Vol 38, No 6 (2017)

In this study, an engineering model was developed to predict the contact fatigue life through contact fatigue test using two-roller machine. DBR (Dark and Bright Ratio) method is used to quantify the failure rate of damaged surfaces of micro-pitting occurance. The variation of failure rates with number of cycles for different Hertzian pressures and rotiaional speeds is demonstrated. We proposed the new regression equation for predicting the contact fatigue life with the film parameter. It is shown that the fatigue life prediction curve by the proposed formula gives very high goodness of fit with the contact fatigue test results.

In this paper, a novel multi-objective optimization of a two-stage spur gearbox is carried out with a comprehensive range of constraints. The first objective function aims to reduce the weight/volume and second aims to minimize the power losses in the gearbox. Various design constraints and tribological constraints such as scuffing and wear are included. By using a specially formulated discrete version of NSGA-II optimization code, these objective functions are minimized for three different gear profiles (unmodified profile, smooth meshing, and high load) and for different SAE oil grades. Optimization is first carried out based on standard single objective minimization using regular constraints based on existing literature and then based on multi-objective optimization with comprehensive constraints which include tribological aspects. Finally, these two cases are compared for different gear profiles and oils. The results indicate that there is a high probability of wear failure, for solutions obtained from single objective minimization. The total power loss is reduced by half when using multi-objective compared to single objective optimization.

To reproduce severe and short-lived contacts like those occurring between the blade tip of an aircraft engine and the abradable material coated inside the engine casing, ballistic benches can be used. These devices allow to achieve a large range of velocities and thus, they allow the study of the abrasion process and the resulting wear of these kind of sealing materials. The Al-based abradable characterized within this work have been grown by atmospheric plasma spraying on a titanium substrate that was afterwards mounted on top of an aluminum projectile. Due to the short-lived forces occurring during these kind of contacts, the dynamic behavior of the device itself strongly affects the measured force signals. In order to overcome these dynamic distortions, we present a compensation approach able to perform a virtual extension of the available dynamometer bandwidth. This approach is based on an inverse-filtering of the response signal by using a 3 × 3 force-to-force transmissibility matrix function. The validated compensation approach was used to characterize the relation between the full three-dimensional measured contact-force and the specimen wear occurring during the abradable/Ti-6-Al-4V high-speed interaction.

A life-size composite brake disc was produced from Si, carbon–carbon composite, copper, and phenol resin. The disc had an outer radius Ø380, inner radius Ø180, and thickness of 36 mm. Chopped carbon fibers were used to reinforce frictional and structural layers. To obtain a preform of each layer, resin and carbon-fibers were mixed and hot-pressed. The preforms were pyrolyzed, and bonded by hot pressing. Finally Si and Cu infiltration in vacuum atmosphere was carried out to obtain a C/C–SiC–Cu_xSi_y composite brake disc. The density of the disc was 2.17 g/cm³. The bending strength was 61 MPa. The heat transfer coefficients in vertical and horizontal directions were 30.7, and 85.2 W/m-°C at 25°C, respectively. Friction coefficients of the C/C–SiC–Cu_xSi_y brake disc were more stable than those of C/C–SiC brake discs. X-ray diffraction analysis showed that Cu formed a compound, Cu₃Si.

To evaluate friction behaviour and adhesion strength of 130 nm-thick zirconia (ZrO₂) film produceded by atomic layer deposition (ALD) on Si substrate, scratch tests were performed at two different scales; micro- and macro-scales. Surface morphology, roughness, crack propagations and interations between the surface and sliding indenter were also investigated. Scratch test was also conducted with Si substrate as a reference. The test results showed that friction coefficient (COF) was influenced by the scale of scratch test regardless of the tested materials. In microscale test, the 130 nm-thick zirconia film failed at the critical load of 96 mN and direct relationship between the generation of micro-cracks and friction coefficient was observed. Based on the Hertzian contact theory and experimental results, the macroscratch width was much greater than that in microscratch test although the maximum contact pressure were comparable in both cases. Further discussion was made with regard to the influence of the contact pressure on COF, crack generation and film removal. Various types of failure mode were identified through analyzing the mechanical response of scratch tracks both at micro- and macroscale tests. This study suggested that 130 nm-thick ALD-ZrO₂ film showed better tribological and adhesion properties at microscale contact than macroscale contact.

The surface quality of the stainless steel affects the efficiency of flexible photovoltaics. Chemical mechanical polishing (CMP) is a finishing process that is used to prepare substrates for electronic devices. The CMP slurry composition is an important factor because additives in the slurry generally improve the polishing performance. However, it is limited to find the optimum conditions for the slurry by only experimental approaches. Thus, this study uses electrochemical analysis and friction monitoring to examine the effects of the abrasive, oxidizer, chelating agent, and pH. Electrochemical and monitoring analysis are useful for validating predictions and understanding interactions between the slurry and the stainless steel surface. Good correspondence was found between the predictions and the polishing results in more accurate. The corrosion rate (CR) obtained from the potentiodynamic polarization curve is proportional to the experimental results, as is the behavior of the curve and the coefficient of friction (COF). After only 3 min CMP, the best performing slurry (abrasive 39 wt %, oxalic acid 1 wt %, H₂O₂ 0.03 wt %, pH 1.5) improved the surface quality of 304 stainless steel by 4 nm. As a result, the proposed methods could help reduce the risks involved in stainless steel CMP slurry and these results could provide a reference for optimizing CMP slurry for flexible 304 stainless steel substrates.