Wear and friction of TiAlN/VN coatings against Al2O3 in air at room and elevated temperatures

TiAlN/VN multilayer coatings exhibit excellent dry sliding wear resistance and low friction coefficient, reported to be associated with the formation of self-lubricating V₂O₅. To investigate this hypothesis, dry sliding ball-on-disc wear tests of TiAlN/VN coatings on flat stainless steel substrates were undertaken against Al₂O₃ at 25 °C, 300 °C and 635 °C in air. The coating exhibited increased wear rate with temperature. The friction coefficient was 0.53 at 25 °C, which increased to 1.03 at 300 °C and decreased to 0.46 at 635 °C. Detailed investigation of the worn surfaces was undertaken using site-specific transmission electron microscopy (TEM) via focused ion beam (FIB) microscopy, along with Fourier transform infrared (FTIR) and Raman spectroscopy. Microstructure and tribo-induced chemical reactions at these temperatures were correlated with the coating’s wear and friction behaviour. The friction behaviour at room temperature is attributed to the presence of a thin hydrated tribofilm and the presence of V₂O₅ at high temperature.     

Wear mechanism of CrN/NbN superlattice coating sliding against various counterbodies

Wear properties of CrN/NbN superlattice coating deposited on the WC-12Co substrate was investigated while using 100Cr6 steel, SiC and Al₂O₃ ball as counterbodies for friction pairs. The value of friction coefficient and wear rate was lowest at ~ 0.01 and 2.6 × 10^− 7 mm³/Nm, respectively, when coating slides against Al₂O₃ ball. In contrast, friction coefficient and wear rate were increased while sliding with steel and SiC ball. The deviation in friction coefficient was described by mechanical and chemical properties of these balls. Hardness of Al₂O₃ and SiC ball was comparable but significant deviation in friction coefficient was observed. That is related to oxidation resistance of these balls which is high for Al₂O₃ compared to SiC ball as evident by Raman analysis of the wear track. However, hardness and oxidation resistance were low for steel ball which shows oxidational wear mechanism.     

WC-Co reinforced NiCoCrAlYTa composite coating: Effect of the proportion on microstructure and tribological properties

The properties of the working surfaces are linked to the safety and lifespan of the modern machines so that variety of coatings are used to protect the parts from breakdown. The NiCoCrAlYTa coating, which has an excellent oxidation resistance, usually undergoes more serious friction and wear due to its lower micro-hardness in contrast to the ceramic coatings. Therefore, the composite coatings reinforced by WC-Co are prepared by HVOF sprayed technology and are also characterized by scanning electron microscope, Raman spectrometer and X-ray diffraction. At the same time, the friction and wear behaviors as well as the mechanisms of different friction pairs are also discussed, in detail. The composite coatings, which mainly consist of γ-(Ni, Co), β-NiAl, γ′-Ni₃Al, WC and W₂C, are dense and uniform. With the increase of WC-17Co, the microhardness of NiCoCrAlYTa/WC-Co composite coating has enhanced from 641.4 HV_300g to 859.7 HV_300g. The wear rates of the composite coatings (10⁻⁵–10⁻⁶ mm³·N⁻¹ m⁻¹) are far lower than those of the as-sprayed NiCoCrAlYTa coating (10⁻⁴ mm³·N⁻¹ m⁻¹). Overall, the mechanical properties and tribological behaviors of the coatings are greatly improved with the addition of WC-Co.     

Effect of graphene platelets on tribological properties of boron carbide ceramic composites

The friction and wear behaviour of hot pressed boron carbide/graphene platelets (GPLs) composites have been investigated using the ball-on-flat technique with SiC ball under dry sliding conditions at room temperature. The hardness and fracture toughness of the investigated materials varied from 18.21 GPa to 30.35 GPa and from 3.81 MPa·m^1/2 to 4.60 MPa·m^1/2, respectively. The coefficient of friction for composites were similar, however the wear rate significantly decreased ~ 77% in the case of B₄C + 6 wt.% GPLs when compared to reference material at a load of 5 N, and ~ 60% at a load of 50 N. Wear resistance increased with increasing GPLs content in regards to the present graphene platelets, which during the wear test pulled-out from the matrix, exfoliated and created a wear protecting graphene-silicon based tribofilm.     

Friction and wear mechanisms in MoS2/Sb2O3/Au nanocomposite coatings

Fundamental phenomena governing the tribological mechanisms in sputter deposited amorphous MoS₂/Sb₂O₃/Au nanocomposite coatings are reported. In dry environments the nanocomposite has the same low friction coefficient as pure MoS₂ (～0.007). However, unlike pure MoS₂ coatings, which wear through in air (50% relative humidity), the composite coatings showed minimal wear, with wear factors of ～1.2–1.4 × 10^?7 mm³ Nm^?1 in both dry nitrogen and air. The coatings exhibited non-Amontonian friction behavior, with the friction coefficient decreasing with increasing Hertzian contact stress. Cross-sectional transmission electron microscopy of wear surfaces revealed that frictional contact resulted in an amorphous to crystalline transformation in MoS₂ with 2H-basal (0 0 0 2) planes aligned parallel to the direction of sliding. In air the wear surface and subsurface regions exhibited islands of Au. The mating transfer films were also comprised of (0 0 0 2)-oriented basal planes of MoS₂, resulting in predominantly self-mated “basal on basal” interfacial sliding and, thus, low friction and wear.     

Chromium carbide–CNT nanocomposites with enhanced mechanical properties

Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr₃C₂ coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr₃C₂ reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH_0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation.     

Plasma electrolytic oxidation of hafnium

This paper presents the results of the investigation of plasma electrolytic oxidation (PEO) of hafnium. Atoms ionized during the PEO micro-discharging were identified using optical emission spectroscopy. The spectral line shape analysis of the hydrogen Balmer line H_β indicated the presence of two types of micro-discharges characterized by electron number densities of around 2.5·10²¹ m^− 3 and 1.3·10²² m^− 3. Scanning electron microscopy and X-ray diffraction were employed to investigate surface morphology and phase composition of the PEO coatings obtained. The coatings were crystalline and composed of monoclinic HfO₂. Diffuse reflectance spectroscopy has shown that HfO₂ coatings have a broad absorption band in the range from 200 nm to 400 nm. Optical band gap of HfO₂ coatings was around 5.4 eV, as estimated from absorption spectra. Photoluminescence measurements show that HfO₂ coatings have broad emission band in the visible region, with a maximum at around 480 nm. The highest photoluminescence was obtained for the excitation wavelength of 270 nm. Intensity of photoluminescence increased with PEO time and is related to an increase of oxygen vacancy defects in HfO₂ coatings formed during the process.     

Novel thermal protection coating based on Zr0.75Ce0.25O2/phosphate duplex system for polyimide matrix composites fabricated via a combined sol–gel/sealing treatment process

Thermal protection coating based on Zr_0.75Ce_0.25O₂/phosphate system was fabricated on polymer–matrix composites via a combined sol–gel/sealing treatment process. Phosphates sealed the cracks and enhanced the adhesion property via chemical bonding and binding. The Zr_0.75Ce_0.25O₂/phosphate duplex coating exhibited good thermal shock resistance and improved thermal oxidation resistance of the substrate. Due to the protection of the duplex coating, the weight loss of the specimen reduced from (4.83 ± 0.12)% to (0.98 ± 0.08)% and the mass ablation rate decreased from 0.088 ± 0.002 mg cm⁻² s⁻¹ to 0.018 ± 0.002 mg cm⁻² s⁻¹ when testing at 810 °C. Coating failure was attributed to the formation of cracks and delamination.     

Microstructure and oxidation resistance of Mo–Si–B coating on Nb based in situ composites

The development of robust high temperature oxidation resistant coatings for Nb–Si based alloy was evaluated for a Mo–Si–B coating system that was applied by a two step process. It is observed that the coating is composed of an outer layer of MoSi₂ containing boride dispersoids and an inner layer of unreacted Mo. The mass gain of substrate and Mo–Si–B coating is 190.08 and 1.28 mg cm⁻² after oxidation at 1250 °C in dry air for 100 h, respectively. The good oxidation resistance of the coating is attributed to the formation of a continuous borosilicate glass coverage.     

Structure and tribological properties of AlCrTiN coatings at elevated temperature

In this study, we analyzed the high temperature tribological behavior of AlCrTiN coatings deposited on WC substrates by low cathodic arc technique. The coatings chemical composition, Al 31 at.%, Cr 16 at.%, Ti 7 at.% and N 46 at.%, and the bonding state were evaluated by X-ray photoelectron spectroscopy. The mechanical properties of the coatings were studied by scratch-test and nanohardness depth sensing indentation. The morphology of the coatings surface, ball scars, wear tracks and wear debris as well as the oxidized samples was examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The structure was analyzed using X-ray diffraction (XRD). Wear testing was carried out using a high temperature tribometer (pin-on-disc) with alumina balls as counterparts. The evaluation of the friction coefficient with the number of cycles (sliding distance) was assessed at different temperatures and the wear rates of the coatings and balls were determined; the maximum testing temperature was 800 °C. The coating showed an excellent thermal stability and wear resistance. The friction reached a maximum at 500 °C and then decreased, whereas the wear rate was negligible up to 600 °C and increased significantly at higher temperatures.     

Preparation and ablation properties of Hf(Ta)C co-deposition coating for carbon/carbon composites

Continuous, uniform Hf(Ta)C coating was co-deposited on carbon/carbon composites by chemical vapor deposition. The phase composition, microstructure and ablation properties of the Hf(Ta)C coating are investigated. Results show that the as-prepared coating is a biphasic coating consisting of HfC and HfTaC₂. The particle-stacked structure is effective to produce a crack free Hf(Ta)C coating and good adhesion between the coating and C/C composites. The Hf(Ta)C coating can effectively protect C/C composites from ablation. After 60 s ablation, the mass and linear ablation rates of coated sample are 0.01 ± 0.02 mg cm⁻² s⁻¹ and 0.46 ± 0.02 μm s⁻¹, respectively.     

Microstructures and properties of Ni based composite coatings prepared by plasma spray welding with mixed powders

The Ni based composite coatings have been obtained by using the plasma spray welding process and mixed powders (NiCrBSi + NiCr-Cr₃C₂ + WC). Their microstructures and properties were studied. The results showed that the coatings consist mainly of γ-Ni, WC, Cr₂₃C₆, Cr₇C₃, Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases, and the Ni₃Si, Cr₅B₃, CrB and FeNi₃ phases mainly segregated between the carbide grains. The carbide contents in the coatings increased with increasing the mass fractions of NiCr-Cr₃C₂ and WC powders in the mixed powders, which results in enhancing the coating hardness. The abrasive wear resistance of the coatings depends on their hardness. The higher the coating hardness, the stronger the wear resistance is. When the mixed powder (15wt%WC + 30 wt% NiCr-Cr₃C₂ + 55wt%NiCrBSi) was used, the composite coating has higher hardness and more excellent wear resistance, and the coating hardness and weight loss after wear tests are 991 HV and 8.6 mg, respectively.     

Effect of C2H2 gas flow rate on synthesis and characteristics of Ti–Si–C–N coating by cathodic arc plasma evaporation

The influences of C₂H₂ gas flow rate on the synthesis, microstructure, and mechanical properties of the Ti–Si–C–N films were investigated. Quaternary Ti–Si–C–N coatings were deposited on WC-Co substrates using Ti and TiSi (80:20 at.%) alloy target on a dual cathodic arc plasma evaporation system. The Ti–Si–C–N coatings were designed with Ti/TiN/TiSiN as an interlayer to enhance the adhesion strength between the top coating and substrate. The Ti–Si–C–N coatings were deposited under the mixture flow of N₂ and C₂H₂. Composition analysis showed that as the C₂H₂ gas flow increased, the Ti, Si and N contents decreased and the carbon content increased in the coatings. The results showed the maximum nanohardness of approximately 40 GPa with a friction coefficient of 0.7 was obtained at the carbon content of 28 at.% (C₂H₂ = 15 sccm). However, as the C₂H₂ gas flow rate increased from 15 to 40 sccm (carbon content from 25.2 to 56.3 at.%), both the hardness and friction coefficient reduced to 20 GPa and 0.3, respectively. Raman analysis indicated the microstructure of the deposited coating transformed from Ti–Si–C–N film to TiSi-containing diamond-like carbon films structure, which was strongly influenced by the C₂H₂ flow rate and is demarcated at a C₂H₂ flow of 20 sccm. The TiSi-containing diamond-like carbon films reveal low-friction and wear-resistant nature with an average friction coefficient between 0.3 and 0.4, lower than both TiSiN and Ti–Si–C–N films.     

Cracking investigation of W and W(C) films deposited by physical vapor deposition on steel substrates

This paper presents the investigation of the cracking of coatings deposited on steel substrates. The coating on substrate systems consisted on pure tungsten films (W) and films of solid solutions of carbon in tungsten [W(C)], which were deposited by direct current reactive magnetron sputtering on stainless steel substrates. The systems were strained uniaxially with a microtensile device adapted to a scanning electron microscope. The mechanical response was analyzed from the experimental results: the straining of the samples showed an evolution of the density of cracks in the coating, which was described trough an empirical equation based on the Weibull distribution function. The density of cracks, which corresponds to the crack saturation of the coating, appeared to vary inversely with coating thickness. Critical parameters relative to their mechanical stability were also determined from the experimental results: the strain energy release rate for crack extension through the film, G^f_c, and the fracture toughness, K^f_Ic, of the coatings. These values are included between 0.2 and 14 J m⁻², and between 0.1 and 2.5 MPa m^−1/2. The fracture resistance of W and W(C) coatings was found to be correlated to their thickness and microstructure.     

Microstructure and properties of intermetallic composite coatings fabricated by selective laser melting of Ti–SiC powder mixtures

Transition metal silicides and carbides are attractive advanced materials possessing unique combinations of physical and mechanical properties. However, conventional synthesis of bulk intermetallics is a challenging task because of their high melting point. In the present research, titanium carbides and silicides composites were fabricated on the titanium substrate by a selective laser melting (SLM) of Ti–(20,30,40 wt.%)SiC powder mixtures by an Ytterbium fiber laser with 1.075 μm wavelength, operating at 50 W power, with the laser scanning speed of 120 mm/s. Phase analysis of the fabricated coatings showed that the initial powders remelted and new multiphase structures containing TiC_x, Ti₅Si₃C_x, TiSi₂ and SiC phases in situ formed. Investigation of the microstructure revealed two main types of inhomogeneities in the composites, (i) SiC particles at the interlayer interfaces and, (ii) chemical segregation of the elements in the central areas of the tracks. It was suggested and experimentally proven that an increase in laser power to 80 W was an efficient way to improve the laser penetration depth and the mass transport in the liquid phase, and therefore, to fabricate more homogeneous composite. The SLM Ti–(20,30,40 wt.%)SiC composites demonstrated high hardness (11–17 GPa) and high abrasive wear resistance (3.99 × 10⁻⁷–9.51 × 10⁻⁷ g/Nm) properties, promising for the applications involving abrasive wear.     

Al2O3/Mo composite and its tribological behavior against AISI201 stainless steel at elevated temperatures

Al₂O₃-reinforced molybdenum (Mo) composites were successfully prepared by powder metallurgy to improve the wear resistance of Mo components at high temperature. The reinforced Al₂O₃ particles are uniformly distributed in the Mo matrix; thus, the Al₂O₃/Mo composite is harder than monolithic Mo. The friction coefficients of both monolithic Mo and the Al₂O₃/Mo composite decrease by 37% and 42%, respectively, at 700 °C compared with those at room temperature (self-lubricating phenomenon). This phenomenon is attributed to the formation of very soft MoO₃ and FeMoO₄ metal oxides on the friction surface at high temperature. The Al₂O₃/Mo composite has better wear resistance than monolithic Mo at both room temperature and at 700 °C. The notable resistance of the composite particularly at 700 °C can be attributed to its increased hardness and the soft tribofilm forming on the worn surface.     

The resistance to sliding along Coulombic shear faults in ice

Sliding experiments were performed along Coulombic shear faults in laboratory-grown freshwater ice over a range of sliding velocities (4 × 10⁻³–8 × 10⁻⁷ m s⁻¹) and temperatures (−3, −10 and −40 °C). The Coulombic failure criterion was used to describe the observed linear relationship between the shear stress along and the normal stress across the fault. From this relationship the coefficient of friction was determined. At each temperature the coefficient of friction peaks at a transitional velocity (∼8 × 10⁻⁶ m s⁻¹). For a given velocity the coefficient of friction increases with decreasing temperature. We propose that the peaked shape of the coefficient of friction vs. sliding velocity graphs are a consequence of a change in sliding behavior from “ductile-like” at low velocities to “brittle-like” at higher velocities. The velocity-strengthening and velocity-weakening friction regimes are attributed to creep and to frictional melting, respectively.     

Mechanical and tribological properties of electrically conductive SiC based cermets

Three different types of SiC based cermets with various content (30, 40, 50 wt.%) of electrically conductive TiNbC phase have been fabricated by hot-pressing without sintering additives. The effect of TiNbC content on the basic mechanical, electrical and tribological properties of SiC-TiNbC cermets was investigated. Tribological properties have been characterized by the ball-on-disc method at the ambient temperature and dry wear conditions with air humidity 35–40% at the load of 5–30 N, sliding distance of 500 m, with the static partner made from SiC. Corresponding wear rate was calculated and wear mechanisms were identified. Resulting materials were relatively hard, with increasing amount of TiNbC the hardness increased from 19.8 ± 1 GPa for 30 wt.% of TiNbC up to 25.4 ± 0.9 GPa at 50 wt.% of TiNbC. The fracture toughness values were independent on TiNbC phase and varied between 2.7 and 2.9 MPa.m^1/2. Similarly, Young's modulus increased from 354 GPa to 435 GPa. It was found that electrical conductivity of SiC cermets was rapidly improved with increased fraction of metallic phases and was three orders of magnitude higher at 30 wt.% TiNbC addition and around four order of magnitude higher at 50 wt.% of TiNbC metallic phase comparing to conventional semiconductive SiC ceramics with electrical conductivity ~ 10 Sm^− 1. Coefficient of friction (between 0.3 and 0.5) and wear resistance (10^− 6–10^− 7 mm³/Nm) were comparable with the wear resistant SiC materials.     

Dry sliding wear behaviour of nickel aluminides coatings produced by self-propagating high-temperature synthesis

Nickel aluminides coatings have been produced by self-propagating high-temperature synthesis using concentrated solar energy, with nickel composition of coatings ranging from 45 to 75 at.%. The dry sliding wear behaviour of coatings has been performed in a pin-on-disk tribometer. NiAl coatings (50 at.% Ni) have been tested against Al₂O₃ and WC–Co balls, while other coatings have been tested against Al₂O₃ balls. In all the coatings a three-body abrasion is produced by the particles detached from the coating surface and then oxidized, which remain between the ball and the coating. NiAl coatings exhibit the lowest wear coefficient while coatings with the highest Ni content have the highest wear coefficients. Wear coefficients show that NiAl coatings or coatings composed mainly of NiAl have a high wear resistance.     

The structure and mechanical and tribological properties of TiBCN nanocomposite coatings

TiBCN nanocomposite coatings were deposited in a closed field unbalanced magnetron sputtering system using pulsed magnetron sputtering of a TiBC compound target with various Ar/N₂ mixtures. TiBCN coatings were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, nanoindentation, Rockwell C indentation and ball-on-disk wear tests. The coatings with a nitrogen content of less than 8 at.% exhibited superhardness values in the range of 44–49 GPa, but also showed poor adhesion and low wear resistance. Improvements in the coating adhesion, H/E ratio and wear resistance were achieved together with a decrease in the coating hardness to 35–45 GPa as the N content in the coatings was increased from 8 to 15 at.%. The microstructure of the coatings changed from a nano-columnar to a nanocomposite structure in which 5–8 nm nanocrystalline Ti(B,C) and Ti(N,C) compounds were embedded in an amorphous matrix consisting of BN, free carbon and CN phases. With a further increase in the N content in the coatings to levels greater than 20 at.%, the inter-particle spacing of the nanocrystalline compounds increased significantly due to the formation of a large amount of the amorphous BN phase, which also led to low hardness and poor wear resistance of the TiBCN coatings.