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Biofilms are an important component in the development of chronic infections, especially when medical devices are involved.This problem presents a huge challenge to the medical community, as standard antibiotics can only eradicate biofilms to a very limited extent.Preventing biofilm formation has led to the development of various coating methods and new materials.These methods aim to coat surfaces in a manner that inhibits biofilm formation.Metallic glassy alloys, especially those containing copper and titanium metals, have emerged as ideal antimicrobial coatings.At the same time, the use of cold spray technology has increased as it is a suitable method for processing temperature-sensitive materials.Part of the purpose of this study was to develop a novel antibacterial film metallic glass composed of ternary Cu-Zr-Ni using mechanical alloying techniques.The spherical powder that makes up the final product is used as a raw material for cold spray coating of stainless steel surfaces at low temperatures.Substrates coated with metallic glass were able to significantly reduce biofilm formation by at least 1 log compared to stainless steel.
Throughout human history, any society has been able to design and promote the introduction of novel materials that meet its specific requirements, which has resulted in improved performance and ranking in a globalized economy1.It has always been attributed to the human ability to develop materials and fabrication equipment and designs for materials fabrication and characterization to achieve gains in health, education, industry, economics, culture and other fields from one country or region to another Progress is measured regardless of country or region. 2 For 60 years, materials scientists have devoted much of their time to focusing on one major concern: the pursuit of novel and cutting-edge materials.Recent research has focused on improving the quality and performance of existing materials, as well as synthesizing and inventing entirely new types of materials.
The addition of alloying elements, the modification of the material microstructure, and the application of thermal, mechanical or thermo-mechanical processing techniques have resulted in significant improvements in the mechanical, chemical and physical properties of a variety of different materials.Furthermore, hitherto unheard compounds have been successfully synthesized at this point.These persistent efforts have spawned a new family of innovative materials, collectively known as Advanced Materials2.Nanocrystals, nanoparticles, nanotubes, quantum dots, zero-dimensional, amorphous metallic glasses, and high-entropy alloys are just some examples of advanced materials introduced into the world since the middle of the last century.When manufacturing and developing new alloys with superior properties, either in the final product or in the intermediate stages of its production, the problem of off-balance is often added.As a result of implementing new fabrication techniques to significantly deviate from equilibrium, a whole new class of metastable alloys, known as metallic glasses, has been discovered.
His work at Caltech in 1960 brought a revolution in the concept of metal alloys when he synthesized glassy Au-25 at.% Si alloys by rapidly solidifying liquids at nearly a million degrees per second 4.Professor Pol Duwezs’ discovery event not only heralded the beginning of the history of metallic glasses (MG), but also led to a paradigm shift in the way people think about metal alloys.Since the earliest pioneering studies in the synthesis of MG alloys, almost all metallic glasses have been produced entirely by using one of the following methods; (i) rapid solidification of the melt or steam, (ii) atomic disordering of the lattice, (iii) solid-state amorphization reactions between pure metal elements, and (iv) solid-state transitions of metastable phases.
MGs are distinguished by their lack of the long-range atomic order associated with crystals, which is a defining characteristic of crystals.In today’s world, great progress has been made in the field of metallic glass.They are novel materials with interesting properties that are of interest not only in solid-state physics, but also in metallurgy, surface chemistry, technology, biology and many other fields.This new type of material exhibits distinct properties from solid metals, making it an interesting candidate for technological applications in a variety of fields.They have some important properties; (i) high mechanical ductility and yield strength, (ii) high magnetic permeability, (iii) low coercivity, (iv) unusual corrosion resistance, (v) temperature independence The conductivity of 6,7.
Mechanical alloying (MA)1,8 is a relatively new technique, first introduced in 19839 by Prof. CC Kock and colleagues.They prepared amorphous Ni60Nb40 powders by grinding a mixture of pure elements at ambient temperatures very close to room temperature. Typically, the MA reaction is carried out between diffusive coupling of the reactant material powders in a reactor, usually made of stainless steel into a ball mill 10 (Fig. 1a, b).Since then, this mechanically induced solid-state reaction technique has been used to prepare novel amorphous/metallic glass alloy powders using low (Fig. 1c) and high energy ball mills, as well as rod mills11,12,13,14,15 , 16.In particular, this method has been used to prepare immiscible systems such as Cu-Ta17, as well as high melting point alloys such as Al-transition metal systems (TM; Zr, Hf, Nb and Ta)18,19 and Fe-W20 , which cannot be obtained using conventional preparation routes.Furthermore, MA is considered one of the most powerful nanotechnology tools for the preparation of industrial-scale nanocrystalline and nanocomposite powder particles of metal oxides, carbides, nitrides, hydrides, carbon nanotubes, nanodiamonds, As well as broad stabilization via a top-down approach 1 and metastable stages.
Schematic showing the fabrication method used to prepare Cu50(Zr50−xNix) metallic glass (MG) coating/SUS 304 in this study.(a) Preparation of MG alloy powders with different Ni concentrations x (x; 10, 20, 30 and 40 at.%) using low energy ball milling technique.(a) The starting material is loaded into a tool cylinder together with tool steel balls, and (b) is sealed in a glove box filled with He atmosphere.(c) A transparent model of the grinding vessel illustrating ball motion during grinding.The final product of the powder obtained after 50 hours was used to coat the SUS 304 substrate using the cold spray method (d).
When it comes to bulk material surfaces (substrates), surface engineering involves the design and modification of surfaces (substrates) to provide certain physical, chemical and technical qualities not contained in the original bulk material.Some properties that can be effectively improved by surface treatments include abrasion resistance, oxidation and corrosion resistance, coefficient of friction, bio-inertness, electrical properties, and thermal insulation, to name a few.Surface quality can be improved by using metallurgical, mechanical or chemical techniques.As a well-known process, a coating is simply defined as a single or multiple layers of material artificially deposited on the surface of a bulk object (substrate) made of another material.Thus, coatings are used in part to achieve some desired technical or decorative properties, as well as to protect materials from expected chemical and physical interactions with the surrounding environment23.
In order to deposit suitable surface protection layers with thicknesses ranging from a few micrometers (below 10-20 micrometers) to over 30 micrometers or even a few millimeters, many methods and techniques can be applied.In general, coating processes can be divided into two categories: (i) wet coating methods, including electroplating, electroless plating, and hot-dip galvanizing methods, and (ii) dry coating methods, including brazing, surfacing , physical vapor deposition (PVD), chemical vapor deposition (CVD), thermal spray techniques and more recently cold spray techniques 24 (Fig. 1d).
Biofilms are defined as microbial communities that are irreversibly attached to surfaces and surrounded by self-produced extracellular polymers (EPS).Superficially mature biofilm formation can lead to significant losses in many industrial sectors, including the food industry, water systems, and healthcare environments.In humans, when biofilms form, more than 80% of cases of microbial infections (including Enterobacteriaceae and Staphylococci) are difficult to treat.Furthermore, mature biofilms have been reported to be 1000-fold more resistant to antibiotic treatment compared to planktonic bacterial cells, which is considered a major therapeutic challenge.Antimicrobial surface coating materials derived from conventional organic compounds have historically been used.Although such materials often contain toxic components that are potentially risky to humans,25,26 it may help avoid bacterial transmission and material destruction.
The widespread resistance of bacteria to antibiotic treatments due to biofilm formation has led to the need to develop an effective antimicrobial membrane-coated surface that can be safely applied27.The development of a physical or chemical anti-adherent surface to which bacterial cells are inhibited to bind and build biofilms due to adhesion is the first approach in this process27.The second technology is to develop coatings that enable antimicrobial chemicals to be delivered precisely where they are needed, in highly concentrated and tailored amounts.This is achieved by developing unique coating materials such as graphene/germanium28, black diamond29 and ZnO-doped diamond-like carbon coatings30 that are resistant to bacteria, a technology that maximizes Toxicity and resistance development due to biofilm formation are significantly reduced.Additionally, coatings that incorporate germicidal chemicals into surfaces to provide long-term protection from bacterial contamination are becoming more popular.Although all three procedures are capable of producing antimicrobial effects on coated surfaces, they each have their own set of limitations that should be considered when developing application strategies.
Products currently on the market are hampered by insufficient time to analyze and test protective coatings for biologically active ingredients.Companies claim that their products will provide users with desirable functional aspects; however, this has been an obstacle to the success of products currently on the market.Compounds derived from silver are used in the vast majority of antimicrobial therapies now available to consumers.These products are developed to protect users from the potentially dangerous effects of microorganisms.The delayed antimicrobial effect and associated toxicity of silver compounds increases the pressure on researchers to develop a less harmful alternative36,37.Creating a global antimicrobial coating that works indoors and out is still proving to be a daunting task.This is because of the associated risks to both health and safety.Discovering an antimicrobial agent that is less harmful to humans and figuring out how to incorporate it into coating substrates with a longer shelf life is a highly sought-after goal38.The latest antimicrobial and anti-biofilm materials are designed to kill bacteria at close range, either through direct contact or after the active agent is released.They can do this by inhibiting initial bacterial adhesion (including counteracting the formation of a protein layer on the surface) or by killing bacteria by interfering with the cell wall.
Fundamentally, surface coating is the process of placing another layer on the surface of a component to enhance surface-related qualities.The goal of surface coating is to tailor the microstructure and/or composition of the near-surface region of the component39.Surface coating techniques can be divided into different methods, which are summarized in Fig. 2a.Coatings can be subdivided into thermal, chemical, physical, and electrochemical categories, depending on the method used to create the coating.
(a) Inset showing the main fabrication techniques used for the surface, and (b) selected advantages and disadvantages of the cold spray technique.
Cold spray technology shares many similarities with conventional thermal spray methods.However, there are also some key fundamental properties that make the cold spray process and cold spray materials particularly unique.Cold spray technology is still in its infancy, but has a bright future.In certain applications, the unique properties of cold spray offer great benefits, overcoming the inherent limitations of typical thermal spray methods.It provides a way to overcome the significant limitations of traditional thermal spray technology, during which the powder must be melted in order to deposit onto the substrate.Obviously, this traditional coating process is not suitable for very temperature-sensitive materials such as nanocrystals, nanoparticles, amorphous and metallic glasses40, 41, 42.Furthermore, thermal spray coating materials always exhibit high levels of porosity and oxides.Cold spray technology has many significant advantages over thermal spray technology, such as (i) minimal heat input to the substrate, (ii) flexibility in substrate coating choices, (iii) absence of phase transformation and grain growth , (iv) high bond strength1,39 (Fig. 2b).In addition, cold spray coating materials have high corrosion resistance, high strength and hardness, high electrical conductivity and high density41.Contrary to the advantages of the cold spray process, there are still some disadvantages to using this technique, as shown in Figure 2b.When coating pure ceramic powders such as Al2O3, TiO2, ZrO2, WC, etc., the cold spray method cannot be used.On the other hand, ceramic/metal composite powders can be used as raw materials for coatings.The same goes for other thermal spray methods.Complicated surfaces and interior pipe surfaces are still difficult to spray.
Given that the current work aims to use metallic glassy powders as raw coating materials, it is clear that conventional thermal spraying cannot be used for this purpose.This is because metallic glassy powders crystallize at high temperatures1.
Most of the tools used in the medical and food industries are made of austenitic stainless steel alloys (SUS316 and SUS304) with a chromium content between 12 and 20 wt% for the production of surgical instruments.It is generally accepted that the use of chromium metal as an alloying element in steel alloys can greatly improve the corrosion resistance of standard steel alloys.Stainless steel alloys, despite their high corrosion resistance, do not exhibit significant antimicrobial properties38,39.This contrasts with their high corrosion resistance.After this, the development of infection and inflammation can be predicted, which is mainly caused by bacterial adhesion and colonization on the surface of stainless steel biomaterials.Significant difficulties may arise due to significant difficulties associated with bacterial adhesion and biofilm formation pathways, which may lead to health deterioration, which may have many consequences that may directly or indirectly affect human health.
This study is the first phase of a project funded by the Kuwait Foundation for the Advancement of Science (KFAS), Contract No. 2010-550401, to investigate the feasibility of producing metallic glassy Cu-Zr-Ni ternary powders using MA technology (Table 1 ) for the production of antibacterial film/SUS304 surface protection coating.The second phase of the project, due to start in January 2023, will examine the electrochemical corrosion characteristics and mechanical properties of the system in detail.Detailed microbiological tests will be carried out for different bacterial species.
In this paper, the effect of Zr alloying element content on glass forming ability (GFA) is discussed based on morphological and structural characteristics.In addition, the antibacterial properties of the coated metallic glass powder coating/SUS304 composite were also discussed.Furthermore, current work has been carried out to investigate the possibility of structural transformation of metallic glass powders occurring during cold spraying within the subcooled liquid region of fabricated metallic glass systems.As representative examples, Cu50Zr30Ni20 and Cu50Zr20Ni30 metallic glass alloys have been used in this study.
In this section, the morphological changes of elemental Cu, Zr and Ni powders in low energy ball milling are presented.As illustrative examples, two different systems consisting of Cu50Zr20Ni30 and Cu50Zr40Ni10 will be used as representative examples.The MA process can be divided into three distinct stages, as shown by the metallographic characterization of the powder produced during the grinding stage (Figure 3).
Metallographic characteristics of mechanical alloy (MA) powders obtained after different stages of ball milling time.Field emission scanning electron microscopy (FE-SEM) images of MA and Cu50Zr40Ni10 powders obtained after low energy ball milling times of 3, 12 and 50 h are shown in (a), (c) and (e) for the Cu50Zr20Ni30 system, while in the same MA Corresponding images of the Cu50Zr40Ni10 system taken after time are shown in (b), (d) and (f).
During ball milling, the effective kinetic energy that can be transferred to the metal powder is affected by the combination of parameters, as shown in Fig. 1a.This includes collisions between balls and powders, compressive shearing of powder stuck between or between grinding media, impact of falling balls, shear and wear due to powder drag between moving ball milling media, and shock wave passing through Falling balls spread through crop loads (Fig. 1a).Elemental Cu, Zr, and Ni powders were severely deformed due to cold welding at the early stage of MA (3 h), resulting in large powder particles (>1 mm in diameter).These large composite particles are characterized by the formation of thick layers of alloying elements (Cu, Zr, Ni), as shown in Fig. 3a,b.Increasing the MA time to 12 h (intermediate stage) resulted in an increase in the kinetic energy of the ball mill, resulting in the decomposition of the composite powder into finer powders (less than 200 µm), as shown in Fig. 3c,d.At this stage, the applied shear force leads to the formation of a new metal surface with fine Cu, Zr, Ni hint layers, as shown in Fig. 3c,d.As a result of layer refinement, solid phase reactions occur at the interface of the flakes to generate new phases.
At the climax of the MA process (after 50 h), the flaky metallography was only faintly visible (Fig. 3e,f), but the polished surface of the powder showed mirror metallography.This means that the MA process has been completed and the creation of a single reaction phase has occurred.The elemental composition of the regions indexed in Fig. 3e (I, II, III), f, v, vi) was determined by using field emission scanning electron microscopy (FE-SEM) combined with energy dispersive X-ray spectroscopy (EDS) (IV).
In Table 2, the elemental concentrations of alloying elements are shown as a percentage of the total weight of each region selected in Fig. 3e,f.When comparing these results with the starting nominal compositions of Cu50Zr20Ni30 and Cu50Zr40Ni10 listed in Table 1, it can be seen that the compositions of these two final products have very similar values ​​to the nominal compositions.Furthermore, the relative component values ​​for the regions listed in Fig. 3e,f do not imply a significant deterioration or fluctuation in the composition of each sample from one region to another.This is evidenced by the fact that there is no change in composition from one region to another.This points to the production of homogeneous alloy powders, as shown in Table 2.
FE-SEM micrographs of the final product Cu50(Zr50−xNix) powder were obtained after 50 MA times, as shown in Fig. 4a–d, where x is 10, 20, 30 and 40 at.%, respectively.After this milling step, the powder aggregates due to the van der Waals effect, resulting in the formation of large aggregates consisting of ultrafine particles with diameters ranging from 73 to 126 nm, as shown in Figure 4.
Morphological characteristics of Cu50(Zr50−xNix) powders obtained after MA time of 50 h.For the Cu50Zr40Ni10, Cu50Zr30Ni20, Cu50Zr20Ni30, Cu50Zr10Ni40 systems, the FE-SEM images of the powders obtained after 50 MA times are shown in (a), (b), (c) and (d), respectively.
Before loading the powders into a cold spray feeder, they were first sonicated in analytical grade ethanol for 15 minutes and then dried at 150°C for 2 hours.This step must be taken to successfully combat agglomeration that often causes many significant problems throughout the coating process.After the MA process was completed, further characterizations were carried out to investigate the homogeneity of the alloy powders.Figure 5a–d show the FE-SEM micrographs and the corresponding EDS images of the Cu, Zr and Ni alloying elements of the Cu50Zr30Ni20 alloy obtained after 50 h of M time, respectively.It should be noted that the alloy powders produced after this step are homogeneous as they do not show any compositional fluctuations beyond the sub-nanometer level, as shown in Figure 5.
Morphology and local elemental distribution of MG Cu50Zr30Ni20 powder obtained after 50 MA times by FE-SEM/energy dispersive X-ray spectroscopy (EDS).(a) SEM and X-ray EDS mapping of (b) Cu-Kα, (c) Zr-Lα and (d) Ni-Kα images.
The XRD patterns of mechanically alloyed Cu50Zr40Ni10, Cu50Zr30Ni20, Cu50Zr20Ni30 and Cu50Zr20Ni30 powders obtained after MA time of 50 h are shown in Fig. 6a–d, respectively.After this stage of milling, all samples with different Zr concentrations showed amorphous structures with characteristic halo diffusion patterns shown in Fig. 6.
XRD patterns of (a) Cu50Zr40Ni10, (b) Cu50Zr30Ni20, (c) Cu50Zr20Ni30 and (d) Cu50Zr20Ni30 powders after MA time of 50 h.All samples without exception showed a halo diffusion pattern, implying the formation of an amorphous phase.
Field emission high-resolution transmission electron microscopy (FE-HRTEM) was used to observe structural changes and understand the local structure of the powders resulting from ball milling at different MA times.FE-HRTEM images of the powders obtained after the early (6 h) and intermediate (18 h) stages of milling for Cu50Zr30Ni20 and Cu50Zr40Ni10 powders are shown in Fig. 7a,c, respectively.According to the bright field image (BFI) of the powder produced after MA​​ 6 h, the powder is composed of large grains with well-defined boundaries of the elements fcc-Cu, hcp-Zr and fcc-Ni, and there is no sign that the reaction phase has formed, as shown in Fig. 7a.Furthermore, the correlated selected area diffraction pattern (SADP) taken from the middle region of (a) revealed a cusp diffraction pattern (Fig. 7b), indicating the presence of large crystallites and the absence of a reactive phase.
Local structural characterization of MA powder obtained after early (6 h) and intermediate (18 h) stages.(a) Field emission high resolution transmission electron microscopy (FE-HRTEM), and (b) the corresponding selected area diffraction pattern (SADP) of Cu50Zr30Ni20 powder after MA treatment for 6 h.The FE-HRTEM image of Cu50Zr40Ni10 obtained after an MA time of 18 h is shown in (c).
As shown in Fig. 7c, extending the MA duration to 18 h resulted in severe lattice defects combined with plastic deformation.During this intermediate stage of the MA process, the powder exhibits various defects, including stacking faults, lattice defects, and point defects (Figure 7).These defects cause the large grains to split along their grain boundaries into subgrains with sizes less than 20 nm (Fig. 7c).
The local structure of Cu50Z30Ni20 powder milled for 36 h MA time has the formation of ultrafine nanograins embedded in an amorphous fine matrix, as shown in Fig. 8a.Local EDS analysis indicated that those nanoclusters shown in Fig. 8a were associated with unprocessed Cu, Zr and Ni powder alloying elements.At the same time, the Cu content of the matrix fluctuated from ~32 at.% (lean area) to ~74 at.% (rich area), indicating the formation of heterogeneous products.Furthermore, the corresponding SADPs of the powders obtained after milling at this stage show halo-diffusing primary and secondary rings of amorphous phase, overlapping with sharp points associated with those raw alloying elements, as shown in Fig. 8b.
Beyond 36 h-Cu50Zr30Ni20 powder nanoscale local structural features.(a) Bright field image (BFI) and corresponding (b) SADP of Cu50Zr30Ni20 powder obtained after milling for 36 h MA time.
Near the end of the MA process (50 h), Cu50(Zr50−xNix), X; 10, 20, 30 and 40 at.% powders invariably have a labyrinthine amorphous phase morphology as shown in Fig. 9a–d .In the corresponding SADP of each composition, neither point-like diffractions nor sharp annular patterns could be detected.This indicates that no unprocessed crystalline metal is present, but rather an amorphous alloy powder is formed.These correlated SADPs showing halo diffusion patterns were also used as evidence for the development of amorphous phases in the final product material.
Local structure of the final product of the MG Cu50 (Zr50−xNix) system.FE-HRTEM and correlated nanobeam diffraction patterns (NBDP) of (a) Cu50Zr40Ni10, (b) Cu50Zr30Ni20, (c) Cu50Zr20Ni30 and (d) Cu50Zr10Ni40 obtained after 50 h of MA.
The thermal stability of the glass transition temperature (Tg), subcooled liquid region (ΔTx) and crystallization temperature (Tx) as a function of Ni content (x) of the amorphous Cu50(Zr50−xNix) system has been investigated using differential scanning Calorimetry (DSC) of properties under He gas flow.The DSC traces of the Cu50Zr40Ni10, Cu50Zr30Ni20 and Cu50Zr10Ni40 amorphous alloy powders obtained after MA time of 50 h are shown in Fig. 10a, b, e, respectively.While the DSC curve of amorphous Cu50Zr20Ni30 is shown separately in Fig. 10c.Meanwhile, the Cu50Zr30Ni20 sample heated to ~700 °C in DSC is shown in Fig. 10d.
Thermal stability of Cu50(Zr50−xNix) MG powders obtained after a MA time of 50 h, as indexed by glass transition temperature (Tg), crystallization temperature (Tx), and subcooled liquid region (ΔTx).Differential scanning calorimeter (DSC) thermograms of (a) Cu50Zr40Ni10, (b) Cu50Zr30Ni20, (c) Cu50Zr20Ni30 and (e) Cu50Zr10Ni40 MG alloy powders after MA time of 50 h.The X-ray diffraction (XRD) pattern of the Cu50Zr30Ni20 sample heated to ~700 °C in DSC is shown in (d).
As shown in Figure 10, the DSC curves of all compositions with different Ni concentrations (x) indicate two different cases, one endothermic and the other exothermic.The first endothermic event corresponds to Tg, while the second is related to Tx.The horizontal span region that exists between Tg and Tx is called the subcooled liquid region (ΔTx = Tx – Tg).The results show that the Tg and Tx of the Cu50Zr40Ni10 sample (Fig. 10a), placed at 526°C and 612°C, shift the content (x) to 20 at.% towards the low temperature side of 482°C and 563°C with increasing Ni content (x), respectively , as shown in Figure 10b.Consequently, the ΔTx of Cu50Zr40Ni10 decreases from 86 °C (Fig. 10a) to 81 °C for Cu50Zr30Ni20 (Fig. 10b).For the MG Cu50Zr40Ni10 alloy, it was also observed that the values ​​of Tg, Tx and ΔTx decreased to the level of 447°C, 526°C and 79°C (Fig. 10b).This indicates that the increase in Ni content leads to a decrease in the thermal stability of the MG alloy.In contrast, the Tg value (507 °C) of the MG Cu50Zr20Ni30 alloy is lower than that of the MG Cu50Zr40Ni10 alloy; nevertheless, its Tx shows a comparable value to the former (612 °C).Therefore, ΔTx exhibits a higher value (87°C), as shown in Fig. 10c.
The MG Cu50(Zr50−xNix) system, taking the MG Cu50Zr20Ni30 alloy as an example, crystallizes through a sharp exothermic peak into the crystal phases of fcc-ZrCu5, orthorhombic-Zr7Cu10 and orthorhombic-ZrNi (Fig. 10c).This amorphous to crystalline phase transition was confirmed by XRD of the MG sample (Fig. 10d), which was heated to 700 °C in DSC.
Figure 11 shows photographs taken during the cold spray process carried out in the current work.In this study, the metal glass-like powder particles synthesized after MA time of 50 h (taking Cu50Zr20Ni30 as an example) were used as antibacterial raw materials, and the stainless steel plate (SUS304) was coated by cold spraying technology.The cold spray method was chosen for coating in the thermal spray technology series because it is the most efficient method in the thermal spray series and can be used for metal metastable temperature sensitive materials such as amorphous and nanocrystalline powders, which are not subject to phase transitions .This is the main factor in choosing this method.The cold spray process is carried out by utilizing high-velocity particles that convert the kinetic energy of the particles into plastic deformation, strain and heat upon impact with the substrate or previously deposited particles.
Field photos show the cold spray procedure used for five consecutive preparations of MG coating/SUS 304 at 550 °C.
The kinetic energy of the particles, and thus the momentum of each particle in the coating formation, must be converted into other forms of energy through mechanisms such as plastic deformation (initial particle and particle-particle interactions in the substrate and particle interactions), voids Consolidation, particle-particle rotation, strain and ultimately heat 39.Furthermore, if not all incoming kinetic energy is converted into heat and strain energy, the result is an elastic collision, which means that the particles simply bounce back after impact.It has been pointed out that 90% of the impact energy applied to the particle/substrate material is converted into local heat 40 .Furthermore, when impact stress is applied, high plastic strain rates are achieved in the contact particle/substrate region in a very short time41,42.
Plastic deformation is generally considered a process of energy dissipation, or more specifically, a heat source in the interfacial region.However, the temperature increase in the interfacial region is usually not sufficient to produce interfacial melting or to significantly promote atomic interdiffusion.No publication known to the authors investigates the effect of the properties of these metallic glassy powders on powder adhesion and deposition that occurs when cold spray methods are used.
The BFI of MG Cu50Zr20Ni30 alloy powder can be seen in Fig. 12a, which was coated on SUS 304 substrate (Figs. 11, 12b).As can be seen from the figure, the coated powders maintain their original amorphous structure as they have a delicate labyrinth structure without any crystalline features or lattice defects.On the other hand, the image indicates the presence of an extraneous phase, as suggested by nanoparticles incorporated into the MG-coated powder matrix (Fig. 12a).Figure 12c depicts the indexed nanobeam diffraction pattern (NBDP) associated with region I (Figure 12a).As shown in Fig. 12c, NBDP exhibits a weak halo diffusion pattern of amorphous structure and coexists with sharp patches corresponding to the crystalline large cubic Zr2Ni metastable plus tetragonal CuO phase.The formation of CuO may be attributed to the oxidation of the powder when traveling from the nozzle of the spray gun to SUS 304 in the open air under supersonic flow.On the other hand, the devitrification of the metallic glassy powders achieved the formation of large cubic phases after cold spray treatment at 550 °C for 30 min.
(a) FE-HRTEM image of MG powder coated on (b) SUS 304 substrate (inset of figure).The index NBDP of the circular symbol shown in (a) is shown in (c).
To verify this potential mechanism for the formation of large cubic Zr2Ni nanoparticles, an independent experiment was performed.In this experiment, the powders were sprayed from a spray gun at 550 °C in the direction of the SUS 304 substrate; however, to elucidate the annealing effect of the powders, they were removed from the SUS304 strip as quickly as possible (about 60 seconds).Another set of experiments was carried out in which powder was removed from the substrate about 180 seconds after deposition.
Figures 13a,b show dark field images (DFI) obtained by scanning transmission electron microscopy (STEM) of two sprayed materials deposited on SUS 304 substrates for 60 s and 180 s, respectively.The powder image deposited for 60 seconds has no morphological detail, showing featurelessness (Fig. 13a).This was also confirmed by XRD, which indicated that the general structure of these powders was amorphous, as indicated by the broad primary and secondary diffraction maxima shown in Figure 14a.These indicate the absence of metastable/mesophase precipitation, where the powder retains its original amorphous structure.In contrast, the powder sprayed at the same temperature (550 °C), but left on the substrate for 180 s, showed the precipitation of nano-sized grains, as indicated by the arrows in Fig. 13b.