Comparative investigation on the adhesion of hydroxyapatite coating on Ti–6Al–4V implant

Comparative investigation on the adhesion of hydroxyapatitecoating on Ti–6Al–4V implant:A review paperE.Mohseni,E.Zalnezhad,A.R.Bushroa nCenter of Advanced Manufacturing and Material Processing,Department of Engineering Design and Manufacture,Faculty of Engineering,University of Malaya,Kuala Lumpur50603,Malaysiaa r t i c l e i n f oArticle history:Accepted17July2013Available online7October2013Keywords:AdhesionHydroxyapatiteCoatingTi–6Al–4V implanta b s t r a c tHydroxyapatite(HA)has been used in clinical bone graft procedures for the past25years.Although abiocompatible material,its poor adhesion strength to substrate makes it unsuitable for major load-bearing devices.Investigations on various deposition techniques of HA coating on Ti–6Al–4V implantshave been made over the years,in particular to improve its adhesion strength to the metal alloy and itslong-term reliability.This review comprehensively analyzes nine techniques mostly used for depositionof HA onto Ti–6Al–4V alloys.The techniques reviewed are Plasma sprayed deposition,Hot IsostaticPressing,Thermal Spray,Dip coating,Pulsed Laser deposition(PLD),Electrophoretic deposition(EPD),Sol–Gel,Ion Beam Assisted deposition(IBAD),and Sputtering.The advantages and disadvantages of eachmethod over other techniques are discussed.The adhesion strength and the factors affecting theadhesion of HA coating on Ti–6Al–4V implants are also compared.&2013Elsevier Ltd.All rights reserved.1.IntroductionBiologicalfixation is defined as the process where prostheticcomponents becomefirmly bonded to the host bone by ongrowthor ingrowth without the use of bone cements[1–3].In the late1960s,the concept of biologicalfixation of load-bearing implantsusing bioactive hydroxyapatite(HA)coatings was proposed as analternative to cementedfixation.Hydroxyapatite(HA:Ca10(PO4)6(OH)2),a pure calcium phosphate phase,is a preferred biomaterialfor both dental and orthopedics use due to its favorable osteo-conductive and bioactive properties[4,5].HA has a similarchemical composition and crystal structure as the apatite in thehuman skeletal system,and is therefore suitable for bone sub-stitution and reconstruction[6].Furthermore,HA has shownsignificant success in implants due to its favorable in vivo behavior[7,8]and the presence of HAfilms prolongs the lifetime ofprostheses[9].However,HA coatings are susceptible to fatiguefailure,making it unsuitable for load bearing implants[10,11].Nevertheless,there is a large demand for implants withexcellent mechanical properties.These implants should possesssimilar properties to the human bones,such as in the value of itsYoung's modulus,which result in less stress shielding effect[12]and extends its service life.The implants can be made intodifferent shapes such as plates,rods,screws and pins[13].Historically,titanium-based alloys are the most common materialfor this purpose since it is known to be a tolerable metal in thehuman body[14].Titanium(Ti)and its alloys are the most commonly usedmetallic materials for medical implants in orthopedic and dentalapplications,due to their low density,high strength,non-toxicityand excellent corrosion resistance[15].However,there have beenreports on inflammatory reaction around these implants as aresult from the creation of an avascularfibrous tissue thatencapsulated the implants[16,17].A coating of hydroxyapatitelayer can be deposited on the metal alloy to assist the osseointe-gration of these implants with surrounding tissues[16].The bond strength between the coating layer and the metalsubstrate is a very critical factor.Separation of the coating layerfrom the implant during service in the human body results inadverse effects on the implants and the surrounding tissue causedby detached particles[18].The main reason of using HA coating onmetallic substrates is to keep the mechanical properties of themetal such as load-bearing ability and,at the same time,to takeadvantage of the coating's chemical similarity and biocompatibil-ity with the bone[19].According to Blind et al.the HA coating allows rapid osteointe-gration as a result of bone tissue bonding properties[20].Thefirstclinical results from HA coatings on titanium dental implants werepromising,showing excellent results,even with poor bone quality.However,after a long period,mechanical failure would occur atthe interface of HA and metallic substrate[21].The HA coatingContents lists available at ScienceDirectjournal homepage:/locate/ijadhadhInternational Journal of Adhesion&Adhesives0143-7496/$-see front matter&2013Elsevier Ltd.All rights reserved./10.1016/j.ijadhadh.2013.09.030n Corresponding author.Tel.:þ+60379675239;fax:þ60379675330.E-mail addresses:ehsanmohseni2008@(E.Mohseni),e.zalnezhad@(E.Zalnezhad),bushroa@.my(A.R.Bushroa).International Journal of Adhesion&Adhesives48(2014)238–257dissolves as a result of poor crystallized structure[22,23],decrease of adherence with the titanium surface and dramatic late implant failure[23,24].Moreover,HA itself has poor mechanical proper-ties,with a bending strength of less than100MPa[25].Thus,it can be concluded that the stability of the HA coating is the most critical factor to ensure the success of this type of implant. Furthermore,the method used to deposit HA powder onto the substrate could influence the coating characteristics such as its adhesion strength and reliability.Several techniques have been used to create the HA coating on metallic implants,such as plasma spraying process[26],thermal spraying[27],sputter coating[28],pulsed laser ablation[29], dynamic mixing[30],dip coating[31],sol–gel[32],electrophoretic deposition[33],biomimetic coating[34],ion-beam-assisted-deposition[35],and hot isostatic pressing[36].Amongst the techniques listed,plasma spraying is the only process which is approved by the Food and Drug Administration(FDA),USA for biomedical coatings due to its excellent coating properties as compared to other coating processes[37].However,plasma sprayed hydroxyapatite coatings suffer from poor mechanical properties on tensile strength,wear resistance,hardness,tough-ness and fatigue.Improvements in plasma spraying techniques over the years have addressed many of these limitations.However, other coating methods are available which can be used as an alternative to conventional techniques.Limitations such as high porosity,poor uniformity in thickness, phase impurity,limited crystallinity,and poor adhesion are com-mon in HA coating.However,low coating adhesion seems to be the major issue,limiting its extensive use for implants at a commercial scale[38–40].Hence,improvement of bonding strength between the metallic substrate and ceramic coating is a general requirement regardless of the techniques used.This review focuses on adhesion strengths between HA coating and Ti–6Al–4V substrate,fabricated using various techniques such as plasma sprayed deposition,hot isostatic pressing,thermal spray,dip coating,pulsed laser deposition(PLD),electrophoretic deposition(EPD),sol–gel,and ion beam assisted deposition(IBAD). Parameters affecting the adhesion of coating and other factors influencing the enhancement of bonding strength of coating sur-face and the substrate are also discussed in detail.2.Coating techniques2.1.Plasma sprayed coating techniquePlasma spraying process involves melting of ceramics or metal powders using the heat of ionized inert gas(plasma).The molten powders are then sprayed onto the surface to be coated,forming the protective layer which provides a barrier against corrosion, wear or high temperatures.The technique offers advantages such as low cost and rapid deposition rate[41,42].In addition,the risk of thermal degradation of the coating and substrate is much less than other high-temperature processes since the gas in the plasma flame is chemically inert and the target can be kept relatively cool [43].However,plasma sprayed coatings suffers from poor adhe-sion between the coatings and substrates[44],and the process may induce structural changes in the microstructure of the coating material[45,46].2.1.1.Plasma sprayed hydroxyapatite(HA)coatingsPlasma spray was thefirst method used for the production of calcium phosphate coatings,such as HA coating,due to its ease of application[26].Plasma sprayed hydroxyapatite(HA)coatings are biocompatible and able to bond directly to the bone[38],thus making plasma spraying a favorable choice amongst the many techniques available for coating HA layers onto metallic substrates[47].Recentstudies on plasma sprayed HA coatings(HACs)on titanium haveshown encouraging results in orthopedic implant applications.Thesestudies reported that the new bone could appose directly onto the HAcoatings and very good adhesion between the HACs and the new bonecan be obtained[48–51].The plasma sprayed HA coatings have alsoassisted in overall quick bone recovery[52].Nevertheless,the brittle nature of the HA coating makes itprone to crack and fracture,non-uniformity in density of coating[53],wear of the coated layer,weak mechanical adhesion to thesubstrate[44,54],and alteration of structure[55].Overall,plasma sprayed coating did not show significantimproved long-life performance,better mechanical integrity andreliability over uncoated implants[56,57].An alternative to plasmaspraying is the pulsed laser deposition(PLD)which enables thestoichiometric transfer of sintered HA yields to form a thin andadherent bioactive coating on titanium substrate surface[58]. 2.1.2.Adhesion of plasma-sprayed hydroxyapatite(HA)coatings onTi–6Al–4VIt is well understood that,the determination of the adhesionbetween the substrate and coating is one of the main concernswhen using plasma spraying techniques[59].It is quite compli-cated that how coating adheres to a substrate and by today it is notcompletely understood.Many theories describe the mechanism ofadhesion,although,there is no single clear interpretation for alladhesion behaviors[60].Many factors seem to affect the adhesion:(1)Van der Waals physical interaction forces mechanical ancho-rage;(2)mechanical anchorage;(3)metallurgical processes and(4)chemical interaction[59].Recent reports on alternative orthopedics implantfixationutilizing plasma sprayed HA coatings(HACs)on Ti–6Al–4V haveshown that the new bone was able to appose directly onto the HAcoatings,which resulted in a very good adhesion between theHACs and the new bone[48–51].From the viewpoint of materialsscience,characteristics of HACs are varied with the sprayingparameters such as phase composition,the microstructure,OH-ion content,crystallinity,and the ration of calcium to phosphorusfor the HACs.Among these parameters,high bonding strength ofHACs can be achieved by high spraying power due to a densermicrostructure caused by the greatest extent of coating melting.Yang et al.experimented on six plasma sprayed HA on Ti–6Al–4V substrates by varying the cooling conditions and thesubstrate temperatures[61].The residual stresses and bondingstrengths were measured by XRD“sin2φ”technique and a standard adhesion test(ASTM C-633).Results of the bondingstrength evaluation shows that the HA coating with the lowestresidual stress exhibited a higher bonding strength(9.1870.72MPa).The deposition stress and thermal stress are the two majorsources of residual stresses in plasma sprayed coating.Depositionstresses are produced during the cooling of sprayed particles aftersolidification.Thermal stresses are generated from differentialthermal contraction during the post-fabrication cooling phaseafter coating[62,63].The residual stresses are present near theinterface of metal substrate and coating[64–66],due to thedifference of thermal expansion coefficients between both materi-als[62,63].These stresses may vary with substrate cooling effects,parameters of spraying[62,63],and coating thickness[67,68].Generally,it is believed that the increased thickness of coatingand the temperature of the specimen during plasma spraying arethe main reasons for the rise in the residual stress.In addition,high-powered,dense plasma sprayed HA coatingswould have stronger bonding strength than those sprayed usinglow power.The result is not solely due to the difference inE.Mohseni et al./International Journal of Adhesion&Adhesives48(2014)238–257239adhesive strength of HA coating.The value for bonding strength reflects the combination of both cohesive(within the coating layers)and adhesive(coating to substrate)strengths of a coating [61].In a similar study,Tsui et al.claimed that the cohesive and adhesive integrity of the coatings influence the long term perfor-mance of HA coated implants considerably[69].The adhesive strength is usually evaluated based on surface roughness,coating properties,residual stress,and the mechanical interlocking between the coating and the substrates,whereas the cohesive strength is determined by coating properties,such as microstruc-ture and crystallinity[61].The bonding strength of HA coatings on metallic substrates can be evaluated using several techniques such as the standard tensile adhesion test[69],interfacial indentation test[58],tensile adhe-sion strength(TAS)[61],and indentation method[63].However, there are limitations on these techniques to accurately measure the adhesion strength,such as a probability of penetration of glue into the coating layer,and a dependence of coating failure to the flaw distribution at the edge of specimen[69].However,Moham-madi et al.have demonstrated that the tensile adhesion strength test measured by the standard adhesion test ISO13779-4,can be used in conjunction with the interface indentation test to predict the effects of different parameters on the adhesion properties of the HA coating by plasma spraying[70].In general,the HA coatings with the densest structure(i.e.lowest porosity,and predominantly amorphous phase)have a higher tensile adhesion strength than those of lower density[61,71].The report by Mohammadi et al.[70]also showed that the tensile adhesion strength was in the range of$25MPa for HA coated on the Ti–6Al–4V.2.2.Hot isostatic pressing techniqueHot isostatic pressing(HIP)is an enabling technology providing an efficient method for the densification of ceramic powders which allows production of net-shape ceramics with superior and consistent properties[72].HIP is an alternative method of producing an HA coating on a Ti substrate in which pressurized gas is used to exert the required load at the desired temperature. This requires a gas-tight metal or glass encapsulation around the porous HA coated implant[73].In the HIP process,pressure and temperature are applied to the workpiece simultaneously[74–77].In hot isostatic pressing,high-pressure levels can be obtained since there is no dependency on rigid tools with limited strength (such as graphite tools in uniaxial hot pressing)to transmit the pressure to the body.Typical operating pressure ranges are 100–320MPa(15–50ksi),with temperatures exceeding above 20001C conducted in large industrial equipment[72].The advan-tages of HIP are better temperature control as compared to uniaxial hot pressing,and a resultant homogeneous material structure and properties.The reduced sintering temperature enables control or even avoidance of grain growth and undesirable reactions.A very high uniformity of properties as well as freedom from directionality can also,if desired,be obtained[72].Some researchers have used HIP treatments to densify plasma sprayed coatings,and results have shown that HIP is useful in reducing the porosity and improving the physical and mechanical properties of ceramic coatings[78].Thus,the most important advantage of the hot isostatic press-ing is the ability to control the size and shape of the product to a very high precision without costly diamond machining operations. Under ideal conditions no change of shape(just a change of scale) of the body occurs.It has an inherent ability to produce parts with exceptionally accurate shape,virtually with no dimensional or shape limitation[72].2.2.1.Hot isostatic pressing of hydroxyapatite(HA)coatingsReports shows that,sort of problems such as porosity and crack appearance are conducted with existing dc plasma sprayed Ha coating on Ti–6Al–4V[79].In medical applications some amount of porosity is needed for bony tissue to grow into the coating for efficientfixation.In addition,the crack propagation needs to be healed for the composite coating to have reasonable mechanical strength during usage.In this sense,HIP introduces its profound advantages by improving the adhesion and physical properties of the plasma sprayed HA coatings as a post-treatment[79].2.2.2.Adhesion of hot isostatic pressing of hydroxyapatite(HA) coatings on Ti–6Al–4VKhor et al.[79]investigated the effect of post-sprayed HIP on plasma sprayed HA on Ti–6Al–4V.Fig.1shows the bond strengths of HA coated Ti–6Al–4V for the plasma sprayed samples,and after HIP treatment at different temperatures with respect to the coat-ing thickness.In general,it was shown that the bonding strength generally improves after HIP.It is also shown that the adhesion strength decreases with increasing coating thickness.The enhancement of the adhesion strength in the20wt%HA coating after HIP is apparent for coating below160μm.However,the result of adhesion strengths for coatings thicker than160μmshow that HIP may have adverse effects on the coating strengths.2.3.Thermal spray coating techniqueThermal spray technology is a group of coating processes that provide functional surfaces to protect or improve the performance of a substrate or component.Many types and forms of materials can be thermal sprayed to provide protection from corrosion, wear,and heat;to restore and repair components;and for a variety of other applications[80].Thermal spraying of biomedical coating is a relatively new class of applications for thermal spray coating as compared with other industrial applications,[81].Thermal spray processes are grouped into three major categories:flame spray, electrical arc spray,and plasma arc spray.These energy sources are used to heat the coating material(in powder,wire,or rod form)to a molten and semi-molten state.The resultant heated particles are accelerated and propelled towards a prepared surface by either process gases or atomization jets.A schematic diagram of thermal spray coating is illustrated in Fig.2.Fig.1.Tensile bond strength result of plasma sprayed Ti–6Al–4V/20wt%hydro-xyapatite coating(as sprayed and HIPed)[79].E.Mohseni et al./International Journal of Adhesion&Adhesives48(2014)238–257 2402.3.1.Thermal spray deposition of hydroxyapatite (HA)coatingsThermal spraying of HAP on implant devices can be compared with plasma spray coating technique,having the advantage of high deposition rate and low cost [82,83].Thermal spray technique has the ability to produce HA layer with thickness from 30to 200m m depending on the coating condition However films deposited by thermal spraying suffers from poor coating –substrate adherence and non-uniform crystallinity which reduces the lifetime of implants [84,85].In addition,thermal spray requires high sintering temperature which may result in crack propagation on the surface of the coating [86–90].2.3.2.Adhesion of thermal spray deposition of hydroxyapatite (HA)coatings on Ti –6Al –4VHsiung et al.[91]have evaluated the applications and char-acterizations of biological coating such as hydroxyapatite on titanium alloy,particularly Ti –6Al –4V,in arti ficial knee joint by thermal spray coating technology.The process involves melting of HA powder and guiding the molten mass via a jet stream of air to form a coating on the substrate,as shown in Fig.3.The thermal spray process conditions of the three coating materials are shown in Table 1,highlighting the important parameters affecting the quality of the coating,such as inert gas compositions,currents,voltage levels,powder feeding rates,and spraying distances.The tensile test is commonly used to evaluate the bond strength in accordance to ASTM C-633standard method [92].A bonding strength of 33.2MPa was obtained by Hsiung et al.[91]for the HA coating on Ti –6Al –4V by thermal spraying technique.In comparison,this result is not satisfactory when compared to other coatings for the same application such as Al 2O 3,ZrO 2.In addition,results of microstructure analysis shows that the HA coatings suffers from spalling,interface separation and high levels of porosity.Several pre and post-treatments of HA coating were also investigated by Hsiung et al.[91].Treatment conditions include high pressure cleaning,ultrasonic cleaning and cryogenic treat-ments [92].Table 2shows the result of the bond strength test using ASTM C-633[93],indicating the bond strengths of samples cleaned with high pressure air are lower as compared with those ultrasonically cleaned,and the bond strengths with cryogenic treatments are better than those without cryogenic treatments.The result shows that the inclusion of ultrasonic cleaning and cryogenic treatments can effectively improve the coating properties.2.4.Dip coating techniqueDip coating involves the deposition of a wet liquid film by withdrawal of a substrate from a liquid coating medium.The complete process of film formation involves several stages,as shown in Fig.3.The process starts by immersion of the substrate in the solution of the coating material.When the substrate is withdrawn from the coating fluid,a coherent liquid film is entrained on the surface of the substrate.A thin layer of coating is formed upon evaporations of solvents and any accompanying chemical reactions in the liquid film.Normally an additional post-treatment such as curing or sintering is required to obtain the final coating.Dip coating technique is similar to sol –gel coating tech-nique,although the process is signi ficantly faster in which a complete transition can be achieved within a few seconds if volatile solvents are used [94].Dip coating is fairly popular in the industry and in laboratory applications due to its low cost,simple processing steps and high coating quality.2.4.1.Dip coating of hydroxyapatite (HA)coatingsHA can be homogenously coated onto metal substrates to obtain coating thickness in the range of 0.05–0.5mm.The surface uniformity of HA can be controlled well using this technique,as can be seen in Fig.4.In addition,the processing time for dip coating can be very short,even for substrate with complex shapes.The coating layer is deposited on the surface of the substrate without decomposition or reaction with the metalsubstrate.Fig.3.Fundamental stages of dip coating (the finer arrows indicate the flow of air)[94].Table 1Thermal spray condition of HA powders [92].Parameters Argon (l/min)Helium (l/min)Current Voltage Powder rate (g/min)Spray distance (mm)Surface speed (m/min)Travers speed (mm)Cooling Setting41607005230115758yesTable 2Bond strength test results with different pretreatment and cryogenic treatment [92].CoatingBonding strength (MPa)Without cryogenic treatment With cryogenic treatment UltrasonicHigh pressure air Ultrasonic High pressure air HA26.5618.9136.6529.30Fig.2.A schematic diagram of thermal spray coating [82].E.Mohseni et al./International Journal of Adhesion &Adhesives 48(2014)238–257241However,this technique requires high sintering post-treatments which may induce crack formations on the surface of the substrate [95].2.4.2.Adhesion of dip coating of hydroxyapatite (HA)coatings on Ti –6Al –4VMavis et al.[96]had developed several compositions of the liquid coating medium for the dip coating of HA on Ti –6Al –4V substrates,using chemically precipitated hydroxyapatite precursor powders.To evaluate the adhesion strength,two steel cylinders 5mm in diameter were attached to both sides (coated and uncoated after the coating layer was ground off)of the dipped strips by a thin layer of glue.The adhesive strengths were determined by measuring the tensile stress needed to separate the cylinders from the strips [97].It is reported that,the HA coatings obtained were highly porous,with bonding strengths of more than 30MPa.2.5.Pulsed laser deposited coating techniqueLaser processing is a rapid and clean process which can be used for surface modi fication and controlled micro-structuring of materials.In biomedical applications,laser has been used to modify the surface texture of materials to improve its bio-functionality [98–102].Pulsed laser deposition (PLD)technique can be used to grow ceramic thin films.By using appropriate laser,thin films such as semiconductor films [103],cuprate supercon-ductor films [103,104],and ferroelectric films [105]can be depos-ited onto substrates.PLD process involves using high power laser energy to vaporize the bulk coating material from a target.The vaporized material is ejected from the target and condenses on the substrate.Repeated laser pulses will result in the deposition of the thin film as a coating on the substrate [106].The formation of thin film by PLS can be separated into the following three stages [103,107,108]:ser radiation interaction with the target.2.Dynamic ablation of the materials.3.Deposition of the ablation materials with the substrate,nuclea-tion and growth of a thin film on the substrate surface.One of the main advantages of PLD technique is the ability to retain the stoichiometry of the target in the deposition films [107].This is due to the high ablation rate which causes all compoundsor elements to evaporate at the same time [106].Conversely,limitations of PLD include the splashing of the particulates deposition on the film.Some methods have been developed to decrease splashing problem since it is a major issues of the PLD [109].One method is to apply a mechanical particle filter that includes a velocity selector acting as a high-velocity pass filter to eliminate slow-moving particulate.The second method is using a smooth,high-density target which can be obtained by polishing the target surface before each coating run.The third method is by applying a lower deposition rate or low energy density.Further-more,the deposited films have only a small area of structural and thickness uniformity,due to the angular distribution of the ablation plume.Several methods have been proposed to scale up the PLD process for large area thin films,such as laser beam rasterizing across a rotating target [106].High quality hydroxyapatite thin films deposited by the PLD was first reported in 1992[105,110]and since then the process have been improved signi ficantly to obtained well adhered and highly crystalline HA thin films under certain conditions [29,111–113].2.5.1.Pulsed laser deposited hydroxyapatite (HA)coatingsPreparing hydroxyapatite thin films by pulsed laser deposition allows accurate control of hydroxyapatite growth parameters at low deposition temperatures and the ability to produce highly crystalline HA coatings [16,112].In-vitro evaluations shows that these HA coatings are stable and osteoinductive [114,115].Nanos-tructured hydroxyapatite layer having unique biological properties can be obtained by selection of suitable parameters for the deposition process [16].2.5.2.Adhesion of pulsed laser deposited hydroxyapatite (HA)coatings on Ti –6Al –4VAdhesion strength of HA coating on metals depends on the microstructure of the substrate,the surface chemistry and the PLD process parameters such as laser power density and substrate temperature.[58,116–118].Various surface modi fication techni-ques have been used to improve the metal –ceramic interface such as nitridation,surface oxidation and ion implantation [119–123].Blind et al.reported that adhesion of pulsed laser deposited HA films on titanium alloy is due to the existence of an oxide,speci fically titanium dioxide,at the interface between the sub-strate and the coating layer [20].Another report suggests that there may be some effects of epitaxy between the oxide and coating [124].Fernández-Pradas et al.[54]commented whether the presence of a titanium oxide interface would favor adhesion of the HA coating to the Ti –6Al –4V substrate is still a cause for debate.Some authors consider that such a layer favours adhesion [125,126].Other studies have attributed the weak adhesion in the first calcium phosphate coatings deposited by PLD at high tem-peratures to the formation of a titanium oxide layer during the process of pressure stabilization [105].A study of the adhesion strength in coatings deposited by ion bombardment on passivated and non-passivated substrates,suggest that this oxide layer should be as thin as possible [127].Koch et al.[16]investigated pulsed laser deposition of hydro-xyapatite on Ti –6Al –4V for medical and dental applications.A pull-off testing method was used to determine the coating-to-substrate adhesion strength.Garcia-Sanz et al.had also examined hydroxyapatite films prepared using pulsed laser deposition using a pull-off test based upon a modi fied ASTM C-633procedure [128].The measured tensile strength of hydroxyapatite grown at 4801C was 58MPa and failure was observed at the coating –substrate interface.Wang et al.obtained tensile bonding strength values within the range of 30MPa and 40MPa forhydroxyapatiteFig.4.SEM micrographs from cross-sectional view of HA coatings (via SOL 2)on Ti –6Al –4Vsubstrates after heating at 8401C [96].E.Mohseni et al./International Journal of Adhesion &Adhesives 48(2014)238–257242。