High repetition rate femtosecond laser heat accumulation and ablation thresholds in cobalt-binder and binderless tungsten carbides

Abstract

Femtosecond (fs) laser ablation has been studied for the potential of fast, high precision machining of difficult-to-machine materials like binderless tungsten carbide. Obstacles that have limited its efficiency include melting from heat accumulation (HA), particle shielding, and plasma shielding. To address HA without shielding effects, high repetition rate (57.4 MHz), ultra-low fluence fs laser irradiation is performed to study the incubation effect and subsequent HA-ablation threshold of fine-grained tungsten carbides. Exposure times on the order of 100 ms were conducted in air with fluences (1.82 to 9.09 mJ/cm2) two orders of magnitude below the single fs pulse ablation thresholds reported in literature (0.4 J/cm2). Heat accumulation at high repetition rate explains the ultra-low fluence melt threshold behavior resulting in melt crowns around ablated holes and grooves. The results of this study aid in predicting heat buildup in high repetition rate laser irradiation for applications that wish to achieve high ablation rates of difficult-to-machine, ultrahard materials and help enable shaping of binderless tungsten carbide for use in applications too extreme for bindered tungsten carbide.