Case-hardening 14Cr14Co13Mo4 martensitic stainless steel needs to be carburized to improve surface performance. Low-pressure carburization has the benefit of having oxidation-free production and being ecofriendly. However, compared with the low-pressure carburization of the low-alloy steel, low-pressure carburization of the 14Cr14Co13Mo4 steel consumes more time and has a risk of network carbides. In order to promote carbon diffusion and avoid network carbide, Fe–Ni films with various thickness were electrodeposited on the 14Cr14Co13Mo4 steel prior to low-pressure carburization. The experimental results show that, under the same carburizing conditions, the surface carbon content decreases and the carburized layer increases with the increase of Fe–Ni film thickness. After the hardening heat treatment, the effective case depth (ECD) of the sample coated with 6.0 μm Fe–Ni film was increased by 29% compared to that of the uncoated sample. The morphology of carbides was a strip-shaped, discontinuous network distribution in the uncoated sample, while in the Fe–Ni coated samples, the carbides changed to a globular, uniformly dispersed distribution. The effect of Fe–Ni film on the low-pressure carburizing of steel is explained by the simulation of the carbon diffusion using DICTRA software. The Fe–Ni films reduce the steel surface carbon content in each boost stage of low-pressure carburizing and release carbon atoms in every diffusion stage. Through this adjustment mechanism, the steel surface carbon content can be reduced and carburized layer growth can be promoted.

Purpose: This study aims to evaluate the rheological properties of aviation lubricating oil under conditions of heavy load, high speed and high temperature and the applicability of the classical rheological model under severe conditions. Design/methodology/approach: A Chinese aviation lubricating oil was used and its traction curves were obtained using a new two-disk tribotester. Its rheological parameters were calculated based on empirical formulae. Moreover, the traction force was calculated based on the classical Eyring rheological model. Findings: The traction curves are obtained with respect to contact pressure, temperature and rolling speed. The rheological parameters are significantly influenced by environmental factors, especially viscosity. The traction force calculated using the Eyring model is consistent with the experimental results. Originality/value: A novel two-disk tribotester was designed using a gas bearing and speed–force closed-loop control to ensure measurement accuracy. The mechanism of rheological properties was analyzed and the applicability of the classical rheological model under severe conditions was verified. It provided an experimental and theoretical basis for expanding the application of classical rheological models under extreme conditions.

In this paper, a two-dimensional semi-analytical method is developed for the mechanical behavior analysis of multilayered solids subjected to surface contact loading, which is indispensable for realizing an optimized tribological performance from the mechanical behavior point of view. Firstly, the explicit analytical frequency response functions of the multilayered solid are derived in a recursive form by analytically solving a system of linear equations established according to the boundary conditions and the interface continuous conditions. Then, the two-dimensional elastic field solution in the subsurface of multilayered solids in the space domain is converted from its corresponding frequency response functions by employing a numerical conversion method based on the inverse fast Fourier transformation. The present method is validated by comparing with the solution given by other methods. Lastly, the stress analysis of multilayered coatings with various structure layouts and various layer number of the multilayers were performed with the present method.

In order to investigate the effect of the structural layout of multilayered coatings on its mechanical behavior, a three-dimensional elastic field solution is developed for multilayered solids subjected to surface point contact loading, which is converted from the elastic field solution in frequency domain by using a numerical conversion algorithm. The elastic field solution in frequency domain is obtained by numerically solving a group of linear equations involving the unknown constants in the general elastic field solution of layered material that is obtained by using Fourier integral transform technique. The present solution is validated by comparing with the exact analytical solution for uncoated solids and finite element solution for solids coated with 30 layers. Lastly, the effect of structural layout of multilayered coatings is further investigated with present solution. The result shows that the gradient structural layout with elasticity modulus decreasing gradually from the top layer to the substrate, which is preferable to a larger friction coefficient for multilayered solids subjected to surface line contact loading, is preferable for a smaller friction coefficient 0.2.