Fig. 5

Raman and FT-IR spectra. a Raman spectra of the sliding surface showing non-polarized (NP) and decomposed polarized spectra (VH, VV; where first letter (V) refers to the vertical polarization direction of the laser, and second letter (H = horizontal, V = vertical) to the orientation of the analyzer). Red bars are normal vibration modes for commercial amorphous silica³¹. b FT-IR spectra of powdered novaculite (starting material), frictional wear powder from two shearing experiments and commercial nano silica. Small absorption peaks at 694 and 1163 cm⁻¹ indicate Si–O bond structures are present in the wear material, although not in long-range order (Inset b-ii)