Abstract

The addition of 0.2–5% nanoscale (40–80 nm) α-phase or microscale (40 μm) γ-phase Al₂O₃ particles in PTFE effectively reduce the matrix wear rate by 99.99%, whereas microscale (> 0.5 μm) α-Al₂O₃ or nanoscale (40–80 nm) γ-Al₂O₃ only reduce PTFE wear by ~ 90% under identical loading, dispersion and testing conditions. This paradoxical material system best illustrates the complexity of tribology and the importance of filler–matrix interactions at small scales. We studied the independent effect of the Al₂O₃/PTFE interface area and alumina structure by systematically varying the particle size over two orders of magnitude for both α- and γ-Al₂O₃/PTFE composites. Detailed characterizations of filler size, surface area and tribofilm’s chemical composition were conducted. The results found: (1) DLS median particle sizes conformed reasonably to vendor reported values and percentages of microscale filler aggregates correlated weakly with wear rates, (2) electron microscopy of the as-worn composite surface suggested a strong relation between the characteristic size of ‘unreinforced’ polymer domain and composite wear rate, (3) third bodies (i.e., transfer films, debris) played an important role in counterface abrasion, (4) wear rate correlated strongly with filler’s specific surface area and ultralow wear was only maintained ~ 0.3–10 m²/g nominal specific filler–matrix area values, (5) ultralow wear coincided with perfluorinated carboxylic salt rich tribofilms which supported a previously proposed wear reduction mechanism that mechanochemically degraded PTFE chelate with alumina and cause crosslinked and wear-resistant tribofilms, (6) tribofilm Al-F bond signal increased with filler surface area and high wear coincided with excessive tribofilm Al-F signal for γ-Al₂O₃/PTFE systems. Based on these results and literature hypothesis, we proposed that (1) the 1 μm α-Al₂O₃ provided the least filler–matrix interface and largest unreinforced polymer domain in PTFE, which lead to the least crosslinked and compartmentalized tribofilms; (2) in γ-alumina filled composites, Al-F bond forms as a product of mechanochemically degraded PTFE but also blocks chelation between the degraded PTFE and alumina fillers, (3) the 20 nm γ-Al₂O₃ provided the most filler–matrix interface which leads to excessive aluminum fluoride that blocked the filler–matrix chelation, prevented the tribofilm crosslinking and lead to high wear rates. This hypothesis was additionally supported by small molecule experiments in this study. However, this study provides no direct insight into how sensitive the filler–matrix tribochemical interaction is to filler phase or aggregate strength (strong, weak or fully dense).

Graphic Abstract

中文翻译：

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复合填料磨损的悖论填料尺寸效应：填料-基体相互作用及其摩擦化学后果

摘要

在PTFE中添加0.2–5％纳米级（40–80 nm）的α相或微米级（40μm）γ相Al ₂ O ₃颗粒可有效降低99.99％的基体磨损率，而微米级（> 0.5μm）的α-Al ₂ ö ₃级或纳米级（40-80纳米）的γ-Al ₂ ö ₃仅由〜减少PTFE的磨损90％下相同负载，分散和测试条件。这种自相矛盾的材料系统最好地说明了摩擦学的复杂性以及小范围填充物-基体相互作用的重要性。我们研究了Al ₂ O ₃的独立作用/ PTFE界面区域和通过在两个数量级系统地改变颗粒尺寸为两个α-和γ-Al系氧化铝结构₂ ö ₃/ PTFE复合材料。进行了填料尺寸，表面积和摩擦膜化学成分的详细表征。结果发现：（1）DLS中值粒径与卖方报告的值和微细填料聚集体的百分比合理地相符，与磨损率之间的相关性很弱；（2）磨损后复合表面的电子显微镜表明，特征尺寸之间存在密切关系'未增强'的聚合物区域和复合材料的磨损率，（3）第三体（即转移膜，碎屑）在对面磨损中起重要作用，（4）磨损率与填料的比表面积密切相关，并且仅保持超低磨损〜 0.3–10 m ²/ g标称特定填充物-基质面积值，（5）超低磨损与富含全氟化羧酸盐的摩擦膜同时发生，该摩擦膜支持先前提出的磨损减少机制，该机制机械降解氧化铝与PTFE的螯合物，并导致交联且耐磨的摩擦膜，（6）的Al-F键的信号与填料的表面积增加和高磨损过度摩擦膜的Al-F信号为一致的γ-Al ₂ ö ₃ / PTFE系统。基于这些结果和文献假设，我们提出：（1）在1微米的α-Al ₂ ö ₃在PTFE中提供了最少的填料-基体界面和最大的非增强聚合物结构域，从而使交联和分隔的摩擦膜最少。（2）在γ氧化铝填充的复合材料，铝-F键形式的机械化学的产物降解的PTFE也退化PTFE和氧化铝填料之间的块的螯合，（3）的20纳米的γ-Al ₂ ö ₃提供最多的填料-基体界面，从而导致过量的氟化铝阻塞了填料-基体的螯合，阻止了摩擦膜的交联并导致高磨损率。该研究中的小分子实验进一步支持了这一假设。但是，这项研究没有提供关于填料-基质摩擦化学相互作用对填料相或聚集体强度（强，弱或完全致密）的敏感性的直接见解。

图形摘要