Materials Science Forum Vols. 638-642

Abstract: The thermal barrier coatings (TBC) enable to lower temperature (at approx. 170°C) of operating elements in a hot section of gas turbine to a range, which enables to operate for a long time in conditions of high temperature influence and prolongs operation of them even three or four times. Usually, the TBC coverings are constructed of four layers: • superalloys on a base of nickel; • outer ceramic zone, from which low thermal conduction is required. It is, in most cases, ZrO2 oxide stabilized with Y2O3 (YSZ – yttria stabilized zirconia), material of one of lowest values of thermal conductivity in high temperature of a rank 2.3 Wm-1K-1 in 1000°C for 100% density and thermal expansion of a rank 11×10-6°C-1, what enables to reduce thermal stresses. Usually, thickness of an outer ceramic layer is within a range 250-375μm; • bond coat of a type Ni(Co)CrAlY or diffusion layer of a type (Ni,Pt)Al; • layer of barrier oxides, accrueting as a result of temperature growth TGO (thermally grown oxide). In the present study characterization of conventional micro-sized YSZ powders stabilized by 8% of yttria and in comparison nano-sized YSZ will be presented. The tests performed showed that the monoclinic phase content in the nanocrystalline powder is approx. 8.8%, and approx. 7.5% in the conventional powder. The chemical composition analysis and test of powder microstructure were also performed. Carbon, sulphur and gas nitrogen contents were determined and the results received were similar. The surface morphology and powder microstructure on the cross-sections were also characterized. The standard powder with spherical shape and relatively smooth surface is predominant. The internal structure is characterized by the presence of tiny sintered particles and low porosity. The nanocrystalline powder tests showed the presence of particles in the form of tiny polyhedrons with bimodal size distribution of particles (approx. 1μm and 10μm).

Abstract: As the top coating, zirconia with 4 mol% yttria was electron beam-physical vapor deposited (EB-PVD) on the bond coating of CoNiCrAlY. The substrates were rotated during EB-PVD process and the rotation speeds were 5 (R5) and 10 rpm (R10). The thickness of the top coating was 0.12 mm. In order to investigate the change of the internal stress in the top coating under a heat cycle, the specimen was heated from a room temperature to 1293K, the internal stress was measured in-situ by a strain scanning method with hard synchrotron X-rays at each temperature step. For the specimen R5, the internal stress increased from about -100 MPa to about 100 MPa with the increase in temperature, then the stress relaxation of the top coating occurred over 1093K. In the cooling process, the internal stress decreased, however, the changing rate of the internal stress was small as compared with the heating process. This was caused by the feather-like structure sintered. For the specimen R10, the internal stress did not show a tension in the heating process, it was caused by the separation between columnar structures.

Abstract: Shot Peening is a well established mechanical surface treatment to induce compressive residual stresses and work hardening into the surface layer of components exposed to cyclic loading. Due to the induced changes in the surface layer, the fatigue limit increases significantly. The concept of local fatigue strength is based on the comparison of the locally effective fatigue limit with locally active loads in order to estimate the maximum outer loading that will not exceed fatigue limit. In this paper an approach of using simulated surface characteristics after shot peening for the determination of the local fatigue strength will be presented. The complex stress distribution due to cyclic bending of notched geometries will also be determined by FEM. Finally the simulated estimation of the fatigue limit of differently notched specimens of AISI 4140 will be verified with experimentally determined fatigue limits.

Abstract: The densification behavior of WC composites based on iron aluminide binder was investigated using laser scanning confocal mi¬croscopy (LSCM). Doped Fe60Al40 alloys with boron levels ranging from 0 to 0.1 wt% were used as the aluminide binders. The aluminide binders were prepared using controlled atmosphere ring grinding and then blended with WC powder. The composite powder compacted in an alumina crucible and held in a platinum holder in the confocal microscope. The temperature increased from ambient temperature up to 1500 °C under high purity argon. The presence of boron was found to facilitate compaction of the composites and improve the wetting between WC and FeAl binder during liquid phase sintering. Increasing the amount of boron in the binder resulted in the melting of binder at lower temperature and increasing of the compacting of the intermetallic tungsten carbide composites.

Abstract: This thesis deals with carbon fiber reinforced plastics (CFRP) composites, an advanced material which is widely used in manufacturing aircrafts because of their unique mechanical and physical properties. The research mainly involved drilling of CFRP. This study is focused on analyzing the thrust force and delamination against drilling parameters namely feed rate, spindle speed and type of tool materials. Also, the optimal parameters were chosen using an optimization method called D optimal. It was observed that the higher the feed rate and spindle speed employed, the higher the thrust force and delamination occur. The split point fibre (SPF) drill gave the lowest values of thrust force and delamination. Based on the optimal parameters, a verification test was conducted and the prediction error was 2.3% and 5.6% for thrust force and delamination respectively. This shows, that the optimal parameters obtained is reliable as it could improve the process considerably. The results of this study could be used as a reference for further research and studies on drilling of CFRP.

Abstract: To develop the small engine used for the wireless control helicopter, a very high power density source will be required. The rotary engine (RE) is advantageous in the points such as a volume of the engine, the exhaust gas emissions, and the vibrations. In this research, in order to develop the small RE, the design of the combustion chamber was reviewed. Additionally, the method of the surface treatment of the rotor housing was investigated by the friction wear test to enable the material substitution for aluminum matrix composites. The friction wear tests of composite materials, the Cr and Ni-plated material were conducted. As a result, the sample with the most excellent abrasion resistance was the composite material reinforced with Al2O3 fibers. The wear-loss of this composite material was 1/10 compared with the Ni-plated material. This composite material formed oil pockets between the fiber and the matrix, and a friction coefficient fell below 0.1. Then, the aluminum rotor housing reinforced partially by Al2O3 fiber was fabricated using a low-pressure infiltration method. The maximum power of the small RE (1 rotor, 30 cc) which built in this housing reached about 5 PS in the range from 13,000 to 15,000 rpm. The inside of the housing used composite materials was not damaged even after 30 hours. On the other hand, the flaking off part was observed on the inside part in the case with the housing used the Ni-plated material. Additionally, the output and the torque of the RE made of composite materials were higher than those of the Ni-plated material. This was considered because the friction coefficient of composite materials was lower than that of the Ni-plated material. Finally, the small unmanned helicopter equipped with RE succeeded in a stable flight.

Abstract: In this paper, the milling process of WO3 and aluminum particles is studied. Influences of rotate rate, milling time and ball-to-powder weight ratio on the morphology and grain size of the particles are studied by scanning electronic microscope and X-ray diffraction techniques. Al particle size decreases firstly and then increases with increasing milling time, while Al grain size decreases gradually as the milling time increases. WO3 particle is distributed uniformly in Al particles after milling for 9h, and has no marked change as further extension of milling time. With increasing the ball-to-powder weight ratio and rotate rate, both particle size and grain size of Al decreases. milling parameters have almost no effect on the grain size of the WO3 particles.

Abstract: The distinctive characteristics of carbon fibre reinforced plastics, like low weight or high specific strength, had broadened their use to new fields. Due to the need of assembly to structures, machining operations like drilling are frequent. In result of composites inhomogeneity, this operation can lead to different damages that reduce mechanical strength of the parts in the connection area. From these damages, delamination is the most severe. A proper choice of tool and cutting parameters can reduce delamination substantially. In this work the results obtained with five different tool geometries are compared. Conclusions show that the choice of an adequate drill can reduce thrust forces, thus delamination damage.

Abstract: This study is to investigate tribological behavior of brake lining materials by hot pressing commercial friction powders with ceramic powders prepared by TEOS / boehmite sol-gel. The stoichiometric ratios of TEOS / boehmite sol-gel were kept constant but calcinated at different temperature to fabricate different homemade ceramic powders. The various phases of ceramic powders such as γ-Al2O3, δ-Al2O3, θ-Al2O3, α-Al2O3, cristobalite and mullite were formed during the preparation process starting from TEOS / boehmite sol-gel solution. The XRD observations reveal the final compositions of these homemade powders were strongly related to the calcining temperature. The brake lining specimens made from TEOS / boehmite sol-gel calcinated at higher temperature show better tribological performance. The brake lining specimen with α-Al2O3 and mullite which were transformed from TEOS / boehmite sol-gel calcinated at 1300°C shows the most stable friction coefficient and the lowest mass loss during wear tests.

Abstract: In order to develop the alumina fiber reinforcements optimized to FRMMCs, the effect of characteristics of alumina fibers on the fabrication process and the characteristics of the alumina fiber reinforced Al alloy composites was investigated. Alumina fibers which have different alumina content were prepared. Alumina content in the fibers was varied from 80% to 100%. Al-4mass%Cu alloy, Al-12mass%Si alloy and Al-10masss%Mg alloy were used as matrix. The FRMMC specimens were fabricated by a low-pressure infiltration process (LPI process). The formability of the preform was improved with increasing alumina content in the fibers. However, broken fibers were observed in the preform when alumina fibers with high alumina content were used. The number of the broken fibers seemed to be increased with increasing alumina content in the fibers. This result could be attributable to a change of fiber strength resulting from a change of alumina content in the fiber. The FRMMC specimens were characterized by using Vickers hardness test. The Vickers hardness of FRMMC specimens depended on the elasticity or the hardness of the fibers. The results obtained suggest that the characteristics of the FRMMCs largely depend on the intrinsic characteristics of the reinforcement fibers.