Nano-tribological properties of topographically undulated nanocrystalline diamond patterns

Surface roughness-controlled nanocrystalline diamond film was fabricated as an undulated line and space pattern on a silicon oxide surface. To simulate a MEMS (Micro-/Electro-Mechanical System) and NEMS (Nano-/Electro-Mechanical System) patterned surface, 800 nm and 1 microm wide lines with a 200 nm wide space pattern were prepared on the substrate using E-beam lithography and an ESAND (Electrostatic Self-assembly of NanoDiamond) seeding layer lift-off process. Through this process, an undulated pattern of a nanocrystalline CVD diamond successfully formed by a conventional micro crystalline diamond growth system. The roughness of the deposited surface was controlled by regulating the size of the seeding nanodiamond particles. Crushing of the nanodiamond aggregates and dispersion of the nanodiamond solution was performed in an attrition milling system. An AFM (Atomic Force Microscopy) probe was used for the wear test and surface profiling of nanocrystalline diamond coatings. 2-D friction coefficient mapping by LFM (Lateral Force Microscopy) scanning showed a low friction coefficient (< 0.1) on the line-patterned diamond surface, and a higher friction coefficient (< 0.3) on a narrow area adjacent to the undulated pattern edges. With prolonged LFM scanning, the high coefficient of friction was reduced to less than 0.1. The bonding status of the nanocrystalline diamond was analyzed with Raman spectroscopy.     

Diamond chemical vapour deposition on seeded cemented tungsten carbide substrates

Diamond particles were deposited onto seeded cemented tungsten carbide (WC-Co) substrates using conventional hot-filament chemical vapour deposition (HFCVD) and time-modulated CVD (TMCVD) processes. The substrates were pre-seeded ultrasonically with diamond particles of different grit sizes. In this investigation, we employ timed methane (CH₄) gas modulations, which are an integral part of our TMCVD process in order to enhance diamond nucleation density. During diamond deposition using the conventional HFCVD process, methane gas flow was maintained constant. The total hydrogen flow into the reactor during TMCVD process was higher than in the HFCVD process. Hydrogen etching can be expectedly more prominent in the TMCVD process than in HFCVD of diamond particles. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results showed that a proper selection of the diamond grit size for seeding using ultrasounds can lead to enhancement in the nucleation density values of about two orders of magnitude (10⁷ to 10⁹ cm^− 2). The TMCVD process using the different seeded substrates can result in high nucleation density values of up to 10¹⁰ cm^− 2.     

大面积印刷纳米金刚石薄膜的场致发射的研究

研制了特定比例的纳米金刚石浆料,采用了丝网印刷工艺在石墨衬底上大面积印制了纳米金刚石场发射薄膜,实验探索了石墨衬底纳米金刚石薄膜的烧结工艺和后处理过程,利用扫描电镜(SEM)观察了纳米金刚石膜的表面形貌,经后处理的薄膜中纳米金刚石露出薄膜表面,纳米金刚石的棱角是天然的发射体.采用本课题组研制的多功能场发射测试台在10-6Pa的真空条件下进行了场发射特性的测试,结果发现石墨上低成本大面积印刷的纳米金刚石薄膜具有均匀稳定的场发射特性,作为电子器件的理想冷阴极发射,可在宇宙飞船、原子反应堆等恶劣条件下工作的平面显示器中得到应用.     

Growth and characterization of diamond particles,diamond films,and CNT-diamond composite films deposited simultaneously by hot filament CVD

It is important to understand the growth of CNT-diamond composite films in order to improve the inter-link between two carbon allotropes, and, in turn, their physical properties for field emission and other applications. Isolated diamond particles, continuous diamond thin films, and thin films of carbon nanotubes (CNTs) having non-uniformly distributed diamond particles (CNT-diamond composite films) were simultaneously grown on unseeded, seeded, and catalyst pre-treated substrates, respectively, using a large-area multi-wafer-scale hot filament chemical vapor deposition. Films were deposited for four different growth durations at a given deposition condition. The changes in surface morphology and growth behavior of diamond particles with growth duration were investigated ex situ using field emission scanning electron microscopy and 2D confocal Raman depth spectral imaging, respectively. A surface morphological transition from faceted microcrystalline nature to nanocrystalline nature was observed as a function of growth duration in the case of isolated diamond particles grown on both unseeded and catalyst pre-treated substrates. However, such a morphological transition was not observed on the simultaneously grown continuous diamond thin films on seeded substrates. 2D confocal Raman depth spectral imaging of diamond particles showed that the local growth of CNTs did not affect the growth behavior of neighboring diamond particles on catalyst pre-treated substrates. These observations emphasize the importance of surface chemical reactions at the growth site in deciding sp² or sp³ carbon growth and the final grain size of the diamond films.     

电镀铬-金刚石复合过渡层提高金刚石膜/基结合力

在铜基体上沉积铬-金刚石复合过渡层, 用热丝CVD系统在复合过渡层上沉积连续的金刚石涂层. 用扫描电镜(SEM)、X射线(XRD)、拉曼光谱及压痕试验对所沉积的镶嵌结构界面金刚石膜的相结构及膜/基结合性能进行了研究. 结果表明, 非晶态的电镀Cr在CVD过程中转变成Cr₃C₂, 由于金刚石颗粒与Cr₃C₂的相互咬合作用, 金刚石膜/基结合力高; 在294 N载荷压痕试验时, 压痕外围不产生大块涂层崩落和径向裂纹, 只形成环状裂纹.     

Polycrystalline diamond on self-assembled detonation nanodiamond: a viable route for fabrication of all-diamond preformed microcomponents

Surface assisted self-assembly of detonation nanodiamond particles (with typical sizes in the range 4-10?nm) has been obtained using different fractions of colloidal aqueous dispersions as starting material. The relationship between dispersion properties and structure/geometry of the aggregates deposited on Si or glass plates has been investigated. A series of differently shaped free-standing nanodiamond structures has been prepared, analysed and used as templates for the growth of polycrystalline diamond layers by the chemical vapour deposition (CVD) technique. The possibility of obtaining textured coating with a relatively strong [Formula: see text] preferred orientation (within a solid angle of about 0.6?srad) is also reported. Overall, the coupling of nanodiamond self-assembling to the CVD diamond growth enables one to produce specimens with complex 3D architectures. The proposed microfabrication methodology could represent a viable route for the production of free-standing all-diamond microcomponents, with tailored shapes and predefined crystalline features, to be used for advanced electronic applications.     

Analysis of the nanodiamond particle fabricated by detonation

In order to comprehensively analyse the structures and the surface states of the nanodiamond particles fabricated by detonation, various apparatus were used to investigate the nanodiamond powder including a high-resolution transmission electron microscope, an energy diffraction spectrometer, an X-ray diffractometer, a Raman spectrometer, a Fourier transform infrared spectrometer and differential scanning calorimeter. The grain size of the nanodiamond particles was in the range of 2–12 nm. However, the average grain size of the nanodiamond was approximately 5 nm. Moreover, the shapes of the nanodiamond particles were spherical or elliptical. The nanodiamond as fabricated was very pure, containing almost only the element of carbon. The contents of the impure element including O, Al and S were very small, which came from the synthesis and purification processes when fabricating the nanodiamond. The surfaces of the nanodiamond particles absorbed many functional groups, such as hydroxy, carbonyl, carboxyl and ether-based resin. The initial oxidation temperature of the nanodiamond powder in the air was about 520°C, which was lower than that of the bulk diamond. However, the oxidation temperature of the nanographite existing in the nanodiamond powder was about 228°C. The graphitisation temperature of the nanodiamond powder in the Ar gas was approximately 1305°C.     

Deaggregation of nanodiamond powders using salt- and sugar-assisted milling

Diamond particles of 5-10 nm in size can be produced in large quantities by denonating oxygen-lean explosives in a closed chamber. They have numerous useful properties and are used in applications ranging from lubricants to drug delivery. Aggregation of diamond nanoparticles is limiting wider use of this important carbon nanomaterial because most applications require single separated particles. We demonstrate that dry media assisted attrition milling is a simple, inexpensive, and efficient alternative to the current ways of deaggregating of nanodiamond. This technique uses water-soluble nontoxic and noncontaminating crystalline compounds, such as sodium chloride or sucrose. When milling is complete, the media can be easily removed from the product by water rinsing, which provides an advantage when compared to milling with ceramic microbeads. Using the dry media assisted milling with subsequent pH adjustment, it is possible to produce stable aqueous nanodiamond colloidal solutions with particles <10 nm in diameter, which corresponds to 1-2 primary nanodiamond particles. The study of milling kinetics and the characterization of the produced nanodiamond colloids led us to conclude that aggregates of less than 200 nm in diameter, observed at the tail of the pore size distribution of milled nanodiamond, are loosely bonded and rather dynamic in nature. Color change observed in ND colloids upon shifting their pH toward the basic end allowed us to demonstrate that the coloration comes from the light interaction with colloidal particles and not from an increase in nondiamond carbon content.     

Characterization of hypervelocity impact craters on chemical vapour-deposited diamond and diamond-like carbon films

Microwave plasma chemical vapour-deposited (CVD) process has been used to grow polycrystalline diamond films over silicon substrates. Diamond-like carbon (DLC) thin films were grown over silicon substrates using a microwave plasma disc reactor. Reactant gases of CH4 and H2 were used in both CVD processes. Some preliminary feasibility tests were performed on the possible applicability of diamond and diamond-like carbon thin films for space-protective applications against artificially simulated electrically actuated plasma drag hypervelocity impact of olivine particles. As-deposited films were analysed by Raman for their chemical nature. The morphology and dimensions of hypervelocity impact craters in diamond and DLC films was also studied by scanning electron microscopy (SEM) and optical microscopy. The velocity of debris particles was determined by high-speed photography using a streak camera. The size of the impact particles was determined by measuring the size of the holes formed in the mylar sheet mounted just above the target diamond and DLC film/silicon and coordinates of the impact sites were determined using the same apparatus. This revised version was published online in November 2006 with corrections to the Cover Date.     

Transformation of onion-like carbon from nanodiamond by annealing

Onion-like carbon (OLC) was synthesized by annealing nanodiamond in low vacuum (1 Pa) at the temperatures from 500 to 11400°C. The high-resolution transmission electron microscope images, X-ray diffraction patterns and Raman spectra showed that, when the annealing temperatures were lower than 900°C, there was no OLC fabricated. The amorphous carbon and the nanodiamond coexisted. The graphitization started from the surfaces of the nanodiamond particles. When the annealing temperatures were higher than 900°C, the OLC was fabricated. At 900°C, OLC began appearing and the size of the OLC particles was smaller than 5 nm. At the annealing temperature of 1400°C all the nanodiamond particles were transformed into OLC. The OLC particles exhibited similarity to the original nanodiamond particles in shape. Based on these results, a mechanism for the OLC synthesis by the method of annealing in vacuum was provided.     

Study on preparation and mechanical property of nanocrystalline NiAl intermetallic

Nanocrystalline NiAl materials were fabricated using mechanical alloying and hot-pressing sintering technique. The crystal structural and microstructure of milled powders during mechanical alloying, and the microstructure and mechanical properties of bulk NiAl intermetallic were characterized. The results show that B2 ordered nanocrystalline NiAl powders were successfully synthesized by solid-state diffusion via the gradual exothermic reaction mechanism during mechanical alloying. Scanning electron microscope image confirmed that the powder particles were flat and flake shape in the early stage of milling, but changed to a spherical shape with the crystallite size about 30 nm after the milling. After sintering, the crystal structure of nanocrystalline NiAl intermetallic was assigned to B2 order NiAl phase with the average crystallite size about 100 nm. The nanocrystalline NiAl intermetallic exhibited prominent room temperature compressive properties, such as the true ultimate compressive strength and the fracture strain were 2143 MPa and 32.2%, respectively. The appearances of vein-like patterns on the fracture surface of NiAl intermetallic materials indicated that the fracture mechanism could be characterized as ductile fracture. It can be concluded that higher sintering density and nanocrystalline of NiAl intermetallic were benefited for the improvement of mechanical properties.     

Implementation of the time-modulated process to produce diamond films using microwave-plasma and hot-filament CVD systems

A newly developed process called time-modulated chemical vapour deposition (TMCVD) was employed to deposit smooth polycrystalline diamond films onto silicon substrates using both microwave plasma CVD (MPCVD) and hot-filament CVD (HFCVD) systems. The distinctive feature of the TMCVD process, which separates it from the conventional diamond CVD process, is that it pulses methane (CH₄) at different flow rates for different time durations into the vacuum reactor during the entire diamond CVD process. Generally, both MPCVD and HFCVD systems produced results that displayed similar trends, except that the growth rate results obtained using the two CVD systems were conflicting. In comparison to the conventional CVD diamond films, the time-modulated films, deposited using both MPCVD and HFCVD techniques, were generally found to be (i) smoother, (ii) consisted of smaller diamond crystallites and (iii) displayed approx. similar film quality. The diamond-carbon phase purity of the as-grown films was assessed using Raman spectroscopy. In addition, the surface roughness, Ra, values of the deposited films were obtained using surface profilometry.     

CATALYST EFFECT FOR DIAMOND NUCLEATION IN THE LOW PRESSURE PROCESS

The fundamental study of diamond nucleation on several kinds of substrates using the hot-filament CVD method, was carried out. To investigate the diamond nucleation density, the substrate which coated with the catalytic materials by the vacuum deposition was utilized. Diamond particles scarcely grew on a normal Si wafer, but a catalytic material promoted the growth of diamond on the Si substrate. The catalytic material increases the diamond nucleation density as compared with any other substrates with a surface treatment. The particles grown on the substrates have been examined by means of x-ray diffraction and scanning electron microscopy, and identified as diamond.     

Design and fabrication of piezoresistive strain gauges based on nanocrystalline diamond layers

The paper reports on design, fabrication and characterization of piezoresistive sensors based on boron doped nanocrystalline diamond (NCD) layers. The shape and position of the piezoresistive element was optimized using finite element 3D modeling. Mechanical and piezoresistive simulations were performed. The piezoresistive sensing boron doped diamond thin films were realized on SiO₂/Si₃N₄/Si substrates by microwave plasma enhanced chemical vapor deposition (CVD) and the piezoresistive structures were formed by reactive ion etching. The extensive study of sensor parameters e.g. deformation sensitivity, edge and contact resistances, temperature dependences gauge factor, temperature coefficient of resistance and bridge output voltage was performed. The highest gauge factor at higher temperatures (GF = 7.2 at 250 °C) was observed for moderate doping level (boron to carbon ratio of 3000 ppm). One of the aims was the extraction of piezoresistive coefficients of fabricated diamond layers for utilization in a finite element piezoresistive solver.     

A possible mechanism of nanodiamond formation during detonation synthesis

A new mechanism of formation of nanodiamond particles during detonation synthesis has been put forward. It includes the following steps: (a) decomposition of trinitrotoluene (TNT) molecules into basic radicals-a radical-like dimer C₂ and CO₃, decomposition of hexogen molecules into C₂ and of benzotrifuroxane molecules into C₂; (b) formation of cyclohexane from C₂ or immediately of radical adamantane molecules; (c) interaction of diamond-like core (adamantane radical) with methyl and other monocarbon radicals; and (d) growth of detonation nanodiamond particles like in a CVD process. It is shown that the nucleation of a radical-like adamantine molecule occurs within a range between the center of the chemical peak zone and the Chapman-Jouguet plane, the growth of diamond particles takes place at the same time and comes to an end at an early stage of isoentropic (Taylor) expansion of detonation gases entrapping solid carbon particles.     

喷射电沉积Cu-Al2O3复合电铸层性能研究

利用喷射电沉积工艺制备了Cu-A l2O3纳米复合铸层,分析了纳米A l2O3颗粒添加量,阴极电流密度以及电铸液喷射速度对复合电铸层中纳米颗粒复合量的影响,采用扫描电镜(SEM)及其附带的能谱仪(EDS)对复合电铸层的微观形貌和铸层成分进行了分析,研究了复合电铸层中纳米颗粒复合量对其显微硬度的影响。结果表明,铜沉积层具有纳米晶微观结构,平均晶粒尺寸约为50 nm;纳米A l2O3颗粒在沉积层中的复合量可达14.43(at%);纳米复合铸层的显微硬度有明显提高,约为普通粗铜的10.5倍.     

High-resolution electron microscopy of detonation nanodiamond

The structure of individual nanodiamond grains produced by the detonation of carbon-based explosives has been studied with a high-vacuum aberration-corrected electron microscope. Many grains show a well-resolved cubic diamond lattice with negligible contamination, thereby demonstrating that the non-diamond shell, universally observed on nanodiamond particles, could be intrinsic to the preparation process rather than to the nanosized diamond itself. The strength of the adhesion between the nanodiamond grains, and the possibility of their patterning with sub-nanometer precision, are also demonstrated.     

金刚石/碳化硅复合梯度膜制备研究

采用微波等离子化学气相沉积(MW-PCVD)制备金刚石/碳化硅复合梯度膜.工作气体为H₂,CH₄和Si[CH₃]₄(四甲基硅烷,TMS),其中H₂∶CH₄=100∶0.6,Si[CH₃]₄为0％-O.05％,沉积压力为3300Pa,基体温度为700℃,微波功率为700W.基体为单晶硅,在沉积前用纳米金刚石颗粒处理.沉积后的样品经扫描电子显微镜(SEM),电子探针显微分析(EPMA),X射线能量损失分析(EDX)表明:沉积膜中的碳化硅含量是随Si[CH₃]₄流量的变化而改变.通过改变Si[CH₃]₄的流量可以制备金刚石/碳化硅复合梯度膜,且梯度膜中金刚石与复合膜过渡自然平滑.     

Mechanically activated crystallization of phase pure nanocrystalline forsterite powders

Pure nanocrystalline forsterite was successfully synthesized via a mechanical activation route assisted with heat treatment. X-ray diffraction (XRD), scanning electron microscopy (SEM) and simultaneous thermal analysis (STA) techniques were utilized to characterize the prepared powders. Results showed that pure nanocrystalline forsterite could be fabricated completely by 10 h of mechanical activation and post-heat treatment at 1000-1200 °C for 1 h. By 10 h mechanical activation, the initial temperature of forsterite crystallization was reduced to about 820 °C. The obtained nanocrystalline forsterite powder had crystallite size about 57 nm according to Williamson-hall approach and particle sizes smaller than 1000 nm. XRD patterns showed that pure forsterite could not be obtained by mechanical activation process alone and that enstatite could be fabricated by increasing the time of milling. Liberation of CO₂ gas increased the rate of forsterite formation by increasing contact surface between grains.     

Tuning the conditions for the deposition of nanocrystalline diamond by hot filament chemical vapour deposition

Although large focus has been placed into the deposition of nanocrystalline and ultra-nanocrystalline diamond films, most of this research uses microwave plasma assisted CVD systems. However, the growth conditions used in microwave systems cannot be directly used in hot-filament CVD systems. This paper, aims to enlarge the knowledge of the diamond film depositing process. H2/CH4/Ar gas mixtures have been used to deposit micro, nano and ultra-nanocrystalline diamond films by hot-filament CVD systems. Additionally, the distance between the filaments array and the substrate was varied, in order to observe its effect and consequently the effect of a lower substrate temperature in the nucleation density and deposition. All the samples were characterized for microstructure and quality, using scanning electron microscopy and Raman spectroscopy.