Thermal Spray Material Selection

       Correct selection of coating materials is the key to ensuring coating performance. When selecting the coating material, the working conditions of the workpiece and the performance of the coating should be considered first, as well as the material, batch, economy of the workpiece and the proposed thermal spraying method. According to the function of the coating, the coating can be divided into corrosion-resistant coating, wear-resistant coating, wear-resistant sealing coating, high temperature thermal barrier coating, insulating or conductive coating, dimensional repair coating.

The failure of surface-coated workpieces during use is usually not caused by a single factor, so there is not necessarily a simple relationship between the satisfaction of the working conditions and the performance of the coating. The working conditions should be analyzed in detail, and one or more coating materials should be determined comprehensively considering the coating structure, physical, chemical, mechanical and other properties according to the references or experimental data.

Wear-resistant coatings are one of the main application areas of surface coating technology. Although there is a rough relationship between coating hardness and wear resistance, hardness does not fully represent the wear resistance of surface coatings. Because different wear types have different requirements for material properties, wear is often accompanied by impact, corrosion, fatigue and temperature.

The selection of surface coating materials cannot blindly pursue high-performance or high-priced coating materials, resulting in unnecessary waste. High-priced and low-priced materials cannot even be used as the standard for selecting coating materials. On the premise, it is particularly important to use cheap coating materials as much as possible in mass production. For example, nickel-based alloys can be coated instead of cobalt-based alloys.

 

When selecting surface coating materials, generally follow the steps below:

(1) Analyze the working conditions and parts performance to understand the failure causes and requirements for coating performance;

(2) List available coating materials;

(3) Analyze the compatibility of the selected material with the base material and the applicable thermal spraying method;

(4) Carry out laboratory or field tests when necessary;

(5) Determine the coating material by comprehensively considering the service life, cost and factory conditions;

(6) Determine the surface coating method and formulate the coating process.

 

01. Corrosion-resistant coating

The selection of corrosion resistant coating materials is complex. The working state, working temperature, working environment and various corrosive media of parts increase the requirements for coating materials. Corrosive environments can be aqueous solutions, gases or various chemical media. It may also have a wide temperature range. In the presence of gases, burning gases or unburned fuels may also react with other substances at the appropriate temperature, creating a very complex corrosive environment. There are basically three anticorrosion mechanisms:

(1) Reject corrosive environment. This is mainly due to the fact that the coating does not penetrate or react with corrosive media.

(2) Prevent electrochemical corrosion. That is, the properties of the base metal and coating materials are used to understand their redox potentials, so as to determine whether the coating on the base is a cathode or an anode, and the potential of the electrodes. Galvanic corrosion can occur if the coating and substrate materials do not match. The table below lists the standard electrode potentials of some metal elements at 25°C.

(3) Corrosion Inhibition Using chemical corrosion inhibitor as the sealant of the coating can achieve a certain protective effect. The method is to fill the porous coating with corrosion inhibitors or inorganic sealants. Corrosive environments can be completely excluded by using thinner coatings. In some cases, corrosion-resistant metals can also be used as anti-corrosion coatings, but in most cases oxide ceramics are used as anti-corrosion coatings.

 

Corrosion-resistant coatings mainly include the following:

(1) Atmospheric corrosion resistant coating

This coating is resistant to corrosion from wind, rain, sunlight and other climatic conditions when the metal or alloy is exposed to outdoor or indoor air, but not immersed in liquid. All quality assessments are based on the results of outdoor exposure. Paints used outdoors are also effective indoors.

Such coatings include:

A. Coatings resistant to industrial atmosphere. This coating can withstand harsh environments with soot or chemical fumes, such as in large cities and heavy industrial areas.

B. Coatings resistant to marine atmospheric corrosion. The coating resists corrosion near the ocean or in salt spray environments, where the atmosphere is salty and humid.

C. Coatings that are resistant to rustic atmospheric corrosion. The rural atmosphere, although free from the industrial smog and salt spray of a marine climate, contains some pollutants.

(2) Corrosion resistant coating

Immersion corrosion refers to the exposure of metals or their coatings to liquids, full or partial or alternating immersion. The coating must be resistant to corrosion above or below the liquid level. There are several types of such coatings:

A. Coatings resistant to edible fresh water. The paint is resistant to fresh water and does not alter the chemical composition of the water, making it inedible.

B. Coatings resistant to non-potable fresh water. The paint is resistant to non-potable fresh water, the water temperature does not exceed 52 ℃, and the PH value is 5-10.

C. Heat resistant fresh water coating. The system is not suitable for drinking, with a temperature of 52-204°c and a pH of 5-10.

D. Salt water resistant coating. The coating can be fully or partially immersed in stationary or moving brine or seawater.

Coatings for the chemical and food industries. The coating resists chemical reactions such as oils, fuels, and solvents, and resists corrosion in a variety of food products without changing the food's chemical composition or taste.

(3) Chemical resistant coating

This coating is resistant to corrosion by various acids, bases, salt solutions, vapors and solids. This coating is mainly composed of various iron, nickel-based and diamond-based alloys, self-fluxing alloys, non-ferrous metals, oxide ceramics, chromium carbide, tungsten carbide and other cermets. Due to the porosity of thermal spray coatings, various coatings must be filled and sealed. The sealant itself must also be resistant to corrosion by chemical media.

 

02. Wear-resistant coating

Part wear is often accompanied by an increase in temperature, whether caused by friction or related to the working environment. Generally, wear-resistant coatings are used in environments that are susceptible to impact wear or corrosion. Therefore, as a wear-resistant coating, it must be hard, not easily broken, and have certain heat and chemical resistance properties. Many carbides, nitrides, oxides, nickel-based or cobalt-based alloys have these properties.

Due to the different use environment, operating temperature and wear conditions, the wear situation is more complicated, which can be summarized as follows:

(1) Adhesive wear

This wear-resistant coating is mainly used for hard and soft bearing surfaces.

A. Soft bearing surface. The coating allows abrasive particles carried in the lubricant to become embedded in the coating and deform the baseline of the bearing surface. Soft bearings are less expensive but must be well lubricated or they will wear out too quickly.

B. Hard bearing surface. The coating can be used as a high wear, high hardness bearing material, can withstand abrasive wear, reduce scratches and scratches, and can be used for high load, low speed hard bearing surfaces.

(2) Abrasive wear

A. Wear-resistant coating. The hardness of the coating is greater than the hardness of the abrasive grains. The working temperature of coatings for high temperature is 540-845 ℃, and the working temperature of coatings for low temperature is below 540 ℃.

B. Wear-resistant coating. The coating resists sliding wear on hard or soft surfaces containing hard abrasives. The coating should be smooth to reduce wear and should have an appropriate coefficient of friction. C. The working temperature of high temperature paint is 540-845℃, and the working temperature of low temperature paint is below 540℃. Coatings that are resistant to low temperature hard surface abrasion include coatings that are resistant to abrasion from fibers and textile threads.

(3) Surface fatigue and wear

Coating against directional movement wear. The coating is able to withstand wear from sliding, rolling or impact on the track and is sufficient to withstand continuous impact wear.

B. Wear-resistant coating. The coating resists vibrational friction caused by small displacements of surfaces in contact with each other. Due to the random motion of the contact friction system, the wear of the coating is difficult to predict. Vibration is the most common phenomenon of this type of wear. Coatings are also divided into two categories: high temperature and low temperature.

C. Anti-cavitation coating. The coating can withstand mechanical shock wear caused by pores in liquid flow, and has high toughness, wear resistance and corrosion resistance.

(4) Erosion wear

The coating resists attack by sharp and hard particles. These particles are transported by gas or liquid and move at a certain speed. When the abrasive erosion angle is less than 45°, the abrasive particles fly along the surface of the abrasive particles, resulting in abrasive wear. At this time, the hardness of the coating is the main requirement, and when the particle erosion angle is greater than 45°, the toughness of the coating is the main requirement. There are also two types of this coating, for high and low temperatures.

 

03. Coating for clearance control of mechanical parts

The mechanical efficiency of a machine powered by gas under pressure depends on the sealing ability of the rotor. High sealing ability reduces or prevents gas leakage. Therefore, a very tight fit clearance is required between the rotor and the stator. It is difficult to make machines with tight clearances because rotating parts can stretch or expand under operating conditions and collide with stationary parts, but the use of wear-resistant seals can solve this problem. The method is to spray a layer of wear-resistant sealing layer on the static parts, and rotate the parts to make the coating form a tight dimensional sealing channel.

Typical abrasion resistant coatings are used on jet engine compressors and turbine casings. The thickness of the coating should be sufficient to allow the rotor blades and casing to overlap each other during engine assembly. When the engine is started, the tip of the blade rubs against the coating, removing part of the coating to form a channel, and the blade itself is not damaged. Since the coating adapts to the radial and axial movement of the blades, the tip of each blade is optimally sealed in the coating. In the design of wear-resistant coatings, two fundamentally opposing requirements must be addressed, namely that the coating must not only be wear-resistant, but must also be resistant to airflow erosion and particle erosion. Therefore, it is necessary to compare the wear resistance, heat resistance and chemical resistance of the coatings. The table below shows several commonly used abrasive seal coat materials and their properties.

 

04. High temperature resistant thermal barrier coating

(1) High temperature resistant coating This coating can improve the high temperature working conditions of the base parts, and can withstand chemical damage caused by chemical or physical decomposition or corrosion at high temperature.

(2) Atmospheric oxidation resistant coating. This coating protects the substrate from damage caused by high temperature oxidation. The melting point of the coating is higher than the working temperature and it has a lower vapor pressure at the working temperature. The coating does not need to withstand mechanical wear.

(3) Gas anti-corrosion coating. This coating protects the substrate from exposure to high temperature corrosive gases. It must be considered that the gas reacts with the coating to prevent the formation of adsorbed oxides, or to form brittle constituents or penetrate the coating to attack the substrate. The coating does not need to withstand mechanical shock or abrasion.

(4) High temperature resistant (above 850℃) erosion coating. This coating can withstand both high temperature and particle erosion. High-velocity particles and high-pressure gases create a variety of harsh environments at high temperatures, so coatings must be able to withstand erosion from the movement of sharp and hard particles. When the particle erosion angle is less than 45°, the particles are worn along the surface of the abrasive particles, so the coating is required to have higher hardness; when the particle erosion angle is greater than 45°, the coating is required to have higher toughness.

(5) Thermal barrier coating. This coating has low thermal conductivity, prevents the base material from reaching the melting point, and has the function of transferring radiant heat.

(6) Corrosion-resistant coating of molten metal, which can withstand corrosion of molten metal without wetting of molten metal. Such as refractory zinc, aluminum, steel, iron and copper coatings.

 

05 Electrically insulating and conductive coatings

Thermal coatings can also be used as conductive materials such as printed wires or contacts for furnace heating elements. Thermal spray coatings of oxides and organic plastics can be used as electrical insulators. The electrical properties of the base material are affected by the spray material. The spray material should generally be selected based on the known properties of the material and its state of use.

(1) The conductive paint must have good conductivity and low resistance.

(2) Dielectric Coatings Such coatings must act as insulators to prevent the passage of current. The strength of the breakdown coating (usually expressed as voltage per unit length) and the allowable conductance are parameters that characterize the dielectric strength.

(3) Shield coating 

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