Reciprocative sliding friction and wear properties of electrical discharge machined ZrO2‐based composites

Hot‐pressed, laboratory‐made, ZrO₂‐based composites with 40 vol. % WC, TiCN or TiN were surface finished by electrical discharge machining in order to compare their reciprocating sliding friction and wear response against WC–6wt%Co cemented carbide in unlubricated conditions. Sliding experiments were performed using a Plint TE77 pin‐on‐flat wear test rig, revealing a strong impact of the secondary phase on the tribological behaviour of the ZrO₂‐based composites. The worn surfaces and wear debris were characterised by scanning electron microscopy, energy dispersive X‐ray analysis and surface topography scanning, pointing out abrasion, polishing and adhesion as main wear mechanisms. The most favourable friction and wear characteristics were encountered with ZrO₂–WC composites compared to the other grades with equal amount of volumetric secondary phase. Copyright © 2009 John Wiley & Sons, Ltd.     

Sliding Wear of Electrically Conductive ZrO<Subscript>2</Subscript>−WC Composites Against WC–Co Cemented Carbide

ZrO₂-based composites with WC addition can be successfully machined by electrical discharge machining (EDM) in demineralised water. ZrO₂ composites with 40 vol.% WC were produced from nanocrystalline and micrometre sized WC starting powders in order to compare their tribological behaviour. Friction and wear data are obtained on wire-EDM’ed ZrO₂–WC composite flats sliding against a WC–Co cemented carbide pin using a small-scale pin-on-plate testing rig. Correlations between wear volume, wear rate and friction coefficient on the one hand and material properties and test conditions on the other hand were elucidated. The experimental results revealed that the grain size of the electro-conductive WC-phase exhibits a strong influence on the friction and wear behaviour of the ZrO₂-based composite.     

EDM machinability and frictional behavior of ZrO2-WC composites

Zirconium dioxide (ZrO₂)-based composites with WC addition are demonstrated to be suitable for electrical discharge machining (EDM) in deionized water. ZrO₂-based composites with 40 vol. % WC, obtained from distinctive WC powder sources, were produced in order to derive correlations between material removal rate (MRR), surface finish, wire EDM parameters and material properties. Dry reciprocating sliding experiments on wire-electrical discharge machined ZrO₂-WC composite samples against WC-Co cemented carbide, performed using a pin-on-plate testing rig, revealed a significant influence of the microstructure of the secondary WC-phase on wire EDM behavior and frictional characteristics.     

Abrasive wear of WC-FeAl composites

The abrasive wear behavior of tungsten-carbide iron-aluminide composite materials was investigated using a pin-on-drum wear-testing machine. Samples were prepared by uniaxially hot pressing blended powders. The wear rates of specimens containing 40 vol.% matrix of atomic composition, Fe₆₀Al₄₀, were measured and results compared with those of conventional WC-10 vol.% Co hardmetal. They were found to be comparable to those of WC-10% Co hardmetal, when abraded by 120 μm SiC papers under identical conditions. The wear resistance of WC-Fe₆₀Al₄₀ composites increased with reduction in WC-grain size and associated with increase in composite hardness. Scanning electron microscopy revealed that the wear surfaces of WC-40% Fe₆₀Al₄₀ composites and WC-Co hardmetal were similar in appearance. The higher hardness and work hardening ability of Fe₆₀Al₄₀ binder, as compared to Co metal, are believed to be responsible for the excellent abrasive wear resistance of WC composites containing iron aluminide binder.     

Process investigation on meso-scale hard milling of ZrO2 by diamond coated tools

This paper investigates the feasible machining of zirconium oxide (ZrO₂) ceramics, in the hard state, via milling by diamond coated miniature tools (from here on briefly indicated as meso-scale hard milling). The workpiece material is a fully sintered yttria stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP). Diamond coated WC mills, 2 mm in diameter, four flutes and large corner radius (0.5 mm) are chosen as cutting tools, and experiments are conducted on a state-of-the-art micro milling machine centre. The influence of cutting parameters, including axial depth of cut (a_p) and feed per tooth (f_z), on the achievable surface quality is studied by means of a one-factor variation experimental design. Further tests are also conducted to monitor the process performance, including surface roughness, tool wear and machining accuracy, over the machining time. Mirror quality surfaces, with average surface roughness Ra below 80 nm, are obtained on the machined samples; the SEM observations of the surface topography reveal a prevailing ductile cutting appearance. Tool wear initiates with delamination of the diamond coating and progresses with the wear of the WC substrate, with significant effect on the cutting process and its performance. Main applications of this research include three dimensional surface micro structuring and superior surface finishing.     

CUTTING PERFORMANCE AND WEAR MECHANISM OF Si3N4-BASED NANOCOMPOSITE CERAMIC CUTTING TOOL IN MACHINING OF CAST IRON

A type of Si₃N₄-based nanocomposites ceramic cutting tool material was prepared by the addition of nano-scale Si₃N_4W whisker and nano-scale TiN particle. Cutting performance of the Si₃N₄/Si₃N_4W/TiN nanocomposite ceramic tool in machining of cast iron was investigated in comparison with a commercial sialon ceramic tool, and the tool wear mechanism was studied. The two types of cutting tools have similar cutting performance at relatively low cutting parameters, while Si₃N₄/Si₃N_4W/TiN nanocomposite tool exhibits a better wear resistance than sialon tool at the relatively high cutting parameters. The wear of sialon ceramic cutting tool is dominated by the plastic deformation, abrasive action, microcracking, pullout of grains and chemical action at the higher cutting parameters. The higher mechanical properties, and the refined matrix grains, intragranular TiN grains and dislocation in the microstructure improve the resistances to plastic deformation, microcracking, and pullout of grains for Si₃N₄/Si₃N_4W/TiN nanocomposite ceramic cutting tool. The wear of Si₃N₄/Si₃N_4W/TiN nanocomposite ceramic cutting tool is dominated by the abrasive and chemical actions at the higher cutting parameters.     

Tribological behaviour and microstructure of TiCxN(1−x) coatings deposited by filtered arc

A series of macroparticle-free TiN, TiC_xN_(1−x) and TiC coatings were deposited on 316 austenitic stainless steel using a titanium target in a filtered arc deposition system (FADS) and reactive mixtures of N₂ and/or CH₄ gases. The surface topography, chemical composition and microstructure of these coatings were characterised by optical microscopy (OM), atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The microhardness has been measured and the adhesion of the coatings has been evaluated. Further, the wear and friction behaviour of the coatings were assessed under controlled test conditions in a pin-on-disc tribometer.The results show a significant increase in surface roughness, microhardness and wear resistance as the CH₄:N₂ gas flow rate ratio is increased. The composition of the coatings was strongly dependent on reactive gas flow rate during deposition. Surface particles were observed on high carbon content coatings and subsequently determined to be carbonaceous particles by using OM, AFM and EDS. At lowest load (10 N), all coatings exhibited low friction and wear. At loads of 15 and 25 N, the higher carbon content TiCN and TiC coatings showed a much lower friction and wear compared to TiN and low carbon TiCN.     

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.     

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.     

Tribological properties of spark-plasma-sintered ZrO2(Y2O3)-CaF2-Ag composites at elevated temperatures

Spark-plasma sintering is employed to synthesize self-lubricating ZrO₂(Y₂O₃) matrix composites with different additives of CaF₂ and Ag as solid lubricants by tailoring the composition and by adjusting the sintering temperature. The friction and wear behavior of ZrO₂(Y₂O₃) matrix composites have been investigated in dry sliding against an alumina ball from room temperature to 800 °C. The effective self-lubrication at different temperatures depends mainly on the content of various solid lubricants in the composites. The addition of 35 wt.% Ag and 30 wt.% CaF₂ in the ZrO₂(Y₂O₃) matrix can promote the formation of a well-covered lubricating film, and effectively reduce the friction and wear over the entire temperature range studied. The friction coefficients at low temperatures were at a minimum value for the composite containing 35 wt.% of silver. At this silver concentration, low and intermediate temperature lubricating properties are greatly improved without affecting high-temperature lubrication by the calcium fluoride in ZrO₂(Y₂O₃) matrix composites. The worn surfaces and transfer films formed during wear process have been characterized to identify the synergistic lubrication behavior of CaF₂ and Ag lubricants at different temperatures.     

Improvement of the Oxidation and Wear Resistance of Pure Ti by Laser-Cladding Ti<Subscript>3</Subscript>Al Coating at Elevated Temperature

Ti₃Al coating was in situ synthesized successfully on pure Ti substrate by laser-cladding technology using aluminum powder as the precursor. The composition and microstructure of the prepared coating were analyzed by transmission electron microscopy, scanning electron microscopy (SEM), and X-ray diffraction technique. Thermal gravimetric analysis was used to evaluate the high-temperature oxidation resistance of the Ti₃Al coating. The friction and wear behavior was tested through sliding against Si₃N₄ ball at elevated temperature of 20, 100, 300, and 500°C. The morphologies of the worn surfaces and wear debris were also analyzed by SEM and three-dimensional non-contact surface mapping. The results show that the Ti₃Al coating with high microhardness, high-temperature oxidation resistance, and high temperature wear resistance. The pure Ti substrate is dominated by severe adhesion wear, abrasive wear, fracture, and severe plastic deformation at lower temperature, and severe adhesion wear, abrasive wear, plastic deformation, oxidation, and nitriding wear at higher temperature, whereas the Ti₃Al coating experiences only moderate abrasive and adhesive wear when sliding against the Si₃N₄ ceramic ball counterpart. In addition, the wear debris of the laser-cladding Ti₃Al coating sliding and Si₃N₄ friction pairs are much smaller than that of pure Ti substrate and Si₃N₄ friction pairs at elevated temperature.     

WC–ZrO2 composites: processing and unlubricated tribological properties

In the present work, we report the processing and properties of WC–6 wt.% ZrO₂ composites, densified using the pressureless sintering route. The densification of the WC–ZrO₂ composites was carried out in the temperature range of 1500–1700 °C with varying time (1–3 h) in vacuum. The experimental results indicate that significantly high hardness of 22–23 GPa and moderate fracture toughness of ∼5 MPa m^1/2 can be obtained with 2 mol% Y-stabilized ZrO₂ sinter-additive, sintered at 1600 °C for 3 h. Furthermore, the friction and wear behavior of optimized WC–ZrO₂ composite is investigated on a fretting mode I wear tester. The tribological results reveal that a moderate coefficient of friction in the range from 0.15 to 0.5 can be achieved with the optimised composite. An important observation is that a transition in friction and wear with load is noted. The dominant mechanisms of material removal appear to be tribochemical wear and spalling of tribolayer.     

Tribological performance of TiN and TiCN coatings on a working tool steel

Although the applications of TiN and TiCN coatings are extensive, their mechanical and tribological properties are influenced by the substrates in which they are deposited. The present work is focused on the tribological performance of TiN and TiCN coatings on a working tool steel. Besides, adhesion and microhardness tests were carried out. The adhesion performance of both coatings resulted in class 1, according to CEN/TS 1071–8 standard, which allows observing the quality of adhesion. The composite microhardness was investigated by the analysis of relative indentation (β). Pin-on-disk tests were performed in dry and lubricated condition at 100 °C against tungsten carbide (WC). Low friction coefficients of μ_k = 0.08 for TiN and μ_k = 0.03 for TiCN were obtained in lubricated conditions. Wear mechanisms were analyzed by scanning electron microscopy (SEM). Abrasive wear was observed as the principal wear mechanism in dry condition, while in lubricated conditions wear signals seem to be scarcely noticeable.     

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.     

Wear performance of C–W2B5 composite sliding against bearing steel

The C–W₂B₅ composite was fabricated by reaction hot-pressing of B₄C and WC powders. The effect of applied load on the friction and wear behavior was investigated by the block-on-ring test in air. In particular, the composition and microstructural characterization of the mechanically mixed layers (MML) and wear debris formed during sliding wear were studied using scanning electron microscopy (SEM) with EDX and X-ray diffraction (XRD). The results revealed that the friction coefficients and wear rates of the composite increased with increasing testing load. The MML and wear debris generated from the sliding system had similar microstructural features and were composed of a mixture of ultrafine grained structures which mainly consisted of W₂B₅, Fe and Fe₂O₃. The formation mechanism of the MML and their influences on wear mechanism of the multiphase material were also studied on the basis of the microstructural observations.     

Tribological properties of ZrO2 (Y2O3)-Mo-BaF2/CaF2 composites at high temperatures

ZrO₂ (Y₂O₃) with different contents of BaF₂/CaF₂ and Mo were fabricated by hot pressed sintering, and the tribological behavior of the composites against SiC ceramic was investigated from room temperature to 1000 °C. It was found that the ZrO₂ (Y₂O₃)-5BaF₂/CaF₂-10Mo composite possessed excellent self-lubricating and anti-wear properties. The low friction and wear were attributed to enhanced matrix and BaMoO₄ formed on the worn surfaces.     

Sliding wear behavior of Fe-Al and Fe-Al/WC coatings prepared by high velocity arc spraying

A comparative study was carried out to investigate the microstructure and tribological behavior of Fe-Al and Fe-Al/WC iron aluminide based coatings against Si₃N₄ under dry sliding at room temperature using a pin-on-disc tribotester. The coatings were prepared by high velocity arc spraying (HVAS) and cored wires. The effect of normal load on friction coefficient and wear rate of the coatings was studied. The microstructure and the worn surfaces of the coatings were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscope (EDS). The results showed that, the main phases in both coatings were iron aluminide (Fe₃Al and FeAl) and α. WC/W₂C particles were embedded in the matrix of the composite coating. With adding WC hard particles, the Fe-Al/WC composite coating exhibited higher wear-resistance than Fe-Al coating. But the friction coefficient of both coatings showed little difference. As the load increased, the friction coefficient decreases slightly due to a rise of friction contact temperature and larger areas of oxide film formation on the worn surface, which act as a solid lubricant. Increasing load causes the maximum shear stress occurring at the deeper position below the surface, thereby aggravating the wear. The coating surface is subjected to alternately tensile stress and compression stress during sliding, and the predominant wear mechanism of the coatings appears to be delamination.     

Impact of Cr3C2/VC addition on the dry sliding friction and wear response of WC–Co cemented carbides

Two grades of WC–10 wt.%Co cemented carbide with or without addition of Cr₃C₂/VC grain growth inhibitor during liquid phase sintering were produced with the goal to investigate their reciprocating sliding friction and wear behaviour against WC–6 wt.%Co cemented carbide under unlubricated conditions. The tribological characteristics were obtained on a Plint TE77 tribometer using distinctive normal contact loads. The generated wear tracks were analyzed by scanning electron microscopy and quantified topographically using surface scanning equipment. The post-mortem obtained wear volumes were compared to the online assessed wear. Correlations between wear volume, wear rate and coefficient of friction on the one hand and sliding distance and microstructural properties on the other hand were determined, revealing a significant influence of Cr₃C₂/VC on the friction characteristics and wear performance.     

Experimental investigations on machinability aspects in finish hard turning of AISI 4340 steel using uncoated and multilayer coated carbide inserts

The present work deals with some machinability studies on flank wear, surface roughness, chip morphology and cutting forces in finish hard turning of AISI 4340 steel using uncoated and multilayer TiN and ZrCN coated carbide inserts at higher cutting speed range. The process has also been justified economically for its effective application in hard turning. Experimental results revealed that multilayer TiN/TiCN/Al₂O₃/TiN coated insert performed better than uncoated and TiN/TiCN/Al₂O₃/ZrCN coated carbide insert being steady growth of flank wear and surface roughness. The tool life for TiN and ZrCN coated carbide inserts was found to be approximately 19 min and 8 min at the extreme cutting conditions tested. Uncoated carbide insert used to cut hardened steel fractured prematurely. Abrasion, chipping and catastrophic failure are the principal wear mechanisms observed during machining. The turning forces (cutting force, thrust force and feed force) are observed to be lower using multilayer coated carbide insert in hard turning compared to uncoated carbide insert. From 1st and 2nd order regression model, 2nd order model explains about 98.3% and 86.3% of the variability of responses (flank wear and surface roughness) in predicting new observations compared to 1st order model and indicates the better fitting of the model with the data for multilayer TiN coated carbide insert. For ZrCN coated carbide insert, 2nd order flank wear model fits well compared to surface roughness model as observed from ANOVA study. The savings in machining costs using multilayer TiN coated insert is 93.4% compared to uncoated carbide and 40% to ZrCN coated carbide inserts respectively in hard machining taking flank wear criteria of 0.3 mm. This shows the economical feasibility of utilizing multilayer TiN coated carbide insert in finish hard turning.     

Tribological behavior and phase transformation of single-crystal silicon in air

The tribological behaviors and phase transformations of single-crystal silicon against Si₃N₄ with different sliding velocities and durations under low contact stress at room temperature were investigated. SEM and Raman analysis indicate that phase transformation was involved and characterized by plastically deformed area which consists of Si-III, Si-XII, and a-Si, especially in the early stage at the low sliding velocity. The plastically deformed layer was wiped off at high-velocity or long sliding duration. TEM analysis indicated that the wear debris consists of amorphous silicon.