Improvement of structural/tribological properties and milling performances of tungsten carbide cutting tools by bilayer TiAlN/TiSiN and monolayer AlCrSiN ceramic films

In this study, bilayer TiAlN/TiSiN and monolayer AlCrSiN ceramic films were grown on carbide cutting tool material by cathodic arc physical vapor coating (CAPVD) method to improve the structural/tribological properties and milling performances. The ceramic films were applied on cylindrical test samples and carbide end mills. The coated materials' structural, mechanical, and tribological properties were determined via scanning electron microscope (SEM), X-ray diffraction meter (XRD), tribometer, microhardness tester, and optical profilometer. DIN 40CrMnNiMo8-6-4 steel workpieces were machined by using a CNC vertical machining center to determine the actual working performance of the coated and uncoated cutting tools. The wear performance of the cutting tools after machining was determined by measuring the flank wear widths and mass losses. The hardness and adhesion results of the coated sample with bilayer TiAlN/TiSiN were higher than the coated sample with monolayer AlCrSiN. According to the scratch test results, the best adhesion results were obtained for TiAlN/TiSiN coating. The critical load value was determined as about 105 N. As a result, the wear rate value of the TiAlN/TiSiN thin film coated sample was lower. After machining, the mass loss of TiAlN/TiSiN coated tools was lower than AlCrSiN coated tools. In addition, the surface roughness value of the workpiece machined by the cutting tool coated with AlCrSiN was higher than the cutting tool coated with TiAlN/TiSiN.     

Effects of ceramic-based CrN,TiN, and AlCrN interlayers on wear and friction behaviors of AlTiSiN+TiSiN PVD coatings

In this study, the wear and friction behavior of cathodic arc physical vapor deposited AlTiSiN+TiSiN coatings on H13 tool steels were investigated by using CrN, TiN and AlCrN interlayers with tribometer tests both under unlubricated and boundary lubricated conditions. 6 mm alumina balls were used as counter surfaces to test ceramic hard coatings. Surface coatings were characterized through nanoindentation, scanning electron microscopy coupled with an energy-dispersive X-ray spectrometer (SEM/EDXS), optical profilometry, and atomic force microscopy (AFM) techniques. The results showed that especially AlTiSiN+TiSiN coating with TiN interlayer resulted in a much more enhanced tribological performance of the tool steels at both unlubricated and the boundary lubricated conditions even at elevated contact pressures.     

Syntheses of nano-multilayered TiN/TiSiN and CrN/CrSiN hard coatings

Nano-structured superhard coatings represent the state-of-the-art in the rapidly increasing worldwide market for protective coatings. In this study, the combination of nano-composite and nano-multilayered structures into the same coating was attempted. Nano-multilayered coatings of TiN/TiSiN and CrN/CrSiN were deposited on tool steel substrates by closed-magnetic-field unbalanced DC magnetron sputter ion plating. The coating structures were characterized using X-ray diffraction, atomic force microscopy, and scanning electron microscopy. Mechanical characterizations were performed including nano-hardness measurement, progressively-increasing-load scratch test, and wear test. TiN/TiSiN coatings have a nano-hardness of 40.2 GPa, whereas CrN/CrSiN coatings have a hardness of 30.9 GPa. TiN/TiSiN coatings also showed a higher critical failure force and scratch fracture toughness as well as better wear resistance and lower acoustic emission signal, indicating less total damage to the coatings.     

Friction behavior of TiN–MoS2/PTFE composite coatings in dry sliding against SiC

To improve the dry friction behavior of traditional hard coatings, MoS₂/PTFE lubricating coatings were prepared on the PVD TiN-coated cemented carbide using spray method. The influences of MoS₂/PTFE lubricating coatings on the primary characteristics of TiN coatings were investigated. Reciprocating sliding tests were carried out with the TiN–MoS₂/PTFE coated specimen (T-M-P) under dry sliding conditions, and the tribological behaviors were compared to those of the TiN-coated one (T-N). The test results indicated that the adhesion force of coatings with substrate for T-M-P specimen increased, the surface micro-hardness, roughness and friction coefficient significantly decreased. Meanwhile, the surface adhesions and abrasion grooves of T-M-P specimen were reduced, and the main wear forms of T-M-P were abrasion wear and coating delamination. The MoS₂/PTFE lubricating coatings can be considered effective to improve the friction properties of traditional hard coatings.     

Mechanical and tribological assessment of composite AlCrN or a-C:Ag-based thin films for implant application

The present study focuses on the comparison of cathodic arc deposited AlCrN (ternary coating) and Ag alloyed a-C (amorphous carbon base coating) on chrome nitride (CrN) medical grade 316 LVM stainless steel. The work comprises of morphological, structural, nanomechanical and tribological evaluation in physiological simulated body fluid (SBF) lubrication following conditions pertaining to simulated hip joint. According to the findings, H/E, H³/E² and E_coating/E_substrate significantly effect the nanomechanical and tribological properties of the coatings. While a-C:Ag/CrN exhibited better L_y value compared to AlCrN/CrN due to better surface quality, the later has shown higher L_c2 value during nanoscratch test attributed to lower H³/E² and higher plastic work done. Inspite of lower friction coefficient, a-C:Ag/CrN observed higher wear rate during simulated tribotest attributed to low hardness, separate graphitic structure due to Ag doping and sudden increase of friction coefficient ascribed to severe abrasive delamination of a-C:Ag top layer. The wear mechanism observed under SEM microscopy indicate severe adhesion of Ti6Al4V counterbody on AlCrN/CrN coated surface. The size of wear debris obtained with AlCrN/CrN-Ti6Al4V tribopair was larger in size compared to a-C:Ag/CrN-Ti6Al4V tribopair. Nevertheless, despite inferior surface quality and lower L_y value and larger wear debris size, AlCrN/CrN coating performed better in nanoscratch (at L_c2 value) and demonstrated lower wear in simulated tribotest in physiological SBF condition.     

Investigation of tribological properties of micro-arc oxidation ceramic coating on Mg alloy under dry sliding condition

To enhance wear resistance of Mg alloy, micro-arc oxidation (MAO) ceramic coatings on Mg substrate were prepared in silicate electrolyte under various currents. It was found that the surface roughness and thickness of MAO coating were increased with the increase of current. The dry tribological tests showed that the friction coefficient and wear resistance of thicker coatings (obtained under currents of 3?A and 4?A) were much higher than that of Mg alloy and the thin coating (obtained under current of 2?A), meanwhile the lifetime of the coating obtained under 4?A was longer than the other coatings under higher load. The wear type of thin MAO coating was slight abrasive wear under low load, whereas translated to severe adhesive wear under high load. While the main wear mechanism of thick MAO coating was slight abrasive wear or scratch under the given test condition, which was attributed to the thick intermediate layer improved load support for the soft substrate. The tribological study indicated that the MAO coating obtained under 4?A current had better wear resistance and life time due to its compact microstructure and thickness.     

Wear-resistant ceramic coatings deposited by liquid thermal spraying

As a surface strengthening and surface modification technology of materials, liquid thermal spray technology has been used in many fields, such as wear and friction reduction, corrosion resistance, and high-temperature oxidation resistance. This article reviews the progress of liquid thermal sprayed coating in wear resistance as well as friction reduction in recent years. The influences of microstructure, composition, phase structure and mechanical properties on the tribological properties of typical coatings (including ceramic coatings and multiphase composite coatings) are investigated. The tribological properties of the coating are determined by the coating characteristics (including microstructure, porosity, mechanical properties, etc.) and the service conditions (working temperature, lubrication state, etc.). Typical ceramic wear-resistant coatings include Al₂O₃, YSZ, HA coatings, etc. The tribological properties of the coating can be significantly improved through process optimization and heat treatment. The comparison of nanostructured and microstructured ceramic-based coating reveals that nanostructured coating reduces wear by absorbing stress. The interaction between different constituent phases improves wear resistance and reduces wear in composite coatings. Finally, various challenges faced by liquid thermal spray are pointed out, and future research focuses are proposed.     

Tribological and mechanical properties of amorphous and semi-crystalline PEEK/SiO2 nanocomposite coatings deposited on the plain carbon steel by electrostatic powder spray technique

One of the main practical limitations of polymer coatings is dependency of their mechanical and physical properties on the crystallinity of polymer matrix. In this research, the effect of the presence of silica nanoparticles on microhardness, interfacial adhesion strength and tribological behavior of amorphous and semi-crystalline polyether–ether–ketone (PEEK) coatings were examined. The coatings were prepared by a combination of ball milling and electrostatic powder spraying methods. The results showed that the semi-crystalline pure PEEK coating had higher hardness, lower adhesion strength, coefficient of friction (COF) and wear rate than the amorphous one. However, the incorporating of PEEK with surface modified silica nanoparticles led to an increase in the coatings microhardness and interfacial adherence. The wear rates of both the semi-crystalline and amorphous nanocomposite coatings were lower than the pure ones but their COF were slightly higher. It was also found that, compared with the pure coatings, the sensitivity of the mechanical and tribological properties of the nanocomposite coatings to the crystalline structure of the PEEK matrix are less pronounced.     

Growth,mechanical properties,and tribological performance of TiAlN/NbN and NbN/TiAlN bilayer coatings

Three different coatings, namely TiAlN, TiAlN (external)/NbN (internal) and NbN (external)/TiAlN (internal), were deposited on cemented carbides by arc ion plating. The comparative investigation conducted in this study elucidates the effect of the NbN layer and coating systems on the growth, mechanical properties, and tribological performance of the coatings. The results showed that the surface of the TiAlN and TiAlN/NbN coatings was smoother when TiAlN served as the external layer. The NbN/TiAlN coating, wherein NbN formed the external layer, had a much rougher but more symmetrical surface. With the introduction of the NbN layer, the increased micro stress induced a lower adhesion strength in the TiAlN/NbN and NbN/TiAlN coatings. The TiAlN/NbN and NbN/TiAlN coatings exhibited higher hardness and hardness/effective elastic modulus (H/E*). During the friction test, when the temperature was elevated to 700 °C, the tribological performance of the monolayer TiAlN coating was the lowest because of the TiO₂-induced breakage of the dense tribo-oxide film. The NbN layer participated in the formation of a NbO_x film at elevated temperatures, which was responsible for the high tribological performance of the two bilayer coatings. When the NbN layer was on the outermost layer and in direct contact with the elevated temperature atmosphere, the NbN/TiAlN coating generated a tribo-oxide film with high integrity, and its coefficient of friction decreased by 27% of that at room temperature. Therefore, the NbN/TiAlN coating exhibited the highest wear resistance at 700 °C.     

Influence of nitrogen gas over microstructural,vibrational and mechanical properties of CVD Titanium nitride (TiN) thin film coating

In this experimental investigation, the influence of different N₂ gas flow rates on different properties (e.g. morphological, mechanical, etc.) of chemical vapor deposited (CVD) Titanium nitride (TiN) coatings has been discussed. The TiN coatings had been grown on Si (100) substrate at elevated temperature (1000 °C) using Titanium dioxide (TiO₂) powder. SEM images reveal a dense uniform microstructure with an irregular surface pattern. The surface roughness of the coatings was found to be increased from 12.42 to 28.56 nm with an increase in flow rate. XRD results indicate a B1 NaCl crystal structure of the film with reduced crystallite size with the increasing N₂ flow rate. Through the corrosion test, it has been observed that due to the variation of N₂ flow rate the corrosion resistance of the films decreases with increasing N₂ flow rate. The mismatch of thermal expansion co-efficient in between Si substrate and TiN thin film reduces with higher N₂ flow rate. The acoustic and optic phonon mode of TiN coatings have been shifted to higher intensities with higher N₂ flow rate. The mechanical properties of the film reveal that the maximum value of hardness (H) and Young's modulus (E) are 30.14 and 471.85 GPa respectively.     

The tribological performance of TiN,WC/C and DLC coatings measured by the four-ball test

An innovative approach to improving the wear resistance and load-carrying capacity of surfaces is by development of novel systems featuring coating treatment. Evaluation of the tribological performance of three physical vapor deposition (PVD) coatings, namely, TiN, WC/C, and DLC (diamond-like carbon), is necessary to determine their suitability as coatings for high-speed and heavy-duty power-transmitting gears. The uncoated and coated steel balls were subjected to four-ball tests under lubricated conditions. An optical microscope and a scanning electron microscope were used to observe wear scars, and energy-dispersive X-ray analysis was performed to determine the chemical compositions of the materials; these methods were also used to analyze the wear mechanisms. The wear performance of the three coatings was compared, and a four-ball method extreme pressure test was performed to determine the last nonseizure load of each tribo-pair. The WC/C and DLC coatings showed excellent tribological performance under high contact pressures and thermal loads, and the benefits of these coatings increased with decreasing performance of the lubricating medium. Therefore, WC/C and DLC coatings are suitable for application in high-speed and heavy-duty gears. Oxidation wear and peeling, fatigue pitting, and adhesive transfer are the main coating failure modes of the TiN, WC/C, and DLC coatings, respectively.     

Microstructure and tribological behavior of Ti3C2Tx MXene reinforced chemically bonded silicate ceramic coatings

MXenes – In recent decades, great attention has been paid to the fast-growing two-dimensional (2D) transition metal carbides and nitrides, in terms of their prominent mechanical and electrical properties. The tribological essence of MXene has not yet been entirely investigated, although researches on MXene were conducted in all aspects of its applications. Hence, a newly compound 2D MXene (Ti₃C₂T_x) is exploited to reinforce the wear resistance of the chemically bonded silicate ceramic coatings, which are utilized to protect component surfaces under severe conditions. The structural features, hardness, and tribological behaviors of the targeted coatings are investigated and analyzed. Results show that the micro-hardness of the coatings increases to 156.9 HV_0.5 when added 1.2 wt% MXene. The increment of microhardness extraordinarily reaches 33.3%, compared with the original. The coating with 1.2 wt% MXene also indicates a 31.6% decrement of the coefficient of friction (COF) and a 73% reduction of the wear rate respectively. Furthermore, fatigue is found to be the main reason of the wear mechanism, through exploring the surface morphologies of wear traces and counterpart balls.     

Nanolayer CrAlN/TiSiN coating designed for tribological applications

With the goal to produce a hard and tough coating intended for tribological applications, CrAlN/TiSiN nanolayer coating was prepared by alternative deposition of CrAlN and TiSiN layers. In the first part of the article, a detailed study of phase composition, microstructure, and layer structure of CrAlN/TiSiN coating is presented. In the second part, its mechanical properties, fracture and tribological behavior are compared to the nanocomposite TiSiN coating. An industrial magnetron sputtering unit was used for coating deposition. X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used for compositional and microstructural analysis. Mechanical properties and fracture behavior were studied by instrumented indentation and focused ion beam techniques. Tribological properties were evaluated by ball-on-disk test in a linear reciprocal mode. A complex layer structure was found in the nanolayer coating. The TiSiN layers were epitaxially stabilized inside the coating which led to formation of dislocations at interfaces, to introduction of disturbances in the coating growth, and as a result, to development of fine-grained columnar microstructure. Indentation load required for the onset of fracture was twice lower for the nanolayer CrAlN/TiSiN, compared to the nanocomposite TiSiN coating. This agrees very well with their mechanical properties, with H³/E² being twice higher for the TiSiN coating. However, the nanolayer coating experienced less severe damage, which had a strong impact on tribological behavior. A magnitude of order lower wear rate and four times lower steady state friction coefficient were found for the nanolayer coating.     

Micromechanical and microtribological properties of BCN thin films near the B4C composition deposited by r.f. magnetron sputtering

Ternary materials with compositions in the B–C–N system offer properties of great interest. In particular, mechanical and tribological properties are expected to be excellent, as they can combine some of the specific properties of BN, B₄C and C₃N₄. In this paper, BCN thin films deposited by r.f. magnetron sputtering are characterized by their micromechanical and microtribological behavior. BCN coatings with different composition were obtained by varying the N₂/Ar proportion in the sputtering gas. Hardness and elastic modulus of the coatings were measured by nanoindentation. The adhesion and friction coefficient against diamond have been evaluated by microscratch and the coatings have been characterized in their wear behavior at the nanometric scale. These mechanical and tribological properties have been related to film composition and structure, which have been studied in a previous work. It is found that the measured wear resistance at the nanometric scale is directly related to the coating microhardness rather than friction behavior or adhesion of the coating to the substrate, which are the determinant factors in the macroscopic scale wear behavior.     

Friction behavior of PTFE-coated Si3N4/TiC ceramics fabricated by spray technique under dry friction

PTFE coatings were deposited on the Si₃N₄/TiC ceramic substrate by using spray technology. The surface and cross-section micrographs, adhesive force of coatings with substrate, surface roughness and micro-hardness of the coated ceramics were examined. The friction and wear behaviors of ceramic samples with and without coatings were investigated through carrying out dry sliding friction tests against WC/Co ball. The test results indicated that the coated ceramics exhibited rougher surface and lower micro-hardness, and the PTFE coatings can significantly reduce the surface friction and adhesive wear of ceramics. The friction performance of PTFE-coated sample was affected by applied load due to the lower surface hardness and shear strength of coatings, and the main wear failure mechanisms were abrasion wear, coating delamination and flaking. It can be considered that deposition of PTFE coatings is a promising approach to improve the friction and wear behavior of ceramic substrate.     

Effect of transition layer on the performance of hydroxyapatite/titanium nitride coating developed on Ti-6Al-4V alloy by magnetron sputtering

A gradient transition multilayer hydroxyapatite/titanium nitride (HA/TiN) coating was prepared on the Ti-6Al-4V alloy by magnetron sputtering. The composition, surface topography, microstructure, adhesion strength and electrochemical properties of the as-deposited coatings were characterized by SEM/EDS, AFM, XRD, FT-IR and electrochemical workstation. The experimental results showed that the single TiN coating deposited at a partial pressure of nitrogen (N₂) of 0.08?Pa had the best internal stress and tribological performance, and its volume loss was only 0.89% of that of Ti-6Al-4V alloy. The introduction of the TiN transition layer greatly improved the wear resistance of the Ti-6Al-4V alloy, and the adhesion strength of the HA layer to the substrate increased from 6.50?±?0.5?N to 11.70?±?1.2?N, an increase of 56%. The HA/TiN coating surface consisted of uniform hemispherical particles with dense structure and invisible defects (micro-cracks and pores). For the HA surface layer, the crystal structure and active hydroxyl (-OH) group was restored after heat treatment. Potentiodynamic polarization experiments indicated that the HA/TiN coating achieved the lowest corrosion current density and the most positive corrosion potential compared to the single TiN layer and Ti-6Al-4V alloy. In summary, it can be conclude that the gradient transition layer can well improve the mechanical properties and electrochemical behavior of the titanium alloy, and largely ensuring the stability of the surface bioactive coating.     

Interfacial processing and adhesion of diamond,diamond-like,and TiN films on metallic and polymeric substrates

Diamond, diamond-like, and titanium nitride (TiN) films have extremely desirable chemical, electrical, and mechanical properties for a variety of applications ranging from corrosion- and erosion-resistant coatings to electronics packaging of microelectronic devices. However, many of these applications are limited by the poor adhesion of these films to metal and polymer substrates. The adhesion of a film is determined primarily by internal stresses in the film, thermal and lattice mismatch, and most importantly by interfacial bonding. We have developed methods based on mechanical interlocking, chemical bonding, grading of interatomic potentials, and the multilayer discontinuous thin films approach to control stresses and strains in thin films. A substantial improvement in adhesion and wear properties is obtained by using these methods selectively. We review issues related to the adhesion of diamond, diamond-like carbon, and TiN films on metal and polymeric substrates.     

Enhanced tribological properties of Y/MoS2 composite coatings prepared by chemical vapor deposition

Chemical vapor deposition (CVD) is an efficient approach to prepare coatings on complex cutting tools. However, MoS₂ with self-lubrication ability and excellent tribological properties fabricated by CVD have been rarely reported in literature. The aim of this study was to deposit pure MoS₂ coatings and yttrium (Y) doped MoS₂ (Y/MoS₂) composite coatings on cemented carbide blades coated with titanium nitride by CVD. The structural and mechanical properties of the coatings were examined by scanning electron microscopy (SEM) and nanoindentation, respectively. The results demonstrated that the microstructure of Y/MoS₂ composite coatings was denser than that of the pure MoS₂ coating. The hardness and the adhesional properties were significantly enhanced for the Y/MoS₂ composite coatings. The tribological performance of the as-deposited coatings were investigated under atmospheric environment. Y/MoS₂ compostite coatings demonstrated an enhanced tribological performance with a stable and low coefficient of friction (COF) over the entire sliding time. In contrast, the COF of pure MoS₂ coating dramatically increased to value above 0.3 after a sliding time of only 30 min. Additionally, the Y/MoS₂ composite coatings showed a decreased wear rate (8.36 ± 0.29 × 10⁻⁷ mm³/Nm) compared to the pure MoS₂ coatings (3.41 ± 0.48 × 10⁻⁵ mm³/Nm) thus reflecting an improvement by two order of magnitude.     

TiCN and metallic coatings for corrosion protection of separator plates in MCFCs

Stainless steel 304 substrates were coated with different materials in order to find a suitable coating material for corrosion protection of separator plates in molten-carbonate fuel cells (MCFCs). Five titanium carbonitride coatings differing in composition and morphology and a titanium monoxide coating were deposited with chemical vapour deposition techniques. Also double-layer coatings of TiN/Au and TiN/Ni were prepared. The coatings were tested on their corrosion protection of separator plates in four different environments: under MCFC-cathode or anode gas, at load or at open circuit conditions. The corrosion behaviour was characterized using cyclic voltammetry. Corrosion rates were determined with electrochemical methods and cross-section analyses of corrosion layers. Titanium nitride coatings showed the best corrosion protection. The titanium carbide and titanium monoxide coating showed respectively less and no protection. The thin gold and Ni-coatings were unstable. Under cathode gas, the most important corrosion protection is given by keeping the cell at load, and then a titanium nitride coating might provide lifetime protection. Under anode gas, corrosion is most severe at load conditions. A titanium nitride coating also gives corrosion protection, but not enough for lifetime protection.