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SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy
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Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy Yakup Say a, Bunyamin Aksakal b,n, Burak Dikici c a
Tunceli Uni. Engineering Faculty, Department of Metallurgical and Materials Engineering, Tunceli 62000, Turkey Yıldız Technical Uni., Faculty of Chemical and Metallurgical and Materials Engineering, Department of Metallurgical and Materials Engineering, 34220 Istanbul, Turkey c Yuzuncu Yil University, Dept. of Mechanical Eng., 65080 Van, Turkey b
ar t ic l e i nf o
a b s t r a c t
Article history: Received 19 January 2016 Received in revised form 2 March 2016 Accepted 17 March 2016
REX-734 stainless steel alloy was coated with hydroxyapatite (HA)–silicon dioxide (SiO2) composite (hybride) coatings by using the sol–gel method. Surface morphologies, adhesion resistances and corrosion behaviors of the coatings were examined and the characterization of the coatings was performed by using XRD, SEM, and EDS. It was observed that porous and crack-free surface morphology enhanced adhesion strength of the coatings. Through the shear tests, it was found that the adhesion resistance was 25.14 MPa in HA coatings and 25.47 MPa in HA–SiO2 coatings. Corrosion analyses of the coatings were performed by using potentiodynamic polarization (PDS) tests. It was determined that corrosion dimples formed on the coating surface and the corrosion resistance in HAþ SiO2 coatings increased with decreasing porosity; however, in the further steps of corrosion, it reached to the base material by causing a pitting effect on the coated surface. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: REX-734 Hydroxyapatite Silica Sol–gel Adhesion Corrosion
1. Introduction Biomaterials are natural or synthetic materials that are used in a live body environment and are designed to carry out or support the functions of the damaged tissues. Due to high mechanical properties and fracture toughness of metal-based implants, they are commonly used in the orthopedic surgery operations (approximately 40%) requiring load-carrying capability [1,2]. The basic characteristic expected from a biomaterial is to be compatible with the surrounding tissues and prevent formation of any clot and inflammation within the tissue. Biomaterials that are implanted in the body are exposed to body fluids containing various anions (Cl , HC O−3 , HP O24−), cations (Na þ , K þ , Ca þ 2, Mg þ 2), and dissolved oxygen [3–5]. Corrosion of the metallic implants causes the loosening of prosthesis and also the formation of free metal ions such as Fe, Ni, Cr [6]. Furthermore, high chloride content in the body fluid may cause corrosion problems in such metallic materials [7]. Commonly used traditional metallic implant materials are lowcarbon stainless steel, cobalt, and titanium alloys. Stainless steels are the materials that should be developed in terms of surface properties as well as corrosion resistances and biocompatibilities and should be more functionalized. A great part of the studies n
Corresponding author. E-mail address: [email protected] (B. Aksakal).
conducted in recent years has focused on the coating of surfaces with different materials in order to develop the corrosion resistances and biocompatibilities of metallic implants. Thus, it can be possible to prevent diffusion of corrosive environments such as water, oxygen, acid into the implant surface and to make the surface stable against such corrosive environments. The previous studies determined that hydroxyapatite (HA, Ca5(PO4)3(OH)) enhanced the biocompatibility characteristics of metallic implants and increased the corrosion resistance due to their bone-like structure and superior biocompatibility characteristics [8]. Being a biocompatible material that can be decomposed and/or absorbed via bacteria, HA can form strong chemical bonds due to its natural bone tissue [9]. Due to the osteoconductive effects in bone recovery, HA is used as both a mass biomaterial and a surface coating for the biomaterials. All the compounds of calcium phosphate are very compatible with the tissues and are used instead of the bone in granular form or a solid block. The apatite compound of calcium phosphate is almost the same as the mineral phase of the bone and the tooth [10]. Additionally, it is determined that bone-like structure of hydroxyapatite has increased bone formation in the practice [11,12]. However, mechanical properties of HA are not sufficient for hard tissue applications requiring strength. Thus, HA is generally preferred to coat the surfaces of the metallic implants in order to obtain bioactive biomaterials having high strength [13]. Various coating techniques can be used for the implant surface modification. These are plasma ion implantation, laser melting,
http://dx.doi.org/10.1016/j.ceramint.2016.03.127 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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laser surface alloying, physical and chemical vapor deposition (PVD–CVD), thermal oxidation, and electrochemical surface modification. However, application of these surface modification techniques may cause complicated procedures and limitations in coating-surface adaptation. Furthermore, many of these techniques are more expensive because their coatings are denser (rapid integration with the tissue surrounding the implant, low porosity ratio required for rapid recovery of the damaged tissue) and they require more complicated devices and applications. Sol–gel is an alternative method in order to obtain the desired coating properties [14]. One of the most superiorities of the sol–gel method is to functionalize the properties of the coating to be applied by changing the particles to be add to the sol and the parameters of the heat treatment. Another main advantage of the method is to achieve a homogeneous coating at requested thicknesses and to obtain the coatings requested at relatively low heat treatment temperatures [15]. The purpose of this study was to perform the HA-matrix SiO2doped composite coatings on REX-734 base materials by using the sol–gel method. The surface morphology of the coated substrates, adhesion strength and corrosion resistance of the coatings were examined.
2. Materials and method
In this study, rod REX-734 samples with an 8 mm diameter in accordance with ASTM F1586 standards were used as substrate material. Table 1 shows chemical composition of REX-734 used as the substrate material. REX-734 samples were cut in the sizes of 10 20 mm. In order to increase adhesion, surfaces of the samples were subject to sanding process. Finally, they were ultrasonically cleaned in distilled water, acetone, and HNO3 and made ready for the coating process. 2.2. Coating In the coatings, HA powder at 10–11 mm particle size as additives P2O5, KH2PO4 and NaCO3 inorganic materials were used. Two types of bioceramic coatings as HA and HA/SiO2 were applied on REX-734 substrate surfaces by using the sol–gel method. Table 2 shows the gel compositions prepared. The sol prepared was ultrasonically homogenized until a homogeneous gel was obtained. After the gel was formed, the samples were coated with Table 1 Chemical composition of REX-734. Element (%) C
Cr
Cu
Mn
Mo
N
Nb
Ni
Si
0.031
20.68
0.13
4.12
2.27
0.38
0.28
9.59
0.47
In order to determine the adhesion resistances of the bioceramic coatings on metallic substrate surface; shear test was performed on the samples (Shimadzu AG-X). Adhesion tests were applied at 2 mm/min rates in accordance with the ASTM C 633 standard. For this purpose, tensile test was applied on the surfaces that were joined with DP460 epoxy (3M) adhesive and the adhesion resistances were calculated by considering the joint surface areas and rupture force values. Then, the data were converted in adhesion resistance – % elongation graphics with the help of data acquisition system software and were evaluated comparatively. At least 3 tests were applied for every sample group and the mean values were considered in the comparisons. Hardness measurements of bioceramic coatings were performed under a load of 300 grf for 10 s by using the micro Vicker (EMCO TEST DURASCAN) method. In order to obtain an optimum value, hardness measurements were repeated for 6 times and mean values were calculated.
The coated corrosion samples were taken into cold bakelite and only the coated surfaces were left as uncovered. Before the bakelite process, the samples were joined with copper wires with an 1.7 mm diameter in order to provide conductivity in the corrosion unit and the conductivity controls were performed after the bakelite process. Corrosion tests were performed at the GAMRY PCI14/750 (USA) corrosion test unit. All the analyses were performed by using the three electrodes technique. In corrosion tests, Ringer solution was used as the electrolyte and the temperature of the corrosion cell was fixed to the body temperature (37 °C) and then the corrosion tests were started. In the tests, saturated silver/ silver chloride electrode (Ag/AgCl) was used as the reference electrode (RE), platinum wire (Pt) was used as the contrary electrode (CE) and the corrosion samples were used as the working electrode (WE). Repeatability was determined by performing at least 2 tests for each sample group. Open circuit potential measurements (OCP) and potentiodynamic polarization measurements (PDS) were applied for the corrosion samples. In OCP tests, change of the mixed potential depending on time (dE–t) was determined and in order to evaluate the localized interactions of the samples, open circuit potential changes were measured. In PDS tests, the polarization curves of the samples were drawn and corrosion characteristics were determined. At first, the samples were kept until they reached to the open circuit potential (OCP) and then all the specimens were potentiodynamically subjected to potentiodynamic polarization measurements (PDS) at the scanning rates of 1 mV/s. The polarization curves were obtained from the samples and their corrosion characteristics were then determined. 2.5. Surface analyses
Table 2 Sol–gel concentrations used during the study.
HA (S1) HA/SiO2 (S2)
2.3. Mechanical tests
2.4. Corrosion
2.1. Material
Samples
2-s immersion periods. The samples coated were kept under room conditions and then they were subject to pre-drying at 130 °C in the furnace and then to the sintering process at 750 °C for 120 min in a vacuum environment.
Coating materials (% mol) HA
P2O5
KH2PO4
NaCO3
SiO2
40 40
30 20
10 10
20 20
– 10
The surface morphologies of the coated samples were performed by using the scanning electron microscope (SEM, JEOL JSM 7001F) and electron dispersive spectroscopy (EDS, Oxford INCA). Also, the coating surfaces were characterized by a X-ray diffraction (XRD, Bruker D8) in which a Kα tube was used. XRD analyses were performed with a pitch rate of 0.02° between 10° and 90° at a wavelength of 1.5406 (λ). Coating thicknesses were measured
Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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from the cross section of the samples by using an optical microscope (OM, Nikon Ecupse MA200).
3. Results On REX-734 alloy, coatings containing HA (C1) and HA/SiO2 (C2) were applied by using the sol–gel method and as a result of the measurements, film thickness was determined as averagely 12 mm in HA coatings and 11 mm in HA/SiO2 coatings. In the hardness measurements; hardness value of the uncoated REX-734 (S0) sample was measured as 427.8 HV. While the hardness value of the coating in C1 group samples in which REX-734 substrates were coated only with HA was 290.2 HV, SiO2 added into the coating caused a slight decrease in the hardness value and the hardness value was calculated as 261.4 HV. Table 3 shows adhesion resistance values obtained as a result of the adhesion tests of the surface coatings. In HA coatings, an adhesion resistance of 25.14 MPa was obtained and it was observed that the addition of SiO2 did not provide a significant increase in adhesion resistance. SEM analyses showed that a porous surface morphology was provided at low crack density and the corrosion caused formation of dimple in the coatings (Fig. 1). XRD diffraction patterns showed that CrO3 and Cr3O8 oxides formed on REX-734 sample surface, which is a Fe–Cr–Ni–Mn alloy, due to the effect of corrosion (Figs. 2 and 3). In results of EDX analysis; the increase in the (O) ratio of the spectra after corrosion also supported the formation of such oxides. Fig. 4 shows the SEM views of uncoated and coated substrates, before and after corrosion. From the obtained surface structures after corrosion, the pitting type of formations detected due to corrosion were marked by arrows. ( Figs. 5 and 6). Fig. 7 shows comparatively the open circuit potential (OCP) changes of bioactive composite coatings applied on REX-734 base material in the Ringer solution under in-vitro conditions. On the other hand, Fig. 8 shows comparatively potentiodynamic (PDS) polarization curves of the samples in the Ringer solution and at body temperature. The coatings were over polarized in the anodic direction by using PDS method. The purpose of this process is to increase the potential on the samples that were exposed to free corrosion for a while and to reveal the corrosion differences. Potentiodynamic polarization method is a very useful method for the examination of the corrosion, starting from the weakening zones and the coating on the metal surface, through electrochemical techniques and for the determination of the quality and stability of the coating. Table 4 shows some corrosion parameters (Eocp, Ecorr, Icorr, βA and βC) calculated from these curves for HA and HA/SiO2 coatings. On the other hand, Table 5 shows corrosion rate and polarization resistance values of the bioceramic coatings.
Pores
Agglomeration
Fig. 1. SEM view of REX-734 coated with HA (before corrosion).
Fig. 2. XRD patterns of specimens (before corrosion). Fe-Ni
Fe-Ni, CrO3 Cr3O8 Cr3O8
Cr3O8 HA HA HA
Fe-Ni Fe-Ni HA
S2
4. Discussion S1
The main purpose of the coating metallic implants with bioceramic materials before implantation is to enhance the tissue– implant interactions. One of the most important problems observed in implant materials during body applications is the loosening in Table 3 Adhesion strength values for HA and HA/SiO2 coatings. Samples
Coatings
Average adhesion strength (MPa)
S0 S1 S2
Uncoated HA HA–SiO2
32.08 25.14 25.47
S0
Fig. 3. XRD patterns of specimens (after corrosion).
between bone–implant interface. When the loosenings to occur in the implant material is needed to be removed from the body without executing its duty and also lead to significant bone tissue damages. If sufficient adhesion is obtained between the coating and implant, bonding is achieved in the coating–tissue interaction in the coatings; then fixation of the implant is provided and the possible
Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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pitting
(a)
(b)
pitting
(c)
(d)
pitting
(e)
(f)
Fig. 4. SEM views of uncoated/coated specimens, before corrosion; (a) uncoated, (b) HA coated, (c) HASiO2 coated, after corrosion; (d) uncoated, (e) HA coated, (f) HASiO2 coated.
loosenings are minimized or prevented. Therefore, in coating–tissue interaction, a porous coating-morphology is expected to integrate faster and better with tissue. In SEM analyses, the pores on the coatings and the connections between the pores were observed (Fig. 1). Cracks observed on the coating surface negatively affected the adhesion resistance of the coatings. The SEM analyses showed that a structure having low crack density was obtained in the coatings (Fig. 4c and e). Fig. 4 shows comparatively SEM images before and after corrosion. Images after corrosion show that the corrosion caused formation of dimple on the surface. In literature, it is indicated that hardness of HA as the solid material is 600 HV [11]. In this study, the reason for decreasing of
this value to 300 HV after the coatings is thought to be be associated with the fact that the coating had a porous surface morphology with low-crack density. It was thought that difference between the metallic substrates and bioceramic coatings in terms of the thermal expansion caused these cracks [16]. As a result of the adhesion test applied on uncoated surface, it was determined that the adhesive had an adhesion resistance of approximately 32 MPa for the REX-734 group. While the adhesion resistance of HA coating (S1) was 25.14 MPa, addition of SiO2 (S2) to HA slightly increased the adhesion resistance (0.33 MPa). Fig. 1 shows the morphology of HA coated REX-734 substrate surface. It is observed that the spherical like formation of the
Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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Fig. 5. EDS spectrums of coatings, before corrosion; (a) uncoated, (b) HA, (c) HASiO2.
coated HA powders and connections (agglomeration) between the powders provided through sintering. Fig. 4c shows that crack-free structure, which was important for the adhesion between the coating and metal, was obtained. Also, pores and regional differences were also observed in the coating. This porous structure facilitates osteointegration in bioceramic coated implants [17]. This structure will facilitate the osteointegration of the tissue with the coating in coating–tissue interaction, provide the preferential orientation of the tissue development within the pores, and also ensure the osteointegration characteristics required between the tissue and the implant owing to the coating. Fig. 4d shows that sphere-like grain structure deteriorated and dimples formed on the surface as a result of the exposure of HA coating to corrosion. When results of the EDX analysis of the obtained morphologies were evaluated, it was observed that an increase in the O ratio and a decrease in Ca–P ratio after corrosion are detected. As a result of the damage caused by the corrosion in the coating, corrosion reached to the substrate material and the formation of Fe, Cr, and Mn-based metal oxides started along with the decrease in the Ca–
P ratio. XRD analyses also showed CrO3 and Cr3O8 formation in the structure after corrosion. HA peaks were present in both XRD diffraction patterns before and after corrosion. Also, when the EDX analysis results were examined, it was observed that a significant decrease occurred in the Ca–P ratio after corrosion. In both EDX and XRD results, oxides occurring after corrosion demonstrated that HA coating layer got damaged due to corrosion and corrosion reached to the substrate material and consequently, the metallic material was exposed to corrosion. The SEM images of such coated surface morphologies are given in Fig. 4e and f. The formation of a less porous structure which is different compared to HA coated samples are the present. Fig. 4e shows a crack-free structure after the coating and Fig. 4f shows that corrosion led to cracks and dimples by causing surface damage in the coating. Additionally, it could be asserted that addition of SiO2 to HA decreased the pore density and reduced the effect of corrosion on the coating surface. When results of EDX analysis were examined, it was observed that addition of SiO2 reduced the decrease in Ca–P ratio, which occurred after corrosion,
Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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Fig. 6. EDS spectrums of coatings, before corrosion; (a) uncoated, (b) HA, (c) HASiO2.
1000
0 -50
-150
-200
S
-250 -300
S
E (mV, Ag/AgCl)
600
S
-100 E (mV, Ag/AgCl)
S
800
400
S
200
S
0 -200 -400 -600
-350
-800
-400 0
200
400
600 t (s)
800
1000
Fig. 7. OCP analysis results of the coated REX-734 alloy.
1200
10
10
10
10
10
I (log, mAcm ) Fig. 8. PDS analysis results of the coated REX-734 alloy.
Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
Y. Say et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 4 Calculated corrosion parameters. Samples
Eocp (mV)
Ecorr (mV)
Icorr βA ( 10 9 A/cm2) ( 10 3 V /dec.)
S0 Uncoated S1 HA S2 Ha/SiO2
97 313 226
153 1030 287 1650 227 795
562.2 88 142
βC ( 10 3 V/ dec.) 187.5 263 232
Table 5 Corrosion speed and polarization values. Samples
S0 S1 S2
Uncoated HA Ha/SiO2
Corr. speed (mpy)
Rp ( 103 ohms cm2)
4640 5940 3439
53.93 18.94 45.73
compared to the coating containing only HA. XRD diffraction patterns before and after corrosion showed that the presence of HA continued after corrosion. Results of EDX analysis before corrosion showed the presence of Silica and HA which is a Ca–P–O compound, in the structure. In HASiO2 coatings, CrO3 and Cr3O8 oxides occurred after corrosion once the corrosion reached to the substrate material as in the HA coatings. One of the essential goals of the bioceramic coatings is to enhance the corrosion resistances of the coated metallic implants. In the direct use of implants, ion releases occurring as a result of the implant–tissue interaction posed a risk in the tissues. Coating of the implants with bioactive bioceramics will provide a coating layer that will protect the implant in terms of risk-free tissue– coating interaction and corrosion instead of the tissue–implant interaction. At the same time, corrosion occurring in the implant materials after fixation may cause the loosening of the implant. Improvement of the corrosion characteristics is quite important to avoid this problem. The OCP curves given in Fig. 7 shows that as a result of corrosion, Eocp values decreased for all the coating types and Eocp value increased depending upon the time in the uncoated base material. However, it was observed that there is no significant change in the Eocp values at the end of a nearly 20-min immersion period. Depending on the coating type, the coated samples became less stabilized into Ringer's solution over time compared to uncoated samples and had lower potential vibration amplitudes compared to uncoated samples. The change in Eocp values of the coatings towards more anodic potentials depending on time within Ringer solution when compared to the initial potentials showed the presence of localized corrosion on the surface. It can be seen from the PDS curves given in Fig. 8 that the potential in the cathodic region increased in all the samples and the currents were regularly decreased. This result suggest the presence of an activation-controlled corrosion mechanism. Thus, the comparison of Icorr sizes can be determinant for the determination of the corrosion tendency. The Icorr value of 1.03 mA in the uncoated sample increased to 1.65 mA with the HA coating. This increase in Icorr value showed that the HA coating increased the corrosion rate of the REX-734 substrate. However, the values at nA level signified that there was no significant difference. Additionally, the decrease in the anodic slope (βA) from 562.2 10 3 V/dec. to 88 10 3 V/dec. due to the HA coating showed that the anodic slope and accordingly the anodic activities decreased on the surface in the HA coated sample compared to the uncoated sample. As a result, it was observed that the HA coating did not provide the increase expected for the corrosion resistance.
7
Similar results have been reported in the HA coatings containing Y2O3–Zr applied on 316L substrates [18] and in the HA–Nb coatings applied by using the plasma spray method [19]. It was thought that the absence of the increase expected in corrosion resistance was caused by the porous structure of the coating. It was considered that the coating morphology in the porous structure negatively affected the corrosion protection of HA coating and also caused localized corrosion on the coated sample surface. These pores were the zones where electrolyte (Ringer solution) was still and in time, the internal sides of these porosities could be covered with oxide-structured corrosion products. If the Cl ions increase in this zone compared to the other zones of the surface, these zones may become the localized regions where surface conductivity and the dissolution of oxide film increase. In other words, these porosities can also act as the preferred corrosion zones. In the HA coated sample, the change of the Eocp values towards more anodic potentials depending on time supported the presence of localized corrosion. Addition of SiO2 decreased the corrosion rate of HA-doped bioactive coatings. In other saying, SiO2 covered the pores on the known porous structure of HA and slowed down the access of corrosive products to the base material and also the separation of these products from the surface. It could be determined from such results that addition of SiO2 was coated with a low-rate porous protective oxide film which was also more stable on the surface. It could be stated that the surface oxide film of S2 sample with similar Eocp ( 226 mV) and Ecorr ( 227 mV) values was more stable when compared to the others. Table 5 shows corrosion rate and polarization resistance values of the bioceramic coatings. Addition of SiO2 applied to the HA coating increased corrosion resistance and consequently decreased the corrosion rate. The reason was associated with the fact that addition of SiO2 applied to the HA coatings decreased the pore density on the surface morphology. The fact that addition of SiO2 decreased the pore density in the HA coating was also supported by the SEM examinations. Porosities observed on bioceramic coatings (Fig. 1) are generally the zones where electrolyte (Ringer solution) is still and in time, the internal sides of these porosities could be covered by the oxide-structured corrosion products. If amount of Cl ions contained by the oxide products increases compared to the other zones of the implant, these regions may become localized regions where oxide film dissolution and surface conductivity increase. In other words, these porosities may also act as the preferred corrosion regions. The microstructural examinations performed after corrosion in HA-based composite coatings showed that localized corrosion distribution in the coatings were not homogeneous throughout the surface. This situation showed the formation of ever-changing micro local batteries on the surface, accordingly, corrosion tracks were observed at different rates and sizes at different locations of the surface. All these dynamic changes appeared with the increase in the rate of change and data dispersion in the electrochemical data (Table 4). This dispersion ratio gradually decreased as a result of the decrease in the void ratio within the coating. The decrease observed in the corrosion current densities in S2 sample supported this result.
5. Conclusions In both coatings, porous surface morphology with low-crack density was achieved, which is important for osteointegration. Addition of SiO2 to HA decreased the pore density, however, increased the corrosion resistance of the coating. It was observed that the samples only coated with HA did not increase the corrosion resistance of REX-734 alloy; however, corrosion rate decreased and polarization resistance increased as a result of
Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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addition of SiO2 to HA. Also the localized corrosion on the surface caused formation of dimple in the coatings. Hardness values in HA and HA–SiO2 coatings having similar coating thickness values gave similar results. It was observed that addition of SiO2 did not cause a significant change in adhesion resistance.
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Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy Yakup Say a, Bunyamin Aksakal b,n, Burak Dikici c a
Tunceli Uni. Engineering Faculty, Department of Metallurgical and Materials Engineering, Tunceli 62000, Turkey Yıldız Technical Uni., Faculty of Chemical and Metallurgical and Materials Engineering, Department of Metallurgical and Materials Engineering, 34220 Istanbul, Turkey c Yuzuncu Yil University, Dept. of Mechanical Eng., 65080 Van, Turkey b
ar t ic l e i nf o
a b s t r a c t
Article history: Received 19 January 2016 Received in revised form 2 March 2016 Accepted 17 March 2016
REX-734 stainless steel alloy was coated with hydroxyapatite (HA)–silicon dioxide (SiO2) composite (hybride) coatings by using the sol–gel method. Surface morphologies, adhesion resistances and corrosion behaviors of the coatings were examined and the characterization of the coatings was performed by using XRD, SEM, and EDS. It was observed that porous and crack-free surface morphology enhanced adhesion strength of the coatings. Through the shear tests, it was found that the adhesion resistance was 25.14 MPa in HA coatings and 25.47 MPa in HA–SiO2 coatings. Corrosion analyses of the coatings were performed by using potentiodynamic polarization (PDS) tests. It was determined that corrosion dimples formed on the coating surface and the corrosion resistance in HAþ SiO2 coatings increased with decreasing porosity; however, in the further steps of corrosion, it reached to the base material by causing a pitting effect on the coated surface. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: REX-734 Hydroxyapatite Silica Sol–gel Adhesion Corrosion
1. Introduction Biomaterials are natural or synthetic materials that are used in a live body environment and are designed to carry out or support the functions of the damaged tissues. Due to high mechanical properties and fracture toughness of metal-based implants, they are commonly used in the orthopedic surgery operations (approximately 40%) requiring load-carrying capability [1,2]. The basic characteristic expected from a biomaterial is to be compatible with the surrounding tissues and prevent formation of any clot and inflammation within the tissue. Biomaterials that are implanted in the body are exposed to body fluids containing various anions (Cl , HC O−3 , HP O24−), cations (Na þ , K þ , Ca þ 2, Mg þ 2), and dissolved oxygen [3–5]. Corrosion of the metallic implants causes the loosening of prosthesis and also the formation of free metal ions such as Fe, Ni, Cr [6]. Furthermore, high chloride content in the body fluid may cause corrosion problems in such metallic materials [7]. Commonly used traditional metallic implant materials are lowcarbon stainless steel, cobalt, and titanium alloys. Stainless steels are the materials that should be developed in terms of surface properties as well as corrosion resistances and biocompatibilities and should be more functionalized. A great part of the studies n
Corresponding author. E-mail address: [email protected] (B. Aksakal).
conducted in recent years has focused on the coating of surfaces with different materials in order to develop the corrosion resistances and biocompatibilities of metallic implants. Thus, it can be possible to prevent diffusion of corrosive environments such as water, oxygen, acid into the implant surface and to make the surface stable against such corrosive environments. The previous studies determined that hydroxyapatite (HA, Ca5(PO4)3(OH)) enhanced the biocompatibility characteristics of metallic implants and increased the corrosion resistance due to their bone-like structure and superior biocompatibility characteristics [8]. Being a biocompatible material that can be decomposed and/or absorbed via bacteria, HA can form strong chemical bonds due to its natural bone tissue [9]. Due to the osteoconductive effects in bone recovery, HA is used as both a mass biomaterial and a surface coating for the biomaterials. All the compounds of calcium phosphate are very compatible with the tissues and are used instead of the bone in granular form or a solid block. The apatite compound of calcium phosphate is almost the same as the mineral phase of the bone and the tooth [10]. Additionally, it is determined that bone-like structure of hydroxyapatite has increased bone formation in the practice [11,12]. However, mechanical properties of HA are not sufficient for hard tissue applications requiring strength. Thus, HA is generally preferred to coat the surfaces of the metallic implants in order to obtain bioactive biomaterials having high strength [13]. Various coating techniques can be used for the implant surface modification. These are plasma ion implantation, laser melting,
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Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i
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laser surface alloying, physical and chemical vapor deposition (PVD–CVD), thermal oxidation, and electrochemical surface modification. However, application of these surface modification techniques may cause complicated procedures and limitations in coating-surface adaptation. Furthermore, many of these techniques are more expensive because their coatings are denser (rapid integration with the tissue surrounding the implant, low porosity ratio required for rapid recovery of the damaged tissue) and they require more complicated devices and applications. Sol–gel is an alternative method in order to obtain the desired coating properties [14]. One of the most superiorities of the sol–gel method is to functionalize the properties of the coating to be applied by changing the particles to be add to the sol and the parameters of the heat treatment. Another main advantage of the method is to achieve a homogeneous coating at requested thicknesses and to obtain the coatings requested at relatively low heat treatment temperatures [15]. The purpose of this study was to perform the HA-matrix SiO2doped composite coatings on REX-734 base materials by using the sol–gel method. The surface morphology of the coated substrates, adhesion strength and corrosion resistance of the coatings were examined.
2. Materials and method
In this study, rod REX-734 samples with an 8 mm diameter in accordance with ASTM F1586 standards were used as substrate material. Table 1 shows chemical composition of REX-734 used as the substrate material. REX-734 samples were cut in the sizes of 10 20 mm. In order to increase adhesion, surfaces of the samples were subject to sanding process. Finally, they were ultrasonically cleaned in distilled water, acetone, and HNO3 and made ready for the coating process. 2.2. Coating In the coatings, HA powder at 10–11 mm particle size as additives P2O5, KH2PO4 and NaCO3 inorganic materials were used. Two types of bioceramic coatings as HA and HA/SiO2 were applied on REX-734 substrate surfaces by using the sol–gel method. Table 2 shows the gel compositions prepared. The sol prepared was ultrasonically homogenized until a homogeneous gel was obtained. After the gel was formed, the samples were coated with Table 1 Chemical composition of REX-734. Element (%) C
Cr
Cu
Mn
Mo
N
Nb
Ni
Si
0.031
20.68
0.13
4.12
2.27
0.38
0.28
9.59
0.47
In order to determine the adhesion resistances of the bioceramic coatings on metallic substrate surface; shear test was performed on the samples (Shimadzu AG-X). Adhesion tests were applied at 2 mm/min rates in accordance with the ASTM C 633 standard. For this purpose, tensile test was applied on the surfaces that were joined with DP460 epoxy (3M) adhesive and the adhesion resistances were calculated by considering the joint surface areas and rupture force values. Then, the data were converted in adhesion resistance – % elongation graphics with the help of data acquisition system software and were evaluated comparatively. At least 3 tests were applied for every sample group and the mean values were considered in the comparisons. Hardness measurements of bioceramic coatings were performed under a load of 300 grf for 10 s by using the micro Vicker (EMCO TEST DURASCAN) method. In order to obtain an optimum value, hardness measurements were repeated for 6 times and mean values were calculated.
The coated corrosion samples were taken into cold bakelite and only the coated surfaces were left as uncovered. Before the bakelite process, the samples were joined with copper wires with an 1.7 mm diameter in order to provide conductivity in the corrosion unit and the conductivity controls were performed after the bakelite process. Corrosion tests were performed at the GAMRY PCI14/750 (USA) corrosion test unit. All the analyses were performed by using the three electrodes technique. In corrosion tests, Ringer solution was used as the electrolyte and the temperature of the corrosion cell was fixed to the body temperature (37 °C) and then the corrosion tests were started. In the tests, saturated silver/ silver chloride electrode (Ag/AgCl) was used as the reference electrode (RE), platinum wire (Pt) was used as the contrary electrode (CE) and the corrosion samples were used as the working electrode (WE). Repeatability was determined by performing at least 2 tests for each sample group. Open circuit potential measurements (OCP) and potentiodynamic polarization measurements (PDS) were applied for the corrosion samples. In OCP tests, change of the mixed potential depending on time (dE–t) was determined and in order to evaluate the localized interactions of the samples, open circuit potential changes were measured. In PDS tests, the polarization curves of the samples were drawn and corrosion characteristics were determined. At first, the samples were kept until they reached to the open circuit potential (OCP) and then all the specimens were potentiodynamically subjected to potentiodynamic polarization measurements (PDS) at the scanning rates of 1 mV/s. The polarization curves were obtained from the samples and their corrosion characteristics were then determined. 2.5. Surface analyses
Table 2 Sol–gel concentrations used during the study.
HA (S1) HA/SiO2 (S2)
2.3. Mechanical tests
2.4. Corrosion
2.1. Material
Samples
2-s immersion periods. The samples coated were kept under room conditions and then they were subject to pre-drying at 130 °C in the furnace and then to the sintering process at 750 °C for 120 min in a vacuum environment.
Coating materials (% mol) HA
P2O5
KH2PO4
NaCO3
SiO2
40 40
30 20
10 10
20 20
– 10
The surface morphologies of the coated samples were performed by using the scanning electron microscope (SEM, JEOL JSM 7001F) and electron dispersive spectroscopy (EDS, Oxford INCA). Also, the coating surfaces were characterized by a X-ray diffraction (XRD, Bruker D8) in which a Kα tube was used. XRD analyses were performed with a pitch rate of 0.02° between 10° and 90° at a wavelength of 1.5406 (λ). Coating thicknesses were measured
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from the cross section of the samples by using an optical microscope (OM, Nikon Ecupse MA200).
3. Results On REX-734 alloy, coatings containing HA (C1) and HA/SiO2 (C2) were applied by using the sol–gel method and as a result of the measurements, film thickness was determined as averagely 12 mm in HA coatings and 11 mm in HA/SiO2 coatings. In the hardness measurements; hardness value of the uncoated REX-734 (S0) sample was measured as 427.8 HV. While the hardness value of the coating in C1 group samples in which REX-734 substrates were coated only with HA was 290.2 HV, SiO2 added into the coating caused a slight decrease in the hardness value and the hardness value was calculated as 261.4 HV. Table 3 shows adhesion resistance values obtained as a result of the adhesion tests of the surface coatings. In HA coatings, an adhesion resistance of 25.14 MPa was obtained and it was observed that the addition of SiO2 did not provide a significant increase in adhesion resistance. SEM analyses showed that a porous surface morphology was provided at low crack density and the corrosion caused formation of dimple in the coatings (Fig. 1). XRD diffraction patterns showed that CrO3 and Cr3O8 oxides formed on REX-734 sample surface, which is a Fe–Cr–Ni–Mn alloy, due to the effect of corrosion (Figs. 2 and 3). In results of EDX analysis; the increase in the (O) ratio of the spectra after corrosion also supported the formation of such oxides. Fig. 4 shows the SEM views of uncoated and coated substrates, before and after corrosion. From the obtained surface structures after corrosion, the pitting type of formations detected due to corrosion were marked by arrows. ( Figs. 5 and 6). Fig. 7 shows comparatively the open circuit potential (OCP) changes of bioactive composite coatings applied on REX-734 base material in the Ringer solution under in-vitro conditions. On the other hand, Fig. 8 shows comparatively potentiodynamic (PDS) polarization curves of the samples in the Ringer solution and at body temperature. The coatings were over polarized in the anodic direction by using PDS method. The purpose of this process is to increase the potential on the samples that were exposed to free corrosion for a while and to reveal the corrosion differences. Potentiodynamic polarization method is a very useful method for the examination of the corrosion, starting from the weakening zones and the coating on the metal surface, through electrochemical techniques and for the determination of the quality and stability of the coating. Table 4 shows some corrosion parameters (Eocp, Ecorr, Icorr, βA and βC) calculated from these curves for HA and HA/SiO2 coatings. On the other hand, Table 5 shows corrosion rate and polarization resistance values of the bioceramic coatings.
Pores
Agglomeration
Fig. 1. SEM view of REX-734 coated with HA (before corrosion).
Fig. 2. XRD patterns of specimens (before corrosion). Fe-Ni
Fe-Ni, CrO3 Cr3O8 Cr3O8
Cr3O8 HA HA HA
Fe-Ni Fe-Ni HA
S2
4. Discussion S1
The main purpose of the coating metallic implants with bioceramic materials before implantation is to enhance the tissue– implant interactions. One of the most important problems observed in implant materials during body applications is the loosening in Table 3 Adhesion strength values for HA and HA/SiO2 coatings. Samples
Coatings
Average adhesion strength (MPa)
S0 S1 S2
Uncoated HA HA–SiO2
32.08 25.14 25.47
S0
Fig. 3. XRD patterns of specimens (after corrosion).
between bone–implant interface. When the loosenings to occur in the implant material is needed to be removed from the body without executing its duty and also lead to significant bone tissue damages. If sufficient adhesion is obtained between the coating and implant, bonding is achieved in the coating–tissue interaction in the coatings; then fixation of the implant is provided and the possible
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pitting
(a)
(b)
pitting
(c)
(d)
pitting
(e)
(f)
Fig. 4. SEM views of uncoated/coated specimens, before corrosion; (a) uncoated, (b) HA coated, (c) HASiO2 coated, after corrosion; (d) uncoated, (e) HA coated, (f) HASiO2 coated.
loosenings are minimized or prevented. Therefore, in coating–tissue interaction, a porous coating-morphology is expected to integrate faster and better with tissue. In SEM analyses, the pores on the coatings and the connections between the pores were observed (Fig. 1). Cracks observed on the coating surface negatively affected the adhesion resistance of the coatings. The SEM analyses showed that a structure having low crack density was obtained in the coatings (Fig. 4c and e). Fig. 4 shows comparatively SEM images before and after corrosion. Images after corrosion show that the corrosion caused formation of dimple on the surface. In literature, it is indicated that hardness of HA as the solid material is 600 HV [11]. In this study, the reason for decreasing of
this value to 300 HV after the coatings is thought to be be associated with the fact that the coating had a porous surface morphology with low-crack density. It was thought that difference between the metallic substrates and bioceramic coatings in terms of the thermal expansion caused these cracks [16]. As a result of the adhesion test applied on uncoated surface, it was determined that the adhesive had an adhesion resistance of approximately 32 MPa for the REX-734 group. While the adhesion resistance of HA coating (S1) was 25.14 MPa, addition of SiO2 (S2) to HA slightly increased the adhesion resistance (0.33 MPa). Fig. 1 shows the morphology of HA coated REX-734 substrate surface. It is observed that the spherical like formation of the
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Fig. 5. EDS spectrums of coatings, before corrosion; (a) uncoated, (b) HA, (c) HASiO2.
coated HA powders and connections (agglomeration) between the powders provided through sintering. Fig. 4c shows that crack-free structure, which was important for the adhesion between the coating and metal, was obtained. Also, pores and regional differences were also observed in the coating. This porous structure facilitates osteointegration in bioceramic coated implants [17]. This structure will facilitate the osteointegration of the tissue with the coating in coating–tissue interaction, provide the preferential orientation of the tissue development within the pores, and also ensure the osteointegration characteristics required between the tissue and the implant owing to the coating. Fig. 4d shows that sphere-like grain structure deteriorated and dimples formed on the surface as a result of the exposure of HA coating to corrosion. When results of the EDX analysis of the obtained morphologies were evaluated, it was observed that an increase in the O ratio and a decrease in Ca–P ratio after corrosion are detected. As a result of the damage caused by the corrosion in the coating, corrosion reached to the substrate material and the formation of Fe, Cr, and Mn-based metal oxides started along with the decrease in the Ca–
P ratio. XRD analyses also showed CrO3 and Cr3O8 formation in the structure after corrosion. HA peaks were present in both XRD diffraction patterns before and after corrosion. Also, when the EDX analysis results were examined, it was observed that a significant decrease occurred in the Ca–P ratio after corrosion. In both EDX and XRD results, oxides occurring after corrosion demonstrated that HA coating layer got damaged due to corrosion and corrosion reached to the substrate material and consequently, the metallic material was exposed to corrosion. The SEM images of such coated surface morphologies are given in Fig. 4e and f. The formation of a less porous structure which is different compared to HA coated samples are the present. Fig. 4e shows a crack-free structure after the coating and Fig. 4f shows that corrosion led to cracks and dimples by causing surface damage in the coating. Additionally, it could be asserted that addition of SiO2 to HA decreased the pore density and reduced the effect of corrosion on the coating surface. When results of EDX analysis were examined, it was observed that addition of SiO2 reduced the decrease in Ca–P ratio, which occurred after corrosion,
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Fig. 6. EDS spectrums of coatings, before corrosion; (a) uncoated, (b) HA, (c) HASiO2.
1000
0 -50
-150
-200
S
-250 -300
S
E (mV, Ag/AgCl)
600
S
-100 E (mV, Ag/AgCl)
S
800
400
S
200
S
0 -200 -400 -600
-350
-800
-400 0
200
400
600 t (s)
800
1000
Fig. 7. OCP analysis results of the coated REX-734 alloy.
1200
10
10
10
10
10
I (log, mAcm ) Fig. 8. PDS analysis results of the coated REX-734 alloy.
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Y. Say et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 4 Calculated corrosion parameters. Samples
Eocp (mV)
Ecorr (mV)
Icorr βA ( 10 9 A/cm2) ( 10 3 V /dec.)
S0 Uncoated S1 HA S2 Ha/SiO2
97 313 226
153 1030 287 1650 227 795
562.2 88 142
βC ( 10 3 V/ dec.) 187.5 263 232
Table 5 Corrosion speed and polarization values. Samples
S0 S1 S2
Uncoated HA Ha/SiO2
Corr. speed (mpy)
Rp ( 103 ohms cm2)
4640 5940 3439
53.93 18.94 45.73
compared to the coating containing only HA. XRD diffraction patterns before and after corrosion showed that the presence of HA continued after corrosion. Results of EDX analysis before corrosion showed the presence of Silica and HA which is a Ca–P–O compound, in the structure. In HASiO2 coatings, CrO3 and Cr3O8 oxides occurred after corrosion once the corrosion reached to the substrate material as in the HA coatings. One of the essential goals of the bioceramic coatings is to enhance the corrosion resistances of the coated metallic implants. In the direct use of implants, ion releases occurring as a result of the implant–tissue interaction posed a risk in the tissues. Coating of the implants with bioactive bioceramics will provide a coating layer that will protect the implant in terms of risk-free tissue– coating interaction and corrosion instead of the tissue–implant interaction. At the same time, corrosion occurring in the implant materials after fixation may cause the loosening of the implant. Improvement of the corrosion characteristics is quite important to avoid this problem. The OCP curves given in Fig. 7 shows that as a result of corrosion, Eocp values decreased for all the coating types and Eocp value increased depending upon the time in the uncoated base material. However, it was observed that there is no significant change in the Eocp values at the end of a nearly 20-min immersion period. Depending on the coating type, the coated samples became less stabilized into Ringer's solution over time compared to uncoated samples and had lower potential vibration amplitudes compared to uncoated samples. The change in Eocp values of the coatings towards more anodic potentials depending on time within Ringer solution when compared to the initial potentials showed the presence of localized corrosion on the surface. It can be seen from the PDS curves given in Fig. 8 that the potential in the cathodic region increased in all the samples and the currents were regularly decreased. This result suggest the presence of an activation-controlled corrosion mechanism. Thus, the comparison of Icorr sizes can be determinant for the determination of the corrosion tendency. The Icorr value of 1.03 mA in the uncoated sample increased to 1.65 mA with the HA coating. This increase in Icorr value showed that the HA coating increased the corrosion rate of the REX-734 substrate. However, the values at nA level signified that there was no significant difference. Additionally, the decrease in the anodic slope (βA) from 562.2 10 3 V/dec. to 88 10 3 V/dec. due to the HA coating showed that the anodic slope and accordingly the anodic activities decreased on the surface in the HA coated sample compared to the uncoated sample. As a result, it was observed that the HA coating did not provide the increase expected for the corrosion resistance.
7
Similar results have been reported in the HA coatings containing Y2O3–Zr applied on 316L substrates [18] and in the HA–Nb coatings applied by using the plasma spray method [19]. It was thought that the absence of the increase expected in corrosion resistance was caused by the porous structure of the coating. It was considered that the coating morphology in the porous structure negatively affected the corrosion protection of HA coating and also caused localized corrosion on the coated sample surface. These pores were the zones where electrolyte (Ringer solution) was still and in time, the internal sides of these porosities could be covered with oxide-structured corrosion products. If the Cl ions increase in this zone compared to the other zones of the surface, these zones may become the localized regions where surface conductivity and the dissolution of oxide film increase. In other words, these porosities can also act as the preferred corrosion zones. In the HA coated sample, the change of the Eocp values towards more anodic potentials depending on time supported the presence of localized corrosion. Addition of SiO2 decreased the corrosion rate of HA-doped bioactive coatings. In other saying, SiO2 covered the pores on the known porous structure of HA and slowed down the access of corrosive products to the base material and also the separation of these products from the surface. It could be determined from such results that addition of SiO2 was coated with a low-rate porous protective oxide film which was also more stable on the surface. It could be stated that the surface oxide film of S2 sample with similar Eocp ( 226 mV) and Ecorr ( 227 mV) values was more stable when compared to the others. Table 5 shows corrosion rate and polarization resistance values of the bioceramic coatings. Addition of SiO2 applied to the HA coating increased corrosion resistance and consequently decreased the corrosion rate. The reason was associated with the fact that addition of SiO2 applied to the HA coatings decreased the pore density on the surface morphology. The fact that addition of SiO2 decreased the pore density in the HA coating was also supported by the SEM examinations. Porosities observed on bioceramic coatings (Fig. 1) are generally the zones where electrolyte (Ringer solution) is still and in time, the internal sides of these porosities could be covered by the oxide-structured corrosion products. If amount of Cl ions contained by the oxide products increases compared to the other zones of the implant, these regions may become localized regions where oxide film dissolution and surface conductivity increase. In other words, these porosities may also act as the preferred corrosion regions. The microstructural examinations performed after corrosion in HA-based composite coatings showed that localized corrosion distribution in the coatings were not homogeneous throughout the surface. This situation showed the formation of ever-changing micro local batteries on the surface, accordingly, corrosion tracks were observed at different rates and sizes at different locations of the surface. All these dynamic changes appeared with the increase in the rate of change and data dispersion in the electrochemical data (Table 4). This dispersion ratio gradually decreased as a result of the decrease in the void ratio within the coating. The decrease observed in the corrosion current densities in S2 sample supported this result.
5. Conclusions In both coatings, porous surface morphology with low-crack density was achieved, which is important for osteointegration. Addition of SiO2 to HA decreased the pore density, however, increased the corrosion resistance of the coating. It was observed that the samples only coated with HA did not increase the corrosion resistance of REX-734 alloy; however, corrosion rate decreased and polarization resistance increased as a result of
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addition of SiO2 to HA. Also the localized corrosion on the surface caused formation of dimple in the coatings. Hardness values in HA and HA–SiO2 coatings having similar coating thickness values gave similar results. It was observed that addition of SiO2 did not cause a significant change in adhesion resistance.
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Please cite this article as: Y. Say, et al., Effect of hydroxyapatite/SiO2 hybride coatings on surface morphology and corrosion resistance of REX-734 alloy, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.127i