Narrow Results

To evaluate the effect of coatings on the fatigue behaviors of turbine rotor steel, TiN and TiAlN films were deposited on the 1Cr-1Mo-0.25V steels by arc-ion plating (AIP) method with and wihtout screen ion filter. The coating thickness were varied with 2.5 μm, 3.5 μm, and 5.0 μm. A Cu-Kα beam source was used as a characteristic X-ray and the crystal plane of (422) was selected to evaluate the residual stresses. In order to clear the relationship between fatigue behavior and residual stress of specimen coated with TiN and TiAlN films, the fatigue tests of specimens with and without coating were carried out at room temperatures respectively. It is shown that the fatigue life of the coated specimen was longer than that of uncoated specimen. The compressive residual stresses on the coatings were higher, and the fatigue crack initiated at an inclusion in the substrate near bond interface. It is known that compressive residual stress caused by hard coating would retard the fatigue crack initiation on the specimen surface, and then led to fatigue strength and fatigue life increasing.

To improve the adhesive strength between the film and substrate, ion bombardment is frequently performed before the deposition of thin film coatings. In this study scratch tests were carried out on aluminum alloy protected with CrN film coated by arc ion plating method. In order to investigate the influence of ion bombardment conditions on the adhesive strength between the aluminum alloy substrate and the CrN coating, the ion bombardment process was performed before CrN coating under several different bias voltages. The properties near the interface were analyzed using SIMS. As a result, the ion bombardment process had an optimum condition and excessive bias voltage reduced the critical load. A Cr rich layer forms near the substrate surface by implantation of Cr ions due to the high incident energy ions. The Cr rich layer is shallow for the high critical load sample, while the low critical load sample has a deep Cr rich layer. It appears that the adhesion strength between the substrate and the film will depend on the depth or intensity of this Cr rich layer.

To investigate the effect of TiN coating on the fatigue strength of high-strength steel, four-point bending fatigue tests were carried out for martensitic stainless steel with TiN film coated using arc ion plating (AIP) method. A 2-μm-thick TiN film was deposited onto the substrate surface under bias voltage of four kinds: V_B = 0, -60, -160 and -260 V. For V_B = 0, -160 V and -260 V, the fatigue limit increased. The highest fatigue limit of σ_max = 900 MPa was obtained for V_B = -160 V. But some samples for V_B = -260 V showed the decrease of fatigue limit due to film delamination during the fatigue test. For V_B = -60 V, the fatigue limit was unchanged by coating. As a result of a coating property analysis, the following conclusions were obtained. Fatigue crack propagation was almost independent of the bias voltage. Fatigue crack initiated from the subsurface in the substrate and the crack initiation behavior depended on the film property of the adhesion, residual stress, elastic modulus, and the film's hardness depended on the bias voltage especially for low fatigue stress level.

The surface properties like roughness etc. strongly influence the fatigue strength of high-tensile steel. To investigate the effect of surface condition and TiN coating on the fatigue strength of high-strength steel, four-point bending fatigue tests were carried out for martensitic stainless steel with TiN film coated using arc ion plating (AIP) method. This study, using samples that had been polished under several size of grind particle, examines the influence of pre-coating treatment on fatigue properties. A 2-µm-thick TiN film was deposited onto the substrate under three kinds of polishing condition. The difference of the hardness originated in the residual stress or thin deformation layer where the difference of the size of grinding particle of the surface polishing. And it leads the transformation of the interface of the substrate and the TiN film and improves fatigue limit.

Hard thin coating directly deposited on soft steel substrate often suffers low adhesion strength and load-bearing capacity. In this work, CrN$x$-type adhesion layers (ALs) were introduced between hard AlCrN coating and soft cool-work tool steel substrate to enhance the adhesion strength and wear performance. The microstructure of CrN$x$ ALs prepared with different nitrogen pressure and its influence on adhesion strength and tribological properties were investigated. The results show that with the nitrogen pressure increase from 0.5 Pa to 3.0 Pa, the phase transformation sequences occurring in ALs are: Cr + Cr₂N $\to$ Cr₂N + CrN $\to$ CrN. The adhesion strength increases with the increase of nitrogen pressure due to the formation of CrN which provides better load support resistance than the fragile Cr₂N and soft Cr. In addition, the adhesion strength of the samples with ALs is always higher than that of the one without ALs. The improvement of adhesion strength for samples with CrN$x$-type ALs is beneficial for improving wear resistance, especially at high wearing loads.