Author: Bill Lenling, TST Coatings
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Thermal spray coatings are engineered for various industrial components to enhance the performance of the parts-coated surfaces. Properties such as wear resistance, corrosion resistance, and electrical insulation can be engineered into a component&#;s design with the addition of a coating. In order to determine how to optimize coatings properties, the microstructure of the coating must be understood. This article is an introduction to understanding thermal spray coatings&#; microstructures.
What is a Thermal Spray Coating?
A definition of a thermal spray is the following: &#;Thermal Spray is generic term for a group of coating processes used to apply metallic or non-metallic coatings. These processes are grouped into three major categories: plasma-arc spray, flame spray, and electric-arc spray. These energy sources are used to heat the coating material (in powder, wire, or rod form) to a molten or semi-molten state. The resultant heated particles are accelerated and propelled toward a prepared surface by either process gases or atomization jets. Upon impact, a bond forms with the surface with subsequent particles causing thickness buildup.&#;
Figure 1 above is a schematic of how a thermal spray process works. As described in the definition, the heat source for melting the powder, wire, or rod feedstock can be generated by electrical energy, plasma-arc, or electric-arc. The heat source can also be through the combustion of liquid or gaseous fuel (flame spray and it subcategories such as HVOF, high velocity oxy-fuel use this process). Figure 2 below shows a part being coated with the HVOF thermal spray process.
Figure 3 below shows a stainless-steel pump housing and rotor that have been coated with a carbide composite coating. The pump is used for pumping food or pharmaceuticals and is made out of an austenitic stainless steel. The addition of a carbide coating to the parts&#; design greatly improves its abrasive wear resistance. The coating increases the hardness of the surface from 30 HRC to over 70 HRC after the coating is applied.
Coating Microstructures
Understanding a coating microstructure is critical in the engineering of a successful coating. Understanding coating properties such as porosity content, hardness, adhesion, and oxide levels can greatly influence the coating performance.
Figure 4 below is a cross-section of a thermal spray metal alloy coating. Those skilled in the art of thermal spray may consider this microstructure of lower quality, but for demonstration purposes, the coating structure shows many of the features that can be found in a thermal spray coating. The coating was deposited using plasma-arc spray. The feedstock material was spherical metal powder.
Surface Preparation
Before a coating is deposited, the surface of the part being coated needs to be prepared to allow the coating to bond to the surface. This is typically done by roughening the surface by grit blasting. Figure 3 shows a jagged interface between the substrate and coating, which was created by grit basting. This surface morphology allowed the coating to anchor itself by the molten particles conforming and solidifying around the rough substrate asperities. Sometimes, however, grit from the grit blast process can get imbedded into the coating by not being cleaned from the surface prior to depositing the coating. An embedded grit feature can be seen in center of the coating/substrate interface. Normally a small degree of embedded grit does not affect the coating&#;s performance, but a larger degree of grit can degrade coating adherence to the part.
Lamellar Structure
When a thermal spray coating is produced, the hot molten or semi-molten particles impinge and solidify on the surface forming a layered structure. Upon impacting the surface the particles flatten and solidify in milliseconds, and the coating buildup is known as a lamellar structure. This structure can be seen in Figure 4. An analogy to the lamellar structure is pancakes randomly stacked upon each other.
Oxidation
Most thermal spray coatings are produced in normal, atmospheric conditions. Since oxygen is typically present when spraying metal materials, the molten particles will react with the oxygen and form an oxide layer around the surface of the particle. The oxide layers can be seen between individual powder splats in the coating structure of Figure 3. Controlling oxide content can be a critical attribute in engineering a coating. When a coating is needed to provide corrosion protection, minimizing oxide content is important because corrosive attack of a coating can be accelerated along metal-oxide boundaries.
Un-melted and Semi-melted Particles
When a powder particle is not melted completely in the spray process, it can still stick to the surface, maintain its shape, and get emended into the coating by subsequent buildup of additional molten particles. These features are known as unmelts and can be seen in the Figure 3 microstructure. Unmelts are usually not desired in a coating structure because they are poorly adhered and can cause the formation of porosity. Poorly adhered unmelts can fall out of a coating structure as the coating is attacked by a wear mechanism. Eliminating unmelts is accomplished by proper sizing of the feedstock powder and optimized process parameters for the thermal spray device.
Porosity
Coating porosity is an important attribute that also needs to be controlled when engineering a coating. The porosity in Figure 3 shows up as the dark area in the microstructure. For most coatings, it is desirable to minimize porosity content to maximize coating&#;s wear and/or corrosion resistance. In some cases, some coating porosity can be beneficial, such as thermal barrier coatings for high temperature protection. In this case, the porosity can provide increased thermal insulation enhancing the performance of the coating.
Composite Coatings
Composite coatings are a commonly produced type of thermal spray coating. An example and cross section of one can be seen in Figure 5 below. This coating is a highly wear resistant coating that is comprised of a ceramic phase, tungsten carbides, and a metallic phase, cobalt. The tungsten carbide is extremely hard and wear resistant. The cobalt wets the carbide and cements the carbide grains together combining to produce a coating with excellent wear resistance and good ductility.
Homogenous Coatings
Another coating that is used on pump components, such as mechanical face seals and sealing surfaces on shafts, is chrome oxide (figure 6 below). Unlike a composite coating, chrome oxide is a single phase, homogenous coating containing chrome oxide only. It is very hard, but also brittle. The brittle nature of the coating does not allow it to work well if a component sees impact. It can be very effective for combating two-body abrasive wear if the coating structure is low in porosity and high in hardness.
Thermal spray coatings contain unique microstructures. A detailed understanding of the microstructures is a critical attribute in the engineering of a coating that will perform optimally. Microstructure examination helps provide an understanding on how the coating material and the coating process and process parameters together create the structure. It is an important tool in engineering optimal properties desired to enhance a components performance.
About TST Engineered Coatings
TST Engineered Coatings designs coatings for enhancing the properties of many different industrial components including pump components. When working with TST, our materials engineers will collaborate with the customer to develop a thorough understanding of the environment the coating will be used, and what properties the coating is engineered to achieve. This assures the optimal coating is developed for the specific component and application. Contact TST Coatings if you have any questions about coatings for your specific application.
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Researchers&#; interest in thermal spraying varies from the very practical to the uncommon and theoretical studies. In between this lies the July issue of Materials Research Bulletin titled &#;Thermal Spray Processing of Materials.&#; The seven articles in this issue include everything from the fundamentals of tools and process selection, through to modeling and dynamics of deposit formation or fabrication of fuel cells, spraying of polymers, or barrier coatings for gas turbine engines.
This article provides a summary of most of the above-mentioned areas.
In the thermal spraying process, particles of 1 to 50 µm are partially melted and accelerated to high velocities by an arc or a flame. The particles get splattered onto a surface, forming a layer whose quality is evaluated by the porosity, oxide content, and bond strength to the substrate. The material sprayed can be ceramic, metal, or polymer. Table 1 illustrates the temperature, velocity, and standard features for five types of sprays.
Table 1. Characteristics of some thermal spray processes. Source: Institute of Materials Engineering Australasia.
Type Temperature C Velocity m/s Features Combustion 40 - 100 Porosity and oxidation High-Velocity Oxy-Fuel 400 - 800 Dense, good adhesion, compressive stress Two-wire arc - 50 - 150 Dense, thick Plasma - 25,000 80 - 300 Ceramics porous Cold spray Ambient 400 - 800 Dense, compressive stresses
The supersonic velocity of the HVOF spray delivers a thin, dense, and well-bonded splat. Variants of the gun are being created for a number of applications. Other conventional approaches involve melting two wires in an arc or feed powder into a high-speed plasma gas stream and, for high-performance applications, using inert or low-pressure gases.
Current advancements include radio-frequency induction plasma with its high temperatures or the opposite process with a high velocity, low temperature, localized spray, which offers pure, dense material with high compressive stresses.
Sprayed layers are anisotropic and resemble a brick wall comprising interlocked splats. The key defects are microcracks or pores in brittle materials, while porosity and oxides characterize metal sprayed layers.
Flaws arise from metal oxidation, unmelted particles, low-velocity impact, isolated large particles, and cold-induced fragmentation of splats. The evaluation for quality needs a range of methods, perhaps including thermal conductivity, porosimetry, or even neutron diffraction in research work.
Automotive uses are a key area of application, including oxygen sensors and piston rings. Although it is now in the research stage, the high-pressure shock (up to 30 GPa) generated while splattering a particle enables developing high-pressure phases, which are stable in the nanocrystalline form at room temperatures, for example, 5 to 10 nm diamond phases developed from a sprayed nickel-graphite powder.
Other applications being developed include the formation of multi-layers for sensors, spray synthesis, and engineering metal layers with resistivity two or more times the bulk resistivity.
Modeling has revealed the complexity of the gas-particle interactions and, for instance, the distribution of non-molten particles in a spray. It also demonstrates how hard it is to ensure that a new spray process stays in the permissible narrow window, and shows why it takes time to formulate new processes.
HVOF modeling is challenging due to the supersonic properties of the gas stream and the work needed to characterize the combustion process. But particle vaporization is not typically as much of a concern as is plasma because the gas temperatures are lower, particles are supplied faster, and the thermal conductivity of the particles is higher in HVOF.
Measurements of in-flight particle size, temperature, and velocity performed to verify model calculations involve using methods such as laser Doppler velocimetry, radiance comparisons, and considerable computation effort. The approaches must include the difference in temperature of several hundred degrees and velocity differences of over 100 m/second.
During the dynamic formation of the coating, tens of microseconds are taken up for splattering, a few milliseconds for layer formation, and a few seconds for the subsequent gun pass. Although the end result can be seen by sectioning a coating, it is somewhat difficult to evaluate the dynamic processes behind the micron size particle splattering itself onto a cold surface.
Firstly, models have to deal with either the splat or the layer piling process, generally with simplifications. It is really challenging to determine mechanical properties such as surface roughness, particle size, direction, shape, and composition at higher temperatures.
The research helps gain insights into residual stresses, which is a major determinant of the spallation, fatigue, and thermal cycling resistance performance of thermal spray coatings.
Residual stresses originate from substrate blasting or quenching of the splat on a cool substrate. Although it is lower than 50 MPa for ceramics due to microcracking, sturdy alloys could have stresses up to 300 MPa.
The discrepancy in thermal expansion coefficients between surface and spray materials also leads to stress, which deteriorates with a higher temperature difference between spray and ambient temperatures, and when temperature gradients exist in the sprayed part.
The deposition of thermal barrier (ceramic) coatings on gas turbine engine components and the spray fabrication of fuel cell components have been discussed in various studies. These ceramic coating properties are governed by the complex interaction of ceramic sintering and creep, elastic modulus kinetics and thermal conductivity, bond coat oxidation and thermal fatigue.
Reports have indicated that the practice of polymer spraying has increased. The materials coated include PE copolymers, ketones, polyethylene, polyester, fluoropolymers, nylon, and liquid crystal polymers. Plasma, combustion, and HVOF processes have all been used, and the operating conditions must be chosen to maintain the window between polymer degradation and poor coalescence of particles.
The coating temperature must be selected to enhance the essential properties as maximum density occurs at a lower temperature than maximum hardness and toughness.
A majority of the polymer applications involve corrosive environments, thus making it crucial to ensure full coalescence and to avoid porosity and splat interfaces. Coating applications include magnetic or wear-resisting polymer composite materials.
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