Detonation spraying

Detonation Spraying

Main article: [[:Thermal spraying]]

First invented in 1955, detonation spraying is one of many forms of thermal spaying techniques used to apply a protective coating at supersonic velocities to a material in order to change its surface characteristics; namely to improve durability of a component. Detonation spraying is performed using a detonation gun (D-gun) and involves the highest velocities (≈3500 m/s shockwave that propels the coating materials) and temperatures (≈4000 °C) of coating materials compared to other thermal spaying techniques, which means detonation spraying is able to apply low porous (below 1%) and low oxygen content (between 0.1-0.5%) coatings.

This allows the application of very hard and dense surface coatings which are useful as wear resistant coatings. For this reason, detonation spraying is commonly used for protective coatings in aircraft engines, plug and ring gauges, cutting edges (skiving knives), tubular drills, rotor and stator blades, guide rails or any other material that is subject to high wear and tear.

History

The process of detonation spraying was first developed in 1955 by H.B. Sargent, R.M. Poorman and H.Lamprey and was subsequently patented. It was first made comercially available as the 'D-Gun Process' by Union Carbide in the same year. It was further developed in the 1960's by the Paton Institute in Kiev (Ukraine), into a technology that is still currently commercially available in the USA by Demeton Technologies (West Babylon).

D-Gun

Detonation spray coatings are applied using a detonation gun (D-gun) which is composed of a long-water-cooled metal barrel containing inlet valves for introducing gases and powders into the chamber. A preselected amount of the desired protective coating material known as feedstock (in powder form of particle size 5–60μm) is introduced into the chamber (powder flow rates of 16–40 g/min). Here oxygen and fuel (generally acetylene) are ignited by a spark plug to create a supersonic shock wave that propels the melted/partially-melted/solid (depending on the type of material used) feedstock out of the barrel and onto the subject being sprayed. The barrel is then cleared using a short burst of nitrogen before the D-gun is ready to be fired again. This is important as the heat from the residual gases can cause the new fuel mixture to combust which would cause an uncontrolable reaction. A small amount of inert nitrogen gas inserted between the two mixtures prior to firing helps to prevent backfiring. D-guns typically operate at firing rates of between 1-10 Hz. Many different mixtures of coating powders and D-gun settings can be used to change the properties of the final coating. Common powder materials used include: alumina-titania, alumina, tungsten carbide-tungsten-chromium carbide mixture with nickel-chromium alloy binder, chromium carbide, tungsten carbide with cobalt binder.

Measurments of surface oxygen content, macro and micro-hardness, porosity, bond strength and surface roughness are used to determine the quality of the coating.

Properties Values Determining Quality of Spay Coatings for D-Gun Spraying

Thickness (μm)	Porosity (%)	Oxygen (%)	Hardness (VHN)	Bond Strength (psi)	Surface Roughness (μm)
75 - 125	0.25 -1	0.1 - 0.5	1350	10 000	3-6

Components

• Spark plug
• Water cooled barrel
• Nitrogen inlet valve
• Fuel inlet valve
• Oxygen inlet valve
• Powder inlet valve
  

Cycle of Operation Overview

1. Mixture of fuel and oxygen is injected into the combustion chamber
2. Powder is introduced into the chamber
3. Nitrogen gas is added to prevent backfiring
4. Mixture is ignited and heated powder is ejected from the barrel onto the target material
5. Barrel is purged by nitrogen gas ready for firing again
6. This process is repeated at a rate of between 1-10 Hz until desired thickness is reached.

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-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 materials 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 sprayed as coatings using the D-gun. These materials used for the feedstock are powders of metals, alloys and cermets; aswell as their oxides. However, mainly high-tech coatings are used, these include ceramics, and complex composites.

Some examples include:

• Al2O3
• Cu–Al
• Cu–SiC
• Al–Al2O3
• Cu–Al2O3
• Al–SiC
• Al–Ti
• TiMo(CN)–36NiCo
• Fe–A

Applications

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
• Instrument engineering
• Tools industry
  • Tubular drills
  • Skiving knives for rubber and plastic
• Shipbuilding industry
• D-gun plated plug and ring gauges

The main functions of detonation spray coatings are to protect against corrosion (due to low oxygen content), abrasion and adhesion under low load.

Limitations

There a few limitations of detonation spraying, these are:

• Detonation spraying creates a coating that is mostly mechanically bonded as opposed to metallurgically bonded which is much stronger
• Detonation spraying is a line-of-sight process meaning that components generally need to be coated before being put to use and the detonation gun needs to be able to access the surface
• The coatings despite being considerably stong in compression are weak under tension.
• The coatings tend to fatigue under pinpoint loading.
• Detonation guns are quite large and loud
• Detonation spraying has to be performed at a location specifically designed for it as the gun is reasonable large and it is a loud process that produces substantial noise.For this reason it is installed in a sound-proof room (concrete walls 45 cm thick).
• The process involves a considerable amount of mechanisation and automation because the operator can't be in the room whilst the D-gun is in operation.

Safety

Noise

The operation of the detonation gun is a very loud process that could cause damage to operators in close proximity. As a result detonation spraying should be performed within a sound proof room and no one should be present in the room during operation.

Heat

Extremely high temperatures are reached by the D-gun (≈4000 °C) whilst in operation. Flamable and explosive fuels (generally acetylene) are used in detonation spraying to produce the supersonic shockwave that propels the powder coating materials onto their target components. This creates a serious burn and explosion hazard. Again no-one should be present in the room whilst the D-gun is in operation

Dust and Fumes

The D-gun atomises the powder feedstock into extremely small particles (80–95% of particles by total number are of size <100 nm). This means proper extraction facilities are required for inhalation safety purposes. Also isolated of the D-gun is recommended to avoid operators breathing in the dangerous dust and fumes. Many of the componds used are toxic to humans if congested.