Effect of Working Pressure on the Performance of HiPIMS Plasma Transport and Cr Coating in the Tube

Owing to the continuously increasing performance requirements, tube components, particularly the inner surface, encounter significant challenges in terms of achieving better anti-corrosion properties, wear resistance, and ablative resistance. Cr film, as a commonly used anticorrosive and wear-resistant coating material, is widely used to protect the inner walls of tubes. The main issue in preparing Cr films via magnetron sputtering in tubes is the difficulty in maintaining the plasma density during long-distance plasma transmission, which limits the effective deposition range of coatings in the tubes. The core problem is the rapid attenuation of plasma density caused by recombination during plasma transport. Low-pressure and high-power impulse magnetron sputtering (HiPIMS) technology can effectively reduce particle-energy loss caused by collisions while generating high-density plasma, thus enabling long-distance plasma transport. A self-designed high-power pulsed magnetron sputtering system under high-vacuum conditions is demonstrated to achieve a stable discharge under low pressure. A #20 steel tube with an inner diameter of 40 mm, an outer diameter of 60 mm, and a length of 120 mm is used as the substrate, and the plasma and coating characteristics at different positions on the steel tube are examined by quartering the steel tube. The effects of the operating pressure on the transport characteristics of the planar Cr target discharge plasma and the deposition of the Cr film in the tube are investigated. The target current, base current, and plasma characteristics at both ends of the tube are investigated using an oscilloscope. The plasma luminescence spectra are obtained under four pressure levels, i.e., 0.1, 0.2, 0.5, and 1.2 Pa. The thickness of the deposited film is characterized by observing the wafer cross-section via scanning electron microscopy, and the phase structure of the coating is analyzed via X-ray diffraction. The nanohardness and wear resistance are evaluated using a nano-indentation instrument and a friction and wear testing machine, respectively. The discharge results show that, under a fixed discharge power and pulse voltage, reducing the operating pressure can weaken the discharge effect of the gas, whereas the discharge intensity of the metal exerts minimal effect. However, owing to the decrease in collision probability under low pressure, the peak current of the tail matrix increases, and the numbers of Cr ions and excited Cr atoms in the tail of the tube increase significantly under low pressure. The long-distance transport capacity of the deposited particles increases significantly under low pressure. The results of film deposition show that the coating thickness decays rapidly with the increase in the target base distance, and that the variation laws of the coating thickness and substrate current are the same. As the operating pressure decreases from 1.2 to 0.1 Pa, the film thickness non-uniformity coefficient of the four groups of coatings decreases from 310% to 205%, and the film uniformity increases significantly. This indicates that low pressure is more favorable for achieving a relatively uniform deposition of the coating in the tube. The Cr coating in the tube primarily presents a (110) preferred orientation, and the grain size at position 1 decreases from 35.77 to 27.1 nm, whereas the grain size decreases from 27.1 nm at position 1 to 16.47 nm at position 3 when the target base distance increases by 0.1 Pa. The surface roughness of the coating decreases significantly at low pressure and is only 2.29 nm at position 1 under 0.1 Pa. At low pressure, the coating exhibits better mechanical properties and its nanohardness increases from 6.37 GPa at 1.2 Pa to 8.36 GPa at 0.1 Pa. The results of friction and wear tests show that the main failure mode of the Cr coating is abrasive wear; however, as the operating pressure increases, the failure mode changes to abrasive wear and adhesive wear, thus exacerbating the coating damage. At 0.1 Pa, the wear width and friction coefficient are the lowest, thus corresponding to the best wear resistance. Low-pressure HiPIMS technology demonstrates significant advantages in long-distance plasma transportation and is expected to further expand the application of magnetron sputtering technology in the preparation of tube inner-wall coatings.