Applications Of Thermal Spray Coating In Artificial Knee .

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Life Science Journal, s of Thermal Spray Coating in Artificial Knee JointsJ.-C. Hsiung* H.-K. Kung*H.-S. Chen*Kuan-Yu Chang***Department of Mechanical Engineering, Cheng Shiu University, Taiwan, ROC**Mackay Memorial Hospital, Taitung Branch, Taiwan, ROC*E-mail: With remarkable medical advances, aging population makes the artificial knee joint replacementincreasingly popular, and the quality of total knee replacement depends on materials selection and the application ofsurface coating. Titanium alloys have been widely used as artificial knee joint material due to their specificproperties such as low density, toxicity and excellent corrosion resistance and biocompatibility. Thermal spraycoating technology is used worldwide in many industrial applications, but the application of thermal spraybiocompatible coatings on the titanium alloy surface in the biomedical field as to improve the knee's functions is arelatively new area. The purpose of this study was to make a comprehensive overview of applications andcharacterizations of titanium alloy surface coating in artificial knee joint by thermal spray coating technology.[J.-C. Hsiung. H.-K. Kung, H.-S. Chen, Kuan-Yu Chang. Applications of Thermal Spray Coating in ArtificialKnee Joints. Life Science Journal. 2012;9(1):457-463] (ISSN:1097-8135). 68KeywordsThermal spray coating; Artificial knee joint; Titanium alloy1. IntroductionWith medical advances, life expectancygradually extends. As people become older, thehuman bones and joints wear badly due to long-termuse and deterioration. Then it leads to pain ormalfunction of joints. When joint disease becomessevere, artificial joint replacement surgery cansignificantly relieve pain and improve joint function.Knee joints are the largest and most important joints,which are vulnerable to various injuries and diseases.Currently there are around 20,000 cases per year forthe artificial knee joint replacement surgery inTaiwan, and this surgery is fairly common inorthopedic surgery [1-3]. Knee structure is mainlycomposed of the femur, tibia, patella and meniscusformed and fixed by the external muscles andligaments. Typical components commonly used inartificial knee joint contain the femoral components,tibial components, patella and tibial bearingcomponent as shown in Figure 1[4]. Existing artificialknee joint biomedical materials are metals, ceramics,polymers and composite materials. Because themetallic materials have excellent mechanicalproperties, easy processing and stability, they havebeen widely used in artificial joints [5-10]. Theselection criteria for biomedical metallic material are:the minimum biological response in the body and tomeet the basic functional requirements of thereplacement and repair tissues. Owing to themechanics and the common needs of the bodyenvironment, three types of metallic materials(stainless steel, cobalt alloys and titanium alloys) aremainly used for the artificial knee joint. Among them,titanium alloys are the preferred medical materialsbecause of density as close to human bones, lowmodulus, corrosion resistance and biocompatibilitybetter than stainless steel and cobalt alloy [11, 12].Thermal spray technology encompasses a groupof coating processes that provide functional surfacesto protect or improve the performance of a substrateor component. Many types and forms of materialscan be thermal sprayed—which is why thermal sprayis used worldwide to provide protection fromcorrosion, wear, and heat; to restore and repaircomponents; and for a variety of other applications [13,14]. Compared with other industrial applications,biomedical coating is a relatively new class ofapplications for thermal spray coating [13-16]. Thermalspray processes are grouped into three majorcategories: flame spray, electrical arc spray, andplasma arc spray. These energies sources are used toheat the coating material (in powder, wire, or rodform) to a molten and semi-molten state. Theresultant heated particles are accelerated andpropelled towards a prepared surface by eitherprocess gases or atomization jets.Titanium alloys with excellent mechanical andbiological properties have been proven to beeffective in clinical medical application. In order toimprove the biological activity, wear resistance andcorrosion resistance of titanium alloy, theimplementation of its surface modification becomesquite important [17, 18]. The types of coating applied inthe artificial knee joint are usually biologicalcoatings. These biological coatings mainly includealuminum oxide (Al2O3), zirconia (ZrO2) andHydroxyapatite (HA). The purpose of this study wasto make a comprehensive overview of applicationsand characterizations of titanium alloy surfacecoating in artificial knee joint by thermal spray457

Life Science Journal, 2012;9(1)coating technology, in order to understand andmaster the titanium surface modification technologyand related mechanisms.Figure 1 The components of an artificial knee joint.2. Experimental Procedures2.1 Experimental MaterialsThe materials used in this study were bars andplates of titanium alloys (Ti-6Al-4V). The bars weremainly used for bond strength test, and the plateswere used in many tests. They were used for the weartest, observing metallographic microstructure,hardness and other tests. The properties of titaniumalloy are shown in Table 1, the sample size of the barisφ25.4mm 30mm and the plate is 75 25 35mm,as shown in Figure 2 (a) and 3 (b).http://www.lifesciencesite.com9MB) was used in this study. Plasma temperatures inthe powder heating region range from 6000 to15,000 , significantly above the melting point ofany known material. Three kinds of powder materialssuch as alumina (Al2O3), zirconia (ZrO2) andhydroxyapatite (HA) are melted respectively, andthen molten particles were guided through by air toform a coating on the substrate, as shown in Figure 3.Al2O3 and ZrO2 ceramic materials are biologicallyinert, while the HA is a bioactive ceramic material.Hydroxyapatite (HA) is calcium phosphate ceramic[Ca10(PO4)6(OH)2] that exhibits strong activity forjoining to bone tissue.The thermal spray process conditions of the threecoating materials were shown in Table 2. Table 2shows the important parameters affecting the qualityof the coating, which includes inert gas composition,current, voltage, powder feeding rate, sprayingdistance and etc. HA powder was used in the inertgases of argon and helium, which is different fromthe Al2O3 and ZrO2 used in the argon and hydrogengas mixture. Before coating, the substrate surfaceswere cleaned, degreased, and roughened in order toimprove the bonding strength.Table 1 The material properties of titanium alloytested.Figure 3 Schematic diagram of thermal spraycoating [13].Table 2 Thermal spray parameters for HA、Al2O3、ZrO2 powders.Figure 2(a) Schematic diagram of sample size forthe bond test (ASTM C633).Figure 2(b) Schematic diagram of sample size forthe wear test (ASTM G65).2.2 Thermal Spray Coating TechnologyThe atmospheric plasma spray (APS) equipmentmade by Sulzer Metro Company (Spray Gun Type:2.3 Characterization of Surface CoatingThe surface coating quality of three kinds ofcoating materials (Al2O3, ZrO2, HA) on Ti-6Al-4Vtitanium alloy substrate by plasma thermal sprayingwas tested and characterized. These tests includecoating appearance inspection, surface roughnessmeasurement, coating thickness measurement, bond458

Life Science Journal, 2012;9(1)strength test (ASTM C633), the porosity test,abrasion test (ASTM G65), hardness test and analysisof metallographic microstructure.3. Results and Discussion3.1 Coating Appearance InspectionThe initial coating quality information can beobtained by visual inspection of appearance afterthermal spray coating. The appearance inspectionfocuses on the existence of macroscopic defects suchas coarse particles, coating spalling, cracks, shadows,or deformation. The results, as shown in Figure 4,showed that there are no coarse particles, coatingspalling, cracks and other defects on the coatingsurface, and the coating layer is smooth, coatingcolor is even without overheating oxidation.Figure 4 Coating appearance inspection of threekinds of coating materials.http://www.lifesciencesite.comof the bond strength test is shown in Figure 6. Bondstrength test results showed that HA coating has thelower bond strength (33.2MPa) than Al2O3 (47.1MPa)and ZrO2 (55.9MPa) coating, as shown in Table 5.Table 4 Coating thickness measurements of threedifferent coating materials under the thermal sprayprocess parameters in Table 2.SpecimenThickness (um)ZrO2237 273Al2O3346 364HA246 260Figure 5(a) ZrO2 coating, magnification 100X.3.2 Surface Roughness MeasurementSurface morphology of the coating can beobtained by surface roughness measurement. Thecentral average roughness (Ra) and the ten-pointaverage roughness (Rz) are commonly used as indexof surface roughness. The results indicated that ZrO2has the larger surface roughness (Ra, Rz) than Al2O3and HA.Table 3 Surface roughness measurements of ZrO2,Al2O3 and 906μmZrO29.973μm71.223μmFigure 5(b) Al2O3 coating, magnification 100X.3.3 Coating Thickness MeasurementThe results of the coating thickness of threecoating materials under the thermal spray processparameters in Table 2 were acquired by themetallographic observation of the specimen andshown in Table 4 and Figure 5 (a) (c).3.4 Bond Strength Test (ASTM C633)The main purpose of the test is to understand theadhesion between coating and substrate. The tensiletest is commonly used to evaluate the bond strengthper ASTM C633 standard method [19]. The schematicFigure 5(c) HA coating, magnification 100X.459

Life Science Journal, 2012;9(1)http://www.lifesciencesite.comTable 5 Bond strength test results of three differentcoatings.Coating typeBond strengthHA33.2MPaAl2O347.1MPaZrO255.9MPaFigure 7(a) The porosity test result of Zirconia (ZrO2)coating, the red zones are the holes.Figure 6 Schematic diagram of bond strength test3.5 Porosity TestThe specimens (HA , Al2O3 , ZrO2w) wereprepared by mounting, grinding and polishingprocesses and observed randomly the selected threeindividual locations using an optical microscope,then analyzed by image analysis software. Theporosity results were shown in Table 6 and Figure 7(a) (c). The results showed that porosity of thecoating of Zirconia(ZrO2) and Alumina(Al2O3) wasless than Hydroxyapatite(HA) coating.Figure 7(b) The porosity test result of Alumina(Al2O3) coating, the red zones are the holes.Table 6 Porosity test results of ZrO2 , Al2O3 and HAcoatings.Coating 1.19Al2O31.551.582.201.54HA5.675.282.875.49Figure 7(c) The porosity test result ofHydroxyapatite (HA) coating, the red zones are theholes.460

Life Science Journal, 2012;9(1)http://www.lifesciencesite.com3.6 Abrasion Test (ASTM G65)The abrasion test is based on the ASTM G65specification [20]. This test is a way of abrasiveparticle wear with rubber wheels and round quartz.The schematic of the abrasion test was shown inFigure 8. The abrasion test (ASTM G65) resultsshowed that HA has lower volume wear rate thanAl2O3 and ZrO2, as shown in Table 7.Figure 8 Schematic diagram of abrasive wear test[13].3.8 Metallurgical Microstructure AnalysisThe thermal spray coating may result in manydefects within the coating because the characteristicsof the technology itself or improper process control.Through metallographic microstructure analysis, thequality of the thermal spray coating can be verified.The defects of three different coatings were shown inTable 9 and Figure 9 (a) (g). Table 9 showed thatthree kinds of coatings are free of cracks, interfacecontamination, not molten particles, clusters ofoxides and strip oxides, but with the phenomenon oflayer spalling and interface separation. These resultsindicated that the adhesive strength between thecoating layers and the bond strength between thecoating and the substrate are weak. There were notany huge holes found in ZrO2 and Al2O3 coating,however huge holes were found in HA coating(Figure 10 (g)). The existence of a small amount ofsmall holes in the coating is usually normal.However, large holes or high porosity will have anadverse impact on mechanical properties of thecoating, such as lower bond strength. The existenceof huge holes and high porosity in HA coating canexplain why the bond strength of HA coating islower, as shown in Table 5.Table 7 Abrasion test results of three differentcoating materials.Table 9 Coating defects of three different coatingmaterials.SpecimenCrackCoating spallingInterface contaminationInterface separationNot molten particlesHuge holesClusters of oxidesStrip oxidesZrO2NYNYNNNNAl2O3NYNYNNNN3.7 Hardness TestThe micro-hardness test method (VickersHardness) was used to measure the hardness ofTi-6Al-4V substrate and the coating layers (atdifferent locations). The results indicated that themicro-hardness values of ZrO2, Al2O3 and HA aremuch higher than that of Ti-6Al-4V substrate asshown in Table 8. In addition, the inside hardness ofcoating is higher than the middle and outer hardness.Table 8 Micro-hardness test results of three 88Middle391409407Inside412411396Ti substrate337332325Figure 9(a) ZrO2 coating, magnification 200X.461HANYNYNYNN

Life Science Journal, 2012;9(1)http://www.lifesciencesite.comFigure 9(b) ZrO2 coating, magnification 200X.Figure 9(e) HA coating, magnification 200X.Figure 9(c) Al2O3 oxide coating, magnification200X.Figure 9(f) Hydroxyapatite coating, magnification200X.Figure 9(d) Al2O3 oxide coating, magnification200X.Figure 9(g) Hydroxyapatite coating, magnification200X.4. ConclusionsIn this study, we have made a concise andcomprehensive overview of applications andcharacterizations of titanium alloy surface coating inartificial knee joint by thermal spray coatingtechnology although the experimental results were462

Life Science Journal, 2012;9(1)preliminary. Due to the different material properties,the process parameters of atmospheric plasmaspraying for the three commonly used biologicalcoating materials (Al2O3, ZrO2, HA) were quitedifferent, experimental results displayed the verydifferent characteristics of surface roughness, coatingthickness, bond strength, porosity, abrasion, hardnessand metallographic microstructure of these threecoating materials. In addition, the coating spallingand interface separation were found in Al2O3, ZrO2and HA coatings from the metallographicmicrostructure analysis, and there is a problem ofhigher level of porosity and huge holes in the HAcoating. Further studies need to be done to verify theeffects of APS process parameters (such as sprayingprogram voltage, current, spraying distance, gas flowand gas composition parameters, feed particle size,feeding rate) and pre-treatment status (cleaning) ofthe substrate surface on the coating quality.AcknowledgementsThe authors would like to thank the financialsupport of National Science Council, Taiwan (Project:Engineering Analysis and Manufacturing of 01-MY3).*Corresponding Author:J.-C. Hsiung, PhDE-mail : Weijhih, Hu Jhihjian, ”What are artificialjoint - materials and types of artificial joints”,Chang Gung Memorial Hospital,Volume 30, No.11,November 2009.2.Huang Syuandi, ” Introduction of development ofnew materials and technologies for artificialjoints”, Kaohsiung Medical University Chung-HoMemorial Hospital,Volume 23 , No. 8,January2004.3.Syu Yeliang, Lyu Dongwu, ”New design of thetotal knee joint”, National Science CouncilReport , 2001.4.Ministry of Economic Affairs , industry andtechnical knowledge Service plan, ”Marketanalysis of metal orthopedic devices”, 2005.5.Jhu Jinguo, Nanjing University of ChineseMedicine ”Artificial joint materials research andclinical application of bone”, 2008.6.T. Saba and A. Rehman. “Effects of ArtificiallyIntelligent Tools on Pattern Recognition”,International Journal of Machine Learning andCybernetics, vol. 4(2), pp. 155-162, 2012.7.Jeng-Nan Lee and Kuan-Yu Chang, An IntegratedInvestigation of CAD/CAM for the Developmentof Custom-made Femoral Stem, Life, Vol 7, No 1, 2009.A. Rehman and T. Saba (2012) “Neural Networkfor Document Image Preprocessing” s10462-012-9337-z.C. I. Nwoye, G. C. Obasi, U. C Nwoye, K. Okeke,C. C. Nwakwuo and O. O Onyemaobi, Model forCalculating the Concentration of Upgraded IronDesignated for Production of Stainless SteelBased Devices Used in Orthopaedics, LifeScience Journal, Volume 7, Issue 4, 2010.Yen Ke Tien and Chang Kuan Yu, Tai chiexercise affects the isokinetic torque but notchanges hamstrings: quadriceps ratios, LifeScience Journal, Vol 6, No 4, 2009.Liou Syuanyong,Biomedical Titanium Alloys andSurface Modification,Chemical Industry Press,2009.Marc Long, H.J. Rack, “Titanium alloys in ��, Biomaterials, 9(1998) 1621-1639.Siao Weidian , Thermal Spray Technology ,CHWA Technology Company, 2006.Wang Haijyun, Materials and applications ofthermal spray, National Defense Industry Press,2008.Robert B. Heimann, “Thermal spraying ofbiomaterials”, Surface and Coatings Technology,201 (2006) 2012-2019.Hong Liang, Bing Shi, Aaron Fairchild, TimothyCale“Applications of plasma coating in artificialjoints: an overview”, Vacuum 73 (2004) 317-326.Fujisawa, I. Noda, Y. Nishio, H. Okimatsu“Thedevelopment of new titanium arc-sprayedartificial joints”, July 1995, Pages 151-157.R. Gadow, A. Killinger, N. ications deposited by different thermal spraytechniques”, surface and coating Technology,2010, 1157-1164.ASTM C633-1(Reapproved 2008), “Standard TestMethod for Adhesion or Cohesion Strength ofThermal Spray Coatings”.ASTM G65-04, “Standard Test Method forMeasuring Abrasion Using the Dry Sand/RubberWheel Apparatus”.1/2/2012463

applications for thermal spray coating [13-16]. Thermal spray processes are grouped into three major categories: flame spray, electrical arc spray, and plasma arc spray. These energies sources are used to heat the coating material (in powder,

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