Cutting tools are widespread across all the industrial sectors, from aerospace and automotive to the biomedical and consumer electronics, and are employed wherever material processing is required. Coatings, such as Diamond-Like Carbon (DLC) and Titanium Nitride (TiN), are extensively applied to enhance the durability and cutting efficiency of Hard Metal (HM) and High-Speed Steel (HSS) cutting tools, particularly under demanding machining conditions. Over time, tools inevitably develop defects and wear, becoming dull and loosing cutting efficiency. To extend their lifespan through resharpening and recoating, an efficient decoating process is essential. Nowadays, chemical and electrochemical processes are mainly employed in industry for coating stripping, while mechanical and physical techniques are less frequently used. In the present research, the application of vacuum and in-liquid plasma technologies was investigated to tackle the critical issue of cutting tools decoating, aiming at a fast removal of the protective film while avoiding the use of harmful chemicals. Low-Energy High-Current Electron Beam (LEHCEB) was employed to remove DLC coatings from WC-Co2 milling inserts, while Electrolytic Plasma Polishing (EPP) was used to remove TiN coatings from AISI M2 drill bits. Pulsed electron irradiation by LEHCEB completely removed the carbon coatings (a-C:H and ta-C types of DLC) due to the high surface temperatures reached and consequent evaporation of the thin film. The electron accelerating voltages were 20, 25 and 30 kV, with an associated energy density of 2.5, 3.3 and 4.9 J cm-2, respectively. The number of pulses was 5, 10 and 20, with a 0.2 Hz repetition frequency. The working pressure (1.8·10-4 Torr) and the ionized gas (Ar) were kept the same. In the optimized conditions the DLC was stripped from the whole cutting tool in 100 seconds, which was faster with respect to the whole topic literature. After decoating, cutting tools where resharpened, coated again and tested. In particular, the adhesion, the microhardness and the wear behaviour were assessed. LEHCEB treatment was performed also on two different WC-Co grades containing 10 and 20 wt.% of cobalt binder, without any coating. After the surface treatment, the microhardness and the corrosion resistance in acidic environment were measured, and an improvement in both surface properties was observed. The EPP technique allowed to remove the nitride coating by combining the anodic dissolution, typical of any electrochemical polishing process, with the effect of plasma interactions occurring at the surface of the cutting tool. An initial optimization of the electrolyte was performed, comparing more than 40 formulations. Once the electrolyte has been selected, the influence of the DC cell voltage (75-350 V), of the electrolyte concentration (3-24 wt.%) and of the temperature (20-95°C) was investigated. Coating thickness and mass loss were measured at regular intervals in between 1 and 15 min. In the optimized treatment conditions, the titanium nitride coating was stripped in around 150 seconds. Moreover, the EPP process showed great reproducibility and the electrolyte did not suffer of aging even after several hours of treatment. In the end, a decoating mechanism was proposed combining results of different characterization techniques such as EDS, GDOES and XPS. In comparison with commercially available techniques, LEHCEB and EPP technologies allowed to remove protective coatings in a shorter time and without the use of any hazardous chemicals.
Gli utensili da taglio sono ampiamente utilizzati in tutti i settori industriali, dall’aerospaziale e automobilistico fino al biomedicale e all’elettronica di consumo, e vengono impiegati ovunque sia richiesta una lavorazione sottrattiva dei materiali. I rivestimenti, come il Diamond-Like Carbon (DLC) e il nitruro di titanio (TiN), sono applicati estensivamente per incrementare la durabilità e l’efficienza di taglio degli utensili in Metallo Duro (HM) e in Acciaio Super Rapido (HSS), in particolare in condizioni di lavorazioni usuranti. Nel tempo, gli utensili sviluppano inevitabilmente difetti e usura, perdendo affilatura ed efficienza di taglio. Per estenderne la vita utile tramite riaffilatura e nuovo rivestimento, risulta essenziale disporre di un processo di rimozione dei rivestimenti danneggiati efficace. Attualmente, in ambito industriale, vengono impiegati principalmente processi chimici ed elettrochimici per la rimozione dei rivestimenti, mentre le tecniche meccaniche e fisiche sono meno diffuse. Nel presente lavoro di ricerca è stata studiata l’applicazione di tecnologie al plasma in vuoto e in liquido per affrontare la problematica critica del decoating degli utensili da taglio, con l’obiettivo di ottenere una rapida rimozione del film protettivo evitando l’utilizzo di sostanze chimiche inquinanti o nocive. Un fascio elettronico a bassa energia e alta corrente (Low-Energy High-Current Electron Beam, LEHCEB) è stato impiegato per la rimozione dei rivestimenti DLC da inserti di fresatura in WC-Co, mentre la tecnica di elettrolucidatura al plasma (Electrolytic Plasma Polishing, EPP) è stata utilizzata per la rimozione dei rivestimenti TiN da punte da per foratura in acciaio AISI M2. L’irraggiamento pulsato con fascio elettronico LEHCEB ha consentito la completa rimozione dei rivestimenti carboniosi (tipologie a-C:H e ta-C) grazie alle elevate temperature superficiali raggiunte e alla conseguente evaporazione del film sottile. Le tensioni di accelerazione degli elettroni utilizzate sono state 20, 25 e 30 kV, con densità di energia associata pari a 2,5, 3,3 e 4,9 J cm⁻², rispettivamente. Il numero di impulsi è stato fissato a 5, 10 e 20, con frequenza di ripetizione di 0,2 Hz. La pressione di lavoro (1,8·10⁻⁴ Torr) e il gas ionizzato (Ar) sono stati mantenuti costanti. Nelle condizioni ottimizzate, il rivestimento DLC è stato completamente rimosso dall’intero utensile da taglio in 100 secondi, risultando più rapido rispetto a quanto riportato negli studi dipsonibili in letteratura. Dopo la rimozione, gli utensili sono stati riaffilati, nuovamente rivestiti e testati. In particolare, sono state valutate l’adesione, la microdurezza e il comportamento a usura. Il trattamento LEHCEB è stato inoltre applicato a due differenti qualità di WC-Co contenenti il 10 e il 20% in peso di legante cobalto, prive di rivestimenti. Dopo il trattamento superficiale, sono stati misurate la microdurezza e la resistenza alla corrosione in ambiente acido, evidenziando un miglioramento di entrambe le proprietà. La tecnica EPP ha permesso di rimuovere il rivestimento nitrurato combinando la dissoluzione anodica, tipica di qualsiasi processo di lucidatura elettrochimica, con l’azione del plasma che si forma sulla superficie dell’utensile. È stata inizialmente effettuata un’ottimizzazione dell’elettrolita, confrontando oltre 40 formulazioni. Una volta selezionato l’elettrolita, è stata studiata l’influenza della tensione continua di cella (75-350 V), della concentrazione dell’elettrolita (3-24 wt.%) e della temperatura (20-95 °C). Lo spessore del rivestimento e la perdita di massa sono stati misurati a intervalli regolari tra 1 e 15 minuti. Nelle condizioni ottimizzate, il rivestimento di nitruro di titanio è stato rimosso in circa 150 secondi. Inoltre, il processo EPP ha mostrato un’elevata riproducibilità e l’elettrolita non ha mostrato fenomeni di invecchiamento nemmeno dopo diverse ore di trattamento. Infine, è stato proposto un meccanismo di rimozione del rivestimento combinando i risultati ottenuti da diverse tecniche di caratterizzazione, quali EDS, GDOES e XPS. Rispetto alle tecnologie commercialmente disponibili, i processi LEHCEB ed EPP hanno consentito di rimuovere i rivestimenti protettivi in tempi più brevi e senza l’utilizzo di sostanze chimiche pericolose.
Decoating of cutting tools exploiting vacuum and in-liquid plasma technologies
Lucchini Huspek, Andrea
2025/2026
Abstract
Cutting tools are widespread across all the industrial sectors, from aerospace and automotive to the biomedical and consumer electronics, and are employed wherever material processing is required. Coatings, such as Diamond-Like Carbon (DLC) and Titanium Nitride (TiN), are extensively applied to enhance the durability and cutting efficiency of Hard Metal (HM) and High-Speed Steel (HSS) cutting tools, particularly under demanding machining conditions. Over time, tools inevitably develop defects and wear, becoming dull and loosing cutting efficiency. To extend their lifespan through resharpening and recoating, an efficient decoating process is essential. Nowadays, chemical and electrochemical processes are mainly employed in industry for coating stripping, while mechanical and physical techniques are less frequently used. In the present research, the application of vacuum and in-liquid plasma technologies was investigated to tackle the critical issue of cutting tools decoating, aiming at a fast removal of the protective film while avoiding the use of harmful chemicals. Low-Energy High-Current Electron Beam (LEHCEB) was employed to remove DLC coatings from WC-Co2 milling inserts, while Electrolytic Plasma Polishing (EPP) was used to remove TiN coatings from AISI M2 drill bits. Pulsed electron irradiation by LEHCEB completely removed the carbon coatings (a-C:H and ta-C types of DLC) due to the high surface temperatures reached and consequent evaporation of the thin film. The electron accelerating voltages were 20, 25 and 30 kV, with an associated energy density of 2.5, 3.3 and 4.9 J cm-2, respectively. The number of pulses was 5, 10 and 20, with a 0.2 Hz repetition frequency. The working pressure (1.8·10-4 Torr) and the ionized gas (Ar) were kept the same. In the optimized conditions the DLC was stripped from the whole cutting tool in 100 seconds, which was faster with respect to the whole topic literature. After decoating, cutting tools where resharpened, coated again and tested. In particular, the adhesion, the microhardness and the wear behaviour were assessed. LEHCEB treatment was performed also on two different WC-Co grades containing 10 and 20 wt.% of cobalt binder, without any coating. After the surface treatment, the microhardness and the corrosion resistance in acidic environment were measured, and an improvement in both surface properties was observed. The EPP technique allowed to remove the nitride coating by combining the anodic dissolution, typical of any electrochemical polishing process, with the effect of plasma interactions occurring at the surface of the cutting tool. An initial optimization of the electrolyte was performed, comparing more than 40 formulations. Once the electrolyte has been selected, the influence of the DC cell voltage (75-350 V), of the electrolyte concentration (3-24 wt.%) and of the temperature (20-95°C) was investigated. Coating thickness and mass loss were measured at regular intervals in between 1 and 15 min. In the optimized treatment conditions, the titanium nitride coating was stripped in around 150 seconds. Moreover, the EPP process showed great reproducibility and the electrolyte did not suffer of aging even after several hours of treatment. In the end, a decoating mechanism was proposed combining results of different characterization techniques such as EDS, GDOES and XPS. In comparison with commercially available techniques, LEHCEB and EPP technologies allowed to remove protective coatings in a shorter time and without the use of any hazardous chemicals.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/247977