Influence of physical properties and chemical composition of sample on formation of aerosol particles generated by nanosecond laser ablation at 213 nm

Abstract

The influence of sample properties and composition on the size and concentration of aerosol particles generated by nanosecond Nd:YAG laser ablation at 213 nm was investigated for three sets of different materials, each containing five specimens with a similar matrix (Co-cemented carbides with a variable content of W and Co, steel samples with minor differences in elemental content and silica glasses with various colors). The concentration of ablated particles (particle number concentration, PNC) was measured in two size ranges (10–250 nm and 0.25–17 µm) using an optical aerosol spectrometer. The shapes and volumes of the ablation craters were obtained by Scanning Electron Microscopy (SEM) and by an optical profilometer, respectively. Additionally, the structure of the laser-generated particles was studied after their collection on a filter using SEM. The results of particle concentration measurements showed a significant dominance of particles smaller than 250 nm in comparison with larger particles, irrespective of the kind of material. Even if the number of particles larger than 0.25 µm is negligible (up to 0.1%), the volume of large particles that left the ablation cell can reach 50% of the whole particle volume depending on the material. Study of the ablation craters and the laser-generated particles showed a various number of particles produced by different ablation mechanisms (particle splashing or condensation), but the similar character of released particles for all materials was observed by SEM after particle collection on the membrane filter. The created aerosol always consisted of two main structures — spherical particles with diameters from tenths to units of micrometers originally ejected from the molten surface layer and µm-sized “fibres” composed of primary agglomerates with diameters in the range between tens and hundreds of nanometers. The shape and structure of ablation craters were in good agreement with particle concentration measurements. Samples with a high crater rim and melted bottom produced the highest number of large spherical particles. The mass values of all produced aerosols were calculated from the measured ablation crater volumes and sample densities.