Research On Hollow Cathode Sintering And Plasma Surface Cr-Mo Co-diffusion Of Ferrous P/m Materials

Powder Metallurgy (P/M) is a very important method of material preparation. With the rapid developing of modern industry, P/M components have played a more and more important role in the whole manufacturing industry. Meanwhile, the requirements on the quality and performance of P/M products are even more extensive and restrict. Sintering is a key process that determines the final performance of P/M parts. So that the development of novel sintering techniques, with typical features of high efficiency, energy saving, environment preserving, has been perused persistently. Hollow cathode discharge plasma sintering is a new technique developed in recent years. Adopting the high density plasma produced by hollow cathode discharge as heating medium, a high sintering temperature may be achieved quickly. In this paper, two typical ferrous structure materials, FN0205 (2%Ni-0.5%C-Fe) and FC0208(2%Cu-0.8%C-Fe), were sintered using this technique. Based on the fundamentals of hollow cathode effect, systemic experiments were carried out. Through the investigations of microstructures, the effective rules of key process parameters on the final sintering density and efficiency were discussed.Another technology trend in P/M manufacturing is applying advanced surface modify techniques to farther improve the performance of common products. So that a study of plasma Cr-Mo co-diffusing on sintered compacts mentioned above was managed. Through a series of experiments, the optimum process parameters were concluded as below: sintering temperature T =1100~1150℃, diffusing time t=3h, working pressure P=35~40Pa, distance between the source cathode to sample d=15mm, voltage of the source cathode VS=950~1050V, voltage of the cathode (sample) VC=500~550V. Prepared in the optimum process conditions, the thickness of the alloying layer of the FN0205 sample is 68μm, and that of the FC0208 sample is 59μm. Both the alloying layers were dense and homogeneous. EDS analysis indicated that the surface content of Cr and Mo of reached about 10 wt% and 5wt% respectively. XRD results showed that the major phases of the alloying layer on FN0205 included Fe, Cr, Cr23C6, Cr7C3 and Mo2C; the major phases of the alloying layer on FC0208 included Fe, Cr, Cr23C6, Fe3Mo2 and Mo2C. The surface micro hardness of both sample were all about 700HV, much high than that of the as sintered parts. Finally, a set of ball-on-disc rubbing was conducted under room temperature and without lubrication. For the FN0205 sample, Cr-Mo co-diffusing had lowered the friction coefficient from 0.53 to 0.33. For the FC0208 sample, Cr-Mo co-diffusing had lowered the friction coefficient from 0.47 to 0.31. The wear resistance of both samples had been improved greatly.