Jan Čupera

Tribological Performance of Ti–Si-Based in Situ Composites

ABSTRACT In this study, a series of Ti–Si-based in situ composites was manufactured by means of a common argon arc melting technique and tribologically evaluated using a sliding ball-on-disc tester under simulated body fluid lubrication. The composite microstructure, mechanical properties, and surface roughness were characterized using light and scanning electron microscopy (SEM), vertical scanning interferometry (VSI), X-ray diffraction (XRD) analysis, and hardness measurements. The evolution of coefficients of friction (COFs) and the appearance of contacting surfaces showed that two the principal wear mechanisms were mixed elastohydrodynamic lubrication (EHL), typically followed by abrasive wear. The mixed EHL was due to the combined effect of serum solution lubrication and surface irregularities, which were produced during the routine surface preparation of samples. The mixed EHL provided the absence of wear and low and stable COFs, which did not depend on the phase composition, microstructure, or hardness of Ti–Si-based alloys. However, in most cases, the change in contact geometry led to the transition from mixed EHL to conventional boundary lubrication, accompanied by increased and unstable friction, adhesive material transfer of metal to the ceramic counterbodies, and abrasive wear. In this respect, the low wear resistance and high adhesion affinity of the titanium matrix of Ti–Si-based alloys should be improved.

Influence of multiple electron-beam remeltings on the characteristics of HVOF and CGDS sprayed conicraly coatings

Nanostructured CoNiCrAlY bond coatings were deposited onto a Ni-based alloy (Inconel 718) by both HVOF and CGDS spraying techniques. Subsequently, the deposits were remelted by an electron beam up to depth of about 100 μm, which resulted in the removal of defects on the substrate to the bond coat interface. This paper examines the influence of the parameters used for EB remelting, including multiple remelting on the microstructural changes, phase modification and the final state of the coatings. The amount of porosity in the coatings and the surface roughness has been evaluated. Scanning electron microscopy and X-ray diffraction were performed in order to characterize the phase modification before and after the applied treatment. The results indicated that multiple remelting improved the coating in terms of porosity, surface roughness decrease, mechanical strength and chemical homogeneity. This study also demonstrates that the CGDS deposition represents a promising alternative for CoNiCrAlY bond coat manufacturing.

Effect of double electron beam remelting on microstructure of HVOF and CGDS bond coat

This work focuses to investigate the influence of the parameters used in electron beam (EB) remelting including the effect of double remelting of CoNiCrAlY coatings fabricated on Nickel based super alloy substrates by using the high velocity oxygen-fuel (HVOF) and cold gas dynamic spraying (CGDS) methods. The microstructures of as sprayed and remelted coatings were investigated by scanning electron microscopy and the phase analysis by X-ray diffraction (XRD). The results obtained show that there are advantages at using the pulsed EB surface modification technique. Double EB treatment provides a smooth surface and low porosity level and at last but not least this study demonstrate that low-temperature processing of CoNiCrAlY bond coat represents an interesting and promising alternative for their manufacturing.

Microstructure Evaluation of Heterogeneous Electron Beam Weld between Stabilised Austenitic and ODS Ferritic Steel

The weldability of advanced heat resistant ODS metallic materials in combination with conventional materials is a prior requirement for their wider use in energy production. The microstructure of ODS steels is composed of alpha iron based matrix with dispersed oxide particles. Due to heating during conventional welding, the microstructure and properties of the resulting weld joints are affected and the joints often become the weakest point of the structure. The electron beam welding with its reduced heat affected zone size may be an answer in this. The presented article is focused on thorough metallographic evaluation of the structure of heterogeneous electron beam welds which combine stabilized austenitic stainless steel with the MA956 ferritic ODS steel. EB welded joints were evaluated by light and analytical electron microscopy including EDS and phase EBSD analyses in the as-welded state and after post-weld heat treatment. Mechanical properties of the weld were evaluated from the results of micro hardness profiles. Achieving an appropriate structure of such welds and correct welding parameters are crucial aspects for future successful application of similar joints in energy industry

Potential of New-Generation Electron Beam Technology in Interface Modification of Cold and HVOF Sprayed MCrAlY Bond Coats

Electron beam (EB) technology treatment was carried out on CoNiCrAlY bond coats deposited on Inconel substrates via cold spray and HVOF techniques in dissimilar thicknesses. Such treatment was carried out with regard to the final materials microstructure, composition, surface roughness, and the quality of the coating-substrate interface. Following a multiple-step optimization of the processing parameters (such as beam pattern configuration, accelerating voltage, longitudinal speed, and multiple beam incidence), two final EB modifications were carried out on both coating types. It was found that the optimized EB treatment could lead to a significant alteration of the interface from a distinctive divide into smooth chemical and structural transition between the materials, significant decrease in surface roughness and porosity, and changes in mechanical properties (increase in Young’s modulus and decrease in hardness of the coating).