聚醚醚酮 (PEEK) 轉移材料在 PEEK 與鋼接觸時的特性
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
當 PEEK 與藍寶石和鋼摩擦時，它會在我們的測試條件下轉移到接觸面上。我們通過磨損過程、接觸溫度和摩擦等離子生成的原位監測來檢查PEEK 轉移層的形成。當摩擦開始時，PEEK表面被鋼球刮擦的凹凸不平，其中一些材料以接觸碎片的形式被夾帶和剪切，同時發生材料轉移。
原位IR熱成像顯示標稱接觸溫度低于 PEEK的Tg，即使局部溫度因夾帶碎片而升高。拉曼研究的結果支持接觸溫度 (100-120°C) 低于 PEEK 的 Tg。因此，單獨的接觸溫度可能不足以產生觀察到的 PEEK 降解。鋼磨痕上薄膜上脆性裂紋的存在也表明變形溫度可能相對較低并且薄膜可能已暴露于紫外線照射。
摩擦表面所經歷的剪切導致它們的摩擦帶電。結果在摩擦過程中產生摩擦原。這種摩擦原具有足夠的能量，與機械剪切一起，可以引起斷鏈并產生自由基。這會促進轉移膜的形成并導致 PEEK 的交聯和降解。我們的結果表明，機械剪切、摩擦加熱和摩擦等離子都有助于摩擦表面上 PEEK 轉移材料的形成和性能。牢記產生紫外線等離子體的可能性，未來聚合物和聚合物復合材料的設計應考慮表面帶電的可能性及其對轉移膜形成和降解的潛在影響。
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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