Key Engineering Materials Vols. 336-338

Abstract: CrN-based multi-component coatings were deposited by a hybrid coating system combining the arc ion plating (AIP) and sputtering technique. In this work, comparative studies on microstructure and mechanical properties of microhardness and wear behaviors among Cr-C-N, Cr-Si-N, and Cr-Si-C-N coatings were systematically conducted. Adding carbon and silicon atoms into CrN coatings had large effects on microstructural change and mechanical properties of CrN coatings. The hardness value of Cr-Si-C-N coatings showed about 44 GPa, while those of Cr-Si-N and Cr-C-N coatings were 34 and 23 GPa, respectively. The average friction coefficient of CrN-based coatings decreased from 0.65 to 0.4 with the incorporation of silicon and carbon content.

Abstract: Si films were deposited by inductively coupled plasma chemical vapor deposition at room temperature with a mixture of SiH4/H2. The microstructure of the film was characterized with Fourier transform of infrared, Raman spectroscopy, atomic force microscopy. We found that SiH4 concentration strongly affects the structure of Si films and nano-crystalline film can be synthesized at room temperature by optimizing the silane concentration. The analysis for optical properties of the films suggested that the optical band gap EOPT of films are distinctively lower than those of amorphous Si films. It has been observed that the EOPT of sample decreases with the increasing of H content in film.

Abstract: A two-step anodization process was used to fabricate wide-range highly ordered porous anodic alumina membrane (PAA) in the electrolyte of oxalic acid through different anodic voltages. The effect of anodic voltage, pretreatments and pore-expanding time on the microstruture of PAA membrane was also studied in the process of two-step anodization. In a certain range, with the increase of anodic voltage, the pore diameter almost increases linearly while the pore density almost decreases linearly. The PAA template with anneal and electropolishing process has better regularity. In a certain time range, the pore diameter increases obviously with the pore-expanding time. It was demonstrated by XRD that the crystal constitution of the PAA without heat treatment was polycrystalline structure which contained Baehmite phase (Al2O3.H2O), Gibbsite phase (Al2O3.3H2O) and θ-Al2O3 phase. After heat treatment at 500°C for 0.5h, only the θ-Al2O3 phase was retained.

Abstract: A novel nanosized multilayer film containing polyoxometalate anion with a pendant supportligand α-[SiW11O39Co(H3P2O7)]7- (abbr. SiW11Co-P2O7) and poly(diallyldimethyl ammonium chloride (PDDA) was fabricated by layer-by-layer self-assembly. The multilayer film was characterized by XPS, UV-vis, AFM and ESR. The mean interface roughness was about 2.0 nm, calculated from an area of 0.5 × 0.5 μm2. The electrochemical property was studied by the cyclic voltammetry, the results indicating that the self-assembly film exhibited favorable electrochemical behavior of polyoxometalate.

Abstract: The thin film deposited by electron beam physical vapor deposition (EB-PVD) on rotating substrate was approached via a kinetic Monte Carlo (KMC) algorithm on a “surface” of tight-packed rows. The motivation is to study the surface morphology distribution of thin film along the substrate radial. Effective deposition rate model and effective incident angle model were established along the substrate radial. Two phenomena are incorporated in the KMC simulation: adatom-surface collision and adatom diffusion. The KMC simulations show that the surface roughness of thin film is small and the changing of surface roughness is small near the side of rotation axis, however, the surface roughness is big near the side of substrate edge, and the surface roughness increases quickly with the increasing of substrate radius when r>300 mm. The simulation results indicate that the effective incident angle is the main factor to cause the changing of surface roughness: the effective incident angle does not reach critical value and the deposition rate difference is small when the radius is less than 300 mm, so the surface roughness of thin film in the scope is small and its changing is small, but when r>300 mm, the effective incident angles increase sharply with radius increasing and all of them are above the critical value, which cause surface roughness of thin film to increase quickly. Experiments reveal that the KMC method can predict surface roughness distribution of thin film deposited by EB-PVD on rotating substrate.

Abstract: In order to improve fracture toughness of ceramics, an intrinsically small frontal process zone (FPZ) size must be expanded. An intra-type nano-structure, where nano-particles are embedded within matrix grains, yields dislocations around the dispersed particles due to residual stresses. These dislocations become sessile dislocations at room temperature, operate as origins of small stress concentration in the matrix, and create nano-cracks in the FPZ. To produce the intra-type nano-structure, we developed a soaking method. This method makes it possible to produce nano-sized metallic particles dispersed within ceramic powders. In this research, alumina-nickel nanocomposite powder was obtained using the soaking method. The powder mixed with α-alumina as a seed was sintered using a pulse electric current sintering technique. The sintered nanocomposites are then annealed to disperse dislocations around the nanoparticles into alumina grains. Results showed that the maximum fracture toughness was 7.6 MPam1/2, which was two times higher than that of alumina.

Abstract: Machinable BN/Si3N4 and electroconductive TiN/Si3N4 nanocomposites were prepared, using powders synthesized through an in-situ nitridation method in flowing ammonia gas. Due to the homogeneous mixing of various components in the powders, nanocomposites with homogeneous second phase distribution in the matrix were obtained. These nanocomposites showed enhanced strengths and distinctive functionalities. BN/Si3N4 nanocomposite with 20-25vol% BN showed a relatively high strength of over 700 MPa and was able to be machined into complicated shapes with diamond bits. Electroconductive TiN/Si3N4 nanocomposite with 25vol% TiN showed a high strength of 1100MPa and low electrical resistivity of 1.1×10-2 ⋅cm, and was promising for electrical discharge machining.

Abstract: SiC nanopowders were surface modified by low temperature plasma polymerization. 5vol% SiC-Al2O3 nanocomposite was prepared by adding the surface modified SiC nanopowders into the matrix of Al2O3.The influence of surface modified SiC nanopowders on the properties and structure of SiCAl2O3 nanocomposite was studied. The surface modified SiC-Al2O3 nanocomposite had higher density and nicer mechanical properties than untreated SiC- Al2O3 nanocomposite because of the improvement of dispersion. The hardness and toughness of the SiC- Al2O3 nanocomposite were higher than monolithic Al2O3 but the ultimate bending strength was not perfect because of retardation of the densification and formation of unsintered pores caused by the induced oxygen during the process of powder treatment.

Abstract: Al2O3 particle dispersed 3Y-TZP nanoceramics doped with TiO2 was fabricated by the vacuum reduction of Ti(OH)4, Al(NO3)3 and 3Y-TZ powders at 873K for 2h and sintered in air at 1573K~1823K for 2-6h. The relation between Al2O3 and TiO2 content, sintering parameters, relative density and grain sizes were studied. Microstructural investigations revealed that, in comparison with direct powder mixing, the reduction method is more efficient in achieving a homogeneous dispersed fine alumina second phase, which results in the reduction of matrix grain size and the increase of the relative density of the 3Y-TZP ceramic. TiO2 addition facilitates the sintering process but may cause significant abnormal grain growth which can be offset by the homogeneity of the powder dispersion. A final 3Y-TZP-TiO2 ceramic matrix with grain size below 500nm dispersed with ultrafine second phase Al2O3 was obtained.

Abstract: Al2O3/24%ZrO2 composite, which is mainly composed of lath-shaped and rod-shaped α-Al2O3 matrix containing ZrO2 nano-micron fibers, was obtained from in-situ growth in the melts produced by combustion synthesis. The results from XRD, SEM and EPMA showed that, besides the rod-shaped and lath-shaped α-Al2O3 grains, there are also t-ZrO2 or m-ZrO2 independent phases and Cr metallic particles at the boundaries of rod-shaped grains. The strength and toughness were measured to be 1186 MPa and 12.8MPa·m0.5 respectively. The procedure for toughening ceramics relies on coordinated action of multiple toughening mechanisms involving reinforcement-induced toughening by ZrO2 nano-micron fibers, deflection-induced toughening, stress-induced transformation toughening, crack-bridging toughening by lath-shaped α-Al2O3 grains as well as deformation-induced toughening by Cr metallic particles, and promises the occurrence of a sharp rising R-curved behavior.