Improving the fatigue behavior of dental implants through processingcommercial purity titanium by equal-channel angular pressing

Roberto B. Figueiredo a,n, Eduardo R. de C. Barbosa b, Xicheng Zhao c, Xirong Yang c,Xiaoyan Liu c, Paulo R. Cetlin d, Terence G. Langdon e,f

a Department of Materials Engineering and Civil Construction, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901, Brazilb Dental Research Center Sao Leopoldo Mandic, Campinas, SP 13045-755, Brazilc School of Metallurgical Engineering, Xi'an University of Architecture and Technology, 710055 Xi'an, Chinad Department of Mechanical Engineering, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901, Brazile Departments of Aerospace & Mechanical Engineering and Material Science, University of Southern California, Los Angeles, CA 90089-1453, USAf Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK

a r t i c l e i n f o

Article history:Received 19 August 2014Received in revised form25 September 2014Accepted 27 September 2014Available online 5 October 2014

Keywords:Commercial purity titaniumDental implantsEqual-channel angular pressingFatigueSevere plastic deformation

a b s t r a c t

An investigation was conducted to evaluate the feasibility of using commercial purity (CP) titanium ofgrade 1 for dental implants after processing by equal-channel angular pressing (ECAP). The material wasprocessed by ECAP for 4 passes at room temperature. Dental implants were machined from theunprocessed material and the material processed by ECAP and their mechanical properties wereevaluated in tensile and compression testing and in fatigue using the conditions specified in the ISO14801 standard for dental implants. The results show processing by ECAP increases the yield stress andthe ultimate tensile stress but reduces the strain hardening rate and hence the overall elongation tofailure. Although processing by ECAP increases the fatigue life using cyclic bending loads, a comparisonwith published data suggests the fatigue behavior CP Ti of grade 1 is slightly less satisfactory than forcommercial implants fabricated from higher grade titanium alloys.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Severe plastic deformation (SPD) techniques are widely used tostrain-harden and refine the grain structure of metallic materials[1,2]. In practice, these processes provide the capability of hard-ening pure metals while avoiding the addition of solutes as in solidsolution hardening or second phases as in precipitation hardening.This is particularly important when processing a biomaterial suchas titanium, which is widely used for orthopedic and dentalimplants [3,4], because the solute atoms and precipitates mayeasily compromise the biocompatibility.

Among the various SPD techniques that are now available [5],including equal-channel angular pressing (ECAP) [6] and high-pressure torsion (HPT) [7], processing by ECAP has to date receivedthe most attention and use. This is because processing by ECAPinvolves repetitively pressing a short bar or rod through a dieconstrained within a channel that is bent through a sharp angleand this processing method is relatively easy to perform in anymechanical testing laboratory. However, there is an inherent

difficulty associated with the processing of commercial purity(CP) titanium because it was shown in early experiments thatthe material exhibits segmented flow when processed at roomtemperature and at a high pressing speed of 25 mm/s [8]. For thisreason, the early reports on processing of CP-Ti by ECAP describedinvestigations in which the processing was performed at tempera-tures of 623 K or higher [9–11] and it was also reported that theultimate tensile strength of the material may be further improvedby subsequently introducing room temperature deformation [9].

The problem of the occurrence of segmentation in ECAPprocessing is a well-established phenomenon which is especiallyprevalent when the material has a low strain rate sensitivity [12].Calculations using finite element modeling, considering ECAP dieswith 901, 1101 and 1351 between channels, showed that alloysprone to segmentation, the so-called difficult-to-work alloys, maybe processed more easily if the channel angle within the ECAP dieis increased to a higher value instead of using the conventionalangle of 901 [13]. Accordingly, later experiments showed that CP-Timay be successfully processed through one pass at room tem-perature (RT) by using a die with a channel angle of 1201 and arelatively low ram speed of 0.5 mm s�1 [14] and subsequently itwas reported that titanium may be processed at room temperatureusing a 1201 die and a faster ram speed of 2 mm s�1 to give a grain

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Materials Science & Engineering A

http://dx.doi.org/10.1016/j.msea.2014.09.0990921-5093/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ55 31 34091751.E-mail address: figueiredo@demc.ufmg.br (R.B. Figueiredo).

Materials Science & Engineering A 619 (2014) 312–318

size of �200 nm after 8 passes [15]. There are also some reportson the processing of CP-Ti at room temperature using an ECAP diewith an angle of 1351 [16,17] and there is a very recent report ofroom temperature processing using a special composite lubricantwith a 901 die at a speed of 3.5 mm s�1 where a grain size of�150 nm was produced after 4 passes [18]. This latter result isidentical to the grain size reported in CP-Ti when processing byHPT at room temperature for 10 turns [19]. All of these resultsconfirm that additional cold deformation is not a necessaryprerequisite for achieving strength enhancement in CP-Ti afterECAP since a high strength may be introduced directly using roomtemperature processing.

The improved mechanical properties attained in CP-Ti permitthe use of this material as a structural biomaterial in dentalimplants. Thus, an early paper reported the production of dentalimplants from CP-Ti processed by ECAP with a subsequent thermalmechanical treatment [20]. Also CP Ti exhibits excellent biocom-patibility after processing by ECAP by comparison with its coarse-grained counterpart [21–23] and, in addition, it has been shownthat titanium processed by ECAP has an increased fatigue life andfatigue limit under constant load testing [24].

It is important in practice to recognize that conventionalmechanical testing is not adequate for providing comprehensiveinformation on the mechanical response of dental implants underloading. This is because dental implants are subjected to multipleloading conditions and also to stress concentration factors.A specific standard for testing dental implants was implementedby the International Organization of Standardization and thisstandard, designated ISO 14801, incorporates the inclined loadingand bone resorption that occurs during the fatigue life of realdental implants. Recently, a report compared the mechanicalresponse of dental implants made from metallic and ceramicmaterials and showed that changing the angle of loading, andconsidering the bone resorption, significantly affected theresponse of these implants [25]. Specifically, the ceramic implantexhibited a better response for parallel loading and without boneresorption but the metallic implant exhibited a better responsewhen inclined loading and bone resorption were also considered[25]. Taking these difficulties into consideration, the presentinvestigation was initiated in order to use the testing conditionsmandated by the ISO14801 standard and to directly evaluate andcompare the fatigue behavior of dental implants fabricated fromconventional CP-Ti of grade 1 both without any SPD processingand after processing by 4 passes of ECAP at room temperature.

2. Experimental material and procedures

The material used in the present experiments was a CP-Ti ofgrade 1 with a composition, in wt%, of 0.100% O, 0.001% H, 0.010%N, 0.007% C and 0.030% Fe. The material was received in a hot-rolled and annealed condition with an initial average grain size of�23 μm. Billets were machined parallel to the rolling directionwith lengths of 70 mm and cross sections of 15�15 mm2.

Some of these billets were processed by 4 passes of ECAP atroom temperature using a die with an angle between the twoparts of the channel of 1201 and an outer arc of curvature of 201.It can be shown that this geometry leads to an imposed strain of�0.6 on each pass through the ECAP die [26]. All billets wereprocessed using route BC in which the billets are rotated by 901 inthe same sense after each pass through the die [27]. Further detailson the ECAP processing were given in an earlier report [15].

Samples for tensile and compression testing were machinedfrom the initial unprocessed material and from the materialprocessed by ECAP through 4 passes. The loading direction wasparallel to the pressing direction for the ECAP samples and parallel

to the rolling direction for the unprocessed samples. For tensiletesting, the specimens had gauge lengths of 25 mm and cross-sectional areas of 3.0�1.4 mm2. For compression testing, thespecimens had lengths of 9 mm and cross-sectional areas of6.0�6.0 mm2. All tests were conducted at room temperatureusing an Instron universal testing machine having a maximumload capacity of 100 kN and equipped with an optical extens-ometer. Marks were placed within the gauge lengths of thesamples prior to testing and the displacements between thesemarks were tracked during testing and converted directly to strain.Direct measurements of the minimum cross-sections wererecorded after the initiation of necking and these measurementswere used to determine the true stress and true strain at all stagesof the tensile deformation.

Representative dental implants were machined from theunprocessed material and from the material processed by ECAP.These implants had lengths of 14 mm and the external diametersof the threaded ends were 3.75 mm. The implants are illustrated inFig. 1. Fatigue testing was conducted specifically to follow therequirements of the ISO 14801 standard. Thus, the implants weremounted in a supporting structure at an angle of 601 with thehorizontal such that the crest of the implant was maintained at adistance of 3 mm from the support surface in order to simulate thebone resorption effect. This configuration is illustrated schemati-cally in Fig. 2. The abutments were attached to the implants with atorque of 35 N cm. A crown was machined from steel with ahemispherical shape at the most distant point having a radius of4 mm. The crown length was such that the distance between thesimulated bone level and the center of the hemisphere was equalto 11 mm. An alternating load was then applied vertically throughthe center of the hemisphere on the crown as required for fatiguetesting under the ISO 14801 standard. The load was adjusted tovary following a sinusoidal pattern such that the minimum loadcorresponded to 10% of the maximum load. The frequency forapplication of the load was 10 Hz.

Fig. 1. Dental implants used in the experiments. Left: implants machined from theas-received material. Right: implants machined from the material processedby ECAP.

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3. Experimental results

Tensile tests were conducted at an initial strain rate, _ε0, of1.0�10�3 s�1 under conditions of a constant rate of cross-headdisplacement and the results are shown in Fig. 3 where theengineering stress is plotted against the engineering strain forthe as-received material and the material processed through4 passes of ECAP. It is readily apparent that processing by ECAPincreases the strength but there is a corresponding reduction inthe overall elongation to failure. The yield stress increases from�340 MPa to �665 MPa through processing by ECAP, the ultimatetensile stress increases from �425 MPa to �735 MPa but theelongation is reduced from �20% to �8%. The uniform elongationalso decreases from �8% to �2% after ECAP.

In Fig. 4 the true stress is plotted as a function of the true strainfor the material before and after ECAP when testing in bothtension and compression. Thus, the strain hardening rate isdecreased after processing by ECAP. However, the maximum strainin tension is �56% after ECAP and this is only slightly lower thanthe value of �62% before processing. This shows that the ductilitydetermined by the reduction in the cross-sectional area is notsignificantly affected by the ECAP processing. It is also observedthat the rate of strain hardening in the material processed by ECAPis lower than in the as-received material. The strain hardeningexponent n was determined for the tensile tests and it was foundthat nE0.12 in the as received condition and there was an averagenE0.04 after processing by ECAP. Examining the compressiondata for the material processed by ECAP, it is apparent that thestrain hardening is higher in compression (nE0.06) than intension due to the frictional effects during testing.

Using the results from the fatigue tests conducted in confor-mity with the ISO 14801 standard, Fig. 5 shows a semi-logarithmicplot of the maximum load applied in fatigue testing against thenumber of cycles to failure. Again, results are shown for both theas-received unprocessed material and the samples processedthrough 4 passes of ECAP. It is readily apparent from thesedata that the material processed by ECAP has the capability ofwithstanding a larger number of loading cycles before failure.This improvement is consistent with published fatigue data formaterials processed by ECAP [24,28].

The fracture surfaces of the implants subjected to fatiguetesting are shown for different conditions in Figs. 6–8. The fracture

surface of the implant in the as-received condition is shown inFig. 6 for a maximum load of 220 N and implants of the materialprocessed by ECAP are shown in Figs. 7 and 8 for maximum loadsof 220 and 330 N, respectively. The loading direction in all images

Fig. 3. Engineering stress vs. engineering strain curves determined by tensiletesting of the as-received material and the material processed by ECAP.

Fig. 4. True stress vs. true strain curves determined by tensile and compressiontesting of the as-received material and the material processed by ECAP.

Fig. 5. Maximum load applied in fatigue testing plotted as a function of thenumber of cycles to failure for implants machined from the as-received materialand the material processed by ECAP.

Fig. 2. Schematic illustration of the apparatus for fatigue testing following ISO14801 standard.

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Fig. 6. Fracture surface of an implant machined from the as-received material and subjected to fatigue testing at a maximum load of 220 N. Different fracture features arealso shown in higher magnification images.

Fig. 7. Fracture surface of an implant machined from the material processed by ECAP and subjected to fatigue testing at a maximum load of 220 N. Different fracture featuresare also shown in higher magnification images.

R.B. Figueiredo et al. / Materials Science & Engineering A 619 (2014) 312–318 315

is horizontal and the sense of loading is from the right to the leftso that the fracture initiation occurred on the right side of theimplants and the final separation and failure occurred on the leftside of the implants. In Figs. 6–8, a lower magnification image ofthe total surface is shown at (a), and (b), (c) and (d) are additionalhigh magnification images of selected areas on the fracturesurfaces where these positions are marked in (a) with the corres-ponding letters.

Fig. 6 shows the fracture surface of the implant of the as-received material subjected to a cyclic load of 220 N which failedafter a total of 32,931 cycles. Thus, the fracture started on the rightside where there was the highest tensile stress during testing andthen it progressed circumferentially around the specimen up toa final and sudden breaking on the left side. The details inFig. 6(b) show the presence of river markings which demonstratethat the crack propagated along specific planes. Fig. 6(d) shows atypical fatigue fracture surface displaying grains with striationsand river markings in the grain boundary transition regions andFig. 6(c) shows the presence of dimples and some flat areasessentially perpendicular to the prevailing maximum tensilestress. For comparison, an implant of the as-received materialwas subjected to a cyclic load of 330 N, failed after 3484 cycles andthen the fracture surface was examined. For this latter sample,there was an absence of any river markings or striations on thefracture surface in the area associated with nucleation andpropagation of the crack and instead profuse dimples wereobserved in the area of sudden breaking.

Fig. 7 shows the fracture surface of an implant of the materialprocessed by ECAP and subjected to a cyclic load of 220 N. For thissample, failure took place after a total of 968,805 cycles, fractureagain started on the right and the cracking progressed around thespecimen to a final breaking on the left side. Fig. 7(b) showssmaller features and an absence of any river markings in thissample and instead the fracture surface displays regions withdimples and again there are some flat areas reasonably perpendicular

to the maximum tensile stress. Fig. 7(c) shows the presence of somesmall dimples and planar features indicating that the crack propa-gates along specific planes.

An implant of the material processed by ECAP was subjectedto a cyclic stress of 330 N and failed after only 13,934 cycles.The fracture surface is shown in Fig. 8(a) and the fracture againstarted on the right with final breaking occurring on the left side.Fig. 8(b) shows the presence of fatigue striations and reasonablyfeatureless regions with some dimples and Fig. 8(c) shows thepresence of small dimples and planar features.

4. Discussion

4.1. Static testing

These experiments show that processing commercially puretitanium by 4 passes of ECAP leads to significant hardening evenwhen using a die with a relatively large channel angle of 1201.In order to compare the present results with the numerous reportsof the mechanical properties of commercially pure titaniumprocessed by ECAP, a detailed summary was prepared showingthe yield stress (YS) and ultimate tensile stress (UTS) together withthe reported maximum elongation to failure for different grades oftitanium processed by ECAP and HPT using different processingconditions: the results are presented in Table 1 [15,16,20,24,29–33].Inspection of these results shows that the elongation obtained forthe processed sample in the present investigation is significantlylower than the elongations reported in other investigations. Thisdifference is due to the specimen geometry since the specimensused in this work exhibit a larger aspect ratio and it is wellestablished that the mechanical properties of ultrafine-grainedand other materials is critically dependent upon the dimensionsof the test samples [34,35].

Fig. 8. Fracture surface of an implant machined from the material processed by ECAP and subjected to fatigue testing at a maximum load of 330 N. Different fracture featuresare also shown in higher magnification images.

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It is apparent from Table 1 that the UTS of the materialprocessed by ECAP in this investigation (�735 MPa) is onlyslightly lower than the maximum value reported for CP-Ti grade1 (�810 MPa) after processing by ECAP [24]. Therefore, it isconcluded that the material used in this work is close to the limitfor strengthening of a CP-Ti grade 1 alloy.

It is also apparent from Table 1 that higher grade titaniumexhibits higher strength when processed by ECAP. Thus, ultimatetensile stresses in the range of �1 GPa were reported in CP-Ti ofgrades 2 [32] and 4 [29] after processing by ECAP and then coldrolling and a similar stress of �940 MPa was reported in CP-Ti ofgrade 2 after processing by HPT through 10 revolutions [33]. All ofthese results confirm that processing by SPD provides the cap-ability of producing CP-Ti with high strength. Furthermore, CP-Tiof grade 1 processed by ECAP exhibits a yield stress and anultimate tensile stress which is in the same range as for theunprocessed CP-Ti of grade 4. This comparison demonstrates thatprocessing by ECAP is able to significantly strengthen titaniumeven when the material has a lower impurity content.

An additional feature of these results is that processing by ECAPat RT leads to a significant reduction in the overall elongation tofailure. This pronounced decrease in elongation is due to theoccurrence of limited strain hardening since the material isalready approaching the strengthening limit after ECAP and thelack of hardening initiates necking in the early stages of the tensiletesting. Nevertheless, the material continues to possess reasonableductility since the reduction in cross-sectional area in tension islarger than 50%. This is in agreement with an earlier report ofductile behavior but a low elongation to failure in titaniumprocessed by ECAP [36].

4.2. Fatigue testing

An early report documented an enhanced fatigue limit andfatigue life in titanium processed by ECAP compared to its coarsegrained counterpart [9]. The fatigue properties were evaluatedthrough a combination of compression/tension cyclic stress-controlled tests and cyclic plastic strain-controlled tests [24].The present results are consistent with these earlier data andshow that ECAP processing also improves the fatigue resistance oftitanium implants when subjected to cyclic bending. However, thisincrease in fatigue resistance may be insufficient for making use ofCP-Ti grade 1 for the fabrication of implants.

In practice, the ISO 14801 standard is designed to reproduce theunfavorable conditions that must be successfully endured by adental implant. Thus, the loading inclination leads to a bendingthat creates a zone of tensile stresses in the implant. The standardalso simulates a total of 3 mm of bone resorption which effectivelyincreases the stress levels by up to 4 times the stresses that areneeded for a perfect positioning of the implants [25]. Early reports[25,37] showed that commercial implants exhibit better fatigueresistance than the CP-Ti grade 1 material tested in the presentstudy. For example, it was reported that implants having sizes of4.1�12 mm2 are capable of withstanding in excess of 106 cycleswith a maximum load of 420 N at different loading frequencies of2 and 30 Hz [37]. Biomedical grade yttria-stabilized zirconia dentalimplants, with sizes of 4�11 mm2, exhibit fatigue limits of�300 N [25]. Furthermore, it was reported that CP-Ti grade3 implants, with sizes of 3.8�10 mm2, also exhibit fatigue limitsof �300 N [25]. By contrast, the present results show that theunprocessed CP-Ti grade 1 implants fail before 105 cycles at alower load of 220 N. Processing the material by ECAP enhances thenumbers of cycles for failure to within the range of 105–106 butnevertheless these numbers are less favorable than those reportedin earlier studies for commercial implants.

An analysis of the fracture surfaces confirms that an increase incyclic loading in the implants from 220 N to 330 N using theunprocessed as-received material leads to an elimination of thefatigue striations and river markings on the fracture surfaces.However, the opposite trend was observed when increasing theload in the implants fabricated from the material processed byECAP. Thus, fatigue striations were clearly observed at 330 N inFig. 8 but they were not clear at 220 N in Fig. 7. River markingswere not observed in the materials processed by ECAP and this isattributed to the much smaller grain size in the processedmaterial. Based on an earlier report using a similar material andprocessing conditions, it is anticipated that the grain size afterECAP in these experiments was �200 nm [15] whereas the grainsize before processing was �23 μm. In practice, it appears that thefracture surface of the unprocessed material tested at 220 N for32,931 cycles exhibits features that are reasonably similar to thematerial processed by ECAP and tested at a higher load of 330 Nfor 13,934 cycles. This similarity is due to the higher strength ofthe processed material.

5. Summary and conclusions

1. Commercial purity titanium of grade 1 was successfully pro-cessed by 4 passes of ECAP at room temperature. Dentalimplants were machined from the unprocessed material andfrom the material processed by ECAP and these implants weretested in fatigue following the requirements of the recom-mended ISO 14801 standard for dental implants. The mechan-ical properties of the implants were also evaluated by tensileand compression testing.

Table 1Summary of the yield stress (Y.S.), ultimate tensile stress (U.T.S.) and elongation tofailure recorded in different grades of titanium before and after processing by ECAPor HPT: data are included for Ti processed at room temperature (RT).

Tigrade

Processingcondition

Y.S.(MPa)

U.T.S.(MPa)

Elongation(%)

Reference

1 Unprocessed 340 425 20.0 This work1 4 passes of ECAP (RT) 665 735 8.0 This work1 8 passes of ECAP 650 810 15.0 Vinogradov et al.

[24]1 Unprocessed 275 407 35.0 Zhao et al. [15]1 1 pass of ECAP (RT) 520 590 16.1 Zhao et al. [15]1 2 passes of ECAP (RT) 565 645 15.8 Zhao et al. [15]1 4 passes of ECAP (RT) 620 655 17.0 Zhao et al. [15]1 8 passes of ECAP (RT) 710 790 19.0 Zhao et al. [15]2 Unprocessed 354 487 30.0 Stolyarov et al. [29]2 10 passes of ECAP 582 645 20.6 Stolyarov et al. [29]2 10 passesþcold

rolling736 928 14.5 Stolyarov et al. [29]

2 Unprocessed 360 600 20.0 Zhang et al. [16]2 1 pass of ECAP (RT) 650 730 7.0 Zhang et al. [16]2 2 passes of ECAP (RT) 700 750 6.0 Zhang et al. [16]2 Unprocessed 307 532 33 Purcek et al. [30]2 12 passes of ECAP 564 771 22.8 Purcek et al. [30]2 Unprocessed 202.6 292.5 44.5 Kang and Kim [31]2 5 passesþextrusion 708.4 791.9 19.7 Kang and Kim [31]2 Unprocessed 310 470 41 Sordi et al. [32]2 6 passes of ECAP 615 790 32 Sordi et al. [32]2 6 passesþcold

rolling915 1030 32 Sordi et al. [32]

2 Unprocessed – 660 40 Wang et al. [33]2 10 turns of HPT (RT) – 940 23 Wang et al. [33]4 Unprocessed 630 740 25.0 Stolyarov et al. [29]4 8 passes of ECAP 750 815 19.0 Stolyarov et al. [29]4 8 passesþcold

rolling1006 1135 10.7 Stolyarov et al. [29]

4 Unprocessed 530 700 25.0 Valiev et al. [20]4 ECAPþTMT 1200 1240 12.0 Valiev et al. [20]

R.B. Figueiredo et al. / Materials Science & Engineering A 619 (2014) 312–318 317

2. The results show that processing by ECAP increases the yieldstress and the ultimate stress but reduces the strain hardeningrate. The elongation to failure after ECAP is reduced due to thelow hardening capability but the reduction in cross-sectionalarea shows the occurrence of good ductility even after ECAP.

3. Processing by ECAP increases the fatigue life of real implantssubjected to cyclic bending loads when using the dentalimplant testing standard. However, a comparison suggests thatthe improvement in the fatigue behavior is insufficient bycomparison with the anticipated behavior from commercialimplants manufactured from higher grade titanium alloys.

Acknowledgments

The authors acknowledge support from FAPEMIG under Grantno. APQ 00144-12, CPGEM, CNPq under Grants no. 483077/2011-9and 301.034/2013-3 and CAPES under Grant no. PVE 037/2012 inBrazil, the Specialized Research Fund for the Doctoral Program ofHigher Education of China under Grant no. 20116120110012 andthe European Research Council under ERC Grant agreement no.267464-SPDMETALS.

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