Chemical Vapor Deposition of Diamond Coatings onto Dental Burrs

Chemistry for Everyone edited by

Products of Chemistry

George B. Kauffman California State University Fresno, CA 93740

Chemical Vapor Deposition of Diamond Coatings onto Dental Burrs Waqar Ahmed,* Htet Sein, and Hussam Rajab Department of Chemistry and Materials, Manchester Metropolitan University, Manchester M1 5GD, United Kingdom; *[email protected] Mark Jackson Department of Mechanical Engineering, Tennessee Technological University, Cookeville, TN 38505

Chemical vapor deposition (CVD) has become an established technology for coating a wide range of metal cutting tools including drills, hacksaws, band saws, razor blades, and inserts. Superior performance and extended lifetimes have been achieved with this technology. CVD involves decomposing chemical precursors on a heated surface resulting in the formation of an adherent coating onto the substrate. To obtain a better fundamental understanding of the CVD process a detailed study of the gas–surface interactions, reaction kinetics, surface chemistry, and chemical interactions needs to be carried out. Well-defined CVD processes, such as the deposition of polycrystalline diamond on silicon, offer good case studies for understanding the chemical kinetics and mass transport processes in heterogeneous systems. The structure and properties of the polycrystalline films are highly dependent on the process parameters such as temperature, pressure, gas flow rates, and reactor geometry. By controlling the

Figure 1. Common type of dental burrs used in the dental surgery and laboratory.

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process parameters, the structure and properties of the surface can be tailored to suit the application. To deposit polycrystalline diamond, a methane and hydrogen mixture is used in a CVD reactor under vacuum conditions. Despite the widespread use of CVD, very little work has been carried out on the surface treatment of biomedical implants and in particular dental tools such as burrs (Figure 1), drills, orthodontic pliers, and tweezers. CVD Diamond Coatings on Dental Burrs In general, diamond dental burrs are used on patients as well as in the dental laboratories to remove material such as enamel, dentine, and fillings (1). Conventional diamond burrs are manufactured by binding hard diamond particles onto the substrate surface using a binder matrix material (Figure 2). There are several limitations to these burrs: the particles embedded in the matrix material wear off quickly rendering the tools ineffective after a short time in operation; there is a heterogeneity of grain shapes and sizes in the diamond dental burrs; and the cutting and trimming effectiveness decreases owing to repeated sterilization. There is also the health hazard problem associated with the binder particles coming off from the burr in the patient’s mouth: the potential release of Ni2+ ions from the metallic binder of the dental diamond burrs into the body fluids, which could be toxic to the patient. The loose Ni2+ particles not only pose a risk to the respiratory system of the patient, the dentist, and the nurse but also contaminate the ceramic pieces during the laboratory manufacturing of dental restorations. There is a growing demand for better quality, long-lasting, and more economical dental tools. An attractive way of overcoming the contamination and health issues is to use a surface-treatment technology for burr manufacture. Several coatings methods have been used in the past for conventional cutting tools including sputtering, evaporation, ion implantation, and plasma-assisted chemical vapor deposition. These treatments have been proven and are widely used to increase the life and performance of metal cutting tools. The choice of technique depends on the substrate material, the surface properties required, and the size and number of components to be coated. Each technique has its own particular advantages and disadvantages. For example, ion implantation can give very hard surfaces without changing the dimensions of the tool, but it is a line-of-sight technique making it diffi-

Journal of Chemical Education • Vol. 80 No. 6 June 2003 • JChemEd.chem.wisc.edu

Chemistry for Everyone

cult to treat a complex-shaped tool such as a dental burr. However, for applications such as silicon chips on flat substrates, ion-implantation is unrivalled for introducing dopants such as phosphorus, boron, and arsenic. CVD is a promising choice for coating the surfaces of dental burrs for several reasons. The major advantage of CVD over the other surface engineering techniques is the ability to coat complex components such as dental burrs, dental drills, pliers, and tweezers uniformly (2–7). It can be used to coat inside trenches, grooves, and holes. Also, it is possible to apply continuous layers of coatings onto the substrate material, thus allowing the tool to last longer. CVD coatings can be done economically and on a large scale. One disadvantage of CVD is that it frequently employs precursors. These precursors often pose a health hazard and may be environmentally unfriendly and flammable. However, good procedures have been developed to handle these gases safely both in the laboratory and on the production line. The decomposition of chemical precursors often requires high temperature A

that limits the use of CVD on heat-sensitive materials. However, modern CVD methods supply additional energy to the precursors enabling lower substrate temperatures to be utilized. For the deposition of diamond, usually methane and hydrogen are activated using a hot filament or microwave power and undergo gaseous reactions at the surface. Gases are then transported via convective and diffusive flow mechanisms to the substrate. Once at the substrate, heterogeneous gas and surface processes give rise to the nucleation and growth of a diamond film under favorable conditions. By optimizing the deposition conditions, the surface properties of the coating can be tailored to suit the application. The fundamental problem of diamond synthesis is the allotropic nature of carbon. Under ordinary conditions graphite, not diamond, is the thermodynamically stable crystalline phase of carbon. Hence, the main requirement of diamond CVD is to deposit carbon with sp3 bonds and simultaneously suppress the formation of graphite sp2 bonds. The proportion of sp3 to sp2 bonds can be increased by establishing high concentrations of nondiamond carbon etchants such as atomic hydrogen. Usually, these conditions are achieved by admixing large amounts of hydrogen to the process gas and by activating the gas either thermally or by plasma. The hydrogen etches the graphite at a much faster rate than diamond, which ensures that the graphite is consumed and diamond growth is encouraged. Hot Filament Chemical Vapor Deposition System A modified hot filament chemical vapor deposition system (HFCVD) was used to uniformly coat dental burrs (Figure 3). This custom designed HFCVD system is composed of a water-cooled, stainless-steel vessel with mass-flow controllers to accurately monitor the quantities of gases flowing into the reactor. The system allows an independent bias voltage to be applied between the substrate and filament. The filament consists of coiled 0.5-mm diameter tantalum wire to activate the reaction mixture of gases. The methane and hydrogen mixture is decomposed by the hot filament into a highly reactive complex “soup” of ions, radicals, and atoms. These decomposed gas-phase species are transported towards

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filament voltage

dental burr H2/CH4

molybdenum holder

filament

pump bias voltage

Figure 2. (A) SEM image of diamond particles embedded in a conventional dental burr. (B) Close-up view of diamond particles embedded in a conventional dental burr.

Figure 3. The HFCVD system used to deposit diamond coatings onto dental burrs.

JChemEd.chem.wisc.edu • Vol. 80 No. 6 June 2003 • Journal of Chemical Education

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the heated substrate by combination of laminar, convective, and diffusive flow mechanisms (Figure 4). Generally, the chemical reactions between the reactive species and the heated substrate result in the formation of a thin diamond film and gaseous byproducts that are pumped away into the exhaust system. The exact details of the homogeneous and heterogeneous chemical reactions are not well understood. The filament temperature is measured using an optical pyrometer. The substrate temperature is measured using a Ktype thermocouple in direct contact with the substrate (3). Plasma-based CVD processes for the deposition of thin film coating were carried out on several types of dental burrs such as tungsten–carbide burrs with different blade geometries (Figure 5). Coating Properties and Tool Performance A critical factor in the performance of the coating is the adhesion between the coating and the substrate. We are investigating ways of enhancing the coating adhesion to the substrate by carrying out several pretreatments of the substrate. Prior to pretreatment, the cutting tools (substrates) are ultrasonically cleaned in acetone for 10 minutes to remove loose residues. The cobalt binder in the cemented tungsten– carbide cutting tool surface can suppress diamond nucleation thus causing deterioration of diamond film adhesion (8–9). To eliminate this problem, it is necessary to pretreat the tungsten–carbide tool surface prior to diamond deposition. Chemical pretreatment enables the removal of cobalt from the surface of the tool. Prior to diamond deposition, the tungsten–carbide substrates are etched using Murakami’s solution1 for 20 minutes in an ultrasonic bath to obtain a rough surface. The surface cobalt is removed by a 10-second acid etch2 followed by washing in an ultrasonic bath with distilled water (Figure 6).

Since dental burrs operate at very high rotational speeds, in the range of 20,000–40,000 rpm, good adhesion between the substrate and coating is critical (5–11). Substrate biasing is another surface pretreatment method that we employ. Biasing is a more controlled method than the commonly used abrasion methods, and it can also enhance diamond nucleation density on various substrates. This is an in situ method where the substrate is negatively biased with respect to the filament. During biasing, a glow discharge is generated and the substrate is exposed to plasma for a period of up to 30 minutes. The substrate is bombarded with positive ions attracted towards the negative substrate creating nucleation sites for subsequent diamond deposition.3 This process is believed to inflict relatively minor damage to the substrate compared to conventional polishing procedures. The method is particularly attractive for applications requiring controlled and reproducible surface sites for nucleation and growth.

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B

Figure 4. Generalized schematic processes of gas–surface reactions in the CVD system.

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Figure 5. (A) SEM image of a tungsten–carbide dental burr with ten cutting edges. (B) SEM image of a tungsten–carbide dental burr with six cutting edges.



HFCVD can be used to fabricate new diamond burrs by applying a continuous coating on the cutting edges. SEM images of the coatings show that good quality diamond coatings can be deposited onto the cutting surface of the dental burrs (Figures 7 and 8A). The coatings adhered very well to the substrate. Trava-Airoldi et al. (12) and Sein et al. (13) have compared the cutting performance of CVD diamond burrs to conventional burrs using a horizontal filament arrangement rather than the vertical one used in this study. Borosilicate glass was drilled repeatedly using a sequence of 50 drilling tests. After 10 drilling operations the geometry of the burr, the sharpness and diameter, was inspected with an optical microscope. A good cutting tool produces accurate cutting with very little or insignificant loss of material from the cutting surfaces and therefore will have an abrading coefficient close to unity. In conventional burrs, the performance dete-

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riorated after 50 drilling operations by at least a factor of ten. In contrast, in the CVD coated burr there was practically no deterioration of performance after 50 drilling operations. Long-term tests on the CVD coated burr showed that even after 1000 drilling operations the abrading factor remained very close to unity, indicating no practical deterioration in the cutting performance. Using 30,000 rpm, we found that the CVD diamond dental burrs do not degrade in performance even after drilling 500 holes in borosilicate glass (Figure 8B). The conventional diamond imbedded burrs revealed that most of the imbedded diamond particles have fallen off after drilling 500 holes (Figure 9). These results demonstrate the promise of CVD as a viable alternative to conventional diamond dental burrs. Additional work on extracted teeth showed that the CVD coated burrs do not deteriorate in performance indicating that performance on live subjects should show similar improvements.

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B B

Figure 6. (A) Surface of a tungsten–carbide dental burr before chemical treatment. (B) Surface of a tungsten–carbide dental burr after chemical treatment.

Figure 7. (A) CVD diamond-coated dental burr shows continuous diamond films on cutting edges. (B) CVD diamond-coated dental burr with different cutting geometry shows diamond can be coated inside trenches and grooves of cutting edges.


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B Figure 9. Close-up view of diamond particles on a conventional embedded dental burr shows significant loss of diamond particles after drilling 500 holes in glass. Note the scale versus Figure 8.

the tool lifetime. The modified vertical filament arrangement used in the HFCVD system shows that continuous and uniform diamond films can be obtained on complex 3-D shape substrates. Testing results showed that the CVD coated burrs lasted much longer than conventional burrs. Further work is required in optimizing the technology for coating multiple components in a single run in order to make CVD more economically viable. Figure 8. (A) Close-up view of diamond particles on a CVD dental burr before drilling in glass. (B) Close-up view of diamond particles on a CVD dental burr after drilling 500 holes in glass.

There is still considerable work to be done to scale up the technology to be able to coat multiple components economically (14). For example, to coat one burr per run with a coating thickness of 500 µm using a HFCVD system costs approximately $8.00. However, if 100 dental burrs could be coated in a single run the cost per burr per run would decrease to a more practical level of about $1.00. Further research in scaling factors and the subsequent impact on the film quality needs to be accomplished for an economically feasible and improved burr. Conclusions CVD technology is a promising technology for coating dental burrs with adherent diamond coatings to improve the their performance and lifetime. The technology eliminates the need to use binder material present in conventional diamond burrs. CVD thus has the potential to overcome problems with contamination of the oral tissues and subsequent infections, to improve the cutting efficiency, and to expand 640

Acknowledgments Norman Jenkinson (MMU) is acknowledged for SEM analysis. Notes 1. Murakami’s solution is composed of 10 g K3Fe(CN)6, 10 g KOH, and 100 mL water. 2. The acid etch solution is composed of 3 mL of 96% H2SO4 solution and 88 mL of 30% H2O2 solution. 3. It is also possible to apply a positive charge to the substrate and bombard the substrate with negatively charged particles.

Literature Cited 1. Nicholson, J. W.; Anestice, H. M. J. Chem. Educ. 1999, 76, 1497–1501. 2. Afzal, A. Effects of Nitrogen on CVD Diamond Growth. Ph.D. Thesis, Manchester Metropolitan University, Manchester, England; 1999. 3. Ahmed, W.; Rego, C.; Cherry, R.; Afzal, A.; Ali, N.; Hassan, I. U. Vacuum 2000, 56, 153–158. 4. Ali, N.; Ahmed, W.; Fan, Q. H.; Rego, C. A.; O’Hare, I. J. Mater. Sci. Tech. 1999, 7, 15–24.


Chemistry for Everyone 5. Ali, N.; Ahmed, W.; Fan, Q. H.; Rego, C. A.; Hassan, I. U.; O’Hare, I. Thin Solid Films 1999, 163, 355–356. 6. Ali, N.; Ahmed, W.; Fan, Q. H.; Rego, C. A. Thin Solid Films 2000, 377, 208–213. 7. Rajab, H.; Ali, N.; Sein, H.; Cherry, R.; Ahmed, W. Materials World 2000, 8, 17–19. 8. Gusev, M. B.; Babaev, V. G.; Khvostov, V. V.; Lopez Ludena, G. M.; Brebadze, A. Y.; Koyashin, I. Y.; Alexenko, A. E. Diamond and Related Materials, 1997, 6, 89–94. 9. Endler, I.; Brash, K.; Leonhardt, A.; Scheibe, H. J.; Ziegele, H.; Fuchs, I.; Raatz, C. Diamond and Related Materials, 1999, 8, 834–839.

10. Ali, N.; Ahmed, W; Fan, Q. H. Surf. Eng. 2000, 16, 421– 426. 11. May, P.; Rego, C. A.; Ashfold, M. N. R.; Rosser, K. N.; Lu, G.; Walsh, T. D.; Holt, L.; Everitt, N. M.; Partridge, P. G. Diamond and Related Materials 1995, 4, 794–797. 12. Trava-Airoldi, V. J.; Moro, J. R.; Corat, E. J.; Goulart, E. C.; Silva, A. P.; Leite, N. F. Surf. Coatings Tech. 1998, 108, 437– 441. 13. Sein, H.; Ahmed, W.; Rego, C. A. Diamond and Related Materials 2002, 11, 731–735. 14. Siegal, S. C.; Fraunhofer, J. A. V. J. Indian Dental Assoc. 1998, 129, 740–745.


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Chemical Vapor Deposition of Diamond Coatings onto Dental Burrs

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