Daoxin Liu

Improvement in fretting wear and fatigue resistance of Ti–6Al–4V by application of several surface treatments and coatings

Application of surface modification methods is expected to be an ideal solution to mitigate fretting damage. In this study, our aim was to improve the fretting wear and fretting fatigue resistance of titanium alloys by using several types of surface treatments and thin films, including shot-peening, ion-beam-enhanced deposition (IBED) CrN films, shot-peening+IBED CrN films as well as IBED CuNiIn films. Results showed that with the application of all the above surface coatings and treatments, the fretting wear and fretting fatigue resistance of Ti–6Al–4V were improved. However, the mechanisms and effects of several surface modification methods to mitigate the fretting damage were quite different. IBED CrN film exhibited the best fretting fatigue performance while the duplex treatment by shot-peening/IBED CrN film exhibited the highest fretting wear resistance. There are four mechanisms which can be used to explain the different fretting performance of these surface treatments and coatings: (1) to induce a compressive residual stress; (2) to decrease the coefficient of friction; (3) to increase the hardness; (4) to increase the surface roughness.

Improvement of the fretting fatigue and fretting wear of Ti6Al4V by duplex surface modification

Abstract Ion-beam-enhanced deposition (IBED) was investigated as a way to increase the fretting fatigue resistance of Ti6Al4V. Both hard CrN coatings with good toughness and soft CuNiIn coatings of low friction have been applied on the base material, and shot peening combined with coatings has also been studied in order to improve the fretting fatigue (FF) resistance. Since these IBED coatings exhibit a good bonding strength even after shot peening, they do not spall off during fretting fatigue and fretting wear tests. When the contact stress is not severe and gross slip contact conditions are operative, both CrN and CuNiIn show a better fretting fatigue resistance than that of shot-peened Ti6Al4V. As the contact stress concentration of fretting fatigue is high, coatings combined with shot peening achieved high levels. The fretting fatigue lifetime is largely dependent on the sliding contact conditions such as contact geometry, sliding distance and contacting materials. Under partial slip, cracks initiate at an early stage limiting the fretting fatigue lifetime, while under gross slip, a much higher fretting fatigue limit is achieved. Compressive residual stresses are particularly important to improve the fretting fatigue lifetime, when crack propagation is predominant during the failure progress.

IMPROVEMENT OF MECHANICAL PROPERTIES OF MARTENSITIC STAINLESS STEEL BY PLASMA NITRIDING AT LOW TEMPERATURE

A series of experiments were carried out to study the influence of low temperature plasma nitriding on the mechanical properties of AISI 420 martensitic stainless steel. Plasma nitriding experiments were carried out for 15 h at 350°C by means of DC-pulsed plasma in 25%N2+75%H2 atmosphere. The microstructure, phase composition, and residual stresses profiles of the nitrided layers were determined by optical microscopy and X-ray diffraction. The microhardness profiles of the nitridied surfaces were also studied. The fatigue life, sliding wear, and erosion wear loss of the untreated specimens and plasma nitriding specimens were determined on the basis of a rotating bending fatigue tester, a ball-on-disc wear tester, and a solid particle erosion tester. The results show that the 350°C nitrided surface is dominated by ɛ-Fe3N and αN, which is supersaturated nitrogen solid solution. They have high hardness and chemical stabilities. So the low temperature plasma nitriding not only increases the surface hardness values but also improves the wear and erosion resistance. In addition, the fatigue limit of AISI 420 steel can also be improved by plasma nitriding at 350° C because plasma nitriding produces residual compressive stress inside the modified layer.

Effect of TiN/Ti multilayer on fretting fatigue resistance of Ti-811 alloy at elevated temperature

Abstract The TiN/Ti multilayer was deposited on Ti-811 alloy surface by magnetron sputtering(MS) technique for improving fretting fatigue(FF) resistance of the titanium alloy at elevated temperature. The element distribution, bonding strength, micro-hardness and ductility of the TiN/Ti multilayer were measured. The effects of the TiN/Ti multilayer on the tribological property and fretting fatigue resistance of the titanium alloy substrate at elevated temperature were compared. The results indicate that by MS technique a TiN/Ti multilayer with high hardness, good ductility and high bearing load capability can be prepared. The MS TiN/Ti multilayer, for its good toughness and tribological behavior, can significantly improve the wear resistance and FF resistance of the Ti-811 alloy at 350 °C.

Effects of Plasma ZrN Metallurgy and Shot Peening Duplex Treatment on Fretting Wear and Fretting Fatigue Behavior of Ti6Al4V Alloy

A metallurgical zirconium nitride (ZrN) layer was fabricated using glow metallurgy using nitriding with zirconiuming prior treatment of the Ti6Al4V alloy. The microstructure, composition and microhardness of the corresponding layer were studied. The influence of this treatment on fretting wear (FW) and fretting fatigue (FF) behavior of the Ti6Al4V alloy was studied. The composite layer consisted of an 8-μm-thick ZrN compound layer and a 50-μm-thick nitrogen-rich Zr–Ti solid solution layer. The surface microhardness of the composite layer is 1775 HK0.1. A gradient in cross-sectional microhardness distribution exists in the layer. The plasma ZrN metallurgical layer improves the FW resistance of the Ti6Al4V alloy, but reduces the base FF resistance. This occurs because the improvement in surface hardness results in lowering of the toughness and increasing in the notch sensitivity. Compared with shot peening treatment, plasma ZrN metallurgy and shot peening composite treatment improves the FW resistance and enhances the FF resistance of the Ti6Al4V alloy. This is attributed to the introduction of a compressive stress field. The combination of toughness, strength, FW resistance and fatigue resistance enhance the FF resistance for titanium alloy.

Effect of Shot Peening and Plasma Electrolytic Oxidation on the Intergranular Corrosion Behavior of 7A85 Aluminum Alloy

The effects of shot peening (SP) and plasma electrolytic oxidation (PEO) on the intergranular corrosion behavior of the novel high strength aluminum alloy 7A85 (AA 7A85) were investigated by electrochemical polarization and electrochemical impedance tests. The intergranular corrosion mechanism of SP, PEO and PEO combined with sealing-treated AA 7A85 was studied by the metallographic analysis, residual stress testing, X-ray diffractometer analysis and scanning electron microscopy. The results show that AA 7A85-T7452 is very sensitive to intergranular corrosion. SP would significantly improve its intergranular corrosion resistance. This is attributed to the combination action of residual compressive stress and grain refinement. PEO would reduce the largest corrosion depth by 41.6%. Moreover, PEO without sealing did not eliminate the intergranular corrosion due to the existence of the micropores and microcracks in the oxide coating. However, PEO combined with the SiO2 sol–gel sealing treatment could effectively protect the AA 7A85-T7452 from intergranular corrosion because of the good corrosion resistance and barrier function of the sealed coating.

Tribology behavior of double-glow discharge Mo layers on titanium alloy in aviation kerosene environment

Abstract In order to improve the tribology behavior in aviation kerosene, molybdenum (Mo) modified layers were fabricated on Ti6Al4V base alloy using a double-glow plasma surface alloying technique. The morphology, microstructure, microhardness and element depth distribution of the Mo modified layers were studied. The tribology properties of Ti6Al4V base alloy, Mo modified layers and 5CrMnMo tool steel sliding with GCr15 steel or QSn4-3 copper alloy counterparts in aviation kerosene were comparatively researched. The effect of roughness on the sliding wear behavior was discussed. The results indicate that the Mo modified layers with polishing treatments not only reduce the friction coefficient of Ti6Al4V base, but also enhance the wear resistance of the counterparts. The Mo modified layers have better tribology behavior than 5CrMnMo steel. It is also found that the wear volume loss of the counterparts is proportional to the value of roughness of Mo modified layers, which is related directly to the ploughing wear between micro convex bodies of the layers and counterparts.

Effects of temperature, slip amplitude, contact pressure on fretting fatigue behavior of Ti811 alloys at elevated temperatures

Effiects of the temperature, slip amplitude, and contact pressure on fretting fatigue (FF) behavior of the Ti811 titanium alloy were investigated using a high frequency fatigue machine and a home-made high temperature apparatus. The fretting fatigue failure mechanism was studied by observing the fretting surface morphology features. The results show that the sensitivity to fretting fatigue is high at both 350 and 500 °C. The higher the temperature, the more sensitive to the fretting fatigue failure is. Creep is an important factor that influences the fretting fatigue failure process at elevated temperatures. The fretting fatigue life of the Ti811 alloy does not change in a monotonic way as the slip amplitude and contact pressure increase. This is owing to the fact that the slip amplitude affiects the action of fatigue and wear in the fretting process, and the nominal contact pressure affiects the distribution and concentration of the stress and the amplitude of fretting slip at the contact surface, and thus further influences the crack initiation probability and the driving force for propagation.

Growth process and corrosion resistance of micro-arc oxidation coating on Mg-Zn-Gd magnesium alloys

A Mg-6Zn-3Gd (mass fraction, %) alloy, noted as ZG63, was coated by different micro-arc oxidation (MAO) processes, and the coating structure and corrosion resistance of the alloy were studied using scanning electron microscopy (SEM), glancing angle X-ray diffractometry (GAXRD) and various electrochemical methods. The micro-arc oxidation process consists of three stages and corresponds with different coating structures. In the initial stage, the coating thickness is linearly increased and is controlled by electrochemical polarization. In the second stage, the coating grows mainly inward and accords with parabolic regularity. In the third stage, the loose coating forms and is controlled by local arc light. The looser coating is mainly composed of MgSiO3 and the compact coating is mainly composed of MgO. From micro-arc oxidation stage to local arc light stage, the corrosion resistance of the coated alloy firstly increases and then decreases. The satisfied corrosion resistance corresponds to the coating time ranging from 6 to10 min.

Effect of the Ultrasonic Surface Rolling Process on the Fretting Fatigue Behavior of Ti-6Al-4V Alloy

The effect of the ultrasonic surface rolling process (USRP) on the rotary bending fretting fatigue (FF) of Ti-6Al-4V alloy was investigated. The reason for the USRP’s ability to improve the FF resistance of Ti-6Al-4V alloy was studied. The results revealed that the USRP induced a compressive residual stress field with a depth of 530 μm and a maximum residual stress of −930 MPa. Moreover, the surface micro-hardness of the USRP sample was significantly higher than that of the untreated base material (BM) sample, and the USRP yielded a 72.7% increase in the FF limit of the alloy. These further enhanced fatigue properties contributed mainly to the compressive residual stress field with large numerical value and deep distribution, which could effectively suppress FF crack initiation and early propagation. The USRP-induced surface work-hardening had only a minor impact on the FF resistance.