Thermally sprayed WC-CoCr and Cr3C2-NiCr hardmetal coatings are widely used in industrial components subject to severe working conditions as a method of protection against wear and corrosion. However, the aforementioned compositions have restrictions that limit their applications: WC-CoCr has thermal expansion incompatibility with steel substrates and oxidizes catastrophically at T > 400-500 °C, while Cr3C2-NiCr offers lower wear protection at low and moderate temperatures. Despite the scarce literature on thermally sprayed NbC-based coatings, promising wear resistance results have been obtained with NbC-based bulk hardmetal formulations over a wide temperature range. In this context, in the present work, different compositions of NbC-based hardmetal coatings were sprayed by the High-Velocity Oxygen Fuel (HVOF) process on stainless steel substrates aiming to achieve an adequate balance of different tribological and electrochemical performances. The production of NbC-based coatings has been developed in two stages: in the first stage, NbC particles were homogeneously dispersed in either 25 vol.% or 40 vol.% of a Ni-20wt.%Cr matrix. In the second stage, NbC particles were dispersed in 40vol.% of a matrix containing Fe-25 wt.%Cr-15 wt.%Mo. Subsequently, an adjustment to the matrix composition was carried out: the content of Mo was reduced to 6 wt.% and the element B was added (1 wt.% on the matrix) aiming to produce a more homogeneous coating and to reduce oxidation during spraying. In both stages, the homogenization of the NbC particles in the binder phase was carried out by mechanical alloying through the process by High-Energy Ball Milling (HEBM). Different sets of thermal spray parameters were employed and the microstructure, Vickers microhardness, abrasion resistance, dry sliding wear resistance (from room temperature up to 600 °C) and corrosion resistance of the coatings were studied. The results were compared to those obtained on reference coatings (WC-CoCr, Cr3C2-NiCr) and other alternative hardmetal coatings (TiC-40NiCr and TiC-25NiCr). Regardless of the process parameters, NbC-based coatings showed low porosity (< 2%) and microhardness values of around 900 HV0.3 (NbC-25NiCr) and 1000 HV0.3 (NbC-40NiCr and NbC-FeCrMo). The NbC-NiCr coatings presented dry sliding wear resistance intermediate between those of conventional WC-CoCr and Cr3C2-NiCr coatings, and comparable to that of TiC-NiCr, despite producing higher friction coefficients. NbC-FeCrMo coatings showed low resistance to sliding wear at low and moderate temperatures (≤ 300 °C), but high resistance at high temperatures, particularly at 400 °C. All the NbC-based coatings exhibited limited three-body abrasion resistance due to the occurrence of some brittle fractures together with grooving due to abrasion. The NbC-NiCr coatings, especially the one with 40vol.% matrix phase, exhibited excellent corrosion resistance: the corrosion current densities (~0.1 μA/cm2) and passive current densities (<1 μA/cm2) were lower than the corresponding values found for all reference coatings. Thus, NbC-NiCr coatings are promising for applications where a good balance between corrosion resistance and wear resistance over a wide temperature range is required, while the NbC-FeCrMo coating is particularly promising for sliding wear applications at 400°C.
I rivestimenti termospruzzati in metallo duro WC-CoCr e Cr3C2-NiCr sono ampiamente utilizzati in componenti industriali soggetti a condizioni di lavoro severe come metodo di protezione contro usura e corrosione. Tuttavia, queste composizioni presentano restrizioni che ne limitano le applicazioni: il WC-CoCr ha incompatibilità di espansione termica con substrati in acciaio e si ossida in modo catastrofico a T>400-500°C, mentre Cr3C2-NiCr offre una minor protezione dall'usura a temperatura ambiente o moderata. Nonostante l’assenza di studi su rivestimenti termospruzzati in NbC, sono stati ottenuti risultati promettenti in termini di resistenza all'usura in un ampio intervallo di temperature usando metalli duri sinterizzati a base di NbC. In questo contesto, nel presente lavoro, diverse composizioni di rivestimenti a base di NbC sono state termospruzzate col processo High-Velocity Oxygen Fuel (HVOF) su substrati di acciaio inossidabile con l'obiettivo di raggiungere un adeguato equilibrio tra le prestazioni tribologiche e di resistenza alla corrosione. La produzione di rivestimenti a base di NbC è stata organizzata in due fasi. Nella prima fase, sono stati prodotti cermet con particelle di NbC disperse in una matrice Ni-20%Cr, con frazione volumetrica del 25% e del 40%.Nella seconda fase, le particelle di NbC sono state disperse in una frazione volumetrica del 40% di una matrice contenente Fe-25wt.%Cr-15wt.%Mo. Successivamente è stato effettuato un aggiustamento alla composizione di quest’ultima matrice, riducendone il contenuto di Mo al 6% e aggiungendo l’1wt.% di B per produrre un rivestimento più omogeneo e per ridurre l'ossidazione durante la spruzzatura. In entrambe le fasi, le particelle di NbC sono state disperse nella fase legante mediante il processo di macinazione ad alta energia (High Energy Ball Milling). Sono stati impiegati diversi set di parametri di termospruzzatura e sono state determinate la microstruttura, la microdurezza Vickers, la resistenza all'abrasione, la resistenza all'usura per strisciamento a secco (fino a 600°C) e la resistenza alla corrosione dei rivestimenti ottenuti. I risultati ottenuti sono stati confrontati con quelli raccolti su rivestimenti in metallo duro di riferimento (WC-CoCr, Cr3C2-NiCr) e su altre composizioni alternative (TiC-40NiCr e TiC-25NiCr). Indipendentemente dai parametri di processo, i rivestimenti a base di NbC hanno mostrato una bassa porosità (< 2%) e valori di microdurezza di circa 900 HV0,3 (NbC-25NiCr) e 1000 HV0,3 (NbC-40NiCr e NbC-FeCrMo). I rivestimenti NbC-NiCr presentano una resistenza all’usura da strisciamento a secco intermedia rispetto ai tradizionali rivestimenti WC-CoCr e Cr3C2-NiCr e paragonabile a quella del TiC-NiCr, nonostante producanro coefficienti di attrito più elevati. I rivestimenti NbC-FeCrMo hanno mostrato una bassa resistenza all'usura da strisciamento a temperature basse e moderate (≤ 300 °C), ma un'elevata resistenza alle alte temperature, in particolare a 400 °C. In entrambe le fasi, i rivestimenti a base di NbC hanno mostrato una resistenza all'abrasione a tre corpi limitata a causa del verificarsi di frattura fragile insieme alla solcatura duttile. I rivestimenti NbC-NiCr, in particolare quello con 40vol.% di matrice, hanno mostrato un'eccellente resistenza alla corrosione, densità di corrente di corrosione (~0,1 μA/cm2) e densità di corrente passivazione (<1 μA/cm2) inferiori ai valori corrispondenti trovati per i rivestimenti di riferimento. Pertanto, i rivestimenti NbC-NiCr sono promettenti per applicazioni in cui è richiesto un buon equilibrio tra resistenza alla corrosione e resistenza all’usura in un ampio intervallo di temperature, mentre il rivestimento NbC-FeCrMo è particolarmente promettente per applicazioni a 400°C.
Sviluppo di rivestimenti in metallo duro a base di carburo di niobio spruzzato termicamente / Larissa Rossi Gehlen , 2024 May 15. 36. ciclo, Anno Accademico 2022/2023.
Sviluppo di rivestimenti in metallo duro a base di carburo di niobio spruzzato termicamente
ROSSI GEHLEN, LARISSA
2024
Abstract
Thermally sprayed WC-CoCr and Cr3C2-NiCr hardmetal coatings are widely used in industrial components subject to severe working conditions as a method of protection against wear and corrosion. However, the aforementioned compositions have restrictions that limit their applications: WC-CoCr has thermal expansion incompatibility with steel substrates and oxidizes catastrophically at T > 400-500 °C, while Cr3C2-NiCr offers lower wear protection at low and moderate temperatures. Despite the scarce literature on thermally sprayed NbC-based coatings, promising wear resistance results have been obtained with NbC-based bulk hardmetal formulations over a wide temperature range. In this context, in the present work, different compositions of NbC-based hardmetal coatings were sprayed by the High-Velocity Oxygen Fuel (HVOF) process on stainless steel substrates aiming to achieve an adequate balance of different tribological and electrochemical performances. The production of NbC-based coatings has been developed in two stages: in the first stage, NbC particles were homogeneously dispersed in either 25 vol.% or 40 vol.% of a Ni-20wt.%Cr matrix. In the second stage, NbC particles were dispersed in 40vol.% of a matrix containing Fe-25 wt.%Cr-15 wt.%Mo. Subsequently, an adjustment to the matrix composition was carried out: the content of Mo was reduced to 6 wt.% and the element B was added (1 wt.% on the matrix) aiming to produce a more homogeneous coating and to reduce oxidation during spraying. In both stages, the homogenization of the NbC particles in the binder phase was carried out by mechanical alloying through the process by High-Energy Ball Milling (HEBM). Different sets of thermal spray parameters were employed and the microstructure, Vickers microhardness, abrasion resistance, dry sliding wear resistance (from room temperature up to 600 °C) and corrosion resistance of the coatings were studied. The results were compared to those obtained on reference coatings (WC-CoCr, Cr3C2-NiCr) and other alternative hardmetal coatings (TiC-40NiCr and TiC-25NiCr). Regardless of the process parameters, NbC-based coatings showed low porosity (< 2%) and microhardness values of around 900 HV0.3 (NbC-25NiCr) and 1000 HV0.3 (NbC-40NiCr and NbC-FeCrMo). The NbC-NiCr coatings presented dry sliding wear resistance intermediate between those of conventional WC-CoCr and Cr3C2-NiCr coatings, and comparable to that of TiC-NiCr, despite producing higher friction coefficients. NbC-FeCrMo coatings showed low resistance to sliding wear at low and moderate temperatures (≤ 300 °C), but high resistance at high temperatures, particularly at 400 °C. All the NbC-based coatings exhibited limited three-body abrasion resistance due to the occurrence of some brittle fractures together with grooving due to abrasion. The NbC-NiCr coatings, especially the one with 40vol.% matrix phase, exhibited excellent corrosion resistance: the corrosion current densities (~0.1 μA/cm2) and passive current densities (<1 μA/cm2) were lower than the corresponding values found for all reference coatings. Thus, NbC-NiCr coatings are promising for applications where a good balance between corrosion resistance and wear resistance over a wide temperature range is required, while the NbC-FeCrMo coating is particularly promising for sliding wear applications at 400°C.File | Dimensione | Formato | |
---|---|---|---|
R.pdf
embargo fino al 15/05/2027
Descrizione: Tesi definitiva - ROSSI GEHLEN LARISSA
Tipologia:
Tesi di dottorato
Dimensione
17.09 MB
Formato
Adobe PDF
|
17.09 MB | Adobe PDF | Visualizza/Apri Richiedi una copia |
Pubblicazioni consigliate
I metadati presenti in IRIS UNIMORE sono rilasciati con licenza Creative Commons CC0 1.0 Universal, mentre i file delle pubblicazioni sono rilasciati con licenza Attribuzione 4.0 Internazionale (CC BY 4.0), salvo diversa indicazione.
In caso di violazione di copyright, contattare Supporto Iris