| Titanium alloys are widely used as new metallic materials in the industries of aerospace and petrochemical, owing to their high specific strength, excellent corrosion resistance, biocompatibility and other superior properties. However, due to their intrinsic properties, titanium alloys usually have low hardness and heat conductivity, which result in adhesion and poor wear resistance. Therefore, for the safe application of components working in the conditions of friction or fretting wear, surface treatments to improve tribological properties are essential.Chosen TiNi and purity Ni as the target materials, TiNi and Ni alloyed layers are prepared on the surface of Ti6A14V alloy (TC4) using plasma surface metallurgy technique. The plasma surface alloying processe is optimized by analyzing the effects of processing parameters on the formation of TiNi and Ni alloyed layers. The characterizations of the alloyed layers indicated that Ni alloyed layer is better than TiNi alloyed layer in wear performance, therefore, the Ni alloyed layer is used to investigate the tribological properties and wear mechanism under the conditions of dry sliding wear at ambient and high temperatures, fretting wear as well as corrosive wear, in comparison with the untreated bare substrate. In addition, the bonding strength and toughness of the Ni alloyed layer and typical hard thin films are also compared by a repeated impact test. The main results are shown as following:(1) The optimized parameters for Ni alloying is processing temperature 900℃, pressure45Pa, alloying time3h, Ni target voltage600-800V, specimen voltage300-600V, the voltage difference between target/specimen350-450V. The Ni alloyed layer, with thickness of50μm, is mainly composed of TiNi, Ti2Ni and Ti. The composition of the Ni alloyed layer exhibits a gradient distribution and the Ni content on the outmost is about90wt.%. The surface hardness of the Ni alloyed layer is677HV0.025, about twice of the untreated Ti6A14V.(2) Compared with hard thin films, the Ni alloyed layer shows better toughness as well as impact resistance and the cohesion is its main failure mechanism.(3) With the increasing of normal load, the friction coefficient of the Ni alloyed layer remains almost unchanged and the wear loss gradually increases. Under different rotation speed, the lowest friction coefficient is obtained at150r/min, while the wear loss is lowest at200r/min. The wear mechanism is predominated from abrasive wear to adhesive wear with the increase of the load. Also, the wear mechanism is changed with different counterface, including GCr15, Si3N4and ZrO2balls.(4) The results of wear tests at temperatures of24℃,200℃and500℃show that the minimums of friction coefficient and wear loss are obtained at500℃and at200℃, respectively. Abrasion, adhesion and oxidation wear are the main wear mechanism for Ti6A14V substrate, accompanied with traces of fatigue. At500℃, the Ni alloyed layer shows adhesive and oxidation wear, while at200℃, it is only slight abraion and oxidation wear.(5) For fretting wear, the friction coefficient of the Ni alloyed layer increases with the increase of frequence, displacement amplitude and time and is reduced by PAO lubrication. Under dry wear condition, the Ti6A14V subtrate exhibits abrasive wear, adhesive wear and fatigue wear, while the Ni alloyed layer is slight abrasive wear. With PAO lubrication, the wear and is characterized as slight abrasive wear for the Ni alloyed layer and the coexist of abrasive and adhesive wear for the substrate.(6) In distilled water,3.5%NaCl and5%HCl solutions, the tribological behaviour of the Ni alloyed layer is superior to Ti6A14V substrate, showing a slight wear, while the wear mechanism of the substrate is dominated by abrasive wear. The effect of corrosion of solutions on wear is insignificant and the material loss is mainly caused by wear.(7) The excellent wear resistance of the Ni alloyed layer is attributed to the effective strengthening effects of Ti2Ni, high toughness of TiNi. |
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