The following outlines are the most common design & application considerations that should be reviewed when implementing SST Cold Spray™:
1. Substrate Surface Features
The bonding mechanism of cold spray does not enable the coating to be self-leveling. While surface roughness can be beneficial to increase the coating bond area, pronounced roughness can lower the deposition efficiency. Coating uneven substrates as the one illustrated in Figure 1 below may result in an irregular coating surface, as illustrated in Figure 2.
Figure 1: Coating Irregular Substrate
The odd coating shape shown in Figure 2 occurs because the Nozzle is not consistently perpendicular to the substrate surface as it moves along. The angles of the surface irregularities change the deposition efficiency and, in effect, amplify the underlying contour of the substrate.
Figure 2: Deposition Deficiency on an Irregular Substrate
To reduce or eliminate the irregular deposition effect, the Nozzle should be retained in an orientation that is perpendicular to the substrate surface in every point, as shown in Figure 3 below. An alternative is to manage this by simultaneously changing the travel pattern or speed to concentrate on filling the depressions. Nozzle position or orientation adjustments can also be used to obtain the same results. In some cases, the Nozzle size or shape can be tailored to assist with the condition of the substrate. In addition, mechanical processing such as sanding can also be used between coating passes to obtain the desired result.
Figure 3: Nozzle Following Substrate's Shape
2. Substrate Edges
Substrates with extreme shapes, such as the sharp edge shown in Figure 4 below, require additional nozzle motion to get purposeful coating deposition. Ideally the nozzle will be perpendicular to the surface so the feedstock powder will not simply be deflected off.
Figure 4: Sharp Angled Substrate
3. Holes and Voids
Holes and voids in the substrate must be filled with an appropriate backing material, as in Figure 5, in order for the powder to have a surface to adhere to; otherwise, the powder will simply pass through. This can of course be a good process trait because masking of holes generally not required.
Figure 5: Coating Through-Holes in Substrate
As with holes, the coating will mask over cracks but they cannot be considered as repaired. Without conventional crack stopping and joining processes the underlying defect will reappear in the coating and may continue to propagate. Cold Gas-Dynamic Spray coatings are ideal for sealing or filling a stabilized repair because it can greatly reduce the heat input or restore the properties of the surface.
It is generally not possible to coat really deep holes such as those illustrated in Figure 6, because the restricted access to the area to be coated does not permit the nozzle to be at a favorable angle to the substrate. If you have a situation like this, please consult SST for recommendations.
Figure 6: Restricted Nozzle Access
4. Material Type
The substrate has to be strong enough, or supported, such that it will resist the impact of the feedstock powder particles. The requirement that the substrate be hard and rigid is rarely a problem in practice. In some cases, the substrate can be a temporary and the consolidated object can be removed from the substrate before or after post processing.
In very limited situations, coatings may be applied on surfaces contaminated with oil, paint, rust or scale. In these cases feedstock powder may be used to clean and activate the substrate before it will begin to adhere. For most substrates however, pre-cleaning is highly recommended. This may involve physical or chemical cleaning if there is grease, contamination, or loose materials on the substrate. The surface may be further prepared with abrasive powder using the Cold Gas-Dynamic Spray equipment.
The substrate and the feedstock powder have to be compatible in service. Compatibility issues relate to the chemical, mechanical, thermal, and electrical properties of the materials. If you have extreme or unusual service conditions process development and experimental testing might be necessary to validate the combination. SST can assist with these requirements.
- Thermal Characteristics
The substrate has to be able to withstand the heat of both the supersonic air jet and the sprayed particles. If the substrate is sensitive to thermal stress, preheating may be necessary to avoid spalling.
1. Nozzle Configuration
SST standard nozzles have been configured for successful application in common applications. Characteristics that should be considered in the application include:
The cross-section, including both the shape and size of the nozzle outlet, will determine the shape and build-up profile of the applied coating: Round nozzles work in general applications or in narrow or small areas requiring thick buildup of coating. Large areas requiring flat coatings are effectively sprayed with oval or rectangular nozzles. Obviously, larger nozzles project larger spray patterns. The standard nozzle shapes and their resulting coating profiles are illustrated in Figure 7 below.
Figure 7 - Cross Section
The length of the nozzle determines the length of time the feedstock powder is accelerated in the carrier gas jet. We have optimized the length for general applications and can make recommend different nozzle lengths for unique access conditions or to suit a specific workpiece configuration.
2. Nozzle Orientation
Nozzle orientation plays an important role in achieving good deposition efficiencies and obtaining high-quality coatings. As shown in Figure 8 below, the spray axis represents the angle between the nozzle axis and the surface of the substrate.
Ideally, the nozzle should be oriented perpendicular to the substrate (spray axis = 90°) in the area being sprayed. The efficiency reduction is reasonably negligible if the spray axis is maintained within about +/- 10° from this orientation.
Figure 8 - Nozzle Orientation
3. Spray Distance
The spray distance is defined as the distance between the nozzle outlet and the substrate as shown in Figure 8. The recommended range for the spray distance is between 5 mm and 15 mm. In general, outside these limits causes the powder consumption to be higher and/or a decrease in process efficiency. A shorter than recommended spray distance or no space between the nozzle outlet and the substrate might obstruct the powder-laden jet.
4. Ambient Conditions
The normal environmental temperature for the cold spray process is +15°C to +35°C (60°F to 95°F). Much colder temperatures will likely require preheating of the substrate; although in moderate cold the hot gas jet of the spray gun can likely provide the necessary heating. Higher temperatures may affect the control electronics but this also can be managed if necessary.
While the coatings can frequently be applied in the presence of moderate humidity, moisture control is recommended to ensure process consistency. In no case should equipment be subjected to, or operated in, a wet environment unless the particular unit has been rated for this application.
The cold gas-dynamic spray process should be conducted in open areas with very good ventilation, or in an approved cabinet or spray booth fitted with good quality explosion-proof ventilation system. Unless enclosed, personnel in the work area must wear respiratory protection appropriate to the application. The process is not intended for use in confined spaces.
The noise level produced by the cold gas-dynamic spray process exceeds 85 dBA so all personnel in the work area should wear suitable hearing protection.
SST standard equipment is designed for operation in Class 2 Group E environment and should not be operated in hazardous explosive or combustible surroundings. The equipment should not be operated in an environment containing active sources of ignition such as flame, high heat, spark, or static electricity.
Twitter Feed @SSTCLine: