*3.3. ToFSIMS Study*

In order to determine the presence or otherwise of particular chemical species, ToFSIMS was employed to provide information about the chemical nature of the outermost molecular layer (1–10 Å) of the PEEK and CaP modified PEEK (HA10, HA300 and HA600) samples via a mass survey. Given the high mass resolution of SIMS it was particularly well suited to polymer surface analysis; however, the analyses of PEEK via ToFSIMS has been scarcely described within the literature [27,28]. The positive and negative ion spectra for the PEEK polymer have been acquired in the relevant *m*/*z* ranges of 0–200. For the purposes of highlighting key results, the positive and negative ion spectra have been shown here between *m*/*z* ranges of 25–110 and 20–100, respectively.

### 3.3.1. ToFSIMS Analysis of PEEK

From the positive survey for the PEEK substrate, as shown in Figure 9a, it can be noted that peaks above an *m*/*z* of 100 were very weakly detected. Specific peaks with a high intensity included *m*/*z* 39, 51, 77, 91, 104, 105, 115, 139, 152, 163, 165, and 195–197, these were all considered to be either indicative of aromaticity or ionically diagnostic of PEEK/PEEK fragments by Pawson et al. [28]. The peak at *m*/*z* 165 has been noted as useful in determining polymer structure as it contains no O [30]. In comparison, for the negative ion spectrum for the PEEK substrate (shown in Figure 9b), a range of peaks indicative of PEEK fragments were detected, including *m*/*z* 25, 41, 49, 73, 108, 121, and 196, which were recorded as corresponding to C2H<sup>−</sup>, C2OH<sup>−</sup>, C4H<sup>−</sup>, C6H<sup>−</sup>, C6H4O2<sup>−</sup>, C7H5O2<sup>−</sup>, and C13H8O2<sup>−</sup>, respectively. The peak at the *m*/*z* ratio of 197 was assigned to the repeating structure of PEEK polymer.

**Figure 9.** ToFSIMS survey spectra for (**a**) positive ions for the PEEK substrate, (**b**) negative ions for the PEEK substrate.

Positive ion intensity surface maps for the PEEK substrate surfaces have been recorded in Figure 10a. The peaks at *m*/*z* 40 and 57 in Figure 10a (i) and (ii) have been known to represent Ca+ and CaOH<sup>+</sup>, respectively. Low levels of both ions and fragments, in comparison to a modified surface have been exhibited. The surface maps recorded for *m*/*z* 104, 139, and 163 (Figure 10a (iii)–(iv), respectively) are thought to be indicative and diagnostic of PEEK. Figure 10b has presented the negative ion intensity surface maps for the PEEK surfaces at the *m*/*z* detailed. The peaks at *m*/*z* 63 and 79 have been known to represent PO2− and PO3<sup>−</sup>, (Figure 10b (i) and (ii)), respectively, low levels of both have been found to be present in comparison to a CaP modified surface. Due to handling and the nature of the sampling technique, some surface contamination was not unexpected. *M*/*z* 108, 121, and 196 peaks (Figure 10b (iii)–(iv)) have been assigned to the C6H4O2<sup>−</sup>, C7H5O2<sup>−</sup>, and C13H8O2− fragments, respectively; these were found to be present in a higher intensity in comparison to the modified surface. The images have indicated homogeneity of the selected ions (150 μm FOV) on the surface of the PEEK material. No unexpected species were detected; the samples were found to be relatively free from contamination.

**Figure 10.** (**a**). ToFSIMS positive ion intensity surface maps for PEEK obtained at selected masses (i) Ca<sup>+</sup>, (ii) CaOH<sup>+</sup>, (iii) *m*/*z* 104, (iv) *m*/*z* 139 and (v) *m*/*z* 163. All images were normalised to the total ion count; (**b**) ToFSIMS negative ion intensity surface maps for PEEK obtained at selected masses (i) PO2<sup>−</sup>, (ii) PO3<sup>−</sup>, (iii) C6H4O2<sup>−</sup>, 108 (iv) C7H5O2− 121, and (v) C13H8O2− 196). All images were normalised to the total ion count.

### 3.3.2. ToFSIMS Analysis of CaP Deposited onto PEEK over Time

The positive ion survey spectra of the CaP thin film sputter deposited onto the PEEK surface for up to 10 min (HA10), in the *m*/*z* 25–110 range is shown in Figure 11a, with characteristic peaks observed at an *m*/*z* ratio of 40 and 57 representative of Ca+ and CaOH<sup>+</sup>, respectively [9]. Isotopes of Ca+ were detected at *m*/*z* 42 and 44, and it was clear from looking at the survey spectra that the CaOH+ ion was dominant in comparison to the neat PEEK spectra. The presence of impurity ion K<sup>+</sup> was noted at *m*/*z* 39. Peaks known to relate to polymers containing aromatic groups were found to be present at *m*/*z* 39, 51, 77, and 91. At *m*/*z* ratios above 100, there were two peaks, namely at 104 and 105, which were thought to be specific to PEEK [28] in this circumstance, and only the peak at 105 was considered to be significant

(>0.4% of the largest peak) [9]. The negative ion survey of the CaP thin films sputter deposited onto the PEEK surface for up to 10 min in the *m*/*z* 20–100 range is shown in Figure 11b, with characteristic peaks within the spectra noted at an *m*/*z* 63 and 79 corresponding to PO2− and PO3<sup>−</sup>. Peaks were also observed at 25, 41, 49, and 73, corresponding to the main negative fragments of PEEK, C2H<sup>−</sup>, C2OH<sup>−</sup>, C4H− and C6H− [27], these have been attributed to either PEEK or surface contamination due to their exposure to atmospheric conditions. Similar results were observed for figure the positive and negative ion spectra for the HA600 sample, as shown in Figure 12a,b, respectively. It is noted that the relative intensities of the PO2− and PO3− ions dominant the negative ion spectra for the HA600 surface, with the contribution from the PEEK significantly diminished. The ToFSIMS surface mapping positive and negative analysis for HA600 has been presented in Figure 13a,b, respectively, with similar results observed when compared to those of the HA10 samples in Figure 10. The positive and negative peak area (normalised by total ion count) bar charts for of the samples analysed here by ToFSIMS are shown in Figure 14a,b.

**Figure 11.** ToFSIMS survey spectra for (**a**) positive ions HA10, (**b**) negative ions HA10.

**Figure 12.** ToFSIMS spectra for (**a**) positive ions HA600 and (**b**) negative ions HA600.

### 3.3.3. ToFSIMS Depth Profile of CaP Sputter Coated PEEK (HA600)

To further investigate the relationship between the sputtered surface and the PEEK substrate, ToFSIMS depth profiling was employed. The depth profile for HA600 modified PEEK in the positive and negative ion modes (1000 s rastering) is shown in Figure 15 (overlaid for both positive and negative ions). There was a decline in intensity of the P and Ca ions as they appeared to tail off in response to the Ar<sup>+</sup> ion bombardment. There was a very slight, ye<sup>t</sup> steady and continuous increase in the intensities correlating to the PEEK fragments, both negative and positive. These ions were expected to be in low intensity, whereas the hydrocarbon was expected to be much more intense. The ion intensity of the C2H− hydrocarbon experienced an initial sharp decline at the very beginning of rastering; this was likely due to surface contamination, and was followed by an incline, followed by an exponential decrease to a less intense but steadier state. A further depth profile, taken using ToFSIMS, is shown in Figure 16a, probing only the PO3− and C4H− ions (as shown in Figure 16b,c) representative of the CaP coating and PEEK substrate, respectively), highlights the fact that the there is significant intermixing of the coating and the PEEK substrate. The depth profilometry of the ToFSIMS sputter crater is illustrated in Figure 17 It was found that the sputter crater was, on average, 2.32 ± 0.19 μm.

**Figure 13.** (**a**) ToFSIMS positive ion intensity surface maps for HA600 obtained at selected masses (i) Ca<sup>+</sup>, (ii) CaOH<sup>+</sup>, (iii) *m*/*z* 104, (iv) *m*/*z* 139 and (v) *m*/*z* 163. All images were normalised to the total ion count; (**b**) ToFSIMS negative ion intensity surface maps for HA600 obtained at selected masses (i) PO2<sup>−</sup>, (ii) PO3<sup>−</sup>, (iii) C6H4O2<sup>−</sup>, 108 (iv) C7H5O2− 121, and (v) C13H8O2− 196). All images were normalised to the total ion count.

**Figure 14.** (**a**) PEEK substrate and HA600 positive ion intensity for the Ca<sup>+</sup>, and CaOH<sup>+</sup>, *m*/*z* 104, 139, and 163 (diagnostic PEEK fragments); (**b**) PEEK substrate and HA600 negative ion intensity for the PO2− and PO3− ions, C6H4O2− (*m*/*z* 108), C7H5O2− (*m*/*z* 121), and C13H8O2− (*m*/*z* 196).

**Figure 15.** ToFSIMS depth profiles of CaP-modified PEEK (HA600). Positive and negative spectra are overlaid, {(**a**) 100,000 counts, (**b**) 1000 counts.}.

**Figure 16.** (**a**) ToFSIMS depth profile for PO3− and C4H− ions, with 2D images for (**b**) the PO3− and (**c**) the C4H− ions.

**Figure 17.** Optical Profilometry cross section of the HA600 depth profile.
