Papers by Author: Jun Du

Abstract: Zirconium aluminum nitride coatings have been deposited onto Ti-6Al-4V substrates by reactive magnetron sputtering in order to investigate the influence of Al-content on the microstructure and mechanical properties. The morphology and microstructure of the coatings were investigated by field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Nanoindentation and Vicker’s indentation methods were employed to measure the hardness and toughness of the coatings, respectively. The results show that a structure of single cubic phase with twinning is formed at Al content of x ≤ 0.23, and a two-phase structure of hexagonal and cubic phase is formed at Al content of x ≥ 0.47. Hardness and toughness of the Zr_1-xAl_xN coatings show similar tendency with the increasing of Al-content. Both of them reach the maximum values at x=0.23 and drop to the minimum values at x=0.47, after that, they slightly increase with the increasing Al-content. The enhanced hardness and toughness achieved at x=0.23 is ascribed not only to single cubic phase structure but also to twinning structure.

Abstract: sp² rich carbon films were produced by using magnetron sputtering deposition. The hardness, friction coefficient and wear volume were elevated by Knoop micro-hardness and pin-on-disk tester; The composition and microstructure of the carbon films have been characterized in detail by combining the techniques of Rutherford Backscattering Spectrum (RBS), X-Ray Photoelectron Spectrum (XPS) and X-Ray Diffraction (XRD); the electrical resistivity was measured by Four Probe Methods (FPM). It is found that: the films hardness are 11~17GPa (HK0.05), the friction coefficients are 0.1-0.2, the wear rate is 10-15m3/Nm; The maximum intensity position in the C1s indicates the chemical bonds are mainly sp²; the electrical resistivity is 1~2×10^-4Ω·m. XRD proves these carbon films are amorphous.

Abstract: A robotic additive manufacturing system of GMAW constrained by electromagnetism has been developed recently. In this paper, the work principle, functions and composition of this system are introduced. A metal part to be manufactured should be constructured three-dimension CAD model firstly, then the delamination process of three-dimension model is carried out. Furthermore, the forming path planning of additive manufacturing is performed and finally the part is fabricated layer-by-layer in virtue of GMAW process. The additive manufacturing system consists of the robot system, GMAW power, a device producing magnetic field, a linear laser system monitoring precision of the forming part, a digital measuring device monitoring forming temperature,central control system and software modules that support various functions. The functions of the additive manufacturing system comprise CAD model construction of parts, discretization of three-dimension model, forming path planning and GMAW deposition forming layer-by-layer, and etc. It is indicated that the exploitation of the additive manufacturing system will provide an effective way for the manufacturing of metal parts.

Abstract: A GMAW additive manufacturing process is performed with H08Mn2Si wire to fabricate a cube forming part. A optical microscope was used to observe the cross section aspect of forming part and the structure mechanism has been analysed. The results demonstrate that the forming part structure presents a characteristic that macrostructure is uniform and microstructure changes circularly. The circularly heating pattern of GMAW surfacing forming results in circular microstructure change and preheating of formed bead to subsequent bead and postheating of subsequent bead to formed bead are primary reason. Electromagnetic stirring effect of longitudinal dc magnetic field to molten pool can break dendritic crystal and refine crystal grain, which is in favor of enhancing forming part performance.

Abstract: Due to the low density and high specific strength, magnesium and its alloys have been extensively used in the automobile and aerospace applications, where the weight reduction is critical. However, they are highly prone to corrosion, which has greatly limited their application in the automotive and aerospace industries. This paper briefly reviews the technologies for improving the corrosion and wear resistance of magnesium alloys and finds that the widespread application of magnesium alloys is still limited by the lack of proper protective coatings. Therefore, there is still a need to explore new materials and methods for the effective protection of magnesium and its alloys.

Abstract: Porous Si₃N₄ ceramics were fabricated by using different species β-Si₃N₄ as seeds, 5 wt.% Y₂O₃ and Lu₂O₃ as sintering additives. Effects of three types of β-Si₃N₄ seeds and two types of sintering additives on the microstructure and mechanical properties of porous Si₃N₄ ceramics were investigated. The results showed rod-like β-Si₃N₄ grains with finer and high aspect ratio developed in the porous microstructure. The microstructure and mechanical properties of specimens added with Lu₂O₃ and β-Si₃N₄ seeds were better than which of those added with Y₂O₃ and β-Si₃N₄ powder. The flexural strength and average aspect ratio of porous Si₃N₄ ceramics were in the range of 297349 MPa and 5.757.26 with the porosity range of 41.07%44.40%.

Abstract: Porous Si₃N₄ ceramics were prepared by conventional sintering method with the initial Si₃N₄ powder added different amounts of β-Si₃N₄ seeds and 5 wt.% Y₂O₃. The porosity, density and flexural strength were tested by Archimedes and three-point bending methods, respectively. Microstructure of porous Si₃N₄ ceramics was studied by scanning electron microscopy. The results showed that the fibrous β-Si₃N₄ grains developed in the bimodal porous microstructure and the average aspect ratio of 6.42 by adjusting the β-Si₃N₄ seeds content up to 3 wt.%. Under the porosity of about 42%, the flexural strength of the materials could reached 315.98 MPa.

Abstract: ZrAlN/Cu coating has been deposited onto Ti-6Al-4V substrates by reactive magnetron sputtering. The morphology of films was investigated by FESEM, the microstructure was studied by XRD, XPS and TEM. The plastic deformation characteristic of thin films were analysised from loading/unloading curves measured by nano-indentation method. Erosion tests were conducted to evaluate anti-erosion ability. Erosion rates were measured and characteristic damage features were identified on the surface of eroded specimens. The mechanisms of erosion are discussed in order to explain the promising performance of materials in erosive conditions. It was found that there is an significant increase of erosion resistance because of the increase of hardness and toughness.

Abstract: Titanium alloys are susceptible to sand erosion, hard zirconium nitride coatings have been deposited onto titanium alloys by Physical vapor deposition (PVD) in order to improve erosion resistance. Al and Cu were added into ZrN coatings to strength and toughing the coating. The microstructure and mechanical properties of ZrAlCuN coating were studied. Erosion tests were conducted to evaluate anti-erosion ability. Erosion rates were measured and characteristic damage features were identified on the surface of eroded specimens. The mechanisms of erosion are discussed in order to explain the promising performance of materials in erosive conditions. It was found that there is an significant increase of erosion resistance because of the increase of hardness and toughness.

Abstract: TiN/carbon multilayer coatings were deposited on M2 and GCr15 wafers by ion beam assistant magnetron spurting deposition (IBMSD). The hardness, elastic modulus, fracture toughness, adhesion strength and tribological properties were investigated. The results show that TiN coating by means of static recoil N+ doped interface preparation and Ar+ beam assistant magnetron spurting has 2 times adhesion strength, 5 times hardness of none-beam-assistant TiN coating. The wear rate of GCr15 wafer with TiN(2h)/Carbon(4h) multilayer coating is 0.36×10-15m3/Nm,1/5 of that of GC15 wafer without coating. The wear rate of M2 wafer with TiN(2h)/Carbon(4h) multilayer coating is 2.8×10-15m3/Nm, 1/8 of that of M2 wafer without coating. The friction coefficient of GCr15 wafter with TiN(2h)/Carbon(4h) multilayer coating is lower than 0.15, and that is 0.04 for M2 wafer. These results indicate that thickness ratio of TiN to carbon is important for the tribological properties of TiN/carbon coating, thick lubricant carbon/thin load-support TiN multilayer coating has better comprehensive tribological properties.