This study experimentally evaluates the lifetime of PTFE coating on Inconel-X750 specimens, a material commonly used in gas foil bearings (GFBs), based on surface roughness using a reciprocating test apparatus from a tribological perspective. The primary objective is to verify the enhancement of coating lifetime for GFBs, specifically focusing on the top foil coatings employing solid lubricants. The surface roughness of Inconel specimens was manipulated in five cases within the range of 0.1–1.0 µm using an electrochemical etching method. A coating of consistent thickness was applied using a Polytetrafluoroethylene (PTFE) solvent-based coating material. Lifetime evaluation was conducted by ASTM G133-05, with normal direction load as a variable to simulate accelerated stress conditions. To simulate the line contact environment of the rotor of the turbo machinery, test specimens were fabricated using a Roll-on-Flat method. A test roller was implemented to maintain the perpendicular angle between the roller and the coated specimens, allowing for automated alignment. The experimental verification demonstrated that the lifetime of the coating reached its maximum in the specific surface roughness range of the test specimen, with a Ra of 0.3 to 0.4 μm (Case: 3). In the SEM (Scanning Electron Microscope) image analysis of the cross-section of Inconel specimens coated with PTFE, it was verified that the region where PTFE particles adhere varies according to the surface roughness of the sample. This results in a tangible increase in the bonding force (mechanical interlocking) between the coating and the Inconel sample. The test results could be utilized to predict the lifetime of Inconel X-750 specimens with PTFE coating through accelerated life test (ALT) evaluation using the Weibull-linear function model. The ALT test results in this study were determined by correlating the actual rotor and bearing interactions under Hertzian-contact pressure conditions. Friction coefficient characteristics and sliding distances observed in a reciprocating test rig were converted into estimations representative of the actual bearing-rotor environment. In conclusion, it was predicted that the cyclic lifetime of GFB at the optimal surface roughness (Case 3) of the Inconel specimens would be approximately 1.4 × 105, confirming an extension of the bearing lifetime by over 18 times compared to Case 1 without surface treatment.

中文翻译：

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考虑气翼轴承最佳表面粗糙度的加速寿命试验

本研究使用往复式测试设备从摩擦学角度，根据表面粗糙度，实验评估了 Inconel-X750 试样（一种常用于气翼轴承 （GFB） 的材料）上 PTFE 涂层的使用寿命。主要目的是验证 GFB 涂层寿命的提高，特别关注使用固体润滑剂的表面箔涂层。使用电化学蚀刻方法在 0.1–1.0 μm 范围内对 Inconel 试样的表面粗糙度进行了 5 种情况的纵。使用聚四氟乙烯 （PTFE） 溶剂型涂层材料涂覆厚度一致的涂层。根据 ASTM G133-05 进行寿命评估，以法向载荷作为变量来模拟加速应力条件。为了模拟涡轮机械转子的线接触环境，使用 Roll-on-Flat 方法制造了试样。使用测试辊来保持辊和涂层试样之间的垂直角度，从而实现自动对准。实验验证表明，涂层的使用寿命在试样的比表面粗糙度范围内达到最大值，Ra 为 0.3 至 0.4 μm（情况：3）。在对涂有 PTFE 的铬镍铁合金试样横截面的 SEM（扫描电子显微镜）图像分析中，验证了 PTFE 颗粒粘附的区域根据样品的表面粗糙度而变化。这导致涂层和 Inconel 样品之间的结合力（机械联锁）明显增加。 通过使用 Weibull 线性函数模型的加速寿命测试 （ALT） 评估，测试结果可用于预测带有 PTFE 涂层的 Inconel X-750 试样的寿命。本研究中的 ALT 测试结果是通过关联赫兹接触压力条件下的实际转子和轴承相互作用来确定的。在往复式试验台中观察到的摩擦系数特性和滑动距离被转换为代表实际轴承转子环境的估计值。总之，预测铬镍铁合金试样在最佳表面粗糙度（情况 3）下 GFB 的循环寿命约为 1.4 × 105，与未进行表面处理的情况 1 相比，轴承寿命延长了 18 倍以上。