Application of CVD diamond (CVDD) to cutting tools is well known, however, sufficient utility in practice has not been achieved yet. The biggest technical problem is delamination caused by strong impact in cutting work piece materials. On the other hand, diamond has many specific properties compared to other hard materials. When used under conditions where strong mechanical impact is induced, CVDD films are expected to show unique behaviors as compared to other conventional hardness films. In order to improve the adhesion of CVDD films, it is very important to reveal the damage processes of CVDD films caused by impact. In this paper, we described the knowledge of damage processes of CVDD films by cyclic impact via observations by a mechanically scanned acoustic microscope. Based on these investigations, we proposed a model for the damage of CVDD films. That is, the damage of CVDD films is triggered by the occurrence of cracks. Extension of cracks to the top surface and delamination around the cracks are gradually extended, and lastly CVDD films are peeled off along the cracks from the substrate.
Nitrogen containing stainless steel coatings were deposited on cemented carbide substrates by arc ion plating at various nitrogen partial pressures using SUS 304 (AISI 304) and SUS 430 (AISI 430) targets. The effect of nickel and nitrogen content on the crystal structure and mechanical properties of coatings were studied. The SUS 304 coatings consisted of nitrogen-supersaturated fct-γ phase with nitrogen content up to about 35at%. Additionally, fine grained Fe3N (ε) phase started to precipitate at nitrogen content above approximately 20at%. The SUS 430 coatings mainly consisted of (Cr, Fe)2N1-x(ε') phase. The hardness of the coatings increased uniformly with the nitrogen content and a maximum hardness value of Hv 1100 was obtained at a nitrogen content of about 35at%. This increase in hardness can be due to the lattice distortion caused by nitrogen atoms at interstitial sites in supersaturated Fe-N solid solution and the precipitation of fine grained nitrides. The tipping resistances of SUS 304 coatings which were determined by scratch tests showed a constant value of more than 200N, independent of the nitrogen content and the precipitation of ε phase. Contrary to this, those of SUS 430 coatings were approximately 20N, which can be due to the formation of ε' phase. Nickel may suppress the formation of ε' phase and contribute to the superior toughness observed. The nitrogen-supersaturated γ phase deposited by arc ion plating is considered to have a structure similar to that of the γN phase deposited by sputtering and YN phase by plasma nitriding.
When diamond is used as a wear-resistant material for tools, its unique wear resistance cannot be fully exploited due to the possibility of brittle fracture. In general, diamond films synthesized by the vapor phase method are polycrystalline, exhibiting columnar crystal growth. In the polycrystalline structure, once cracks occur on the surface of the film, they tend to propagate through the columnar particles, leading to a decrease in toughness. In this study, using the hot-filament chemical vapor deposition (CVD) method, diamond secondary nuclei are grown on a substrate by applying bias current to the substrate repeatedly and intermittently; multi-layered diamond films, in which the continuity of grain growth is suppressed, are synthesized. The interfaces formed by the multi-layer structure are expected to prevent crack propagation. To confirm this effect, mechanical characteristics, such as the bending strength of the multi-layer diamond film, are evaluated. The results indicate that the bending strength of the multi-layer diamond film is approximately 30% higher than that of a conventionally-produced diamond film.
The effects of ion-energy values on mechanical properties, including frictional durability, nano-indentation hardness and micro-wear, of nitrogen-ion-beam irradiated DLC (Diamond-Like-Carbon) films at various energies have been investigated. These properties of the films were examined by means of the oscillating-type minute scratch test, and the nano-indentation test together with the micro-scratch test utilizing an atomic force microscope. The obtained results are summarized as follows: (1) The result obtained by the minute scratching test shows that the critical load at which film break out occurs tends to increase with ion-beam irradiation, that is, the frictional durability is improved. The degree of this improvement depends on the ion energy value, and the load increase with increasing ion energy. (2) The nano-indentation hardness of irradiated films does not show a remarkable difference in comparison with that of the film without irradiation under these irradiating conditions. (3) The result obtained by the micro-scratch test shows that micro-wear tends to decrease with ion-beam irradiation. The micro-wear depth depends on the ion energy value, and the depth decreases with increasing energy.
Diamond like carbon (DLC) films were deposited on various polymers by magnetron type RF sputtering, and the effects of DLC coating on frictional properties were investigated by a ball on disk type tribometer. DLC coatings on high stiffness and heat resistant polymers such as LCP (liquid crystal polymer), PPS (polyphenylene sulfide) and PES (polyether sulfone) reduce the friction in both dry and water lubrication conditions. Especially, DLC films coated on PES and PPS decrease the friction coefficient to one fifth and one third to those of untreated polymers, respectively, in dry conditions. In water lubrication, DLC film coating can decrease the friction coefficient to less than a half of that of untreated polymers. DLC films coated LCP, PPS and PES show the low friction variation in both dry and water lubrication conditions. DLC film coating on PES decreases the wear to an undetectable level in both dry and water lubrication conditions.
The hardness and microstructure of Ti-Si-N films were investigated. The hardness of the films increased with the increasing Si content of the films. The maximum hardness of more than Hk 4500 was obtained within the range of 0.3 to 0.5 of Si/(Si+Ti) ratio. The X-ray diffraction peaks of TiN in the Ti-Si-N films became broad with the Si content of the films. The TiN crystallite sizes calculated from the FWHM values of the TiN diffraction peaks decreased with the increment of the Si content of the films. It became clear that when the Si/(Si+Ti) ratio of the films was more than 0.1, TiN crystallite sizes decreased to less than 6nm. The thin amorphous grain boundaries of Si-N and the TiN crystallites at a size of about 5nm were observed by high resolution transmission electron microscopy. It was assumed that the high degree of hardness of the Ti-Si-N nanocomposite films was due to a halt of the propagation of dislocations by an amorphous phase on the TiN grain boundaries in addition to nano-scale TiN crystallites which was brought about by inhibition of the crystal growth of the TiN by an amorphous Si-N phase.
Thin films of amorphous carbon (a-C) and its nitride (a-C: N) have been prepared by shielded arc ion plating. Their tribological properties were evaluated based on the ball-on-disk method using a SUJ 2 ball. Compared with TiN, both a-C and a-C: N films showed excellent wear-resistivities. Frictional coefficients of the a-C and a-C: N films were almost half of that of the TiN film. Furthermore, wear of the SUJ 2-balls with the films were much smaller than that with the TiN film. The a-C and a-C: N films are promising for tribological coatings. Although, difference in wear-resistivity between the a-C and a-C: N films was negligible, the a-C: N films less wore the SUJ 2-balls than the a-C films did. This is probably due to a higher chemical reactivity of a-C to Fe than that of a-C: N.