Tribological properties of flame sprayed Fe–Ni–RE alloy coatings under reciprocating sliding

Fe–Ni–RE self-fluxing alloy powders were flame sprayed onto 1045 carbon steel. The tribological properties of Fe–Ni–RE alloy coatings under dry sliding against SAE52100 steel at ambient conditions were studied on an Optimol SRV oscillating friction and wear tester in a ball-on-disc contact configuration. Effects of load and sliding speed on tribological properties of the Fe–Ni–RE coatings were investigated. The worn surfaces of the Fe–Ni–RE alloy coatings were examined with a scanning electron microscopy(SEM) and an energy-dispersive spectroscopy(EDS). It was found that the Fe–Ni–RE alloy coatings had better wear resistance than the SAE52100 steel. An adhered oxide debris layer was formed on the worn surface in friction. Area of the friction layer varied with variety of sliding speed, but did not vary with load. The oxide layer contributed to decreased wear, but increased friction. Wear rate of the material increased with the load, but dramatically decreased at first and then slightly decreased the sliding speed. The friction coefficient of the material was 0.40-0.58, and decreased slightly with the load, but increased with sliding speed at first, and then tended to be a constant value. Wear mechanism of the coatings was oxidation wear and a large amount of counterpart material was transferred to the coatings.     

Tribological behaviour and wear mechanism of MoS2–Cr coatings sliding against various counterbody

MoS₂–Cr coatings with different Cr contents have been deposited on high speed steel substrates by closed field unbalanced magnetron (CFUBM) sputtering. The tribological properties of the coatings have been tested against different counterbodies under dry conditions using an oscillating friction and wear tester. The coating microstructures, mechanical properties and wear resistance vary according to the Cr metal-content. MoS₂ tribological properties are improved with a Cr metal dopant in the MoS₂ matrix. The optimum Cr content varies with different counterbodies. Showing especially good tribological properties were MoS₂–Cr8% coating sliding against either AISI 1045 steel or AA 6061 aluminum alloy, and MoS₂–Cr5% coating sliding against bronze. Enhanced tribological behavior included low wear depth on coating, low wear width on counterbody, low friction coefficients and long durability.     

Tribological properties of plasma sprayed and laser remelted 75/25 Cr3C2/NiCr coatings

A CO₂ laser was used to fuse based Ni–Cr Cr₃C₂ coatings for the purpose of homogenizing their microstructures and eliminating their porosity. Tests of layer control and wear resistance were carried out on the samples treated with the laser. The results have shown that laser remelting improves the microstructure of the coatings, increases the coating microhardness, and improves coating–substrate adherence. The dry sliding wear behaviour was characterized by the existence of two periods. During the first period the square of the wear volume is proportional to the sliding distance. During the second, the wear volume is proportional to the sliding distance.     

Structure and mechanical properties of NiAl and Ni3Al-based alloys

The investigation of Ni–Al–Fe–Ti–B alloys was carried out to determine the influence of iron and small titanium and boron additions on the phase composition, microstructure and mechanical characteristic, particularly with respect to high-temperature deformation conditions. These alloys, containing Al 35.8 at% and Fe 3.6–8.6–17.6 at% were prepared from high-purity components and Al master alloy containing Ti₂B particles. The influence of alloying additions of chromium and iron on the mechanical properties of directionally solidified Ni–Al–Cr–Fe alloy was investigated. Additions of both Cr 8 at% and Fe 2 at% result in higher strength than exhibited by unalloyed Ni₃Al. However, the ductility is reduced by the formation of the β′ phase. The typical, lamellar structure of Ni–20Al–8Cr–2Fe alloy undergoes coagulation during a high-temperature deformation process. The sequence of structural changes of NiAl and Ni₃Al-based alloys has been correlated with mechanical characteristics of high-temperature deformation process, determined in uni-axial compression tests. Two ranges of work hardening have been identified on the stress–strain curves of these alloys. It has been found that the first range of the deformation of Ni–Al–Fe–Ti–B alloys corresponds to the intergranular slip system operating within individual grains, while the second one is connected with transgranular slip. In the directionally solidified Ni–20Al–8Cr–2Fe alloy similar work hardening curves were observed in relation to the microstructural evolution from the lamellae shape, through elliptical shape into circular shape.     

Effect of CeO2 on the microstructure and wear behavior of thermal spray welded NiCrWRE coatings

NiCrWRE alloy coatings with the addition of CeO₂ were produced on a medium-carbon steel substrate by thermal spray welding. The bonding strength of the coatings was measured by tensile testing and reached 300–350 MPa. The wear behavior of the coatings was investigated under dry sliding wear conditions, and was compared with that of NiCrW coatings produced without the addition of CeO₂. The results show that the hardness and wear resistance of the thermal spray welded coatings are significantly increased with the addition of CeO₂. Analysis of the worn surfaces revealed that the coatings with CeO₂ addition show improved abrasive wear resistance over the coatings without CeO₂.     

Wear behavior of low-cost, lightweight TiC/Ti–6Al–4V composite under fretting: Effectiveness of solid-film lubricant counterparts

The wear behavior of low-cost, lightweight 10 wt% titanium carbide (TiC)-particulate-reinforced Ti–6Al–4V matrix composite (TiC/Ti–6Al–4V) was examined under fretting at 296, 423, and 523 K in air. Bare 10 wt% TiC/Ti–6Al–4V hemispherical pins were used in contact with dispersed multiwalled carbon nanotubes (MWNTs), magnetron-sputtered diamond-like carbon/chromium (DLC/Cr), magnetron-sputtered graphite-like carbon/chromium (GLC/Cr), and magnetron-sputtered molybdenum disulfide/titanium (MoS₂/Ti) deposited on Ti–6Al–4V, Ti–48Al–2Cr–2Nb, and nickel-based superalloy 718. When TiC/Ti–6Al–4V was brought into contact with bare Ti–6Al–4V, bare Ti–48Al–2Cr–2Nb, and bare nickel-based superalloy 718, strong adhesion, severe galling, and severe wear occurred. However, when TiC/Ti–6Al–4V was brought into contact with MWNT, DLC/Cr, GLC/Cr, and MoS₂/Ti coatings, no galling occurred in the contact, and relatively minor wear was observed regardless of the coating. All the MWNT, DLC/Cr, GLC/Cr, and MoS₂/Ti coatings on Ti–6Al–4V were effective from 296 to 523 K, but the effectiveness of the MWNT, DLC/Cr, GLC/Cr, and MoS₂/Ti coatings decreased as temperature increased.     

Frictional behaviour of oxygen diffusion hardened titanium in dry sliding against Co–28Cr–5W–4Fe–3Ni–1Si cobalt alloy

The results of conformal pin-on-disc tribological tests of a hard layer of the solid solution of oxygen in α-titanium sliding against a Co–28Cr–5W–4Fe–3Ni–1Si cobalt alloy counterspecimen are presented. The α-Ti(O) layer was diffusely produced over 2–8 h of oxidising in the superficial zone of a technical quality titanium specimen.The friction and wear responses of the system were recorded and the wear mechanisms were studied. Investigations of the material structure and chemical constitution in micro-areas of the titanium specimen, cobalt alloy counterspecimen and wear debris formed in dry sliding were performed with a Philips XL20 microscope equipped with an EDAX analyser. Crushing of the α-Ti(O) layer, lowering of the wear rate after comminution of the hard α-Ti(O) layer, local tack spots and fine powder wear particles, mostly Ti oxides, were detected at the beginning of each test. Gradual brittle fracture and decay by pulverising of the α-Ti(O) particles embedded in both mating surfaces, which occurred during the test, led to the increase of the wear rate of the couple and domination of microcutting and tack spots spalling after their partial oxidation. Finally, after the disappearance of the α-Ti(O) loose particles, adhesive junctions, metal transfer and smearing become leading wear mechanisms.     

Erosive wear of hardfaced Fe–Cr–C alloys at elevated temperature

Many engineering components are subjected to erosive wear at elevated temperature. As erosive wear at elevated temperature is governed by the synergistic effect of erosive wear and oxidation, it is possible to modify surfaces of the components in order to achieve improved performances. In view of the above, two different types of hardfacing alloys of Fe–Cr–C were designed incorporating Nb, Mo and B to ensure improved performances at elevated temperature. In order to achieve the above objective, mild steel was hardfaced with these alloys under optimised gas metal arc welding (GMAW) condition. The microstructures of the hardfaced coating was characterised with the help of optical microscopy (OM) and scanning electron microscopy (SEM). The mechanical properties of these coatings were obtained by means of micro indenter. Erosive wear of these coatings was evaluated for four different temperatures, for two different impact angles and at one impact velocity. The morphologies and the transverse sections of the worn surfaces are examined with SEM. The erosive wear of these coatings were compared with conventional M2 tool steel. Results indicate that erosion rate of these coatings increases with increase of test temperature and impact angles. Among various coatings, Fe–Cr–C coating containing higher amount of Nb, Mo and B exhibits best erosion resistance particularly at elevated temperature.     

Preparation and tribological properties of the carbon nanotubes–Ni–P composite coating

A method to prepare the carbon nanotubes (CNTs)–Ni–P composite coating with different mass content of CNTs on the surface of 45^# steel by electroless plating was proposed. The transmission electron microscopy (TEM) and the scanning electron microscopy (SEM) were used to observe the appearance of the as-prepared CNTs and the CNTs–Ni–P composite coating, and then the roughness of the coating surface was also analyzed by atomic force microscopy (AFM). Furthermore, the wear and friction behavior of the CNTs–Ni–P composite coating were investigated under oil-lubricated condition, Due to the self-lubrication property and the unique antifriction structure, CNTs can greatly improve the wear resistance of the CNTs–Ni–P composite coating, where the wear resistance of the CNTs–Ni–P composite coating is optimized with the intermediate mass content of 2 kg/m³ CNTs.     

Wear study of Ni–WC composite coating modified with CeO2

In the present investigation, Ni–WC composite powder was modified with the addition of CeO₂ in order to form a new composition of Ni–WC–CeO₂. The Ni–WC and Ni–WC–CeO₂ compositions were used for coating deposition by high-velocity oxy-fuel (HVOF) spraying process so as to study the effect of CeO₂ addition on microstructure, distribution of various elements, hardness, formation of new phases, and abrasive wear behavior. Further, the effect of load, abrasive size, sliding distance, and temperature on abrasive wear behavior of these HVOF-sprayed coatings was investigated by response surface methodology. To investigate the abrasive wear behavior of HVOF-sprayed coatings four factors such as load, abrasive size (size in micrometers), sliding distance (meters), and temperature (°C) with three levels of each factor were investigated. Analysis of variance was carried out to determine the significant factors and interactions. Investigation showed that the load, abrasive size, and sliding distance were the main significant factors while load and abrasive size, load and sliding distance, abrasive size and sliding distance were the main significant interactions. Thus an abrasive wear model was developed in terms of main factors and their significant interactions. The validity of the model was evaluated by conducting experiments under different wear conditions. A comparison of modeled and experimental results showed 4–9% error. The abrasive wear resistance of coatings increases with the addition of CeO₂. This is due to increase in hardness with the addition of CeO₂ in Ni–WC coatings.     

The friction and wear of electroless Ni–P matrix with PTFE and/or SiC particles composite

In this paper, the friction behaviour and wear mechanism of electroless Ni–P matrix with PTFE and/or SiC particles composite coating are investigated by virtue of ring-on-disk wear machine at a high load of 150 N. The worn surface, wear debris and the composition changes after wear were characterized using scanning electron microscopy (SEM) and energy-dispersive analysis of X-ray (EDAX). By comparison with Ni–P and Ni–P–SiC coatings, the results indicated that the combination of a PTFE-rich mechanical mixed layer (PRMML) formed on the worn surface and hard SiC were responsible for the good tribological properties of the hybrid Ni–P–PTFE–SiC composites at high load. After heat treatment at 400 °C for 1 h, the wear rate of Ni–P matrix composites decreased with corresponding increase in microhardness. During sliding, an obvious decrease in the temperature rise with PTFE addition was attributed to the good anti-friction of PTFE.     

The tribological behavior of Ni–Cu–Ag-based PVD coatings for hybrid bearings under different lubrication conditions

In a cryogenic environment, components like bearings with interacting surfaces in relative motion (tribosystems) often generate undesired heat and experience high wear. Because the properties of conventional bearing materials like stainless steel cannot be applied to this temperature range, the PVD coating based on metal–metal pairs with better frictional properties must be employed. To test the suitability of the Ni–Cu–Ag-based PVD coatings of hybrid bearings for liquid rocket engine turbopumps and to obtain reliable coating material data in the extreme environment, the tribological behaviors of coatings under the cryogenic fluid (liquid oxygen and liquid nitrogen) and water lubricated conditions are studied, respectively. In the paper, the specimens are in a vibrocryotribometer with the ball-on-plane contact type, and various running conditions in terms of lubricants, contacting loading, and contacting velocity are examined. The simulated experiment for testing the actual tribological performance of Ni–Cu–Ag-based PVD coatings for hybrid bearings is tested. The results of the tests indicate that the coatings can be suitable for cryogenic tribosystems of turbopumps. In the cryogenic environment, the volume wear rate of coatings increases rapidly with the contacting loading, but 15 min later, the volume wear volume of coatings turns into 2.5–15×10⁻⁴ mm³. Besides, under the liquid oxygen condition in simulating the liquid rocket engine turbopumps environment, the friction coefficients are 0.03–0.1.     

Investigation of micro-scale abrasion–corrosion of WC-based sintered hardmetal and sprayed coating using in situ electrochemical current-noise measurements

The micro-scale wear–corrosion interactions of WC-based sintered hardmetals and sprayed coatings are typically investigated by comparing the wear-rates in corrosive environments with neutral (pH 7) conditions and inferring electrochemical activity. However, for a greater understanding of the wear–corrosion interactions, there is a need to examine the repassivation kinetics during micro-abrasion tests under different pH conditions. This paper details in situ electrochemical current-noise measurements performed using a modified micro-abrasion tester to elucidate these wear–corrosion interactions for pH 7–13 conditions for sintered WC–5.7Co–0.3Cr and sprayed WC–10Co–4Cr specimens. Electrochemical measurements and SEM micrographs of worn surfaces are used to detail the degradation process. Discussion will focus on the wear–corrosion interactions present under neutral and alkaline conditions for sintered and sprayed specimens and the influence of microstructure on the electrochemical activity will be detailed.     

Comparative evaluation of ambient temperature friction behaviour of thermal sprayed Cr3C2–25(Ni20Cr) coatings with conventional and nano-crystalline grains

Due to their improved hardness and toughness properties, nano-crystalline materials have become attractive for a wide variety of applications. However, as the processing of nano-crystalline materials is quite expensive, efforts have been made to develop coatings using nano-crystalline grains. The present study investigates the ambient temperature friction behaviour of Cr₃C₂–25(Ni20Cr) coatings with nano-crystalline grains (CNG). In the technical process, both, Cr₃C₂–25(Ni20Cr) coatings with conventional grains (CCG), and nano-crystalline grains were deposited using an HVOF technique. Micro-structural characteristics and mechanical properties of the coatings were determined. Friction behaviour of the coatings as a function of different test conditions was evaluated. The specimens worn were examined by a scanning electron microscope (SEM). The results showed that the coefficient of friction of the CNG against 100Cr6 steel is lower than that of CCG against 100Cr6 steel. The principal types of material removal of CNG on the one hand, and CCG on the other, differ. Maps showing the prevalence of different wear mechanisms for different wear conditions were established for both types of coatings.     

Sliding Wear Behavior of HVOF-sprayed Cr3C2–NiCr/CeO2 Composite Coatings at Elevated Temperature up to 800 °C

Four types of Cr₃C₂–NiCr coatings containing different fractions of CeO₂ additive were deposited using high velocity oxy-fuel spraying. Hardness tester, X-ray diffractometer, contact surface profiler, and scanning electron microscope equipped with energy dispersive spectrometer were employed to characterize the microhardness, phase composition, surface roughness, and microstructure of as-sprayed coatings. At the same time, the friction and wear behavior of the as-sprayed coatings sliding against Si₃N₄ ball at room temperature and elevated temperature of 400 or 800 °C under unlubricated condition was evaluated using an oscillating friction and wear tester. The worn surfaces of the composite coatings and Si₃N₄ counterpart balls were analyzed by means of scanning electron microscopy, X-ray diffraction, and three dimensional non-contact surface profiler. The friction and wear mechanisms of the coatings with and without CeO₂ additive were comparatively discussed. Results show that the composite coatings doped with CeO₂ had better wear-resistance than that without CeO₂, and the coating containing 4 wt% CeO₂ showed the best wear-resistant property. The improved wear-resistant properties of the composite coatings doped with CeO₂ were attributed to the refined microstructure and improved mechanical properties induced by CeO₂.     

Effects of Chromium Addition on Microstructure and Abrasion Resistance of Fe–B Cast Alloy

This research work studies the effects of chromium on microstructure and abrasion resistance of Fe–B cast alloy. The results show that eutectic boride changes from continuous network to less continuous and matrix changes from pearlite to martensite with the increase in chromium content in the alloy. Meanwhile, an increase in chromium addition in the alloy leads to an increase in the chromium content in M₂B-type boride because chromium can enter boride by substituting for iron in Fe₂B. Under two-body wear, Fe–B cast alloy exhibits excellent wear resistance. When alloys are tested against soft abrasive, chromium can markedly improve the wear resistance of Fe–B cast alloy, whereas excessive chromium can reduce the wear resistance. The wear resistance of Fe–B cast alloy increases first and then decreases with the increase in chromium. But when tested against hard abrasive, since the hardness of SiC is much higher than that of M₂B boride, an increase in chromium content marginally increases the wear resistance. Weight losses of Fe–B cast alloy increase with the increase in the load and exhibit the linear relationship.     

Microstructure and tribological characteristics of ZrO2–Y2O3 ceramic coatings deposited by laser-assisted plasma hybrid spraying

ZrO₂–Y₂O₃ ceramic coatings were deposited on AISI 304 stainless steel by both a low-pressure plasma spraying (LPPS) and a laser-assisted plasma hybrid spraying (LPHS). Microstructure and tribological characteristics of ZrO₂–Y₂O₃ coatings were studied using an optical microscope, a scanning electron microscope, and an SRV high-temperature friction and wear tester. The LPHS coatings exhibit distinctly reduced porosity, uniform microstructure, high hardness and highly adhesive bonding, although more microcracks and even vertical macrocracks seem to be caused in the LPHS coatings. The ZrO₂ lamellae in the LPHS coatings before and after 800°C wear test consist mainly of the metastable tetragonal (t′) phase of ZrO₂ together with small amount of c phase. The t′ phase is very stable when it is exposed to the wear test at elevated temperatures up to 800°C for 1 h. The friction and wear of the LPHS coatings shows a strong dependence on temperature, changing from a low to a high wear regime with the increase of temperature. At low temperatures, friction and wear of the LPHS coatings is improved by laser irradiation because of the reduced connected pores and high hardness in contrary to the LPPS coating. However, at elevated temperatures, the friction and wear of the LPHS coatings is not reduced by laser irradiation. At room temperature, mild scratching and plastic deformation of the LPHS coatings are the main failure mechanism. However, surface fatigue, microcrack propagation, and localized spallation featured by intersplat fracture, crumbling and pulling-out of ZrO₂ splats become more dominated at elevated temperatures.     

Microstructure and Dry-Sliding Wear Behavior of Thermal Sprayed and Fused Ni-Based Coatings with the Addition of La2O3

A Ni-based alloy with 1.5 wt% of La₂O₃ powders was thermal sprayed onto steel substrate. The microstructure and dry sliding wear behavior of the coatings were studied by XRD, field emission gun scanning electron microscope (FEGSEM) and SEM analyses. The microstructure of the coating with 1.5 wt% of La₂O₃ differs widely from the coating without La₂O₃; the typical microstructure with 1.5 wt% of La₂O₃ is composed of net-like dendrite (Cr, Fe)₂₃C₆ and Cr₇C₃, cellular-dendrite Fe₂₃(C, B)₆, γ-Ni + Ni₅Si₂ interdendritic lamellar eutectic. Interestingly, significant amounts of net-like (Cr, Fe)₂₃C₆ and Cr₇C₃ hard phases as a wear-resistant skeleton were formed and uniformly dispersed in the coating. Meanwhile, blocky and rod-like hard-phase CrB scattered in the coating can also contribute to improving the wear resistance. The novel microstructure, therefore, is beneficial for wear resistance. Friction and wear tests without lubricant show that the friction coefficients of the coating are less than 0.57. There is an approximately linear relationship between friction coefficients and sliding speed. The wear rate slightly increases with an increase of load, and the wear rate of the coating slightly decreases with sliding speed.     

Dry sliding wear of Cu–15Ni–8Sn alloy

Using a pin-on-disc apparatus, the wear behavior of Cu–15Ni–8Sn alloy aged for different periods of time at 400 °C was investigated under dry condition. The results showed the wear rate was inversely proportional to the hardness of the alloy, but the maximum wear resistance was not consistent with maximum hardness. The alloy contained about 10% (volume) cells precipitated along grain boundaries had the lowest wear rate. The friction coefficient was constant for different hardness. SEM micrographs of the debris and pin revealed that the removal process of surface material involved subsurface deformation, crack nucleation, crack propagation and delamination of the material.     

Wear characterization of electrodeposited Ni–WC composite coatings

The present contribution reports the tribological properties of Ni–WC composite coatings, electrodeposited on steel substrate. Commercial WC particles with an average size of 5 μm were codeposited with Ni on a mild steel substrate using a Watts bath at 50°C. The effect of plating variables on deposition behavior was studied. The amount of WC in the deposited layer decreased and plating efficiency increased with an increase in current density from 0.1 to 0.3 A/cm². The tribological properties of the coatings were studied using a small amplitude reciprocating friction wear tester. The addition of WC in Ni increases the microhardness of the electrodeposited coatings. An important result is that the presence of embedded WC particles in the electrodeposited coatings results in a much lower coefficient of friction (COF) of 0.34, when compared with pure Ni (COF 0.62) and mild steel (COF 0.54).