Thermal spray coating defects and their causes
One of the most common defects of thermal spray coatings is insufficient bonding strength between the coating and the substrate, manifesting as localized or large-scale flaking of the coating, which seriously impacts its protective function and service life. The main causes of this defect include improper substrate surface pretreatment, such as the presence of oil, rust, or scale. These impurities hinder direct contact between the coating and the substrate, reducing bonding strength. For example, if the steel substrate is not thoroughly degreased before spraying, residual rolling oil will carbonize at high temperatures, forming an isolation layer, causing the coating’s bonding strength to drop from the normal 50 MPa to below 20 MPa. Insufficient surface roughness is also a major factor. An overly smooth surface cannot form an effective mechanical bond. For example, the roughness Ra value of an aluminum alloy substrate after mechanical grinding must reach 3.2-6.3 μm. If it is only 1.6 μm, the bonding strength will decrease by 40%. Improper spraying process parameters can also lead to poor bonding. For example, in flame spraying, the flame temperature is too low, resulting in insufficient powder melting, or in plasma spraying, the spray gun is too far away, resulting in insufficient kinetic energy of the particles, preventing a strong bond. An investigation into an accident in which the coating on a wind turbine blade bolt peeled off revealed that an oxide film was re-formed on the surface due to the failure to spray paint in time after sandblasting, resulting in insufficient bonding strength and eventual peeling under vibration load.
Excessive porosity is another common defect in thermal spray coatings. This refers to an excessively high volume fraction of pores within the coating (typically required to be less than 5%, and less than 2% for precision coatings). This can reduce the coating’s corrosion resistance, wear resistance, and density. The main causes of porosity include incomplete melting of powder particles, which prevents them from spreading completely to form a continuous coating. For example, in arc spraying, excessive wire feed speed can cause particles to be sprayed onto the substrate surface before they are fully melted, resulting in a loose structure. Improper spraying parameters can also lead to increased porosity. For example, in supersonic flame spraying, insufficient gas flow reduces the flame velocity, resulting in insufficient kinetic energy from particle impact, preventing them from fully deforming and filling the voids. Alternatively, in plasma spraying, excessively large powder particle size (over 50μm) prevents the large particles from melting, forming pores upon cooling. Furthermore, excessively thick coatings can easily trap gas between layers. For example, a nickel-based alloy coating up to 1mm thick can have a porosity of up to 8% if sprayed in a single application, but this can be reduced to 3% using three separate spraying applications. Analysis of the corrosion resistance of a chemical pipeline coating found to be unqualified was due to a porosity of up to 10%. The corrosive medium penetrated into the substrate through the pores, causing corrosion under the coating and bubbling and shedding within 6 months.
Coating cracking is a serious defect, including surface cracks, through-cracks, and interface cracks, which can lead to a decrease in the coating’s water resistance or even complete failure. Surface cracking is often caused by shrinkage stress generated during the coating’s cooling process exceeding the material’s tensile strength. For example, cemented carbide coatings (such as tungsten carbide – cobalt) are highly brittle. If they are not subjected to low-temperature tempering after spraying to relieve stress, they are prone to developing a network of cracks during room temperature storage. Through-cracks often result from a significant difference in the thermal expansion coefficient between the coating and the substrate, generating cyclical stresses during temperature cycling. For example, a zirconium oxide coating ( 8×10⁻⁶/ °C) sprayed onto a steel substrate (with a thermal expansion coefficient of 12×10⁻⁶/ °C) can develop through-cracks after 100 cycles at 100-600 °C . Interface cracking is associated with insufficient bonding strength. When the internal stress of the coating exceeds the interfacial bonding strength, debonding cracks will form at the coating-substrate interface. Analysis of cracking in the coating of an aircraft engine blade revealed that the lack of a transition layer prevented stress release between the zirconia coating and the nickel-based alloy substrate. This resulted in through-cracks during the test run, leading to overheating and damage to the blade. Furthermore, defects such as sharp corners and scratches on the substrate surface can cause stress concentration and serve as crack initiation points.
Uneven coating thickness can lead to localized performance deficiencies, cracking in thick areas, and ineffective protection in thin areas, compromising the overall coating quality. Causes of uneven coating thickness include unstable spray gun movement speed. For example, inconsistent spraying techniques during manual spraying can lead to varying coating deposition amounts on different parts of the workpiece. Alternatively, inconsistent spray gun-to-workpiece distance can lead to thicker coatings at closer locations and thinner coatings at farther distances. Inadequate equipment precision can also contribute to uneven coating thickness. For example, trajectory control errors exceeding ±2mm in automated spray robots can cause coating thickness deviations exceeding 20%. Fluctuations in powder feed rate are also a significant factor. For example, uneven wire feed in arc spraying or intermittent powder feed in flame spraying can lead to unstable coating deposition. During coating thickness testing on a certain automotive wheel hub, it was discovered that due to an incorrect robot spray trajectory programming, the hub had a thickness of 150μm at the edge, but only 50μm in the center (the required 80-120μm). This resulted in cracking at the edges and premature wear in the center.
Inclusions and foreign matter in coatings are hidden defects that affect performance. These impurities can disrupt the coating’s continuity and become vulnerable points to corrosion or wear. Inclusions primarily arise from contamination in the spraying environment, such as dust and metal debris from the workshop air entering the coating, or from impurities such as oxides and sulfides in the spraying material itself. For example, if nickel-based alloy powder contains more than 1% nickel oxide inclusions, the corrosion resistance of the sprayed coating will decrease by over 50%. Foreign matter is often introduced by improper operation, such as sand particles left on the surface after sandblasting and becoming encapsulated in the coating after spraying, or metal particles from worn spray gun nozzles that enter the coating. Inspections of the coating on the cooling water pipes of a nuclear reactor revealed that due to insufficient cleanliness in the spraying environment, micron-sized silicate particles were present in the coating. Under high-pressure water flow, these particles became a source of wear, leading to premature failure of the coating. Furthermore, cross-contamination when spraying different materials, such as failure to thoroughly clean the powder feed lines during powder changes, can also result in inclusions of dissimilar materials, impacting coating performance.
