Sensitisation to immunologically active elements like chromium, cobalt or nickel and debris particle due to wear are serious problems for patients with metallic implants. We tested the approach of using a hard and thick ceramic coating as a wear-resistant protection of titanium implants, avoiding those sensitisation and foreign body problems. We showed that the process parameters strongly influence the coating porosity and, as a consequence, also its hardness.
Sensitisation to chromium (1%), cobalt (2.4–3%), nickel (13.1%) of patients who received endoprosthetic surgery is a growing problem . The ionic release of these immunologically active elements from the implant material can lead to allergic responses of the host like eczema, swelling, sterile osteomyelitis, aseptic loosening and in worst cases also to the loss of the implant , . Furthermore, particular debris of articulating joints can lead to wear problem in metal-on-metal  but also in metal-on-polymer systems . CoCr alloys are used for endoprosthetic devices since the sixties  and today 80% of the artificial joints are made of CoCr. It cannot simply be replaced by titanium because its wear-resistivity is not sufficient , even though titanium is biocompatible and its mechanical strength is high enough . One approach is to compensate the wear resistance by applying a hard ceramic coating onto the titanium parts, in particular onto the sliding surface.
Advanced plasma spray (PS) method allows coating of complex geometries and is not limited to small implants sizes. Coatings with thicknesses of several hundreds of microns are feasible while other coating technologies like PVD or thermal oxidation  are limited to few microns. Whereas commercial implant surfaces in direct contact with bone have been coated by titanium-spraying (TPS) since decades and show good long-term survival rates , plasma sprayed coatings of the Al-oxide- and Ti-oxide-based system are so far not used as slide bearing of implants but for research purpose  or photocatalytic application . In this study, the morphology, hardness and the porosity of biocompatible advanced plasma spray coating consisting of a mix of oxides were investigated.
2 Material and methods
2.1 Sample production
Discs (ø25 mm, height 6 mm) made of CoCr were sandblasted with corundum and coated by advanced plasma spraying. Different process parameters were use in order to optimize the coating towards porosity and hardness.
2.2 Sample preparation
The coated samples were prepared for subsequent microscopic and materialographic characterization. The specimens were fixed in a polymeric 2-component cold embedding material. Cross sections were then produced on the TegraPol 21 grinding and polishing system with a Tegra Force 5 head from Struers. SiC grinding paper from BUEHLER followed by polishing paper from the same manufacturer has been used.
2.3 SEM/EDX analysis
The scanning electron microscope (SEM) TM3030 Plus (acceleration voltage U0 = 15 kV, backscattered electron and secondary electron detectors, Hitachi) with the integrated energy dispersive X-ray spectroscopy (EDX) system (Quantax 70, Bruker) enables the materialographic analysis.
2.4 Roughness analysis
The FORM Talysurf PGI 800 device from Taylor Hobson, equipped with a standard 2 μm, 90° diamond tip, a cut-off λC = 2.5 mm/0.08 mm and a measuring length l = 15 mm has been used for roughness determination.
2.5 Analysis of the porosity
The porosity of the coating was optically determined in polished cross sections according to ASTM F1854  with light microscope Axiovert 40 MAT ZEISS, AxioCam MRc CCD camera, Epiplan x10/0.20HD ocular, AxioVision 40 V.188.8.131.52 Software used for image analysis. Five microscopic images of every sample were analysed (np = 5).
2.6 XRD analysis
The crystallographic phases were characterized with the diffractometer D2 PHASER (Bruker) and the Diffrac EVA software V.4.0, applying Co-Kα radiation (λCo = 1.79 Å), angular θ-range 20–120°, power 300 W, sample φ-rotation of 15°/min. The ICDD database served for the identification of the phases.
2.7 Hardness testing
The hardness of the coating was determined by the Zwick Roell ZHV 10 system at a load of 2.94 N and a holding time of 15 s. Four spots were measured for each specimen (nh = 4).
3.1 SEM/EDX analysis
While the native PS surface shows a rough morphology, see Figure 1A, the polished surface is smooth and only a few pores can be seen, see Figure 1B. The coating homogeneously consists of titanium, aluminium and oxygen, see EDX mapping in Figure 1C. No foreign elements were detected. The elementary composition is given in Table 1.
|Ti||49.9 ± 0.2|
|Al||7.9 ± 0.1|
|O||42.2 ± 0.2|
The cross section through the coating is shown in Figure 2. The coating in tight contact with the rough substrate shows a layered microstructure originating from the deposition process and a low porosity. A thickness of about 100 μm was estimated.
3.2 Roughness analysis
The roughness values of the native PS coating and the polished specimen are shown in Table 2.
|Roughness||Native [μm] (n=9)||Polished [μm] (n=9)|
|Ra||8.15 ± 2.07||0.016 ± 0.005|
|Rt||65.54 ± 13.98||0.67 ± 0.33|
|Rz||53.72 ± 10.75||0.13 ± 0.04|
3.3 XRD analysis
The crystallographic phases of the coating were identified as different modifications of TiO2 (Brookite, Anatase, Rutile) and Al2O3. No signals from the metallic substrate were observed.
3.4 Hardness testing
3.5 Analysis of the porosity
The optical quantification of the porosity and its relation to the observed hardness is illustrated in Figure 4. Porosity values between 0.43% and 12.95% were found, depending on the applied process parameters.
The investigation of the hard plasma spray process coatings indicate a trend between the hardness and the porosity as expected. According to  the hardness is related to the abrasive wear resistance. The quality of the coating can be optimized using specific process parameters to reduce surface roughness and porosity while simultaneously increasing the hardness, both important prerequisites to prevent friction. The proven feasibility of polishing the hard coating to surface roughness in the sub-micrometer range and expose a low-porous sliding surface opens perspectives for new biomedical applications. This novel biocompatible hard-coating could enhance the wear resistivity of titanium implants that are free from Co, Cr and Ni.
Research funding: This study was supported by the Hightech Zentrum Aargau AG. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The conducted research is not related to either human or animal use.
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