Thermal spray: Better performance for less

Carol Reeve
Independent Contractor
Ellison Surface Technologies Inc.
Hebron, Ky.
www.ellisonsurfacetech.com

Edited by Jean Hoffman

Thermal-spray technology improves surface properties of parts letting engineers produce designs with less-expensive, lighter, or weaker base materials. Thermal-spray coatings of metallic, carbide, ceramic, or cermet material are said to perform as well or better than parts produced in the same materials for a significantly lower investment. Thermally coated parts are also more resilient where extreme heat, wear, or corrosion can degrade conventional coatings or resurfacing processes.

Designed for grueling environments, thermal-spray coatings handle a wide range of applications from jet-engine turbines and galvanized steel to golf-club heads and hospital beds. They excel when heat and wear resistance are the primary demands. But they can also boost chemical and oxidation resistance, provide electrical and frictional properties, and restore parts to original dimensional specifications.

Applications
Since the 1950s, aerospace has accounted for most uses of thermal-spray coatings. Applied to resist wear, corrosion, and heat, these coatings perform under the intense conditions required of aircraft engines and landing gear.

In the steel industry, processing-line rolls are prone to wear and dross development that shortens their work life. Conditions on galvanizing lines are the harshest — sheets feed through pots of molten zinc at around 850°F. Here, directional sink and stabilizer rolls coated with tungsten carbide resist wear and corrosion.

Much as in the steel industry, paper mills coat rolls in paper-converting lines. Besides mitigating wear, coatings also aid release and protect the quality of the paper product.

Oil and gas industries employ thermal-spray coatings in compressors, turbines, and pumps. Wear-resistant coatings on shafts, sleeves, and piston rods extend equipment life and ensure smooth operation.

Across all industries, thermalspray coatings can be used for restorative work. Worn tooling is just one example. Tools or dies worn beyond acceptable tolerance can be restored to original specifications via thermal-spray coatings. Also common is the use of thermal spray as an economical alternative to chrome plating.

The Process
Preparation of the substrate is vital to success. For a strong bond, the part surfaces must be cleaned and slightly roughened, most often through grit blasting. Pretreatment masking methods include heat-resistant tape, special paint, or customdeveloped metal fixtures that shield areas of a part from the coating.

Part size and shape generally pose no problems for the process. But coating capability is limited to line-of-site surfaces. Deep, narrow channels or hidden areas are difficult to coat with a consistent thickness. In addition, some coatings may chip with severe or repeated impact and do not wear well in such environments. Thermal-spray technology is better suited for frictional and abrasive wear conditions.

Multiple parts can be sprayed simultaneously, and very small parts can be sprayed in a vacuum to eliminate contamination worries.

Before activating the coating feed, the spray gun sometimes passes over the parts, heating and expanding the part surface. Pressurized gas or air propels the atomized molten coating onto the substrate. The coating forms a mechanical bond with the substrate as the part cools and contracts.

The spray technique employed will depend on the coating. Commonly available thermal-spray processes include HVOF (high-velocity oxygen fueled), plasma spray, electric arc, and flame spray.

HVOF generally provides the strongest bonds. The coating material, in the form of a powder, feeds directly into a combined stream of oxygen and a fuel gas. It heats to a molten state and discharges through an expansion nozzle at velocities exceeding Mach II (over 1,600 mph). HVOF has proven to be the best way to spray high-density, high-hardness, and highbond-strength carbides and metallic materials. Coating thickness ranges from 0.0005 to 0.1 in. It provides material densities exceeding 99%, hardness above 1,000 Vickers, and tensile adhesion strengths over 12,000 psi.

Plasma thermal spray is used for coatings with high melting points, such as ceramic oxides. Plasma spray uses an enclosed arc which is struck between watercooled electrodes in the presence of an inert gas. The gas ionizes into a hot plasma into which metallic or ceramic powders are injected. The electric spray gun propels the molten material onto the substrate at a high velocity. As the coating cools it mechanically bonds to the substrate.

In addition to metal deposition, plasma spray also is used to deposit thermalbarrier coatings (TBCs), which resist heat to 2,100°F.

Electric arc, also called twin wire arc, is a thermal-spray technique for metallic coatings with high internal porosity (10 to 15%). It permits fast deposition for thick coatings or large surface areas and can be used in situ for large applications such as bridges and architectural structures. A wire of aluminum, copper, nickel, zinc, or stainless steel feeds into the electric arc gun. The method can deposit coatings up to 0.25 in. thick.

Flame spray or metallizing introduces low-melting-point alloys in powder or wire form into a mixture of oxygen and an inflammable gas. A carrier gas propels the molten metal onto the prepared substrate. Coating deposition for flame spray is similar to electric arc.

For quality-control purposes, a test coupon can be coated with the part to ensure the accuracy and effectiveness of the coating application. A chip taken from the coupon is analyzed and tested for tensile strength, density, hardness, porosity, and other properties. The coupon can also be tested in the physical environment in which the part will function such as a zinc bath in a galvanizing line.