*Featured image: ‘Cold Spray’ SMART (Surface Modification and Additive Research Technologies) Laboratory established by Indian Institute of Technology Madras in collaboration with General Electric (GE). This is the first of its kind High-Pressure Cold Spray (HPCS) facility being installed in any academic institute in India.
The birth of new technologies is often characterised as the result of dedicated methodological research, analysis, and hard work. However, this is not always the case. Historically, serendipitous discoveries have led to novel ideas that eventually resulted in valuable scientific inventions, such as the discovery of penicillin by Alexander Fleming in 1928 while studying staphylococcus or the microwave effect by P.L Spencer in 1945 while working near a radar tube.
One noteworthy accidental discovery took place in the mid-1980s when scientists at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences (ITAM of RAS) in Novosibirsk discovered the cold spray deposition method while studying the interaction of two-phase flows with immersed body surfaces .
Two-phase flows are interactive flows of two distinct phases. They can be of the transient, separated, or dispersed type. In this case, it was a particle-carrying gas (dispersed two-phase flow).
Given the effects of two-phase flows on the flow field and flow parameters and their influence upon reaching a body’s surface on the latter’s state and aerodynamic properties, Dr. Antolli Papyrin and his colleagues carried out several wind tunnel experiments on models subjected to a supersonic two-phase flow.
During their work, however, they observed an unexpected event, where upon exceeding a critical particle velocity at relatively low temperatures, a shift from target surface erosion to a quickly increasing particle deposition took place .
Upon further studying this phenomenon and identifying its future prospects, they developed it into a coating technology and went on to name it Cold Spray, with reference to other thermal spray deposition technologies requiring much higher temperatures to operate .
Cold spray (supersonic particle deposition) is the latest addition to the family of thermal spray technology, which has been used since the past century to deposit thick coatings on surfaces in aeronautical, navy, electrical and other applications. Also known as cold gas dynamic spraying, cold spray is simply an extension to the thermal spray technology which helps to obtain properties and applications the conventional technology could not achieve.
Cold Spray (CS) is a solid-state deposition process that uses solid powder particles, accelerated to supersonic velocities by the help of a converging-diverging de Laval nozzle towards a target substrate . This technology follows a similar mechanism as the thermal spray-based processes of HVOF/HVAF coating – especially in terms of increasing particle velocity – yet differs from them with its relatively low temperature, which is generally below the melting point of the sprayed particle material.
Advantages and Disadvantages
Its significant advantage as compared to traditional thermal spraying methods lies in its independence of the thermal energy of the powder feedstock. Instead, it relies merely on the particles’ high kinetic energy, which circumvents the need for conventional material melting and rapid solidification upon impact. This also diminishes the formation of heat-affected zones (HAZ), which results in less induced stresses.
The powder particles in CS undergo extreme plastic deformation due to their ballistic impact, leading to a bonding mechanism known as “Adiabatic shear instability,” which adheres the particles to the substrate when reaching or exceeding the critical impact velocity, as their kinetic energy is transformed into thermal and mechanical deformation .
Other advantages of CS include :
- Improved mechanical properties and fatigue life of the coating
- No toxic fumes emitted
- Retain of initial phases of particles
- No separate substrate surface preparation required (Grit Blasting)
- High hardness, low solidification stress, and thick coatings
- No inter-metallic formation when coating different metals
- Low defects and oxidation
- Faster powder feed rates and 100% reuse of particles
- High density, hardness, and low porosity
- Precise control of gas temperature
- Increased operational safety due to the absence of high-temperature particle jets
However, CS has its drawbacks. Its critical disadvantages can be seen in:
- High cost due to high gas consumption, especially of helium
- Inefficiency for hard and brittle materials
Process parameters and components
Cold spray is basically used for metals, ceramics, polymers, and metal matrix and ceramic composites. The average particle size is between 1 μm and 50 μm. Further improvements in CS have allowed for a wider range of particle size, reaching up to 250 μm . Particle velocity basically ranges between 500 m/s and 1500 m/s. Air, helium, and nitrogen gases are the commonly used gases in CS.
Gas control module, Powder feeder, electric heater, and de Laval nozzle comprise the main elements of a cold spray system.
In order to ensure good coating and part quality, some key parameters should be regarded meticulously:
- Gas pressure, which plays an important role in maintaining particle velocity
- Substrate material on which the sprayed material is to be coated
- Minimum distance from the nozzle to the substrate
- Type of gas used (i.e nitrogen or helium)
- Design and orientation of the nozzle
- Powder feedstock quality and morphology
- Temperature at which the process is operated
Main Techniques of Cold Spray
Cold spray process is mainly classified into High Pressure CS and Low Pressure CS.
High-pressure cold spray
In HPCS, powder particles are introduced before the nozzle. High pressure increases the gas density, which helps in accelerating the powder particles to much higher speeds. Subsequently, the speeds are further increased via the higher expansion ratio nozzle .
HPCS can work with nitrogen and helium as particle-carrier gas at pressures above 1.5MPa, with a flow rate of 2 m3/min. The benefits of HPCS can be observed in better resulting mechanical properties and in its compatibility with nitrogen and air as gas mediums. It has the ability to form thicker and denser coatings, in addition to free-standing structures with low oxide and porosity content (~1%).
Low-pressure cold spray
On the other hand, LPCS, presented by Van Steenkiste in 2002 , is an improved CS process, where particles are introduced downstream of the nozzle throat.
Here, compressed gas is used at a pressure lower than the ambient pressure (0.5-1.0 MPa) with a flow rate that is less than 2 m3/min. Larger particles are key in this process (up to 250 μm), allowing for the reduction of oxide formation due to their relatively smaller surface-to-volume ratio.
Metal, ceramic, and mixture powder particles can be used in LPCS.
Applications and Future Prospects
Cold spray coating process is mainly used for coatings, repair and manufacturing of components. High density, superior deposition rate and higher-grade adhesion have made the cold spray technology attractive for applications in near-net-shape part manufacturing and repair.
The most interesting application of cold spray is additive manufacturing . Cold spray stands out among new AM techniques such as SLM (Selective Laser Melting), EBM (Electron Beam Melting), and Direct Metal Deposition, thanks to its high speed, low cost, and repeatability.
It is usually utilized where rapid manufacturing of low-tolerance parts is involved. It has a high bonding strength, low material waste, low energy consumption, and can manufacture larger parts than other AM methods.
The materials used to produce components or hybrid parts with CS include titanium, zinc, stainless steel, aluminium, and superalloys; even exotic materials such as niobium and metal matrix composites are possible options.
Another valuable application area of cold spray is the repair of damaged components. Cold spray has emerged as an effective alternative to traditional thermal-based processes, which induce residual stress in components that may lead to part failure.
Its ability to repair in-situ components in a short time and at a low cost, without disturbing the encompassing structure, has propelled CS to become a prominent player in repairing automotive and aircraft components made of materials such as magnesium, titanium, and superalloys.
One of the great advantages that CS has in component repair is the restoration of dissimilar materials which helps enhance the mechanical properties of the part, such as corrosion and wear resistance.
Other future applications where the cold spray process is expected to expand its reach into include smart structures, biomedical applications, and photovoltaic applications.
- Cold spray coatings, such as NiCrAlY bond coats, have also been used as thermal barriers on turbines of advanced jet engines.
- Hydroxyapatite (HAP) material has already been used for biomedical applications by traditional thermal spray processes but will pave the way for new medical research with the cold spray process.
- Stents, anastomosis chips, and embolic protection devices are some of the potential applications where cold spray technology could be used .
GE (General Electric), VRC Metal Systems, Oerlikon Metco, and ASB Industries are the companies at present that are heavily involved in this technology, with an estimated increase of 6.7% overall thermal spray market comprising of powders and equipment.
After Dr. Papyrin and his colleagues’ accidental discovery over 30 years ago, cold spray technology has gained much popularity among scientists and inventors with the number of subsequent patents filed around the world.
Today, it has become a vital process in the manufacturing world, and with its integration into numerous applications, it may well be argued that such a technology has a very promising future.
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