The Annealing Behavior of the Subsurface Zone Induced by Friction in Bismuth Detected by Positron Lifetime Technique

The annealing behavior of the subsurface zone (SZ) in pure bismuth induced by dry sliding was studied using the positron lifetime measurement. This measurement allows us to detect the SZ and its recovery, and recrystallization processes. The comparative measurements of the sample exposed to compression revealed the thermal stability of the SZ. The compressed sample rebuilt its structure due to the recovery and recrystallization processes at the temperature of 60 °C, whereas the sample exposed to dry sliding does it at higher temperature of 260 °C, which is close to the melting point. The isothermal annealing at the temperature of 100 °C confirmed these results. The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that the concentration of defects at the worn surface gradually decreases, but at the depth between 50 and 170 μm, the generation of new defects takes place at the temperature of 75, 100 and even at 175 °C. At the temperature of 175 °C, the defects still are extended up to the depth of about 60 μm from the worn surface. The results were qualitatively confirmed by the measurements of the Vickers microhardness depth profile. Similar annealing behavior of the SZ was observed in pure magnesium.     

Positron Studies of Subsurface Zone Created in Sliding Wear in Bismuth

The report about subsurface zone (SZ) created during dry sliding in pure bismuth is presented. The positron lifetime measurements revealed the well defined subsurface zone where the concentration of created during sliding vacancy clusters decreases exponentially with the depth from the worn surface increases. The experimental results are quite well described by the two state trapping model which allows to determine the positron trapping rate and hence deduce about the depth dependency of vacancy voids concentration. Despite of the low recrystallization temperature ca. 115 °C in this semimetal, determined also by the positron lifetime measurements, no significant effect of temperature rise due to sliding on the SZ constitution is observed.     

Positron Studies of Subsurface Zone in Pure Iron Induced by Sliding

Positron annihilation spectroscopy and microhardness measurements were performed for pure iron samples after dry sliding. Well-defined depth profiles of edge dislocations decorated by vacancies and vacancy clusters were observed in the subsurface zone (SZ) of the samples studied. The vacancy clusters contained more than ten vacancies and their concentration decreased with the depth from the worn surface. Like in other metals studied, the depth profile of the defects detected exhibited the exponential decay with the depth. The total depth of the SZ was correlated with the load applied during sliding but did not exceed 220 μm. The defect profile in the SZ was revealed also in raw iron samples after dry sliding and blasted iron samples. In the case of raw iron, the profile induced by sliding overlaps defect structure which already exists in a raw material.     

Effects of microstructure and experimental parameters on high stress abrasive wear behaviour of a 0.19 wt% C dual phase steel

The present investigation is aimed at understanding the influence of the size and quantity of ferrite plus martensite on mechanical and abrasive wear properties in a 0.19 wt% C dual phase steel. The results indicate that the mechanical properties like strength, ductility and impact, as well as abrasion resistance of the samples are greatly influenced by the material and test conditions. For example, the samples involving prior annealing showed higher ductility but less strength over the normalized specimens. Also, the increasing intercritical annealing temperature led to superior strength associated with reduced ductility. The wear rate increased with load and abrasive size due to a larger depth of cut made by the abrasive medium. The wear rate decreased as sliding distance increased. The steel subjected to prior normalizing treatment attained superior wear resistance to that of the one subjected to prior annealing treatment. The wear rate also decreased with increasing intercritical annealing temperature from 765 to 805 °C with an exception that the steel treated at 805 °C exhibited wear rate comparable to the one treated at 765 °C when tested against coarser size (40 μm) abrasive.     

Studies of high temperature sliding wear of metallic dissimilar interfaces II: Incoloy MA956 versus Stellite 6

The development of wear surfaces formed during limited debris retention sliding wear of Incoloy MA956 against Stellite 6 between room temperature and 750 °C, and sliding speeds of 0.314 and 0.905 m s⁻¹ (7 N applied load, 4522 m sliding distance) were investigated. At 0.314 m s⁻¹, mild oxidational wear was observed at all temperatures, due to oxidation of Stellite 6-sourced debris and transfer to the Incoloy MA956; this debris separated the Incoloy MA956 and Stellite 6 wear surfaces. Between room temperature and 450 °C, the debris mainly took the form of loose particles with limited compaction, whilst between 510 °C and 750 °C the debris were compacted and sintered together to form a Co–Cr-based, wear protective ‘glaze’ layer. The behaviour was identical to that previously observed on sliding Nimonic 80A versus Stellite 6 at 0.314 m s⁻¹.At 0.905 m s⁻¹, mild oxidational wear was only observed at room temperature and 270 °C and dominated by Incoloy MA956-sourced debris. At 390 and 450 °C, the absence of oxide debris allowed ‘metal-to-metal’ contact and resulted in intermediate temperature severe wear; losses in the form of ejected metallic debris were almost entirely Incoloy MA956-sourced. This severe wear regime was also observed from 510 up to 630 °C, but increasingly restricted to the early stages of wear by development of a wear protective Incoloy MA956-sourced ‘glaze’ layer. This ‘glaze’ layer formed so rapidly at 690 °C and 750 °C, that severe wear was all but eliminated and wear levels were kept low.The behaviour observed for Incoloy MA956 versus Stellite 6 at 0.905 m s⁻¹ contrasts sharply with that previously observed for Nimonic 80A versus Stellite 6, in that the Incoloy MA956-sourced high Fe–Cr debris formed a protective oxide ‘glaze’, whilst the Nimonic 80A-sourced Ni and Cr oxides formed an abrasive oxide that at high sliding speeds assisted wear. The data indicates that the tendency of oxide to form a ‘glaze’ is readily influenced by the chemistry of the oxides generated.     

Influence of temperature on erosion by a cavitating liquid jet

The influence of water temperature on cavitation erosion has previously been studied using a vibratory apparatus, but no researches have been conducted at a constant cavitation number in flow condition. This study deals with the influence of the water temperature on cavitation erosion using a cavitating jet apparatus. The optimum stand-off distance at 25 °C was 11, 15, 21 and 25 mm at cavitation number of σ = 0.03, 0.025, 0.02 and 0.015, respectively, and was almost the same as that of a guideline in the ASTM G134 standard. The optimum stand-off distance at 75 °C is similar to that at 25 °C. The erosion rate increases with the liquid temperature and reaches a peak, followed by a decrease. The relative temperature was defined as 0 °C for freezing temperature and 100 °C for boiling temperature of pressurized water. The peak appears at the approximate average of freezing and boiling relative temperatures. The erosion rate increases by 1%/ °C between 5 and 45 °C of relative temperatures, and decreases by 2%/ °C between 45 and 80 °C.     

Hardness properties and high-temperature wear behavior of nitrided AISI D2 tool steel, prior and after PAPVD coating

Wear experiments in the range of 25–600 °C have been conducted on samples of D2 tool steel in different conditions involving unnitrided, nitrided and nitrided and coated with Balinit^® A (TiN) and Balinit^® Futura (TiAlN) deposited industrially at Balzers (Amherst, NY, USA), by means of PAPVD. The results indicate that coating the nitrided D2 tool steel substrate with these two films gives rise to an improvement of 97% (TiN) and 99% (TiAlN) in the wear behavior at the test temperature of 300 °C, in comparison with the uncoated substrate. However, at a temperature of 600 °C, besides oxidation of the coatings, the mechanical strength of the substrate decreases giving rise to fracture and delamination of the films. At this temperature the uncoated substrate exhibited the highest resistance to sliding wear, presumably due to the formation of a well bonded surface glazed layer which gives rise to a significant reduction in the friction coefficient. The indentation experiments that were conducted with the nitrided steel substrate and the coated systems indicates that the nitriding process applied to the D2 steel prior to PAPVD coating provides a satisfactory load support which contributes to the improvement of the coated systems capability to withstand indentation loads at room temperature. In this regard, the coated system with a TiAlN coating displayed a better behavior than that shown by the system with a TiN coating. An experimental procedure is proposed in order to predict the hardness profile of the nitrided tool steel, along the cross section of the material, just from hardness measurements taken on the surface of the sample, employing different indentation loads.     

Application of Positron Annihilation Studies of the Subsurface Zones Beneath the Surface Exposed to Normal Loading

This paper presents an experimental study of the subsurface zones in copper induced by normal loading of a spherical indenter, detected by the positron annihilation technique. The obtained depth dependencies of the S-parameter were compared with the principal stress distributions beneath the ball predicted by the elastic theory. Our studies show that the depth-concentration profile of open-volume defects is not uniform and exhibits a local minimum. Nevertheless the concentration profile decays with increasing depth, reaching a value for residual defect concentration at a depth where the von Mises criterion for yield is fulfilled. For comparison, we performed studies of subsurface zones which occurred in compressed copper samples and a sample whose surface was blasted by silicon carbide particles. The detected zones exhibit quite different features in both cases. A similarity between the dependency of the S-parameter on the depth obtained for the case when the surface was blasted and when the surface was exposed to sliding contact during the friction process was found.     

The detection of subsurface zones in aluminium-based alloys 2017A and 6101A using a positron annihilation technique

This paper presents positron lifetime studies of the subsurface region in aluminium-based alloys 2017A and 6101A, the surfaces of which were exposed to dry sliding. In the case of the 2017A alloy, the total range of the subsurface zone below the worn surface detected using the positron annihilation technique was larger than 120 μm and this value was hardly affected by load and sliding distance. For the 6101A alloy, the subsurface zone ranged from 30 to 500 μm and depended on the applied load and only to some extent on the sliding distance. The obtained results are significantly different from the results achieved for pure aluminium, thus alloying has considerable effect on the subsurface zone formation. The positron lifetime depth distribution characterizing the subsurface zone was correlated with the microhardness profile.     

The subsurface zone in magnesium alloy studied by positron annihilation techniques

This paper presents the positron lifetime and Doppler broadening of annihilation line studies of the subsurface region in a magnesium-based alloy which was exposed to dry sliding. The total range of the subsurface zone below the worn surface detected using these techniques was lower than 100 μm and was hardly affected by the applied load and sliding distance. The obtained results are significantly different from the results achieved for pure magnesium, thus alloying has considerable effect on the subsurface zone formation. The positron lifetime profile of the subsurface region was well correlated with the microhardness profile. The significant result was that the weak long-lived component indicated ortho-positronium formation has been found at the depth lower than 30 μm. This indicates the formation of voids below the worn surface.     

Reciprocating sliding wear of 9% Cr steel in carbon dioxide at elevated temperatures

Experiments were conducted on the initial stages of reciprocating sliding wear of a 9% chromium steel in an environment of carbon dioxide at temperatures in the range 200 to 550°C. At ambient temperatures of 290°C and above, an initial severe wear mode was followed by a transition to mild oxidational wear. At any given ambient temperature above 290°C, the distance of sliding required to reach such a transition was found to depend on load and mean sliding speed, although the dependency on speed was not simple. When a transition occurred, most of the surfaces were covered with a stable oxide film which consisted of an agglomerate layer of wear debris being mainly of oxide at the surface and mainly at the metal boundary. This film was supported by a work hardened layer extending for about 30 μm into the bulk of the metal. A surface model is proposed to explain the mechanism of formation of the supportive oxide layer; predictions of volume of material removed and final oxide coverage at the transition are in close agreement with experimental values     

Wear transitions and stability of polyoxymethylene homopolymer in highly loaded applications compared to small-scale testing

Wear and failure mechanisms of polyoxymethylene homopolymer (POM-H) loaded above its yield strength are studied on test samples with a 22500 mm² sliding area at 8–150 MPa contact pressures. Test results are compared to small-scale cylinder-on-plate tests. Plastification of the sliding surfaces at high loads is favourable for low friction, while different wear mechanisms compared to small-scale testing are induced. Small-scale tests show a transition from mild adhesive/abrasive wear to severe wear due to softening, which is characterised by the formation of shear lips. Softening of large-scale sliding surfaces does not cause overload but it contributes to stable wear rates. Overload of large-scale samples is characterised by the transition from softening to melting and degradation. The dimensional stability of polymer elements is influenced by creep and it is verified that deformation of small-scale samples and large-scale samples loaded at 8 MPa is recovered after sliding, while it remains as permanent deformation for large-scale tests at 16–150 MPa. The wear transitions are further analysed by optical microscopy and available temperature models. The flash temperature concept can be applied for small-scale tests and large-scale tests up to 8 MPa. Calculated flash temperatures indicate softening and are in agreement with visual observations of the polymer surfaces. Flash temperatures for large-scale tests at 16–150 MPa indicate melting and degradation that was not visually observed on the polymer surfaces. The bulk temperature model prevails during large-scale sliding and only indicates melting at 150 MPa. Thermal analysis of the worn polymer surfaces confirms that crystallisation happened during small-scale sliding and large-scale sliding up to 55 MPa, occurring between 120 and 150 °C. Thermo-oxidative degradation above 200 °C is evidenced at 150 MPa.     

Sliding abrasion resistance assessment of metallic materials for elevated temperature mineral processing conditions

The multiplicity of harsh environments in mining, processing and transporting ore and related waste, cause severe wear, extremely high maintenance costs and lost production.Elevated temperature processing is one of the conditions that influence the performance of possible materials of construction. This takes the forms of reduced hardness and strength, deleterious changes in the structure and properties of materials during protracted exposure and increased oxidation and corrosion.Drag chain conveying of hot solids e.g. in smelting, typically results in three-body sliding abrasion and adhesive wear of connecting pins and hole surfaces in link assemblies and of moving paddles that impel the particulates in enclosed channels. Selected materials have been assessed for this type of service under reciprocating sliding abrasion contact conditions using an adapted Cameron-Plint TE77 wear rig at 20 °C and 350 °C. These include the current carburised low alloy steel, other steels, Cr white irons and Co-based alloys in bulk, overlay and surface treated forms.Examination of wear scars, using scanning electron microscopy, identified the main wear mechanisms affecting the highly resistant powder metallurgical (PM) tool steels and HVOF coating as micro-scratching and as indentation leading to micro-fracture. Materials with lowest resistance displayed evidence of significant material removal by micro-ploughing. The formation of oxide layers on some samples during testing appeared to be beneficial.     

Hot wear properties of ceramic and basalt fiber reinforced hybrid friction materials

In the present study, hybrid friction materials were manufactured using ceramic and basalt fibers. Ceramic fiber content was kept constant at 10 vol% and basalt fiber content was changed between 0 to 40 vol%. Mechanical properties and friction and wear characteristics of friction materials were determined using a pin-on-disc type apparatus against a cast iron counterface in the sliding speeds of 3.2–12.8 m/s, disc temperature of 100–350 °C and applied loads of 312.5–625 N. The worn surfaces of the specimens were examined by SEM. Experiments show that fiber content has a significant influence on the mechanical and tribological properties of the composites. The friction coefficient of the hybrid friction materials was increased with increasing additional basalt fiber content. But the specific wear rates of the composites decreased up to 30 vol% fiber content and then increased again above this value. The wear tests showed that the coefficient of friction decreases with increasing load and speed but increases with increasing disc temperature up to 300 °C. The most important factor effecting wear rate was the disc temperature followed by sliding speed. The materials showing higher specific wear rates gave relatively coarser wear particles. XRD studies showed that Fe and Fe₂O₃ were present in wear debris at severe wear conditions which is indicating the disc wear.     

Study of tribofilms formed during dry sliding of Ta2AlC/Ag or Cr2AlC/Ag composites against Ni-based superalloys and Al2O3

Herein we show that dry sliding, in air, in the 25–550 °C temperature range, of the novel Ta₂AlC/20 vol.% Ag and Cr₂AlC/20 vol.% Ag composites against Ni-based superalloys (SAs) and alumina led to the formation of steady-state tribofilms whose thicknesses and compositions varied depending on sliding conditions. At elevated temperatures, under both isothermal and thermocyclic conditions, relatively thick (>0.5 μm) well-compacted “glaze” lubricious tribofilms were developed as a result of joint action of tribo-chemical and tribo-mechanical factors involving repeated tribo-oxidation, mixing, fracturing, sintering, etc. They were mainly composed of oxidized constituents from both counterparts (if slid vs. SA) or solely from MAX-Ag ones (vs. Al₂O₃) and possessed a fine multi-layered microstructure, i.e. a more oxidized thin outermost layer and a less oxidized carbide-containing thicker inner layer. During thermocycling the tribofilms adapted to 25–550 °C temperature variations and preserved their primary macro- and microstructure, hardness, good adhesion to the matrix and lubricating characteristics. Consequently, the tribological properties of MAX-Ag/SA tribocouples did not deteriorate, but slightly improved with sliding distance. A possible mechanism of tribofilm generation and their transformations at various temperatures is discussed. The effect of SA and alumina counterparts on the formation and degradation of the tribofilms are also discussed.     

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.     

Subsurface Zone in Aluminium Studied by Positron Lifetime Spectroscopy

The subsurface zone in the well-annealed pure 99.999% aluminium exposed to the sliding of a graphite pin was studied using positron lifetime spectroscopy. This experimental method allowed to detect the dislocation lines, dislocation lines with opposite jogs and single vacancies that occur during sliding. The type of defect changes upon increase of the sliding time and depends on the distance from the damaged surface. It was established that the total depth of defect distribution depends on the applied load and extends from 150 to 450 m.     

Fretting corrosion of lubricated tin plated copper alloy contacts: Effect of temperature

The fretting corrosion behaviour of lubricated tin plated copper alloy contacts at ambient and elevated temperatures is addressed in this paper. At 27 °C, lubrication is very effective and the contact resistance remains stable for several thousand fretting cycles whereas at elevated temperatures (155 °C) the performance of lubricated contact is not appreciable. Surface profile and surface roughness confirm that the lubricated contacts have a smoother profile and experience a lesser damage at the contact zone at ambient as well as at elevated temperatures. The mechanism of fretting corrosion of tin plated contacts appears to be similar with and without lubrication at all the temperatures studied. The difference in performance of the lubricated contacts at ambient and elevated temperatures is due to the faster wear rate of tin coating at elevated temperatures. Oxidation of the contact zone of the lubricated contacts is prevented at all temperatures studied. The study concludes that lubrication is effective in improving the life of the tin plated copper alloy contacts under fretting conditions at ambient temperatures whereas at elevated temperatures lubrication provides only a marginal improvement in performance. The decrease in performance of lubricated tin plated contacts at elevated temperatures is due to the higher wear rate of tin coating and not due to evaporation of the lubricant.     

Effect of temperature on friction and oscillating sliding wear of dense and porous 3Y-TZP zirconia ceramics

Microstructures of 3 mol% Y₂O₃-ZrO₂ (3Y-TZP) with systematically varying porosity up to about 15% were produced by sintering. Hardness and fracture toughness of the ceramics as well as the amount of tetragonal, cubic and monoclinic phase were measured. Wear tests were carried out on the different self-mated microstructures under dry reciprocating sliding contact using ring-on-block geometries in air at five different contact temperatures up to 500°C. The microstructures and worn surfaces were extensively analysed using scanning electron microscopy (SEM) and X-ray diffraction techniques. The experimental results revealed a reduction of the amount of wear (independent of porosity) by more than one order of magnitude compared with room temperature if the test temperature was increased to 250°C. Between room temperature and 250°C, wear increased with increasing porosity while at 500°C the highest wear was measured on the dense structure. Microscopic observations showed that plastic deformation, surface layers consisting of compacted wear debris and also intercrystalline, transcrystalline or delamination type fracture influenced friction and wear.     

Sliding wear behaviour of Ca α-sialon ceramics at 600 °C in air

As an extension of a previous investigation on the wear behaviour of Ca α-sialon ceramics of differing microstructures at room temperature, wear testing was conducted at 600 °C in air to explore the effects of microstructure, contact pressure and sliding speed on the wear behaviour. Under all loading conditions from 1 MPa to 1 GPa, a constant high friction coefficient was observed and a severe wear process was dominant, in which the sliding contact induced cracks were observed in different microstructures. Wear particles were generated along the wear track, but no tribofilm was detected. Increasing the sliding speed from 10 to 23 cm/s was found to significantly increase wear rate. However, variations in microstructure had little impact. That is, large elongated-grained α-sialon exhibited only a slightly lower wear rate than fine equiaxed-grained α-sialon.