R.R. Segbi, DDS, MS Effects Of Ion Exchange On Hardness And . - Quintpub

Efíects of ion ExchangeTable 4 Means and Standard Deviations of Crack Lengths and FractureTougtiness Values of Control and Ion-Reinforced Dentai Ceramic SurfacesCrack length (c (Mm)IDcodeVNCOPDIMMCEVEXV-NweFracture toughness (% ChangeIon .0637.8140.6242.9442.1046.62 231.010.980.970.930.88 002.191.981.771.901.66 0.16 0.06 0.05 0,12 0.13 0.18 0.15 0.07 0.185939-1621161028210489Solid horizomai and verticai iines connect group means that are nol significantly difierent at n 0.05.ment {P .068) and tfiat the interaction betweenion treatment and material type was not significant{P— .245), The results of ihe multiple comparisonstest for tfie hardness data are summarized in Table3. Vertical lines connect group means that werenot significantiy different at the a 0.05 level.The mean and standard deviations of the measured crack lengths (c) and resulting fracture toughness ¡Kiel values for the control and ion-treatedgroups and the percent change in K, are summarized in Table 4. The results of the analysis ofvariance indicated that the ion treatment significantly affected {P .01 ) the fracture toughness ofthe material surface and that a strong interactionexisted between material type and ion treatment( P .001]. The results of the multiple-comparisonstest for the fracture-toughness data are summarized in Table 4. Vertical and horizontal lines connect group means that were not significantlydifferent at the a 0.05 level. With the exceptionof material DI, the mean K,ç values were significantly different for the ion-treated groups than forthe untreated control groups. The mean K, valtiesincreased with an improvement in fracture toughness ranging from 39% to 116% for the ion-treatedgroups. Material DI showed no significant difference in either the hardness or fracture toughness.netitrality but occupy a larger volume in the glassmatrix directly adjacent to the surface (Fig 2). Theincreased molar volume results in a two-dimensional state of surface compression because theexpansion of the stjrface structure is restrained bythe underlying bulk material." The replacement ofsmaller mobile sodium ions in the glass phase withlarger potassium ions is the most c o m m o nexchange process and is the type used in this system. This type of process has been shown by anumber of researchers to be an effective means ofi m p r o v i n g the breaking strength of dentalWhile these studies provide an adequate assessment of the effect of this process on the overallproperties of a material, measurements made withindentation procedures provide improved meansof assessing the degree and uniformity of the effectdirectly at the surface where cracks are initiated.The results reflect a compilation of measurementsmade at vastly different locations on several different samples for each group. The low standarddeviations of the group means relative to the treatment results are an indication of the uniformity ofthe effect on the surface with respect to its apparent ability to resist crack propagation.The changes in K, values ranged from - 1 % to116% for the materials tested. The type of ceramicbeing chemically strengthened affected the treatment results as indicated by the significant interaction term. Material factors that can effect theexchange process include the conceritration andmobility of the exchangeable ions in the matrixhe glass iransrtion temperature of the ceramic, andthe time-temperature history of the process Theconcentration of sodium ions present in theceratT ,c material alone does not appear to corre spond with the amount of strengthening that takesDiscussionThe fracture of glasses and ceramics is alwaysinitiated by a tensile stress and can often be tracedto the propagation of surface flaws through thebulk material." A practical means of increasing thetensile strength of ceramics is to create a compressive skin on the surface. The ion-exchangeprocess achieves a surface compressive layer byexchanging surface ions that maintain electricalnai of Prosthodontii312Volume 5, Number 4.

Seghi et alEffects of ion Fuchangeplace. This is evident by the fact that the ceratnicscotitaining low amounts of sodium (WC and V-N)did tiot have lower K, increases (%] than did theconventional feldspathic systems. This supportsthe idea that the mobility of the ionic species inthe material is more critical than is its total concentration. The lack of statistical difference foundbetween control and treatment groups for materialDI indicates the inability of this ion-exchange process to enhance its resistance to crack propagation.This is related to its compositional lack of sodiumor equivalent ions (similar size and charge) available for exchange. Further investigations areneeded to evaluate the amount of mobile ionsavailable for exchange in each material system.This information may help to explain the differences in effects found between material groups.The kinetics of the ion-exchange process arecontrolled by the opposing phenomena of diffusion and stress relaxation. Both of these processesare related directly to the time and thermal historyas well as to the glass-transition temperatures ofthe materials. While the same time-temperattiretreatment was used for all materials in this study,compositional variations between products mayrequire different treatment schedules to maximizethe effectiveness of the ion-exchange strengthening process. The kinetics of this ion-exchange system are currently under investigation for severalmaterials.An obvious limitation of this process is the factthat the strengthening mechanism is confined tothe surface of the material. Microprobe analysis hasshown the depth of ion penetration to be as greatas 100 to 300 fim below the surface,"' While thisprocess has been shown to improve the breakingstrength of mildly abraded ceramic surfaces,'-''significant grinding on tbe treated surface of a restoration would likely negate the effects.The effect of moisture on crack propagation isnow considered to be a significant factor in thefailure in ceramics. Various researchers"-' haveshown that static fatigue or delayed failure inceramics occurs as a result of a reaction betweenwater and the glass surface. It is believed that staticfatigue results from a stress-induced chemical reaction between water and the surface of the giass atthe crack tip. ' It has been suggested' that staticfatigue should not occur in chemically strengthened glasses until the residual stresses are overcome by tensile stresses that are greater thannormal static-fatigue thresholds. This would implythat static-fatigue failures cotild be delayed orreduced and further investigations are needed. Significant crack growth in the materials used in thisVolume5, Number 4, 1992Before cfiemical treatment Slllton Ion9 C v9cr IonAfter chemical treatmentFig 2 After chemical treatment there is an increased molarvolume of the surface relative to the interior resulting in astale of surface oompression because the expansion of thesurface is restrained by the underlying material. study was not observed in the short, 5-minute termfollowing indentation; this suggests that the effectis not significant within the time frame of thesemeasurements. The effects of moisture on thegrowth of these cracks over a longer period, however, may provide interesting and valuable information about differences in stress-corrosionsusceptibility.The fact that the ion treatment did not significantly affect the surface hardness in this study issomewhat contrary to what was expected and tothe product information provided by the manufacturer. This is most likely due to the magnitudeof the indentation loads used in this study to calculate hardness values. The hardness of thesespecimens has been reported previously usingthe Knoop indenter and a 500-g load force. Whilethe statistical analysis showed that only one iontreated group was statistically greater than itsuntreated control, the mean hardness values ofmost of the treated surfaces were greater than weretheir respective controls. It is hypothesized that thetreatment effect on hardness would continue tobecome more pronounced under lighter loadforces. The ion treatment is confined to the surfaceof the material. Lighter loads confine the indentation area to a more superficial volume of materialand, therefore, are likely to be more heavily influ-313The International Iournal of Prosthodontics

Effects of Ion ExchangeSeghi et alenced by the surrounding stress field. The characteristic ability of a particular chemically treatedsurface to resist a range of indentation forces mayprovide a basis for assessing the depth and degreeof the residual surface stress present from the ionstrengthening process.3.4.Perhaps most critical to the issue of improvedclinical performance is a better understanding ofthe mode and location of the clinical failure site.It has been suggested that crack initiation beginsfrom flaws located on the internal surface of allceramic dental crowns, "''' and there is some scientific evidence that supports this view."' ' Theresults of such studies could greatly influenceapplication techniques wilh respect to location.The clinical significance of this process on the longterm performance of ceramic restorations is difficult to assess and clinical trials must be performedto support its routine use.5.6.7.8.9.0.Conclusions1.Indentation techniques were used to evaluatethe effects of ion strengthening on the hardnessand fracture toughness of restorative dentalceramics. Within the limitations of the presentinvestigation, the following conclusions can bemade:1.The ion-exchange strengthening method usedin this investigation resulted in a significantimprovement in the apparent fracture toughness of all feidspathic and reinforced feldspathic-porcelain surfaces tested.2.The Dicor material did not show a significantdifference in fracture toughness with the ionexchange treatment.3.No significant difference in hardness could bedetected between the ion-treated and controlgroups for any of the ceramic materials.2.3.4.5.AcknowledgmentsThe authors thank the University of California at Los Angelesfor its support of this project.ReferencesAnstis GR, Chantikul P, Lawn BR, Marshall DB: A criticalevaluation of indentation techniques for measuring l racture (ougliness: I, Direct crack measurements, i Am CeramSoc 19B1;64:533-S38.Chantikul P, Anstis GR, Lawn BR, Marshall DB: A criticalThe International1 of Prosthodontics314evaluation of indentation techniques for measuringture toughness: II. Strength method. ; Am Ceraml98i;64:539-543.Morena R, Lockwood PE, Fairhurst CW: Fracture tougnness of commercial dental porcelain. Dent Mater1986;2:S8-62.Jones DW, Rizalla AS, King HW, Sutow EJ: Fracture toughness and dynamic modulus of titrasilicic-mic3 glassceramic, / Can Ceram Soc 19a8;S7:39-46,Rosenstiel SF, Porter SS: Apparent fracture toughness ofdental porcelain with a metal substructure. Dent Mater19BB,4:187-19O.Seghi RR, Crispin BJ, Mito W: The effect of ¡on exchangeon the flexural strength of feldspathic porcelains. Int IProsthodont 1990,3:130-134.Seghi RR, Daher T, Capu(o A: Relative flexural strength ofdental restorative ceramics. Denl Mater 1990,6:181-184.Kingery WD, Bowen HK, Uhlmann DR: Introduction toCeramics, ed 2. New York, John Wiley and Sons, 1975,PP 768-944.Southan DE: Strengthening modern dental porcelain byion-exchange. Austr Dent I 197O;15:5O7-51O,Dunn B, Levy MN, Reisbick MH: Improving the fractureresistance of dental ceramic. ; Dent Res 1977,56:12091213.Jones DW: The strength and strengthening mechanismsof dental ceramics, in McLean JW (ed): Dental Ceramics:Proceedings of the First International Symposium onCeramics. Chicago, Quintessence PublCo. 1983, pp 130135.Piddock V, Qualtrough AJE, Brough I: An investigation ofan ion strengthened paste for dental porcelain, Int I Prosthodont 1991,4:132-137.Southan DE: Effect of surface injury on chemicallystrengthened dental porcelain. Quintessence Int1987;18:575-58O.Curney C, Pearson S: The effect of the surrounding atmosphere on the delayed fracture of glass. Proc Phys Soc B1949;62:469-476.Fox PG:Thethermodynamicstability of oxides in aqueoussolutions and its relevance to static fatigue in silicateglasses. Fhys Chem Classes 19aO;21:161-166.Wiederhorn SM: A chemical interpretation of staticfatigue, ; Am Ceram Soc 1972;55:81 -85.Fox PG: The thermodynamic stability of oxides in aqueoussolutions and its relevance to static fatigue in silicateglasses. Phys Chem Glasses l98O;Z1:Ib1-165.Ritter JF: Static fatigue acid-etched, soda-lime-silica glassrods. Phys Chem Glasses 1970,11:16-17.Brajevic F, Seghi RR, Denry I: Effect of ion exchange reinforcement on surface indentation properties of dentalceramics. / Denl Res 1991;70:290, abstr no, 193.Seed IR, McLean JW, Holtz P: The strengthening of aluminous porcelain with bonded foils, / Dent Res1977;56:1067-1069.Southan DE, Jorgensen KD: Faulty porcelain jacketcrowns. Austr Dent I 1972;17,336-340.Derand T: Analysis of stresses in loaded models of porcelain crowns. Odont Revy 1974;25:189-206.Kelly JR, Campbell SD, Bowen HK: Fracture-surface analysis of dental ceramics. / Prosthet Derxt 1989;62:536-541.Volume S. Number 4. 1992

College of Dentistry, Ohio State University, 305 W 12th Avenue, Columbus, Ohio 43210. "Research Associate, Department of Restorative Dentistry, College of Dentistry, Ohio State University. "Graduate Prosthodontic Student, Medical College of Geor-gia, Augusta, Georgia.