聚醚醚酮 (PEEK) 转移材料在 PEEK 与钢接触时的特性
Polyetheretherketone (PEEK) is a high performance polymer that can be an alternative to metal for some moving components in unlubricated conditions. During rubbing, PEEK is transferred to the counterface. The formation and properties of PEEK transfer films on steel and sapphire are studied by in-situ observations of PEEK wear process, contact temperatures and triboemission, as well as FTIR and Raman spectroscopies ex-situ. Our results suggest that frictional heating alone may not be sufficient to generate PEEK degradation observed in the transfer materials. Triboplasma observed during rubbing, together with mechanical shear, may promote generations of radicals and degradation of PEEK, which subsequently influence the properties of PEEK transfer film and performance of polymer-metal tribopair.
Keywords：Polyetheretherketone；Transfer film formation；In situ triboplasma；In situ contact temperature
当 PEEK 与蓝宝石和钢摩擦时，它会在我们的测试条件下转移到接触面上。我们通过磨损过程、接触温度和摩擦等离子生成的原位监测来检查PEEK 转移层的形成。当摩擦开始时，PEEK表面被钢球刮擦的凹凸不平，其中一些材料以接触碎片的形式被夹带和剪切，同时发生材料转移。
原位IR热成像显示标称接触温度低于 PEEK的Tg，即使局部温度因夹带碎片而升高。拉曼研究的结果支持接触温度 (100-120°C) 低于 PEEK 的 Tg。因此，单独的接触温度可能不足以产生观察到的 PEEK 降解。钢磨痕上薄膜上脆性裂纹的存在也表明变形温度可能相对较低并且薄膜可能已暴露于紫外线照射。
摩擦表面所经历的剪切导致它们的摩擦带电。结果在摩擦过程中产生摩擦原。这种摩擦原具有足够的能量，与机械剪切一起，可以引起断链并产生自由基。这会促进转移膜的形成并导致 PEEK 的交联和降解。我们的结果表明，机械剪切、摩擦加热和摩擦等离子都有助于摩擦表面上 PEEK 转移材料的形成和性能。牢记产生紫外线等离子体的可能性，未来聚合物和聚合物复合材料的设计应考虑表面带电的可能性及其对转移膜形成和降解的潜在影响。
When PEEK is rubbed against sapphire and steel, it is transferred to the counterfaces under our test conditions. The formation of PEEK transfer layers was examined by in-situ monitoring of the wear process, contact temperature, and triboplasma generation. As rubbing starts, the PEEK surface is initially ploughed by the asperities of the steel ball. Some of these materials are entrained and sheared in the contact. Debris form, as well as materials transfer occurs.
The chemistry of PEEK transferred materials on wear scars differ from that of pristine PEEK. The thin film, which are formed between the thicker transfer films and the counterface, is mainly amorphous carbon aceous materials. FTIR results of other PEEK transferred materials suggest scission of PEEK chains occurs at various positions in the ether and ketone groups. In addition, opening of the aromatic rings, substitution, crosslinking, along with loss of crystallinity, and co-planarity of the rings are observed. Carbonate and carboxylic acid may form and react with steel or sapphire surface through an acid-base reaction, forming the thin and robust transfer films.
In-situ IR thermography shows that the nominal contact temperature is below PEEK Tg even though local temperature is raised by the entrainment of debris. Results from Raman studies support that the contact temperature (100-120°C) is below the Tg of PEEK. Hence contact temperature alone may not be sufficient to generate the PEEK degradations observed. The presence of brittle cracks on the thin film on the steel wear scar also suggests that the deformation temperature may be relatively low and the film may have exposed to UV irradiation.
The shear experienced by the rubbing surfaces leads to their triboelectrification. As a result, triboplasma is generated during rubbing. This triboplasma has sufficient energy, which together with the mechanical shear, can cause chain scission and generate radicals. This promotes transfer film formation and leads to crosslinking and degradation of PEEK. Our results show that mechanical shear, as well as frictional heating and triboplasma all contribute to the formation and properties of the PEEK transferred materials on the rubbing counterface. Keeping the possibility of UV plasma generation in mind, the design of future polymer and polymer composites should take the possibility of surface charging and the potential effect it may have on transfer film formation and degradation into considerations.
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