Material transport phenomena in the impact wear of titanium alloys

Results are presented from compound impact wear studies performed with titanium alloys of different β phase content and morphology. The “material pair” consisted of a 17-4 PH stainless steel counterface and a flatended titanium alloy specimen. Each material pair was exposed to variations in relative transverse sliding velocity and number of repetitive load cycles. Testing was conducted primarily at a single level of nominal peak normal impulsive stress.Both optical and scanning electron microscopy were used to monitor changes in worn surface and subsurface regions. Energy-dispersive X-ray studies of the initial and worn surfaces comprising the material pair clearly indicated the nature of the material transport between the opposing surfaces. Wear debris were studied by optical microscopy and by powder X-ray techniques.Utilizing the reciprocating impact wear testing apparatus, it was determined that material transport appears to be a controlling factor. The type of transport (i.e. material passing from specimen to counterface or vice versa) was found to vary under differing test conditions. Such findings may contribute to the understanding of wear for systems other than those characterized by repetitive impulsive contact.Material removal is minimal at particular levels of relative transverse sliding velocity, and these levels are not necessarily affected by the magnitude of the nominal level of stress. It appears that the nature and quantity of the constituents (α, β) in the titanium alloys are critical in establishing wear behavior for the material pairs investigated.     

Specimen-counterface bulk hardness effects in impact wear of 17-4 PH steel pairs

Impact sliding wear tests have been performed with 17-4 PH steel as both specimen and counterface material. Tests were designed to explore the influence of bulk hardness; this was done by contacting hard pins against soft discs and vice versa. Additional wear tests were conducted with specimen and counterface of equal hardness. The entire program employed four distinct microstructural conditions in the hardness range from 30 to 44 HRC. The results indicate that for the test conditions explored the wear rate of the specimens (pins) depends on the “hardness pair” while the counterface (disc) wear does not. It was further shown that changes in specimen geometry (mushrooming) are of major relevance. The subsurface sections from the pins and discs formed characteristic zones of plastic deformation and mechanochemical mixing. In this, the hardest pin-softest disc and softest pin-hardest disc pairs developed essentially the same near-surface zone on the counterface, irrespective of initial microstructural condition.     

Effect of counterface balls on the friction layer of Ni3Al matrix composites with 1.5 wt% graphene nanoplatelets

Research on the friction layer is needed to minimize friction- and wear-related mechanical failures in moving mechanical assemblies. Dry sliding tribological tests of Ni₃Al matrix composites (NMCs) with 1.5 wt% graphene nanoplatelets (GNPs) sliding against different counterface balls are undertaken at the condition of 10 N–0.234 m s^?1 in this study. When sliding against GCr15 steel, a uniform and thick friction layer is formed, resulting in a lower friction coefficient (0.29–0.31) and wear rate (2.0–3.1 × 10^?5 mm³ N^?1 m^?1). While sliding against Al₂O₃ and Si₃N₄, the formation and stability of the friction layers are restricted in the severe wear regime, and the NMCs exhibit higher friction coefficients and wear rates. Therefore, various counterface balls have a great effect on the stability and thickness of the friction layer, thus affecting the tribology performance of NMCs. The result also shows that GNPs exhibit enrichment and self-organized microstructures in the friction layer. In addition, the friction layer is also found to be divided into two layers, protecting the subsurface from further damage and reducing shear.     

Adhesive wear and frictional characteristics of UHMWPE and HDPE sliding against different counterfaces under dry contact condition

Abstract

The current work evaluates the wear and frictional performance of ultrahigh molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE) sliding against different metal counterfaces, stainless steel(SS), mild steel (MS) and aluminium (Al), under dry contact condition. The experiments were conducted using pin on disc machine at different sliding distances (0–40·32 km), 15 N applied load and 2·8 m s^–1 sliding velocity. Interface temperatures and frictional forces were measured simultaneously during the sliding, while specific wear rates were determined for every 1·68 km sliding distance. Based on the optical microscopy of the worn surface and wear track, frictional and wear results were analysed and discussed. The experimental results showed that the type of counterface material significantly influences both frictional and wear performances of the selected polymers. This was mainly due to the film transfer characteristics. Higher temperature and friction coefficient for UHMWPE and HDPE were evident when sliding took place against Al counterface. Sliding the polymers against stainless steel showed low friction coefficients compared to other counterfaces.     

Formation of a Metallic Amorphous Layer During the Sliding Wear of Ti/TiN Nanolaminates

This paper describes experimental studies of metallic/ceramic nanolaminate performance under sliding contact and identifies the formation of an amorphous layer between the nanolaminate and counterface. Nanolaminates used for this study had either 20- or 100-nm-thick alternating layers of Ti and TiN, resulting in a total thickness of ~1-μm films. The structure of the Ti and TiN layers was confirmed using X-ray diffraction [(111)_TiN and (002)_Ti], and compositions were determined using electron energy loss spectroscopy (EELS)—Ti and TiN_0.7. Variation of the individual layer thicknesses within Ti/TiN nanolaminates was shown to influence both the deformation observed through the nanolaminate thickness and also the friction coefficient between the nanolaminate and 440C steel counterface during linear reciprocating wear. During sliding, the 100-nm-layered nanolaminate had a lower coefficient of friction (0.25 ± 0.01) than the 20-nm-layered nanolaminate (0.56 ± 0.06). An amorphous titanium layer developed during sliding at the interface between the 100-nm nanolaminate and steel counterface. EELS confirmed that this layer did not contain any nitrogen and recrystallization occurred near the in-contact surface. While phase changes under compressive loading have been reported for other systems, this is the first report to indicate this response within a titanium layer.     

Wear mechanisms in electroless nickel coatings

Electroless nickel coatings are susceptible to severe adhesive wear against plain carbon steel under dry sliding contact, because the low interfacial and high surface free energies of this material combination produce highly compatible surfaces. Controlled heat treatment eliminates severe wear by inducing crystallization of amorphous nickel to Ni₃P, which is incompatible with steel. Electroplating the counterface with chromium also eliminates severe adhesive wear owing to the efficient barrier properties of its surface oxide but a stainless steel counterface exacerbates the problem because of its single-phase microstructure and the low durability of its oxide film. The reciprocating diamond scratch test gives a different ranking order of wear rate from the Falex, pin-on-flat and Taber tests, because of its dependence on fracture toughness as the dominant material parameter rather than adhesive transfer. Wear mechanisms observed in the investigation include adhesive transfer, oxidation, three-body abrasion, micromachining, plastic deformation and delamination. In general, no correlation is found between wear rate and hardness; this is primarily because of changes in the mechanism of wear effected by processing, counterface materials or sliding conditions.     

The wear characteristics of ultrahigh molecular weight polyethylene against a high density alumina ceramic under wet (distilled water) and dry conditions

In recent years there has been growing interest in the use of high density alumina ceramic material for the femoral ball in association with ultrahigh molecular weight polyethylene (UHMWPE) for the acetabular component in total replacement hip joints.The wear characteristics of UHMWPE pins sliding against a high density alumina ceramic disc in the presence of distilled water in a tri-pin-on-disc machine have been revealed in very long-term experiments reported in this paper. A total sliding distance in excess of 6000 km was achieved and very low mean wear coefficients of the order of 10^?8 mm³ N^?1 m^?1 were recorded.Experiments were also carried out over a shorter sliding distance under dry conditions and the average wear coefficient of 2 × 10^?7mm³N^?1m^?1 was consistent with earlier findings. In these dry tests, comet-like streaks of polyethylene were transferred to the ceramic counterface, but no such transfer was noted during the wet tests. When distilled water was added to the test chamber after a considerable period of dry sliding, the wear coefficient rapidly decreased to about 10^?8 mm³ N^?1 m^?1 and the streaky transfer film disappeared from the ceramic counterface.The possibility of hydrodynamic action between the wear face on the pins and the counterface was investigated by reversing the direction of sliding. Surface topography changes on both the pins and the discs and friction and bulk temperatures of the pins were recorded throughout the tests.It is concluded that the excellent dry wear coefficients of UHMWPE sliding on alumina ceramic counterfaces are about twenty times greater than those experienced by the same materials in the presence of distilled water. The tribological advantage of the ceramic with respect to stainless steel having a similar surface roughness has been confirmed in dry sliding involving UHMWPE, but further work is required to determine whether or not the same advantage can be achieved under wet conditions.     

Friction and wear of titanium alloys sliding against metal, polymer, and ceramic counterfaces

Recent advances in lower-cost processing of titanium, coupled with its potential use as a light weight material in engines and brakes has renewed interest in the tribological behavior of titanium alloys. To help establish a baseline for further studies on the tribology of titanium against various classes of counterface materials, pin-on-disk sliding friction and wear experiments were conducted on two different titanium alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo). Disks of these alloys were slid against fixed bearing balls composed of 440C stainless steel, silicon nitride, alumina, and polytetrafluoroethylene (PTFE) at two speeds: 0.3 and 1.0 m/s. The friction coefficient and wear rate were lower at the higher sliding speed. Ceramic sliders suffered unexpectedly higher wear than the steel slider. The wear rates, ranked from the highest to the lowest, were alumina, silicon nitride, and steel, respectively. This trend is inversely related to their hardness, but corresponds to their relative fracture toughness. Comparative tests on a Type 304 stainless steel disk supported the fracture toughness dependency. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses confirmed the tendency of Ti alloys to transfer material to their counterfaces and suggested possible tribochemical reactions between the ceramic sliders and Ti alloy disks. These reaction products, which adhere to the ceramic sliders, may degrade the mechanical properties of the contact areas and result in high wear. The tribochemical reactions along with the fracture toughness dependency helped explain the high wear on the ceramic sliders.     

Tribological properties of a Fe3Al material in sulfuric acid corrosive environment

The tribological properties of a Fe₃Al material in an aqueous solution of 1 mol/l H₂SO₄ corrosive environment sliding against a Si₃N₄ ceramic ball are studied using an Optimol SRV oscillating friction and wear tester in a ball-on-disc contact configuration. We investigate the effects of load and sliding speed on tribological properties of the Fe₃Al material. The worn surfaces of the Fe₃Al material are examined by a scanning electron microscope (SEM) and an X-ray photoelectron spectroscope (XPS). It is found that the Fe₃Al material exhibits better wear resistance than 1Cr18Ni9Ti stainless steel in the sulfuric acid corrosive environment. The wear rate of the Fe₃Al material is on the order of 10^?13 m³/m and increases with increasing load, but does not vary below the sliding speed of 0.08 m/s then dramatically increases with increasing sliding speed. The friction coefficient of the Fe₃Al material is in the range of 0.1–0.28, and slightly increases with increasing load, and does not vary with the increase of sliding speed. The Fe₃Al material occurs tribochemical reaction with the H₂SO₄ aqueous solution in the friction process. Wear mechanism of the Fe₃Al material is dominated by microploughing and corrosive wear.     

Dry sliding wear behaviors of La2O3–WSi2–MoSi2 composite against alloy steel

A molybdenum disilicide (MoSi₂) matrix composite with the addition of WSi₂ and La₂O₃ (RWM) was fabricated as a wear resistant material by self-propagating high temperature synthesis (SHS) and hot pressing (HP). This composite was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The wear resistance of MoSi₂ against steel is significantly improved by the addition of both WSi₂ and La₂O₃, and it is attributed to the increase in hardness and toughness of the composite. It is found that the wear behavior of the RWM is sensitive to sliding speed, load and hardness of the counter-face material. When worn against a steel with a lower hardness (A), the wear rate of RWM increases with an increase of sliding speed, and increases initially and then decreases with an increase of load. The material removal mechanisms varied from ploughing wear at low load and speed to serious adhesive wear at high load and speed. When worn against a steel with a higher hardness (B), the wear resistance of the RWM improved and the material removal mechanism were brittle fracture wear at low speed and adhesive wear at high speed.     

Third Body Behavior During Dry Sliding of Cold-Sprayed Al-Al2O3 Composites: In Situ Tribometry and Microanalysis

Reciprocating sliding wear experiments were conducted on cold-sprayed pure aluminum and Al–22.6 wt% Al₂O₃ coatings using a custom-built in situ tribometer. Using a transparent sapphire counterface for the wear tests, the dynamic behavior of third body material in the contact was optically observed. The presence of Al₂O₃ particles led to greater stability of the transfer films adhering to the sapphire counterface, as well as greater stability of the friction coefficient and lower wear rates. Ex situ microanalysis of material in the wear tracks and transfer films suggests that the presence of Al₂O₃ particles promoted strain localization during sliding. This produced more uniform third body microstructures and protected the underlying aluminum matrix from deformation, which slowed the rate of transfer to the counterface.     

Effect of Sliding Speed and Applied Load on Dry Sliding Tribological Performance of TiAl Matrix Self-lubricating Composites

More durable, low-friction self-lubricating materials in modern industry are greatly needed for tribological systems. The current paper presents the tribological performance of TiAl matrix self-lubricating composites (TSC) containing MoS₂, hexagonal BN and Ti₃SiC₂ designated as MhT against GCr15 steel counterface under several sliding speeds from 0.2 to 0.8 m s^?1 and applied loads from 6 to 12 N. The results suggested that MhT played an important role in decreasing friction coefficients and wear rates. The covering percentage of transfer layers on worn surfaces varied with the changing of sliding speeds and applied loads, hence resulting in the distinct friction and wear characteristics of TSC. TSC containing 10 wt% MhT exhibited the best excellent tribological performance at 10 N–0.8 m s^?1, which could be due to the formation of the best compaction and largest coverage of transfer layer on the worn surfaces.     

Dry sliding wear behaviour of polyamide 66 and polycarbonate composites

A study has been made of the reciprocating dry sliding wear behaviour of polyamide 66 and polycarbonate containing glass fibres, ultra high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE/2% Si oil). Studies have been conducted at sliding loads of 2 kg and 10 kg at an average velocity of 0.33 m s⁻¹ against a hardened stainless steel counterface with a surface roughness of 0.3 μm.It has been shown that additions of 10–15% of filler/reinforcement lead to greatly improved sliding wear behaviour. PTFE/2% Si oil filled polyamide 66 has been shown to have the best overall wear performance whilst the high glass filled variants of polyamide 66 and polycarbonate have the best combination of wear resistance and mechanical strength. These findings are discussed with reference to composite constitution and properties, thermal effects and counterface interactions. Explanations are advanced to account for the differences in behaviour inter alia the composite materials.     

Design of experiments approach to the study of tribological performance of Cu-concentrate-filled PPS composites

The tribological performance of copper-concentrate (CC) mineral deposit as the filler in polyphenylene sulfide (PPS) was studied as a function of the filler proportions and sliding test variables. CC is a complex mixture of CuS, Fe_xO_y, SiO₂, Al₂O₃, and other trace materials. The design of experiments based upon L₉ (3⁴) orthogonal arrays by Taguchi was used. Sliding tests were performed in the pin-on-disk configuration against a hardened tool steel (55-60 HRC) disk. The improvement in wear resistance of PPS was considerable with the use of fillers. The lowest steady state wear rate of 0.0030 mm³/km was obtained for PPS+20%CC+15%PTFE composition. It was two orders of magnitude lower than that of unfilled PPS. The variations in steady state coefficient of friction with the changes in filler proportions and sliding test variables were small. The transfer film was studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM). X-ray photoelectron microscopy (XPS) was used to detect chemical reactive species developed during sliding, especially in the interface between transfer film and its counterface. Wear particles and the polymer worn surfaces were analyzed by energy dispersive spectroscopy (EDS) for elemental distribution.     

Effect of transfer film structure, composition and bonding on the tribological behavior of polyphenylene sulfide filled with nano particles of TiO2, ZnO, CuO and SiC

The tribological behavior of polyphenylene sulfide (PPS) filled with inorganic nano particles was studied. The fillers investigated were TiO₂, ZnO, CuO and SiC whose sizes varied from 30 to 50 nm. The polymer composites were compression molded with varying proportions of these fillers. Wear and friction tests were performed in a pin-on-disk configuration at a sliding speed of 1.0 m/s, nominal pressure of 0.65 MPa, and counterface roughness of 0.10 μm Ra. The polymer composite pins slid against hardened tool steel counterfaces. The transfer films of the composite materials formed on the counterfaces during sliding were studied by optical microscopy and X-ray photoelectron spectroscopy (XPS) and the adhesion between the transfer film and counterface was measured in terms of the peel strength. It was found that the wear rate of PPS decreased when TiO₂ and CuO were used as the fillers but increased with ZnO and SiC fillers. The optimum wear resistance was obtained with 2 vol.% CuO or TiO₂. These filled composites had the coefficients of friction lower than that of the unfilled PPS. The wear behavior of the composites is explained in terms of the topography of transfer film and adhesion of transfer film to the counterface as observed from peel strength studies. There is a good correlation observed between the transfer film-counterface bond strength and wear resistance.     

Brush-on-disc simulation tribotesting of materials for gas turbine-compliant seal components

A brush-on-disc tribometer was used for high-speed, high-temperature sliding experiments on materials simulating conditions in turbine flexible element gas path seals. Of the polymeric, ceramic and metallic material pairs tested only superalloy wires sliding against ceramic surfaces were promising candidates. After extensive running-in, nickel-based wires wore at a rate close to 10⁻⁸ mm³/N m when sliding under moderate loads at speeds to 100 m/s and temperatures to 500°C against a Cr₃C₂/NiCr coated counterface whose wear was also very low. Thus, useful seal life under engine operating conditions may result, depending upon allowable initial and minimum effective brush-rotor interferences.     

The effect of sliding time and speed on the wear of composite materials

The friction and wear properties of an Al 201 alloy and a unidirectionally oriented graphite fiber-aluminum matrix composite (T50-Al 201) were investigated. The experiments were conducted on a pin-on-disc type friction machine. The diameter of the pin was 0.22 cm and the load 4.46 N. The sliding velocity varied between 0.17 and 0.43 m s^?1. The disc counterface was of commercially pure iron. It has been found that the friction coefficient μ and the wear rate W_L of the composite material decrease as the sliding time is increased until a steady state value is reached. The steady state wear rate is proportional to the reciprocal of the sliding speed in accord with a recently proposed model. Scanning electron microscopy and Auger electron spectroscopy observations indicate that the high initial values of μ and W_L are due to a high degree of matrix adhesion to the counterface accompanied by fiber breaking and transfer. The low steady state values of μ and W_L are due to the formation of a film that impedes adhesion and confers some degree of self-lubrication. It is suggested that the observed variation of W_L with sliding speed is related to changes in the degree of subsurface damage as the velocity is varied.     

Boric Acid Self-Lubrication of B2O3-Filled Polymer Composites

Utility of boric oxide particles in PTFE and epoxy composite materials, in sliding contact with stainless steel, is explored. Boric oxide filler can provide PTFE with a two-decade reduction in wear rate, to 10^?5 mm₃/N-m. With adequate ambient humidity reduced wear rate can be achieved without inducing counterface abrasion, and the friction of PTFE is further reduced slightly. In such environments, boric oxide fillers can also reduce friction coefficient of epoxy from μ>0.7 to as low as μ=0.07. This lubrication mechanism results from replenishment of lubricous boric acid lamellar solid provided to the sliding interface by reaction of boric oxide with ambient water. Maintenance of the lubricating effect depends upon a sufficient rate of boric acid formation, relative to subsequent removal by wear. It is demonstrated that this formation/removal balance is affected by relative humidity and volume fraction of boric oxide filler, as well as normal load and sliding speed.     

Sliding contacts for the pharmaceutical industry: failure analysis and dry sliding tests for the replacement of hard Cr on AISI 316L steel

Several alternatives were compared for the replacement of hard electroplated Cr coating to improve the tribological properties of the AISI 316L austenitic stainless steel for pharmaceutical packaging applications, including low temperature carburizing (LTC), thermal spray coatings (Al₂O₃-13TiO₂, WC-17Co), substitution of the AISI 301 reference counterface with polymeric materials (PTFE, UHMWPE, PEEK). In dry sliding block on ring tests, the LTC AISI 316L cylinders lead to the lowest wear rates of the AISI301 sliders under low loads (up to 10 N). When considering the polymer vs. uncoated AISI 316L couple, PEEK and UHMWPE lead to lower friction and comparable wear rates with respect to the reference couple (AISI 301 vs. Hard Cr coated-AISI 316L) in the whole range of tested loads.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 2. The analysis of wear mechanism

This paper describes the tribological performance of diamond-like carbon (DLC) coatings deposited on AISI 440C steel substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process. A variety of analytic techniques were used to characterize the coatings, such as Raman spectroscopy, atomic force microscopy (AFM) and nano-indentation. The sliding wear and friction experiments were carried out by the conventional ball-on-disk tribometry against 100Cr6 steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). All the wear tests were conducted under dry sliding condition in ambient air for a total rotation cycle of 1 × 10⁵ (sliding distance ∼2.2 km). Surfaces of the coatings and the steel balls were examined before and after the sliding wear tests. The DLC coatings that had been tested all showed relatively low values of friction coefficient, in the range of 0.1-0.2 at a steady-state stage, and low specific wear rates (on the order of 10⁻⁸ mm³/Nm). It was found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Plastic deformation became more evident on the coating surface during the sliding wear test at higher contact stresses. The friction-induced transformation of the coating surface into a graphite-like phase was revealed by micro-Raman analysis, and the flash temperature of the contact asperities was estimated. It was suggested that the structural transformation taking place within the wear tracks was mainly due to the formation of compact wear debris layer rather than the frictional heating effect. On the other hand, an adherent transfer layer (tribolayer) was formed on the counterface, which was closely related to the steady-state friction during sliding and the wear mechanisms. Fundamental knowledge combined with the present tribological study led to the conclusion that adhesive wear along with abrasion was probably the dominant wear mechanism for the DLC/steel sliding systems. Additionally, fatigue processes might also be involved in the wear of the coatings.