Friction and Wear Properties of Amorphous Carbon Nitride Coatings in Water Lubrication

The friction and wear properties of amorphous carbon nitride(a-CN x)coatings in water lubrication were reviewed.The influences of mating materials and tribological variables such as normal load(W)and sliding speed(V)on the friction and wear properties of the a-CN x coatings were analyzed.It was indicated that the specific wear rate of the a-CN x coatings was related to the hydration reaction of mating materials with water.If the mating materials were easily hydrated,the specific wear rate of a-CN x coatings was low.The water-lubricated properties of the a-CN x coatings were better in comparison to the a-C coatings.The a-CN x/Si-based non-oxide ceramics tribo-pairs exhibited the lowest friction coefficient and wear rate.To describe their friction and wear properties at the normal loads of 3—15Nand the sliding speeds of 0.05—0.5m/s,the wear-mechanism maps for the a-CN x/SiC(Si3N4)tribo-pairs in water were developed.

High-resolution characterization of hexagonal boron nitride coatings exposed to aqueous and air oxidative environments

Hexagonal boron nitride （h-BN） is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature, which facilitates blocking the flow of electrons, thereby preventing the occurrence of galvanic reactions. Nevertheless, this may not be the case when an h-BN-protected material is exposed to aqueous environments. In this work, we analyzed the stability of mono and multilayer h-BN stacks exposed to H202 and atmospheric conditions. Our experiments revealed that monolayer h-BN is as inefficient as graphene as a protective coating when exposed to H202. Multilayer h-BN offered a good degree of protection. Monolayer h-BN was found to be ineffective in an air atmosphere as well. Even a 10-15 layers-thick h-BN stack could not completely protect the surface of the metal under consideration. By combining Auger electron spectroscopy and secondary ion mass spectrometry techniques, we observed that oxygen could diffuse through the grain boundaries of the h-BN stack to reach the metallic substrate. Fortunately, because of the diffusive nature of the process, the oxidized area did not increase with time once a saturated state was reached. This makes multflayer （not monolayer） h-BN a suitable long-term oxidation barrier. Oxygen infiltration could not be observed by X-ray photoelectron spectroscopy. This technique cannot assess the chemical composition of the deeper layers of a material. Hence, the previous reports, which relied on XPS to analyze the passivating properties of h-BN and graphene, may have ignored some important subsurface phenomena. The results obtained in this study provide new insights into the passivating properties of mono and multilayer h-BN in aqueous media and the degradation kinetics of h-BN-coated metals exposed to an air environment.

Hydrogen Embrittlement of Nitrogenating Layer on Martensitic Alloys

Nitriding of the surface in martensitic stainless steels is commonly carried out to improve their wear resistance. The process of plasma nitriding in stainless steel is influenced by two mechanisms: physical diffusion through the surface and chemical gas-metal reaction. The inner nitriding interaction involves the simultaneous penetration and formation of a solid solution, as well as the interaction of nitrogen with specific alloying elements, resulting in the development of homogeneous and heterogeneous structures. Our study concludes that the observed intergranular hydrogen embrittlement and crack formation during the surface nitridation process of AMS 5719 martensite alloy steel can be attributed to the ammonium concentration of approximately 50% at a temperature of 530˚C.

Online preparation of high-quality BN coatings with atomic diffusion based on carbon-free water-soluble precursor

Efficient and environmentally friendly production of high-quality continuous fiber coatings using current preparation methods is highly challenging due to issues such as scale and batch processing restrictions,low deposition rate,high energy consumption,and utilization of multiple environmentally hazardous steps.To address these challenges,we propose a stable and efficient wet chemical deposition coating method for high-throughput online continuous preparation of boron nitride(BN)coatings on ceramic fibers under an ambient environment.Our process involves surface modification,in-situ wet chemical deposition,and heat treatment,and all seamlessly connecting with the ceramic fiber preparation process through continuous stretching.Hydrophilic groups were introduced via surface modification enhancing wettability of the fiber surface with impregnating solution.An in-situ reaction and atom migration improve uniformity and binding of the coating.As a result,outstanding impregnation and adhesion properties are achieved.A comprehensive analysis to evaluate the impact of the BN coatings was conducted,which demonstrates that the BN-coated fibers exhibit a remarkable 36%increase in tensile strength,a 133%increase in fracture toughness,and enhanced temperature resistance of up to 1600℃.It provides a secure and efficient platform for cost-effective production of functional and high-quality coatings through targeted surface modification and rapid stretching impregnation.

The first order phase-transition of polycrystal solid surfaces with nanothickness

The fundamental equations of thermodynamics of a film have been used for describing a fundamental property of solid crystalline materials i.e. the first-order phase transition on the grain boundaries by the formation of two-dimensional liquid. The generalized equation that is obtained is used for calculating the premelting temperature of any metal, which has a value in the range of 0.55-0.86 of the melting point. The experimental diffusion coefficient of nitrogen in steel at premelting temperature is the same as in the liquid phase. The described phenomenon of phase transition on the grain boundaries decreases in case of radical modification of the existing process engineering of handling metals. It also provides a precise physical explanation to the super plasticity of fine-structure metal alloys.

Improved corrosion resistance of dental Ti50Zr alloy with(TiZr)N coating in fluoridated acidic artificial saliva

The electrochemical corrosion behavior of the dental Ti50 Zr alloy with and without nanocrystalline(TiZr)N coating was comparatively investigated in artificial saliva solutions with different pH values and fluoride ion concentrations. The chemical stability of the passive films on the coated and non-coated Ti50 Zr alloy was evaluated by calculating passive film thickness. The chemical compositions and valence structures of the passive films were analyzed by X-ray photoelectron spectroscopy(XPS). The results show that the(TiZr)N-coated alloy displays distinctly decreased corrosion rate and increased impedance compared with Ti50 Zr alloy in nonfluoridated and fluoridated acidic solutions. Particularly, in the solution of pH = 3.9 and 0.15% NaF-containing, the corrosion protection efficiency of(TiZr)N coating reaches90%. The excellent corrosion resistance of the coated alloy is attributed to that the nanocrystallines in(TiZr)N coating decreases micropores and crack defects, which strongly impedes the corrosive ions diffusion and electrode process at Ti substrate/coating interface. Meanwhile,(TiZr)N coating shows good passivation behavior in acidic solution and active–passive transition behavior in fluoridated acidic solution. The coated Ti50 Zr alloy with high chemical stability has potential application prospect for dental implants.

Plasma electrolytic oxidation of the magnesium alloy MA8 in electrolytes containing TiN nanoparticles

The formation of protective multifunctional coatings on magnesium alloy MA8 using plasma electrolyt- ic oxidation （PEO） in an electrolytic system containing nanosized particles of titanium nitride was investigated. Electrochemical and mechanical properties of the obtained layers were examined. It was established that microhardness of the coating with the nanoparticle concentration of 3 gl-1 increased twofold （4.2 ± 0.5 GPa）, while wear resistance decreased （4.97 × 10-6 mm3 N-1 m-1）, as compared to re- spective values for the PEO-coating formed in the electrolyte without nanoparticles （2.1 ± 0.3 GPa, 1.12 × 10.5 mm3 N-1 m-1）.