# Regolith-Rich PEEK Composite Bricks: Steps Towards Space-Ready Lunar Construction Materials
## 4 Discussion on the Material Properties
### Impact of the Process Parameters  
Figure 4 presents a summary of the first bending campaign, showing the strengths (Figure 3(a)) and moduli (Figure 3(b)) measured across the samples. The bars represent the average values for each run, with error bars indicating the standard deviation. The bars correspond to the data after removing (possible) outliers, which are excluded based on the two standard deviations criterion. In all cases, each sample set includes a minimum of five specimens as required by ASTM standards for mechanical characterization. Both bar charts are divided into three sections, each representing a different binder weight percentage (wt%). At the same binder weight percentage, the compaction pressure increases from left to right as described in Table 3. Significant variability is observed, with bending strengths ranging from 0.6 to 14.9 MPa and moduli from 66.0 to 1877.5 MPa, as both properties are strongly influenced by binder content and compaction pressure.  
Figure 5 follows an analogous approach while presenting a summary of the compression campaign, showing the strengths (Figure 4(a)) and moduli (Figure 4(b)) measured across the tested samples. Similarly, the compression properties are significantly affected by the compaction pressure and binder weight percentage. There is notable variability across the  
Figure 4: Experimental results: bending strength and modulus per DoE run.  
full sample set, with compression strengths ranging from 1.9 to 25.0 MPa, and moduli from 35.5 MPa to 558.0 MPa.  
To separate the individual effects of the two processing parameters on the mechanical properties, Figures 6 and 7 present the main effects plots for bending and compression properties. The vertical axes display the response variables, with blue representing the bending and compression strengths, and red indicating the bending and compression moduli. The horizontal axes show the levels of the control factors, specifically the compaction pressure and the binder weight percentage (wt%). The analysis confirms that both parameters are critical in determining the mechanical properties of the samples, as adjusting them enables tailoring the material properties by enhancing the strengths and moduli. When ranking the two parameters, the effect of the binder is far more pronounced than that of the compaction pressure. The trend appears monotonic across all response variables, following a consistent and overlapping pattern. Within the considered ranges, there is no clear indication of an asymptote or saturation point where the positive effects of increased binder content or compaction pressure diminish. This suggests that even higher mechanical properties could be achieved with further increases in these parameters. However, caution must be exercised in pushing these limits: binder weight percentage cannot be increased indefinitely, as minimizing the use of Earth-sourced raw materials is crucial. Similarly, compaction pressure should not be excessive, especially when considering applications for large-scale components.