Machinability improvement in milling of SiCf/SiC composites based on laser controllable ablation pretreatment

The poor machinability of SiC_f/SiC composites greatly limits its application and promotion. The laser-induced ablation products of SiC_f/SiC composites are powdery, loose and porous. Milling of laser ablated samples demonstrated that the force and heat were almost negligible when milling ablation products. Accordingly, a laser ablation pretreatment milling (LAPM) process of SiC_f/SiC composites was proposed. Under the LAPM process, after the laser ablation treatment with controllable depth, the cutting allowance could be achieved in only one pass, which greatly improved the machining efficiency compared with the conventional milling process. The material removal rate was greatly improved on the premise of ensuring the machining quality. Taking the milling of tensile specimens as an example, compared with conventional milling, the total processing time of the specimen was reduced by 31.29 % by LAPM process. Therefore, LAPM provides a potential feasible process scheme for greatly improving the machinability and machining efficiency of SiC_f/SiC composites.     

Experimental investigation on fatigue property at room temperature of C/SiC composites machined by rotary ultrasonic milling

Rotary ultrasonic milling technology (RUM), as a surface strengthening machining method, was proposed to utilize in processing of C/SiC composites for enhancing anti-fatigue performance innovatively. Static tensile, intermittent fatigue and residual strength test were carried out. Due to constant impingement of high-frequency and low-amplitude vibration, surface residual compressive stress was formed near 2 GPa maximally. Axial thermal residual stress in fiber achieved -662.4 MPa proved by loading-unloading test. The peak value of fatigue damage parameter was reduced significantly. RUM surface restrains most of interface cracks because of residual compressive stress, and hinder the growth of fiber cracks for better machined surface quality. The damage accumulation, the first stress redistribution and fiber reinforcement stage were delayed. Average damage rate was decreased by 80.5 %. Residual tensile strength of RUM C/SiC was improved after fatigue, up to 95.8 % of tensile strength. The strengthening effect from RUM on fatigue property of C/SiC is significant and valuable.     

Investigations on continuous-wave laser and pulsed laser induced controllable ablation of SiCf/SiC composites

To lay a foundation for the feasibility exploration of laser-induced ablation-assisted machining for SiC_f/SiC composites, combined with numerical simulation and experiments, the laser-induced ablation mechanism of SiC_f/SiC composites was studied, and the relationship between laser parameters and ablation depth was analyzed. The laser-induced ablation products of SiC_f/SiC composites mainly consisted of recrystallized 3C-SiC and amorphous SiO₂, which were powdery and porous. According to the stratification characteristic, the ablation products were divided into attached smoke dust layer, sublimate recrystallization layer, heat-affected layer, and unaffected layer from the surface to the inside of the material. By adjusting the laser parameters (significant factors were the scanning speed and the scanning spacing), the depth of laser-induced ablation was adjustable and controllable. The ablation depth was greater in continuous-wave (CW) mode due to the continuous energy input. Therefore, CW laser is more suitable for generating larger and various ablation depths to match various cutting allowances.     

Experimental study on laser assisted ultrasonic elliptical vibration turning (LA-UEVT) of 70% SiCp/Al composites

SiC_p/Al composites are more and more used in aerospace, military industry and other industries. However, the surface integrity of materials is poor, and the cutting force is large as the anisotropy of materials in the traditional machining (TM) process, which hinders the application of ceramic particle reinforced metal matrix composites. With the requirement of high dimensional accuracy, high efficiency and low damage for materials in these fields, non-traditional machining technology has become a research hotspot. Laser assisted machining (LAM) is a non-contact special machining method. Its advantages in machining SiC_p/Al composites have been proved by experiments, but there are still processing defects such as thermal cracks. Therefore, to further improve the machining quality of 70% SiC_p/Al composites with high volume fraction, a new machining method combining ultrasonic elliptical vibration turning (UEVT) and laser heating assisted turning (LAT) is proposed. High frequency intermittent machining and the adjustment of laser temperature influence on materials can be realized by adjusting the ultrasonic amplitude. Combining the characteristics of the two processing techniques, the feasibility study of the new machining method was studied by turning experiments. In this paper, compared with TM and LAT, the removal mechanism of materials and the effects of different laser heating temperatures and ultrasonic vibration on cutting force, surface quality, subsurface damage and chip morphology are explored. The results show that LA-UEVT can effectively reduce the cutting force and surface roughness, improve the plastic removal ability, and inhibit surface and subsurface damage. And the material removal process is mainly in the form of small particle crushing and particle pressing, which improves the stability of cutting force in the cutting process.     

A study of laser machining combined with dynamic chemical etching in micro-hole drilling of 2.5D SiCf/SiC composites

In this study, we used a novel method of laser machining combined with dynamic chemical liquid etching (LMDCE) to drill holes in 2.5D SiC_f/SiC ceramic matrix composites (CMC-SiC). A chemical solution that could quickly remove the recast layer without damaging the substrate was selected. Severe recast layer and microcrack defects were observed when laser machining was performed in the air. The surface of the material was highly carbonized due to the thermal effect of the laser. The effect of different defocus amounts and scanning speeds on the hole taper was studied. A lower scanning speed can ensure that a smaller taper is obtained by the microhole. The bore diameter of the holes processed with a defocusing amount of 0 or −1 mm is more uniform. The results show that with the assistance of a dynamic chemical solution, the fibers break neatly into needle-like shapes, the thermal effect of the laser on the ceramic substrate is significantly weakened, the microhole shows good roundness, and there are no recast layers and oxides on the sample surface. In addition, microcracks are significantly reduced, and high-quality microholes without a heat-affected zone (HAZ) are machined. The method provides a new idea on how to eliminate machining defects and achieve higher-quality micromachining for ceramic matrix composites.     

Effects of initial density during laser machining assisted CVI process and its influence on strength of C/SiC composites

Novel laser-assisted chemical vapor infiltration (LA-CVI) technique has been used to improve the density and strength of carbon fiber reinforced SiC composites (C/SiC). Initial density of C/SiC before laser machining played an important role in determining the final density and strength of composites. Results show that final density and bending strength of lower initial density composites were better than that of higher initial density samples after LA-CVI process, while the porosity exhibited opposite behavior. Micro-CT and COMSEL simulation results verify that after LA-CVI process, dense band width of C/SiC with initial density of 1.5 g/cm³ was twice as that of C/SiC with initial density of 1.8 g/cm³. This led to crack propagation bypassing the micro-hole. In conclusion, low initial density when laser machining was carried out resulted in better properties of final composites.     

Influences of clearance angle and point angle on drilling performance of 2D Cf/SiC composites using polycrystalline diamond tools

C_f/SiC composite (carbon fiber reinforced silicon carbide ceramic matrix composites) is a kind of advanced composite material constituted by SiC as matrix and carbon fiber as reinforcing phase. C_f/SiC composites are being extensively used in the modern aerospace industry owing to their excellent physical and mechanical properties. The current work aims to investigate influences of clearance angle and point angle on drilling performance of 2D C_f/SiC composites using PCD (polycrystalline diamond) tools in terms of thrust force, drilling torque, hole surface quality, and material removal mechanism. PCD drill bits with different point angles and clearance angles were used in the experiment. The obtained results indicate that the 150° point angle is beneficial to improve the hole surface quality, and larger clearance angle is helpful to reduce the damage of exits. Additionally, the variation of clearance angle has little effect on the roughness of the machined surface. During the drilling process, the dominated material removal mechanisms are matrix removal, fiber breakage, and matrix-fiber debonding. The brittle fracture mode of carbon fibers, which directly affects surface roughness, can be divided into micro-brittle fractures in carbon fiber and macro-brittle fractures. Besides, the damage identification method of hole entrance and exit based on image processing technology is helping to improve the efficiency of machining quality evaluation.     

Investigation on ultrasonic vibration-assisted femtosecond laser polishing of C/SiC composites

This paper proposed a novel ultrasonic vibration-assisted femtosecond laser polishing method for C/SiC composites. The effect of near-field convection enhancement of ultrasonic vibration can improve the cooling of ablated particles and reduce their tendency of bonding to the material surface, reducing surface oxidation and improving the machined surface quality, removal depth and material removal rate. Through optimizing defocusing distance and scanning speed, a specific relationship between ultrasonic amplitude, pulse energy density, and spot overlap rate was established, obtaining a smooth and flat surface without defects. The residual stress of carbon fibers was investigated, and found that the coupling effect of ultrasonic energy and laser energy fields can enhance the residual compressive stress of carbon fibers. The formation of typical features of fiber fracture and pulling-out, banded pits, voids and deposition, was explained. This paper proposes new research ideas for better understanding of the removal mechanism of C/SiC composites using ultrasonic vibration-assisted femtosecond laser.     

Modeling of fatigue failure for SiC/SiC ceramic matrix composites at elevated temperatures and multi-scale experimental validation

The fatigue failure of ceramic matrix composites at elevated temperatures was predicted using the micromechanics method. Multiple micro-damage models were developed to describe the evolution processes of matrix cracking, interface wear, and fiber fracture during fatigue loading. On this basis, the fatigue life was calculated. To validate the fatigue failure model, multi-scale experiments were conducted. In the macroscale, the S-N curve was obtained by the fatigue test. In the microscale, multiple in-situ measuring methods were developed through which the matrix crack density, the interfacial shear stress, and the percentage of fracture fibers were obtained. Both the macroscale and microscale experimental results were in good agreement with the predicted results. Therefore, the fatigue failure model developed in the present work is accurate.     

Mechanical properties of the SiCf/SiC composites reinforced with KD-I and KD-II fibers fabricated assisted by a microwave heating method

SiC_f/SiC composites using KD-I and KD-II SiC fibers braided preforms as the reinforcements were fabricated by applying the polymer impregnation and pyrolysis (PIP) technique with a microwave heating assistance. The microwave heating temperature was 1100 °C, 1200 °C, 1300 °C, and 1400 °C, respectively. Microstructures, flexure properties, and fracture behaviors of the composites were investigated. The KDIISiC_f/SiC composites exhibited higher flexure properties and improved non-brittle fracture characteristics than those of the KD-ISiC_f/SiC composites. The differences in the flexural properties, fracture behaviors and microstructures between the KD-I and KDIISiC_f/SiC composites were discussed based on the tensile properties of the SiC filaments and the interfacial bonding statues in the composites.     

Fabrication of gradient C/C-SiC-MoSi2 composites with enhanced ablation performance

C/C-SiC-MoSi₂ composites with gradient composition and microstructure were prepared by a novel vacuum filtration infiltration (VFI) process with a later hydrothermal densification. The composition distribution, microstructure, density, porosity, thermal conductivity and ablation properties of the composites were investigated. Results show that the distributions of SiC and MoSi₂ are homogeneous and gradient along the cross-section of the composites, respectively. From the inner part to the outer part of the composites, the increase in density and thermal conductivity is achieved. The outer part of the composites exhibits enhanced ablation performance. After being exposed to the oxyacetylene flame at 2000 °C for 30 s, the linear and mass rates of the as-prepared composites are only 0.0051 mm/s and 0.76 mg/s.     

Preparation and analysis of micro-holes in C/SiC composites and ablation with a continuous wave laser

Pulsed lasers and continuous wave (CW) lasers play an important role in modern machining technology, especially in machining materials with high hardness, such as C/SiC ceramic composites. However, for pulse lasers, the high costs have been a huge obstacle to their application in machining C/SiC composites. In this work, a CW laser was used to machine micro-holes with less than 1 mm diameter, and the effects of different laser output powers to micro-holes quality and the methods which are used to clean residue were examined. One can conclude that, when the power output is 50 W, the micro-hole surface morphology is better than that of the holes formed with two other output power values. During laser machining, the SiC matrix will be ablated and removed from a carbon fiber layer-by-layer. After cleaning with distilled water and HF, oxide residues can be removed and the expected micro-holes would be successfully constructed.     

Mechanical,thermal, and dielectric properties of SiCf/SiC composites reinforced with electrospun SiC fibers by PIP

Electrospun unidirectional SiC fibers reinforced SiC_f/SiC composites (e-SiC_f/SiC) were prepared with ∼10% volume fraction by polymer infiltration and pyrolysis (PIP) process. Pyrolysis temperature was varied to investigate the changes in microstructures, mechanical, thermal, and dielectric properties of e-SiC_f/SiC composites. The composites prepared at 1100 °C exhibit the highest flexural strength of 286.0 ± 33.9 MPa, then reduced at 1300 °C, mainly due to the degradation of electrospun SiC fibers, increased porosity, and reaction-controlled interfacial bonding. The thermal conductivity of e-SiC_f/SiC prepared at 1300 °C reached 2.663 W/(m∙K). The dielectric properties of e-SiC_f/SiC composites were also investigated and the complex permittivities increase with raising pyrolysis temperature. The e-SiC_f/SiC composites prepared at 1300 °C exhibited EMI shielding effectiveness exceeding 24 dB over the whole X band. The electrospun SiC fibers reinforced SiC_f/SiC composites can serve as a potential material for structural components and EMI shielding applications in the future.     

Mechanical and dielectric properties of SiCf/SiC composites fabricated by PIP combined with CIP process

2D KD-1 SiC fiber fabrics were employed to fabricate SiC_f/SiC composites by an improved polymer infiltration and pyrolysis (PIP) process, combined with cold isostatic pressing (CIP). The effect of CIP process on the microstructure, mechanical and dielectric properties of SiC_f/SiC composites was investigated. The infiltration efficiency was remarkably improved with the introduction of CIP process. Compared to vacuum infiltration, the CIP process can effectively increase the infiltrated precursor content and decrease the porosity resulting in a dense matrix. Thus SiC_f/SiC composites with high density of 2.11 g cm⁻³ and low porosity of 11.3% were obtained at 100 MPa CIP pressure, together with an increase of the flexural strength of the composites from 89 MPa to 213 MPa. Real part (ε′) and the imaginary part (ε″) of complex permittivity of SiC_f/SiC composites increase and vary from 11.7-i9.7 to 15.0-i12.8 when the CIP pressure reaches 100 MPa.     

Material deformation and removal mechanism of SiCp/Al composites in ultrasonic vibration assisted scratch test

The material removal process of SiCp/Al composites is a result of synergetic deformation and interaction among Al matrix, SiC particles and interface. The non-homogeneity of microscopic mechanical properties due to the inherent polyphase heterogeneity of SiCp/Al composites will directly affect the removal mechanism and surface integrity in the machining process. This paper aims to gain further insight of the material deformation and removal mechanism of SiCp/Al composites in ultrasonic vibration assisted machining process. The elastic modulus and hardness of SiCp/Al composites were determined through the indentation test by loading on Al matrix and SiC particles, respectively. Due to the interaction effects of the three phases during the deformation process, when the indenter is on a single phase, the influence of the other phases cannot be neglected and is reflected in the P-h curves. Scratch force, friction coefficient and material removal behavior were investigated in traditional scratch (TS) and ultrasonic vibration assisted scratch (US) tests. In most cases, with the assistance of ultrasonic vibration, scratch force and friction coefficient in US process are smaller than those in TS process, and the reduction of them is modeled and analyzed. The material removal behavior of SiCp/Al composites is similar to metal at the macroscale, and a high material removal rate is achieved in US process. SiC particles tend to maintain the structural integrity rather than be fractured or pulled out in US process. The scratched surface in TS process is damaged to a greater degree than that subjected to US process.     

Influence of pyrolysis and melt infiltration temperatures on the mechanical properties of SiCf/SiC composites

In order to improve the degree of matrix densification of SiC_f/SiC composites based on liquid silicon infiltration (LSI) process, the microstructure and mechanical properties of composites according to various pyrolysis temperatures and melt infiltration temperatures were investigated.Comparing the microstructures of SiC_f/C carbon preform by a one-step pyrolysis process at 600 °C and two-step pyrolysis process at 600 and 1600 °C, the width of the crack and microcrack formation between the fibers and matrix in the fiber bundle increased during the two-step pyrolysis process. For each pyrolysis process, the density, porosity, and flexural strength of the SiC_f/SiC composites manufactured by the LSI process at 1450–1550 °C were measured to evaluate the degree of matrix densification and mechanical properties. As a result, the SiC_f/SiC composite that was fabricated by the two-step pyrolysis process and LSI process showed an 18% increase in density, 16%p decrease in porosity, and 150% increase in flexural strength on average compared to the composite fabricated by the one-step pyrolysis process.In addition, among the SiC_f/SiC specimens fabricated by the LSI process after the same two-step pyrolysis process, the specimen that underwent the LSI process at 1500 °C showed 30% higher flexural strength on average than those at 1450 or 1550 °C. Furthermore, under the same pyrolysis temperature, the mechanical strength of SiC_f/SiC specimens in which the LSI process was performed at 1500 °C was higher than that of the 1550 °C although both porosity and density were almost similar. This is because the mechanical properties of the Tyranno-S grade SiC fibers degraded rapidly with increasing LSI process temperature.     

Effect of SiCnws on flexural strength of SiCf/HfC-SiC composites after impact and ablation

SiC nanowires (SiC_nws) modified SiC_f/HfC-SiC composites were prepared by precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) methods. The microstructure, flexural strengths, impact and impact-ablation tests of the composites with and without SiC_nws were investigated. The results showed that after introducing SiC_nws, not only the retention rate of HfC ceramic produced by PIP was increased obviously, but also the fracture displacement of the modified composites was reduced due to the enhancement effect of SiC_nws at interface between SiC fiber and matrix. After impact and impact-ablation, the strength retention of SiC_nws modified composites was 91.6 % and 69.1 % respectively, higher than that of the composites without SiC_nws (85.2 % and 54.8 %). As the impact resistance of the modified composites was improved by the pull-out and bridging of SiC_nws, the ablation resistance of the impacted composites was enhanced as well.     

New ablation evolution behaviors in micro-hole drilling of 2.5D Cf/SiC composites with millisecond laser

2.5D C_f/SiC composite has been a key heat-resistant ceramic matrix composite (CMC) in aerospace field due to the special structure characteristics. Against the existing research almost focus on 'ablation behavior' in laser processing of CMC, this paper put forward the 'ablation evolution behavior' for the first time, and reveals the 'ablation evolution behavior' in micro-hole machining of 2.5D C_f/SiC composites with millisecond laser. The results show that 0° fiber experiences the ablation evolution from filiform connection, flat ellipse, needle-like to convex structure. And the 90° fiber suffers the ablation evolution from cylindrical structure, lotus lead shape, bud-shape to closed shape. The core of the 90° fibers undergoes the topography structures from cylindrical protrusion, spherical protrusion to needle-like. Except for the honeycomb structure studied in existing research, four new recast layer structures, named transverse strip, longitudinal strip, shell structure, and multi-layer structure are found and analyzed. Further, the mechanism analysis reveals that the recast layer contains both oxidized characteristics and highly carbonized characteristics. In addition, the microstructural analysis shows that three types of particle topography adhere to the recast layer, that are spherical micro-protrusion (20–48 μm), bubble particles (5–15 μm), and sub-micron particles (<1 μm).     

多孔Al2O3/SiC、MoSi2/SiC复合材料的制备及吸波性能

以Si、Al₂O₃、MoSi₂微粉和生物竹材为原料,采用包埋烧结法分别制备出SiC多孔材料、Al₂O₃/SiC、MoSi₂/SiC复合材料。采用XRD、SEM及波导法测试其物相组成、显微结构及吸波性能。结果表明：MoSi₂/SiC复合材料的厚度为2 mm时有明显的吸波特性,有效吸收带宽在X波段的9.65~12.4 GHz频率范围内达2.75 GHz,且最低反射损耗为-38.27 dB。Al₂O₃/SiC复合材料孔道内的Al₂O₃与SiC晶须交缠,形成大量电偶极矩,产生介电损耗;MoSi₂/SiC复合材料除介电损耗外还存在电阻损耗,使得复合材料电磁损耗增加,是较有前途的结构功能吸波材料。     

Oxidation and thermal shock resistant properties of Al-modified environmental barrier coating on SiCf/SiC composites

SiC_f/SiC ceramic matrix composites (CMCs) are being widely used in the hot-sections of gas-turbines, especially for aerospace applications. These CMCs are subjected to surface recession if exposed to heat-corrosion. In this research, an alternative environmental barrier coating (EBC) is introduced to protect the SiC_f/SiC CMC from high temperature degradation that is, Al film was deposited on the surface of SiC_f/SiC CMC followed by heat-treatment in a vacuum. After that, a dense Al₂O₃ overlay was in-situ synthesized on the surface of CMC, and in this process the microstructure evolution of SiC_f/SiC CMC was analyzed. The oxidation and thermal shock resistance were characterized, showing that the Al-modified SiC_f/SiC CMC has a better oxidation resistance, because the dense Al₂O₃ overlay can hinder oxygen diffusion from environment. What is more, the water-quenching testes show that the Al-modified SiC_f/SiC CMC has a good spallation resistance.