coated dental articles according to various embodiments of the invention are shown in fig1 - 5 . in each instance , the article includes a substrate that is covered by a thin coating of hard carbon that is substantially continuous , and preferably is continuous . fig1 illustrates an article of the invention that in this instance comprises a bracket 10 used in orthodontic treatment . the bracket 10 includes a base 12 adapted for direct bonding to a tooth surface , and twin tiewings 14 , 16 are connected to the base 12 . each of the tiewings 14 , 16 has a slot 18 to receive an arch wire . recesses 20 behind each side of the tiewings 14 , 16 are adapted to receive an elastomeric or a wire ligature that also straddles the arch wire in order to retain the arch wire in the slots 18 . the bracket 10 has an inner body or substrate that can be a cast or machined structure , but preferably is made using a sintering technique wherein the entire substrate is initially formed as a pressed &# 34 ; green &# 34 ; preform stainless steel powder , and then heated to a sintering temperature to yield the final product . the entire exterior surface of the bracket 10 is covered with a hard carbon coating 22 that will be described in more detail below . in fig2 a dental article according to another embodiment of the invention comprises a buccal tube 30 that is covered with a hard carbon coating 40 . the buccal tube 30 is adapted to be secured to the molar teeth , and has a channel 32 to receive the ends of an arch wire . a round passage 34 next to the channel 32 is provided for receiving a headgear wire or a lip bumper wire , while a rectangular passage 36 on the opposite side of the channel 32 is adapted to receive a segmented arch wire . in addition , a hook 38 is provided for optional use of an interarch elastomeric force module to correct malocclusions . the buccal tube 30 has a metal body or substrate beneath the outer coating 40 . the substrate is preferably made using the sintering technique described above . the substrate of the buccal tube is somewhat similar to the buccal tube described in u . s . pat . no . 4 , 927 , 362 . a coated dental article in fig3 comprises an arch wire 50 used in orthodontic treatment in conjunction with brackets and buccal tubes such as the bracket 10 and the buccal tube 30 . the arch wire 50 has a metal core or substrate 52 that is round in cross - section , and that is surrounded by a relatively thin hard carbon coating 54 . preferably , the substrate 52 is stainless steel or a nickel - titanium alloy . fig4 illustrates a coated dental article that comprises an orthodontic band 60 according to another embodiment of the invention . the band 60 includes a stainless steel substrate that is essentially the same as commercially available bands . the substrate is covered with a hard carbon coating 62 . bands such as band 60 are adapted to encircle the teeth and be cemented in place , and provide support for orthodontic brackets that are not directly bonded to teeth . a coated dental article shown in fig5 comprises a metal crown substrate 70 that is covered with a hard carbon coating 72 . as illustrated , the substrate is shaped to receive a prepared tooth 74 . an aesthetic porcelain layer 76 is bonded to the coating 72 to make a crown . the substrate 70 can also be used for a bridge . although the major extent of the coating 72 is covered by the porcelain layer 76 , a gingival edge of the porcelain layer 76 is tapered and stops short of the gingival edge of the coating 72 covering the underlying portions of the substrate 70 to enable the dentist to seat the crown without undue injury to the gingiva . the coating 72 covers the gingival edge of the substrate 70 and provides a nickel and chromium barrier between the substrate 70 and the saliva or the gingiva 78 . the hard carbon coating of the article of the present invention means a polycrystalline diamond coating , a diamond - like amorphous hydrogen - free carbon coating , a diamond - like hydrogenated amorphous carbon coating or combinations thereof . the hard carbon coatings of this invention will generally show an intensity in the range of about 1300 - 1500 cm - 1 using raman spectroscopy when the coating is at least about 0 . 15 microns thick . conventionally , diamond is characterized by sp 3 bonding between atoms and graphite is characterized by sp 2 bonding between atoms . diamond - like materials exhibit both sp 2 and sp 3 bonding . the hard carbon coatings of the present invention will exhibit at least some sp 3 bonding . a number of deposition processes may be utilized to coat a substrate , and are described in more detail in an article entitled &# 34 ; diamond and diamondlike films : deposition processes and properties &# 34 ;, authored by c . v . deshpandey and r . f . bunshah ( j . vac . sci . technol . a , vol 7 , no . 3 , may / jun 1989 ). these techniques include chemical vapor deposition processes including chemical transport method , hot filament thermal chemical vapor deposition ( cvd ) techniques , and electron assisted chemical vapor deposition ( eacvd ), along with plasma assisted chemical vapor deposition ( pacvd ) and ion beam assisted / enhanced deposition techniques . a presently preferred method of establishing a polycrystalline diamond coating to make a dental article of the invention is plasma assisted chemical vapor deposition . preferably , diamond - like amorphous hydrogen - free carbon coatings are applied to a substrate via a cathodic arc technique or an ion beam deposition , while diamond - like hydrogenated amorphous carbon coatings are applied via a plasma assisted chemical vapor deposition technique . preferably , the diamond - like coatings of the invention have a thickness in the range of about 0 . 05 - 30 microns , more preferably in the range of about 0 . 1 - 3 microns and most preferably in the range of about 0 . 1 - 0 . 3 microns . polycrystalline diamond coatings of the articles of the invention preferably have a thickness in the range of about 0 . 1 - 50 microns , more preferably in the range of about 1 - 10 microns , and most preferably in the range of about 2 - 3 microns . presently , the most preferred embodiment of the invention has a coating thickness of 0 . 2 microns and consists essentially of diamond - like amorphous hydrogen - free carbon . relatively thin diamond - like coatings are advantageous in that such coatings are less expensive than polycrystalline diamond coatings . additionally , thicker coatings may adversely affect the article ; for example , a relatively thick coating applied to an orthodontic arch wire substrate may adversely affect the amount of available force that can be utilized to shift the teeth for a wire of a given , overall cross - sectional area . preferably , surface contaminants , such as oxides or nitrides , on the substrate are removed by a cleaning process before the carbon deposition process is initiated . ion sputtering techniques may be used for the cleaning process . when surface oxides are removed from metal substrates the incident carbon may react with the bare metal and form a carbide - like material between the substrate and the developing pure carbon coating . the carbide - like material forms a transition layer that may enhance the adhesion between a hard carbon coating and the substrate . in addition , a primer such as a layer of titanium may enhance the bond between the coating and the substrate . the hard carbon coating of the dental article of the invention provides a diffusion resistant barrier that substantially reduces leaching of chromium and nickel from stainless steel and other alloys . in addition , the coating provides enhanced resistance to corrosion of metallic substrates when exposed to the oral environment . further , hard carbon coatings are substantially clear , and as a result are more aesthetic than a hard coating made , for example , from carbide or nitride . the hard carbon coated dental articles present a relatively low coefficient of friction , a particular advantage when the article is associated with sliding mechanics in orthodontic treatment . for example , when the coating 54 on the arch wire 50 is in engagement with the slots 18 of the bracket 10 , reduced friction is presented to longitudinal sliding movement of the arch wire 50 relative to the bracket 10 in comparison to uncoated similar articles . reduced friction permits the tooth associated with bracket 10 to more easily shift along the length of the arch wire 50 under the influence of induced forces as selected by the orthodontist . test samples were made using a cathodic - arc deposition process to apply a diamond - like amorphous hydrogen - free carbon coating to substrates . the deposition process used was essentially the same as described in an article entitled &# 34 ; properties of the diamond - like carbon film produced by the condensation of a plasma stream with an rf potential &# 34 ;, authored by v . e . strel &# 39 ; nitskii et al ( sov . phys . tech . phys . 23 ( 2 ), feb . 1978 ). a dc bias of - 25 to - 35 volts was measured on the sample holder . the arc discharge was initiated with a mechanical striker and maintained at 100 amps with a miller welding power supply ( intelliweld 650 ), operating in the current control mode . the arc voltage varied between 40 and 50 volts , and produced a carbon plasma which was the source of carbon for the coating . the plasma was transported to the substrates using a magnetic field with a magnitude ranging from 25 to 100 gauss . flat samples were prepared by mounting a flat substrate in a vacuum chamber of the deposition apparatus in an orientation such that a test surface of the sample was facing the carbon cathode . orthodontic bracket substrates to make bracket samples were mounted in the test chamber by placing the tooth - facing side of the bracket base in a direction away from the carbon cathode such that the tiewings and arch wire slots faced the cathode . orthodontic arch wire substrates for making coated arch wire samples were mounted in the vacuum chamber by clamping the ends of straight sections of arch wire in an orientation such that the longitudinal axis of the arch wire was perpendicular to the chamber axis ( that extended in a direction from the midpoint of the arch wire substrate to the cathode ). the arch wire substrates were secured to a plate that was rotated about the chamber axis . prior to coating each substrate , a kauffman argon ion beam source ( commonwealth scientific corporation , 3 cm ion source ) was directed to each substrate for approximately 5 minutes in order to sputter clean each substrate and remove surface oxides and other contaminants . an ion energy of 1000 volts and a beam current of 30 to 40 milliamps was used . the distance between the ion source and the sample was approximately 15 cm . unless otherwise noted , each substrate was then exposed 12 times to the carbon plasma for five second intervals for a total deposition time of 60 seconds . a 0 . 2 mm flat metal sheet made of an alloy having a relatively high nickel content ( similar to alloys sold under the trademark &# 34 ; permaloy &# 34 ;) was cut into a square substrate measuring about 10 cm × 10 cm and coated according to the procedure set out in sample preparation . the coated sample was then cut into two square test samples measuring about 1 . 3 cm × 1 . 3 cm . a perkin - elmer auger system ( phi model no . 595 ) was used to obtain an auger spectrum from each sample at various depths in the coating . initially , the elemental composition of the near - surface region was determined . next , the near - surface region was removed by sputtering with a 3 , 000 volt , 75 μ a / cm 2 argon ion beam for 30 seconds . the elemental composition of the newly exposed near - surface region was determined from another auger spectrum and recorded . the sputtering and auger analysis cycle was repeated until the coating was removed from the substrate . the concentration vs . sputtering time profiles ( using a spline - fitting algorithm ) for a control sample are set out in fig6 wherein a is a curve representing carbon , b is a curve representing nickel and c is a curve representing chromium . since each time interval of sputtering removed additional surface regions , the total sputtering time somewhat correlates to the depth of the surface of the sample that was removed . fig6 is therefore somewhat representative of the relative concentrations of carbon , nickel and chromium at various depths in the sample . another sample was annealed at 250 ° c . for 24 hours in a gravity oven ( stabil - therm , model ov - 12a , by blue - m ). a mercury thermometer protruded from the top of the oven and was used to monitor the temperature . the auger spectrum was determined for the near - surface region of the annealed sample and again after exposure to ion beam sputtering in the manner described above in connection with the control sample , and the results are set out in fig7 ( where curves d , e , f represent carbon , nickel and chromium respectively ). the data in fig6 and 7 show that the concentration of nickel and chromium is not greater in the annealed sample after any given sputtering time than the concentration of nickel and chromium in the non - annealed sample after the same sputtering time had elapsed . this shows that the coating presented a substantial barrier to the diffusion of nickel and chromium . since diffusion is a thermally activated process , it is concluded that the hard carbon coating of a dental article would substantially prevent chromium and nickel from diffusing to the surface and contacting the patient &# 39 ; s saliva or oral tissue when subjected to temperatures normally encountered in the oral cavity . sliding friction was compared between various combinations of uncoated and coated orthodontic brackets and arch wires . coated brackets were prepared according to sample preparation , using a central maxillary ceramic bracket (&# 34 ; transcend &# 34 ; brand brackets , cat . # 2001 - 801 , 3m unitek ) made from alumina , and a metal injection molded upper right cuspid , miniature twin bracket ( cat . # 017 - 206 , 3m unitek ) made from astm type 630 ( 17cr - 4ni ) stainless steel . coated arch wire samples were prepared according to sample preparation from an 18 cm section of 0 . 43 mm × 0 . 63 mm ( 0 . 017 in .× 0 . 025 in .) stainless steel arch wire (&# 34 ; hi - t ii twist flex &# 34 ; brand arch wire ; cat . # 256 - 725 ; 3m unitek ) and nickel - titanium arch wire ( nitinol se brand arch wire , cat . # 297 - 002 , 3m unitek ). elastomeric ligatures (&# 34 ; alastic &# 34 ; brand ligatures , cat . # 406 - 011 , 3m unitek ) were used to ligate the arch wires into the arch wire slots of the brackets . one end of each arch wire was attached in turn to an instron model 1123 tensile testing apparatus . the bracket was mounted in an orientation such that the arch wire was pulled in a direction aligned with the portion of the wire ligated to the bracket . each arch wire was pulled by the instron apparatus through the arch wire slot of the corresponding bracket at a rate of 0 . 5 mm per minute . the force in grams required to pull the arch wire a total distance of 0 . 152 mm was recorded on a strip chart recorder having a chart speed of 25 . 4 cm per minute . the work done to overcome friction was determined and is set out in table i for the various bracket / arch wire combinations . table i__________________________________________________________________________work done to overcome friction , in gm - cm ceramic bracket ceramic bracket metal bracket metal bracket with with with with stainless steel nickel - titanium stainless steel nickel - titanium arch wire arch wire arch wire arch wire__________________________________________________________________________uncoated bracket 1227 1422 889 1276uncoated arch wirecoated bracket 828 1295 1007 1180uncoated arch wireuncoated bracket 782 1229 875 1246coated arch wirecoated bracket 807 1066 887 1228coated arch wire__________________________________________________________________________ the data show that significantly less frictional force is encountered between ceramic brackets and either stainless steel or nickel - titanium arch wire when either the bracket or the arch wire is coated with a hard carbon coating . slight improvement in the relative frictional force between the metal bracket and the nickel - titanium arch wire was observed when either was coated . no significant improvement in the frictional force between a metal bracket and a stainless steel arch wire was observed when either or both was coated . no explanation is known for the higher frictional force as noted in table i between a coated metal bracket and an uncoated stainless steel arch wire . potentiodynamic polarization resistance measurements were made to compare the corrosion resistance of stainless steel samples to stainless steel samples coated with hard carbon . a sheet of stainless steel , type ss430 , was cut into three flat samples of about 1 . 9 cm × 1 . 9 cm × 1 . 9 mm . one sample was left uncoated and two samples were coated in accordance with the procedures set out in sample preparation . the testing procedure was carried out according to astm standard practices g3 - 89 , g59 - 78 ( 1984 ), and g102 - 89 except that no hydrogen flow was employed . contact area of the sample with the electrolyte solution was limited by a polytetrafluoroethylene o - ring to a circular area of 1 . 0 cm 2 . experiments were run using a flat electrode cell ( eg & amp ; g parc ) with a silver / silver nitrate reference and 1 . 0n sulfuric acid bridge solution . software sold under the trademark &# 34 ; soft corr &# 34 ; ( eg & amp ; g parc m342 ) was used on an ibm ps / 2 model 70 computer to control a potentiostat ( eg & amp ; g parc m273 ). results of the tests indicated a current density of 0 . 28 μ / cm 2 for the uncoated sample , 0 . 05 μ / cm 2 for the sample coated for 17 seconds , and 0 . 02 μ / cm 2 for the sample coated for 30 seconds . the data show that the coated samples had significantly greater resistance to corrosion than the uncoated sample , and that the corrosion resistance may increase with increased thickness of the coating . wear testing was undertaken to determine wear characteristics of hard carbon coated and uncoated samples along with wear characteristics of opposing tooth enamel in an apparatus simulating the oral environment . two flat samples of polycrystalline alumina ( ceralloy # 514f ) were obtained and one sample was coated with a hard carbon coating according to sample preparation . wear testing was carried out using the apparatus and procedure described in an article entitled &# 34 ; an artificial oral environment for testing dental materials &# 34 ; by r . delong and w . douglas ( ieee transactions on biomedical engineering , vol . 38 , no . 4 , apr . 1991 pp . 339 - 345 ). the apparatus was utilized for 300 , 000 cycles . no wear was observed on either the hard carbon coated ceramic sample or the uncoated sample . however , a volume decrease on the enamel was determined to be 0 . 0402 ± 0 . 0075 mm 3 when opposed to the uncoated sample , and 0 . 0308 ± 0 . 0062 mm 3 when opposed to the hard carbon coated sample . the volume change was determined using a three dimensional contact stylus digitizing profiliometer system . the data show that the wear on opposing enamel was slightly less for the hard carbon coated sample than the uncoated sample , although the difference was slight and may or may not be significant .
a dental article is coated with a hard carbon coating of polycrystalline diamond , diamond - like amorphous hydrogen - free carbon , diamond - like hydrogenated amorphous carbon , or combinations thereof . the hard carbon coating presents a barrier to nickel and chromium that might otherwise diffuse from an underlying metal substrate , and as such is useful for patients exhibiting sensitivity to nickel and chromium .