Design, fabrication, and testing of CVI - SiC/SiC turbine blisk under di ﬀ erent load spectrums at elevated temperature

: In this article, design, fabrication, and testing of SiC/SiC turbine blisk with the ﬁ ber ’ s preform of Spider Web Structure ( SWS ) subjected to di ﬀ erent load spec - trums at elevated temperature are conducted. Micro - CT scans are conducted to show the ﬁ bers preform and defects inside the SWS - SiC/SiC turbine blisk. For 2D plain - woven SiC/SiC composite under monotonic tensile loading at an elevated temperature of T = 900°C in air atmosphere, the composite ultimate tensile strength is σ uts = 200 MPa with the fracture strain ε f = 0.36%. For SWS - SiC/SiC turbine blisk under the rotation testing, the ﬁ rst and second order natural frequency of the SWS - SiC/ SiC turbine blisk are tested using the laser vibration meter. Relationships between the rotation speed, internal damage, and the natural frequency degradation of the SWS - SiC/SiC turbine blisk are established. Under the maximum rotation speed of n = 17,000 rpm at the exhaust temperature of T = 930°C, no damage occurred in the SWS - SiC/SiC turbine blisk. However, multiple coating spalling occurred due to the thermal - chemical coupling failure of the coating under ﬂ ame impingement. The ﬁ rst natural frequency of the SWS - SiC/SiC turbine blisk decreases by 5% with the increase in the rotating speed from n max = 85,000 rpm to n max = 105,000 rpm, which indicates that there is an internal damage in the SWS - SiC/SiC turbine blisk, which leads to the decrease in the sti ﬀ ness of the blisk.


Introduction
With the development of aerospace, there is a continuous and inexhaustible demand for aeroengine performance.It is considered that improving the structural efficiency and unit thrust are two important ways to improve the thrust-weight-ratio of the aeroengine.The first way to improve the structural efficiency is to solve the problem by using lighter and stronger structural materials; and the second way to improve the unit thrust is to increase the temperature in front of the turbine, and finally the turbine rotor material needs to be upgraded to a more heat-resistant material to solve the problem [1,2].At present, the turbine inlet temperature (TIT) has already approached 1,600-1,700°C, which is beyond the bearing capacity of single crystal alloy turbine blade.The TIT of the next generation turbofan aeroengine is expected to approach 1,830-1,930°C.It is obvious that the high temperature alloy materials are far from meeting these requirements.Therefore, the development of lightweight, high specific strength, and high temperature resistant materials for the hot-section components is the only way for future high-performance aeroengines [3].
Ceramic-matrix composites (CMCs) have the characteristics of lightweight, high temperature resistance, impact resistance, and long service life.The density of CMCs is only 1/4-1/3 of that of Ni-based superalloy, and the working temperature is 400-500°C higher than that of Ni-based superalloy.Moreover, it can reduce cooling air by approximately 20%, reduce NO x and CO emissions, and significantly improve the aeroengine efficiency [4][5][6].C or SiC can be used for the matrix of CMCs.C-matrix can bear higher temperature than the SiC-matrix, which is suitable for the application on rocket engine with higher working temperature and shorter working duration.SiC-matrix possesses better resistance of oxidation and is suitable for the application on the hot-section components in aeroengines with long working duration.The main methods for the fabrication of CMCs include chemical vapor infiltration (CVI) [7,8], polymer infiltration and pyrolysis process (PIP) [9][10][11][12][13], and melt infiltration (MI) [14].In the CVI process, the SiC matrix is deposited on the fibers' surface by gaseous precursors at low temperature (i.e., 800-1,100°C) and low pressure (i.e., 1-10 kPa).In the PIP process, the dense SiC matrix is obtained through multiple infiltration and pyrolysis of the liquid precursor at temperature range of 700-1,600°C.The density of the SiC matrix increases with the cycles of the infiltration and pyrolysis.The MI process can be divided into reactive melt infiltration (RMI) and non-reactive melt infiltration (NRMI).In the process of RMI, the molten Si is infiltrated into the porous C-matrix preform and the SiC-matrix is formed as the C reacts with the Si.However, in the process of NRMI, the molten Si is infiltrated in the porous of the matrix without reaction with the matrix.In the abovementioned fabrication method, the SiC matrix possesses better mechanical properties in the CVI process due to the low fabrication temperature and pressure.The CVI processes have already been adopted to fabricate the hot-section components in military or commercial aeroengines.Outer flaps for the M53-2 and M88-2 SNECMA aeroengines have already been designed and fabricated using the CVI-C/SiC composites at intermediate temperatures (i.e., 650-700°C) with weight saving of approximately 50% compared with superalloy counterpart.NASA designed and tested a CVI-C/SiC turbine blisk for the rocket engine [15].The diameter of the CVI-C/SiC turbine blisk was 19 cm and the thickness was 2 cm.The blisk can bear the temperature as high as 1,093°C, which is much higher than Ni-based alloy (i.e., 649°C).During the rotating testing, cracks occur in the turbine blade.However, the turbine blisk can continue work with cracks in the blade.For higher temperature above 800°C, the SiC/SiC composites have been applied on different hot section components due to a low thermal residual stress in the SiC matrix.General electric (GE) has successfully applied SiC/SiC combustor in F-414 engine, which is the first application of SiC/SiC stationary components.On February 10, 2015, GE demonstrated the SiC/SiC low-pressure turbine blades on the F-414 turbofan verification aeroengine and achieved a complete success; this work first introduced the SiC/SiC rotating parts into the area with the highest aeroengine temperature and the worst working conditions.In addition, the LEAP-X engine that CFM first flew in 2016 also adopted the SiC/SiC high-pressure turbine outer ring and low-pressure turbine guide vane [16].Ogawa [17] had designed a 3D-C/C turbine blisk with outer diameter of 200 mm, inner hole diameter of 80 mm, and thickness of 12-13 mm.The preform of the disk component is made by 3D polar orthogonal weaving, the high modulus carbon fiber is used in the disk surface, and the high strength carbon fiber is used in the thickness direction; in order to increase the strength of the blade, the fiber's content in the radial direction and thickness direction of the preform was increased to two times of the original fiber content at the outer side of the disk (i.e., the diameter is greater than 130 mm).The results show that the turbine disk rupture speed reaches n = 60,000 rpm, and the speed along the blade tip is approximately v = 565 m•s −1 .The simulation results show that the maximum stress is expected to be σ max = 450 MPa at the center of the turbine disk.The failure position is at the position of turbine disk diameter d = 130 mm, that is, the position where the fiber volume content changes abruptly.For small gas turbine engines, CIAM (Central Institute of Aviation Motors, Russia) is focusing on the research and development of CMC guide vanes, combustion chambers, high pressure turbine blisk, and other components, and the assessment and verification plan is gradually implemented [18].During operation of CMC hotsection components, internal damage occurs and affects the mechanical performance, reliability, and safety of these components.It is necessary to develop damage detecting method to determine internal defects and damage propagation during operation.There are many types of nondestructive testing (NDT) methods applied to the internal defects and environmental damage in CMCs, including X-ray [19], infrared thermal imaging [20], microanalysis [21], micro-CT [22], acoustic emission (AE) [23,24], electrical resistance (ER) [25], and ultrasonic [23,26], etc.The natural frequency is one of the main characteristics of vibration characteristics.Bai et al. [27] investigated the natural frequency of delaminated advanced grid stiffened composite plates by hump resonance method.Li [28] performed vibration analysis of damaged SiC/SiC composite.When the natural frequency of SiC/SiC composite decreased by approximately 5.47%, the SiC/SiC composite was seriously damaged and in a dangerous state.However, in the research mentioned above, the high temperature engineering application evaluation for CVI-SiC/SiC blisk has not been performed.
This article focuses on the design, fabrication, and testing on CVI-SiC/SiC turbine blisk subjected to different load spectrums at elevated temperature.The fiber preform for the turbine blisk is Spider Web Structure (SWS).Micro-CT scans are conducted to show the fiber preform and internal defects in the SWS-SiC/SiC turbine blisk.Monotonic tensile test is conducted for the 2D plain-woven CVI-SiC/SiC composite at an elevated temperature of T = 900°C in air atmosphere to investigate the damage evolution behavior.The SWS-SiC/ SiC blisk is tested under the rotation test of different load spectrums at elevated temperature.During the rotation testing, the first and second order natural frequency of SWS-SiC/SiC turbine blisk are tested using the laser vibration meter.Relationships between the rotation speed, internal damages, and the natural frequency degradation of the SWS-SiC/SiC turbine blisk are analyzed.

Materials fabrication and experimental procedures 2.1 Design and fabrication of CVI-SiC/SiC turbine blisk
In view of the characteristics of high circumferential and radial stress and low axial stress of turbine blisk, the Spider Web Structure (SWS) was adopted for the fiber's preform for the SiC/SiC turbine blisk.The circumferential fibers are arranged in concentric circles or spirals with the center of the disk as the origin, and the radial fibers are distributed in a radial plane.As the distance between the fiber and the origin gradually increases, the area within the included angle of adjacent radial fibers will gradually increase.At this time, yarn needs to be added at the appropriate position within the included angle of adjacent radial fibers, in order to increase the local fiber volume content to ensure the uniform fiber content in the unit layer and reduce the internal stress concentration of the disk.The CVI process was adopted to fabricate the 2D plain-woven Cansas-3303 TM (Liya New Material Co., China) SiC/SiC composite and SWS-SiC/SiC turbine blisk.The diameter of the SiC fiber is d = 14-16 μm, the elastic modulus is E f = 250 GPa, the tensile strength is σ f = 3.1 GPa, and the density is approximately ρ f = 2.75 g•cm −3 .
To protect the SiC fibers and increase the load transfer capacity between the fibers and the matrix [3,29], the pyrocarbon (PyC) interphase was deposited on the surface of the SiC fibers using the chemical vapor deposition (CVD) method.The CVD temperature was T = 1,000°C using the Ar as the protective gas.The deposition duration is 30 h and the interphase thickness is 350 nm.After deposition of the interface, the SiC matrix was deposited on the fiber's preform using the CVI method at 1,100°C.The SiC/SiC turbine blisk was further coated with SiC coating of approximately 20 μm thick to prevent oxidation at elevated temperature.The final density of the SWS-SiC/SiC turbine blisk is ρ = 2.5 g•cm −3 .
Figure 1 shows the micro-CT scan photo of the 2D plain-woven CVI-SiC/SiC composite.The final porosity volume of the composite was approximately V p = 5%, and the fiber volume fraction in the composite was approximately V f = 67%.The porosity distributed between fiber bundles or crossover sections inside of the composite.Figure 2 shows the SWS fiber's preform, SWS-SiC/SiC turbine blisk, and the corresponding micro-CT scan photo.The SWS-SiC/SiC turbine blisk is composed of three parts: the boss for clamping and transferring torque, the body connecting the center boss and the outer blade, and 24 turbine blades.The outer radius is R = 100 mm, and the inner radius is R = 15 mm.The density of the turbine blisk is approximately ρ = 2.5 g•cm −3 , and the porosity is  approximately 5%, and the thermal expansion coefficient is approximately 4.64 ± 0.12 × 10 -6 •K −1 .

Experimental procedures
Monotonic tensile test at an elevated temperature of T = 900°C in air atmosphere was conducted on an Instron 8801 servo hydraulic load-frame.The high-temperature furnace was heated by resistance wire with temperature error ±1°C and heated to the target temperature for 10 min to ensure uniform temperature field in the test section of the sample.The tensile test was conducted under the stress control with the loading rate 120 MPa•min −1 .The dogbone shaped specimens, with dimensions of 127 mm in total length, 30 mm length, 3 mm thickness, and 6 mm width in the gage section, as shown in Figure 3, were used for the monotonic tensile test at 900°C in air.
Aeroengine load spectrum is the basis of engine structural strength design and service life analysis, and its main value is to provide decision-making basis for aeroengine design [30,31].The load spectrum of this test is "start-idle-cruise-max-idle-stop," as shown in Figure 4(a), which aims to simulate the actual load spectrum conditions of aeroengine under different working conditions to test and verify the performance, structural integrity, and life of SWS-SiC/SiC turbine blisk under high temperature environment.The SWS-SiC/SiC turbine blisk is assembled with disk/shaft, as shown in Figure 4(b), and then the rotor dynamic balance of the assembly is corrected.The upper limit of the dynamic unbalance tolerance of the SWS-SiC/SiC turbine blisk is set to be 0.3 g•mm, and the dynamic balance correction test of the assembly is carried out at the rotation speed n = 4,000 rpm.The dynamic unbalance tolerance of the SWS-SiC/SiC turbine blisk is  within the allowable range of the maximum unbalance, which means that the unbalance caused by the rotor assembly has been basically eliminated.
The high temperature test of SWS-SiC/SiC turbine blisk under load spectrum is conducted on the micro turbine engine developed by the AECC Hunan Aviation Powerplant Research Institute.The micro turbine engine is mainly composed of the engine inlet, compressor, combustion chamber, turbine, and tail nozzle.The temperature was monitored using infrared temperature sensor with the accuracy of ±3°C.During the rotation testing, the natural frequency of the SWS-SiC/SiC turbine blisk is tested using the laser vibration meter, as shown in Figure 4(c) and (d).

Experimental results and discussion
In this section, the high temperature monotonic tensile test for 2D plain-woven CVI-SiC/SiC composite and rotating test for SWS-SiC/SiC turbine blisk subjected to different loading spectrums are conducted.High temperature tensile damage evolution and fracture behavior is analyzed, and the relationships between the rotation speed, internal damages, and the natural frequency degradation of the SWS-SiC/SiC turbine blisk are established.stress range for Stage II is between the proportional limit stress (i.e., σ pls = 128.3MPa) and matrix cracking saturation stress (i.e., σ sat = 145 MPa).• Stage III, the secondary linear damage stage with slow degradation of composite's tangent modulus, due to the gradual fibers broken with the increasing applied stress.The corresponding stress range for Stage III is between the matrix cracking saturation stress (i.e., σ sat = 145 MPa) and composite's tensile strength (i.e., σ uts = 200 MPa).

Rotating test of SWS-SiC/SiC turbine blisk under different load spectrums
Two rotating tests under different load spectrums are conducted for the SWS-SiC/SiC turbine blisk.During the rotating test, the first and second order natural frequency of the SWS-SiC/SiC turbine blisk are tested to reflect the internal damage evolution inside the SWS-SiC/SiC turbine blisk.Relationships between the rotation speed, composite's internal damage, and the degradation of the natural frequency are analyzed.

First testing
According to the given load spectrum, the micro turbine engine with the SWS-SiC/SiC turbine blisk is tested.
During the test run, it was observed that the test speed failed to reach the target speed and the exhaust temperature is too high during the start-up and acceleration phase, as shown in Figure 6.To avoid the damage of rotor components due to overheat, the micro turbine engine was stopped manually.The target rotation speed is set to be n = 112,000 rpm, and the actual test rotation speed approached n = 17,000 rpm, which is far less than the target value, mainly attributed to two reasons, as following: • The integral turbine blisk is made of SWS-SiC/SiC composite material and the outer ring of the turbine blisk is made of GH3625 alloy material.The thermal expansion coefficient of the two materials is quite different, as shown in Figure 7.The thermal expansion coefficient of SWS-SiC/SiC integral turbine blisk is much less than that of the outer ring of turbine, and the deformation of the SWS-SiC/SiC turbine blisk caused by elevated temperature is also less than that of the outer ring of GH3625 alloy material, leading to the large rotating and stationary clearance during turbine engine operation, and increase the leakage loss and cause the serious decline of aeroengine efficiency, so the micro turbine engine does not reach the target speed.• In the test, the machining dimension of SWS-SiC/SiC turbine blisk blade is approximately 0.18 mm thicker than the designed blade profile, and the thickness distribution is uneven, which leads to poor micro turbine engine performance.
After the completion of a test run, it was found that the SWS-SiC/SiC turbine blisk had multiple coating spalling, as shown in Figure 8.The spalling of the coating is due to the thermal-chemical coupling failure of the coating under flame impingement [32].

Second testing
In the second rotating test run, the method of speed gradient rising is adopted.The target rotating speed is set to be n = 85,000 rpm.After reaching the specified target speed, the blisk is increased by 5,000 rpm each time until the designed target speed of 112,000 rpm is achieved.After each test, the natural frequency of the turbine blisk is measured by laser vibration meter, and the internal damage state in the disk was qualitatively analyzed by the change in the natural frequency.
Figure 9 shows the test and target rotating speed and exhaust temperature during the second testing with the maximum rotation speed n max = 90,000, 95,000, 100,000, 105,000, and 107,100 rpm, and n max = 100,000 for 5 min.Figure 10 shows the change in first and second order natural frequency of the SWS-SiC/SiC turbine blisk with increasing rotation speed.When the maximum rotation speed increases from n max = 85,000-105,000 rpm, the first order natural frequency decreases from f = 14,426 to 13,681 Hz, and the second order natural frequency decreases from f = 15,394 to 14,188 Hz.
In the second test run, the test rotating speed is gradually increased from n max = 85,000 rpm to n max = 107,100 rpm at the maximum speed.The internal damage state of the SWS-SiC/SiC turbine blisk was monitored through the natural frequency measuring after different maximum speeds.For the subsequent flight test, the endurance performance of the engine at n = 100,000 rpm speed is also investigated.Experimental results show that the first natural frequency of the SWS-SiC/SiC turbine blisk decreased by 5% with the increase in the rotating speed from n max = 85,000 rpm to n max = 105,000 rpm, which indicates that there is some damage in the disk, which leads to the decrease in the stiffness of the blisk.

Comparison and discussion
Min et al. [33] performed vibration experiments on C/SiC turbine blisk at room temperature for measuring the frequency and damping properties.The blisk was designed with a polar fiber preform with the fiber volume content of approximate 40%.However, in the research of Min et al. [33], the high-temperature rotation test has not been conducted.The relationships between the rotation speed, internal damage, and natural frequency have not been established.In the present research, the SWS-SiC/ SiC turbine blisk has been designed, fabricated, and tested at elevated temperature for different load spectrums to establish the relationship between the rotation speed, internal damage, and natural frequency.
Figure 11 shows the internal defect analysis of the SWS-SiC/SiC turbine blisk observed under micro-CT scanning.The fiber's preform of Spider Web Structure (SWS) was composed of spiral circumferential fibers and radial fibers.The internal porosity and defects were mainly distributed at the crossover section of the fiber's yarns.The distribution of the internal porosity and defects affected the stiffness and strength of SWS-SiC/SiC turbine blisk.With increase in the rotating speed, the circumferential and radial stresses increase and cause the matrix crack propagation, interface crack deflection, and fibers' fracture inside the structure.It is necessary to control the distribution, volume, and location of the internal porosity and defects in SWS-SiC/SiC turbine blisk to decrease the uncertainty of the rotating experiments.

Summary and conclusion
In this article, design, fabrication and testing of SWS-SiC/ SiC turbine blisk subjected to different load spectrums were conducted.Micro-CT scans were performed to analyze the internal fiber's preform, defects, and porosity.Monotonic tensile test of 2D plain-woven CVI-SiC/SiC composite was conducted at an elevated temperature of T = 900°C to analyze the tensile damage and fracture behavior.For the SWS-SiC/SiC turbine blisk under the rotating test, the first and second order natural frequencies of the SWS-SiC/SiC turbine blisk were tested using the laser vibration meter.Relationships between the rotation speed, internal damages, and the natural frequency degradation of the turbine blisk were established.• For the 2D plain-woven CVI-SiC/SiC composite at an elevated temperature of T = 900°C in air condition, the tensile curve exhibited nonlinear behavior with the composite's proportional limit stress σ pls = 128 MPa, the composite's tensile strength σ uts = 200 MPa, and the fracture strain ε f = 0.36%.Evolution of related damage parameter, i.e., tangent modulus, is divided into three stages, i.e., linear, nonlinear, and secondary-linear regions.• For the SWS-SiC/SiC turbine blisk, under the maximum rotation speed of n = 17,000 rpm at the exhaust temperature of T = 930°C, no damages occurred in the SWS-SiC/SiC turbine blisk.However, multiple coating spalling occurred due to the thermal-chemical coupling failure of the coating under flame impingement.The first natural frequency of the SWS-SiC/SiC blisk decreased by 5% with the increase in the rotating speed from n max = 85,000 rpm to n max = 105,000 rpm, which indicates that there was damage in the blisk, leading to the decrease in the stiffness of the turbine blisk.

Figure 3 :
Figure 3: Photo of the dog-bone specimen for tensile test of 2D CVI-SiC/SiC composite at 900°C in air atmosphere.

Figure 5
Figure5shows the monotonic tensile curve and related composite's tangent modulus evolution with strain curve of 2D plain-woven CVI-SiC/SiC composite at an elevated temperature of T = 900°C in air atmosphere.The tensile curve at 900°C exhibit nonlinear damage behavior due to internal multiple damage mechanisms of matrix cracking, interface debonding, and fiber's failure.Through the analysis of the tensile curve, the composite's initial elastic modulus is E c = 122 GPa, the composite's proportional limit stress is σ pls = 128 MPa with the corresponding strain ε pls = 0.12%, the composite's saturation matrix cracking stress is σ sat = 145 MPa with the corresponding strain ε smc = 0.16%, and the composite's tensile strength is σ uts = 200 MPa with the fracture strain ε f = 0.36%.The composite's tensile curve and related tangent modulus can be divided into three stages, including:• Stage I, the linear-elastic stage with the high composite's tangent modulus.The corresponding stress range for Stage I is between the initial loading stress to the proportional limit stress (i.e., σ pls = 128.3MPa).• Stage II, the nonlinear damage stage due to multiple damage mechanisms (e.g., matrix cracking, interface debonding, and fiber's failure) with the rapid degradation of composite's tangent modulus.The corresponding

Figure 5 :
Figure 5: (a) Tensile curve; and (b) tangent modulus versus strain curve of 2D plain-woven CVI-SiC/SiC composite at an elevated temperature of T = 900°C in air atmosphere.

Figure 7 :
Figure 7: Comparison of thermal expansional coefficient of GH3625 alloy material and SWS-SiC/SiC composite.

Figure 6 :
Figure 6: Test and target speeds and the exhaust temperature during the first testing of SWS-SiC/SiC turbine blisk.