ContentslistsavailableatScienceDirect
Applied
Surface
Science
j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c
Mechanical,
structural
and
dissolution
properties
of
heat
treated
thin-film
phosphate
based
glasses
Bryan
W.
Stuart,
Miquel
Gimeno-Fabra,
Joel
Segal,
Ifty
Ahmed,
David
M.
Grant
∗AdvancedMaterialsResearchGroup,FacultyofEngineering,UniversityofNottingham,UK
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received12December2016
Receivedinrevisedform6April2017
Accepted15April2017
Availableonline18April2017
Keywords:
Heattreatment
Phosphateglass
Mechanicalproperties
Ionleaching
Osseointegration
Thinfilms
a
b
s
t
r
a
c
t
Hereweshowthedepositionof2.7mthickphosphatebasedglassfilmsproducedbymagnetron sput-tering,followedbypostheattreatmentsat500◦C.Variationsindegradationpropertiespreandpostheat treatmentwereattributedtotheformationofHematitecrystalswithinaglassmatrix,ironoxidationand thedepletionofhydrophilicP-O-Pbondswithinthesurfacelayer.Asdepositedandheattreatedcoatings showedinterfacialtensileadhesioninexcessof73.6MPa;whichsurpassedISOandFDArequirementsfor HAcoatings.Scratchtestingofcoatingsonpolishedsubstratesrevealedbrittlefailuremechanisms, ampli-fiedduetoheattreatmentandinterfacialfailureoccurringfrom2.3to5.0N.Coatingsthatweredeposited ontosandblastedsubstratestomimiccommercialimplantsurfaces,didnotsufferfromtensilecracking ortracksidedelaminationshowingsubstantialinterfacialimprovementstobetween8.6and11.3N.An exponentialdissolutionratewasobservedfrom0to2hforasdepositedcoatings,whichwas elimi-natedviaheattreatment.From2to24hionreleaseratesorderedP>Na>Mg>Ca>Fewhilstallcoatings exhibitedlineardegradationrates,whichreducedbyfactorsof2.4–3.0followingheattreatments.
©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Hydroxyapatite(HA)emerged asanosteoconductivecoating in the 1980s and remains an industrial surface treatment for orthopaedicintegration[1].Secondgenerationbioceramics facil-itatedinterfacialbondingbetweenthehosttissueandimplant[2]. PlasmasprayedHAisaceramiccoatinglayerwithaCa/Pratioof 1.67,similarin compositiontothecorticalbone.Bonelike lay-ersonmetallichipstemsordentalscrews promoteadhesionof osteoblastcellsandproteinattachmentforboneregenerationand osseointegration[3–5].
Plasma sprayed HA layers have been known to delaminate entirelyattheinterface,preventingcompleteosseointegrationvia thecoatinglayerorcausingintegrationdirectlywiththeimplant surface.AstudybyBloebaumetal.foundparticlesofHA upto 75m,andmetallicparticlesduetowearoftheunderlyingimplant embeddedintheacetabularcup[6].Despiteimprovements,aseptic looseningremainsthebiggestcauseofimplantfailure,responsible for≈40%ofrevisions[7].
Thekeyfactorforbioactivityisthestimulationofosteoblastsfor collagensecretionandsubsequentmineralisationofbone,assisted
∗Correspondingauthor.
E-mailaddress:[email protected](D.M.Grant).
bycreatinganenvironmentforosteoconductionatthesurfaceof theimplantdevice[8].Collagenproteinfibrilsarelaiddownby osteoblast cellsalong thebonesurface,facilitating regeneration fromthecreation ofa extracellular matrix(ECM) [3].Ca andP withintheformednetoftheECMmaymineralisetoform crys-tallisedboneintheformofHA. PhosphateBased Glasses(PBG) representathirdgenerationbiomaterialwhichisfullyresorbable in aqueous media and could repair the surrounding tissue by activatingacontrolledcellularresponsethroughthedeliveryof potentiallytherapeuticions[1].ResearchonPBGshasincludedMg
[9],Ca[10,11],Sr[12,13],F[14],whichhavebeenusedforbone tissuegeneration,Ti[15–17],Fe[18,19]toimprovedurability,Cu
[20]andAg[21,22]fortheirantibacterialproperties.
Pre-existingthermaltechnologieshavebeenusedtoproduce silicatebasedglass(SBG)coatingswithmixedsuccess.The inher-ent temperaturesassociated with thethermal processes, make theproductionofeitheradherentoramorphousglasses imprac-ticalwithoutdelamination,crackingorcrystallisation.Thethermal expansionmismatchleadstointerfacialstressesandpoor adhe-sion[23,24].Forexample,Bioglass®45S5wasplasmasprayedwith failureoccurringfromthermallyinducedresidualstressesattheTi coatinginterface[25].Bolellietal.utilisedsuspensionhighvelocity flamespraying[26]whilstGomez-Vegaetal.formed25–150m thickcoatingsviaanenamellingprocess[24].
http://dx.doi.org/10.1016/j.apsusc.2017.04.110
Thincoatingsin particularmaybedesiredtopreventbrittle failuresassociatedwithshearduringimplantation.A comprehen-sivereviewbyMohsenietal.concludedthatmagnetronsputtering producedthegreatestinterfacialadhesionofHAonTi6Al4Vafter investigating9depositionmethodsincludingplasmaspraying,hot isostaticpressing,thermalspraycoating,dipcoating,pulsedlaser deposition,electrophoreticdeposition,solgel,ionbeamassisted depositionandPVDsputtering[27].
RFmagnetronsputteringofSBGwasexploredbyMardareetal.
[28]andbyStanetal.[29].Themanufacturingcomplications asso-ciatedwithPBGcoatingsduringsputteringhasbeendemonstrated elsewhere[30],whilsta previouspublicationshowedstructural variabilitybetweencompositionallyequivalentmeltquenchedand PVDcoatings,showinggreaterbulkandsurfacepolymerisationin coatingcompositions[31].
ThemeltquenchedPBGcompositionP2O5-40MgO-24CaO-16 Na2O-16 Fe2O3-4mol%(denotedMQ:P40)hasbeenextensively researched for the production of degradable glass fibres [32]. Hassanet al. demonstrated good cytocompatibility withMG63 osteoblastcells,indicatingcomparableresultstothetissueculture plasticcontrolsamples[33].Thereforetheworkpresentedhere investigates the sputtered coating composition; P2O5-40 MgO-24CaO-16Na2O-16Fe2O3-4mol%appliedtoTi6Al4V,specifically exploringtheeffects ofsurface topographyandpostdeposition annealingonstructural,mechanical,degradationandionrelease properties.
2. Methodology
2.1. TargetpreparationT1:P51.5Fe5andmeltquenched:P40
Pre-calculated (mol%) proportionsof precursor saltsnamely sodium dihydrogen phosphate (NaH2PO4), calcium hydrogen phosphate (CaHPO4), magnesium phosphate dibasic trihydrate (MgHPO4·3H2O), iron phosphate dehydrate (FePO4·2H2O) and phosphorouspentoxide(P2O5)(SigmaAldrich,U.K.), were thor-oughlymixedthenpreheatedat400◦Ctodehydrate.Themixture wasthenmeltedinaPt:Rh(90/10wt%)crucibleat1200◦Cfor2h inair.Thetargetswereformedbyquenchingthemoltenmixture at450◦Cfollowedbyslowcoolingtoroomtemperature.Thetarget mouldandthesubsequenttargetmeasured75±2mmindiameter and6±1mminthickness.SeeTable1fortargetcomposition.MQ: P40wassimilarlycastinto9mmdiameterrodsandcutinto7mm cylindersusingadiamondsaw.
2.2. RFmagnetronsputtering
Thecoatingsweredepositedviaacustomin-housedesigned PhysicalVapourDeposition(PVD)rigusingRF(13.56MHz) mag-netron sputtering, with a PBG target (denoted T1: P51.5). The chamber was pumped down to a vacuum using a combina-tion of a rotary (Edwards E2M-18) and turbo molecularpump (Edwards EXT250) to a base pressure <7×10−3Pa. Phosphate glassdepositionwasperformedat60±1Wpower(13.6kWm−2) for1165mintoacoatingthicknessof2.67±0.09mata pres-sureof10±0.05mTorrof99.99%pure-shieldargon.Thedistance betweentargetandsubstratewasalsoheldconstantat4±0.5cm. Prior todeposition, targetswere sputter cleaned for 30min at 30W and increased to 60W for a further 30min. The target glasshasbeenlabelled(T1), melt quenchedglass rodsas(MQ) andcoatingsas(C).Nominalcompositionshavebeendenotedby (N)whilstaspreparedcompositionsassessedviaEDX composi-tionalanalysishave beendenotedby(AP). Seecompositionsin
Table1.
2.3. Postdepositionannealing
PBGcoatingsC2:P40HT30andC3:P40HT120weredepositedas amorphousandsubsequentlyheattreatedinaLentontubefurnace, with99.99%pureshieldargonto500◦Cat10◦Cmin−1.CoatingsC2: P40HT30andC3:P40HT120wereheldfordwelltimesof30min and120minrespectively.Allsampleswerelefttocoolnaturallyto roomtemperature.
2.4. Thermomechanicalanalysis
Thermal Expansion coefficient (TEC) was measured using ThermoMechanicalAnalysis(TMA)viaaTMAQ400.50mmlong, 9mmdiameterMQ:P40glassrodswereheatedupto400◦Cata rateof10◦Cmin−1.TheTECmeasurementswereobtainedfroma best-fitlinebetween50and300◦C.Allexpansionmeasurements representtheStandardErrorMeanofn=3samples.
2.5. X-raydiffraction
Samples of theglass targets were ground to a fine powder for X-Ray diffraction(XRD) analysis (BrukerD8, Cu K␣source:
=1.5418 ˚A,40kV,40mA)conductedovera2rangefrom15◦to 65◦withastepsizeof0.04◦in2,andadwelltimeof5s.In addi-tionglancingangleXRDwasperformedonthedepositedcoatings utilisingastepsizeof0.02◦in2.
2.6. EnergydispersiveX-rayspectroscopy
The compositions of the sputtering targets and sputtered coatingsweredeterminedviaa Phillips XL30SEM-EDX.Energy DispersiveX-RaySpectroscopy(EDX)wasconductedata work-ing distanceof 10mm at a minimum of 200,000 counts and a beamvoltageof15kV.Theelectronbeamcurrentwasoptimised byincreasingthespotsizetoobtainaminimumacquisitionrateof 4000countss−1whilstmaintaininganacquisitiondeadtime<30%.
2.7. Scanningelectronmicroscopy
Thecoatingcrosssectionsandsurfaceimageswereobtainedvia ScanningElectronMicroscopy(SEM)usingaPhillipsXL30-ESEM. Theworkingdistancewaskeptconstantat10mm,withabeam voltageof15kV.TheprecisionoftheEDXsystemwasanalysedover arepeatedspot(n=5),whilstcoatinghomogeneitywasassessed over (n=12) randomsample areas.Additionally, batch-to-batch variation(n=4)wascalculatedandtheestimatederrorispresented inTable1.
2.8. Fouriertransforminfraredspectroscopyattenuatedtotal reflectance
ABrukerTensorFourierTransformInfraredSpectrometer(FTIR) withanAttenuatedTotalReflectance(ATR)attachmentwasused forallInfraredabsorptionmeasurements.Aspectralresolutionof 4cm−1overthewavenumberrange500–4000cm−1 wasset.All spectraobtainedrepresenttheaverageof64scansoverthesample area.
2.9. Focussedionbeamscanningelectronmicroscope
Table1
Aspreparedtarget,meltquenchedandcoatingcompositionsmol%.Nominalcomposition(N)andasprepared(AP)wereanalysedbyEDX.
Glasscodenominal(N)vs.asprepared(AP) P2O5 MgO CaO Na2O Fe2O3
mol%
T1:P51.5(N) 51.50 18.50 14.00 11.00 5.00
T1:P51.5(AP) 49.81±0.11 19.31±0.08 13.31±0.09 13.15±0.11 4.42±0.53 MQ:C1:C2:C3:P40(N) 40.00 24.00 16.00 16.00 4.00 MQ:P40(AP) 39.64±0.08 24.49±0.08 15.59±0.07 16.73±0.06 3.55±0.06 C1:P40AD(AP) 40.23±0.19 23.83±0.15 15.37±0.15 16.75±0.14 3.82±0.53 C2:P40HT30(AP) 40.71±0.17 22.30±0.13 16.59±0.14 15.95±0.15 4.45±0.53 C2:P40HT120(AP) 41.47±0.37 22.58±0.21 15.89±0.16 15.93±0.31 4.13±0.53
2.10. X-rayphotoelectronspectroscopy
X-RayPhotoelectronSpectroscopy(XPS)wasconductedusing aVGScientificEscaLabMarkIIwithanAlk␣non-monocromatic X-Raysourceincidenttothesamplesurfaceat≈30◦.Scanswere collectedat20mAand12kVemissions.Thesurveyandhigh resolu-tionspectrawerescanned3and5timesrespectively,utilisingstep sizesof1.0and0.2eVfordwelltimesof0.2and0.4srespectively. AnalysisandpeakfittingwasconductedusingCasaXPS.All spec-trawerechargedcorrectedfortheC1semissionto284.8eV.For compositionalquantification,theareaunderthepeakofthemost intensephotoelectronemissionforeachelementwascalculated andnormalisedtotheoverallarea,utilisingtheirRelative Sensi-tivityFactorsacquiredfromtheScofieldRSFlibrary.Forthehigh resolutionspectra,relativepeakpositioningandpeakareasdueto spinorbitsplittingwereconstrainedbasedontherelationshipof theelectronsubshell.
2.11. Atomicforcemicroscopy
AtomicForce Microscopy (AFM) micrographsand roughness measurementswereacquiredintappingmodeviaaBruker Dimen-sionIcon.AFMtips;modelTAP525A0.01–0.025Ohm-cmAntinomy (n)dopedSiwereused.Allmeasurementswereacquiredovera scanareaof20.0×20.0m2.AFManalysissoftwareGwyddionwas
usedforanalysis.
2.12. Samplepreparationforcoatingdeposition
Substrates for coating were 1.5mm thick, 10mm diameter Ti6Al4V(grade5)discswireerodedfromsheet.Thediscswerewet polishedtoa6mfinishusingsiliconcarbidepaperfromgrades P200-P4000andfurtherpolishedona0.25mchemomet finish-ingpadwiththeapplicationofcolloidalsilica.Thepolishedand sandblastedsubstrateroughness’sweremeasuredas7±1nmand 696±57nmrespectively(n=9)viaAFM.Sampleswerethen ultra-sonicallycleanedfor10mininacetone,followedbydistilledwater. 100mthickborosilicatecoverslideswerealsocoatedto mea-surethecoatingthicknessesachieved.Priortocoating,theslides wereplatinumcoatedfor90sat1.2keVinaPolaronsputtercoater toenhancetheinterfacialcontrastbetweencoatingandglassfor microscopyanalysis.
2.13. Degradationofcoatingsandionrelease
ThePBGcoatingsweredepositedontobothpolishedand sand-blastedTi6Al4V(Grade5)substratesbyRFmagnetronsputtering.
ForSEManalysisthecoateddiscsweresubmergedin15mLof distilledwater(dH2O)andincubatedat37±2◦C.Isolatedcoated
sampleswereobservedat16h.ThepHwasmeasuredpreandpost degradationtoensurethesolutiondidnotbecomesaturated. Sam-plescouldnotbere-submergedfollowingconductivecoatingfor imagingduetocontaminationinhibitingdegradation.
Mass loss per unit projected surface area was recorded for coatingsonsandblastedsubstrates,asbulkcoatingliftoffor delam-inationhadbeenobservedonpolishedC1:P40AD.Measurements werecollectedusingaMettlerToledoprecisionscaleaccurateto 0.01mg.Allsampleswereweighedpriortodepositionandpost depositiontodeterminethemassofthecoatings.Sampleswere subsequently submerged in15mLof solutionand incubatedat 37±2◦C(n=3).Ateachtimepointthediscswereremoved,and placedinanovenat50◦Cfor1htoremovesurfacemoisture,prior tomasslossmeasurements.Thesolutionwaschangedafter1and 3days.
Toassessionrelease,isolatedsamples(n=3)fortimepointsof2, 16,24and48hweresubmergedin15mlofultrapureMilli-Qwater. Multi-element analysis of solutions wasundertaken by ICP-MS (Thermo-FisherScientific iCAP-Q)utilisinginternal andexternal calibrationstandards, whilstsampleswere processedusing the QtegraTMsoftware(Thermo-FisherScientific).
2.14. Scratchadhesiontesting
ScratchAdhesiontestingwasconductedinaccordancewiththe BSEN1071-3:2005standard,usingaBrukerscratchtesteranda RockwellC Indenter.Progressiveloadswereappliedovera dis-tanceof 3mmwithapreloadof0.5Nfollowedbyprogressive loadingto30Noveraperiodof180s.Allvaluesrepresentthe aver-ageofn=8scratches.Thecriticalloads(Lc)weredefinedasthe initialappearanceofthefailuremechanismalongthescratchpath ortrackside.
2.15. Pulloffadhesiontesting
The pull off test was conducted in accordance with ASTM-D4541-09 using a precision adhesion testing apparatus (PAT Handy) manufactured by DFD instruments. Pull off stubs of 2.81mmindiameterwereusedwithasamplenumberof(n=8). Athermally curingepoxyresin (DFDE1100S)wasappliedover thearea ofthe stuband pressed onto thecoating surface.The stub/substrateswereplacedonaheatedplateat140◦Cfor60min tocure.
3. Results
3.1. Compositionandstructuralanalysis
3.1.1. EDX
The coating composition P2O5-40 MgO-24 CaO-16 Na2O-16
Fe2O3-4mol%wasinvestigatedtoobservetheeffectofheat
treat-ment on thecoating topography, crystallinity, degradation and mechanicalproperties.Elementalmol%compositionsofMQ,C1, C2and C3wereobservedtovaryby amaximum of1.24, 1.53, 1.22, 0.82, 0.63mol% for P2O5, MgO,CaO, Na2O, Fe2O3,
Fig.1.XRDofC1:P40ADandC2:P40HT30andC3:P40HT120coatings.Substrate signalindicated()titanium(ICDDPDF-00-001-1197).Acrystallinephase corre-spondingto(䊉)Hematite(Fe2O3)(ICDDPDF-01-085-0599)emergedfollowingC2:
P40HT30andC3:P40HT120.Amorphoushumpswerepresentinallsamplesfrom 15◦to40◦(2).ThedepositiontargetT1:P51.5wasentirelyamorphous.
method,consistencyofthecoatingprocessandbatch reproducibil-ity[30,34].
3.1.2. XRD
GlancingangleXRDsuggestedanamorphousmicrostructurefor C1:P40AD.SignalfromtheTisubstrateproducedpeaksassociated withtitaniumICDDPDF-00-001-1197.Inadditionasingle diffrac-tionpeakemergedinC2:P40HT30andC3:P40HT120locatedat ≈33◦(2)(seeFig.1Insertformagnifiedslowscan).Thispeakwas attributedtothereflection(104)ofHematite(Fe2O3)(seeFig.1). Anamorphousdiffractionhumpremainedpresentfrom15◦to38◦ (2)followingheattreatment.Structuralanddegradation proper-tiesofC1:P40ADandMQ:P40,havebeenpreviouslypublished
[30,35].
3.1.3. FTIR
IRabsorptionofthethreecoatingsC1:P40AD,C2:P40HT30and C3:P40HT120containedpeaksassociatedwithvibrationalmodes locatedat≈1280,1200,1100,1055,1012,912,769,530cm−1.The positionrangesandpeakdesignationsaredetailedinFig.2.C1: P40ADshowedweakerrelativeabsorptionat1280and530cm−1 attributedtoQ2(PO
2)groupsandQ0(PO43−)respectivelywhilst C2:P40HT30andC3:P40HT120containedQ2 (PO
2)shouldersat 1280cm−1.ThebandattributedtoQ0(PO
43−)increasedwith inten-sityfollowingC2:P40HT30andC3:P40HT120andmayhavebeen attributedtoHematite.AccordingtoMinittietal.Hematitebands arelocatedat315,461and560cm−1,overlappingthePO
43−band at500–565cm−1[36,37].
Thenotabledeclineintherelativeintensitiesofthepeaksat900, 769,and530cm−1foralldegradedsamplesmayindicatepossible cleavageoftheP-O-Pbonding,areductioninQ0speciesor propor-tionalincreaseinHematite(Fe2O3)withrespecttotheamorphous phase(Fig.2).
3.1.4. XPS
XPSsurveyspectraofC1,C2andC3showedthesurface ele-mentalcompositionswithinthefirst5–10nmofthecoatinglayer (Fig. 3A, C, E) [38]. Following heat treatment at the annealing temperatureof500◦Cforboth30minand120min,thepeaks pre-viouslypresentinC1:P40ADforP2pandP2sat132–140eVand 185–195eV(Fig.3A)respectivelywereabsentfromthespectra ofC2:P40HT30(Fig.3C)andC3:P40HT120(Fig.3E)showinga
phosphorousdeficiencyinthesurfacesoftheheattreated coat-ings.Quantificationofthecompositioninat%showedanincrease inFecontentfrom1.4%to22.4%and22.8%followingHT30and HT120min(Table2).Theat%and mol%havebeenpresentedin
Table2.
ThehighresolutionscanforFe2pforC1:P40ADcouldnotbe deconvolutedwithasufficientlevelofconfidencebasedonlow signalintensityandthepresenceofcloselyspacedbinding ener-giesattributedtothemultiplechemicalstates(Fig.3B,DandF). Guptaetal.showedover20peakslocatedbetween708.3–713.6eV whichwereattributedtoFeO,Fe2O3,Fe3O4,FeOOH[39].Thepeak positionsweredeterminedbyanalysisofstandardreference mate-rials.Theasymmetricalpeakshapetowards709.0eVandthebreath of theFe2p3/2 spinorbital suggestedthe combinedpresence ofFe2O3,Fe3O4,andFeOOH[39].Guptaetal.suggestedpeak1 positionsat709.8eV,710.2eV,710.2eVrespectivelywithpeak4 positionsat712.3,713.6and713.2eVrespectively[39].
Fe2pshowedimprovedresolutionfollowingheattreatment, with no apparent difference between C2: P40HT30 and C3: P40HT120. The satellite regions here were observed between 715.0–724.0and730.0–737.0eV.Fe2phasaspinorbitsplit(Fe2p 1/2),generatingspectralpeakslocatedbetween721.0and737.0eV (Fig.3DandF)[40,41].Thedefinedasymmetricpeakpositionsof 710.4and710.6eV,thenarrowingoftheFe2p3/2and1/2peaksand thesatellitepeaklocationsof7.9/7.8and8.7/8.0eVfromthemain spectralpeaksforC2:P40HT30/C3:P40HT120respectively(Fig.3D andF)suggestedtheprominenceofFe2O3.Grosvenoretal.located satellitepeaksforHematite(␣-Fe2O3),Magnetite(␥-Fe2O3), Lipi-docrocite(␥-FeOOH),Magnetite(Fe3O4)tobespaced8.3,8.5,8.0 and5.9eVfromtheirelectronspinorbitalFe2p3/2.
3.2. Adhesionandmechanicalproperties
3.2.1. Scratchtesting
ThemechanicalpropertiesofcoatingsC1:P40AD,C2:P40HT30 andC3:P40HT120weredeterminedbyprogressivescratchand tensilepullofftesting(seeFigs.4–6).Thescratchwear mecha-nismshavebeenlabelledinaccordancewiththestandard;BSEN 1071-3:2005inorderof criticalload(Lcx) occurrenceandwere attributedtotheinitialindentation(scratchLc1),(tensilecracking Lc2),(tracksidedelaminationLc3),(bucklingandbucklespallation Lc4)and(InterfacialdelaminationLc5)(Fig.4A).
Fig.2. IRabsorptionspectraforasmanufacturedcoatingsandcoatings16hpostdegradationindH2O.(Inserttable)attributedIRabsorptionmodes.
Fig.3.XPSsurveyspectraandhigh-resolutionspectraofFe2p.(A)C1:P40ADsurvey(B)C1:P40ADFe2p.Broadpeakpositionssuggestedmultipleoxidationstatesassociated
withFefollowingdeposition(C)C2:P40HT30survey(D)C2:P40HT30Fe2p(E)C3:P40HT120survey(F)C3:P40HT120Fe2p.(DandF)FollowingC2:P40HT30andC3:
Table2
SurfacecompositionsdeterminedbyXPSofC1:P40AD,C2:P40HT30andC3:P40HT120.Column(at%)andcolumn(*at%)showthenormalisedcompositionswithoutthe
contributionofadventitiouscarbonandtheabsolutequantificationincludingthecarbonC1semissionrespectively.
C1:P40AD C2:P40HT30 C3:P40HT120
mol% at% *at% mol% at% *at% mol% at% *at%
(P2p)P2O5/Phosphorous 44.1 21.6 19.2 3.8 1.6 1.2 2.9 1.3 1.1
(Na1s)Na2O/Sodium 8.2 4.0 3.5 12.9 5.5 4.2 12.5 5.6 4.8
(Ca2p)CaO/Calcium 22.0 5.4 4.8 5.2 1.0 0.7 8.0 1.8 1.5 Mg(2s)MgO/Magnesium 22.9 5.6 5.0 25.4 5.4 4.1 25.8 5.6 4.8 (Fe2p)Fe2O3/Iron 2.9 1.4 1.3 52.7 22.4 16.9 50.8 22.8 19.6
(O1s)Oxygen – 62.0 55.0 – 64.1 48.4 – 62.9 54.1
(C1s)Carbon – – 11.2 – – 24.6 – – 14.0
Fig.4.ScratchadhesiontestingofC1:P40AD,C2:P40HT30andC3:P40HT120.ThecharacteristicwearmechanismshavebeenlabelledinC1:P40ADas(A)ScratchLc1,(B),
TensilecrackingLc2,(C)TracksidedelaminationLc3,(D)TracksidespallationduetobucklingLc4,(F)InterfacialdelaminationLc5,(G)ParticulatebuildUp.
3.2.2. Pullofftesting
Theinterfacial mechanical strengthsof C1,C2,and C3were assessedbytensilepull offtesting.Asa control,theepoxywas usedtoglueastubtoaTi6Al4Vsubstrate(Fig.6C).Inallpulloff cases(n=8)duringtesting oftheepoxy,failure occurred cohe-sivelyatanaverageloadof≈76.8MPa.Thefailureloadsranged froma(≈59.2–87.2MPa).CoatingsC1:P40AD,C2:P40HT30andC3: P40HT120failedat≈79.3,78.6and73.6MParespectivelyshowing ranges(60.8–105.6),(64.0–99.2)and(70.4–80.8)MPa.Microscopic observationofthefailurelocationsuggestedepoxycohesive fail-ureinallcases.Forcomparativepurposes(Fig.6insert“Coating Failure”)representsthecomplete interfacialfailurea of490nm thicksilicateglasscoating(SiO2-50CaO-31Na2O-17P2O5-2mol%), which was observed to fail at 14.0±1.0MPa with a range of (8.8–17.6MPa).
FIB-SEM cross sections (Fig. 6A-C) were milledthrough the coating/substrate interface to observe through thickness alter-ationsofthecoatingfeaturesduetoheattreatment.C1:P40AD (Fig.6A,D)followedthetopographicalfeaturesofthesubstrateand demonstratedcompleteinterfacialadherence.Partialcoatingvoid formationanddelaminationwasobservedforC2:P40HT30(Fig.6B, E).C3:P40HT120showedconsistentdelaminationofthecoating alongtheinterfaceonbothpolishedandsandblasted substrates (Fig.6C,F).
3.3. Degradationandionrelease
Fig.5. ScratchadhesiontestingofC1:P40AD,C2:P40HT30andC3:P40HT120.Scratchtestcriticalloadsandfailuremechanismsforcoatingson(A)PolishedTi6Al4V,
(B)SandblastedTi6Al4V.
Fig.6.(Tableinsert)TensilefailureloadsandrespectivefailuremodesforC1:P40AD,C2:P40HT30andC3:P40HT120.Allcoatingstrengthsexceededthestrengthofthe
epoxy.FIBSEMcrosssectionalmicrographs.PolishedandSandblasted(A,D)C1:P40AD,completeinterfacialadherence,(B,E)C2:P40HT30,isolateddelamination,(C,F)
C3:P40HT120,consistentdelaminationalongthesubstrate/coatinginterface.
Table3
Dissolutionandionreleaseratesindistilledandultra-purewaterrespectively.Dissolutionrateswerecalculatedbetween2–24hand2–96hwhilstionreleaserateswere
calculatedbetween2–24hforC1:P40AD,C2:P40HT30andC3:P40HT120.Ionreleasehoweverwascontinuedtothe48htimepointinFig.7.
2h 2–24h 2–96h Ionrelease(PPMh−1)(r2)2–24h
×10−4mgmm2 (×10−4mgmm2h−1)(r2) Na Mg P Ca Fe
C1:P40AD 2.9±0.4 2.05±0.35 (0.92)
Fullydegraded by48h
0.08±0.01 (0.98)
0.08±0.01 (0.98)
0.34±0.1 (0.94)
0.07±0.02 (0.95)
0.05±0.02 (0.89) C2:P40HT30 0.7±0.3 0.61±0.10
(0.93)
0.68±0.02 (0.99)
0.02±0.01 (0.92)
0.02±0.00 (0.99)
0.10±0.00 (0.99)
0.02±0.00 (0.99)
0.01±0.00 (0.99) C3:P40HT120 0.3±0.1 0.85±0.06
(0.99)
0.84±0.01 (0.99)
0.02±0.01 (0.72)
0.02±0.00 (0.99)
0.10±0.01 (0.99)
0.02±0.01 (0.85)
Fig.7. DegradationofcoatingsC1:P40AD,C2:P40HT30andC3:P40HT120.(A)Dissolutionratesupto96hindistilledwaterandIonreleaseprofilesfollowingdissolution inultra-purewaterupto48h(B)C1:P40AD(C)C2:P40HT30and(D)C3:P40HT120.
2h of degradation followed by a linear profile from 2 to 24h (Fig.7A).Bythe2htimepointthecomparativemasslosseswere (2.9,0.7and0.3)×10−3mgmm−2forC1,C2,andC3respectively, suggesting anincreased stability in thefirst 2hof degradation duetoheattreatment(Fig.7A).From2to24hdegradationrates were(2.1.0.6and0.9)×10−4mgmm−2h−1respectively(Fig.7A). C1: P40ADwas fully resorbed by the 48h time point. Contin-ueddegradationupto96hof C2:P40HT30and C3:P40HT120, followedsimilardegradation profiles,degradingatrates of(0.7 and0.8)×10−4mgmm−2h−1(Fig.7A).Linearregressionr2values rangedfrom0.92to0.99supportingalinearcorrelation.SeeTable3
forcomparativedissolutionrates.
Linearionreleaseprofileswereobservedforallcoatingsand ionsofNa,P,Mg,CaandFeoverthe48htimeperiodinultra-pure waterwiththeexceptionofC1:P40ADwhichwasfullyresorbed bythe48htimepoint.Thereforeionreleaseslopeswerecalculated from2to24h.r2valuesrangedfrom0.85to0.89withtheexception ofC3:P40HT120whichhadanoutlierat16hforNarelease.
For the three coatings the ion release rates ranked P>Na>Mg>Ca>Fe whilst the observed exponential degrada-tionprofileforC1:P40ADwasreflectedbytheinitialsurgeofions releasedbythe2htimepoint(Fig.7B).
Ion release rates ranged from 0.08 to 0.05ppmh−1, 0.01–0.02ppmh−1 and 0.01–0.02ppmh−1 for C1, C2 and C3. Heattreatmentledtoareductioninionreleaseratesbyfactorsof (4.0,3.6)-Na(4.1,4.3)-Mg(3.7,3.9)-P(3.0,3.4)-Caand(7.7,5.4)-Fe forC2:P40HT30andC3:P40HT120respectively.SeeTable3for comparativeionreleaserates.
3.4. Surfacetopography
Electron micrographs of the coating surfaces pre and post heattreatmentsandpreand16hpostdegradationonboth pol-ishedand sandblastedsubstrateshavebeenpresentedinFig.8.
In Fig.8A, C1:P40ADshowed nonotablesurface features. Fol-lowingheattreatmentofC2:P40HT30thecoatingcolourbecame blue/violet(Fig. 8Binset). (Fig. 8B, C for C2: P40HT30 and C3: P40HT120 polished, showed surface features, consistent with expansion and contraction of thesurface layerduring heating. Additionally expansion craters were observed to be dispersed across the coating surfaces. An increase in surface roughness from C1: P40AD toC2: P40HT30 and C3: P40HT120 was sup-portedbyAFMmeasurements(Fig.8M)forwhichn=4400m2 regionswerechosenforanalysis.RoughnessRavaluesincreased from (8 to 27 to 44nm) respectively for coatings on polished substrates.Incontrastcoatingsdepositedontosandblasted sub-stratesshowednoremarkablesurfacechangesortrendsrelating to Ra values of 519, 321, and 485 following heat treatment (Fig.8G,H,I,M).Coatingsonsandblastedsubstratesshowed sur-facesmoothingfromthecalculatedsubstrateRavalueof696nm whilstnosignificantvariationwasobservedfromthepolished sub-strateroughnessof7nm.Notably,surfaceareaanalysisbyAFM revealedthatsandblastedsubstrateshadanareaof18.5%greater thantheprojected400m2,leadingtoan21.5%increaseforC1: P40AD.
Fig.8.Micrographsofcoatingsurfacepreandpostdegradationofasmanufacturedonpolishedsubstrates(A)(B)(C)onsandblastedsubstrates(G)(H)(I)and16hdegradation
onpolishedsubstrates(D)(E)(F)onsandblastedsubstrates(J)(K)(L)C1:ADC2:P40HT30C3:P40HT120respectively.(M)AFMroughnessandsurfaceareameasurements(N)
HematitecrystalsformedatP40HT120onpolishedsubstrates.
4. Discussion
4.1. CompositionandstructuralanalysisbyEDX,XPS,FTIR,and XPS
Apostannealingstageinglassesisconventionallyconducted withintheprocessingwindow, definedastherange fromglass transitiontemperaturetotheonsetofcrystallisation.Such treat-mentshouldallowresidualstressesformedduringquenchingto diffusefromtheglass,thereforeleadingtoimprovedmechanical propertiesduetomolecularrelaxation[42].
PBG’sareamorphous,covalentlyboundsolids,composedofa polymericlikebackboneofphosphatetetrahedralPO4.The inclu-sion of ionic species depolymerises the network and enhances durability via network cross linking [43,44]. Previous research showedthatcompositionallyequivalentsputteredcoatingswere structurallydifferenttotheirmeltquenchedcounterparts,showing increasedpolymerisationofthebulkmaterialwhichwasconfirmed via31PNMRandXPS[31,35].Thisincreaseinpolymerisationled togreatersolubilityofsputteredcoatings[31,35].Thereforethe
aimof this studywastoobservetheeffects of postdeposition annealing(PDA)onthemechanicalandsolubilitybehaviourofPBG coatings.
TheXRDresultssuggestedthatC1:P40ADwasdeposited amor-phous. Followingthe heat treatments of C2: P40HT30 and C3: P40HT120 the formation and subsequent growth of Hematite crystalswithinanamorphousmatrixwasobserved.Alow inten-sitypeakforHematitewaslocatedaround33◦ 2followingC2: P40HT30 and C3: P40HT120. The broad hump associated with amorphous diffractionremained, suggesting theformation of a glass-ceramic structure. This was supported by IR absorption (Fig.2)revealingpeakslocatedbetween500and565cm−1; consis-tentwithPO43−withinthephosphatestructureandthepossible presence ofHematite. Wangetal. reportedvibration bandsfor aHematite (␣-Fe2O3)standardbetween567and584cm−1.The increasedIRpeakintensityinC2:P40HT30andC3:P40HT120in comparisontotheC1:P40ADsupportedthegrowthofHematite crystals[37].
Fig.9.Diagramdepictingsuggestedstructuralandcompositionalchangesfrom(A)C1:P40ADto(B)C2/C3:P40HT30/120followingheattreatment.(C)Migrationofcations,
phosphorusdepletionandgrowthofHematite.
showed no peaks associated with phosphorus following C2: P40HT30andC3:P40HT120whilsttheproportionofFeincreased from1.4%to22.4%and22.8%(seeTable2andFig.3A,C,E).The volatilityofphosphorousmayhaveledtoasurfacedeficiency[45]. Boydet al.suggestedthatforCa:P filmswhich underwentpost depositionannealingat500◦C,theCa:Pwasreportedtoincrease, explainedbyvolatilephosphorousevaporatingfromthesurfaceat lowerannealingtemperatures[46].FromXPS,thebroadspectral rangeoftheFe2ppeakinC1:P40ADsuggestedthatIrononthe surface(0–10nm)mayhaveexistedinmultipleoxidationstates, FeO,Fe2O3,Fe3O4andFeOOHwhichwasthecauseofbroadening oftheFe2phighresolutionspectra(seeFig.3B)[41].FeOOH, how-everwasnotsupportedashydroxylgroupswereabsentfromall IRspectra(seeFig.2).FollowingC2:P40HT30andC3:P40HT120, thedefinedpeakshifttowardsthelowerbindingenergiesof710.4 and710.6eVcombinedwiththerelativedistances betweenthe mainspectralpeaksandsatellitepeakssuggestedtheprominence ofFe2O3(seeFig.3D,F)[39,41].
ThecomplementarytechniquesofFTIR,XPS,andXRDconfirmed IronoxidationonthesurfaceandthecrystallisationofFe2O3 as Hematite.SEMimagingat64,000timesmagnificationshowed crys-tallitegrowths(Fig.8N).AsdepictedinFig.9,diffusionofcations intothebulkglassduringannealingmayhavepromotedthe migra-tionof Fe2+and3+ ions into thevacancy sites [47]. Minitti et al. reportedtheformationof<1mthickHematitecoatingson oxi-disedbasalticglassat700◦Cwhilstshowingnosuchformation at350◦C [36].The mechanism wasbelievedto betheremoval ofelectronsfromFe2+byoxidationandre-oxidationatthe sur-facewithFetoformFe3+similartotheeffectobservedhere.As suggestedbyCookand Cooperetal.forIroncontainingsilicate glass,migrationofoxygenallowsoutwardmigrationofFetofill theelectronholesinthesurfacematerialleadingtothegrowth
ofHematiteobservedhere[47,48].Wisniewskietal.demonstrated theformationofHematiteandMagnetitecrystalsinIroncontaining soda-limeglasswhenannealedat480◦C(30◦Cbelowthemeasured Tgof510◦C)[49].Thismigrationofoxygentothesurfacecouldhave causedtheobservedcratersformedpostheattreatment(Fig.8Band C).
Colourchangesofthesamplestobluish/violetinFig.8inserts followingheattreatmentmayhavebeenduetoaredoxreaction betweentheTi6Al4Vsubstrateandoxygenwithintheglass[50,51]. Thisinterferenceeffectastheoxidelayergrewwasalsoobserved byKumaretal.forTi6Al4Vsurfacesheattreatedat500◦C[52].
4.2. Adhesionandmechanicalproperties
Magnetronsputteringenabledfilmdepositionintothe topog-raphyofthesubstrate,leadingtoenhancedadhesion[53,54].Such adherence has been observed via FIB-SEM interfacial observa-tionofdeposited phosphateglasses onTi6Al4Vsubstrates[35]. CoatingthicknessvariationfromC1:P40ADwasrecognisedfrom lightinterferencefringepatternsasvisiblelightreflectedoffthe substrateand through thesample(Fig.8A). Analysisof coating thicknessbySEMcrosssectionindicatedastandarderrorof90nm for(n=5)locationsacrossasample.Ideally,tominimisevariation, thesampleshouldberotateduniformlywithrespecttotheplasma. Thecapabilitiesofthespecificrigdesignusedforthisdeposition didnotallowforsamplerotation.
fail-uresmaymoreoftenbecausedbyshear,ratherthanpuretension. Thelimitationofthepulloffmethodisthetensilestrengthofthe epoxy,howeveritremainsthestandardisedquantitative assess-mentofcoating/substratemechanicalstrength.ISO-0-137792Part 2“CoatingsofHydroxyapatite”statesthattheminimum interfa-cialtensilestrengthshouldbenolessthan15.0MPa.Callahanetal. statedinthe1994FDAdraftguidanceforCa/Pcoatingsonmedical implantsthattheminimuminterfacialstrengthapprovedforFDA commerciallyapprovedcoatingswas50.8MPa(Fig.6A)[56,57].
Interfacialintegrityofcoatingsonpolishedsubstrates follow-ingheattreatment,inthecaseofC2:P40HT30andC3:P40HT120 appearedtobecompromisedbycoating/substratedelamination; asobservedinFIB-SEMcrosssections(seeFig.6BandC).Scratch testsshowedvariationinLc5 (interfacialdelamination)from5.0 to2.3to4.4NforC1,C2andC3respectively.Thefailuremodes forLc1-Lc5rangedover a4.0NforC1:P40AD,andwere,in con-trast,closelyspacedrangingover1.3and2.0NforC2:P40HT30and C3:P40HT120demonstratingadecreaseinductilitydueto catas-trophiccoatingfailureatthepointofinitialindentationLc1.C3: P40HT120,showedincreasedLc1(initialscratch)to2.0NandLc5 to4.4N.Allcoatingsexhibitedbrittlefailuremechanismshowever heattreatmentseeminglyledtoincreasedbrittlenessandhardness withprolongeddwelltimeof120min.
Coatingsonsandblastedsubstratesshowednosignsof catas-trophic failure, specifically the absence of spallation due to tracksidedelamination.Theinterfacialpropertieswereimproved asdelaminationoccurredat8.6–11.3NforC1,C2andC3 respec-tively(B).Toqueetal.appliedupto2.5Nloadson1.3–2.0mCa/P sputteredfilmsdepositedonto316Lstainlesssteelandtheirresults showedinterfacialdelaminationat800–900mN.Postdeposition annealingsimilarlyledtoareductionininterfacialfailureloads asshownhereforpolishedsubstrates,intherangeof450–550mN
[54].AdditionallyChengetal.comparedpulloffandscratchtestsof solgelHAcoatingsonTi6Al4Vandreportedscratchdelamination ofupto487mNandpulloffstrengthsofupto22MPa[58].
Themeasurableinterfacialfailurestrengthatthecoating sub-strateinterfaceisconsiderablylowerthantheanticipatedtensile propertiesoftheglassitselftofailcohesivelyintension. Cozien-cazucet al.tested thetensileproperties ofMQ:P40 formedas glassfibres,bothaspreparedandupto3ddegradationindistilled waterandreportedtensilestrengthsof480±150,decreasingto 370±60MPa.Followingannealingat444◦C(5◦CbelowtheTg)a reductionintensilestrengthto290±50MPawasobservedandan increaseinstrengthfollowing3ddegradationto460±140MPa
[59].Forcoatingsonpolishedsubstratesasimilarreductionin fail-ureproperties duetocoating embrittlementwasalsoobserved followingannealing.Sglavoetal.alsoshowedrangingtensile prop-ertiesofPBGopticalfibresofbetween200and400MPa[60].
Allcoatingspreparedandtestedhereshowedsufficient min-imumadhesivepropertiesaccordingtoISOandFDAregulations onpullofftesting(Fig.6);theactualfailureloadshoweverwere unattainableduetolimitationsoftheepoxyglueused.Ongetal. tested ion beam sputtered Ca:P films comparing the pull off strength of depositedamorphous, water quenched and furnace cooledcoatingsandreportedpullofstrengthsof38.0±8.2MPa, 17.0±6.5MPaand 9.0±9.0MParespectively withfailureeither occurringat thecoating interfaceorcohesively [61].45S5 Bio-glasshasbeenplasmasprayedwithlittlesuccessasfailurewas showntooccurfromthermallyinducedresidualstressesatthe Ti coating interface with adhesion strength of approximately 8.56±0.57MPa [25]. The use of a bonding coat (60wt% Al2O3 40wt%TiO2)toaccommodatethethermalexpansioncoefficient (TEC)mismatchofBioglassandthesubstrate,improvedbonding strengthto27.17±2.24MPawithfailureotherwiseoccurringby coatingdelamination[25].
Similarstudiesreportingpulloffvalueswerealsoperformed byMardareetal.andStanetal.whoproducedsilicatebioactive glassesbysputtering.Mardareetal.reportedadhesionstrengthof 41.0±4.5MPafor300nmbioactiveglassontitaniumsubstrates. Post-depositionheattreatmentofthecoatinglayerat900–1000◦C caused crystallisationand subsequentreduction in adhesion to 16.3±1.9MPa [28]. Mardare’s work was conducted above the phase transitiontemperatureof titaniumin whichthetitanium changesfromHCCtoBCCstructureandreducesinvolume,inducing stressinthematerials[28,62].
Stanetal.reportedpulloffinterfacialstrengthsofasdeposited silicate glasscoatingsof29.2±7.0MPaand attributedlessthan optimaladhesiontotheTECmismatchbetweendepositedcoatings andsubstratesduringdeposition[63].Improvementsinadhesion byreducingtheTECofthecoatingthroughcompositionalchanges ledto production of a glasscoating withTEC of 10×10−6K−1, appliedtotitaniumsubstrateswithTECof9.2–9.6×10−6K−1[64]. The adhesionstrength for this glass wasmeasuredas >75MPa (limited by the epoxy) [65]. Whilst bonding was dramatically improvedbytailoringTECbyalteringcomposition,thisrestricted theflexibilityofcoatingcompositionsthatcouldbeapplied. Fur-ther publications demonstrated improvedpull off to>85.0MPa (againlimitedbythestrengthoftheepoxy)followingan anneal-ingprocessduringwhichStanetal.suggestedthatheattreatment causedanoticeablediffusionoftitaniumintotheglassatthe inter-face,formingchemicalbondingleadingtoimprovedadhesion[29]. Additionallyexplorationofgradedbufferlayersprovedtobe effec-tivemethodsforadhesivestrengthening[29,63,66,67].
TEC couldnotbeassessedin thin films;thereforetheMQ5: P40 Fe4 was cast as rods for comparison and measured as 13.6±0.2×10−6K−1.Thesevalueshoweverarenotdirectly com-parableasitwaspreviouslyshownthat structuresandthermal propertiesvaried incoatingsand melt quenchedglasses. Elmer etal.citedTECinTI6Al4Vbetween8.5and10.0×10−6K−1[68]. ThehigherTECofPBGwithrespecttotheTi6Al4Vsubstratemay have causedtheobserved ripples inthecoating shown inSEM micrographs(Fig.8B, C)and theinterfacialdelaminationshown inFIB-SEMsections(Fig.6B,C,E,F)followingC2:P40HT30andC3: P40HT120.
4.3. Degradationandionrelease
DegradationofPBGoccursbyhydrationduringwhichH+ions interchangewithcationstoformP-OH−andsubsequent disentan-glementofthepolymerlikechainsbyhydrolysis[69,70].Theas depositedPBGcoating;C1:P40AD,wasfoundtobehighly solu-bleinthefirst2h,degradingexponentially,priortostabilisingto alinearregimethereafter[35]asobservedinFig.7A.Howeverthe thermalannealingprocessesledtoareductionininitialsolubilityas thecomparativemasslosseswere2.9,0.7and0.3×10−3mgmm−2 atthe2htimepointforC1,C2andC3respectively.Thiseffectmay havebeendue toreoxidationofIronasFe3+toformhydration resistantFe-O-PbondsandthereductionofsolubleP-O-Pbonds followingheattreatmentatthesurfaceofthematerialasshown byXPS(Fig.3B,C)[18,35,47].
widenandfollowingC3:P40HT120visiblepittingassociatedwith degradation wasfairly uniform, indicating that prolonged heat treatmentled to a more diffused, homogenous bulk structure. Choukaetal.andCozien-Cazucetal.bothshowedpitting corro-sionofCa:PandPBGfibresfollowingdegradationinTris-Buffered HClanddistilledwaterrespectively,whilstalsoreportingimproved durabilityofannealedfibres.Choukaetal.alsosuggestedthat dura-bilitywasproportionaltoannealingtemperature[71,72].
TheeffectsofionreleaseforCa,P,Mg,andFeontherapeutic potentialhavebeenreported[73–77].ForCa2+2–8mM(80–320 PPM)wasdeemedsufficienttofacilitateosteoblastproliferation, whilst6–8mM(240–320PPM)wasreportedasoptimal,causing cytotoxicityabove10mM(400PPM)[73].Phosphateion concen-trationsfrom2–10mM(62–310PPM)werefoundtostimulategene expressionforosteoblast differentiation[74].Heet al.reported thatMg2+from1to3mM(24–73PPM)improvedosteoblastcell viability,alkalinephosphataseandosteocalcinexpression[75].A toxiclimitforFewasreportedas0.4mM(22PPM)[76].Sondietal. demonstratedtheantimicrobialeffectivenessofAgnanoparticles onE.coli,using10–60gcm−3(0.01–0.06PPM),showing70–100% growthinhibition[77].IncomparisonFigure7showedthat cumu-lativeionreleaseby48hin15mlofultra-purewaterrangedinC1: P40AD,C2:P40HT30andC3:P40HT120,from(1–3),(5–13),(1–3), (1–2)and(1–2)PPMforNa,P,Mg,CaandFerespectively.However foraccuratecomparisonofionreleaseeffects,considerationof liq-uidvolumesshouldbecarefullyconsideredalongwiththemedia usedfortesting. Forexample,dissolutionvolumesvaryfroml tomlduringinvitrostudiesofMG63Osteoblastcells,oftenwith workingvolumesof0.2–0.3ml,thereforeionconcentrationscould beupto75×higherascomparedtothe15mlusedhere,providing sufficientionicconcentrationsfortherapeuticefficacy.
ProductionofPBGthinfilmshasbeenshowntobeapromising methodfordeliveringacontrolledreleaseofionsatadissolution site,whilsttheabilitytofurthercontrolsuchpropertiesthrough heattreatmenthasbeenconfirmed.
Thesefindingsrepresentastepforwardinunderstandingthe mechanical and degradation properties of fully resorbable PBG coatings, intended for application on orthopaedic implants to facilitateosseointegration. Theability toincorporatea range of therapeuticionsusingthemagnetronsputteringprocesscouldlead toastratifiedapproachtopromotingorthopaedicimplant integra-tion.
5. Conclusions
RFmagnetronsputteredPBG coatingsweredeposited amor-phous onto both polished and sandblasted Ti6Al4V substrates. Coatingsweresubsequentlyheattreatedfordwelltimesof30and 120minat500◦C.Similarstructuralchanges wereobservedfor P40HT30andP40HT120,associatedwiththeformationofFe2O3 HematitecrystalswhilstXPSshowedthereductionof phospho-rousfrom(21.6–1.3)at%,coupledwithanincrease inIronfrom (1.4–22.8)at%withinthesurfacelayers.
Pull off adhesion tests were limited by the strength of the epoxy,thereforeallcoatingsshowedaveragestrengthsinexcess of 73.6MPa, exceedingthe internationalISO standard and FDA requirements.Scratchadhesiontestsrevealedbrittlefailuremodes for coatings on polished substrates. Although heat treatment improvedtheoveralladhesionvalues,coatingsfailed catastroph-icallyat thelocation ofinitialindentation,suggestingthat heat treatment led to increased hardness at the expense of coat-ing embrittlement. In contrast coatings applied to sandblasted substrates were mechanically durable, showing no trackside delaminationortensilecrackingwhilstloadsassociatedwith inter-facial delamination were increased to between 8.6 and 11.3N
howevervariationduetoheattreatmentwasnotstatistically sig-nificant.
CoatingsonsandblastedsubstratesweredegradedindH2Ofor upto96handassessedforionreleaseinultrapurewaterupto48h. C1:P40ADwasfullyresorbedbeyond24h,heattreatedcoatings remainedbeyondthe96htimepoint.Heattreatmentimproved thedurabilityofcoatingsbystabilisingtheinitialt1/2 solubility profileinthefirst2hofsubmersionandultimatelyreduced lin-eardegradationratesby2.4–3.0±0.35forC2:P40HT30andC3: P40HT120respectively.Heattreatmentledtoareductioninion releaseratesbymaximumfactorsof3.9,4.0,4.3,3.4and7.7for P,Na,Mg,CaandFerespectively.Releaseratesrangedfrom0.08 to0.05ppmh−1,0.01–0.02ppmh−1and0.01–0.02ppmh−1forthe C1:P40AD,C2:P40HT30andC3:P40HT120coatingsrespectively.
Acknowledgements
ThisworkwassupportedbytheEngineeringandPhysical Sci-ences Research Council [grantnumber EP/K029592/1]; and the Centrefor InnovativeManufacturing in MedicalDevices (MeDe Innovation).TheauthorswouldliketogratefullyacknowledgeSaul VazquezReinaforassistancewithICP-MSanalysisandDr.Emily SmithforreviewingtheXPSresultsaswellastheNanoscaleand MicroscaleResearchCentreattheUniversityof Nottinghamfor SEMaccess.
References
[1]M.Navarro,A.Michiardi,O.Castano,J.A.Planell,Biomaterialsinorthopaedics, JRSocInterface5(27)(2008)1137–1158.
[2]S.Zhang,BiologicalandBiomedicalCoatingsHandbookProcessingand Characterization,CRCPress,Florida,2011.
[3]B.Clarke,Normalboneanatomyandphysiology,Clin.J.Am.Soc.Nephrol.3 (Suppl3)(2008)S131–S139.
[4]E.Jimi,S.Hirata,K.Osawa,M.Terashita,C.Kitamura,H.Fukushima,The currentandfuturetherapiesofboneregenerationtorepairbonedefects,Int. J.Dentist.2012(2012).
[5]J.Sieniawski,W.Ziaja,K.Kubiak,M.Motyka,Microstructureandmechanical propertiesofhighstrengthtwo-phasetitaniumalloys,Titanium
Alloys-AdvancesinPropertiesControl(2013)69–80.
[6]R.D.Bloebaum,D.Beeks,L.D.Dorr,C.G.Savory,J.A.DuPont,A.A.Hofmann, Complicationswithhydroxyapatiteparticulateseparationintotalhip arthroplasty,Clin.Orthop.Relat.Res.298(1994)19–26.
[7]NationalJointRegistry.England,WalesandNorthernIreland,2013.
[8]K.Mediaswanti,C.Wen,E.P.Ivanova,C.Berndt,F.Malherbe,V.Pham,J.Wang, Areviewonbioactiveporousmetallicbiomaterials,J.Biomim.Biomater. TissueEng.18(104)(2013)2.
[9]I.Ahmed,A.Parsons,A.Jones,G.Walker,C.Scotchford,C.Rudd, CytocompatibilityandeffectofincreasingMgOcontentinarangeof quaternaryinvertphosphate-basedglasses,J.Biomater.Appl.24(6)(2010) 555–575.
[10]E.A.A.Neel,W.Chrzanowski,S.P.Valappil,L.A.O’Dell,D.M.Pickup,M.E.Smith, R.J.Newport,J.C.Knowles,Dopingofahighcalciumoxidemetaphosphate glasswithtitaniumdioxide,J.Non-Cryst.Solids355(16)(2009)991–1000.
[11]A.Hoppe,N.S.Guldal,A.R.Boccaccini,Areviewofthebiologicalresponseto ionicdissolutionproductsfrombioactiveglassesandglass-ceramics, Biomaterials32(11)(2011)2757–2774.
[12]E.A.A.Neel,W.Chrzanowski,D.M.Pickup,L.A.O’Dell,N.J.Mordan,R.J. Newport,M.E.Smith,J.C.Knowles,Structureandpropertiesof
strontium-dopedphosphate-basedglasses,J.Roy.Soc.Interface(2008),rsif. 2008.0348.
[13]J.Massera,L.Petit,T.Cardinal,J.-J.Videau,M.Hupa,L.Hupa,Thermal propertiesandsurfacereactivityinsimulatedbodyfluidofnewstrontium ion-containingphosphateglasses,J.Mater.Sci.Mater.Med.24(6)(2013) 1407–1416.
[14]D.S.Brauer,M.N.Anjum,M.Mneimne,R.M.Wilson,H.Doweidar,R.G.Hill, Fluoride-containingbioactiveglass-ceramics,J.Non-Cryst.Solids358(12–13) (2012)1438–1442.
[15]A.Kiani,N.J.Lakhkar,V.Salih,M.E.Smith,J.V.Hanna,R.J.Newport,D.M. Pickup,J.C.Knowles,Titanium-containingbioactivephosphateglasses,Philos. Trans.SeriesAMath.Phys.Eng.Sci.370(1963)(2012)1352–1375.
[16]T.Kasuga,T.Hattori,M.Niinomi,Phosphateglassesandglass-ceramicsfor biomedicalapplications,Phosphor.Res.Bull.26(0)(2012)8–15.
[18]X.Y.Yu,D.E.Day,G.J.Long,R.K.Brow,Propertiesandstructureofsodium-iron phosphateglasses,J.Non-Cryst.Solids215(1)(1997)21–31.
[19]I.Ahmed,C.A.Collins,M.P.Lewis,I.Olsen,J.C.Knowles,Processing, characterisationandbiocompatibilityofiron-phosphateglassfibresfortissue engineering,Biomaterials25(16)(2004)3223–3232.
[20]E.A.Neel,I.Ahmed,J.Pratten,S.N.Nazhat,J.C.Knowles,Characterisationof antibacterialcopperreleasingdegradablephosphateglassfibres,Biomaterials 26(15)(2005)2247–2254.
[21]A.G.Avent,C.N.Carpenter,J.D.Smith,D.M.Healy,T.Gilchrist,Thedissolution ofsilver-sodium-calcium-phosphateglassesforthecontrolofurinarytract infections,J.Non-Cryst.Solids328(1–3)(2003)31–39.
[22]I.Ahmed,D.Ready,M.Wilson,J.C.Knowles,Antimicrobialeffectof silver-dopedphosphate-basedglasses,J.Biomed.Mater.Res.A79(3)(2006) 618–626.
[23]A.Al-Noaman,S.C.Rawlinson,R.G.Hill,Bioactiveglass-stoichimetric wollastoniteglassalloystoreduceTECofbioactiveglasscoatingsfordental implants,Mater.Lett.94(2013)69–71.
[24]J.Gomez-Vega,E.Saiz,A.Tomsia,Glass-basedcoatingsfortitaniumimplant alloys,J.Biomed.Mater.Res.46(4)(1999)549–559.
[25]G.Goller,Theeffectofbondcoatonmechanicalpropertiesofplasmasprayed bioglass-titaniumcoatings,Ceram.Int.30(3)(2004)351–355.
[26]G.Bolelli,V.Cannillo,R.Gadow,A.Killinger,L.Lusvarghi,J.Rauch, Microstructuralandinvitrocharacterisationofhigh-velocitysuspension flamesprayed(HVSFS)bioactiveglasscoatings,J.Eur.Ceram.Soc.29(11) (2009)2249–2257.
[27]E.Mohseni,E.Zalnezhad,A.R.Bushroa,Comparativeinvestigationonthe adhesionofhydroxyapatitecoatingonTi-6Al-4Vimplant:areviewpaper,Int. J.Adhes.Adhes.48(2014)238–257.
[28]C.C.Mardare,A.I.Mardare,J.R.F.Fernandes,E.Joanni,S.C.A.Pina,M.H.V. Fernandes,R.N.Correia,Depositionofbioactiveglass-ceramicthin-filmsby RFmagnetronsputtering,J.Eur.Ceram.Soc.23(7)(2003)1027–1030.
[29]G.E.Stan,C.O.Morosanu,D.A.Marcov,I.Parsuk,F.Miculescu,G.Reumont, EffectofAnealingUpontheStructureandAdhesionPropertiesofSputtered Bio-Glass/TitaniumCoatings,Appl.Surf.Sci.255(22)(2009)9132–9138.
[30]B.Stuart,M.Gimeno-Fabra,J.Segal,I.Ahmed,D.M.Grant,Preferential sputteringinphosphateglasssystemsfortheprocessingofbioactive coatings,ThinSolidFilms(2015).
[31]B.W.Stuart,J.J.Titman,M.Gimeno-Fabra,I.Ahmed,D.M.Grant,Insightsinto structuralcharacterisationandthermalpropertiesofcompositionally equivalentvapour-condensedandmeltquenchedglasses,Mater.Design (2016).
[32]P.Haque,I.Ahmed,A.Parsons,R.Felfel,G.Walker,C.Rudd,Degradation PropertiesandMicrostructuralAnalysisof
40P2O5—24MgO—16CaO—16Na2O—4Fe2O3PhosphateGlassFibres,JNCS375 (2013)99–109.
[33]M.S.Hasan,I.Ahmed,A.J.Parson,G.S.Walker,C.A.Scotchford,Material characterisationandcytocompatibilityassessmentofquinternaryphosphate glasses,J.Mater.Sci.Mater.Med.23(10)(2012)2531–2541.
[34]I.Ahmed,A.J.Parson,C.D.Rudd,S.N.Nazhat,J.C.Knowles,P.Guerry,M.E. Smith,Comparisonofphosphate-basedglassesintherange
50P2O5–(50–x)CaO–xNa2Opreparedusingdifferentprecursors,Eur.J.Glass Sci.Technol.A49(2)(2007)63–72.
[35]B.W.Stuart,M.Gimeno-Fabra,J.Segal,I.Ahmed,D.M.Grant,Degradationand characterizationofresorbablephosphate-basedglassthin-filmcoatings appliedbyradio-frequencymagnetronsputtering,ACSAppl.Mater.Interfaces 7(49)(2015)27362–27372.
[36]M.E.Minitti,M.D.Lane,J.L.Bishop,Anewhematiteformationmechanismfor Mars,Meteorit.Planet.Sci.40(1)(2005)55–69.
[37]Y.Wang,A.Muramatsu,T.Sugimoto,FTIRanalysisofwell-defined␣-Fe2O3 particles,ColloidsSurf.Physicochem.Eng.Aspects134(3)(1998)281–297.
[38]S.Zhang,L.Li,A.Kumar,MaterialsCharacterizationTechniques,Taylor& FrancisGroup,2009.
[39]R.P.Gupta,S.K.Sen,Calculationofmultipletstructureofcorep-vacancy levels,PhRvB10(1)(1974)71–77.
[40]A.Grosvenor,B.Kobe,N.McIntyre,Studiesoftheoxidationofironbywater vapourusingX-rayphotoelectronspectroscopyandQUASESTM,Surf.Sci.Rep. 572(2)(2017)217–227.
[41]A.Grosvenor,B.Kobe,M.Biesinger,N.McIntyre,Investigationofmultiplet splittingofFe2pXPSspectraandbondinginironcompounds,Surf.Interface Anal.36(12)(2004)1564–1574.
[42]K.J.Rao,StructuralChemistryofGlasses,Elsevier,2002.
[43]J.C.Knowles,Phosphatebasedglassesforbiomedicalapplications,J.Mater. Chem.13(10)(2003)2395–2401.
[44]J.R.Jones,Reviewofbioactiveglass:fromHenchtohybrids,ActaBiomater.9 (1)(2013)4457–4486.
[45]A.Boyd,M.Akay,B.Meenan,Influenceoftargetsurfacedegradationonthe propertiesofrfmagnetron-sputteredcalciumphosphatecoatings,Surf. InterfaceAnal.35(2)(2003)188–198.
[46]A.R.Boyd,H.Duffy,R.McCann,B.J.Meenan,Sputterdepositionofcalcium phosphate/titaniumdioxidehybridthinfilms,MSEngC(28)(2008)228–236.
[47]R.F.Cooper,J.B.Fanselow,D.B.Poker,Themechanismofoxidationofa basalticglass:chemicaldiffusionofnetwork-modifyingcations,Geochim. Cosmochim.Acta60(17)(1996)3253–3265.
[48]G.B.Cook,R.F.Cooper,Ironconcentrationandthephysicalprocessesof dynamicoxidationinanalkalineearthaluminosilicateglass,Am.Mineral.85 (3–4)(2000)397–406.
[49]W.Wisniewski,R.Harizanova,G.Volksch,C.Russel,Crystallisationofiron containingglass-ceramicsandthetransformationofhematitetomagnetite, CrystEngComm13(12)(2011)4025–4031.
[50]P.Kofstad,High-temperatureoxidationoftitanium,J.LessCommonMet.12 (6)(1967)449–464.
[51]M.V.Diamanti,B.DelCurto,M.Pedeferri,Interferencecolorsofthinoxide layersontitanium,ColorRes.Appl.33(3)(2008)221.
[52]S.Kumar,T.S.Narayanan,S.G.S.Raman,S.Seshadri,Thermaloxidationof Ti6Al4Valloy:microstructuralandelectrochemicalcharacterization,Mater. Chem.Phys.119(1)(2010)337–346.
[53]Y.D.Zhao,Y.T.Qian,W.C.Yu,Z.Y.Chen,Surfaceroughnessofaluminafilms depositedbyreactiver.f.sputtering,ThinSolidFilms286(1–2)(1996)45–48.
[54]J.A.Toque,M.K.Herliansyah,M.Hamdi,A.Ide-Ektessabi,I.Sopyan,Adhesion failurebehaviorofsputteredcalciumphosphatethinfilmcoatingsevaluated usingmicroscratchtesting,J.Mech.Behav.Biomed.Mater.3(4)(2010) 324–330.
[55]K.L.Mittal,AdhesionMeasurementofFilms&Coatings:Volume2.2001.
[56]T.JCallahan,J.B.Gantenberg,B.E.Sands,Calciumphosphate(Ca-Pcoating)
draftguidanceforpreparationoffoodanddrugadministration(FDA)
sumbissionsfororthopaedicanddentalendosseuousimplants1196,A.S.,Ed.
AmericanSocietyforTestingandMaterials:Philadelphia,1994.
[57]ISO,ISO13779-2:2008:Implantsforsurgery–Hydroxyapatite–Part2:
Coatingsofhydroxyapatite.2008.
[58]K.Cheng,C.Ren,W.Weng,P.Du,G.Shen,G.Han,S.Zhang,Bondingstrength offluoridatedhydroxyapatitecoatings:acomparativestudyonpull-outand scratchanalysis,ThinSolidFilms517(17)(2009)5361–5364.
[59]S.Cozien-Cazuc,A.Parsons,G.Walker,I.Jones,C.Rudd,Effectsofaqueous agingonthemechanicalpropertiesofP40Na20Ca16Mg24phosphateglass fibres,JMatS43(14)(2008)4834–4839.
[60]V.M.Sglavo,E.Mura,D.Milanese,J.Lousteau,Mechanicalpropertiesof phosphateglassopticalfibers,Int.J.Appl.GlassSci.5(1)(2014)57–64.
[61]J.L.Ong,L.C.Lucas,W.R.Lacefield,E.D.Rigney,Structure,solubilityandbond strengthofthincalciumphosphatecoatingsproducedbyionbeamsputter deposition,Biomaterials13(4)(1992)249–254.
[62]E.Stan,A.C.Popa,A.C.Galca,G.Aldica,J.M.F.Ferreira,Strongbonding betweensputteredbioglass?ceramicfilmsandTi-substrateimplantsinduced byatomicinter-diffusionpost-depositionheat-treatments,Appl.Surf.Sci.280 (2013)530–538.
[63]G.E.Stan,A.C.Popescu,I.N.Mihailescu,D.A.Marcov,R.C.Mustata,L.E.Sima, S.M.Petrescu,A.R.Ianculescu,R.Trusca,C.O.Morosanu,Onthebioactivityof adherentbioglassthinfilmssynthesizedbymagnetronsputteringtechniques, ThinSolidFilms518(21)(2010)5955–5964.
[64]C.Berbecaru,G.E.Stan,S.Pinac,D.U.Tulyaganov,J.M.F.Ferreira,The bioactivitymechanismofmagnetronsputteredbioglassthinfilms,Appl.Surf. Sci.258(2012)9840–9848.
[65]G.E.Stan,S.Pina,D.U.Tulyaganov,J.M.Ferreira,I.Pasuk,C.O.Morosanu, Biomineralizationcapabilityofadherentbio-glassfilmspreparedby magnetronsputtering,J.Mater.Sci.Mater.Med.21(4)(2010)1047–1055.
[66]G.E.Stan,D.A.Marcov,I.Pasuk,F.Miculescu,S.Pina,D.U.Tulyaganov,J.M.F. Ferreira,Bioactiveglassthinfilmsdepositedbymagnetronsputtering technique:theroleofworkingpressure,Appl.Surf.Sci.256(23)(2010) 7102–7110.
[67]G.E.Stan,D.Bojin,Adherantglass-ceramicthinlayerwithbioactivepotential depositedbymagnetronsputteringtechniques,U.P.B.Sci.Bull.,SeriesB72(2) (2010)187–196.
[68]J.Elmer,T.Palmer,S.Babu,E.Specht,Insituobservationsoflatticeexpansion andtransformationratesof␣andphasesinTi–6Al–4V,Mater.Sci.Eng.A 391(1)(2005)104–113.
[69]B.C.Bunker,G.W.Arnold,J.A.Wilder,Phosphateglassdissolutioninaqueous solution,J.Non-Cryst.Solids64(3)(1984)291–316.
[70]H.Gao,T.Tan,D.Wang,Dissolutionmechanismandreleasekineticsof phosphatecontrolledreleaseglassesinaqueousmedium,J.ControlRelease 96(1)(2004)29–36.
[71]J.Choueka,J.L.Charvet,H.Alexander,Y.H.Oh,G.Joseph,N.C.Blumenthal, W.C.LaCourse,Effectofannealingtemperatureonthedegradationof reinforcingfibersforabsorbableimplants,J.Biomed.Mater.Res.29(11) (1995)1309–1315.
[72]S.Cozien-Cazuc,A.J.Parsons,G.S.Walker,I.A.Jones,C.D.Rudd,Real-time dissolutionofP40Na20Ca16Mg24phosphateglassfibers,J.Non-Cryst.Solids 355(50–51)(2009)2514–2521.
[73]S.Maeno,Y.Niki,H.Matsumoto,H.Morioka,T.Yatabe,A.Funayama,Y. Toyama,T.Taguchi,J.Tanaka,Theeffectofcalciumionconcentrationon osteoblastviability,proliferationanddifferentiationinmonolayerand3D culture,Biomaterials26(23)(2005)4847–4855.
[74]G.R.Beck,E.Moran,N.Knecht,Inorganicphosphateregulatesmultiplegenes duringosteoblastdifferentiation,includingNrf2,Exp.CellRes.288(2)(2003) 288–300.
[75]L.He,X.Zhang,B.Liu,Y.Tian,W.Ma,Effectofmagnesiumiononhuman osteoblastactivity,Braz.J.Med.Biol.Res.49(7)(2016).
[76]T.Diamond,R.Pojer,D.Stiel,A.Alfrey,S.Posen,Doesironaffectosteoblast function?StudiesInvitroandinpatientswithchronicliverdisease,Calcif. TissueInt.48(6)(1991)373–379.
