Solutions We Provide
APT has rich experience, knowledge and resources in thermal spray coatings and can provide comprehensive solutions for specific surface problems or needs.
The APT thermal spray coating system offers coating solutions in terms of wear, thermal effects and protection, corrosion resistance, dimensional control and electrical properties.
APT has rich experience, knowledge and resources to provide thermal spray solutions for various industries, improving efficiency, extending service life and reducing operating costs.
Wear includes various forms of material removal; thermal effects and protection can insulate heat or promote uniform heat conduction; corrosion resistance covers different types of corrosion processes; dimensional control can be used for clearance control, repair and specific surface profile requirements; electrical properties include conduction, insulation, magnetism, etc.
Wear Solutions
Provide coating solutions for various forms of wear, such as adhesion caused by “cold welding”, fretting, two-body abrasion, three-body abrasion, erosion, impact, cavitation, fatigue and stress cracking.
When surfaces slide relative to each other, there is a tendency for one material to transfer onto the counterface. This is adhesive wear, and in the most severe case, it will lead to galling and may cause the machine to seize up due to cold welding.
Solution: Adhesion is usually combated by using dissimilar materials and harder surfaces.
Fretting is the result of damage caused by low-amplitude vibration and/or small oscillations between two surfaces in contact with each other. This kind of damage is often unnoticed for a long time.
Solution : in anti-fretting applications, such as the thermally sprayed copper-nickel-indium coating used for the roots of gas turbine blades.
A harder object can cut or spall a softer contact surface. Abrasion is divided into two main mechanisms. In two-body abrasion, a hard and rough surface scratches, cuts or spalls a softer surface. In three-body abrasion, a hard third body damages one or both of the two sliding surfaces. This is usually caused by grit or dirt getting between the sliding interfaces.
Solution : hard and well-adhered coatings and diffusion layers can effectively reduce this kind of wear. In order to effectively reduce wear, the protected surface must be at least 20% harder than the abrasive.
Three – body abrasion occurs when a hard third body, such as grit, sand, or other hard particles, gets between two sliding or contacting surfaces. These particles act as an abrasive medium and damage one or both of the surfaces. This type of abrasion is different from two – body abrasion, where the damage is mainly caused by the direct interaction of two surfaces with different hardness.
Solution : Using hard and well – adhered coatings can provide a protective barrier. Ceramic coatings, such as titanium nitride (TiN) or chromium nitride (CrN), are often used. These coatings have a high hardness value and can resist the penetration and scratching of the abrasive third – body particles.
“Erosion” is caused by the impact of particles in gases or liquids on the surface of components. The severity of this type of wear depends to a large extent on the velocity, hardness and impact angle of the particles.
Solution : For impacts at large angles, coatings with a certain degree of flexibility or very thick coatings applied by welding or thermal spraying should be chosen. For impacts at small angles, very hard coatings are preferred.
Cavitation wear occurs on surfaces exposed to fluids, where entrained bubbles burst on or near the surface. The bursting releases a jet of fluid that impacts the surface, causing a severe “hammering” effect locally.
Solution : to combat intense cavitation by using tough materials, usually cobalt-based, with a strong work-hardening ability.
“Impact” is defined here as one object suddenly striking another with great force. The repeated action of the impact force usually leads to the weakening and cracking of the base material. For application scenarios where impact is to be endured, hard, thick and particularly tough materials are required. This is best achieved through systems that are metallurgically bonded to the base, such as plasma transferred arc overlays (PTA overlays) or fused thermal spray coatings.
Solution : in anti-fretting applications, such as the thermally sprayed copper-nickel-indium coating used for the roots of gas turbine blades.
Fatigue and stress cracking usually occur due to the constantly changing mechanical stresses that mechanical components endure during their use. When mechanical stresses act on the components repeatedly, they will cause changes in the microstructure inside the material, gradually accumulate damage, and eventually lead to the phenomena of fatigue and stress cracking.
Solution : Usually, it is best to solve this problem through mechanical redesign. Meanwhile, thermal spray coatings, especially high-velocity oxy-fuel (HVOF) coatings, can introduce compressive stresses within the coatings. Such coatings can be used on components that are subjected to high stress cycles to help reduce the occurrence of fatigue and stress cracking.
Thermal Effects and Protection
Used for heat insulation or promoting uniform heat conduction, including thermal conduction and thermal barrier coatings.
Thermal spray coating systems can improve thermal conductivity. Thermally sprayed copper and thermally sprayed aluminum are some examples of heat-conducting surfaces. These coatings can not only promote heat transfer but also evenly distribute heat to achieve higher efficiency.
Thermal barrier coatings are designed to slow down heat transfer and insulate the substrate. The coating systems currently in use usually consist of two parts: a metallic bond coat (usually MCrAlY), which also serves as an antioxidant layer; and a ceramic top coat (usually yttrium-stabilized zirconia). These coating systems have a low thermal conductivity and a high coefficient of thermal expansion. These coating systems can extend the service life of components or enable the components to operate at higher temperatures, thereby improving thermal efficiency.
Corrosion Resistance Performance
Deal with different types of corrosion, such as galvanic corrosion, pitting corrosion, high-temperature corrosion, etc.
Galvanic corrosion is a phenomenon that occurs when different metals are in an electrolyte environment and will attack the metal with more anodic characteristics. Galvanic corrosion can occur not only when components made of two different metals are in such a state, but also on a single component, that is, when the individual grains of an alloy have different potentials. Usually, the electrolyte is a salt solution, an acidic or alkaline solution, or an atmospheric environment.
Pitting corrosion does not occur over large areas but in small cracks or pits. These small cracks or pits are formed due to insufficient oxygen supply or the presence of high concentrations of anions such as chloride ions. These factors will prevent the formation of a passive film on the surface. Pitting corrosion is usually a local corrosion phenomenon caused by a specific chemical environment, in which small areas are severely eroded while most of the surrounding surface remains relatively unaffected.
High-temperature corrosion is a form of non-galvanic corrosion in which metals are exposed to an oxidizing environment (such as sulfur, oxygen or other oxidizing compounds) for a long time under high-temperature conditions. This type of corrosion is of great concern in gas turbine engines and even in automotive internal combustion engines.
Dimensional Control Applications
Meet the requirements of dimensional control, repair and specific surface profiles, including clearance control, salvage and repair, embossing and engraving, etc.
In rotating equipment such as compressors, gas turbines and turbochargers, dimensional changes occur between rotor and stator components due to thermal and mechanical effects during operation. These dimensional changes can affect the seals, opening up clearances between blade tips and housings in gas path systems and between seals and casings in labyrinth seal systems. In these applications, a clearance control system can be installed, which consists of sacrificial elements and cutting components. Special thermal spray coatings (known as abradable coatings) and honeycomb seals form effective sacrificial systems.
It is a well-known practice to use thermal spray coatings to repair components with machining errors or wear. The materials used for these coatings are either metallurgically similar to the substrate material or a material that can provide additional desirable surface characteristics such as wear resistance. These coatings can be applied thickly and are easily machined to the required dimensions and surface finish.
In the plastic film or paper processing industries, to create an attractive irregular matte-appearance surface, the foil or paper is pressed between a textured embossing roll and a pressure roll. Applying thermal spray coatings on new or old embossing rolls can produce surfaces with a uniform and random pattern. In the printing industry, thermal spray coatings are an important part of anilox rollers. A dense and uniform chromium oxide coating is applied on the roller, and then a fine pattern is engraved with a laser. These coated rollers are used for ink transfer during the printing process and have wear resistance and corrosion resistance to today’s water-based ink formulations. For corrugating rolls, wear-resistant thermal spray or thin-film coatings are used to maintain their surface profiles. Rolls with a length exceeding four meters (13 feet) are often coated.
Electrical Performance Coatings
Can achieve electrical properties such as conduction, insulation and magnetism, and can be applied to large areas such as corona rolls or local sensor applications.
Metal coatings are applied to non-conductive or poorly conductive substrates to enhance conductivity. Combinations of conductive and non-conductive coatings can also be applied. Such coatings are used to prevent electrostatic loading, shield electromagnetic radiation, or simply to improve the conductivity in electrical contact areas.
Ceramic materials are excellent electrical insulators when applied to metal substrates. This means that when ceramic materials are coated on metal substrates, they can effectively prevent the passage of electric current. These coatings are stable at high temperatures, indicating that in a relatively high-temperature environment, the ceramic coatings will not undergo significant performance changes or damage. Moreover, they can resist molten metals and mechanical wear, which means that even when in contact with molten metals or subjected to mechanical friction, the ceramic coatings can maintain the integrity of their performance and structure.
Coatings of ferromagnetic materials such as iron, nickel, and cobalt are precisely applied to very small surfaces to generate an induced voltage. This induced voltage can be detected by sensors, thereby determining the precise position of moving parts. For example, in a contaminated environment, magnetic coatings can be used on piston rods. The surface profile of corrugating rolls is maintained by wear-resistant thermal spray or thin-film coatings. Rolls with a length exceeding four meters (13 feet) are usually coated.
