聚醚醚酮 (PEEK) 轉(zhuǎn)移材料在 PEEK 與鋼接觸時的特性
DOI：10.1016/j.triboint.2019.02.028
文章鏈接：
https://www.sciencedirect.com/science/article/abs/pii/S0301679X1930091X
摘要：
聚醚醚酮(PEEK)是一種高性能聚合物，可在無潤滑條件下替代某些運動部件的金屬。在摩擦過程中，PEEK被轉(zhuǎn)移到配合面。通過對PEEK磨損過程、接觸溫度和摩擦發(fā)生的原位觀察，以及FTIR和拉曼光譜異位分析，研究了PEEK轉(zhuǎn)移膜在鋼和藍(lán)寶石上的形成和性能。我們的結(jié)果表明，單獨的摩擦加熱可能不足以產(chǎn)生在轉(zhuǎn)移材料中觀察到的PEEK降解。在摩擦過程中觀察到的摩擦，連同機械剪切，可能會促進自由基的產(chǎn)生和PEEK的降解，進而影響PEEK轉(zhuǎn)移膜的性能和聚合物-金屬摩擦對的性能。
關(guān)鍵詞：聚醚醚酮；轉(zhuǎn)移膜形成；原位摩擦等離子體；原位接觸溫度
Abstract:
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
圖1摩擦測試實驗裝置示意圖
圖2 原始PEEK圓盤、PEEK磨損軌跡和鋼上各種PEEK轉(zhuǎn)移材料的FTIR光譜。
圖2 50Hz下PEEK藍(lán)寶石接觸成像的快照，顯示了原位PEEK的磨損過程
圖2 干凈鋼球、原始PEEK圓盤、PEEK磨損軌跡以及鋼磨損疤痕上的轉(zhuǎn)移膜和薄膜的拉曼光譜（785nm激發(fā)）
圖3使用紅外成像進行原位溫度測量的快照
結(jié)論：當(dāng) PEEK 與藍(lán)寶石和鋼摩擦?xí)r，它會在我們的測試條件下轉(zhuǎn)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(jiān)測來檢查PEEK 轉(zhuǎn)移層的形成。當(dāng)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(fā)生材料轉(zhuǎn)移。
PEEK轉(zhuǎn)移材料在磨損疤痕上的化學(xué)性質(zhì)不同于原始PEEK的化學(xué)性質(zhì)。在較厚的轉(zhuǎn)移膜和反面之間形成的薄膜主要是無定形碳質(zhì)材料。其他PEEK轉(zhuǎn)移材料的FTIR結(jié)果表明PEEK 鏈的斷裂發(fā)生在醚和酮基團的不同位置。此外，觀察到芳香環(huán)的打開、取代、交聯(lián)以及結(jié)晶度的損失和環(huán)的共面性。碳酸鹽和羧酸可以通過酸堿反應(yīng)形成并與鋼或藍(lán)寶石表面反應(yīng)，形成薄而堅固的轉(zhuǎn)移膜。
原位IR熱成像顯示標(biāo)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(jié)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(chǎn)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(jīng)歷的剪切導(dǎo)致它們的摩擦帶電。結(jié)果在摩擦過程中產(chǎn)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(chǎn)生自由基。這會促進轉(zhuǎn)移膜的形成并導(dǎo)致 PEEK 的交聯(lián)和降解。我們的結(jié)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(zhuǎn)移材料的形成和性能。牢記產(chǎn)生紫外線等離子體的可能性，未來聚合物和聚合物復(fù)合材料的設(shè)計應(yīng)考慮表面帶電的可能性及其對轉(zhuǎn)移膜形成和降解的潛在影響。
Conclusions:
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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