226 J. Dent. 1991; 19: 226-229

Properties of a titanium nitride coating for dental instruments

J. G. Steele, J. F. McCabe* and I. E. Barnes Department of Operative Dentistry and *Dental Materials Science Unit, The Dental School, University of Newcastle upon Tyne, UK

ABSTRACT Adhesion of tooth-coloured restorative materials to the instruments used to place them is a clinical problem. This paper examines the low-stick properties and the durability of a 5 pm coating of titanium nitride on stainless steel when used with two composites (a hybrid and a microfilled) and a glass polyalkenoate (ionomer) cement. Titanium nitride-coated dies were compared to polished stainless steel for adhesion to the unset restorative material before and after a period of wear, and also for properties of surface hardness, contact angle with unfilled resin and frictional coefficient. The results demonstrated that, while the coated instruments were significantly harder and showed a lower coefficient of friction and a higher contact angle with resin, they were slightly ‘stickier’ than highly polished stainless steel, and this difference persisted even after wear. However, the material is very hard, resistant to damage, and appears to have little potential to discolour the restorative materials with which it is used. It is concluded that a titanium nitride coating confers no additional advantage as regards low-stick properties, over clean, polished stainless steel used for dental instruments.

KEY WORDS: Dental instruments, Composite placement

J. Dent. 199 1; 19: 226-229 (Received 10 January 1991; reviewed 1 March 1991; accepted 3 April 1991)

Correspondence should be addressed to: Mr J. G. Steele, Department of Operative Dentistry, The Dental School, Framlington Place, Newcastle upon Tyne NE2 46W, UK.

INTRODUCTION

Adhesion of unset composite material to dental instru- ments during placement is a common problem, which leads to difficulty in adapting the material to cavity walls, problems of achieving a reasonable surface finish and theoretical reductions in the physical properties of the material due to porosity and void formation. Another potential disadvantage of placing composite with conven- tional stainless steel dental instruments is that of abrasion of the metal surface leading to contamination and discolouration of the composite with metallic particles. However, the validity of this as a clinical problem has been questioned (Kanter er al., 1979; Deubert and Jenkins, 1982).

Methods recommended to overcome these difficulties have included modification of the placement technique using conventional instruments (Chadwick et al., 1990), using a syringe to inject the material into the cavity (Sturdevant, 1985) and the use of various low-stick surfaces on the placement instruments, including Delrin and Teflon (Kanter et al., 1979; Sturdevant, 1985).

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Aluminium oxide and titanium nitride (TiN) coatings on metal instruments have also been tested as potential low- stick surfaces (Kooner et al., 1989). Other authors have recommended using cotton wool soaked in resin to compress the composite into the cavity (Craig, 1970) or the use of unfilled resin or alcohol on the instruments as a separator (Kanter et al., 1979; Sturdevant, 1985; Tjan and Glancy, 1988). Tjan and Glancy (1988) recently found a detrimental effect on bond strength between increments where alcohol was applied to the instrument as a separator, but found no such disadvantage with unfilled resin. However, many of the techniques and materials mentioned above remain empirical and their relative advantages and shortcomings are uncertain.

The ideal solution to this problem would seem to be the use of an instrument constructed of a material which has inherent low-stick properties of its own and which will not break down or deteriorate quickly with usage. Stainless steel instruments with a 5 pm TiN coating have recently become available (Prima Instruments, Surrey, UK). This material has proved to be useful in engineering and

Steele et al.: Titanium nitride-coated instruments 227

industry on account of its low coefficient of friction. Initial work on dental instruments showed promising results when the wear characteristics were compared with aluminium oxide-coated instruments in a study (Kooner et al., 1989), but the low-stick properties of TiN as a coating for dental instruments have not yet been fully investigated.

This study was undertaken to determine the low-stick qualities of a TiN-coated stainless steel surface compared to an uncoated surface.

MATERIALS AND METHODS

The adhesion to various restorative dental materials of two conical, round-tipped, highly polished, uncoated stainless steel dies was compared to two tips of identical design with a 5 urn TiN coating, identical to that of commercially available low-stick instruments recom- mended for use with composites. The dies were cones of base diameter 9.5 mm and height 9.0 mm, with a rounded tip of diameter 1.5 mm. The cone was attached to a cylindrical base with a threaded hole tapped to allow attachment to a testing machine.

The composite material was placed in a cavity (7 mm diameter and 5.5 mm depth) cut out of a resin block which was slightly undercut to ensure retention of the material. The composite was rendered flush with the top of the well using a flat, plastic instrument. The block was secured in an Instron Universal Testing Machine and a die, attached to a tension load cell, lowered to just contact the flat surface of the composite. The tip was then inserted to a depth of 2.5 mm and allowed to equilibrate to zero force, before being withdrawn. The force required to separate the two materials was recorded. Each test was repeated 20 times for each tip. Between each test the die was immersed in acetone for at least a minute before being gently wiped with a soft tissue.

The restorative materials against which the tip was tested were a hybrid urethane dimethacrylate composite resin (Occlusin, ICI Dental, Macclesfield, UK), a micro- tilled combined BisGMA and urethane dimethacrylate composite (Heliosit, Vivadent, Schaan, Lichtenstein), and a pre-encapsulated glass polyalkenoate (ionomer) restora- tive material (Ketac-til, Espe, Seefeld/Oberbay, Germany). Testing against the light-cured composites was under- taken in a dimly lit room in all cases to prevent any alteration in the properties of the composite during the procedure, and the same batch of product was used for all tests. The glass polyalkenoate material was tested in the unset state exactly 1 min after the start of mixing.

Each die was subjected to an identical wear regime after the initial testing of fresh dies was complete, and the adhesion test was then repeated. The wear regime consisted of the rotation of a die tip, mounted on a domestic electric drill, in a well of cured composite (matched exactly to the tip dimensions). The same brand of composite, in an unset state, was used as an abrasive

paste between die surface and cured composite. Both the microfilled and the hybrid composites were used against different dies in this way. The force applied was controlled at 25 N by mounting the whole assembly on a load cell, and the depth of each well, the time of application of the force and the rotation speed of the tip (minimum setting on the domestic drill) was constant for all dies. Each wear sequence consisted of five I-min cycles with the abrasive being replaced after every second cycle. Each tip surface was gently cleaned with acetone and a soft tissue. The adhesion testing process was then repeated as before.

The cnntact angle between both surfaces and untilled UEDMA resin were measured by applying a drop of the material onto the flat end surface of a cylindrical tip, which had been precleaned in acetone. The contact angle made between the liquid and the two test surfaces was measured after 30 s using a calibrated microscope. Measurements were made at 10 evenly spaced points around the circumference of the drop. The surface was then cleaned with acetone for 1 min and wiped dry with a clean tissue before repeating the test with fresh material.

The frictional coefficient was measured for both surfaces against a clean dry Perspex surface. A cylindrical die was placed, smooth flat end down, onto the surface and the Perspex raised gradually at one end until smooth movement of the die down the slope occurred. The frictional coefficient was calculated by taking the tangent of the angle of the slope when the first smooth movement occurred. Ten flat-ended cylindrical tips were tested in this way with each test repeated 10 times.

Five fresh cylindrical tips of each type were surveyed for roughness average using a protilometer (Planer Surfometer S101.63, Planer Products Ltd, Sunbury on Thames, UK). The tips were then subjected to the wear regime described above before being resurveyed. In these cases the wear regime was carried out only with the hybrid composite, in view of the relevance of the adhesion results described below.

Statistical analysis of paired data was carried out by means of a paired t-test. The data on adhesion to, and roughness of, the two surfaces before and after wear was analysed by means of a two-way analysis of variance.

RESULTS

The results are presented in Table I. Adhesion testing with the microtilled composite and

the glass polyalkenoate cement before and after the relevant wear sequence revealed cohesive failure of the restorative material, rather than separation at the die/ restorative interface, in all cases. This renders all values of force to separate tip from material in these cases of little relevance. Only the hybrid composite material separated from the tip surface, allowing comparative measurements of adhesive strength to be made.

Two-way analysis of variance demonstrated a signifi- cant effect of both coating and a period of wear on the

228 J.Dent. 1991; 19:No.4

Tab/e 1. Mean values for physical properties

Contact angle (UEDMA)

Frictional coefficient Vickers hardness Pre-wear adhesion (Occlusin) force (N)

Post-wear adhesion (Occlusin) force (N)

Roughness average, pre-wear (pm)

Roughness average, post-wear (pm)

TiN coating Stainless steel n Mean Sd. Mean s.d. P

20 34.75 1.13 33.4 2.56 < 0.05

100 0.326 0.020 0.37 1 0.045 < 0.001 10 751.9 41.6 591.6 25.4 < 0.001

20 0.717 0.079 0.539 0.063 < 0.01

20 0.757 0.065 0.584 0.078 < 0.01

5 0.058 0.01 1 0.046 0.027 < 0.05

5 0.062 0.011 0.084 0.01 1 < 0.01

adhesion of unset material to the die surface (P < 0.01 in both cases), but the interaction between the two was not significant. The presence of a TiN coating on the die significantly increased the force required to separate the composite from the tip in both the worn and the unworn state (P < 0.01). The wear regime demonstrated a signifi- cant overall effect on the roughness average (P < 0.05) as did the interaction between wear and the presence of a coating. However, the effect of coating on its own was not a significant factor overall (P > 0.05). TiN coating showed a rougher surface than uncoated stainless steel before the wear regime, but this was not significant at the 0.05 level. Although there was a net increase in roughness of both surfaces after wear, this was greater for the uncoated stainless steel leading to a significantly rougher surface of this material after the wear period (P < 0.05).

The TiN-coated surfaces had a mean contact angle with unfilled UEDMA resin 1.35” greater than those of the uncoated surfaces, significant at the 0.05 level of prob- ability. It is unlikely, however, that such a small difference would significantly affect the clinical performance of the surface. The coefficient of friction was found to be significantly lower for the TiN-coated surface at the 0.001 level of probability. The Vickers hardness of the coated surface was significantly higher for the TiN coating, again at the 0.001 level of probability.

DISCUSSION

TiN-coated composite placement instruments are widely available, and at least one recent textbook recommends their use (Kidd et al., 1990). However, the properties of the material as a low-stick coating for dental instruments do not appear to have been fully assessed in any clinical or laboratory trial.

This study demonstrates that, when used with micro- filled composite or glass ionomer materials, any theoretical benefit conferred by the coating is clinically invalid as the adhesion of the restorative material to the instrument surface was greater than the cohesive strength of the unset material. Only when used with a hybrid

composite could the efficacy of the low-stick surface be assessed. The higher forces required to separate the composite from the coated die rather than from the polished stainless steel indicate that there appears to be no benefit in using coated instruments rather than new stainless steel ones from the point of view of material adhesion. Following a short sequence of wear with the same composite both surfaces demonstrated an increase in adhesive strength, but the stainless steel material surprisingly remained relatively less ‘sticky’.

Measurements of roughness average showed that the fresh stainless steel instruments were rather variable in this respect, some of those measured were very smooth with roughness averages as low as 0.02 urn, whilst others were as high as 0.8 urn. The TiN-coated surfaces were generally rougher but were more consistent in their roughness average (0.04-0.07 urn). The TiN dies were slightly less well polished than the instruments typically are, and this may have had some effect on the physical properties. The wear regime used was rather short but nevertheless the resistance to damage of the coated surface was clearly demonstrated (Figs 1-4). The overall roughness average of the smoother stainless steel surfaces increased following the wear sequence, however those surfaces which were rougher to begin with showed less increase. The roughness average of the TiN-coated surface remained almost unchanged after the wear regime. This is in contrast to the work of Kooner et al. (1989) who showed a decrease in roughness average of TiN-coated surfaces with clinical usage. This finding was published as an abstract, but the findings have not been published in full. In the clinical situation further damage by a variety of agents may lead to the stainless steel surface becoming relatively more or less low-stick than the coated surface, but there was no evidence of this within the limitations of this study, the adhesive force of composite to the two surfaces increasing by a similar amount for both materials. Nevertheless, the TiN coating proved to be extremely hard and resistant to damage, a factor which could be expected to lead to a longer working life for such instruments, and those who have used the instruments clinically confirm that this is the case.

Steele et al.: Titanium nitride-coated instruments 229

Fig, 7. Surface of stainless steel die before wear, showing only minimal surface damage.

Fig. 2. Surface of stainless steel die after wear regime with hybrid composite. The filler component of the composite has caused extensive damage to the surface.

Fig. 3. Surface of the TiNcoated die before wear. Fig. 4. Surface of the TiN-coated die after wear regime with hybrid composite. The surface appears almost unchanged.

The small but statistically significant potential advantages demonstrated by the TiN-coated surfaces when the basic properties of contact angle and frictional coefficient were considered, appeared to confer no benefit when the low-stick properties of the two surfaces were compared. It is likely that so many other variables are involved that any benefit is obscured.

Acknowledgements

The authors would like to acknowledge Mr T. E. Carrick for his invaluable help with the experimental work.

References Chadwick R. G., McCabe J. F. and Walls A W. G. (1990) The

effect of placement technique upon the compressive strength and porosity of a composite resin. J Dent. 17,230-233.

Craig C. G. (1979) The placement of composite resin restorations. Aust. Dent. J. 15, 271-280.

Deubert L. W. and Jenkins C. B. G. (1982) Tooth Coloured Filling Materials in Clinical Practice, 2nd edn. Bristol, Wright, pp. 81-85.

Further work into the effect of various polishing and wear regimes on the properties of TiN may yet show an advantage as far as low-stick properties are concerned. Discolouration of composites by metal particles from the instruments which are used to place them has been quoted as a potential problem (Deubert and Jenkins, 1982) although some authors feel that this is not a significant clinical problem (Kanter et al., 1979). In our study gross grey discolouration was observed in the unset composite used as an abrasive during the wear regime, but this was only noted with the stainless steel instruments. If clinicians find that discolouration of composite by the placement instrument is a clinical problem, transfer to TiN-coated instruments may provide a solution.

In conclusion, the TiN coating appears to confer no additional advantage as regards low-stick properties, over clean, highly polished stainless steel used for dental instruments.

Kanter J., Roski R. E. and Gough J. E. (1979) Evaluation of insertion methods for composite resin restorations. J. Prosthet. Dent. 41,45-50.

Kidd E. A. M., Smith B. G. N. and Pickard H. M. (1990) Pickard’s Manual of Operative Dentistry, 6th edn. Oxford, Oxford University Press, p. 88.

Kooner S., Robinson P. B., Rogers P. S. et al. (1989) The durability of ceramic coated dental instruments. .I Dent. Res. 68, 604.

Sturdevant C. M. (1985) The Art and Science of Operative Dentistry. 2nd edn. St Louis, Mosby, pp. 304-306,365.

Tjan A. H. L. and Glancy J. F. (1988) Effects of four lubricants used during incremental insertion of two types of visible light cured composites. .I Prosthet. Dent. 60, 189-194.