Thermal spraying is advancing on several fronts as the technology matures. Spray processes are becoming colder. High performance systems enable a higher powder throughput, thus making production faster and more efficient. The current spectrum of substrate materials is much broader. The new materials prevent thermal stresses from arising during the production process. Yet at the same time, to achieve the best results, more applications are requiring the use of special cooling methods to increase the cooling efficiency and, in turn, optimize the process.
Whether the substrate should be cooled in thermal spraying and, if so, with what, is frequently the topic of discussion. Cooling is not necessary for all applications, but it may be very important in some. A few examples show clearly that cooling should be viewed as an important parameter in the application as a whole.
Zircon oxide can be applied as a thick layer with high adhesion values if a coating temperature of 1,112°F (600°C) is set. Such a coating temperature allows the thermal spray operation to obtain a layer that is porous against thermal fatigue if the component temperature does not exceed 212°F (100°C). Oxide ceramic materials in particular often require a higher temperature in order to not solidify in shock on the substrate. Yet if the component walls are thin, it is important to maintain a narrow temperature field so that the conditions are always the same. Only in this way can it be guaranteed that the coating will be homogenous throughout the coating thickness.
Because of its low thermal conductivity, the danger with titanium is of rapid overheating and consequent material changes. With titanium, the temperature stability of alloy elements is most likely to be affected. Pure titanium starts to creep even at room temperature. Titanium containing vanadium (V), chromium (Cr) or carbon (C) as the alloy elements retains its solidity up to 608°F (320°C). Other elements such as aluminum (Al) or tin (Sn) prevent the diffusing-in of nonmetal atoms such as oxygen (O), nitrogen (N) or carbon, which cause brittleness. With the very high local temperatures reached in high velocity flame spraying, these ranges are reached quickly. Here, cooling with carbon dioxide (CO2) could create the ideal conditions for coating these components in one step.
In the field of new materials, it is mainly the plastics that deserve attention. In order to apply an adhesive layer on a carbon-fiber-reinforced material with plasma spraying, trials were carried out with carbon dioxide, air cooling and without cooling, with identical parameters. Figure 1 shows the result. In the tests with air and without cooling, carbon fibers came loose from the matrix. With carbon dioxide cooling, an intact join between the carbon-fiber-reinforced plastic and the nickel chromium (NiCr) was created.
Metallurgical reactions at high temperatures such as oxidations during copper coating reduce the quality of the coatings. Particular attention must be paid to the material of the substrate. Excessively high temperatures, even if only localized, may cause structural changes that then change the material properties.
Comparison of Cooling Media Used During Thermal Spray
Air. The most widespread cooling medium in practice is air. The important thing for thermal spray operations is that the air must be dry and grease free. Investment in a cooling dryer and an oil separator, which must both be sufficiently dimensioned, is generally unavoidable. One reason for this is that in large operations with mechanical production, in order to lubricate the valves of pneumatically controlled machines, oil is often mixed with the compressed air from network pipes.
Using air, the heat is dissipated in an ideal way by means of convection. The continuous flow of compressed air over large areas of the surface yields the best efficiencies and dissipates the heat accordingly.
With special nozzles modified to the application or with air showers working on the injector or vortex principle, air can be distributed over a large surface on the entire component in order to keep the temperature within a suitable range. If injector air showers are used, the ambient air also should be taken into account because it is sucked in and comes into direct contact with the component.
The air-consumption levels of cooling systems must always be considered in the calculation. With all of the methods that are used, air consumption rates of 20 to 45 m3/hr are not uncommon. Figure 2 shows the air consumption of air jets, and figure 3 shows the air consumption of cooling strips and injector air showers.
In order to apply a curtain of air across a component, a strip should be prepared in which the individual holes are not more than 0.394 to 0.787” (1 to 2 cm) apart. Every hole should be as small as possible. In the comparison shown, a strip that was 7.874” (20 cm) long with 16 holes with a diameter of 0.031” (0.8 mm) was used.
Water. In special cases, water also can be used for cooling. One example is when hollow-component geometry allows it. However, if components are affixed to an external water cooler, care must be taken to ensure that contact with the latter is always ideal.
Nitrogen. With certain coating systems, air is replaced by nitrogen to prevent oxidation caused by cooling. However, changing to carbon dioxide would definitely have a more favorable effect.
Although cooling with liquid nitrogen may be the most effective method due to the extremely low temperature of the gases, physics does not allow such low temperatures to be used. No direct contact between the liquid nitrogen and the component is possible. A natural gas cushion, which functions like an insulation layer, always forms between the two media (figure 4). The nitrogen behaves like a droplet of water skittering across the hot plate of a stove. The phenomenon is called the Leidenfrost effect, and it plays a major role in the behavior of all liquid gases.
By contrast, the Leidenfrost effect phenomenon does not occur with carbon dioxide. The carbon dioxide snow particles with cold gas are propelled onto the surface and penetrate this barrier.
Carbon Dioxide. Carbon dioxide is provided in a liquid state. In the pressurized tanks or the gas cylinders, the liquid and gaseous phases are in balance as a function of the temperature. In the carbon dioxide phase diagram, a pressure of approximately 5.7 MPa is shown for room temperature (68°F [20°C]). If liquid carbon dioxide is expanded into the atmosphere, a mixture of carbon dioxide foam and cold carbon dioxide gas is produced (figure 5).
If carbon dioxide is used as a cooling medium, specially developed hardware is used. The nozzles take the carbon dioxide in a bundled jet and direct it to the point at which the temperature is to be lowered. The cooling performance of carbon dioxide is controlled via the nozzle size and, thus, via the carbon dioxide consumption (table 1).
Setup and Results from Cooling Tests
In order to compare air and carbon dioxide directly with each other, a copper sphere (figure 6) was heated to 212°F (100°C). A thermo element in the center of the component with a length of 3.937” (100 mm) and a diameter of 1.575” (40 mm) recorded the temperature drop on the time scale. This test was used to compare all cooling media and to show their respective efficiencies.
The first test compared carbon dioxide with an air injector. This air injector has the advantage of increasing the amount of cooling air due to a vacuum effect, which enlarges the previously mentioned convection effect. For the test, a small amount of 35 m3/hr compressed air is used. Figure 7 shows a total airflow of 300 m3/hr at 4-bar air pressure.
The test shows that air needs up to five times more time to reach a temperature of 68°F compared with a medium-sized carbon dioxide nozzle (Type 25) at a rate of 640 g/min (figure 8).
The second test shows the results using liquid nitrogen (N2) compared with carbon dioxide. As described above, cooling with liquid nitrogen may be the most-effective method due to the gas’s extremely low temperature. Physics does not allow this low temperature to be utilized. The problem is that no direct contact between the liquid nitrogen and the component is possible. Again, a natural gas cushion, the Leidenfrost effect, serves as an insulation layer.
Figure 9 shows the time needed for the test with nitrogen and three different carbon dioxide nozzles with different carbon dioxide consumption rates. What can be seen is that with nitrogen, very short cooling times can be reached; however, in comparison with carbon dioxide, three times more nitrogen needs to be used for similar cooling efficiency. The use of equal consumptions of the cooling media achieved a 50 percent time reduction using carbon dioxide for the same cooling results.
In conclusion, in most cases, air will remain the cheapest and most common medium for cooling during thermal spraying. There are different ways to optimize the cooling for applications with special needs in terms of the temperature management or cost reductions due to shorter spray times. Because of its specific advantages, which include time savings and optimum cooling performance, the use of carbon dioxide opens up new applications for thermal spraying.