Collaborative Research: Understanding Run-In and Superlubricity of Diamond-Like Carbon Coatings from a Tribochemical Perspective

智能手机屏幕是一个令人印象深刻的例子，说明新技术往往依赖于表面工程，比如创造耐用、坚硬和光滑的表面的能力。许多关键的经济部门可以从减少表面磨损和摩擦中受益，包括汽车、医疗设备、计算机部件和国防部门，因此这项研究直接和积极地影响了美国的经济福利和国家安全。最近，一种被称为类钻石碳(DLC)的硬质表面涂层被发现可以达到一种被称为“超级润滑性”的极致滑润性。然而，这种理想的特性对环境条件非常敏感，例如周围大气中的水或氢气量。对环境条件的敏感性限制了涂层在许多潜在应用中的有效性。这项合作研究旨在了解控制DLC超润滑性的化学反应，并发现将其高性能扩展到更广泛条件的方法。宾夕法尼亚州立大学和圣奥拉夫学院的合作者专门使用不同的最先进的方法来测量摩擦，这些方法相互补充。这项研究的更广泛的影响延伸到通过支持学生参与研究来培养美国的下一代科学家。教育活动将包括为宾夕法尼亚州立大学招募人数不足的少数族裔和圣奥拉夫大学的本科生进行外联，在圣奥拉夫大学教授与摩擦有关的物理本科生，为宾夕法尼亚州立大学的研究生课程教授类金刚石表面性质，在专业会议上提供表面表征教程，以及学生参与国际合作。由于它的非晶态性质，类金刚石中的碳原子在键长和键角上有非常广泛的分布。DLC中的共价键是在高能沉积过程中形成的，如果不在极高的温度下进行热处理，就不能松弛或重排。这意味着DLC中许多键的长度和角度明显偏离理想的sp2和sp3杂化的最小能量结构。与参数接近理想结构的键相比，这些键更弱，活性更强。键参数和反应活性的这种广泛分布是每个DLC涂层及其沉积条件特有的固有参数。这项研究的主要论点是高度扭曲的碳-碳键的存在促进了机械力化学诱导的多晶型转变为剪切界面上的石墨化结构域。在这个假设下，磨合过程可以归因于剪切诱导的扭曲的碳网络向石墨化(或类石墨烯)结构域的机械力化学转变。转变过程也会受到气相分子撞击反应的影响。这些与周围气体分子的表面反应是影响类金刚石磨合和超润滑性的外在参数。机械辅助热激活(Arrhenius类型)模型将与结构表征相结合，以研究内部和外部参数如何促进或阻碍DLC界面剪切诱导的机械力化学转变为具有超低剪切阻力的石墨微区。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Smartphone screens are an impressive example of how new technologies often rely on surface engineering, such as the ability to create durable, hard and slippery surfaces. Many key economic sectors can benefit from minimized wear and friction of surfaces, including the automotive, medical device, computer component and defense sectors, so that this research directly and positively impacts the economic welfare and national security of the United States. Recently, a hard surface coating known as diamond-like carbon (DLC) has been found to achieve an extreme level of slipperiness called "superlubricity". This desirable property is, however, very sensitive to environmental conditions such as the amount of water or hydrogen in the surrounding atmosphere. Sensitivity to environmental conditions limits the effectiveness of the coating for many potential applications. This collaborative research aims to understand the chemical reactions that govern superlubricity in DLC and to discover ways to extend its high performance to a wider range of conditions. The collaborators at Penn State and St. Olaf College specialize in measuring friction using different state-of-the-art methods that complement each other. The broader impacts of this study extend to training the next generation of scientists in the United States by supporting student participation in the research. Educational activities will include outreach for underrepresented minority recruitment at Penn State and undergraduates at St. Olaf, instruction of undergraduates in friction-related physics at St. Olaf, instruction in DLC surface properties for a graduate course at Penn State, surface characterization tutorials in professional conferences, and student participation in international collaborations. Due to its amorphous nature, the carbon atoms in DLC have very broad distributions in bond length and angle. The covalent bonds in DLC are formed during the high-energy deposition process and cannot be relaxed or rearranged without annealing at extremely high temperatures. This means that the lengths and angles of many bonds in DLC deviate significantly from the minimum-energy structure of ideal sp2 and sp3 hybridization. Those bonds are weaker and more reactive compared to the bonds with parameters close to the ideal structures. Such broad distributions in bond parameters and reactivities are intrinsic parameters specific to each DLC coating and its deposition conditions. The main thesis of this study is that the presence of highly-distorted carbon-carbon bonds facilitates a mechanochemically-induced polymorphic transition to graphitic domains at the shearing interface. Under this hypothesis, the run-in process can be attributed to the shear-induced mechanochemical transformation of the distorted carbon networks to graphitic (or graphene-like) domains. The transformation process will also be affected by reactions with molecules impinging from the gas phase. These surface reactions with surrounding gas molecules are extrinsic parameters affecting the run-in and superlubricity of DLC. A mechanically-assisted thermal activation (Arrhenius-type) model will be combined with structural characterization to study how the intrinsic and extrinsic parameters facilitate or hamper the shear-induced mechanochemical transformation of the DLC interface to graphitic domains with ultra-low shear resistance.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.