Effect of laser parameters and atmosphere in the structuring of aluminum nitride

The characteristics and formation mechanism of V-shaped groove formed by laser-activated metallization of aluminum nitride (AlN) ceramic under different shielding gas environments of air, nitrogen, and argon are investigated, using a novel analysis way which is based on the intensity distribution of the focused laser beam. It is found that the width of the V-shaped groove is slightly different under different gas environments, and the depth of the V-shaped groove is nearly the same, which means that the energy thresholds required for laser decomposition of AlN ceramic are different in various gas environments: air > nitrogen > argon, and the energy required for laser etching AlN ceramic is the same. It is also found that the nonlinearity in the curves of depth and width versus different machining parameters, which are explained in detail.     

Influence of oxygen on the joining between copper and aluminium nitride

Aluminium nitride has been developed for electronic ceramic packaging applications because of its high thermal conductivity and high electrical resistivity. To improve the heat dissipation at the metal/ceramic interface, a high quality bonding between the substrate and the copper conductor is needed. This process requires a previous step of AIN pre-oxidation of the substrates by oxygen gas at 1200 °C, in order to form a thin layer of Al₂O₃ at the surface of AlN. The junction between Cu and the substrate is carried out at 1075 °C in controlled oxygen atmosphere which promote the oxidation of the copper and the formation of an eutectic phase which can form a strong junction with AlN via the layer of Al₂O₃. The goal of the current work is to study the influence of oxygen supplied by gaseous phase to form the exact amount of the eutectic phase needed to get a strong junction. First, in order to fix conditions for joining, the wetting behaviour of copper has been studied using the sessile drop method. The influence of oxygen brought by surrounding gas is given in terms of wettability of the liquid, interfacial tension and chemical reactions. According to previous results, copper foils and copper cylinders have been directly joined to AIN substrates. Interfacial reactions, mechanical and thermal properties have been investigated.     

Ultrasonic-assisted wetting and soldering of AlN ceramic by using a nonactive solder (Sn9Zn) in air

AlN ceramic was successfully wetted and then joined with nonactive Sn9Zn eutectic solder assisted by ultrasonication in air. The effect of ultrasonic time on the formation of joint was studied. Results indicated that the defect-free joint can be obtained at an ultrasonic time of 5 s. Two regions, namely, AlN/Sn (s,_s) and AlN/Zn (s,_s), were found in the bonding interface. Zn and O accumulated in the AlN/Sn (s,_s) interface. An amorphous and nanocrystalline layer of ZnO formed in the hard-wet AlN surface. And Zn (s,_s) directly bonded with AlN. The low temperature and fast bonding of the AlN was attributed to the high pressure and temperature caused by cavitation effect. The shear strength of the joint increased from 10.6 MPa to 30.7 MPa when the ultrasonic treatment time increased from 5 s to 150 s. With the prolongation of ultrasonic time, more AlN ceramic particles entered the solder and acted as the reinforcing phase.     

Improvement for Ti3SiC2/Cu joint brazed using composite fillers with abnormal expansion ceramic particulates

Reducing the residual stresses and improving the mechanical strength of large-scale ceramic/metal brazing joints is an important problem that must be solved for its practical engineering application. Using composite filler with solid-state phase transformation ceramic particulates, it is theoretically feasible to relieve the residual stress and improve the mechanical properties of ceramic/metal brazed joints. In this study, Cu mesh, Ag–28Cu–2Ti (wt.%), and yttria-stabilized zirconia (0.6 mol.% YSZ solid-state phase transformation ceramic particulates) composite power fillers were used in the brazing of Ti₃SiC₂ ceramic and pure copper. The microstructure of joints and YSZ particulates in the interface was investigated and confirmed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (STEM), and transmission electron microscopy (TEM). In addition, the effect of YSZ particulates content on the mechanical properties of joints was investigated and evaluated by the shear strength. The results show that the interfacial phases were mainly Ti₅Si₃, TiC, Ti_xCu, Ag (s, s), Cu (s, s), and YSZ particulates. Moreover, most of YSZ particulates undergo the solid-state phase transformation from tetragonal zirconia (t-ZrO₂) to monoclinic zirconia (m-ZrO₂) during the cooling process of brazing. The abnormal volume expansion of the solid-state phase transformation reduced the thermal mismatch between Ti₃SiC₂ ceramic and filler, thereby reducing the residual stress in the interface of joint. When using composite filler with 6 wt.% YSZ particulates, the shear strength of Ti₃SiC₂/Cu joint reached the maximum. The maximum average shear strength of the joints was 80.2 MPa, which was about 103.6% more than the joint without YSZ particulates.     

Mechanical performances of AlN/Al metallized ceramic substrates fabricated by transient liquid phase bonding and pre-oxidation treatment

For the miniaturization of high-power electronic components, AlN/Al is a promising metallized ceramic substrate due to its superior mechanical and thermal performances. Numerous bonding processes have been proposed for fabricating the metallized ceramic substrate. Unfortunately, the influences of various bonding techniques on the mechanical performance of AlN/Al metallized ceramic substrate remain undetermined to date. The objective of this study was thus to investigate the effects of the transient liquid phase (TLP) technique and pre-oxidation treatment on the bonding, microstructure, and mechanical strength of the AlN/Al metallized ceramic substrate.The results indicated that the three-layered AlN/Al/AlN specimen could be effectively bonded by the TLP process and pre-oxidation treatment. However, the bending strengths of the specimens fabricated by the two techniques were obviously divergent. The bending strength of raw AlN substrate was 333 MPa. In contrast, the bending strengths of the three-layered specimens with AlN substrates pre-oxidized at 1050 °C, 1150 °C, and 1250 °C were 292 MPa, 250 MPa, and 224 MPa, respectively. Raising the pre-oxidation temperature of the AlN substrate from 1050 °C to 1250 °C obviously increased the thickness of the Al₂O₃ layer and deteriorated the bending strength, for the fracture propagated along the Al₂O₃ layer and the Al₂O₃/AlN interface. For the TLP bonding, the Cu film deposited on the AlN substrate contributed to the generation of Al–Cu transient liquid and to bonding. The bending strength of the three-layered specimens fabricated by TLP at 650 °C was 417 MPa, which was 25% and 43% better than those of the raw AlN substrate and the three-layered specimens prepared by the pre-oxidation treatment, respectively.     

Onset of selective laser flash sintering of AlN

Flash sintering uses a combination of heating and electric fields to rapidly densify ceramics. Previously, it has been shown that a scanning laser can be used to initiate flash sintering in localized regions on an yttria-stabilized zirconia (YSZ) sample in a process known as selective laser flash sintering (SLFS). In this work, we show using a combination of measurements of electric current flowing through the sample and observations of necks formed between powder particles that aluminum nitride (AlN) can also undergo SLFS. Scan conditions required to initiate SLFS are characterized over a range of laser powers and laser scan speeds in a dry nitrogen environment. It is shown that initiation of SLFS in AlN is governed by both the local input energy density per scan and heat dissipation and a numerical model is developed to predict temperatures during SLFS. Assuming the minimum temperature along the conductive path determines the onset of SLFS, the minimum temperature and time required is 450–670 K in 2–0.25 s for the pressed AlN pellets used in this study for laser scan speeds of 33–300 m/s, laser powers of 10–30 W, and an applied electric field of 3000 V/cm.     

Effects of doping Al-metal powder on thermal,mechanical and dielectric properties of AlN ceramics

In this work, the influence of Al-metal powder addition upon that thermal, mechanical and dielectric properties of aluminium nitride (AlN) ceramic was studied. The findings show that adding Al-metal powder improves not only the mechanical and thermal properties of the AlN ceramic but also has no negative impact on its dielectric properties. Based on Y₂O₃ as sintering aid, the AlN ceramic with 1.0 wt% Al doping were 14.35% higher thermal conductivity, 11.73% higher flexural strength and 59.50% higher fracture toughness than those doped without Al, respectively. This study showed that the addition of Al-metal powder may favor the purifying of the AlN lattice and the formation of homogenous and isolated second phase, which would increase the AlN–AlN interfaces and improve the thermal conductivity. Furthermore, the grain boundaries of AlN ceramics might be strengthened by the isolated second phases due to the thermal mismatch between the second phases and AlN grains, thus strengthening and toughening the AlN ceramic doped with Al. However, the large additive amount of Al powder (>1.0 wt%) was not help the isolation and homogenization of the second phase, giving a deterioration in an AlN ceramic's mechanical and thermal properties. These results suggest that the introduction of an appropriate dose of aluminium metal powder is a simple method that can be used to improve the AlN ceramic's mechanical and thermal properties simultaneously.     

Fast joining of 8YSZ to NiCrFe medium-entropy alloy by using an electric field

In this study, yttria-stabilized zirconia (8YSZ, 8 mol%) was flash joined to Ni-Cr-Fe medium-entropy alloy (MEA) within seconds under an electric field (E-field), where the current density was above the threshold (about 100 mA·mm⁻²) at 750–1000 °C. The maximum joint strength of 103 MPa was achieved at joining temperature of 900 °C under current density of 100 mA·mm⁻² for 30 s. The impedance spectrum of the joined sample revealed that there was no impedance barrier to charge transfer between the metal electrode and the ceramic, indicating that the interface was well bonded. Scanning electron microscopy and transmission electron microscopy results show that the joint was formed mainly due to the fast diffusion of Ni into 8YSZ and reduced Zr into MEA. The diffusion of these elements does not change the crystal structure of the ceramic and metal, and leads to only slight increase in the lattice spacing of MEA.     

Cu/LaCrO3 joining by local melt infiltration through laser cladding

Direct bonding of copper and porous LaCrO₃ without an extra filler interlayer was successfully completed using local and fast Cu‐infiltration through laser cladding. This significantly reduced the susceptibility of the ceramic to cracking. A high‐speed camera investigation into the wetting and infiltration behavior of a Cu‐melt into LaCrO₃ with a porosity of ~63 vol% was performed. By adjusting the focal distance with a constant laser power of 300 W, the Cu‐melt was rapidly infiltrated into the ceramic preform in 10 seconds. This was completed under atmospheric air conditions, without added inert gas. The joining process developed can be used to fabricate ceramic/metal joints with targeted (micro‐) structure properties by adjusting the infiltrated melt and the infiltration depth, which would be suitable for many applications, such as multifunctional devices, solid oxide fuel cells or heating elements.     

Microstructures and mechanical property of ZrCx joints diffusion bonded using Ti/Ta/Ti as the interlayer

In present study, homogeneous joint of ZrC_x ceramic was achieved by diffusion bonding using Ti/Ta/Ti as the interlayer. The effect of bonding temperature and time on the microstructure and mechanical property of the joints was uncovered. The homogeneous joints can be formed at 1400 °C for 2 h and at 1500 °C for 1 h, respectively. The Ti/Ta/Ti interlayer prefers to form Ti-Ta solid solutions rather than to form carbides with ZrC_x ceramic during the bonding process, which is more readily to be dissolved with the base ceramic, thus contributing to the formation of the homogeneous joints. The mechanical property of the homogeneous joints can be comparable to that of the base ceramics due to the similar composition of the joint with the base ceramics. The unique microstructure feature and mechanical property of the homogeneous joints illustrate the great potential of our method for joining transition metal carbides.     

金属基耐高温陶瓷涂层抗热冲击性能的研究

为提高金属基陶瓷涂层的抗热冲击性能，以无机胶粘剂磷酸二氢铝、耐磨陶瓷骨料氧化铝、碳化硅和氧化镁混合后涂覆于金属表面制得陶瓷涂层。通过交替加热及冷却试验测试该陶瓷涂层的抗热冲击性能，并与其他人的研究数据进行比较。所得涂层抗热冲击次数超过10次，超过了其他人的实验数据，这是由于涂层与基体在界面处相互扩散形成过渡层。另外，宏观上的机械联锁有利于提高涂层与基体在界面处的结合，从而提高了其抗热冲击性能。     

Lap joint formed by friction stir spot welding between SiC and magnesium alloy containing aluminum

A lap joint between plates of SiC and AZX612-magnesium alloy containing aluminum was formed by friction stir spot welding using a drilling machine. The joint interface was analyzed by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy. Aluminum from the AZX612-magnesium alloy segregated along the joint interface suggesting that SiC and AZX612-magnesium alloy could be strongly joined by friction stir spot welding due to the formation of aluminum compounds. SiC and A1050-aluminum alloy plates were joined by friction stir spot welding and a tensile-shear test was performed. At 245 N, the lap joint fractured in the SiC matrix rather than the joint interface. These results confirmed that SiC and A1050-aluminum can be strongly joined due to the formation of aluminum compounds.     

氮化铝陶瓷覆铜基板的研制

通过高温热氧化的方法,在ＡＩＮ陶表面形成一薄层ＡＩ２Ｏ３作为过渡层,成功地将铜与ＡＩＮ陶瓷键合在一起,研制出性能优越的ＡＩＮ陶瓷覆铜基板。研究了ＡＩＮ热氧化时间及温度对键合质量的影响,提出子较佳的儿得的键合力可达１１０Ｎ／ｃｍ,同时,运用扫描电镜（ＳＥＭ０、电子能谱（ＥＤＸ）对键合结构作了分析和研究。ＡＩＮ衬底上的氧化物相对键合过程中上重要作用。     

Factors governing interfacial reactions in liquid metal/non-oxide ceramic systems: Ni-based alloy–Ti/sintered AlN system

Wetting of ceramics by liquid metals is often promoted using alloying elements which form at the interface, by reaction with the ceramic, continuous layers of a better wetted compound. These layers can, in turn, improve or be detrimental to the mechanical performance of the interface, depending on their microstructure and thickness.The aim of this investigation is to determine the factors governing the growth kinetics in metal/non-oxide ceramic systems in which strong reactivity is often observed. The study system consists of a predominantly covalent ceramic, AlN, and a Ni-based liquid alloy containing Ti. Experiments are performed by varying the temperature, Ti content of the alloy and level of vacuum in the furnace. Point experiments were also carried out for a Au based alloy–Ti/AlN and AgZr/AlN couples.     

The effect of boron nitride nanosheets on the mechanical and thermal properties of aluminum nitride ceramics

The boron nitride nanosheets (BNNSs)/aluminum nitride (AlN) composites were prepared by hot press sintering at 1600°C. The microstructure, mechanical properties, and thermal conductivity of the samples were measured, and the effect of adding BNNSs to AlN ceramics on the properties was studied. It is found that the addition of BNNSs can effectively improve the mechanical properties of AlN. When the additional amount is 1 wt%, the bending strength of the sample reaches the maximum value of 456.6 MPa, which is 23.1% higher than that of the AlN sample without BNNSs. The fracture toughness of the sample is 4.47 MPa m^1/2, a 68.7% improvement over the sample without BNNSs. The composites obtained in the experiment have brilliant mechanical properties.     

Aluminum nitride‐single walled carbon nanotube nanocomposite with superior electrical and thermal conductivities

Development of aluminum nitride (AlN)‐single walled carbon nanotube (SWCNT) ceramic‐matrix composite containing 1‐6 vol% SWCNT by hot pressing has been reported in this article. The composites containing 6 vol% SWCNT are dense (~99% relative density) and show high dc electrical conductivity (200 Sm^?1) and thermal conductivity (62 Wm^?1K^?1) at room temperature. SWCNTs contain mostly metallic variety tubes obtained by controlled processing of the pristine tubes before incorporation into the ceramic matrix. Raman spectroscopy and field emission scanning electron microscopy (FESEM) of the fracture surface of the samples show the excellent survivability of the SWCNTs even after high‐temperature hot pressing. The results indicate the possibility of preparation of AlN nanocomposite for use in plasma devices and electromagnetic shielding.     

Research of laser remelting on the thermal-mechanical behaviors and heat treatment of yttria-stabilized zirconia coatings

In this study, the thermal and mechanical behaviors were investigated by simulating laser remelting of atmospheric plasma-sprayed yttria-stabilized zirconia coatings, and the molten depth and regions of stress concentration were compared between simulation and experiment. The heat  treatment process of the remelted coating was also simulated. The crack formation mechanism in the YSZ coating remelted by laser and the heat-treatment effect on residual stress were investigated. Results showed that the simulated results were consistent with the experimental measurements, and the residual thermal stress was the main cause of cracks formation. The coating remelted by a laser power of 1500 W and a scanning rate of 9 mm/s possessed less residual concentrated stress and segmented cracks. Heat treatment released concentrated stress, which was still accurate for the ceramic coating. If the coatings were slowly heated to demonstrate heat treatment after laser remelting, the cracks in the remelted layer decreased correspondingly.     

Controlled anisotropy in 3D printing of silica-based ceramic cores through oxidization reaction of aluminum powders

Ceramic cores are key components to form inner hollow structures in aero-engine blades, and 3D printing is an ideal molding technology for ceramic cores. In this work, silica-based ceramic cores are fabricate via 3D printing of digital light processing (DLP) stereolithography, and the anisotropy in microstructure and property are controlled by aluminum powders. The ceramic cores without aluminum powders exhibit anisotropic microstructure with interlayer gaps, which get narrower and disappear with doping of 7.5–10 wt% of aluminum powders, due to the volume expansion during oxidization reaction of aluminum powders filling the interlayer gaps. The anisotropy in mechanical property is rely on the printing direction, and the ratio of strength in different directions (σ_V/σ_H) is put forward to value the mechanical anisotropy; the ratios rise from 0.40 to 0.92 at room temperature and 0.51 to 0.97 at 1540 °C, as 7.5 wt% of aluminum is doped, and the optimized ceramic cores show high-temperature strengths of 16.6 MPa and 16.1 MPa in different printing directions. Even though ceramic cores with 10 wt% of aluminum show uniform microstructure and higher σ_V/σ_H ratio, the weak particle bonding within printing layers limits their mechanical property, and the strengths decrease to 13.8 MPa and 13.4 MPa at 1540 °C. This work inspires a new technique to excellent high-temperature mechanical properties with anisotropy control in 3D printing of ceramic cores.     

Addition of h-BN for enhanced machinability and high mechanical strength of AlN/Mo composites

Ceramic/metal composites, as high-temperature structural materials, have limited applications because of their poor machinability. AlN/Mo/Mo₂B machinable composites were fabricated by adding h-BN to AlN/Mo composites by plasma-activated sintering at various temperatures and at an axial pressure of 30 MPa under argon atmosphere. The sample sintered at 1500 °C exhibited excellent machinability for cemented carbide tools, with a bending strength of approximately 500 MPa and a Vickers hardness value of 9.34 GPa. The effects of sintering temperature on the microstructure, mechanical properties, and machinability were investigated, and the reactions introduced by adding h-BN were studied. These reactions reinforced the composite and newly introduced a Mo₂B phase, which showed better ability to deflect cracks and prevent them from moving deep inside the composite, thus improving its machinability. This research provides a possible solution for improving the machinability of ceramic/metal composites without sacrificing the strength.