Tribological behaviors of Ti–6Al–4V modified by chemical methods

In order to improve the tribological properties of titanium-based implants, sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂) solutions, sol–gel hydroxyapatite (HA) film, thermal treatment and combined methods of NaOH solution/HA film, H₂O₂ solution/HA film are used to modify the surfaces of Ti–6Al–4V (coded TC4). The chemical states of some typical elements in the modified surfaces were detected by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of modified surfaces sliding against an AISI52100 steel ball were evaluated on a reciprocating friction and wear tester. As the results, complex surfaces with varied components are obtained. All the methods are effective in improving the wear resistance of Ti–6Al–4V in different degrees. Among all, the surface modified by the combined method of NaOH solution/HA film gives the best tribological performances. The friction coefficient is also greatly reduced by the modification of NaOH solution. The order of the wear resistance under 3 N is as following: Ti–NaOH–HA>Ti–NaOH>Ti–HA>Ti–H₂O₂–HA>Ti–H₂O₂ >Ti–500; under 1 N is Ti–HA, Ti–NaOH–HA>Ti–NaOH. For Ti–H₂O₂, a very low friction coefficient and long wear life over 2000 passes is obtained under 1 N. SEM observation of the morphologies of worn surfaces indicates that the wear of TC4 is characteristic of abrasive wear. Differently, abrasion, plastic deformation and micro–crack dominate the wear of Ti–HA; slight abrasive wear dominate the wear mechanism of Ti–NaOH and microfracture and abrasive wear for Ti–NaOH–HA and Ti–H₂O₂–HA, while the sample modified by thermal treatment is characterized by sever fracture. The superior friction reduction and wear resistance of HA films are greatly attributed to the slight plastic deformation of the film. NaOH solution is superior in improving the wear resistance and decreasing the friction coefficient under relative higher load (3 N) and H₂O₂ is helpful to reduce friction and wear under relatively lower load (1 N). Combined method of Ti–NaOH–HA is suggested to improve the wear resistance of Ti–6Al–4V for medial applications under fretting situations.     

A Study of Friction and Wear Behavior of Nitrided Layer on 2cr13 Steel in Vacuum

Tribological characteristics and wear mechanisms of gas-nitrided layer on a 2Cr13 steel in vacuum were investigated using a pin-on-disk type tribometer under self-mating dry sliding conditions with various normal loads and sliding velocities. The wear mechanisms involved were investigated by microscopic observations of the worn surfaces, the wear debris, and the corresponding cross sections. Experimental results show that for both sliding velocities of 0.2 and 1.6 m s⁻¹, friction forces are relatively stable in the case of lower loads (≤50 N), whereas become unstable and show high fluctuations under higher loads (>50 N). Wear mechanisms of the nitrided layer in vacuum are different for the lower and the higher sliding velocities. In the former case, mild abrasive wear dominates. In the latter case, a transition takes place from mild adhesive wear to severe adhesive or even delamination wear, with increasing normal load from 10 to 90 N.     

Microstructure and Tribological Properties of a HfB<Subscript>2</Subscript>-Containing Ni-Based Composite Coating Produced on a Pure Ti Substrate by Laser Cladding

A HfB₂-containing Ni-based composite coating was fabricated on Ti substrates by laser cladding, and its microstructure and tribological properties were evaluated during sliding against an AISI-52100 steel ball at different normal loads and sliding speeds. The morphologies of the worn surfaces were analyzed by scanning electron microscopy (SEM) and three-dimensional non-contact surface mapping. The results show that wear resistance of the pure Ti substrate and NiCrBSi coating greatly increased after laser cladding of the HfB₂-containing composite coating due to the formation of hard phases in the composite coating. The pure Ti substrate sliding against the AISI-52100 counterpart ball at room temperature displayed predominantly adhesive wear, abrasive wear, and severe plastic deformation, while the HfB₂-containing composite coating showed only mild abrasive wear and adhesive wear under the same conditions.     

Several plasma diffusion processes for improving wear properties of Ti6Al4V alloy

Different kinds of diffusion processes, plasma nitriding, oxidizing and oxynitriding as of a combination of other two, have been applied to Ti6Al4V alloy to evaluate the effect of treatment times (1 and 4 h) and temperatures (650 and 750 °C) on wear properties of the alloy. It was observed that a hard modified layer was produced on the surface of the alloy after each diffusion process. While TiN and Ti₂N phases form in the modified layer with plasma nitriding, mainly TiO₂ phase forms after plasma oxidizing treatment. The wear tests performed at different normal loads showed that all treated samples, except for nitrided and oxidized at 650 °C for 1 h, exhibited higher wear resistance than untreated Ti6Al4V alloy. The plasma nitrided samples showed adhesive wear. On the other hand, while the plasma oxidizing samples displayed adhesive wear at lower loads, wear mechanism changed to abrasive wear as the load increased because the oxide film which covers the surface was broken during the sliding at higher loads.     

Fretting wear behavior of nanocrystalline surface layer of copper under dry condition

Unlubricated fretting tests were performed with a nanocrystalline surface layer of a 99.99 wt.% copper fabricated by means of surface mechanical attrition treatment (SMAT), in comparison with a coarse-grained (CG) copper. The measured friction and wear data show that the fretting wear resistance is markedly enhanced with the nanocrystalline surface layer relative to the CG counterpart. The friction coefficient and wear volume of the SMAT Cu are lower than that of the CG Cu. For both samples, the friction coefficients and wear volumes increase with an increasing applied load and fretting frequency. A rapid increase of the friction coefficient and wear volume under an applied load above a critical value (30 N for the SMAT Cu and 20 N for the CG Cu) is noticed, corresponding to the formation of a continuous oxide layer between two contact surfaces. Also two sharp increases of the friction coefficient and wear volume at fretting frequencies of 50 Hz and 175 Hz were observed for the SMAT and the CG Cu. The former is correlated with the formation of a continuous oxide layer, while the latter corresponds to wearing away of the oxide layer.     

Characterization and tribological investigation of sol–gel ceramic films on Ti–6Al–4V

SiO₂, TiO₂, and hydroxyapatite (HA) thin films with good biocompatibility were grown on Ti–6Al–4V (coded as TC4) substrate by sol–gel and dip-coating processes from specially formulated sols, followed by annealing at 500 °C The chemical states of some typical elements in the target films were detected by means of X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM) are applied to characterize the original unworn films. The tribological properties of thin films sliding against an AISI52100 steel ball were evaluated on a reciprocating friction and wear tester. As the result, the target films composed of nano-particles ranging from 30 nm to 100 nm around were obtained. All the sol–gel ceramic films are superior in resisting wear compared with the TC4 substrate. Among all, HA film shows the best resistance while SiO₂ film shows the worst wear resistance both under higher (3 N) and lower load (1 N). TiO₂ shows good wear resistance under lower load (1 N). SEM observation of the morphologies of worn surfaces indicates that the wear of TC4 is characteristic of abrasive wear. Differently, abrasion, plastic deformation and micro-fracture dominate the wear of ceramic films. The superior friction reduction and wear resistance of HA film is greatly due to the slight plastic deformation of the film. It is supposed that the deformation of the HA film is closely related to the special arrangement of the nano-particles and microstructure. HA film is recommended for clinical application from the point of wear resistance view.     

Fretting wear behavior of nanocrystalline surface layer of pure copper under oil lubrication

Lubricated fretting tests in mineral oil were performed with a nanocrystalline surface layer on a pure bulk Cu prepared by surface mechanical attrition treatment (SMAT) against a WC-Co ball. It was found that the nanocrystalline surface layer exhibited a markedly enhanced fretting wear resistance and higher friction coefficient relative to the coarse-grained (CG) form. The wear volume of the SMAT Cu is one order of magnitude lower than that of the CG Cu. The friction coefficient of the SMAT Cu increases with an increasing load and frequency, while for the CG Cu, the friction coefficient increases with an increasing fretting frequency up to 100 Hz and thereafter decreases. The higher hardness of the SMAT Cu is suggested to be the main factor causing its improved wear resistance and higher friction coefficient. A discontinuous metal transfer layer can be found on the WC-Co ball only after fretting against the SMAT Cu, which may partly account for the higher wear resistance of the SMAT Cu in comparison with the CG Cu.     

Torsional Wear Behavior of MC Nylon Composites Reinforced with GF: Effect of Angular Displacement

The torsional wear behavior of monomer cast nylon (MC nylon) composites reinforced with glass fiber was studied with a self-made torsional friction tester. The worn surface of MC nylon composites was investigated with a scanning electron microscope. The worn surface of the steel disk was observed with a 3-D profiler. The experimental results indicated that the shape of torque–angular displacement (T–θ) curves changed from elliptic shape to quasi-parallelogram with the angular displacement increased from 5° to 30°. The serious wear characterized with a deep groove occurred at the position of about 1.5–4 mm radius of contact zone on the steel surface. The mass of MC nylon samples increased after torsional wear test. The torsional contact area can be divided into three zones: (a) a central stick zone, (b) an intermediate mixed-slipping annulus, and (c) a peripheral sliding annulus. The most serious wear occurred in the intermediate annulus because of the higher contact stress and mixed slip regime. The main wear mechanism of MC nylon samples was adhesive wear and abrasive wear. Plastic deformation of asperities was the character in the central zone. Slight adhesive wear was the main wear mechanism in the peripheral annulus.     

Adhesive and abrasive wear behaviour of TiC alloyed,sintered, austenitic stainless steels

Abstract

This study examines abrasive and adhesive wear behaviour of austenitic stainless steel and its TiC alloyed composite produced through powder metallurgy technique. Abrasive wear tests have been carried out using a pin on disc wear tester under loads of 10, 20 and 30 N. For adhesive wear tests, a block on ring wear tester has been used under loads of 20, 40, 60 and 80 N. A possible correlation between the hardness, microstructure and wear behaviour of the samples has been investigated. The abrasive wear tests have revealed that the highest rate of mass loss occurred in the austenitic matrix stainless steel sample; also, mass losses decreased with an increased rate of reinforcing material in the composite. In adhesive wear tests, interparticle spacing developed from severe wear and extreme plastic deformation under heavy loads; however, at low loads, oxidation type wear was shown to be dominant.     

Effect of electrical current on tribological behavior of copper-impregnated metallized carbon against a Cu–Cr–Zr alloy

A block-on-slip ring-type wear tester was used to investigate the tribological behavior of copper-impregnated metallized carbon against a Cu–Cr–Zr alloy under 2 to 6 N applied load and 0 to 20 A electrical current. The sliding speed was maintained at 25 km/h. The wear loss of copper-impregnated metallized carbon increased with greater electrical current. Under a certain applied load, the wear loss with electrical current was minimized. The tribo-layer had an apparent effect on the friction coefficient. The wear mechanisms were complex, consisting of adhesive wear, abrasive wear and arc erosion.     

Comparisons of dry sliding tribological behaviors between coarse-grained and nanocrystalline copper

Dry sliding tribological behaviors of nanocrystalline (NC) and coarse grained (CG) Cu were studied by using a ball-on-plate tribometer with a counterface ball of cemented tungsten carbide. The results showed that prior to oxidation and delamination, the steady-state friction coefficients (FCs) of NC and CG Cu are comparable (～0.35). As oxidation with delamination of wear surface occur, the FC for either CG or NC Cu increases gradually, approaching a steady-state FC (～0.63). The wear resistance of the NC Cu was enhanced by at least one order of magnitude under the measured loads ranging from 5 N to 25 N in comparison with the CG counterpart, which is mainly attributed to the higher hardness of the NC layer.     

Friction and wear behaviors of nanocrystalline surface layer of pure copper

Surface mechanical attrition treatment (SMAT) was employed to fabricate a nanocrystalline surface layer on a pure copper plate. The grain size is about 10 nm in the top layer and increases with an increasing depth from the treated surface. The tribological behavior of the nanocrystalline surface layer was investigated under dry conditions. Experimental results show that the load-bearing ability is markedly enhanced with the nanocrystalline surface layer relative to the coarse-grained form. The friction coefficient of the nanocrystalline layer is lower than that of the coarse-grained copper when the applied load is below 20 N. With increase of the load, the difference in wear resistance between the SMAT and the conventional Cu decreases. When the load exceeds 40 N, for the SMAT Cu, there occurs a transition of wear regime from local damage to delamination of a mechanical mixed layer. There is an abrupt increase of the wear volume, which corresponds to the wearing away of the nanocrystalline layer. The enhanced wear properties of the nanocrystalline surface layer are correlated with the stability of the mechanical mixed layer and the high hardness of the nanocrystalline structure.     

Wear of aligned silicon nitride under dry sliding conditions

The wear behaviour of textured silicon nitride (Si₃N₄) ceramics with aligned microstructures was analyzed under abrasive wear conditions. Dry reciprocating self-mated ball-on-flat wear tests were performed to study the influence of different microstructural plane/orientation combinations on the Si₃N₄ tribological behaviour. Textured materials showed superior wear resistance than non-textured reference Si₃N₄ for the whole range of loads and contact pressures, 5–50 N and 1.7–3.6 GPa, respectively, with an increase of about 70% for the maximum applied load. Within textured materials, the plane perpendicular to the extruding direction exhibited a 50% higher wear resistance (4 × 10^?5 mm³ N^?1 m^?1) than the parallel plane where the elongated grains were aligned. The severe wear process involved debonding, fracture and debris formation mechanisms. The progress of this sequence depended on the particular microstructure of each plane/orientation combination. A relationship between abrasive wear resistance and selected microstructural parameters has been established.     

Comparative study of micro- and nano-ZnO reinforced UHMWPE composites under dry sliding wear

This is a comparative study between ultra-high molecular weight polyethylene (UHMWPE) reinforced with micro-zinc oxide (ZnO) and nano-ZnO under different filler loads. These composites were subjected to dry sliding wear test under abrasive conditions. The micro- and nano-ZnO/UHMWPE composites were prepared by using a hot compression mould. The wear and friction behaviours were monitored using a pin-on-disc (POD) test rig. The pin-shaped samples were slid against 400 grit SiC abrasive papers, which were pasted, on the stainless steel disc under dry sliding conditions. The worn surfaces and transfer film formed were observed under the scanning electron microscope (SEM). Experimental results showed that UHMWPE reinforced with micro- and nano-ZnO would improve the wear behaviour. The average coefficient of friction (COF) for both micro- and nano-ZnO/UHMWPE composites were comparable to pure UHMWPE. The weight loss due to wear for nano-ZnO/UHMWPE composites are lower compared to micro-ZnO/UHMWPE and pure UHMWPE. The optimum filler loading of nano-ZnO/UHMWPE composites is found to be at 10 wt%. The worn surface of ZnO/UHMWPE composites shows the wear mechanisms of abrasive and adhesive wear. Upon reinforcement with micro- and nano-ZnO, the abrasive and adhesive wear of worn surfaces transited from rough to smooth.     

Micro-scale abrasive wear testing of duplex and non-duplex (single-layered) PVD (Ti,Al)N, TiN and Cr–N coatings

A micro-scale abrasive wear test, based on ball-cratering, has been used to evaluate the wear resistance of duplex and non-duplex (Ti,Al)N, TiN and Cr–N coatings. The term duplex is used here when plasma nitriding is followed by PVD coating. Coatings without the plasma nitriding stage are termed single-layered. Coating properties were evaluated by surface profilometry, hardness and scratch testing. All duplex coatings showed higher micro-abrasive wear resistance than their single-layered counterparts, with the duplex (Ti,Al)N coating achieving the best performance. After a certain number of ball revolutions, the coating material became worn through, exposing the substrate material. After this point, the presence of a hard nitrided case diminished the scratching action of the SiC abrasive particles. The experimental results also indicate that the choice of the PVD coating plays an important role in improving the micro-abrasive wear resistance. Apart from single-layered and duplex Cr–N coatings, all the other coating systems provided a higher micro-abrasive wear resistance than the uncoated substrate (hardened AISI H13 steel). The poor abrasive wear resistance recorded for the single-layered and duplex Cr–N coatings could be attributed to the hardness of the Cr–N being much lower than that of the SiC abrasive particles, which caused tearing of the coating with subsequent delamination. The wear pattern observed was found to change from surfaces characterised by grooves (uncoated substrate, single-layered TiN and Cr–N systems and duplex Cr–N system) to surfaces which exhibited multiply indented surfaces (single-layered and duplex (Ti,Al)N systems), indicating a transition between wear mechanisms. This transition was found to be dependent on the ratio between the hardness of the SiC abrasive particles and surface (coating) or subsurface hardness. By decreasing this ratio, the ability of the SiC abrasive particles to scratch the composite surface was reduced and the resistance to micro-scale abrasion was improved.     

Study of TiAlSiN coatings post-treated with N and C + N ion implantations. Part 2: The tribological analysis

Ion implantation has found to be an effective approach to modify surface properties of materials. The present research investigates the effect of (1) nitrogen (N), and (2) carbon subsequently with nitrogen (C + N) implantations on the mechanical and tribological properties of the titanium–aluminium–silicon–nitride (Ti–Al–Si–N) coatings. Superhard TiAlSiN coatings produced by magnetron sputtering, of approximately 2.5 μm thickness, were post-treated by implantations of N or C + N at an energy level of 50 keV. The dose range was between 5 × 10¹⁶ and 1 × 10¹⁸ ions cm^?2. After implantation, the tribological performance of the coatings was investigated by a ball-on-disk tribometer against WC–6 wt.%Co ball under dry condition in ambient air. The wear performance of the samples was examined by a variety of characterization techniques, such as secondary electron microscopy (SEM), 3D profilometry, atomic force microscopy (AFM), and micro-Raman. The results showed that the wear performance of the samples depended strongly on the implanted elements and doses. There was slight improvement on the samples implanted with N whereas significant improvement was found on the C + N implantations. Particularly, the friction coefficient of the sample with 5 × 10¹⁷ C⁺ cm^?2 and 5 × 10¹⁷ N⁺ cm^?2 could reach 0.1. In addition, the specific wear rate of the sample was extremely low (0.85 × 10^?7 mm³/Nm), which was nearly two orders of magnitude below that of the un-implanted coating. The speculation of the mechanical and tribological analyses of the samples indicates that the improvement of the N implanted and C + N implanted TiAlSiN samples could be due to a combined effect of improved hardness, plus enhanced adhesive and cohesive strength. In addition, the improved performance of the C + N implanted samples could be explained by the formation of lubricating implanted-layer, which existed mostly in sp² C–C and C–N forms. The formation of such implanted layer could lead to a change of wear mode from strong abrasive wear to mostly adhesive wear, and result in a drop of friction coefficient and wear rate.     

Comparison of wear properties of tool steels AISI D2 and O1 with the same hardness

Two commercial cold work tool steels, AISI D2 and O1, were heat treated in order to obtain the same hardness 700 HV (60 HRc) and were subsequently tested in three different modes of wear, namely in adhesion, three-body and two-body abrasion, by using pin-on-disk, dry sand/rubber wheel apparatus and pin abrasion on SiC, respectively. Even though AISI O1 and D2 steel are heat treated to the same hardness, they perform differently under the three modes of wear examined. The results show that the steel microstructures play the most important role in determining the wear properties. For relatively low sliding speeds AISI O1 steel performs up to 12 times better than AISI D2 steel in adhesive wear. For higher sliding speeds, however, this order is reversed due to oxidation taking place on the surface of the AISI D2 steel. The wear rate of both tool steels in three-body and two-body abrasion wear is proportional to the applied load. In three-body abrasive wear, AISI D2 exhibits a normalised wear rate about two times lower than the AISI O1 tool steel, and this is due to the presence of the plate-like hard carbides in its microstructure. Both tool steels perform 3–8 times better in three-body abrasive wear conditions than in two-body abrasive wear.     

Friction and wear behaviour of AISI 5140 steel under rectangular wave loading conditions

Abstract

The tribological characteristics and wear mechanisms of AISI 5140 steel at ambient temperature were investigated using a home built ball on disc tribometer under constant normal loading and rectangular wave loading respectively. The worn surface and wear debris collected from the disc were studied using scanning electron microscope. Results show that the friction coefficients under the constant normal force correspond to the traditional theory. The coefficients all exhibited increased normal loads, whereas under the rectangular wave condition, the highest coefficient appeared when the peak value of the periodically alternative load was 90?N. The different underlying wear mechanism under different loading conditions was explored. It was found that abrasive wear was the main mechanism in the constant loading, whereas severe plastic deformation and adhesive wear were the main wear mechanism in the rectangular wave loading cases. This phenomenon can be attributed to the role of debris in the ‘lubrication’ process under the rectangular wave loading conditions.     

Influence of sliding frequency on reciprocating wear of mold steel with different microstructures

The wear resistance of a low alloy plastic mold steel has been studied under pin-on-flat reciprocating configuration against AISI 52100 steel pins, under variable sliding frequency. The as-received material (HTO; 33 HRC) was heat treated under variable conditions to obtain different microstructures and hardness (HT1, quenched 880 °C, 58 HRC; HT2, tempered 550 °C, 43.4 HRC; HT3, tempered 300 °C, 52 HRC; HT4, annealed, 26 HRC). Under low sliding frequency (1 Hz), no significant differences in the wear resistance of the different materials are observed. Only at 8 Hz, a relationship between hardness and wear resistance is found. The softer annealed material HT4 shows an increasing wear rate under increasing frequency, while the quenched steel HT1 gives the lowest wear at the highest frequency. Wear mechanisms have been studied from SEM and EDS observations. Only HT4 shows a transition from the abrasive and oxidative wear mechanisms found in all cases to an adhesive wear mechanism under the highest frequency.     

Improvement of the Oxidation and Wear Resistance of Pure Ti by Laser-Cladding Ti<Subscript>3</Subscript>Al Coating at Elevated Temperature

Ti₃Al coating was in situ synthesized successfully on pure Ti substrate by laser-cladding technology using aluminum powder as the precursor. The composition and microstructure of the prepared coating were analyzed by transmission electron microscopy, scanning electron microscopy (SEM), and X-ray diffraction technique. Thermal gravimetric analysis was used to evaluate the high-temperature oxidation resistance of the Ti₃Al coating. The friction and wear behavior was tested through sliding against Si₃N₄ ball at elevated temperature of 20, 100, 300, and 500°C. The morphologies of the worn surfaces and wear debris were also analyzed by SEM and three-dimensional non-contact surface mapping. The results show that the Ti₃Al coating with high microhardness, high-temperature oxidation resistance, and high temperature wear resistance. The pure Ti substrate is dominated by severe adhesion wear, abrasive wear, fracture, and severe plastic deformation at lower temperature, and severe adhesion wear, abrasive wear, plastic deformation, oxidation, and nitriding wear at higher temperature, whereas the Ti₃Al coating experiences only moderate abrasive and adhesive wear when sliding against the Si₃N₄ ceramic ball counterpart. In addition, the wear debris of the laser-cladding Ti₃Al coating sliding and Si₃N₄ friction pairs are much smaller than that of pure Ti substrate and Si₃N₄ friction pairs at elevated temperature.