J. Dent. 1992;
nitride coatings in clinical
P. R. Mezger and N. H. J. Creugers Trikon, institute for Dental Clinical Research, Department Nijmegen, Dental School, Nijmegen, The Netherlands
of Oral Function and Prosthetic
ABSTRACT Titanium and its alloys are increasingly important in dentistry. Thin titanium nitride (TiN) coatings improve properties of metallic material for industrial purposes. Recently TiN coatings have been advocated in dentistry. In a survey of the literature, aspects of biological, mechanical and corrosive behaviour are assessed. Moreover, specific problems for clinical application are indicated. It is concluded that as long as the integrity of the TiN coating is not guaranteed under critical conditions, one should be cautious with its clinical application, and in no situation can it be used to improve deficient dental alloys. KEY WORDS: J. Dent. 1992)
nitride coating, Dental materials
Correspondence should be addressed to: Dr P. R. Mezger, TRIKON, Institute for Dental Clinical Research, University of Nijmegen, Department of Oral Function and Prosthetic Dentistry, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
INTRODUCTION (TIN) coatings are used to improve metallic surface properties for industrial purposes. This layer of a few microns thickness renders the surface scratch-proof and is considered to be chemically stable. Its high hardness, low coefficient of friction and resistance to adhesive wear are favourable, for example, for drills and cutting tools. Moreover, its pleasing ‘golden’ appearance is used for ornamental purposes (watches, trinkets). The chemical vapour deposition (CVD) process for industrial TiN coatings has been in use for more than 20 years. The high substrate temperature (1OOO’C) makes this technique unsuitable for dental alloys. In dentistry the more recent physical vapour deposition (PVD) processes may be used. Under vacuum, evaporated titanium is applied on thoroughly cleaned metallic surfaces while admitting nitrogen gas. Substrate temperatures vary from 200 to 500°C. The quantity of nitrogen gas admitted determines the colour of the coating. In dentistry the possibilities of TiN coatings are evaluated, in particular because of the excellent biological properties of titanium Extremely
Q 1992 Butterworth-Heinemann 0300-5712/92/060342-03
and the aesthetic aspect of the ‘golden coloui. Elimination of electro-galvanic corrosion due to dissimilar casting alloys in one mouth (e.g. complicated removable partial dentures with precision attachments, telescopic crowns and soldering joints) is another incentive cited for TiN dental research. Masking of the greying effect of silver-coloured dental alloys by a ‘gold-coloured’ layer turns out to be a ‘hot’ item in dentistry. The gilding process produces a non wearresistant, porous surface, so a TiN coating may be a viable alternative. Crowns, partial fixed and removable prostheses made from casting alloys ranging from silverpalladium (Ag-Pd), cobalt-chrome-molybdenum (CoCr-Mo), nickel-chromium (Ni-Cr) to stainless steel received TiN coatings (Aleksandrov et al., 1984; Laetzsch et al., 1984; Buth et al., 1989; Buth, 1991). Published research mostly focuses on biological, mechanical and corrosive aspects of the coating. A survey of literature on the medical/dental application of TiN coatings is hindered by limited accessibility of studies published in non-international journals (Japan, former USSR and E. Germany).
BIOCOMPATIBILITY In medical/dental literature the biocompatibility of TiN is compared to that of totally different materials. Therefore, reference materials are included in the following survey. TiN coatings on a Ag-Pd and Co-Cr-Mo alloy showed similar biological behaviour as the base materials (Laetzsch et al., 1986; Laetzsch and Pilgrim, 1988). Compared to stainless steel, TiN was more compatible in cell culture and animal experiments (Suka, 1986). The repair of mandibular bone of dogs around implants coated with TiN was more rapid than around the Co-Cr base material (Panikarovskii et al., 1985). Three-day subcutaneous reactions for TIN and hydroxyapatite in rats were less favourable than for TiO, and pure Ti. However, after 84 days all implanted materials were completely encapsulated by matured fibrous connective tissue and no active proliferation was evident. The fibrous membrane thickness around all solid implants was favourably low and only hydroxyapatite showed a significantly thicker capsule compare to TiN (Satomi et al., 1988). On the other hand, in the femur of dogs, hydroxyapatite showed more promising results than TiN coatings (Hayashi et al., 1989). In general, biocompatibility of TiN in cell culture and animal tests has been evaluated positively, in agreement with preliminary clinical observations (Buth, 1991). Longitudinal clinical information, however, has been deficient up to now.
Wear and yield strength of the TIN and underlying alloy combination are the most common mechanical properties tested. Compared to dental Ag-Pd and Co-Cr-Mo alloy samples, wear characteristics of TiN-coated specimens were superior (Laetzsch et al., 1986). Research using a knee joint simulator did not show any signs of wear or scratching on a Co-Cr-Mo or a TiN-coated surface (unlike a Ti-6Al-4V alloy) (Peterson et al., 1988). The thickness of the TiN coating on stainless steel determined resistance to frictional wear. Only a 3 urn thick layer could protect the substrate from wear. The 1.5 urn layer disappeared while a 4.5 ym coating turned out to be too brittle. Application of a TiN coating resulted in a decrease in yield strength, though the grain structure of the substrate did not change (Suka, 1986). After bending the TiN coating on Co-Cr-Mo removable partial dentures showed cracking. Submersion testing of these TiN-coated removable partial dentures in an artificial saliva solution resulted in signs of corrosion (Wirz, 1989). The extreme hardness of TiN contrasts with the substrate material. In particular, the more or less elastic part of partial removable denture clasps may suffer from localized cracking off of the coating (Benzer. 1991). In conclusion, wear characteristics of TIN are good but seem to depend on the coating thickness. The extreme
TiN in dentistry
hardness of the TiN layer may lead to microcracking cases of elastic deformation of the substrate.
CORROSION The good corrosion resistance of titanium and its alloys is related to the formation of a protective (passive) surface layer. The corrosion behaviour of the TiN and substrate combination depends on the presence or absence of porosity in the coating. Scratching, wear or selective grinding may (also) lead to mechanical perforation of the surface. Thus the coating will be locally disturbed inducing attack on the substrate. For instance, porosity and cracking of a TiN layer induced signs of pitting and crevice corrosion in a corrosion-resistant Co-Cr-Mo alloy substrate (Wirz, 1989). Defects in the TiN layer were caused by carbides, dust particles and surface roughness of a Co-Cr-Mo alloy surface which was meticulously cleared prior to coating (Wisbey et al., 1987). Potentiodynamic testing in 0.17 M NaCl of TiN-coated, polished Co-Cr-Mo samples showed an even more favourable pitting potential. This effect was absent on 40-60 mesh grit-blasted specimens. In a 0.17 M NaCl and 2.7 X 1O-3 M EDTA solution, release of Co, Cr and to some extent MO decreased after coating. However, a certain amount of Ti-ion release from the coating itself was noted (Wisbey et al., 1987). The lack of porosity of a TiN coating on (316 L) stainless steel was established by measuring the current change during potentiostatic polarization in a 1.5 M H,SO, solution at an extreme potential of 1100 mV (SCE). Galvanic coupling experiments showed that strong polarization of the coating reduced dissolution of the (anodic) steel (Gluszek et al., 1990). In spite of this favourable result it was concluded that further research was necessary to assess the effectiveness of the coating on edges and cavities and mechanical and biological aspects (Gluszek et al., 1990). Recently it was indicated (without relevant references) that TiN coatings were (electro)chemically unstable in time in body electrolytes, resulting in a more or less unstoichiometric oxide (Thull, 1991). As an alternative coating a titanium-niobium-oxinitride ([TiNb]GN) was advocated as a means to reduce the number of casting alloys exposed in the mouth. Corrosion testing of a large but not proportional choice of the casting alloy spectrum was executed in a physiological NaCl solution at 37 “C. In spite of the presence of pores extending to the substrate (an estimate of l-5% of the specimen surface), cyclic polarization only indicated a rise in current density possibly related to crevice corrosion after coating for the one high-palladium alloy investigated. Galvanic coupling resulted in pH-dependent quite low current densities (Thull, 1991). In summary, the corrosion behaviour of TiN coatings seems to be acceptable. However, the surface layer turns out to be porous to some extent. The use of a coating to
J. Dent. 1992; 20: No. 6
improve the behaviour of non-corrosion-resistant alloys is therefore not recommended (Peterson et al., 1988; Wirz and Schmidli, 1991).
Clinical application of TiN coatings in dentistry enhances specific problems. For instance, partial removable framework design must absolutely exclude occlusal interference. -
Selective grinding cannot be done without damage to the coating. - For resin-bonded bridges a TiN layer reduces bonding possibilities and even silane application is hardly effective in overcoming this problem (Wirz, 1991). - After porcelain fusing, the metallic part of a crown or bridge cannot be selectively coated with TiN yet (Grohmann and Hegi, 1989).
Indirect dental restorations are the result of collaboration between the dentist and the dental technician. The industrial PVD process is supplied by a third party. Quality control in this longer chain may become a problem. This was confirmed by a study investigating TIN coatings from different sources. Marked differences in composition of the coating (addition of Wolfram or a secondary gold layer for aesthetic reasons), as well as variation in the layer thickness and corrosion behaviour occurred (Wirz, 1989). Apart from technical, functional and aesthetic reasons, patent protection may be the reason for modification of a coating composition. The aesthetic importance of a (wear-resistant) goldcoloured layer in clinical dentistry is questionable and probably time and culture dependent. The TiN coating certainly has remarkable properties. However, improvements are necessary, especially with respect to the continuity of the coating (absence of pores or microcracking). Moreover, longitudinal clinical research is needed before dental application may be recommended. References Aleksandrov N. M., Alekseev L. I., Levchenko S. I. et al. (1984) Experience made of stainless Zh. 2, 58-59.
in coating bridge and clasp prostheses steel with titanium nitride. Voen. Med.
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