Plasma etching as a surface engineering technique for SiC/SiC composites to improve joint strength

Silicon carbide fiber reinforced silicon carbide (SiC/SiC) composites have unique properties that make them suitable for demanding applications. These materials have to be joined with other materials or with the same type of material to produce the final component. This work investigates the effectiveness of a fluorine-based plasma process on SiC/SiC composites as a surface engineering technique to manufacture a brush-like texturized surface. The studied pre-treatment induced an interlocking effect at the composite/joining material interface, thus improving the joint strength of the joined components.Several plasma conditions were considered and the most promising one (CF₄, 20 sscm, 200 W, 30 min) was selected to assess the effect on the mechanical performance of SiC/SiC brazed with a commercial brazing alloy (Cusil-ABA®). The selected plasma pre-treatment led to a 55% increase in the apparent shear strength of the joined SiC/SiC composites.     

Microstructure and mechanical properties of C/C composite/Ti6Al4V joints with a Cu/TiCuZrNi composite brazing alloy

C/C composites and Ti₆Al₄V sheets were joined using a novel Cu/TiCuZrNi composite braze and the effect of Cu foil thickness on the microstructure and mechanical properties of the joints were investigated. A composite joining material, consisting of a TiC particle reinforced brazing alloy formed in situ during the brazing process was obtained. Before the joining process, the C/C composite surface was modified by solid state reaction with chromium to form a chromium carbide coating. This carbide reaction layer can react with the Ti-based brazing alloy and form evenly distributed TiC particles. The maximum apparent shear strength of as-received joint was 39±8.5 MPa obtained by using a 40 μm thick Cu foil together with the TiCuZrNi alloy; this value was 70% higher than the mechanical strength obtained for joints brazed with the TiCuZrNi alloy only.     

温度及表面金属化对铝基复合材料与可伐合金真空钎焊的影响

采用Al63-Cu22-Ti5-Si10钎料,研究了55%SiCp/6063Al复合材料和可伐合金之间的真空钎焊工艺,分析了钎焊温度和复合材料表面镀层材料对接头抗剪强度和显微硬度的影响规律,并对接头的显微组织进行了研究。结果表明钎焊温度和复合材料表面镀层对接头的力学性能影响很大,在同样的焊接参数下,复合材料表面镀铜的试样,其抗剪强度要高于无镀层和镀镍的试样,镀铜试样的最高抗剪强度为92.8 MPa。当钎焊温度从560℃增加到580℃时,接头的抗剪强度逐渐降低再上升,经过不同表面处理的试样均在钎焊温度为560℃时达到最大抗剪强度。钎焊温度相同时,镀铜试样的显微硬度均最高,而镀镍试样的显微硬度最低。焊缝组织致密,没有出现孔洞和未润湿等钎焊缺陷,钎焊完成后,接头中镀层被钎料取代而消失。在保温时间为30 min、真空度6.5×10-3 Pa条件下,采用Al63-Cu22-Ti5-Si10钎料,55%SiCp/6063Al复合材料与可伐合金真空钎焊合理钎焊温度为560℃,合理镀层为镀铜。     

Ceramic protection plates brazed to aluminum brake discs

Aluminum alloys are light-weight and one of the most interesting material solutions to optimize the strength/weight ratio to reduce car weight; however they are also relatively soft and therefore cannot be used for intensive wear applications. We developed an aluminum alloy part combined with hard and wear-resistant Al₂O₃-based ceramic plates on the surface for demanding mechanical parts for automotive industry such as disc brakes.Tribological tests of various engineering ceramic materials were performed in order to find a ceramic material with a combination of coefficient, wear resistance and thermal energy dissipation for the car brakes. Al₂O₃-based ceramic showed promising properties, as well as being cost effective.Two different approaches to braze ceramic on aluminum were investigated. A two-step brazing process using Cu-Sn-Ti-Zr filler alloy and a single step ultrasonic active soldering with Sn-Ag-Ti filler alloy. Larger areas of aluminum could be covered with a segmented brake design in which many ceramic plates were joined surface. Comparable tribological properties to those of the bulk ceramic material were achieved.     

Microstructure Control and Wear of Al2O3-SiC-(Al, Si) Composites Made by Melt Oxidation

Ceramic matrix composites of Al₂O₃-SiC-(Al, Si) have been fabricated by directed melt oxidation of aluminum alloys into SiC particulate preforms. The proportions of AI₂O₃, alloy, and porosity in the composite can be controlled by proper selection of SiC particle size and the processing temperature. The wear resistance of composites was evaluated in pin-on-disk experiments against a hard steel substrate. Minimum wear rate comparable to conventional ceramics such as ZTA is recorded for the composition containing the highest fraction of alloy, owing to the development of a thin and adherent tribofilm with a low coefficient of friction.     

An analysis of deformation mechanism in the Si3N4–AgCuTi + SiCp–Si3N4 joints by digital image correlation

The composite filler shows more advantages than traditional brazing alloys, and has been widely introduced into joining ceramics or ceramics to metals. However, the underlying formation and strengthening mechanisms remained uncertain in the joint. In current research, SiCp (p = particle) was incorporated in Ag–Cu–Ti brazing alloy for joining Si₃N₄ ceramic. Nanoindentation method was introduced for probing the mechanical properties of reaction phases between the brazing alloy and SiCp. A novel formation mechanism model was thus proposed. In addition, optical microscope (OM) in conjunction with digital image correlation (DIC) techniques has been first applied to elucidate the deformation mechanism in the joint with and without SiC incorporation. The following reasons were believed to strengthen the brazed joint: load-transfer ability of SiCp, plastic relaxation in the brazing layer and CTE reduction of the composite filler.     

Preparation of aluminum/silicon carbide metal matrix composites using centrifugal atomization

This paper describes the development of a new technique to produce metal matrix composites (MMCs) by injecting silicon carbide particles into molten aluminum just prior to centrifugal atomization. A centrifugal atomization apparatus has been constructed for this study. Silicon carbide particles are injected during atomization of 6061 aluminum alloy to form metal matrix composite powder. The prepared aluminum/silicon carbide powder contains 18 vol.% of SiC particles and 1.2 vol.% of voids. The particle grain size is almost independent from the particle size.     

In situ formation of SiC in Al–40Si alloy during high-pressure solidification

The in situ formation of SiC in Al–40Si alloys during the fabrication of SiC/Al–Si composites by high-pressure solidification were investigated. The results demonstrate that the in situ formation of SiC occurs by a gradual conversion of Al₄C₃ and Al₄SiC₄ to SiC. In situ SiC can be formed in an Al–40Si alloy solidified under a pressure of 3 GPa at a temperature of 1373 K. The SiC particles (SiC_p) formed in situ was compatible with the α-Al matrix and the Si phases. The relative density of the SiC/Al-38.6Si composite was 99.9%. The bending strengths of the Al–40Si alloy and the SiC/Al-38.6Si composite obtained by solidification under a pressure of 3 GPa were 200.8 MPa and 322 MPa, respectively, which represents an enhancement of 60.3% as a result of reinforcement by the in situ-formed SiC.     

Compressive strength and wear properties of SiC/Al6061 composites reinforced with high contents of SiC fabricated by pressure-assisted infiltration

Pressure-assisted infiltration was used to synthesize SiC/Al 6061 composites containing high weight percentages of SiC. A combination of PEG and glass water was used to fabricate SiC preforms and the effect of the presence of glass water on the microstructure and mechanical properties of the preforms was evaluated by performing compression tests on the preforms. Also, the compressive strength and the hardness of the SiC/Al composites were investigated. The results revealed that the glass water improved the compressive strength of the preforms by about five times. The microstructural characterization of the composites showed that the penetration of the aluminum melt into the preforms was completed and almost no porosity could be seen in the microstructures of the composites. Moreover, the composite containing 75 wt% SiC exhibited the highest compressive strength as well as the maximum hardness. The results of the wear tests showed that increasing the SiC content reduces the wear rate so that the Al-75 wt% SiC composite has a lower wear rate and a lower coefficient of friction than those of Al-67 wt% SiC composite. This indicated higher wear resistance in these composites than the Al alloy due to the formation of a tribological layer on the surface of the composites.     

Development and tribological studies of a novel metal-ceramic hybrid brake disc

Ceramic matrix composite (CMC) friction materials show promising tribological properties. Typically, carbon ceramic brake discs consist of a C/SiC rotor which is joined to a brake disc bell. Within this work, a novel metal-ceramic hybrid brake disc, consisting of C/SiC friction segments which are mounted by screws onto an aluminum carrier body, was designed and investigated. A prototype was built which was tribologically tested with three different brake pad materials, LowMet reference, modified SF C/SiC as well as C/C. A constant starting sliding velocity of 20 m/s and braking pressures of 1, 2, and 3 MPa were investigated. To simulate emergency braking conditions 10 consecutive brake applications were carried out in close succession for each brake pad material and braking pressure. The C/C brake pad material showed the highest average coefficient of friction followed by the LowMet and C/SiC material. However, the wear rates of the C/C and LowMet material were orders of magnitude higher compared to the C/SiC material.     

C/SiC composite-Ti6Al4V joints brazed with negative thermal expansion ZrP2WO12 nanoparticle reinforced AgCu alloy

Brazing C/SiC composites to Ti6Al4V alloy is associated with the problem of high residual stress inducing low joining strength. To overcome this problem, negative thermal expansion Zr₂P₂WO₁₂ (ZWP) nanoparticles were introduced into AgCu brazing alloy to obtain robust C/SiC-Ti6Al4V joints. Microstructures and mechanical properties of the joints brazed with different ZWP contents were investigated. Results indicated that 3 wt% ZWP nanoparticles dispersed homogeneously among brazing seam and compatible with brazing alloy. The width of reaction layer at C/SiC side was reduced sharply. Meanwhile, the finite element analysis showed that residual stress was reduced by 52.9 MPa and stress concentration among reaction layer was eliminated. The average shear strength of the joints brazed with AgCu + 3 wt% ZWP increased to 146.2 MPa, which was 70.8% higher than that of joints brazed without ZWP.     

Mechanical properties and wear behavior of Al5083 matrix composites reinforced with high amounts of SiC particles fabricated by combined stir casting and squeeze casting; A comparative study

Combined stir casting and squeeze casting processes were used to fabricate Al5083 matrix composites reinforced with 20, 25, and 30 wt% SiC_p. The microstructure, mechanical properties and wear behavior of the composites fabricated by combined stir casting and squeeze casting were compared with those fabricated by stir casting. The results revealed that the combined casting method improved the distribution of SiC particles through the reduction of the agglomeration of SiC particle and reduced the porosities of the samples from 2.32% to 1.29% in the sample containing 30 wt% SiC. These modifications led to the enhancement of mechanical properties i.e. increased the hardness to 85 BHN and the compressive strength to 350 MPa for the sample containing 30 wt% SiC fabricated by the combined casting method. In addition, the wear resistance of the samples fabricated by the combined casting method improved because of the reduced size of the wear debris as well as the smaller worn region. The dominant wear mechanism of all the composite samples fabricated by both methods was the delamination of the tribological layer while adhesion wear was dominant in the monolithic Al alloy.     

On the effects of particle size and preform porosity on the mechanical properties of reaction-bonded boron carbide infiltrated with Al-Si alloy at 950 °C

The infiltration of boron carbide preforms with Al alloys at relatively low temperature prevents the formation of the undesired Al₄C₃ phase. In the present study the effect of boron carbide powder particle size on the mechanical properties and phase composition of composites infiltrated with Al-20%Si alloys at 950 °C was investigated. According to XRD analysis, the infiltrated composites contain Al₈C₇B₄, AlB₂ and AlB₁₂, as well as non-reacted Al and Si that originated from solidification of Al-Si alloy. The presence of small amounts of SiC was noted in specimens fabricated from fine boron carbide powder. No evidence for the formation of non-desired aluminum carbide phase was obtained. Infiltration of ceramic preforms with virtually the same green density generated composites with an elastic modulus and bending strength that continuously decreased from 270 GPa and 405 MPa to 195 GPa and 345 MPa for powder with 90% particles close to 3 μm and powder with 90% particles close to 180 μm, respectively. These results ambiguously confirm that boron carbide particle size strongly affects mechanical properties of reaction-bonded composites infiltrated with Al-Si alloy at 950 °C and reflect the amount of newly formed ceramic phases appearing during infiltration and the presence of defects at the metal-ceramic interface.     

Effect of interface oxidation treatment on the interfacial reactions and wear properties of co-continuous SiC3D/Cu composites

Co-continuous SiC_3D/Cu composites are interesting wear-resistant materials with a unique structure. However, their interface reaction limits their practical applications. In this study, co-continuous SiC_3D/Cu composites were fabricated by pressureless infiltration, and their interfaces were modified with different interface oxidation states by high-temperature oxidisation and atmospheric treatment. The effects of the different interface oxidation treatments on the interfacial reaction and wear behaviour of the co-continuous composites were investigated. The results revealed that the surface oxidisation of the three-dimensional SiC skeleton by high-temperature oxidisation prevented the interface reaction between the SiC strengthening phase and the Cu alloy matrix phase, thus improving the interface bonding and wear properties of the composites. In addition, the surface oxidisation of the Cu alloy matrix phase prevented the occurrence of the interface reaction and improved the interface bonding of the composites. However, the Cu alloy was oxidised and the hardness of the matrix phase in the composites decreased when the infiltration atmosphere was changed into an air environment. Finally, the co-continuous SiC_3D/Cu composite with oxidised SiC and unoxidised Cu showed the best wear-resistance performance (approximately 9.77 × 10^-7 cm³·m^?1·N^?1).     

Surface engineering of SiCf/SiC composites by selective thermal removal

Surface engineering based on the Selective Thermal Removal (STR) of SiC fibers from SiC_f/SiC composites was used to obtain a brush‐like surface in view of joining processes. As observed through 3D confocal microscopy, the thermal treatment led to a selective removal of the surface fibers so that the specific surface increased. Wetting tests were performed using a Ag–Cu–Ti brazing alloy. The STR led to a negligible increase in the contact angles, which ranged from 16° to 21° for as‐received composites and increased to 28° for composites after STR. Microstructural observations showed that the brazing alloy perfectly adhered to the brush‐like surfaces were giving a mechanical interlocking expected to enhance the strength of the joint.     

Effect of Al addition on the single and cyclic ablation properties of C/C–SiC composites

This article focuses on the damage behavior and mechanism of aluminum addition on reactive melt infiltrated C/C–SiC composites in single and cyclic ablation environments. Plasma ablation tests were performed on C/C–SiC composites containing 20 wt % and 40 wt % aluminum respectively. Coupled with TMA, XRD, SEM and EDS, the results showed that composites with 40 wt % Al had better ablation resistance during the cyclic ablation, while the composites with 20 wt % Al had excellent ablation damage resistance during a single ablation. This difference was due to higher number of microcracks formed inside the composites containing 40 wt % Al than 20 wt % Al, the lower specimen surface temperature during ablation, and the thermal stresses can be released by pore crack expansion during gas reciprocal loading. While in the single continuous loading of gas, the 20 wt % Al composite formed a protective oxide layer with smaller pores and fewer gas and oxygen entry channels, resulting in good resistance to ablation.     

Oxidation behaviour of C/C–SiC brake discs tested on full-scale bench rig

C/C–SiC composites are considered to be strong candidates for the new generation of high-speed train brake discs. To achieve a better application, it is necessary to improve understanding of the oxidation behaviour of C/C–SiC brake discs after a full-scale bench test rig. In this study, full-scale braking bench tests for C/C–SiC self-mated brake pairs were conducted under a braking speed of 350–420 km/h and a braking pressure of 17–28 kN. Moreover, the oxidation behaviour and mechanisms of the C/C–SiC brake discs during the practical braking process were investigated. The results indicate that the oxidation behaviour is highly dependent on the friction surface region of the C/C–SiC brake disc owing to the distribution of microcracks, the formation of friction films, the difference in temperature, and the contact content with O₂. Specifically, the oxidation depths of the friction layer on the inner circumferential surface, middle friction surface, and outer circumferential surface were 278.3, 252.1, and 359.9 μm, respectively. Furthermore, the oxidation reaction preferentially occurs in the active area of the C fibre and pyrolytic carbon (PyC) during the braking process.     

Thermodynamic Brazing Alloy Design for Joining Silicon Carbide

Using solution thermodynamic theory, a Ni-Cr Si alloy, based on the Ni/Cr ratio of AWS BNi-5 (Ni-18Cr-19Si (atom%)), was designed for brazing SiC ceramics. The optimum thermodynamic composition was computed to be Ni-14.3Cr-36Si (atom%). Brazing experiments were conducted to assess the effect of changing the Si content away from this composition on the joint microstructures. For alloys containing less than 36 atom% Si, an excessively vigorous joining reaction occurred, resulting in the formation of a porous reaction zone at the brazing alloy/SiC interface, the amount of porosity decreasing as the brazing alloy composition approached the thermodynamically predicted optimum. It was found that the most likely mechanism for the formation of the porous zone was CO evolution during the SiC decomposition reaction. The best microstructures were attained for 40 atom% Si alloy joints, closely agreeing with the thermodynamic model, whereas higher Si content alloys exhibited localized debonding of the brazing alloy from the SiC.     

Studies on Fractures of Friction Stir Welded Al Matrix SiC-B<Subscript>4</Subscript>C Reinforced Metal Composites

Studies pertaining to joining of Al alloy metal matrix composites reinforced with B₄C and SiC by solid state friction stir welding (FSW) are presented in this paper. FSW tool dimensions are designed and fabricated to suit the weld sample dimensions and subsequently, the implications of the tool pin profile on the weldability is investigated. Through experimental recordings, the heat generated during the friction stir joining process of composites is estimated by developing relative equations. Maintaining the tool traverse speed constant, the rate of rotation and its effects on the tensile strength at the joints are investigated which reveals reduced ductility. The study emphasizes that when the speed is maintained between 100–400 mm/min, the tensile strength is at its optimal maximum while speeds higher or lower than the optimal range indicate detrimental effects on the tensile strength. This is followed by fracture studies on samples welding with varying traverse speed and rate of welding. Traverse speed appears to govern the fracture modes while brittle fracture is predominantly noticed indicating the importance of feeding optimal heat input during joining.     

碳/碳化硅复合材料刹车盘/片热应力场分析

采用有限元方法分析二维正交碳纤维增强碳化硅(C/SiC)复合材料制成的汽车刹车盘/片在刹车过程中引起的非线性热力耦合行为,主要研究在强制对流和热辐射作用下刹车结构的温度变化,讨论不同材料属性对刹车温度场的影响以及在温度场和膨胀系数耦合下C/SiC刹车盘/片中热应力和形变情况。数值结果表明:在双重散热条件下需要更多时间用于降温,而垂直于刹车面的热导率分量对温度传导或者降温影响较大;对于C/SiC刹车盘/片每一次刹车行为等效于一次热应力的加载和卸载,而每次产生的热应力可能突破C/SiC的极限弹性强度引起的残余塑性形变,而这种不断累积的残余效果继而引起C/SiC刹车盘/片失效。