Research On Preparation And Properties Of Co-based Alloy Layer On TC4 Alloy

Titanium alloys are widely used in aerospace, chemical, marine, medical and civil due to its good specific strength, less density, strong corrosion resistance, good biocompatibility, etc. But its wear resistance and high-temperature oxidation resistance are poor. Therefore, it was proposed in this dissertation to prepare Co-based modified surface layer on TC4 alloy by using the double glow plasma surface alloying technique.The Co-based modified surface layer on TC4 alloy has been obtained by using the double glow plasma surface alloying technique. The effects of processing parameters on modified layer was investigated and the optimized parameters were obtained as follows: processing temperature 860℃, holding time 3h, discharge pressure 40Pa, Source Voltage 850⁹00V, Cathode Voltage 350⁴00V, the distance between source and cathode 16mm. The morphology, composition and microstructure of the modified layer were analyzed by means of SEM, EDS and XRD, respectively. The results indicated that the as-fabricated modified layer was 16μm thick, including three layers of deposition, transition and diffusion. The modified layer consisted of Co, Cr and Ti phases, with the gradient distribution of Co elements from the surface to substrate. The adhesive strength was tested by using the scratch method, and the result exhibited the good adhesion strength between the modified layer and the substrate.Ball-on-disc wear experiments were performed to study the tribological behavior of TC4 and the as-prepared modified layer. The effects of temperature and load on friction coefficient, abrasion loss, wear volume and grinding crack pattern were investigated respectively. The results showed that the wear resistance of the modified layer was improved under any loads at any temperatures, but it was much more obviously at room temperature than at high temperature.Box type resistance furnace was used to do the high-temperature oxidation experiment. The oxidation dynamics curves showed that oxidation mass gain of the as-prepared modified layer is much lower than that of TC4 substrate. This indicated that the high-temperature oxidation resistance of the modified layer was improved obviously. The reason is that the oxide layer of TC4 substrate is mainly consisted of TiO₂ whereas the oxide layer of modified layer is mainly composed of Cr₂O₃, and Cr₂O₃ can obstruct from the further incursion of oxygen due to its compact texture.