Evaluation of Failure Micromechanisms of Advanced Thermal Spray Coatings by In Situ Experiment

Identification of failure mechanisms of thermal spray coatings by means of traditional fractography of failed parts is often troublesome. Reason for this is a highly inhomogeneous character of the coating microstructure and harsh in-service conditions which may hinder evidentiary fractographic marks. In this study, failure evolution of advanced thermal barrier coating (TBC) prepared by plasma spraying was studied in-situ at high magnification in a scanning electron microscope under well-defined laboratory conditions of three-point bending (3PB).

Surface Evolution and Lubricant Distribution in Deep Drawing

Deep drawing is one of the most important processes applied in industrial production. Here the Finite-Element-Method (FEM) is an important tool in the development and optimization process. One aspect to optimize simulations is to consider real friction behavior. Thus the friction phenomenon has to be describable. In addition to contact normal pressure and velocity the surface topography and the lubricant amount have a great influence on friction. This paper illustrates the influence of surface evolution in real, inhomogeneous processes on the lubricant distribution. For this a rectangular cup with four different corner radii is used to evaluate local surface topographies and lubricant amounts in deep drawing. The lubricant amount is measured by fluorescence technique and the surface topography is evaluated by a confocal white-light microscope. Due to hydrodynamic effects the lubricant is squeezed out and displaced to adjacent regions. Further hydrostatic pressures built up in closed lubricant pockets force the lubricant to stay in the forming zone to bear a part of the load. In free forming zones without contact between the sheet and tool the surface roughens due to grain dislocations in the microstructure. This paper also presents the results of lubricant distribution and surface evolution by varying the initial lubricant amounts and drawing depth. It can be recognized that the different corner radii of the rectangle cup have a great influence on the surface evolution and lubricant distribution. Moreover it can be clearly seen that surface parameters correlate with the lubricant amount. By means of the described evaluation it is also possible to correlate these values with load histories consisting of contact pressures and strain evolution, evaluated in FEM. All the results contribute to a better understanding of the friction behavior in deep drawing and point out the inhomogeneous character of friction.

Research of SiCP/2A50 Composite Materials Creeper Treads Prepared by Liquid Forging

Microstructure of creeper treads is inhomogeneous when they are produced by way of common casting, which lead to lower mechanical properties. Compound loading and feeding are introduced into the technology of liquid forging. Defects of filling incompletely, shrinkage porosities and cavities are usually appeared in raised part of creeper treads. With feeding amount increased above 5mm, plastic deformation also would be increased, so the microstructure and mechanical properties of raised part were improved and those defects disappeared meanwhile. SiCP/2A50 composite materials were used in order to enhance abrasive resistance and extend their service life. Creeper tread produced by compound loading and feeding could obtain homogeneous and fine microstructure. Average tensile strength and hardness of specimen reached 360MPa and HV115.97 respectively after (500°C×3h) solution treatment, (20min) quenching and (160°C×10h) artificial aging. The results show that inhomogeneous character of microstructure and mechanical properties of creeper treads could be overcome by compound loading and feeding.

Two Dimensional Imaging of the Laterally Inhomogeneous Au/4H-SiC Schottky Barrier by Conductive Atomic Force Microscopy

We present a novel approach based on conductive atomic force microscopy (c-AFM) for nano-scale mapping of the Schottky barrier height (SBH) between a semiconductor and an ultrathin (1-5 nm) metal film. The method was applied to characterize the uniformity of the Au/4H-SiC Schottky contact, which is attractive for applications due to the high reported (∼1.8 eV) SBH value. Since this system is very sensitive to the SiC surface preparation, we investigated the effect on the nano-scale SBH distribution of a ∼2 nm thick not uniform SiO2 layer. The macroscopic I-V characteristics on Au/SiC and Au/not uniform SiO2/SiC diodes showed that the interfacial oxide lowers the average SBH. The c-AFM investigation is carried out collecting arrays of I-V curves for different tip positions on 1μm×1μm area. From these curves, 2D SBH maps are obtained with 10- 20 nm spatial resolution and energy resolution <0.1 eV. The laterally inhomogeneous character of the Au/SiC contact is demonstrated. In fact, a SBH distribution peaked at 1.8 eV and with tails from 1.6 eV to 2.1 eV is obtained. Moreover, in the presence of the not uniform oxide at the interface, the SBH distribution exhibits a 0.3 eV peak lowering and a broadening (tails from 1.1 eV to 2.1 eV).