Simulations of the Dependence of Gas Physical Parameters on Deposition Variables during HFCVD Diamond Films

During the growth of the hot filament chemical vapor deposition （HFCVD） diamond films, numerical simulations in a 2-D mathematical model were employed to investigate the influence of various deposition parameters on the gas physical parameters, including the temperature, velocity and volume density of gas. It was found that, even in the case of optimized deposition parameters, the space distributions of gas parameters were heterogeneous due primarily to the thermal blockage come from the hot filaments and cryogenic pump effect arisen from the cold reactor wall. The distribution of volume density agreed well with the thermal round-flow phenomenon, one of the key obstacles to obtaining high growth rate in HFCVD process. In virtue of isothermal boundary with high temperature or adiabatic boundary condition of reactor wall, however, the thermal roundflow was profoundly reduced and as a consequence, the uniformity of gas physical parameters was considerably improved, as identified by the experimental films growth.

MAX phase forming mechanism of M-Al-C(M=Ti,V,Cr)coatings:In-situ X-ray diffraction and first-principle calculations

The interesting hybrid properties of ceramics and metals induced by unique nano-laminated structures make the M_(n+1)AX n(MAX)phase attractive as a potential protective coating for vital structural compo-nents in harsh systems.However,an extremely narrow phase-forming region makes it difficult to prepare MAX phase coatings with high purity,which is required to obtain coatings with high-temperature anti-oxidation capabilities.This work describes the dependence of the phase evolution in deposited M-Al-C(M=Ti,V,Cr)coatings as a function on temperature using in-situ X-ray diffraction analysis.Compared to V_(2)AlC and Cr_(2)AlC MAX phase coatings,the Ti_(2)AlC coating displayed a higher phase-forming tempera-ture accompanied by a lack of any intermediate phases before the appearance of the Ti_(2)AlC MAX phase.The results of the first-principle calculations correlated with the experience in which Ti_(2)AlC exhibited the largest formation energy and density of states.The effect of the phase compositions of these three MAX phase coatings on mechanical properties were also investigated using ex-situ Vickers and nano-indenter tests,demonstrating the improved mechanical properties with good stability at high temperatures.These findings provide a deeper understanding of the phase-forming mechanism of MAX phase coatings to guide the preparation of high-purity MAX phase coatings and the optimization of MAX phase coatings with expected intermediate phases such as Cr_(2)C,V_(2)C etc.,as well as their application as protective coat-ings in temperature-related harsh environments.

用PECVD工艺制备功能装饰氧化硅薄膜的性能

用PECVD技术制备氧化硅薄膜,研究了生成样品的位置对薄膜成分、结构和性能的影响,探讨了制备兼具高透光性和耐刮擦性的功能装饰氧化硅薄膜的方法。结果表明,在阳极位置生成的薄膜具有Si(CH3)nO有机氧化硅结构,在380~780 nm波长范围内透光率高达90%~98%,但是薄膜的结构疏松,硬度仅为2 GPa。提高制备温度可使薄膜硬度提高至6 GPa,但是透光率略有降低;在阴极位置生成的薄膜具有无机氧化硅复合非晶碳结构,薄膜结构致密,硬度可达15 GPa,但是在380~780 nm波长范围内透光性差;增加O2反应气体可促使碳与氧反应生成二氧化碳,非晶碳结构消失,薄膜透光率提高到99%,但是硬度降低到9 GPa。

Influence of Substrate Negative Bias on Structure and Properties of TiN Coatings Prepared by Hybrid HIPIMS Method

TiN coatings were deposited using a hybrid home-made high power impulse magnetron sputtering(HIPIMS)technique at room temperature.The effects of substrate negative bias voltage on the deposition rate,composition,crystal structure,surface morphology,microstructure and mechanical properties were investigated.The results revealed that with the increase in bias voltage from-50 to-400 V,TiN coatings exhibited a trend of densification and the crystal structure gradually evolved from(111) orientation to(200)orientation.The growth rate decreased from about 12.2 nm to 7.8 nm per minute with the coating densification.When the bias voltage was-300 V,the minimum surface roughness value of 10.1 nm was obtained,and the hardness and Young’s modulus of TiN coatings reached the maximum value of 17.4 GPa and 263.8 GPa,respectively.Meanwhile,the highest adhesion of 59 N was obtained between coating and substrate.

Preparation of Ti_2AlC MAX Phase Coating by DC Magnetron Sputtering Deposition and Vacuum Heat Treatment

Due to the excellent corrosion resistance and high irradiation damage resistance,Ti 2AlC MAX phase is considered as a candidate for applications as corrosion resistant and irradiation resistant protective coating.MAX phase coatings can be fabricated through firstly depositing a coating containing the three elements M,A,and X close to stoichiometry of the MAX phases using physical vapor deposition,followed by heat treatment in vacuum.In this work,Ti-Al-C coating was prepared on austenitic stainless steels by reactive DC magnetron sputtering with a compound Ti （50）Al （50） target,and CH4 used as the reactive gas.It was found that the as-deposited coating is mainly composed of Ti 3AlC antiperovskite phase with supersaturated solid solution of Al.Additionally,the ratio of Ti/Al remained the same as that of the target composition.Nevertheless,a thicker thermally grown Ti 2AlC MAX phase coating was obtained after being annealed at 800℃ in vacuum for 1 h.Meanwhile,the ratio of Ti/Al became close to stoichiometry of Ti 2AlC MAX phases.It can be understood that owing to the higher activity of Al,it diffused quickly into the substrate during annealing,and then more stable Ti 2AlC MAX phases transformed from the Ti 3AlC antiperovskite phase.

Structural design of Cr/GLC films for high tribological performance in artificial seawater:Cr/GLC ratio and multilayer structure

In this paper, graphite-like carbon （GLC） films with Cr buffer layer were fabricated by DC magnetron sputtering technique with the thickness ratio of Cr to GLC films varying from 1：2 to 1：20. The effect of Cr]GLC modulation ratio on microstructure, mechanical and tribological properties in artificial seawater was mainly investigated by scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, nano-indenter and a reciprocating sliding tribo-meter. The propagation of defects plays an important role in the evolution of delamination, which is critical to wear failure of GLC films in artificial seawa- ter. Designing the proper multilayer structure could inhibit the defects propagation and thus protect the basis material. The multilayer Cr/GLC film with optimized ratio of 1：3 demonstrates a low average friction coefficient of 0.08 ± 0.006 and wear rate of （2.3 ± 0.3） × 10^-8 mm3/（N m） in artificial seawater, respectively.

Erosion behavior and failure mechanism of Ti/TiAlN multilayer coatings eroded by silica sand and glass beads

Severe erosion by hard particles is a crucial problem to engine blades when aircraft take off and land in harsh environments, especially for the developed lightweight titanium alloy components. Here, we deposited the Ti/TiAlN multilayer coatings with various cycles on Ti–6 Al–4 V substrates by a home-made hybrid multisource cathodic arc system. The effects of the silica sand and glass beads on erosion behavior of the coatings were focused. Results showed that the Ti/TiAlN multilayer coatings eroded by the silica sand exhibited the predominant "layer by layer" failure mechanism. In particular, increasing the number of cycles led to the dramatic increase in erosion rate for Ti/TiAlN multilayer coatings, due to the deterioration of their mechanical properties. Different from the silica sand case, however, the erosion rate of the coatings treated by glass beads indicated faint dependence upon the number of cycles, where the coating failure was dominated by the "piece by piece" failure mechanism. Noted that the Ti layers along with the formed interfaces enhanced the erosion resistance of the coatings, although the failure mechanisms were differently eroded by silica sand and glass beads. Meanwhile, the Ti layers and interfaces hindered the propagation of radial cracks and restrained the lateral cracks within one single TiAlN layer.

Microstructure and Tribological Behavior of Self-lubricating(Si:N)-DLC/MAO Coatings on AZ80 Magnesium Substrate

The combined micro arc oxidation （MAO） and a hybrid beam deposition process was used to deposit duplex （Si：N）-DLC/MAO coatings on AZ80 magnesium alloy. The microstructure and composition of the duplex coatings were analyzed by Raman spectroscopy, X-ray photoelectron spectroscope （XPS）, scanning electron microscope （SEM） and atomic force microscopy （AFM）. Tribological behaviors of the coatings were studied by ball-on-disk friction test. It was found that the ID/IG ratio of the （Si：N）-DLC （diamond-like carbon） top films increases with decreasing C2H2/N2 ratio. The （Si：N）-DLC top film with SigN4 was formed on the MAO coated sample as the C2H2/N2 ratio was 10sccm：5sccm, which showed an increasing critical load compared with the pure DLC directly deposited on the Mg alloy substrate. As a result, the （Si：N）-DLC/MAO coating exhibited an advanced wear protection for the substrate.

Effect of Negatively Charged Ions on the Formation of Microarc Oxidation Coating on 2024 Aluminium Alloy

The present study deals with the effect of negatively charged ions on the ceramic coating formation on 2024 aluminium alloy during microarc oxidation （MAO） process. On the basis of the experimental results, two steps （the formation of an incipient film without arc presence and the growth of a ceramic coating with arc discharge） of MAO process have been observed. For comparison, four different negatively charged ions studied. It is proved that negatively charged ions strongly participated in the formation of an incipient film with high impedance value at the first step. The growth of ceramic coating depends on the combination between AI of the substrate and O from the electrolyte, and the negatively charged ions are little consumed. As an anodic oxide coating is prepared on the sample surface instead of the incipient film, the first step occurs easily and the growth of ceramic coating is accelerated. Furthermore, the mechanism of negatively charged ions in the formation of the MAO coating has been proposed.

Phase orientation improved the corrosion resistance and conductivity of Cr_(2)AlC coatings for metal bipolar plates

In view of the M_(n+1)AX_(n)(MAX)phase coatings benefting the adaptive passivation flm for good corrosion resistance and high electronic density of states for excellent electrical conductivity,here,we reported the Cr_(2)Al C MAX phase coatings with different preferred orientations by a homemade technique consisting of vacuum arc and magnetron sputtering.The dependence of surface and interface microstructural evolution upon the corrosion and electrochemical properties of deposited coating was focused.Results showed that all the Cr_(2)Al C coatings with different phase orientations greatly improved the performance of stainless steel(SS)316 L substrate.Specifcally,the lowest value of interface contact resistance(ICR)reached to 3.16 mΩcm^(2)and the lowest corrosion current density was 2×10^(-2)μA cm^(-2),which were much better than those of bare SS316L.The combined studies of electrochemical properties and theoretical calculations demonstrated that the Cr_(2)Al C coatings with preferred(103)orientation were easier to form oxide passivation flm on their surface to increase the corrosion resistance.

Insights on high temperature friction mechanism of multilayer ta-C films

In this work,the high temperature friction mechanism of the tetrahedral amorphous carbon(ta-C)film was elucidated.The multilayer ta-C film with alternating hard and soft sub-layers exhibited a low friction coefficient of 0.14 at 400℃ before a sudden failure occurred at 4600 cycles.The wear failure was attributed to the gradual consumption of the ta-C film at the contact region.The design of a hard or soft top layer effectively regulated the high temperature friction properties of the multilayer ta-C.The addition of a hard top layer contributed to a low friction coefficient(0.11)and a minor wear rate(4.0×10^(-7)mm^(3)/(N m)),while a soft top layer deteriorated the lubrication effect.It was proposed that the passivation of dangling bonds at the sliding interface dominated the low-friction mechanism of the ta-C film at high temperature,while the friction induced graphitization and the formation of sp^(2)-rich carbonaceous transfer layer triggered C-C inter-film bonding,resulting in serious adhesion force and lubrication failure.Moreover,the multilayer ta-C film with hard top layer obtained excellent friction performance within 500℃,while the high temperature induced oxidation and volatilization of carbon atoms led to the wear failure at 600℃.

Mechanism of contact pressure-induced friction at the amorphous carbon/alpha olefin interface

Combining an amorphous carbon(a-C)film with a lubricating oil can significantly improve the friction performance and lifetime of moving mechanical components.However,the friction mechanism is not well understood owing to a lack of information regarding the structure of the interface when exposed to high contact pressure.Here,we select linear alpha olefin,C5H10,as a lubricant and study the evolution of the structure of the a-C/C5H10/a-C sliding interface under contact pressure via reactive molecular dynamics simulation.Our results suggest that introducing C5H10 into the a-C/a-C interface reduces the friction coefficient by up to 93% compared with no lubricant,although the lubricating efficiency strongly depends on the contact pressure.In particular,increasing the contact pressure not only induces the binding of the lubricant with a-C,but also facilitates the dissociation of the C5H10 carboncarbon skeleton by specific scissions,which governs the friction behavior.These results disclose the underlying lubrication mechanism and could enable the development of new and effective lubricating systems with long lifetimes.

Accelerated deterioration mechanism of 316L stainless steel in NaCl solution under the intermittent tribocorrosion process

In this research,the tribocorrosion behavior of 316L stainless steel in simulated seawater was investigated under continuous and intermittent sliding at open circuit potential.The tribocorrosion mechanism was discussed in terms of wear morphologies,mechanical property as well as chemical composition.Meanwhile,microstructure evolution inside the wear track and open circuit potential recorded after sliding were analyzed to quantify the repassivation kinetics and evaluate the impact of the regenerated passive film on wear.The results showed that the wear rate increased under intermittent sliding when the pause time is long enough to repassivate after sliding.Repeated sliding promoted the refinement of the grain inside the sliding area,which was beneficial to the generation of the thicker and more compact passive film inside the wear track.The ruptured passive film often acted as abrasives during subsequent sliding.Therefore,the accelerated material loss under intermittent sliding was attributed to the periodic mechanical removal of the thickened passive film and the enhanced abrasive wear inside the wear track.

Long-term tribocorrosion resistance and failure tolerance of multilayer carbon-based coatings

Current tribocorrosion research of metallic materials and their surface protective coatings mainly focuses on their short-term properties,with test time of 0.5‒2.0 h and a sliding distance 50‒500 m,which may significantly deviate from the practical long-term service condition and thus cause a catastrophe of marine equipments.In this study,three carbon-based multilayer coatings(Ti/DLC,TiC_(x)/DLC,and Ti‒TiC_(x)/DLC)were deposited on S32750 substrates,and both short-term and long-term tribocorrosion behaviors were investigated.The experimental results indicate that the coatings substantially improve the tribocorrosion resistance of the S32750 stainless steel.During the short-term tribocorrosion test,TiC_(x)/DLC exhibited the best tribocorrosion resistance owing to its high hardness.During the long-term tribocorrosion test,however,Ti‒TiC_(x)/DLC coating indicated the best anti-tribocorrosion performance owing to its excellent fracture toughness together with high hardness.Moreover,under 5 N,Ti‒TiC_(x)/DLC can withstand a long-term test of more than 24 h.Additionally,under a higher load of 20 N,the Ti‒TiC_(x)/DLC with a corresponding sliding distance of approximately 1,728 m maintained a low friction coefficient of approximately 0.06.However,the coating was completely worn out;this is attributable to the formation of tribocorrosion products consisting of graphitized carbon and nanocrystalline Fe_(x)O_(y).