Diamond films were deposited by microwave plasma CVD using H2 and CH4 gas mixture over various substrate materials such as Si, Pd, Be, Cu, Mo, AIN, SiO2, Si3N4, Al2O3, Sapphire, Quartz, Ni-base alloys, single crystal Ni, boron nitride, and Ti. We have used a Z-axis pull stud test to determine adhesion strength of diamond film to some of the substrates. Our observations on the adhesion of diamond films to the above substrates are reported. A method will be described to evaluate Young's modulus of CVD diamond films using fabricated diamond cantilever beams.

The commercial diamond grits and the diamond films deposited by a microwave plasma chemical vapor deposition system were used to study the wetting characteristics of few commercial brazing alloys, e.g., Cu, Ni, Ag-Cu-Zn-Cd-Ni and Ag-Cu-In. The substrates for brazing experiments include AISI-304 stainless steel, AISI-1045 steel and cemented WC insert. The results shows that the wettability of the brazing alloys and the bonding strength between diamond and substrate are strongly influenced by the compositions of the brazing alloys and the surface conditions of diamond and substrate. The wettability of Cu with diamond grit or diamond film depends on the brazing substrate, and is in order of AISI-304 > WC > AISI-1045. The brazing alloys with Cu or Ag as the main component often show a poor brazing strength for diamond/plain carbon steel or diamond/WC.

The fracture, fatigue and indentation properties of pyrolytic carbon, both as a monolithic material and as a coating on a graphite substrate, are described in light of its use for biomedical implant applications, specifically for the manufacture of mechanical heart valve prostheses. From the perspective of determining properties that are important for the prediction of safe structural lifetimes in such prostheses, it is found that by traditional engineering standards, pyrolytic carbon has low damage tolerance, i.e., fracture toughness values between 1 and 3 MPa√m and susceptibility to subcritical crack growth by both cyclic fatigue and stress-corrosion cracking (static fatigue). Subcritical crack-growth rates are evaluated in simulated physiological environments for both through-thickness “long” cracks, and for physically “small” surface cracks, the latter measurements being performed for cracks initiated at hardness indents. The unusual deformation characteristics of indentation in pyrolytic carbon are described based on instrumented microhardness indentation and scanning probe microscopy (AFM/STM) studies.

The crack initiation sequence during Vickers indenter penetration into the (100) plane of diamond was observed. It was revealed that for <110> orientation, two median cracks are initiated during the loading half of the cycle. In unloading, the median cracks grow toward the surface and turn into half-penny cracks. It was proposed to use half-penny cracks as initials and evaluate fracture toughness in second loading from a crack-starting load for these cracks. The length of initial half-penny cracks was measured after first loading cycle from opposite side of the sample. Test results obtained by the reloading technique agree well with the data on conventional fracture toughness determination by indentation from the length of cracks after unloading.

Annealing of low inclusion, single crystals of synthetic Type I diamond at 1200–1700°C and 50–60kbar, was observed to increase average crystal compressive fracture strength by as much as 20%. The increase in strength is associated with the healing of lattice dislocations, coalescence of small (< 400 nm) metal inclusions, and dissociation of aggregatenitrogen. Photoluminescence and FTIR spectroscopic data indicate H3 and “A”-type defect centers are unstable relative to NVx and “C”-type defect centers under the conditions of these experiments. Dissociation rate is greater in the (111) growth sector than in the (100). Changes in residual lattice strain of the samples could not be detected using Raman spectroscopy and optical microscopy. A model reaction of the type, 2V + N2V = NV2 + NV, in which N = nitrogen and V = vacancy (i.e., N2V = H3 center) is proposed which provides a qualitative thermodynamic understanding of the observations reported in this study consistent with previous results of other researchers. In addition, photoluminescence examination of smaller crystals produced from a quench of early stage non-equilibrium growth reveal a lower concentration of aggregate nitrogen than those quenched from late stage growth suggesting that nitrogen may aggregate via in situ annealing processes. Ultimately, understanding mechanisms to improve synthetic single crystal diamond strength will lead to improved tools which utilize such crystals (e.g., drill bits and saws for cutting stone and concrete) which constitute a market of ≈ $2b worldwide.

In this work, the fracture toughness of monolithic pyrolytic carbon and sandwich composites of this carbon on a graphite substrate were measured by a disk-shaped compact specimen and by using a tensile specimen. On this material, a large number of specimens that had small cracks induced by a Vickers diamond indenter were tested under cyclic stress. The existence of a true threshold stress intensity factor range for non-propagation of the cracks was demonstrated. Statistical confidence on pre-cracked specimens was obtained through testing a large number of specimens to a very long lifetime of the order of 109 cycles.

The features of diamond that make it unique are: 1] its efficient tetrahedral structure which is stabilized by resonance of the bonding electrons among adjacent bonds; 2] the exceptionallly high ratios of its shear moduli to its bulk modulus; 3] the large gap between the top of its valence band and the bottom of its conduction band which makes dislocation mobility very low in it; 4] its high surface energy. Although diamond is nearly elastically Isotropic, the ratio of its C44 shear constant to its bulk modulus is 1.3 which is the highest measured value for this ratio. It is quite unusual for it to be greater than unity. This high shear stiffness appears to be associated with the bimodal distribution of electrons along the lengths of the C-C bonds. Also, for centrally directed interatomic forces, the ratio C44/C12is expected to be unity. But In diamond it is 4.6, indicating that the behavior of diamond cannot be described even approximately by pair-potentials. Because of its unique strength diamond can perform exceptionally in mechanical devices such as: pressure vessels, ultracentrifuges, springs, flywheels, and cutting (or drawing) tools.

The fracture toughness, stress corrosion and cyclic fatigue properties of polycrystalline chemical vapor deposited (CVD) diamond have been investigated on thick (˜100 to 300 μm) free-standing films. Specifically, the fracture toughness, Kc, of diamond was determined using indentation methods and for the first time by the tensile testing of pre-notched fracture-mechanics type compact-tension samples. Measured Kc values were found to be between 5 and 7 MPa-m1/2 by either method and to be apparently independent of grain size and shape. Studies on subcritical crack growth (i.e., at stress intensities less than Kc) indicated that CVD diamond is essentially immune to stress-corrosion cracking under sustained loads in room air, water and acid environments. Corresponding experiments to examine susceptibility to cyclic fatigue are currently being performed using indentation-precracked cantilever beams cycled in three-point bending.

Progress made in the field of CVD diamond has enabled to strengthen and diversify mechanical applications of these coatings. Although CVD diamond is presently mainly in the laboratory stage, it is possible to recognize the main applications in the field of tribology as follows: cutting tools, metal working tools, bearings and mechanical components. In these 4 areas diamond films can be beneficial, but there are still several problems related to nucleation, adhesion and surface roughness of the coating, to be solved.

Microwave plasma enhanced chemical vapor deposition was used to grow diamond films with different morphologies and surface roughnesses. With the proper choice of deposition parameters (111) faceted, octahedral, flat (100) and microcrystalline diamond films were obtained. Scanning electron microscopy, atomic force microscopy and stylus profilometry were used to assess the surface topography for each type of film. Raman spectroscopy and x-ray diffraction were also used to determine the purity of the diamond phase and growth orientation of the films, respectively. Single pass friction and wear tests were conducted on each film in order to determine the effect of surface morphology on the coefficient of friction and wear of the counterface materials and/or diamond films. Counterface materials included alumina, tungsten carbide, zirconia, and the (100) face of a synthetic diamond single crystal. Results showed a decrease in the coefficient of friction as the film roughness decreased. Specific wear of the non-diamond counterface materials showed a marked decrease for the flatter and smoother diamond surfaces. For diamond on diamond, the coefficient of friction also decreased as film topography became smoother. Wear of the diamond films occurred by fracture or shearing of asperity tips which was most severe for the rougher films. Control of diamond morphology is shown to be of paramount importance in tribological applications in order to reduce abrasive wear, material transfer, and diamond film fracturing.

The frictional behaviors of 304 stainless steel disks coated with C60 microparticles, both containing benzene and free of benzene, have been studied under different loads and sliding speeds with a pin-on-disk configuration in ambient air atmosphere at room temperature. The results indicated that the coating containing benzene, benzene-solvated C60 microparticles (C60·4C6H6), reduced friction as well as wear. The coated samples showed a 50–70% reduction in friction coefficient in comparison to uncoated samples. Neither the coated nor the uncoated sample showed significant change in friction coefficient for different sliding speeds. Under different loads, the uncoated sample had almost the same friction coefficient. However, with the increase of load, the friction coefficient of C60·4C6H6-coated disk showed a minimum value of 0.25 at 25 g load and then reached the uncoated values beyond 50 g load. The coefficient of friction of the disk coated with benzene-free C60 showed a slight increase with load, reaching the value of uncoated 304 stainless steel disk at about 40 g. The reduced friction of the solvated-C60 coated 304 stainless steel is probably due to the lowered shear strength of the hcp structure of C60·4C6H6 molecular crystal in which the benzene molecules are intercalated. The results of this study suggest the importance of the presence of second component, in addition to C60, in the coating materials in order for them to form a preferred crystal structure with low shear strength as far as using C60 as a solid lubricant is concerned.

Uniform amorphous hydrogenated carbon (a-C:H) films with surface roughnesses ranging between 1 nm and 4 nm were produced by radio frequency self biased plasma enhanced chemical vapor deposition (rf PECVD) on > Silicon substrates using 100% methane precursor gas mixture, rf power densities ranging between 0.11 W/cm2 and 1.07 W/cm2, and pressures ranging between 0.67 Pa and 40 Pa. Reciprocating sliding friction experiments were conducted on the a-C:H films with hemispherical, silicon nitride pins in dry nitrogen and in 60% relative humidity. The coefficients of friction and the wear rates of the a-C:H were very low in dry nitrogen, ranging from 0.03 to 0.05, and from 1.1 × 108 mm3/Nm to 2.3 × 10−6 mm3/Nm, respectively. In 60% relative humidity, the initial coefficients of friction were approximately 0.30. However, the steady state coefficients of friction of the a-C:H films ranged from 0.10 and 0.30, depending on the deposition conditions. The wear rates ranged from 2.0 × 10−9 mm3/Nm to 8.9 x 10−8 mm3/Nm in 60% relative humidity. Raman microprobe spectroscopy and Auger electron spectroscopy (AES) revealed that sliding friction was transforming the a-C:H films into a material primarily composed of sp2 bonded carbon with increasing short range order. Qualitatively, the amount of wear which occurred corresponded to the extent that the structural changes progressed. The a-C:H films were further characterized by scanning electron microscopy (SEM) and surface profilometry.

Diamond exists in many forms including natural diamond, synthetic diamond produced at high temperatures and pressures, polycrystalline diamond composites (PCD's) and most recently chemical vapour deposited (CVD) diamond. The liquid impact behaviour of these materials has been characterised in terms of a single and multiple impact damage threshold velocity which was obtained using a jet technique capable of reaching velocities of 600 m s−1. The multiple impact damage threshold velocity is a highly reproducible parameter which is invaluable in comparing the rain erosion resistances of different materials.

The sand erosion resistances of the various forms of diamond have also been evaluated using a specially designed sand blasting apparatus at velocities up to 270 m s-1. The extent of damage is quantified by mass and transmission loss as a function of impact velocity.

The results of these experiments reveal that the exceptional mechanical properties of diamond cause it to have an equally exceptional response to erosion.

Stable clean and hydrogenated diamond (100) and (111) surface reconstructions found by density-functional molecular-dynamics (DF-MD) are characterized by their vibrational properties. For sufficiently large surface slab supercells we have calculated the surface top-three-layer projected vibrational spectra and reproduced the main features obtained from experiments. The various reconstructions of the diamond surface yield rich spectra of surface modes involving excitations of dimers, trimers and chains. We classify most characteristic surface modes and discuss correlations to peaks observed in surface sensitive experiments. The vibrational spectra represent signatures of the considered surfaces that might be used to understand and to classify as grown diamond surfaces.

The reconstruction of the diamond {1111} surface is re-examined by means of density functional theory based tight-binding molecular dynamics. Evidence is found for competition between a graphitizing tendency leading to an unreconstructed but relaxed 1 × 1 surface and a π-bonded chain-like 2 × 1 reconstruction. The implications of the possible co-existence of these two distinct surface phases for diamond growth are discussed.

In contrast to the form in which other carbon fibers are produced, vapor grown carbon fiber (VGCF) is produced from gas phase precursors in the form of individual fibers of discrete lengths. VGCF can be harvested as a mat of semi-aligned, semicontinuous fibers, with occasional fiber branching and curling. The use of VGCF mats as reinforcement result in composites which exhibit unique microstructure and physical properties that are not observed in other types of carbon composites. This paper describes the processing of VGCF mats reinforced carbon composites, and its unique microstructure and properties. Utilization of fiber tensile properties, as well as thermal conductivity, in the composites is discussed. Comparison of experimental results from various VGCF composites to theory indicates that mechanical properties are more strongly affected by characteristics of VGCF mat than are thermal conductivity. The implications of this relationship favors applications for thermal management where structural demands are less stringent.

We have studied the structural properties of hydrogenated carbon films deposited by plasma enhanced chemical vapor deposition (PECVD). The substrate holder in reaction chamber could be biased and be heated. The Raman peak intensity at 1350 cm−1 was increased by reducing CH4 flow rate. The film structure changed from soft a-C:H to hard carbon with decreasing CH4 flow rate, resulted from increased self-bias. The 1520 cm−1 peak shifts to higher frequency by reducing the CH4flow rate, probably resulted from the increased internal stress.

Tungsten and silicon carbide fibres have been coated with diamond using the HFCVD technique. The diamond volume fraction varied between 26% and 73%. Resonance in bending tests gave a Young's modulus of 880 GPa for the diamond coating. Tensile testing indicated that the diamond fracture strength was between 600 MPa and 2000 MPa, depending on the coating thickness, and thus the grain size, of the diamond. The strain to failure of the diamond coating in bending was approximately 0.15% for 25 μm thick films.

Comparison of behavior observed during deadhesion of CVD diamond films and compositional analysis of their microstructure allows a correlation between microstructure and mechanical behavior to be made. Diamond films were grown in a DC Triple Torch Reactor using a mixture of methane and hydrogen on molybdenum substrates. To address the issue of voids at the interface, a metal binder was electroplated after the nucleation of individual diamond crystals on the substrates. The films were completed by growing a layer of diamond upon the composite layer. Interfacial composition and structure of the films are characterized by a x-ray, Auger, and Raman spectroscopy. A carbide may act to enhance the adhesion via chemical bonding between the substrate and film. Diffusion of the binder into the substrate and film is also important for mechanical properties and is confirmed by x-ray mapping. It is suggested that a diamond like carbon layer acts to enhance the adhesion of the film to the substrate.

The surface properties of diamond-like carbon (DLC) films deposited by a vacuum arc technique on smooth silicon wafers are presented with specific emphasis given to stiction reduction in microelectromechanical systems (MEMS). The low deposition temperatures afforded by the vacuum arc technique should allow for easy integration of the DLC films into the current fabrication process of typical surface micromachines by means of a standard lift-off processing technique. Using X-ray photoelectron spectroscopy (XPS), contact angle analysis, and atomic force microscopy (AFM), the surface chemistry, microroughness, hydrophobicity, and adhesion forces of DLC-coated Si(100) surfaces were measured and correlated to the measured water contact angles. DLC films were found to be extremely smooth and possess a water contact angle of 87°, which roughly corresponds to a surface energy of 22 mJ/m2. It is shown that the pull-off forces measured by AFM correlate well with the predicted capillary forces. Pull-off forces are reduced on DLC surfaces by about a factor of five compared to 10 nN pull-off forces measured on the RCA-cleaned silicon surfaces. In the absence of meniscus forces, the overall adhesion force is expected to decrease by over an order of magnitude to the van der Waals attractive force present between two DLC-coated surfaces- To further improve the surface properties of DLC, films were exposed to a fluorine plasma which increased the contact angle to 99° and lowered the pull-off force by approximately 20% over that obtained with as-deposited DLC. The significance of these results is discussed with respect to stiction reduction in micromachines.