Papers by Author: Fan Xiu Lu

Abstract: As an emerging brand new type of engineering material for a variety of important high technology applications, the deep understanding of the mechanical behavior of freestanding diamond films has become an emergent task of vital importance. Of the many deposition methods dc arc plasma jet has been regarded as the most promising technique for large area high quality and low cost production of freestanding diamond films. In the present paper, recent progress in mechanical properties of freestanding diamond films mainly by high power dc arc plasma jet with rotating arc root and gas recycling is reviewed. Testing methods for fracture strength and fracture toughness are discussed. Experimental data are presented and compared to that by MWCVD. Dc arcjet diamond films start to oxidize at about 700°C, however, oxidation up to 800°C for 10 min does not affect the fracture strength. Fracture mechanism is discussed. The strange mechanical behavior of freestanding diamond films is explained. It is surprising that CVD diamond film is such a kind of material which is strong, but full of different size of defects. It is hoped that the present paper will be helpful for those who wish to understand and use this brand new type of engineering material.

Abstract: Diamond film coated hard metal cutting tools are indispensible for high efficiency machining of materials which are difficult to cut by ordinary tools, and are successfully used in the dry cutting of high silicon content Al-Si cast alloys, graphite, carbon reinforced composite (CRFC) and metal matrix composite (MMC) , ceramics, and many other materials. In the present presentation, a novel process of High Current Extended DC Arc (HCEDCA) plasma CVD for mass production of diamond film coated hard metal cutting tools is presented. Besides, a novel process for the pretreatment of the hard metal cutting tool substrate, which involves the idea of “surface engineering” consisting of boronizing and alkaline and acidic etching is also discussed, by which the adhesion of the diamond film coating to the hard metal substrate can be greatly enhanced. Highly adherent and uniform diamond film coatings are successfully obtained. Diamond film coated WC-6wt%Co indexable tool bits, drills, endmill samples have been produced and been shown having excellent cutting performance by field cutting tests in dry cutting of Al-12%Si cast alloy and Al-15% SiC MMC materials.

Abstract: The plasma sprayed gradated coating with the bottom layer of NiCrAlY and the top layer of (ZrO2+CaO) was prepared on the 1Cr18Ni9Ti steel surface by the plasma spraying technique. The phase structure and morphology of the gradated coating were analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The high temperature oxidation resistance of the plasma sprayed samples at 800°C was investigated. The oxidation kinetics curve was worked out. The wear-resistance behavior of the graduated coating under dry sliding against GCr15 steel was evaluated on a ball-on-disc test rig. The results show that the thickness of gradated coating is about 320μm. The plasma sprayed gradated coating on stainless steel surface can improve the high temperature oxidation resistance of stainless steel. The oxidation rate of stainless steel is less than that of plasma sprayed gradated coating. The oxidation coating of plasma sprayed sample was very dense and is not easily exfoliated. The dense oxidation coating prevents the stainless steel from more oxidation. The friction coefficient of the 1Cr18Ni9Ti substrate was about 0.33 on dry sliding against the GCr15 steel, while the graduated coating experienced abated friction coefficient to 0.21 on the same testing condition.

Abstract: As quasi-thermodynamic equilibrium plasma, DC Arc Plasma has the advantage of very high gas temperature and thus the very high degree of activation of the precursors for diamond film deposition. The present paper reviews the progresses in the R&D of the novel high power dc arc plasma jet CVD system with rotating arc and operated at gas recycling mode for large area high quality diamond film deposition, developed at the University of Science and Technology Beijing (USTB) in the mid 1990s of the 20th century. Thanks to the continuous efforts made in the technological improvement in the past 15 years, considerable progresses have been achieved in the commercialization of this high power dc arcjet CVD system, which is now capable of mass production of large area high quality freestanding diamond films for optical, thermal, and mechanical (tool) applications. The present status in the commercialization and the property level of the resultant diamond films in optical, thermal, mechanical, dielectric, oxidation resistance, sand erosion resistance, and laser damage threshold etc. are presented. Based on the same high power dc arcjet technology, a novel high current extended dc arc plasma (HCEDCA) CVD system has been developed which successfully changed the diamond film deposition mode from 2D planar deposition in to 3D deposition (as confined by two hollow (virtue) columns). It is demonstrated to be advantageous for mass production of diamond thin film coated WC-Co cutting tools. Recent results in the R&D of thin diamond film coated WC-Co drills and end mills, and the results in field tests are discussed.

Abstract: A multiple fields’ coupled model of new magnetic controlled DC plasma torch, which was used for CVD diamond film, was presented. In this model, the effects of electric field and magnetic field on the flow field and temperature field were taken into account, and the fluid dynamics equations were modified by the addition of some source terms relating to electromagnetic field, such as Lorentz force, joule heating, and radiative cooling. Conversely, the generalized ohm’s law was used to solve the current density, which reflected the effects of flow field and temperature field on the electric field and magnetic field. In addition, the rest Maxwell’s equations and external solenoid magnetic field equation were also modeled. In order to know the effect of external magnetic field on the torch, the current intensity of external solenoid was chosen to simulate its influence on the flow and heat transfer in the torch. Results show that external magnetic field plays a part in stirring the plasma, which is advantageous for the preparation of diamond film. The larger the external solenoid current intensity is, the better the uniformity of the temperature and velocity of plasma is.

Abstract: Diamond coatings on pure titanium substrates are of interest for tribological and biomedical implants. However, due to the different thermal expansion coefficients of the two materials, the complex nature of the interlayer formed during diamond deposition, and the difficulty in achieving very high nucleation density, it is hard to deposit adherent thin diamond layers on titanium. The aim of the present research was to successfully produce smooth and well adherent nanocrystalline diamond (NCD) film on a pure Ti substrate using the microwave plasma chemical vapor deposition (MWPCVD) method. The influence of Argon addition to CH4/H2 plasma on the crystallinity, morphology and growth of the diamond film deposited by MWPCVD was investigated using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), Xray diffraction (XRD) and Raman spectroscopy.

Abstract: The deposition of a well adherent diamond film on titanium and its alloys is always complicated due to the different thermal expansion coefficients of the two materials, the complex nature of the interlayer formed during diamond deposition, and the difficulty in achieving very high nucleation density. In this work well-adherent and smooth nano-crystalline diamond film was successfully deposited on pure titanium substrate by microwave plasma assisted chemical vapor deposition (MWPCVD) method in CH4/H2 environment. It is found that the average grain size was less than 20 nm with a surface roughness value as low as 28nm. Of particular interest in this study was the exceptional adhesion of approximately 2μm-thick diamond film to the metal substrate as observed by indentation testing up to 150 kg load. Experimental results on growth mechanism and obtaining good adhesion are discussed.

Abstract: Oxidation ceramic coating was directly synthesized on LY12 aluminium alloy by micro-arc oxidation (MAO) process in Na2SiO3 electrolyte solution with the Na2WO4-KOH-Na2EDTA addition. The corrosion resistance of the coating was tested using CS300P electrochemical corrosion workshop in 3.5% NaCl solution. Using the scanning electron microscopy (SEM) and X-ray diffraction (XRD), the cross-section microstructure, the surface morphology and the phase structure of the micro-arc oxidation ceramic coating were analyzed. The results showed that the corrosion resistance of the micro-arc oxidation ceramic coating in 3.5% NaCl solution was enhanced remarkably, the corrosion velocity was obviously slowed down. The thickness of micro-arc oxidation ceramic coating was about 11μm. The final phases in the coating were found to be α-Al2O3 and γ-Al2O3. The mechanism of the oxidation ceramic coating formation was investigated too.

Abstract: Free-standing CVD diamond films were prepared under the substrate temperature in the range of 850-1050oC. Macro- and micro-textures of the films were investigated based on the SEM observation as well as on the ODF and EBSD analysis. It was found that certain growth selection process appeared during diamond deposition which, however, did not lead to a strong film texture. It is indicated that strong fluctuation of growth ratio V<100>/V<111> and frequent growth twinning during film deposition resulted in randomization effect of grain orientations, which can be transformed by adjusting the parameters of film preparation.

Abstract: The residual stress and crystallographic texture of diamond films were investigated in the present work. The diamond films were synthesized on (100) silicon wafer by Microwave Plasma Chemical Vapor deposition (MPCVD). Then the residual stresses of the films were measured by X-ray diffractometer equipped with the two-dimensional detector. The residual stresses can be classified into two categories, i.e., the intrinsic stresses and the thermal stresses. It was shown that the thermal stresses were compressive in the temperature range studied and the intrinsic stresses were tensile. The crystallographic textures of the films were measured by X-ray diffractometer with the method of pole figure and orientation distribution function (ODF). The experimental results suggest that the crystallographic textures of the films depend upon the deposition temperature and methane flow rates, and the components and intensity of crystallographic textures have effect on the residual stresses in diamond films to a certain extent.