## 1 Introduction  
As a type of emerging composite, SiC CMCs have been widely employed in the aerospace and nuclear industries owing to their unique material parameters including a better strength to weight ratio, high toughness, superior resistance to heat effects, and low density [1; 2; 3]. For example, CMCs are composed of braided SiC fibers whose hardness is 9.5 on the Mohs scale. Furthermore, the CMCs could maintain their high strength property even under a high-temperature load of 1500 K, which is 150 K higher than Nickel-based superalloys [4]. It is worth noting that the density of CMCs material is only 30% of superalloys, which is beneficial for its application in the aerospace field [5]. Due to their excellent material performance, researchers have shown an increased interest in application studies of CMCs. However, a series of challenges have been raised by the outstanding material properties for CMCs machining.  
Due to the anisotropic woven SiC fibers, ultrahigh hardness, and strength, it is difficult to obtain a promising result for conventional machining techniques. Various manufacturingdeficiencies exist, such as pulling out of SiC fibers, delamination, and burrs in the processing area [6]. Differing from conventional machining, special processing techniques were introduced into CMCs machining with non-contact including laser processing, water jet machining, electric discharge processing, and LWJ [7]. The heat-affected zone (HAZ) and recast layer in the machining area can hardly be avoided during laser and electric discharge processing [8; 9]. The pullout of SiC fibers is always accompanied by water jet machining on account of super high water pressure (100-400 Mpa) [10]. Among these techniques, the LWJ may achieve a satisfactory processing result for CMCs with high precision due to its unique properties. This technology was first introduced in the material processing field in 1996 by Dr. [PERSON], with the successful coupling of the water jet and focused laser spot [11]. LWJ could be considered as a kind of \"cold machining\" technique owing to the forced cooling and scouring effect of the water jet, so that characteristics such as low taper, high depth-to-width/ diameter ratio, less/no HAZ, and recast layer could be easily acquired [12; 13]. In particular, there is a great advantage for LWJ compared to a conventional laser in that focusing the laser spot on the material in processing is no longer necessary. Moreover, all the compact sections of the water jet play a role as machine tools that could reach a maximum length of 100 mm [14].  
According to the literature results up to now, there has been no detailed investigation on removal mechanism as well as high-quality processing of CMCs under LWJ machining in any systematic way. In this study, the three-dimensional FEM describing a representative microstructure of CMCs was established to explore the ablation mechanism during LWJ machining. The temperature distribution and its evolution rule on a temporal and spatial scale were analyzed through a numerical simulation. Then, the effect of cooling and scouring brought about by a water jet was validated by a series of relevant experiments. The influences of different laser pulse energies on the ablation depth were also revealed with the interpretation of FEM. Single-row scribing experiments were put forward to discover the limit value of groove depth. On this basis, a multi-row scribing experiment was established to realize the thorough cutting of the CMCs substrate with the absence of a recast layer, pulling out of SiC fibers, and delamination. In addition, the mechanism of CMCs machining with LWJ was summarized according to theoretical and experimental research.