Cold standard - cold spray technology
While there is a reliance on thermal spraying in industries such as aerospace, medicine and electronics, the coating technology is not without its flaws. A colder alternative could be the answer, say Tiziana Marrocco and Dave Harvey from UK research organisation TWI.
Cold gas dynamic spray technology, commonly referred to as cold spray, is well established in a number of industrial applications. Its uses range from electrically conducting oxide-free copper coatings for electronics and sensors to induction-heating coatings for domestic cookware. The technology is increasingly used in aerospace, including helicopter gearbox and propeller repair, and aluminium alloy coatings for internal surfaces of aircraft landing gear. Cold spray is used for corrosion protection with dense, pure low-oxygen zinc, aluminium, titanium or tantalum, as well as stainless steel and nickel coatings for dimensional restoration and repair. Titanium alloy coatings for medical implants make use of the technology, and the printing industry employs cold spray in copper coatings for rollers. Currently under development are more challenging repair applications for aluminium, magnesium and nickel-based alloy aerospace components.
Cold spray technology is similar to thermal spraying, with the main difference that it does not melt or oxidise the powder feedstock. Instead the solid-state process converts the kinetic energy of powder particles into interfacial deformation and localised heat upon impact with a substrate. This produces a combination of mechanically interlocked and metallurgically bonded particles, forming a coating or a spray-formed shape. The powder particles require a minimum of critical energy (related to impact velocity, material type and temperature) for effective bonding. The accumulation of multiple layers of cold spray particles can produce deposits in excess of 20mm thick (above).
Cold spray has low- and high-pressure process variants. In low-pressure cold spray (5–20bar), pressurised air, nitrogen or helium is heated to 550°C and fed through a converging–diverging de Laval nozzle, where gas accelerates to 600m.s-1. The feedstock is then introduced downstream of the nozzle throat in the divergent section. This system is suited to spraying a limited range of metals such as aluminium, tin, copper and zinc. Several commercial low-pressure systems exist on the market, and while these are simple, portable and inexpensive to operate, they are restricted to a maximum of 20 bar delivery pressure for practicality and safety.
High-pressure systems normally use nitrogen as the propellant at pressures of up to 55 bar. The gases are accelerated to supersonic speeds (up to 1,200m.s-1) by heating to 1,000°C and passing them through a de Laval nozzle. The feedstock powder, typically 10–50¬m, is introduced in the high-pressure side of the nozzle prior to the nozzle throat. The energy levels attained in high-pressure systems provide higher deposition efficiencies and a wider range of depositable materials such as stainless steels, nickel, tantalum, titanium and niobium. This is, however, at the expense of greater equipment complexity, higher costs and limited portability.
When the required coating properties cannot be achieved with nitrogen, helium is occasionally used. It is expected that improved nozzle design and powder development will open up the possibility of depositing higher quality coatings at more affordable prices using nitrogen gas. Not only will this allow cold spray to compete with thermal spray for more demanding engineering applications, but the cost will also be significantly lower than that of laser powder deposition.
Hot or not?
While thermal spraying might currently be the more cost-effective option, coatings produced often display oxidation or thermal decomposition of thermally sensitive phases or particles.
The quality of thermal spray coating depends predominantly on the degree of melting and the velocity of the particles. In high velocity oxy-fuel (HVOF) spraying, for example, high particle velocities allow dense adherent coatings to be achieved even if the powder is only partially melted.
In cold spraying, velocities are increased even further and particle temperatures remain below the melting point. As a result, the cold spray coatings do not exhibit the detrimental effects associated with melting of the feedstock. Compared to its thermal counterpart, cold spray offers some interesting possibilities:
- low substrate heating
- thermally sensitive coating materials or those prone to in-flight oxidation (for example copper, titanium and tantalum) can be deposited without significant degradation, resulting in superior corrosion or oxidation resistance, lower coefficients of friction and/or high thermal and electrical conductivity
- thick coatings can be built up in layers
- deposition of highly dissimilar materials not normally amenable to thermal spray techniques, such as nanophase, intermetallic and amorphous materials
- the peening effect of impinging particles on previously deposited layers can produce potentially beneficial compressive residual stresses
- high density, high hardness and cold-worked microstructures are typical
- substrates may only need minimal surface preparation/masking
- high powder-feed rates increase productivity
- the absence of high-temperature gas jets, radiation and explosive gases improves safety
Cold spray of materials such as copper and polymers offer innovative possibilities for cost-effective and environmentally friendly alternatives to technologies such as electroplating, soldering and painting. However, the process is not without its limitations. Hard, brittle materials such as ceramics cannot be sprayed without the use of ductile binders. Substrates must be resilient or well supported to accept the coating, and low ductility substrates typically result in low bond strengths. In the as-sprayed condition, some coatings display limited ductility. With cold spray being a line-of-sight process, internal surfaces and complex shapes often prove difficult to spray. Furthermore, international standards relating to cold spray are mostly still under development.
Commercial high-pressure systems are able to deliver gas pressures and temperatures up to 40 bar and 800°C respectively, allowing the application of cold spray in coatings for electronic devices, corrosion protection and component repairs. Titanium and its alloys have been spray-formed with deposition efficiencies greater than 95% and porosity less than 1% after post-deposition treatment (below).
Cohesive strength is comparable to the ultimate tensile strength of the parent material in the as-sprayed and post-treated conditions. Aluminium has been cold sprayed onto magnesium alloy and carbon steel substrates, achieving adhesive bond strengths of 38 and 48MPa respectively. These values exceed those of conventional wire-flame spraying (7–10MPa) and twin wire arc spraying (15–25MPa). A typical metallographic section of a cold sprayed aluminium coating reveals a microstructure superior to wire flame spray, with low porosity and a wellbonded interface.
The introduction of ceramic reinforcements such as silicon carbide improves abrasion and erosion resistance, while employing a second powder feed unit facilitates controlled co-deposition of metallic and ceramic powders. This allows the co-deposition of materials with very different melting points, such as aluminium and ceramics.
Cold spray is still undergoing significant technical developments and industry awaits publication of the first standard, the first step towards international standardisation of the process. However, the new commercial systems coming to market will no doubt increase the range of materials that can be deposited, extending its application to many more industries.
Nickel-based coatings on log saw blades
Due to the highly abrasive nature of paper fibres, log saw blades require frequent re-sharpening. TWI was approached by a Sheffield-based manufacturer of high-quality circular blades and asked to assess whether cold-sprayed nickel (Ni) could offer protection from reciprocating sliding wear and micro-abrasion. After preliminary trials, a blade coated with Ni was trialled. Initial results have been encouraging, although further optimisation is required.
Left: Cold spray of Ni for log saw blades. Right: Ni particles embedded on the blade surface.