QUANTITATIVE DETERMINATION OF CHANGES IN TEXTURE (ROUGHNESS) PRODUCED ON TilE SURFACE OF CAST

NICKELLCHRQMIUM ALLOY AFT~R EXPOSURE TO VARIOUS SURFACE TREATMENT PROCEDURES

M,alik Ravi*, Jain Deshraj**

This in vitro study evaluated the quantitative determination of changes in ~exture (roughness) produced in surface of cast Ni-Cr aUoy. after exposure to various surface treatment procedures.

MATERIAL AND METHODS: Ni-Cr base metal alloy was sel~ctyd for the study. Samples were qividcd into four groups and ten pairs of samples were included in t';ach group.

Group I: NO surfac~ treatment and act as con­trol group. Oroup II: Specimens received sl,lrface treatment with 50 IJ.m A1203. Grol,1p UI: Electrolyti­cally etch~d. Group IV: Laser surface treatment with

. Nd: Y AG laser.

One representative sall1p.l~ f{l,)In ~ach group was studied under s~anning electron microscope at various lDugnifications and surface roughne!'ls was'assessed with the help of stylus type measuring instrumel\t. Data were ana~yzed with on<;l tailed't' test.

GrOllP 11. IlIllod Group TV silmplr.s shnwr:n sig­nificant increall~ in the average roughfless as com­par~d tQ coptrol group (Group I). Ro~ghness of Group III si~niflcantly greater than of Oroup Il sam­ples (P , 0.001) and less than that of Group IV. RO~ght\~!lfi 6f Group. V 19 ~lgtllhOL\llth' lcM UlI'Ul tlJltt of Group II s~ples.

Studies revealed that all surface treatment pro­c~dures ar\< capable for remat'kably enhancing sur­face roughness of cast Ni-Cr alloy. Surface treat. ment with al\lminum oxide and with Nd:YAG laser produce nearly equjvalcnt Rurtilr,61 rnV/lhll"llf! P\lt )(\(jp

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. 140 JPFA. Vol. 24. D~ceniber, lULU

than that of electrochemical etching.

Electrochemical et<;:hing procedure was found to produce greater sl,uface roughness than air-abra­sion and laser ablation.

INTRODUCTION

Resin bonded fixed partial dentures have gained considerable popularity since the technique of splinting Tnllndibular anterior teeth was described by Rochette (1-3) in 1973.

In contrast to the conventional fixed prosthe­sis, this prosthesis relies on adhesive bonding be­tween ~tched enamel iUld m~tal castings. Th~y are held in place by resin, which locks mechanically into «hemically etched enllme! with microscopic under­cuts in the castings (4, ~).

The lllellli resin bonqing Clln be Clllssifieclils r.i­thl!r mechanical or Chemi<;aI,<o'J) Mechanical bond­ing is subdivided into micromechanical retention, whidl \l ~s ~h:.hiIlg 10 create microscopic porositics, and IlIAerfl-IH(':dlilllic<11 reteulion which WM etching to cre"te micfQsQopil; rnrnflitir:s (Inti m1lljTO.mq'e chanipal retention which relies on visible wldcrcuts, ~sually with a mesh or pitted metal (10). Chemical bonding generally employs tin plating of me~al framework and specific resin adhesivcs for metal and enamel. (11,16)

The metal alloys are prot~cted l?y a passiv\ltlnl'!; mdti~ l~yf.'lr. Thi~ oxide lilycir iR c\aim.,ct to P~ jm p0l14nt in f.;:IJ 1I1il1~ bUJ1Ys with adhesive rellIn lilting AtCiit In 1ft _ lIllHlILJldl'\llll~ ~eelllS lU llitHy lhl,! yellt­tive oxide coating even though a Qlicro retentive

surface is provide.d. Electrochemical etching pro­cedure, on the other h<wd, is extremely technique sensitive. Great care and costly equipment is re­quired to mcoiporate the desired precision (19).

Quantitative changes in teeth after 'laser beam impact have been reported in several recent laboratory studies. Effects caused by absorption of focu!ied laser at the surface of solids are heat­ing, melting, vaporization, atomization, excitation and ionization. The optical ener~y is turned into heat at point at which light is absorbed. The intense looal hl'ating causes :.\ rapid rise in surface tcm­perature of materi~l. Heat is condqcted into inte­rior of target and a thin molten layer js formed below the surface. Further heating causes vapori zation leaving ~raters with raised edges and splat­tered molten particles surrounding the crater (20) .

This reported obs~rvations initiated the qse of la­ser for ablading the alloy surf~ce to produce mi­oro r"ton~i\!1jt ourf/loo

This stuqy explores laser ablation itS it pussibk me~~uJ \Jt .\",hlevitl~ d lUi\.;1U ictculivt; !:'UICl:\CC aut.! COJT1pareS tile etlect (If three ditlerent surface treat­IIJC!llt proceuuJ'e!;.

MATE~ANDMffiTHODS

The present study is an in-vitro evaluation of changes in surface t9pography of cast Ni-Cr alloy produced as a result of exposure to air abrasion, e1ec~ochemical etching and laser ablation.

Ni-Cr base metal alloy was selected for this study due to its wide appli9ation ill fabrication of fixed prostqesis. .

~quare patt~rns 1 cm x 1 em wert: cut frum 4 cm ,. lO C1I1 A 2.5 nUll ucrylic rtlsin sheet~. These pa~­h .. rnai w",n: ~plU\!'J, 111Vl:lil",u with pl.IUliph"tu bondlld inveRtm\,nt material by vaouum investment teoh­nique in Muffle furnace using fresh Ni-Cr alloy for each casting operatiop. Cjlstings w~re divested .and the residual surface investm~nt was removed with Ilandbla&ting ~i~ with 110 llm alull1innm oxid6 Gond an~ casti!lgs were cleaned'jn ultrasonic cleaner with dl&hlled water. .

Depending upon type·of surface treatment used, the samples were divided into four grpups and ter pairs of samples were includ~d in each gro1,lp. Each pair of sample contains one mOllnted and one un­mounted' sample . .

Half specimens out of 80 were then mounted in circular bases of autopolymerizin~ resin. After mounting the exposed surf/ice 9f 1\11 specimens receive,d metallo~raphic' treatment with series wet emery p~per of 200-, ;320- , 420- and 600- grit respectively and then polished with 51lm diamond polishing plate on a 5<) 11m. The elltirt: spt:cimen were then ultrasonically cleaned with soap solution for 15 min and rinseq witl) 4is­tilled water, dried and stored in respective .pqll­tainers,

Gropp I: 20 specimens r~ceived no surface treatment (Control group)

Croup 0: 20 Specimens received sllrfflce Ut:atlut:ut with 50 ,.un al~minllm oxide for 10 s~c al P1t:SSUtc ~r 6U psi, at dt!itahCe of 10 !hm from the nO;lzle aftel' maskin~ ' th~ surface witll q, water proof T etlon tape except for a 5 mml area in center.

Gruup III: 20 Specimens were electrolyt~ca\ly etched with 0.5 N nitric acid solutions at 3 volts of direct cut'renl [01 5 mill. A stitinless steel plate WaS used as cathode and specimens were made anoqe. Metal membrane formed during etching was re- ' moved by ultrasonically cleaning the samplys in 18% HCI for 10 minutes.

Group {V: 20 Specimens were exposed to ia­ser radiation ofQ-switched Nd: YAG l~sers of 100 mira; plll lin fillTntinn, h!lvin8 wllvl;'lf,'nAth of 1,0(,4 micron were used. After tocusing 14e beam, tPe spot

. size obtained WaS 100 microns.

First, we made an Auto-CAD plot fjl~ for the required etching and then fed it throug4 the compu­tilr,' Th~ Ilpfl(1im(lrw woro ultrpGonioqlly oleanod ill distilled water for ten minutt{s both before and af­ter irradiation.

JPFA, Vol. 24, Decf!mber, 2010 141

Fig. 1 Control group (Group I)

Fig. 2 Air abraded samples (Group 11)

Fig. 3 e,e~troche1!'ic{l1l1 etched group (Group III)

", ...

142 JPFA, Vol" 24, December, 2010

I,

Fig. 4 Laser abladed samples (Gr,oup IV)

Fig. 5 Profile of control samples on T(l(V surf-50.

Fig. 6 Profile of air abraded samples on Taly Surf~50.

Fig. 7 Profile of electrochemically etched Sampie,Y on Taly Surf-50.

Fig. 8 Prufile uf laser abladed samples on Taly Surf-50.

One representative sample from eac:h eroup was selected randomly and was studied under scanning electron microscope to assess surface topography. Surface roughness was assessed for representative samples from each group with the help of a stylus type meas\lring instrument (Taly Surf 50, Taylor

Hobson) and Ra Values were computed at; a~eragcs in microns. Statistical Analysis

The data collected after measuring the surface roughness of 10 samples each from all four groups were analyzed using 'one tniled't' tcst. -

RESULT

Table Showing Statistical Analysis of data collected after testing Mean Surface Roughness Values (Ra) of all Four Groups.

SanwleNo. Group-I Group-JI Grollp-III Group-lV

11 10 10 1u 10 Mean 0.224 1.396 2.272 1.3270 S.U . 0,0493 0.270 11 0.2833 0.00:341

"

, Group I = r.nntrol group: Group II = Air abraded; Group III = Electrochemically etched; Group IV "" Laser {/b/{/dIJd

t I vs 2 71.3185 P < 0.001 Significant Increase

t 1 vs 3 124.6248 p < 0.001 Significant In<;rense

t 1 vs 4 67.1197 P < 0.001 -

Significant Increase

t 2 vs 3 7.0741 P < 0.001 Signific.ant Increase,

t2 vs4 0.8070 p> 0.10 Insignificant Increase

t 3 vs 4 10.5947 P <; 0.001 Significant Increase

Group IT, TTl and IV _sampl~s show significant increase in roughness a~ compared to control group. On ' comparing we find that roughness of group III samples is significantly greater than that of group II samples (P < 0.001). Roughness of group IV samples is significantly greater than that of group III but less than that of group II samples (P > 0.10).

JPFA, Vol. 24, December, 2010 143

DISCUSSION

Reliable bonding between metal prosthesis and topth surface is obtained after chemically etch the tooth surface and failure site is most often at metal cement interface (1 , 2). Surface treatment of the metal to enhance the bonding and to strengthen this weak liqk in the bond module has been studied extellsively (3,7,9,21,22). Micro perforations have been provided to better retention than macro retentive arrange­ments. (19)

Air abrasio~ an4 electrochemical etching have been tried for the purpose of roughening metal sur­faces and are now aCCepted laboratory techniques. Etching provides excellent undercuts for locking the resin tags, but at the same time removes high-en­ergy surface regions from mt(tal surface, thus de­creasing me~l to resin bond strength. (18) Aluminum oxide roughens the surface. It also elevates the re­action to resinous cements and created micromechanical retention to improve bond strenath. Althouah electrolyte etching is presently considered the best way to bond resin LO tht: sur­tilce of a metal casting, there are a number of dis ad­vanlagt:s such as curr~nt density, inter-electrode distance. Finally, it is difficult, if not iritpossible to detenIline v.sually whether the surface has been properly etched. (II)

In this experiment, we are using effect of laser for producing porosities on m~tal liUrfaces. Since Nd:YAG lasers are capable of providing the high energy output, whi~h is used for the geuct'atiug pits in the metal.

An appropria~e quantitative judgme~t of the ruuglUlt:~s pnx,luct:d as a rt:S~dt of surface treatment WilS achieved by using a stylu~ ~ surface rough, ness testing Pl8chine and recording the Ra (rough­ness p~eter) villues, Ra is the universally recog­nized, and most used internaHonal parameter of roughnes!!. It is arithmetic mean ,of dep~re. of pro­file from mean line, The graph obtained by the pa­rameter studied givell a visual impressipn of the amount of r9ughness produced in profile. This and the Ra values obq.ined from samples uf t:ach group

144 JPFA, . Vol. 24, D(!cemher, 20W

provided logical basis for the comparison of dif­ferent amount of surfaces roughness produced for each group (from manufacturer's instruction manual). Since this instrument is stylus type, the sm~llest depression measured cannot be smaller than stylus tip . Therefore, the use of more accurate lao ser based interferometers is neede~ for enhancing aCCuracy.

The extremely consistent Ra values obtained for the group N samples suggest that this method of surface treatment is the most accurate procedure for achieving extremely even surface topography. Since all parameters of laser power, scan length, dwell time and increment were kept constant for all exposures with the help of Auto-CAD setup, such results were expected. These results reinforced pur belief that as far as this technique was concerned: all parameters can be exceedingly well controlled according to requirement. Ex4au&tive ~xp~rim~nts with different laser intensities and other variabl~ should bc carricd out to pinpoint tho OplttuuUi pl,l­rameters. Other high I'0wer lasers can be tried for the !lnfTIc P\lrpOS" find "ost effectiveness allrl ~ase: of handling determined.

All the three methods were found to ~ubstnn­tially augment the surface roughnells. These find. ings corr~late with all the previous studies carried OUt with air ahr(lsion ilnc1 e:le:r.troGhe:mit;,al etchmg. The innovative technique oflaser ablatioG clescribed an~ used in this experiment is nniqlH:'! in its effi­ciency, regularity of pitting and reproducibility.

CONCLUSION

In this in-vitro expcrim~l1tal study, afteI' I,;umpi­latilJli amI statistical analysis of qata, we came to the following conclusions:

1 : All the three surface treatment procedures are capable for remarkably enhancing the surfac~ roughness of cast Ni-Cr alloy (P < 0.001),

2. The surface treatment with 50 micron aluminum oxide sand and with Nd: YAG lasers prod\1ced ncarly equivalent surface ·roughness.

3. The sllrface tr\!atment with but electrochemical etching enhance greater surface roughness than 50 IIlicron aluminum oxide sand.

4. The surface treatment with but electro9hemical etching enhance greater surface roughness than Nd: YAG lasers.

5, Nd: YAG lasers provide an effective surface treatment prpcedure for fixed prosthesis.

6. Remarkably consistent results with Nd: YAG la­sers may be obtained with the precise focusitlg and Auto-CAD system.

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20ll The Silver Jubil~e Yea .. of JPF A PubUc~ti()n

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