Tohru Arai

Tribological properties and characterization of diamond-like carbon coatings with silicon prepared by plasma-assisted chemical vapour deposition

Abstract Amorphous C-Si coatings containing hydrogen (a-C-Si-H) were formed by a d.c. glow discharge method from reactant gases of CH4, SiCl4, H2, and argon. Coatings with a smooth surface, a hardness of HV 2000 and adhesion to steel substrates were obtained below 550 °C at a deposition rate of 1–4 μm h-1. The ball-on-disk test, against steel with no lubricant in ambient atmosphere, revealed that the friction coefficient μ of the a-C-Si-H coatings changed markedly with the carbon content. μ abruptly decreased from 0.43 to 0.16 at around 60 at.% C (composition excluding hydrogen) and reached a minimum value as low as 0.04 in a range of about 75–85 at.% C content. Laser Raman spectroscopy indicated that the low-friction a-C-Si-H coatings consisted of diamond-like carbon with silicon (DLC-Si). The μ value of 0.04 was below one-third that of DLC coatings without silicon, where μ = 0.12–0.20. The wear of both the DLC-Si and the counter steel ball was very small.

Evaluation of adhesion strength of thin hard coatings

Abstract Extensive research has been carried out in order to study the feasibility of various methods for evaluating the adhesion strength of thin hard coatings, produced by a wide variety of coating methods, to steels. Indentation, scratch, hammering, rolling with slip, and coining and metal stamping tests were employed and some of them were concluded to be of no use for such thin hard coatings which have large adhesion strength. Carbide and nitride coatings produced by high temperature processes such as chemical vapour deposition (CVD) and salt bath immersion showed less failure by all testing methods than those produced by low temperature processes such as physical vapour deposition, low temperature CVD and chromium plating. However, TiN coatings formed by plasma-assisted CVD at low temperature showed less failure than those formed by sputtering and ion plating.

Carbide coating process by use of molten borax bath in Japan

Pore-free and smooth surface carbide layers can be formed on metals and carbon immersed in a molten borax bath above 1073 K. The carbide layers, consisting of VC, NbC, TiC, or Cr7C3, are formed by the reaction between the carbon atoms in the substrate and the carbide forming element atoms dissolved into the fused borax from additive powders such as ironvanadium, ironniobium, iron-titanium, ironchromium, and chromium. Core hardening of steel substrates can be simultaneous or by reaustenitizing after carbide coating. Carbide coated steels show excellent resistance to wear, seizure, corrosion and oxidation. The carbide layers are adherent to the substrate and do not exfoliate in severe service as in cold forming. The process effectively improves performance of dies, tools, machine parts, and is applied in various fields of Japanese industries.

Growth behavior of chromium carbide and niobium carbide layers on steel substrate, obtained by salt bath immersion coating process

Abstract The nucleation and growth behavior, microstructure, and preferred orientation of chromium carbide and niobium carbide coatings formed using a borax bath were studied, focusing on the effects of the carbide species, coating temperature and substrate steel. The growth behavior of these carbides is the same as that of vanadium carbide found in previous research. However, large differences in the morphology, topography, preferred orientation, etc., were found between the carbides species.

Diffusion carbide coatings formed in molten borax systems

Carbide coatings can be formed on carbon containing materials such as steels, nickel and cobalt alloys, cemented carbides, and carbon itself, by immersion in a molten borax based bath, often used as a basis for bonding. Coatings, thus formed, consist of VC, NbC, TaC, TiC, and Cr7C3. They are superior to coatings formed by conventional surface hardening techniques in resistance to wear, seizure, oxidation, and corrosion. Selected additives to a borax bath can produce either a carbide layer, a borided layer (the boride of the main element in the substrate), or no layer at all. An understanding of additive behavior as dictated by the free energies of carbide formation and oxide formation of elements added to molten borax will explain such behavior.Materials carbide coated by the process have excellent tribological, mechanical, and chemical properties, and are very similar to Titanium carbide coatings produced by chemical vapor deposition, in performance in industrial applications. Unlike some conventional surface hardening techniques, the improved surface properties are not achieved at the expense of the mechanical strength; there is no significant reduction in strength between conventionally hardened steel and that coated by this process.

Fatigue strength at a number of cycles of thin hard coated steels with quench-hardened substrates

Abstract Bending fatigue tests were carried out to clarify the effects of thin hard coatings on the fatigue strength at a number of cycles for steels. Coatings of carbides and nitrides produced by thermoreactive deposition and diffusion, chemical vapour deposition and physical vapour deposition, processes and of electroplated chromium on various steels were tested as well as hardened steels. The results thus obtained revealed that the fatigue strength at a number of cycles highly depended on hardness and residual stress at the substrate surface as in the case of the endurance limit and that the existence of the coatings suppressed the cracking in the substrates to increase the fatigue strength after loading cycles. Effects of coating thickness, surface roughness, notch shape, and post-heat treatment were also examined.

Growth behavior of vanadium carbide coatings on steel substrates by a salt bath immersion coating process

Abstract Nucleation and growth behavior, micro-structure, and preferred orientation of vanadium carbide coatings formed using a borax bath were studied, focussing on the effects of coating temperature and substrate steel. The coatings were formed through three stages; nucleation and growth of relatively coarse grains, sub-micron sized grain formation on these, and growth into thick coatings by successive deposition of coarse grains. Preferred orientation was highly influenced by temperature and substrate steel.

Effect of N2-to-TiCl4 flow rate ratio on the properties of TiN coatings formed by d.c. discharge plasma-assisted chemical vapor deposition

Abstract Attempts have been made to clarify the effect of the N 2 -to-TiCl 4 flow rate ratio on properties of TiN coatings which were formed on M2 steel and pure iron at a temperature of 550 by a d.c. glow discharge method from mixed gases of TiCl 4 , N 2 , H 2 and argon. The deposition rate of the coatings saturated at an N 2 -to-TiCl 4 flow rate ratio of 8 to 9. The composition and hardness of the coatings remained unchanged with the N 2 -to-TiCl 4 flow rate ratio except for the maximum ratio of 14.3, whereas the preferred orientation and microstructure changed markedly from (111) to (200), and from columnar to non-oriented. The coatings deposited at an N 2 -to-TiCl 4 flow rate ratio of 3 to 6, which resulted in the (200) preferred orientation and fine grained dense structure, showed greater adhesion strength.

Tool materials and surface treatments

Abstract Cost in metal forging associated mainly with tooling manufacture, tooling maintenance and equipment downtime is successfully reduced by employment of newly developed surface coating methods, namely the thin hard coatings: CVD, PVD and TRD, and new tool materials of which chemical composition and microstructure are well designed for tooling for metal forging. Japanese forging industry has benefitted a great deal from both thin hard coatings and the new tool materials for these ten to twenty years. By the thin hard coating the surface properties of tools are improved to a high level that any further modifications of tool materials themselves in chemical composition and manufacturing method cannot match. Good use of the thin hard coating is indispensable for the future of the metal forging industry.

Effect of excess carbon on the hardness of SiC and TiC coatings formed by plasma-assisted chemical vapor deposition

On etudie egalement les etats de liaison des atomes de carbone en exces dans leurs revetements, par spectroscopie Raman