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This paper addresses the relation between mechanical properties and tribological behaviour of self-lubricating steels produced by in-situ dissociation of SiC. Literature shows that an increase in t...

The production of self-lubricating composites containing second phase particles is one of the most promising choices for controlling friction and wear in energy efficient modern systems. Initially, we present a new microstructural model/processing route able to produce a homogeneous dispersion of in situ generated, discrete, solid lubricant particles in the volume of sintered composites. The high mechanical and tribological performances of the composites are a result of the combination of matrix mechanical properties and structural parameters, such as the degree of continuity of the metallic matrix, the nature, the amount, and the lubricant particle size and shape which determine the mean free path between solid lubricant particles and the active area covered by each lubricant particles. This new route was achieved by in situ formation of graphite nodules due to the dissociation of a precursor (SiC particles) mixed with metallic matrix powders during the feedstock preparation. Thermal debinding and sintering were performed in a single thermal cycle using a plasma-assisted debinding and sintering (PADS) process. Nodules of graphite (size ≤20 μm) presenting a nanostructured stacking of graphite foils with thickness of a few nanometers were obtained. Micro-Raman spectroscopy indicated that the graphite nodules are composed of a so-called turbostratic 2D graphite which has highly misaligned graphene planes separated by large interlamellae distance. The large interplanar distance and misalignment among the graphene foils has been confirmed by transmission electron microscopy and is, probably, the origin of the remarkably low dry friction coefficient (0.06). The effects of precursor content (0 to 5 wt% SiC) and of sintering temperature (1100 °C, 1150 °C and 1200 °C) on tribolayer durability and average friction coefficient in the lubricious regime (μ < 0.2) are presented and discussed. In addition, the effect of the metallic matrix composition (Fe-C; Fe-C-Ni; Fe-C-Ni-Mo) is presented. Friction coefficient decreased and durability drastically increased with the amount of graphite formed during sintering, whereas friction coefficient was little affected by sintering temperature. However, the durability of the tribolayer was greatly increased when lower sintering temperatures were used. The addition of alloying elements considerably reduced wear rate and friction of specimens and counter-bodies. Friction coefficient values as low as 0.04 were obtained for the Fe-C-Ni-Mo composites. We also analyzed the effect of precursor content and of sintering temperature on the tribological behavior under constant normal load sliding tests. Again, the presence of graphite nodules significantly reduced the friction coefficients and wear rates, whereas the sintering temperature hardly affected these parameters. The results were compared with those caused by other forms of graphite (nodules in nodular cast iron and powder graphite) and were discussed in terms of the crystalline structure of the analyzed graphite using micro-Raman spectroscopy. Chemical analyses of the wear scars using scanning electron microscopy (SEM – EDX) and Auger electron spectroscopy (AES) showed a tribolayer that was composed predominantly of carbon and oxygen. This tribolayer is removed and restored during sliding and is continuously replenished with graphite. Finally, the strong effect of surface finishing is presented and discussed.

As a major steel production technology, cold rolling processes have a strong influence on the final quality of rolled products. Traditionally, cold rolling mill rolls present a controlled surface topography that is imprinted onto the sheet during forming and are often coated, particularly using hard chrome deposition. However, wear of cold rolling rolls can affect tribological performance during rolling. Since the nature of tribological phenomena is highly systemic and evolve with sliding time, it is very challenging to reproduce wear in cold rolling rolls using laboratory tests. This work aimed to establish a methodology to reproduce wear mechanisms found in real cold rolling rolls. Due the high cost and dimensions of the rolls, the replication technique was used. Replicas were obtained from four major rolling mill Brazilian companies before and after a normal rolling campaign. The wear mechanisms in the replicas were evaluated using SEM and laser interferometry. To reproduce these mechanisms in laboratory, reciprocating sliding tests were used, which mimic well the change in the direction of friction force that occurs during rolling. The tests used spherical and cylindrical counter bodies under different normal loads and test times to verify which tribological conditions allowed to reproduce the wear mechanisms found in the replicas. These tests indicated the use of reciprocating sliding tests with cylindrical counter bodies in a line contact, where the evolution in wear can be assessed by tests with different test times. This methodology has a great potential to evaluate the tribological performance of different surface modifications used in cold rolling mill rolls.

In recent years, the ferritic stainless steels have been proposed as a competitive alternative option for the bio-fuel and concrete mixer industries. The goal of the present work is to evaluate, using a recently proposed new methodology, the effect of chromium content with and without stabilization (Ti and Nb) on the tribocorrosion of ferritic stainless steels. For comparative purposes, a carbon steel (0.15 wt% C) was also studied. A reciprocating motion tribometer associated with a potentiostat assured that the potential scan during the tests was used under a constant load and sliding velocity. To this end, a recently introduced approach was applied. It consisted of performing sliding tribological tests to strictly evaluate the mechanical wear, potentiodynamic corrosion tests to determine the corrosion resistance in the absence of mechanical wear, and tribocorrosion tests, which combine both mechanical wear and corrosion degradation from chemical/electrochemical effects. Concerning the tribological performance, the 16% Cr steel showed a higher friction coefficient and lower wear rate than the 11% Cr steel. Carbon steel, in turn, presented the lowest friction coefficient and the highest wear rate among all materials. The corrosion tests showed that a higher chromium content was associated with a greater corrosion resistance. The similarities of the wear mechanism between the tribocorrosion and the tribological tests were verified and it can possibly explain the obtained synergy (S), which was slightly positive (11% Cr (S = 0.009 mg/min), 11% CrTi steel (S = 0.031 mg/min) and 16 % Cr steels (S = 0.005 mg/min)), as the tribocorrosion worsened the wear resistance of the materials, although insignificantly. The exception is the carbon steel, whose synergy effect was drastically higher than all other materials with chromium content (S = 2.674 mg/min).

In this study, powder technology was used to obtain Fe-SiC composites in which SiC particles act as precursors to generate a large amount of turbostratic graphite dispersed in the composite matrix. The selection of the alloy composition was studied employing Thermo-Calc® software to obtain the temperature and composition range for the stabilization of the graphite phase in iron with a high yield. The extracted turbostratic 2D graphite particles were dispersed in mineral oil in order to evaluate the potential of these particles as a lubricating oil additive. The structure and morphology of the extracted graphite were examined by Raman spectroscopy and transmission electron microscopy (TEM), indicating the highly disordered nature of turbostratic graphite. Reductions in the friction coefficient and wear rate of a tribological pair were observed when compared to the pure mineral oil and mineral oil with commercial graphite particles added. The misorientation and increase in interplanar distances of turbostratic 2D graphite induce a low degree of interaction between these atomic planes, which contributes to the low-friction coefficient and the lower wear rate obtained for this system.

In the present work, we overview the multidisciplinary development of a regular, lubricated, hermetic compressor which works in an on-off cycle, circular motion, single-speed, many tribological contacts, into an innovative, linear motion, variable displacement, single tribological contact, oil-less hermetic compressor presenting high versatility in terms of refrigerator design, sustainability and improved efficiency. The original approach encompassed the development of new surface engineering procedures applying purpose-oriented phases to soft substrates. Particular emphasis is given to the in situ fluorination of the tribolayer formed in the piston-cylinder tribopair. Although chlorinated halocarbons (CFC) are efficient refrigerants from a thermodynamic point of view, they do have serious environmental implications that have forced the refrigeration industry to switch to more environmentally friendly hydrofluorocarbon (HFC) based refrigerants. The first and most successful alternative to the CFCs was tetrafluorocarbon, particularly tetrafluoroethane (CF3CH2F), aka R134a refrigerant. The current trend to downsizing mechanical systems, smaller clearances, and increased speeds leading to greater energy efficiencies associated with miscibility issues imposing the use of costly, fully synthetic lubricants with the R134a refrigerant gas led to the introduction of a new hermetic compressor design, the Wisemotion®, the first, and until now, unique, oil-free hermetic compressor on the world market. In this context different types of multi-layers, their thickness, substrate material, processing routes, etc., have been studied and optimized. Si-rich hydrogenated DLC (a:C-H) presented enhanced tribological properties when tested under fluorine-rich atmospheres, and semi-industrial scale tests have been carried out to understand this point further. A homemade tribological emulator was developed allowing close-to-real tribopair, atmosphere, and imposed mechanical conditions used in an oil-free commercial hermetic compressor. The tests were carried under different stroke frequencies (5, 20, and 40 Hz) and atmospheres (R134a, ambient air, and argon). Results showed a strong influence of both atmosphere and stroke frequencies. The friction coefficients were significantly lower (~3.8X) for the refrigerant gas atmosphere, attributed to the fluorine and highly disordered graphitic structures rich tribolayers. Under the high frequency (40 Hz), the energy input seems to be a deterrent to the formation of stable tribolayers, and the DLC coating shatters on the first few sliding meters.

In cold rolling, a textured roll can be used to imprint a desired surface topography onto the sheet during rolling. This work proposes the use of diamond-like carbon (DLC) coatings to protect the surface topography of the rolls in replacement of the carcinogenic hard chrome. For that, hydrogenated amorphous carbon (a-C:H) was deposited on plasma nitrided tool steel, both for ground and textured specimens. Changes in surface topography due to DLC coating were assessed using a confocal microscope. Coating adhesion was evaluated using the method VDI 3198. The specimens were characterized using X-ray diffraction (XRD), microhardness test and scanning electron microscopy (SEM). The coating was characterized using Raman spectroscopy (RS) and X-ray photoelectron spectroscopy (XPS). The results showed a soft multilayer coating consisting of a plasma nitrided layer for load support, a Si-rich interlayer to improve adhesion and an a-C:H top layer. DLC deposition reduced the roughness of the textured specimens. The coating resulted in relatively stable friction and good durability, with small damage and negligible wear even under dry sliding.

In the pursuit of suitable alternative substrate materials for DLC coatings, grey cast iron (GCI) and nodular cast iron (NCI) stand out due their vast industrial applications, consequence of its sufficient mechanical strength associated with low cost. However, graphite flakes and nodules introduce critical discontinuities on these metallic surfaces. In the present work, the effect of graphite morphology on the tribological behaviour of DLC (a-C:H) obtained via PECVD is analyzed. Scuffing resistance was accessed using dry reciprocating sliding tests, where GCI presented a friction coefficient of 0.06 during a lubricious regime whereas NCI presented μ=0.14. Thus, active friction reduction mechanisms were investigated using additional interrupted tests. The wear tracks were analyzed revealing how tribolayers morphology and chemical nature modified friction.

Consistent and concise literature about self-lubricating metallic composites containing MoS2 is very scarce in particular those analyzing the possible reactions during processing, which are often neglected or even unknown in a variety of publications. In some situations, the composites present a poor lubricating behavior not related to environmental parameters or surface properties, but due to the simple fact that the solid lubricant added is not there anymore, i.e. it has reacted with the matrix or other additives. This review compiled publications from 1945 up to 2017 about the research developments in metal matrix self-lubricating composites containing MoS2. Information on the tribological properties of such composites according to the varied matrixes, contents, processing conditions, testing temperatures and atmospheres are presented.

In this work, new carbide-derived carbons (CDCs) generated by the solid-state reaction between Fe and B4C were produced as nanostructured powders. Thermodynamic calculations were used to define the composition range and the process window for the reaction. Initially, samples with Fe + 5 wt%B4C were produced by traditional powder metallurgy techniques to analyse the reaction between the materials. As CDC nanoflakes were found, the amount of B4C was increased in order to improve the graphite productivity and an acid treatment was used to extract the nanoflakes. The results showed that it is possible to use this method to obtain powders of nanostructured turbostratic graphite in high quantity (yield of ∼87%). This is the first method ever presented that allows obtaining 2D turbostratic graphite nanoflakes derived from the reaction between Fe and B4C as powders with potential applications in tribology and other fields yet to be explored.

Recently, we presented the tribological evaluation of self-lubricating sintered steels produced by taking advantage of the powder injection molding process, the recently introduced plasma-assisted debinding and sintering process, and the in situ formation of solid lubricant particles. This new processing route promotes the in situ generation of nanostructured turbostratic graphite particles during silicon carbide dissociation. In this work, we present the influence of surface finishing on the tribological behavior of self-lubricating composites sintered at 1150°C with (3 and 5 wt%) and without SiC additions. We discuss the effects of the surface topography (Ra) on the friction coefficient and wear rates of specimens and counterbodies. The tribological behavior was analyzed using linear reciprocating sliding tests (constant load of 7 N, 60-min duration). It was shown that the reduction in surface roughness increased both the friction coefficients and wear rates of specimens and counterbodies, proba...

Purpose The progressive wear of cutting tools used in industrial cutlery production results in excessive burr formation and reduces tool service life. This paper aims to investigate the effects of the sheet surface finish on tool wear and service life during blanking. Design/methodology/approach Two alternative surface finish techniques were proposed and initially implemented under laboratorial conditions and compared with conventional acid pickling. Those surface finish techniques were then implemented on an industrial scale to improve the service life of cutting tools. Industrial blanking tests characterized the effect of sheet surface finish on tool life. Findings In the first technique, called skin pass, an additional mechanical pass under controlled conditions reduced the height of the surface peaks and resulted in partial embedding of the carbides into the surface. The second technique, called electrochemical pickling, removed solely the surface carbides, leaving behind a smoother surface without carbides. Real industrial blanking tests identified that the use of skin pass reduced burr formation and increased tool life by around 300 per cent when compared with conventional acid pickling. With electrochemical pickling, burr formation was further reduced and tool life increased further by 300 per cent when compared with skin pass. Research limitations/implications First, this work proposes an alternative surface finishing technique (electrochemical pickling) to be used after annealing of stainless steel. Second, the work clearly shows the presence of protruding surface carbides when conventional surface finishing techniques are used, which do not exist after acid pickling. Practical implications When electrochemical pickling is implemented on an industrial scale, the life of blanking tools is substantially improved. Originality/value Although the sheet surface finish is widely recognized to affect metalforming processes, the literature lacks studies on the effect of sheet surface finish on tool wear during blanking. First, this work proposes an alternative surface finishing technique (electrochemical pickling) to be used after annealing of stainless steel. Second, the work clearly shows the presence of protruding surface carbides when conventional surface finishing techniques are used, which do not exist after acid pickling. Third, when electrochemical pickling is implemented on an industrial scale, the life of blanking tools is substantially improved.

This work is a review of previous works, presenting and discussing the most important results obtained by an ongoing research program towards the development of innovative, low-cost, self-lubricating composites with a low friction coefficient and high mechanical strength and wear resistance. Special emphasis is given to uniaxial die pressing of solid lubricant particles mixed with matrix powders and to metal injection moulding associated with in situ generation of solid lubricant particles. Initially, a microstructural model/processing route (powder injection moulding followed by plasma-assisted debinding and sintering) produced a homogeneous dispersion of in situ generated solid lubricant particles. Micrometric nodules of graphite with diameter smaller than 20 μm were formed, constituting a nanostructured stacking of graphite foils with nanometric thickness. Micro Raman analysis indicated that the graphite nodules were composed of turbostratic 2D graphite having highly misaligned graphene planes separated by large interlamellae distance. Large interplanar distance between the graphene foils and misalignment of these foils were confirmed by transmission electron microscopy and were, probably, the origin of the outstandingly low dry friction coefficient (0.04). The effect of sintering temperature, precursor content, metallic matrix composition and surface finish is also reported. Furthermore, the influence of a double-pressing/double-sintering (DPDS) technique on the tribological performance of self-lubricating uniaxially die-pressed hBN + graphite-Fe-Si-C-Mo composite is also investigated. Moreover, the tribological behaviour of die-pressed Fe-Si-C matrix composites containing 5, 7.5 and 10 wt% solid lubricants (hBN and graphite) added during the mixing step is analysed in terms of mechanical properties and wear mechanisms. Finally, the synergy between solid lubricant particles dispersed in a metallic matrix and fluid lubricants in a cooperative mixed lubrication regime is presented.

The production of self-lubricating composites containing second phase particles is one of the most promising choices for controlling friction and wear in energy efficient modern systems. To gain a better understanding of the wear behaviour of such materials, Fe-Si-C matrix composites containing solid lubricants added during the mixing step were studied in this work. The samples were produced using powder metallurgy route, with total contents of 5, 7.5 and 10% in terms of volume of hexagonal boron nitride (hBN) and graphite mixtures as the solid lubricants. The composite's tribological properties were evaluated under reciprocating sliding conditions and their mechanical properties were tested using tensile tests. Additionally, after interruptions at different stages of the reciprocating tests, the wear scars were characterised by Raman spectroscopy and scanning electronic microscopy, to evaluate the evolution of the wear with test time. Higher total solid lubricant contents greatly increased the scuffing resistance of the composites, but decreased the mechanical properties. Furthermore, increasing the hBN content reduced both properties. Among the composites studied, the samples containing 1% vol hBN and 9% vol graphite showed the best combination of mechanical and tribological properties.

Objetivo: Caracterizar o fluxo de conhecimento em colaboracoes universidade-industria, no nivel dos individuos, identificando os fatores que facilitam e dificultam o fluxo. Design/Metodologia/Abordagem: O estudo investiga o fenomeno a partir de uma perspectiva qualitativa, por meio de entrevistas semi-estruturadas e analise tematica. Resultados: Apresentou-se uma representacao para descrever o fluxo do conhecimento em colaboracoes universidade-industria, no nivel dos pesquisadores, como um processo iterativo em niveis e etapas. Identificou-se que um dos componentes chave para o fluxo sao as interacoes entre pesquisadores de diferentes niveis e que disponibilidade, distância, contexto e a atuacao de intermediadores influenciam a realizacao destas interacoes.Originalidade/Valor: Poucos estudos analisam o fluxo de conhecimento no nivel dos individuos (micro nivel). O trabalho evidenciou a importância da analise de fluxo de conhecimento no micro nivel, mostrando aos gestores de colaboracao universidade-industria que e fundamental garantir a participacao dos diferentes pesquisadores nos varios niveis de interacao.

The shape and dimensions of the component play an essential role in plasma nitriding processes, a surface treatment crucial for the powder metallurgy industry since salt bath and gas nitriding poorly interact with porous structures. When the geometry of the components contains cavities, holes, or even parallel flat surfaces, the hollow cathode effect (HCE) can be activated, and, as a consequence, microstructural transformations that develop columnar nitride compound layers and the formation of a N-rich eutectoid, the braunite, may occur. The tribological properties of nitride compound layers with microstructural characteristics induced by the hollow cathode effect were evaluated in this work. Hollow cylinders of sintered iron were plasma nitrided and the tribological behaviour was assessed via reciprocating dry sliding tests. Unlike the friction coefficient, which is high (~0.4) regardless of the surface treatment, the wear rate is strongly increased by the presence of columnar morphology (2.4x) and braunite layer (2.1x), which are at the origin of brittle mechanisms such as crack propagation and interaction, resulting in the detachment of the compound layer.

Nanostructured turbostratic graphite derived from the solid-state dissociation of carbides is a new kind of carbide-derived carbon (CDC) consisting of heavily misaligned graphene layers. A recently developed processing route was used to obtain 2D turbostratic graphite nanoparticles via solid-state reactions between boron and chromium carbides. In this work, the tribological performance of these new CDC nanoparticles with different degrees of crystallinity was evaluated. Vacuum impregnation of a tailored sintered porous matrix (Astalloy CrL + 0.6%C) was used to obtain self-lubricating specimens containing both CDC and commercial crystalline 3D graphite, used as a reference. The results showed that the CDC particles promoted low friction coefficients (~0.1) and wear rates up to 70% lower than that of commercial crystalline graphite particles. The study of the wear scars through SEM and an extensive Raman spectroscopy analysis indicated that, although carbon crystallinity plays a fundamental role in the tribological behaviour, it is not the only variable to be considered, as the slow release of solid lubricant from the pores during the tests was shown to be crucial for the stability of a lubricious tribolayer, which reduces adhesion and wear.

In the present study, the main critical factors affecting the production of high tribological performance coatings on substrates containing near surface graphite were evaluated. Plasma nitriding followed by deposition of a hydrogenated diamond like carbon film (a-C:H) on gray cast iron (GCI) and nodular cast iron (NCI) substrates were carried out using a single thermal cycle. The surface topography evolution was evaluated via Scanning Electron Microscopy (SEM) and White Light Interferometry (WLI). As expected, there is a huge variation in surface topography and morphology according to graphite shape. An intense modification on the topography of GCI was observed, in particular wedges formation after nitriding. In order to enable a more in-depth understanding, the surface evolution after plasma nitriding of cast irons was carefully investigated and discussed in light of EDS linescans and XRD. Micro Raman spectroscopy and TEM analysis indicated that the DLC has an average amount of clustered sp 2 bonding in the structure. Additionally, the tribological performance of multifunctional coatings was evaluated via durability tests in ambient air, which revealed friction coefficients as low as 0.06 during steady-state lubrication regime for gray cast iron substrates, whereas the values were of 0.14 for nodular cast iron substrates. Crucial differences were found between friction coefficient behaviours: the GCI presented lubricious tribolayers covering most part of wear track whereas NCI presented topographic factors inhibiting tribolayers formation, a key factor that might explain the friction coefficient variation.

The performance of dry self-lubricating bulk materials is directly related to microstructural aspects such as solid lubricant chemical composition and distribution. In this paper, dry powder mixtures were prepared from iron powder and 9-16.5 vol.% of solid lubricants (graphite and MoS2), both combined and isolated. The results showed that interactions and reactions occurred during processing, either between the solid lubricants or between the lubricants and the matrix, generating carbides and sulfides. On account of that, the lubricant distribution in the microstructure is greatly altered, and the microhardness, friction coefficient and wear rate are increased. The best results were achieved by adequate powder particle size, solid lubricant content and sintering temperature control. In the composite containing 9%MoS2 + 2.5%C, values of friction coefficient and wear rate lower than 0.08 and 8 × 10−6 mm3 N−1 m−1 were reached.

The surfaces of dental implants are generally modified as an attempt to improve osseointegration. Three surface characteristics of implants are regarded as relevant in the process of bone formation: chemical composition, surface energy and topography. The topography of threaded implants is difficult to measure by conventional equipments with mechanical contact, so optical measuring equipments are preferred. However, implants with large thread angles may have the topography overestimated by the optical measuring equipment due to errors caused by scattering of the reflected light. This paper proposes a new methodology to measure implants with internal thread angle larger than 70° via laser interferometry, positioning the implant with an inclination angle in relation to the equipment base. The results were compared with results for flat discs, produced with the same surface treatment of the implant. Regarding surface energy of implants, most studies use measurements for flat discs, but it is questionable whether the results represent the implant itself. Here, two new methods were proposed to characterize the surface energy of real implants: a qualitative method related to the phenomenon of capillarity and a quantitative method with different liquids of known surface tension. It was found that the topography of the thread flanks is considerably different from the topography of the discs and it is believed that the geometry of the implant influences the surface treatment. The proposed quantitative methodology to analyze the surface energy allowed to detect differences between two commercial implants tested.

Purpose This paper aims to synthesize nanostructured flower-like molybdenum disulphide (MoS2) particles and evaluate their suitability to act as additives for lubricants. Design/methodology/approach A hydrothermal method without any surfactant successfully produced 250 nm mean diameter Nanostructured flower-like MoS2 particles. The resulting product was then solvothermally treated with absolute ethanol producing lipophilic powder particles which after being dispersed in two different lubricants were tribologicaly characterized by using a pin-on-disk tribometer geometry with reciprocal movement under immersion. Findings The influence on the stability and the coefficient of friction of the nano-oils is shown to be strongly dependent on their molecular structure. After 1 h of tribological testing, a ramified polyolester oil with 1 Wt.% of MoS2 nanoflowers did not show decrease in its average coefficient of friction, while an additive naphthenic oil with the same concentration of nanoparticles showed a decrease of 86 per cent. Originality/value In this paper, nanostructured flower-like MoS2 synthesized by the hydrothermal method was dispersed in lubricating oils to study its stability and tribological properties. To ensure good stability of the nano-oils, the particles were solvothermally treated with absolute ethanol to dehydrate and lipophilize them. A ramified polyolester-based nano-oil with 1 Wt.% of MoS2 nanoflowers showed no decrease in its steady state coefficient of friction, while a naphthenic-based nano-oil with the same concentration of nanoparticles presented a remarkable 86 per cent reduction.

When multifunctional surface engineering processes that combine purpose-oriented phases are applied to soft substrates, a combination of high wear resistance, high load support and low coefficients of friction can be achieved. In this study, the effects of different nitrided layers on the tribological behaviour of a diamond-like carbon (DLC) film deposited on a SAE 1040 steel were investigated. The nitriding was carried out under different temperatures and gas mixtures to create three distinct nitrided layers: two compound layers with predominant ε and γ' phases and a diffusion layer. All of the surfaces were then coated with DLC deposited via plasma-enhanced chemical vapour deposition (PECVD). The tribological tests indicated that the best performance was achieved for a specific combination of hardness, surface roughness and nitride type. The best load-bearing capacity between the DLC coating and the soft substrate was achieved when the nitrided layer was primarily a diffusion layer.

During the sintering process of self-lubricating steels, the dissociation of Silicon Carbide (SiC) in the iron matrix forms turbostratic graphite nodules which provide the lubricating properties of these materials. Previous works have explored the turbostratic nature of the graphite nodules and the influence of the SiC content in the microstructure and tribological properties of these materials, however, the thermodynamics and kinetics of SiC dissociation have not been fully explored yet. This study uses the versatility of die pressing to study the influence of particle sizes, sintering temperature and carbon content in the dissociation of SiC. The resulting materials were analysed by optical microscopy and scanning electron microscopy (SEM). The evolution of the size and morphology of pores and graphite nodules was investigated through image analysis software and simulations with the DICTRA® software were used to a better assessment of the kinetical aspects of SiC dissociation. Also, the thermodynamic influence of the particle size in the equilibria of the Fe–SiC system is discussed. Results indicate that SiC particle size rather than Fe/SiC size ratio governs the dissociation process, sintering temperature causes coalescence and rounding of pores as well as of graphite nodules. In addition, excessive activity of carbon in the system causes the precipitation of graphite flakes, which are not desired for technological applications.

The shape and geometry of the component play an essential role in surface plasma treatments. Certain plasma conditions can activate the hollow cathode effect (HCE) in parts with cylindrical geometry and, consequently, intensify the discharge by increasing the current density [1]. Due to this fact, a local surface over-heating can occur and cause some structural and microstructural transformation, developing columnar nitride compound layers and the appearance of braunite [2]. In this work, the tribological properties of nitride compound layers with microstructural characteristics induced by the hollow cathode effect (HCE) were evaluated. Hollow cylinders of sintered iron (7,5 mm in inner diameter and 9 mm in height) were plasma nitrided in three different conditions in order to obtain: a columnar compound layer (550 °C, 90%N2 + 9%H2 +1%CH4); a compound layer with presence of braunite (570 °C, 30%N2 + 70%H2); a compact and homogeneous compound layer with no microstructural defects (540 °C, 30%N2 + 70%H2). Pressure, output voltage and treatment time were kept constant in all treatments. The layer microstructures were analyzed by optical microscopy (OM) and the phase identification was conducted by X-ray diffraction (XRD). Reciprocating dry sliding tests were carried out to evaluate wear resistance and friction coefficient. White light interferometry was used to evaluate the surface topography and measure the wear volumes. The wear rate is affected by microstructural characteristics of the nitride layers induced by the HCE. Columnar nitride compound layer and the presence of braunite are undesired characteristics to achieve good wear resistance. Further-more, the friction coefficient was not influenced.