5.6 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Detonation spraying | 2/3 | https://en.wikipedia.org/wiki/Detonation_spraying | reference | science, encyclopedia | 2026-05-05T10:47:14.991060+00:00 | kb-cron |
For cases where a very strong mechanical bond is required (such as for components that are going to be used to machine with) the components surface is often machined to create grooves for the coating to bond to. Dovetail grooves offer strong positive bonding but can be laboursome and costly. A cheaper method is to cut simple partially open grooves, yet this method produces an inferior final bond strength. The edges and corners of a component present possible weak points in the coating structure, as they can break off from the component. To increase the bond strength at these points the corners and edges of the component should be rounded off. If the coating does not need to reach the edges of a component, then an undercut can be used (as shown in the diagram to the right) to secure the coating to the substrate. Although undercuts can also be used in other scenarios. Coatings often have a tendency to shrink after being applied due to the cooling process. This means steps need to be taken in order to minimise the negative effects of shrinking. If not, the coating can suffer from stress due to tension which will weaken the coating and in some cases may cause it to peel off. The fact coatings shrink can be used to increase the bond strength if applied wisely. Coating over the entire external surface of a component means that the coating will shrink around the component when cooled providing a sort of gripping force that will increase the mechanical bond strength. This is also the case if a flat component is sprayed over the edges, the coating will grip the surface like a clamp; again increasing bond strength. Internal coatings suffer from the effect of shrinking in that they will be pulled away from the surface of the component. To counter this the component can be heated to reduce the relative shrinking effects on cooling. Components should be dry machined (without oils) to avoid oils being deposited on the component before spraying. If this is unavoidable then the substrate will need to be cleaned again prior to detonation spraying.
=== Bond Coating === After a surface has been abrasion blasted and/or machined a thin layer of molybdenum, nickel-chromium alloys or nickel aluminide can be sprayed before the final detonation spray coating to improve the bond strength. This is known as a bond coating. Bond coatings are often used when spray coating materials of ceramic composites are being applied. The component may need to be machined and/or abrasion blasted slightly deeper for the purpose of allowing space for the bond coating and spray coating to fit flush on the component surface. Areas that are not to be sprayed must be covered in stop-off chemicals (chemicals that stop the spray from bonding) or tape. The chemicals and tape are then removed after the coating has cooled.
== Detonation Spray Coatings == Detonation spraying produces coatings of very high chemical bond strength and hardness. Coatings are of low porosity, oxygen content and have a low to medium surface roughness. This is achieved due to the extremely high temperatures and velocities produced by the detonation gun during surface coating application. These properties make detonation spraying the standard of comparison for all other thermal spray coatings (wire arc, plasma, flame, HVAF, HVOF, Warm, Cold). There are many factors that determine the final detonation gun coating properties. Primarily, surface properties are determined by the type and properties of the powdered feedstock used (composition and particle size) but they are also affected by the settings used on the D-gun. These are powder flow rate, firing rate, distance from gun to target, how the D-gun is moved around to apply the coating, size of barrel, amount and composition of fuel and oxygen mixture. Detonation spraying is able to apply protective coatings to relatively sensitive and delicate materials. This is due to the nature of the application of detonation gun coatings, being very quick and having the heat source removed from the target material. This allows for a large range of suitable applications for detonation spraying.
=== Types of Materials === Many materials are able to be sprayed as coatings using the D-gun. These materials used for the feedstock are powders of metals, alloys and cermets; as well as their oxides. However, mainly high-tech coatings are used, these include ceramics, and complex composites. Characteristics such as strength, hardness, shrink, corrosion resistance and wearing quality of possible spraying materials are factored into the decision of selecting a coating material. Some examples include:
Al2O3 Cu–Al Cu–SiC Al–Al2O3 Cu–Al2O3 Al–SiC Al–Ti TiMo(CN)–36NiCo Fe–A
== Applications == The main functions of detonation spray coatings are to protect against corrosion (due to low oxygen content), abrasion and adhesion under low load. This means that detonation spraying produces hard durable coatings that are suitable for:
Various components of general machinery: shafts, seals, bushings, bearings, seals Aviation: rotor and stator blades engine components guide rails Oil and gas industry: bushings and sealing rings of ESP units gate valves shut-off valves working surface of drill tools Space rocket industry Electronic and radio industry Engineering of instruments Tools industry Tubular drills Skiving knives for rubber and plastic Shipbuilding industry D-gun plated plug and ring gauges
== Limitations == There are a few limitations of detonation spraying, these are: