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- Wear (11)
- Friction (9)
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- Niobium carbide (NbC) (5)
- NbC (4)
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Eingeladener Vortrag
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The differences between the binary phase diagrams W-C and Nb-C illuminate several parameters for tailoring the properties of NbC. In the region of homogeneity of NbCx, with Nb4C3 and Nb6C5 other Niobium carbide phases occur. Properties, like micro-hardness, hot hardness, sliding wear, elastic modules and toughness can be tailored by the C/Nb Ratio, secondary carbides and the type of binder. Supporting results from different grades with varying C/Nb ratio or binder types will be illuminated. Thus, the NbC system offers to producers a wider parameter window, than WC.
The current study reports on the influence of the Addition of 5–15 vol% VC or/and Mo2C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo2C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents.
The liquid phase sintered NbC-12 vol% Ni cermet had a modest Vickers hardness (HV30) of 1077 ± 22 kg/mm2 and an indentation toughness of 9.1 ± 0.5 MPa·m1/2. With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm2, whereas the toughness increased to 11.3 ± 0.1 MPa·m1/2. Addition of 5 and 10 vol% Mo2C into a NbC-12 vol% Ni mixtures generated the same values in HV30 and KIC when compared to VC additions. A maximum flexural strength of 1899 ± 77 MPa was obtained in the cermet with 20 vol% Ni binder and 4 vol% VC+4 vol% Mo2C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400 °C from 0.1 to 10 m/s.
The aim of this research is to characterize a surface that has experienced low wear on asperities. The procedure used is similar to an Abbott-Firestone approach and based on functional bearing of projected area, surface area and material volume. Surface features extracted from surface height maps provide more comprehensive information about topography changes due to wear. Thus, they were used to detect low wear volumes on the top of asperities.
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2, alumina and antimony impregnated carbon.
Tribology in hot steam
(2017)
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2, alumina and antimony impregnated carbon.
The cause and type of solid particle erosion varies across different industries and locations in plants, for instance the particles could be volcanic ash in aero-engines, fly ash in boilers, exfoliated scale in steam turbines or mineral matter in oil excavation. In all cases the durability of materials can be improved through better surface engineering and coatings, but the development of these is restricted also due to lack of generic models. In this respect, the simulation of a single impact with finite element methods using 100.000 knots would take about 15 minutes. In order to simulate an entire particle erosion experiment, up to one billion of such impacts would have to be considered. In order to simulate an entire particle erosion experiment, the current work presents an alternative approach capable of calculating about half a billion impacts on a substrate in less than six hours on a standard PC. The predictions of the simulation will be compared to a steel sample at 600°C eroded by aluminum oxide particles with a velocity around 210 m/s.
Niobium is today largely available. The initial NbC grade was substoichiometric, SPS sintered and cobalt bonded. The NiMo-bonded stoichiometric NbC1.0 grades enable the substitution of cobalt by nickel, SPS by conventional sintering and NbC0.88 by NbC1.0 in view of functional properties. Nickel bonded NbC grades have improved toughnesses versus cobalt NbC grades, but lose hardness. NiMo and NiMo2C bonded NbC1.0 grades compensated the loss in hardness while keeping the toughness.
Niobium is today largely available. The initial NbC grade was substoichiometric, SPS sintered and cobalt bonded (NbC0.88-12Co SPS). The NiMo-bonded stoichiometric NbC1.0 grades enable the subtituion of cobalt by nickel, SPS by conventional sintering and NbC0.88 by NbC1.0 in view of functional properties. Nickel bonded NbC grades have improved toughnesses versus cobalt bonded NbC grades, but lose hardness. NiMo and NiMo2C bonded NbC1.0 grades compensated the loss in hardness while keeping the toughness.
The frictional response of molybdenum bis(C11-14 branched and linear alkyl) carbamodithioate oxo thioxo complex (Mo-dttc) versus bismuth dimethyldithiocarbamate (Bi-dtc) and bismuth dodecylbenzene Sulfonate (Bi-ddbsa) slip-rolling under mixed/boundary lubrication (T=120°C, n=10 millions of cycles, P0max=2.25 GPa) in PAO-based formulation against three different steel alloys were compared.
The tribotesting categories have different scopes with associated "pros" and "cons" and results obtained in one category cannot be simply transferred to another. The impact of design overrules in general the influence of the metallurgy or materials choice. The "cheap" test methods aim pre-screening and assessment of basic properties as well as use mainly for the test specimens SAE E52100 (100Cr6, SUJ2) ball bearing steel and carbon steels. The main test geometries are ball-on-flat/(disk) or block-on-ring apart 4-ball geometry. The tribological test conditions. This enabled "dynamic" testing, where during a test the "critical" solicitations were repeatedly and freely passed.
The slip-rolling 2Disk tribometer from Optimol Instruments represented the technological evolution in tribotesting of the well-known AMSLER-type tribometer marketed in 1922. Both use two discs with the same outer diameter rolling against each other on their circumference. In the AMSLER, the lower disc dips into an oil reservoir, whereas in the 2Disk the oil is injected into the contact. The oil supply of the 2Disk is controlled by pump and temperature by a heat exchanger.
The current study reports on the effect of the sintering temperature and secondary carbide (VC, Mo2C and TiC) additions on the microstructure and concommitant Vickers hardness (HV30) and fracture toughness of Ni-bonded NbC cermets. All cermets were prepared by pressureless sintering in vacuum. Detailed microstructural investigation was performed by electron probe microanalysis (EPMA) and X-ray diffraction (XRD) analysis. Sintering results indicated that both the sintering temperature and secondary carbide additions had a significant effect on the properties of NbC-Ni cermets. Nickel pools and residual pores were observed in the cermets sintered at temperatures ≤ 1340 °C. Increasing of the sintering temperature up to 1420 or 1480 °C resulted in fully densified NbC-Ni based cermets composed of homogeneous contrast cubic NbC grains for the single carbide (VC or Mo2C) modyfied system, whereas core-rim structured NbC grains were observed with the additon of TiC + VC or TiC+Mo2C. The secondary carbide doped cermets with 5–10 vol.% VC/Mo2C and 10 vol.% TiC showed a significantly improved hardness and fracture toughness, as compared to the plain NbC-Ni cermets.