles; at which point the fatigue part of the experiment was discontinued. The strength of the sample was then determined, using the three-point-bendtest. The sample failed at 140F10 N, signifying that the diamond had a strength of 280F20 MPa; very close to the average strength (see Table 1).
Fig. 5. Load required to initiate a crack.
The process was repeated for another sample at a higher fraction of the average strength. This time the growth side was stressed from 15 to 125 N at a rate of 20 Hz. This was 89% of the average growth side strength. The amplitude of the loading cycle was 55 N, centred around a mean load of 70 N. The sample survived the specified 107 cycles. The strength of the sample was then determined, using the three point-bend-test and the sample failed at 138F10 N corresponding to a strength of 276F20 MPa (again, close to the average strength measurements). The process was repeated on another sample of CVD diamond cycle loaded, stressing the nucleation side, from 10 to 290 N at a rate of 20 Hz. This was 85% of the average nucleation side strength. The amplitude of the loading cycle was 140 N, centred around a mean load of 150 N. The sample survived the specified 107 cycles, and it was still intact after 2107 cycles. The strength of the sample was then determined, using the three point-bend-test and the sample failed at 356F10 N corresponding to a strength of 712F40 MPa. Again close to the average strength; see Table 1. Finally, the process was repeated on another sample of CVD diamond cycle loaded, stressing the nucleation side,
Fig. 4. Schematic of the four-point-bend test apparatus. The distance between the outer and inner spherical contact points in 4 mm.
A.R. Davies et al. / Diamond & Related Materials 14 (2005) 6–10
Fig. 6. Load-cell reading, recorded over ~3 days.
from 10 to 320 N at a rate of 20 Hz. This was 95% of the average nucleation side strength. The amplitude of the loading cycle was 155 N, centred around a mean load of 165 N. The sample survived the specified 107 cycles. The sample was then broken using the three point-bend-test and the sample failed at 356F10 N corresponding to a strength of 712F40 MPa. 2.3. Slow crack growth The fact that CVD diamonds do not readily suffer slow crack growth was confirmed using a simple experiment that made use of a four-point bend (double torsion) test. Rectangular plates of CVD diamond were mounted between four spherical loading points, as shown in Fig. 4. This geometry was used in related experiments to measure the fracture toughness, K IC, of this grade of CVD diamond. A value of 8.3F0.4 MPa m1/2 was obtained [25]. In order to asses the vulnerability of CVD diamond to slow crack growth, a plate of CVD diamond was stressed using the geometry illustrated in Fig. 4. The load was increased until a crack grew from the notch, indicated by an audible cracking sound and a sudden drop in load. This occurred at a load of 112 N, see Fig. 5. The apparatus was then left for over 3 days, at a load of 109 N and with the load recorded every 10 min. The humidity was 75%. The load remained effectively constant over the 3 days, see Fig. 6. The slight fluctuations observed are thought to have arisen in the load-cell as a result of temperature variations over the test period. If the crack had extended, the load would have fallen over the entire length of the experiment.
3. Discussion and conclusions Fatigue in a material is usually the result of dislocation motion or, so-called, atmospheric fatigue (stress/corrosion cracking). Dislocation motion in diamond only takes place at room temperature under extreme conditions (see, for example, Ref. [26]), and is an unlikely mechanism here. A much more likely fatigue process with glasses and ceramics
9
is the second type of process which involves a combination of chemistry plus stress at the crack tip. This is well-known with glasses where the Si–O band is replaced by the weaker Si–OH silanol bond (see, for example, Refs. [27,28]). Stress corrosion is also known in sapphire, aluminas [29], in silicon nitride [30] in zinc sulphide [31] and many other ceramics. The present paper has used two types of experiment to look for fatigue in CVD diamond. The first showed that samples could be subjected to 107 cycles at up to 95% of their failure strength without loss of strength by fatigue taking place. The second experiment showed that a crack which initiated at a load of 112 N could sustain a load of 109 N for many days without the crack extending in a normal environment (75% humidity, T~293 K). This does not mean that other environmental conditions would not cause crack extension, and this needs investigation. CVD is often used as a coating. The present research suggests that the CVD would not fatigue. However, a coated system involves the coating, an interface and a substrate and in applications involving repeated cycling the fatigue of the system would need to be studied. However, the conclusion is that CVD diamond does not exhibit fatigue in the normal laboratory environment and that its strength is not reduced by cyclic loading. This provides a significant advantage for CVD diamond over other materials.
Acknowledgements This research has been supported by grants from De Beers Industrial Diamond (now Element Six) under contract with the Japan Atomic Energy Research Institute (JAERI) and DERA (now QinetiQ), Malvern. We thank Dr. C.S.J. Pickles (Element Six) and Dr. T. Mollart (QinetiQ) for their advice and comments, and R. Marrah for his technical assistance.
References [1] R.S. Sussman, J.R. Brandon, G.A. Scarsbrook, C.G. Sweeney, T.L. Valentine, A.J. Whitehead, C.J.H. Want, Diamond Relat. Mater. (1994) 303 – 312. [2] J.E. Field, Wear 233–235 (1999) 1 – 12. [3] C.S.J. Pickles, Diamond Relat. Mater. 11 (2002) 1913. [4] C.S.J. Pickles, J.R. Brandon, S.E. Coe, R.S. Sussmann, in: P. Vincenzini (Ed.), Adv. in Sci. and Tech. 21 (1998) 435. [5] O. Braz, A. Kasugai, K. Sakamoto, K. Takahashi, T. Imai, M. Thumm, Int. J. Infrared Millim. Waves 18 (1997) 1495. [6] A. Kasugai, K. Sakamoto, K. Takahashi, M. Tsuneoka, T. Kariya, T. Imai, O. Braz, M. Thumm, J.R. Brandon, R.S. Sussman, A. Beale, D.C. Ballington, Rev. Sci. Instrum. 69 (1998) 2160. [7] K. Takahashi, K. Sakamoto, A. Kasugai, T. Imai, J.R. Brandon, R.S. Sussman, Rev. Sci. Instrum. 71 (2000) 4139. [8] K. Takahashi, E. Ishitsuka, C.P. Moeller, K. Hayashida, A. Kasugai, K. Sakamoto, K. Hayashi and T. Imai, Fusion Eng. Design 66–68 (2003) 473.
10
A.R. Davies et al. / Diamond & Related Materials 14 (2005) 6–10
[9] P.D. Warren, D.A. Hills, R.G. Roberts, J. Hard Mater. 5 (1994) 213. [10] J.E. Field, The Properties of Diamond, Academic Press, London, 1979. [11] J.E. Field, The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992. [12] J.E. Field, C.S.J. Pickles, Diamond Relat. Mater. 5 (1996) 625. [13] V.R. Howes, Proc. Phys. Soc. 80 (1962) 78. [14] V.R. Howes, in: R. Berman (Ed.), Physical Properties of Diamond, Clarendon Press, Oxford, 1965, p. 174. [15] A.L. Ruoff, J. Wanagel, Science 198 (1977) 1037. [16] N. Ikawa, S. Shimada, Tech. Rep. Osaka Univ. 33 (1983) 347. [17] N. Ikawa, S. Shimada, H. Tsuwa, Ann. CIRP 34 (1985) 117. [18] M.J. Matthewson, J.E. Field, J. Phys. E, Sci. Instrum. 13 (1980) 355. [19] G.H. Jilbert, PhD thesis, University of Cambridge (1997). [20] Z. Jiang, F.X. Lu, W.Z. Tang, S.G. Wang, Y.M. Tong, T.B. Huang, J.M. Liu, Diamond Relat. Mater. 9 (2000) 1734. [21] A.R. Davies, PhD thesis, University of Cambridge (2002).
[22] A.R. Davies, J.E. Field, Wear 256 (2004) 153 – 158. [23] R.P. Brown, in: G. Godwin (Ed.), Handbook of Plastics Test Methods, 2nd ed., Pitman Press, 1981, Limited in Association with the Plastics and Rubber Institute. [24] J.B. Wachtman, Mechanical Properties of Ceramics, Wiley, USA, 1996. [25] A.R. Davies, J.E. Field, K. Takahashi and K. Hada, J. Mater. Sci. 39 (2004) 1571. [26] C.A. Brookes, in: J.E. Field (Ed.), The Properties of Natural and Synthetic Diamond, Academic Press, London, 1992, pp. 515 – 547. [27] S.W. Wiederhorn, J. Am. Ceram. Soc. 50 (1967) 407. [28] B.R. Lawn, Fracture of Brittle Solids, 2nd ed., Cambridge University Press, UK, 1993. [29] A. Evans, J. Mater. Sci. 7 (1972) 1137. [30] A. Evans, S.M. Wiederhorn, J. Mater. Sci. 9 (1974) 270. [31] C.S.J. Pickles, J.E. Field, J. Mater. Sci. 29 (1994) 1115.
Tensile and fatigue strength of free-standing CVD diamond A.R. Daviesa, J.E. Fielda,*, K. Takahashib, K. Hadab a
Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, UK b Japan Atomic Energy Research Institute (JAERI), 801-1, Naka, Ibaraki 311-0193, Japan Received 12 June 2003; received in revised form 1 June 2004; accepted 3 June 2004 Available online 30 July 2004
Abstract Chemical vapour deposited (CVD) diamond is finding increased application and it is important to study its strength and fatigue properties. The present paper describes research on a batch of di-electric (optical) grade CVD material. The procedure in the experiments was to use some samples from a batch to obtain an average failure load. Other samples were then stress cycled to a chosen fraction of this average load. If they survived 107 cycles, they were considered to be immune to fatigue at that load, and their strengths were measured using a three-point bend test. Some samples survived at least 95% of their critical fracture stress for 107 cycles without fatiguing. In other experiments, a 4-point (double torsion) rig was used to stress samples with macro-flaws in different environments. The observation that fatigue does not reduce the strength of CVD diamond in normal environments adds to its attractiveness for various applications. D 2004 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Strength; Fatigue and stress corrosion
1. Introduction Chemically vapour deposited (CVD) diamond is finding increased application where the exceptional properties attained by diamond are required. For example, in applications where high wear or erosion resistances are important, or as optical, infra-red and microwave bwindowsQ in aggressive environments [1–4]. The stimulus for the present research was the use of CVD diamond as a window in a millimetre wave heating system for fusion research. Attractions are the low loss target (tand=2105 at the 100 GHz band), high thermal conductivity (1800–2000 W m1 K1) and high mechanical strength [5–7]. Additionally, the window can also act as a barrier to tritium and radioactive dusts from the heating system [7,8]. Clearly it is important for the window design to have data on the mechanical and fatigue properties. This paper reports
* Corresponding author. Tel.: +44 1223 337 336; fax: +44 1223 350 266. E-mail address: [email protected] (J.E. Field). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.06.015
experiments on the cyclic loading of CVD diamond and its response to fatigue. There are several techniques for measuring the strength of a material. In the case of single crystal diamond, the choice is almost exclusively restricted to indentation methods due to the small specimen size and cost of samples. However, indentation stresses a small volume of material and only part of the flaw distribution is sampled [9]. Thus the strength value obtained depends on the indenter size with the measure strength value increasing as the indenter size decreases; see the reviews of Field [10,11], Field and Pickles [12] and the data of Howes [13,14], Ruoff and Wanagel [15] and Ikawa et al. [16,17]. A further disadvantage with diamond indenting diamond is that the tip of the indenter is susceptible to damage. Chemical vapour deposited (CVD) diamond is potentially easier to study since it can be obtained in disc or plate geometries of up to millimetre thickness. The bursting disk technique is an excellent method for determining the strength of most ceramic materials since the maximum tensile stresses are at the center of the disk and are much smaller near the sample edges [18]. Jilbert [19]
A.R. Davies et al. / Diamond & Related Materials 14 (2005) 6–10
was able to obtain a sufficient number of CVD diamond samples to utilise the bursting disk test. She obtained strength measurements for the nucleation and the growth side, and showed that the nucleation side (smaller grain size) was consistently stronger. These strength measurements were reliable since they were measured from samples that failed in the center, negating any edge effects. Her data are reviewed in Field and Pickles [12]. However, the bursting disk test is often impractical for routinely measuring the strength of CVD diamond due to the fact that only small specimen sizes are readily available. Instead, a three-point bend test is often employed since it may be performed on smaller sections [4,20]. The volume stressed in the three-point bend test is larger than in an indentation test but usually smaller than using the bursting disc methods. A further potential disadvantage with the three-point test is that specimen edges are stressed and unless the specimen sides are polished, as well as its faces, misleading results may be obtained. This is certainly the case with most brittle materials. However, recent research [3,4,21,22] has shown that the strength of CVD diamond is not affected by surface polishing. This is because the bintrinsicQ flaws are so large (the equivalent flaw size, assuming a half-penny geometry, may be 100 Am or more in radius depending on grain size; see [3,19,22]). That a material can be so strong with such large intrinsic flaws shows the potential of CVD if the material could be produced with smaller flaws!
2. Experimental 2.1. Strength The strength of di-electric (optical) grade CVD diamond was determined using a three-point bend test [4]. The geometry and sample size are shown in Fig. 1. The three-point bend test apparatus itself, is illustrated in Fig. 2. The two halves of the three-point-bend test apparatus were held together by four spring-repelled screws, ensuring that the specimens were constrained vertically but not
Fig. 1. Schematic of three-point bend test arrangement.
7
Fig. 2. Three-point-bend test apparatus; photograph and schematic.
horizontally. The side components were fixed to a base stand and the central loading beam was attached to an Instron loading machine. Since the apparatus was immobile, the loading points could be accurately positioned on the specimen with good reproducibility. Note that load bpointsQ are in reality line contacts extending across the top and bottom surfaces of the specimen. The loading beams were made from toughened steel drill blanks of 1 mm diameter. The two lower beams were positioned 10 mm apart, allowing a proportion of the specimen to protrude at the sides. The Instron loading device was first calibrated using a 2kg and a 5-kg weight, and the friction force was measured to be less than 0.01 N when the loading rate was V0.1 mm per minute. The load rate used during the experiments was 0.02 mm per minute. Typical loading curves are shown in Fig. 3. The load at the point of failure for each sample was used to calculate the fracture stress, r. The fracture stress for each CVD diamond sample was determined by inserting the failure load into Eq. (1) [23,24]. r¼
3PL ; 2H 2 W
ð1Þ
where P is the mid-point load failure load, L is the span interval between the two outer support beams, and H and W
Fig. 3. Load to failure for di-electric grade CVD diamonds, in a three-point bend test. The curves are shifted along the time axis to avoid overlap.
8
A.R. Davies et al. / Diamond & Related Materials 14 (2005) 6–10
Table 1 Strengths of CVD diamond samples
Average strength
Strength of nucleation side (MPa)
Strength of growth side (MPa)
727F20 526F20 712F20 813F20 660F20 690F95
305F20 266F20 312F20 235F20 262F20 280F30
are the specimen thickness and width, respectively. The strengths of 1.89 mm thick, di-electric grade CVD diamonds are shown in Table 1. The flaws within the bulk determine the strength of CVD diamond [19,20]. These intrinsic flaws are limited to the size of the grains, and are therefore smaller on the nucleation side. Correspondingly, the CVD diamond samples are stronger if the nucleation side is stressed. 2.2. Fatigue The apparatus for the three-point-bend test was also used in the fatigue studies. However, the middle loading point was connected to an Instron Fatigue loader, model 8872. A 1.89 mm thick sample of CVD diamond was cycle loaded, stressing the growth side, from 10 to 100 N at a rate of 20 Hz. This was 70% of the average growth side strength. The amplitude of the loading cycle was 45 N, centred around a mean load of 55 N. Not only did the sample survive the specified 107 cycles, but it was still intact after 1.5107 cycles; at which point the fatigue part of the experiment was discontinued. The strength of the sample was then determined, using the three-point-bendtest. The sample failed at 140F10 N, signifying that the diamond had a strength of 280F20 MPa; very close to the average strength (see Table 1).
Fig. 5. Load required to initiate a crack.
The process was repeated for another sample at a higher fraction of the average strength. This time the growth side was stressed from 15 to 125 N at a rate of 20 Hz. This was 89% of the average growth side strength. The amplitude of the loading cycle was 55 N, centred around a mean load of 70 N. The sample survived the specified 107 cycles. The strength of the sample was then determined, using the three point-bend-test and the sample failed at 138F10 N corresponding to a strength of 276F20 MPa (again, close to the average strength measurements). The process was repeated on another sample of CVD diamond cycle loaded, stressing the nucleation side, from 10 to 290 N at a rate of 20 Hz. This was 85% of the average nucleation side strength. The amplitude of the loading cycle was 140 N, centred around a mean load of 150 N. The sample survived the specified 107 cycles, and it was still intact after 2107 cycles. The strength of the sample was then determined, using the three point-bend-test and the sample failed at 356F10 N corresponding to a strength of 712F40 MPa. Again close to the average strength; see Table 1. Finally, the process was repeated on another sample of CVD diamond cycle loaded, stressing the nucleation side,
Fig. 4. Schematic of the four-point-bend test apparatus. The distance between the outer and inner spherical contact points in 4 mm.
A.R. Davies et al. / Diamond & Related Materials 14 (2005) 6–10
Fig. 6. Load-cell reading, recorded over ~3 days.
from 10 to 320 N at a rate of 20 Hz. This was 95% of the average nucleation side strength. The amplitude of the loading cycle was 155 N, centred around a mean load of 165 N. The sample survived the specified 107 cycles. The sample was then broken using the three point-bend-test and the sample failed at 356F10 N corresponding to a strength of 712F40 MPa. 2.3. Slow crack growth The fact that CVD diamonds do not readily suffer slow crack growth was confirmed using a simple experiment that made use of a four-point bend (double torsion) test. Rectangular plates of CVD diamond were mounted between four spherical loading points, as shown in Fig. 4. This geometry was used in related experiments to measure the fracture toughness, K IC, of this grade of CVD diamond. A value of 8.3F0.4 MPa m1/2 was obtained [25]. In order to asses the vulnerability of CVD diamond to slow crack growth, a plate of CVD diamond was stressed using the geometry illustrated in Fig. 4. The load was increased until a crack grew from the notch, indicated by an audible cracking sound and a sudden drop in load. This occurred at a load of 112 N, see Fig. 5. The apparatus was then left for over 3 days, at a load of 109 N and with the load recorded every 10 min. The humidity was 75%. The load remained effectively constant over the 3 days, see Fig. 6. The slight fluctuations observed are thought to have arisen in the load-cell as a result of temperature variations over the test period. If the crack had extended, the load would have fallen over the entire length of the experiment.
3. Discussion and conclusions Fatigue in a material is usually the result of dislocation motion or, so-called, atmospheric fatigue (stress/corrosion cracking). Dislocation motion in diamond only takes place at room temperature under extreme conditions (see, for example, Ref. [26]), and is an unlikely mechanism here. A much more likely fatigue process with glasses and ceramics
9
is the second type of process which involves a combination of chemistry plus stress at the crack tip. This is well-known with glasses where the Si–O band is replaced by the weaker Si–OH silanol bond (see, for example, Refs. [27,28]). Stress corrosion is also known in sapphire, aluminas [29], in silicon nitride [30] in zinc sulphide [31] and many other ceramics. The present paper has used two types of experiment to look for fatigue in CVD diamond. The first showed that samples could be subjected to 107 cycles at up to 95% of their failure strength without loss of strength by fatigue taking place. The second experiment showed that a crack which initiated at a load of 112 N could sustain a load of 109 N for many days without the crack extending in a normal environment (75% humidity, T~293 K). This does not mean that other environmental conditions would not cause crack extension, and this needs investigation. CVD is often used as a coating. The present research suggests that the CVD would not fatigue. However, a coated system involves the coating, an interface and a substrate and in applications involving repeated cycling the fatigue of the system would need to be studied. However, the conclusion is that CVD diamond does not exhibit fatigue in the normal laboratory environment and that its strength is not reduced by cyclic loading. This provides a significant advantage for CVD diamond over other materials.
Acknowledgements This research has been supported by grants from De Beers Industrial Diamond (now Element Six) under contract with the Japan Atomic Energy Research Institute (JAERI) and DERA (now QinetiQ), Malvern. We thank Dr. C.S.J. Pickles (Element Six) and Dr. T. Mollart (QinetiQ) for their advice and comments, and R. Marrah for his technical assistance.
References [1] R.S. Sussman, J.R. Brandon, G.A. Scarsbrook, C.G. Sweeney, T.L. Valentine, A.J. Whitehead, C.J.H. Want, Diamond Relat. Mater. (1994) 303 – 312. [2] J.E. Field, Wear 233–235 (1999) 1 – 12. [3] C.S.J. Pickles, Diamond Relat. Mater. 11 (2002) 1913. [4] C.S.J. Pickles, J.R. Brandon, S.E. Coe, R.S. Sussmann, in: P. Vincenzini (Ed.), Adv. in Sci. and Tech. 21 (1998) 435. [5] O. Braz, A. Kasugai, K. Sakamoto, K. Takahashi, T. Imai, M. Thumm, Int. J. Infrared Millim. Waves 18 (1997) 1495. [6] A. Kasugai, K. Sakamoto, K. Takahashi, M. Tsuneoka, T. Kariya, T. Imai, O. Braz, M. Thumm, J.R. Brandon, R.S. Sussman, A. Beale, D.C. Ballington, Rev. Sci. Instrum. 69 (1998) 2160. [7] K. Takahashi, K. Sakamoto, A. Kasugai, T. Imai, J.R. Brandon, R.S. Sussman, Rev. Sci. Instrum. 71 (2000) 4139. [8] K. Takahashi, E. Ishitsuka, C.P. Moeller, K. Hayashida, A. Kasugai, K. Sakamoto, K. Hayashi and T. Imai, Fusion Eng. Design 66–68 (2003) 473.
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