预沉积ZnO薄膜对ZnO−MoS2/ZnO复合涂层结构与性能影响

目的 通过优化原子层沉积工艺获取不同厚度ZnO薄膜，研究ZnO薄膜晶体取向对ZnO?MoS2涂层生长结构的影响，获得具有优异摩擦学性能的ZnO?MoS2/ZnO复合涂层。方法 采用原子层沉积法在不锈钢基体上预沉积不同厚度的ZnO薄膜，再用射频磁控溅射技术继续沉积ZnO?MoS2涂层，制备ZnO?MoS2/ZnO固体润滑复合涂层。结果 X射线衍射分析发现，预沉积ZnO薄膜有诱导后续ZnO?MoS2涂层沉积生长的作用，预沉积100 nm厚ZnO薄膜的ZnO?MoS2/ZnO复合涂层显示出宽化的MoS2 (002)馒头峰，其截面形貌显示为致密的体型结构，获得的摩擦因数最低(0.08)，纳米硬度最高(2.33 GPa)，硬度/模量比显示该复合涂层的耐磨损性能得到提升;X射线光电子能谱分析结果表明，复合涂层表面游离S与空气中水发生反应程度大约为原子数分数5%，显示复合涂层耐湿性能较好;基于原子层沉积ZnO薄膜生长及其对后续ZnO?MoS2涂层生长的影响分析，提出了ZnO?MoS2/ZnO复合涂层磨损模型，阐明了ZnO薄膜对复合涂层结构及摩擦学性能的影响，并以该模型解释了200 nm厚 ZnO薄膜上沉积ZnO?MoS2涂层出现的摩擦因数由高到低的变化趋势及最终磨损失效现象。结论 合适的原子层沉积制备的ZnO薄膜有利于MoS2 (002)取向生长，可有效提升ZnO?MoS2/ZnO复合涂层的摩擦学性能;控制ZnO薄膜厚度，可实现ZnO薄膜与基底及ZnO?MoS2层间界面之间的优化结合，以制得具有较好摩擦学性能及使用寿命的ZnO?MoS2/ZnO复合涂层。

Influence of Pre-deposition ZnO Thin Film on Structure and Properties of ZnO-MoS2/ZnO Composite Coating

Crystalline films produced by atomic layer deposition, high binding forces between the film and the substrate induced by chemical adsorption or chemical bonding on the substrate surface will affect the tribological properties of composite coatings. For optimized tribological properties of ZnO-MoS2/ZnO composite coatings, ZnO thin layers of various thicknesses were first manufactured by atomic layer deposition. ZnO-MoS2 coatings were then deposited via radio frequency magnetron sputtering to obtain ZnO-MoS2/ZnO composite solid lubrication coatings. The effects of pre-deposition ZnO thin films on the structure of the subsequent ZnO-MoS2 coatings and the tribological properties of the composite coatings were analyzed. The X-ray diffraction analysis indicates that the pre-deposited ZnO film may induce the deposition and growth of the subsequent ZnO-MoS2 coating. The composite coating with a ZnO film thickness of 100 nm has a wide MoS2 (002) peak. In the meantime, the cross-sectional morphology of the composite coating shows a more compact structure. The obtained composite coating has the lowest coefficient of friction (0.08) and the highest nano-hardness (2.33 GPa). H/E shows that the wear resistance of the composite coating is also improved. The results of X-ray photoelectron spectroscopy show that approximately 5 at.% free S on the surface of the composite coating under atmospheric conditions, indicating improved moisture resistance. Atomic layer deposition and radio frequency magnetron sputtering methods reveal that pre-deposited ZnO thin film has a strong effect on the structures of subsequent ZnO-MoS2 coatings, providing insights into the effect of ZnO in MoS2-based solid lubrication coatings. According to the analysis of the growth of ZnO thin film and its influence on the growth of subsequent ZnO-MoS2 coating, a worn model for ZnO-MoS2/ZnO composite coating was put forward. The schematic diagram assumed that factors of ZnO thin film-substrate bonding force, brittleness of ZnO thin film, and chemical adsorption between ZnO-MoS2 coating and ZnO thin film layers resulted in lubrication properties of the composite coatings. A brittle fracture in the ZnO thin film was likely to occur as the thickness increased. Therefore, an overly thick ZnO film must be avoided. The chemical binding force of the ZnO thin film-substrate was generally greater than the adherence of the subsequent ZnO-MoS2 coating to the ZnO thin film layers, therefore reducing the mismatch between the ZnO-MoS2 coating and the ZnO thin film would be benefit to the lubrication of composite coatings. The influence of ZnO thin film on the structure and tribological properties of the composite coating has been clarified. In short, both the nanocrystalline coating and adhesive force between the ZnO thin film and the substrate reduced the coefficient of friction and improved the wear-resistance of the ZM100 coating. The trend of friction coefficient from high to low and the ultimate wear failure of ZnO-MoS2 coatings deposited on ZnO thin films with a thickness of 200 nm were also explained. Overall, the tribological properties of ZnO-MoS2/ZnO composite coatings can be improved effectively by the optimized atomic layer deposition process. The optimal interface combination between ZnO thin film, substrate and ZnO-MoS2 can be achieved by controlling the thickness of ZnO thin film, so as to fabricate ZnO-MoS2/ZnO composite coatings with better tribological properties and long service life.