Chemical vapour deposition (CVD) diamond coated tools performance in machining of PEEK composites

This paper presents a study about the chemical vapour deposition (CVD) diamond coated tool performance in machining unreinforced PEEK and composite PEEK CF30 (reinforced with 30% of carbon fibres).

The experimental procedure consisted of turning operations, during which cutting forces and surface roughness obtained in composite workpieces were measured.

The obtained results showed a best cutting performance for CVD diamond coated tool in machining PEEK composites, particularly in terms of cutting forces and power consumption, when compared with polycrystalline diamond (PCD) and cemented carbide (K10) cutting tools. This fact is very important due to the minor production costs of CVD diamond coated tools in comparison with PCD tools.     

Cutting forces and wear analysis of Si3N4 diamond coated tools in high speed machining

Si₃N₄ tools were coated with a thin diamond film using a Hot-Filament Chemical Vapour Deposition (HFCVD) reactor, in order to machining a grey cast iron. Wear behaviour of these tools in high speed machining was the main subject of this work. Turning tests were performed with a combination of cutting speeds of 500, 700 and 900 m min⁻¹, and feed rates of 0.1, 0.25 and 0.4 mm rot⁻¹, remaining constant the depth of cut of 1 mm. In order to evaluate the tool behaviour during the turning tests, cutting forces were analyzed being verified a significant increase with feed rate. Diamond film removal occurred for the most severe set of cutting parameters. It was also observed the adhesion of iron and manganese from the workpiece to the tool. Tests were performed on a CNC lathe provided with a 3-axis dynamometer. Results were collected and registered by homemade software. Tool wear analysis was achieved by a Scanning Electron Microscope (SEM) provided with an X-ray Energy Dispersive Spectroscopy (EDS) system. Surface analysis was performed by a profilometer.     

Effect of the microstructure of Si3N4 on the adhesion strength of TiN film on Si3N4

The effect of the microstructure of silicon nitride, which was used as a substrate, on the adhesion strength of physical vapor deposited TiN film on Si₃N₄ was investigated. Silicon nitride substrates with different microstructures were synthesized by controlling the size (fine or coarse), the phase ( or β) of starting Si₃N₄ powder, and sintering temperature. The microstructure of Si₃N₄ was characterized in terms of grain size, aspect ratio of the elongated grain, and β-to- phase ratio. For a given chemical composition but different mechanical properties, such as toughness, elastic modulus, and hardness of Si₃N₄ were obtained from the diverse microstructures. Hertzian indentation was used to estimate the yield properties of Si₃N_4, such as critical loads for yield (P_y) and for ring cracking (P_c). The effect of the microstructure of Si₃N₄ on adhesion strength evaluated by scratch test is discussed. TiN films on Si₃N₄ showed high adhesion strengths in the range of 80–140 N. Hardness and the P_y of Si₃N₄ substrate were the primary parameters influencing the adhesion strength of TiN film. In TiN coating on Si₃N₄, substrates with finer grain sizes and higher phase ratios, which show high hardness and high P_y, were suitable for higher adhesion strength of TiN film.     

Dense and near-net-shape fabrication of Si3N4 ceramics

With silicon nitride significant progress has been made in order to search for fully dense, strong, reliable structural ceramics to find wide use in applications at high temperatures which are allowing new and innovative solutions to component design problems. Taking into account that more and more ceramic components based on Si₃N₄ are being used in the aerospace and automobile industries, it is a great challenge to fabricate such complex-shaped components with high reliability and with defect-free microstructures such as pores, inclusions or any other inhomogeneity at acceptable costs. On the other side, the high hardness of Si₃N₄ ceramics is almost always cost prohibitive to shape components by hard machining. It is therefore great effort exhibited in the development of near-net-shape fabrication processes that can produce complex-shaped components with a minimum of machining as well as to minimize the number and size of microstructural defects within design limits. In this review, the fabrication of near-net-shape Si₃N₄ ceramics is given in detail. All kinds of these techniques (injection molding, gelcasting, robocasting, mold shape deposition, rapid prototyping) and their advantages and disadvantages are explained.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

Thermal stability and crystallization kinetics of sputtered amorphous Si3N4 films

The crystallization of thin silicon nitride (Si₃N₄) films deposited on polycrystalline SiC substrates was investigated by X-ray diffractometry as a function of annealing time. The amorphous Si₃N₄ films were produced by means of reactive r.f. magnetron sputtering. Annealing at temperatures between 1300 and 1700 °C led to the formation of crystalline films composed of -Si₃N₄ and β-Si₃N₄. The fraction of β-Si₃N₄ in the films reaches approximately 40% at temperatures above 1550 °C. Both polymorphic modifications were formed simultaneously during the crystallization process. A transformation of -Si₃N₄ to β-Si₃N₄ could not be observed in the time and temperature range investigated. The crystallization process of amorphous Si₃N₄ can be described according to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism, assuming a three-dimensional, interface controlled grain growth from pre-existing nuclei. The rate constants show an Arrhenius behaviour with an activation enthalpy of approximately 5.5 eV.     

Influence of the surface constitution on the as-sintered strength of Si3N4 micro specimens

In order to study the significance of as-sintered surfaces for the strength of Si₃N₄ micro components, micro bending specimens with dimensions of 240 μm × 240 μm × 1400 μm were prepared from sintered reaction-bonded silicon nitride. The processing parameters sintering temperature and dwell time were varied, and two different types of powder bed were applied during sintering. The characteristic strength σ₀ - determined by micro-3-point-bending tests - varied from about 500 to 1200 MPa. Strength values of about 1000 MPa and higher were observed when powder beds were applied which were newly prepared and doped with the sintering additives Y₂O₃, Al₂O₃, and MgO. In that case, the mass loss during sintering was negligible or even a small mass gain occurred. Samples sintered in used and undoped powder beds by contrast showed mass loss and an enrichment of the secondary phase YN-melilite (Y₂Si₃O₃N₄) at the surface; for these samples σ₀ was found to be restricted to about 500-700 MPa.Contrary to expectations, it was observed that the residual porosity - ranging from less than 1% up to 11% - does not act as a predominant strength-determining factor. Instead, the more or less pronounced formation of surface-near defects, due to the decomposition and volatilization of Si₃N₄ during sintering, is considered to be decisive for the achievable strength. Mass loss and YN-melilite formation are indicators for defect creating reactions. Due to the increased surface-to-volume ratio of micro components, the affected surface zone in particular determines the overall mechanical properties.     

A simulation of the reaction bonding of Si3N4 to generate porous microstructures

Summary A stochastic numerical technique is presented that has been used to study the formation of reaction-bonded porous silicon nitride (RBSN). Like many reaction-bonded materials, RBSN is a structural ceramic that has a naturally occurring porosity of approximately 25% resulting from the unique kinetics of formation. Since such porosity is usually unavoidable, it is of interest to discover its relevant measures, such as the size distribution and degree of homogeneity of pores, and the factors that govern the formation of these pores, in order to understand and optimize the crack propagation resistance of such materials. The probing of the fracture toughness of the generated microstructures by computational means is described in the companion paper [E G. Haubensak, V V Bulatov and A.S. Argon, J. Comput.-Aided Mater. Design, 2 (1995) 205]. The primary purpose of the simulated reaction was the generation of realistic microstructures incorporating the major features of reaction bonding. In this manner it is possible to generate objective, systematic variations of similar microstructures. With such microstructures, crack nucleation and propagation mechanisms can be observed, and processing parameters, which control fracture toughness, can be identified. In the present study, we outline the details of the reaction kinetics model, demonstrate its ability to generate realistic microstructures, and report some additional observations on some topological phenomena resulting from the reaction, such as those controlling the formation of large pores in the final structure. In yet a separate communication of a related experimental study we present the microstructures of the actual RBSN. We will use the findings of the latter study here for making some comparisons.     

基于TiC_(0.5)N_(0.5)/Si_3N_4复相陶瓷基体的PVD及CVD涂层刀具性能

首先,以15vol%或25vol%的TiC0.5N0.5粉体为导电第二相,利用热压烧结法制备了TiC0.5N0.5/Si3N4复相陶瓷;然后,分别通过物理气相沉积(PVD)和化学气相沉积(CVD)技术在TiC0.5N0.5/Si3N4陶瓷刀具表面沉积了CrAlN和TiN/Al2O3/TiN涂层;最后,通过对TiC0.5N0.5/Si3N4刀具进行连续切削灰铸铁实验,研究了TiC0.5N0.5含量和涂层类型对刀具磨损特征的影响,并探讨了刀具的磨损机制。结果表明:TiC0.5N0.5含量的增加有利于提高TiC0.5N0.5/Si3N4复相陶瓷刀具基体的硬度和电导率,但对耐磨性和切削寿命的影响较小;采用PVD技术沉积CrAlN涂层时,随着TiC0.5N0.5含量的增加,涂层的厚度、结合强度和硬度都得到提高,涂层刀具的磨损性能显著提高,切削寿命也明显延长;而采用CVD技术沉积TiN/Al2O3/TiN涂层时,TiC0.5N0.5含量的变化对涂层的厚度、结合强度和硬度基本没有影响,TiN/Al2O3/TiN涂层刀具整体切削性能变化不大。CrAlN涂层和TiN/Al2O3/TiN涂层都可明显改善TiC0.5N0.5/Si3N4复相陶瓷刀具的耐磨性和切削寿命;相对于TiN/Al2O3/TiN涂层,CrAlN涂层具有更高的涂层硬度和粘着强度,但TiN/Al2O3/TiN涂层具有较大的涂层厚度,TiN/Al2O3/TiN涂层刀具表现出更加优异的耐磨性和切削寿命。TiC0.5N0.5/Si3N4复相陶瓷刀具的磨损机制以机械摩擦导致的磨粒磨损为主,伴随有少量的粘结磨损。     

Mass spectrometric study of gas-phase chemistry in the hot-wire CVD processes of SiH4/NH3 mixtures

The gas-phase reaction products of SiH₄, NH₃ and their mixtures from a hot-wire CVD chamber were investigated using laser ionization time-of-flight mass spectrometry. Both vacuum ultraviolet laser single photon ionization and laser-induced electron impact ionization were used. The main products observed from a 50% NH₃/He sample were H₂ and N₂. The study of an NH₃/SiH₄ mixture (P_NH3:P_SiH4 = 100:1) has shown that the NH₃ dissociation on the filament was suppressed by the presence of SiH₄ in the system. Signals from Si(NH₂)₄ and Si(NH₂)₃ species were identified as products from the 100:1 NH₃/SiH₄ mixture. The spectrum for a 1:1 NH₃/SiH₄ mixture was dominated by mass peaks characteristic of SiH₄ chemistry in the reactor, i.e. H₂, Si₂H₆, and Si₃H₈, at low temperatures. The extent to which the decomposition of NH₃ is suppressed is enhanced with more SiH₄ molecules in the system.     

Improvement in light output intensity of InGaN/GaN multiple-quantum-well blue light-emitting diode by SiO2/Si3N4 distributed Bragg reflectors and silver back mirror

In this article, the light output intensity of InGaN/GaN multiple-quantum-well (MQW) light emitting diodes (LEDs) is improved by using SiO₂/Si₃N₄ distributed Bragg reflectors (DBRs) as window layer and Ag back mirror. The SiO₂/Si₃N₄ DBRs can take several advantages, such as high reflectance with less number of DBR, passive characteristics, and high reliability due to growth in one pump down growth system. The experimental results indicated that InGaN/GaN LEDs with the 3-pair of SiO₂/Si₃N₄ DBRs show a maximum light output intensity of 64 mcd under 20 mA driving current and an improvement of 42% as compared to that of InGaN/GaN LEDs without SiO₂/Si₃N₄ DBRs. In addition, the turn-on voltage, forward resistance, and full width at half maximum (FWHM) of the emission spectra for InGaN/GaN LEDs with the 3-pair of SiO₂/Si₃N₄ DBRs and Ag back mirror are 3.23 V, 16 Ω, and 22.4 nm under 20 mA forward current.     

Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.     

Study on particle-stabilized Si3N4 ceramic foams

The stability of bubbles and the microstructures of sintered Si₃N₄ ceramic foams produced by direct foaming method were investigated. The bubbles produced by short-chain amphiphiles (propyl gallate) have higher stability as compared with that produced by long-chain surfactants (TritonX-114). Si₃N₄ ceramic foams using short-chain amphiphile are particle-stabilized one, the pore cells are spherical and closed, and cell surfaces are smooth and dense. The pore cells of sintered Si₃N₄ ceramic foams using TritonX-114 foaming are coarse and large, and pore cells are polyhedral. High gas-pressure sintering is conducive to the development of the whisker-like microstructures in Si₃N₄ ceramic foams. The sintered Si₃N₄ ceramic foams with the whisker-like microstructure are quite promising for improving the mechanical strength of the ceramics by a simple and safe way.     

Properties of polyimide/Al2O3 and Si3N4 deposited thin films

Polyimide (PI) nanocomposites with different proportions of Al₂O₃ were prepared via two-step reaction. Silicon nitride (Si₃N₄) was deposited on PI composite films by a RF magnetron sputtering system and used as a gas barrier to investigate the water vapor transmission rate (WVTR). The thermal stability and mechanical properties of a pure PI film can be improved obviously by adding adequate content of Al₂O₃. At lower sputtering pressure (4 mTorr), the PI/Al₂O₃ hybrid film deposited with Si₃N₄ barrier film exhibits denser structure and lower root mean square (RMS) surface roughness (0.494 nm) as well as performs better in preventing the transmission of water vapor. The lowest WVTR value was obtained from the sample, 4 wt.%Al₂O₃-PI hybrid film deposited with Si₃N₄ barrier film with the thickness of 100 nm, before and after bending test. The interface bonding, Al-N and Al-O-Si, was confirmed with the XPS composition-depth profile.     

Microstructure and hardening mechanisms in a-Si3N4/nc-TiN nanostructured multilayers

Amorphous/nanocrystalline Si₃N₄/TiN nanostructured multilayer films were fabricated by radio-frequency reactive magnetron sputtering. The microstructure and properties of these films were measured using an X-ray diffractometer, X-ray photoelectron spectroscope, high-resolution transmission electron microscopy and nanoindenter. The superhardness effect was found in Si₃N₄/TiN multilayers. The hardness of Si₃N₄/TiN multilayers is affected not only by modulation periods, but also by layer thickness ratio and deposition temperature. The hardness value is about 40% higher than the value calculated from the rule of mixtures at a deposition temperature of 500 °C and a layer thickness ratio (l_Si3N4/l_TiN) of 3/1. The hardening mechanisms in this system are discussed in the light of our experimental results. Results of calculation of the theoretical stress distribution in the multilayers suggests that alternating stress fields caused by thermal mismatching between Si₃N₄ and TiN is one of the main reasons for the superhardness effect observed in Si₃N₄/TiN multilayers.     

Hydrogen etching of Si3N4 layers with plasma assisted hot wire CVD

In order to develop sustainable processes for clean manufacturing environment for thin film or other solar cell production, we studied the hydrogen etching of silicon nitride (Si₃N₄) films on flat crystalline silicon (c-Silicon) substrates. With an arrangement primarily constructed for hot wire CVD (HWCVD) deposition of thin silicon films also cleaning processes with atomic hydrogen were studied with a simplified three wire assembly. The three filaments could be biased independently by different potential. A variation of hydrogen pressure and flow was performed to find out conditions of high etching rates for the Si₃N₄ layers. The etching rate was simply determined by measuring the time for total removal of the film, since this could be easily detected by the change of the anti-reflection property. Etching rates of 0.1 nm/s have been obtained under 15 Pa and a flow of 50 sccm. An intensive study was carried out of the direct current (DC) plasma hot wire CVD conditions.     

氮化硅层状陶瓷界面性能对力学性能的影响

通过加入Si₃N₄调节隔离层的性质, 研究了Si₃N₄/ BN 层状复合材料在不同界面结合情况下的力学性能变化规律。结果表明, 随着隔离层中Si₃N₄含量的提高, 基体片层之间的结合力不断增强。在结合力很强时, 层状材料具有与块状材料相同的破坏方式, 强度出现极值, 达到1000M Pa以上; 在结合力较弱时, 隔离层能够反复偏折裂纹, 层状材料表现出高达20M Pa?m1/2 的表观断裂韧性和约600M Pa 的强度。      

旋转超声磨削Si3 N4陶瓷用金刚石磨具磨损行为研究

热压烧结Si₃N₄陶瓷材料常应用于航天飞行器中关键耐高温零部件,但由于高硬度和低断裂韧性,其加工效率和加工表面质量难以满足制造需求。为了提高热压烧结Si₃N₄陶瓷旋转超声磨削加工质量,减小由于金刚石磨具磨损带来的加工误差,开展了磨具磨损行为研究。基于热压烧结Si₃N₄陶瓷旋转超声磨削加工实验,分析了金刚石磨具磨损形式;基于回归分析建立了金刚石磨具磨损量数学模型,揭示了加工参数及磨具参数与金刚石磨具磨损量间映射关系;并研究了磨损形式与磨具磨损量及加工表面粗糙度影响规律。结果表明：磨粒磨耗是旋转超声磨削Si₃N₄陶瓷用金刚石磨具最主要磨损形式,比例超过50%;主轴转速和磨粒粒度对磨具磨损量影响最为显著;且磨损量较小时,加工表面粗糙度值反而增加。以上研究可为提高旋转超声磨削Si₃N₄陶瓷加工精度和加工质量提供指导。     

Effect of holding time on microstructure and mechanical properties of Si3N4/Si3N4 joints brazed with Au58.7Ni36.5V4.8 filler alloy

Si₃N₄ ceramics were brazed using Au–Ni–V metal foils at 1423 K for different holding times. Effect of holding time on microstructure and mechanical properties of the joints was investigated. The results indicate that a reaction layer of VN exists at the interface between Si₃N₄ ceramic and filler alloy. With increasing holding time from 0 to 90 min, thickness of the VN reaction layer increases from 0.4 to 2.8 μm, obeying a linear relation. Mechanism of the interfacial reaction was discussed by calculating the formation of free energy of VN. No specific orientation relationship exists between VN reaction layer and Si₃N₄ ceramic. In addition, Ni₃Si intermetallic compound appears in the joint when the holding time increases to 90 min, resulting in the deterioration of the joint strength.     

Synthesis and characterization of C3N4 nanowires and pseudocubic C3N4 polycrystalline nanoparticles

C₃N₄ nanowires and pseudocubic C₃N₄ polycrystalline nanoparticles have been synthesized by the reaction between C₃N₃Cl₃ and NaN₃ with Zn powder as catalyst. The process was carried out using a constant-pressure benzene thermal method at 40 MPa and 220 °C. The prepared nanowires have a diameter range of 3-6 nm and length range of 100-200 nm, while the diameters of the nanoparticles range from 10 nm to 40 nm. The as-prepared samples were characterized by X-ray powder diffraction (XRD), Fourier transform spectroscopy (FTIR), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS).