Milling parameters effects on WC-12%Co alloy thermal spray coatings

Treball de Fi de Grau Grau en Enginyeria en Tecnologies Industrials (GETI) Milling parameters effects on WC-12%Co alloy thermal spray coatings REPORT Author: Tomàs Minguell Llovera Supervisor: Crisanto José Villalobos Gutiérrez Call: June 2024 Escola Tècnica Superior d’Enginyeria Industrial de Barcelona Pàg. 2 Report Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 3 Resum Aquest treball de fi de grau investiga els paràmetres òptims de mòlta per produir pols d'aliatge WC-Co 12% amb diferents concentracions de nanotubs de carboni (CNT en anglès) per al seu ús en recobriments mitjançant projecció tèrmica d'alta velocitat amb oxicombustible (HVOF). L'objectiu és identificar les condicions que minimitzin la porositat i millorin les propietats mecàniques i tèrmiques dels recobriments. L'estudi avalua tres diferents relacions de cossos demolidors a pols (BPR en anglès) de 0:1, 1:1 i 2:1, combinades amb concentracions de CNT del 15%, 35% i 45%. Utilitzant aquests paràmetres, s'han creat i analitzat nou mostres dividides en tres grups. S'ha mesurat la porositat dels recobriments resultants de les mostres i s'han examinat les distribucions de mida i forma de les partícules utilitzant imatges de microscòpia electrònica d'escombrat d'emissió de camp (FESEM en anglès). L'anàlisi ha revelat que la relació BPR de 1:1 amb una concentració de CNT del 45% (mostra M-01-45) i la relació BPR de 2:1 amb una concentració de CNT del 15% (mostra M-02-15) produeixen els resultats més favorables. Aquestes mostres mostren una porositat mínima i una baixa desviació estadística en la forma de les partícules, indicant la producció de pols uniformes i d'alta qualitat adequats per a aplicacions de projecció tèrmica. La mostra M-01-45 presenta una porositat lleugerament menor, per la qual cosa s'ha seleccionat com la mostra amb condicions òptimes de mòlta. En contrast, la relació BPR de 0:1 resulta en una alta desviació en la forma de les partícules i recobriments menys uniformes, posant de relleu la necessitat de mitjans de mòlta en el procés d'aliatge mecànic. La relació BPR de 2:1, tot i que millora la distribució de la forma de les partícules, generalment té una major desviació estadística però també resulta en recobriments de baixa porositat. Això la converteix en una altra opció adequada per a aplicacions de projecció tèrmica d'alta qualitat a més de la relació de 1:1. En conclusió, les condicions òptimes de mòlta per produir pols d'aliatge WC-Co 12% d'alta qualitat per a recobriments HVOF s'han identificat amb una relació BPR de 1:1 amb una concentració de CNT del 45%, tot i que una relació BPR de 2:1 amb una concentració de CNT del 15% també dona resultats pràcticament òptims. Aquests paràmetres asseguren baixa porositat i distribucions uniformes de partícules, millorant les propietats mecàniques i tèrmiques dels recobriments. Aquesta investigació proporciona una base sòlida per al desenvolupament continu de recobriments de projecció tèrmica d'alt rendiment, amb implicacions importants per a indústries que requereixen tractaments de superfícies duradors i fiables. Pàg. 4 Report Resumen Este trabajo de fin de grado investiga los parámetros óptimos de molienda para producir polvos de aleación WC-Co 12% con diferentes concentraciones de nanotubos de carbono (CNT en inglés) para su uso en recubrimientos mediante proyección térmica de alta velocidad con oxicombustible (HVOF). El objetivo es identificar las condiciones que minimicen la porosidad y mejoren las propiedades mecánicas y térmicas de los recubrimientos. El estudio evalúa tres diferentes relaciones de cuerpos demoledores a polvo (BPR en inglés) de 0:1, 1:1 y 2:1, combinadas con concentraciones de CNT de 15%, 35% y 45%. Usando estos parámetros, se han creado y analizado nueve muestras divididas en tres grupos. Se ha medido la porosidad de los recubrimientos resultantes de las muestras y se han examinado las distribuciones de tamaño y forma de las partículas utilizando imágenes de microscopía electrónica de barrido de emisión de campo (FESEM en inglés). El análisis ha revelado que la relación BPR de 1:1 con una concentración de CNT del 45% (muestra M-01-45) y la relación BPR de 2:1 con una concentración de CNT del 15% (muestra M-0215) producen los resultados más favorables. Estas muestras muestran una porosidad mínima y una baja desviación estadística en la forma de las partículas, indicando la producción de polvos uniformes y de alta calidad adecuados para aplicaciones de proyección térmica. La muestra M-01-45 presenta una porosidad ligeramente menor, por lo que se ha seleccionado como la muestra con condiciones óptimas de molienda. En contraste, la relación BPR de 0:1 resulta en una alta desviación en la forma de las partículas y recubrimientos menos uniformes, poniendo de relieve la necesidad de medios de molienda en el proceso de aleación mecánica. La relación BPR de 2:1, aunque mejora la distribución de la forma de las partículas, generalmente tiene una mayor desviación estadística pero también resulta en recubrimientos de baja porosidad. Esto la convierte en otra opción adecuada para aplicaciones de proyección térmica de alta calidad además de la relación de 1:1. En conclusión, las condiciones óptimas de molienda para producir polvos de aleación WCCo 12% de alta calidad para recubrimientos HVOF se han identificado con una relación BPR de 1:1 con una concentración de CNT del 45%, aunque una relación BPR de 2:1 con una concentración de CNT del 15% también da resultados prácticamente óptimos. Estos parámetros aseguran baja porosidad y distribuciones uniformes de partículas, mejorando las propiedades mecánicas y térmicas de los recubrimientos. Esta investigación proporciona una base sólida para el desarrollo continuo de recubrimientos de proyección térmica de alto rendimiento, con implicaciones importantes para industrias que requieren tratamientos de superficies duraderos y fiables. Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 5 Abstract This thesis delves into the optimal milling parameters for producing WC-Co 12% alloy powders with varying concentrations of carbon nanotubes (CNTs) for use in High-Velocity Oxy-Fuel (HVOF) thermal spray coatings. The objective is to identify conditions that minimize porosity and enhance the mechanical and thermal properties of the coatings. The study evaluates three different ball-to-powder ratios (BPR) of 0:1, 1:1, and 2:1, combined with CNT concentrations of 15%, 35%, and 45%. Using these parameters, nine samples were created and analyzed divided in three groups. The samples' resulting coating porosities were measured, and the particle size and shape distributions were examined using images from Field Emission Scanning Electron Microscopy (FESEM). The analysis revealed that the 1:1 BPR with a 45% CNT concentration (sample M-01-45) and the 2:1 BPR with a 15% CNT concentration (sample M-02-15) produced the most favorable results. These samples exhibited minimal porosity and low statistical deviation in particle shape, indicating uniform and high-quality powders suitable for thermal spray applications. Sample M-01-45 yielded a slightly lower porosity, so it was selected as the one with optimal milling conditions. In contrast, the 0:1 BPR resulted in a high deviation in particle shape and less uniform coatings, underscoring the necessity of milling media in the alloying process. The 2:1 BPR, although improving particle shape distribution, usually showed higher statistical deviation, also yielded low porosity coatings. This makes it another suitable option for high-quality thermal spray applications besides 1:1 ball-to-powder ratio. In conclusion, the optimal milling conditions for producing high-quality WC-Co 12% alloy powders for HVOF coatings were identified as a 1:1 BPR with 45% CNT concentration, although 2:1 BPR with 15% CNT concentration also yielded almost optimal results. These parameters ensure low porosity and uniform particle distributions, enhancing the mechanical and thermal properties of the coatings. This research provides a robust foundation for the continued development of high-performance thermal spray coatings, with significant implications for industries requiring durable and reliable surface treatments. Pàg. 6 Report Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 7 Contents RESUM ____________________________________________________ 3 RESUMEN _________________________________________________ 4 ABSTRACT _________________________________________________ 5 CONTENTS _________________________________________________ 7 ABBREVIATIONS AND SYMBOLS ______________________________ 8 LIST OF FIGURES ___________________________________________ 9 LIST OF TABLES ___________________________________________ 11 1. PREFACE _____________________________________________ 13 2. INTRODUCTION ________________________________________ 14 2.1. Motivation ................................................................................................. 14 2.2. Scope ....................................................................................................... 14 2.3. Prerequisites ............................................................................................ 15 2.3.1. Technical prerequisites ............................................................................... 15 2.3.2. Legal Prerequisites ..................................................................................... 15 2.4. Objectives ................................................................................................ 16 3. THEORETICAL BACKGROUND ___________________________ 17 3.1. Theoretical fundamentals ......................................................................... 17 3.2. Antecedents ............................................................................................. 18 3.3. State of the Art ......................................................................................... 21 4. METHODOLOGY AND EQUIPMENT________________________ 26 5. RESULTS _____________________________________________ 27 6. PLANNING ____________________________________________ 43 7. ECONOMIC ASSESSMENT _______________________________ 44 8. ENVIRONMENTAL ASSESSMENT _________________________ 46 9. SOCIAL AND GENDER EQUALITY ASSESSMENT ____________ 47 10. CONCLUSIONS ________________________________________ 48 11. BIBLIOGRAPHY ________________________________________ 49 Pàg. 8 Report Abbreviations and symbols CNT: Carbon NanoTubes MWCNTs: Multiwalled Carbon NanoTubes SWCNT: Single Walled NanoTube HVOF: High-velocity oxygen fuel TFG: Treball fi de grau (bachelor’s thesis) SDG: Sustainable development goals WC-Co: tungsten carbide-cobalt alloy BPR: Ball to powder ratio (Relación de Cuerpos demoledores-polvo metálico) FESEM: Field-Emission Scanning Electron Microscope Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 9 List of figures Image 1: Sample R-00-15 ............................................................................................... 27 Image 2: Sample R-00-35 ............................................................................................... 28 Image 3: Sample R-00-45 ............................................................................................... 28 Image 4: Sample R-01-15 ............................................................................................... 29 Image 5: Sample R-01-35 ............................................................................................... 30 Image 6: Sample R-01-45 ............................................................................................... 30 Image 7: Sample R-02-15 ............................................................................................... 31 Image 8: Sample R-02-35 ............................................................................................... 32 Image 9: Sample R-02-45 ............................................................................................... 32 Image 10: Sample M-00-15 ............................................................................................. 35 Image 11: Sample M-00-35 ............................................................................................. 35 Image 12: Sample M-00-45 ............................................................................................. 36 Image 13: Sample M-01-15 ............................................................................................. 37 Image 14: Sample M-01-35 ............................................................................................. 37 Image 15: Sample M-01-45 ............................................................................................. 38 Image 16: Sample M-02-15 ............................................................................................. 39 Image 17: Sample M-02-35 ............................................................................................. 39 Image 18: Sample M-02-45 ............................................................................................. 40 Figure 1: Size and shape distribution on sample M-00-15 .............................................. 35 Figure 2: Size and shape distribution on sample M-00-35 .............................................. 36 Figure 3: Size and shape distribution on sample M-00-45 ............................................... 36 Figure 4: Size and shape distribution on sample M-01-15 ............................................... 37 Pàg. 16 Report except for referenced works and analyzed images. These will adhere to standard APA citation and referencing procedures. The research will be conducted in full compliance with all relevant university procedures and regulations. 2.4. Objectives The overall objective of this thesis is to evaluate the effect of mixing parameters, specifically the percentage of carbon nanotubes (CNT) and the ball-to-powder ratio (BPR), on a mechanically alloyed mixture of tungsten carbide-cobalt (WC-Co) powders with carbon nanotubes. This evaluation involves comparing the distribution of shapes and sizes of the powders after the mixing process and the porosity of the resulting coating after thermal spraying. Furthermore, the aim is to establish a relationship between the characteristics of the powders, in terms of shape and size distribution, and the porosity of the deposited coatings. This will allow the determination of the best milling conditions to produce the tungsten carbide-cobalt alloy, using the concentration of carbon nano tubes and the ball-to-powder ratio as controlled variables, to optimize its use on thermal spray processes. Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 17 3. Theoretical background 3.1. Theoretical fundamentals This section serves as the foundational knowledge base for comprehending the context and principles underlying the research. This foundational knowledge is involved in finding the optimal conditions to produce an enhanced mechanical alloy of tungsten carbide-cobalt, mainly to be used in thermal spray processes, through the incorporation of carbon nanotubes. Mechanical alloying is a powder processing technique used to create homogeneous mixtures of various materials, particularly metals, by repetitive deformation, fracture, and cold welding of particles during high-energy ball milling. This process involves the use of a ball mill, a mechanical device that imparts energy to the materials through the impact of grinding balls. The resultant microstructural changes include grain size reduction and the formation of solid solutions or compounds. It produces an alloy powder and requires pure powder of the materials to be alloyed. Tungsten carbide (WC) combined with cobalt (Co) forms a widely used alloy known for its exceptional hardness, wear resistance, and toughness. WC-Co alloys exhibit high hardness, making them suitable for various applications. However, a fundamental challenge in materials science is the trade-off between hardness and toughness, as increasing one property often comes at the expense of the other. This balance is particularly critical in tribological applications, where wear resistance and durability are paramount. Additionally, the alloys' propensity for crack propagation, especially through the interface between the carbide and cobalt phases, is a critical concern, affecting parameters such as structural integrity and performance. Tribological materials are those engineered to balance friction, wear, and lubrication in the interaction of surfaces. They are designed with specific properties that cater to the challenging demands of high-performance applications, where the reduction of frictional forces, prevention of wear, and effective management of lubrication are of paramount importance. Tribological materials are instrumental in enhancing the efficiency, longevity, and reliability of complex machinery, such as engines, bearings, and industrial equipment, where surface interactions and the mitigation of friction-related challenges are central concerns. They are often used in applications involving friction, wear, and lubrication. Balancing the properties of hardness and toughness in tribological materials is a recurring challenge. High hardness is essential for resistance to wear and abrasion, while toughness is crucial for withstanding impact and preventing crack propagation. Optimizing tribological properties Pàg. 18 Report involve achieving the right balance between hardness and toughness, as excessive hardness can lead to brittleness, while excessive toughness may compromise wear resistance. Carbon nanotubes are nanoscale cylindrical structures composed of carbon atoms arranged in a hexagonal lattice. These structures are known for their exceptional mechanical, electrical, and thermal properties. In this research. CNTs are considered as potential reinforcing agents to enhance the mechanical properties and thermal conductivity of materials. Achieving uniform dispersion of CNTs in the alloy matrix is crucial for optimizing properties. In this research, the aim is to address the challenge of optimizing the tribological properties of tungsten carbide-cobalt alloys. This endeavor involves enhancing the toughness of the WC-Co alloy through the addition of carbon nanotubes in the mechanical alloying process. Subsequent sections will delve into experimental methodologies and analysis to allow this research. 3.2. Antecedents Ball mills have been extensively studied by various researchers, yielding favorable results that have spurred further investigation. Bakshi reported significant improvements in both modulus of elasticity and creep resistance by 39% and 27%, respectively, using a ball mill (Bakshi, 2008). Mixed multi-walled carbon nanotubes (MWCNTs) at fixed 10-hour intervals with WC-10%Co powders at varying concentrations (0.4%, 0.8%, 1.6%), concluding that the 0.4% MWCNT coating exhibited optimal performance based on evaluated characteristics (Bakshi, 2008). Mixing times of CNTs with WC-12%Co powders significantly influence outcomes, identifying an optimal mixing time of 36 hours using a ball mill, followed by evaluation using the High Velocity Oxi-Fuel (HVOF) technique for coating application (Picas, 2011), and effective dispersion of CNTs in the composite was achieved with a 48hour mixing time using a horizontal jar mill (Laha T. L., 2007). Finally, more recent investigation provided insights into the weight ratio between milling balls and powder, ranging from 1:1 to 10:1, respectively, for ball mill mixing operations (Loos, 2015). Multi-walled carbon nanotubes (MWCNTs) have been investigated in conjunction with WCCo to enhance mechanical properties. Other conducted studies emphasizing WC-Co's tribological use as a coating material, offering a combination of high toughness, hardness, and good resistance (Rodriguez, Effects of the dispersion time on the microstructure and wear resistance of WC/Co-CNTs HVOF sprayed coatings, 2014). Attempts to improve mechanical and wear properties of WC-Co include reinforcement with MWCNTs using thermal spraying techniques. In their work, 0.35 wt% of MWCNTs were dispersed in an ethanol solution using a jar mill with microcrystalline WC-12%Co powders, varying mixing times. The coating's microstructure was characterized using scanning electron microscopy Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 19 (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and X-ray diffraction (XRD). Microhardness was measured using Vickers indentation, and abrasive wear resistance was evaluated using a dry sand rubber wheel test according to ASTM G65-00 standards. Results indicated an optimal mixing time of 36 hours for achieving uniform dispersion of CNTs. It was demonstrated that the composite mixed for 36 hours increased wear resistance by 80% and 70% compared to conventional and nanostructured coatings, respectively. Zhang and collaborators used CNT-WC-Co composites and demonstrated that even a small amount of CNTs can improve compound density, with marginal grain size increase (Ma, 2014). Their sintered CNT-WC-Co composite exhibited a superior hardnesstoughness ratio compared to pure WC-Co cermet manufactured under the same process. Other research illustrated experimental results showing that in WC-10Co-0.5wt%CNTs, there was approximately 15% higher hardness and 40% higher fracture toughness compared to pure WC-10Co (Xu, 2015). Carbon nanotubes (CNTs) belong to the group of one-dimensional nanostructured materials. Discovered in 1991 by Iijima, carbon nanotubes have spurred extensive research due to their unique electrical and mechanical properties (Mamalis, 2004). CNTs are allotropes of carbon characterized by a cylindrical nanostructure, resembling a rolled-up graphite sheet with either open ends or capped with half a C60 molecule at each end (Mamalis, 2004). The cylindrical wall of a nanotube can consist of a single layer, forming a single-walled carbon nanotube (SWCNT), or multiple concentric layers, known as multiwalled carbon nanotubes (MWCNTs), with interlayer spacing of approximately 0.34 nm, close to that of graphite (Iijima, 1991) (Kuchibhatla, 2007). Carbon nanotubes vary in length from hundreds of nanometers to several hundred micrometers and can have diameters ranging from 0.37 to 100 nm (Kuchibhatla, 2007). Due to their excellent electrical conductivity, CNTs find applications in molecular sieves, nano-test tubes, and hydraulic actuators (Kuchibhatla, 2007). Ensuring efficient adhesion of CNTs to the material they reinforce is crucial for their involvement under load, ensuring uniform load distribution within the CNT to prevent the outermost layer from peeling off (Mamalis et al., 2004). Recent research has extensively explored the use of MWCNTs as reinforcement materials, given their exceptionally high modulus of elasticity (300-950 GPa) and tensile strength (11-63 GPa). Other research emphasized the challenges in achieving homogeneous distribution and effective retention of CNTs in the matrix post-consolidation (Laha T. L., 2007). In their study, MWCNTs were used as reinforcement for Al-Si hypereutectic structures obtained via plasma thermal spraying and HVOF (High Velocity Oxy-Fuel) thermal spraying. A mixture of Al-Si powders and 10% by weight of MWCNTs was ball-milled for 48 hours to promote homogenous mixing. They observed that in HVOF-sprayed coatings, darker zones on the surface corresponded to carbon presence, more prominent compared to plasma-sprayed surfaces, attributed to higher velocities (700-1400 m/s vs. 350-1000 m/s) causing cluster explosion in the HVOF process. Moreover, HVOF-sprayed composites exhibited a denser, compact structure with higher residual stress, modulus of elasticity, and hardness compared to plasma-sprayed counterparts, successfully retaining physically intact CNTs Pàg. 20 Report within the sprayed composite. Similarly, MWCNTs were used as reinforcement in a nanocomposite aluminum coating using cold spraying (Bakshi, 2008). They employed a turbula mixer to blend aluminum powder and eutectic aluminum-silicon alloy powders with surface-dispersed MWCNTs for one hour. The resulting coatings with 0.5% and 1% by weight of MWCNTs successfully retained and embedded within the matrix. However, they reported CNTs experiencing length reduction due to impact-induced fractures between projected particles. The coatings achieved higher elasticity modulus values of 229 GPa for 0.5% by weight and 191 GPa for 1% by weight, attributed to the reinforcing effect of MWCNTs and their high concentration in the measured regions. Carbon nanotubes (CNTs) are a group of one-dimensional nanostructured materials. Discovered by Iijima in 1991, these cylindrical nanostructures of carbon have sparked extensive research due to their unique electrical and mechanical properties (Mamalis, 2004). They consist of rolled-up graphite sheets with cylindrical walls, either open-ended or capped with half a C60 molecule at each end. CNTs can exist as single-walled (SWCNTs), comprising a single graphene layer, or multi-walled (MWCNTs), with multiple concentric graphene cylinders, exhibiting an interlayer spacing close to graphite (Iijima, 1991) (Kuchibhatla, 2007). These nanotubes vary widely in length from hundreds of nanometers to several hundred micrometers, with diameters ranging from 0.37 to 100 nm (Kuchibhatla, 2007). Their exceptional electrical conductivity makes them suitable for applications such as molecular sieves, nano-test tubes, and hydraulic actuators (Kuchibhatla, 2007). Ensuring efficient adhesion of CNTs to the material they reinforce is crucial for uniform load distribution and to prevent peeling of the outermost layer under load (Mamalis et al., 2004). Recent research has extensively explored the use of MWCNTs as reinforcements due to their exceptionally high modulus of elasticity (300-950 GPa) and tensile strength (11-63 GPa). Laha et al. (2007) highlighted the challenges in achieving homogeneous distribution and effective retention of CNTs in the matrix post-consolidation. They used MWCNTs as reinforcement in Al-Si hypereutectic structures obtained through plasma thermal spraying and HVOF (High Velocity Oxy-Fuel) thermal spraying. A mixture of Al-Si powders and 10% by weight of MWCNTs was ball-milled for 48 hours to promote homogenous mixing. They observed that HVOF-sprayed coatings exhibited darker zones on the surface corresponding to carbon presence, more pronounced compared to plasma-sprayed surfaces due to higher particle velocities (700-1400 m/s vs. 350-1000 m/s), causing cluster explosions in the HVOF process. Moreover, HVOF-sprayed composites displayed a denser, compact structure with higher residual stress, modulus of elasticity, and hardness compared to plasma-sprayed counterparts, successfully retaining physically intact CNTs within the sprayed composite (Laha T. L., 2007). Similarly, (Bakshi, 2008) employed MWCNTs as reinforcement in a nanocomposite aluminum coating via cold spraying. Using a turbula mixer, they blended aluminum powder and eutectic aluminum-silicon alloy powders with surface-dispersed MWCNTs for one hour. The resulting coatings with 0.5% and 1% by weight of MWCNTs successfully retained and embedded within the matrix. However, they reported CNTs experiencing length reduction due to impact-induced Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 21 fractures between projected particles. The coatings achieved higher modulus of elasticity values of 229 GPa for 0.5% by weight and 191 GPa for 1% by weight, attributed to the reinforcing effect of MWCNTs and their high concentration in the measured regions (Bakshi, 2008). Tungsten carbide cobalt (WC-Co) is one of the most widely used mechanical alloys in industry, belonging to the subgroup of composite materials known as cemented carbides. These materials combine hard carbide particles such as WC and VC, bonded together by a soft metal binder like cobalt (Venter, 2013) (Żórawski, 2012). This combination produces materials that are both hard and wear resistant. Cemented carbide components are typically manufactured through a series of stages: stoichiometric mixing and milling, spraydrying of fine powder, compaction into component forms, and sintering at high temperatures (Venter, 2013). WC-Co mechanical alloys can be synthesized using thermal spray techniques for creating high-performance tribological coatings on machine elements or tools (Rodriguez, Effects of the dispersion time on the microstructure and wear resistance of WC/Co-CNTs HVOF sprayed coatings, 2014). Guo et al. (2014) reported that WC-Co coatings have been successfully applied using the High Velocity Oxy-Fuel (HVOF) thermal spraying technique, involving the combustion of liquid or gaseous fuel with large quantities of oxygen, one of the most advanced processes for producing unique cermet-type coatings. They found that cermet particles in the HVOF process could be rapidly and uniformly heated near or up to their melting point by the flame jet's heat. The melted or semi-melted particles impinge on a substrate, resulting in a coating with minimal material decomposition, low oxidation, low porosity, and excellent coating adhesion. WC, one of the hardest carbides, has a melting temperature close to 3043 K (Mohanty, 2012). Some authors (Ma, 2014) have found that the characteristics of feedstock powders prior to projection significantly affect the mechanical properties and wear resistance of WC-Co coatings, indicating that small grains in WC-Co powders favor mechanical properties and wear resistance of the coating. However, they also noted that when WC grains are in the nanoscale, severe particle decomposition occurs during coating preparation, resulting in decreased wear resistance of the coating. 3.3. State of the Art Thermal spraying encompasses a series of processes where a material, whether metallic, ceramic, or composite, is heated using combustible gases, electric arc, or plasma, and then accelerated to high speeds before being projected onto a prepared surface. Upon impact, these particles flatten to form discs called splats, adhering to the treated surface, and increasing its hardness beyond its original state. The technique was commercially developed by engineer M.U. Schoop, who conducted experiments projecting molten lead bullets at high temperatures onto a stone wall, observing their flattening and spreading upon impact (Rodriguez, Effects of the dispersion time on the microstructure and wear resistance Pàg. 22 Report of WC/Co-CNTs HVOF sprayed coatings, 2014). Schoop subsequently designed a nozzle that circulated high-temperature gas to propel molten metallic material in a similar manner. One of the critical phenomena involved in thermal spraying is residual stress, which develops as a particle cool from above its melting point to ambient temperature, forming a splat (Cheh Tan, 1997). In thick coatings, it has been reported that residual stress increases linearly with coating thickness, and this shear stress can lead to cracking (al., 1990). However, Kitahara (S., 1974) noted that coatings typically experience tensile stresses because of residual stress. Another phenomenon affecting the quality of thermal spray coatings is the bond strength between the substrate and the sprayed material. Due to rapid cooling of the sprayed material, diffusion between the substrate and sprayed powders does not occur, resulting in predominantly mechanical bonding rather than metallurgical or chemical bonding (Cheh Tan, 1997). Molten particles deform the surface roughness of the substrate, creating mechanical anchoring (K.T, 1982). Calculations by Moss A.R. and Young W.J. (W.J, 1974) demonstrated that even a tin particle sprayed by flame traveling at just 100 m/s possesses sufficient thermal and kinetic energy to exceed the plastic flow stress of a mild steel with grit-blasted surface. The grit-blasting action breaks the oxide layer, providing a clean region for metal-to-metal contact. Cheh Tan, (Cheh Tan, 1997) also suggested that oxidation can increase the hardness of thermal spray coatings. Furthermore, differences in deposition angle, spray distance, and residual stress can cause variations in hardness measurements between test samples and coated parts. Among these parameters, spray distance exerts the greatest influence on coating hardness in thermal spraying. High-Velocity Oxygen-Fuel (HVOF) thermal spraying, developed in the 1930s and widely commercialized since then, offers significant advantages over conventional spraying techniques. It boasts higher deposition rates, thereby reducing application time while producing durable coatings with excellent cohesion, hardness, and wear resistance due to the uniform distribution of deposited particles (Tabbara, 2009). This high-energy process employs specially designed guns that combust oxygen and a fuel gas (such as hydrogen, propane, or propylene) within a chamber, resulting in a gas flow much greater than conventional thermal spray guns. The flame jet can reach supersonic velocities of approximately 2000 m/s, accelerating particles to speeds up to 800 m/s, with peak temperatures in the process ranging from 2700 to 3200 K (Mohanty, 2012). Coating powder particles are introduced into the gun chamber through a carrier gas or directly after a convergent-divergent nozzle to propel them at high speeds without overheating, a distinctive feature of this process. Typically, feedstock powder particles range from 15 μm to 45 μm in size, softened or brought to a pasty state by the hot gas as they are propelled onto the substrate to form coatings that can reach thicknesses in the millimeter range. Due to the high velocities and impact force of the particles on the substrate, HVOF coatings exhibit lower porosity and Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 23 stronger bonding compared to coatings produced by other methods (D.C., 1992) (W.J., 1993) (K.V., 1986) (X., 1993). However, when particles meet air during spraying, heterogeneous mixing occurs due to oxide inclusion and porosity (D.W., 1991) (K.T., 1991). As explained by Cheh Tan, during flight from the gun to the substrate, particles chemically and physically interact with the surrounding environment (Cheh Tan, 1997). Additional benefits of the HVOF process, according to Cheh Tan, compared to other commercial processes, include: 1. Short particle exposure time during flight due to high velocity. 2. Formation of a small oxide layer on the particle surface due to brief exposure. 3. Uniform and efficient particle heating due to turbulence. 4. Lower maximum temperature compared to plasma or arc processes. Cheh Tan (Cheh Tan, 1997) also indicates that for coatings of hard metals, thicknesses typically range from 0.1 to 0.2 mm, with thicknesses greater than 0.3 mm possible but not recommended due to high residual stresses. It is challenging to build coatings thicker than 1.5 mm using WC-Co powders, as they tend to crack under residual tensile stresses generated during thermal spraying (G.K., 1994) (P., 1993). However, coatings up to 2.5 mm thick of WC-Co on stainless steel have been successfully produced using HVOF and a liquid CO2 cooling system to control spraying temperature. The entire HVOF thermal spraying process can be divided into three fundamental steps to achieve desired coating quality: a) Substrate Surface Preparation: The integrity of the deposit heavily depends on the substrate surface condition (Cheh Tan, 1997). Research by Lin, C.K. and Berndt, C.C. indicates that coatings sprayed onto rough surfaces generally exhibit better adhesion compared to smooth surfaces (Lin, 1994). For thin coatings applied using high-energy techniques like HVOF, fine ceramic materials (Al2O3, SiC, etc.) are typically sprayed onto the substrate to prepare the surface. However, studies by Paredes, R.S.C., using HVOF to project aluminum particles found that reducing surface roughness increased adhesion (Paredes, 2006). Ivosevic also used HVOF to project Nylon 11 particles onto a 4140 steel substrate, finding that increased substrate roughness promoted splat instability, resulting in radial splashing and splat rupture on rough surfaces (Ivosevic, 2006). b) Preheating Process: Immediately before powder projection, the substrate must be preheated to remove moisture and condensation from its surface. This preheating helps reduce internal stresses caused by different thermal expansion coefficients Pàg. 24 Report between the substrate and the coating. Preheating temperatures around 250°C are required to mitigate stress gradients, but excessive preheating can lead to oxide formation on the substrate surface, which can degrade coating quality. According to Cheh Tan, residual stress can be significantly reduced with increasing preheating temperature (Cheh Tan, 1997). c) Post-Spraying Treatment Process: Newly deposited surfaces are rarely ready for use and often require polishing or treatment to achieve the desired roughness for each application. Thermal treatment can also alter coating phases, reduce porosity, or improve specific coating properties. Cheh Tan conducted studies using HVOF to spray WC-Co particles onto a stainless-steel substrate, transferring all samples immediately after spraying to a 450°C furnace for thermal treatment (Cheh Tan, 1997). They found that adhesion resistance between the substrate and the coating was higher for samples treated in the furnace after thermal spraying. HVOFdeposited coatings can also be machined using carbide tools, but improper angles and pressures may lead to excessive material removal and damage to the substrate-coating bond. HVOF is a high-energy process that use specially designed guns that burn oxygen and a fuel gas (hydrogen, propane, or propylene). Combustion occurs within a designed chamber inside the gun, resulting in a gas flow significantly superior to conventional thermal spray guns. The flame jet can reach supersonic speeds of approximately 2000 m/s, propelling particles up to 800 m/s, with maximum process temperatures around 2700-3200 K (Mohanty, 2012). Coating powder particles, typically 15-45 μm in size, are introduced into the gun via a gas in the combustion chamber or after the convergent-divergent nozzle to propel them at high speeds without overheating. This characteristic distinguishes HVOF from other processes. Due to the high velocities and strong impact of the projected particles, HVOF-produced coatings are less porous and have higher bond strength compared to other methods (D.C., 1992) (W.J., 1993) (K.V., 1986). However, interaction with air during projection can lead to heterogeneous mixing, including oxide inclusions and porosity (D.W., 1991) (K.T., 1991). During flight from the gun to the substrate, particles interact chemically and physically with the surrounding environment (Cheh Tan, 1997). Cheh Tan highlighted several additional benefits of HVOF over other commercial processes: 1. Short particle exposure time during flight due to high speed. 2. Minimal oxide layer formation on particle surfaces due to short exposure time. 3. Uniform and efficient particle heating due to high turbulence. 4. Lower maximum temperature compared to plasma or arc processes. Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 25 For hard metal coatings, thicknesses typically range between 0.1 to 0.2 mm, with greater thicknesses possible but not recommended due to high residual stress, which can lead to cracking (Cheh Tan, 1997). For instance, coatings up to 2.5 mm thick of WC-Co on stainless steel have been produced using HVOF with a liquid CO2 cooling system to control the spraying temperature (G.K., 1994) (P., 1993) The HVOF technique can produce high-performance coatings for wear-resistant applications. Critical properties for such coatings include high hardness and crack propagation resistance (Houdkova, 2013) (Erickson, 2001) (Holmberg, 1998). The degree of particle decomposition and coating strength are directly related to process parameters like speed, temperature, and particle state, which are influenced by variables such as particle size, gas flow rate, and fuel type (Kamnis, 2008). For example, smaller WC-Co particles (<5 μm) may melt and solidify before impact, while larger particles may not fully liquefy. HVOF is a versatile and efficient thermal spraying process that provides high-quality, wearresistant coatings. Optimizing process parameters such as powder feed rate, spray distance, oxygen-fuel ratio, and combustion chamber pressure is crucial for achieving desirable coating properties. Future advancements in HVOF technology and material science will likely further enhance its application scope and effectiveness. Pàg. 32 Report Image 8: Sample R-02-35 Image 9: Sample R-02-45 Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 33 The porosity values are shown in a matrix with each combination of Ball-to-Powder Ratio (BPR) and carbon nanotube (CNT) concentration. This matrix provides a clear comparison of the porosity levels across the different samples, helping to identify the optimal milling parameters for achieving the lowest porosity and the most even distribution of CNTs within the WC-Co 12% alloy. Table 1: Coating Porosity values CODE 15 35 45 00 21,00% 37,32% 12,13% 01 19,17% 10,84% 6,22% 02 6,45% 13,23% 8,29% The matrix shows that the porosity levels vary significantly depending on both the BPR and the CNT concentration. As expected, the samples with a higher BPR generally exhibit lower porosity, indicating a more effective mechanical alloying process due to the increased milling energy. Similarly, the CNT concentration influences the porosity, with certain concentrations leading to better particle distribution and less porous structures. Based on the porosity results, it can be observed that the combination of a moderate BPR and an optimal CNT concentration yields the most favorable results in terms of reduced porosity. For instance, the sample with a BPR of 1:1 and 45% CNTs shows the lowest porosity at 6.22%, while the sample with a BPR of 0:1 and 35% CNTs has a significantly higher porosity of 37.32%. This suggests that there is a synergistic effect between the milling parameters and the CNT concentration that enhances the overall quality of the alloy. Further analysis and characterization using FESEM will provide additional insights into the microstructural features of these samples, corroborating the porosity measurements and aiding in the selection of the optimal parameters for HVOF coatings. This approach ensures that the chosen parameters not only minimize porosity but also promote a uniform distribution of CNTs, ultimately enhancing the mechanical and thermal properties of the WC-Co 12% alloy. Following the porosity analysis, the next step involves studying the size and shape of the metal powder. Both parameters are critical indicators of powder quality and play a significant role in determining the overall performance of the WC-Co 12% alloy. A low statistical dispersion of particle size ensures consistent mechanical properties and uniform coating thickness when applied through the HVOF process. Similarly, the shape of the metal powder particles influences the coating's microstructure and adhesion. Achieving powders with a narrow size distribution and spherical shape is ideal, as these characteristics are typically associated with high-quality powders suitable for thermal spray applications. The main objective here is to achieve uniformity in shape and size distribution, seeking a low statistical dispersion to ensure consistent and reliable performance. Pàg. 34 Report For this study, FESEM images and particle size distribution analysis will be used to assess the morphology and size range of the metal powders. The goal is to correlate the optimal milling conditions by comparing the results from the coatings analysis with the powder parameter distributions. The optimal milling parameters should yield both a low-porosity coating and powders with a uniform shape and low statistical dispersion. A low statistical dispersion in size distribution contributes to a uniform melting during the HVOF process, leading to a dense and cohesive coating. Conversely, an uneven size distribution can result in varying melting rates, causing defects such as porosity and poor adhesion. Additionally, spherical particles contribute to a smoother and more uniform coating. The primary goal of this analysis is to establish a correlation between the optimal milling conditions identified through coating analysis and those determined by the powder parameter distributions. By achieving this correlation, it will be possible to define a set of milling parameters that ensure both high-quality coatings and powders, enhancing the mechanical and thermal properties of the WC-Co 12% alloy. The focus is on achieving uniformity in the shape and size distribution of the metal powders, aiming for a low statistical dispersion to guarantee the best performance possible. The study of the metal powders is crucial for understanding the impact of different milling parameters on the quality of the WC-Co 12% alloy. By examining the size and shape distributions of the powders, the optimal milling conditions that result in uniform particles with low statistical dispersion can be identifed. Such uniformity is essential for ensuring consistent coating quality in HVOF applications. The analysis is divided into three groups based on the Ball-to-Powder Ratio (BPR) and varying Carbon Nanotube (CNT) concentrations: 0:1 BPR, 1:1 BPR, and 2:1 BPR. Each group is examined for its particle size and shape distribution, providing insights into the milling process's effectiveness. The first group of metal powders was prepared with a BPR of 0:1, indicating no additional milling balls were used. This set of experiments allows us to observe the baseline characteristics of the WC-Co 12% alloy powders with different CNT concentrations. The distributions of particle size and shape for CNT concentrations of 15%, 35%, and 45% are presented and analyzed, using pixels on the sample image as measurement unit for particle size. Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 35 Image 10: Sample M-00-15 This sample analysis yields the following size and shape distributions: Figure 1: Size and shape distribution on sample M-00-15 Image 11: Sample M-00-35 Pàg. 36 Report Figure 2: Size and shape distribution on sample M-00-35 Image 12: Sample M-00-45 Figure 3: Size and shape distribution on sample M-00-45 While particle shape shows a high deviation in all 3 CNT concentrations, particle size is more centered around a medium value. This indicates that the 0:1 ball-to-powder ratio is not suited for high quality thermal spray processes, as the wide range of particle shape contributes to uneven coatings. Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 37 The second group of metal powders was prepared with a BPR of 1:1, where an equal weight of milling balls and powder was used. This configuration increases the milling energy, potentially resulting in more uniform particle distributions. Pixels on the sample image are used as measurement unit for particle size. Image 13: Sample M-01-15 Figure 4: Size and shape distribution on sample M-01-15 Image 14: Sample M-01-35 Pàg. 38 Report Figure 5: Size and shape distribution on sample M-01-35 Image 15: Sample M-01-45 Figure 6: Size and shape distribution on sample M-01-45 When using 1:1 ball-to-powder ratio, particle size concentrate around a medium value and shape distribution approximates a normal distribution. This makes it a good parameter value, as it allows to better use statistical deviation as a powder evaluation criterion. The best CNT concentration values are 35% and 45%, as they better approximate a normal distribution and low statistical deviation. Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 39 The third group of metal powders was prepared with a BPR of 2:1, where the weight of milling balls was twice that of the powder. In this case, higher milling energy is expected to further enhance the uniformity of the particle distributions. Pixels on the sample image are used as measurement unit for particle size. Image 16: Sample M-02-15 Figure 7: Size and shape distribution on sample M-02-15 Image 17: Sample M-02-35 Pàg. 40 Report Figure 8: Size and shape distribution on sample M-02-35 Image 18: Sample M-02-45 Figure 9: Shape and size distribution on sample M-02-45 When using 2:1 ball-to-powder ratio particle size is less concentrated around a medium value and, while approximating a normal distribution, particle shape statistical deviation is high. This make it less appropriate parameters to be used on thermal spray, as the variability of particle size and shape contributes to uneven and less compact coatings. Milling parameters effects on WC-12%Co alloy thermal spray coatings Pág. 41 The tables below summarize the key parameters for the size and shape distributions of the metal powders across all samples, focusing on the statistical dispersion as a measure of uniformity. The particle sized has been measured using the surface in pixels on the sample image. Table 2: Statistical deviation on size distribution CODE 15 35 45 00 1903,0 555,3 580,4 01 232,4 1183,9 464,8 02 294,9 152,3 385,4 Table 3: Statistical deviation on shape distribution CODE 15 35 45 00 0,1761 0,1807 0,1799 01 0,1780 0,1497 0,1702 02 0,1550 0,1619 0,1863 Table 4: Mean of shape distribution CODE 15 35 45 00 0,6499 0,7042 0,5959 01 0,5510 0,6306 0,5951 02 0,5725 0,5968 0,5799 While evaluating the metal powders, it was observed that particle size does not follow a normal distribution. Consequently, statistical deviation in particle size is not a very useful metric for quality assessment. A focus on the shape distribution, which more closely Pàg. 48 Report 10. CONCLUSIONS The research conducted on WC-Co 12% alloy with varying concentrations of carbon nanotubes (CNTs) and different ball-to-powder ratios (BPR) has provided significant insights into the optimal conditions for producing high-quality metal powders for HVOF thermal spray coatings. The study aimed to identify the optimal milling parameters by examining the resulting powder distributions and coating properties. Firstly, it was observed that ball-to-powder ratio and carbon nano-tube concentrations play a crucial role in determining the quality of the metal powders. The 0:1 BPR, which involved no additional milling balls, resulted in high variance in particle shape and less uniform coatings. This highlights the importance of incorporating milling media to achieve more consistent particle distributions. Conversely, the 1:1 BPR yielded more favorable results, with particle shapes and sizes approximating a normal distribution. This ratio proved to be effective in minimizing porosity and achieving uniform coatings, particularly with CNT concentrations of 35% and 45%. The 2:1 BPR, despite showing some improvement in particle shape distribution, often resulted in higher statistical deviation, making it less suitable for high-quality thermal spray applications in several combinations. In terms of specific samples, the most promising results were obtained from samples M-0145, M-02-15, and M-02-45. These samples demonstrated a combination of low porosity and favorable statistical indicators, suggesting superior quality for HVOF coatings. On the other hand, samples M-01-15 and M-02-35, while showing potential, had issues with the substrate that resulted in higher porosity. These samples could still be worth further investigation to explore their potential as viable options. Based on the porosity and statistical indicators, the optimal milling parameters for producing WC-Co 12% alloy powders for HVOF coatings were identified as those used in samples M01-45 and M-02-15. These conditions ensured minimal porosity and consistent particle distributions, thereby enhancing the mechanical and thermal properties of the coatings. Among these two options M-01-45 yielded a slightly lesser coating porosity, making it the sample with optimal milling conditions. In summary, the optimal milling parameters for producing high-quality WC-Co 12% alloy powders for HVOF coatings involve a 1:1 BPR and CNT concentrations around 45%. These conditions ensure low porosity and uniform particle distributions, enhancing the mechanical and thermal properties of the coatings. Future research should focus on further refining these parameters and exploring the long-term performance of the coatings in real-world applications. This study provides a robust foundation for the continued development of highperformance thermal spray coatings, with significant implications for industries requiring durable and reliable surface treatments. 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