Hard polyurethane coatings on compliant polycarbonate: An application of the 3D deformation response model to scratch visibility

The present investigation deals with the design of a transparent protective coating and its application on flat substrates in polycarbonate. The experimental analyses looked into the formulation of the coating material, the best strategy to deposit it as well as the characterization of the coated substrates. Visual appearance and morphological features of the coatings were studied by combined scanning electron microscope and contact gauge surface profiler. Their scratch and wear endurance were assessed by progressive and constant load scratching procedure and dry sliding linear reciprocating tribological tests. Imaging analyses were also used to evaluate the deformation response of the coating material to scratch and wear. Analytical modeling was developed accordingly, thus allowing to establish a strict relation between the design criteria of the coatings, the overall (coating + substrate) material performance and the loading conditions. The experimental findings showed the organic coatings were able to significantly improve the micro-mechanical and tribological response of the bare polycarbonate, thus making it available for a large share of applications where high performant, scratch and wear resistant materials are an ineluctable pre-requisite.     

Analysis of instrumented scratch hardness and fracture toughness properties of laser surface alloyed tribological coatings

The mechanical characteristics of ceramic matrix composite (CMC) coatings are widely different from the same materials in bulk form or the individual constituents and are very important to be assessed to carry out application oriented studies on CMC coatings with novel compositions. In the present work, a composite coating of TiB₂, TiN and SiC is fabricated in-situ through a combination of high temperature chemical reaction and laser surface alloying. The formation of the surface layer is due to the laser-assisted chemical reaction followed by laser melting. A mixture of TiO₂, SiO₂, hBN and graphite in stoichiometric proportions is used as the precursor for the chemical reaction. The presence of all the reaction products in the CMC coatings developed is confirmed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). A thorough evaluation of various mechanical properties achieved more insight into the CMC coatings developed. Hardness and fracture toughness of the coatings are measured with a scratch tester. The property evaluations are performed in a similar way for two more coatings fabricated with precursor mixtures containing more than a stoichiometric amount of SiC and hBN respectively. For comparison, a number of composites fabricated through various other routes are characterized afresh with the same set of techniques. Coatings formed with SiC in precursor show higher values of scratch hardness (14.37 GPa), microhardness (24.37 GPa) and fracture toughness (6.63 MPa-m^1/2).     

Tribological behavior of graphene reinforced chemically bonded ceramic coatings

To investigate tribological behavior of graphene reinforced chemically bonded ceramic coatings at different temperatures, tribological tests at room temperature, 200 °C and 500 °C were carried out. Results show that the fracture toughness and the hardness of the coating are improved with the introduction of graphene. Besides, the friction coefficient of the coating decreases with the addition of graphene at the room temperature and 200 °C. The coating without graphene achieves the similar friction coefficient at all temperatures. However, the coating with graphene achieves the lowest friction coefficient at 200 °C, and achieves the highest at 500 °C. In addition, the wear rate of the coating decreases with the increase of graphene. Besides, the wear rate at 200 °C is almost similar with that at room temperature. In contrast, the wear rate at 500 °C is much larger than those at room temperature and 200 °C. The mechanisms for graphene to decrease the friction coefficient and improve the wear resistance of chemically bonded ceramic coatings at evaluated temperatures are clarified.     

Correlation between the wear resistance, and the scratch resistance, for nanocomposite coatings

The nanoscratch test, used in this study, quantitatively characterizes the scratch behaviour of coatings. Some of the obtained parameters are plastic and elastic deformation, critical load to start the scratch, chipping or other change in the scratch mechanism of the coating. The knowledge of the scratch mechanism allows the optimisation of the material behaviour. In the present study the scratch resistance with the wear resistance has been correlated for nanocomposite coatings. Knowledge of mechanical properties extracted from the scratch test, supports the optimization of the coating against wear. The study was applied to nanocomposite coatings with different polymer matrices, different percentages and nature of nanoparticles.     

Visual appearance and scratch resistance of high performance thermoset and thermoplastic powder coatings

A comparative evaluation of electrostatic spray and ‘hot dipping’ fluidized bed to deposit two different organic paints belonging to the class of thermoplastic (PPA571, an alloy of acid modified polyolefins) and thermoset (TGIC-free transparent pigmented bronze polyester) powders was performed. Visual appearance of the investigated coatings was evaluated by colour, gloss and coating thickness measurements as well as by the determination of the surface morphologies. Micro-mechanical performance of the coatings was assessed by progressive load scratch tests.     

Dependence of mechanical and tribological performance on the microstructure of (CrAlTiNbV)Nx high-entropy nitride coatings in aviation lubricant

The (CrAlTiNbV)N_x coatings are fabricated by controlling the substrate bias via magnetron sputtering method. Under low substrate bias, the coating tends to form a loose columnar crystal structure with (200) preferred orientation. However, when the substrate bias increases, the coating transforms into a dense nanocrystalline structure, and the (111) orientation is enhanced. As the substrate bias increases, the residual stress and hardness of the coating gradually increase, while the adhesion strength decreases slightly. Friction tests show that the coating possesses the lowest average friction coefficient (about 0.06) and wear rate (8.7 × 10^-9 mm³/N·m) in 4050# aviation lubricant, which is achieved under the substrate bias of -96 V and -126 V, respectively.     

Performance of polyester and modified polyester coil coatings exposed in different environments with high UV radiation

Coil coating is a continuous strip coating technology, which enjoyed rapid growth. It provides a cost-effective method of applying a coating to a metal substrate with a low solvent emission, and nowadays paints of different nature are applied this way on a range of metal substrates, for a large variety of products and uses.

Coil coatings can be more or less degraded by high UV radiation. However, differences in distribution of light intensity as a function of wavelength between exposure environments, can lead to significant differences in the degradation results, depending on coating nature. In this work, the behaviour of polyester and silicone polyester coatings exposed in three different high UV environments (natural atmosphere and two accelerated UV tests), was studied. Coatings of two different colours (blue and brown) were considered for each paint system. Colour change, gloss and chalking measurements and, visual inspections of coating defects were carried out. In order to explain their corresponding performance, chemical changes on coil coated exposed surfaces were studied by to Fourier transform infrared spectroscopy equipped with a photoacustic detector (PAS/FTIR).     

A focused review on the tribological behavior of C/SiC composites: Present status and future prospects

Silicon carbide-based ceramic matrix composites have received extensive attention in recent years. Many excellent reviews have reported on the tribological behavior of carbon fiber-reinforced carbon and silicon carbide dual matrix (C/C-SiC) composites. However, a systematic overview of the tribological properties of carbon fiber-reinforced silicon carbide (C/SiC) composites does not exist. This review focuses on C/SiC composites and summarizes the key factors, including internal factors (constituent content, graphitization process, material structure and fiber direction), and various test conditions (pressure and speed, dry and wet, temperature, and counterparts) that affect their tribological behavior. Their wear mechanisms under different conditions are elaborated. Finally, some potential future development directions for improving the performance of C/SiC composites are proposed to provide high-quality ceramic matrix composites for engineering applications. These directions include structural modification, matrix modification, coating technology, laser surface texturing, and material genome method.     

Influence of Mo doping on the tribological behavior of Ti3AlC2 ceramic at different temperatures

(Ti_0·8Mo_0.2)₃AlC₂ solid solutions were successfully synthesized from Ti, Al, TiC, and Mo powders using the in situ hot-pressing sintering method. The tribological properties of (Ti_0·8Mo_0.2)₃AlC₂ and the reference Ti₃AlC₂ in the temperature range 25–800 °C were evaluated in ambient air with the counterpart of Al₂O₃ balls. The results show that (Ti_0·8Mo_0.2)₃AlC₂ has improved lubricating properties and wear resistance above 400 °C compared with Ti₃AlC₂. This can be contributed to the formation of tribo-oxidation films containing MoO₃ and MoO_3-x. Structural characterization of the tribo-oxidation films was conducted using SEM, EDS, Raman spectroscopy, and XPS to evaluate the effect of Mo doping on the wear mechanisms of Ti₃AlC₂ in detail.     

A new approach to characterize scratch and mar resistance of automotive coatings

Mar damage is a major customer concern of the automotive coatings industry. Our study of mar performance can be separated into two distinct areas, a detailed understanding of the damage formation mechanism, and an investigation of the relationship between the damage morphology and appearance. We have developed a nanoscratch technique that can measure important physical quantities, such as penetration depth, normal force and tangential force during the formation of the scratch. Mar resistance of three coatings was evaluated and compared based on the damage mechanisms: plastic flow and fracture. The different deformation mechanisms result in different damage morphologies and a corresponding change in visual impact of the scratch. Statistical surveys of appearance of well-defined scratches indicate that in very short observation times, scratches where fracture has occurred are much more visible than those made by plastic deformation alone. However, with sufficient time and strong illumination a significant percentage of observers could see plastic deformation as well.     

Effect of substrate rotational speed during deposition on the microstructure,mechanical and tribological properties of a-C:Ta coatings

Rotational speed has an important influence on the performance of coating materials. The a-C:Ta composite coatings were prepared by controlling the substrate rotational speed during deposition process using PVD technique. The results showed that the coating transformed from dense structure to columnar structure. Due to the changes of deposition time and the vapor incident angle of the sputtering ions, the sp² hybrid structure increased and the C–Ta bonds contents decreased as a function of the rotational speed, which led to the improvement of adhesion force. The average friction coefficient of the composite coatings did not fluctuate significantly for the amorphous carbon matrix and the transfer films formed during friction, while the wear rates were gradually increased. The sample at 0.5 rpm possessed the lowest wear rate, which was mainly associated with the cooperative behavior of the dense structure and the formation of TaC nanoclusters in the composite coating.     

An investigation of the friction and wear behaviors of polyphenylene sulfide filled with solid lubricants

Composites of polyphenylene sulfide (PPS) filled with solid lubricant particles of graphite (C), molybdenum disulfide (MoS₂), and polytetrafluoroethylene (PTFE) were prepared by compression molding. The size of the solid lubricant particles was 3‐;5 µm. The friction and wear behaviors of the composites were examined with a pinon‐disk test rig. The worn composite pin surfaces and the transfer films formed on the counterface were analyzed with scanning electron microscopy. An X‐ray photo‐electron spectroscope (XPS) was used to characterize the chemical states of the elements in the transfer film. It has been found that graphite and PTFE as the fillers increase the wear resistance of PPS considerably, while MoS₂ as the filler decreases the wear resistance of PPS greatly. The fillers promote the decomposition of PPS and generate compounds, which accounts for the changes in the wear resistance of the composites.     

A comparative study of duty cycle and argon / methane flow ratio effect on the tribological behavior of DLC coatings over nitrided Astaloy Mo-based steel

Surface properties of Astaloy Mo-based steel were enhanced by using DLC deposition. The specimens were formed by double-sided compaction and heated for 30 min at 1393 K, in the NH₃ atmosphere. Following this, the plasma nitriding process was applied to improve the adhesion of the DLC coating. Afterward, the DLC coating was performed by Pulsed DC PACVD. Surface characteristics were studied by changing the duty cycle and the Argon/Methane flow ratio. The reciprocating method was carried out to evaluate wear behavior. Field emission scanning electron microscopy equipped with EDS and Raman spectroscopy, hardness tester, nanoindentation test and surface roughness tester were used to evaluate the chemical structure, wear mechanisms of DLC coatings. This study proved that hardness reached up to 12.2 ± 1.11 GPa and the wear behavior was enhanced significantly by the DLC coating deposition. The mass loss increased with a rise in the duty cycle. Increasing the Argon/Methane ratio from 4:1 to 6:1 caused a decrease in the mass loss of DLC coatings. Burnishing, pulling out and adhesive wear were the dominant mechanisms.     

Effect of oxidation inhibitor on the low energy tribological behavior of carbon-carbon composites

In this study, the tribological performance of carbon-carbon composites impregnated with different amounts of MoSi₂ as an oxidation inhibitor were investigated. The results of the friction tests indicated that the carbon-carbon composites underwent an abrupt transition of the coefficient of friction at the frictional temperature range of 150-180°C. And the composites made with MoSi₂, exhibited lower frictional coefficient and wear rate in comparison with the composites made without MoSi₂. The composites made with 4 wt% MoSi₂ showed an improvement in activation energies for wear resistance when compared with the other composites under the present condition. These results were probably due to the consequence that the friction and wear properties of carbon-carbon composites are sensitive to the friction temperature and can be largely dependent on the adhering force between fibers and matrix-MoSi₂, the reduction of porosity, and the formation of a lubricative, powdery, debris film, formed on the friction surfaces of the carbon samples.     

Wear behavior of Fe3Al-TiN-TiB2 HVOF coatings: A comparative study between in situ and ex situ powder processing routes

In the present study, the tribological properties of High Velocity Oxy-Fuel (HVOF) coatings prepared from Fe₃Al-based composite powders were investigated. The iron aluminide matrix of the composite powders was reinforced with TiN and TiB₂ particles made using two different processing routes: a) an in situ method where fine ceramic particles were formed in the matrix by the reaction between Ti and BN, and b) an ex situ method where preformed coarse TiN and TiB₂ particles were added to the matrix. The tribomechanical performance of the coatings was assessed using indentations and pin-on-disc wear tests. Compared to ex situ samples, the Fe₃Al-based coatings strengthened with in situ ceramic particles exhibit higher microhardness and wear resistance regardless of the sliding velocity. The presence of voids, cracks and scratches/grooves in the wear track of the in situ coatings and the coating material transferred to the corresponding counterpart suggest that coatings with fine reinforcing particles fail predominantly via delamination and adhesive wear mechanisms. In the case of the ex situ coatings, the presence of a significant amount of hard ceramic particles within the wear track indicates that abrasive wear plays a dominant role in the degradation mechanism. Oxidation wear also contributed to material removal at high sliding velocity since transfer materials inside the wear track contain a high oxygen content compared to the unworn region regardless of the coating type.     

Effect of nano-sized calcium carbonate on cure kinetics and properties of polyester/epoxy blend powder coatings

Today's strict environmental laws pose significant challenges for coating's formulators to look for eco-friendly products. Powder coatings, particularly polyester/epoxy blends have demonstrated their ability as alternatives to traditional solvent-borne coatings. Recently, the use of nanoparticles such as nano-CaCO₃ (nCaCO₃) has been suggested as a beneficial strategy towards powder coating application with improved properties. Here, we study the effect of nCaCO₃ on morphology, cure behavior, adhesion and hardness of polyester/epoxy systems. The nanoparticles shape, size and dispersion state were investigated through X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) methods. Furthermore, isothermal cure characterization of the neat and filled systems was performed using a torque rheometer. The most important finding based on the rheological studies was the catalytic effect of nCaCO₃ on cure reaction of polyester/epoxy, leading to the shorter curing time. Moreover, the kinetic analyses of rheograms revealed a marked decrease in the activation energy of the cure process upon raising nCaCO₃ content. Interestingly, pull-off adhesion and hardness tests showed that the hardness and adhesion strength were dramatically increased by the addition of nCaCO₃ into the polyester/epoxy system compared to pure blend resin. Therefore, considering the strong competition in powder coating market, the use of nCaCO₃ as a commercial and inexpensive nanofiller is necessary not only to reduce the dwell time which has benefits in terms of the energy consumption and economics, but also to improve the performance of final polyester/epoxy coating.     

Friction and wear properties of Si3N4-hBN ceramic composites using different synthetic lubricants

This paper presents a tribological investigation of Si₃N₄-hBN composite ceramics using synthetic lubricants. The friction and wear properties of Si₃N₄-hBN ceramic composites sliding against TC4 titanium alloy (Ti6Al4V) were investigated via pin-on-disc tests. An axial compressive load of 10?N was applied with a sliding speed of 0.73?m/s. Three different lubrication conditions including simulated body fluid (SBF), physiological saline (PS) and bovine serum (BS) were used. For SBF lubrication, the friction coefficients and wear rates of Si₃N₄-hBN/Ti6Al4V pairs were varying with the increase of hBN contents. When using 20?vol% hBN, the average friction coefficient and wear rate of Si₃N₄ (0.28 and 3.5?× 10^?4 mm³ N^?1 m^?1) were as good as that of the pure Si₃N₄ (0.34 and 3.69?× 10^?4 mm³ N^?1 m^?1). Meanwhile, the processability of the Si₃N₄ material would be improved by adding hBN. It was worth to mention that when using 30?vol% hBN, the tribological performance of bearing combination deteriorated with extensive wear from the ceramic pin. This may due to the reduction of mechanical property caused by adding hBN and the occurring of tribochemical reaction. According to the worn surface examination and characterization, the main wear mechanism was abrasive and adhesion wear. Scratch grooves were observed on the metal disc, and metallic transform layers were seen on the ceramic pin. Moreover, surface lubrication film consisting of TiO₂, SiO₂·nH₂O, Mg(OH)₂, and H₃BO₃ were formed on the metal disc when using SBF lubrication and 20?vol% hBN content. Among the three lubrication conditions, SBF generally led to the best tribological performance. No surface lubrication film was found during BS and PS lubrications. This may be resulted from the absence of essential ions to promote the formation of surface lubrication film (PS lubrication) and the formation of a protein barrier on the surface of the metal disc (BS lubrication).     

Microstructural,mechanical and tribological properties of nanostructured YSZ coatings produced with different APS process parameters

Plasma sprayed ceramic coatings can be used in turbine engines as thermal barrier or abradable coatings, in order to improve the durability of the components as well as the efficiency. The presence of nanostructures, deriving from partial melting of agglomerated nanostructured particles, represents an interesting technological solution in order to improve their functional characteristics. In this work nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by air plasma spraying (APS). The influence of the main process parameters on their microstructural, mechanical and tribological properties was investigated by scanning electron microscopy (SEM), indentation techniques at micro- and nano-scale and wear tests, respectively. Their porous microstructure was composed of well melted overlapped splats and partially melted nanostructured areas. This bimodal microstructure led to a bimodal distribution of the mechanical properties. An increase of plasma power and spraying distance was able to produce denser coatings, with lower content of embedded nanostructures, which exhibited higher elastic modulus and hardness as well as lower wear rate.     

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.     

Remarkable improvement in tribological behavior of plasma sprayed carbon nanotube and graphene nanoplatelates hybrid reinforced alumina nanocomposite coating

Addition of 0.5?wt% of graphene nanoplatelates (GNPs) and 1?wt% carbonnanotube (CNTs) in plasma sprayed Al₂O₃ coating showed the reduction of 93.25% in wear volume loss and 90.94% in wear rate. This could be attributed to the simultaneous effect of enhanced densification, presence of the transferred layer from the counterpart, strong interface between Al₂O₃, GNP and CNTs and toughening offered by the GNPs and CNTs. The lowest COF value of 0.27 was recorded on addition of 0.5?wt% of GNP in Al₂O₃ coating, which could be attributed to the graphitic lubrication on the worn track during the wear.