Papers by Author: Jakob Kübarsepp

Abstract: Cr₃C₂-Ni cermets exhibit high hardness and excellent corrosion, oxidation, abrasive and erosion resistance. However, nickel is toxic and carcinogen and because of that great efforts were made to displace or replace nickel in the composition of cermets. Therefore, in the present research chromium carbide-based cermets with FeCr-type ferritic binder was fabricated and investigated. Composites were sintered at different conditions: vacuum and spark plasma sintering. Spark plasma sintered cermets demonstrated acceptable structure and mechanical characteristics. Chemical composition of chromium carbide-based iron alloy bonded cermets was analyzed by energy-dispersive X-ray spectroscopy and structural analysis was carried out using X-ray diffraction. Sintered cermet consists of two main phases: α-Fe and (Cr,Fe)₇C₃ complex dicarbide. Mechanical characterizations – hardness and fracture toughness – was performed. Also oxidation rates were determined.

Abstract: Friction stir welding is a novel and promising joining process and most common welding tool failure is transformation of geometry caused by wear. In our point of view, this is adhesion wear. The lathe testing of adhesion wear was conducted to compare wear resistance of following materials: Co-25wt%Cr, TiC-25wt%Ni/Mo and WC-6wt%Co. According to characteristics of investigated materials, they are capable to become alternatives for conventional frictional stir welding (FSW) tool materials. Adhesive wear tests were performed by turning aluminium alloy AW6082-T6 at low speed – travel length, turning speed and feed rate were selected to simulate FSW conditions. The adhesive wear was determined as the change of the geometry of the cutting edges of the tool measured using SEM images. Most promising tool material in terms of adhesion wear resistance is WC-Co hardmetal. The two main stages of wear were distinguished: at first, the appearance of intensive adhesion wear followed by steady state wear. Surface fatigue wear complements development of the adhesive wear.

Abstract: In this work the influence of sintering method on the surface fatigue of carbide composite was studied. The research focuses on WC-15wt%Co hardmetals prepared using different powders and different sintering techniques and as a result different microstructure: conventional WC+Co powder and novel reactive powder type W+C+Co are sintered using vacuum furnace, compression sintering (sinterhipping) and spark plasma sintering (SPS method). As tungsten carbide is a common material for cold forming punches and surface degradation causes punch failure [1], the tool life can be significantly extended by material surface fatigue life improvement. It is expected that SPS production route of WC-15wt%Co hardmetal will conclude in better microstructure, more even average grain size distribution and smaller residual porosity, and respectively better mechanical and surface fatigue properties compared to conventional production routes. There are some expectations related to the reactive sintering production routes, as this technique promotes the fine microstructure and better mechanical properties.

Abstract: In this work, a catalyst-free direct deposition of multi-layered graphene closed shells around highly aligned alumina nanofibers with aspect ratio of 10⁷ is demonstrated for the first time. A single – step chemical vapor deposition process of specified parameters was used for development of hybrid structures of carbon shells around the core alumina nanofibers. Transmission electron microscopy and Raman spectroscopy were used to confirm formation of graphene layers and to understand the morphology of the various structures. The developed routine for growth of peculiar carbon nanostructures opens new opportunities for deposition of the tailored carbon structures on dielectric substrates.

Abstract: Sintering behavior of ZrC-ZrO2 ceramic composites processed by vacuum and spark plasma sintering routines as well as mechanical properties of the obtained materials were studied in the present paper. It has been shown that both methods of the composites fabrication are suitable for producing ZrC-ZrO₂ composites; however, SPS-ed composition gives somewhat higher hardness of the bulk.

Abstract: This paper discusses the effect of advanced sintering technologies – sinter + hot isostatic pressing (HIP) and sinter/HIP on the performance of TiC-based cermets. The performance was evaluated by strength (transverse rupture strength), adhesive wear resistance and durability. Adhesive wear was performed by a turning method (turning of mild steel at low speed) while durability was evaluated by functional testing – wear of tools during blanking of sheet metal. It was found that sinter/HIP ensures a higher positive effect than sinter + HIP in different cycles.

Abstract: The present study describes the wear and mechanical behaviour of some carbide composites (TiC-base cermets and WC-base hardmetals) in cyclic loading applications (blanking of sheet metal). Adhesive wear as well as fatigue endurance were tested, followed by XRD measurements. The results show that the blanking performance of a carbide composite is dependent on its level of resistance to adhesion wear and fatigue sensitivity. XRD measurement revealed that fatigue damage is preceded by plastic strain in both the ductile binder and the brittle carbide phase.