Chemical vapour deposition (CVD) is an established technology for coating a wide range of metal cutting tools, including drills, hacksaws, band saws, razor blades and inserts. In some cases, CVD can increase the life of a cutting tool by as much as 20 times the life of an uncoated tool. In addition the cutting efficiency, cutting speeds and the quality of cutting of the workpiece are significantly improved by surface engineering. The technology can be used to deposit a wide variety of coatings such as TiN, TiAlN, multilayers, graded coatings and novel new nanocomposite coatings, for a broad range of applications. Yet in one area there has been little CVD work carried out - the surface treatment of biomedical implants and dental tools such as burs, orthodontic pliers and tweezers, all of which can benefit in terms of quality, safety and cost from the application of a CVD coating.
Diamond Coated Dental Tools
Diamond-coated dental burs are commonly used on patients, as well as in dental laboratories. Dental burs are used for several purposes, including the preparation of teeth cavities and, in many cases, preparation of the teeth themselves for crown and bridgework and partial dentures. Dental burs are also used extensively in the dental laboratory for cutting, drilling, grinding, trimming and polishing of various types of materials such as dentures and metal frameworks.
The burs are made by fixing hard diamond particles onto a substrate surface using a binder matrix material. Dental burs are currently specified by the dimensions of the bur head and the length of the shaft. There is no specification for the grinding surface. In the case of diamond coated burs in particular, there is no standardisation of the grit size or quality of the diamond particles used. The average grit size of diamond particles can vary widely from 50-300µm.
Limitations and Problems Associated with Diamond Coated Dental Tools
There are certain problems with the long-term quality and effectiveness of dental tools, and burs in particular. For example, the particles on some dental tools wear off quite quickly, rendering the tools ineffective after only a short time in operation. With diamond coated dental burs, the cutting and trimming effectiveness decreases owing to repeated sterilisation, disinfection and cleaning, processes which employ elevated temperatures and acidic environments. One significant finding was the discovery of the corrosive action on carbon steel burs of a phosphoric acid based cleaning solution, which was in routine use at the time. In another instance, three cases of tungsten carbide bur separation, one of which resulted in the patient swallowing the separated bur head, have been described. This spate of bur heads separating from the shank was associated with a cold sterilising solution used for disinfection.
Coating particles from dental burs also present a health hazard should they come away from the bur in the patient's mouth - for example, there is a potential release of Ni2+ ions from the metallic binder of the diamond coated dental burs into the body, which could possibly be toxic to the patient. This aspect not only poses a risk to the respiratory system of the patient, the dentist and the nurse, but also causes contamination of the ceramic during the laboratory manufacturing of dental restorations.
Alternative Coating Techniques
Owing to these limitations described above, there is a growing demand for better quality, long-lasting and more economical dental tools. An attractive way of catering for this demand and overcoming contamination/health issues is to use a surface treatment technology. Several methods can be used for coating, including sputtering, evaporation, ion implantation and plasma-assisted chemical vapour deposition (CVD), figure 1.
Figure 1. Generalised schematic of the processes in a diamond CVD reactor
Each method has its 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, which makes it difficult to use when treating a complex shaped tool such as a dental bur. For other applications, such as treating silicon chips on fiat substrates, ion implantation is unrivalled for introducing controlled amounts of dopants such as phosphorus, boron and arsenic.
Advantages of CVD
However, CVD is likely to be the future choice for surface coating of dental burs. The major advantage of CVD over the other surface engineering techniques is its ability to coat, uniformly, complex components such as dental burs, dental drills, pliers and tweezers. Additionally, it is possible to apply continuous layers of coatings onto the substrate material, and so make the tool last longer. Another benefit is that CVD coatings can be applied economically and on a large scale with minimal cost towards the equipment used.
Disadvantages of CVD
One disadvantage of CVD is that it frequently employs precursors that can pose a health hazard, are environmentally unfriendly and flammable. For the deposition of diamond coatings, the CVD process involves decomposition of chemical precursor gases, usually methane and hydrogen, which are activated and undergo gaseous reactions. They are then transported via convective and diffusive flow mechanisms to the substrate. Once there, heterogeneous gas/surface processes give rise to the nucleation and growth of a diamond film if the conditions are favourable. By optimising the deposition conditions, the surface properties of the coating can be tailored to suit application.
The fundamental problem of diamond synthesis is caused by the allotropic nature of carbon. Under ordinary conditions graphite, not diamond, is the thermodynamically stable crystalline phase of carbon. So the main requirement in diamond CVD is to deposit carbon with sp3 bonds and simultaneously suppress the formation of graphitic sp2 bonds. This is done 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 using a plasma.
In general, the adhesion of coatings such as diamond, graded coatings, multilayers and nanocomposites, applied by processes including CVD, ion assisted deposition and plasma CVD on complex surfaces is rather poor. Possible methods of improving coating/substrate adhesion include abrasion of the substrate with various powders, substrate biasing, pulsed biasing and the use of interlayer materials.
New Concepts – High Frequency CVD
Using a modified high frequency CVD (HFCVD) figure 2, incorporating a hot filament CVD system in a water cooled stainless steel vessel with controlled gas flow rates, the system allows independent bias to be applied between the substrate and filament. The filament consists of flat coiled tantalum wire of diameter 0.5mm to activate the reaction mixture. The process can be adapted to cater for the deposition of thin film coatings on several types of dental burs such as tungsten carbide, diamond coated, and stainless steel burs.
Figure 2. Schematic of a modified HFCVD system that could be used for producing diamond coatings on dental burs.
HFCVD can be used to fabricate new diamond burs by applying a continuous coating on the cutting edges. The technology eliminates the need to use binder material present in conventional diamond burs. As a result it has potential for overcoming problems with contamination of the oral tissues (and subsequent infections), improving the cutting efficiency and increasing tool life.
Methods to Enhance Coating/Substrate Bonding
Coating/substrate adhesion can be enhanced by carrying out several pre-treatments of the substrate. Among these treatments is the roughening of the substrate surface using various powder mixtures such as diamond, alumina, and silicon carbide. A recent study has indicated that the controlled roughening of the surface of the substrate can increase the diamond nucleation density of the coating material.
Substrate biasing is another surface pre-treatment method that can be employed. Biasing is a much more controllable technique than abrasion and it can also enhance diamond nucleation density on various substrates. It is an in-situ method in which the substrate is either negatively or positively biased with respect to the filament. During biasing a glow discharge is generated and the substrate is exposed to a plasma for a period of up to 30 minutes. The substrate is bombarded with ions, creating nucleation sites for subsequent diamond deposition. 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.