P. Kulu’s research while affiliated with Tallinn University of Technology and other places

Good cementing properties, fast setting and strong thermal performance make calcium-aluminate a valuable raw material for use in the production of different types of new refractory materials, e.g., heat conductive/storage materials. The main aim of the study was to determine thermal properties of novel Nb-slag based materials with different fillers, and to clarify optimal composition and technology. Process of preparation of studied materials was following: mixing of components, casting to moulds and hardening of materials. To estimate potential application areas, we studied the following thermal properties of CA-based materials: thermal behaviour, coefficient of thermal expansion (CTE) and conductivity. For this small cylindrical specimens were cut out from produced materials, and plates sized 300 × 300 mm were used for conductivity studies. Different compositions of CA-based materials, the hardening process, and the influence of mechanical activation on the strength were analyzed. Best thermal properties similar to the analogous reference material were obtained by quartz sand and granite sand as filler materials. The thermal conductivity of the novel CA-based material is 1.5 times higher and the bending strength is about 3 times higher compared to commercial thermoplates.

This paper studies possible uses of Cr3C2–Ni cermet prepared via mechanically activated synthesis as reinforcement in submerged arc welded (SAW) and plasma transferred arc welded (PTAW) Fe- and Ni-based hardfacings. The microstructure and phase composition of the hardfacings were analysed by scanning electron microscopy and X-ray diffraction, respectively. Vickers hardness (HV30) was measured and two-body (ASTM G132) and three-body (ASTM G65) abrasive wear were tested. Cermet particles were found to be entirely dissolved in the SAW hardfacings, while in the PTAW hardfacings they were partially retained. The main phases in the Fe-based hardfacings were γ-Fe and α-Fe along with Cr7C3 in the PTAW hardfacing. The Ni-based SAW and PTAW hardfacings were mostly comprised of different carbides (Cr3C2, CrC, NiC0.22). Addition of cermet particles increased the hardness of the hardfacings 1.1–3.3 times, the effect being more pronounced in the Fe-based hardfacings and PTAW hardfacings. Under two-body abrasive wear conditions, composite hardfacings exhibited 1.2–7.8 times lower wear and under three-body abrasive wear conditions 1.4–9.4 times lower wear than the unreinforced hardfacings. The wear resistance of the PTAW hardfacings was improved considerably, while the effect of the matrix alloy was insignificant. Microcutting was the major wear mechanism under both two-body and three-body abrasive wear conditions, accompanied by microploughing in the SAW hardfacings in the former case and by the pull-out of carbide particles in the PTAW hardfacings in the latter case. The principle of the wear mechanism remained unaltered in the presence of cermet particles, but the wear was less severe.

High-velocity oxy-fuel sprayed, iron alloy-based powder coatings, reinforced with tungsten carbide–cobalt (WC–Co) and titanium carbide–nickel molybdenum (TiC–NiMo) cermet particles, are compared under high-temperature abrasive–erosive wear conditions. Both WC–Co and TiC–NiMo particles underwent fracture, as well as dissolution, during the spraying process, but in the case of WC–Co particles this process was remarkably less intensive. Under the low impact angle conditions, the WC–Co particle-reinforced coating exhibited 1.1 times lower wear than the TiC–NiMo particle-reinforced coating because of the larger amount of the reinforcement remaining. Under the normal impact angle conditions, the WC–Co particle-reinforced coating showed 1.2 times lower wear than the TiC–NiMo particle-reinforced coating because of the resulting larger size of the WC–Co reinforcement and a more ductile matrix.

Hardness and toughness have a major effect on wear resistance. The material wear rate depends directly on hardness (at sliding wear), as well as on material toughness (at impact wear). The main goal of the present work was to study the abrasive wear behaviour of different wear resistant steels, hardmetals/cermets and hardfacings. The following steels were under the study: a) hardenable boron and non-alloy steel C 45 subjected to different heat treatments; b) hardened steels Hardox 400...600. WC-Co hardmetals and TiC-NiMo cermets and composite metal-matrix coatings were studied. The abrasive wear tests were performed using two methods: a) abrasive rubberwheel wear; b) abrasive impact wear. The dependence of wear resistance on hardness and toughness of steels is clarified, the wear maps are composed. In combined abrasive/impact wear conditions, composite materials/coatings with multimodal reinforcements show promising results in increasing of abrasive wear resistance of rapidly wearing wear parts.

The formation of nanoscopic ripple patterns on top of material surfaces has been reported for different materials and processes, such as sliding against polymers, high-force scanning in atomic force microscopy (AFM), and surface treatment by ion beam sputtering. In this work, we show that such periodic ripples can also be obtained in prolonged reciprocating sliding against nanocrystalline diamond (NCD) films. NCD films with a thickness of 0.8 µm were grown on top of silicon wafer substrates by hot-filament chemical vapor deposition using a mixture of methane and hydrogen. The chemical structure, surface morphology, and surface wear were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and AFM. The tribological properties of the NCD films were evaluated by reciprocating sliding tests against Al2O3, Si3N4, and ZrO2 counter balls. Independent of the counter body material, clear ripple patterns with typical heights of about 30 nm induced during the sliding test are observed by means of AFM and SEM on the NCD wear scar surfaces. Although the underlying mechanisms of ripple formation are not yet fully understood, these surface corrugations could be attributed to the different wear phenomena, including a stress-induced micro-fracture and plastic deformation, a surface smoothening, and a surface rehybridization from diamond bonding to an sp 2 configuration. The similarity between ripples observed in the present study and ripples reported after repeated AFM tip scanning indicates that ripple formation is a rather universal phenomenon occurring in moving tribological contacts of different materials.

The main goal of the present work was to study the abrasive wear behaviour of different wear resistant steels, hardmetals/ cermets and advanced hardmetal particle reinforced PTA-hardfacings. The following wear resistant steels were under the study: (a) hardenable B-steel, (b) hardened steels Hardox 400...600 and as reference carbon steel C45. Different WC-Co hardmetals and TiC-NiMo cermets were studied. Experimental PTA-welded hardfacings consist of Ni- And Fe-based self-fluxing alloy and Cr-Ni-steel matrices and recycled WC-Co hardmetal as reinforcements were investigated. As reference hardfacing NiCrSiB + WC/W2C composition was studied. The abrasive wear tests of steels, hardmetals and hardfacings were performed using two abrasive wear methods: (a) abrasive rubber-wheel wear according to ASTM standard G65 and abrasive impact wear (b). The dependence of wear resistance on hardness and toughness of steels is clarified, and recommendations for steels selection for realistic wear conditions and applications are proposed. However, in extreme abrasive/impact wear conditions, produced hard particle reinforced hardfacings, have shown promising results in increasing of wear resistance of rapidly wearing wear parts.

Composite powder coatings (65/75 vol.% NiCrSiB self-fluxing alloy + 35/25 vol.% TiC-NiMo/Cr 2 C 3 -Ni cermet) were obtained by high velocity oxy-fuel spraying (HVOFS). Despite a 1.1 – 1.2 increase in microhardness, cermet particles reinforced coatings demonstrated a 1.1 times lower hardness and a 4.2 – 4.5 times higher wear in comparison with the unreinforced one. The wear mechanism changed from adhesive wear to surface fatigue and abrasive wear with addition of cermet particles.

Main focus was on the deposition of carbon nanofibers (CNFs) onto the hard nanocomposite (nc-Ti1 − xAlxN)/(a-Si3N4) (nACo®) coating surface and the investigation of the structure and tribological properties of CNFs. The alcohol chemical vapor deposition (ACCVD) method was employed to prepare CNFs and the deposition temperatures were 600 and 700 °C, respectively. Prior to the CNF deposition, Ni catalyst was deposited onto the nACo® surface using the magnetron sputtering. The influence of the deposition temperature on the carbon nanofibers structure was investigated by Raman spectroscopy and scanning electron microscopy (SEM). The higher order degree of CNF structure is observed with increasing deposition temperature. Tribological tests were carried out under fretting contact conditions against Al2O3 ball. It is shown that the coefficient of friction (COF) decreases from 1.0 to 1.2 for the clean nACo® surface to 0.2–0.4 for the CNF layers deposited on the nACo® surface. The roughness of the nACo® surface was varied and a higher durability of the CNF layers deposited on the rougher nACo® surface is found.