Cincinnati Thermal Spray, Inc.
Coatings, Technology, Solutions
WHAT IS THERMAL SPRAY
Thermal spray refers to a set of processes that utilize heat energy to produce coatings using feedstocks that include metals, cermets, ceramics and plastics. The feedstock is deposited in a molten or semi-molten state in layers and adheres to the work piece by mechanical means. The coating material may be in powder or wire form.
The CTS Thermal Spray Processes
Thermal spray as practiced by CTS includes four processes: combustion spray, plasma spray, arc spray and HVOF spray. All of these processes produce coatings that are serviceable for various applications. The selection of a process takes into consideration all the requirements of the application.
Combustion coatings are typically not as well bonded as coatings produced with other processes, but can often be applied in very thick coatings due to their low coating stress.
Plasma coatings are outstanding for ceramics and other high melting point materials.
Arc coatings are excellent for large areas or onsite applications.
HVOF coatings are excellent for wear resistance or where very dense coatings are required, such as chemical corrosion applications.
Each of the processes has its place in applying thermal spray coatings.
The Steps of Thermal Spray Application
The typical thermal spray application consists of several basic steps:
All of these steps are necessary in most applications in order to produce a serviceable coating.
Many well known applications exist in order to drastically reduce rusting and extend the structure's service life. For example, coatings of zinc or aluminum can be applied to bridge structures. There are also many applications designed for industrial machinery components. Molybdenum coatings on shifter forks will improve lubricity and extend service life. Another example is the use of zinc or aluminum coatings on acetylene or oxygen bottles to prevent rusting.
There are highly technical applications like thermal barriers utilizing complex alloys and high tech ceramics and garden variety applications using steels or stainless steels to build up worn areas.
Thermal spray is well recognized as a tool for component performance enhancement in many industries including:
CTS is a strong participant in many of these markets.
Designing components with thermal spray coatings as one of the features is a method for optimizing performance of machinery by the original equipment manufacturer. For example, pump manufacturers often specify specialized coatings called self-fluxing alloys as a means to increase service life of components, especially pump sleeves. These coatings are applied using the combustion powder process. The coating is then melted onto the part. A diffusion layer is formed with the substrate and the coating becomes a part of the component. The coating is then ground to size and the component exhibits superior wear life as a result of the hard self-fluxing alloy overlay. CTS is an expert at the application of these self-fluxing alloy coatings.
Land based gas turbine manufacturers design in thermal barrier protection to many of the components that are exposed to hot gases, particle erosion and severe thermal cycling. Many other industries also use this method to ensure maximum service life for their critical components and wear parts.
Thermal spray coatings can also be used to rebuild worn parts and can be used to make components better than new by analyzing the wear patterns and operating environment, then prescribing coatings to extend service life.
In addition to thermal spray coatings, CTS offers dry film lubricants and release coatings, all of which can enhance the surface properties of our coatings, or can sometimes improve the performance of components even without using thermal spray coatings.
It is important to note that coatings are applied selectively where they are needed and that they protect components from exposure to heat, chemicals, molten metal, abrasion, adhesive wear, galling, fretting, or something as simple as weathering. The limit to the applications that are possible is your imagination.
Combustion thermal spray includes coatings produced using powder or wire as a feedstock.
Wire Combustion Process
The wire combustion process uses a wire, either solid or cored, and continuously drives the wire into a combustion flame (usually oxygen/acetylene). The wire may be driven into the flame using either an electric motor or an air turbine which transmits power to drive rollers through a geared transmission. Compressed air is used to propel the molten droplets onto the surface to be coated. The coating is built up layer by layer until the desired coating thickness has been achieved. Coating feedstock is limited to materials that can be drawn into wire form. A very limited number of cored wires are also available, but development of new materials for combustion wire spraying stopped long ago.
Today's combustion wire applications are frequently driven by aging specifications. Combustion wire coatings tend to be less well-bonded and more porous than coatings produced with other processes. Bond coats are often used; special layers applied directly on the work piece to improve the bond. The coatings are quite economical and can be sprayed quite thick producing very serviceable coatings that last for many years. Even with bond coats, combustion wire coatings are still less well bonded than coatings applied with other thermal spray processes. The arc spray process has overtaken many of the applications that were once performed using combustion wire.
Combustion wire coatings are normally applied using oxygen/acetylene gases; for lower melting point materials, Mapp and other relatively lower BTU gasses may be used. The coatings available for combustion wire include virtually any material that can be drawn into a wire.
Commonly used combustion wire materials:
Exotic Materials used:
Combustion Powder Process
The combustion powder process uses a powder feed stock that is blown into the combustion flame using inert gas (normally nitrogen), or powder is delivered through a gravity feed. The flame melts and accelerates the particles onto the work piece. A much broader range of materials may be used to produce coatings with powder than with wire. Combustion powder coatings are typically fairly well bonded and fairly dense. Special techniques can be developed to improve properties. Frequently the combustion powder process is used at CTS to apply coatings containing graphite. The relatively low flame temperature allows maximum graphite retention in the coating.
The combustion powder process produces coatings that are well bonded, fairly dense and serviceable. Coatings can be applied quite thick because of the relatively low stress involved in the production of these coatings.
Coating materials available for the combustion powder process include self-fluxing alloys, most of the pure metals, many alloys, carbides, ceramics and a class of coatings called self-bonding. Self-bonding materials exhibit high bond strengths and excellent machinability or grindability. Properly applied, they can be machined to a feather edge.
Spray and Fuse
Spray and fuse is terminology used to describe coatings applied with the combustion powder process that are subsequently heated to about 2000°F and melted to the substrate material. These coatings are typically nickel or cobalt based and form hard carbide and boride phases during processing, thus are considered hard facing applications. Spray and fuse as a process has several advantages over conventional weld overlay hard facing techniques. The component being sprayed is heated all at once during the fusing operation instead of being heated in a localized area as happens in welding. Therefore, distortion of the component is minimized. Since the coating thickness can be controlled to a much better extent than is possible in weld overlay, it is possible to use a minimum of material. Since that material is fairly smooth after fusing, machining/grinding time can be minimized. Coatings applied using the spray and fuse process are 100% dense and show a diffusion layer with the substrate. They are specified and used where there is high potential for corrosion or corrosion combined with severe wear. Therefore they are often used in such machine elements as pump sleeves and valves. They may also be used in impact applications such as hammers.
Electric arc, sometimes also called twin wire arc, is a process that uses two wires, electrically charged and run into a dead short. The heat produced by the electricity in the short is sufficient to melt most materials. Compressed gas (usually air) is then used to project the molten particles onto the work piece.
Arc coatings are typically well bonded, although bond coats (special coating layers used to improve the strength of subsequent layers) are often used. Coatings can range from relatively porous to quite dense. Arc spraying is very versatile in terms of applying metals and some cermets. Arc is especially useful for thick overlays or coatings on large areas. It's also especially effective as a process for coating projects done onsite.
There has been a great deal of development of materials for arc spraying in recent years. Cored wires have been produced resulting in coatings containing ceramics and cermets that greatly enhance performance characteristics of the coatings making arc spray an even more versatile process.
All materials that can be drawn into wire form may be sprayed with the arc process. In addition, development of a broad range of cored wires (wires that are made of a conductive sheath filled with a different material) has substantially broadened the scope of the arc spray process. Materials exist that combine the metal sheath material with various carbide cermets and with ceramics. These materials often outperform traditional arc spray materials especially in the area of wear resistance and chemical corrosion resistance.
High Velocity Oxygen Fuel or HVOF is the most recently developed of the thermal spray processes in use at CTS. Combustible gas and oxygen are used to produce a flame that is directed at very high velocity through a gun by use of a converging, diverging nozzle arrangement. Powder is injected into the hot gas stream and softened, then impacted with very high kinetic energy onto the work piece layer by layer until the desired thickness is built up. The resultant coatings approach theoretical density and exhibit outstanding performance characteristics.
HVOF coatings are the densest among the processes and also the most securely bonded. Bond strength usually exceeds the strength of the glue used in the tensile bond test, normally above 10,000 psi. Coating porosity for many materials is 1% or less. These are the most wear and corrosion resistant coatings of all the processes.
Coatings of tungsten carbide materials are optimized using HVOF because the HVOF flame is not hot enough to cause the formation of complex phases of carbides. Tungsten carbide coatings produced with the HVOF process are among the most wear resistant coatings possible with thermal spray technology. These coatings routinely outwear chrome plating by factors in excess of five to one.
Coatings of high alloys are another popular use for HVOF. Oxidation resistant, heat resistant alloys such as Hastelloys®, Inconels®, Stellites®, Tribaloys® and some proprietary alloys make up a key area of HVOF coatings. All of these materials protect against temperature, wear, chemical attack or some combination. HVOF is also used frequently as a component rebuilding process utilizing steel and stainless steel as coating materials.
Hastelloy® is the registered trademark name of Haynes International, Inc.
Inconel® is the registered trademark name of Special Metals Corporation.
Stellite® is the registered trademark name of Deloro Stellite Company.
Tribaloy® is the registered trademark name of Deloro Stellite Company.
The plasma spray process uses inert gas, usually nitrogen or argon, excited by a pulsed DC arc to ionize the gas and produce a state of matter called plasma. Other gases (mainly hydrogen and helium) are often introduced in small quantities in order to increase the ionization. The plasma gases are introduced at high volume and high velocity and are ionized to produce a plume that ranges in temperature from about 12,000° to 30,000°F. Powder feedstock is then injected into this hot gas stream (called a plume), heated very quickly, and deposited onto the work piece.
Plasma is the most versatile of the processes with the ability to spray almost any feedstock in powder form. Extensive experience with this process makes it the one most used at CTS. We spray thousands of pounds of a broad range of materials through our plasma systems. Almost all of these applications are sprayed robotically.
Materials that are very difficult to melt, virtually any ceramic or refractory metal, can be used to produce serviceable coatings using the plasma spray process. Plasma coatings are very dense, well bonded structures.
Plasma spray is the most versatile of the thermal spray processes. Excellent coatings of metals, ceramics, cermets, refractory metals, and plastics can all be produced using the plasma process. Of all the potential coating choices, plasma is best suited to apply ceramic coatings because of their high melting temperature.
Plasma spray is especially effective in applying hard to melt materials like refractory metals and ceramics although it is certainly possible to apply the other classes of materials such as metals, alloys, and plastics using plasma. The extremely high temperatures and gas velocities, second only to HVOF, produce well bonded dense coatings of the broadest range of materials of any of the processes.