Wear behaviour of iron boride coatings produced by VPS technique on carbon steels

The vacuum plasma spray (VPS) technique is a useful tool for designing the characteristics of the coatings and, thus, the tribological properties of coated components. In the present paper, the wear properties of iron boride coatings produced by means of VPS technique on AISI 1040 steel samples were evaluated as a function of their microstructural characteristics. One coating type was obtained by using Fe₂B pure powder, the other with differentiated FeB + α-Fe blends, with the FeB content increasing and α-Fe content decreasing from the matrix to the surface. Wear tests were performed by means of a tribometer in block-on-ring configuration, without lubricant and in air, by using 40- and 60-N coupling loads and 0.8- and 1.6-m s⁻¹ sliding velocities. On Fe₂B coated samples, wear is essentially oxidative until the failure of the coating, the fragments of which cause a third body abrasion. On the FeB + α-Fe coated samples the wear mechanism is mainly oxidative and the coating totally wears out without spalling as a consequence of its graded structure, which succeeds in both improving the adhesion of the coating to the substrate and reducing the residual stress at the coating–substrate interface.     

Effects of sliding velocity and normal load on friction and wear characteristics of multi-layered diamond-like carbon (DLC) coating prepared by reactive sputtering

Friction and wear tests were performed to investigate effects of sliding velocity and normal load on tribological characteristics of a multi-layered diamond-like carbon (DLC) coating for machine elements. The DLC coatings which consist of sequentially deposited gradient Cr/CrN, W-doped DLC (a-C:H:W) and DLC (a-C:H) layers were formed on carburized SCM 415 Cr–Mo steel disks using a reactive sputtering system. The tests against AISI 52100 steel balls were performed under various sliding velocities (0.0625, 0.125, 0.25, 0.5, 1 and 2 m/s) and normal loads (6.1, 20.7 and 49.0 N) in ambient air (relative humidity=26±2%, temperature=18±2 °C). Each test was conducted for 20 km sliding distance without lubricating oil. The results show that friction coefficients decrease with the increase in sliding velocity and normal load. Wear rates of both surfaces decrease with the increase in normal load. The increase in sliding velocity leads initially to the increase in wear rates up to the maximum value. Then, they decrease, as the sliding velocity increases above specific value that corresponds to the maximum wear rate. Through surface observation and analysis, it is confirmed that formation of transfer layers and graphitized degree of wear surfaces of DLC coatings mainly affect its tribological characteristics.     

Microstructure and wear behaviour of Ni-based surface coating on copper substrate

The Ni-based surface coatings were prepared by a vacuum infiltration casting technique on copper substrate. The surface coatings were fabricated through copper melt penetrating into thin preforms whose thickness could change. By optimizing the processing parameters, compact surface coatings were achievable as confirmed through SEM observation. The surface coating was mainly composed of solid solution of Ni, solid solution of Cu and CrB. The macro-hardness of the coating was about HRC 58, and the micro-hardness of the coating shows a gradient distribution. The average micro-hardness of the coating was about HV450. Wear behaviour was investigated by using block-on-ring dry sliding linear contact at several loads (50 N–300 N) and two different sliding speeds (0.424 m/s and 0.848 m/s). Wear rate and friction coefficient were estimated using a method founded upon the PV factor theory. The surface oxidation predominated as the principle wear mechanism at low load. Meanwhile, adhesion and oxidation mechanism were observed when the coatings were tested at higher load more than 200 N. Friction coefficient decreased with increasing load and sliding speed.     

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.     

Tribological performance of α-Fe(Cr)-Fe2B-FeB and α-Fe(Cr)-h-BN coatings obtained by laser melting

α-Fe(Cr)-h-BN and α-Fe(Cr)-Fe₂B-FeB coatings on X30Cr13 stainless steel are synthesized by laser melting with incorporation of hexagonal boron nitride, or by alloying of boron. The additive powders are deposited on steel before pulsed irradiation by Nd-YAG laser beam. The solidification structures of the obtained coatings are investigated by optical microscopy and X-ray diffractometry. The mechanical properties are investigated by nanoindentation and the tribological behaviour is characterized on pin-on-disc tribometer, under dry-sliding conditions with different loads and a temperature range 25–500 °C. h-BN-α-Fe(Cr) and Fe₂B-α-Fe(Cr) coatings have average hardnesses 10.0 and 14.5 GPa, respectively, while hardness of untreated stainless steel is 4.2 GPa. In comparison with this untreated steel, the sliding contact on ceramic (ruby) of such coating shows a lower coefficient friction and a definitively better wear resistance.     

Sliding wear resistance of reactive plasma sprayed Ti–TiN coatings

The reactive plasma spraying (RPS) of titanium powders in a nitrogen containing plasma gas produces thick coatings characterised by microdispersed titanium nitride phases in a titanium matrix. In this paper, the wear resistance properties of Ti–TiN coatings deposited on carbon steel substrates by means of RPS technique are studied. Wear tests were performed in block-on-ring configuration and dry sliding conditions, at different applied loads (45 and 100 N) and sliding velocities (in the range 0.4–2.0 m s⁻¹) by using hardened and stress relieved AISI O2 disks as counterpart. At low applied load the wear volumes are low, and tend to slightly increase as the sliding velocity increases. At high applied load and low sliding velocities the highest wear volumes for the coated samples are observed, due to adhesion in the contact area with the tool steel counterpart and decohesion of coating particles. As the sliding velocity is increased, the wear volume of the coated samples tends to decrease owing to oxidation phenomena.     

Preparation,microstructure and tribological behavior of laser cladding NiAl intermetallic compound coatings

Nickel aluminide (NiAl) intermetallic compound coatings were in situ synthesized from pre-placed mixed powders of Ni and Al by laser cladding. The phase composition and microstructure of the NiAl coatings were studied by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The effects of laser cladding parameters on the microstructure and friction and wear behavior of the NiAl coatings were investigated. It has been found that laser power density had a crucial influence on the microstructure and friction and wear behavior of NiAl coatings. Namely, the NiAl coatings synthesized under a lower power density have more dense and fine microstructure, and lower friction coefficient and wear rate. Besides, the friction and wear behavior of the laser cladding NiAl coatings is highly dependent on applied normal load and sliding speed; and the resulting coatings sliding against Si₃N₄ in a ball-on-disc contact mode is more suitable for tribological application at a moderate normal load of 3–7 N and sliding speed of 0.16–0.21 m/s.     

Wear characteristic of in situ synthetic TiB2 particulate-reinforced Al matrix composite formed by laser cladding

In order to improve the wear resistance of an aluminum alloy, an in situ synthesized TiB₂ particulate-reinforced metal matrix composite coating was formed on a 2024 aluminum alloy by laser cladding with a powder mixture of Fe-coated boron, Ti and Al was successfully achieved using a 3-kW CW CO₂ laser. The chemical composition, microstructure and phase structure of the composite clad coating were analyzed by energy dispersive X-ray spectroscopy (EDX), SEM, TEM and XRD. The nanohardness and the elastic modulus of the phases of the coating have been examined. The dry sliding wear behaviour of the coating was investigated using a pin-on-ring machine under four loads, namely 8.9, 17.8, 26.7, and 35.6 N. It has been found that the wear characteristics of cladding were completely dependent on the content and morphology of the TiB₂ particulate and intermetallic in the microstructure and the applied load. At the lowest load (8.9 N), with increasing content of TiB₂ particulate and intermetallic, the wear weight loss of the laser cladding was decreased. At higher loads (17.8, 26.7, and 35.5 N), the 2024 Al alloy exhibited superior wear resistance to the particle-reinforced metal matrix composite cladding.     

Comparison of self-mated hardmetal coatings under dry sliding conditions up to 600 °C

Hardmetal coatings prepared by high velocity oxy-fuel (HVOF) spraying represent an advanced solution for surface protection against wear. In the current systematic study the high-temperature oxidation and unidirectional sliding wear in dry and lubricated conditions were studied. Results for a series of experiments on self-mated pairs in dry conditions as part of that work are described in this paper. Coatings with nominal compositions WC-10%Co4%Cr, WC-(W,Cr)₂C-7%Ni, Cr₃C₂-25%NiCr, (Ti,Mo)(C,N)-29%Ni and (Ti,Mo)(C,N)-29%Co were prepared with an ethylene-fuelled DJH 2700 HVOF spray gun. Electrolytic hard chromium (EHC) coatings and bulk (Ti,Mo)(C,N)-15%NiMo (TM10) hardmetal specimens were studied for comparison. The wear behaviour was investigated at room temperature, 400 and 600 °C. For the coatings sliding speeds were varied in the range 0.1–1 m/s for a wear distance of 5000 m and a normal force of 10 N. In some cases the WC- and (Ti,Mo)(C,N)-based coatings showed total wear rates (sum of wear rates of the rotating and stationary samples) of less than 10^?6 mm³/Nm, i.e., comparable to values typically measured under mixed/boundary conditions. Coefficients of friction above 0.4 were found for all test conditions. The P × V values as an engineering parameter for coating application are discussed. The microstructures and the sliding wear behaviour of the (Ti,Mo)(C,N)-based coatings and the (Ti,Mo)(C,N)-15%NiMo hardmetal are compared.     

Synthesis of boride coatings on steel using plasma transferred arc (PTA) process and its wear performance

Boriding of the surface of a tool steel using boron powder and the plasma transferred arc process was investigated. It was shown that this method is an easy and effective technique in producing uniform alloyed layers with a thickness of about 1.5 mm and a hardness between 1000 and 1300 HV.The microstructure of the borided surfaces consists of primary Fe₂B-type borides and a eutectic mixture of borides and martensite. Some cracks are observed in the eutectic regions but they do not seem to critically affect the behaviour of the coatings in sliding wear.The wear rate of pin on disc tests is primarily affected by the applied load and it lies between 10⁻⁵ mm³/m for low loads and 10⁻² mm³/m for high loads. Two distinct regimes of mild and severe wear are obtained separated by a critical load. Mild wear is due to the load supporting effect of borides and severe wear is due to their breakage above a critical load. The wear rate is not significantly affected by the sliding velocity and is consistent with the friction coefficient.The friction coefficient varies from 0.13 to 0.23 and depends strongly on the oxidation status of the wear track. The sliding velocity affects the sliding distance where the coefficient of friction reaches equilibrium.     

Mechanical and tribological behavior of W/WCN bilayers grown by pulsed vacuum arc discharge

This study investigates W/WCN coatings grown by means of a repetitive pulsed vacuum arc discharge while varying the substrate temperature from room temperature (RT) to 200 °C. The coatings were grown using a cathode with W (6N) and WC (6N) targets for the W and WCN layers, respectively, and an anode that contains stainless-steel 304 as a substrate. The films grown at RT and at 150 °C possessed a lower percentage of carbon obtained from energy dispersive spectroscopy. This characteristic was related to the hardness, the coefficient of friction and the adherence. These films also possessed the best mechanical and tribological properties. Several wear mechanisms were identified, such as delamination and plowing out of abrasive and adhesive particles.     

Tribological characterization of tungsten nitride coatings deposited by reactive magnetron sputtering

The structure, hardness, friction and wear of tungsten nitrides prepared by d.c. reactive magnetron sputtering were investigated. The coatings were deposited with different nitrogen to argon ratios; the total pressure was kept constant. The tribological tests were performed on a pin-on-disc tribometer in terrestrial atmosphere with 100Cr6 steel, Al₂O₃ and Si₃N₄ balls as sliding counter-bodies. The wear tracks, the ball-wear scars and the wear debris were analysed by scanning electron microscopy in order to characterize the dominant wear mechanisms.The coatings exhibited different phases as a function of the nitrogen content: films with low N content exhibited the α-W phase; β-W phase was dominant for nitrogen contents from 12 to 15 at.% and β-W₂N was observed for nitrogen content higher that 30 at.%. The mechanical and tribological properties of the tungsten nitride coatings were strongly influenced by the structure. The hardness and the Young's modulus values were in the ranges (29–39 GPa) and (300–390 GPa), respectively; the lowest values correspond to the coatings with the highest nitrogen content. Generally, the friction and wear rate of tungsten nitride coatings sliding against ceramic balls increased with nitrogen content reaching a maximum at 12 at.%; further increase of the nitrogen content led to a decrease of the friction and wear. The sliding with the steel balls did not wear the coatings under the selected testing conditions.     

The tribological characteristics of TiCN coating at elevated temperatures

The tribological behaviour of TiCN coating prepared by unbalanced magnetron sputtering is studied in this work. The substrates made from austenitic steel were coated by TiCN coatings during one deposition. The measurements were provided by high temperature tribometer (pin-on-disc, CSM Instruments) allowing measuring the dependency of friction coefficient on cycles (sliding distance) up to 500 °C. The evolution of the friction coefficient with the cycles was measured under different conditions, such as temperature or sliding speed and the wear rate of the ball and coating were evaluated. The 100Cr6 balls and the Si₃N₄ ceramic balls were used as counter-parts. The former were used at temperatures up to 200 °C, the latter up to 500 °C. The wear tracks were examined by optical methods and SEM. The surface oxidation at elevated temperatures and profile elements composition of the wear track were also measured.The experiments have shown considerable dependency of TiCN tribological parameters on temperature. Rise in temperature increased both friction coefficient and the wear rate of the coating in case of 100Cr6 balls. The main wear mechanism was a mild wear at temperatures up to 200 °C; fracture and delamination were dominating wear mechanisms at temperatures from 300 to 500 °C.     

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.     

Mechanical and unlubricated tribological properties of titanium-containing diamond-like carbon coatings

Titanium-containing diamond-like carbon (Ti-DLC) coatings were deposited on steel with a close-field unbalanced magnetron sputtering in a mixed argon/acetylene atmosphere. The morphology and structure of Ti-DLC coatings were investigated by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Nanoindentation, nanoscratch and unlubricated wear tests were carried out to evaluate the hardness, adhesive and tribological properties of Ti-DLC coatings. Electron microscopic observations demonstrated the presence of titanium-rich nanoscale regions surrounded by amorphous carbon structures in Ti-DLC coating. The Ti-DLC coatings exhibit friction coefficients of 0.12–0.25 and wear rates of 1.82 × 10^?9 to 4.29 × 10^?8 mm³/Nm, depending on the counterfaces, sliding speed and temperature. The Ti-DLC/alumina tribo-pair shows a lower friction coefficient than the Ti-DLC/steel tribo-pair under the identical wear conditions. Increasing the test temperature from room temperature to 200 °C reduces the coefficient of friction and, however, clearly increases the wear rate of Ti-DLC coatings. Different wear mechanisms, such as surface polishing, delamination and tribo-chemical reactions, were found in the tribo-contact areas, depending on different wear conditions.     

Friction and wear behavior of flame-sprayed PEEK coatings

Polyetheretherketone (PEEK) becomes of great interest to applications as bearing and slider materials. In this paper, PEEK coatings with three kinds of crystallinities were deposited using the flame spray process. Employing a uniform design experiment, the friction and wear behavior of the three PEEK coatings were systematically investigated under dry sliding conditions against a 100C6 counterbody on a ball-on-disc arrangement for several loads and sliding velocities. For the three coatings, the friction coefficient significantly followed the normal distribution. The average friction coefficients appeared to decrease while increasing the sliding velocity, but were insensitive to the applied load in the range of investigation. Among the three coatings, the higher the crystallinity of the coating, the lower its average friction coefficient was. The wear rate of the coating with the lowest crystallinity decreased with an increase in the load and a decrease in the sliding velocity. The wear rate of the coating with the intermediate crystallinity decreased with an increase in the load, but increased with an increase in the sliding velocity at lower loads, and then decreased with an increase in the velocity at higher loads. The wear rate of the coating with the highest crystallinity decreased with the increase of both the load and the sliding velocity. The wear mechanisms of the different coatings are explained in terms of plastic deformation, plogh marks and fatigue tearing.     

Influence of surface treatment of elastomers on their frictional behaviour in sliding contact

To reduce friction of elastomer parts moving against a metal counter body in dry conditions, two different surface treatment techniques were applied on elastomer parts: laser cladding and plasma treatment at atmospheric pressure.Polyamide 11 (PA 11) based coatings were produced on thermoplastic polyurethane (TPU) substrates by laser cladding. During ball-on-disc tribotesting the effect of a PA 11 coating was identical to that of a PA 11 + 9% MoS₂ coating: friction of the TPU substrate was reduced with 40%. The incorporation of 15 wt% PTFE in the PA 11 coating resulted in a further decrease of the frictional force. A reduction of 80% of the frictional force of the TPU substrate was measured. The surface of the coatings before and after tribotesting was analysed.The plasma treatment of HNBR was done using a Plasmaspot^® to form a plasma polymerised coating based on two different types of siloxanes. A reduction of 74–80% of the initial friction coefficient was measured in two different tribotest rig configurations: ball-on-disc and disc-on-disc. The resulting wear tracks were analyzed by SEM and EDX.     

Wear behavior of electroless Ni–P–B4C composite coatings

In this research, the wear of electroless Ni–P and Ni–P–B₄C composite coatings was reviewed. Auto catalytic reduction of Ni in nickel sulfate and sodium hypophosphate bath including suspended B₄C particles with different concentration was used to create composite coatings with 12, 18, 25 and 33 vol.% of B₄C particles. Coatings 35 μm thick were heat treated at 400 °C for one hour in an argon atmosphere and the wear resistance and friction coefficient of heat-treated samples were determined by block-on-ring tests. All wear tests were carried out at 24 °C, 35% moisture, 0.164 m/s sliding speed and about 1000 m sliding distance. Graphs show that an electroless Ni–P–B₄C composite coating with 25 vol.% of B₄C had the best wear resistance against a CK45 steel counterface.     

Wear characteristics of a diffusion bonded sintered steel with short term surface treatments

A pin on disc machine was used to investigate the tribological behavior of a diffusion bonded sintered steel, with and without surface treatments of steam oxidation and manganese phosphating, over a wide range of speed (0.2–4 m/s) and applied load (4–500 N) in conditions of dry sliding and starved lubrication by oil impregnation of the porous structure of the materials. Besides the calculated wear rates, the wear mechanisms were determined by examination of the components of the rubbing system (sintered pin, disc and generated debris). A transition from a mild to a severe wear regime was identified, denoted by sharp changes of the wear rate. A transient wear regime, interposed between the mild and severe wear regimes, was detected. The rubbing surface quality degradation was in terms of material displacement around the pin circumference due to a delamination wear mechanism. Such regime was detected for the base sintered steel in dry sliding at 1 m/s for the load range 60–80 N and for both surface treatments in oil impregnated sliding at 0.5 m/s for the load range 200–300 N. Oil impregnation of the base sintered steel expanded the mild wear regime towards higher loads throughout the whole sliding speed range compared to dry sliding. For the lower speeds of 0.2 and 0.5 m/s, manganese phosphated samples in dry sliding exhibited higher transition loads compared to the base sintered steel. The lower oil impregnability of the surface treated samples, due to the sealing of porosity by steam oxidation, led to slightly lower transition loads in oil impregnated sliding, compared to the base sintered steel.     

Wear characteristics of Cr3C2–NiCr and WC–Co coatings deposited by LPG fueled HVOF

Wear behavior of the HVOF deposited Cr₃C₂–NiCr and WC–Co coatings on Fe-base steels were evaluated by the pin-on-disc mechanism. The constant normal load applied to the pin was 49 N and sliding distance was 4500 m with velocity of 1 m/s, at ambient temperature and humidity. The specific wear rate of WC–Co coating was 3 mm³/N m and Cr₃C₂–NiCr coating was 5.3 mm³/N m. SEM/EDAX and XRD techniques were used to analyze the worn out surface and wear debris. The Fe₂O₃ was identified as the major phase in the wear debris. The wear mechanism is mild adhesive wear in nature.