## 1 Introduction  
Fused silica (FS) has excellent properties, such as high transmittance and high temperature resistance, as well as a low thermal expansion coefficient. As a result, FS has been widely used in high-power laser systems, the semiconductor industry, and other high-tech fields [1; 2; 3]. During precise machining, handling, transporting, and cleaning, and in the service process, the FS surface is inevitably subjected to physical contact, which causes surface damage. The surface damage is known to affect the performance and usage; for instance, the surface damage of FS caused by chemical mechanical polishing (CMP) can suppress the laser-induced damage threshold (LIDT, which is the maximum laser fluence that a material can withstand without exhibiting any permanent damage), limiting the service lifetime of FS optics used in high-fluence laser systems [4]. The subsurface damage caused by polishing and grinding, or revealed by subsequent cleaning methods, can also act as a precursor that can lead to surface damage under laser irradiation, limiting the material's optical performance and working lifetime [5]. Thus, to improve the performance and lifetime of FS optics, it is of great importance to understand the damage mechanism of the FS surface under various contact conditions and thus find possible ways to enhance the surface properties of FS. In humid air, water molecules impinging from the gas phase are inevitably adsorbed on the FS surface, which can potentially alter the physical and chemical properties of the FS surface. It is well-known that without physical contact, FS can react with dissociated OH groups in water to form hydroxylated groups [6]. With physical contact such as indentation and scratching, the surface damage of FS is known to be sensitive to the presence of water [7]. It was reported that the hardness of FS did not change with loading time when the indentation tests were conducted in water-free conditions, such as in toluene, acetonitrile, formamide, and hydrazine, but it decreased with loading time when the indentation tests were conducted in conditions involving water, such as in humid air and liquid water [8]. Further analyses suggested that the adsorbed water on the FS surface can diffuse into the FS subsurface during the indentation process, causing more damage on the FS surface via water-induced hydrolysis reactions of the Si-O-Si network under normal stress conditions [8]. The indentation tests not only showed that the reciprocating scratch (wear) resistance of the FS surface decreased along the surface's normal direction with environment humidity, but that it also decreased along the tangential direction. This can be explained because the adsorbed water molecules impinging from the gas phase can facilitate the hydrolysis of the Si-O-Si network of FS under shear stress conditions [9]. Apart from the ambient environments conditions such as relative humidity (RH) and temperature, the water reactivity on the material surface also varies with surface hydrophilicity, which essentially alters the contact damage behavior. For instance, the nanowear of hydrophilic single crystalline silicon (100) was more serious than that of hydrophobic and pristine silicon (100) [10]. Moreover, with the increase in RH, the nanowear of hydrophilic silicon increased to a maximum value at \(\sim\)50% RH and then decreased to no discernible wear in water [11], while the nanowear of hydrophobic silicon increased with RH and the maximum wear was found in water [12]. All these behaviors can be explained by the variation in the adsorbed water layer thickness on the silicon surface, which is dependent on the surface hydrophilicity and RH and influences the formation of interfacial Si-O-Si bonds at sliding interfaces [10; 11; 12]. When the surface groups of silica are mainly Si-O-Si bonds, the surface is hydrophobic, but when the coverage of Si-OH groups is sufficiently high, the surface is hydrophilic [13].