Study On Surface Alloying Processing By Double Glow Plasma Alloying Technology To Improve The Tribological Properties Of Ti6Al4V Alloy

Titanium alloys have high specific strength, superb high temperature properties and corrosion resistance. However, insufficient tribological properties have always been the major obstacle in the application of titanium alloys. Surface treatment techniques are very effective ways to improve titanium alloy's tribological properties. Double glow plasma surface alloying technique is an environment-friendly plasma surface alloying process which can produce uniform and continuous diffusing layer on surface of metallic materials. In this study, Mo-C, Mo-N, Mo-Cr surface-modification layers were prepared on surface of Ti6A14V alloy by double glow plasma surface alloying technique. The component, structure, mechanical properties and tribological properties of these surface-modification layers were investigated; the results show that the optimal process is Mo-N alloying treatment. Since the service condition of titanium alloys is often a synergism of corrosion and wear, the performance of Mo-N modified layer in corrosive fretting-wear condition was also investigated. Following are the main results:(1) The component and structure of the three surface-modification layers of Ti6A14V have been studied. The modification layers are diffusing layers with gradient profile of chemical components and hardness distributions.(2) The micro-hardness tests show that Mo-N and Mo-Cr modification layers enhance the surface strength of Ti6A14V substrate obviously. The maximum of hardness for Mo-Cr and Mo-N modification layers are 1208HK_0.025 and 941HK_0.025, 3.09 times and 2.41 times as that of Ti6Al4V substrate respectively. However, the surface hardness of Mo-C modification layer is only 350HK_0.025 , lower than that of Ti6Al4V substrate 390HK. Toughness is evaluated by scratch and impact tests. Although with lower surface hardness, Mo-C modification layer shows the maximum toughness, while Mo-Cr modification layer shows a brittleness tendency obviously. The tribological properties and mechanism of the three modification layers were studied by ball-on-disc test. Mo-Cr modification layer and Mo-N modification layer increase the wear resistance of Ti6Al4V substrate remarkably. Under un-lubricated condition, Mo-N modification layer shows the best wear resistance with a corundum ball count-part. But Mo-C alloying on Ti6Al4V shows no improvement on tribological properties. The excellent wear resistance of Mo-N modification layer comes from the cooperation of their strong enough load-bearing capacity, toughness and surface hardness.(3) Mo-N modification layer improves fretting-wear resistance of Ti6Al4V, for its lower friction coefficient and excellent fretting-wear resistance as sliding against corundum. Different mechanisms are responsible for fretting-wear of Ti6Al4V and Mo-N modification layer. The mechanism for Ti6Al4V is mainly adhesion and abrasion. The Mo-N modification has significantly improved Ti6Al4V alloy adhesion resistance and its fretting-wear mechanism is mainly abrasion.(4) The corrosion-wear behaviors of Ti6Al4V alloy and Mo-N modification layers were also studied. In 0.02M Na₃PO₄, 3.5 wt.% NaCl, 0.5M H₂SO₄ and Hanks' solutions, Mo-N modification layer shows smaller friction coefficients and higher wear resistance than that of Ti6Al4V substrate. The excellent corrosion-wear performances of Mo-N modification layer come form their strong corrosion resistance and wear resistance. Electrochemistry tests show that the corrosion-wear mechanism for the Mo-N modified titanium alloy is similar to that on the untreated titanium alloy in Hanks' solution during the steady-state phase, i.e. a periodic removal and growth of a passive film in the active wear track. Nevertheless, the current noise levels for the Mo-N modified titanium alloy are much smaller than that measured on untreated titanium alloy in the same solution.(5) The mechanism of double glow superposed enhancing discharge is analyzed theoretically. The second electrode mechanism is oppugned. To explain the phenomena of enhancing discharge current because of double glow superposed discharge, a suppose is put forward, in which the strong increasing of discharge current is considered for the influence of the first Townsend ionization coefficient (a). Furthermore, mass transfer from Source Electrode to Workpiece Electrode is studied in which atom is the main status of transferring particles.