Role Of Third Body Research Articles

Abrasive wear behavior of PTFE for seal applications under abrasive-atmosphere sliding condition

Abrasive wear is a common failure phenomenon that often limits the service life of sealing elements. Evaluation and comparison of the abrasion resistance of polytetrafluoroethylene (PTFE) were conducted using Al2O3 particles with sizes in the range 5 to 200 µm on a pin-on-flat tribo-tester under dry reciprocating sliding conditions at room temperature. Based on the examined worn surface characteristics of both PTFE and 316L stainless steel (as a counterpart) and the analyzed coefficient of friction (COF) evolutions, the wear mechanism and particle size effect have been explored in detail. The results demonstrate that the abrasive size is the main contributing factor, which can drastically impact the wear mechanism and tribological properties of tribo-pairs. The COF exhibits different evolution characteristics (trends) for different abrasive sizes. For moderate particle sizes, the COF trends become more complicated and the most evident wear of the metallic counterpart is evident. The activity behaviors of abrasives are dominated by the particle size. Particles can becomes embedded in one of the tribo-pair materials to plough-cut the counterpart, thus causing two-body abrasive wear. The abrasives can also behave as free rolling bodies, which play the role of third body to realize three-body “PTFE-abrasive-316L” abrasion. When abrasives are involved in the wear process, both the wear rate and COF of the metallic counterpart increase, but the material removal rate of the PTFE is reduced. The results obtained can offer guidelines regarding the design and protection of seals.

Role of Third Bodies in Friction and Wear of Cold-Sprayed Ti and Ti–TiC Composite Coatings

Titanium (Ti) and Ti-based alloy wear performance is often poor unless coating or lubricants are used. An alternative is to use hard phase reinforcement. Cold spray is a relatively new method to deposit composite coatings, where here we report the deposition of a Ti–TiC coating and its sliding wear behavior. Mixtures of mechanically blended Ti–TiC with various TiC content were injected into a de Laval nozzle and sprayed onto substrates. Two composite coatings and a pure Ti coating are reported here, where the as-sprayed compositions of the composites were 13.8 and 33.4 vol% TiC. Reciprocating dry sliding wear was performed using a custom-built in situ tribometer. All tests were conducted with a sliding speed of 3 mm/s and at a normal load of 0.5 N. Using a transparent sapphire hemisphere of 6.25 mm as counterface, dynamic behavior of third bodies was directly observed. It was found that adhesive transfer of Ti was the primary wear mechanism for the Ti coating, with oxidative and abrasive wear also occurring for longer sliding cycles. The superior wear resistance of the composite coatings compared to Ti was related to dual function of TiC particles, where they reinforced the Ti matrix and facilitated the formation of a stable and protective tribofilms. The tribofilms contained carbonaceous material that provided easier shear and lower friction. The formation of these tribofilms was highly dependent on the TiC particles, which contained excess carbon compared to the equilibrium composition. Higher TiC content was more effective in quickly developing and sustaining the tribofilms.

Role of Third Body on Bolted Joints' Self-Loosening

Bolted joints are frequently subjected to self-loosening (gradual loss of clamping force) causing multiple failures, especially leaking and breaking of mechanical systems. Such physical phenomena would occur whatever the considered coating (Ag, MoS2, Zn–Ni and others). To enlighten this phenomenon, which remains rather misunderstood due to the confined nature of bolted joint contacts, a coupled experimental-numerical approach is adopted on a bolted joint with silver coating. Indeed, from tribological expert assessments of disassembled joints without loosening, a local view of nut/screw threads contacts is proposed, using discrete element method. This method becomes essential in tribology since it offers the ability to model the dynamic behavior of a contact interface. The model is based on a Non-Smooth Contacts Dynamics approach. The case of third body formed in contacts during tightening process, which has been ignored so far, is placed at the focus of self-loosening phenomenon.

Some Considerations on the Role of Third Bodies during Automotive Braking

Some Considerations on the Role of Third Bodies during Automotive Braking

In situ tribology of nanocomposite Ti–Si–C–H coatings prepared by PE-CVD

Abstract In situ tribometry was used to examine the role of third bodies for the friction and wear behaviour of five hard Ti–Si–C–H coatings produced by plasma-enhanced chemical vapour deposition. Coatings exhibited nanocomposite structures consisting of nanocrystalline titanium carbide surrounded by amorphous carbon containing H and Si (the latter being present in all coatings but one). From nanoindentation testing, the hardness for the coatings was between 22 and 35 GPa. An inverse correlation was found between hydrogen content, which was determined by Raman spectroscopy, and hardness. A custom-built in situ tribometer, equipped with a hemispherical sapphire slider, was used for reciprocating pin-on-flat wear tests. The buried interface was examined in situ with video microscopy, allowing for observation of third body formation (transfer films, wear debris) and velocity accommodation modes. The formation of a stable transfer film, which was dependent on coating chemistry and humidity level, resulted in increased wear resistance compared to coatings that did not form a transfer film, regardless of the coatings’ mechanical properties. Results from this study suggest an approach to optimising the tribological performance for hard protective coatings that does not rely solely on mechanical properties but is complemented by investigations on the third body behaviour.

Role of third bodies in friction and wear of protective coatings

The literature on protective tribological coatings often focuses on correlations with measurable coating properties (composition, structure, and mechanical) but ignores the mechanisms of friction and wear. In fact, long-lived coatings often survive because of third bodies that form inside the moving contact. This article reviews earlier studies of third body processes carried out by mainly ex situ methods and reports more recent studies investigating third body processes using in situ techniques. Direct evidence that third bodies control friction and wear processes has been obtained with a tribometer incorporating in situ optical microscopy and Raman spectroscopy. Videotapes and Raman spectra of the sliding contact were recorded during reciprocating sliding tests performed in both dry and humid air with transparent hemispheres (glass or sapphire). Third body processes were correlated directly to friction and wear behavior of three low friction coatings: amorphous Pb–Mo–S; diamond-like carbon (DLC); and annealed boron carbide. In all three cases, the friction behavior could be explained in terms of the relative motion between a transfer film on the hemisphere and the wear track. With amorphous Pb–Mo–S, the transfer film was MoS2; with DLC, it was a graphite-like carbon; and with annealed boron carbide, it was either a mix of H3BO3 and carbon (at μ=0.08) or, when the H3BO3 wore away, carbon alone (at μ=0.2). Friction rises with Pb–Mo–S and DLC in humid air were ascribed to a change in interfacial shear strength; friction spikes and fluctuations with DLC were associated with periodic loss of transfer film thickness. For all three coatings, interfacial sliding between transfer film and wear track on coating was the dominant velocity accommodation mode.

Friction Behavior of Boric Acid and Annealed Boron Carbide Coatings Studied by In Situ Raman Tribometry

The sliding friction behavior of boric acid and annealed boron carbide coatings has been investigated by in situ Raman tribometry. Reciprocating sliding tests were performed in air (∼20% relative humidity) at 1 mm/s with a transparent sapphire hemisphere dead weight loaded against the coated substrates. A homebuilt micro-Raman tribometer recorded the friction force as well as allowed both visualization and Raman spectroscopy of the contacts during sliding. Friction coefficients for the boric acid films remained low (μ ∼ 0.06) throughout sliding until failure (μ ∼ 0.7), at which time boric acid was lost from the contact and substrate material (aluminum) transferred to the slider. Friction coefficients for the annealed boron carbide, known to form boric acid surface films upon cooling, were initially low (0.08), then gradually increased and leveled out at about 0.2. Raman spectroscopy revealed a mixture of boric acid and carbon initially; but as the friction coefficient rose, the boric acid intensity decreased to zero while the carbon intensity increased. Correlations between friction coefficients and amounts of solid lubricating material in the contact are presented, and the role of third bodies in controlling friction is discussed.

Role of Third Bodies in Friction Behavior of Diamond-like Nanocomposite Coatings Studied by In Situ Tribometry

The friction and wear behavior of amorphous diamond-like nanocomposite (DLN) coatings (C:H:Si:0) in low speed, dry sliding contact has been investigated using a home-built in situ tribometer. Tests were performed under reciprocating sliding against sapphire and steel hemispheres in dry (∼4% RH) and moist (35 to 52% RH) air at contact stresses of 0.7 and 1.1 GPa. In situ visual observations identified how third body processes affected the friction behavior of DLN coatings. For all test conditions, a transfer film began to form on the sapphire hemisphere during the first cycle, and the coefficient of friction dropped during further buildup of the transfer film. Video analysis showed most of the sliding motion took place between the transfer film on the hemisphere and the DLN coating. This velocity accommodation mode, interfacial sliding, was responsible for steady-state friction coefficients between 0.03 and 0.05 with lower values obtained at high stress and low RH. When the transfer film broke up the friction began to spike (0.1 ≤ μ ≤ 0.5); during the spikes, shearing of third bodies also contributed to velocity accommodation, as debris attached, detached and recirculated between the hemisphere and the track. Subsequently, the friction coefficient fluctuated between low and high values due to reformation and breakup of the transfer film, respectively.