**3. Results**

### *3.1. Chemical and Structural Characterisation*

### 3.1.1. Film Morphology and Thickness

Figure 1A shows a cross-section of a HA thin film on a CPTi disc annealed at 600 ◦C. Selected Area Electron Diffraction (SAED) confirmed that the substrate and the coating was indeed CPTi and polycrystalline HA. Moreover, the coating can be seen to be uniform in thickness, free of voids or defects and 185 ± 4 nm in thickness. Figure 1B shows that the SiHA3 thin film measured 216 ± 5 nm in thickness; films became thicker with increasing silicon content. Local recrystallisation of the HA films was noted, however, this may be a result of the e-beam interaction.

**Figure 1.** Bright field TEM image of (**A**) the tungsten/HA/Ti lamellar showing the HA thin film deposited on a CPTi substrate with associated selected area electron diffraction (SAED) patterns of the crystalline W protective coating, polycrystalline HA coating and crystalline Ti substrate and (**B**) the tungsten/SiHA3/Ti lamellar showing an amorphous SiHA3 coating on a CPTi substrate and associated SAED.

Surface micrographs of all films exhibited similar morphologies in as deposited and heat-treated states following the topography of the CPTi substrates. Figure 2A shows a representative as deposited HA film, showing a smooth dense coating without voids or defects. After a heat treatment at 600 ◦C (Figure 2B), all HA and silicon containing films looked similar in morphology with no notable differences with respect to silicon addition. At 700 ◦C, films became notably more textured with distinct features rising from the surfaces (Figure 2C).

**Figure 2.** Representative SEM micrographs displaying the surface morphology of HA and SiHA thin films where (**A**) is an as deposited thin film on a CPTi substrate, (**B**) annealed at 600 ◦C in flowing argon for 2 h and (**C**) 700 ◦C in flowing argon for 2 h. Images taken from HA samples.

### 3.1.2. Energy Dispersive X-Ray Analysis (EDX)

Table 1 demonstrates averaged Ca/P ratios of all coatings, and plasma sprayed targets. All samples were significantly higher than the stoichiometric value of bulk HA (1.67), however, they were significantly lower than the Ca/P ratio of the target material. The standard error of the mean was found to increase with increasing silicon content. Silicon content was seen to increase from 1.8 to 13.4 wt.%, with increasing target power densities from 6.6 × 10−<sup>4</sup> to 3.3 × <sup>10</sup>−3, respectively.

**Table 1.** A summary of combined silicon content of HA and SiHA thin films batches as measured by EDX. Values displayed are the mean ± standard error of the mean, where *n* = 6. Ca/P content calculated from EDX and XPS measurements.


### 3.1.3. X-Ray Diffraction (XRD) Analysis

XRD spectra (Figure 3) show representative XRD plots of as deposited films onto silicon (100) wafers heat treated at both 600 (Figure 3A) and 700 ◦C (Figure 3B). All as deposited (unannealed) coatings revealed an amorphous nature with a distinct hump at 27.5◦. Following heat treatment at 600 ◦C, HA, SiHA1 and SiHA2 films recrystallised forming a single-phase HA structure matching ICDD card 09-432. Preferential orientation was seen along the (002) reflection when compared to a randomly orientated sample. The peak intensity of films decreased after inclusion of silicon. This was seen by peak broadening along the (002), (211), (112) and the (300) planes. Surprisingly, the SiHA3 samples remained amorphous after heat treatment at both 600 and 700 ◦C. The silicon addition to SiHA3 clearly had an effect on the recrystallisation transitional temperature of the HA structure. Full recrystallisation was not achieved for the SiHA2 at 700 ◦C with clear broadening of the FWHM compared to the full crystalline HA.

Consequently, a sequential heat treatment investigation (Figure 3C) was conducted on the SiHA3 coating to determine the temperature of recrystallisation. Figure 3C shows that the structure of SiHA3 did not alter after heat treatment up to 700 ◦C. At 800 ◦C, a single phase HA structure matching ICDD card number 09-432 was observed. Preferential orientation along the (002) plane was no longer observed, with the (211) plane being the most intense. For further heat treatments up to 1000 ◦C, the intensity of the (002) increased, and peaks sharpened, indicating crystal growth.

**Figure 3.** (**A**) and (**B**) Representative XRD plots of as deposited HA (unannealed), and HA and SiHA thin films on silicon (100) wafers heat treated at 600 ◦C and 700 ◦C in argon, respectively. (**C**) XRD plots of successive heat treatments on SiHA3 on silicon (100) wafer. Planes refer to ICDD 09-432 HA.

In addition, approximate crystallite size was calculated using the Scherrer equation. Table 2 lists calculated crystallite sizes of annealed HA and SiHA thin films from the XRD data presented. The crystallite size of all coatings increased with increasing annealing temperature. For example, the crystallite size of HA was ca. 78 and 89 nm following annealing at 600 and 700 ◦C, respectively. Respective values were seen to decrease with increasing silicon content. Crystallite size could not be calculated for SiHA3 at either annealing temperature, as both films were amorphous.


**Table 2.** A summary of HA crystallite size (nm) calculated by the Scherrer equation for HA and SiHA thin films sputtered onto silicon (100) single crystal wafer using the <001> planes.
