Thermal spraying process and quality control
The thermal spraying process is a complex, multi-step collaborative process encompassing four core stages: substrate pretreatment, spray material preparation, spray application, and post-processing. The quality of each stage directly impacts the performance of the final coating. Substrate pretreatment is fundamental to the process, aiming to remove surface oil, rust, and scale, increase surface roughness, and provide a good foundation for coating adhesion. It involves three steps: degreasing, rust removal, and roughening. Degreasing typically involves using alkaline detergents or organic solvents. Surface grease is removed by immersion, spraying, or ultrasonic cleaning to ensure a continuous, unbroken water film on the surface after degreasing. Rust removal can be achieved by pickling or sandblasting to thoroughly remove scale and rust products. Roughening typically involves sandblasting to achieve a surface roughness Ra of 3.2-6.3μm, creating a uniform concave-convex structure. Spray material preparation varies depending on the process type. Powder materials require drying (120°C for 2 hours) to remove moisture and screening to remove large particles. Wire material requires straightening and descaling to ensure smooth feeding. Spraying is a key step, requiring appropriate process parameters based on the material type. For example, flame spraying requires controlling the oxygen-acetylene ratio and flame power, while plasma spraying requires adjusting current, voltage, and gas flow to ensure adequate powder melting and acceleration. Post-processing includes sealing, heat treatment, and finishing. Sealing agents (such as epoxy resin) fill the coating’s pores and improve its density; heat treatment eliminates coating stress; and finishing involves grinding to achieve the required dimensional accuracy and surface finish.
Quality control of the thermal spraying process must begin at the source, establishing a raw material quality control system to ensure that the performance of the sprayed materials meets design requirements. Raw material quality control includes testing for chemical composition, particle size distribution, morphology, and purity. The chemical composition of metal powders must be verified by spectral analysis. For example, the chromium content of nickel-based alloy powders should be within ±0.5%. Powder particle size should be measured using a laser particle size analyzer to ensure that the distribution is within the process requirements (e.g., 10-50μm for plasma spray powders). Morphology, observed through scanning electron microscopy, must be spherical or quasi-spherical with good flowability (apparent density ≥2.5g/cm³). Purity must exceed 99.5%, and impurity content (such as oxygen and sulfur) must be controlled below 0.1%. Wire materials must be tested for diameter deviation (±0.05mm), straightness, and surface quality to ensure the absence of defects such as cracks and folds. Each batch of raw materials is sampled and tested, and unqualified materials are strictly prohibited from being put into use. The failure analysis of a certain aircraft engine coating showed that the use of titanium alloy powder with excessive oxygen content (oxygen content 0.3%, standard ≤0.15%) caused the coating to become more brittle and eventually crack under vibration load.
Controlling process parameters during the spraying process is crucial for quality control. Optimal parameter ranges must be determined through process testing, and rigorous monitoring and adjustment are required during production. Key parameters vary across different spraying processes. For flame spraying, flame temperature (2000-3000°C), spray distance (100-200 mm), and powder feed rate (5-20 g/min) must be controlled; for arc spraying, current (200-400 A), voltage (20-40 V), and spray speed (100-300 mm/s) must be controlled; for plasma spraying, power (20-60 kW), plasma gas flow (30-60 L/min), and powder feed rate (10-30 g/min) must be controlled. Parameter fluctuations must be kept within acceptable limits. For example, plasma spray current fluctuations must not exceed ±5 A, otherwise uneven powder melting will occur. Automated control systems monitor parameters in real time, such as using infrared thermometers to monitor substrate temperature to ensure it does not exceed 200°C (to prevent overheating). Laser thickness gauges measure coating thickness in real time, allowing for timely adjustment of spray speed. An automotive parts factory used a PLC system to control arc spraying parameters, reducing coating thickness deviation from ±15μm to ±5μm, significantly improving coating consistency.
Coating performance testing is a crucial tool for verifying process quality. A comprehensive performance evaluation based on application requirements is required to ensure that the coating meets design specifications. Mechanical performance testing includes bond strength (measured by tensile testing according to GB/T 8642), hardness (measured by a Vickers hardness tester according to GB/T 4340), and wear resistance (measured by volume loss through abrasion testing according to GB/T 12444). Protective performance testing includes salt spray resistance (neutral salt spray testing according to GB/T 10125), corrosion resistance (immersion testing to measure corrosion rate), and high-temperature resistance (high-temperature oxidation testing to measure weight gain). Structural performance testing includes porosity (metallographic or image analysis methods, required to be ≤5%), thickness (eddy current thickness gauge or metallographic method, deviation ≤10%), and surface roughness (Ra value, as per design requirements). 100% coating testing is performed on critical components, while sampling testing is performed on general components (sampling ratio ≥10%). Products that fail the test must be reworked or scrapped. During an inspection of the steel structure coating on an offshore platform, it was discovered that 10% of the samples showed signs of rust after 480 hours of salt spray testing. Tracing back, it was discovered that this was caused by insufficient sandblasting roughness. The coating quality was ensured through rework.
The quality control system for thermal spraying processes must integrate process control and continuous improvement, continuously improving process stability through standardized operations and data analysis. Detailed work instructions should be developed to specify the operating procedures, parameter ranges, and testing requirements for each process. Operators must undergo training and pass assessments before taking up their posts. A quality traceability system should be established to record raw material batches, process parameters, test results, and operator information, ensuring full traceability from raw materials to finished product. Regular process audits should be conducted to analyze the causes of nonconforming products and implement corrective and preventive measures. For example, to address fluctuations in coating bond strength, sandblasting parameters can be optimized to maintain a stable roughness within the required range. Statistical process control (SPC) methods should be introduced to statistically analyze key parameters (such as spray current and coating thickness) and calculate the process capability index (CPK ≥ 1.33). Adjustments should be made promptly when abnormal fluctuations occur. By implementing SPC, one spray coating plant increased its coating qualification rate from 90% to 98%, significantly reducing production costs. Only by establishing a comprehensive quality control system can a stable and reliable thermal spray process be ensured , producing high-quality coatings that meet application requirements.
