Effects of Sintering Temperature on Structural, Morphological, and Mechanical Properties of Co-Cr-Mo Alloys Coated with ZrO2 Ceramic Films

1Department of Dentistry, Chi Mei Hospital, Liouying No. 201, Taikang Vil., Liuying Dist., Tainan City 73657, Taiwan (R.O.C.) 2Department of Mechanical and Automation Engineering, Da-Yeh University, No. 168, University Rd., Dacun, Changhua City 515, Taiwan (R.O.C.) 3Bachelor Program for Design and Materials for Medical Equipment and Devices, Da-Yeh University, No. 168, University Rd., Dacun, Changhua City 515, Taiwan (R.O.C.) 4Department of Materials Science and Engineering, Da-Yeh University, No. 168, University Rd., Dacun, Changhua City 515, Taiwan (R.O.C.) 5Division of Prosthodontics, Department of Dentistry, Chi Mei Medical Center, No. 901, Zhonghua Rd., Yongkang Dist., Tainan City 710, Taiwan (R.O.C.)

In this study, ZrO 2 thin films were coated on Co-Cr-Mo alloys by screen printing and spraying, which was followed by sintering at a high temperature of 750, 900, or 1100 °C to enhance the characteristics of the coated alloys. Through X-ray diffraction (XRD) measurements, scanning electron microscopy (SEM), surface roughness measurements, and microscale hardness testing, the structural, morphological, and mechanical characteristics of the Co-Cr-Mo alloys coated with ZrO 2 films were investigated in detail. The experimental results revealed that both the microhardness and smoothness of the ZrO 2 ceramic films prepared by screen printing and spraying methods clearly improved with increasing sintering temperature. It was also found that the increase in the sintering temperature contributes to the increased thickness and density of the films, leading to enhanced mechanical properties of the Co-Cr-Mo alloys. After sintering at 1100 °C, the ZrO 2 film prepared by spraying had the smoothest surface (surface roughness: 0.70 μm) and the highest hardness (767 HV 0.5 ). The results confirm that ZrO 2 coatings on Co-Cr-Mo alloys have high potential for medical implant applications. In addition, the sintering process is helpful for improving the mechanical properties. The ZrO 2 materials are used in many sensor applications, and sensing applications can be realized using ZrO 2 /Co-Cr-Mo materials. In the future, we will study the sensing performance of ZrO 2 sensors fabricated on Co-Cr-Mo alloys as well as implants made of the coated alloys.

Introduction
In recent years, Co-Cr-Mo alloys have been widely used for medical implant applications including the replacements of human joints (hip joint and knee) and dental treatments. (1)(2)(3) In particular, as dental applications, Co-Cr-Mo alloys are practically applied in the base plates in complete dentures, bridgeworks, and dental implants. This is because of the many advantages of Co-Cr-Mo alloys, including good mechanical properties, excellent wear resistance, superior corrosion resistance, and extremely high biocompatibility with the human body. It is well known that the high biocompatibility of Co-Cr-Mo alloys is correlated with their superior corrosion resistance. As Co-Cr-Mo alloys are manufactured, a very thin passive oxide film composed of Cr 2 O 3 is formed spontaneously on their surface, resulting in their excellent corrosion resistance. Although the passive oxide film generated on the surface of Co-Cr-Mo alloys can suppress the dissolution of metal ions, when Co-Cr-Mo alloys are immersed in various solutions and biological environments, the Co element is easily dissolved. This will result in the instability of the oxide film in the human body. Despite the many advantages of Co-Cr-Mo alloys given above, the release of metal elements from orthopedic implants into body fluids is unavoidable. The metal elements released from Co-Cr-Mo alloys accumulate at the interface between the body tissue and the implant. Then, they migrate through the tissue, which is very detrimental to the human body.
To overcome these disadvantages of applying Co-Cr-Mo materials in implants, it is advantageous to coat them with a ceramic film. (4)(5)(6) Many ceramic films, including TiN, TiC, and ZrO 2 , have been deposited on the surface of Co-Cr-Mo alloys. (7)(8)(9) Because of its biocompatibility and bioactivity, ZrO 2 is one of the most promising ceramic materials for biomedical applications. In addition, ZrO 2 also possesses excellent chemical stability and high hardness, resulting in good protective properties. Moreover, ZrO 2 materials are also used in many sensing applications such as chemical sensors and gas sensors. As well as medical applications, Co-Cr-Mo alloys have also been applied in engineering, such as aero engine gas turbines, and biomedical engineering. Through the deposition of ZrO 2 films on Co-Cr-Mo, its functionality is expected to be increased. ZrO 2 ceramic films have been prepared on Co-Cr-Mo alloys by various techniques including plasma spraying, electrolytic coating, and sol-gel spin coating. To further enhance the mechanical characteristics of ZrO 2 films, sintering processes have usually been used. Nevertheless, up to now, there has been almost no research on the deposition of ZrO 2 films on Co-Cr-Mo alloys by sintering to improve their mechanical characteristics.
In this study, to expand the biomedical applications of Co-Cr-Mo alloys and enhance their mechanical properties, the ZrO 2 coating and sintering techniques were both used. Two coating methods, i.e., spraying and screen printing, were employed to deposit ZrO 2 ceramic films on Co-Cr-Mo alloys. Then, the samples were sintered in a thermostatic furnace at 750, 900, and 1100 °C. The effects of the coating method and sintering temperature on the surface morphology, crystal structure, and mechanical properties of the ZrO 2 coatings on Co-Cr-Mo implant alloys were investigated in detail from the viewpoint of their future application as sensing and implant materials.

Experimental Method
In our work, the substrates employed for the ZrO 2 growth were Co-Cr-Mo (ASTM F1537, ASTM F799) standard forging alloys for surgical implants. The composition of the Co-Cr-Mo alloy was C (0.04%), Mn (0.81%), Si (0.16%), Cr (27.58%), Ni (0.14%), Mo (5.48%), Co (64.99%), and N (0.16%). The disc-shaped Co-Cr-Mo alloy samples had a diameter of 31.75 mm and a thickness of 6 mm. To obtain a low surface roughness (R a ), the Co-Cr-Mo samples were polished with sandpaper and diamond paste in sequence. Before the ZrO 2 coating process, the samples were ultrasonically cleaned in acetone and isopropyl alcohol to remove organics and other impurities. The ZrO 2 slurry used for spraying and screen printing was prepared by grinding ZrO 2 ceramic powder, then mixing it well in a solution containing Y 2 O 3 powder, a dispersant, and an adhesive (or thickener) to form the ZrO 2 slurry.
As mentioned above, spraying and screen printing methods were both used for coating the ZrO 2 on the Co-Cr-Mo alloy. During the spraying process, the ZrO 2 slurry was spurted from a spray gun and coated on the Co-Cr-Mo alloy 20 times. On the other hand, both the same strength (squeegee pressure: 2 kgw) and the same printing direction were used for ZrO 2 coatings on Co-Cr-Mo alloys in the screen printing technique. After coating the ZrO 2 layer, the sample was subsequently placed in a high-temperature furnace to perform the sintering experiments. Three sintering temperatures, 750, 900, and 1100 °C, were set for these samples and the sintering time was fixed at 1 h. In addition, the heating and cooling rates for the sintering process were fixed at 240 and 60 °C/h, respectively.
The crystal structures of the ZrO 2 -coated Co-Cr-Mo samples were determined by X-ray diffraction (XRD) (PANalytical, X'Pert Pro MRD). In the XRD measurement, the Cu Kα line (λ = 1.541874 Å) was employed as the source and Ge (220) was adopted as the monochromator. The surface morphologies of samples were observed by scanning electron microscopy (SEM) (S-3000H, Hitachi). The composition of samples was investigated by energy dispersive X-ray spectroscopy (EDS). The hardness of samples, defined as the resistance offered by the material to indentation, i.e., to permanent deformation and cracking, was analyzed using a Vickers micro hardness test machine. Figure 1 shows the θ-2θ XRD patterns of the ZrO 2 films sprayed on Co-Cr-Mo alloys after sintering at 750, 900, and 1100 °C. It was found that γ(111), γ(200), and γ(220) diffraction peaks were located at 2θ positions of 43.91, 51.23, and 75.47°, respectively, for all samples. These three diffraction peaks were indexed to the Co metal phase of the Co-Cr-Mo alloy. As the sintering temperature was increased to 750 °C, two diffraction peaks with very low intensities appeared: tetragonal ZrO 2 (101) and monoclinic ZrO 2 (021), located at the 2θ positions of 29.79 and 38.64°, respectively. When the ZrO 2 -coated Co-Cr-Mo sample was sintered at 900 °C, the diffraction peak of tetragonal ZrO 2 (101) disappeared. However, a phase of tetragonal ZrO 2 (110) appeared (the peak located at a 2θ position of 34.91°), while the phase of monoclinic ZrO 2 (021) was still weak. When the sintering temperature was further increased to 1100 °C, only the tetragonal (110) phase existed in the ZrO 2 film (the monoclinic ZrO 2 (021) phase disappeared). Figure 2 shows the θ-2θ XRD patterns of the screen-printed ZrO 2 layers on Co-Cr-Mo alloys after sintering at 750, 900, and 1100 °C. It can be seen that the diffraction peaks of γ(111), γ(200), and γ(220) also appeared for these samples. When the ZrO 2 -coated Co-Cr-Mo sample was sintered at 750 °C, there was no diffraction peak of the ZrO 2 phase. Upon further increasing the sintering temperature to 900 and 1100 °C, both the tetragonal ZrO 2 (110) and monoclinic ZrO 2 (021) phases appeared in these two samples. Compared with the XRD results shown in Fig. 1, the intensities of the tetragonal ZrO 2 (110) and monoclinic ZrO 2 (021) diffraction peaks appearing in the Co-Cr-Mo alloys coated with the screen-printed ZrO 2 layers (sintered at 900 and 1100 °C) are clearly higher than those for the samples coated with the sprayed ZrO 2 layers. Figures 3(a)-3(c) show plan-view SEM images of screen-printed ZrO 2 on Co-Cr-Mo substrates after sintering at 750, 900, and 1100 °C, whereas plan-view SEM images of sprayed ZrO 2 on Co-Cr-Mo substrates after sintering at 750, 900, and 1100 °C are shown in Figs. 3(d)-3(f), respectively. At the sintering temperature of 750 °C, there were no thick-plate structures, and equiaxed rhombohedral columnar structures formed on the sample surface for both deposition methods. There were also a few thin-plate structures on the surfaces of these two samples. When the sintering temperature was increased to 900 and 1100 °C, a large number of thick-plate and equiaxed rhombohedral columnar structures were precipitated and formed on the sample surface for both deposition methods, whereas these microstructures were not observed in the ZrO 2 slurry (not shown here). This indicates that the thick-plate and equiaxed rhombohedral columnar structures were formed during the sintering at 900 and 1100 °C. Moreover, these structures became more prevalent with increasing sintering temperature.  while the thicknesses of the sprayed samples were 1.52, 2.17, and 2.68 μm, respectively. The screen-printed samples were thicker than the sprayed samples. The ZrO 2 film thickness also increased with the sintering temperature. Figures 5(a)-5(c) show the R a values of screen-printed ZrO 2 on Co-Cr-Mo substrates after sintering at 750, 900, and 1100 °C, while the R a values of ZrO 2 sprayed on Co-Cr-Mo substrates after sintering at 750, 900, and 1100 °C are shown in Figs. 5(d)-5(f), respectively. For the sintering temperatures of 750, 900, and 1100 °C, the R a values of the ZrO 2 films prepared by screen printing were 1.06, 1.50, and 1.04 μm, respectively. The sprayed ZrO 2 films had R a values of 1.02, 1.46, and 0.70 μm, after sintering at 750, 900, and 1100 °C, respectively. The average R a values of the Co-Cr-Mo alloys coated with ZrO 2 films were all lower than that of the original Co-Cr-Mo substrate slab (R a : 1.92 μm). Moreover, the samples prepared by spraying were smoother than those prepared by screen printing. Regardless of whether ZrO 2 films are coated by screen printing or spraying, a smoother ZrO 2 film can be achieved after sintering at 1100 °C. In particular, the sprayed ZrO 2 film sintered at 1100 °C had the smoothest surface (R a : 0.70 μm).

Results and Discussion
The microhardness values of the samples were measured using a small Vickers hardness machine and are summarized in Table 1. With increasing sintering temperature, the microhardness of the ZrO 2 ceramic films increased, with the sample sintered at 1100 °C having the highest value. The enhanced hardness was attributed to the increased density of the ZrO 2 film after the high-temperature sintering process.

Conclusion
In our work, ZrO 2 films were coated on Co-Cr-Mo substrates by screen printing and spraying. Regardless of whether screen printing or spraying was used, there were many thickplate and equiaxed rhombohedral columnar structures formed on the sample after sintering at 900 and 1100 °C, whereas only a few thin-plate structures were formed during sintering at 750 °C. After sintering, the surface of the sample coated with the ZrO 2 film was smoother than the original Co-Cr-Mo substrate. With increasing sintering temperature, the hardnesses of the ZrO 2 films deposited by screen printing and spraying were both improved (especially for the sample sintered at 1100 °C), resulting from the enhanced density of the ceramic ZrO 2 films. According to our experimental results, the sprayed ZrO 2 film sintered at 1100 °C possessed the smoothest surface and the highest hardness, which will be helpful for expanding the applicability of medical implants of Co-Cr-Mo alloys. In the future, we will study the sensing performance of ZrO 2 sensors fabricated on Co-Cr-Mo alloys.