Advances in Thermal Spray Technology

Variations of thermal-spray techniques and the diversity of sprayable materials, along with advanced spray-control systems, have created new opportunities for the thermal-spray industry. Use of the process has grown well beyond the initial stage, and while its continuing importance in maintenance and repair is assured, thermal spray has a great deal more to offer. Thermal-spray technology has entered a new phase of development. Largely accepted by the gas-turbine industry, the process is rapidly gaining recognition as a viable process in "front-end" design in other industries.

Plasma spray: taming a complex process

All thermal-spray processes use a device (the gun) to melt and propel a coating material at high velocities onto a substrate where solidification occurs rapidly (l million degrees per second), forming either a protective coating or a bulk shape. There are basically three types of thermal spray guns: plasma, combustion-flame, and two-wire electric arc. The consumable coating material (feedstock) is in the form of powder, wire, or rod, and combustion or electrical power supplies the energy to achieve melting and acceleration.

Plasma-arc spraying uses a thermal plasma (the highest temperature heat source), and is the most versatile thermal-spraying process. The thermal plasma, a dense, highly ionized gas, has a sufficiently high enthalpy density to melt and deposit powders of virtually any metal alloy or refractory ceramic, as well as combinations of materials.

Traditional DC thermal-plasma units can spray powders at high velocities (>200 m/sec), yielding good coating densities, potentially approaching theoretical density. Plasma spraying results in fine, essentially equiaxed grains, without extensive columnar boundaries, of particular advantage in certain ceramics applications (thermal-barrier coatings, for example). Coatings are chemically homogeneous; there is no (or controllable) change in composition with thickness. It is possible, however, to change from depositing a metal, to a continuously varying metal-ceramic mixture, to a ceramic-rich mixture, and finally to a completely ceramic outer layer, using programmed automation without intermediate delays in spraying or in part handling.

Off-the-shelf plasma-spray equipment offers the capability of high coating-feedstock throughput (3 kg/hr), and special high-power guns can achieve a feedstock (e.g., alumina) throughput of over 25 kg/hr. Aside from normally spraying in air, it is possible, and sometimes essential, to plasma spray in a reduced-pressure environment chamber. Underwater spraying also is possible.

The plasma flame is maintained by a steady, continuous-arc discharge of flowing inert gas, generally argon plus a small percentage of an enthalpy-enhancing diatomic gas, such as hydrogen. Feedstock powder {10 to 70 μm diameter} is carried by an inert gas into the emerging plasma flame. The particles melt in transit without vaporizing excessively, are accelerated, and impinge on the substrate where they flatten and solidify at cooling rates similar to those achieved in rapid-solidification processes.

Much of the heat contained within the particles being deposited, as well as the heat of solidification and the heat of the plasma flame, is removed by conduction through the substrate. Consequently, precautions must be taken to prevent thermal degradation of substrate properties, or to prevent a metal substrate and/or coating from becoming excessively oxidized. Both the substrate and coating contract upon cooling, which can generate high residual stresses if a significant difference in coefficients of thermal expansion exists; these stresses can lead to coating delamination.

While there are hundreds of parameters that influence the plasma-arc spraying process, about 12 have been identified as having the strongest influence on coating properties and the survivability of the coating system. Improved control of these parameters was the focus of many developments that have occurred during the past few years, and is the focus of many current developments. These include incorporating empirical or real-time feedback looping, redesigning fundamental gun components and feedstock powders (e.g. chemical composition, size distribution, and shape), and rethinking power-supply design.

There also have been major changes in gas-handling equipment. Mass-flow control and metering are replacing traditional analog gages, which enable digital output with feedback potential. Data logging is gaining acceptance; flawed areas within a coating are now attributable to an "event" in gas flow, for example. Similar control schemes have been adopted for the powder-feed operation, including a variety of devices that display instantaneous powder-feed rates. Powder feeders also have changed, with fluidized-bed feeders becoming common; these feeders permit smooth flow (less pulsing) of a wider range of powder types.

In the area of power supply, controlled de-power supply systems incorporating heat exchangers have been designed specifically for use with plasma guns, and are becoming the standard in the industry. And while not yet commonly practiced throughout the industry, monitoring and logging current, voltage, cooling-system temperature at various locations (including the gun), gas parameters, and feed rate is a relatively straightforward task.

A revolutionary development in plasma-spray technology that occurred in the 1980s is reduced-pressure atmosphere chamber spraying. Plasma spraying essentially in the absence of oxygen allows the coating/substrate system to be maintained at a high temperature during processing, resulting in interfacial diffusion, which produces a true metallurgical bond.

Chamber plasma spraying is expected to be capable of producing coatings having unique properties in a wide range of applications. For example, it is possible to chamber spray refractory oxides to obtain fully dense, well-bonded coatings. It also is relatively easy to add a high-temperature metal alloy to the oxide to obtain a composite having good high-temperature wear resistance. Chamber spraying also can produce good coatings of reactive metals, such as titanium and zirconium.

An extension of the technique involves spraying the interior of large pipes or tanks for handling chemicals, using the vessel itself as a reduced-pressure inert-gas chamber by excluding air during spraying. Enhanced coating characteristics (e.g., density and adhesion strength) and accompanying improved coating properties achieved in chamber spraying are related to increased particle velocity and the high temperature of the coating/substrate system attained during spraying.

Another variation of chamber spraying is reverse-arc sputtering. The technique involves electrically connecting the target substrate to the spray-gun system, which establishes a transferred arc at the surface, thus effecting a highly efficient sputter-cleaning process. This surface pretreatment combined with the high coating/substrate temperature results in excellent coating adhesion.

Versatility through process variety

Combustion-flame spraying generally uses an oxyacetylene flame to melt and spray either powder or wire feedstock. Due to its lower flame temperature and particle velocity compared with plasma spraying, flame spraying produces a less dense coating having lower adhesion strength. However, flame spraying is simpler in principle and operation, and system and production costs are lower than for plasma spraying. An additional consideration is the possible use of less-skilled operators because the process is more forgiving.

Commercially available wire combustion flame guns can be used to spray virtually any welding wire including composite wires.

A variation of combustion-flame spraying is the spray-and-fuse method of surface hardening. This well-established technique enables flame-spray deposition of a hard-facing material, for example, with subsequent flame fusing. Although the process lacks some control, it is highly effective and is widely used.

The hypervelocity oxyfuel (HVOF) gun represents a major development in thermal-spray technology. Developed by several companies to obtain well-bonded, dense coatings, HVOF guns have in common a method to burn oxygen and fuel and carry the combustion products through a nozzle with subsequent free expansion. This arrangement results in hypersonic flame gas velocities, and by introducing the feedstock powder "up-wind", powder particles attain high heat and supersonic velocities, this permits particle flattening upon striking the substrate, thus forming a dense coating. Special particle-size distributions are required for HVOF spraying, creating challenges and significant opportunities for powder producers.

HVOF sprayed metallic coatings often have properties superior to those of plasma-sprayed coatings, and equal to or superior to coatings produced using the detonation gun. The aircraft industry is especially interested in the HVOF spraying process for producing wear-resistant coatings. Refinements in the process are expected in the future, which may extend its application into areas traditionally dominated by plasma spraying.

Two-wire, electric-arc spraying represents an important method to achieve low-cost application of metallic coatings. Most welding wires can be electric-arc sprayed at high throughput (from 30 to 50 kg/hr). During the process, two consumable wires, through which an electric current is passed, form an electric arc at the point where they intersect. The arc melts the wires and the molten metal is atomized by a continuous flow of either high-velocity compressed air or nonoxidizing gases, such as carbon dioxide, nitrogen, or argon.

Coatings formed using air atomization are relatively dense and have good adhesion. Those formed using inert-gas atomization (which can be carried out in a reduced-pressure chamber) are very dense and well-bonded to the substrate.

The Sonarc process combines two-wire, electric-arc and HVOF spraying; molten metal at the arc is atomized and rapidly propelled to the substrate by the HVOF flame. The introduction of hard reinforcement particles (e.g. alumina or silicon carbide) into the flame makes it is possible to form either a metal-matrix composite coating or a free-standing bulk shape. The high particle velocities attainable in the Sonarc process result in extremely dense composite materials.

New powders create new opportunities

The enhanced quality and variety of feedstock powders is contributing significantly to the advancement of thermal-spray technology. New processes are being used to economically produce special metal-alloy and ceramic formulations (e.g. cemented chromium and tungsten carbides). For example, GTE Products Corp. has developed a new microatomization process in which metal is melted using a plasma torch and molten droplets are propelled at high velocities against a rapidly rotating substrate. The droplets are fragmented and rapidly solidified resulting in spherical powders tens of micrometers in diameter, which can be used as feedstock for plasma and HVOF spraying. Spherical powders are especially necessary in plasma and HVOF spraying to obtain even, nonpulsing powder injection into the flame.