Stephan Siegmann

Fundamental scales of adhesion and area-scale fractal analysis

Abstract This paper advances the discrete bonding model (DBM) for adhesion on rough substrate surfaces. The DBM supposes that adhesive strength can be determined from the relative, or normalized, area of the substrate surface at some fundamental scale. Relative area can be determined as a function of scale from a measured surface by area–scale fractal analysis, which uses repeated virtual tiling exercises. To the limits of the resolution of the measured surface the fundamental scale of adhesion is found for four substrates treated by grit blasting and thermal spray coatings. This experimental support for the DBM suggests that there is good potential for using relative area determined by area–scale fractal analyses in establishing functional correlations, or functionality of surface textures, in a broad range of applications.

The role of nano-particles in the field of thermal spray coating technology

Nano-particles play not only a key role in recent research fields, but also in the public discussions about health and safety in nanotechnology. Nevertheless, the worldwide activities in nano-particles research increased dramatically during the last 5 to 10 years. There are different potential routes for the future production of nano-particles at large scale. The main directions envisaged are mechanical milling, wet chemical reactions or gas phase processes. Each of the processes has its specific advantages and limitations. Mechanical milling and wet chemical reactions are typically time intensive and batch processes, whereas gas phase productions by flames or plasma can be carried out continuously. Materials of interest are mainly oxide ceramics, carbides, nitrides, and pure metals. Nano-ceramics are interesting candidates for coating technologies due to expected higher coating toughness, better thermal shock and wear resistance. Especially embedded nano-carbides and-nitrides offer homogenously distributed hard phases, which enhance coatings hardness. Thermal spraying, a nearly 100 years old and world wide established coating technology, gets new possibilities thanks to optimized, nano-sized and/or nano-structured powders. Latest coating system developments like high velocity flame spraying (HVOF), cold gas deposition or liquid suspension spraying in combination with new powder qualities may open new applications and markets. This article gives an overview on the latest activities in nano-particle research and production in special relation to thermal spray coating technology.

Investigation of hydroabrasive erosion of laser alloyed nickel and titanium based intermetallic matrix composites with TiC reinforcement

Abstract B2–NiTi and Ti3Al based single and dual phase intermetallic matrix composites (IMC) with TiC reinforcement were laser alloyed using Ni/Al–VC, Al–VC and Al–Cr3C2 powder mixtures. A sufficient amount of Ti from the substrate reacted with the injected alloy powder mixture to form Ti based intermetallics and TiC. The coatings were characterised by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and energy dispersive spectroscopy (EDS). The erosion resistance of the alloyed layers was characterised by high speed particle laden liquid impact erosion tests. The high work hardening ability of Ti3Al IMC strengthened by TiC reinforcement improved the erosion resistance up to a factor of 1˙39. Compared to the single phase IMC, the dual phase IMC coatings were found not to be suitable for applications similar to the present test conditions due to possibly preferential removal of secondary precipitates. An attempt has been made to identify the influence of different depth sensing indentation (DSI) derived mechanical properties to the erosion resistance of the alloyed layers. The correlation indicates the combined contribution of hardness, strain hardening exponent, surface elasticity and yield strength on improving the erosion resistance.