3.1.4. RHEED Analysis

All as deposited samples were amorphous (not shown), with RHEED being conducted mainly to ascertain crystallinity and phase purity. After annealing at 600 ◦C, diffraction rings could be observed (Figure 4A–D). Due to the low accuracy of the RHEED measurements and the large number of HA diffractions, it proved difficult to index the rings. Therefore, only the most intense rings have been indexed with confidence, with the first ring corresponding to the (002) plane and the second broader ring corresponding to the (211), (112) and (300) planes. With increasing silicon content, the number of rings present decreased. The SiHA1 sample, Figure 4B, exhibited only two hazy rings corresponding to the d-spacings at 2.9 and 2.7 Å. The first ring was assigned to the (002) plane and the second broader ring is a combination of the (211), (112) and the (300) planes. The SiHA2 samples, Figure 4C, displayed the same rings as above but with lower intensity. The SiHA3 samples, however, showed no rings, indicating that these samples were amorphous.

The samples annealed at 700 ◦C (Figure 4E–H) demonstrated sharper diffraction rings compared to 600 ◦C annealing. Ring intensity increased for the HA sample with no new observed peaks (Figure 4E). The SiHA1 sample (Figure 4F) detailed the presence of additional rings related to a combination of HA and rutile, which are shown in (Figure 5). For the higher silicon content coatings, (SiHA2 and SiHA3), the number of rings decreases, reverting back to a HA system, however, it may be seen that in SiHA2 (Figure 4G) some rings relating to rutile remain. This was also the case for the SiHA3 (Figure 4H) sample, however, the rings were more defuse.

**Figure 4.** RHEED diffraction patterns of HA and SiHA thin films sputtered onto CPTi discs and annealed at 600 ◦C and 700 ◦C for A-D and E-H, respectively. Images were obtained at 200 keV. (**A**) and (**E**) HA, (**B**) and (**F**) SiHA1, (**C**) and (**G**) SiHA2 and (**D**) and (**H**) SiHA3.

**Figure 5.** Comparisons of RHEED patterns for (**A**) SiHA1 film on a CPTi disc annealed at 700 ◦C in flowing argon for 2 h and (**B)** CPTi sample annealed in air at 750 ◦C for 1 h. All indexed planes match to ICDD card 76-1939 (Rutile) unless indexed with a superscript a plus diagonal arrows indicating possible HA reflections matching to ICDD card 09-432 (HA).

### 3.1.5. X-Ray Photoelectron Spectroscopy (XPS)

Ca 2p, P 2p and O 1s high resolution XPS spectra are shown in Figure 6. A calcium doublet was observed separated by 3.55 eV and fitted with two components at peak positions of 347.5 and 351.0 eV for Ca 2p1/2 and Ca 2p3/2 respectively [30]. Calcium was in low concentrations in the as deposited films (4.3–6.8 at.%), but increased after annealing at both temperatures (12.1–18.0 at.%). P 2p peaks were fitted with a doublet [31], with separation energy of 0.84 eV. Phosphorus content decreased with

both increasing annealing temperature and silicon content. Interestingly, no phosphorus was seen on any of the HA thin films annealed at 600 ◦C, but was seen on HA samples annealed at 700 ◦C.

**Figure 6.** Representative high-resolution XPS spectra for HA, SiHA3 and 600 ◦C annealed SiHA3 samples, demonstrating the binding energy data for O 1s, Ca 2p, P 2p and Si 2p spectra, where appropriate.

The O 1s peak for the as deposited samples was fitted with two components at 531.0 eV and 532.5 eV, corresponding to PO4 [32] groups and C–O [32] or SiO2 groups [33], respectively. The second C–O component became larger with increasing silicon content. Therefore, it is thought that this is related to SiO2 binding [33]. After annealing at 600 or 700 ◦C, the O 1s peak could only be fitted to a single component at 531.0 eV, which corresponded to PO4 bonds. Furthermore, a reduction in oxygen content was seen in all films after annealing, however, no di fferences were observed between the oxygen content of films annealed at 600 or 700 ◦C. In the HA samples annealed at 600 ◦C (which showed no phosphorus present on the surface), no shift in the binding energy of the component at 531 eV was seen.

The Si 2p silicon peak was fitted to a single component. As already shown by the EDX data, the silicon content of thin films increased with increasing power density applied to the silicon target. These values, however, were in poor agreemen<sup>t</sup> with the EDX being consistently lower. In the as deposited samples the chemical shift for the Si 2p were found to depend on the silicon content of the film, with lower binding energies measured for the samples with lower silicon concentrations. After annealing at both temperatures all Si 2p peak positions were in the region of 101.5. Silicon content did not vary after annealing at 600 ◦C, compared with the as deposited samples, however after a heat treatment of 700 ◦C, only very small quantities were seen on all of the SiHA samples.

The Ca/P ratio decreased with increasing silicon content from 1.43 in the HA samples to 1.03 in the SiHA3 samples for the as deposited samples. After heat treatments of 600 ◦C the Ca/P ratio increased. This increase was higher for higher concentration silicon containing HA films. Following annealing at 700 ◦C a further increase in Ca/P ratio was seen.

### 3.1.6. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR was used to assess the chemical bonding in RF magnetron sputtered thin films. Figure 7A shows infrared spectra for as deposited HA and SiHA thin films sputtered onto CPTi. The HA, SiHA1 and SiHA2 films exhibited four distinct bands at wavenumbers 1147, 1028, 950 and 617 cm<sup>−</sup>1, which are indicative of υ3 P–O stretching. SiHA3 samples showed a reduction in the number of phosphate bands, with only peaks at 1147, 950 and 617 cm<sup>−</sup><sup>1</sup> present.

After heat treatments at 600 ◦C (Figure 7B), the HA films showed sharper peaks with an additional phosphate band at 1080 cm<sup>−</sup><sup>1</sup> when compared to the as deposited HA film. Moreover, a small OH peak was seen at 3643 cm<sup>−</sup>1. The SiHA1 sample showed all phosphate bonds exhibited by the recrystallised HA sample, however, bands were slightly broader, with a new peak at 820 cm<sup>−</sup>1. The SiHA2 sample showed broader phosphate bands, and the intensity of the peak at 820 cm<sup>−</sup><sup>1</sup> was reduced. The SiHA3 sample only showed three broad phosphate bands at 1147, 950 and 617 cm<sup>−</sup>1. This spectrum was very similar to the spectrum of the as deposited coating. Due to similarity of the produced spectra, the 700 ◦C heat treatment is not shown.

**Figure 7.** FTIR Spectra of (**A**) as deposited HA and SiHA thin films on CPTi substrates and (**B**) HA and SiHA thin films on CPTi substrates heat treated at 600 ◦C for 2 h in argon.
