Intellectual Merit: The main objective of this BRIGE project is to investigate the microwear mechanism of surface protective carbon films under extremely high speed contact by accounting for thermomechanical and chemical stability of materials. An integrated research will be carried out through (1) analytical modeling and computational simulations of rough surface sliding contact, and (2) systematic experiments of high speed contact to measure the wear, phase transition, and oxidation of film and substrate materials. The unique features of the proposed research are to: (a) incorporate the in-situ changes of thermomechanical and chemical film properties into the analysis of wear behaviors, and (b) quantitatively measure the physical and chemical degradation of materials after high speed contact experiments. In analytical modeling, inhomogeneous contact stress and frictional heat flux will be obtained from an improved rough surface contact model. Individual flash temperatures of surface asperities (i.e., multi-points of heat source) will be applied into a theory of heat transfer to calculate the final temperature of carbon film and substrate. The phase transition changes the strength of carbon film, while the oxidation affects the adhesion energy of carbon atoms on substrate. In computational simulations, these transient material properties will be incorporated into finite element analysis code to investigate the in-situ wear performance of carbon film. To verify and improve the proposed analytical/computational researches, systematic experiments will be performed.
Broader Impacts: It is expected that the proposed research will clearly answer the question of why carbon films experience critical wear under extremely high-speed contact even with mild contact load. The outcomes of this research will thus deliver scientific solutions for carbon films to (a) improve the reliability and accuracy of a system, and thus (b) reduce energy loss from unexpected friction and wear. The exciting research outcomes will be brought into the education of college students and K-12 outreach program. The exciting research outcomes will be brought into the education of college students and the K-12 outreach program. An undergraduate course in the area of tribology and interface engineering will be created including classroom lectures and hands-on laboratory experience, and undergraduate students will readily participate in actual research activities. A simplified mock-up experiment will be developed for K-12 students, which will increase their understanding on thermomechanical contact behavior in solid materials. In particular, to broaden the participation of underrepresented groups in engineering and science, a systematically designed mock-up will be provided in summer camps and lab experience for female and Hispanic K-12 students. Currently two female students are involved in the PI?s research. The PI will keep efforts on recruitment of the underrepresented students through close interaction with the Hispanic Student Society in Texas Tech University.
Carbon films are widely used as a surface protective coating for many engineering and scientific devices. When thin carbon film is performing high speed surface contact, the resulting temperature rise by friction could be high enough to change the atomic structure of carbon atoms increasing the sp2/sp3 bond ratio. In other words, under dynamic surface contact condition, thin carbon films could develop significant graphitization process (or thermal softening) by frictional heat generation, and accordingly its mechanical strength could become much lower than the original state. Therefore, if the thermally degraded carbon film is brought into continued dynamic surface contact, it can produce critical surface damage or wear failure even at much lower contact stress value. 1. Scientific outcomes in relation to intellectual merit: In this project, an improved thermomechanical contact model has been established to estimate the in-situ contact stress and temperature rise on thin carbon films under high speed surface contact. As one of the unique features in this modeling, the frictional heat flux could be used as the medium that connects the continuum contact mechanics and the theory of heat transfer. When the heat flux was applied into the theory of heat transfer, another key parameter (i.e., heat partition factor) was taken into account. Under dynamic surface contact, the temperature rise by friction is highly dependent on the heat partition factor, because two different heat sources (i.e., moving and stationary heat sources) make different thermal activities on each contacting bodies. In this modeling, the effective heat partition factor could be calculated from the thermal & mechanical properties and dynamic contact parameters of the contacting bodies, which was used to calculate the surface flash temperature rise on the two contacting bodies. From the parametric simulation results, it was observed that even though the contact stress is not high enough to make immediate fracture of thin carbon films, its surface temperature could be significantly increased beyond the critical value (~ 300 C) to initiate the graphitization process on thin carbon films. To verify the change of atomic structure of thin carbon films by high speed surface contact, amorphous carbon film of a few nanometers thickness was prepared using the ion beam deposition process, and then accelerated dynamic contact experiments have been performed causing a scratch and a burnishing types of damage on the carbon films’ surfaces. For the damaged carbon film samples, XPS and Micro-Raman measurements were carried out to examine the thermomechanical degradation process of carbon atoms. First, from the XPS measurement, it could be observed that the orbital energy value of sp2 bond on the scratch or burnished area significantly increased. This indicates that the scratch or burnishing area would have experienced significant graphitization process during dynamic surface contact. Second, micro-Raman spectra was obtained at the scratch or burnishing area, whose results were compared to those at non-contact area. To characterize the change of sp2 and sp3 bonds in carbon films, three Raman parameters were used in this study, including the G-peak position, G-peak width, and intensity ratio of G & D peaks (=I(D)/I(G)). It was found that the scratch or burnishing area showed higher G-peak position, lower G-peak width, and higher I(D)/I(G) values, all of which correspond to the increase of sp2 bonds (or graphitization) of carbon films. In summary, from this research program, we clearly found that when thin carbon films are performing high speed surface contact, its physical/chemical properties can be degraded by frictional heat generation thereby leading to critical surface failure during the continued surface contact. The scientific results from those modeling and experimental researches could answer for the historically long standing question ‘why do carbon films experience catastrophic wear failure under high speed contact even with mild contact load?’ Therefore, the outcomes of this research can deliver fundamental solutions for carbon film to (a) improve the long-term reliability and thus (b) reduce energy loss of engineering devices from unexpected friction and wear. 2. Broader impact: Two students from underrepresented group (one woman doctoral student and one Nepalese undergraduate student) were directly involved in this program, where they have performed an interdisciplinary research on thermomechanical contact behavior of carbon film. In particular, with the financial support of this NSF award, the doctoral woman student successfully finished her degree program and joined a technology leading company after her graduation. Moreover, the scientific findings and instrumental techniques obtained from this project were added to the PI’s teaching including ‘Mechanics of Solids’ (undergraduate course) and ‘Contact Mechanics of Engineering Materials’ (graduate course). Considering more than 50% of the class enrollment in Texas Tech University is Hispanic, the research outcomes from this project would have been well exposed to those underrepresented group students during the PI’s lecturing, which accordingly could improve students’ interests in emerging technologies in engineering and science.
