Plasma Nitriding Treatment By DC Pulsed Hollow Cathode Discharge

Nitriding is an important chemical heat treatment method. Chemical composition and structure of the surface of the processed workpieces are modified after forming a nitrided layer, which makes the workpieces achieve high hardness, friction performance and fatigue resistance. Moreover, nitriding treatment is very suitable to strengthen precision workpiece due to small deformation. Also, nitriding can serve as an intermediate process of coatings, especially for hard coatings on low hardness alloys (e.g. titanium, aluminum, and magnesium alloys). Nowadays, as the development of ion nitriding technique, the method for nitriding the outside surface is mature. However, for nitriding the inner surface of tube-shaped specimens, problems arouse such as uneven nitrided layer, poor nitriding efficiency. To solve this problem, a new method experiment is designed:DC pulsed hollow cathode discharge plasma is applied in nitriding processing. The tube-shaped specimen serves as cathode and plasma is generated inside. To ensure following-up mechanical property testing, we replace the tube-shaped specimen with a flat one.The material used in the experiment is AISI304stainless steel, fabricated into flat hollow cathode. Its inner surface is nitrided with DC-pulsed hollow cathode. Four sets of parameters are considered:current, pressure, N2content and processing time. The nitriding effect on thickness, phase structure, hardness and friction property of nitrded layer is measured under different parameters, and the optimum process parameters are optimized. Metallographic microscope is used to observe the thickness and morphology; XRD is used to analyze the phase structure; micro Vickers is used to measure the surface hardness; and friction-wear tester to test the friction property of the nitrded layer.The results of the experiment show that plasma nitriding using DC pulsed hollow cathode discharge is an efficient way, which provides high nitriding rate, thick nitrided layer. The thickest layer up to80μm is achieved after4hours’nitriding. The nitrided layer is mainly S phase with CrN phase separating out. The phase structure and thickness of nitrided layer guarante excellent mechanical property. After nitriding, surface hardness of specimen increases, with maximum value up to12times as the original one. Friction property is also promoted:friction coefficient decreases, grinding crack turns to be vague. The fittest nitriding currency is0.3A. The forming of nitrided layer is inhibited for too low current and the layer is too thin for a higher current. The best pressure is3Torr; the performance of the nitrided layers declines slightly when pressure is not enough, while under a high pressure, nitrided layer will be difficult to form. Optimum N2content is33%. A higher rate of N2results in no formation of layer. The optimum nitriding time is2hours. A short processing time leads to a thinner layer. When processing time is too long, internal stress will exceeds the yield stress of specimen, leading to plastic deformation, thus release of lattice distortion energy, hardness and friction property will decrease.