Thermal Spray Goes Cool

Most coatings have a functional purpose as well as a decorative one. Paint protects a car's body against the elements while adding razzle-dazzle to its appearance. Some coatings, though, are mainly functional, and as a class, thermal spray coatings are the most functional of all.

Traditionally, thermal spray coating applies metals, ceramic, plastics, and mixtures of these materials at temperatures ranging from 4,500 to 30,000 ^oF. Such coatings provide surer footing on oil-rig platforms and increase the durability of auto transmission components. Thermal spray coatings are typically used to impart improved wear, corrosion, and abrasion resistance in applications where curb appeal does not matter as much as performance under rugged conditions where traditional polymer paints alone just won't do.

But a nascent technology, cold spray, can be used to apply functional coatings at or near room temperature. Cold spray is less energy and heat intensive than existing processes and provides a novel means of coating or altering a variety of materials. One new entrant, Dow Corning, has launched a business to commercialize and license cold plasma spray technology. Industrial gas firm Praxair, long involved in the thermal spray services and equipment business, has commercialized a slightly different cold spray technology.

Traditional thermal spray coating is a substantial business. Markus Heusser, head of Sulzer Metco Thermal Spray, pegs the global market for thermal spray powders and equipment at just less than $1 billion annually. Sulzer Metco is a Switzerland-based thermal spray services provider as well as a maker of thermal spray equipment. Overall, Heusser says, the market grows at about the same rate as gross domestic product, which means slower growth rates in Western Europe and the U.S., and higher growth rates in the rapidly expanding economies of Asia.

The market for thermal spray coatings is even larger if thermal spray applicator sales are included. Alejandro Pena, vice president of thermal spray consumables for Praxair Surface Technologies, places the global market at about $2.5 billion.

Gordon England, who heads an eponymous thermal spray consulting firm in Surrey, England, says metals and ceramics can be applied to various surfaces using a variety of thermal spray techniques. For instance, wire flame spray and wire arc spray techniques are used to coat large metallic structures such as bridges with anticorrosive coatings. "Basically, anything you can't put into a galvanizing bath" is a candidate for these spray techniques, he says.

The equipment used for wire arc spray coating resembles welding equipment, in which two electrodes form an electric arc. In the wire arc process, air or an inert gas such as nitrogen is fed into a spray gun. The gas passes over melting wires and propels molten droplets onto the target. Heat source temperatures can run as high as 15,000 ^oF.

Wire flame spray coating is a little different. A spark is used to ignite an oxygen and fuel mixture in a spray gun that is holding a metal rod. After the rod turns molten, it is disintegrated by a stream of air and then propelled onto the object to be coated. Temperatures are generally below 4,500 ^oF.

Both wire arc and wire flame spray techniques have been around for a while, England says. A more recent development is the high-velocity oxygen fuel (HVOF) gun, which allows application of "extreme wear" coatings such as tungsten carbide or chromium carbide. The paper industry uses such coatings to increase the useful life of paper machine rollers. The HVOF gun mixes oxygen and a fuel to achieve temperatures of about 5,000 ^oF.

More advanced are high-temperature plasma spray guns, says England, who calls such equipment a "universal tool to spray virtually any material." The high temperature and control over coating thickness offered by plasma spray makes it a favorite to apply ceramic coatings, he adds. Potential applications include coating jet engine turbine blades and airplane landing gears. Other plasma spray uses include application of a biocompatible coating, such as hydroxyapatite, over artificial joint implants.

Plasma spray equipment uses two electrodes to form a plasma gas consisting of argon and hydrogen or argon and helium. The gas melts metal, ceramic, and carbide powders at temperatures that may exceed 30,000 ^oF.

The technique to spray metal, polymers, and other particles at or slightly above room temperature is still experimental, England says. This cold spray process flings powders at high velocity to produce a coating that flattens and adheres to the substrate. Using metal powders, the process produces what England calls a "very clean," oxide-free, electrically conductive coating. Metal, metal composites, paper, and plastics can serve as substrates for such coatings, he adds.

But the pressure and volume of gas used in cold spray technologies "is alarmingly large," England says. Where helium is used for cold spray, manufacturers "need to think about building gas recovery systems, otherwise gas costs will be very high," he says. Alternatively, they could use less expensive gases such as nitrogen.

According to England, Russian scientists originally developed cold spray technology in the mid-1980s as an alternative to HVOF thermal spray. Although he says industrial users are interested in the process, he considers it largely experimental at this time.

Dow Corning is one firm, however, that hopes to take cold spray technology from lab curiosity to the industrial production line. In August, it began to offer industrial clients its PlasmaStream technology and equipment, which uses a jet of cold plasma to cover a substrate with a uniform coating just tens of nanometers thick.

According to Program Manager Andy Goodwin, Dow Corning began working on cold spray processes as far back as 1996 when it undertook a research project with the University of Durham, in England. As work with the university progressed, the company found the technology increasingly promising. To bring it to the industrial market, however, the firm needed to focus on developing the appropriate equipment.

In 2001, Goodwin recalls, Dow Corning bought equipment know-how from an Irish firm and then set up shop in Cork, Ireland, to come up with a commercially viable cold spray plasma process. This summer, with several patents in hand, Dow Corning and equipment fabrication partners brought out new cold plasma spray equipment.

Equipment in the SE-1000 series comes in widths of 1-8 feet depending on customer needs, says Gary Lord, a Dow Corning commercial manager. SE-1000 series equipment is suited for coating textiles and plastic films, generally for customers who want to put a protective coating on textiles, metal, or filtration products.

SE-2000 series equipment is designed to coat three-dimensional objects including polymers, metals, ceramics, or glass with hydrophilic, adhesion promoting, antimicrobial, and even slick coatings. The basic component of the series is a coating head that can be placed on a robotic arm in a production line or above a conveyor belt.

Dow Corning's equipment aerosolizes a coating material in a gas such as helium or argon. A "small amount of electrical energy" is introduced to create a plasma that is emitted from the gun, Goodwin explains.

The plasma, which can be as low as 77 ^oF, hurls the coating material onto the substrate. Upon impact, the coating cross-links and forms a covalent bond with the substrate, Goodwin says. Coating materials include fluorocarbons that impart oil and chemical resistance, silicones for water resistance, and acrylics or alkoxysilanes for adhesion promotion.

Praxair's cold spray technology, in contrast, does not involve creation of a plasma. David Brackins, vice president of business development, says this relatively new technology instead involves the acceleration of particles in a gas stream whose temperature reaches a few hundred degrees Fahrenheit. The kinetic energy created upon impact results in interparticle bonding and adhesion of coating particles to the workpiece.

The technique, Brackins says, is particularly useful for depositing unoxidized metallic materials such as copper, aluminum, titanium, and zinc. Praxair doesn't supply cold spray equipment, but it does provide the service to customers who need to restore worn high-value parts such as aircraft propellers and landing gears that would be damaged using higher temperature coatings.

Heusser of Sulzer Metco won't comment on any plans the firm may have to enter the market for cold spray equipment and supplies. He will only say that the company continues "exploring all kinds of technologies."

But others are already eager to enter the market. One firm, Ktech Corp., now offers to sell customers a cold spray system. The Albuquerque, N.M., engineering and research firm can set customers up with a laboratory-scale system that operates with helium, nitrogen, or air at temperatures ranging from 68 to nearly 1,300 ^oF.

Although they are intriguing, high-cost, high-technology cold spray systems aren't always justified, consultant England says, and in many cases, high-temperature thermal spray coatings will do just fine. Because "at the end of the day you want to spend the least amount of money to get the best job," he says.

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