Studies of high temperature sliding wear of metallic dissimilar interfaces III: Incoloy MA956 versus Incoloy 800HT

Wear variations of Incoloy MA956 slid against Incoloy 800HT between room temperature and 750 °C, and sliding speeds of 0.314, 0.654 and 0.905 m s⁻¹ were investigated using a ‘reciprocating block-on-cylinder’ (low debris retention) configuration.Three forms of wear depending largely on sliding temperature were observed: ‘severe wear’ with high transfer between room temperature and 270 °C, ‘severe wear’ with low transfer between 390 and 570 °C and ‘glaze formation’ (retarded by increased sliding speed) at 630 °C and above. The differences in wear behaviour are discussed, with wear behaviour mapped and wear surfaces at 750 °C (0.314 and 0.905 m s⁻¹) cross-sectioned and profiled.     

Studies of high temperature sliding wear of metallic dissimilar interfaces IV: Nimonic 80A versus Incoloy 800HT

Wear variations of Nimonic 80A slid against Incoloy 800HT between room temperature (RT) and 750 °C, and sliding speeds of 0.314 and 0.905 m s⁻¹ were investigated using a ‘reciprocating-block-on-cylinder’, low debris retention configuration. These were considered alongside previous observations at 0.654 m s⁻¹.Different wear types occurring were mapped, including high transfer ‘severe wear’ (RT and 270 °C, also 0.905 m s⁻¹ at ≤570°C), low transfer ‘severe wear’ (0.314 m s⁻¹ at 390 °C to 510 °C oxide abrasion assisted at 510 °C), and ‘mild wear’ (0.314 m s⁻¹ at ≥570 °C; 0.905 m s⁻¹ at ≥630 °C). Wear surfaces at 750 °C were cross-sectioned and profiled.     

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.     

Studies of high temperature sliding wear of metallic dissimilar interfaces

The evolution of microstructures in the glaze layer formed during limited debris retention sliding wear of Nimonic 80A against Stellite 6 at 750 °C and a sliding speed of 0.314 m s⁻¹ (7 N applied load, 4522 m sliding distance) was investigated using scanning electron microscopy (SEM), energy dispersive analysis by X-ray (EDX), X-ray diffraction (XRD), scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM). The collected data indicate the development of a wear resistant nano-structured glaze layer. The process of ‘fragmentation’ involving deformation, generation of dislocations, formation of sub-grains and their increasing refinement causing increasing misorientation was responsible for the formation of nano-structured grains. The rapid formation of this glaze layer from primarily cobalt–chromium debris transferred from (and also back to) the surface of the Stellite 6, kept wear of both the Nimonic 80A and Stellite 6 to very low levels.However, increasing the sliding speed to 0.905 m s⁻¹ (750 °C) suppressed glaze formation with only a patchy, unstable glaze forming on the Stellite 6 counterface and an absence of glaze development on the Nimonic 80A sample (the Nimonic 80A surface was covered with at most, a very thinly smeared layer of oxide). The high levels of oxide debris generated at 0.905 m s⁻¹ instead acted as a loose abrasive assisting wear of especially the Nimonic 80A. This behaviour was attributed to a change in oxide chemistry (due to the dominance of nickel and chromium oxides generated from the Nimonic 80A) resulting in poor oxide sintering characteristics, in combination with increased mobility and reduced residency of the oxide debris at 0.905 m s⁻¹.     

Development of a simple ‘temperature versus sliding speed’ wear map for the sliding wear behaviour of dissimilar metallic interfaces

The variation in wear behaviour during limited debris retention sliding wear of Nimonic 80A versus Stellite 6 (counterface) between room temperature and 750 °C, at sliding speeds of 0.314, 0.654 and 0.905 m s⁻¹, was investigated. At 0.314 m s⁻¹, mild oxidational wear was observed at all temperatures, due to transfer and oxidation of Stellite 6-sourced debris to the Nimonic 80A and resultant separation of the Nimonic 80A and Stellite 6 wear surfaces. Between room temperature and 450 °C, this debris mostly remained in the form of loose particles (with only limited compaction), whilst between 510 and 750 °C, the particles were compacted and sintered together to form a wear protective ‘glaze’ layer.At 0.654 and 0.905 m s⁻¹, mild oxidational wear due to transfer and oxidation of Stellite 6-sourced debris was only observed at room temperature and 270 °C (also 390 °C at 0.654 m s⁻¹). At 390 °C (450 °C at 0.654 m s⁻¹) and above, this oxide was completely absent and ‘metal-to-metal’ contact resulted in an intermediate temperature severe wear regime—losses in the form of ejected metallic debris were sourced almost completely from the Nimonic 80A. Oxide debris, this time sourced from the Nimonic 80A sample, did not reappear until 570 °C (630 °C at 0.654 m s⁻¹), however, were insufficient to eliminate completely severe wear until 690 and 750 °C. At both 0.654 and 0.905 m s⁻¹, the oxide now preventing severe wear at 690 and 750 °C tended not to form ‘glaze’ layers on the surface of the Nimonic 80A and instead supported continued high wear by abrasion. This abrasive action was attributed to the poor sintering characteristics of the Nimonic 80A-sourced oxide, in combination with the oxides’ increased mobility and decreased residency.The collected data were used to compose a simple wear map detailing the effects of sliding speed and temperature on the wear of Nimonic 80A slid against Stellite 6, at these speeds and temperatures of between room temperature and 750 °C.     

Influence of Cr content on tribological properties of Ni3Al matrix high temperature self-lubricating composites

High temperature self-lubricating composites Ni₃Al-BaF₂-CaF₂-Ag-Cr were fabricated by powder metallurgy technique. In this paper the effect of Cr content on tribological properties at a wide temperature range starting from room temperature to 1000 °C was investigated. It was found that Ni₃Al matrix composite with 20 wt% Cr exhibited low friction coefficient of 0.24-0.37 and a wear rate of 0.52-2.32×10⁻⁴ mm³ N⁻¹ m⁻¹. Especially at 800 °C it showed the lowest friction coefficient of 0.24 and a favorable wear rate of 0.71×10⁻⁴ mm³ N⁻¹ m⁻¹. This implied that 20 wt% Cr was the optimal Cr content and its excellent tribological performance could be attributed to the balance between strength and lubricity.     

Influence of flow conditions on the corrosion of AISI 304L stainless steel

Hydrodynamic and electrochemical noise measurements (ENMs), of AISI 304L stainless steel, were made in a pipe test section of 28 mm inside diameter for a range of flow regimes from laminar to turbulent. Mean flow velocities through the test section were controlled at 0.04, 0.07, 0.11, 0.36, 1.8 and 2.7 m s⁻¹, equivalent to Reynolds numbers of 1000, 2000, 3000, 10 000, 50 000 and 75 000, respectively. Standard hydrodynamic parameters were employed to characterise and evaluate the complex interrelationship between the mass transfer rate of oxygen and momentum transfer through turbulence to the metal/solution interface. For AISI 304L stainless steel, pitting typically occurs in the form of metastable pits which either repassivated before achieving stability or grow to become stable pits. Metastable pitting was evident under all flow regimes. The fluid flow, whether laminar or turbulent, had little overall effect on the nucleation rates of metastable pitting events. Conversely, stable pit growth was most evident during laminar flow immediately before the transition to turbulent flow and close to the critical velocity (∼1.5 m s⁻¹).     

Erosive wear of advanced composites based on WC

WC based and yttria stabilized zirconia doped (13 vol%) cermets with different metal binders (Co, Ni or Fe) have been successfully produced and tested for performance in erosive media (silica abrasive particles; particle velocity of 80 ms⁻¹ and impact angles of 30° and 90°). An increase in fracture toughness and erosion resistance was shown to be influenced by tetragonal zirconia transformability. Wear performance of the newly developed cermets is found to be highly dependent on sintering conditions.     

Investigation into the wear behaviour of Stellite 6 during rotation as an unlubricated bearing at 600 °C

The wear behaviour of Stellite 6 was studied during rotational sliding in a bespoke bearing rig at 600 °C for times between 2 min and 12 h. Six stages of wear were identified: (i) formation of a mixed oxide ‘glaze’, (ii) cobalt and chromium elemental diffusion to the ‘glaze’ surface forming chromium- and cobalt-dominated oxide layers, (iii) oxygen diffusion into the ‘glaze’ leading to a chromium-dominated oxide layer at the ‘glaze’/substrate interface, (iv) spallation of the ‘glaze’ through chemical failure, (v) re-formation of the ‘glaze’ and (vi) elemental diffusion within the ‘glaze’, again resulting in discrete oxide layer formation.     

Thermal diffusivity measurement of Au nanofluids of very low concentration by using photoflash technique

In this paper, the application of photoflash technique to measuring the thermal diffusivity of gold nanofluids of very low concentration at room temperature was presented. The nanofluid samples were prepared from the pulse laser ablation procedure. The thermal diffusivity was obtained by fitting the theoretical temperature signal to the experimental data, and it was found to increase linearly from 1.47 × 10⁻³ cm² s⁻¹ to 1.68 × 10⁻³ cm² s⁻¹ as the concentration increased from 1.11 mg/L to 3.18 mg/L. The increase in thermal diffusivity in these multidispersed nanofluids was attributed to the higher nanoparticle concentration as well as to the increasing presence of the smaller size nanoparticles.     

Measurements of plastic strain around indentations caused by the impact of round and angular particles, and the origin of erosion

The aim of this paper is to examine plastic strain distributions around indentations and to consider the mechanisms of erosion damage caused by solid particle impact. A WC ball and an angular SiC particle of 3 mm in diameter were used to compare the effect of particle shape on plastic strain. Measurements of principal shearing strain distributions around the indentations were performed on surfaces of aluminum, iron and cast iron at impact angles of 20°, 30°, 40°, 60° and 90° at impact velocities from 50 to 200 m s⁻¹. It was found that the impact angle dependence was roughly consistent with the maximum principal shearing strain and erosion damage data, which have been published in previous papers and obtained during additional works in this study. The surface topography of the impact craters suggested that depth, contact area and volume of indentation are affected by the particle density and the hardness of both particle and target material. Measurements of volume ratio of lips to craters proved that material removal did not necessarily occur at a single impact of the WC ball, but occurred at the impact of the angular SiC particle at low impact angles. It is concluded that the origin of erosion is probably attributed to the conjoint actions of high plastic strains followed by subsequent removal and the cutting process caused by particle impact.     

Investigations of microstructures and defect structures in wear affected region created on Nimonic 80A during high temperature wear

The microstructures of a wear induced surface glazed layers formed during sliding wear of Nimonic 80A against Stellite 6 at 20–750 °C using a speed of 0.314 m s^-1 under a load of 7 N have been investigated using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) in combination with energy dispersive X-ray (EDX) analysis. The defects formed in the glazed layers were measured by positron lifetime spectroscopy. The results indicate the formation of a wear resistant nanostructured glazed layer. Positron lifetime and Doppler-broadening measurements demonstrated that the defects (mainly dislocations) existed in the glazed layers at low temperatures which increasing wear test temperature led to decrease in defects density. Positron measurements also suggested that, at the annealing temperature (1200 °C), the presence of dislocations might lead to the formation of ordered or partially ordered regions in Nimonic 80A.     

Ni3Al matrix high temperature self-lubricating composites

A Ni₃Al matrix high temperature self-lubricating composite Ni₃Al-BaF₂-CaF₂-Ag-Cr was fabricated by the powder metallurgy technique, and tribological behavior at a wide temperature range from room temperature to 800 °C was investigated. The results indicated that the composite exhibited low friction coefficients (0.30-0.36) and wear rates (0.65-2.45×10⁻⁴ mm³ N⁻¹ m⁻¹). It was found that the low friction coefficient was attributed to the synergistic effects of Ag, fluorides and chromates formed in the tribo-chemical reaction at high temperatures. The low wear rate of the composite was due to the high strength and the excellent lubricating properties.     

High-temperature erosion-oxidation of uncoated and FB-CVD aluminized and aluminized-siliconized 9Cr-1Mo steel under fluidized-bed conditions

Fluidized-bed combustion is one of the methods to generate energy in a clean and efficient way from a variety of fuels. However, conditions in fluidized-bed boilers: high temperature, oxidizing atmosphere and impacts by fluidized sand particles, can cause significant degradation of some boiler components, such as heat exchangers, by a combination of oxidation attack and erosion wear. Protective coatings, deposited mainly by thermal spraying and diffusion techniques, are considered a solution to extend the lifetime of such components. This paper allows evaluation whether diffusion coatings, applied using a fluidized-bed chemical vapour deposition (FB-CVD) technique, could be used to provide protection for 9Cr-1Mo steel against high-temperature erosion-oxidation.In this paper, the results from laboratory studies of the erosion-oxidation behaviour of uncoated, aluminized and aluminized-siliconized 9Cr-1Mo steel, subjected to air, at temperatures of 550-700 °C and impacts by 200 μm silica sand particles, are presented. The tests were carried out in a fluidized-bed rig, using speeds of 7.0-9.2 m s⁻¹ and angles of 30° and 90°. Erosion-oxidation damage was characterized by measurement of the mean thickness changes using a micrometer and examination of worn surfaces by scanning electron microscopy. The results show that the coatings, particularly the aluminized-siliconized coating, protect the 9Cr-1Mo steel for some period of the test under the given conditions, but, once the coatings are penetrated, aluminizing and aluminizing-siliconizing are no longer effective in preventing erosion-oxidation of the substrate. The interactions between erosion and oxidation processes are discussed and explanations for differences in behaviour of uncoated and coated specimens are presented. Finally, the challenges in developing thicker coatings to provide longer term protection of the steel against erosion-oxidation are considered.     

The corrosion wear behaviour of selected stainless steels in potash brine

Tests were conducted at 25 and 85 °C to evaluate the corrosion wear resistance of selected stainless steels in potash brine using a reciprocating motion wear apparatus. Four materials were tested: Ferralium 255 (UNS S32550), AL6XN (UNS N08367), 254SMO (UNS S31254) and AISI 1018 (UNS G10180) for comparative purposes. The evaluation methods employed included weight loss analysis, optical microscopy and scanning electron microscopy (SEM). The results show that Ferralium 255 has superior corrosion wear resistance in potash brine environment compared to AISI 1018 plain-carbon steel and the other stainless steels tested. Wear surface analysis using SEM shows evidence of brittle fracture damage, which is attributed to the presence of Cl⁻.     

Mechanical properties of the alloy Ti-6Al-4V irradiated with swift Kr ion

The radiation damage effect on the friction coefficient, wear, and microhardness of the alloy Ti-6Al-4V after 250 MeV krypton ion irradiation was studied. Tribological measurements were made in air, oxygen, and argon atmospheres and in a vacuum. The smallest friction coefficient for the irradiated samples occurred in argon and the vacuum. The wear of the unirradiated samples and those irradiated with low fluences (<10¹³ cm⁻²) increased in the vacuum and argon atmosphere. Wear was significantly reduced after irradiation with the fluence of 10¹⁴ cm⁻². The microhardness of the alloy Ti-6Al-4V increased by over 25% after irradiation with a large fluence.     

Friction and wear behavior of electroless Ni-based CNT composite coatings

Ni-based carbon nanotube (CNT) composite coatings with different volume fraction (from 5 to 12 vol.%) of CNTs were deposited on medium carbon steel substrates by electroless plating. The friction and wear behavior of the composite coatings were investigated using a pin-on-disk wear tester under unlubricated condition. Friction and wear tests were conducted at a sliding speed of 0.0623 m s⁻¹ and at an applied load of 20 N. The experimental results indicated that the friction coefficient of the composite coatings decreased with increasing the volume fraction of CNTs due to self-lubrication and unique topological structure of CNTs. Within the range of volume fraction of CNTs from 0 to 11.2%, the wear rate of the composite coatings showed a steadily decreasing trend with increasing volume fraction of CNTs. Because of the conglomeration of CNTs in the matrix, however, the wear rate of the composite coatings increased with further increasing the volume fraction of CNTs.     

Grain boundary enrichment in the FePt polymorphic A1 to L10 phase transformation

A series of Fe_54±1Pt_46±1 thin films have been sputter-deposited and annealed at various times and temperatures to facilitate the A1 to L1₀ polymorphic phase transformation. The annealing times span one minute to tens of minutes over temperatures of 300–800 °C. The films were characterized by X-ray and electron diffraction and atom probe tomography. This time–temperature regime provides ‘snap-shots’ into the compositional segregation evolution at the grain boundaries during the polymorphic phase transformation. The as-deposited A1 phase showed a preferential segregation of Pt to the grain boundaries. The reduction of Pt enrichment at the boundaries was observed for all L1₀ ordered films.     

Accurate conductance measurements of a pinhole orifice using a constant-pressure flowmeter

A pinhole orifice with a known conductance can be used as a secondary flow standard. Commercially available laser-drilled pinhole orifices with diameters ranging from 1.0 μm to 50 μm can have molecular-flow conductances ranging from about 0.1 μL/s to 200 μL/s for N₂ at 23 °C. Gas flows of 10⁻¹¹–10⁻⁶ mol/s can easily be produced by applying an upstream pressure in the range of 1–10⁵ Pa. Accurate measurements of the orifice conductance as a function of pressure are required to use the pinhole orifice as a basis of a flowmeter. We use a constant-pressure flowmeter to make accurate measurements of the conductance of a 20 μm orifice as a function of pressure for gas flows of Ar and N₂ into vacuum. We present results of these conductance measurements for an orifice with a nominal diameter of 20 μm. The N₂ conductance of this orifice ranged from 30 μL/s to 60 μL/s over the range of pressures investigated, and was measured with an uncertainty of better than 0.2% (k = 2) for upstream pressures greater than 10 Pa.     

Metrological characterization of the new 1 MN force standard machine of NPL India

Since November 2010, NPL India’s force scale has been complemented in the range from 10 kN to 1 MN by a further force standard machine. This automatically working 1 MN force standard machine utilizes a lever amplification of a 100 kN mass stack and enables low relative expanded uncertainties of smaller than 9 × 10⁻⁵ on the lever, and 2 × 10⁻⁵ on the deadweight side. In this paper, the constructional design of the machine is described. According to the new EURAMET Calibration Guide, a model for the uncertainties is developed.Supplementary to this, results from comparison measurements of the new NPL India machine with PTB´s force standard machines are presented.