Laser Induced Breakdown Spectroscopy (LIBS)

Laser-induced breakdown spectroscopy (LIBS) uses a highly energetic laser pulse as an excitation source, to optically breakdown, vaporize and convert elements in the sample to plasma state. The amount of sample subjected to breakdown (termed ‘ablation’) and vaporization to plasma, is in the range of nanograms to picograms. The plasma cools, within microseconds and various atoms emit characteristic spectral lines that can identify them. The detector needs to be gated to capture full spectral information within the microseconds duration. The principle is illustrated in Figure 1.

Adhesive bonded fibre-polymer composites are important for applications such as light-weighting of structures and components in the transportation sector. They also find application in consumer goods, industrial and infrastructural applications. Composites can be bonded to each other or to metals or non-metals using adhesives. Adhesive bonds between surfaces require the surfaces to be free from contaminants. Even trace contaminants can reduce bond strength and result in structural failure under load.

Carbon Fibre Reinforced Polymers (CFRP) are an important category of composites, due to their light weight, strength and corrosion resistance. Due to its high sensitivity LIBS can be used to detect ultralow concentrations of contamination on CFRP samples.  

In this case study, the LIBS technique was used for detection of low levels of silicone contaminants in CFRP composites. Sample (30.5 cm × 30.5 cm) CFRP panel surfaces were contaminated in a controlled manner with polydimethylsiloxane (PDMS). The panels were analyzed by LIBS before and after laser ablation, providing information on the ability of LIBS to remove silicone as well as to detect very low levels of silicone.

LIBS ablation was performed using an Nd:YVO4 laser system with pulse energies below 30 μJ, duration about 10 picoseconds, lasing wavelength of 350 nm and frequency about 400 kHz. For contamination studies, wavelengths ranging from 370 nm to 900 nm with a 10 nm step size were used at three incident angles: 65°, 70°, and 75°. From this data, the PDMS layer thickness  was inferred. SEM micrographs of ablation depth of the surface at increasing energy of single pulse laser shots are seen in Figure 2. The craters produced show no melted or redeposited material around the crater border. Above15 μJ, an inner crater starts forming due to the increasing energy and the crater deepens faster than it widens. In contrast to the crater contour, the inner crater has a circular shape. In all the LIBS measurements, the pulses do not interact with the carbon fibres underneath the resin surface layer, as shown in the SEM micrographs.

Figure 1:  The principle of LIBS and the type of spectrum for Si.

Figure 2:  Single laser pulse shot, SEM micrographs of ablated craters at a) 7.5 μJ (3.77 J/cm2), b) 15 μJ (7.53 J/cm2), c) 25 μJ (12.55 J/cm2), and d) 30 μJ (15.06 J/cm2).

The figures 3 and 4 show peak heights for Carbon and Silicon, before and after ablation. The reduction in height of Si peak after ablation is obvious. The ability of LIBS to detect low levels of Si and to remove contaminant by ablation from CFRP is also established

Figure 3: LIBS spectra before laser ablation for the wavelength from 240 nm to 300 nm. The relevant peaks are 247.9 nm for C I, and 251.6 nm and 288.2 nm for Si I.

Figure 4: LIBS spectra after laser ablation for the wavelengths from 240 nm to 300 nm. The relevant peaks are 247.9 nm for C I, and 251.6 nm and 288.2 nm for Si I.

Reference:

Rodolfo Ledesma, Frank Palmieri, John Connell, William Yost, and James Fitz-Gerald, Surface characterization of carbon fiber reinforced polymers by picosecond laser induced breakdown spectroscopy, Spectrochim Acta Part B At Spectrosc. 2018 Feb; 140: 5–12.