Thermal spraying is a surface enhancement technology with a wide range of industrial applications. It can form wear-resistant, corrosion-resistant, and high-temperature-resistant coatings on the surfaces of metals, ceramics, or composite materials. As a result, the performance of these devices and components is enhanced and their service life can be extended.
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Thermal spray powders are crucial materials used in thermal spraying technology. The advantages of using spray powders include the fact that they are not limited by wire forming processes, have low production costs, and are widely available. Additionally, the components can be mixed in any proportion. Choosing the right thermal spray powder has a very obvious impact on the process effect.
Different types of powders have different characteristics and applicable fields. Therefore, when choosing a specific spray powder, you must first determine which type of powder can meet the application requirements. The following are the main types of thermal spray powders and their respective applicable industries.
' Nickel-based alloys: Inconel 625 and Inconel 718, suitable for high-temperature and corrosive environments, commonly used in turbine and aerospace engine components.
' Cobalt-based alloys: Such as the Stellite series, known for excellent wear and heat resistance, often used in valves, pumps, and cutting tools.
' Aluminum-based alloys: Lightweight and oxidation-resistant, commonly used for thermal barrier coatings and corrosion protection.
' Molybdenum and molybdenum-based alloys: Used for high-temperature and wear-resistant coatings, such as in automotive parts and industrial equipment.
' Alumina (Al2O3): Provides electrical insulation and wear resistance and is widely used in electronic components and mechanical parts.
' Zirconia (ZrO2): Known for excellent thermal shock resistance, often used for thermal barrier coatings, such as in gas turbines and aerospace engine components.
' Tungsten carbide (WC): Extremely hard and wear-resistant, widely used in cutting tools and wear-resistant parts.
' Tungsten carbide-cobalt (WC-Co): Combines the toughness of metals with the hardness of ceramics, used for wear-resistant coatings, such as in oil drilling equipment and mining machinery.
' Chromium carbide-nickel chromium (Cr3C2-NiCr): Used for high-temperature wear and corrosion-resistant coatings, such as in turbine blades and chemical equipment.
' Polyethylene (PE) and Polytetrafluoroethylene (PTFE): Used for corrosion-resistant and sliding coatings, such as in chemical containers and bearings.
' Composite powders: Combining metals and ceramics, such as nickel-aluminum/alumina composites, used for wear and oxidation-resistant coatings.
' Self-lubricating materials: Such as molybdenum disulfide (MoS2) or graphite, are used for friction-reducing and lubricating coatings, such as in bearings and sliding parts.
' Thermal barrier coating materials: Such as yttria-stabilized zirconia (YSZ), used in gas turbines and aerospace engines for thermal barrier coatings.
Physical properties of Powders are a key factor in determining thermal spraying process parameters. Therefore, when selecting thermal spray powders, it is important to carefully study the physical properties of the powders. Powders with the same composition can have significantly different effects on the thermal spraying process because of differences in physical properties. The main powder properties affecting the thermal spraying process include shape, particle size, particle size distribution, specific surface area, adsorption, and flowability.
Thermal spray powders must have good flowability to be continuously, uniformly, and smoothly fed into the spray flame. Spherical powders undoubtedly have the best flowability. On the contrary, irregularly shaped powders have reduced flowability because of internal friction and "bridging" phenomena between particles.
Spherical powders have the smallest specific surface area and receive heat evenly, resulting in less oxidation and contamination by other impurities during the high-temperature spray process. Therefore, the shape of thermal spray powders is preferably spherical or spheroidal (nearly spherical).
Fig 1. Particle morphologies of alloy powder (a) spherical and (b) irregular in shape1
Particle size refers to the dimensions of the powder particles. The size of the particles directly affects powder delivery, heating conditions, and the density of the coating during thermal spraying.
The choice of particle size is closely related to the inherent properties of the material, such as melting point, thermal conductivity, specific heat capacity, and density, as well as the temperature of the heat source used in thermal spraying and the flight speed and dwell time of the particles in the spray flame. Generally, materials with high melting points, high densities, large specific heat capacities, and poor thermal conductivity should use finer powders for the same heat source, and vice versa.
For different heat sources, such as plasma spraying and flame spraying, the former's higher flame temperature and longer jet length allow for coarser powders compared to flame spraying. Similarly, water-stabilized plasma flames have higher temperatures and longer jets compared to gas-stabilized plasma flames, favoring the spraying of coarser powders.
Fig 2. Fine powder vs coarse powder mass flow rate results2
Particle size distribution refers to the ratio of different particle sizes in the powder. This parameter statistically represents the ratio distribution of particle sizes. A wider particle size distribution indicates a large difference in size between large and small particles. Using powders with a wide particle size distribution in the spray flame can result in finer particles melting first, while larger particles remain unmelted or incompletely melted, affecting the coating quality.
To achieve uniform and dense coatings, narrow particle size distribution powders should be selected. However, this increases the cost of powder materials. Additionally, coatings formed by spraying powders with uniform or similar particle sizes may have porosity between layers, reducing the coating density.
An ideal particle size distribution should meet the process requirements and coating performance needs. Incorporating a certain amount of finer powder with appropriately coarse particles can produce coatings where fine particles fill the gaps between coarse particles, resulting in a relatively dense coating.
Fig 3. Powders with different particle size distributions3
Flowability refers to the ability of the powder to flow and is a crucial indicator of the suitability of the powder for thermal spraying. It is typically measured using a Hall flowmeter, with units of seconds per 50 grams.
The physical significance is the time required for 50 grams of a specific powder to flow through a standard funnel with a defined aperture. The lower the flow time, the better the powder's flowability. Good flowability ensures uniform powder delivery, while poor flowability can cause clogging in the delivery channel.
Generally, alloy powders have better flowability, ranging from 20 to 45 seconds per 50 grams. Ceramic powders typically have poorer flowability, ranging from 50 to 120 seconds per 50 grams. Thermal spray powders should have good flowability to ensure smooth flow and uniform powder delivery in the delivery channel.
Spherical thermal spray powders, a new type of spray powder, not only have excellent flowability but also provide superior coating performance. However, poor sphericity, surface oxidation, and moisture absorption can reduce the flowability of the powder.
Stanford Advanced Materials (SAM) is a premium supplier of metal powders, providing high-quality spherical powders suitable for thermal spraying at very competitive prices. Powders including spherical Inconel 625 and spherical Inconel 718, Cobalt-based alloys, Aluminum-based alloys, and Molybdenum-based alloys are also available.
The selection of appropriate thermal spray powders, considering their shape, particle size, particle size distribution, and flowability, is critical to achieving high-quality coatings. Understanding these properties and their impact on the thermal spraying process enables industries to enhance the performance and lifespan of their components, ensuring reliability and efficiency in various applications.
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1. Gülsoy, H & Gülsoy, Nagihan & Calışıcı, Rahmi. (). Particle morphology influence on mechanical and biocompatibility properties of injection molded Ti alloy powder. Bio-medical materials and engineering. 24. -73. 10./BME-.
2. Kroeger, Jens & Poirié, Thomas & Moghimian, Pouya & Marion, Frédéric & Larouche, Frederic. (). Flow rate ranges for spherical metallic powders for additive manufacturing. Progress in Additive Manufacturing. 7. 10./s-021--7.
3. Masaya Higashi, Naoya Kanno, Effect of initial powder particle size on the hot workability of powder metallurgy Ni-based superalloys, Materials & Design, Volume 194, , , ISSN -, https://doi.org/10./j.matdes...
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The meaning and characteristics of thermal spraying
Thermal spraying technology refers to a series of processes, in these processes, fine and dispersed metal or non-metallic coating materials, in a melting or semi-melting state, deposited on the surface of the prepared matrix, forming a spray coating layer. Is the use of a certain heat source (such as arc, plasma spraying, etc.) to powder or filament-shaped metal or nonmetallic materials heated to molten or semi-molten state, and then with the help of the flame itself or compressed air at a certain speed to spray to the surface of the pretreated matrix, deposition and formation of a surface coating with various functions of a technology.
The characteristics of thermal spraying :
(1)The substrate is not limited, can be sprayed on a variety of substrates.
(2) Spraying coating materials are very wide, can be used to spray almost all solid engineering materials.
(3) The temperature rise of the substrate material is small in the spraying process, and no stress and deformation are generated.
(4) Flexible and convenient operation process, not limited by the shape of the workpiece, convenient construction.
(5) coating thickness can be from 0.01 to a few millimeters.
(6) coating performance is diverse, can form wear resistance, corrosion resistance, heat insulation, oxidation resistance, insulation, electrical conductivity, radiation and other coating with a variety of special functions.
(7) Strong adaptability and good economic benefits.
Thermal spraying methods include detonation spraying. Wire arc spraying. Flame spraying. High velocity oxy-fuel coating spraying (HVOF).
Due to the advantages of convenient operation, strong adaptability and good economic effect, thermal spraying is widely used in chemical industry (all kinds of containers, reactors, pipelines, pumps, valves and sealing parts), transportation (such as gear shaft, mud pump water seal neck, mud door, mud impeller, sand scraper, etc.). Electric power industry (ball mill, turbine rotor journal and generator rotor journal, cylinder joint surface), metallurgy (blast furnace tuyere, guide roller and delivery roller), etc.
Thermal spraying process:
Thermal spraying workpiece: workpiece surface pretreatment - workpiece preheating - spraying - coating post treatment
Workpiece surface pretreatment: in order to make the coating and the substrate material is well combined, the substrate surface must be clean and rough, there are many ways to purify and coarsely surface, the selection of methods should be based on the suitable requirements of the coating and the substrate material, shape, thickness, surface original condition and construction conditions and other factors.
The purpose of purification treatment is to remove all the dirt on the surface of the workpiece, such as oxide skin, oil stains, etc. The purpose of coarsening treatment is to increase the contact surface between the coating and the matrix material and increase the mechanical biting force between the coating and the matrix material. For some coating materials which are not well bonded with the substrate, a transition layer of coating materials which are bonded with the substrate material should be sprayed, called the bottom layer of the bond, and the thickness is generally 0.8-0.18μm.
Preheating: The purpose is to eliminate moisture and moisture on the surface of the workpiece, improve the interface temperature when spraying particles contact with the workpiece, and improve the binding strength of the coating and the collective.
Spraying
Which spraying method is used for spraying mainly depends on the selected spraying material, the working condition of the workpiece and the requirements for coating quality. The pretreated workpiece should be sprayed in the shortest possible time, and the spraying parameters should be determined according to the coating material, the performance of the spray gun and the specific situation of the workpiece. The optimized spraying conditions can improve the spraying efficiency, and obtain the high-quality coating with high density and high binding strength.
After coating processing: thermal spraying coating sometimes cannot be used directly, mustturningry out a series of post-processing, such as for under impact load or high stress abrasion artifacts, in order to improve the bonding strength of coating, to remelting the coating layer, make the combination of porous and substrate with only mechanical coating into and the density of the metallurgical bonding coating to the substrate.
There are dimensional accuracy requirements, to the coating for mechanical processing. Because the spray coating has different characteristics with the general metal and ceramic materials. The strength of the coating itself is low, can not withstand a large cutting force; In the coating.
Mechanical processing of workpiece after thermal spraying
The above mentioned dimension accuracy requirements of the workpiece, the need for mechanical processing, to ensure the size and accuracy requirements. However, the workpiece hardness after thermal spraying is generally above HRC50, and part of the thermal spraying coating is above HRC65, and there are hard points in the coating, which requires the tool material not only to have high hardness, but also to have good wear resistance and impact resistance.
According to the coating thickness and processing allowance of thermal spraying materials and the substrate material to choose the turning or grinding processing; Due to the low grinding efficiency, the thermal spraying workpiece with small machining allowance and spray thickness is generally selected. On the other hand, when the allowance is large, it is a more economical choice to replace the grinding with turning.
Thermal spraying are widely used in metal surface strengthening, workpiece after thermal spraying, the surface hardness can reach 50 to 55 HRC, general high up to 70 HRC, and most of the coating on the surface of the demand is higher, the surface after spray to cold working, some large machining allowance or restricted to large cylindrical grinding machine equipment, often used in car generation grinding machining method.
1. The type of thermal spraying materials
(1) Types of thermal spraying materials:
Thermal spraying commonly used spraying powder including nickel base, iron base and cobalt base alloy powder, according to different coating hardness, respectively applied to repair and protection of mechanical parts; Spray thickness can be from 0.01 to a few millimeters; The hardness is generally above 50HRC.
General machining allowance and spraying thickness can be used for grinding; On the contrary, instead of grinding car is an economic choice.
2. Cutting tool selection for turning thermal sprayed workpieces
(1) When the hardness of the thermal spray coating is less than HRC45, the carbide tool material can be selected. The commonly used grades are YS30, YG3X, etc., and the line speed can be selected as νc=40 m/min.
(2) When the hardness of the thermal spray coating is greater than HRC45, ceramic or CBN cutters can be used; ceramic cutters and welded composite CBN cutters are often used in finishing due to their poor impact resistance (the cutting depth is preferably within 0.3mm).
(3) When the hardness of the thermal spray coating is greater than HRC55, CBN tools can have better tool life and processing efficiency.
3. Cutting parameters of the tool
(1) When using carbide cutting tools, the cutting depth is usually aO=0.15~0.6mm. The higher the hardness of the surface layer, the smaller the aP should be. aP=0.05'0.1mm during fine turning. The feed rate is generally taken as f=0.1'1mm/r, and f should be smaller during fine turning. The first speed is generally selected according to the hardness of νc=40 m/min; when the hardness of thermal spraying is HRC45-65,choose the carbide tool and the linear speed is better at νc=10 m/min.
(2) When using ceramic cutting tools, the cutting depth is generally controlled within 0.2mm, and the feed rate is about 0.1mm/r; however, the linear speed can be increased to about 3 times that of the original cemented carbide.
--EDITOR:Doris Hu/Anna Wang
---POST: Doris Hu
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