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1.
Polycrystalline diamond (PCD) cylindrical tool-bits used in oil well drilling are susceptible to fracture due to the hostile environment of randomly occurring impact loads to which they are subjected. The fact that the tool-bits fail after repeated use suggests the possibility of fatigue type processes in PCD. The study of stress fields on the surface of the PCD thus becomes crucial in the quest to have extended lives for these tool-bits. Since the diamond Raman peak reveals both the nature and magnitude of the stress present in the material, this technique can be employed as a non-destructive measurement tool to investigate these stress fields. Raman stress measurements at room temperature were carried out using a 36 point mapping array in area close to the size of the PCD samples. The mapping points provided histograms of the magnitude and nature of these small individually stressed regions showing a general compressive stress for the lower numbers of fatigue cycles which deteriorates to a high proportion of tensile regions. The data are also illustrated by 2-D surface maps as an alternative mode of presentation again confirming the change from surface stresses being dominantly compressive to dominantly tensile with exposure to the higher numbers of fatigue cycles. Whereas a general compressive stress is desirable in the PCD layer as it inhibits the propagation of cracks, on the contrary tensile stresses facilitate the formation of cracks ultimately leading to catastrophic failure of the tool-bits.  相似文献   

2.
Tian-Bao Ma  Yuan-Zhong Hu  Hui Wang 《Carbon》2009,47(8):1953-329
The shear-induced graphitization of amorphous carbon (a-C) films in sliding contact with a diamond counterface is investigated by molecular dynamics (MD) simulations. The gradual formation of a graphene-like sp2 dominant layer on the a-C film surface is observed after steady-state sliding has been achieved, which provides direct evidence for the experimental observations of friction induced graphitization of a-C film. After the graphitized layer is formed, the relative sliding occurs between the graphitized atomic layers. During the shearing process, the biaxial stress in the graphitized layer experiences a transition from highly compressive (42 GPa) to tensile (−3 GPa). It is the relaxation of the local biaxial stress that leads to the sp3-to-sp2 structural transformation.  相似文献   

3.
Surface residual stresses caused by grinding and polishing of alumina are thought to influence materials properties but have previously been measured only by low spatial resolution techniques which sample average stresses. In this work confocal Cr3+ fluorescence microscopy has been used to investigate the spatial distribution of the residual stresses. A model for the residual stresses, accounting for both surface plastic deformation and “pullout” of material from the surface by brittle fracture, was developed to help in analysing the results. After coarse diamond grinding, the results showed that the residual stresses fluctuate greatly with position. Large tensile stresses (up to ~600 MPa) were found below the plastically deformed surface layer in regions between the “pullouts”. These tensile stresses are expected to aid crack propagation and further surface pullout. They arise because pullout removes parts of the plastically deformed surface layer. The stresses beneath the pullout sites themselves were compressive, but the largest compressive stresses (≈?1.5 GPa) were within the plastically deformed surface regions and extended to a depth of 1.3 μm. The plastically deformed surface layer was much shallower following polishing with 3 μm diamond paste but the compressive stress within it was of similar magnitude to that in the plastically deformed surface layer caused by grinding.  相似文献   

4.
Vacuum-sealed cavities featuring diamond membranes are fabricated using plasma-activated direct bonding technology. A chemical mechanical polished (CMP) silicon dioxide interlayer, deposited on diamond with a high temperature oxide (HTO) process at 850 °C in a low pressure chemical vapor deposition (LPCVD) furnace, is employed for successful direct bonding and vacuum cavity formation. The circular cavities are defined on the thermally grown oxide of the phosphorus-doped Si wafer (4-in, < 100>, 1.2 Ω/sq) using reactive ion etching (RIE). The same microfabrication steps are applied for low residual stress (i.e. < 50 MPa) nanocrystalline (NCD) and ultrananocrystalline (UNCD) diamonds to determine and compare membrane characteristics. For both diamond types, successful microfabrication of membranes is demonstrated using the optimized process flow. Profilometer measurements of membrane deflection are compared with finite element modeling (FEM), and indicate a Young's modulus of 1000 GPa for NCD and 850 GPa for UNCD. Furthermore, FEM analysis suggests the residual stress of UNCD membrane is approximately 100 MPa tensile, whereas NCD one does not show any significant residual stress (< 50 MPa). Our results show that NCD is a more promising choice than UNCD as a membrane material for electromechanical transducers.  相似文献   

5.
The high temperature compressive strength behavior of zirconium diboride (ZrB2)-silicon carbide (SiC) particulate composites containing either carbon powder or SCS-9a silicon carbide fibers was evaluated in air. Constant strain rate compression tests have been performed on these materials at room temperature, 1400, and 1550 °C. The degradation of the mechanical properties as a result of atmospheric air exposure at high temperatures has also been studied as a function of exposure time. The ZrB2-SiC material shows excellent strength of 3.1 ± 0.2 GPa at room temperature and 0.9 ± 0.1 GPa at 1400 °C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB2-SiC-C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB2-SiC composites contain significant matrix microcracking due to residual thermal stresses, and show poor mechanical properties and oxidation resistance. After exposure to air at high temperatures an external SiO2 layer is formed, beneath which ZrB2 oxidizes to ZrO2. A significant reduction in room temperature strength occurs after 16-24 h of exposure to air at 1400 °C for the ZrB2-SiC material, while for the ZrB2-SiC-C composition this reduction is observed after less than 16 h. The thickness of the oxide layer was measured as a function of exposure time and temperatures and the details of oxidation process has been discussed.  相似文献   

6.
Neutron diffraction, Raman spectroscopy, and x-ray diffraction were employed to measure the stresses generated in the ZrB2 matrix and SiC dispersed particulate phase in ZrB2-30 vol% SiC composites produced by hot pressing at 1900 °C. Neutron diffraction measurements indicated that stresses begin to accumulate at ∼1400 °C during cooling from the processing temperature and increased to 880 MPa compressive in the SiC phase and 450 MPa tensile in the ZrB2 phase at room temperature. Stresses measured via Raman spectroscopy revealed the stress in SiC particles on the surface of the composite was ∼390 MPa compressive, which is ∼40% of that measured in the bulk by neutron diffraction. Grazing incidence x-ray diffraction was performed to further characterize the stress state in SiC particles near the surface. Using this technique, an average compressive stress of 350 MPa was measured in the SiC phase, which is in good agreement with that measured by Raman spectroscopy.  相似文献   

7.
Residual Stresses in Polycrystalline Diamond Compacts   总被引:5,自引:0,他引:5  
The effects of residual stress on the integrity of polycrystalline diamond compact (PDC) cutters was investigated. When the compact cooled from the sintering temperature to room temperature, very high radial compressive stresses were induced in the diamond table, and (generally) much lower radial tensile stresses were induced in the cemented tungsten carbide backing. The magnitudes of these residual stresses were not affected very much by the diameter of the compact. However, the residual stresses were affected significantly by the thickness ratio of the carbide layer to the diamond layer. The higher this ratio the greater was the radial compressive stress in the diamond and the lower was the radial tensile stress in the carbide. This same effect was obtained by sintering a relatively thin layer of tungsten carbide on top of the diamond table.  相似文献   

8.
In this paper, a brittle–ductile transition lapping mechanism is proposed for the mechanical lapping of single crystal diamond cutting tools. The critical depths of cut for brittle–ductile transition and the dynamic critical tensile stress are regarded as the references to analyze the influence of contact accuracy between the rapid rotating scaife and lapped tool surface and the effects of tool face orientation on the sharpened cutting edge radius, respectively. The experimental results indicate that the vibrations of lapping setup, surface quality of scaife, dynamic balance and motion accuracy of the spindle, lapping compression force and lapping velocity all have enormous influences on the contact accuracy so as to affect the lapped cutting edge radius of diamond cutting tools. Under the optimal settings of each influencing factor for a robust contact accuracy, a perfect diamond cutting tool is sharpened in ductile lapping mode with a cutting edge radius of 30–40 nm and a surface roughness Ra of 0.7 nm on the tool rake face. On the other side, different orientation settings of tool faces enable diamond cutting tools edge to have different micro mechanical strength. Under the same configurations of the lapping parameters, the higher the micro mechanical strength of tool cutting edge is, the smaller the sharpened cutting edge radius would be.  相似文献   

9.
The plasma treatment of rapid heating was introduced for increasing fracture strength of free-standing diamond films. The effects of plasma high temperature annealing treatment on surface morphology, internal stress, vacancy defects, impurities and fracture strength of free-standing diamond films were investigated by scanning electron microscopy (SEM), Raman, positron annihilation technique (PAT) and mechanical property testing. It showed that the fracture strength of the diamond films increases up to 70% for lower fracture diamond films with treating temperature (1500-1600 °C). The graphitization in surface and interior of diamond films would be produced by high temperature treatment more than 1700 °C. Fracture strengths of diamond films could be enhanced after high temperature treatment and the main factor of that was compressive stress state in diamond films induced by graphitization. The impurity of N was segregated and integrated with vacancy cluster to become [N-V]0 and [N-V].  相似文献   

10.
The residual stresses introduced in MgO crystals by grinding on {100} surfaces in 〈100〉 directions were measured using photoelastic techniques. Grinding was conducted with two wheels; a 100-grit diamond wheel removed material by brittle fracture, and a 46-grit alumina wheel caused plastic flow and burnishing. Both wheels introduced a discrete, highly deformed layer adjacent the machined surface. In all cases the machined surfaces were under a residual tensile stress which became compressive within the deformed region. Beneath the deformed layer the residual stress patterns were distinctly different. In crystals ground with the alumina wheel the stresses became tensile again within 0.5 mm of the ground surface, whereas the subsurface stresses in crystals ground with the diamond wheel remained compressive to distances ≥1 mm. These residual stress distributions are discussed in terms of a simple model based on the superposition of mechanically and thermally induced stresses.  相似文献   

11.
L.R. Bradley  C.R. Bowen  D.C. Johnson 《Carbon》2007,45(11):2178-2187
This study examines the shear properties of a 2D PAN-CVI carbon/carbon composite whose reinforcement layers are formed from a non-woven duplex cloth comprising a continuous fibre layer needled to a short fibre felt layer. Composites of three lay-ups were tested in several orientations using the Iosipescu (V-notched beam) shear test. Material anisotropy means that shear failure stresses and shear moduli are required in several orientations for structural analysis. It was necessary to mitigate the effect of specimen twisting by using strain gages on both sides of the specimens. The approximate ranges of measured shear failure stresses were 30-45 MPa in-plane and 8-22 MPa interlaminar. Shear moduli ranged between 6-16 GPa in-plane and 1-3 GPa interlaminar. While the presence of flaws in these composites leads to scatter in properties, the effect of bulk density on interlaminar properties was clearly observed.  相似文献   

12.
The creep behaviour of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) membrane material was investigated in the temperature range 600-950 °C using tubular specimens. Oxygen partial pressure (4 and 2 × 10−6 mbar), as well as compressive stress (20-63 MPa) were varied. The steady state strain rates from compressive creep tests under axial loading were described by a standard creep equation considering as exponential fitting parameters the influence of temperature, stress, oxygen partial pressure and grain size. Differences in activation energy in the low and high temperature regime appeared to be associated with the disappearance of a second phase. After the creep tests, no significant changes in grain morphology were observed. In addition to the compressive tests, C-ring shaped specimens were machined from the tubes and deformed at 900 °C in a combined tensile-compressive creep mode that revealed the formation of pores along grain boundaries normal to the local tensile stresses that ultimately might lead to creep rupture.  相似文献   

13.
The mechanical properties of polycrystalline diamond coatings with thickness varying from 0.92 to 44.65 μm have been analysed. The tested samples have been grown on silicon substrates via microwave plasma enhanced chemical vapour deposition from highly diluted gas mixtures CH4-H2 (1% CH4 in H2). Reliable hardness and elastic modulus values have been assessed on lightly polished surface of polycrystalline diamond films.The effect of the coating thickness on mechanical, morphological and chemical-structural properties is presented and discussed. In particular, the hardness increases from a value of about 52 to 95 GPa and the elastic modulus from 438 to 768 GPa by varying the coating thickness from 0.92 to 4.85 μm, while the values closer to those of natural diamond (H = 103 GPa and E = 1200 GPa) are reached for thicker films (> 5 μm). Additionally, the different thickness of the diamond coatings permits to select the significance of results and to highlight when the soft silicon substrate may affect the measured mechanical data. Thus, the nanoindentation experiments were made within the range from 0.65% to 10% of the film thickness by varying the maximum load from 3 to 80 mN.  相似文献   

14.
A study of microwave plasma (MPCVD) diamond deposition on Si3N4–TiN composites with different TiN amounts (0–30 vol.% TiN) is performed. These ceramic composites are requested in order to obtain a suitable material to be cut by electrodischarge machining (EDM), aiming their use as substrates for cutting tools and tribological components. TiN is an electrical conductor, contrarily to Si3N4, but it is characterized by a higher thermal expansion coefficient value than Si3N4 and diamond. The estimated thermal stresses are found to be low and tensile (0.90 GPa) when using the monolithic Si3N4 substrate, but compressive for the Si3N4–TiN composites, and even relatively high in magnitude (− 1.9 GPa) for the Si3N4–30 vol.% TiN composite. Brale indentation assessed the adhesion strength of diamond on the different substrate grades. Optimal behaviour (very low residual stress; no film delamination under 1000 N) is observed for the Si3N4–9 vol.% TiN substrate, corresponding to the lowest thermal mismatch and minimal residual stress magnitude.  相似文献   

15.
Shuqing Wu  Suobo Zhang  Wenmu Li 《Polymer》2005,46(19):8396-8402
Copoly(phenylene-imide)s were synthesized by Ni(0)-catalyzed coupling of aromatic dichlorides containing imide structure and 2,5-dichlorobenzophenone. The route offered the flexibility of incorporating different ratios of benzophenone and imide groups in the polymer backbone in a controlled manner. The resulting copolymers exhibited high molecular weights (high inherent viscosity), and a combination of desirable properties such as good solubility in dipolar aprotic solvents and cresols, film-forming capability and good mechanical properties. Wide-angle X-ray diffractograms revealed that the polymers were amorphous. These copolymers had glass transition temperatures between 209 and 319 °C and 10% weight loss temperatures in the range of 502-543 °C in nitrogen atmosphere. The tough polymer films, obtained by casting from N-methylpyrrolidone solution, had a tensile strength range of 83-156 MPa and a tensile modulus range of 1.6-3.6 GPa.  相似文献   

16.
Ultra-high molecular weight isotactic polypropylene (UHMW-iPP) reactor powders have been successfully ultra drawn below melting temperature (Tm) by a combination of calendar rolling and tensile drawing techniques. Two UHMW-iPP reactor powders having different MWs were synthesized by using the same Ziegler-Natta catalyst system at 70 °C in hexane. The resultant tensile properties increased with increasing draw ratio, due to orientation-crystallization during tensile draw, which was indicated by DSC and WAXD measurements. The film drawn under optimum conditions exhibited the maximum tensile modulus of ∼25 GPa, independent of sample MW, corresponding to 70% of the ultimate modulus of iPP crystal. However, the higher maximum tensile strength of ∼1.0 GPa was achieved for the reactor powder having the higher MW, which is three times as high as those of commercial high-strength iPP tapes. Such a fact that high performances have been achieved by processing from reactor powder state below Tm implies that crystallization with less entanglement occurs during polymerization. When drawability and resultant properties were compared among different iPP reactor powders prepared under different conditions, it was clarified that they were predominantly affected by not only MW but also by the reactor powder morphology, especially surface smoothness.  相似文献   

17.
Thermal stress analytical model of three-layer bonded structure under temperature field based on physical mechanical law was established in this paper. Thermal stress distribution of bonded structure composed of silicone chip, polyimide film and composite under temperature gradient from −120 °C to 100 °C was also investigated. It is concluded that thermal stress at −120 °C is larger than that at 100 °C. The maximal and minimal normal stresses of bonded structure are in the polyimide film and silicone chip, respectively. Maximal shear stress is in adhesive layer between the polyimide film and the composite.  相似文献   

18.
Lead indium niobate, Pb(In1/2Nb1/2)O3 (PIN), is an interesting ferroelectric due to a transition from a disordered to an ordered state by long-time thermal annealing. However, the temperature related to the maximum dielectric constant (Tmax) of PIN in relaxor phase is low (at 1 kHz, Tmax = 66 °C). In this study, lead titanate PbTiO3 (PT) was added to PIN with compositions (1 − x)PIN–xPT (for x = 0.1–0.5) to increase their Tmax. The influence of stress on dielectric properties of (1 − x)PIN–PT ceramics was then investigated. The dielectric properties were measured under various uniaxial compressive stresses up to 400 MPa. The results showed the reduction of dielectric constant in 0.9PIN–0.1PT with superimposed compression load. For other compositions, dielectric constants first increased with compressive stress, then decreased when the stress was further increased up to 400 MPa. The loss tangent of all composition was found to decrease with increasing compressive stress.  相似文献   

19.
High pressure-temperature (P-T) phases of the ZrxHf1−xO2 (x = 0.5) solid-solution have been stabilised in a CO2 laser heated diamond anvil cell. At room-temperature the monoclinic to orthorhombic-I structural transformation is initiated at 5-8 GPa. The X-ray diffraction (XRD) studies show these two phases coexist to above ∼15 GPa. A progressive increase in the orthorhombic-I phase abundance occurs, to culminate in full conversion at ∼20 GPa. At this lower threshold of ∼20 GPa transformation to the orthorhombic-II (cotunnite) structure can be initiated by heating in the range of 600-1200 °C. Substantial conversion to the cotunnite phase occurs in the same temperature range at 25-30 GPa. Raman signatures have been assigned to the two orthorhombic high-pressure phases, aided by the qualitative assessment of the complementary XRD data. Decompression experiments show that phase mixture composites of these high pressure structures, possibly with enhanced tribological properties, can be recovered to ambient conditions.  相似文献   

20.
High strain rate tensile impact properties of aliphatic polyketone terpolymers were investigated and related to the polymer chain structure. Aliphatic polyketones were used as a model system, by changing the termonomer content and type. Aliphatic polyketone is a perfectly alternating copolymer and the structure was changed with the addition of a few mol% of termonomer: propylene, hexylene and dodecene. Studied were the thermal properties with DSC and DMTA, tensile behaviour, notched tensile impact behaviour, notched Izod properties and the temperature development during deformation. The perfectly alternating copolymer had a melting point of 257 °C, a Tg at 15 °C, a high crystallinity (48%), a high yield stress (77 MPa) and yield strain (31%) but a relatively low fracture strain (85%) and an impact strength (notched Izod) of 13 kJ/m2. Increasing the propylene content to 6%, lowered the melting temperature to 224 °C, without changing the Tg. The modulus and yield stress were lowered but the impact strength improved. Increasing the length of the termonomer while keeping the Tm at 224 °C lowered the Tg, the modulus, the yield stress but strongly improved the impact resistance. The longer termonomers, with a lower yield stress, reduced the necking behaviour. The temperature increase in front of the notch was about 85 °C. By adding termonomers to aliphatic ketones, the notched impact behaviour improved significantly at the cost of modulus and yield stress.  相似文献   

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