Search

Purpose This paper aims to present a comprehensive perspective on how tribology and sustainability are related and intertwined and are linked to CO2 emissions. This paper emphasizes on how tribological aspects affect everybody’s life and how tribological research and progress can improve energy efficiency, sustainability and quality of life. Design/methodology/approach Based upon available data and predictions for the next 50 years, the potential of tribological research and development is addressed. Findings The effects of tribological design can significantly increase energy savings and reduce CO2 emissions. Taking advantage of tribological technologies and applying them to current infrastructure would have the largest energy savings coming from the transportation and power generation at 25% and 20%, respectively. Implementing these technologies can also cut down global CO2 emissions by about 1,460 megatons of CO2 per year in the immediate future and 3,140 megatons of CO2 per year in the long term. The extraction and processing of resources inevitably generates CO2. Doubling the lifetime of machine components and the use of circular economy reduces the material footprint with associated reductions in CO2. Originality/value This perspective summarizes concisely the interrelation of tribology and sustainability with CO2. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-09-2020-0356/

© 2019 The Author(s) Basics about physical, mechanical and tribological properties of straight NbCs, including paths to tailor these properties, were layed down in Metal Powder Report, Vol. 71(4), 2016 and Vol. 71(5), 2016. The object is now to give an up-date on the progress in the key properties, like HV30, K IC and strength, followed by machining and tribological results of the respective NbC grades. The sales of niobium carbide (NbC) have grown in the last years from ∼350 tons p.a. to 500 tons p.a. recently, but NbC is still a hidden carbide and mainly used as grain growth inhibitor in hard metals. Another driver is the intention of hardmetal producers and downstream manufacturers to substitute tantalum carbide (TaC) by NbC in order to compensate the increased cobalt prices, which likely will persist on midterm. ispartof: Metal Powder Report vol:74 issue:2 pages:82-89 status: published

Self-mated magnesia stabilized zirconia (Mg-PSZ) ceramic sliding couples have been investigated at 100 N load (P0max= 1324 MPa) in oscillating sliding conditions in different humidity conditions in air and in hot steam. Temperatures have been varied up to 400 °C and pressures up to 6 bars. The results show that the wear behaviour of MgO-ZrO2 under high Hertzian contact pressures is strongly dependent on temperature and is similar for both dry oscillating and oscillating in hot steam. However, although the evolution in wear rates on temperature is similar and the wear rates of MgO-ZrO2 plunged above 300 °C in hot steam and air by nearly three orders of magnitude, SEM micrographs revealed in hot steam at 400 °C smooth wear tracks. In contrast, hot steam enhanced the tribochemistry of self-mated alumina couples and reduced wear rates. Hot steam decreased the coefficients of friction of MgO-ZrO2 with increasing temperature, but not the wear rates.

Cet article fait suite a une enquete de la Societe allemande de tribologie menee en 2014 visant a determiner les activites liees a la tribologie au sein des universites allemandes. D'eminents experts du secteur automobile et de l'industrie de la lubrification ainsi que des instituts de recherche rendent compte de l’etat des lieux des activites tribologiques en Allemagne et donnent un apercu de l'avenir de la tribologie. Cette etude tend a demontrer dans quelle mesure la recherche et les developpements dans le secteur de la tribologie peuvent contribuer au changement d’orientation de la politique energetique, a la preservation des ressources naturelles et a la reduction des emissions polluantes.

The use of materials for tribological applications in hot steam environment becomes more important in the future. Unfortunately, experimental data recorded under hot steam conditions are scarce mainly due to suitable testing equipment. This work examines the wear behaviour of α-aluminium oxide by combining thermodynamic modelling with advanced wear testing as well as analytical methods to get a better understanding of this structural ceramic materials wear behaviour and its possible use in high temperature steam environment. Self-mated α-alumina sliding couples have been investigated under oscillating sliding conditions in air, in water and in hot steam up to temperatures of 300 °C and ambient pressures up to 4 bars. The applied normal load was 100 N corresponding to an initial contact pressure of 3.1 GPa. The results show that the wear behaviour is directly dependent on the amount of water in the environment. Two competing wear mechanisms are identified. Following the fragmentation of alumina grains, abrasion is predominant in the low water vapour regime, i.e. dry sliding in laboratory air. Tribochemical wear is predominant, if enough water is available to form aluminium hydroxide and a gel-type reaction layer, i.e. when sliding in water or in steam. This gel layer has been confirmed as a possible explanation by electron backscattered diffraction. However, severe fracture and fragmentation of grains is observed which is attributed to initial wear at high Hertzian contact pressure during the running-in period. Thermodynamic calculations for identification of the possible phase equilibria support the experimental finding by Raman spectroscopy that only diaspore as crystalline hydroxide phase is present in the wear scar.

NbC-xTi (C0.7N0.3)-10Ni-7.5VC (vol%) based cermets with 0, 5, 10, 15 or 25 vol% Ti (C0.7N0.3) were prepared by conventional pressureles liquid phase sintering at 1420°C in vacuum. Detailed microstructural investigation was performed by SEM, EPMA and XRD analysis. Sintering results indicated that the partial replacement of NbC by Ti (C0.7N0.3) had a significant effect on the carbide grain growth, microstructure, hardness as well as fracture toughness of the fully densified NbC-based cermets. The Ti (C0.7N0.3)-free NbC cermet was composed of homogeneous cubic (Nb,V)C solid solution grains, whereas core-rim structured NbC grains were observed in cermets with Ti (C0.7N0.3) addition. All sintered cermets with  15 vol% Ti (C0.7N0.3) were composed of a fcc solid solution Ni binder and a cubic core-rim solid solution (Nb,V,Ti)C phase with a Nb-rich core and a Ti-rich rim. 3.8 vol% of residual pristine Ti (C0.7N0.3) was present in the cermets with 25 vol% Ti (C0.7N0.3) addition. The 15 vol% Ti (C0.7N0.3) starting powder based cermet exhibited the finest average NbC grain size of 1.48 μm, with a core-rim structure and an interesting combination of hardness (1486 kg/mm2) and fracture toughness (8.7 MPa.m1/2).

The current study reports on the effect of the sintering temperature and secondary carbide (VC, Mo2C and TiC) and Mo additions on the NbC grain growth, microstructure evolution as well as concomitant 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 backscatter diffraction (EBSD), electron probe microanalysis (EPMA) as well as X-ray diffraction (XRD) analysis. Sintering results indicated that the sintering temperature, especially secondary carbide additions had a significant effect on the properties of NbC-Ni cermets. Nickel pools and residual porosities were observed in the cermets sintered at temperatures ≤ 1340 °C. Increasing of the sintering temperature above 1380 °C resulted in fully densified NbC-Ni based cermets composed of homogeneous composition cubic NbC grains for the single carbide (VC or Mo/Mo2C) modified system, whereas core-rim structured NbC grains were observed with the addition of TiC + VC or TiC + Mo2C. The secondary carbide doped cermets with 5–10 vol% VC/Mo2C and 10 vol% TiC showed a clearly improved hardness and fracture toughness, as compared to the plain NbC-Ni cermets.

Niobcarbid (NbC) stellt aufgrund vorteilhafter Eigenschaften, wie zum Beispiel eine geringe Diffusionsneigung, in der Zerspanung eine vielversprechende Alternative zu konventionellen Schneidstoffen wie Wolframcarbid (WC) dar. Als Substitution der Hartstoff-Phase WC im Hartmetall wird NbC in der trockenen Drehbearbeitung von eisenbasierten Werkstoffen untersucht. Diesbezüglich wurden zwei NbC-Substrate mit WC hinsichtlich ihres Werkzeugverschleißes quantitativ verglichen. Im Gegensatz zu WC-Werkzeugen wird das Potential von NbC bei zunehmender Schnittgeschwindigkeit vc durch eine steigende Prozessstabilität deutlich. Due to its advantageous properties, such as a low diffusion tendency, niobium carbide (NbC) represents a promising alternative to conventional cutting materials such as tungsten carbide (WC) in machining operations. As a substitution of WC in carbide substrates, NbC is tested in dry turning of iron-based materials. In this regard, two different NbC substrates are compared to WC. In contrast to WC tools NbC shows its potential of an increased process stability at higher cutting speeds vc.

Downsizing (power-to-weight ratio) and higher speeds lead to a rise in Hertzian contact pressures in combination with an increase in surface or oil temperatures. Under such conditions, commonly used bearing steels, such as 100Cr6, reach their limits, creating a demand for alternative slip-rolling resistant steel alloys. The present work therefore compares the slip-rolling performance of various steel types with Maraging- and PM-type steel alloys such as e.g. CSS-42L™, ASP2012, BIMAX42+, in the Hertzian contact pressure range up to P0max of 4 GPa. Through-hardened 100Cr6H (AISI 52100), case-hardened 20MnCr5 (AISI 5120H) and nitrogen alloyed Cronidur30 (AMS 5898) still compete in terms of slip-rolling and wear resistance and load carrying capacity, whereas Maraging- and PM-type steel alloys offer superior strength and toughness properties.

The current study reports on the influence of the addition of 5–15 vol% VC or/and Mo 2 C 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/Mo 2 C 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 (HV 30 ) of 1077 ± 22 kg/mm 2 and an indentation toughness of 9.1 ± 0.5 MPa·m 1/2 . With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm 2 , whereas the toughness increased to 11.3 ± 0.1 MPa·m 1/2 . Addition of 5 and 10 vol% Mo 2 C into a NbC-12 vol% Ni mixtures generated the same values in HV 30 and K IC 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% Mo 2 C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m 1/2 , 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.

Les biolubrifiants sont definis a l´echelle internationale, ou un certain consensus sur des criteres ecotoxicologiques clefs voit le jour. Les lois et decrets ont permis de reduire les risques pour l'environnement, de proteger les ressources naturelles et d´ameliorer la qualite des eaux. Cependant, des lubrifiants « neutre a l'environnement » existent et ont acquis une petite part du marche des lubrifiants. Cet article compare les differentes exigences ecotoxicologiques avec leurs evolutions historiques, analyse les proprietes des differentes familles de biolubrifiants et detaille leurs differentes applications.

High temperature tribology is considered to begin from a minimum temperature of 300–350 °C, where organic base oils and polymers begin to decompose, until a temperature of 1000 °C. In this field of tribology, tests are typically run under dry or solid-state friction, unless a solid lubricant is used, since most lubricants will oxidize or break down when exposed to these extreme temperatures. Therefore, this form of tribotesting is useful to determine the friction, wear, and other tribological characteristics of coatings, ceramics, alloys, cermets, and similar materials. Additionally, high temperature tribology is important to further understand the frictional interactions and adhesive behavior of contacts that operate at these high temperatures. When considering measurements of the tribological parameters in a high temperature application, the standard Schwingung, Reibung, Verschleiž (SRV) (Oscillating, friction, wear, in English) reciprocating, linear-oscillatory tribometer can be modified for testing temperatures of up to 1000 °C by using a high temperature heating block. With this configuration, the instrument can accurately monitor many parameters of the tribosystem, such as coefficient of friction, electrical resistance, zero stroke point, sliding speed, and others. As a result, the SRV instrument is shown to be a powerful tool for high temperature tribotesting. This paper will provide an overview of this high temperature tribology test rig and will discuss its versatility and efficacy, and will show how it can effectively be implemented in both research and practical applications for the development of various coatings and other high temperature tribological contacts.

The current study reports on the effect of the addition of Al metal and secondary refractory carbides on the microstructure and mechanical properties of Ni bonded NbC matrix cermets. Powder mixtures were pressurelessly sintered for 1 h at 1420°C in vacuum. Microstructural and elemental mapping of the sintered cermets were performed by electron probe microanalysis (EPMA) to reveal the effect of the additions on the NbC grain morphology, grain growth and binder composition. Results indicated that NbC grain growth was suppressed and a homogeneous NbC grain size distribution was obtained in the cermets with the addition of Al and secondary carbides, i.e. Mo2C + VC + WC, as compared to the pure Ni binder. The liquid phase sintered NbC-12 vol% Ni cermet had a Vickers hardness (HV10) of 1130 ± 22 kg/mm2 and indentation toughness of 11.8 ± 0.4 MPa m1/2. With the addition of 4 vol% WC-4 vol% VC-4 vol% Mo2C, the hardness increased to 1490 ± 15 kg/mm2, whereas the corresponding toughness decreased to 9.2 ± 0.4 MPa m1/2. Addition of 4 vol% Mo2C into WC-13.8 vol% Ni-52.3 vol% NbC mixture further increased the hardness to 1620 ± 19 kg/mm2 in combination with a moderate fracture toughness of 7.7 ± 0.2 MPa m1/2.

Downsizing in mechanical systems requires effective lubrication strategies to ensure that increased contact stresses do not cause critical material failure during operation. Additionally, eco-toxicological considerations are becoming increasingly important. In this regard, the goal of this investigation is to transfer the running-in phase into the final step of the mechanical finishing process through the targeted pre-conditioning of novel, high toughness steel bearings without thermo-chemical treatments and compare these to conventional, case-hardened steels. Pre-conditioning involved implementation of the ecologically sustainable, bismuth-based additives to generate chemically reactive tribofilms on slip-rolling contacts by using a formulation with a high concentration of tribofilm forming additive. Generated tribofilms were analyzed by Raman spectroscopy to elucidate their molecular composition and, ultimately, determine the reaction mechanisms of bismuth-based tribofilm formation. Tribofilm-protected samples were subjected to slip-rolling endurance testing in a factory fill engine oil without pre-conditioning additives to determine the influence of pre-conditioned tribofilms on friction behavior and wear performance. It was observed that pre-conditioned tribofilms from the bismuth-based additives were able to yield lower coefficients of friction (COF) and profilometric wear coefficients than for steels without pre-conditioning. Moreover, COF values under mixed/boundary conditions approaching and even less than 0.04 were achieved, thereby rivaling DLC-coated alloy equivalents.

Niobium carbide is a hardly explored carbide but its functional profile shows a high potential for wear protection and tribological applications, which are currently dominated by tungsten carbide. Surprisingly little information is available on niobium carbide (NbC). Niobium carbide can be either synthesized by carbothermal conversion of Nb2O5 or be metallurgically grown and leached out. Furthermore NbC hardmetal grades can be bonded by all known metallic binders and processed and sintered in exactly the same way as WC-based hardmetals. Niobium is today largely available. NbC can be efficiently produced, provides comparably low friction in many relevant tribo-contacts and displays low wear. NbC and Nb2O5 have so far no REACH classification related to human toxicology and are not listed as substances of very high concern contrary to WO3 and Co3O4. This contribution demonstrates the key characteristics of NbC and discusses its sustainability and reliable value chain.

The origin of the intrinsic wear resistance of NbC-based materials is investigated through an assessment of the tribological performance of fully dense, crack-free spark plasma sintered Nb2O5 (here as a reduced polymorph: monoclinic Nb12O29 or NbO2.416). The most likely wear mechanism on NbC is the tribo-oxidation to Nb2O5. The unlubricated (dry) friction and wear behavior of alumina (99.7%) mated against rotating disks of crack-free niobium(V)oxide (Nb2O5) under unidirectional sliding (0.03–10 m/s; 22 °C and 400 °C) and oscillation (f=20 Hz, dx=200 mm, 2/50/98% rel. humidity, n=105/106 cycles) will be presented. The microstructure and mechanical properties of the crack-free Nb2O5 are assessed. The tribological data obtained are benchmarked with different NbC grades, ceramics, cermets and thermally sprayed coatings.

The goal of reducing CO 2 emissions in the automobile industry has led to the development of increasingly efficient lightweight material solutions that yield enhanced performance. In light of this goal, this current work involves the optimization of the pre-conditioning of novel, high toughness steel bearings without thermo-chemical treatments, with the aim of transferring the running-in phase into the final step of the mechanical finishing process. A case-hardened gear steel and two novel non-case-hardened steels were evaluated. Pre-conditioning was carried out by running cylindrical sample disks against tungsten carbide rollers on a twin-disk test rig to generate cold work hardening in the contact zone of the cylindrical disks. Subsequent analyses of the pre-conditioned samples indicated strong increases in localized hardness and stable compressive residual stresses. Cold work hardened sample disks were then run against untreated spherical counterbodies of identical alloy in slip-rolling endurance testing to evaluate changes in friction behavior and wear performance as a result of pre-conditioning. Initially, the non-case-hardened alternative alloys experienced premature critical material failure in endurance testing, which indicated that an optimization of pre-conditioning parameters would be necessary. Consequently, new tungsten carbide rollers with more gradual radius of curvature were implemented. Additionally, lubricant temperature and rotational speed were increased to optimize the material residual stress profiles. Endurance testing of optimally pre-conditioned samples showed that the optimization ultimately yielded strong reductions in both friction and wear: coefficient of friction values of under 0.04 were reached, rivaling alloy-equivalent DLC vs. DLC contacts, and wear coefficients of less than 1/10 of those for the untreated alloy pairings were achieved.

The effects of rapid pulse electric current sintering (PECS), substitution of WC by NbC and Co by Ni, and carbide additives (TiC and Mo2C) on the microstructure, elastic modulus, B3B transverse rupture strength (TRS) and high temperature sliding wear on WC-Co, WC-Ni, NbC-Co and NbC-Ni cermets were studied. Additions of x% Mo2C and y% TiC (where x and y were 14 GPa hardness and ~10 MPa.m1/2 fracture toughness (KIC) and ball-on-three-balls (B3B) TRS > 1600 MPa. The sintering techniques had negligible effect on the samples' elastic and shear modulus, and all WC-based samples had higher elastic modulus than all NbC-based samples (by ~120 GPa). High temperature sliding wear tests were carried out using a ball-on-disk tribometer, with a 10 N force, at a sliding speed of 1.34 m/s for 0.8 km (10 min) and 2.4 km (30 min), using 100Cr6 (AISI 52100) steel balls at 400 °C and 0% humidity. For the 2.4 km sliding distance, all the WC cermets had lower wear volumes than NbC cermets, with LPS WC-0.5Cr3C2-10Co having the lowest wear volume. Additions of TiC and Mo2C to NbC-12Ni improved the sliding wear resistance, with TiC having the greater effect, reducing the sample wear rate by over 30% from 15.1 × 10−6 mm3/N·m to 9.4 × 10−6 mm3/N·m after sliding distance of 2.4 km. Generally, the LPS samples had lower wear volumes than the corresponding SPS samples, due to higher K1c and TRS.

An attempt to improve the machining performance of NbC-Ni cutting inserts by rapid pulse electric current sintering (PECS), TiC and Mo2C additions and laser surface modification (LSM) was done. Use of a nickel binder and additions TiC and Mo2C to liquid phase sintered (LPS) NbC based samples led to comparable hardness (>13 GPa) and KIC (~10 MPa.m1/2) to LPS WC-Co/Ni samples. The laser surface modification (LSM) technique produced a ~2.5 μm thick self-carbide coating, increasing the surface hardness of all the samples. Laser surface modification was done to improve abrasion and attrition wear resistance. Face-milling of grade 17 grey cast iron (BS 1452/GG35) was conducted at 100–500 m/min cutting speeds (vc) and 0.25–1.5 mm depths of cut (ap). The insert wear was measured after every pass, and analyzed by annular dark field scanning transmission electron microscopy (ADF-STEM). During roughing, WC-Co based inserts had the lowest flank wear rate (FWR) values, with the WC-10Co (LPS) insert having a FWR of 10.15 μm/min after 20 min cutting time. However, during semi-finishing and finishing, NbC-4TiC-12Ni (PECS) and NbC-4Mo2C-4TiC-12Ni (PECS) inserts had the lowest FWR values, showing up to six times longer tool life than the WC-Co (LPS) inserts based inserts and 12 times longer life than the WC-Ni based inserts. Generally, LSM improved the NbC inserts' tool life, reducing the FWR values in all NbC based inserts in all cutting tests.

Under unidirectional sliding, the Stribeck-type curve traverses different lubrication regimes. Under mixed/boundary conditions, tribofilms formed by additives in lubricants represent a key mechanism for preventing wear but are difficult to investigate because of the complex composition of lubricants, their small thickness, and their amorphous nature. The formation of a hydrodynamic fluid film depends on the topography and the viscometric fluid properties. Low coefficients of friction do not consequently indicate that hydrodynamic fluid films were formed. The electric contact resistance serves here as a complementary indicator to identify the hydrodynamic fluid film and tribofilm formation under transient or static sliding conditions. The frictional work (WR) is a useful quantity to assess the contribution to the frictional losses of all lubrication regimes in a Stribeck-type curve.

Carbidic Austempered Ductile Iron (CADI) microstructures containing eutectic carbides can be produced by the addition of carbide stabilizing elements, such as chromium. Carbides formed from the addition of Cr are eutectic of M3C type. The presence of such hard phases can enhance the abrasion wear resistance of ductile iron. A new CADI can be produced by the addition of Nb. Niobium carbide particles are formed in the beginning of solidification and remain stable once they are insoluble in solid iron matrix. The dry sand abrasive wear resistance of ductile irons alloyed with 1.0, 1.8, and 2.4 wt% Nb were tested in both “as-cast” and “heat treated” conditions using standard ASTM G65. Results were compared to abrasive wear data obtained on ductile iron alloyed with 1 wt% Cr, CADI (1 wt% Cr), and the basic composition of iron without carbide stabilizing elements. In the “as-cast” condition, the addition of Nb did not lead to a reduction in wear, while CADI with Nb is a promising substitute for CADI with Cr addition, because both materials showed very similar values of abrasion resistance. Micro-ploughing and micro-cutting mechanisms were observed on the worn surfaces of ductile irons. Abrasive wear resistance of these alloys was correlated with the volume fraction of carbides.

Sub-stoichiometric oxides formed by tribo-oxidation were often analyzed in tribofilms, like γ-Ti3O5, Ti5O9 and Ti9O17 and Mo0,975Ti0,025O2 as well as double oxides, like β-NiMoO4 or NiTiO3. Thus, the contribution of sub-oxides with planar defects (TinO2n-1, Tin-2Cr2O2n-1) or as block-structures (Nb3n+1O8n-2) to the tribological profile of carbides by using monolithic oxides is of scientific interest and has model character. The tribological profiles of these model oxides under dry unidirectional sliding have shown, that sub-oxides have a contribution to the tribological behavior of carbides and cermets, when they are tribo-oxidatively formed, because their tribological profiles as monolithic materials are homologuous in part or totally, or compete with hardmetals or cermets, depending from the operating conditions regarded.

Magnéli-type vanadium oxides form the homologous series VnO2n-1 and exhibit a temperature-induced, reversible metal–insulator first order phase transition (MIT). We studied the change of the adhesion force across the transition temperature between the cleavage planes of various vanadium oxide Magnéli phases (n = 3 … 7) and spherical titanium atomic force microscope (AFM) tips by systematic force–distance measurements with a variable-temperature AFM under ultrahigh vacuum conditions (UHV). The results show, for all investigated samples, that crossing the transition temperatures leads to a distinct change of the adhesion force. Low adhesion corresponds consistently to the metallic state. Accordingly, the ability to modify the electronic structure of the vanadium Magnéli phases while maintaining composition, stoichiometry and crystallographic integrity, allows for relating frictional and electronic material properties at the nano scale. This behavior makes the vanadium Magnéli phases interesting candidates for technology, e.g., as intelligent devices or coatings where switching of adhesion or friction is desired.

The aim of this work was to correlate the overall carbon content in NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo starting powders with the resulting microstructure, hardness, and fracture toughness of Ni-bonded NbC cermets. A series of NbC-Ni, NbC-Ni-VC and NbC-Ni-Mo cermets with different carbon content were prepared by conventional liquid phase sintering for 1 h at 1420 °C in vacuum. Microstructural analysis of the fully densified cermets was performed by electron probe microanalysis (EPMA) to assess the effect of carbon and VC or Mo additions on the NbC grain growth and morphology. A decreased carbon content in the starting powder mixtures resulted in increased dissolution of Nb, V, and Mo in the Ni binder and a decreased C/Nb ratio in the NbC based carbide phase. The Vickers hardness (HV30) and Palmqvist indentation toughness were found to decrease significantly with an increasing carbon content in the Mo-free cermets, whereas an antagonistic correlation between hardness and toughness was obtained as a function of the Mo-content in Mo-modified NbC cermets. To obtain optimized mechanical properties, methods to control the total carbon content of NbC-Ni mixtures were proposed and the prepared cermets were investigated in detail. ispartof: Metals vol:8 issue:3 pages:1-13 status: published

The influence of friction and wear on the function and structure of tribological systems is determined by various types of tribological tests. This article introduces the general categories of tribological testing and describes the basic objectives of testing. It reviews the results of tribological tests, where the system-dependent characteristics of friction and wear data can be expressed in different forms, such as tribographs, transition diagrams, and tribomaps. A summary of various methods of surface analysis is presented in a table. The article discusses the relationship between wear and reliability in terms of exponential distribution, Weibull distribution, and gamma distribution. It concludes with information on the effects of interaction on failure probability.

The running-in phase of mechanical systems is critical from a tribological standpoint, though poorly understood. Microcracks accelerate material failure and wear during this phase of heightened friction. With this in mind, the ultimate goal of this current work is to transfer the running-in phase into the final step of the mechanical finishing process through the targeted pre-conditioning of novel, high toughness steel bearings without thermo-chemical treatments and compare these to conventional, case-hardened steels. This pre-conditioning involved the targeted implementation of two specific lubricant packages, the first with CaCO 3 as the active ingredient and the second with MoDTC as the active ingredient, to generate chemically reactive tribolayers (tribofilms) on twin disk testing rigs. Pre-conditioning was carried out up to 10 4 load cycles (approximately 25 min). The films generated in pre-conditioning were analyzed by SEM-EDX with Element-Mapping, Raman spectroscopy, and XPS to elucidate their molecular composition and concentration on the sample surfaces. The combination of these methods of analysis gave a clear indication that 10 4 cycles were sufficient to generate stable chemical tribofilms. CaO and CaCO 3 were the main components of the tribofilm from the first lubricant package, while MoS 2 , MoO 2 and MoO 3 were the main components from the second lubricant package. Tribofilm-protected samples were then subjected to slip-rolling endurance testing ( T =+120 °C, 10,000,000 cycles, approximately 19 days in a factory fill engine oil) to determine any changes in friction behavior or wear performance. Some significant reductions in coefficients of friction at the end of endurance testing were observed, though in certain cases, no definitive improvement was observed. In contrast, very strong reductions in wear were observed across the entire spectrum of materials and testing loads. In some cases, sample surface wear reduction from pre-conditioning via tribofilms reached over 90%. The observed improvements to friction behavior and wear performance are indicative of a technically simple, cost- and energy-efficient pre-conditioning method that may prove to be competitive with existing thermochemical treatments for steel alloys.

The automotive industry places significant importance on downsizing components to achieve greater efficiency. The goal of reducing CO 2 emissions has led to the development of lightweight materials that are also able to enhance performance. In light of these aspirations, the aim of this study is to characterize two novel, high-performance steels, as well as a “classical” gear steel for comparison with each other and currently applied materials. The gear steel underwent carburization and subsequent deep freezing treatments in an attempt to yield discrete sample groups with respect to residual austenite. The high-performance steels were heat treated as recommended by their respective manufacturers, and were not carburized. Elemental analyses were conducted by multiple methods to ensure accurate results. Residual austenite contents of the steels and the depth profiles of residual stresses were determined by X-ray diffraction (XRD). Hardness profiles were taken from the testing surfaces into the material core. The carburization of 20MnCr5 led to higher hardness and the greater concentration of carbon in the carburization zone more representative of a hardened SAE E52100, or 100Cr6/102Cr6, than of a non-carburized 20MnCr5. Residual austenite contents ranging from approximately 6–14 vol% were generated, though effectively providing only two, rather than the desired four discrete sample groups. Residual stresses from machining and carburization were measured directly at the sample surface, and from carburization alone below the surface. The high-performance steels fulfilled manufacturer expectations in terms of elemental content, hardness between 50 and 55 HRC and strongly martensitic microstructure character. Finally, slip-rolling endurance testing ( T =+120 °C, 10,000,000 cycles, approximately 19 days in a factory fill engine oil) was carried out on all materials, whereby coefficient of friction distributions during testing and wear coefficients after testing were calculated. Testing was performed up to and including P 0Mean =1.94 GPa ( P 0Max =2.91 GPa, F N =2000 N). Ultimately, the non-carburized high-performance steels showed competitive wear performance and better friction behaviour than the carburized 20MnCr5, which has been attributed to their work hardening capability.

The tribological profile of alumina (99.7%) mated against rotating disks made in binder-less niobium carbide (NbC) and cobalt-bonded NbC were determined under unidirectional sliding tests (0.1 m/s to 8.0 m/s; 22 °C and 400 °C) as well as in oscillation tests (f = 20 Hz, Δx = 0.2 mm, 2/50/98% rel. humidity, n = 105/106 cycles) under unlubricated (dry) conditions. In addition, the microstructure and mechanical properties of binderless NbC and NbC bonded with 8% cobalt were determined as well. The reason for testing hot-pressed NbC was to avoid side effects generated by sintering additives and/or second phases. The tribological data obtained were benchmarked with different ceramics, cermets and thermally sprayed coatings. NbC and cobalt-bonded NbC exhibited low wear rates under dry sliding associated with high load carrying capacity. The tribological profile established revealed a strong position of NbC bearing materials under tribological considerations and for closed tribo-systems against traditional references, such as WC, Cr3C2 and (Ti,Mo)(C,N).

3 different base-stocks (poly-α-olefin, polypropylene glycol and unsaturated TMP ester) and additionally formulations with 2 different additives (1 mass% ZnDTP or 1 mass% dibenzyl disulfide (DBDS)) were used as lubricants to evaluate the tribological properties of A6061-T6 treated by means of plasma electrolytic oxidation in a SRV tribometer (Toil = 80°C, FN = 50 N, ν = 50 Hz, 2 mm stroke, 2 hrs). The base oils of PPG and the TMP-ester had better lubricities than the PAO with/without the additives. ZnDTP tend to reduce wear, but when added to PAO and PPG increased the friction. The unsaturated TMP-ester +1 mass% DBDS showed 45% lower friction than PAO + ZnDTP (reference), whereas PPG + DBDS had 30% lower friction.