Metal pipes are widely used in mechanical, chemical, and energy industries. However, under harsh service conditions, their inner surfaces are prone to wear, corrosion, and oxidation, often resulting in premature failure. Among various inner-surface strengthening techniques, plasma-enhanced chemical vapor deposition (PECVD) demonstrates distinct advantages in forming uniform coatings on the inner walls of elongated or curved metal pipes. Depositing diamond-like carbon (DLC) films on the inner surfaces using PECVD, due to their high hardness, excellent wear resistance, and chemical stability, is considered an ideal method for surface reinforcement. In recent years, most studies focused on the effects of single process parameters on PECVD deposition behavior, while systematic investigations of multiple parameters and their interactions remained limited. Orthogonal experimental design provides an effective approach for analyzing the significance of multiple factors and optimizing the deposition process efficiently. In this study, a multifunctional coating system equipped with a pulsed direct-current bias power supply is employed to deposit DLC films on the inner walls of 304 stainless steel pipes with an inner diameter of 40 mm, wall thickness of 2.5 mm, and length of 420 mm, with methane and argon as precursor gases. The microstructure and morphology of the films are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). Film thickness is measured with a profilometer, while adhesion strength, residual stress, and mechanical properties are evaluated with a Rockwell hardness tester, a residual stress analyzer, and a nano indenter, respectively. Variance analysis is conducted to determine the effects of three major process parameters, methane flow rate, bias voltage, and duty cycle, on six indices: deposition rate, surface roughness, deposition area, adhesion area, hardness, and elastic modulus. A weighted-sum model is applied to perform multi-objective optimization of the deposition process. The results show that duty cycle has the most significant influence on inner-wall temperature, average power, adhesion strength, hardness, elastic modulus, deposition area and thickness uniformity, followed by bias voltage and then methane flow rate. For residual stress, duty cycle has the greatest effect, followed by methane flow rate and bias voltage. Deposition rate is primarily affected by methane flow rate, followed by duty cycle and bias voltage. Surface roughness is most strongly influenced by bias voltage, followed by methane flow rate and duty cycle. Multi-objective optimization identifies the optimal process parameters as a methane flow rate of 10 cm3/min bias voltage of 500 V, and duty cycle of 5%. Under these conditions, the deposited DLC films exhibit a deposition rate of 7.8 nm/min, surface roughness of 0.43 nm, deposition area of 88.18%, adhesion grade of HF3, and a nano-hardness and elastic modulus of 16.256 GPa and 135.668 GPa, respectively. This study systematically reveals the influence of methane flow rate, bias voltage, and duty cycle on the deposition behavior, microstructural characteristics, and mechanical properties of DLC films deposited on the inner walls of pipes via PECVD. By integrating variance analysis with multi-objective optimization, the study provides a rapid and systematic method for identifying critical process parameters that govern film performance. The findings offer theoretical support and practical guidance for the optimization of PECVD processes for inner-surface pipe applications, contributing to the development of high-performance, durable DLC coatings for metal pipes operating under severe service conditions. In summary, this study systematically reveals the effects of PECVD process parameters on the deposition behavior and overall performance of DLC films on the inner walls of pipes. It establishes a process screening method that combines variance analysis with multi-objective optimization, providing an important theoretical basis and technical guidance for the rapid optimization and controllable deposition of DLC coatings on inner surfaces of pipes.
Key words
inner wall of steel pipe /
diamond-like carbon film /
preparation process /
orthogonal experiment /
analysis of variance
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Funding
Guangdong Provincial Science and Technology Program (2023B1212060045); Guangdong Academy of Sciences Special Project (2022GDASZH-2022010201)
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