Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
1999-12-16
2002-11-19
Meeks, Timothy (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S576000, C427S577000, C427S585000, C427S255180, C427S255310, C427S255380, C427S255394
Reexamination Certificate
active
06482476
ABSTRACT:
BACKGROUND OF THE INVENTION
Thanks to their superior mechanical properties such as hardness, low coefficient of friction, thermal stability at high temperature, oxidation resistance, inertness to most common chemicals, and electrical and optical properties, ceramic materials are technologically important materials for many applications, especially for high temperature, high power, electronic barrier coatings, and cutting tool applications. For example, tungsten carbide (WC) including WC-Co alloys, silicon nitride, aluminum oxide, silicon oxide, titanium nitride, and titanium carbide are all very popular materials in the market, particularly in machining business.
As used herein, a “cerarnics material” includes nitrides, oxides, borides, sulfides, carbides, suicides, carbon, hydrogenated carbon materials and the like.
For example, titanium nitride has been exceptionally popular among other ceramic materials. Due to its beautiful golden color, it has been widely used for decoration and marketing purposes. It is also very hard—1770 kg/mm2, stable in air at temperature as high as 650-degC. It is relatively corrosion- and wear-resistant with relatively low friction coefficient, making it very popular in cutting tool industry. Various high temperature PVD and CVD titanium nitride coated cutting/machine tools (such as inserts, drills, saws, punches, dies, molds, pins, knifes), fuel injector components are currently available in the commercial markets. In fact, most of the above components are made of high speed tool steel. The material itself is expensive relative to lower grade steel materials. Taken diesel fuel injector components as an example, the diesel fuel injection operation is carried out at very high pressure, normally at about 150,000 psi (pounds per square inch) for diesel fuel engine. The injection is also carried out at very high speed and in very short cycle time. The excessive failures are resulted due to fatigue from repeated impact, extensive wear and erosion caused by abrasive particles. In order to prolong the life of the fuel injectors, high speed tool steel is commonly used, with extensive post-treatments, including heat treating which increases hardness; tempering to relieve stresses and restore toughness; surface nitriding; and finally coating with titanium nitride. The entire post treatment costs approximately three times as much as the combined cost for material and fabrication process (including finish grounding). The titanium nitride coating alone costs almost half of the total components. The key for cost-reduction is to lower post-treatment cost, especially coating cost.
NITRIDING, CARBURIZING, AND CARBONITRIDING
There are basically two major kinds of nitriding methods: gas phase nitriding and liquid bath nitriding. In the gas phase nitriding process, nitrogen-containing gases, mostly ammonia or nitrogen are used. Under high temperature or high energy radiation, e.g. DC electric, RF radiation, ammonia or nitrogen are dissociated into nitrogen atoms. The highly energetic and active nitrogen atoms reacts with metal surface and diffused into the subsurface to form metal-nitrides.
If hydrocarbon is also introduced along with nitrogen and/or ammonia, the process is then called carbonitriding or nitrocarburizing. In the like manner, surface carburizing is performed with hydrocarbon containing gases.
The other method uses liquid bath—actually a molten nitrogen-containing salt bath. The molten salt reacts with the metal surface to form nitrides, carbides, and oxides.
These nitriding, carburizing and carbonitriding processes may form up to millimeter deep nitride, carbide material over extended period of time.
HEAT TREATMENT OF METAL AND ALLOY MATERIALS
In general, heat treatment refers to a defined process which heats and cools a solid metal or alloy in a controlled manner in order to improve specific properties, such as hardness, strength, ductility, magnetic susceptibility, toughness, machinability, fabricability, and even corrosion resistance. According to this definition, heat treating has long been practiced since ancient metalsmith or blacksmith made bronze tools and iron-based swords. Metals are still heat treated today, for the same reasons as ancient days—namely, to enhance or maximize mechanical properties of the materials. Optimizing hardness, strength, and other mechanical characteristics by heat treating techniques remains as primary interest of many equipment designers. Depends on composition and nature of the materials to be treated and their applications, heat treating may include austenitizing, quench hardening, annealing, tempering, normalizing, austempering, stress reliving, precipitation hardening, sintering, as well as surface enhancement techniques which utilizes thermochemical treatment such as carburizing, carbonitriding, nitriding, nitrocarburizing, etc.
COMMON PRACTICES FOR CRITICAL APPLICATIONS
For applications involving extensive wear, the most common practices today are to chose high grade (relatively expensive) steel as starting material, for example, high speed tool steel which offers better mechanical properties such as a good combination of hardness and toughness. After the components are machined to desired size, shape and surface finish, heat treatment is carried out to harden and temper the parts. In most cases, quench hardening may increase hardness of the treated part significantly, however, it makes the part brittle. Tempering is therefore necessary to increase toughness of the material, however, it sacrifices the hardness. It is difficult to balance between toughness and hardness because a part treated for maximum toughness is often too soft, whereas the same part treated for maximum hardness becomes too brittle. Surface nitriding is therefore employed to further improve the hardness and wear-resistance. Due to the increasing safety and environmental concerns, gas nitriding and plasma nitriding have been widely used today. In the plasma nitriding process, nitrogen-containing gas (nitrogen, ammonia) is dissociated and ionized. The nitrogen ions bombard metal surface, react and diffuse into the subsurface to form nitrided layer. The high energy plasma used in plasma nitriding (or ionitriding) also reduces substrate temperature.
After nitriding, a thin film of titanium nitride is often further applied to increase the hardness of the components. Since current titanium nitride coating techniques require high temperature, all problems associated with this technique, as described in the previous sections, become inevitable.
For certain applications such as machining, ceramic materials are often employed by utilizing the hardness of ceramic materials. For example, tungsten carbide based cutting inserts, drills, dies, and so on are gaining popularity. However, ceramic materials are very difficult to machine. Materials such as tungsten carbide often lack good tribological quality. Therefore, a layer of tribological coating such as TiN or composites would improve wear related performance.
These and other limitations of the prior art methods are addressed by the techniques of the present invention, which do not depend on a high grade material, which do not require fill range of heat treatment, which do not require high temperature and which do not adversely affect mechanical properties of the base materials.
TITANIUM NITRIDE COATING AND TECHNIQUES
The most popular process for titanium nitride coating is PVD method, as described in previous section. Its major limitations include (1) high substrate temperature which anneals substrate and causes dimension change and distortion; (2) complicated equipment and control operation; (3) extended preheating and cooling period; (4) long cycle time; and (5) coating-substrate adhesion is not as good as the coating produced with high temperature CVD method.
Chemical vapor deposition (CVD) has also been developed, though it is not as popular as PVD. A comprehensive review may be found in (F.S. Galasso, Chemical Vapor Deposited Materials, CRC press, FL, (1991)).
Although Musher and Gordon (“Atmospher
Liu Shengzhong Frank
Meeks Timothy
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