Tribological Characteristics of Al2O3 Nanocomposites for Joint

Journal of Biomechanical Science and Engineering

Vol. 3, No. 3, 2008

Tribological Characteristics of Al2O3 Nanocomposites for Joint Prostheses*

Carlos MORILLO**, Yoshinori SAWAE***and Teruo MURAKAMI*** ** Department of Intelligent Machinery and Systems, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, Japan E-mail:[email protected] *** Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku , Fukuoka, Japan

Abstract In this study the tribological characteristics of a sliding pair of Al2O3 nanocomposites against Al2O3 were investigated. Al2O3 nanocomposites are proposed as a candidate material for the fabrication of prostheses. Nanopowders of Al2O3 (AKP 50, 300 nm), Y-TZP; tetragonal zirconia polycrystal stabilized with 8mol % of yttria (TZ-8YS, 100 nm) and TiO2 (PS-25, 50 nm) were mixed and hot pressed. A ball-on-plate tribometer was used for the wear test. The nanocomposite plate specimen was rubbed against the Al2O3 ball at a frequency of 1 Hz for 1 h, with a load of 49 N. Distilled water and fetal bovine serum solution (FBSS) were used as lubricants. Mechanical properties were estimated using the indentation method. It was found that the specific wear rate of Al2O3 nanocomposites was about 2–6x10-8 mm3/Nm and the coefficient of friction from 0.3 to 0.5 for FBSS. The nanocomposites containing 15mol% Y-TZP (ATZ150) had the lowest wear rate (1.77x10-8 mm3/Nm) for FBSS. Worn surfaces were observed using SEM. Furthermore, AFM and XPS were used to study the effect of the serum lubricant on the articulating surface of the Al2O3 nanocomposites, and AlPO4 was found on the wear track of the samples tested in FBSS. Key words: Biotribology, Nanocomposite Ceramics, Hip Prostheses, Surface Analysis.

1. Introduction High-density, high-purity (>99.5%) Al2O3 was the first bioceramic widely used clinically for its properties such as excellent corrosion and wear resistance, good biocompatibility, low friction, and high mechanical strength (1). In the case of alumina-on-alumina, hip joints are characterized by full fluid film lubrication with good machining, good fit and proper lubricant (2). The main problem of alumina is its relatively low fracture toughness, which makes it prone to catastrophic failure.

*Received 28 May, 2008 (No. 08-0390) [DOI: 10.1299/jbse.3.356]

Ceramic nanocomposites are defined as ceramic composites with more than one solid phase, in which at least one of the phases has dimensions in the nanoscale range (50–100 nm). The drive toward the use of nanoscale ceramics and ceramic nanocomposites is the improvement in properties such as strength, hardness, and wear resistance that they can offer. Whereas several studies have shown that the mechanical properties of conventional ceramics, such as strength, hardness, and wear resistance can be improved by reduction in grain size, particularly to the nanoscale dimension, a greater improvement can be achieved by the use

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of ceramic nanocomposites. Some prototype nanocomposite materials suitable for prosthetic orthopedic bearings are described below (3). An Al2O3/TiO2 nanocomposite with high toughness, high hardness, and high wear resistance was reported recently for potential use as the bearings in THA and TKA (4). The materials consisted of a fine-grained Al2O3 matrix containing 0–25 mol% nanosize TiO2 particles (50 nm). The optimum properties were obtained for composites containing 10 mol% TiO2 nanoparticles. Nanocomposites with this composition had a fracture toughness of 3.7 MPam1/2, which was 25% higher than that for the unreinforced Al2O3 matrix, and a Vickers hardness of 13 GPa, which was lower than the value of 17.5% measured for the Al2O3. However, wear testing, carried out with a ball-on-disk tribometer, using Si3N4 balls and distilled water at 37ºC for 100 h, showed that the wear volume of the Al2O3/TiO2 nanocomposite was approximately five times lower than that for the Al2O3, and the specific wear rate around 10-4 mm3/Nm. In order to improve the mechanical properties, specifically the fracture toughness; 0-20 mol% Y-TZP was added to the composition of Al2O3/10mol%TiO2, the maximum fracture toughness achieved was 3.31 MPam½, for a composition of 15mol%. But in this case the environment used was air and the specific wear rate was also around 10-4 mm3/Nm (5). Since 1997 Niihara has studied the mechanical properties of Al2O3 nanocomposites with the addition of 12mol%Ce-TZP (6), (10-12)mol%Ce-TZP (7) and (0-2) mol%TiO2; at that moment a high mechanical strength was achieved. Tanaka et al (8) proposed the same Ce-TZP/Al2O3 nanocomposites with small amounts of TiO2 as a candidate material of hip joint, but it has not yet been used clinically, and studies are ongoing. Modern Al2O3 bearings are suggested as safe and reliable if used with THA components with proven design and durability, the risk of failure is still unacceptably high, and so understanding the origins and mechanisms of failure, as well as the prevention of failure is important. Failure of ceramic bearings in vivo commonly results from slow crack growth under the static or repetitive loading experienced in the body, until fracture occurs. This can be understood as a corrosion-assisted crack propagation process that leads to a loss of strength as the time of loading increases (3). Wear is the removal of material, with the generation of wear particles that occurs as a result of the relative motion of two opposing surfaces under load. Although the mechanical consequences of wear, such as the progressive thinning of the UHMWPE acetabular or tibial liner, can limit the functional life of a prosthetic joint replacement, the clinical problems are more often due to implant loosening caused by the release of an excessive number of wear particles into the biological environment (3). In this work TiO2 was added because fully stoichiometric or oxygen-deficient rutile shows sensitivity in the shear strength, when it is free of pore and the shear strength around 20 MPa (9). The addition of TiO2 makes it a self-lubricating triboceramic useful for extreme environmental applications. In the case of Y-TZP it is widely known that the addition of tetragonal zirconia stabilized with rare earths can enhance the mechanical properties of an alumina matrix (3). For these reasons the purpose of this work is to fabricate Al2O3 nanocomposites with the addition of TiO2 and Y-TZP, to obtain high wear resistance, and low friction materials for hip prostheses and to analyze the phenomena occurring on the surface tested in different lubricants. The amount of Y-TZP was varied to study the influence on the tribological behavior and find the optimum composition which has higher tribological properties. This

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material did not show an increase in fracture toughness in an earlier work (5), but exhibited higher wear resistance. Wear tests in biological environment should be carried out. This nanocomposite is a promising candidate material, for long lifetime prosthesis, giving the patient a better quality of life. To explain the phenomenon of wear, techniques as Scanning Electron Microscopy (SEM), Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectrometry (XPS) were used.

2. Materials 2.1 Sample preparation Al2O3 (AKP-50,300nm, Sumitomo Chemical Co., LTD, Japan), TiO2 (Aeroxide, P25, 21nm, Degussa, Germany) and Y-TZP (TZ-8YS, 100nm, Tosoh, Japan) were used as starting powders; the mixture has a composition of 10 mol% of TiO2; 0, 2.5, 5, 7.5, 15 and 20 mol% of Y-TZP (8mol% Y2O3) and Al2O3 as the remainder as shown in Table 1. Wet milling of starting powders was carried out in a polyethylene jar with alumina balls and ethanol for 24 h. Ethanol was extracted using a rotary evaporator after dried in a stove. The mixture was dry-milled again for 12 h in a polyethylene jar. The powder mixtures were compacted in a graphite sleeve coated with boron nitride, and hot pressed at 1500º C, 15 MPa, for 1 hour in vacuum. After sintering samples were metallographically prepared, they were cut and polished until reaching a mirror-like surface (Ra = 0.02 – 0.03 μm), using diamond disks and slurries of 9, 3 and 1 μm. Nanocomposite samples were used as plate specimen in wear test.

Table 1 Composition of the nanocomposites Sample AT ATZ25 ATZ50 ATZ75 ATZ150 ATZ200

TiO2 (mol%) 10 10 10 10 10 10

ZrO2 (mol%) 0 2.5 5 7.5 15 20

Al2O3 (mol%) 90 87.5 85 82.5 75 70

2.2 Wear test Wear tests were carried out in a reciprocating ball-on-plate tribometer, for 4 h, at a sliding speed of 20 mm/s, with a sliding distance 288 m, a load of 49 N and a frequency of 1 Hz. Counterfaces used were alumina balls of ¼ inch diameter. Distilled water and fetal bovine serum solution (FBSS) diluted in water at 30vol % with sodium azide were the lubricants used in wear tests. The wear test of each nanocomposite was iterated three times. In order to measure the coefficient of friction a load cell was used. A profilometer was used to read the wear track depth and a planimeter to measure the wear area. From this data wear volume was calculated. Specific wear rate (SWR) was obtained from formula 1 1 where w is wear volume, P the load and S the sliding distance.

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2.3 Mechanical properties, characterization and surface analysis For the evaluation of the mechanical properties the indentation method was used. The crack length was measured using an optical microscope. Fracture toughness was estimated using the following formula (10),

0.0074

2

where c is the average crack length (mm) from the center of the indentation and P is the applied indentation load (N) . The applied load was 98 N for 15 s. Crystalline phases of compounds were determined by X-ray diffraction analysis with CuKα from 5 to 90º at 40kVA, 40 mA at 5º/min using bulk specimens of every sample. Phases were determined with a PCPDFWIN program, and the ICDD data base. The worn surfaces were coated with Pt and observed in a Scanning Electron Microscope. Surface analysis was carried out using Atomic Force Microscope (Nanoscope IIIa, Dimension 3000, US). The imaging was performed in Tapping Mode using commercial tapping mode with etched silicon probes. The measurements were performed under ambient conditions. For XPS, spectrometer samples were analyzed in ultra high vacuum. The sample transfer was made in air. Samples were analyzed at room temperature.

3. Results 3.1 Material properties of nanocomposites Figure 1 shows diffraction patterns of AT and ATZ25 samples. α-Al2O3, Ti2O3 and Zr5Ti7O24 were the phases determined.

Figure 1 X-ray diffraction pattern of a) AT and b) ATZ25 samples.

One phase that would be present is rutile, because at temperatures between 400-1200°C, anatase begins to transform into rutile (11) (12); nevertheless, the phase present is Ti2O3, possibly because TiO2 was reduced during the sintering process.

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Figure 2 X-ray diffraction patterns of c) ATZ50 and d) ATZ75 samples. Figures 2 and 3 show the same phases but with an increase of the intensity peak corresponding to Zr5Ti7O24; indicating an increase in ZrO2 content.

Figure 3 X-ray diffraction patterns of e) ATZ150 and f) ATZ200 samples.

Table 2 Mechanical properties of alumina nanocomposites with different amount of zirconia. Samples AT ATZ25 ATZ50 ATZ75 ATZ150 ATZ200

Y-TZP (mol %) 0 2.5 5 7.5 15 20

Vickers Hardness (HV) 1972 ±176 2058 ± 200 1902 ± 104 1849 ± 117 1943 ± 151 1844 ± 140

Fracture toughness (MPam½) 3.30 ± 0.56 3.43 ± 0.55 2.96 ± 0.32 3.69 ± 0.40 2.83 ± 0.35 3.37 ± 0.38

The values of the Vickers Hardness are around to 2000 HV, these values are higher than the correspondent to zirconia-alumina composites, but the values of fracture toughness are a little bit lower (4). 3.2 Frictional characteristics of nanocomposites In Figures 4 and 5 the coefficients of friction of Al2O3 nanocomposites with different amounts of zirconia are shown, in distilled water and FBSS, respectively. Final values are

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approximately 0.3 and 0.5 for FBSS and approximately 0.4 to 0.7 for distilled water. The sample that had the lowest coefficient of friction for both lubricants was 15mol% of Y-TZP (ATZ150).

Coefficient of friction

0.8

0.6

0.4 AT ATZ25 ATZ50 ATZ75 ATZ150 ATZ200

0.2

0.0

0

2000

4000

6000

8000

10000 12000 14000

Cycles

Figure 4 Coefficient of friction of alumina nanocomposites with different amount of zirconia against alumina in distilled water.

0.8

Coefficient of friction

0.7 0.6 0.5 0.4 AT ATZ25 ATZ50 ATZ75 ATZ150 ATZ200

0.3 0.2 0.1 0.0 0

2000

4000

6000

8000

10000 12000 14000

Cycles

Figure 5 Coefficient of friction of alumina nanocomposites with different amount of Y-TZP against alumina in FBSS.

In Figure 5 it can be observed that in the first stage a phenomenon denominated start-up state (2). After approximately 6000 cycles, the coefficients of friction stabilize reaching a steady state; this trend is thought to be due to running-in. 3.3 Wear of nanocomposites Figure 6 shows the variation of the specific wear rate of the samples with different amounts of Y-TZP. In the samples AT, ATZ75, ATZ150 ATZ200, there is a reduction of the specific wear rate in the presence of FBSS, and Al2O3 nanocomposites with an appropriate amount of Y-TZP (ATZ150) have the lowest specific wear rate (1.77x10-8 mm3/Nm) for FBSS, as shown in Figure 6. In Figures 7 and 8 SEM images of the worn surfaces of the alumina nanocomposites are shown, which explain the wear mechanisms. Wear of alumina usually suggests brittle fracture by crack propagation. Some of the worn surfaces tested in distilled water like AT, ATZ25, ATZ50 and ATZ200 showed a more roughened surface, evidencing severe wear; some grains were taken out and reveal severe damage throughout the sliding direction (14) with wavy wear scars; evidencing adhesive wear (15). ATZ75 showed the highest wear but

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exhibited a relatively smooth surface which might have been improved by running in process after initial severe rubbing with large friction variation shown in Figure 4. ATZ150 in water showed a polished surface.

Water FBSS

2.0e-7

3

Specific Wear Rate (mm /N*m)

2.5e-7

1.5e-7

1.0e-7

5.0e-8

0.0 AT

ATZ25

ATZ50

ATZ75

ATZ150

ATZ200

Samples

Figure 6 Specific wear rates of alumina nanocomposites against alumina in different media. Bar represents average and error bar represents standard deviation.

Figure 7 SEM images of worn surfaces of alumina nanocomposites with different amount of zirconia a) 0 mol%; b) 2,5mol%, c) 5mol%; d) 7,5mol%, e) 15mol% and f) 20mol% tested against alumina in distilled water. (Sliding direction) SEM images of worn surfaces tested in FBSS (Fig 8a-d) showed less roughened surfaces than in water. There are indications of mild plastic flow, grain boundary relief, pits and microcracks along the grain boundaries (16). In Figure 8d a very smooth surface is shown and remains of the original microstructure are present. In contrast, wavy wear scars are present in the sample ATZ150, which indicates slight adhesive wear and plastic deformation in the articulating surface, but showed minimum wear. The surface of ATZ200 exhibits roughened features.

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Figure 8 SEM images of worn surfaces of alumina nanocomposites with different amount of zirconia a) 0 mol%; b) 2,5mol%, c) 5mol%; d) 7,5mol%, e) 15mol% and f) 20mol% tested against alumina tested in FBSS. (Sliding direction) Samples with more than 7.5 mol% of Y-TZP in serum solution show a decrease in the specific wear rate, for that reason an analysis in the surface has been carried out to understand the phenomena occurred on the surface. FESEM images of worn surfaces of ATZ75 samples at higher magnification (Figures 9a and 9b) were compared to describe the wear mechanism in detail, since the effect of test lubricant was the most significant for ATZ75. The worn surface tested in distilled water exhibit severe wear, where it can be seen that the material had been pulled-out from grain boundaries; in contrast, a polished surface with few small cracks is characteristic of the sample in FBSS.

Figure 9 FESEM images of worn surfaces of Al2O3 nanocomposites (ATZ75) a) in water and b) in bovine serum

AFM worn surface images of flat areas of ATZ75 samples in distilled water and in FBSS, were compared in Figure 10. In flat areas there are not large differences.

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Figuree 10 AFM images of worn surfaces of Al2O3 nanocom mposites (ATZ75) against alumina a) in water and b) in a bovine serum m solution.

Figuure 11 XPS pattern of alumina nanocomposites wear trrack in a) distilled water and b) bovine fetal serum solutionn. In Figgure 11 XPS patterns of Al 2p3/2 from wear tracks of ATZ75 sample tested in distilled water and FBSS are shown. It can be observed that peaks of o AlO(OH) appeared in the wear tracks of samples tested in distilled water, but the peak of AlPO4 appeared with a higher intensity in wear track of the sample tested in FBSS. These peaks were detected in the curve fittingg calculation mode. Aluminun hydroxide (AlO(OH))) was reported as a product of a

tribocchemical reaction on alumina surface in water enviroonment (17).

4. Disscussion Two types t of lubricating media were chosen in this workk, i.e. distilled water and FBSS dilutedd at 30vol %. It can be observed (Figure 5) that the cooefficient of friction decreases in the preesence of fetal bovine serum solution for most of the nanocomposites. n The decrease of frictioon appears to be due to the existence of the proteinns and lipids in the fetal bovine serum m solution that produces a better mixed lubrication in thhe sliding wear test. Furtheermore, the AlPO4 peak that was found in the XPS patttern of the sample ATZ75 tested in FBSS can have an influence on the tribological behavvior of alumina nanocomposites. Rabinnowicz (18) explained that phosphate anodized films can prevent the galling. Also a mixturre of AlPO4, BaF2 and CaF2 had been proposed as a solid, self lubricating bearing materiial (19). But, further research is needed to understand the t role of AlPO4 as a lubricious film inn biological environments. As shown in Figure 6, thee specific wear rate decreases for most of o nanocomposites in the presence of FBSS, except inn the case of ATZ25 and ATZ50 samples, in which there is an increase in wear in FBSS even e with a lower coefficient of frictioon. The lower friction in FBSS is thought to be caussed by the presence of AlPO4, in

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addition to protein adsorbed films. It is well known that the addition of nanometer particles to ceramic matrices enhances the mechanical properties (20). Lee et al (4)(5) have found that these materials present better tribological properties, higher wear resistance and low friction properties when mechanical properties were enhanced with the increase in the amount of alloy elements or dopants. Nanocomposites with more than 7.5 mol % of Y-TZP show a lower specific wear rate in FBSS than in water. It can be observed that the specific wear rate in Figure 6 of the samples ATZ25 and ATZ50, revealed higher average value of the specific wear rate in the presence of FBSS. However, it was not significant. The changes in surface properties exerted during the articulation may be responsible for the enhancement of the running in process and reduction of friction in FBSS. In earlier works (19) it had been found that an increase in wear of Al2O3 against Al2O3 in FBSS is due to the prevention of a hydrated film but wear of Y-TZP against Al2O3 was reduced when fetal bovine serum solution is present. The Al2O3 nanocomposites containing Y-TZP are expected that lower wear is maintained in the presence of serum constituents. Also, the reduction in the wear rate on alumina nanocomposites/alumina compared with alumina/alumina (19) is due to different effects of the nanophase (21). First, the matrix grain size of the alumina nanocomposites (5) is smaller than the monolithic alumina. Second, grain boundary fracture is reduced (21). Nevertheless, it can be observed that on the worn surface of ATZ75 sample tested in water, material had been pulled–out from the grain boundaries. However in serum solution a mild worn surface was observed. SEM and FESEM images of worn surfaces confirm that the wear mechanism differs according to the lubricants. In the case of samples tested in distilled water, the samples AT and ATZ50 presented an area where the material as a large grain size has been removed due to the wear process, and in samples with more than 7.5 mol% of Y-TZP, the surfaces are smoother. In contrast, for the samples tested in FBSS, the roughening was suppressed compared with water lubrication, since the reaction between proteins and the surface reduce the sensitivity of the shear strength restraining the pulling-out of material from the grain boundaries as shown in Figure 9b. A clear difference between worn surfaces of the Figures 9a and 9b is observed, and it seems that the wear mechanism in water is containing brittle and large scale fracture within grain boundaries, although the microscopic adhesive wear within grains is predominant in FBSS. In this work the sample with 15 mol% of Y-TZP showed the best tribological behavior, with the lowest coefficient of friction and specific wear rate in FBSS. For nanocomposites with 20 mol% of Y-TZP, the rubbing surfaces in both water and FSBB became rougher than 15 mol%. Excessive addition of Y-TZP appears to reduce thermal conductivity, which might enhance temperature increase. In order to clarify the wear mechanism of this nanocomposite the effect of the addition of zirconia to alumina should be studied. In this study higher fracture toughness was not attained in the alumina nanocomposites containing TiO2 and Y-TZP. In the case of fracture toughness shown in Table 2, according to Nawa(7) et al, when 1mol% of TiO2 is added to 12Ce-TZP/30vol%Al2O3 nanocomposites, mechanical strength, as well as fracture toughness, tend to decrease because Ti ions dissolve in the zirconia network and restrain the tetragonal-monoclinic transformation. Also this dissolution of Ti ions may lead to the formation of Zr5Ti7O24. Perhaps this is one of the reasons why the values of fracture toughness did not increase up to 6-7 MPam1/2 as have

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been found by Niihara (20). Further studies to develop appropriate nanocomposites with both higher wear resistance and high fracture toughness are required. Even though there was not any enhanced of fracture toughness with the addition of TiO2 and Y-TZP, the values of this property are comparable with those of biomaterials that are currently used, the same with the Hardness Vickers. Also the values for the specific wear rate are lower than the values presented by Saikko et al (22).

5. Conclusions Alumina nanocomposites, with an appropriate composition, are a promising candidate material in the fabrication of hip joint replacements, due to its low specific wear rate, approximately 10-8 mm3/Nm. The sample with a composition of 15 mol% Y-TZP shows the best tribological behavior as the lowest coefficient of friction and specific wear rate in the presence of FBSS. The formation of chemical species such as AlPO4 in FBSS in addition to protein adsorbed films on the wear track appears to have an influence on the friction of the alumina nanocomposites. In FBSS the coefficient of friction decreases, and samples with more than 7.5 mol% of Y-TZP showed lower specific wear rate than in water.

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