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Effect of Sintering Temperature in Physical-Mechanical Behaviour and in TitaniumHydroxyapatite Composite Sinterability W.R.Weinand&, F.F.R. Gonçalves* and W.M. Lima& &
Physics Department; *Course in Mechanical Engineering/ State University of Maringá. Av. Colombo, 5970 – CEP: 87020-900 - Maringá PR/ Brazil;Phone: 00 55 44 261 4330.
[email protected] and
[email protected] Key words: composite, biomaterial, sinterability, physical-mechanical property. Abstract. Mechanical alloying (MA) has been successfully used to produce alloys and composites with a high homogeneity degree. In current research, titanium (Ti) powder was mixed with 40, 50% volume of hydroxyapatite (HAp). MA was performed without atmosphere control, at room temperature, for 4.5 hours of milling time, at rotation speed of 300 rpm. Samples of material were compacted in cylindrical form at 350 MPa and sintered in 2.0 flux air (l/min) at 1000, 1100 and 1200oC during 1 hr. The material’s morphological and microstructural characterization, in powder form and in sintered material, was performed by scanning electronic microscope and X-ray diffractometry. Thermal treatments revealed that sintering temperature affects the microstructure, microhardness and the composition of the composites evaluated by EDS. Introduction Crystalline salts deposited in the bone’s organic matrix are mainly composed of calcium and phosphorus, with hydroxyapatite (HAp: Ca10(PO4)6.(OH)2) as predominant crystalline salt. However, synthetic and natural hydroxyapatite has low fracture toughness due to their brittleness and low mechanical resistance. This factor is an obstacle in implants that must support high loads [1, 2]. Since FeCr alloy fibers have been used to reinforce the hydroxyapatite to solve this problem, reinforcement produced a high fracture toughness of Hydroxyapatite [3]. Results encouraged other researchers to use 316L stainless steel fibers. However, a good densification of the composite was impossible when it underwent thermal treatment in air. This was due to the fact that whereas steel fibers oxidized, high temperature sintering was ineffective too, since HAp phases decomposed [4]. Calcium-stabilized zirconium was added to hydroxyapatite in another attempt. However, this method needed an additional process to produce the composite [5, 6]. On the other hand, Kokubo et al. [7, 8] deployed massive titanium, whose external surface consisted of a bioactive surface gotten by titanium immersion in a NaOH solution and later thermally treated. Other processes are being searched to improve the biocompatible materials’ resistance mechanics. The process of covering metallic surfaces with hydroxyapatite is one of the mostly used processes, whereas the commonest techniques are electrochemical deposition, electrophoreses deposition, ionic implantation (sputtering) and plasma spray. The plasma spray technology is commonly deployed but serious problems arise when it has to be applied uniformly in material with complex geometry. Moreover, due to the temperature’s uncertain precise control during deposition, thermal decomposition, functional group OH- loss and hyidroxyapatite transformation may occur [9 - 13]. Another process under analysis consists of the production of composite using powder metallurgy. In this technique, original materials are in powder form and mixed in diverse ratios. The mixture may simply be homogenized or milled in a high-energy mill. Afterwards, it will be undergo sintering process [1416].
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Current study describes the preparation and the characterization, evaluates the possible sintering temperature effect on sinterability, microstructure and physical-mechanical properties of the titanium-hydroxyapatite composites obtained by mechanical alloying (MA), followed by sintering. The physical-mechanical characterization will be evaluated according pre-established norms [18, 19] and microstructural characteristics according to scanning electronic microscopy (SEM) coupled to a semi-quantitative chemical analysis (EDS). Experimental methods Flake-shaped Titanium (Ti) produced by CTA (ITA/Br), milled for size reduction, and hydroxyapatite powder (HAp - DFI/UEM-Br), obtained by fish bone calcination at 900ºC, for 8 hours, ground in a high energy mill, for 8 hours [17], were deployed as precursory items. These less than 37µm particles were respectively mixed at ratios 60/40 (TH4) and 50/50 (TH5) % in volume. After homogenization, mixtures TH4 and TH5 underwent a grounding process in a Fritsch Planetary Mono Mill (Pulverisetti 6), for 4.5 hours. During milling, a 6/1 ball/mass ratio was employed at 300rpm. TH4 and TH5 were uniaxially compacted at 350MPa after the milling process. Samples with 10mm diameter and 2mm thick were produced. Compacted materials were sintered in a tubular furnace, at room atmosphere, at 1000ºC, 1100ºC and 1200ºC for 1 hour, at 5oC/min heating rate. The morphological and the microstructural characterization of the material, in powder form and in sintered material, were carried by X-ray diffractometry techniques and by scanning electronic microscopy (SEM). The composites’ chemical composition was obtained by semi-quantitative chemical analysis (EDS). Density and microhardness were evaluated by Standard MPIF 42 [17] and ASTM 20 E-384-89 [18], respectively. Vicker microhardness measures were undertaken by Microhardness Tester Leica VMHT – MOT, with a 200gf load for a 15-min loading time at room temperature. In each composite five indents were done. Results and discussion
ο (012)
♦(222)
♦(213)
♦ HAp ο Ti
ο (002) ο (011)
♦(211) ♦(112) ♦(300) ♦(202) ο (010)
♦(002)
HAp 50%
♦(210)
After milled for 4.5 hours, precursory elements and composites TH4 and TH5 in powder form were analyzed by X-ray diffractometry. Figure 1 shows the results. The crystallographic planes of the precursory elements are kept in the two composites, mainly TH5, in which relative peaks to hydroxyapatite planes are more intense than those in TH4. Such behaviour is probably due to TH5 with its greater hydroxyapatite amount than TH4’s. However, in both composites, the width at half the Hydroxyapatite peak height is slightly bigger when compared to reference Hydroxyapatite. On the other hand, possible reactions of solid state due to milling process failed to modify the Hydroxyapatite structure.
Intensity (A. U.)
Coordenação
HAp 40%
HAp
Ti
20
30
40
50
60
2θ (Degrees)
Fig.1. Representative XRD patterns of the precursory elements and of the two composites.
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Figures 2 and 3 show respectively TH4 and TH5 microstructures from scanning electron microscopy (SEM). Microphotographs show three distinct morphological regions. Dark regions are high oxygen concentrations with probable formation of TiO2. These regions also show the presence of phosphorus, calcium and small traces of sodium and magnesium. Titanium predominates in the gray region, whereas oxygen, phosphorus and calcium are detected too at a lesser ratio. The shining phase corresponds to the presence of high titanium concentration. Generally, there is a good distribution of HAp and titanium components due to processes during the composites’ milling and sintering. In Figures 2a and 2c, or rather, sintered TH4 at 1000oC and 1200oC, certain points were selected in which semi-quantitative chemical analysis for EDS was undertaken. Table 1 shows results of this analysis.
(a) (b) Fig.2. Evolution of TH4 microstructure as a function of sintering temperature : (a) 1000oC; (b) 1100oC; (c) 1200oC. Table1. Distribution of elements in sintered TH4 at selected points. Sintering temperature 1000oC. Points Elements and weight concentration ( % ). analysed O Na Mg P Ca 1 26.92 0.40 3.47 10.87 2 15.46 0.19 3.21 6.30 3 22.56 0.34 1.03 4 9.43 0.78 3.63 5 24.99 0.64 4.36 6 14.09 0.77 1.99 7 20.06 0.27 1.67 3.84 8 36.46 0.43 0.28 2.24 4.06 o Sintering temperature 1200 C. 1 3.15 3.75 23.95 2 4.23 8.51 34.91 3 13.91 0.16 0.59 1.36 4 27.02 0.89 2.04 5 29.87 0.68 1.70 6 28.72 4.68 7.43 7 16.59 0.68 5.63 18.88
(c)
Ti 58.34 74.83 76.06 86.15 70.01 83.16 74.16 56.54 69.15 52.36 83.97 70.06 67.75 59.17 58.22
The selected dark points show the predominance of titanium and oxygen in sintered TH4 at 1000oC. A slight alteration occurred in its behaviour when sintered at 1200oC. In this case, a higher amount of calcium may be observed in TH4’s dark region sintered at 1000oC. In the gray regions of
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TH4 sintered at 1200oC oxygen and titanium are predominant with low contents of phosphorus and calcium, whereas sodium and magnesium do not exceed 0.68%. The chemical composition of both composites was evaluated through a semi-quantitative chemical general analysis (EDS). Figure 3 represents microstructure evolution of TH5 as a function of sintering temperature. TH5 microstructure in Figures 3a and 3b is similar to that in TH4. Table 2 shows results of semi-quantitative chemical general analysis of Ti-HAp by EDS. Sintering temperature increase, mainly in TH4 at 1200oC, caused a double amount of oxygen than that at a temperature of 1000oC. TH5 does not show such behaviour and oxygen level remains practically constant. The analysis indicates the presence of Mg and Na. However, Na is only observed in TH4 when sintered at 1200oC. These elements in the composites under analysis are due to HAp, as detected in the analyses of atomic absorption by Lima et al [19].
(a) (b) (c) Fig.3. Evolution of TH5 microstructure as a function of sintering temperature: (a)1000oC; (b) 1100oC; (c) 1200oC. Table 2. Semi-quantitative chemical general analysis of Ti-HAp by EDS. Composition (wt %) Sintering Materials Temperature Ti Ca P O 1000ºC 70.86 10.14 4.48 14.11 THS4 1100ºC 71.13 10.38 4.45 13.58 1200ºC 56.29 7.36 3.17 32.37 1000ºC 55.05 11.90 4.80 27.79 THS5 1100ºC 57.01 11.67 5.03 25.81 1200ºC 55.54 12.59 5.21 26.66
Mg 0.40 0.46 0.41 0.46 0.48 -
Na 0.41 -
Density and microhardness have been evaluated so that the composites’ microstructure might be correlated to their physical and mechanical behaviour. Figure 4a shows TH4’s and TH5’s densities. TH4 presents a density decrease due to sintering temperature. This behaviour may be explained by taking into account the semi-quantitative chemical general analysis of the composite in Table 3, in which increase of sintering temperature causes a significant variation of titanium, oxygen and calcium concentration. Moreover, in the case of high oxygen levels a density reduction occurs in TH4 and in TH5. On the other hand, TH5 density alone is affected by oxygen concentration, since other elements show only slight variations with sintering temperature. In addition, sintering temperature provides a development in porosity, which affects the composites’ physical and mechanical properties. At sintering temperature 1000oC, the semi-quantitative chemical general analysis reveals that oxygen amount in TH5 is approximately double that in TH4. Associated to actual porosity, this fact lessens TH5 density. In other sintering temperatures, TH5 density is slightly higher than that of TH4. In these temperatures, a higher sinterability of the composite occurs with slight oxygen level variation. Moreover, for the three temperatures, all TH5 components fail to present significant concentration variations.
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Figure 4b shows relative results Vicker microhardness due to sintering temperature for TH4 and TH5. Predominance of titanium amount in TH4 provides a greater microhardness than the TH5 one. However, microhardness reduction may be observed in both composites due to sintering temperature. Such behaviour may be correlated to high oxygen concentration and to the porosity produced in sintering. A greater microhardness of TH4 when compared to that in TH5 may be due to a higher amount of titanium used in the preparation of the composites. 2,1 2,87
Composites THS4
Microhardness (GPa)
Density (g/cm3)
2,86 2,85 2,84 2,83 2,82 2,81 2,80 1000
1100
1200 o
Temperature ( C)
Composites: THS4
2,0
THS5
THS5
1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1000
1100
1200
Temperature (ºC)
(a) (b) Fig. 4. (a) Density of composites TH4 and TH5 due to sintering temperature. (b) Microhardness of composites TH4 and TH5 due to sintering temperature. Conclusion Results and analyses carried in current research lead to conclusions related to the composites’ microstructure and to the influence of sintering temperature in physical and mechanical properties. • The milling process produces a reduction in peak intensity of HAp components without modifying the crystalline structure of the components used in the composites’ production. • Sintering temperatures alter the composites’ chemical composition and provide an approximately 46% oxygen variation in sintered TH4 at 1000oC, mainly in its dark regions. Moreover, sintering at 1200oC produces an increase in the amount of calcium in these regions. Abrupt changes in chemical composition have not been observed in TH5. Further, a slight reduction in the concentration levels of the diverse elements occurs at the points of possible porosity that constitute the composite. Oxygen levels are practically constant at all sintering temperatures. • Since sintering temperatures affect the morphology of porosity and the distribution of oxygen levels, density is reduced in TH4 and in TH5. The density of TH5 alone is affected by oxygen concentration since all other elements show very slight variations with sintering temperature. • The predominance of the amount of titanium in TH4 provides a greater microhardness than TH5’s. However, an identical trend for both composites has been observed, due to sintering temperature. An increase in sintering temperature reduces microhardness. Acknowledgements The authors would like to thank Prof. Dr. Eduardo Radovanovic of the Scanning Electronic Microscopy Lab /UEM and to Prof. Dr. Antonio Carlos Saraiva da Costa of the X-ray Diffractometry Lab of the Department of Agronomy /UEM for their invaluable service in carry out
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the various calculations contained in this research; to Prof. Dr. Jose Sasaki of the Department of Physics/UFC for suggestions in the interpretation of Diffractometry data. References [1] Rao, R.R., Roopo, H. N., Khannan, T. S., J. Mater. Sci. Mater. Med. 8 9 (1997) 511. [2] Klein, C.P.A.T., Patka, P., Vander Lubbe, H.B.M., Wolke, C.G., De Groot, K. “Plasma-Sprayed coatings of tetracalcium phosphate, hydroxyapatite, and α-TCP on titanium alloy: an interface study”. J. Biomed Matter Res 1991; 25:S53. [3] De Groot, K., De Putter, C., Diessen, H. “Mechanical failure of artificial teeth made of dense calcium hydroxyapatite”. Sci Ceram 1981;11; 433-437. [4] Miao, X. “Observation of microcracks formed in HA-316L composites”, Materials Letters, 57, (2003), 1848-1853. [5] Kassuga, T., Yoshida, M., Ikushima, A., Tuchiya, M., Kusakari, H. “Stability of zirconia toughened bioactive glass-ceramics: in vivo study using dogs”. J. Mater. Sci: Mater Med 1993; 4: 36-39. [6] Takaqi, N., Mochida, M., Uchida, N., Saito, K., Uematsu, K. “Filter cake forming and a hot isostatic pressing for TZP-dispersed hydroxyapatite composite”, J.Mater Sci:Mater Med 1992; 3: 199-203. [7] Kokubo, T., Miyaji, F., Kin, H.M., Nakamura, T. “Spontaneous formation of bonelike apatite layer on chemically treated titanium metals”. J. Am Ceram Soc 1996, 79: 1127-1129. [8] Kin, H.M., Miyaji, F., Kokubo, T., Nakamura, T. “Preparation of bioactive Ti and its alloys via simple chemical surface treatment”. J. Biomed. Mater Res. 1996; 32: 409-417. [9] Sridhar, T.M; Mudali, K.U e Subbaiyan M. “Sintering atmosphere and temperature on hydroxyapatite coated type 316L stainless steel”. Corrosion Science, v. 45, p. 2337-2359, 2003. [10] Andrade, M. C; Filgueiras M. R. T. e Ogasawara; T. “Hydrothermal Nucleation of Hydroxyapatite on Titanium surface”. Journal of the European Ceramic Society, v.22, p. 505510, 2002. [11] Xiong, L. e Leng, Y. “TEM study of calcium phosphate precipitation on bioactive titanium surfaces”. Biomaterials, Article in press, 2003. [12] Fu, L; Khor, K e Lim, J.P. “Processing, microstructure and mechanical properties of ytria stabilized zirconia reinforced hydroxyapatite coatings”. Materials Science Engineering A, v. A316, p. 46-51, 2001. [13] Wang, J; Layrolle, P; Stigter, M e De Groot, K. “Biomimetic and aletrolytic calcium phosphate coatings on titanium alloy: Physicochemical characteristics and cell attachament”. Biomaterials, v. 25, p. 583-592, 2003. [14] Ning, C.Q. e Zhou, Y. “In vitro bioactivity of a biocomposite fabricated from HA and Ti powders by metallurgy method”. Biomaterials, v. 23, p. 2909-2915, 2001. [15] Chenglin, C; Jingchuan, Z. e Shidong, W. “Hydroxyapatite-Ti functionally graded biomaterial fabricated by powder metallurgy”. Mat. Sci. Engineering A, v. A271, p. 95-100, 1999. [16] Yang; Y; Kim; K.H; Agrawal, C.M. e Ong, L. J. “Interaction of hydroxyapatite-titanium at elevated temperature in vacuum environment. Biomaterials”, Article in press, 2003. [17] Determination of Density of Compacted: MPIF standard no 42, 1986. [18] Standard Test Method for Micro hardness of Materials. Current edition approved Oct. 1989. Published April 1999. ASTM Committee E-4 on Metallographic and subcommittee E04.05 on Micro hardness. [19] Lima,W. M; Weinand, R.W; Santos, O.A.A. Santos; Paesano.Jr, A. e Ortega, F. H.M.O. “Effect of the Calcination Time of Fish Bone in the Synthesis of Hydroxyapatite”. In: Fourth International Latin-American Conference on Powder Technology – PTECH 2003, 2003, Guarujá, Brazil. Proceedings PTECH 2003. p. 1238 - 1243.
