
Alumina (20% vol) – yttrium stabilized tetragonal zirconia polycrystal nano-composite disks with the dimension of Ø12 × 1.2 mm were used. Nano-sized Al2O3 (α-Al2O3 purity > 99/99%, average particle size of 13 nm, sigma aldrich, U.S.A) and Y-TZP (Y-TZP, Particle size < 100 nm, sigma aldrich, U.S.A) powders were used as the starting materials. After sieving, the powders were uniaxially pressed into disks with the dimension of Ø12 × 1.2 mm under a pressure of 150 MPa [19]. An A-3Y-TZP nano-composite containing 20 vol.% alumina was sintered at 1270°C for 170 min in air and used as the substrate. The A-Y-TZP 20 nanocomposite disks were obtained in the biomaterial laboratory of Isfahan University of Technology. The sintered A-3Y-TZP20, with the dimensions of Ø12 × 1.2 mm and the average grain size of < 400 nm, was almost fully dense with the theoretical density of above 96%, as measured by the kern system (ALS series). The A-3Y-TZP20 substrates were sand blasted using alumina particles (with the size of ~60 μm). Surface roughness was measured by mitutoyo system. For the deposition of 3Y-TZP-HA nano-composite coat on A-3Y-TZP20 nano-composite substrate, the precursors of calcium nitrate [Ca(NO3)2.4H2O, Merck, Germany] and phosphorus pentoxide [P2O5, Merck, Germany] were used as the starting materials [18]. Toward this purpose, stoichiometric amounts of the precursors were separately dissolved in ethanol under stirring condition for 20 min. Then, the Ca-containing solution was slowly added to a P-containing solution and stirred for 1h. To prevent crack formation in the coating, a certain amount of oxalic acid (2% weight solution) was added and the solution was stirred for another 1h. Finally, a solution containing 10 wt.%3Y-TZP (with the particle size of < 100 nm, Sigma-Aldrich, USA) was incorporated into the former solution and stirred for 4h to prepare the 3Y-TZP-HA sol. The [Ca]/[P] ratio of the 3Y -TZP-HA sol was adjusted to 1.67. For comparison, HA sol without 3Y-TZP was also prepared by the same procedure, except for the last step. Prior to coating deposition, the substrates were ultrasonically cleaned by immersing in acetone, alcohol and distilled water for 20 min, after which the substrates were dried in the oven at 120°C for 24h. The 3Y-TZP-HA coats were deposited onto A-3Y-TZP20 substrates by a dip coating process with a speed of 30 mm per min. The obtained coats were aged at room temperature for 24h, dried in the oven at 80°C for another 24 h, and finally, heat treated at 600°C for 1h. The heating and cooling rates were adjusted to 1°C/min.
Phase composition of the coatings, after heat treatment, was determined by x-ray diffraction (XRD) (Philips diffractometer, 40 kV, Cu Kα radiation: λ = 0.15418 nm) in the 2θ range of 20-80°, with the step size of 5°/sec. The surface morphology of the coatings was examined by scanning electron microscopy (SEM) (Phillips, XL30). Young’s modulus, hardness and fracture toughness of the coatings were measured by nanoindentation testing (NHT compact platform, NHT-epx, CSM) over a load range of 30 mN and a penetration depth equal to 0.1 of coating thickness [20]. Fracture toughness (KIC) of the coatings was obtained by measuring the accurate size of the cracks formed during nanoindentation [21]. Toward this purpose, the indented surface was examined by an in-situ atomic force microscopy (AFM) to capture the features of cracks. Then, the fracture toughness of the samples was calculated using the following equation
KIC = α [E/H]1/2 [P/ c 3/2]
To evaluate the dissolution behavior, the 3Y-TZP-HA and HA coatings deposited on A-3Y-TZP20 nano-composite and calcined at 600°C were immersed in a physiological saline solution (0.9% NaCl) and aged in water bath at 37°C, by following the Kim’s studies [24]. At predetermined incubation periods, the sample was removed and the concentration of Ca and P ions was determined using inductively coupled plasma-atomic emission spectroscopy(ICP-AES) ICPS-100IV, Shimadzu). For comparison, a pure HA coating was also examined in terms of dissolution behavior.
Sand blasting was used to modify the implant surface using alumina particles. The surface roughness parameters of sand blasted substrates were measured to be: Ra: 2.836 µm, Rq: 3.671 µm, and Rz: 17.256 µm.
Figure 1 shows the XRD patterns of HA and 3Y-TZP-HA coatings after heat treatment. In the XRD –patterns of coatings after heat treatment, without applying on the substrate (see Figure1), only the calcium oxide phase was present in the HA coat. The other peaks in the XRD-patterns of 3Y-TZP-HA(B) and HA(A) coatings are shown in Figure 1.

Figure 1: XRD patterns of (A) HA and (B) 3Y-TZP-HA coating after heat treatment at 600°c for 1h without applying on A-3Y-TZP20 substrate.
To achieve free crack and pit coating, the morphology of HA(A) and 3Y-TZP-HA(B) coatings heat treated at 600, 700 and 900°C with aging times of 24, 18 and 12h, respectively, were evaluated by SEM. As shown in Figure 2, the existence of cracks and pits was clearly observed for the coatings heat treated at 900°C, while by reducing the temperature to 700°C and increasing the aging time to 18h, the minimal pits and cracks took place. Further decreasing the temperature to 600°C and increasing the aging time to 24h resulted in the formation of a free-crack 3Y-TZP-HA and HA coatings on A-3Y-TZP20 substrate (Figure 2). According to TEM image shown in Figure 3, the presence of the nano-sized 3Y-TZP-HA coating with the grain size of < 50 nm was evident in the 3Y-TZP-HA nano-composite coat.

Figure 2: SEM images from HA (A) and 3Y-TZP–HA(B) coating after heat treatment at 900°c, 700°c and 600°c with the aging time 12h, 18h and 24h, respectively.
Comparison of young’s modulus and hardness means in HA and HA-3Y-TZP groups showed that p-values were significant (0.04 for young’s modulus and 0.00 for hardness). In other words, the HA-3Y-TZP nano-composite coat showed higher young’s modulus and hardness (Table 1). Figure 4 shows the effect of nanoindentor on 3Y-TZP-HA nanocomposite coating. Also, Figure 5 presents the variation of hardness and young’s modulus of coatings versus zirconia content. It could be observed that the young’s modulus and hardness of the coatings were increased in the ranges of 149.6-214 GPa and 5.981-12.34 GPa, by increasing the zirconia content from 0 to 10wt%, thereby indicating superior mechanical properties of 3Y-TZP-HA coating, in comparison to those of HA coating. This improvement in mechanical behavior could be attributed to the presence of ZrO2 phase combined with the formation of nano-structured 3Y-TZP-HA coat. As shown in Figure 6, the fracture toughness of coatings was increased in the range of 1.48 - 3.53 MPa.m1/2 with increasing the zirconia content from 0 to 10 wt.%.
Variable | Young’s modulus (G Pa) | Hardness (Kg/mm2) |
Group | ||
HA coat (mean ± S.D) | 149 ± 23.8 | 609.28 ± 8.78 |
HA-3Y-TZP coat (mean ± S.D) | 214.02 ± 5.92 | 1258.2 ± 14.2 |
P* ≤ 0.05 | 0.04 | 0.00 |
Table 1: Statistical results from the comparison of the young’s modulus and hardness means between HA and HA-3Y-TZP coats.
P* - value is significant at level of ≤ 0.0 5

Figure 5: Variation of young’s modulus and hardness of nanocomposite coatings versus 3Y-TZP content.
Figure 7 and Figure 8 show the dissolution behavior of HA and 3Y-TZP-HA coatings with respect to Ca and P ions release in a physiological saline solution (0.9% NaCl) as a function of time. It could be observed that the dissolution rate of 3Y-TZP-HA coat was lower than that of the pure HA coat.

Figure 7: Comparison between the dissolution behavior and ca ion release of HA and 3Y-TZP-HA coatings after heat treatment at 600°c for 1h.

Figure 8: Comparison between the dissolution behavior and p ion release of HA and 3Y-TZP-HA coatings after heat treatment at 600°c for 1h.
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